The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Abstract

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 μm and greater than 0.01 μm 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, 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. 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. Laser particle counter (LPC) results show that particles > 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 μm 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.

Large-scale spatial trends in recent firn chemistry along an east-west transect through central Greenland

“…The relative fractions of the total dry deposition to the overall inputs of sulphate, nitrate, calcium and chloride for the average accumulation rate at Summit, for example, are 37, 30, 61 and 51%, respectively. These numbers are in close agreement with direct observations made by Bergin et al (1995) during the summer season at this site (except for nitrate for which no gas phase dry depositioln flux was reported}. This strongly suggests our approach to be an alternative way to determine average dry deposition rates, adding significant temporal representativeness.…”

“…Despite surface rime accumulation rates AA may only contribute a few per cent to the overall snow deposition A, the last term on the right-hand side of equation (6) may substantially increase the mean firn concentration of submicron aerosol. For example, in surface rime deposited at Summit (T99) NH2 and SO~-concentrations are found to be enhanced over the typical fresh snow values by roughly a factor of 7 and 5, respectively (Bergin et al, 1995). Similar enrichments are observed in cloud water with respect to (unrimed) falling snow at high elevation Alpine sites during the winter season (Brantner et al, 1994).…”

“…J, = gw + Jd + AJd (5) with Jw, Jd denoting the common wet and dry deposition fluxes, respectively. Here AJd comprehends a series of supplementary depositional (and evasional) processes not directly associated with precipitationforming events, as for example firn ventilation (Harder et al, 1991), scavenging by or evaporation from drifting ice crystals (Pomeroy et al, 1991), surface rime or frost deposition (Bergin et al, 1995) and reevaporation from surface snow (Wolff, 1995). If irreversibly deposited species like sulphate are considered, reevaporation effects may be neglected and, furthermore, for the sake of simplicity the net snow accumulation rate A is set equal to the precipitation rate implying that frost or surface rime accumulation AA is balanced by water vapour sublimation.…”

“…Furthermore, increased interest in microscale effects on firn chemistry like remobilisation of gaseous species (Wolff, 1995), migration during firnification , firn ventilation (Harder et al, 1991), fog deposition (Bergin et al, 1995), etc. has given rise to uncertaintites in the straightforward interpretation of ice core records with respect to atmospheric conditions.…”

Mercury in the Arctic tundra snowpack: temporal and spatial concentration patterns and trace gas exchanges

“…Trace gas exchanges between the atmosphere and the tundra are modulated by sinks and sources below and within snowpack, by snow diffusivity, snow height, and snow porosity (Dominé and Shepson, 2002;Lalonde et al, 2002;Monson et al, 2006). The snowpack accumulates nutrients, pollutants, and impurities that are deposited by snowfall and dry deposition processes, all of which can subsequently be transported to underlying ecosystems during snowmelt (Bergin et al, 1995;Uematsu et al, 2000).…”