Measurements were made onboard the NOAA R/V Discoverer during the First Aerosol Characterization Experiment (ACE 1) to understand the optical properties of a minimally perturbed natural aerosol system in terms of its chemical and physical properties. ACE 1 took place during November and December of 1995 in the Southern Ocean region south of Australia. Reported here are observations at a wavelength of 550 nm of the submicron and supermicron aerosol scattering coefficient, &sgr;sp; single scattering albedo, ω0; and the hemispheric backscattered fraction and mass scattering efficiencies of non-sea-salt sulfate, sea salt, and the total aerosol. Also presented is the ¿ngstr¿m exponent, ¿, for the 450 and 700 nm wavelength pair. Variations in these parameters were found to be a strong function of the relative concentrations and size distributions of the dominant aerosol chemical components. Both the submicron and supermicron aerosol mass were composed primarily of water-soluble ionic species. This is in agreement with an experiment-average single scattering albedo of 0.99 (-0.4, +1.0%). Of the submicron ionic mass, 80¿10% was sea salt, 16¿8% was non-sea-salt sulfate, and 4¿3% was methanesulfonate. Sea salt composed 99¿0.7% of the supermicron ionic mass. The magnitude of scattering by both submicron and supermicron aerosol was controlled by sea salt. The backscattered fraction for the submicron aerosol averaged 0.11¿0.02 and was controlled by the tailing of coarse mode sea-salt mass into the submicron size range. Calculated mass scattering efficiencies for submicron non-sea-salt sulfate ion averaged 1.5¿0.74 m2 g-1 (at 30 to 45% relative humidity) with the highest values corresponding to continentally influenced air masses where sulfate aerosol surface mean diameters and surface area concentrations were the largest. Mass scattering efficiencies for submicron sea salt were higher (averaging 4.2¿0.96 m2 g-1) due to the tailing of coarse mode sea salt into the particle size range most efficient for light scattering. Given the similar lifetimes of submicron non-sea-salt sulfate and sea salt in the marine boundary layer, it is evident that sea salt controls the aerosol optical properties in this Southern Ocean region. ¿ 1998 American Geophysical Union