As a result of over-reduction of nitrogen oxides, light and medium duty vehicles and newer heavy duty vehicles on U.S. roadways emit significant levels of ammonia (NH₃). Herein we provide updated spatial distribution and inventory data for on-road NH₃ emissions for the continental U.S. On-road NH₃ emissions in urbanized regions are typically 0.1 – 1.3 t/km²/yr. By comparison, NH₃ emissions in agricultural regions generally range from 0.4 – 5.5 t/km²/yr, with a few hotspots as high as 6 – 11 t/km²/yr. We have identified 500 counties that receive at least 30% of the NH₃ emissions from on-road sources. Counties with higher vehicle NH₃ emissions than from agriculture include 41% of the U.S. population. Within CONUS the percent of wet inorganic N deposition from the NADP/NTN as NH₄⁺ ranged from 37 to 83% with a mean of 59.5%. Only 13% of the NADP sites across the U.S. had less than 45% of the N deposition as NH₄⁺ based on data from 2014-2016, illustrating the near-universal occurrence of NH₄⁺ deposition across the United States, regardless of the primary sources of NH₃ emissions.

In four urban sites in Oregon and Washington the NH₄-N:NO₃-N ratio in throughfall averaged 1.0 (range of 0.8 – 1.3) compared to an average ratio of 2.3 in bulk deposition (range of 1.8 – 2.9) measured in open canopy-free areas. In bulk deposition at urban sites in the LA Basin deposition of NH₄-N and NO₃-N were highly similar to each other. Ratios of NH₄-N:NO₃-N in throughfall under shrubs in the Los Angeles Air Basin ranged from 0.7 to 1.5 across a spatially extensive network of chaparral and coastal sage scrub sites with high but varying degrees of urban influence. The NH₄-N:NO₃-N ratio at ten sites in the Lake Tahoe Basin averaged 1.4 and 1.6 in bulk deposition and throughfall, respectively. The ratio in throughfall at Valhalla, immediately adjacent to the city of South Lake Tahoe, was 1.9. Annual throughfall deposition and bulk deposition of NH₄-N was strongly correlated with summertime NH₃ concentrations in the Tahoe Basin. Values of δ¹⁵NH₄⁺ in bulk deposition and throughfall samples in the Tahoe Basin were predominantly within the range of -5.0 to -0.9‰, indicative of tailpipe NH₃ emissions. The relative importance of urban and on-road NH₃ emissions versus emissions from agriculture varies regionally. In some areas both are important and should be considered when evaluating the principal sources of N deposition to affected ecosystems.