Atmospheric Chemistry and Physics, vol. 12, issue 22 (2012) pp. 11213-11227
Atmospheric concentrations of inorganic gases and aerosols (nitrate, sulfate, and ammonium) are simulated for 2009 over the United States using the chemical trans-port model GEOS-Chem. Predicted aerosol concentrations are compared with surface-level measurement data from the Interagency Monitoring of Protected Visual Environ-ments (IMPROVE), the Clean Air Status and Trends Net-work (CASTNET), and the California Air Resources Board (CARB). Sulfate predictions nationwide are in reasonably good agreement with observations, while nitrate and ammo-nium 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 hetero-geneous hydrolysis of N 2 O 5 is over-predicted based on cur-rent values of the N 2 O 5 uptake coefficient, γ , onto aerosols. After reducing the value of γ by a factor of 10, predicted ni-trate 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 predic-tions with satellite measurements of ammonia from the Tro-pospheric Emissions Spectrometer (TES) indicates that am-monia emissions in GEOS-Chem are underestimated in Cal-ifornia and that the nationwide seasonality applied to ammo-nia emissions in GEOS-Chem does not represent California very well, particularly underestimating winter emissions. An ammonia sensitivity study indicates that GEOS-Chem sim-ulation 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 re-produce observed nitrate concentrations in southern Califor-nia and in other ammonia sensitive areas of California. How-ever, 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 po-tential regional overestimate of GEOS-5 mixed layer depths in the central valley due to unresolved topography in this re-gion.
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