Sulfur, sea salt, and radionuclide aerosols in GISS ModelE

We simulate a suite of aerosols in the new GISS ModelE GCM: present-day sulfur species and the natural species sea salt, radionuclides ⁷Be (stratospheric source), and ²¹⁰Pb (derived from ²²²Rn from soils). The natural species are used to test the model and to help diagnose the anthropogenic sulfur species. Model improvements over previous versions include increased vertical resolution, addition of a stratiform dissolved species budget (DSB), better tracer coupling to the boundary layer, and an improved relative humidity-dependent radiative scheme. The DSB reduces the loads of most soluble species since it increases stratiform precipitation scavenging. We compare the model with extensive surface and aircraft concentration measurements in the troposphere and stratosphere. We compare three different formulations of the ²²²Rn emissions and find that a scheme with reduced radon flux at high latitudes of the Northern Hemisphere for all seasons gives the best ²¹⁰Pb results. Although the ²²²Rn emissions appear to be approximately the right order of magnitude, model ²¹⁰Pb is too large. Conversely, our ⁷Be source is too small (since concentrations in the upper troposphere and stratosphere are deficient), while the ⁷Be surface concentrations are as observed. These radionuclide results suggests that model scavenging (by moist convection) is deficient. Our new model uses increased natural sulfur emissions; however, the DSB causes sulfate to be generally less than observed and indicates a need for additional sulfur oxidation mechanisms. Model radiative forcing for sulfate and sea salt are -0.54 and -1.1 W m^-2, respectively. The anthropogenic sulfate forcing is -0.25 W m^-2, less than the -0.68 W m^-2 in our previous model mainly owing to an 18% decrease in industrial emissions.