We have implemented fully interactive tropospheric gas-phase chemistry and sulfate aerosol modules into the new generation state-of the-art Goddard Institute for Space Studies (GISS) modelE general circulation model (GCM). The code has been developed with a unique flexibility to perform simulations in coupled or off-line (decoupled) mode. Both modes use identical chemical calculations, but the decoupled simulation relies on previously saved off-line oxidant and aerosol concentration fields whereas the coupled simulation is fully interactive. Here we describe the application of the model to isolate the impacts of the two-way chemistry-aerosol coupling on the predictions of sulfate aerosol and ozone pollution and to provide insights into the mechanisms that drive the different predictions between coupled and off-line models. On annual and global scales, the differences between the coupled and off-line simulations are small, but larger deviations do occur on regional and seasonal scales. The chemistry-aerosol coupling leads to similar to 20% increases in surface sulfate over SO2 source regions in the northern hemisphere summer due to higher H2O2 levels and aqueous-phase oxidation rates in the coupled model. Compensating sulfate decreases occur in downwind regions and in the upper troposphere due to depleted SO2. At middle to high tropospheric altitudes in the northern hemisphere, ozone and OH are increased in the coupled model relative to the off- line model by similar to 10% due to reductions in sulfate loading and subsequent suppression of N2O5 heterogeneous hydrolysis. The use of off- line versus coupled models has implications for the simulation of the intercontinental transport of pollutants and their precursors.
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