Abstract

Measurements of methyl chloride (CH3Cl) in Antarctic polar ice and firn air are used to describe the variability of atmospheric CH3Cl during the past 300 years. Firn air results from South Pole and Siple Dome suggest that the atmospheric abundance of CH3Cl increased by about 10% in the 50 years prior to 1990. Ice core measurements from Siple Dome provide evidence for a cyclic natural variability on the order of 10%, with a period of about 110 years in phase with the 20th century rise inferred from firn air. Thus, the CH3Cl increase measured in firn air may largely be a result of natural processes, which may continue to affect the atmospheric CH3Cl burden during the 21st century.

Abstract

The extraction and analysis of air from the snowpack (firn) at the South Pole provides atmospheric concentration histories of biogenic greenhouse gases since the beginning of the present century which confirm and expand on those derived from studies of air trapped in ice cores. Furthermore, calculations based on the inferred atmospheric concentrations of oxygen and carbon dioxide indicate that–in contrast to the past few years—the terrestrial biosphere was neither a source nor sink of C02 between ~1977 and 1985.

Abstract

Before ice core chemistry can be used to estimate past atmospheric chemistry it is necessary to establish an unambiguous link between concentrations of chemical species in the air and snow. For the first time a continuous long?term record of aerosol properties (aerosol light scattering coefficient, ? sp , and Ångström exponent, å) at the South Pole are compared with the chemical record from a high resolution firn core (?10 samples per year) covering the period from 1981 to 1991. Seasonal signals in å, associated with winter minima due to coarse mode seasalt and summer maxima due to accumulation mode sulfate aerosol, are reflected in the firn core SO4 2?/Na+ concentration ratio. Summertime ratios of ? sp and aerosol optical depth, ? to corresponding firn core sulfur concentrations are determined and the ‘calibrations’ are applied to sulfur concentrations in snowpits from a previous study. Results show that ? sp estimates from snowpit sulfur concentrations are in agreement with atmospheric measurements while ? estimates are significantly different, which is likely due to the lack of understanding of the processes that mix surface air with air aloft.

Abstract

[ 1] The Dobson spectrophotometer is the primary standard instrument for ground-based measurements of total column ozone. The accuracy of its data depends on the knowledge of ozone absorption coefficients used for data reduction. We document an error in the calculations that led to the set of absorption coefficients currently recommended by the World Meteorological Organisation (WMO). This error has little effect because an empirical adjustment was applied to the original calculations before the coefficients were adopted by WMO. We provide evidence that this adjustment was physically sound. The coefficients recommended by WMO are applied in the Dobson network without correction for the temperature dependence of the ozone absorption cross sections. On the basis of data measured by Dobson numbers 80 and 82, which were operated by the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory at the South Pole, we find that omission of temperature corrections may lead to systematic errors in Dobson ozone data of up to 4%. The standard Dobson ozone retrieval method further assumes that the ozone layer is located at a fixed height. This approximation leads to errors in air mass calculations, which are particularly relevant at high latitudes where ozone measurements are performed at large solar zenith angles (SZA). At the South Pole, systematic errors caused by this approximation may exceed 2% for SZAs larger than 80°. The bias is largest when the vertical ozone distribution is distorted by the ``ozone hole'' and may lead to underestimation of total ozone by 4% at SZA = 85° ( air mass 9). Dobson measurements at the South Pole were compared with ozone data from a collocated SUV-100 UV spectroradiometer and Version 8 overpass data from NASA's Total Ozone Mapping Spectrometer ( TOMS). Uncorrected Dobson ozone values tend to be lower than data from the two other instruments when total ozone is below 170 Dobson units or SZAs are larger than 80°. When Dobson measurements are corrected for the temperature dependence of the ozone absorption cross section and accurate air mass calculations are implemented, data from the three instruments agree with each other to within +/- 2% on average and show no significant dependence on SZA or total ozone.

Abstract

Atmospheric monitoring stations are operated at Barrow, Alaska; Mauna Loa, Hawaii; American Samoa; and South Pole by the Geophysical Monitoring for Climatic Change program to measure the characteristics of gaseous and aerosol species under background conditions. A nearly continuous record of light-scattering coefficient and condensation nuclei concentration measurements is available for Barrow since 1971, Mauna Loa since 1974, Samoa since 1977, and South Pole since 1974. The Barrow light-scattering data exhibit a strong annual cycle with a maximum in winter and spring (the Arctic haze) and a minimum in summer. The Barrow condensation nuclei data exhibit a strong semiannual cycle with a maximum coinciding with that of light scattering and an additional maximum about August. The Mauna Loa light-scattering data show a strong annual cycle with a maximum in April or May caused by long-range transport of Asian desert dust. The Mauna Loa condensation nuclei data show no significant annual cycle. The Samoa light-scattering and condensation nuclei data are representative of a clean marine atmosphere and exhibit no significant annual or diurnal cycle. The South Pole light-scattering data show a complicated annual cycle with a maximum in the austral summer and a minimum about April. The austral winter is dominated by events most likely caused by the transport of sea salt in the troposphere from the coastal regions to the interior of the Antarctic continent. The South Pole condensation nuclei data show a repeatable annual cycle with a maximum in the austral summer and a minimum in the austral winter. Linear least squares trend analyses show no significant trend compared to the standard error about the regression line at any station.

Abstract

Aerosol absorption (?ap) has been measured continuously using aethalometers at Barrow, Alaska (1986 to present); Mauna Loa, Hawaii (1990 to present); and south pole, Antarctica (1987–1990). These three stations are part of a network of baseline monitoring stations operated by the Climate Monitoring and Diagnostics Laboratory (CMDL) of the National Oceanic and Atmospheric Administration (NOAA). Condensation nucleus (CN) concentration and multiwavelength aerosol scattering (?sp) have also been measured continuously for many years at these stations. Aethalometer measurements are usually reported in terms of atmospheric black carbon aerosol (BC) concentration using the calibration suggested by the manufacturer. Here we deduce the in situ ?ap(550 nm) from aethalometer measurements by assuming that the aerosol absorption on the aethalometer filter is enhanced by a factor of 1.9 over that in the atmosphere. This is consistent with using 19 m2 g?1 for the specific absorption of BC on the aethalometer filter and 10 m2 g?1 for the in situ specific absorption of BC in the atmosphere (the ratio of the two specific absorptions is 1.9). Although these values of specific absorption may vary significantly for different environments, the ratio might be expected to be relatively constant. The single-scattering albedo, defined by ? = ?sp/(?sp + ?ap), has been estimated from the simultaneous measurements of ?ap and ?sp. Furthermore, assuming a 1/? dependence for ?ap in the 450 to 700-nm wavelength region, multiwavelength ?sp measurements allow the estimation of the wavelength dependence of ?. Each station shows a considerable annual cycle in ?ap, ?sp, and ?. The maximum in the Barrow annual cycle is caused primarily by the springtime Arctic haze phenomenon; the maximum in the Mauna Loa annual cycle is caused by the springtime Asian dust transport; and the maximum in the south pole annual cycle is caused by late winter transport from southern midlatitudes. It was found that annual mean values are ?ap = 4.1 × 10?7 m?1 (?41 ng m?3 BC) and ? = 0.96 for Barrow; ?ap = 5.8 × 10?8 m?1 (?5.8 ng m?3 BC) and ? = 0.97 for Mauna Loa; and ?ap = 6.5 × 10 ?9 m?1 (?0.65 ng m?3 BC) and ? = 0.97 for south pole. It was also found that the wavelength dependence of ? may be important at Barrow and south pole, but not important at Mauna Loa.

Abstract

Some results are given regarding the aerosol measurement program conducted by the NOAA at their atmospheric monitoring observatory at Amundsen-Scott Station, South Pole. The program consists of the continuous measurement of condensation nuclei (CN) concentration and aerosol scattering extinction coefficient. A time series of sodium, chlorine, and sulfur concentrations shows that the sulfur and CN records are similar and that the sodium, chlorine, and extinction coefficient records are similar. Large episodes of sodium are measured at the ground in the austral winter and are apparently caused by large-scale warming and weakening of the surface temperature inversion. The CN data show an annual cycle with a maximum exceeding 100 per cubic centimeter in the austral summer and a minimum of about 10 per cubic centimeter in the winter. The extinction coefficient data show an anual cycle markedly different from that of CN with a maximum in late winter, a secondary maximum in summer, and a minimum in May.

Abstract

The Geophysical Monitoring tor Climatic Change (GMCC) program of the National Oceanic and Atmospheric Administration (NOAA) operates an atmospheric monitoring observatory at Amundsen-Scott Station, South Pole. Long-term measurements of carbon dioxide, ozone, aerosols, and other background pollutants are obtained to understand their possible effects on the earth's climate. The aerosol measurement program consists of the continuous measurement of condensation nucleus (CN) concentration and aerosol scattering extinction coefficient (?sp). During 1982 Nuclepore-filter aerosol samples were taken with 8-h time resolution for analysis by the proton induced X-ray emission (PIXE) technique. A time series of sodium, chlorine, and sulfur concentrations shows that the sulfur and CN records are similar and that the sodium, chlorine, and ?sp records are similar. Episodes of sodium are measured at the ground in the austral winter and are apparently caused by large-scale transport from coastal regions and vertical transport to the surface during times of surface warming and weakening of the surface temperature inversion. These episodes are characterized by increases in sodium concentration, CI/Na ratio, ?sp, and particle size, and decreases in nonseasalt sulfur concentration, suggesting a decrease in atmospheric acidity with a displacement of sulfuric acid aerosol. An analysis of back trajectories suggests transport times of several days from the Antarctic coast.

Abstract

A four wavelength nephelometer was installed at South Pole station in January 1979 in order to measure the volumetric aerosol scattering coefficient during the austral winter of 1979. The monthly mean aerosol light scattering at 550 nm wavelength for May was 1.23×10?7m?1, the lowest monthly mean value ever recorded at any GMCC station. This implies a total aerosol mass loading of 39 ng m?3, in agreement with aerosol mass measurements at South Pole station. Condensation nuclei concentrations in 1979 range between monthly means of 185 cm?3 in February and 10 cm?3 in July.

Abstract

NOAA's Geophysical Monitoring for Climatic Change program has been operating Pollak photoelectric and General Electric automatic condensation nuclei counters at the South Pole since 1974. The G.E. counter is a modified version of the older Cat. No. 112L428G1 and has shown good stability with sensitivity to below 10 nuclei cm?3. The data are broken into time periods corresponding to intervals where the G.E. counter was operational and the calibration remained unchanged. G.E.—Pollak data pair calibration points are generated using a variability criterion determined from the raw G.E. counter one minute voltage means. The scaling equations are found by using a linear least squares analysis. The coefficients of the scaling equations have a minimum of a 90% confidence level and in most cases, the level is substantially higher. The equations, also, normally have a minimum multiple correlation coefficient of at least 0.9, where a value of 1.0 would signify that the linear least squares analysis has given a perfect fit of G.E. counter data to the Pollak observations. The regression equations are then applied to the G.E. counter hourly data to present the results in their final form for archiving.

Abstract

This paper presents a comparison of the stratospheric water vapor measurements made by the satellite-borne sensors the Stratospheric Aerosol and Gas Experiment II (SAGE II), the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS), and the Spacelab 3 Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. LIMS obtained data for 7 months between November 1978 and May 1979; ATMOS was carried on Shuttle and observed eight profiles from April 30 to May 6, 1985 at approximately 30°N and 50°S; and, SAGE II continues to make measurements since its launch in October 1984. For both 30°N and 50°S in May, the comparisons between SAGE II and ATMOS show agreement within the estimated combined uncertainty of the two experiments. Several important features identified by LIMS observations have been confirmed by SAGE II: a well-developed hygropause in the lower stratosphere at low- to mid-latitudes, a poleward latitudinal gradient, increasing water vapor mixing ratios with altitude in the tropics, and the transport of dry lower stratospheric water vapor upward and southward in May, and upward and northward in November. A detailed comparative study also indicates that the two previously suggested corrections for LIMS, a correction in tropical lower stratosphere due to a positive temperature bias and the correction above 28 km based on improved emissivities will bring LIMS measurements much closer to those of SAGE II. The only significant difference occurs at high southern latitudes in May below 18 km, where LIMS measurements are 2–3 ppmv greater. It should be noted that LIMS observations are from 16 to 50 km, ATMOS from 14 to 86 km, and SAGE II from mid-troposphere to 40 km. With multiyear coverage, SAGE II observations should be useful for studying tropospheric-stratospheric exchange, for stratospheric transport, and for preparing water vapor climatologies for the stratosphere and the upper troposphere.

Abstract

The distribution and variations of atmospheric CO2 from 1981 to 1992 were determined by measuring CO2 mixing ratios in samples collected weekly at a cooperative global air sampling network. The results constitute the most geographically extensive, carefully calibrated, internally consistent CO2 data set available. Analysis of the data reveals that the global CO2 growth rate has declined from a peak of ~2.5 ppm yr^-1 in 1987-1988 to ~0.6 ppm yr^-1 in 1992. In 1992 we find no increase in atmospheric CO2 from 30° to 90°N. Variations in fossil fuel CO2 emissions cannot explain this result. The north pole-south pole CO2 difference increased from ?3 ppm during 1981-1987 to ~4 ppm during 1988-1991. In 1992 the difference was again ~3 ppm. A two-dimensional model analysis of the data indicates that the low CO2 growth rate in 1992 is mainly due to an increase in the northern hemisphere CO2 sink from 3.9 Gt C yr^-1 in 1991 to 5.0 Gt C yr^-1 in 1992. The increase in the north pole-south pole CO2 difference appears to result from an increase in the southern hemisphere CO2 sink from ~0.5 to ~1.5 Gt C yr^-1.

Abstract

Because 90% of the CO2 from fossil fuel combustion is emitted in the Northern Hemisphere, annual mean atmospheric CO2 mixing ratios are higher at middle and high northern latitudes than in the Southern Hemisphere. The observed CO2 latitude gradient varies interannually and has generally increased as fossil fuel CO2 emissions have increased. Back extrapolation of the measured CO2 latitude gradient to zero fossil fuel emissions gives a latitude gradient with the Northern Hemisphere lower than the Southern. A linear regression of Mauna Loa minus South Pole annual mean differences versus fossil fuel emissions for 1958 through 1996 gives a slope of 0.5 ?mol mol?1 (abbreviated as ppm CO2) (Gt C)?1 (? = 0.03) and an intercept (at zero fossil fuel emissions) of ?0.8 ppm (? = 0.2). Shorter data records yield similar results with larger uncertainties. We argue that this extrapolated gradient does not represent preindustrial conditions but is more correctly viewed as a decadal average gradient due to natural sources and sinks that underlie the anthropogenic perturbation. We interpret the extrapolated gradient as evidence for a contemporary Northern Hemisphere sink that has been proposed on the basis of other measurement and model approaches. The slopes (ppm CO2 per gigaton of C from fossil fuel burning) calculated from sufficiently long records tend to agree with model calculations based on fossil fuel emissions, suggesting that any trend in the Northern Hemisphere sink, during the period of the measurements, has been small relative to the trend in fossil fuel emissions.

