Aerosol measurements were made on three cruises in the mid-Pacific along longitude 140¿W from 55¿N to 70¿S for a total of about 90 days in 1992 and 1993. The three data sets document the aerosol concentration and general features of its number-size distribution in the marine boundary layer (MBL) and their variation with latitude and meteorological conditions. Mean concentration varied from 300 cm-3 in the tropics to 500 cm-3 in the midlatitudes outside of continental air masses. Infrequent short-term spikes in concentration ranged up to 2000 cm-3. Two dominant modes were observed, the Aitken and accumulation, with mean diameters of 25 to 60 nm and 150 to 200 nm, respectively. An intermittent ultrafine mode was noted at diameters less than 25 nm. The concentration and dominance of one mode over another depended on the relative strength of the entrainment of ultrafine and Aitken particles from the free troposphere (FT) into the MBL compared to the rate of growth of Aitken mode into accumulation mode particles and removal rate of the accumulation mode. In general, aging times were shorter in the subtropics, longer in the tropics, and variable in the midlatitudes. The rate of new particle formation within the MBL itself was either low and did not contribute significantly to the observed number concentration or, if the rate was high, it occurred infrequently and was not observed in these experiments. ¿ American Geophysical Union 1996