FILTERING OF AIR THROUGH SNOW AS A MECHANISM FOR AEROSOL DEPOSITION TO THE ANTARCTIC ICE-SHEET

Aerosol particles serve as cloud condensation nuclei worldwide, and th ey affect the Earth's radiation budget both directly and indirectly. T hese particles consist mostly of sulfate compounds. Ice core measureme nts can be used to infer past variations of atmospheric sulfate concen tration, but to do so requires knowledge of the deposition mechanisms. Significant ''dry'' deposition may occur by filtering when air moves through the snow due to changes in pressure caused by wind blowing ove r a rough surface (wind pumping). The filtering efficiency of snow was measured at South Pole Station, using an optical particle counter and a condensation nucleus counter. The number size-distribution of ambie nt aerosol peaks at a dry particle diameter of 0.13 mu m, the volume s ize-distribution at 0.17 mu m. Less than 5% of the particles have diam eters > 0.3 mu m Diffusion from interstitial air to snow grains appear s to be the primary mechanism of dry deposition for particles < 0.6 mu m in diameter, but another mechanism, probably impaction, becomes sig nificant for larger particles. Aerosol deposition by filtering occurs with an e-folding time of 1-3 s depending on particle size, correspond ing to an e-folding depth of 0.5-1 cm for an estimated air velocity of 0.4 cm s(-1) within the surface snow. Even for long residence times, a small number of particles (< 0.1%) are found in filtered air, sugges ting a small degree of new particle formation or reentrainment. Howeve r, both the e-folding depth and the reentrainment rate are small enoug h that smoothing of the sulfate records in ice cores should be negligi ble. Three mathematical models for filters agree in describing filteri ng by snow as dominated by diffusion, but all underpredict the filter efficiency. Capture of aerosol particles is found to be 2-3 times as r apid as that assumed by Cunningham and Waddington [1993], supporting t heir conclusion of nearly total removal of particles from air entering the snow. Blowing snow might also be expected to collect aerosol part icles; however, a calculation suggests that deposition to blowing snow on the Antarctic Plateau is insignificant. If wind pumping and diffus ion contribute significantly to total deposition, the flux of air into the snow and the residence time of the air within the snow control th e deposition rate. Both air flux and residence time are functions of w ind speed and surface roughness, so that the aerosol flux to the snow depends on these factors as well as atmospheric concentration, complic ating the interpretation of paleoclimate records for aerosol-bound sub stances.