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AbstractA simple model is presented to estimate atmospheric concentrations of chemical species that exist primarily as aerosols based on snow core/ice core chemistry at Summit, Greenland. The model considers the processes of snow, fog, and dry deposition. The deposition parameters for each of the processes are estimated for SO4 2? and Ca2+ and are based on experiments conducted during the 1993 and 1994 summer field seasons. The seasonal mean atmospheric concentrations are estimated based on the deposition parameters and snow cores obtained during the field seasons. The ratios of the estimated seasonal mean airborne concentration divided by the measured mean concentration ( C ¯ a,est / C ¯ a,meas ) for SO4 2? over the 1993 and 1994 field seasons are 0.85 and 0.95, respectively. The C ¯ a,est / C ¯ a,meas ratios for Ca2+ are 0.45 and 0.90 for the 1993 and 1994 field seasons. The uncertainties in the estimated atmospheric concentrations range from 30% to 40% and are due to variability in the input parameters. The model estimates the seasonal mean atmospheric SO4 2? and Ca2+ concentrations to within 15% and 55%, respectively. Although the model is not directly applied to ice cores, the application of the model to ice core chemical signals is briefly discussed.
AbstractExperiments were performed during the period May-July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO4 2? were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 ?m and greater than 0.01 ?m were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO4 2? is the dominant aerosol anion species, with Na+, NH4 +, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO4 2? have similar mass size distributions. Furthermore, MSA and SO4 2? have mass in both the accumulation and coarse modes. A limited number of samples for NH4 + indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/? 0.011 cm/s for NH4 + to 0.110 cm/s +/? 0.021 cm/s for Ca. SO4 2? dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 ?m in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 ?m are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO4 2? serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3 ?, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4 +. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO4 2? and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4 + and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.
AbstractThe temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground. The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between similar to9.00 and 18.00 h local time with the formation of shallow mixing heights of similar to70-250 m above the surface. The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37-76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. A similar to0.1-3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime.
AbstractSurface 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.
AbstractCauses for the unusually high and seasonally anomalous ozone concentrations at Summit, Greenland were investigated. Surface data from continuous monitoring, ozone sonde data, tethered balloon vertical profiling data, correlation of ozone with the radionuclide tracers Be-7 and Pb-210, and synoptic transport analysis were used to identify processes that contribute to sources and sinks of ozone at Summit. Northern Hemisphere (NH) lower free troposphere ozone mixing ratios in the polar regions are similar to 20 ppbv higher than in Antarctica. Ozone at Summit, which is at 3212m above sea level, reflects its altitude location in the lower free troposphere. Transport events that bring high ozone and dry air, likely from lower stratospheric/higher tropospheric origin, were observed similar to 40% of time during June 2000. Comparison of ozone enhancements with radionuclide tracer records shows a year-round correlation of ozone with the stratospheric tracer Be-7. Summit lacks the episodic, sunrise ozone depletion events, which were found to reduce the annual, median ozone at NH coastal sites by up to similar to 3 ppbv. Synoptic trajectory analyses indicated that, under selected conditions, Summit encounters polluted continental air with increased ozone from central and western Europe. Low ozone surface deposition fluxes over long distances upwind of Summit reduce ozone deposition losses in comparison to other NH sites, particularly during the summer months. Surface-layer photochemical ozone production does not appear to have a noticeable influence on Summit's ozone levels. (C) 2007 Published by Elsevier Ltd.
AbstractThe uptake of atmospheric ozone to the polar, year-round snowpack on glacial ice was studied at Summit, Greenland during three experiments in 2003, 2004, and 2005. Ozone was measured at up to three depths in the snowpack, on the surface, and above the surface at three heights on a tower along with supporting meteorological parameters. Ozone in interstitial air decreased with depth, albeit ozone gradients showed a high variation depending on environmental conditions of solar radiation and wind speed. Under low irradiance levels, up to 90% of ozone was preserved up to 1 m depth in the snowpack. Ozone depletion rates increased significantly with the seasonal and diurnal cycle of solar irradiance, resulting in only 10% of ozone remaining in the snowpack following solar noon during summertime. Faster snowpack air exchange from wind pumping resulted in smaller above-surface-to-within snowpack ozone gradients. These data indicate that the uptake of ozone to polar snowpack is strongly dependent on solar irradiance and wind pumping. Ozone deposition fluxes to the polar snowpack are consequently expected to follow incoming solar radiation levels and to exhibit diurnal and seasonal cycles. The Summit observations are in stark contrast to recent findings in the seasonal, midlatitude snowpack [Bocquet, F., Helmig, D., Oltmans, S.J., 2007. Ozone in the mid-latitude snowpack at Niwot Ridge, Colorado. Arctic, Antarctic and Alpine Research, in press], where mostly light-independent ozone behavior was observed. These contrasting results imply different ozone chemistry and snowpack–atmosphere gas exchange in the snow-covered polar, glacial conditions compared to the temperate, mid-latitude environment.
