Marine boundary layer aerosol size distributions observed during June 1992 are described. Measurements were made from on board the National Oceanic and Atmospheric Administration ship Malcolm Baldrige in support of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange field experiment in the area surrounding 32 ¿N latitude and 25 ¿W longitude. Measurements obtained from three instruments, the differential mobility particle sizer, the active scattering aerosol spectrometer probe, and the forward scattering spectrometer probe, were used to investigate the contributions of the different aerosol modes to total number, surface area, and volume concentrations. It was observed that aerosol populations in air masses that appear to have originated from remote marine areas have a bimodal number distribution with total number concentrations ranging from 200 to 800 particles cm-3. In air masses that have been affected by anthropogenic emissions, the aerosol size distributions become monomodal and total number concentrations increase to 500 to 1800 cm-3. The data also suggest that air mass boundaries can be very sharp and that there may be some mesoscale variability in the aerosol population within an air mass. ¿ American Geophysical Union 1996