Abstract

Observations of OH, NO, and actinic flux at the South Pole surface during December 1998 suggest a surprisingly active photochemical environment which should result in photochemical production of ozone. Long?term South Pole in situ ozone data as well as sonde data also appear to support this conclusion. Other possible factors contributing to ozone variability such as stratospheric influence and the origin of air transported to the South Pole are also explored. Based on box model calculations it is estimated that photochemistry could add 2.2 to 3.6 ppbv/day of ozone to surface air parcels residing on the Antarctic polar plateau. Although the oxidizing potential of the polar plateau appears to be exceptionally high for a remote site, it is unlikely that it has a significant impact on surrounding regions such as the Southern Ocean and the Antarctic free troposphere. These new findings do suggest, however, that the enhanced oxidizing power of the polar plateau may need to be considered in interpreting the chemical history of climate proxy species in ice cores.

Abstract

[1] Time series of cloud solar transmission and cloud occurrence frequency are developed for the past 27 years at four globally remote and climatically diverse surface locations. A new methodology is developed that objectively segregates times of cloud-free conditions from those times when clouds are detected in high-time-resolution total solar irradiance observations that are obtained from pyranometers. The methodology for cloud detection depends on the magnitude and short-term variability of observed departures from clear-sky conditions. Expected clear-sky irradiances are based on interpolated clear-sky observations. Results of the new cloud detection methodology are compared to four independent cloud detection methods. An effective cloud transmission is determined as the ratio of observed irradiance in the presence of clouds to that expected in the absence of clouds. Selective forward scattering by clouds toward the observation site results in computed effective transmissions frequently being >1.0. It is shown that conditional temporal averaging of effective cloud transmission over periods of three days or more virtually eliminates the unrealistic cloud transmissions exceeding 1.0. Such temporal averaging of the surface measurements is advantageous for comparing against other area-wide cloud transmission estimates, such as those determined from satellite or by numerical climate models. The cloud occurrence frequency and the effective solar transmission for long-term observational records are summarized into monthly and annual averages, and their long-term variability is investigated. Temporal variations in frequency distributions of transmission are used to determine which clouds are responsible for changes in mean cloudiness. A statistically significant upward trend in cloud occurrence frequency, from 76 to 82% between 1976 and 2001, is detected at Barrow, Alaska, where clouds having solar transmission near 0.2 exhibit the largest increase. At the South Pole, decadal timescale oscillations in both cloud characteristics are detected, but no particular cloud category is identified as the source of that oscillation.

Abstract

Incoming global solar irradiance measured at the surface at the South Pole unexpectedly decreased steadily by 15% from 1976 through 1987 during the late austral summer season, whereas no trend is apparent for September through December. February's irradiance trend, ? 1.24% yr?1 on the average, is statistically significant at greater than the 99.9% confidence level. The irradiance observations were made continuously with the same calibrated sensor and are confirmed by a second simultaneous solar irradiance measurement series. Associated changes in seasonal sky cover (clouds) and surface air temperature were also observed. Seasonally increasing cloud cover is directly associated with the decreasing irradiance trends, whereas temperatures show a warming trend significant only in March, followed by a cooling trend significant only in May. Cloudiness and temperature records for 32 years suggest that the observed cloudiness trend began in the late 1970s, while the temperature trends become apparent only in the early 1980s. The observed sensitivity of total global solar irradiance to the change in sky cover is roughly six percent per one-tenth and is shown to vary spectrally. Although the annual averages of solar irradiance at the South Pole display an overall decrease between 1976 and 1989, the most recent years in this period show some recovery from earlier declines. Likewise, the downward trends in January and February irradiance diminished in 1988 and 1989.

Abstract

Associations between the season-to-season changes in CO2 concentration and the sea-surface temperature in the eastern equatorial Pacific, the tropospheric air temperature, and the precipitation in the tropics are explored. The CO2 records at Mauna Loa and the South Pole from the Scripps Institution of Oceanography and the GMCC/NOAA program, as well as the GMCC records at Barrow, Alaska and American Samoa were used after the annual cycle and the growth due to fossil fuel emission has been removed. We find that the correlation between CO2 changes and each of the other variables changes with time. In particular, the period from about 1968 to about 1978 was the period of highest correlation, which was also the period when the climate variables were best correlated with each other. The air temperature and the precipitation were as well correlated with CO2 changes as was SST. Also, there are individual seasons when the CO2 changes are much better correlated with the climate variables than at other seasons. Furthermore, El Niño events, while the source of the largest signal in the CO2 record, are by no means the same from one event to the next. We take these results as further confirmation that the apparent effect of SST on the CO2 record comes less from changes in the equatorial eastern Pacific Ocean than from climate changes throughout the globe. Climate effects on the terrestrial biosphere seem a likely source of much of the interannual variation in atmospheric CO2.

Abstract

Polar stratospheric clouds were frequently observed by lidar at the Amundsen?Scott South Pole Station during May–October 1988. The dependence of the bakscattering cross section on the temperature can be referred to transitions of the HNO3/H2O system: it appears possible to distinguish the pure trihydrate from the mixed ice?trihydrate phase in the composition of the aerosol and, in some cases, to bracket the HNO3 and H2O content of the ambient gas, and to provide indications on the size of the particles.

Abstract

SINCE the discovery of the springtime Antarctic ozone depletion1–2 a great deal of attention has been given to processes related to aerosol formation and heterogeneous chemistry at low tem-peratures. Polar stratospheric cloud (PSCs),3 are frequent features of the southern winter atmosphere, and may provide a site for heterogeneous reactions that lead to ozone destruction4,5. Models have been proposed6–8 for the formation of PSCs based on the condensation of HNO3 (see ref. 9 for a review). An optical radar (lidar) has been operated at Amundsen Scott (South Pole), Antarctica, since the austral summer 1987–88. Observations made during the 1988 polar night show the presence of PSCs. Here we report ozonesonde measurements made quasi-simultaneously at the South Pole which indicate sharp minima of the ozone concentration in the vicinity of the PSCs. Although definitive information is unavailable for unambiguous interpretation of the results, the data may be viewed as evidence either for the role of dynamics in transporting air of different composition in conditions of substantial stability, or for processes leading to ozone destruction during the polar winter; the latter may include heterogeneous chemical reactions taking place in the absence of photolysis.

Abstract

An optical radar—lidar—has been operational at the Amundsen-Scott South Pole Station since summer 1987–1988. The observations were specially directed to the detection of aerosol layers and polar stratospheric clouds (PSCs). The lidar utilized a Nd-YAG laser followed by a second harmonic generator, and a 0.5-m diameter Cassegrain receiving telescope. Results obtained during the period May-October 1988 are summarized. Some 10,000 profiles of the lidar echoes, each the result of 1-min averaging, were obtained. Data sets consisting of profiles of the scattering ratio and of the backscattering cross section B a , based on half-hour averaging, are presented. The data can be related to profiles of the atmospheric temperature T, usually obtained on a daily basis at South Pole. Stratifications appear to have two distinct types of structures: one structure shows only a modest variation with height; the other is characterized by sharp features, with large changes of the cross section with height. The basic results, the relationship between B a and T, and their statistical relevance are considered in this paper. The microphysical interpretation, the attribution of these structures to PSC Type I and Type II, respectively involving the condensation of nitric acid trihydrate and of water ice, and the seasonal evolution of the phenomena are treated in a companion paper.

Abstract

This report provides information on a potentially important new atmospheric tracer of large-scale behaviour at the Earth's surface, the oxygen isotope composition of CO2. We use measurements on flask air collected from Alaska, Hawaii, Samoa, Tasmania, coastal Antarctica and the South Pole. Recently, we examined 1982–84 measurements of 18O in CO2 extracted in situ from marine air at Cape Grim, Tasmania1. Here we report on a comparison of Cape Grim flask and in situ data that gives a measure of precision and serves to demonstrate a marked improvement over previous infrequent measurements. While previous data2,3 suggests a north-south gradient, our flask data establish a strong, asymmetric meridional gradient. We argue that this reflects the oxygen isotope ratio in ground water, via mechanisms involving gross catalysed CO2 exchange with leaf (and possibly soil) water. Very large CO2 fluxes are involved, of the order of 200 Gt carbon (C) yr-1.

Abstract

Hourly carbon dioxide concentrations at the south pole were obtained by nondispersive infrared analyzers for the years 1975–1978 and 1980–1982. A spectral analysis of the ambient CO2 variability showed very little power for periods shorter than 5 days. Our data showed good agreement with other data sets for the range of the annual fluctuation from 1977 to 1982 and disagreements for 1976. The estimated annual CO2 increase (about 0.6 to 2 ppm yr?1) and ranges of seasonal fluctuation were insensitive to the data selection methods used. After 1979, seasonal fluctuations apparently decreased.

Abstract

Abstract. We measured the global distribution of tropospheric N2O mixing ratios during the NASA airborne Atmospheric Tomography (ATom) mission. ATom measured concentrations of ∼ 300 gas species and aerosol properties in 647 vertical profiles spanning the Pacific, Atlantic, Arctic, and much of the Southern Ocean basins, nearly from pole to pole, over four seasons (2016–2018). We measured N2O concentrations at 1 Hz using a quantum cascade laser spectrometer (QCLS). We introduced a new spectral retrieval method to account for the pressure and temperature sensitivity of the instrument when deployed on aircraft. This retrieval strategy improved the precision of our ATom QCLS N2O measurements by a factor of three (based on the standard deviation of calibration measurements). Our measurements show that most of the variance of N2O mixing ratios in the troposphere is driven by the influence of N2O-depleted stratospheric air, especially at mid- and high latitudes. We observe the downward propagation of lower N2O mixing ratios (compared to surface stations) that tracks the influence of stratosphere–troposphere exchange through the tropospheric column down to the surface. The highest N2O mixing ratios occur close to the Equator, extending through the boundary layer and free troposphere. We observed influences from a complex and diverse mixture of N2O sources, with emission source types identified using the rich suite of chemical species measured on ATom and the geographical origin calculated using an atmospheric transport model. Although ATom flights were mostly over the oceans, the most prominent N2O enhancements were associated with anthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomass burning, especially in the tropical Atlantic outflow from Africa. Enhanced N2O mixing ratios are mostly associated with pollution-related tracers arriving from the coastal area of Nigeria. Peaks of N2O are often associated with indicators of photochemical processing, suggesting possible unexpected source processes. In most cases, the results show how difficult it is to separate the mixture of different sources in the atmosphere, which may contribute to uncertainties in the N2O global budget. The extensive data set from ATom will help improve the understanding of N2O emission processes and their representation in global models.

Abstract

In December 1986 an aethalometer was installed at the NOAA/GMCC South Pole Observatory to measure concentrations of the combustion effluent tracer species aerosol black carbon (BC) with a time resolution of one hour. We present data covering a 1?yr period from December 1986 through November 1987. The hourly data show infrequent events in which the concentrations increased greatly for periods of a few hours. We attribute these events to local contamination and identified them as such in the database. The remaining background data then yield daily average BC concentrations generally ranging from 50 pg m?3 to 5 ng m?3, with a minimum in the early austral winter. The results imply long?range transport of this aerosol species, and suggest a minimum value of the order of 10 pg m?3 for its global background concentration.

Abstract

An analysis of 5-day midtropospheric flow patterns for the South Pole during 1985-1989 is presented. Cluster analysis was used to summarize trajectories by year and by month. The results indicate that flow from the east was most often anticyclonic and light, occurring 8-18% of the time. Westerly flow patterns were the strongest and most frequent (37-51% occurrence). They were consistently cyclonic, usually reflecting storms in the Ross Sea area, the average center of the circumpolar vortex. Strong northerly flow occurred more often in 1987 than in other years. Year-to-year variability was also evident in southwesterly flow, which was enhanced in 1988, and weaker in 1987, compared with other years. The lightest winds over the South Pole occur during January, while the most vigorous long-range transport to South Pole occurs from July through October. Selected isentropic trajectories were examined to determine errors inherent in the isobaric estimates. Isentropic trajectories from the east showed little vertical motion and good agreement with isobaric ones. Over west Antarctica, however, isentropic trajectories consistently showed positive vertical motion. As a result, their isobaric counterparts were too long and overestimated the cyclonic curvature in the flow. Preferred transport from the west with warm-air advection results from the circumpolar vortex being asymmetrical, and the average isotherms, though roughly circular, being offset to the east of the South Pole.

Abstract

In 2010, 25 years of regular, year-round ozone soundings at South Pole station, Antarctica, were completed. These measurements provide unique information about the seasonality, trends, and variability of ozone depletion in the polar stratosphere at high vertical resolution. Here, we focus on the observed loss rates, and their changes since the measurement series began. The fastest loss rates occur between the end of August and end of September between 50 hPa and 30 hPa. Loss rates at these pressure levels increased by approximately 40% from the late 1980s to the late 1990s and have remained stable within estimated uncertainties since then. To estimate the time frame when a reduction in ozone loss rates will be observable outside the range of dynamical variability at the South Pole, we scale the estimated loss rates to the future projected concentrations of equivalent effective stratospheric chlorine (EESC). If a linear relationship between ozone loss rates and EESC is assumed, we project that a change in lower stratospheric ozone loss rates at South Pole station will be first detectable in the 2017–2021 time period.

Abstract

Surface ozone records from ten polar research stations were investigated for the dependencies of ozone on radiative processes, snow-photochemisty, and. synoptic and stratospheric transport. A total of 146 annual data records for the Arctic sites Barrow, Alaska; Summit, Greenland; Alert, Canada; Zeppelinfjellet, Norway; and the Antarctic stations Halley, McMurdo, Neumayer, Sanae, Syowa, and South Pole were analyzed. Mean ozone at the Northern Hemisphere (NH) stations (excluding Summit) is similar to 5ppbv higher than in Antarctica. Statistical analysis yielded best estimates for the projected year 2005 median annual ozone mixing ratios, which for the Arctic stations were 33.5 ppbv at Alert, 28.6 ppbv at Barrow, 46.3ppbv ppb at Summit and 33.7ppbv at Zeppelinfjellet. For the Antarctic stations the corresponding ozone mixing ratios were 21.6 ppbv at Halley, 27.0 ppbv at McMurdo, 24.9 ppbv at Neumayer, 27.2 ppbv at Sanae, 29.4 ppbv at South Pole, and 25.8 ppbv at Syowa. At both Summit (3212m asl) and South Pole (2830m asl), annual mean ozone is higher than at the lower elevation and coastal stations. A trend analysis revealed that all sites in recent years have experienced low to moderate increases in surface ozone ranging from 0.02 to 0.26 ppbv yr(-1), albeit none of these changes were found to be statistically significant trends. A seasonal trend analysis showed above-average increases in ozone during the spring and early summer periods for both Arctic (Alert, Zeppelinfjellet) and Antarctic (McMurdo, Neumayer, South Pole) sites. In contrast, at Barrow, springtime ozone has been declining. All coastal stations experience springtime episodes with rapid depletion of ozone in the boundary layer, attributable to photochemically catalyzed ozone depletion from halogen chemistry. This effect is most obvious at Barrow, followed by Alert. Springtime depletion episodes are less pronounced at Antarctic stations. At South Pole, during the Antarctic spring and summer, photochemical ozone production yields frequent episodes with enhanced surface ozone. Other Antarctic stations show similar, though less frequent spring and summertime periods with enhanced ozone. The Antarctic data provide evidence that austral spring and summertime ozone production in Antarctica is widespread, respectively, affects all stations at least through transport events. This ozone production contributes to a several ppbv enhancement in the annual mean ozone over the Antarctic plateau; however, it is not the determining process in the Antarctic seasonal ozone cycle. Although Summit and South Pole have many similarities in their environmental conditions, this ozone production does not appear to be of equal importance at Summit. Amplitudes of diurnal, summertime ozone cycles at these polar sites are weaker than at lower latitude locations. Amplitudes of seasonal ozone changes are larger in the Southern Hemisphere (by similar to 5 ppbv), most likely due to less summertime photochemical ozone loss and more transport of ozone-rich air to the Arctic during the NH spring and summer months.

