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GMD Publications for 2016

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B
Berg, Larry K., Jerome D. Fast, James C. Barnard, Sharon P. Burton, Brian Cairns, Duli Chand, Jennifer M. Comstock, Stephen Dunagan, Richard A. Ferrare, Connor J. Flynn, Johnathan W. Hair, Chris A. Hostetler, John Hubbe, Anne Jefferson, Roy Johnson, Evgueni I. Kassianov, Celine D. Kluzek, Pavlos Kollias, Katia Lamer, Kathleen Lantz, Fan Mei, Mark A. Miller, Joseph Michalsky, Ivan Ortega, Mikhail Pekour, Ray R. Rogers, Philip B. Russell, Jens Redemann, Arthur J. Sedlacek, Michal Segal-Rosenheimer, Beat Schmid, John E. Shilling, Yohei Shinozuka, Stephen R. Springston, Jason M. Tomlinson, Megan Tyrrell, Jacqueline M. Wilson, Rainer Volkamer, Alla Zelenyuk and Carl M. Berkowitz, (2016), The Two-Column Aerosol Project: Phase I-Overview and impact of elevated aerosol layers on aerosol optical depth, Journal of Geophysical Research: Atmospheres, 121, 1, 336-361, 10.1002/2015JD023848

Abstract

The Two-Column Aerosol Project (TCAP), conducted from June 2012 through June 2013, was a unique study designed to provide a comprehensive data set that can be used to investigate a number of important climate science questions, including those related to aerosol mixing state and aerosol radiative forcing. The study was designed to sample the atmosphere between and within two atmospheric columns; one fixed near the coast of North America (over Cape Cod, MA) and a second moveable column over the Atlantic Ocean several hundred kilometers from the coast. The U.S. Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) was deployed at the base of the Cape Cod column, and the ARM Aerial Facility was utilized for the summer and winter intensive observation periods. One important finding from TCAP is that four of six nearly cloud-free flight days had aerosol layers aloft in both the Cape Cod and maritime columns that were detected using the nadir pointing second-generation NASA high-spectral resolution lidar (HSRL-2). These layers contributed up to 60% of the total observed aerosol optical depth (AOD). Many of these layers were also intercepted by the aircraft configured for in situ sampling, and the aerosol in the layers was found to have increased amounts of biomass burning material and nitrate compared to aerosol found near the surface. In addition, while there was a great deal of spatial and day-to-day variability in the aerosol chemical composition and optical properties, no systematic differences between the two columns were observed.

Bian, Lingen, Zhiqiu Gao, Yulong Sun, Minghu Ding, Jie Tang and Russell C. Schnell, (2016), CH4 Monitoring and Background Concentration at Zhongshan Station, Antarctica, Atmospheric and Climate Sciences, 06, 01, 135-144, 10.4236/acs.2016.61012

Abstract

Background CH4 concentration and seasonal variations measured at Zhongshan Station (69°22'2''S, 76°21'49''E, 18.5 m) in Antarctica from 2008 through 2013 are presented and discussed. From 2008-2013 CH4 was measured in weekly flask samples and started on line measurement by Picarro CO2/CH4/H2O analyzer from March, 2010-2013. These CH4 measurements show the expected growth period of CH4 concentration during February (Antarctic spring) with a peak in September (fall). Irrespective of wind direction, CH4 concentrations distribute evenly after the removal of polluted air from station operations, accounting for 1% of the data. The mean daily cycle of CH4 concentration in all four seasons is small. The monthly mean CH4 concentration at Zhongshan station is similar to those at other stations in Antarctica showing that CH4 observed in Antarctica is fully mixed in the atmosphere as it is transported from the northern through the southern hemisphere. The annual CH4 increase in recent years at Zhongshan station is 4.8 ppb·yr-1.

Bodeker, G. E., S. Bojinski, D. Cimini, R. J. Dirksen, M. Haeffelin, J. W. Hannigan, D. F. Hurst, T. Leblanc, F. Madonna, M. Maturilli, A. C. Mikalsen, R. Philipona, T. Reale, D. J. Seidel, D. G. H. Tan, P. W. Thorne, H. Vömel and J. Wang, (2016), Reference Upper-Air Observations for Climate: From Concept to Reality, Bulletin of the American Meteorological Society, 97, 1, 123-135, 10.1175/BAMS-D-14-00072.1

Abstract

The three main objectives of the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) are to provide long-term high-quality climate records of vertical profiles of selected essential climate variables (ECVs), to constrain and calibrate data from more spatially comprehensive global networks, and to provide measurements for process studies that permit an in-depth understanding of the properties of the atmospheric column. In the five years since the first GRUAN implementation and coordination meeting and the printing of an article (Seidel et al.) in this publication, GRUAN has matured to become a functioning network that provides reference-quality observations to a community of users.

This article describes the achievements within GRUAN over the past five years toward making reference-quality observations of upper-air ECVs. Milestones in the evolution of GRUAN are emphasized, including development of rigorous criteria for site certification and assessment, the formal certification of the first GRUAN sites, salient aspects of the GRUAN manual and guide to operations, public availability of GRUAN’s first data product, outcomes of a network expansion workshop, and key results of scientific studies designed to provide a sound scientific foundation for GRUAN operations.

Two defining attributes of GRUAN are 1) that every measurement is accompanied by a traceable estimate of the measurement uncertainty and 2) that data quality and continuity are maximized because network changes are minimized and managed. This article summarizes how these imperatives are being achieved for existing and planned data products and provides an outlook for the future, including expected new data streams, network expansion, and critical needs for the ongoing success of GRUAN.

Butler, J. H., S. A. Yvon-Lewis, J. M. Lobert, D. B. King, S. A. Montzka, J. L. Bullister, V. Koropalov, J. W. Elkins, B. D. Hall, L. Hu and Y. Liu, (2016), A comprehensive estimate for loss of atmospheric carbon tetrachloride CCl4 to the ocean., Atmospheric Chemistry and Physics Discussions, , 1-27, 10.5194/acp-2016-311

Abstract

Abstract. Extensive undersaturations of carbon tetrachloride (CCl4) in Pacific, Atlantic, and Southern Ocean surface waters indicate that atmospheric CCl4 is consumed in large amounts by the ocean. Observations made on 16 research cruises between 1987 and 2010, ranging in latitude from 60° N to 77° S, show that negative saturations extend over most of the surface ocean. Corrected for physical effects associated with radiative heat flux, mixing, and air injection, these anomalies were commonly of the order of −5 % to −10 %, with no clear relationship with temperature, productivity, or other gross surface water characteristics other than being more negative in association with upwelling. The atmospheric flux required to sustain these undersaturations is 11 (7–14) Gg y−1, a loss rate implying a partial atmospheric lifetime with respect to the oceanic loss of 209 (157–313) y and that ~ 16 (10–21) % of atmospheric CCl4 is lost to the ocean. Although CCl4 hydrolyses in seawater, published hydrolysis rates for this gas are too slow to support such large undersaturations, given our current understanding of air–sea gas exchange rates. The even larger undersaturations in intermediate depth waters associated with reduced oxygen levels, observed in this study and by other investigators, strongly suggest that CCl4 is ubiquitously consumed at mid-depth, presumably by microbiota. Although this subsurface sink creates a gradient that drives a downward flux of CCl4, the gradient alone is not sufficient to explain the observed surface undersaturations. Since known chemical losses are likewise insufficient to sustain the observed undersaturations, this suggests a possible biological sink for CCl4 also in surface or near-surface waters of the ocean.

