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| Titel |
Simulation of nitrate, sulfate, and ammonium aerosols over the United States |
| VerfasserIn |
J. M. Walker, S. Philip, R. V. Martin, J. H. Seinfeld |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 22 ; Nr. 12, no. 22 (2012-11-27), S.11213-11227 |
| Datensatznummer |
250011625
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| Publikation (Nr.) |
 copernicus.org/acp-12-11213-2012.pdf |
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| Zusammenfassung |
| Atmospheric concentrations of inorganic gases and aerosols (nitrate, sulfate,
and ammonium) are simulated for 2009 over the United States using the
chemical transport model GEOS-Chem. Predicted aerosol concentrations are
compared with surface-level measurement data from the Interagency Monitoring
of Protected Visual Environments (IMPROVE), the Clean Air Status and Trends
Network (CASTNET), and the California Air Resources Board (CARB). Sulfate
predictions nationwide are in reasonably good agreement with observations,
while nitrate and ammonium are over-predicted in the East and Midwest, but
under-predicted in California, where observed concentrations are the highest
in the country. Over-prediction of nitrate in the East and Midwest is
consistent with results of recent studies, which suggest that nighttime
nitric acid formation by heterogeneous hydrolysis of N2O5 is
over-predicted based on current values of the N2O5 uptake coefficient,
γ, onto aerosols. After reducing the value of γ by a factor of
10, predicted nitrate levels in the US Midwest and East still remain higher
than those measured, and over-prediction of nitrate in this region remains
unexplained. Comparison of model predictions with satellite measurements of
ammonia from the Tropospheric Emissions Spectrometer (TES) indicates that
ammonia emissions in GEOS-Chem are underestimated in California and that the
nationwide seasonality applied to ammonia emissions in GEOS-Chem does not
represent California very well, particularly underestimating winter
emissions. An ammonia sensitivity study indicates that GEOS-Chem simulation
of nitrate is ammonia-limited in southern California and much of the state,
suggesting that an underestimate of ammonia emissions is likely the main
cause for the under-prediction of nitrate aerosol in many areas of
California. An approximate doubling of ammonia emissions is needed to
reproduce observed nitrate concentrations in southern California and in other
ammonia sensitive areas of California. However, even a tenfold increase in
ammonia emissions yields predicted nitrate concentrations that are still
biased low in the central valley of California. The under-prediction of
nitrate aerosol in the central valley of California may arise in part from an
under-prediction of both ammonia and nitric acid in this region. Since
nitrate aerosols are particularly sensitive to mixed layer depths, owing to
the gas-particle equilibrium, the nitrate under-prediction could also arise
in part from a potential regional overestimate of GEOS-5 mixed layer depths
in the central valley due to unresolved topography in this region. |
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