Measurements and analyses of the aerosol size distributions and optical properties found in the marine boundary layer (MBL) during the 1983 USNS Lynch cruise from Charleston, South Carolina, to Scotland via Canary Islands are presented. The data given are the most extensive and accurate measurements of the submicron marine aerosol size distribution to date and are supplemented by extensive meteorological observations. Eight detailed case studies of the evolution of the size distribution that occurred under different meteorological conditions are presented and discussed. The data indicate that repeated cycling of MBL air through nonprecipitating clouds at the top of the MBL is a major factor in shaping the size distribution and that new particle formation by heteromolecular, homogeneous nucleation is the most likely mechanism for sustaining the particle concentration below 0.04-μm radius. Calculations of the scattering and extinction coefficients and optical depth of the MBL as a function of wavelength directly from the measured size distribution and MBL vertical structure are compared to measured values of the scattering coefficient and optical depth. These measured and calculated optical properties correlate well throughout the cruise and the results give a relatively consistent picture of the relationship between the aerosol size distribution and electromagnetic properties in the MBL.