Passive microwave remote sensing over the Greenland Summit in support of the GISP2 project
Shuman, C.A., R.B. Alley, S. Anandakrishnan, K.M. Cuffey, and C.R. Stearns

Processes affecting the ice sheet surface at the Greenland Summit are progressively recorded in an ice core from the region. Passive microwave remote sensing, with its broad spatial and detailed temporal coverage, provides a sensitive means of analyzing specific surface processes and parameters as well as a means of extending local observations over regional scales. Two aspects of this research in support of the GISP2 project are outlined below.

The development of hoar complex layers at or near the snow surface across central Greenland provides a visible means of identifying the summer period of accumulation in a snow pit or an ice core. Hoar complex development, through a combination of grain growth, density reduction, and an increase of centimeter-scale surface roughness, generates distinctive variations in polarized passive microwave radiation. Polarization ratio trends across the Summit region show a gradual decline as a hoar complex develops and an abrupt increase when new snow and/or wind scour resurfaces the region. This characteristic signal pattern has allowed identification of hoar events across the dry firn facies over the Greenland Summit using satellite data. The current widespread development of hoar layers suggests that ice sheet facies migration associated with climate change would not affect their occurrence at Summit. The long-term passive microwave record allows monitoring of the temporal development of these annual summer markers which influences their interpretation in the GISP2 core. Field evidence, in the form of daily surface observations of hoar complex development and decay at the GISP2 site, as well as temporally-constrained snow pit profiles, have been used to correlate the satellite signals to the development of this critical summer marker.

Decadal-length, near-surface temperature records from the Summit region can be obtained from satellite- derived passive microwave brightness temperatures and short-term automatic weather station (AWS) data. A combination of short-term AWS temperature trends and emissivity modeling can be used to calibrate the brightness temperature data from a specific passive microwave channel through a modified Rayleigh-Jeans approximation. The resulting near-surface temperature records, extending back almost two decades, can then be compared to other proxy temperature records such as stable isotopes or used in climate modeling of the Greenland Summit.

Results from automatic weather stations on the Greenland crest
Stearns, C.R. and G. A. Weidner

The Greenland automatic weather station (AWS) record started in May 1987 with a single site on the Greenland Crest (Cathy, 72.3oN, 38.0oW, 3210 m, May 87 to May 89). The final AWS sites totaled six on and around the Greenland Crest. The site names, latitude, longitude, elevation and start date are: GISP2, 72.58oN, 38.46oW, 3205 m, Jun. 89; Kenton, 72.28oN, 38.82oW, 3185 m, Jun. 89; Klinck, 72.31oN, 40.48oW, 3105 m, Aug. 90; Barber, 71.67oN, 38.17oW, 3170 m, Jul 90; Julie, 72.57oN, 34.63oW, 3100 m, Aug. 91; Matt, 73.48oN, 37.62oW, 3100 m, Aug. 91. Snow accumulation measurements were started in June 1992 at GISP2 and extended to Kenton site in June 1993 and Klinck site in June 1994. The AWS units measure wind speed, wind direction, air temperature, and relative humidity at a nominal height of 3.6 m, air pressure at the electronics enclosure, and the vertical air temperature difference between 3 and 0.5 meters. A snow temperature profile to 10 m is measured at GISP2. The data are transmitted at 200 second intervals to the ARGOS System on board the polar orbiting satellites of the NOAA series. The data are updated at 10 minute intervals and between 80 and 144 data points for each AWS unit are received every 24 hours. Between the months of November to April the wind systems and the acoustic depth gauge apparently collect hoarfrost and substantial amounts of data are lost. Then in April or May the systems will start functioning again.

The AWS wind speed, vertical temperature difference, and relative humidity are used to estimate the sensible and latent heat fluxes from the snow surface. The GISP2 snow temperature profile is used to estimate the heat flux into the snow. The AWS sites are located so that the divergence and the vorticity of the airflow around the Greenland Crest can be determined. Comparisons will be made between the snow accumulation measured by the acoustic depth gauge and the divergence of the wind field. Tables of monthly means, maximums and minimum values of air pressure and air temperature, mean and maximum wind speed, resultant wind speed and direction, and monthly means of sensible and sensible heat will be presented. Examples of the relationship between snow accumulation, the divergence and vorticity field and the synoptic weather patter will be give. The method used to forecast the weather for the Greenland Crest will be described along with an example of the forecast.

Beryllium-7 and lead-210 in the atmosphere and surface snow over the Greenland ice sheet in the summer of 1989
Dibb, J. E.
Journal of Geophysical Research, Vol. 95, No. D13, p. 22,407-22,415, 1990

The concentrations of 7Be and 210Pb were measured in surface air and fresh and aging snow samples from Summit (72 20' N, 38 45' W) and Dye 3 (65 10' N, 44 45' W) Greenland, during June and July, 1989. The aerosol concentrations of these radionuclides showed rapid variations at both sites, but were nearly twice as high, on average, at Summit. Concentrations in the 16 fresh snowfall events that were sampled also showed wide variability, but the averages were the same at the two sites. The apparent difference in air-snow fractionation and the lack of coherence in the concentration in air time series between the two sites indicate previously unsuspected complexity in atmospheric dynamics over the ice sheet. Improved understanding of atmospheric processes, and how the results of those processes are recorded in snow and ice, are crucial for full interpretation of the information about past atmospheric chemistry and climate contained in the snow and ice of glaciers around the world.

