ATMOSPHERIC NEAR-SURFACE NITRATE AT COASTAL ANTARCTIC SITES

Records of atmospheric nitrate were obtained by year-round aerosol sam pling at Neumayer and Dumont D'Urville stations, located in the Atlant ic and Pacific sector of coastal Antarctica, respectively. Where possi ble, evaluation of the nitrate records is mainly based on concurrently measured radioisotopes (Be-10, Be-7, Pb-210) as well as delta(15)N in nitrate nitrogen. Observations made at these land two other coastal A ntarctic sites [Savoie et al., 1993]) reveal a uniform nitrate backgro und near 10 ng m(-3) persisting throughout coastal Antarctica between approximately April and June. The dominant seasonal nitrate maximum, w hich occurred between spring and midsummer and ranged from 20 to 70 ng m(-3), tended to increase with latitude. An estimate based on Neumaye r mineral dust concentrations suggests that an average of less than 5% of the observed atmospheric nitrate load may be associated with conti nental tropospheric sources, while a separate estimate based on Pb-210 records implies a much higher proportion of up to 60%. Stratospheric nitrate influx rates seen at coastal sites, deduced from Neumayer Be-1 0/Be-7 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 oxidat ion by almost a factor of 5 and are found to be in close agreement wit h the observed surface nitrate flux, implying again that the continent al source contribution is relatively unimportant. Consideration of nit rate reemission from near-surface snow layers reveals a minor effect o f this flux on the global Antarctic troposphere but possibly a substan tial influence on the nitrate load of a persistent surface inversion l ayer. Evaluation of the mean seasonal nitrate pattern, based on concur rent Be-10, Pb-210 and delta(15)N records at Neumayer and on tritium i n precipitation at Halley, suggests that the period of significant enh ancement above the background mainly reflects inputs of stratospheric nitrate with secondary peaks in winter and late summer most likely dom inated by PSC sedimentation and stratospheric air mass intrusions, res pectively.