THE CONTRIBUTIONS OF SNOW, FOG, AND DRY DEPOSITION TO THE SUMMER FLUXOF ANIONS AND CATIONS AT SUMMIT, GREENLAND

Experiments were performed during the period May-July of 1993 at Summi t, 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 su rfaces (symmetric airfoils) as well as impactor data. Real-time concen trations of particles greater than 0.5 mu m and greater than 0.01 mu m were measured. Snow and fog samples from nearly all of the events occ urring during the field season were collected. Filter sampler results indicate that SO42- is the dominant aerosol anion species, with Na+, N H4+, and Ca2+ being the dominant cations, Impactor results indicate th at MSA and SO42- have similar mass size distributions. Furthermore, MS A and SO42- have mass in both the accumulation and coarse modes. A lim ited number of samples for NH4+ indicate that it exists in the accumul ation 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 NH4to 0.110 cm/s +/- 0.021 cm/s for Ca.SO42- dry deposition velocity esti mates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as muc h as 60% greater than values calculated using the airborne size distri bution data. The rough agreement between the airfoil and impactor-esti mated 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 mu m in diameter effi ciently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 mu m are not as greatly af fected 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 depos ition, which suggests efficient scavenging of HNO3(g) by snowflakes. T he contribution of snow deposition to the seasonal inventories of aero sols 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 25% an d 32% for SO2-4 and MSA. The dry deposition contribution to the season al 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 in to consideration the spatial variability caused by blowing and driftin g snow. Overall, results indicate that snow deposition of chemical spe cies is the dominant flux mechanism during the summer at Summit and th at all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.