C
Chambers, Scott D., Alastair G. Williams, Franz Conen, Alan D. Griffiths, Stefan Reimann, Martin Steinbacher, Paul B. Krummel, L. Paul Steele, Marcel V. van der Schoot, Ian E. Galbally, Suzie B. Molloy and John E. Barnes, (2016), Towards a Universal “Baseline” Characterisation of Air Masses for High- and Low-Altitude Observing Stations Using Radon-222, Aerosol and Air Quality Research, 16, 3, 885-899, 10.4209/aaqr.2015.06.0391

Abstract

We demonstrate the ability of atmospheric radon concentrations to reliably and unambiguously identify local and remote terrestrial influences on an air mass, and thereby the potential for alteration of trace gas composition by anthropogenic and biogenic processes. Based on high accuracy (lower limit of detection 10–40 mBq m–3), high temporal resolution (hourly) measurements of atmospheric radon concentration we describe, apply and evaluate a simple two-step method for identifying and characterising constituent mole fractions in baseline air. The technique involves selecting a radon-based threshold concentration to identify the “cleanest” (least terrestrially influenced) air masses, and then performing an outlier removal step based on the distribution of constituent mole fractions in the identified clean air masses. The efficacy of this baseline selection technique is tested at three contrasting WMO GAW stations: Cape Grim (a coastal low-altitude site), Mauna Loa (a remote high-altitude island site), and Jungfraujoch (a continental high-altitude site). At Cape Grim and Mauna Loa the two-step method is at least as effective as more complicated methods employed to characterise baseline conditions, some involving up to nine steps. While it is demonstrated that Jungfraujoch air masses rarely meet the baseline criteria of the more remote sites, a selection method based on a variable monthly radon threshold is shown to produce credible “near baseline” characteristics. The seasonal peak-to-peak amplitude of recent monthly baseline CO2 mole fraction deviations from the long-term trend at Cape Grim, Mauna Loa and Jungfraujoch are estimated to be 1.1, 6.0 and 8.1 ppm, respectively.

Chirkov, M., G. P. Stiller, A. Laeng, S. Kellmann, T. von Clarmann, C. D. Boone, J. W. Elkins, A. Engel, N. Glatthor, U. Grabowski, C. M. Harth, M. Kiefer, F. Kolonjari, P. B. Krummel, A. Linden, C. R. Lunder, B. R. Miller, S. A. Montzka, J. Mühle, O&apos, S. Doherty, J. Orphal, R. G. Prinn, G. Toon, M. K. Vollmer, K. A. Walker, R. F. Weiss, A. Wiegele and D. Young, (2016), Global HCFC-22 measurements with MIPAS: retrieval, validation, global distribution and its evolution over 2005–2012, Atmospheric Chemistry and Physics, 16, 5, 3345-3368, 10.5194/acp-16-3345-2016

Abstract

We report on HCFC-22 data acquired by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) in the reduced spectral resolution nominal observation mode. The data cover the period from January 2005 to April 2012 and the altitude range from the upper troposphere (above cloud top altitude) to about 50 km. The profile retrieval was performed by constrained nonlinear least squares fitting of modelled spectra to the measured limb spectral radiances. The spectral ν4-band at 816.5 ± 13 cm−1 was used for the retrieval. A Tikhonov-type smoothing constraint was applied to stabilise the retrieval. In the lower stratosphere, we find a global volume mixing ratio of HCFC-22 of about 185 pptv in January 2005. The rate of linear growth in the lower latitudes lower stratosphere was about 6 to 7 pptv year−1 in the period 2005–2012. The profiles obtained were compared with ACE-FTS satellite data v3.5, as well as with MkIV balloon profiles and cryosampler balloon measurements. Between 13 and 22 km, average agreement within −3 to +5 pptv (MIPAS – ACE) with ACE-FTS v3.5 profiles is demonstrated. Agreement with MkIV solar occultation balloon-borne measurements is within 10–20 pptv below 30 km and worse above, while in situ cryosampler balloon measurements are systematically lower over their full altitude range by 15–50 pptv below 24 km and less than 10 pptv above 28 km. MIPAS HCFC-22 time series below 10 km altitude are shown to agree mostly well to corresponding time series of near-surface abundances from the NOAA/ESRL and AGAGE networks, although a more pronounced seasonal cycle is obvious in the satellite data. This is attributed to tropopause altitude fluctuations and subsidence of polar winter stratospheric air into the troposphere. A parametric model consisting of constant, linear, quasi-biennial oscillation (QBO) and several sine and cosine terms with different periods has been fitted to the temporal variation of stratospheric HCFC-22 for all 10°-latitude/1-to-2-km-altitude bins. The relative linear variation was always positive, with relative increases of 40–70 % decade−1 in the tropics and global lower stratosphere, and up to 120 % decade−1 in the upper stratosphere of the northern polar region and the southern extratropical hemisphere. Asian HCFC-22 emissions have become the major source of global upper tropospheric HCFC-22. In the upper troposphere, monsoon air, rich in HCFC-22, is instantaneously mixed into the tropics. In the middle stratosphere, between 20 and 30 km, the observed trend is inconsistent with the trend at the surface (corrected for the age of stratospheric air), hinting at circulation changes. There exists a stronger positive trend in HCFC-22 in the Southern Hemisphere and a more muted positive trend in the Northern Hemisphere, implying a potential change in the stratospheric circulation over the observation period.

D
de Boer, Gijs, Scott Palo, Brian Argrow, Gabriel LoDolce, James Mack, Ru-Shan Gao, Hagen Telg, Cameron Trussel, Joshua Fromm, Charles N. Long, Geoff Bland, James Maslanik, Beat Schmid and Terry Hock, (2016), The Pilatus unmanned aircraft system for lower atmospheric research, Atmospheric Measurement Techniques, 9, 4, 1845-1857, 10.5194/amt-9-1845-2016

Abstract

This paper presents details of the University of Colorado (CU) “Pilatus” unmanned research aircraft, assembled to provide measurements of aerosols, radiation and thermodynamics in the lower troposphere. This aircraft has a wingspan of 3.2 m and a maximum take-off weight of 25 kg, and it is powered by an electric motor to reduce engine exhaust and concerns about carburetor icing. It carries instrumentation to make measurements of broadband up- and downwelling shortwave and longwave radiation, aerosol particle size distribution, atmospheric temperature, relative humidity and pressure and to collect video of flights for subsequent analysis of atmospheric conditions during flight. In order to make the shortwave radiation measurements, care was taken to carefully position a high-quality compact inertial measurement unit (IMU) and characterize the attitude of the aircraft and its orientation to the upward-looking radiation sensor. Using measurements from both of these sensors, a correction is applied to the raw radiometer measurements to correct for aircraft attitude and sensor tilt relative to the sun. The data acquisition system was designed from scratch based on a set of key driving requirements to accommodate the variety of sensors deployed. Initial test flights completed in Colorado provide promising results with measurements from the radiation sensors agreeing with those from a nearby surface site. Additionally, estimates of surface albedo from onboard sensors were consistent with local surface conditions, including melting snow and bright runway surface. Aerosol size distributions collected are internally consistent and have previously been shown to agree well with larger, surface-based instrumentation. Finally the atmospheric state measurements evolve as expected, with the near-surface atmosphere warming over time as the day goes on, and the atmospheric relative humidity decreasing with increased temperature. No directional bias on measured temperature, as might be expected due to uneven heating of the sensor housing over the course of a racetrack pattern, was detected. The results from these flights indicate that the CU Pilatus platform is capable of performing research-grade lower tropospheric measurement missions.