A two year record of the climate on the Greenland crest from an automatic weather station
Weidner, G. A., and C. R. Stearns
Proceedings of the International Conference on the Role of Polar Regions on Climate Change, Vol. 1, June 11-15, 1990 Fairbanks, Alaska

An automatic weather station (AWS) was installed on the Greenland Summit (72.30N, 38.00W, 3210 m) in May 1987. The AWS unit operated for two years until May 1989 when it was moved to Fresh Air Site (72.82N, 38.82W, 3185 m), an air sampling site, where it is still operating. The AWS data were transmitted to the ARGOS data collection system on the NOAA polar orbiting satellites. The AWS unit measures wind speed and direction, air temperature, and the relative humidity at a nominal height of three meters, air pressure at the height of the electronics enclosure, and the vertical air temperature difference between 3.0 m and 0.5 meters. The latent and sensible heat from the snow surface to the air were estimated using the wind speed, vertical air temperature difference, and the relative humidity. The data are compared with that from two earlier stations, Eismitte (70.90N, 40.70W, 3000 m) from September 1930 through August 1931 (Wegener's expedition) and Station Central (70.92N, 40.64W, 2993 m) from September 1949 through August 1951 (Victor's Expedition). The winds observed at Cathy Site were quite similar to those observed at the two previous stations. Also, the large fluctuations in temperature observed during the winter months at the two historic stations were sensible and latent heat flux occurred in October.

Snow temperature profiles and heat fluxes measured on the Greenland crest by automatic weather stations
Stearns, C. and G. Wiedner
Proceedings of the International Conference on the Role of the Polar Regions in Global Change, Vol. I, pp. 223-226, June 11-15, 1990, Fairbanks, Alaska

In June 1989 three automatic weather station (AWS) units were installed on the Greenland crest at the GISP2 (78.58N, 38.46W, 3205 m) and GRIP (78.57N, 37.62W, 3230 m) ice coring sites and at Kenton (73.28N, 38.80W, 3185 m), the air sampling site. The purpose of the AWS units is to measure the local meteorological variables, including snow temperatures at various depths, in support of ice coring studies. The AWS units measure wind speed and direction, air temperature, and relative humidity at a nominal height of 3.6 meters, air pressure at the electronics enclosure, and air temperature difference between 3.6 m and 0.5 m. The AWS units at GISP2 and GRIP also measure solar radiation, and seven snow temperatures from the surface to a depth of approximately 4 m in the snow. The data are updated at 10- minute intervals and transmitted to the ARGOS data collection system on board the NOAA series of polar-orbiting satellites. The air temperature and snow temperatures are presented as a function of time for the period form June 8, 1989 to August 31, 1990 and as tautochrones at 30-day intervals. The heat flux into the snow is determined from the daily mean snow temperature between the day after and the day before using the volumetric heat capacity of the snow assuming a snow density of 300 kg m-3. The daily mean heat flux into the snow between the highest and the lowest levels of snow temperature is presented as a function of time.

The polar automatic weather station project of the University of Wisconsin.
Stearns, C. and G. Wiedner
Proceedings of the International Conference on the Role of the Polar Regions in Global Change, Vol. I, p. 58-62, June 11-15, 1990, Fairbanks, AK.

Initial findings of recent investigations of air-snow relationships in the summit region of the Greenland ice sheet
Dibb, J. E., J.-L. Jaffrezo, and M. Legrand
Journal of Atmospheric Chemistry, Vol. 14, p.167-180, 1992

The concentrations of 7Be, 210Pb and major ions have been measured in aerosol and snow samples collected near Summit, Greenland (7220'N, 3845'W) in the summers of 1989 and 1990. Comparison to previous results from free tropospheric sampling of the North American Arctic indicates that some aerosol-associated species are as much as 50% depleted in near surface air over the Greenland Ice Sheet. It is shown that local atmospheric processes, particularly isolation of air masses beneath a near surface inversion, can exert dominant influence on the chemistry of surface-level air. These findings illustrate the extreme caution that must be taken if the results of surface-based atmospheric sampling are to be used to examine the relationship between the chemistry of the atmosphere and snow falling from it. Depth profiles of 7Be in the surface layers of the snowpack near Summit suggest that up to half of the annual accumulation of snow may occur in the two to three month late spring-early summer period. If this is generally true for the Summit region, previous regional studies of snow chemistry that assumed linear dependence of age on depth to convert depth profiles to time series will have to be reassessed. However, spatial heterogeneity of near surface snow chemistry, that is currently not well understood, makes interpretation of the 7Be profiles tentative at present

The accumulation of 210Pb at Summit, Greenland since 1855
Dibb, J. E.
Tellus, Vol. 44B, p. 72-79, 1992

Detailed (202 samples) profiles of total beta and 210Pb activity were determined from a 32 m firn core collected at the GISP2 site (72 20' N, 38 45' W) near Summit, Greenland in 1989. The beta radioactivity profile verifies year by year dating based on recognition of annual hoar layers back to 1955, and lends a high degree of confidence to the 74 year age assigned to the bottom of the core by this stratigraphic dating technique. The decay corrected activity of 210Pb at the time of deposition shows considerable short term variability, but no clear seasonal or annual periodicity. 210Pb activity in surface snow at this site has averaged 0.7 pCi kg-1 since 1927, but the period 1915 - 1927 is characterized by a steady decline from higher levels. The average annual accumulation of 210Pb has markedly declined since at least 1870. Similar observations at Dye 3 suggest that 210Pb accumulation has decreased throughout this century over much of the Greenland Ice Sheet. If the records of 210Pb in the firn on the Greenland Ice Sheet are mainly reflecting the northern hemisphere atmospheric burden of 210Pb, these results will demand careful reassessment of 210Pb-based radiochronologies. However, our current lack of understanding of the linkages between atmospheric and snow chemistry makes the widespread applicability of these findings an open question.

