Aerosol properties vary with the aerosol type, loading, or atmospheric conditions. We found that a dynamical aerosol model that describes the variability of aerosol properties as a function of aerosol optical thickness can address this issue. Ambient urban/industrial aerosol is measured in the mid-Atlantic region of the United States during the summer of 1993 by automatic Sun/sky radiometers. The volume size distributions inverted from the measured sky radiance represent the entire column of ambient particles. These data depict an aerosol regime in which accumulation mode particle size and volume increases as optical thickness increases. A dynamical aerosol model that is a function of optical thickness is constructed by fitting lognormal functions to the volume size distributions. It accounts for 60% of the variance in the aerosol accumulation mode size distributions. The accumulation mode is represented as a linear combination of two modes: one with volume modal radius 0.11 μm and the other with modal radius 0.21 μm. The model is shown to recreate the correct independently measured sky radiance in the backscattering direction and to predict the phase function within model uncertainties of data collected in subsequent years. The model is shown to be unbiased with respect to the time of day and to reproduce the wavelength dependence measured by the independent Sun measurements. The dynamic model has substantially different properties than a static continental model except for the limited optical thickness range of 0.0--0.25. ¿ 1998 American Geophysical Union