de Boer, Gijs, Scott Palo, Brian Argrow, Gabriel LoDolce, James Mack, Ru-Shan Gao, Hagen Telg, Cameron Trussel, Joshua Fromm, Charles N. Long, Geoff Bland, James Maslanik, Beat Schmid and Terry Hock, (2016), The Pilatus unmanned aircraft system for lower atmospheric research, Atmospheric Measurement Techniques, 9, 4, 1845-1857, 10.5194/amt-9-1845-2016

Abstract

This paper presents details of the University of Colorado (CU) “Pilatus” unmanned research aircraft, assembled to provide measurements of aerosols, radiation and thermodynamics in the lower troposphere. This aircraft has a wingspan of 3.2 m and a maximum take-off weight of 25 kg, and it is powered by an electric motor to reduce engine exhaust and concerns about carburetor icing. It carries instrumentation to make measurements of broadband up- and downwelling shortwave and longwave radiation, aerosol particle size distribution, atmospheric temperature, relative humidity and pressure and to collect video of flights for subsequent analysis of atmospheric conditions during flight. In order to make the shortwave radiation measurements, care was taken to carefully position a high-quality compact inertial measurement unit (IMU) and characterize the attitude of the aircraft and its orientation to the upward-looking radiation sensor. Using measurements from both of these sensors, a correction is applied to the raw radiometer measurements to correct for aircraft attitude and sensor tilt relative to the sun. The data acquisition system was designed from scratch based on a set of key driving requirements to accommodate the variety of sensors deployed. Initial test flights completed in Colorado provide promising results with measurements from the radiation sensors agreeing with those from a nearby surface site. Additionally, estimates of surface albedo from onboard sensors were consistent with local surface conditions, including melting snow and bright runway surface. Aerosol size distributions collected are internally consistent and have previously been shown to agree well with larger, surface-based instrumentation. Finally the atmospheric state measurements evolve as expected, with the near-surface atmosphere warming over time as the day goes on, and the atmospheric relative humidity decreasing with increased temperature. No directional bias on measured temperature, as might be expected due to uneven heating of the sensor housing over the course of a racetrack pattern, was detected. The results from these flights indicate that the CU Pilatus platform is capable of performing research-grade lower tropospheric measurement missions.
F
Feingold, Graham, Allison McComiskey, Takanobu Yamaguchi, Jill S. Johnson, Kenneth S. Carslaw and K. Sebastian Schmidt, (2016), New approaches to quantifying aerosol influence on the cloud radiative effect, Proceedings of the National Academy of Sciences, , 201514035, 10.1073/pnas.1514035112

Abstract

The topic of cloud radiative forcing associated with the atmospheric aerosol has been the focus of intense scrutiny for decades. The enormity of the problem is reflected in the need to understand aspects such as aerosol composition, optical properties, cloud condensation, and ice nucleation potential, along with the global distribution of these properties, controlled by emissions, transport, transformation, and sinks. Equally daunting is that clouds themselves are complex, turbulent, microphysical entities and, by their very nature, ephemeral and hard to predict. Atmospheric general circulation models represent aerosol−cloud interactions at ever-increasing levels of detail, but these models lack the resolution to represent clouds and aerosol−cloud interactions adequately. There is a dearth of observational constraints on aerosol−cloud interactions. We develop a conceptual approach to systematically constrain the aerosol−cloud radiative effect in shallow clouds through a combination of routine process modeling and satellite and surface-based shortwave radiation measurements. We heed the call to merge Darwinian and Newtonian strategies by balancing microphysical detail with scaling and emergent properties of the aerosol−cloud radiation system.

H
Hallar, A. Gannet, Elisabeth Andrews, Nicolas Bukowiecki, Daniel A. Jaffe and Neng-Huei Lin, (2016), Overview of the Special Issue “Selected Papers from the 2nd Atmospheric Chemistry and Physics at Mountain Sites Symposium”, Aerosol and Air Quality Research, 16, 3, 471-477, 10.4209/aaqr.2016.02.0077

Abstract

Mountain sites provide a unique window on atmospheric chemistry and physics. These sites allow for continuous observations at high elevation, often in the free troposphere, where many important processes occur. Observations at these mountain sites allow for studies on long-range transport of pollution, cloud and precipitation processes, boundary layer ventilation and long-term observations of climate relevant gases and aerosols. However operating at mountain sites presents a unique set of challenges, and for this reason scientists doing research at these sites sought a forum to share knowledge on both the science and challenges of working at these sites. 

Hallar, A. Gannet, Ross Petersen, Ian B. McCubbin, Doug Lowenthal, Shanhu Lee, Elisabeth Andrews and Fangqun Yu, (2016), Climatology of New Particle Formation and Corresponding Precursors at Storm Peak Laboratory, Aerosol and Air Quality Research, 16, 3, 816-826, 10.4209/aaqr.2015.05.0341

Abstract

Thirteen years of measurements of ultrafine (3–10 nm diameter) aerosols are presented from a remote high elevation (3210 m a.s.l.) site in Colorado, Storm Peak Laboratory. Previous work has shown that frequent new particle formation (NPF) occurs regularly at the site (52% of days). This long-term climatology of ultrafine aerosols clearly shows a seasonal dependence on new particle formation at Storm Peak Laboratory, reaching a maximum during the spring season and a minimum in summer. Recent sulfur dioxide data indicates a strong source region west of Storm Peak Laboratory, and this wind direction corresponds to the predominant wind direction observed during NPF events.