Field observations, measurements, and preliminary results from a study of wet deposition processes influencing snow and ice chemistry at Summit, Greenland
Borys, R. D., D. Del Vecchio, J.-L. Jafrezzo, J. Dibb, and D. L. Mitchell
Precipitation Scavenging and Atmosphere-Surface Exchange, Vol. 3, S.E. Schwartz and W.G.N. Slinn, Coord., Hemisphere Pub. Corp., p. 1705-1718, 1992

During the summers of 1989 and 1990, investigations were conducted on the nature of wet deposition to the Greenland ice sheet at Summit (3025 m MSL). Ice particle habits and growth processes, the formation of surface hoar frost and the formation and physical properties of fogs were examined. The chemical composition of the ice particles, hoar frost and fogs was also determined. From these simple first measurements, an assessment of the contribution from snow, frost and cloud deposition of water and chemical species is made. Results indicate that direct deposition of cloud or fog water may be significant in determining the growth and chemistry of the ice cap. Surface deposition of frost and fog may be responsible for 5% to 10% of the measured ice accumulation rates and from 27% to 44% of the chemical deposition of certain species.

Air flow and dry deposition of non-sea salt sulfate in polar firn: Paleoclimatic implications
Cunningham, J. and E.D. Waddington
Atmospheric Environment, Vol. 27A, No. 17/18, p. 2943-2956, 1993

Non-sea salt sulfate aerosol (NSS) is an important factor for the Earths albedo because it backscatters solar radiation and is the major cloud condensation nucleus over oceans. At Vostok, Antarctica, NSS concentration shows an increase in glacial period ice of 20-46% that cannot be attributed to changes in accumulation rate. The additional NSS may be due to enhanced dry deposition of NSS by topographic windpumping during the windy glacial periods. We model the volume flux of air into snow due to barometric pressure changes and air flow over surface microrelief. The Gormley-Kennedy equation approximately describes how aerosols advocated into the snow pack are removed from the air by diffusion to the snow matrix. Barometric pressure and wind speed data from several polar sites have been used to quantify the vertical volume flux of air and mass flux of NSS. Model results indicate that air flow over small sastrugi, wind carved snow dunes commonly found on ice caps, is the dominant dry deposition mechanism for NSS. Paleo wind speed and surface roughness can significantly influence the aerosol record in ice cores.

Atmosphere-snow transfer function for H2O2: Microphysical Considerations
Conklin, M.A., A. Sigg, A. Neftel, and R. C. Bales
Journal of Geophysical Research, Vol. 98, No. D10, p. 18,367-18,376, 1993

H2O2 analyses of polar ice cores show an increase in concentration from 200 years to the present. In order to quantitatively relate the observed trend in the ice to atmospheric levels, the atmosphere-snow transfer behavior and post-depositional changes must be known. Atmosphere- snow transfer was studied by investigating uptake and release of H2O2 in a series of laboratory column experiments in the temperature range -3C to -45C. Experiments consisted of passing H2O2-containing air through a column packed with 200-um diameter ice spheres, and measuring the change in gas-phase H2O2 concentration with time. The uptake of H2O2 was a slow process requiring several hours to reach equilibrium. Uptake involved incorporation of H2O2 into the bulk ice as well as surface accumulation. The amount of H2O2 taken up by the ice was greater at the lower temperatures. The sticking coefficient for H2O2 on ice in the same experiments was estimated to be on the order of 0.02-0.5. Release of H2O2 from the ice occurred upon passing H2O2-free air through the packed columns, with the time scale for degassing similar to that for uptake. These results suggest that systematic losses of H2O2 from polar snow could occur under similar conditions, when atmospheric concentrations of H2O2 are low, i.e., in the winter.

Soluble acidic species at Summit, Greenland
Dibb, J. E., R. W. Talbot, and M. H. Bergin
Geophysical Research Letters, Vol. 21, No.15, p. 1627-1630, July 15, 1994

Thirty six mist chamber samples were collected 1.5 m above the snow surface at Summit, Greenland (72o 25' N, 36o 40' W, 3200 m elevation) between 20 June and 20 July, 1993. Mean concentrations of gas phase formic, acetic, and nitric acids (49 + 28, 32 + 17 and 0.9 + 0.6 nmol m -3 STP, respectively) exceeded the concentrations of aerosol-associated formate, acetate and nitrate by 1 - 3 orders of magnitude. On the average, SO2 concentrations (0.9 + 0.6 nmol m -3 STP) were approximately 1/3 those of aerosol sulfate, but this ratio varied widely due largely to variations in the concentration of aerosol sulfate. The relative abundances of these species in surface snow were opposite of those in the atmosphere; nitrate concentrations in snow were 3- 20-fold more abundant than sulfate, while concentrations of formate and acetate were less than sulfate. The observed partitioning of these acidic species between the gas and aerosol phases over the Greenland ice sheet was similar to previous studies at lower latitudes, but the relationships between concentrations in the atmosphere and those in surface snow are poorly understood.

Polycyclic aromatic hydrocarbons in the Polar Ice
Jaffrezo, J.-L., M. P. Clain and P. Masclet
Atmospheric Environment, Vol. 28A, p. 1139-1145, 1994

Sampling of surface snow for PAH analysis took place at the Summit of the Greenland Ice Sheet in summer 1991, with 28 samples collected in a 3 m snowpit covering the previous 4 years of deposition. All concentrations were below detection limits (a few pg kg-1) in the soluble phase, while concentrations of 13 PAH were determined in the insoluble fraction. These are essentially the same that are present in the aerosol at this location. The total amount of PAH is on the order of a few hundred pg kg-1 on average, lower than concentrations from precipitation samples of remote sites at sea level. The concentration profiles in the ice indicate summer minima for all species, attributed to lower emissions, shorter distances of transport and reactions in the atmosphere at that time of the year. The maxima take place in spring for many compounds, in phase with that of sulfate, and is tentatively attributed to the influence of Arctic Haze. However, concentration increases are already seen in winter, particularly for 3-ring species that peak at that time. This situation could reflect the larger emissions in winter, but indicates also differential scavenging among the compounds and specific transport pathways. Finally, the snowpit profile shows that some modification occurs after deposition, with for example a 90% decrease of benzo pyrene concentrations in the course of 4 years. The rate of change seems in rough agreement with the atmospheric reactivity index of the compounds.