AbstractContinuous surface-layer ozone flux measurements over the polar, year-round snowpack at Summit, Greenland, resulted in deposition velocities (v(d)) that were smaller than most previous assumptions and model inputs. Substantial seasonal differences were seen in the ozone v(d) behavior. Spring, daytime ozone v(d) values showed low variability and were consistently <= 0.01 cm s(-1). During summer, ozone fluxes displayed distinct diurnal cycles, and evidence for regular occurrences of bi-directional behavior. Summer, daytime v(d) ranged between similar to 0.01 to 0.07 cm s(-1). Maximum summertime downward fluxes (ozone deposition) coincided with the hours of maximum solar radiation, i.e., noon afternoon. During summer nighttime hours upward ozone fluxes were observed. These upward fluxes were interpreted as ozone production in a shallow layer near and above the snow surface with resulting upward ozone fluxes out of the shallow surface layer. Comparisons with published observations from temperate, midlatitude sites suggest different controls and behavior of ozone fluxes, and that ozone fluxes over snow depend on a myriad of parameters, including solar irradiance, snow chemical and physical properties, snowpack depth, and the type of substrate underneath the snow. Citation: Helmig, D., L. D. Cohen, F. Bocquet, S. Oltmans, A. Grachev, and W. Neff (2009), Spring and summertime diurnal surface ozone fluxes over the polar snow at Summit, Greenland, Geophys. Res. Lett., 36, L08809, doi:10.1029/2008GL036549.
AbstractAs a part of the IPY project POLARCAT (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate Chemistry, Aerosols and Transport), this paper studies the sources of equivalent black carbon (EBC), sulphate, light-scattering aerosols and ozone measured at the Arctic stations Zeppelin, Alert, Barrow and Summit during the years 2000–2007. These species are important pollutants and climate forcing agents, and sulphate and EBC are main components of Arctic haze. To determine where these substances originate, the measurement data were combined with calculations using FLEXPART, a Lagrangian particle dispersion model. The climatology of atmospheric transport from surrounding regions on a twenty-day time scale modelled by FLEXPART shows that the stations Zeppelin, Alert and Barrow are highly sensitive to surface emissions in the Arctic and to emissions in high-latitude Eurasia in winter. Emission sensitivities over southern Asia and southern North America are small throughout the year. The high-altitude station Summit is an order of magnitude less sensitive to surface emissions in the Arctic whereas emissions in the southern parts of the Northern Hemisphere continents are more influential relative to the other stations. Our results show that for EBC and sulphate measured at Zeppelin, Alert and Barrow, northern Eurasia is the dominant source region. For sulphate, Eastern Europe and the metal smelting industry in Norilsk are particularly important. For EBC, boreal forest fires also contribute in summer. No evidence for any substantial contribution to EBC from sources in southern Asia is found. European air masses are associated with low ozone concentrations in winter due to titration by nitric oxides, but are associated with high ozone concentrations in summer due to photochemical ozone formation. There is also a strong influence of ozone depletion events in the Arctic boundary layer on measured ozone concentrations in spring and summer. These results will be useful for developing emission reduction strategies for the Arctic.
AbstractThe seasonal variation in atmospheric transport patterns to Summit, Greenland, is examined using a 44-year record of daily, 10-day, isobaric back trajectories at the 500-hPa level. Over 24,000 modeled trajectories are aggregated into distinct patterns using cluster analysis. Ten-day trajectories reaching Summit are longest during winter, with 67% extending upwind (westward) as far back as Asia or Europe. Trajectories are shortest during summer, with 46% having 10-day origins over North America. During all seasons a small percentage (3–7%) of trajectories originate in west Asia/Europe and follow a meridional path over the Arctic Ocean before approaching Summit from the northwest. Trajectories at the 700-hPa level tend to be shorter than at 500 hPa, with many of the 700-hPa trajectories from North America tracking over the North Atlantic and approaching Summit from the south. The long-range transport climatology for Summit is similar to a year-round climatology prepared for Dye 3, located 900 km to the south [Davidson et al., 1993b]. An analysis of several aerosol species measured at Summit during summer 1994 reveals examples of the usefulness and also the limitations of using long-range air trajectories to interpret chemical data.
AbstractThe interannual variations in atmospheric transport patterns to Summit, Greenland, are studied using twice-daily, three-dimensional, 10day backward trajectory data corresponding to the summers (1June-31August) of 1989-98.While previous trajectory climatology studies have been prepared for Summit, the present work considers both the horizontal and vertical components of transport. A three-dimensional residence-time methodology is employed to account for both horizontal and vertical components of transport. The vertical transport component is quantified by passing all trajectories through a three-dimensional grid and tracking the time spent (i.e. the residence time) in each gridcell. This method also allows inspection of trajectory altitude distributions corresponding to transport from upwind regions of interest.The three-dimensional residence-time methodology is shown to be a valuable tool for diagnosing the details of long-range atmospheric transport to remote locations. For Summit, we find that the frequent transport from North America tends to occur at low altitudes, whereas transport from Europe is highly variable. Mean summertime flow patterns are described, as are anomalous patterns during 1990, 1996 and 1998.
Measurements of cloud properties over Summit,Greenland are necessary to document the full range of cloud conditions and characteristics throughout the Arctic. A new lidar system has been developed to measure depolarization and backscatter of clouds in the lower troposphere and aerosols in the upper troposphere and lower stratosphere. This lidar uses recent developments in optical methods to more accurately characterize system polarization effects. This allows the system to more accurately measure aerosols and cloud polarization ratios through methods that minimize systematic biases. The lidar is located at Summit, Greenland as part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit project and NOAA’s Global Monitoring Division’s stratospheric lidar network.