Regional evaluation and estimates of atmospheric nitrogen deposition for United States hydrologic units and ecoregions

Atmospheric nitrogen deposition is one of the major contributors to aquatic nitrogen loading in many regions of the United States. In this work, we evaluate the performance of the Comprehensive Air Quality Model with Extensions (CAMx) in predicting dry and wet deposition of nitrogen-containing species in different regions of the US. We compare our estimates to the measurements and model-measurement estimates available from the National Atmospheric Deposition Program (NADP). In addition, we provide estimates of the mass of oxidized and reduced nitrogen depositing to all levels of hydrologic units and ecoregions for the year 2011. Our estimated deposition fluxes align with the trends seen in recent analyses, including higher reduced nitrogen in the Upper Midwest and higher oxidized nitrogen in the Eastern United States and urban areas. Normalized mean bias factor and normalized mean absolute error factor are significantly higher over the Great Lakes. The dry deposition scheme based on Zhang (2001; 2003) showed substantially better performance for oxidized nitrogen deposition throughout the United States than the scheme based on Weseley (1989) and Slinn and Slinn (1980). Normalized mean bias factor and normalized mean absolute error factor are low in the central US for oxidized nitrogen dry deposition. This work provides accessible estimates of nitrogen deposition based on dry and wet deposition for reduced and oxidized species for water quality and ecosystem managers to use when updating the atmospheric loading in their models.