Sulfate and MSA in the air and snow on the Greenland ice sheet
Jaffrezo J. L., C. I. Davidson, M. Legrand, and J. E. Dibb
Journal of Geophysical Research, Vol. 99(D1), p. 1241-1253, 1994

Sulfate and methanesulfonic acid (MSA) concentrations in aerosol, surface snow and snowpit samples have been measured at two sites on the Greenland Ice Sheet. Seasonal variations of the concentrations observed for these chemical species in the atmosphere are reproduced in the surface snow and preserved in the snowpit sequence. The amplitude of the variations over a year are smaller in the snow than in the air, but the ratios of the concentrations are comparable. The seasonal variations for sulfate are different at the altitude of the Ice Sheet compared to those observed at sea level, with low concentrations in winter and short episodes of elevated concentrations in spring. In contrast, the variations in concentrations of MSA are similar to those measured at sea level, with a first sequence of elevated concentrations in spring and another one during summer, and a winter low resulting from low biogenic production. The ratio MSA/sulfate clearly indicates the influence of high latitudes source for the summer maximum of MSA, but the large impact of anthropogenic sulfate precludes any conclusion for the spring maximum. The seasonal pattern observed for these species in a snowpit sampled according to stratigraphy indicates a deficit in the accumulation of winter snow at the summit of the Greenland Ice Sheet, in agreement with some direct observations. A deeper snowpit covering the years 1985-1992 indicates the consistency of the seasonal pattern for MSA over the years, which may be linked to transport and deposition processes.

Fluxes of chemical species to the Greenland ice sheet at Summit by fog and dry deposition
Bergin M. H., J.-L. Jaffrezo, C. I. Davidson, R. Caldow, J. Dibb
Geochim. Cosmoschim. Acta, Vol. 58, p. 3207-3215, 1994

Experiments were performed during June-July 1992 to determine the impact of dry deposition and fog deposition on surface snow chemical inventories. The fluxes of SO42-, NO3-, Cl-, MSA, Na+, Ca2+, and Al were measured by collecting deposited fog on a flat polyethylene plate. Dry deposition fluxes of SO42- were measured using an aerodynamic surface. Real time concentrations of atmospheric particles greater than 0.5 um and 0.01 um were measured using continuous monitors. Filter samplers were used to determine daily average atmospheric SO42- and MSA concentrations. Also, daily surface snow samples were taken and analyzed for SO42-, NO3-, Cl-, Na+, Ca2+ and NH4+. The real-time concentration data indicate that fog efficiently scavenges particles greater than 0.5 um from the atmosphere. Results also show that condensation nuclei (CN) are not as greatly affected by fog as the larger particles. Fog fluxes of SO42-and NO3- have similar values and are approximately 4 times greater than those of Cl-, an order of magnitude greater than those of MSA, Na+, and Ca2+, and two orders of magnitude greater than those of Al. The fog deposition flux of SO42- appears to be much greater than the dry deposition flux, based on experimental data. This indicates that dry deposition has a negligible effect on surface snow SO42-concentrations on days when there is fog. Such a finding is consistent with significant increases in surface snow SO42-, NO3-, and NH4+ inventories seen after fog events. Cl- surface snow inventories are affected by fog but not as greatly. Variation in surface snow chemical inventories makes it difficult to obtain quantitative estimates of daily chemical fluxes. Surface snow Ca2+ and Na+ are relatively unaffected by post snowfall processes due to low atmospheric concentrations relative to the amount of material in fresh snow. Model results suggest that the fog fluxes have been underestimated by the current sampling technique.

Changes in continental and sea-salt atmospheric loadings in central Greenland during the most recent deglaciation
Alley, R.B., R.C. Finkel, K. Nishiizumi, S. Anandakrishnan, C.A. Shuman, G.R. Mershon, G.A. Zielinski and P.A. Mayewski
Journal of Glaciology, Vol. 41, p. 503-514, 1995

By fitting a very simple atmospheric-impurity model to high-resolution data on ice accumulation and contaminant fluxes in the GISP2 ice core, we have estimated changes in the atmospheric concentrations of soluble major ions, insoluble particulates and 10Be during the transition from glacial to Holocene conditions. For many species, changes in concentration in the ice typically overestimate atmospheric changes, and changes in flux to the ice typically underestimate atmospheric changes, because times of increased atmospheric contaminant loading also are times of reduced snowfall. The model interpolates between the flux and concentration records by explicitly allowing for wet- and dry-deposition processes. Compared to the warm Preboreal that followed, we estimate that the atmosphere over Greenland sampled by snow accumulated during the Younger Dryas cold event contained on average four to seven times the insoluble particulates and nearly seven times the soluble calcium derived from continental sources, and about three times the sea salt, but only slightly more cosmogenic 10Be.

The contributions of wet, fog, and dry deposition to the summer SO42- flux at Summit, Greenland
Bergin, M.H., C.I. Davidson, J.L. Jaffrezo, J.E. Dibb, R. Hillamo, H.D. Kuhns, T. Makela
Ice Core Studies of Global Biogeochemical Cycles, Annecy, France, R. Delmas (editor). NATO Advanced Sciences Institutes Series 1, Vol. 30, p. 121-138, 1995

Experiments were performed during May-July of the 1993 field season at Summit, Greenland. Real time concentrations of particles greater than 0.5 mm and greater than 0.01 m were measured with continuous monitors. Filter samplers were used to determine the daily average aerosol SO42- concentrations, and impactors were used to determine mass size distributions. Dry deposition velocities for SO42- were estimated using surrogate surfaces (symmetric airfoils) and the airborne size distribution data. Snow and fog samples from nearly all of the events occurring during the field season were collected on polyethylene trays. Impactor and real time concentration data indicate that particles > 0.5 m efficiently serve as nuclei to form fog droplets. Results also show that condensation nuclei > 0.01 m (CN) are not as greatly affected by fog. Dry deposition velocity estimates using the airfoils are in the range 0.023 cm/s to 0.062 cm/s, 60% greater than values calculated using the airborne size distribution data with a model for deposition to snow. This could be due to differences in the boundary layer resistances of the airfoils and the modeled snow surface; furthermore, calculations using the impactor results assume no particle growth in the viscous sub layer. The contribution of wet, fog, and aerosol dry deposition to the seasonal SO42- inventory is estimated as 58% 6%, 25% 4%, and 17% 7%, respectively. These values do not take into consideration the spatial variability caused by the blowing and drifting of surface snow. Results indicate that all three processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.