Abstract

Vertical profile measurements of ozone, water vapor, and meteorological conditions, as well as surface and tower measurements of these parameters during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) yielded their vertical (between the surface and 500 m) and temporal distribution in the boundary layer at South Pole (SP) during December 13–30, 2003. Ozone in the surface and lower planetary boundary layer above SP was frequently enhanced over lower free tropospheric levels. During stable atmospheric conditions (which typically existed during low wind and fair sky conditions) ozone accumulated in the surface layer to reach up to twice its background concentration. These conditions were correlated with air transport from the N–SE sector, when air flowed downslope from the Antarctic plateau towards the SP. These data provide further insight into the vigorous photochemistry and ozone production that result from the highly elevated levels of nitrogen oxides (NOx) in the Antarctic surface layer.

Abstract

The vertical distribution of nitric oxide (NO) was investigated by profiling from a tethered balloon platform during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole (SP), Antarctica. The lower atmosphere was probed between the surface and 120 m height by pulling air from an inlet attached to the balloon through a thin-wall, 135 m-long Teflon sampling line and by analyzing NO in this airflow with a ground-borne monitor. Losses and conversion of NO during the 2-4-min residence time in the sampling line were on average on the order of 6-16%, providing a feasible approach for the measurement of vertical NO profiles under SP conditions. NO was found to be highly variable within the lowest 100 m of the atmosphere. Greatly enhanced NO mixing ratios were constrained to a shallow (20-50 m height) air layer nearest to the surface, above which NO rapidly dropped to its mixed boundary layer background levels. Concurrent measurements of ozone and meteorological conditions provide insight into linkages between the ongoing snowpack and boundary layer nitrogen oxides (NOx = NO + NO2) and ozone chemistry. Since [OH] and [HO2] are non-linearly coupled to absolute levels of NOx, their concentrations and the rate of ozone production are expected to similarly show appreciable changes on small vertical scales during conditions with enhanced [NOx]. (C) 2007 Elsevier Ltd. All rights reserved.

Abstract

Since 1986, the Earth System Research Laboratory and its predecessors have been making weekly balloon ozone soundings at the South Pole Station in Antarctica. During the springtime ozone hole period, the sounding frequency is 10 increased to 2–3 per week. The 2007 springtime minimum total column ozone at South Pole was 125 Dobson units, with the layer between 14 and 21 km showing a typical 95% loss of ozone. In contrast, the 2006 minimum total column ozone was 93 Dobson units and showed 99% ozone destruction in the 14–21 km layer. Owing to variations in meteorology and stability of the polar 15 vortex, year to year variations in the severity of the ozone hole of this magnitude are expected. Analysis of the ozone loss rate in September indicates large interannual variability suggesting a dynamic component. Detailed analysis of the 22-year record is used to search for early signs of the beginning of ozone hole recovery. The conclusion is that up to the year 2007, no definitive signs of the beginning of ozone hole recovery have been detected at South Pole Station.

Abstract

During 1994, springtime Antarctic ozone measured at the south pole did not reach the record lows recorded during the 1993 ozone hole period when a value of 91±5 DU was observed. A low value of 102 DU was recorded on October 5, 1994, but such values were not sustained as in 1993. The recovery of total ozone in 1994 was mainly the result of moderation of ozone destruction in the 10–14 km region, probably related to diminishing stratospheric aerosol from the Pinatubo eruption, and may have also been partially related to disturbance of the vortex earlier than normal. As in 1993, ozone profiles at the minimum showed nearly complete destruction of ozone between 15 and 20 km in 1994. In this region, the rate of decline of ozone in September was at least as fast or somewhat faster than in 1992 and 1993 indicating continuing saturation of the ozone destroying chemistry, which is expected as stratospheric chlorine amounts continue to rise. As in 1993, ozone was again observed to be reduced in the 22–24 km region, suggesting that the ozone hole has now probably extended to a region unaffected prior to 1992.

Abstract

On October 12, 1993, a balloon?borne ozone detector recorded a total ozone value of 91±5 Dobson Units (DU) at the U.S. Amundsen?Scott Station at the south pole. This is the lowest value of total ozone ever recorded anywhere, 13% below the previous low of 105 DU at the south pole in October of 1992 [Hofmann and Oltmans, 1993]. A region with a thickness of 5 km, from 14 to 19 km, was totally devoid of ozone as compared to only about half this thickness for the ozone void in 1992. Sub?100 DU total ozone values were observed on several soundings during 1993 whereas the 105 DU value was observed on only one occasion in 1992. The vertical profile of ozone indicates that the main reason for the record low ozone values in 1993 was an approximately 1 km upward extension of the ozone hole caused by unusual ozone loss in the 18–23 km region. Temperatures in this region were unusually low in September and October. Thus, the extension of the ozone hole may have been the result of the prolonged presence of polar stratospheric clouds at 18–23 km combined with the continued presence of sulfate aerosol from the Pinatubo eruption and, finally, increased chlorine levels. This scenario resulted in elevated ozone loss in a region where the ozone loss process is normally not saturated.

Abstract

Ten years of ozonesonde data at the south pole are used to investigate trends and search for indicators that can be used to detect Antarctic ozone recovery in the future. These data indicate that there have been no systematic winter temperature trends at altitudes of 7–25 km and thus no expected changes in stratospheric cloud particle surface area, which would affect heterogeneous chemistry. Springtime ozone depletion has been very severe since about 1992, with near-total loss of ozone in the 14- to 18-km region, but has lessened somewhat in 1994 and 1995. probably because of the decay of the sulfate aerosol from the Mount Pinatubo eruption which was present at 10–16 km. Sulfate aerosol particles from the Pinatubo eruption resulted in new ozone depletion in 1992 and 1993 in the 10- to 12-km region where it is too warm for polar stratospheric clouds (PSCs) to form. The volcanic aerosol also augmented depletion related to PSCs at 12–16 km. Although ozone depletion was not as severe in 1995 as in 1993, the depleted region remained intact longer than ever, with record low values throughout December in 1995. Since about 1992, a pseudo-equilibrium seems to have been reached in which springtime ozone depletion, as measured by the total column or the ozone in the 12- to 20-km main stratospheric cloud region, has remained relatively constant. Independent of volcanic aerosol, ozone depletion has extended into the upper altitudes at 22–24 km since about 1992. There is some indication that ozone depletion has also worsened at the bottom of the depletion region at 12–14 km. Extensions of the ozone hole in the vertical dimension are believed to be the result of increases in man-made halogens and not due to changes in particle surface area or dynamics. A quasi-biennial component in the ozone destruction rate in September, especially above 18 km, is believed to be related to variations in the transport of halogen-bearing molecules to the polar region. A number of indicators for recovery of the ozone hole have been identified. They include an end to springtime ozone depletion at 22–24 km, a 12- to 20-km mid-September column ozone loss rate of less than about 3 Dobson Units (DU) per day, and a 12- to 20-km ozone column value of more than about 70 DU on September 15. It is estimated that if the Montreal protocol and its amendments, banning and/or limiting substances that deplete the ozone layer, is adhered to, recovery of the Antarctic ozone hole may be conclusively detected from the aforementioned changes in the vertical profile of ozone as early as the year 2008. Future volcanic eruptions would affect ozone at 10–16 km, making detection more difficult, but indicators such as depletion in the 22- to 24-km region will be immune to these effects.

Abstract

Local ozone reductions approaching 50% in magnitude were observed during the Antarctic spring in the 11–13 and 25–30 km altitude regions over South Pole and McMurdo Stations in 1991. These reductions, at altitudes where depletion has not been observed previously, resulted in a late September total ozone column 10–15% lower than previous years. The added depletion in the lower stratosphere was observed to coincide with penetration into the polar vortex of highly enhanced concentrations of aerosol particles from volcanic activity in 1991.

Abstract

Vertical profiles of stratospheric water vapor and ozone were measured together at McMurdo and South Pole Stations in Antarctica, and at Kiruna, Sweden, on several occasions during the austral spring of 1990 and the boreal winter of 1991. The Antarctic data indicated that major dehydration had occurred on a continental scale over the winter stratospheric cloud formation period leaving only 2 to 3ppmv water vapor between 11 and 19km. Measurements before and after movement of the boundary of the polar vortex across McMurdo detected increases in both water vapor and ozone in the 17 to 20km region. This injected layer was still observed at South Pole Station a month later suggesting continental proportions. In early November, with the vortex still intact, South Pole measurements indicated a substantial degree of inhomogeneity in both water vapor and ozone in the lower stratosphere. In comparison, stratospheric water vapor measurements in the Arctic gave values of 4 to 5 ppmv indicating the absence of the gross stratospheric dehydration effects obvious in the Antarctic, and they did not reveal significant structure except on one occasion with very cold temperatures (?90°C) at 25km and nacreous cloud displays.

Abstract

Unusual stratospheric ozone levels were observed in the Antarctic stratosphere in 1992. The rate of ozone decrease during formation of the springtime ozone hole and the severity of ozone loss in the lower stratosphere were greater in 1992 as compared to previous years. Total ozone reached an all time low of about 105 Dobson units on October 11 at South Pole Station. On this day, the balloon-borne instrument encountered an apparent ozone void between altitudes of 14 and 18 km. Ozone profiles showed evidence of unusual ozone depletion in autumn, before polar stratospheric cloud existence temperatures were reached. Satellite measurements indicated that the 1992 ozone hole was about 25% larger in geographical extent than in previous years. The possible effects of the eruption of the Pinatubo volcano in the Philippine Islands in 1991 are investigated, and it is concluded that the sulfuric acid droplets, which formed in the stratosphere following the eruption and were trapped in the south polar vortex, are the most likely source of the anomalous Antarctic ozone depletion in 1992.

Abstract

Abstract. The detection of increasing global CFC-11 emissions after 2012 alerted society to a possible violation of the Montreal Protocol on Substances that Deplete the Ozone Layer (MP). This alert resulted in parties to the MP taking urgent actions. As a result, atmospheric measurements made in 2019 suggest a sharp decline in global CFC-11 emissions. Despite the success in the detection and mitigation of part of this problem, regions fully responsible for the recent global emission changes in CFC-11 have not yet been identified. Roughly two thirds (60 ± 40 %) of the emission increase between 2008–2012 and 2014–2017 and two thirds (60 ± 30 %) of the decline between 2014–2017 and 2019 were explained by regional emission changes in eastern mainland China. Here, we used atmospheric CFC-11 measurements made from two global aircraft surveys – the HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) in November 2009–September 2011 and the Atmospheric Tomography Mission (ATom) in August 2016–May 2018, in combination with the global CFC-11 measurements made by the US National Oceanic and Atmospheric Administration during these two periods – to derive global and regional emission changes in CFC-11. Our results suggest Asia accounted for the largest fractions of global CFC-11 emissions in both periods: 43 (37–52) % during November 2009–September 2011 and 57 (49–62) % during August 2016–May 2018. Asia was also primarily responsible for the emission increase between these two periods, accounting for 86 (59–115) % of the global CFC-11 emission rise between the two periods. Besides eastern mainland China, temperate western Asia and tropical Asia also contributed significantly to global CFC-11 emissions during both periods and likely to the global CFC-11 emission increase. The atmospheric observations further provide strong constraints on CFC-11 emissions from North America and Europe, suggesting that each of them accounted for 10 %–15 % of global CFC-11 emissions during the HIPPO period and smaller fractions in the ATom period. For South America, Africa, and Australia, the derived regional emissions had larger dependence on the prior assumptions of emissions and emission changes due to a lower sensitivity of the observations considered here to emissions from these regions. However, significant increases in CFC-11 emissions from southern hemispheric lands were not likely due to the observed increase of north-to-south interhemispheric gradients in atmospheric CFC-11 mole fractions from 2012–2017.

Abstract

Abstract. Bromine released from the decomposition of short-lived brominated source gases contributes as a sink of ozone in the lower stratosphere. The two major contributors are CH2Br2 and CHBr3. In this study, we investigate the global seasonal distribution of these two substances, based on four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission. Observations of CH2Br2 in the free and upper troposphere indicate a pronounced seasonality in both hemispheres, with slightly larger mixing ratios in the Northern Hemisphere (NH). Compared to CH2Br2, CHBr3 in these regions shows larger variability and less clear seasonality, presenting larger mixing ratios in winter and autumn in NH midlatitudes to high latitudes. The lowermost stratosphere of SH and NH shows a very similar distribution of CH2Br2 in hemispheric spring with differences well below 0.1 ppt, while the differences in hemispheric autumn are much larger with substantially smaller values in the SH than in the NH. This suggests that transport processes may be different in both hemispheric autumn seasons, which implies that the influx of tropospheric air (“flushing”) into the NH lowermost stratosphere is more efficient than in the SH. The observations of CHBr3 support the suggestion, with a steeper vertical gradient in the upper troposphere and lower stratosphere in SH autumn than in NH autumn. However, the SH database is insufficient to quantify this difference. We further compare the observations to model estimates of TOMCAT (Toulouse Off-line Model of Chemistry And Transport) and CAM-Chem (Community Atmosphere Model with Chemistry, version 4), both using the same emission inventory of Ordóñez et al. (2012). The pronounced tropospheric seasonality of CH2Br2 in the SH is not reproduced by the models, presumably due to erroneous seasonal emissions or atmospheric photochemical decomposition efficiencies. In contrast, model simulations of CHBr3 show a pronounced seasonality in both hemispheres, which is not confirmed by observations. The distributions of both species in the lowermost stratosphere of the Northern and Southern hemispheres are overall well captured by the models with the exception of southern hemispheric autumn, where both models present a bias that maximizes in the lowest 40 K above the tropopause, with considerably lower mixing ratios in the observations. Thus, both models reproduce equivalent flushing in both hemispheres, which is not confirmed by the limited available observations. Our study emphasizes the need for more extensive observations in the SH to fully understand the impact of CH2Br2 and CHBr3 on lowermost-stratospheric ozone loss and to help constrain emissions.

Abstract

[1] We carried out a systematic study of the seasonal cycle and its latitudinal variation in the nitrous oxide (N2O) data collected by National Oceanic and Atmospheric Administration - Global Monitoring Division (NOAA-GMD) and the Advanced Global Atmospheric Gases Experiment (AGAGE). In order to confirm the weak seasonal signal in the observations, we applied the multitaper method for the spectrum analysis and studied the stations with significant seasonal cycle. In addition, the measurement errors must be small compared with the seasonal cycle. The N2O seasonal cycles from seven stations satisfied these criteria and were analyzed in detail. The stations are Alert (82 degrees N, 62 degrees W), Barrow (71 degrees N, 157 degrees W), Mace Head (53 degrees N, 10 degrees W), Cape Kumukahi (19 degrees N, 155 degrees W), Cape Matatula (14 degrees S, 171 degrees W), Cape Grim (41 degrees S, 145 degrees E) and South Pole (90 degrees S, 102 degrees W). The amplitude ( peak to peak) of the seasonal cycle of N2O varies from 0.29 ppb (parts-per-billion by mole fraction in dry air) at the South Pole to 1.15 ppb at Alert. The month at which the seasonal cycle is at a minimum varies monotonically from April ( South Pole) to September ( Alert). The seasonal cycle in the Northern Hemisphere shows the influence of the stratosphere; the seasonal cycle of N2O in the Southern Hemisphere suggests greater influence from surface sources. Preliminary estimates are obtained for the magnitude of the seasonally varying sources needed to account for the observations.

