An on-line simulation of aerosol sulfate, nitrate, ammonium, and water in the Goddard Institute for Space Studies general circulation model (GCM II-prime) has been used to estimate direct aerosol radiative forcing for the years 1800, 2000, and 2100. This is the first direct forcing estimate based on the equilibrium water content of a changing SO42--NO3--NH4+ mixture and the first estimate of nitrate forcing based on a global model of nitrate aerosol. Present-day global and annual average anthropogenic direct forcing is estimated to be -0.95 and -0.19 W/m2 for sulfate and nitrate, respectively. Simulations with a future emissions scenario indicate that nitrate forcing could increase to -1.28 W/m2 by 2100, while sulfate forcing declines to -0.85 W/m2. This result shows that future estimates of aerosol forcing based solely on predicted sulfate concentrations may be misleading and that the potential for significant concentrations of ammonium nitrate needs to be considered in estimates of future climate change. Calculated direct aerosol forcing is highly sensitive to the model treatment of water uptake. By calculating the equilibrium water content of a SO42--NH4+ aerosol mixture and the optical properties of the wet aerosol, we estimate a forcing that is almost 35% greater than that derived from correcting a low relative humidity scattering coefficient with an empirical f(RH) factor. The discrepancy stems from the failure of the empirical parameterization to adequately account for water uptake above about 90% relative humidity. These results suggest that water uptake above 90% RH may make a substantial contribution to average direct forcing, although subgrid-scale variability makes it difficult to represent humid areas in a GCM. ¿ 2001 American Geophysical Union