Current status of atmospheric studies at Summit (Greenland) and implications for future research
Jaffrezo, J.-L., J.E. Dibb, R.C. Bales, and A. Neftel
Ice Core Studies of Global Biogeochemical Cycles, Annecy, France, R. Delmas (editor). NATO Advanced Sciences Institutes Series 1, Vol. 30, p. 427-458, 1995

The present paper is an overview of the some of the basic questions raised by the recent experiments in Greenland, in the field of air-snow transfer of gases and particles. After an introduction that indicates the frame of the ATM programme, we successively present results of studies concerning the transport of chemical species from source regions to the atmosphere of the Greenland Ice Sheet, their deposition processes, and the post deposition effects that can modify the chemical signal recorded in the snow layers. It shows that a comprehensive approach of all of these questions calls for complementary investigations in research areas covering several scales of space and time, from large scale meteorology to microscale physics. In many cases, the field studies performed during DGASP and ATM increased significantly our knowledge of the processes involved in the transfer of chemical species from source regions to the deep ice, and allowed the determination of the most important parameters controlling these processes. But many areas are still poorly understood, that we try to point out.

H2O2 and HCHO in polar snow and their relation to atmospheric chemistry
Neftel, A., R.C. Bales, and D.J. Jacob
Ice Core Studies of Global Biogeochemical Cycles, Annecy, France, R. Delmas (editor). NATO Advanced Sciences Institutes Series 1, Vol. 30, p. 249-264, 1995

Our ability to infer past atmospheric concentrations of hydrogen peroxide (H2O2) and formaldehyde (HCHO) from concentrations measured in polar firn and ice cores is limited by our lack of understanding of the atmosphere-snow transfer functions for the two species. Continuous H2O2 records going back into the last century have been obtained from Dye 3 and Summit in central Greenland and Siple in west Antarctica, with some measurements preserved back into the last glaciation. HCHO records also extend back into the last glaciation in these same cores. Surface-snow and pit studies clearly show that H2O2 and HCHO concentrations in snow change in the days to weeks after the snow is deposited. It is thought that buried layers reflect seasonal average values, mediated by the amount and seasonal pattern of water accumulation and the temperature. Observed surface air concentrations of H2O2 and HCHO at Summit, Greenland in summer are about 3-4 times those predicted by photochemical modeling; evaporation from the snow may explain the discrepancy. Development of process models of phase exchange in the snow and firn is essential for interpreting the ice core records of H2O2 and HCHO in terms of trends in the oxidizing power of the atmosphere.

H2O2 in snow, air and open pore space in firn at Summit, Greenland
Bales, R. C., M. Losleben, J. McConnell , K. Fuhrer , and A. Neftel
Geophysical Research Letters, Vol. 22, No. 10, p. 1261-1264, 1995

Measurements of H2O2 in firn and firn gas down to a 1.7-m depth showed a consistent trend, with higher firn-gas concentrations generally associated with higher concentrations in the firn at the same depth. However, firn to firn-gas concentration ratios still exhibited a seasonal dependence, suggesting that at least for summer layers equilibrium has not yet been reached. The time to reach equilibrium between firn and firn gas is at least weeks. Snowfall and fog deposit several times more H2O2 than the surface snow will retain at equilibrium, supporting the idea that surface snow is a temporary reservoir for H2O2 . Thus from an equilibrium standpoint, the snowpack should be a source of atmospheric H2O2 in the summer as well as fall, resulting in higher daytime concentrations than would occur based on just atmospheric photochemical reactions. But firn-gas measurements reported here were generally lower than those in the atmosphere, suggesting that degassing is too slow to significantly influence atmospheric H2O2 levels.

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland
Bergin, M.H., J.-L. Jaffrezo, C. I. Davidson, J. E. Dibb, S. N. Pandis, R. Hillamo, W. Maenhaut, H. D. Kuhns, and T. Makela
Journal of Geophysical Research, Vol. 100, No. D8, p. 16,275-16,288, August 20, 1995

Experiments 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 SO42- were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 um and greater than 0.01 um were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42- is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO42- have similar mass size distributions. Furthermore , MSA and SO42- 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+, Mg2+, and Ca2+ 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. SO42- 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. L aser particle counter (LPC) results show that particles > 0.5 um in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 um are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42- 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 SO42- 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.

Diel variations of H2O2 in Greenland: A discussion of the cause and effect relationship
Bales, R. C., J.R. McConnell, M.V. Losleben, M. H. Conklin,
K. Fuhrer , A. Neftel , J.E. Dibb , J.D. W. Kahl, C. R. Stearns
Journal of Geophysical Research, Vol. 100, No. D9, p. 18,661-18,668, September 20, 1995

