General circulation model assessment of direct radiative forcing by the sulfate-nitrate-ammonium-water inorganic aerosol system

An on-line simulation of aerosol sulfate, nitrate, ammonium, and water in t he Goddard Institute for Space Studies general circulation model (GCM II-pr ime) has been used to estimate direct aerosol radiative forcing for the yea rs 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 aeros ol, Present-day global and annual average anthropogenic direct forcing is e stimated to be -0.95 and -0.19 W/m(2) for sulfate and nitrate, respectively . Simulations with a future emissions scenario indicate that nitrate forcin g could increase to -1.28 W/m(2) by 2100, while sulfate forcing declines to -0.85 W/m(2). This result shows that future estimates of aerosol forcing b ased solely on predicted sulfate concentrations may be misleading and that the potential for significant concentrations of ammonium nitrate needs to b e considered in estimates of future climate change. Calculated direct aeros ol forcing is highly sensitive to the model treatment of water uptake. By c alculating the equilibrium water content of a SO42--NH4+ aerosol mixture an d the optical properties of the wet aerosol, we estimate a forcing that is almost 35% greater than that derived from correcting a low relative humidit y scattering coefficient with an empirical f(RH) factor. The discrepancy st ems from the failure of the empirical parameterization to adequately accoun t for water uptake above about 90% relative humidity. These results suggest that water uptake above 90% RH may make a substantial contribution to aver age direct forcing, although subgrid-scale variability makes it difficult t o represent humid areas in a GCM.