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Published June 20, 1999 | public
Journal Article

Global concentrations of tropospheric sulfate, nitrate, and ammonium aerosol simulated in a general circulation model


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 determines 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 concentration 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 with 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 continents than over 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 because of changes in relative humidity and aerosol composition. Spatial variations in aerosol composition and water uptake have implications for direct and indirect aerosol radiative forcing.

Additional Information

This work has been supported by a graduate fellowship from the Fannie and John Hertz Foundation as well as by the National Aeronautics and Space Administration Earth Observing System Interdisciplinary Science program (NASA EOS-IDS). We would also like to acknowledge the Center for Advanced Computing Research at Caltech for computational resources, Loretta Mickley and fellow researchers at Harvard University for nitric acid concentration fields from the Harvard CTM, and Lex Bouwman at the National Institute of Public Health and the Environment in Bilthoven, Netherlands, for providing us with the GEIA ammonia emissions inventory in advance of publication. The EMEFS data utilized in this study were collected and prepared under the sponsorship of the Electric Power Research Institute. EMEP data were obtained from the Norwegian Institute for Air Research We also thank two anonymous reviewers for helpful comments and constructive criticism.

Additional details

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October 25, 2023