COMPOSITIONAL VARIATIONS OF SEA-SALT-MODE AEROSOL-PARTICLES FROM THE NORTH-ATLANTIC

Individual sea-salt-mode aerosol particles collected during the Atlant ic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) experiment in June 1992 were studied using transmission electron microscopy in both imaging and analysis modes. The set of eig ht samples provided an opportunity to compare ''clean,'' ''intermediat e,'' and ''dirty'' oceanic aerosols. In the clean samples, major speci es include NaCl, mixed-cation (Na, Mg, K, and Ca) sulfates, and in som e particles, NaNO3. The same compounds also occur in intermediate samp les, but compositional groups can be distinguished that are characteri zed by low- and high-Cl losses from sea salt. In these samples, most C l loss is compensated by NaNO3 formation. Several compositional groups occur in the dirty samples; these include, in addition to the particl e types in clean and intermediate samples, Na2SO4 (with minor Mg, K, a nd Ca), (NH4)(2)SO4, and silicates. The uniform compositions of sea-sa lt-mode particles in the clean samples suggest that the same process w as acting on all particles. Their excess sulfate and nitrate probably formed through the oxidation of SO2 in the sea-salt aerosol water and by reactions between NOx and NaCl. On the other hand, distinct composi tional groups in the dirty samples reveal that long-range transport of continental air masses resulted in the mixing of aerosols that were e xposed to different conditions. In addition to O-3 oxidation, cloud pr ocessing may have contributed to the formation of excess sulfate in th ese samples.