Atmospheric near-surface nitrate at coastal Antarctic sites

Records of atmospheric nitrate were obtained by year-round aerosol sampling at Neumayer and Dumont D'Urville stations, located in the Atlantic and Pacific sector of coastal Antarctica, respectively. Where possible, evaluation of the nitrate records is mainly based on concurrently measured radioisotopes (10Be, 7Be, 210Pb) as well as δ15N in nitrate nitrogen. Observations made at these (and two other coastal Antarctic sites [Savoie et al., 1993]) reveal a uniform nitrate background near 10 ng m−3 persisting throughout coastal Antarctica between approximately April and June. The dominant seasonal nitrate maximum, which occurred between spring and midsummer and ranged from 20 to 70 ng m−3, tended to increase with latitude. An estimate based on Neumayer mineral dust concentrations suggests that an average of less than 5% of the observed atmospheric nitrate load may be associated with continental tropospheric sources, while a separate estimate based on 210Pb records implies a much higher proportion of up to 60%. Stratospheric nitrate influx rates seen at coastal sites, deduced from Neumayer 10Be/7Be records for stratospheric air mass intrusions and from tritium for the sedimentation of polar stratospheric clouds (PSC), exceed the theoretical stratospheric odd nitrogen production rate from N2O oxidation by almost a factor of 5 and are found to be in close agreement with the observed surface nitrate flux, implying again that the continental source contribution is relatively unimportant. Consideration of nitrate reemission from near-surface snow layers reveals a minor effect of this flux on the global Antarctic troposphere but possibly a substantial influence on the nitrate load of a persistent surface inversion layer. Evaluation of the mean seasonal nitrate pattern, based on concurrent 10Be, 210Pb, and δ15N records at Neumayer and on tritium in precipitation at Halley, suggests that the period of significant enhancement above the background mainly reflects inputs of stratospheric nitrate with secondary peaks in winter and late summer most likely dominated by PSC sedimentation and stratospheric air mass intrusions, respectively.

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