On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition

Abstract

We provide updated spatial distribution and inventory data for on-road NH₃ emissions for the continental United States (U.S.) On-road NH₃ emissions were determined from on-road CO₂ emissions data and empirical NH₃:CO₂ vehicle emissions ratios. Emissions of NH₃ from on-road sources in urbanized regions are typically 0.1–1.3 t km^− 2 yr^− 1 while NH₃ emissions in agricultural regions generally range from 0.4–5.5 t km^− 2 yr^− 1, with a few hotspots as high as 5.5–11.2 t km^− 2 yr^− 1. Counties with higher vehicle NH₃ emissions than from agriculture include 40% of the U.S. population. The amount of wet inorganic N deposition as NH₄⁺ from the National Atmospheric Deposition Program (NADP) network ranged from 37 to 83% with a mean of 58.7%. Only 4% of the NADP sites across the U.S. had < 45% of the N deposition as NH₄⁺ based on data from 2014 to 2016, illustrating the near-universal elevated proportions of NH₄⁺ in deposition across the U.S. Case studies of on-road NH₃ emissions in relation to N deposition include four urban sites in Oregon and Washington where the average NH₄-N:NO₃-N ratio in bulk deposition was 2.3. At urban sites in the greater Los Angeles Basin, bulk deposition of NH₄-N and NO₃-N were equivalent, while NH₄-N:NO₃-N in throughfall under shrubs ranged from 0.6 to 1.7. The NH₄-N:NO₃-N ratio at 7–10 sites in the Lake Tahoe Basin averaged 1.4 and 1.6 in bulk deposition and throughfall, and deposition of NH₄-N was strongly correlated with summertime NH₃ concentrations. On-road emissions of NH₃ should not be ignored as an important source of atmospheric NH₃, as a major contributor to particulate air pollution, and as a driver of N deposition in urban and urban-affected regions.

Citation

Fenn, Mark E.; Bytnerowicz, Andrzej; Schilling, Susan L.; Vallano, Dena M.; Zavaleta, Erika S.; Weiss, Stuart B.; Morozumi, Connor; Geiser, Linda H.; Hanks, Kenneth. 2018. On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition. Science of The Total Environment. 625: 909-919. https://doi.org/10.1016/j.scitotenv.2017.12.313.