Increasing anthropogenic emissions of nitrogen released into the atmosphere contributes to water eutrophication. In this study we quantified seasonal dry and wet deposition of major nitrogen-containing species, including ammonia (NH₃), nitric acid (HNO₃), nitrogen oxides (NOx), particulate ammonium (PNH₄) and particulate nitrate (PNO₃) onto hydrologic regions in the contiguous United States. We used a regional chemical transport model, the Comprehensive Air Quality Model with eXtension (CAMx) to obtain deposition information in the contiguous United States.

Our results show that gaseous species dominant dry deposited mass. Ammonia (NH₃), nitric acid (HNO₃), and nitrogen dioxide (NO₂) each contribute 40% to 60%, 20% to 40%, and approximately 10% of the total nitrogen dry deposition, respectively. For wet deposition the major contributors are nitric acid (HNO₃) and particulate ammonium (PNH₄). HNO₃ contributes 40%-60% of the total nitrogen wet deposited mass, PNH₄ 20%-40%, and NH₃ and particulate nitrate (PNO₃) together about 10%. Spring and summer consistently receive the more deposited nitrogen than fall or winter for both wet and dry deposition. The maximum deposition of spring is over twice that of fall due to more frequent rainfall, which removes the nitrogen-containing species from the atmosphere increasing the loading to hydrologic regions. Increased vegetation in spring and summer increases dry deposition.

Over the different hydrologic regions, the annual mass of nitrogen deposited through dry deposition ranges from around 110 to 910 kgN/km², over twice that of wet deposition, which ranges from about 40 to 390 kgN/km². The Eastern United States, including Ohio, Mid Atlantic, Great Lakes, Upper Mississippi, Lower Mississippi, Tennessee, and New England hydrologic regions alongside the California, Texas-Gulf, and Arkansas-White-Red hydrologic regions receive the most deposition during the year. These spatial trends exist because nitrogen sources are mainly vehicular emissions, power plants, and fertilizer use and these hydrologic regions are highly populated.

In addition, we have estimated the amount of deposited nitrogen from outside each region to provide additional insight into the regional sources of nitrogen in each hydrologic region. We will conclude with a discussion about which regions contribute disproportionately to nitrogen deposition and which regions are disproportionately impacted by nitrogen emissions.