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Abstract |
Year-round composition of bulk and size-segregated aerosol was examined at a coastal Antarctic site (Dumont d'Urville). Sea-salt particles display a summer depletion of chloride relative to sodium, which reaches ?10%. The mass chloride loss is maximum on 1- to 3-?m-diameter particles, nitrate being often the anion causing the chloride loss. The summer SO42?/Na+ ratio exceeds the seawater value on submicron particles due to biogenic sulfate and on coarse particles due to ornithogenic (guano-enriched soils) sulfate and to heterogeneous uptake of SO2 (or H2SO4). HCl levels range from 47 ± 28 ng m?3 in the winter to 130 ± 110 ng m?3 in the summer, being close to the mass chloride loss of sea-salt aerosols. In the winter, sea-salt particles exhibit Cl?/Na+ and SO42?/Na+ mass ratios of 1.9 ± 0.1 and 0.13 ± 0.04, respectively. Resulting from precipitation of mirabilite during freezing of seawater, this sulfate-depletion-relative sodium takes place from May to October. From March to April, warmer temperatures and/or smaller sea ice extent offshore the site limit the phenomenon. A range of 14–50 ng m?3 of submicron sulfate is found, confirming the existence of nssSO42? in the winter at a coastal Antarctic site, highest values being found in the winters of 1992–1994 due to the Pinatubo volcanic input. Apart from these three winters, nssSO42? levels range between 15 and 30 ng m?3, but its origin is still unclear (quasi-continuous SO2 emissions from the Mount Erebus volcano or local wintertime dimethyl sulfide [DMS] oxidation, in addition to long-range transported by-product of DMS oxidation). |
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