Changes in the cloud-free albedo of the Earth, and therefore direct aerosol radiative forcing, due to aerosol can be parameterized as a function of one extensive quantity that depends on the amount of aerosol present (e.g., the aerosol optical depth, &tgr;) and two intensive quantities that depend on the internal characteristics of the aerosol (the aerosol single-scattering albedo, ωo, and the aerosol asymmetry parameter, g). Airborne measurements of the dry aerosol scattering and absorption coefficients and dry particle size distributions made aboard the University of Washington's research aircraft during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) field campaign off the east coast of the United States in July 1996 are used in conjunction with measurements of the effects of humidity on aerosol light scattering to determine an optically equivalent model for the physical and optical properties of the ambient aerosol. This model is validated and used to derive values of ωo and g for the ambient (wet) aerosol. The average and standard deviation of the column-mean, ambient values of ωo and g at a wavelength of 550 nm, determined from 12 vertical profiles, are 0.958¿0.050 and 0.703¿0.092, respectively. A pronounced increase in g with increasing relative humidity is observed. Using the aerosol model, the mean instantaneous change in the local albedo induced by the aerosol is found to be 0.027¿0.018 at a wavelength of 550 nm (compared to an albedo of about 0.05 for the ocean). ¿ 2001 American Geophysical Union