Atmospheric hydrogen peroxide ( H2O2) measurements at Summit, Greenland in May-June, 1993 exhibited a diel variation, with afternoon highs typically 1-2 ppbv and nighttime lows about 0.5 ppbv lower. This variation closely followed that for temperature; specific humidity exhibited the same general trend. During a 17-day snowfall-free period, surface snow was accumulating H2O2, apparently from nighttime co-condensation of H2O and H2O2. Previous photochemical modeling [Neftel et al., 1994] suggests that daytime H2O2 should be about 1 ppbv, significantly lower than our measured values. Previous equilibrium partitioning measurements between ice and gas phase [Conklin et al., 1993] suggest that air in equilibrium with H2O2 concentrations measured in surface snow (15-18 uM) should have an H2O2 concentration 2-3 times what we measured 0.2-3.5 m above the snow surface. Using a simple eddy diffusion model, with vertical eddy diffusion coefficients calculated from balloon soundings, suggested that atmospheric H2O2 concentrations should be significantly affected by any H2O2 degassed from surface snow. Field measurements showed the absence of either high concentrations of H2O2 or a measurable concentration gradient between inlets 0.2 and 3 m above the snow, however. A surface resistance to degassing, i.e., slow release of H2O2 from the ice matrix, is a plausible explanation for the differences between observations and modeled atmospheric profiles. Degassing of H2O2 at a rate below our detection limit would still influence measured atmospheric concentrations and help explain the difference between measurements and photochemical modeling. The cumulative evidence suggests that surface snow adjusts slowly to drops in atmospheric H2O2 concentration, over time scales of at least weeks. The H2O2 losses previously observed in pits sampled over more than one year are thought to occur later in the summer or fall.

Interpreting natural climate Signals in Ice Cores
Bales, Roger C., Eric W. Wolff
EOS Transactions, American Geophysical Union, Vol. 76, No. 47, 482-483, p. 482-483, November 21, 1995

Polar ice caps preserve information about atmospheric composition over the past tens of thousands to hundreds of thousands of years. They contain a rich history of the Earth's volcanic activity, terrestrial dust sources, sea ice location, terrestrial and marine biological activity, pollution, and atmospheric oxidation capacity. Differences in concentrations of CO2 and CH4 in air extracted from ice of various ages, changes in temperature inferred from d18O in ice, and differences in the dust or acid loading of ice are all used to deduce major changes in the global environment. These temporal patterns of physical properties and chemical species that are recorded in ice offer an opportunity to study the cause and effect relationships of environmental change.

Conceptual framework for interpretation of exchange processes
Bales, R. C. and J. Choi
Chemical Exchange between the Atmosphere and Polar Snow. E. Wolff and R. Bales (eds.), NATO ASI Series, vol. 43, p. 319-338, 1996

This contribution presents a conceptual and mathematical model describing the exchange of reactive gases between the atmosphere and polar snow, with with particular emphasis on chemical and microphysical processes controlling the rate of atmosphere-snow transfer. The aim is t develop a physically based "transfer function" for atmosphere-snow exchange. Two central questions are addressed. First, what do we know about the processes governing the atmosphere-snow transfer for chemical species that are "reversibly deposited" to snow? Reversibly deposited refers to species that have a significant presence in both the gas and condensed phases, and can undergo reversible exchange between the snow and the overlying atmosphere. Second, what more do we need to know in order to extract relevant climatic information about these species from snow and ice-core records? The main focus is on hydrogen peroxide, for which the most complete information is available. The degree to which a chemical species undergoes post-depositional exchange between snow and the atmosphere depends on both characteristics of the site and the species. At the macroscale, the extent to which previously deposited snow equilibrates with the atmosphere is controlled by the time between subsequent snow deposition events and the depth of air-snow exchange. Forced ventilation of snow due to uneven surface features and blowing snow enhance the exchange that would occur with diffusion alone. At the grain scale, diffusion of species into and out of snow grains limits the rate at which equilibrium is established. Deeper in the firn, diffusion in the open pore space should be more limiting than bulk diffusion in ice. Water d18O is the least mobile volatile species, followed by hydrogen peroxide and strong acids HNO3 and HCl, followed by formaldehyde and organic acids. Much of the spatial variability in concentrations of aerosol-derived species and d18O is absent or damped out for hydrogen peroxide due to its more-rapid snow-air exchange. The even greater mobility of formaldehyde and organic acids results in the loss of seasonal signals in firn, but should also result in these species reaching equilibrium with the atmosphere much sooner. Our ability to mathematically describe the extent of snow-atmosphere equilibrium is limited by both a lack of site-specific, year-round data for model testing (accumulation, physical properties, ventilation, concentration changes) and a lack of knowledge of microphysical parameters (partition and diffusion coefficients).

Composite Temperature Record from the Greenland Summit, 1987-1994: Synthesis of Multiple Automatic Weather Station Records and SSM/I Brightness Temperatures
Shuman, C.A., M.A. Fahnestock, R.A. Bindschadler, R.B. Alley, and C.R. Stearns
American Meteorological Society, Vol. 9, p. 1421-1428, June 1996

Air temperature (TA) records from automatic weather stations (AWS) in Central Greenland and associated Special Sensor Microwave/Imager (SSM/I) brightness temperature (TB) data (37 GHz, vertical polarization) have been used to create a composite, daily, monthly, and annual average temperature record of the Greenland summit fro the period 1987-1994. The record is derived primarily from near-surface temperatures from a single station; AWS Cathy (May 1987 to May 1989), which was moved 28 km and became AWS Kenton (starting in June 1989 and continuing). The Cathy daily average TA record has been converted to the equivalent basis of Kenton by a technique based on the ratio of the contemporaneous daily average TB data from the two locations. The accuracy of this technique has been statistically tested using 16 months of contemporaneous TA and TB data from the GISP2 and Kenton AWS. The resulting composite temperature record provides a multiyear data set for comparison to other climate records from the Greenland summit.

