Aerosol optical thickness (AOT) measurements at two remote Pacific Ocean locations in the northern hemisphere reveal an absolute minimum in the solar spectrum of magnitude ~0.02 at 550 nm and an ¿ngstr¿m exponent of about 1. Along with similar results from measurements at continental sites reported previously, this minimum indicates the presence of background aerosol at both continental and marine sites. A measure of marine boundary layer AOT is obtained by taking the difference between values measured at sea-level and at altitude on Mauna Loa, Hawaii. This then is compared with AOT calculated by a Lagrangian boundary-layer microphysical aerosol model using sea-salt and sulfate aerosols and with meteorology input from a mesoscale model to provide estimates of the sea salt flux that are within a factor of 7 of a hybrid Monahan-Smith formulation.