Abstract

Vertical boundary-layer ozone profiles were measured from a tethered balloon platform during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole Station, Antarctica. Electrochemical concentration cell (ECC) ozonesondes were used in obtaining 128 ascent and descent profile measurements to about 500 m height during 13–30 December 2003. Various data checks and intercomparisons were done to confirm the accuracy of the ozonesondes. The ozonesondes compared well to a surface ozone ultra-violet (UV) absorption monitor located next to the tether balloon site. During the 18-day period, ozonesonde measurement checks at the surface averaged 0.2±1.0 ppbv higher than the continuous ozone measurements under ambient concentrations ranging from 18 to 51 ppbv. This agreement was also consistent when compared to the nearby NOAA UV-monitor sampling at 17 m above ground level during well-mixed conditions near the surface. In addition to the single ECC sonde profiles, five dual ECC ozonesondes were run on the tether platform. Four release balloon-borne ozonesondes were also launched during the project. Under very sharp ozone gradient events, the release ozonesonde (with a rise rate of 4–6 m s?1) passed through the gradient layer too quickly to capture the detail as measured by the controlled tethersonde at 0.3 m s?1 ascent/descent rate. Another method of ozone profiling was also done utilizing the UV monitor at the tether site and a 135-m-long Teflon sampling line with a sampling inlet mounted to and raised with the tethered balloon. The ECC ozonesonde averaged about 0.7±0.8 ppbv lower than the long-line sampling method from eight profiles.

Abstract

Balloon-borne ozonesondes launched weekly from South Pole Station (1986-2021) measure high-vertical-resolution profiles of ozone and temperature from the surface to 30-35 km altitude. The launch frequency is increased in late winter before the onset of rapid stratospheric ozone loss in September. Ozone hole metrics show that the yearly total column ozone and 14-21 km partial column ozone minimum values and September loss rate trends have been improving (less severe) since 2001. The 36-year record also shows interannual variability, especially in recent years (2019-2021). Here we show additional details of these 3 years by comparing annual minimum profiles observed on the date when the lowest integrated total column ozone occurs. We also compare the July-December time series of the 14-21 km partial column ozone values to the 36-year median with percentile intervals. The 2019 anomalous vortex breakdown showed stratospheric temperatures began warming in early September followed by reduced ozone loss. The minimum total column ozone of 180 Dobson units (DU) was observed on 24 September. This was followed by two stable and cold polar vortex years during 2020 and 2021 with total column ozone minimums at 104 DU (1 October) and 102 DU (7 October), respectively. These years also showed broad near-zero-ozone (loss saturation) regions within the 14-21 km layer by the end of September which persisted into October.Validation of the ozonesonde observations is conducted through the ongoing comparison of total column ozone measurements with the South Pole ground-based Dobson spectrophotometer. The ozonesondes show a more positive bias of 2 +/- 3 \% (higher) than the Dobson following a thorough evaluation and homogenization of the long-term ozonesonde record completed in 2018.

Abstract

The impact and significance of uncertainties in model calculations of stratospheric ozone loss resulting from known uncertainty in chemical kinetics parameters is evaluated in trajectory chemistry simulations for the Antarctic and Arctic polar vortices. The uncertainty in modeled ozone loss is derived from Monte Carlo scenario simulations varying the kinetic (reaction and photolysis rate) parameters within their estimated uncertainty bounds. Simulations of a typical winter/spring Antarctic vortex scenario and Match scenarios in the Arctic produce large uncertainty in ozone loss rates and integrated seasonal loss. The simulations clearly indicate that the dominant source of model uncertainty in polar ozone loss is uncertainty in the Cl2O2 photolysis reaction, which arises from uncertainty in laboratory-measured molecular cross sections at atmospherically important wavelengths. This estimated uncertainty in J(Cl2)O(2) from laboratory measurements seriously hinders our ability to model polar ozone loss within useful quantitative error limits. Atmospheric observations, however, suggest that the Cl2O2 photolysis uncertainty may be less than that derived from the lab data. Comparisons to Match, South Pole ozonesonde, and Aura Microwave Limb Sounder (MLS) data all show that the nominal recommended rate simulations agree with data within uncertainties when the Cl2O2 photolysis error is reduced by a factor of two, in line with previous in situ ClOx measurements. Comparisons to simulations using recent cross sections from Pope et al. (2007) are outside the constrained error bounds in each case. Other reactions producing significant sensitivity in polar ozone loss include BrO + ClO and its branching ratios. These uncertainties challenge our confidence in modeling polar ozone depletion and projecting future changes in response to changing halogen emissions and climate. Further laboratory, theoretical, and possibly atmospheric studies are needed.

Abstract

THE 13C/12C ratio of atmospheric carbon dioxide has decreased by approximately 0.6 over 22 yr according to new direct measurements reported here. Our results offer a way of establishing whether 13C/12C ratios of tree rings1?6 are representative of atmospheric 13CO2 variations. We have carried out both isotopic (at Groningen) and concentration (at La Jolla) measurements of atmospheric CO2 on air samples obtained during 1977 and 1978 at three widely spaced locations: La Jolla, California (33°N, 117°W), Fanning Island (4°N, 159°W) and the South Pole. Sampling, instrumental, and analytical procedures closely matched a similar study carried out 22 yr earlier by Keeling7,8

Abstract

A significant negative correlation exists between June-August sea surface temperatures (SSTs) in the eastern equatorial Pacific and 15-31 October total ozone values at South Pole, Antarctica. SSTs in the eastern equatorial Pacific were anomalously warmer by 0.67-degrees-C during 1976-1987 compared with 1962-1975. Quasi-biennial oscillation (QBO) easterly winds in the equatorial Pacific stratosphere were generally stronger after 1975 than they were before that time. Prior to the early-to-mid 1970s the trend in global ozone was generally upward, but then turned downward. Total ozone at Hawaii and Samoa, which had been decreasing at a rate of about 0.35% yr-1 during 1976-1987, showed recovery to mid-1970s values in 1988-1989 following a drop in SSTs in the eastern equatorial Pacific to low values last observed there prior to 1976. During 15-31 October 1988, total ozone at South Pole, which had decreased from about 280 Dobson units (DU) prior to 1980 to 140 DU in 1987, suddenly recovered to 250 DU, though substantial ozone depletion by heterogeneous photochemical processes involving polar stratospheric clouds was still evident in the South Pole ozone vertical profiles. These observations suggest that the downward trend in ozone observed over the globe in recent years may have been at least partially meteorologically induced, possibly through modulation by the warmer tropical Pacific ocean waters of QBO easterly winds at the equator, of planetary waves in the extratropics, of the interaction of QBO winds and planetary waves, and of Hadley Cell circulation. A cursory analysis of geostrophic wind flow around the Baffin Island low suggests a meteorological influence on the observed downward trend in ozone over North America during the past decade. Because ozone has a lifetime that varies from minutes to hours in the primary ozone production region at high altitudes in the tropical stratosphere to months and years in the low stratosphere, changes in atmospheric dynamics have the potential for not only redistributing ozone over the globe, but also changing global ozone abundance.

Abstract

An air sampling apparatus is described which standardizes sampling height at a field station at 10 m or more above ground level and which minimizes loss of particles and destruction and contamination of sampled trace atmospheric gases as air is conducted through the apparatus to various monitoring instruments. Basic design features render the apparatus useful for air sampling under widely varying climate conditions, and at station altitudes ranging from sea level to more than 4 km. Four systems have been built, and have been used sucessfully since 1977 at the NOAA Geophysical Monitoring for Climatic Change program baseline stations at Point Barrow, Alaska; Mauna Loa, Hawaii; American Samoa, South Pacific; and South Pole, Antarctica.

Abstract

Ozone trends, derived from 1979-1996 Dobson spectrophotometer total ozone data obtained at five U.S. mainland midlatitude stations, averaged -3.4, -4.9, -2.6, -1.9, and -3.3%/decade for winter, spring, summer, and autumn months, and on an annual basis, respectively. At the lower latitude stations of Mauna Loa and Samoa, corresponding-period annual ozone trends were -0.4 and -1.3%/decade, respectively, while at Huancayo, Peru, the 1979-1991 annual trend was -0.9%/decade. A linear trend approximation to ozone changes that occurred since 1978 during austral daylight times at Amundsen-Scott (South Pole) station, Antarctica, yielded a value of -12%/decade. By combining 1979-1996 annual trend data for three U.S. mainland stations with trends for the sites derived from 1963-1978 data, it is estimated that the ozone decrease at U.S. midlatitudes through 1996, relative to ozone present in the mid-1960s, was -6.7%. Similar analyses incorporating South Pole data obtained since 1963 yielded an ozone change at South Pole (daylight observations) through 1996 of approximately -25%. South Pole October total ozone values in 1996 were lower than mid-1960s October ozone values by a factor of two. Trend data are also presented for several shorter record period stations, including the foreign cooperative stations of Haute Provence, France; Lauder, New Zealand; and Perth, Australia.

Abstract

Ozone and temperatures measured in 1986 and 1987 at South Pole, Antarctica, are compared, with emphasis on observations made at the time of formation of the Antarctica ozone hole. In early October 1987, total ozone decreased at South Pole to a record low of 127 Dobson units (DU), compared with the early October 1986 value of 158 DU. Electrochemical concentration cell (ECC) ozonesonde soundings made during both years showed the ozone depletion at 11–23 km in 1987 to be greater in vertical extent and magnitude and to proceed more rapidly. As in 1986, two exponential ozone decrease rates occurred in 1987 at 17 ± 1 km, with half-lives of 19.5 and 4.5 days (compared with half-lives of 35 and 12 days observed in 1986). By early October 1987, nearly all ozone was depleted from a 4-km-thick atmospheric layer centered at 17 km. At the time of ozone hole formation, stratospheric temperatures were colder, but tropospheric temperatures were warmer, in 1987 compared to 1986. Because polar vortex breakdown occurred 3 weeks later in 1987 than it did in 1986, stratospheric temperatures in the heart of the ozone depletion region were 10°–40°C colder in mid- to late November 1987.

Abstract

Vertical-profile ozone distributions and variations, as well as the annual course of total ozone, are described for South Pole, Antarctica, from observations made throughout 1986 with electrochemical concentration cell (ECC) ozonesondes and a Dobson spectrophotometer. Ozone decrease in the stratosphere in September and October months was the highlight of the measurements. From mid-August to October 7, total ozone decreased by about 40%, with the bulk of the decrease occurring between 12 and 21 km. Within this region the maximum rate of ozone decrease, with an exponential decay half-life of 11 days, was observed between September 20 and October 15, when column ozone and ozone volume mixing ratio at 16 ± 1 km decreased by 78% and column ozone between 12 and 21 km decreased by 50%. This time interval was characterized by virtual cessation of ozone flux into the stratosphere above South Pole that might have resulted either from movement of the polar vortex or from ozone advection from lower latitudes. In contrast, ECC ozonesonde data obtained in 1971 show ozone to have arrived at South Pole above 30 mbar in mid-September and above 100 mbar in late September, 6 and 3 weeks, respectively, earlier than it did in 1986. Supporting evidence for a temporal change in the timing of ozone transport to Antarctica by atmospheric circulation comes from 1974–1986 surface ozone observations at South Pole, which show a negative surface ozone growth rate during summer months, a positive growth rate during winter months, and an increase in the amplitude of the annual cycle by a factor of 2. A correspondence is shown between El Niño-related highs in sea surface temperature anomalies in the equatorial Pacific Ocean and lows in October–December total ozone averages that were observed at South Pole in the 1960s and 1970s. Such ozone lows occurred during years of increased circumpolar vortex stability, late stratospheric warming, and late ozone arrival in Antarctica. It is suggested that the 1982–1983 El Niño, which was of unprecedented intensity, similarly affected the transport of ozone to Antarctica, thereby contributing to the observed springtime ozone decrease. The transport of air parcels from 50°–60°S latitude into the tropopause and low stratosphere region of Antarctica, as suggested by 10-day, isobaric back trajectory analyses, may also contribute to the Antarctica springtime ozone decrease and warrants further investigation.

Abstract

The significant climate feedback of stratospheric water vapor (SWV) necessitates quantitative estimates of SWV budget changes. Model simulations driven by the newest European Centre for Medium-Range Weather Forecast reanalysis ERA5, satellite observations from the Stratospheric Water and OzOne Satellite Homogenized data set, Microwave Limb Sounder, and in situ frost point hygrometer observations from Boulder all show substantial and persistent stratospheric moistening after a sharp drop in water vapor at the turn of the millennium. This moistening occurred mainly during 2000–2006 and SWV abundances then remained high over the last decade. We find strong positive trends in the Northern Hemisphere and weak negative trends over the South Pole, mainly during austral winter. Moistening of the tropical stratosphere after 2000 occurred during late boreal winter/spring, reached values of ∼0.2 ppm/decade, was well correlated with a warming of the cold point tropopause by ∼0.4 K/decade and can only be partially attributed to El Nino-Southern Oscillation and volcanic eruptions.

Abstract

The Antarctic ozone hole is a seasonal depletion of the ozone layer over Antarctica occurring every austral spring since the early 1980s. The depletion depends on the amount of ozone depleting substances (ODS) and the meteorological conditions in the Antarctic lower stratosphere. The structure and evolution of the 2018 Antarctic ozone hole and its relation to previous years were studied using global reanalysis temperatures (from MERRA-2), the NOAA ozonesonde record collected at Amundsen–Scott South Pole Station, satellite observations, and insights derived from a global chemical transport model.

Abstract

Downwelling and upwelling shortwave and longwave radiation components from six active polar sites, taking part of the Baseline Surface Radiation Network (BSRN), were selected for the period of the last International Polar Year (March 2007 to March 2009), and included in the BSRN-IPY dataset, along with metadata and supplementary data for some of the stations. Two sites, located at Svalbard archipelago (Ny Ålesund) and Alaska (Barrow), represent Arctic sea-level conditions. Four Antarctic stations represent both sea-level (Dronning Maud Land and Cosmonaut Sea) and high-elevation conditions (South Pole and East Antarctic Plateau). The BSRN-IPY dataset content and quality are discussed. The dataset is now available at doi:10.1594/PANGAEA.737668, and can be used for free after accepting the BSRN data release guidelines. The dataset has been summarized as monthly averages and subject to further evaluation according to strict criteria not previously applied.