Hossaini, R., P. K. Patra, A. A. Leeson, G. Krysztofiak, N. L. Abraham, S. J. Andrews, A. T. Archibald, J. Aschmann, E. L. Atlas, D. A. Belikov, H. Bönisch, R. Butler, L. J. Carpenter, S. Dhomse, M. Dorf, A. Engel, L. Feng, W. Feng, S. Fuhlbrügge, P. T. Griffiths, N. R. P. Harris, R. Hommel, T. Keber, K. Krüger, S. T. Lennartz, S. Maksyutov, H. Mantle, G. P. Mills, B. Miller, S. A. Montzka, F. Moore, M. A. Navarro, D. E. Oram, P. I. Palmer, K. Pfeilsticker, J. A. Pyle, B. Quack, A. D. Robinson, E. Saikawa, A. Saiz-Lopez, S. Sala, B.-M. Sinnhuber, S. Taguchi, S. Tegtmeier, R. T. Lidster, C. Wilson and F. Ziska, (2016), A multi-model intercomparison of halogenated very short-lived substances (TransCom-VSLS): linking oceanic emissions and tropospheric transport for a reconciled estimate of the stratospheric source gas injection of bromine, Atmospheric Chemistry and Physics Discussions, , 1-49, 10.5194/acp-2015-822

Abstract

The first concerted multi-model intercomparison of halogenated very short-lived substances (VSLS) has been performed, within the framework of the ongoing Atmospheric Tracer Transport Model Intercomparison Project (TransCom). Eleven global models or model variants participated, simulating the major natural bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2), over a 20-year period (1993-2012). The overarching goal of TransCom-VSLS was to provide a reconciled model estimate of the stratospheric source gas injection (SGI) of bromine from these gases, to constrain the current measurement-derived range, and to investigate inter-model differences due to emissions and transport processes.Models ran with standardised idealised chemistry, to isolate differences due to transport, and we investigated the sensitivity of results to a range of VSLS emission inventories. Models were tested in their ability to reproduce the observed seasonal and spatial distribution of VSLS at the surface, using measurements from NOAA’s long-term global monitoring network, and in the tropical troposphere, using recent aircraft measurements - including high altitude observations from the NASA Global Hawk platform. The models generally capture the seasonal cycle of surface CHBr3 and CH2Br2 well, with a strong model-measurement correlation (r ≥ 0.7) and a low sensitivity to the choice of emission inventory, at most sites. In a given model, the absolute model-measurement agreement is highly sensitive to the choice of emissions and inter-model differences are also apparent, even when using the same inventory, highlighting the challenges faced in evaluating such inventories at the global scale. Across the ensemble, most consistency is found within the tropics where most of the models (8 out of 11) achieve optimal agreement to surface CHBr3 observations using the lowest of the three CHBr3 emission inventories tested (similarly, 8 out of 11 models for CH2 Br2). In general, the models are able to reproduce well observations of CHBr3 and CH2 Br2 obtained in the tropical tropopause layer (TTL) at various locations throughout the Pacific. Zonal variability in VSLS loading in the TTL is generally consistent among models, with CHBr3 (and to a lesser extent CH2 Br2) most elevated over the tropical West Pacific during boreal winter. The models also indicate the Asian Monsoon during boreal summer to be an important pathway for VSLS reaching the stratosphere, though the strength of this signal varies considerably among models. We derive an ensemble climatological mean estimate of the stratospheric bromine SGI from CHBr3 and CH2 Br2 of 2.0 (1.2-2.5) ppt, ≫ 57% larger than the best estimate from the most recent World Meteorological Organization (WMO) Ozone Assessment Report. We find no evidence for a long-term, transport-driven trend in the stratospheric SGI of bromine over the simulation period. However, transport-driven inter-annual variability in the annual mean bromine SGI is of the order of a ±5%, with SGI exhibiting a strong positive correlation with ENSO in the East Pacific.

Hu, Lei, Stephen A. Montzka, Ben R. Miller, Arlyn E. Andrews, John B. Miller, Scott J. Lehman, Colm Sweeney, Scot M. Miller, Kirk Thoning, Carolina Siso, Elliot L. Atlas, Donald R. Blake, Joost de Gouw, Jessica B. Gilman, Geoff Dutton, James W. Elkins, Bradley Hall, Huilin Chen, Marc L. Fischer, Marikate E. Mountain, Thomas Nehrkorn, Sebastien C. Biraud, Fred L. Moore and Pieter Tans, (2016), Continued emissions of carbon tetrachloride from the United States nearly two decades after its phaseout for dispersive uses, Proceedings of the National Academy of Sciences, 113, 11, 2880-2885, 10.1073/pnas.1522284113

Abstract

National-scale emissions of carbon tetrachloride (CCl4) are derived based on inverse modeling of atmospheric observations at multiple sites across the United States from the National Oceanic and Atmospheric Administration’s flask air sampling network. We estimate an annual average US emission of 4.0 (2.0–6.5) Gg CCl4 y−1 during 2008–2012, which is almost two orders of magnitude larger than reported to the US Environmental Protection Agency (EPA) Toxics Release Inventory (TRI) (mean of 0.06 Gg y−1) but only 8% (3–22%) of global CCl4 emissions during these years. Emissive regions identified by the observations and consistently shown in all inversion results include the Gulf Coast states, the San Francisco Bay Area in California, and the Denver area in Colorado. Both the observation-derived emissions and the US EPA TRI identified Texas and Louisiana as the largest contributors, accounting for one- to two-thirds of the US national total CCl4 emission during 2008–2012. These results are qualitatively consistent with multiple aircraft and ship surveys conducted in earlier years, which suggested significant enhancements in atmospheric mole fractions measured near Houston and surrounding areas. Furthermore, the emission distribution derived for CCl4 throughout the United States is more consistent with the distribution of industrial activities included in the TRI than with the distribution of other potential CCl4 sources such as uncapped landfills or activities related to population density (e.g., use of chlorine-containing bleach).

Höpner, F., F. A.-M. Bender, A. M. L. Ekman, P. S. Praveen, C. Bosch, J. A. Ogren, A. Andersson, Ö. Gustafsson and V. Ramanathan, (2016), Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012, Atmospheric Chemistry and Physics, 16, 2, 1045-1064, 10.5194/acp-16-1045-2016

Abstract

A detailed analysis of optical and microphysical properties of aerosol particles during the dry winter monsoon season above the northern Indian Ocean is presented. The Cloud Aerosol Radiative Forcing Experiment (CARDEX), conducted from 16 February to 30 March 2012 at the Maldives Climate Observatory on Hanimaadhoo island (MCOH) in the Republic of the Maldives, used autonomous unmanned aerial vehicles (AUAV) to perform vertical in situ measurements of particle number concentration, particle number size distribution as well as particle absorption coefficients. These measurements were used together with surface- based Mini Micro Pulse Lidar (MiniMPL) observations and aerosol in situ and off-line measurements to investigate the vertical distribution of aerosol particles.

Air masses were mainly advected over the Indian subcontinent and the Arabian Peninsula. The mean surface aerosol number concentration was 1717 ± 604 cm−3 and the highest values were found in air masses from the Bay of Bengal and Indo-Gangetic Plain (2247 ± 370 cm−3). Investigations of the free tropospheric air showed that elevated aerosol layers with up to 3 times higher aerosol number concentrations than at the surface occurred mainly during periods with air masses originating from the Bay of Bengal and the Indo-Gangetic Plain. This feature is different compared to what was observed during the Indian Ocean Experiment (INDOEX) conducted in winter 1999, where aerosol number concentrations generally decreased with height. In contrast, lower particle absorption at the surface (σabs(520 nm) = 8.5 ± 4.2 Wm−1) was found during CARDEX compared to INDOEX 1999.

