Urban and On-Road Emissions:  Underappreciated Sources of Atmospheric Ammonia

Mark Fenn1, Susan Schilling2, Andrzej Bytnerowicz3, Michael Bell4, James Sickman5, Ken Hanks6 and Linda Geiser7

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.


1USDA Forest Service, PSW Research Station, mfenn@fs.fed.us
2USDA Forest Service, PSW Research Station, sschilling@fs.fed.us
3USDA Forest Service, PSW Research Station, abytnerowicz@fs.fed.us
4National Park Service, Air Resources Division, michael_d_bell@nps.gov
5Environmental Sciences Department, University of California, Riverside, james.sickman@ucr.edu
6USDA Forest Service, PSW Research Station, bougeotte1473@yahoo.com
7USDA Forest Service, NFS-WO, Wildlife, Fish & Rare Plants, Planning, lgeiser@fs.fed.us