Abstract

From a global perspective, the annual average surface temperature in 2004 was the fourth highest value observed since regular instrumental records began in 1880. Global surface air temperatures in 2004 were 0.44°C (0.79°F) above the 1961–90 mean, according to both the U.S. and U.K. archives. Observations of the global annual mean temperature in 2004 from the combined lower and middle troposphere was 0.38°C (0.68°F)—the fourth warmest year in the 47-yr archive of worldwide radiosonde observations, and the ninth warmest year out of the past 26 based on satellite measurements. The average precipitation anomaly over global land areas in 2004 was 10.7 mm above average, which was 1% above the 1961–90 mean, and the first year since 2000 that the global mean value was wetter than average. Northern Hemisphere sea ice extent was the third lowest on record for the year, dating back to 1973. The annual snow cover extent over Northern Hemisphere land areas was 25.1 million km2, which was the 25th most extensive snow cover during the period of record. Levels of carbon dioxide (CO2) continue to increase in the atmosphere at the NOAA/Climate Modeling and Diagnostics Laboratory (CMDL) Mauna Loa Observatory on the Big Island of Hawaii; CO2 rose approximately 1.3 parts per million (ppm) in 2004, to reach a preliminary value of 377.6 ppm. However, the 2004 increase was below the long-term average increase of 1.5 ppm yr?1. A minimum ozone concentration of 138 Dobson units (DU) was measured on 4 October 2004 at South Pole station, which was above the 1986–2004 average minimum value of 117 ±26 DU. Sea levels continued to rise globally, increasing at a rate of 2.8 ±0.4 mm yr?1 based on satellite altimeter measurements. The satellite measurements since 1993 have recorded a significantly higher rise in sea level than the overall twentieth-century rate of 1.8 ±0.3 mm yr?1, determined from tide gauge observations during the past century. The climate of 2004 was influenced by the development of a weak El Niño (i.e., ENSO warm event) in the western and central equatorial Pacific Ocean during the second half of the year. A series of westerly wind bursts during July–October, which were initiated by Madden–Julian oscillation activity in the tropical western Pacific, generated several Kelvin waves in the oceanic mixed layer that aided in the formation of the warm event. Only limited regional-scale impacts associated with El Niño occurred during the boreal autumn, because the event did not develop basinwide. Tropical cyclone activity was above average in the North Atlantic, west North Pacific, and South Indian Ocean basins in 2004. The hurricane season was extremely active in the North Atlantic basin, with a total of 15 named storms, nine hurricanes, and six major hurricanes in 2004. Nine of these tropical cyclones struck the Atlantic and Gulf of Mexico coasts of the United States, with three of these landfalling as major hurricanes. The first documented hurricane developed in the South Atlantic Ocean (cyclone “Catarina”), which made landfall along the southern coast of Brazil in late March. The west North Pacific typhoon season was also very active, with 10 tropical systems making landfall in Japan, breaking the previous record of 6 during a single season. In the South Indian Ocean, Tropical Cyclone Gafilo devastated Madagascar, making landfall as a category 5 supercyclone. From a regional perspective, the annual mean temperature across Europe as a whole in 2004 was 0.98°C above the 1961–90 base period average, with temperature anomalies in excess of 1°C measured across parts of northwest Europe and Scandinavia. Temperatures were also warm across South America and parts of Asia. The annual average temperature in Russia was 0.8°C above the long term mean, but temperatures in 2004 were anomalously cold in Asian Russia. Drought conditions continued across western North America, although conditions improved in the southwest United States and California late in the year, while the multiyear drought persisted in parts of the Pacific Northwest and Northern Rockies. Drought conditions also persisted across a majority of the Greater Horn and southern Africa. Monsoonal rains were deficient across the Indian subcontinent in 2004; only 87% of the long period average rainfall was recorded. In contrast, above-normal rainfall across parts of Southwest Asia helped ease some of the long-running drought conditions in the region.

Abstract

We describe the development of an automated gas chromatography-isotope ratio mass spectrometry (GC-IRMS) system capable of measuring the carbon isotopic composition of atmospheric methane (?13CH4) with a precision of better than 0.1‰. The system requires 200 mL of air and completes a single analysis in 15 min. The combination of small sample size, fast analysis time, and high precision has allowed us to measure background variations in atmospheric ?13CH4 through the NOAA Climate Monitoring and Diagnostics Laboratory Cooperative Air Sampling Network. We then present a record of ?13CH4 obtained from six surface sites of the network between January 1998 and December 1999. The sites are Barrow, Alaska (71°N); Niwot Ridge, Colorado (40°N); Mauna Loa, Hawaii (20°N); American Samoa (14°S); Cape Grim, Tasmania (41°S); and the South Pole (90°S). For the years 1998 and 1999, the globally averaged surface ?13C value was ?47.1‰, and the average difference between Barrow and the South Pole was 0.6‰. Consistent seasonal variations were seen only in the Northern Hemisphere, especially at Barrow, where the average amplitude was 0.5‰. Seasonal variations in 1998, however, were evident at all sites, the cause of which is unknown. We also use a two-box model to examine the extent to which annual average ?13C and CH4 mole fraction measurements can constrain broad categories of source emissions. We find that the biggest sources of error are not the atmospheric ?13C measurements but instead the radiocarbon-derived fossil fuel emission estimates, rate coefficients for methane destruction, and isotopic ratios of source emissions.

Abstract

[1] Carbonyl sulfide ( COS) and other trace gases were measured in firn air collected near South Pole (89.98degreesS) and from air trapped in ice at Siple Dome, Antarctica (81.65degreesS). The results, when considered with ambient air data and previous ice core measurements, provide further evidence that atmospheric mixing ratios of COS over Antarctica between 1650 and 1850 A. D. were substantially lower than those observed today. Specifically, the results suggest annual mean COS mixing ratios between 300 and 400 pmol mol(-1) (ppt) during 1650 - 1850 A. D. and increases throughout most of the twentieth century. Measurements of COS in modern air and in the upper layers of the firn at South Pole indicate ambient, annual mean mixing ratios between 480 and 490 ppt with substantial seasonal variations. Peak mixing ratios are observed during austral summer in ambient air at South Pole and Cape Grim, Tasmania (40.41degreesS). Provided COS is not produced or destroyed in firn, these results also suggest that atmospheric COS mixing ratios have decreased 60 - 90 ppt ( 10 - 16%) since the 1980s in high latitudes of the Southern Hemisphere. The history derived for atmospheric mixing ratios of COS in the Southern Hemisphere since 1850 is closely related to historical anthropogenic sulfur emissions. The fraction of anthropogenic sulfur emissions released as COS ( directly or indirectly) needed to explain the secular changes in atmospheric COS over this period is 0.3 - 0.6%.

Abstract

Concern for stratospheric ozone depletion has prompted a phase out in production and use of fully halogenated chlorofluorocarbons (CFCs). Partially hydrogenated CFCs (HCFCs) are among the compounds used as replacements. HCFCs are preferred to CFCs because model calculations predict that HCFCs will have shorter atmospheric lifetimes and release less reactive chlorine to the stratosphere. The major HCFC in use today is HCFC-22. This study reports the results of measurements of HCFC-22 in air samples collected at the South pole. A mean atmospheric mixing ration of 95 ppt was determined for HCFC-22. 13 refs., 3 figs.

Abstract

The Antarctic Tropospheric Chemistry Investigation (ANTCI) was carried out from late November to December 2003 with both extended ground-based and tethered balloon studies at Amundsen Scott Station, South Pole. ANTCI 2003 was the first of two Antarctic field studies with the primary goal of further exploring the active photochemistry of the South Pole region that was first identified in the previous Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) program. Since ISCAT was fully ground-based, ANTCI 2003 goals included expanding chemical studies both vertically upward to investigate mixing and horizontally to better understand large-scale plateau NOx production and transport. Thus, in addition to ground-based experiments at South Pole, Twin Otter aircraft sampling took place out to hundreds of kilometers in several directions from the South Pole. These were designed to specifically address the issue of how representative past South Pole chemical measurements are of the surrounding high plateau region. The Twin Otter was also used to make transects along the coast both north and south of McMurdo Station. The present paper summarizes the overall setting and results of this investigation and highlights the many new findings that were obtained.

Abstract

Vertical profile measurements of ozone, water vapor, and meteorological conditions, as well as surface and tower measurements of these parameters during the 2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) yielded their vertical (between the surface and 500 in) and temporal distribution in the boundary layer at South Pole (SP) during December 13-30, 2003. Ozone in the surface and lower planetary boundary layer above SP was frequently enhanced over lower free tropospheric levels. During stable atmospheric conditions (which typically existed during low wind and fair sky conditions) ozone accumulated in the surface layer to reach up to twice its background concentration. These conditions were correlated with air transport from the N-SE sector, when air flowed downslope from the Antarctic plateau towards the SP. These data provide further insight into the vigorous photochemistry and ozone production that result from the highly elevated levels of nitrogen oxides (NOx) in the Antarctic surface layer. (C) 2007 Elsevier Ltd. All rights reserved.

Abstract

Recent data from the Earth Radiation Budget Experiment (ERBE) have raised the question as to whether or not the addition of clouds to the atmospheric column can decrease the top-of-the-atmosphere (TOA) albedo over bright snow-covered surfaces. To address this issue, ERBE shortwave pixel measurements have been collocated with surface insolation measurements made at two snow-covered locations: the South Pole and Saskatoon, Saskatchewan. Both collocated datasets show a negative correlation (with solar zenith angle variability removed) between TOA albedo and surface insulation. Because increased cloudiness acts to reduce surface insulation, these negative correlations demonstrate that clouds increase the TOA albedo at both snow-covered locations.

Abstract

The Antarctic ozone hole is a severe ozone depletion that regularly appears in austral spring. In 2016, Antarctic stratospheric ozone depletion was less severe compared to the 1991–2006 average (a period of peak chlorine and bromine over Antarctica), but ozone levels were still low compared to pre-1990 levels. Figure 6.11a displays the ozone column between 12 and 20 km derived from NOAA South Pole balloon profiles averaged over 21 September to 16 October (the period with the largest ozone depletion). The 2016 South Pole ozone column was ~6 Dobson units (DU) higher than the 1991–2006 average (horizontal dashed line in the figure), and all ozone column means through the ozone minimum seasons since 2009 have been higher than this 1991–2006 average.

Abstract

The Antarctic ozone hole is showing weak evidence of a decrease in area, based upon the last 15 years of ground and satellite observations. The 2014 Antarctic stratospheric ozone depletion was less severe compared to the 1995–2005 average, but ozone levels were still low compared to pre-1990 levels. Figure 6.12a displays the average column ozone between 12 and 20 km derived from NOAA South Pole balloon profiles. The 2014 South Pole ozone column inventory was relatively high with respect to a 1991–2006 average (horizontal line), and in fact, all of the 2009–2014 ozone column inventories were higher than the 1991–2006 average. The 1998–2014 period shows a positive secular trend (blue line), excluding 2002 (the year with a major stratospheric sudden warming; Roscoe et al. 2005).

Abstract

Longer-term (i.e., 20-40 years) tropospheric ozone (O₃) time series obtained from surface and ozonesonde observations have been analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time series model, running 15-year trends, and changes in the distribution by month in the O₃ mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (˜30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10-15 years. It is uncertain if the flattening of the O₃ change over Japan reflects the impact of O₃ transported from continental East Asia in light of reported O₃ increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and mid-latitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and O₃ has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on O₃ precursors have likely been a factor in the leveling off or decline from earlier O₃ increases.

Abstract

Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid?1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.

Abstract

Long-term surface-ozone and ozone-profile measurements are used to investigate the character of summertime ozone behavior at South Pole. Summer ozone profiles show a significant gradient more than 40% of the time in which mixing ratios at the surface are at least eight parts per billion by volume (ppbv) higher, and may exceed 20 ppbv higher, than mixing ratios several hundred meters above the surface. These ozone gradients are linked to very stable conditions in the boundary layer. The frequency of occurrence of these surface-ozone enhancements has varied with time with the most recent 10-year period showing a greater number of occurrences. Although the summer enhancements have influenced the overall long-term pattern of change in surface ozone, they are not the only factor. The earlier decline in surface-ozone amounts that continued into the mid 1990s was influenced by changes in other seasons as well. Surface-ozone measurements from the 1960s show that summer enhancements were a significant feature of the record at South Pole during this period. Measurements at a lower elevation inland location (Byrd Station), not on the Antarctic Plateau, do not show large summer ozone chemical production events indicating that this phenomenon is primarily confined to the plateau. (C) 2007 Elsevier Ltd. All rights reserved.

Abstract

From a network of sites, primarily in the Atlantic and Pacific Ocean regions, measurements of the surface ozone concentration yield information on the seasonal, synoptic, and diurnal patterns. These sites, generally removed from the effects of local pollution sources, show characteristics that typify broad geographical regions. At Barrow, AK; Mauna Loa, HI; American Samoa; and South Pole, data records of 15–20 years show trends that in all cases are a function of season. This dependence on season is important in understanding the causes of the long-term changes. At Barrow, the summer (July, August, September) increase of 1.7% per year is probably indicative of photochemical production. At South Pole, on the other hand, the summer (December, January, February) decrease is related to photochemical losses and enhanced transport from the coast of Antarctica. At all the sites there is a pronounced seasonal variation. In the Southern Hemisphere (SH), all locations which run from 14 to 90°S show a winter (July– August) maximum and summer minimum. In the Northern Hemisphere (NH) most of the sites show a spring maximum and autumn minimum. At Barrow (70°N) and Barbados (14°), however, the maxima occur during the winter, but for very different reasons. At many of the sites, the transport changes associated with synoptic scale weather patterns dominate the day-to-day variability. This is particularly pronounced at Bermuda and the more tropical sites. In the tropics, there is a very regular diurnal surface ozone cycle with minimum values in the afternoon maxima early in the morning. This appears to result from photochemical destruction during the day in regions with very low concentrations of nitrogen oxides. At Niwot Ridge, CO, and Mace Head, Ireland, there is clear evidence of photochemical ozone production in the summer during transport from known regional pollution sources.

Abstract

Examination of the relationship between tropical stratosphere zonal wind and ozone indicate a variable response in latitude with Northern Hemisphere tropics and polar regions and Southern Hemisphere mid-latitudes showing the strongest response with relatively weaker response at Northern Hemisphere mid-latitudes and the Southern Hemisphere tropics. In tropical regions, the west winds and ozone maxima are in phase while at higher latitudes, a more nearly out-of-phase relationship prevails. At subtropical and middle latitudes, the QBO in ozone does not appear to change phases with altitude. These features are suggestive of an interaction between the tropical zonal winds and poleward transport of horizontal eddies in conjunction with the annual poleward transport of ozone.

Abstract

Surface ozone measurements made in Antarctica during the 1960's show a pronounced annual variation with a summer minimum and winter to late winter maximum. Furthermore, the maximum in surface ozone precedes that in total ozone by from 3 to 5 months, the indication being a loose coupling between the Antarctic stratosphere and troposphere. The annual cycle in surface ozone, instead of reflecting changes in the Antarctic stratosphere, may be a consequence of the variation in low-level meridional transport of ozone from higher latitudes into the Antarctic continent by synoptic scale disturbances. As might be expected from a consideration of Antarctic geography and meteorology, no significant diurnal variations occur in surface ozone. The nonperiodic surface ozone fluctuations observed during the late spring and summer months at South Pole station are most likely caused by sporadic breakdowns of the low-level inversion layer. At the lower latitude stations the day-to-day variations in surface ozone are in all likelihood associated with changes in weather systems.