Layers with source region specific single-scattering albedo (SSA) values were derived by combining vertical in situ particle absorption coefficients and scattering coefficients calculated with Mie theory. These SSA layers were utilized to calculate vertical particle absorption profiles from MiniMPL profiles. SSA surface values for 550 nm for dry conditions were found to be 0.94 ± 0.02 and 0.91 ± 0.02 for air masses from the Arabian Sea (and Middle East countries) and India (and Bay of Bengal), respectively. Lidar-derived particle absorption coefficient profiles showed both a similar magnitude and structure as the in situ profiles measured with the AUAV. However, primarily due to insufficient accuracy in the SSA estimates, the lidar-derived absorption coefficient profiles have large uncertainties and are generally weakly correlated to vertically in situ measured particle absorption coefficients.

Furthermore, the mass absorption efficiency (MAE) for the northern Indian Ocean during the dry monsoon season was calculated to determine equivalent black carbon (EBC) concentrations from particle absorption coefficient measurements. A mean MAE of 11.6 and 6.9 m2 g−1 for 520 and 880 nm, respectively, was found, likely representing internally mixed BC containing particles. Lower MAE values for 880 and 520 nm were found for air masses originating from dust regions such as the Arabian Peninsula and western Asia (MAE(880 nm)  = 5.6 m2 g−1, MAE(520 nm)  = 9.5 m2 g−1) or from closer source regions as southern India (MAE(880 nm)  = 4.3 m2 g−1, MAE(520 nm)  = 7.3 m2 g−1).

K
Kiedron, P. W. and J. J. Michalsky, (2016), Non-parametric and least squares Langley plot methods, Atmospheric Measurement Techniques, 9, 1, 215-225, 10.5194/amt-9-215-2016

Abstract

Langley plots are used to calibrate sun radiometers primarily for the measurement of the aerosol component of the atmosphere that attenuates (scatters and absorbs) incoming direct solar radiation. In principle, the calibration of a sun radiometer is a straightforward application of the Bouguer–Lambert–Beer law V = V0eτ ⋅ m, where a plot of ln(V) voltage vs. m air mass yields a straight line with intercept ln(V0). This ln(V0) subsequently can be used to solve for τ for any measurement of V and calculation of m. This calibration works well on some high mountain sites, but the application of the Langley plot calibration technique is more complicated at other, more interesting, locales. This paper is concerned with ferreting out calibrations at difficult sites and examining and comparing a number of conventional and non-conventional methods for obtaining successful Langley plots. The 11 techniques discussed indicate that both least squares and various non-parametric techniques produce satisfactory calibrations with no significant differences among them when the time series of ln(V0)'s are smoothed and interpolated with median and mean moving window filters.

Kooijmans, Linda M. J., Nelly A. M. Uitslag, Mark S. Zahniser, David D. Nelson, Stephen A. Montzka and Huilin Chen, (2016), Continuous and high precision atmospheric concentration measurements of COS, CO2, CO and H2O using a quantum cascade laser spectrometer (QCLS), Atmospheric Measurement Techniques Discussions, , 1-36, 10.5194/amt-2016-50

Abstract

Carbonyl sulfide (COS) has been suggested as a useful tracer for Gross Primary Production as it is taken up by plants in a similar way as CO2. To explore and verify the application of this novel tracer, it is highly desired to develop the ability to perform continuous and high precision in situ atmospheric measurements of COS and CO2. In this study we have tested a quantum cascade laser spectrometer (QCLS) for its suitability to obtain accurate and high precision measurements of COS and CO2. The instrument is capable of simultaneously measuring COS, CO2, CO, and H2O after including a weak CO absorption line in the extended wavelength range. An optimal background and calibration strategy was developed based on laboratory tests to ensure accurate field measurements. We have derived water vapor correction factors based on a set of laboratory experiments, and found that line interference with H2O dominates over the dilution effect for COS. This interference can be solved mathematically by fitting the COS spectral line separately from the H2O spectral line. Furthermore, we improved the temperature stability of the QCLS by isolating it in an enclosed box and actively cooling its electronics with the same thermoelectric chiller used to cool the laser. The QCLS was deployed at the Lutjewad atmospheric monitoring station (60 m, 6°21'E, 53°24'N, 1 m a.s.l.) in the Netherlands from July 2014 to April 2015. The measurements of an independent calibration standard showed a mean difference with the assigned cylinder value within 3.3 ppt COS, 0.05 ppm for CO2 and 1.7 ppb for CO over a period of 35 days. The different contributions to uncertainty in measurements of COS, CO2 and CO were summarized and the overall uncertainty was determined to be 7.1 ppt for COS, 0.22 ppm for CO2 and 3.4 ppb for CO for one second data. The comparison of in situ QCLS measurements with measurements from flasks and a cavity ring-down spectrometer showed a difference of −3.5 ± 8.6 ppt for COS, 0.12 ± 0.77 ppm for CO2 and −0.9 ± 3.8 ppb for CO.

Kräuchi, Andreas, Rolf Philipona, Gonzague Romanens, Dale F. Hurst, Emrys G. Hall and Allen F. Jordan, (2016), Controlled weather balloon ascents and descents for atmospheric research and climate monitoring, Atmospheric Measurement Techniques, 9, 3, 929-938, 10.5194/amt-9-929-2016

Abstract

In situ upper-air measurements are often made with instruments attached to weather balloons launched at the surface and lifted into the stratosphere. Present-day balloon-borne sensors allow near-continuous measurements from the Earth's surface to about 35 km (3–5 hPa), where the balloons burst and their instrument payloads descend with parachutes. It has been demonstrated that ascending weather balloons can perturb the air measured by very sensitive humidity and temperature sensors trailing behind them, particularly in the upper troposphere and lower stratosphere (UTLS). The use of controlled balloon descent for such measurements has therefore been investigated and is described here. We distinguish between the single balloon technique that uses a simple automatic valve system to release helium from the balloon at a preset ambient pressure, and the double balloon technique that uses a carrier balloon to lift the payload and a parachute balloon to control the descent of instruments after the carrier balloon is released at preset altitude. The automatic valve technique has been used for several decades for water vapor soundings with frost point hygrometers, whereas the double balloon technique has recently been re-established and deployed to measure radiation and temperature profiles through the atmosphere. Double balloon soundings also strongly reduce pendulum motion of the payload, stabilizing radiation instruments during ascent. We present the flight characteristics of these two ballooning techniques and compare the quality of temperature and humidity measurements made during ascent and descent.

M
McNorton, J., M. P. Chipperfield, M. Gloor, C. Wilson, W. Feng, G. D. Hayman, M. Rigby, P. B. Krummel, O&apos, S. Doherty, R. G. Prinn, R. F. Weiss, D. Young, E. Dlugokencky and S. A. Montzka, (2016), Role of OH variability in the stalling of the global atmospheric CH4 growth rate from 1999 to 2006, Atmospheric Chemistry and Physics Discussions, , 1-24, 10.5194/acp-2015-1029

Abstract

The growth in atmospheric methane (CH4) concentrations over the past two decades has shown large variability on a timescale of many years. Prior to 1999 the globally averaged CH4 concentration was increasing at a rate of 6.0 ppb/yr, but during a stagnation period from 1999 to 2006 this growth rate slowed to 0.6 ppb/yr. Since 2007 the growth rate has again increased to 4.9 ppb/yr. These changes in growth rate are usually ascribed to variations in CH4 emissions. We have used a 3-D global chemical transport model, driven by meteorological reanalyses and variations in global mean hydroxyl (OH) concentrations derived from CH3CCl3 observations from two independent networks, to investigate these CH4 growth variations. The model shows that between 1999 and 2006, changes in the CH4 atmospheric loss contributed significantly to the suppression in global CH4 concentrations relative to the pre-1999 trend. The largest factor in this is relatively small variations in global mean OH on a timescale of a few years, with minor contributions of atmospheric transport of CH4 to its sink region and atmospheric temperature. Although changes in emissions may be important during the stagnation period, these results imply a smaller variation is required to explain the observed CH4 trends. The contribution of OH variations to the renewed CH4 growth after 2007 cannot be determined with data currently available.

