HYGROSCOPIC GROWTH OF AEROSOL-PARTICLES IN THE MARINE BOUNDARY-LAYER OVER THE PACIFIC-OCEAN AND SOUTHERN-OCEAN DURING THE FIRST AEROSOL CHARACTERIZATION EXPERIMENT (ACE-1)

The hygroscopic properties of submicrometer aerosol particles were stu died with a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) in the remote marine tropospheric boundary layer (MBL) over the Pacif ic and Southern Oceans in connection with the southern hemisphere mari ne First Aerosol Characterization Experiment (ACE 1) in October-Decemb er 1995. The H-TDMA was placed on board the ship RN NOAA Discoverer an d measured the hygroscopic diameter growth of individual aerosol parti cles when taken from a dry state to a relative humidity (RH) of 89-90% . Measurements were performed for the particles with dry diameters of 35, 50, 75, and 150 (or 165) nm. The natural aerosol present in the re mote MBL largely consists of two types, a sea-salt component and a non -sea-salt (nss) sulfate component. Since their hygroscopic behavior is significantly different, the H-TDMA. could clearly distinguish betwee n these two types and thus make in situ measurements of the mixing sta te of the MBL aerosol. During the ACE 1 intensive campaign in the Sout hern Ocean south of Australia, the hygroscopic diameter growth factors at RH = 90% for the nss-sulfate aerosol particles were 1.62, 1.66, an d 1.78 at dry particle diameters of 35, 50, and 150 nm, respectively, and for time periods with remote marine air masses. These values excee d those normally found in continental polluted environments. The growt h factors for the externally mixed sea-salt particles were even higher (2.12 and 2.1 for 50 and 150 nm). The corresponding values for the Pa cific Ocean (at RH = 89%) for the nss-sulfate particles were 1.56, 1.5 9, 1.61, and 1.63 for 35, 50, 75, and 165 nm. Particle deliquescence a nd RH hysteresis between RH = 68-90% was only observed in air masses n orth of the South Pacific Gyre, and then only for the Aitken mode part icles (particle diameters similar to 20-80 nm). The occurrence of exte rnally mixed sea-salt particle!; could be linked to conditions with hi gh wind speeds in connection with frontal passages or low pressure sys tems. Nevertheless, the number of externally mixed 150 nm sea-salt par ticles was found to be poorly correlated with local wind speed, probab ly due to a rather long life-time of these submicrometer particles. Pa rticles with hygroscopic growth factors significantly less than those of the nss-sulfate particles (denoted less hygroscopic particles) were only present during periods with anthropogenic influence.