PHYSICAL-PROPERTIES OF MARINE BOUNDARY-LAYER AEROSOL-PARTICLES OF THEMID-PACIFIC IN RELATION TO SOURCES AND METEOROLOGICAL TRANSPORT

Aerosol measurements were made on three cruises in the mid-Pacific alo ng longitude 140 degrees W from 55 degrees N to 70 degrees S for a tot al of about 90 days in 1992 and 1993. The three data sets document the aerosol concentration and general features of its number-size distrib ution in the marine boundary layer (MEL) and their variation with lati tude and meteorological conditions. Mean concentration varied from 300 cm(-3) in the tropics to 500 cm(-3) in the midlatitudes outside of co ntinental air masses. Infrequent short-term spikes in concentration ra nged up to 2000 cm(-3). Two dominant modes were observed, the Aitken a nd accumulation, with mean diameters of 25 to 60 nm and 150 to 200 nm, respectively. An intermittent ultrafine mode was noted at diameters l ess than 25 nm. The concentration and dominance of one mode over anoth er depended on the relative strength of the entrainment of ultrafine a nd Aitken particles from the free troposphere (FT) into the MBL compar ed to the rate of growth of Aitken mode into accumulation mode particl es 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 i tself was either low and did not contribute significantly to the obser ved number concentration or, if the rate was high, it occurred infrequ ently and was not observed in these experiments.