Individual sea-salt-mode aerosol particles collected during the Atlantic 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 eight samples provided an opportunity to compare ''clean,'' ''intermediate,'' and ''dirty'' oceanic aerosols. In the clean samples, major species include NaCl, mixed-cation (Na, Mg, K, and Ca) sulfates, and in some particles, NaNO3. The same compounds also occur in intermediate samples, but compositional groups can be distinguished that are characterized by low- and high-Cl losses from sea salt. In these samples, most Cl loss is compensated by NaNO3 formation. Several compositional groups occur in the dirty samples; these include, in addition to the particle types in clean and intermediate samples, Na2SO4 (with minor Mg, K, and Ca), (NH4)2SO4, and silicates. The uniform compositions of sea-salt-mode particles in the clean samples suggest that the same process was 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 compositional groups in the dirty samples reveal that long-range transport of continental air masses resulted in the mixing of aerosols that were exposed to different conditions. In addition to O3 oxidation, cloud processing may have contributed to the formation of excess sulfate in these samples. ¿ American Geophysical Union 1995