Modeling of the processing and removal of trace gas and aerosol species by Arctic radiation fogs and comparison with measurements
Bergin, M.H., S.N. Pandis, C.I. Davidson, J.-L. Jaffrezo, J.E. Dibb, A. G. Russell, and H.D. Kuhns
Journal of Geophysical Research, Vol. 101(D9), p. 14,465-14,478, 1996

A Lagrangian radiation fog model is applied to a fog event at Summit, Greenland. The model simulates the formation and dissipation of fog. Included in the model are detailed gas and aqueous-phase chemistry, and deposition of chemical species with fog droplets. Model predictions of the gas-phase concentrations of H2O2, HCOOH, SO2, and HNO3 as well as the fog fluxes of S(VI), N(V), H2O2, and water are compared with measurements. The predicted fluxes of S(VI), N(V), H2O2, and fog water generally agree with measured values. Model results show that heterogeneous SO2 oxidation contributes to approximately 40% of the flux of S(VI) for the modeled fog event, with the other 60% coming from preexisting sulfate aerosol. The deposition of N(V) with fog includes contributions from HNO3 and NO2 initially present in the air mass. HNO3 directly partitions into the aqueous-phase to create N(V), and NO2 forms N(V) through reaction with OH and the nighttime chemistry set of reactions which involves N2O5 and water vapor. PAN contributes to N(V) by gas-phase decomposition to NO2, and also by direct aqueous-phase decomposition. The quantitative contributions from each path are uncertain since direct measurements of PAN and NO2 are not available for the fog event. The relative contributions are discussed based on realistic ranges of atmospheric concentrations. Model results suggest that in addition to the aqueous-phase partitioning of the initial HNO3 present in the air mass, the gas-phase decomposition of PAN and subsequent reactions of NO2 with OH as well as nighttime nitrate chemistry may play significant roles in depositing N(V) with fog. If a quasi-liquid layer exists on snow crystals, it is possible that the reactions taking place in fog droplets also occur to some extent in clouds, as well as at the snow surface.

The deposition of particles and gases to ice sheets
Davidson, C.I., M.H. Bergin, H.D. Kuhns
Chemical Exchange Between the Atmosphere and Polar Snow. E. Wolff and R. Bales, NATO ASI Series, Vol. 43, p. 275-306, 1996

The glacial record as proven to be a most valuable source of information about climate change, major geologic events, and variations in global cycling of chemical species. This information has been obtained by noting variations in chemical concentrations and in ice properties with depth in ice sheets worldwide. Virtually all of the chemical constituents found in the worlds glaciers, aside from those near bedrock, originally came from the atmosphere. Yet published studies of ice core data have seldom incorporated information about the atmosphere or air-to-snow transfer into their interpretations.

Overview of field data on the deposition of aerosol-associated species to the surface snow of polar glaciers, particularly recent work in Greenland
Dibb, J. E.
Chemical Exchange Between the Atmosphere and Polar Snow. E. Wolff and Roger C. Bales (eds.), NATO ASI Series, Vol. 43, p. 249-274, 1996

This contribution presents a review of recent field experiments investigating the relationship between the composition of snow falling onto polar glaciers and the composition of aerosols in the overlying atmosphere. The limited data existing prior to the late 1980s indicated that aerosol removal processes should cause fractionation between the composition of aerosols and snow. Uncertainties regarding the relative importance of ice-nucleation scavenging, in-cloud riming of snow flakes and dry deposition over polar ice sheets precluded assessment of the impact the likely fractionation would have on efforts to reconstruct temporal variations in aerosol chemistry from ice core chemistry records. Two large international experiments on the Greenland ice sheet after 1988 focused on these, and other, issues central to understanding air-snow exchange processes. These experiments confirmed that ice-nucleation scavenging is the major process incorporating aerosol-associated species into polar snow. This process enriches snow in large aerosols relative to the aerosol population aloft. Dry deposition was found to be of minor importance at the present time, but also enriches the snow in large aerosols and the species associated with them. Since the large aerosols over Greenland are predominantly derived from sea-salt and dust, while SO4-, NH4+ and several pollutant trace metals are concentrated in submicron aerosols, snow chemistry presents a biased view of aerosol composition. However, it would appear to be possible to incorporate such a bias into efforts to reconstruct aerosol chemistry records from ice cores, were it not for several complicating factors. Spatial variability in snow chemistry will likely impose an inherent limit on the temporal resolution that will be possible in such reconstructions. Variations in the seasonal pattern of snow accumulation over time have the potential to greatly complicate interpretation of ice core chemistry records. Unfortunately, it is not clear if seasonal changes in snow accumulation could be recognized and quantified from ice core records, if they have occurred. Deposition of aerosols and associated species by fog droplets was found to be of greater importance than expected. This mechanism does not fractionate the atmospheric aerosol population to the same extent as snowfall and dry deposition. Variations in the relative contributions of fog versus snow over time (on seasonal to millennia scales) would thus alter the relationship between the composition of aerosols and the snowpack. Here again, it is uncertain whether such changes could be recognized in an ice core and accounted for in the reconstruction of past aerosol composition and loading.

Biomass burning signatures in the atmosphere and snow at Summit, Greenland: An event on 5 August, 1994
Dibb, J. E., R.W. Talbot, S. I. Whitlow, M.C. Shipham, J. Winterle, J. McConnell, and R.C. Bales
Atmospheric Environment, Vol. 30, No. 4, p. 553-561, 1996

Two recent reports have suggested that thin layers of ice in Greenland cores with anomalously high concentrations of NH4+, K+ and HCOO - represent deposition from biomass burning plumes advocated over Greenland. These interpretation were based primarily on the similarity between the suite of enriched species in the ice and several recent characterizations of biomass burning plumes from various regions around the globe. In August, 1994 a biomass burning plume was transported to Summit, Greenland (72oN 38oW) from the Hudson Bay lowlands region of Canada. Gas phase, aerosol and snow samples impacted by this plume had large enhancements of HCOOH/HCOO -, CH3COOH/CH3COO -, NH3/NH4+ and K+. Several other species that have been reported to be enriched in some biomass burning plumes were also enriched in at least one of the three phases (gas, aerosol and snow) at Summit. Comparisons between the plume at Summit and biomass burning plumes sampled in 1990 over the Hudson Bay lowlands suggest that the carboxylic acids may be significantly enhanced by secondary production during the 3 - 4 days of transport between Canada and Greenland. It also appears that gas to particle conversion during transport may modify the partitioning of the carboxylates, nitrate, and perhaps ammonium and inorganic sulfur between the gas and aerosol phases in the plume. The relative enrichments of these species differs considerably between the atmosphere and snow, but the signal in snow was quite similar to the composition of the anomalous samples previously described in the ice cores.

