Sulfate in air and snow at the South Pole: Implications for transport and deposition at sites with low snow accumulation

Air and surface snow were sampled at Amundsen-Scott Station at the South Po le from July through December of 1992. Four-day averages of non-sea-salt su lfate (nss-SO4=) aerosol show a strong seasonal trend, increasing by a fact or of about 30 from winter to summer as oceanic biogenic sources become mor e active and atmospheric transport pathways change. Three-dimensional sampl ing of small-scale surface topography (sastrugi) provides evidence supporti ng wind pumping and filtration of aerosol by snow as a significant mechanis m for dry deposition at this site. The estimated monthly flux of nss-SO4= t o the snow surface also increases from winter to spring, but by only a fact or of 2. suggesting that the efficiency of deposition for this aerosol-born e species from the near-surface air to the snow is greater in winter. The s trong surface-based temperature inversion in winter inhibits vertical motio n and may limit the rate of delivery of aerosol to the boundary layer from the free troposphere. Because the snow surface is a sink for aerosol, near- surface measurements of aerosol in the stable inversion layer may not be re presentative of the free troposphere. Air and snow data in summer (when the inversion is weak) are used to estimate a tropospheric residence rime of 4 -20 days for nss SO4=.