Airborne measurements of the optical properties of aerosols near the coasts of California and Washington are presented. Parameters examined include the particle light-scattering coefficient (for both total scattering and backscattering), the particle mass light-scattering efficiency, the hygroscopic growth factor for particle light scattering, and aerosol optical depths computed from vertical profiles of the light-scattering coefficient. In several instances, the measured values of these parameters differed from commonly accepted values, derived primarily from more polluted airsheds. For example, the aerosol mass scattering efficiency was found to be 2.1¿0.3 m2 g-1. ¿ American Geophysical Union 1996