Considerations on the origin and properties of the Antarctic aerosol

Information pertaining to aerosols and to microparticles preserved in snow and ice on the high plateau of Antarctica is assembled and interpreted with a diffusive transport model. The boundary conditions for the problem involve the existence of a two‐mode microparticle size distribution function. The smaller (Aitken) mode likely consists of converted products from nucleation of trace gases; the particles are less than 2.3 days old and are produced locally over the ice caps at a production rate of 4 × 10 −21 g cm −3 s −1 . The Aitken particles are present in concentrations of 10²–10³ cm −3 throughout the troposphere, except in the lowest few hundred meters within the turbulent boundary layer, where they are depleted. The Aitken particles are probably composed of converted sulfates and hydrocarbons. Large particles ∼0.4 µm in radius are also found in Antarctica, in concentrations of 0.5 cm −3 ; these particles dominate the aerosol mass. They are of great interest because they have apparently come from long distances and because they precipitate onto the surface and are incorporated into the polar ice in varying rates that can show a connection with past changes in terrestrial climate. The large particles are estimated to have come from (1) unidentified sulfate sources, (2) oceanic sources surrounding the continent, (3) arid regions in the southern hemisphere (Australia, Kalahari Desert, Atacama Desert), (4) extraterrestrial sources, and (5) oases on the continent—in that order of importance. Turbulent diffusion seems to be the primary particle transport mechanism, and diffusion models support the plausibility of the mentioned terrestrial sources and quantify their contributions. The transport by eddy diffusion from continental sources apparently is operating mainly within the middle and upper troposphere. Particles are removed over Antarctica by diffusing into the turbulent boundary region or by encountering clouds or hydrometeors. The major removal mechanisms are impaction on the surface (2.0 × 10 −14 g cm −2 s −1 ), impaction onto snowflakes (1.6 × 10 −14 g cm −2 s −1 ), impaction onto ice crystals (0.2 × 10 −14 g cm −2 s −1 ), and nucleation (0.6 × 10 −14 g cm −2 s −1 ). The Aitken particles are removed predominantly by diffusing to ice crystals (0.006 × 10 −14 g cm −2 s −1 ) and by diffusing across the laminar layer to the surface (0.015 × 10 −14 g cm −2 s −1 ). The many near‐surface removal mechanisms deplete aerosol mass loading by a factor of 5–10 in the turbulent boundary layer (making it a poor place to collect samples from). Changes in atmospheric circulation during the last third of the latest Wisconsin ice age may have increased desertification in the southern hemispheric mid‐latitude zones and caused greater amounts of dust to be deposited on the polar ice cap.