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Adams, P.J., J.H. Seinfeld, and D.M. Koch, 1999: Global concentrations of tropospheric sulfate, nitrate, and ammonium aerosol simulated in a general circulation model. J. Geophys. Res., 104, 13791-13823, doi:10.1029/1999JD900083.
Global sulfate aerosol composition is simulated online in the Goddard Institute for Space Studies general circulation model II' (GISS GCM II-prime). Four sulfur species, hydrogen peroxide, gas-phase ammonia, and particulate ammonium are the prognostic tracer species, the emissions, transport, and deposition of which are explicitly simulated. Nitric acid fields are prescribed based on a global chemical transport model. An online thermodynamic equilibrium calculation is used to determine the partitioning of ammonia and nitrate between gas and aerosol phases, and the quantity of aerosol water based on the temperature, relative humidity, and sulfate in each GCM grid cell. The total global burden of sulfate, nitrate, ammonium, and aerosol water is 7.5 Tg, and is most sensitive to changes in sulfur emissions. Tropospheric lifetimes for ammonium and ammonia are 4.2 and 0.9 days, respectively; the tropospheric ammonium burden is 0.30 Tg N, compared to 0.14 Tg N for ammonia. Simulated ammonium concentrations are generally within a factor of 2 of observations. Subgrid variability in measured concentrations hinders comparison of observations to predictions. Ammonium nitrate aerosol plays an important role in determining total aerosol mass in polluted continental areas. In the upper troposphere and near the poles, cold temperatures allow unneutralized nitric acid to condense into the aerosol phase. Acidic aerosol species tend to be neutralized by ammonia to a greater degree over the continents than oceans. The aerosol is most basic and gas-phase ammonia concentrations are highest over India. Water uptake per mole of sulfate aerosol varies by two orders of magnitude due to changes in relative humidity and aerosol composition. Spatial variations in aerosol composition and water uptake have implications for direct and indirect aerosol radiative forcing.
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