Müller, Rolf, Anne Kunz, Dale F. Hurst, Christian Rolf, Martina Krämer and Martin Riese, (2016), The need for accurate long-term measurements of water vapor in the upper troposphere and lower stratosphere with global coverage, Earth's Future, 4, 2, 25-32, 10.1002/2015EF000321

Abstract

Water vapor is the most important greenhouse gas in the atmosphere although changes in carbon dioxide constitute the “control knob” for surface temperatures. While the latter fact is well recognized, resulting in extensive space-borne and ground-based measurement programs for carbon dioxide as detailed in the studies by Keeling et al. (1996), Kuze et al. (2009), and Liu et al. (2014), the need for an accurate characterization of the long-term changes in upper tropospheric and lower stratospheric (UTLS) water vapor has not yet resulted in sufficiently extensive long-term international measurement programs (although first steps have been taken). Here, we argue for the implementation of a long-term balloon-borne measurement program for UTLS water vapor covering the entire globe that will likely have to be sustained for hundreds of years.

O
Ortega, Ivan, Sean Coburn, Larry K. Berg, Kathy Lantz, Joseph Michalsky, Rich Ferrare, Johnathan Hair, Chris Hostetler and Rainer Volkamer, (2016), The CU 2D-MAX-DOAS instrument - part 2: Raman Scattering Probability Measurements and Retrieval of Aerosol Optical Properties, Atmospheric Measurement Techniques Discussions, , 1-34, 10.5194/amt-2015-385

Abstract

The multiannual global mean of aerosol optical depth at 550 nm (AOD550) over land is ~0.19, and that over oceans is ~0.13. About 45% of the Earth surface shows AOD550 smaller than 0.1. There is a need for measurement techniques that are optimized to measure aerosol optical properties under low AOD conditions. We present an inherently calibrated retrieval (i.e., no need for radiance calibration) to simultaneously measure AOD and the aerosol phase function parameter, g, based on measurements of azimuth distributions of the Raman Scattering Probability (RSP), the near-absolute Rotational Raman Scattering (RRS) intensity. We employ Radiative Transfer Model simulations to show that solar azimuth RSP measurements are insensitive to the vertical distribution of aerosols, and maximally sensitive to changes in AOD and g under near molecular scattering conditions. The University of Colorado two dimensional Multi-AXis Differential Optical Absorption Spectroscopy (CU 2D-MAX-DOAS) instrument was deployed as part of the Two Column Aerosol Project (TCAP) at Cape Cod, MA, during the summer of 2012 to measure direct sun spectra, and RSP from scattered light spectra at solar relative azimuth angles (SRAA) between 5° and 170°. During two case study days with (1) high aerosol load (17 July, 0.3 < AOD430 < 0.6) and (2) near-molecular scattering conditions (22 July, AOD430 < 0.13) we compare RSP based retrievals of AOD430 and g with data from a co-located CIMEL sun photometer, Multi-Filter Rotating Shadowband Radiometer (MFRSR), and airborne High Spectral Resolution Lidar (HSRL-2). The average difference (relative to DOAS) for AOD430 is: +0.012 ± 0.023 (CIMEL), –0.012 ± 0.024 (MFRSR), –0.011 ± 0.014 (HSRL-2), and +0.023 ± 0.013 (CIMEL – MFSRS); and yields the following expressions for correlations between different instruments: DOASAOD = –(0.019 ± 0.006) + (1.03 ± 0.02)·CIMELAOD (R2 = 0.98), DOAS = –(0.006 ± 0.005) + (1.08 ± 0.02)·MFRSRAOD (R2 = 0.98), and CIMELAOD = (0.013 ± 0.004) + (1.05 ± 0.01)·MFRSR = 0.99). The average g measured by DOAS on both days was 0.66 ± 0.03, with a difference of 0.014 ± 0.05 compared to CIMEL. Active steps to minimize RSP in the reference spectrum help to reduce the uncertainty in RSP retrievals of AOD and g. As AOD decreases, and solar zenith angle (SZA) increases the RSP signal-to-noise ratio increases. At AOD430 ~ 0.4 and 0.10 the absolute AOD errors are ~0.014 and 0.003 at 70° SZA, and 0.02 and 0.004 at 35° SZA. Inherently calibrated, precise AOD and g measurements are useful to better characterize the aerosol direct effect in urban polluted and remote pristine environments.
P
Parrish, D. D., I. E. Galbally, J.-F. Lamarque, V. Naik, L. Horowitz, D. T. Shindell, S. J. Oltmans, R. Derwent, H. Tanimoto, C. Labuschagne and M. Cupeiro, (2016), Seasonal cycles of O in the marine boundary layer: Observation and model simulation comparisons , Journal of Geophysical Research: Atmospheres, 121, 1, 538-557, 10.1002/2015JD024101

Abstract

We present a two-step approach for quantitatively comparing modeled and measured seasonal cycles of O3: (1) fitting sine functions to monthly averaged measurements and model results (i.e., deriving a Fourier series expansion of these results) and (2) comparing the phase and amplitude of the statistically significant terms between the models and measurements. Two and only two sine terms are sufficient to quantify the O3 seasonal cycle in the marine boundary layer (MBL) in both the measurements and the model results. In addition to the expected fundamental (one sine cycle per year), a second harmonic term (i.e., two sine cycles per year) is identified as a ubiquitous feature of O3 in the MBL. Three chemistry climate models (Community Atmosphere Model with chemistry, GFDL-CM3, and GISS-E2-R) approximately reproduce many features of the measured seasonal cycles at MBL surface sites throughout the globe, with some notable quantitative disagreements, but give divergent results that do not agree with O3 sonde measurements above the MBL. This disagreement and divergence of results between models indicate that the treatment of the MBL dynamics in the chemistry-climate models is not adequate to reproduce the isolation of the MBL indicated by the observations. Within the MBL the models more accurately reproduce the second harmonic term than the fundamental term. We attribute the second harmonic term to the second harmonic of opposite phase in the photolysis rate of O3, while the fundamental term evidently has many influences. The parameters derived from the Fourier series expansion of the measurements are quantitative metrics that can serve as the basis for future model-measurement comparisons.

