Nitrogen Deposition to the United States: Distribution, Sources, and Processes 3 4

32 33 We simulate nitrogen deposition over the US in 2006-2008 by using the GEOS-Chem 34 global chemical transport model with 1/2° × 2/3° horizontal resolution over North 35 America and adjacent oceans. US emissions of NO x and NH 3 in the model are 6.7 and 2.9 36 Tg N a-1 respectively, including a 20% natural contribution for each. Ammonia emissions 37 are a factor of 3 lower in winter than summer, providing a good match to US network 38 observations of NH x (≡ NH 3 gas + ammonium aerosol) and ammonium wet deposition 39 fluxes. Model comparisons to observed deposition fluxes and surface air concentrations 40 of oxidized nitrogen species (NO y) show overall good agreement but excessive 41 wintertime HNO 3 production over the US Midwest and Northeast. This suggests a model 42 overestimate N 2 O 5 hydrolysis in aerosols, and a possible factor is inhibition by aerosol 43 nitrate. Model results indicate a total nitrogen deposition flux of 6.5 Tg N a-1 over the 44 contiguous US, including 4.2 as NO y and 2.3 as NH x. Domestic anthropogenic, foreign 45 anthropogenic, and natural sources contribute respectively 78%, 6%, and 16% of total 46 nitrogen deposition over the contiguous US in the model. The domestic anthropogenic 47 contribution generally exceeds 70% in the east and in populated areas of the west, and is 48 typically 50-70% in remote areas of the west. Total nitrogen deposition in the model 49 exceeds 10 kg N ha-1