A quantitative source function for sea spray aerosol produced by waves breaking in the surf zone was determined from data collected with optical particle counters at both sides of the surf zone at two locations on the Californian coast. Three optical particle counters were used to measure profiles at the base of a pier; a fourth instrument was used at the end of the pier. Careful calibration and intercomparisons of the instruments were made to avoid systematic errors. Aerosol concentrations measured downwind from the surf, in wind speeds of up to 9 m s-1, were 1--2 orders of magnitude higher than those upwind. Surf aerosol concentration gradients and plume heights vary with particle size and with wind speed. The derived surf aerosol source functions are compared with current estimates for the open ocean, taking into account the different proportions of the ocean surface covered by whitecaps. Application of a simple transport model indicates that surf-produced sea spray contributes significantly to the aerosol concentrations at fetches up to at least 25 km. This has implications for, for example, heterogeneous chemistry and electro-optical propagation. ¿ 2000 American Geophysical Union