| In order to better understand and quantify the direct and indirect effect of aerosols on climate, an earlier general circulation model (GCM) simulation of tropospheric sulfate has been extended by incorporating aerosol thermodynamics and microphysics. The thermodynamic simulation allows the prediction of nitrate, ammonium, and aerosol water concentrations. It is estimated that nitrate contributes as much to total aerosol mass as sulfate on regional scales in parts of Europe and North America. The direct radiative forcing associated with the sulfate-nitrate-ammonium-water mixture is estimated to be −1.14 W m−2 for the present day. Based on a future emissions scenario, this could increase to as much as −2.13 W m−2 by the year 2100, an increase that results from increased nitrate concentrations. Although currently a minor contributor to aerosol direct radiative forcing, nitrate is predicted to exceed sulfate in its contribution by the end of the century for this emissions scenario. It is also found that direct radiative forcing estimates are highly sensitive to aerosol behavior at relative humidity above 90%, highlighting the shortcomings of global models in their treatment of aerosol water uptake under partly cloudy conditions. The microphysical simulation allows the prediction of tropospheric aerosol number concentrations and size distributions, key parameters in determining the indirect effect of aerosols on clouds. A two-moment sectional algorithm is used to simulate the microphysical processes of condensation/evaporation and coagulation. It has been tested by performing a simulation of sulfate microphysics. Predicted aerosol number concentrations generally agree with observations to within 25%. The microphysical simulation also reproduces key features of the tropospheric aerosol such as increasing number concentrations with altitude and land-sea contrasts in cloud condensation nuclei concentrations. It is found that there are important uncertainties in the source rates of new particles to the atmosphere, whether from in situ nucleation or emissions of particulates, that can significantly impact predicted aerosol number and cloud condensation nuclei concentrations. |
