PK Quinn, DJ Coffman, MJ Hamilton, VN Kapustin, TS Bates, DS Covert (NOAA/PMEL, Seattle, WA 98115; 206-526-6892; email: quinn@pmel.noaa.gov)
A goal of the first Aerosol Characterization Experiment (ACE 1) was to determine the effect of the chemical composition of the aerosol on its optical properties in a remote marine atmosphere. To this end, simultaneous measurements were made onboard the NOAA ship Discoverer of chemical mass size distributions of the major cations and anions, gravimetrically-determined sub- and supermicron aerosol mass, number size distributions from 20 nm to 5 µm, and light scattering and backscattering by the submicron and total aerosol at 450, 550, and 700 nm.
Ionic species (as determined by ion chromatography) made up about 80% of the submicron and 100% of the supermicron aerosol mass. The remoteness of the region led to relatively low nss SO4= concentrations (0.1 to 0.6 mg m-3) and NH4+ to nss SO4= molar ratios less than 1. Highest nss SO4= concentrations corresponded to air masses that had recently passed over Tasmania or Australia. Large mass concentrations of seasalt in the coarse mode led to significant tailing into the accumulation mode such that seasalt comprised about 40% of the ionic mass at diameters less than 0.25 µm and 75% at diameters between 0.25 and 0.5 µm.
Non-seasalt SO4= aerosol scattering coefficients as well as mass scattering efficiencies were highest in either continental air masses where the sulfate aerosol surface area was highest or in marine air masses where the sulfate aerosol geometric surface mean diameter was largest. Because seasalt dominated mass concentrations in both the optically active accumulation mode and the coarse mode, it was the dominant light scatterer.