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Sea‐salt aerosol particles in the lowest tens of meters above the ocean are, typically, more than three‐fourths water on a volume basis. Calculations herein indicate that aqueous‐phase conversion of sulfur dioxide dissolved in the water associated with sea‐salt particles (sea‐salt aerosol water) supported the production of 2–8 nmol m−3 of nonsea‐salt sulfate (nssSO4=) during the Marine Aerosol and Gas Exchange (MAGE) experiment intensives. This production is based on ozone oxidation of dissolved SO2 in sea‐salt aerosol water and accounts for sulfur gas and ozone mass transfer limitations as a function of sea‐salt particle size. Measurements showed that 1–15 nmol m−3 of nssSO4= was actually present in the sea‐salt particle mode except for four enhanced concentration cases due to continental sulfur input. The range in predicted, as well as observed, nssSO4= was primarily due to variability in sea‐salt aerosol water volume. The nssSO4= produced by ozone oxidation of sulfur dioxide, being in the sea‐salt particle mode with observed volume geometric median diameter of 3.5–5 μm, is dry deposited at a fairly rapid rate. The remainder, being large‐particle cloud condensation nuclei, may contribute little to cloud albedo over the global oceans. The two papers following this one, Kim et al. (this issue) on aerosol size distribution and water content and Parfai et al. (this issue) on compositional variations of sea‐salt‐mode aerosol particles observed by electron microscopy, complement and support results presented here. All three present results of the MAGE experiment and precede a forthcoming special issue of the Journal of Geophysical Research‐Atmospheres on MAGE (B. Huebert, guest editor).

A high‐volume sampler and a two‐stage cascade impactor were used aboard the NOAA ship Mt. Mitchell to obtain a fine‐particle‐fraction and two coarse‐particle‐fraction size cuts of atmospheric marine boundary layer aerosol samples. Ion chromatography analysis yielded NO3−, S04=, Na+, and Cl− concentrations and, by calculation, excess sulfate (XSO4) and chloride deficit (Cld) concentrations. Statistical analysis of the July 1988 sample sets from the Bermuda and U.S. east coast areas showed that most of the observed coarse‐particle Cld in the Bermuda‐area samples may be associated with the equivalence sum of NO3− and XSO4. However, the coarse‐particle Cld observed in the U.S. east coast samples is substantially greater than the equivalence sum of NO3− and XSO4 observed at this location. It is concluded that for the east coast sampling, about 13% of the observed 16.9 nmol XSO4 m−3 displaced Cl− in sea salt aerosol particles. For the Bermuda area, 15‐20% of the observed 11.1 nmol XSO4 m−3 displaced Cl− in sea salt aerosol particles (nearly 30% of the observed Cld) despite a relatively low sea salt mass concentration under light wind speed conditions. Under more typical sea state and wind speed conditions for the western North Atlantic Ocean, substantially more XSO4 may displace chloride ions in sea salt aerosol particles.

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