R
Rhoderick, George C., Duane R. Kitzis, Michael E. Kelley, Walter R. Miller, Bradley D. Hall, Edward J. Dlugokencky, Pieter P. Tans, Antonio Possolo and Jennifer Carney, (2016), Development of a Northern Continental Air Standard Reference Material, Analytical Chemistry, , , 10.1021/acs.analchem.6b00123

Abstract

The National Institute of Standards and technology (NIST) recently began to develop standard mixtures of greenhouse gases as part of a broad program
mandated by the 2009 United States Congress to support research in climate change. To this end, NIST developed suites of gravimetrically assigned primary standard mixtures (PSMs) comprising carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in a dry-natural air balance at ambient mole fraction levels. In parallel, the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, charged 30 aluminum gas cylinders with northern hemisphere
air at Niwot Ridge, Colorado. These mixtures, which constitute NIST Standard Reference Material (SRM) 1720 Northern Continental Air, were certified by NIST for ambient mole fractions of CO2, CH4, and N2O relative to NIST PSMs. NOAAassigned values are also provided as information in support of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) Program for CO2, CH4, and N2O, since NOAA serves as the WMO Central Calibration Laboratory (CCL) for CO2, CH4, and N2O. Relative expanded uncertainties at the 95% confidence interval are <±0.06% of the certified values for CO2 and N2O and <0.2% for CH4, which represents the smallest relative uncertainties specified to date for a gaseous SRM produced by NIST. Agreement between the NOAA (WMO/GAW) and NIST values based on their respective calibration standards suites is within 0.05%, 0.13%, and 0.06% for CO2, CH4, and N2O, respectively. This collaborative  development effort also represents the first of its kind for a gaseous SRM developed by NIST.

S
Sanchez-Romero, A., J.A. González, J. Calbó, A. Sanchez-Lorenzo and J. Michalsky, (2016), Aerosol optical depth in a western Mediterranean site: An assessment of different methods, Atmospheric Research, 174-17, 70-84, 10.1016/j.atmosres.2016.02.002

Abstract

Column aerosol optical properties were derived from multifilter rotating shadowing radiometer (MFRSR) observations carried out at Girona (northeast Spain) from June 2012 to June 2014. We used a technique that allows estimating simultaneously aerosol optical depth (AOD) and Ångström exponent (AE) at high time-resolution. For the period studied, mean AOD at 500 nm was 0.14, with a noticeable seasonal pattern, i.e. maximum in summer and minimum in winter. Mean AE from 500 to 870 nm was 1.2 with a strong day-to-day variation and slightly higher values in summer. So, the summer increase in AOD seems to be linked with an enhancement in the number of fine particles. A radiative closure experiment, using the SMARTS2 model, was performed to confirm that the MFRSR-retrieved aerosol optical properties appropriately represent the continuously varying atmospheric conditions in Girona. Thus, the calculated broadband values of the direct flux show a mean absolute difference of less than 5.9 W m− 2 (0.77%) and R = 0.99 when compared to the observed fluxes. The sensitivity of the achieved closure to uncertainties in AOD and AE was also examined. We use this MFRSR-based dataset as a reference for other ground-based and satellite measurements that might be used to assess the aerosol properties at this site. First, we used observations obtained from a 100 km away AERONET station; despite a general similar behavior when compared with the in-situ MFRSR observations, certain discrepancies for AOD estimates in the different channels (R < 0.84 and slope < 1) appear. Second, AOD products from MISR and MODIS satellite observations were compared with our ground-based retrievals. Reasonable agreements are found for the MISR product (R = 0.92), with somewhat poorer agreement for the MODIS product (R = 0.70). Finally, we apply all these methods to study in detail the aerosol properties during two singular aerosol events related to a forest fire and a desert dust intrusion.

Sharma, Nimmi C.P. and John E. Barnes, (2016), Boundary Layer Characteristics over a High Altitude Station, Mauna Loa Observatory, Aerosol and Air Quality Research, 16, 3, 729-737, 10.4209/aaqr.2015.05.0347

Abstract

The unique boundary layer at Mauna Loa Observatory (3396 meters) is examined with a combination of radiosondes launched from the observatory and a novel aerosol profiling technique called CLidar or camera lidar. This boundary layer is influenced by a combination of radiation winds, due to the heating and cooling of the surrounding lava, and off-island winds. Typically an upslope surface wind forms after sunrise as the ground heats up. The reverse occurs after sunset as the ground cools and a temperature inversion, tens of meters thick forms. Aerosol increases for the first 90 to 160 meters and then decreases to free tropospheric levels. The 90 to 160 m aerosol peak indicates the vicinity of the upslope/downslope interface in the air flow. An upper transition is seen in the aerosol gradient at about 600 meters above the observatory (4000 m Above Sea Level). This transition is also seen in radiosonde potential temperature data. The sondes indicate that the air above the nighttime downslope surface region usually has an upslope component. Some of this counter-flowing air can be entrained in the downslope air, possibly influencing the sampling of aerosols and trace gases at the observatory.

Sheridan, Patrick, Elisabeth Andrews, Lauren Schmeisser, Brian Vasel and John Ogren, (2016), Aerosol Measurements at South Pole: Climatology and Impact of Local Contamination, Aerosol and Air Quality Research, 16, 3, 855-872, 10.4209/aaqr.2015.05.0358

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. 

T
Tarasick, D. W., J. Davies, H. G. J. Smit and S. J. Oltmans, (2016), A re-evaluated Canadian ozonesonde record: measurements of the vertical distribution of ozone over Canada from 1966 to 2013, Atmospheric Measurement Techniques, 9, 1, 195-214, 10.5194/amt-9-195-2016

Abstract

In Canada routine ozone soundings have been carried at Resolute Bay since 1966, making this record the longest in the world. Similar measurements started in the 1970s at three other sites, and the network was expanded in stages to 10 sites by 2003. This important record for understanding long-term changes in tropospheric and stratospheric ozone has been re-evaluated as part of the SPARC/IO3C/IGACO-O3/NDACC (SI2N) initiative. The Brewer–Mast sonde, used in the Canadian network until 1980, is different in construction from the electrochemical concentration cell (ECC) sonde, and the ECC sonde itself has also undergone a variety of minor design changes over the period 1980–2013. Corrections have been made for the estimated effects of these changes to produce a more homogeneous data set.

The effect of the corrections is generally modest. However, the overall result is entirely positive: the comparison with co-located total ozone spectrometers is improved, in terms of both bias and standard deviation, and trends in the bias have been reduced or eliminated. An uncertainty analysis (including the additional uncertainty from the corrections, where appropriate) has also been conducted, and the altitude-dependent estimated uncertainty is included with each revised profile.

The resulting time series show negative trends in the lower stratosphere of up to 5 % decade−1 for the period 1966–2013. Most of this decline occurred before 1997, and linear trends for the more recent period are generally not significant. The time series also show large variations from year to year. Some of these anomalies can be related to cold winters (in the Arctic stratosphere) or changes in the Brewer–Dobson circulation, which may thereby be influencing trends.

In the troposphere, trends for the 48-year period are small and for the most part not significant. This suggests that ozone levels in the free troposphere over Canada have not changed significantly in nearly 50 years.