Abstract

Thirteen years of ozone soundings at the Antarctic Belgrano II station (78° S, 34.6° W) have been analysed to establish a climatology of stratospheric ozone and temperature over the area. The station is inside the polar vortex during the period of development of chemical ozone depletion. Weekly periodic profiles provide a suitable database for seasonal characterization of the evolution of stratospheric ozone, especially valuable during wintertime, when satellites and ground-based instruments based on solar radiation are not available. The work is focused on ozone loss rate variability (August–October) and its recovery (November–December) at different layers identified according to the severity of ozone loss. The time window selected for the calculations covers the phase of a quasi-linear ozone reduction, around day 220 (mid-August) to day 273 (end of September). Decrease of the total ozone column over Belgrano during spring is highly dependent on the meteorological conditions. Largest depletions (up to 59%) are reached in coldest years, while warm winters exhibit significantly lower ozone loss (20%). It has been found that about 11% of the total O₃ loss, in the layer where maximum depletion occurs, takes place before sunlight has arrived, as a result of transport to Belgrano of air from a somewhat lower latitude, near the edge of the polar vortex, providing evidence of mixing inside the vortex. Spatial homogeneity of the vortex has been examined by comparing Belgrano results with those previously obtained for South Pole station (SPS) for the same altitude range and for 9 yr of overlapping data. Results show more than 25% higher ozone loss rate at SPS than at Belgrano. The behaviour can be explained taking into account (i) the transport to both stations of air from a somewhat lower latitude, near the edge of the polar vortex, where sunlight reappears sooner, resulting in earlier depletion of ozone, and (ii) the accumulated hours of sunlight, which become much greater at the South Pole after the spring equinox. According to the variability of the ozone hole recovery, a clear connection between the timing of the breakup of the vortex and the monthly ozone content was found. Minimum ozone concentration of 57 DU in the 12–24 km layer remained in November, when the vortex is more persistent, while in years when the final stratospheric warming took place "very early", mean integrated ozone rose by up to 160–180 DU.

Abstract

The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere^{1, 2, 3}. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane^{4, 5, 6}. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004–2011 the model predicts an annual mean NH–SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns^{11, 12, 13}. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Abstract

Molecular hydrogen (H₂) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H₂ are expected to rise as the use of H₂ as an energy source increases. Documenting past variations in atmospheric H₂ will help to validate current understanding of the global H₂ budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H₂ prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H₂ levels over the 20th century. Inversion of the measurements indicates that H₂ over South Pole has increased from 350–540 ppb from 1910–2000. A biogeochemical box model indicates that the atmospheric burden of H₂ increased by 37% over that time. The rise in H₂ is consistent with increasing H₂ emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation.

Abstract

The atmospheric history of molecular hydrogen (H ) from 1852 to 2003 was reconstructed from measurements of firn air collected at Megadunes, Antarctica. The reconstruction shows that H levels in the southern hemisphere were roughly constant near 330 parts per billion (ppb; nmol H mol air) during the mid to late 1800s. Over the twentieth century, H levels rose by about 70% to 550 ppb. The reconstruction shows good agreement with the H atmospheric history based on firn air measurements from the South Pole. The broad trends in atmospheric H over the twentieth century can be explained by increased methane oxidation and anthropogenic emissions. The H rise shows no evidence of deceleration during the last quarter of the twentieth century despite an expected reduction in automotive emissions following more stringent regulations. During the late twentieth century, atmospheric CO levels decreased due to a reduction in automotive emissions. It is surprising that atmospheric H did not respond similarly as automotive exhaust is thought to be the dominant source of anthropogenic H The monotonic late twentieth century rise in H levels is consistent with late twentieth-century flask air measurements from high southern latitudes. An additional unknown source of H is needed to explain twentieth-century trends in atmospheric H and to resolve the discrepancy between bottom-up and top-down estimates of the anthropogenic source term. The firn air–based atmospheric history of H provides a baseline from which to assess human impact on the H cycle over the last 150 y and validate models that will be used to project future trends in atmospheric composition as H becomes a more common energy source.

Abstract

The National Oceanic and Atmospheric Administration program of Geophysical Monitoring for Climatic Change (GMCC) has operated a background atmospheric monitoring observatory in Antarctica at the United States Amundsen-Scott South Pole Station since 1972. The program objectives at South Pole, as at the other three GMCC observatories, include the determination of concentrations, variations with time, and other properties of atmospheric trace gases and aerosol particles which can potentially impact climate. In addition, GMCC monitors solar radiation and meteorological factors to determine long-term means and any trends that could be associated with climatic changes as well as to examine correlations between meteorological and air chemistry parameters. This discussion emphasizes the long-term GMCC South Pole air chemistry record for carbon dioxide, total ozone, surface ozone, methane, halocarbons, nitrous oxide, and aerosol concentrations. Comparisons of South Pole findings with other global GMCC data are also given. The total ozone discussion includes the results of recent GMCC ozonesonde operations and an assessment of Dobson ozone spectrophotometer data taken at South Pole by NOAA since 1964. These data sets are directly applicable to Antarctic "ozone hole" investigations, and current findings related to this phenomenon are discussed.

Abstract

ncurrent backscatter and ozone measurements were made with near-simultaneous frost point soundings over the South Pole when the center of the 1990 winter vortex was at or very near that location. The initial water vapor concentration in the stratosphere was approximately 5ppmv and decreased to approximately 1.5-2.0 ppmv as cooling took place. By mid-July the stratospheric temperature had decreased to the frost point and heavy polar stratospheric cloud (PSC) activity was observed presumably due to the condensation of water vapor. The lowest water vapor concentrations observed correspond to saturated air at the lowest temperatures encountered. The slow recovery of the water vapor concentration during spring warming indicates that the 12 to 22 km altitude region in the vortex is not readily penetrated by outside air. The observed large decrease in PSC backscatter above approximately 14 km before the stratosphere began to warm is consistent with loss of particles by sedimentation leading to significant dehydration and denitrification. The region of PSC activity in July is noted to be in the same region in which ozone depletion and the persistent dehydration is observed later in the year. At the end of August heavy PSC activity was observed in the lower stratosphere and upper troposphere, consistent with earlier observations from NASA aircraft. These lower clouds were in a region that apparently was still experiencing cooling. No compelling evidence was found supporting earlier claims that PSC layers are anti-correlated with ozone inside the vortex.

Abstract

Simultaneous polar stratospheric cloud (PSC) and ozone measurements were made over South Pole Station using a two-wavelength backscattersonde. This instrument produces aerosol profiles similar to those obtained with a ground-based lidar system but with higher vertical resolution. In one sounding, depolarization of the PSCs was also measured. The backscattersondes were supplemented with occasional frost point soundings. The measurements made before the appearance of PSCs do not show clear evidence of particle deliquescence, suggesting that the background sulfate particles may be frozen solids rather than liquids. PSCs began appearing at approximately 20 km when the temperature at that altitude dropped to -80-degrees-C (193 K). Initially, there was apparent evidence of supersaturation (with respect to nitric acid trihydrate) associated with some type I PSCs, while other examples indicated that the condensation of nitric acid was in quantitative agreement with that expected from the saturation vapor pressure and available nitric acid vapor. The apparent supersaturated layers (which occurred within the first 2 weeks of the onset of PSCs) can alternatively be interpreted as denitrified regions. The wavelength dependence of the backscatter is used to deduce rough particle sizes, and in particular, type Ia and Ib population types can be readily identified by the backscattersonde when not occurring as mixed systems. The mode radius of the first observed PSCs of the season was approximately 0.5 mum. In the polarization sensitive sounding, two varieties of type I PSCs were observed, one of which exhibited significant depolarization and another which produced very little depolarization. This observation would be consistent with the classification of types Ia and Ib, respectively. At the precise time that sunlight was returning to the stratosphere near South Pole Station, a strong inverse correlation in the structure of PSCs and ozone mixing ratio was observed. Using trajectory analysis, it is argued that the effect is probably the result of chemical depletion rather than transport processes. This chance observation is consistent with enhanced ozone depletion occurring in association with sunlit PSCs during the early spring.

Abstract

From a global perspective, the annual average surface temperature in 2004 was the fourth highest value observed since regular instrumental records began in 1880. Global surface air temperatures in 2004 were 0.44 degrees C (0.79 degrees F) above the 1961-90 mean, according to both the U.S. and U.K. archives. Observations of the global annual mean temperature in 2004 from the combined lower and middle troposphere was 0.38 degrees C (0.68 degrees F)-the fourth warmest year in the 47-yr archive of worldwide radiosonde observations, and the ninth warmest year out of the past 26 based on satellite measurements. The average precipitation anomaly over global land areas in 2004 was 10.7 mm above average, which was similar to 1% above the 1961-90 mean, and the first year since 2000 that the global mean value was wetter than average. Northern Hemisphere sea ice extent was the third lowest on record for the year, dating back to 1973. The annual snow cover extent over Northern Hemisphere land areas was 25.1 million km(2), which was the 25th most extensive snow cover during the period of record. Levels of carbon dioxide (CO2) continue to increase in the atmosphere at the NOAA/Climate Modeling and Diagnostics Laboratory (CMDL) Mauna Loa Observatory on the Big Island of Hawaii; CO2 rose approximately 1.3 parts per million (ppm) in 2004, to reach a preliminary value of 377.6 ppm. However, the 2004 increase was below the long-term average increase of 1.5 ppm yr(-1). A minimum ozone concentration of 138 Dobson units (DU) was measured on 4 October 2004 at South Pole station, which was above the 1986-2004 average minimum value of 117 +/- 26 DU. Sea levels continued to rise globally, increasing at a rate of 2.8 +/- 0.4 mm yr(-1) based on satellite altimeter measurements. The satellite measurements since 1993 have recorded a significantly higher rise in sea level than the overall twentieth-century rate of 1.8 +/- 0.3 mm yr(-1), determined from tide gauge observations during the past century. The climate of 2004 was influenced by the development of a weak El Nino (i.e., ENSO warm event) in the western and central equatorial Pacific Ocean during the second half of the year. A series of westerly wind bursts during July-October, which were initiated by Madden-Julian oscillation activity in the tropical western Pacific, generated several Kelvin waves in the oceanic mixed layer that aided in the formation of the warm event. Only limited regional-scale impacts associated with El Nino occurred during the boreal autumn, because the event did not develop basinwide. Tropical cyclone activity was above average in the North Atlantic, west North Pacific, and South Indian Ocean basins in 2004. The hurricane season was extremely active in the North Atlantic basin, with a total of 15 named storms, nine hurricanes, and six major hurricanes in 2004. Nine of these tropical cyclones struck the Atlantic and Gulf of Mexico coasts of the United States, with three of these landfalling as major hurricanes. The first documented hurricane developed in the South Atlantic Ocean (cyclone ``Catarina''), which made landfall along the southern coast of Brazil in late March. The west North Pacific typhoon season was also very active, with 10 tropical systems making landfall in Japan, breaking the previous record of 6 during a single season. In the South Indian Ocean, Tropical Cyclone Gafilo devastated Madagascar, making landfall as a category 5 supercyclone. From a regional perspective, the annual mean temperature across Europe as a whole in 2004 was 0.98 degrees C above the 1961-90 base period average, with temperature anomalies in excess of 1 degrees C measured across parts of northwest Europe and Scandinavia. Temperatures were also warm across South America and parts of Asia. The annual average temperature in Russia was 0.8 degrees C above the long term mean, but temperatures in 2004 were anomalously cold in Asian Russia. Drought conditions continued across western North America, although conditions improved in the southwest United States and California late in the year, while the multiyear drought persisted in parts of the Pacific Northwest and Northern Rockies. Drought conditions also persisted across a majority of the Greater Horn and southern Africa. Monsoonal rains were deficient across the Indian subcontinent in 2004; only 87% of the long period average rainfall was recorded. In contrast, above-normal rainfall across parts of Southwest Asia helped ease some of the long-running drought conditions in the region.

Abstract

From a global perspective, the annual average surface temperature in 2004 was the fourth highest value observed since regular instrumental records began in 1880. Global surface air temperatures in 2004 were 0.44 degrees C (0.79 degrees F) above the 1961-90 mean, according to both the U.S. and U.K. archives. Observations of the global annual mean temperature in 2004 from the combined lower and middle troposphere was 0.38 degrees C (0.68 degrees F)-the fourth warmest year in the 47-yr archive of worldwide radiosonde observations, and the ninth warmest year out of the past 26 based on satellite measurements. The average precipitation anomaly over global land areas in 2004 was 10.7 mm above average, which was similar to 1% above the 1961-90 mean, and the first year since 2000 that the global mean value was wetter than average. Northern Hemisphere sea ice extent was the third lowest on record for the year, dating back to 1973. The annual snow cover extent over Northern Hemisphere land areas was 25.1 million km(2), which was the 25th most extensive snow cover during the period of record. Levels of carbon dioxide (CO2) continue to increase in the atmosphere at the NOAA/Climate Modeling and Diagnostics Laboratory (CMDL) Mauna Loa Observatory on the Big Island of Hawaii; CO2 rose approximately 1.3 parts per million (ppm) in 2004, to reach a preliminary value of 377.6 ppm. However, the 2004 increase was below the long-term average increase of 1.5 ppm yr(-1). A minimum ozone concentration of 138 Dobson units (DU) was measured on 4 October 2004 at South Pole station, which was above the 1986-2004 average minimum value of 117 +/- 26 DU. Sea levels continued to rise globally, increasing at a rate of 2.8 +/- 0.4 mm yr(-1) based on satellite altimeter measurements. The satellite measurements since 1993 have recorded a significantly higher rise in sea level than the overall twentieth-century rate of 1.8 +/- 0.3 mm yr(-1), determined from tide gauge observations during the past century. The climate of 2004 was influenced by the development of a weak El Nino (i.e., ENSO warm event) in the western and central equatorial Pacific Ocean during the second half of the year. A series of westerly wind bursts during July-October, which were initiated by Madden-Julian oscillation activity in the tropical western Pacific, generated several Kelvin waves in the oceanic mixed layer that aided in the formation of the warm event. Only limited regional-scale impacts associated with El Nino occurred during the boreal autumn, because the event did not develop basinwide. Tropical cyclone activity was above average in the North Atlantic, west North Pacific, and South Indian Ocean basins in 2004. The hurricane season was extremely active in the North Atlantic basin, with a total of 15 named storms, nine hurricanes, and six major hurricanes in 2004. Nine of these tropical cyclones struck the Atlantic and Gulf of Mexico coasts of the United States, with three of these landfalling as major hurricanes. The first documented hurricane developed in the South Atlantic Ocean (cyclone ``Catarina''), which made landfall along the southern coast of Brazil in late March. The west North Pacific typhoon season was also very active, with 10 tropical systems making landfall in Japan, breaking the previous record of 6 during a single season. In the South Indian Ocean, Tropical Cyclone Gafilo devastated Madagascar, making landfall as a category 5 supercyclone. From a regional perspective, the annual mean temperature across Europe as a whole in 2004 was 0.98 degrees C above the 1961-90 base period average, with temperature anomalies in excess of 1 degrees C measured across parts of northwest Europe and Scandinavia. Temperatures were also warm across South America and parts of Asia. The annual average temperature in Russia was 0.8 degrees C above the long term mean, but temperatures in 2004 were anomalously cold in Asian Russia. Drought conditions continued across western North America, although conditions improved in the southwest United States and California late in the year, while the multiyear drought persisted in parts of the Pacific Northwest and Northern Rockies. Drought conditions also persisted across a majority of the Greater Horn and southern Africa. Monsoonal rains were deficient across the Indian subcontinent in 2004; only 87% of the long period average rainfall was recorded. In contrast, above-normal rainfall across parts of Southwest Asia helped ease some of the long-running drought conditions in the region.

Abstract

As an oxidant and a precursor for other highly reactive oxidants, ozone plays an important role in tropospheric photochemistry. In the upper troposphere, ozone absorbs infrared radiation and is thus an effective greenhouse gas1. Here we show that surface ozone concentrations at the South Pole in the austral summer decreased by 17% over the period 1976–90. Over the same period, solar irradiance at the South Pole in January and February decreased by 7% as a result of a 25% increase in cloudiness. We suggest that the trend in the summer ozone concentrations is caused by enhanced photochemical destruction of ozone in the lower troposphere caused by the increased penetration of ultraviolet radiation associated with stratospheric ozone depletion, coupled with enhanced transport of ozone-poor marine air from lower latitudes to the South Pole.

