In situ measurements of atmospheric aerosols have been made from an airborne platform over the remote southern hemisphere ocean in November and December 1995 as part of the First Aerosol Characterization Experiment (ACE 1). A subset of these measurements have been evaluated during three of the flights to characterize the aerosol microphysical and optical properties in the cloud-free, marine boundary. The relationship between the microphysical and optical characteristics in the size range from 0.3 to 20 μm and relative humidity was evaluated. A new technique is introduced by which the scattering coefficient is derived directly from the optical particle counter measurements. The results of this study indicate that changes in particle volume, effective radius, and optical scattering are strongly related to changes in relative humidity (RH). The observations are in very good agreement with laboratory studies of particle volume changes as a function of relative humidity. ¿ 1998 American Geophysical Union