A mixed aerosol dynamical model which simulates the growth of mixed aerosol populations of sulfate, black carbon, organic carbon, and sea salt is described, and results from the implementation of this model in the Tracer Model 2 (TM2) off-line chemical transport model are presented. We represent the aerosol size distribution by eight modes: nucleation mode sulfate, Aitken mode sulfate, accumulation mode sulfate, pure fossil fuel black carbon, a mixed mode of fossil fuel black carbon, organic carbon, and sulfate, pure biomass burning black carbon, a mixed mode of biomass burning black carbon, organic carbon, and sulfate, and a sea-salt mode. The model reproduces both observed zonal average marine aerosol number concentrations and observed sulfate mass/accumulation mode number concentration ratios from the North Atlantic but does less well at reproducing number concentrations at individual sites and consistently overpredicts nucleation and Aitken mode concentrations in the free troposphere. A comprehensive validation of the model is not possible with the available data, but qualitatively, it is consistent with current understanding. The model shows that the accumulation mode at the surface is dominated by the mixed modes over the fossil fuel and biomass source regions, the pure sulfate mode in peripheral continental and marine areas and sea salt in the southern oceans. A preindustrial study showed that there is not a consistent positive linear relationship between the anthropogenic increase in aerosol mass burden and the corresponding increase in aerosol number burden, and regionally there may be an inverse relationship. ¿ 2001 American Geophysical Union