Ten Hoeve, John E. and John A. Augustine, (2016), Aerosol effects on cloud cover as evidenced by ground-based and space-based observations at five rural sites in the United States, Geophysical Research Letters, 43, 2, 793-801, 10.1002/2015GL066873

Abstract

Previous studies of the second aerosol indirect (lifetime) effect on cloud cover have estimated the strength of the effect without correcting for near-cloud contamination and other confounding factors. Here we combine satellite-based observations with a multiyear ground-based data set across five rural locations in the United States to more accurately constrain the second indirect aerosol effect and quantify aerosol effects on radiative forcing. Results show that near-cloud contamination accounts for approximately 40% of the satellite-derived aerosol-cloud relationship. When contamination is removed and the effect of meteorological covariation is minimized, a strong physical aerosol effect on cloud cover remains. Averaged over all stations and after correcting for contamination, the daytime solar and total (solar + IR) radiative forcing is −52 W/m2 and −19 W/m2, respectively, due to both direct and indirect aerosol effects for aerosol optical depths (τ) between 0 and 0.3. Averaged diurnally, the average total radiative forcing is +16 W/m2.

V
Vollmer, Martin K., Jens Mühle, Cathy M. Trudinger, Matthew Rigby, Stephen A. Montzka, Christina M. Harth, Benjamin R. Miller, Stephan Henne, Paul B. Krummel, Bradley D. Hall, Dickon Young, Jooil Kim, Jgor Arduini, Angelina Wenger, Bo Yao, Stefan Reimann, Simon O'Doherty, Michela Maione, David M. Etheridge, Shanlan Li, Daniel P. Verdonik, Sunyoung Park, Geoff Dutton, L. Paul Steele, Chris R. Lunder, Tae Siek Rhee, Ove Hermansen, Norbert Schmidbauer, Ray H. J. Wang, Matthias Hill, Peter K. Salameh, Ray L. Langenfelds, Lingxi Zhou, Thomas Blunier, Jakob Schwander, James W. Elkins, James H. Butler, Peter G. Simmonds, Ray F. Weiss, Ronald G. Prinn and Paul J. Fraser, (2016), Atmospheric histories and global emissions of halons H-1211 (CBrClF), H-1301 (CBrF), and H-2402 (CBrFCBrF) , Journal of Geophysical Research: Atmospheres, 121, 7, 3663-3686, 10.1002/2015JD024488

Abstract

We report ground-based atmospheric measurements and emission estimates for the halons H-1211 (CBrClF2), H-1301 (CBrF3), and H-2402 (CBrF2CBrF2) from the AGAGE (Advanced Global Atmospheric Gases Experiment) and the National Oceanic and Atmospheric Administration global networks. We also include results from archived air samples in canisters and from polar firn in both hemispheres, thereby deriving an atmospheric record of nearly nine decades (1930s to present). All three halons were absent from the atmosphere until ∼1970, when their atmospheric burdens started to increase rapidly. In recent years H-1211 and H-2402 mole fractions have been declining, but H-1301 has continued to grow. High-frequency observations show continuing emissions of H-1211 and H-1301 near most AGAGE sites. For H-2402 the only emissions detected were derived from the region surrounding the Sea of Japan/East Sea. Based on our observations, we derive global emissions using two different inversion approaches. Emissions for H-1211 declined from a peak of 11 kt yr−1 (late 1990s) to 3.9 kt yr−1 at the end of our record (mean of 2013–2015), for H-1301 from 5.4 kt yr−1 (late 1980s) to 1.6 kt yr−1, and for H-2402 from 1.8 kt yr−1 (late 1980s) to 0.38 kt yr−1. Yearly summed halon emissions have decreased substantially; nevertheless, since 2000 they have accounted for ∼30% of the emissions of all major anthropogenic ozone depletion substances, when weighted by ozone depletion potentials.

W
Weigel, K., A. Rozanov, F. Azam, K. Bramstedt, R. Damadeo, K.-U. Eichmann, C. Gebhardt, D. Hurst, M. Kraemer, S. Lossow, W. Read, N. Spelten, G. P. Stiller, K. A. Walker, M. Weber, H. Bovensmann and J. P. Burrows, (2016), UTLS water vapour from SCIAMACHY limb measurementsV3.01 (2002–2012), Atmospheric Measurement Techniques, 9, 1, 133-158, 10.5194/amt-9-133-2016

Abstract

The SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) aboard the Envisat satellite provided measurements from August 2002 until April 2012. SCIAMACHY measured the scattered or direct sunlight using different observation geometries. The limb viewing geometry allows the retrieval of water vapour at about 10–25 km height from the near-infrared spectral range (1353–1410 nm). These data cover the upper troposphere and lower stratosphere (UTLS), a region in the atmosphere which is of special interest for a variety of dynamical and chemical processes as well as for the radiative forcing. Here, the latest data version of water vapour (V3.01) from SCIAMACHY limb measurements is presented and validated by comparisons with data sets from other satellite and in situ measurements. Considering retrieval tests and the results of these comparisons, the V3.01 data are reliable from about 11 to 23 km and the best results are found in the middle of the profiles between about 14 and 20 km. Above 20 km in the extra tropics V3.01 is drier than all other data sets. Additionally, for altitudes above about 19 km, the vertical resolution of the retrieved profile is not sufficient to resolve signals with a short vertical structure like the tape recorder. Below 14 km, SCIAMACHY water vapour V3.01 is wetter than most collocated data sets, but the high variability of water vapour in the troposphere complicates the comparison. For 14–20 km height, the expected errors from the retrieval and simulations and the mean differences to collocated data sets are usually smaller than 10 % when the resolution of the SCIAMACHY data is taken into account. In general, the temporal changes agree well with collocated data sets except for the Northern Hemisphere extratropical stratosphere, where larger differences are observed. This indicates a possible drift in V3.01 most probably caused by the incomplete treatment of volcanic aerosols in the retrieval. In all other regions a good temporal stability is shown. In the tropical stratosphere an increase in water vapour is found between 2002 and 2012, which is in agreement with other satellite data sets for overlapping time periods.

Y
Yokouchi, Yoko, Takuya Saito, Jiye Zeng, Hitoshi Mukai and Stephen Montzka, (2016), Seasonal variation of bromocarbons at Hateruma Island, Japan: implications for global sources, Journal of Atmospheric Chemistry, , , 10.1007/s10874-016-9333-9

Abstract

High-frequency measurements of dibromomethane (CH2Br2) and bromoform (CHBr3) at Hateruma Island, in the subtropical East China Sea, were performed using automated preconcentration gas chromatography/mass spectrometry. Their baseline concentrations, found in air masses from the Pacific Ocean, were 0.65 and 0.26 ppt, respectively, in summer and 1.08 and 0.87 ppt, respectively, in winter. Air masses transported from Southeast Asia were rich in bromocarbons, suggesting strong emissions in this area. The passage of cold fronts from the Asian continent was associated with sharp increases in observed concentrations of bromocarbons derived from coastal regions of the continent. Comparison of the relationships between [CH2Br2]/[CHBr3] and [CHBr3] in the Hateruma Island data with those in monthly mean data from 14 globally distributed U.S. National Oceanic and Atmospheric Administration ground stations suggested that these gases are produced primarily from a common process on a global scale.