Abstract

The Atmospheric Research Observatory (ARO), part of the National Science Foundation’s (NSF’s) Amundsen-Scott South Pole Station, is located at one of the cleanest and most remote sites on earth.  NOAA has been making atmospheric baseline measurements at South Pole since the mid-1970's. The pristine conditions and high elevation make the South Pole a desirable location for many types of research projects and since the early 2000's there have been multiple construction projects to accommodate both a major station renovation and additional research activities and their personnel. The larger population and increased human activity at the station, located in such close proximity to the global baseline measurements conducted at the ARO, calls into question the potential effects of local contamination of the long-term background measurements. In this work, the long-term wind and aerosol climatologies were updated and analyzed for trends. Winds blow toward the ARO from the Clean Air Sector ~88% of the time and while there is some year-to-year variability in this number, the long-term wind speed and direction measurements at South Pole have not changed appreciably in the last 35 years. Several human activity markers including station population, aircraft flights and fuel usage were used as surrogates for local aerosol emissions; peak human activity (and thus likely local emissions) occurred in the 2006 and 2007 austral summer seasons. The long-term aerosol measurements at ARO do not peak during these seasons, suggesting that the quality control procedures in place to identify and exclude continuous sources of local contamination are working and that the NSF’s sector management plan for the Clean Air Sector is effective. No significant trends over time were observed in particle number concentration, aerosol light scattering coefficient, or any aerosol parameter except scattering Ångström exponent, which showed a drop of ~0.02 yr^–1 over the 36-year record. The effect of discrete local contamination events in the Clean Air Sector is discussed using one well-documented example. 

Abstract

Ozonesonde observations from Syowa and the South Pole over more than 40 years are described and intercompared. Observations from the two sites reveal remarkable agreement, supporting and extending the understanding gained from either individually. Both sites exhibit extensive Antarctic ozone losses in a relatively narrow altitude range from about 12 to 24 km in October, and the data are consistent with temperature-dependent chemistry involving chlorine on polar stratospheric clouds as the cause of the ozone hole. The maximum October ozone losses at higher altitudes near 18 km (70 hPa) appear to be transported to lower levels near the tropopause on a timescale of a few months, which is likely to affect the timing of the effects of ozone depletion on possible tropospheric climate changes. Both sites also show greater ozone losses in the lowermost stratosphere after the volcanic eruption of Mt. Pinatubo, supporting the view that surface chemistry can be enhanced by volcanic perturbations and that the very deep ozone holes observed in the early 1990s reflected such enhancements. Sparse data from the Syowa station in the early 1980s also suggest that enhanced ozone losses due to the El Chichon eruption may have contributed to the beginning of a measurable ozone hole. Observations at both locations show that some ozone depletion now occurs during much if not all year at lower altitudes near 12-14 km. Correlations between temperature and ozone provide new insights into ozone losses, including its nonlinear character, maximum effectiveness, and utility as a tool to distinguish dynamical effects from chemical processes. These data also show that recent changes in ozone do not yet indicate ozone recovery linked to changing chlorine abundances but provide new tools to probe observations for the first such future signals.

Abstract

Methane has been measured in air samples collected at approximately weekly intervals at 23 globally distributed sites in the NOAA/GMCC cooperative flask sampling network. Sites range in latitude from 90° S to 76° N, and at most of these we report 2 years of data beginning in early 1983. All measurements have been made by gas chromatography with a flame ionization detector at the NOAA/GMCC laboratory in Boulder, Colorado. All air samples have been referenced to a single secondary standard of methane-in-air, ensuring a high degree of internal consistency in the data. The precision of measurements is estimated from replicate determinations on each sample as 0.2%. The latitudinal distribution of methane and the seasonal variation of this distribution in the marine boundary layer has been defined in great detail, including a remarkable uniformity in background levels of methane in the Southern Hemisphere. We report for the first time the observation of a complete seasonal cycle of methane at the South Pole. A significant vertical gradient is observed between a sea level and a high altitude site in Hawaii. Globally averaged background concentrations in the marine boundary layer have been calculated for the 2 year-period May 1983–April 1985 inclusive, from which we find an average increase of 12.8 ppb per year, or 0.78% per year when referenced to the globally averaged concentration (1625 ppb) at the mid-point of this period. We present evidence that there has been a slowing down in the methane growth rate.

Abstract

Atmospheric aerosols affect the Earth's radiation budget through interactions with solar and terrestrial radiation. Various committees involved with assessing global climate change recognize that aerosols can significantly impact the earth’s radiation balance. In particular, the Scientific Committee on Antarctic Research has recommended the establishment of an international network of solar spectrophotometers to monitor aerosol optical depth (AOD) at high latitudes. Although such a network now exists, better coordination is needed in order to provide research quality data to the scientific community. The U.S. National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL) is collaborating with other institutes to assimilate AOD data from all polar observatories into a central archive for analysis. Historically, in situ aerosol data have been collected at CMDL baseline observatories located near Barrow, Alaska (BRW) and at South Pole, Antarctica (SPO), and since January 2000 continuous photometric measurements (during sunlit periods) have been made at these sites. An overview of past and current CMDL efforts to monitor AOD is given and some recent results are presented. Significant differences between the magnitudes and spectral signatures of AOD measured at BRW and SPO highlight the importance of assimilating similar data sets from other locations to better characterize polar aerosols spatially and temporally. Initial efforts should focus on defining natural cycles of AOD at a number of high latitude sites. Once these cycles are understood, more accurate assessments of climate forcing due to anthropogenic aerosol perturbations are possible. Through international cooperation this work can be expedited.

Abstract

From a global perspective, the annual average surface temperature in 2004 was the fourth highest value observed since regular instrumental records began in 1880. Global surface air temperatures in 2004 were 0.44 degrees C (0.79 degrees F) above the 1961-90 mean, according to both the U.S. and U.K. archives. Observations of the global annual mean temperature in 2004 from the combined lower and middle troposphere was 0.38 degrees C (0.68 degrees F)-the fourth warmest year in the 47-yr archive of worldwide radiosonde observations, and the ninth warmest year out of the past 26 based on satellite measurements. The average precipitation anomaly over global land areas in 2004 was 10.7 mm above average, which was similar to 1% above the 1961-90 mean, and the first year since 2000 that the global mean value was wetter than average. Northern Hemisphere sea ice extent was the third lowest on record for the year, dating back to 1973. The annual snow cover extent over Northern Hemisphere land areas was 25.1 million km(2), which was the 25th most extensive snow cover during the period of record. Levels of carbon dioxide (CO2) continue to increase in the atmosphere at the NOAA/Climate Modeling and Diagnostics Laboratory (CMDL) Mauna Loa Observatory on the Big Island of Hawaii; CO2 rose approximately 1.3 parts per million (ppm) in 2004, to reach a preliminary value of 377.6 ppm. However, the 2004 increase was below the long-term average increase of 1.5 ppm yr(-1). A minimum ozone concentration of 138 Dobson units (DU) was measured on 4 October 2004 at South Pole station, which was above the 1986-2004 average minimum value of 117 +/- 26 DU. Sea levels continued to rise globally, increasing at a rate of 2.8 +/- 0.4 mm yr(-1) based on satellite altimeter measurements. The satellite measurements since 1993 have recorded a significantly higher rise in sea level than the overall twentieth-century rate of 1.8 +/- 0.3 mm yr(-1), determined from tide gauge observations during the past century. The climate of 2004 was influenced by the development of a weak El Nino (i.e., ENSO warm event) in the western and central equatorial Pacific Ocean during the second half of the year. A series of westerly wind bursts during July-October, which were initiated by Madden-Julian oscillation activity in the tropical western Pacific, generated several Kelvin waves in the oceanic mixed layer that aided in the formation of the warm event. Only limited regional-scale impacts associated with El Nino occurred during the boreal autumn, because the event did not develop basinwide. Tropical cyclone activity was above average in the North Atlantic, west North Pacific, and South Indian Ocean basins in 2004. The hurricane season was extremely active in the North Atlantic basin, with a total of 15 named storms, nine hurricanes, and six major hurricanes in 2004. Nine of these tropical cyclones struck the Atlantic and Gulf of Mexico coasts of the United States, with three of these landfalling as major hurricanes. The first documented hurricane developed in the South Atlantic Ocean (cyclone ``Catarina''), which made landfall along the southern coast of Brazil in late March. The west North Pacific typhoon season was also very active, with 10 tropical systems making landfall in Japan, breaking the previous record of 6 during a single season. In the South Indian Ocean, Tropical Cyclone Gafilo devastated Madagascar, making landfall as a category 5 supercyclone. From a regional perspective, the annual mean temperature across Europe as a whole in 2004 was 0.98 degrees C above the 1961-90 base period average, with temperature anomalies in excess of 1 degrees C measured across parts of northwest Europe and Scandinavia. Temperatures were also warm across South America and parts of Asia. The annual average temperature in Russia was 0.8 degrees C above the long term mean, but temperatures in 2004 were anomalously cold in Asian Russia. Drought conditions continued across western North America, although conditions improved in the southwest United States and California late in the year, while the multiyear drought persisted in parts of the Pacific Northwest and Northern Rockies. Drought conditions also persisted across a majority of the Greater Horn and southern Africa. Monsoonal rains were deficient across the Indian subcontinent in 2004; only 87% of the long period average rainfall was recorded. In contrast, above-normal rainfall across parts of Southwest Asia helped ease some of the long-running drought conditions in the region.

Abstract

Both the increasing concentrations of greenhouse gases and potential changes in cloud distributions are likely to affect the surface energy budget of the polar regions. Changes in the polar atmosphere are linked to dynamical processes that control the transport of mass, heat, and moisture from lower latitudes and in turn, feed back into the global circulation. Radiation and meteorological data collected at the South Pole during the 1986 austral winter are analyzed to gain a better understanding of the relationships between cloud radiative effects, transport processes and the vertical distribution of temperature and wind. An algorithm is developed to characterize the quasi-permanent surface-based temperature inversion and the “warm” radiatively active layer above it. Mean winter temperature and wind profiles for clear and overcast conditions are combined with surface radiation measurements and synoptic circulation patterns to study the mechanisms that cause periodic weakening of the inversion. Results support previous studies that ascribe this weakening to (1) warm air advection, (2) downward vertical mixing of sensible and latent heat, and (3) longwave cloud radiative heating. The integrity of the inversion depends on the combined effects of all three mechanisms. Parameters representing the intensity of the inversion and the bulk wind shear through the lower troposphere are suggested as appropriate indices for the detection of climate change in the region of the Antarctic Plateau.

Abstract

The precision and accuracy of trends and seasonal cycles of CO2, as determined from grab samples, was investigated. First, the statistical aspects of infrequent (weekly) sampling were studied by simulating, via a partially random procedure, parallel time series of CO2 flask samples. These simulated flask series were compared to the continuous analyzer records from which they had been derived. The second approach to studying the uncertainties of flask records was to compare real flask results with simultaneous hourly mean concentrations of the in situ analyzers at the Geophysical Monitoring for Climatic Change observatories at Point Barrow, Mauna Loa, Samoa, and the south pole. The latter comparisons emphasized experimental, rather than statistical, errors. The uncertainties and sampling biases depend on the site and on the period of averaging. For monthly means the uncertainty varies from 0.2 to 0.6 ppm (one standard deviation, parts per million by volume), being largest for Barrow. Sampling biases for monthly means at Barrow and Mauna Loa are significant, up to 0.5 ppm. Experimental errors are the dominant error source for annual averages, and spurious interannual variations can be up to 0.4 ppm.

Abstract

We present a comparison of chemistry-transport models (TransCom-N₂O) to examine the importance of atmospheric transport and surface fluxes on the variability of N₂O mixing ratios in the troposphere. Six different models and two model variants participated in the inter-comparison and simulations were made for the period 2006 to 2009. In addition to N₂O, simulations of CFC-12 and SF₆ were made by a subset of four of the models to provide information on the models' proficiency in stratosphere–troposphere exchange (STE) and meridional transport, respectively. The same prior emissions were used by all models to restrict differences among models to transport and chemistry alone. Four different N₂O flux scenarios totalling between 14 and 17 TgN yr⁻¹ (for 2005) globally were also compared. The modelled N₂O mixing ratios were assessed against observations from in situ stations, discrete air sampling networks and aircraft. All models adequately captured the large-scale patterns of N₂O and the vertical gradient from the troposphere to the stratosphere and most models also adequately captured the N₂O tropospheric growth rate. However, all models underestimated the inter-hemispheric N₂O gradient by at least 0.33 parts per billion (ppb), equivalent to 1.5 TgN, which, even after accounting for an overestimate of emissions in the Southern Ocean of circa 1.0 TgN, points to a likely underestimate of the Northern Hemisphere source by up to 0.5 TgN and/or an overestimate of STE in the Northern Hemisphere. Comparison with aircraft data reveal that the models overestimate the amplitude of the N₂O seasonal cycle at Hawaii (21° N, 158° W) below circa 6000 m, suggesting an overestimate of the importance of stratosphere to troposphere transport in the lower troposphere at this latitude. In the Northern Hemisphere, most of the models that provided CFC-12 simulations captured the phase of the CFC-12, seasonal cycle, indicating a reasonable representation of the timing of STE. However, for N₂O all models simulated a too early minimum by 2 to 3 months owing to errors in the seasonal cycle in the prior soil emissions, which was not adequately represented by the terrestrial biosphere model. In the Southern Hemisphere, most models failed to capture the N₂O and CFC-12 seasonality at Cape Grim, Tasmania, and all failed at the South Pole, whereas for SF₆, all models could capture the seasonality at all sites, suggesting that there are large errors in modelled vertical transport in high southern latitudes.

Abstract

An updated set of time series of derived aerosoloptical depth (AOD) and Ångström’s exponent α from a number of Arctic and Antarctic stations was analyzed to determine the long-term variations of these two parameters. The Arctic measurements were performed at Ny-Ålesund (1991–2010), Barrow (1977–2010) and some Siberian sites (1981–1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurementsperformed at Mirny (1982–2009), Neumayer (1991–2004), and Terra Nova Bay (1987–2005), and at South Pole (1977–2010). Analyses of daily mean AOD were made in the Arctic by (i) adjusting values to eliminate volcanic effects due to the El Chichón, Pinatubo, Kasatochi and Sarychev eruptions, and (ii) selecting the summer background aerosol data from those affected by forest fire smoke. Nearly null values of the long-term variation of summer background AOD were obtained at Ny-Ålesund (1991–2010) and at Barrow (1977–2010). No evidence of important variations in AOD was found when comparing the monthly mean values of AOD measured at Tiksi in summer 2010 with those derived from multi-filter actinometer measurementsperformed in the late 1980s at some Siberian sites. The long-term variations of seasonal mean AOD for Arctic Haze (AH) conditions and AH episode seasonal frequency were also evaluated, finding that these parameters underwent large fluctuations over the 35-year period at Ny-Ålesund and Barrow, without presenting well-defined long-term variations. A characterization of chemical composition, complex refractive index and single scattering albedo of ground-level aerosol polydispersions in summer and winter–spring is also presented, based on results mainly found in the literature.

