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.