A simple model to estimate atmospheric concentrations of aerosol chemical species based on snow core chemistry at Summit, Greenland
Bergin, M.H., C.I. Davidson, J.E. Dibb, J.-L. Jaffrezo, H.D. Kuhns, and S.N. Pandis
Geophysical Research Letters, In Press

A simple model is presented to estimate atmosph eric 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 SO42- 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 for SO42- over the 1993 and 1994 field seasons are 0.85 and 0.95, respectively. The 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 SO42- 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.

Measuring snow accumulation on the Greenland crest
Stearns, C. R., and G. A. Weidner:
4th AMS Conference on Polar meteorology and Oceanography, 15-20 January 1995, Dallas, In Press

Recent increase in H2O2 concentration at Summit, Greenland
Anklin, Martin and Roger C. Bales
Journal Geophysical Research, Submitted

Prior measurements of hydrogen peroxide H2O2 in Greenland ice suggested a 50% increase of the H2O2 concentration during the last 200 years, where most of the increase occurred between 1960 and 1988 (Sigg and Neftel, 1991). In this work we present data from two shallow cores drilled at Summit, Greenland in 1995 that confirm the H2O2 increase found earlier and that show a further increase of the H2O2 concentration since 1988, leading to an overall increase of 60% during the last 150 years. The new shallow cores were drilled 6 years after Erocore, which allowed us to identify the influence of the firification process on the mean annual H2O2 concentration recorded in the firn. We found that the H2O2 concentration in the upper snow/firn decreased until the layer was buried with at least 1 m of snow and that the mean annual H2O2 concentrations in deeper layers stayed essentially unchanged. Besides the increase in the mean annual concentration, the annual amplitude between winter minima and summer maxima has tripled since 1970. Since there has been no significant change n temperature during either the last 150 years or last 25 years, it is unlikely that the increasing H2O2 concentrations are temperature related. A small part of the increase of both the mean annual concentration and annual amplitude of H2O2 in recent years could be due to increasing UV-B radiation caused by the depletion of stratospheric ozone; but a combination of changes intropospheric chemistry are apparently also involved.

Airborne methane sulfonate, sulfate, and calcium at Summit Greenland: the effects of different back trajectory lengths and elevations
Kuhns, H.D., C. Davidson, J.L. Jaffrezo, M. Bergin, J. Kahl
Atmospheric Research, Submitted

Atmospheric concentrations of MSA, SO42- , and Ca2+ at Summit, Greenland during summer in 1993 and 1994 are compared with isentropic back trajectory calculations. Concentrations of SO42- and Ca2+ are small when the air originates close to sea level at the coast of Greenland; concentrations are higher when the air comes from higher elevations. When transport time from the coast is short, airborne concentrations of MSA and SO42- are highly variable, while a narrower range of concentrations is observed for longer transport times. Ca2+ concentrations, on the other hand, are equally variable for short and long transport times suggesting hat this species is well mixed in the Arctic free troposphere relative to MSA and SO42- . These results are significant for interpreting ice core records since the relative proximity and geographic size of chemical sources affect the chemical signal in the ice core record.

Processes controlling aerosol Methanesulfonate, sulfate and Calcium loading at Summit, Greenland
Kuhns, H.D., C.I. Davidson, J.L. Jaffrezo, M.H. Bergin, and J.W. Kahl
Atm. Envir. Submitted

Temporal and Spatial Variability of Snow Accumulation in central Greenland
Kuhns, H.D., C. Davidson, J. Dibb, C. Stearns, M. Bergin, and J.L Jaffrezo
Jouurnal of Geophysical Research, Submitted

Snow accumulation records from central Greenland are explored to improve the understanding of the accumulation signal in Greenland ice core records. Results from a :forest" of 100 bamboo poles and automated accumulation monitors in the vicinity of Summit as well as shallow cores collected in the Summit and Crete areas are presented. Based on these accumulation data, a regression has been calculated to quantify the signal to noise variance ratio of ice core accumulation signals on a variety of temporal (1 week to 2 years) and spatial (20 m to 200 km) scales. Results are consistent with data obtained from year round automated accumulation measurements deployed at Summit which suggest that it is impossible to obtain regional snow accumulation data with seasonal resolution using 4 accumulation monitors positioned over a length scale of ~30 km. Given this understanding of temporal and spatial dependence of noise in the ice core accumulation signal, the accumulation records from 17 shallow cores are revisited. Each core spans the time period from 1964 to 1983. By combining the accumulation records, the regional snow accumulation record has been obtained for this period. the results show that 9 of the 20 years can be identified as having an accumulation different from the 20 year mean with 99% confidence. The signal to noise variance ratio for the average accumulation signal sampled at annual intervals is 5.8 0.5. The averaged accumulation time series will be useful to climate modelers attempting to validate their models with accurate regional hydrologic data sets.

Temperature history and accumulation timing for the snow pack at GISP2, central Greenland
Shuman, C.A., R.B. Alley, M.A. Fahnestock, R.A. Bindschadler, J.W.C. White, and J. Winterle
Journal of Glaciology, Submitted

Previous research has documented a close association between high-resolution snow pit profiles of hydrogen and oxygen stable isotope ratios and multi-year special sensor microwave/imager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989 to 1991 field seasons indicated that D and d18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To further test this new technique, additional stable isotope data were obtained from a 2 m snow pit constructed during late June 1005 near the GISP2 site.

This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable isotope records with SSM/I brightness temperatures. The subannual variation of the dD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time- constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of the chemistry data. The slope of the multi-year T vs. d correlation was evaluated along with the subannual variation in the amount, rate, and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and the paleoclimatic signal variations in the GISP2 and GRIP deep cores.