The long-term variation in Antarctic AOD was estimated to be stable, within ±0.10% per year, at the three coastal sites, and nearly null at South Pole, where a weak increase was only recently observed, associated with an appreciable decrease in α, plausibly due to the formation of thin stratospheric layers of ageing volcanic particles. The main characteristics of chemical composition, complex refractive index and single scattering albedo of Antarctic aerosols are also presented for coastal particles sampled at Neumayer and Terra Nova Bay, and continental particles at South Pole.

Abstract

The stable isotopic composition of atmospheric CO2 is being monitored via measurements made at the University of Colorado-Institute of Arctic and Alpine Research, using air samples collected weekly by the Global Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmospheric CO2 mixing ratios, offer the potential to characterize quantitatively the mechanisms operating in the global carbon cycle, by recording the isotopic signatures imparted to CO2 as it moves among the atmosphere, biosphere, and oceans. This data set increases the number of measurements of atmospheric CO2 isotopes by nearly an order of magnitude over those previously available. We describe the analytical techniques used to obtain and calibrate these data and report measurements from 25 land-based sites, and two ships in the Pacific Ocean, from samples collected during 1990-1993. The typical precision of our mass spectrometric technique is 0.03‰ for δ13C and 0.05‰ for δ18O. Collecting the flask samples without drying leads to loss of δ18O information at many sites. The seasonal cycle in δ13C at sites in the northern hemisphere is highly correlated with that of the CO2 mixing ratio, with amplitudes approaching 1‰ at high latitudes. The seasonal cycle in δ18O is of similar amplitude, though variable from year to year and lags the other species by 2-4 months. Interhemispheric differences of the 1992 and 1993 means of the isotopic tracers are in strong contrast: the north pole-south pole difference for δ13C is ~0.20‰, which though highly quantitatively significant is dwarfed by the ~2‰ difference for δ18O. In contrast to the record of atmospheric δ13C during the 1980s we observe no significant temporal trend in annual mean δ13C during 1990-1993.

Abstract

Measurements of HCl and CH4 taken by the aircraft laser infrared absorption spectrometer (ALIAS) on the ER-2 high-altitude research aircraft during the Southern Hemisphere winter of 1994 have been used to examine the abundance of HCl as a fraction of total inorganic chlorine. The fractional abundance of HCl shows a threshold behaviour as a function of temperature history; on a 10 day timescale, the abundance dropped sharply in those air parcels experiencing a temperature < 195 K, but little or no change was seen in parcels which stayed warmer than this temperature. The behaviour mirrors well the temperature behaviour calculated for the transformation of HCl into reactive forms (Cl-2, HOCl) from laboratory studies of sulfate aerosols and polar stratospheric clouds. During the course of the winter, the fractional abundance of HCl outside the vortex decreased from its values in late May by about a third, while inside it dropped to near zero by early August. Some recovery was evident in October. Examples of the peel-off of low-HCl air equatorward of the wind maximum were evident in early June. Meteorological trajectories are used to show, in a case study of a flight in early August, that air parcels which experienced temperatures of < 195 K, and as a result had low fractional HCl abundances, did so largely poleward of the maximum in the polar night jet stream. Encountering temperatures of < 195 K during the previous 10 days was a necessary and sufficient condition for the transformation of HCl into reactive forms by heterogeneous reactions. The trajectories further showed that air arriving from sub-tropical latitudes had higher fractional HCl abundances than the air in the middle latitudes, and much higher fractions than the air at high latitudes. The resulting picture is one in which the fractional abundance of HCl in air at mid latitudes was the result of mixing of air from sub-tropical latitudes with air mainly from polward of the jet stream core which has experienced temperatures < 195 K. The sensitivity of the fractional abundance of HCl to the assumption that no HCl enters the stratosphere via the tropical tropopause is examined in the light of an observed profile near the equator with a volume fraction of 0.4 ppb HCl, zero ClO and tropospheric mixing ratios of CFCs at the tropical tropopause.

Abstract

We report CH4 mixing ratios and ?13C of CH4 values for remote air at two ground-based atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)-Tropics A. Surface values of ?13C-CH4 ranged from ?47.02 to ?47.52‰ at Niwot Ridge, Colorado (40°N, 105°W), and from ?46.81 to ?47.64‰ at Montaña de Oro, California (35°N, 121°W). Samples for isotopic analysis were taken from 2° to 27°S latitude and 81° to 158°W longitude and from sea level to 11.3 km in altitude during the PEM-Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At ?11 km, ?13C-CH4 was ?1‰ greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in ?13C-CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two-dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model-calculated values for ?13C-CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups.

Abstract

Abstract. Optical properties of surface aerosols at Dome C, Antarctica, in 2007–2013 and their potential source areas are presented. Scattering coefficients (σsp) were calculated from measured particle number size distributions with a Mie code and from filter samples using mass scattering efficiencies. Absorption coefficients (σap) were determined with a three-wavelength Particle Soot Absorption Photometer (PSAP) and corrected for scattering by using two different algorithms. The scattering coefficients were also compared with σsp measured with a nephelometer at the South Pole Station (SPO). The minimum σap was observed in the austral autumn and the maximum in the austral spring, similar to other Antarctic sites. The darkest aerosol, i.e., the lowest single-scattering albedo ωo≈0.91, was observed in September and October and the highest ωo≈0.99 in February and March. The uncertainty of the absorption Ångström exponent αap is high. The lowest αap monthly medians were observed in March and the highest in August–October. The equivalent black carbon (eBC) mass concentrations were compared with eBC measured at three other Antarctic sites: the SPO and two coastal sites, Neumayer and Syowa. The maximum monthly median eBC concentrations are almost the same (∼3±1 ng m−3) at all these sites in October–November. This suggests that there is no significant difference in eBC concentrations between the coastal and plateau sites. The seasonal cycle of the eBC mass fraction exhibits a minimum f(eBC) ≈0.1 % in February–March and a maximum ∼4 %–5 % in August–October. Source areas were calculated using 50 d FLEXPART footprints. The highest eBC concentrations and the lowest ωo were associated with air masses coming from South America, Australia and Africa. Vertical simulations that take BC particle removal processes into account show that there would be essentially no BC particles arriving at Dome C from north of latitude 10∘ S at altitudes <1600 m. The main biomass-burning regions Africa, Australia and Brazil are more to the south, and their smoke plumes have been observed at higher altitudes than that, so they can get transported to Antarctica. The seasonal cycle of BC emissions from wildfires and agricultural burning and other fires in South America, Africa and Australia was calculated from data downloaded from the Global Fire Emissions Database (GFED). The maximum total emissions were in August–September, but the peak of monthly average eBC concentrations is observed 2–3 months later in November, not only at Dome C, but also at the SPO and the coastal stations. The air-mass residence-time-weighted BC emissions from South America are approximately an order of magnitude larger than from Africa and Oceania, suggesting that South American BC emissions are the largest contributors to eBC at Dome C. At Dome C the maximum and minimum scattering coefficients were observed in austral summer and winter, respectively. At the SPO σsp was similar to that observed at Dome C in the austral summer, but there was a large difference in winter, suggesting that in winter the SPO is more influenced by sea-spray emissions than Dome C. The seasonal cycles of σsp at Dome C and at the SPO were compared with the seasonal cycles of secondary and primary marine aerosol emissions. The σsp measured at the SPO correlated much better with the sea-spray aerosol emission fluxes in the Southern Ocean than σsp at Dome C. The seasonal cycles of biogenic secondary aerosols were estimated from monthly average phytoplankton biomass concentrations obtained from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor data. The analysis suggests that a large fraction of the biogenic scattering aerosol observed at Dome C has been formed in the polar zone, but it may take a month for the aerosol to be formed, be grown and get transported from the sea level to Dome C.

Abstract

In 1994 an intensive program of balloon-borne frost point measurements was performed at McMurdo, Antarctica. During this program a total of 19 frost point soundings was obtained between February 7 and October 5, which cover a wide range of undisturbed through strongly dehydrated situations. Together with several soundings from South Pole station between 1990 and 1994, they give a comprehensive picture of the general development of the dehydration in the Antarctic stratospheric vortex. The period of dehydration typically starts around the middle of June, and a rapid formation of large particles leads to a fast dehydration of the vortex. The evaporation of falling particles leads to rehydration layers, which have significantly higher water vapor concentrations than the undisturbed stratosphere. Through the formation of these rehydration layers in the early stages of the dehydration we can estimate a particle fall speed of ? km/d and thus a mean particle size of 4 ?m. Ice saturation was observed over McMurdo in only two cases and only well after the onset of the dehydration. From the inspection of synoptic maps it then follows that a small cold region inside the vortex seems to be sufficient to dehydrate the entire vortex. Above 20 km the dehydration is completed by the end of July. From the descent of the upper dehydration edge we can estimate a mean descent rate inside the vortex of 1.5 km/month. In McMurdo we observed occasional penetration of the vortex edge in cases where the vortex edge was close to McMurdo, however, these cases seem to have little effect on the bulk of the vortex. A sounding from November 3, 1990, at South Pole shows that the dehydration may persist into November and indicates that there is no significant transport into the vortex throughout winter and early spring.

Abstract

Earth system models project that the tropical land carbon sink will decrease in size in response to an increase in warming and drought during this century, probably causing a positive climate feedback. But available data are too limited at present to test the predicted changes in the tropical carbon balance in response to climate change. Long-term atmospheric carbon dioxide data provide a global record that integrates the interannual variability of the global carbon balance. Multiple lines of evidence demonstrate that most of this variability originates in the terrestrial biosphere. In particular, the year-to-year variations in the atmospheric carbon dioxide growth rate (CGR) are thought to be the result of fluctuations in the carbon fluxes of tropical land areas. Recently, the response of CGR to tropical climate interannual variability was used to put a constraint on the sensitivity of tropical land carbon to climate change. Here we use the long-term CGR record from Mauna Loa and the South Pole to show that the sensitivity of CGR to tropical temperature interannual variability has increased by a factor of 1.9 ± 0.3 in the past five decades. We find that this sensitivity was greater when tropical land regions experienced drier conditions. This suggests that the sensitivity of CGR to interannual temperature variations is regulated by moisture conditions, even though the direct correlation between CGR and tropical precipitation is weak. We also find that present terrestrial carbon cycle models do not capture the observed enhancement in CGR sensitivity in the past five decades. More realistic model predictions of future carbon cycle and climate feedbacks require a better understanding of the processes driving the response of tropical ecosystems to drought and warming.

Abstract

We describe a new 4D-Var inversion framework for nitrous oxide (N₂O) based on the GEOS-Chem chemical transport model and its adjoint, and apply it in a series of observing system simulation experiments to assess how well N₂O sources and sinks can be constrained by the current global observing network. The employed measurement ensemble includes approximately weekly and quasi-continuous N₂O measurements (hourly averages used) from several long-term monitoring networks, N₂O measurements collected from discrete air samples onboard a commercial aircraft (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container; CARIBIC), and quasi-continuous measurements from the airborne HIAPER Pole-to-Pole Observations (HIPPO) campaigns. For a 2-year inversion, we find that the surface and HIPPO observations can accurately resolve a uniform bias in emissions during the first year; CARIBIC data provide a somewhat weaker constraint. Variable emission errors are much more difficult to resolve given the long lifetime of N₂O, and major parts of the world lack significant constraints on the seasonal cycle of fluxes. Current observations can largely correct a global bias in the stratospheric sink of N₂O if emissions are known, but do not provide information on the temporal and spatial distribution of the sink. However, for the more realistic scenario where source and sink are both uncertain, we find that simultaneously optimizing both would require unrealistically small errors in model transport. Regardless, a bias in the magnitude of the N₂O sink would not affect the a posteriori N₂O emissions for the 2-year timescale used here, given realistic initial conditions, due to the timescale required for stratosphere–troposphere exchange (STE). The same does not apply to model errors in the rate of STE itself, which we show exerts a larger influence on the tropospheric burden of N₂O than does the chemical loss rate over short (< 3 year) timescales. We use a stochastic estimate of the inverse Hessian for the inversion to evaluate the spatial resolution of emission constraints provided by the observations, and find that significant, spatially explicit constraints can be achieved in locations near and immediately upwind of surface measurements and the HIPPO flight tracks; however, these are mostly confined to North America, Europe, and Australia. None of the current observing networks are able to provide significant spatial information on tropical N₂O emissions. There, averaging kernels (describing the sensitivity of the inversion to emissions in each grid square) are highly smeared spatially and extend even to the midlatitudes, so that tropical emissions risk being conflated with those elsewhere. For global inversions, therefore, the current lack of constraints on the tropics also places an important limit on our ability to understand extratropical emissions. Based on the error reduction statistics from the inverse Hessian, we characterize the atmospheric distribution of unconstrained N₂O, and identify regions in and downwind of South America, central Africa, and Southeast Asia where new surface or profile measurements would have the most value for reducing present uncertainty in the global N₂O budget.

Abstract

Insoluble trace gases are trapped in polar ice at the firn-ice transition, at approximately 50 to 100 m below the surface, depending primarily on the site temperature and snow accumulation. Models of trace gas transport in polar firn are used to relate firn air and ice core records of trace gases to their atmospheric history. We propose a new model based on the following contributions. First, the firn air transport model is revised in a poromechanics framework with emphasis on the non-homogeneous properties and the treatment of gravitational settling. We then derive a nonlinear least square multi-gas optimisation scheme to calculate the effective firn diffusivity (automatic diffusivity tuning). The improvements gained by the multi-gas approach are investigated (up to ten gases for a single site are included in the optimisation process). We apply the model to four Arctic (Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, Berkner Island, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites and calculate their respective depth-dependent diffusivity profiles. Among these different sites, a relationship is inferred between the snow accumulation rate and an increasing thickness of the lock-in zone defined from the isotopic composition of molecular nitrogen in firn air (denoted δ¹⁵N). It is associated with a reduced diffusivity value and an increased ratio of advective to diffusive flux in deep firn, which is particularly important at high accumulation rate sites. This has implications for the understanding of δ¹⁵N of N₂ records in ice cores, in relation with past variations of the snow accumulation rate. As the snow accumulation rate is clearly a primary control on the thickness of the lock-in zone, our new approach that allows for the estimation of the lock-in zone width as a function of accumulation may lead to a better constraint on the age difference between the ice and entrapped gases.

Abstract

The HIAPER Pole-to-Pole Observations (HIPPO) programme has completed three of five planned aircraft transects spanning the Pacific from 85°N to 67°S, with vertical profiles every approximately 2.2° of latitude. Measurements include greenhouse gases, long-lived tracers, reactive species, O2/N2 ratio, black carbon (BC), aerosols and CO2 isotopes. Our goals are to address the problem of determining surface emissions, transport strength and patterns, and removal rates of atmospheric trace gases and aerosols at global scales and to provide strong tests of satellite data and global models. HIPPO data show dense pollution and BC at high altitudes over the Arctic, imprints of large N2O sources from tropical lands and convective storms, sources of pollution and biogenic CH4 in the Arctic, and summertime uptake of CO2 and sources for O2 at high southern latitudes. Global chemical signatures of atmospheric transport are imaged, showing remarkably sharp horizontal gradients at air mass boundaries, weak vertical gradients and inverted profiles (maxima aloft) in both hemispheres. These features challenge satellite algorithms, global models and inversion analyses to derive surface fluxes. HIPPO data can play a crucial role in identifying and resolving questions of global sources, sinks and transport of atmospheric gases and aerosols.