Nitrogen deposition to the United States: distribution, sources, and processes

We simulate nitrogen deposition over the US in 2006–2008 by using the GEOS-Chem global chemical transport model at 1/2×2/3 horizontal resolution over North America and adjacent oceans. US emissions of NOx and NH3 in the model are 6.7 and 2.9 Tg N a−1 respectively, including a 20 % natural contribution for each. Ammonia emissions are a factor of 3 lower in winter than summer, providing a good match to US network observations of NHx (≡NH3 gas + ammonium aerosol) and ammonium wet deposition fluxes. Model comparisons to observed deposition fluxes and surface air concentrations of oxidized nitrogen species (NOy) show overall good agreement but excessive wintertime HNO3 production over the US Midwest and Northeast. This suggests a model overestimate N2O5 hydrolysis in aerosols, and a possible factor is inhibition by aerosol nitrate. Model results indicate a total nitrogen deposition flux of 6.5 Tg N a−1 over the contiguous US, including 4.2 as NOy and 2.3 as NHx. Domestic anthropogenic, foreign anthropogenic, and natural sources contribute respectively 78 %, 6 %, and 16 % of total nitrogen deposition over the contiguous US in the model. The domestic anthropogenic contribution generally exceeds 70 % in the east and in populated areas of the west, and is typically 50–70 % in remote areas of the west. Total nitrogen deposition in the model exceeds 10 kg N ha−1 a−1 over 35 % of the contiguous US.