H I G H L I G H T S• This study documents the increasing importance of on-road emissions of NH 3.• NO x emissions control mechanisms frequently result in NH 3 production and emissions.• NH 3 is an important driver of N deposition in urban-affected areas and near roadways.• NH 4 -N:NO 3 -N ratios in urban deposition are indicative of elevated NH 3 emissions.• On-road NH 3 emissions exceed agricultural emissions where 40% of the U.S. resides. We provide updated spatial distribution and inventory data for on-road NH 3 emissions for the continental United States (U.S.) On-road NH 3 emissions were determined from on-road CO 2 emissions data and empirical NH 3 :CO 2 vehicle emissions ratios. Emissions of NH 3 from on-road sources in urbanized regions are typically 0.1-1.3 t km −2 yr −1 while NH 3 emissions in agricultural regions generally range from 0.4-5.5 t km
G R A P H I C A L A B S T R A C T a b s t r a c t a r t i c l e i n f o, with a few hotspots as high as 5.5-11.2 t km −2 yr −1. Counties with higher vehicle NH 3 emissions than from agriculture include 40% of the U.S. population. The amount of wet inorganic N deposition as NH 4 + 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 b45% of the N deposition as NH 4 + based on data from 2014 to 2016, illustrating the near-universal elevated proportions of NH 4 + in deposition across the U.S. Case studies of on-road NH 3 emissions in relation to N deposition include four urban sites in Oregon and Washington where the average NH 4 -N:NO 3 -N ratio in bulk deposition was 2.3. At urban sites in the greater Los Angeles Basin, bulk deposition of NH 4 -N and NO 3 -N were equivalent, while NH 4 -N:NO 3 -N in throughfall under shrubs ranged from 0.6 to 1.7. The NH 4 -N:NO 3 -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 NH 4 -N was strongly correlated with summertime NH 3 concentrations. On-road
Inorganic nitrogen (N) and sulfur (S) deposition in bulk throughfall and fog were determined at two sites located at opposite ends (42 km apart) of a pollution gradient in the San Bernardino Mountains. Plot-level averages for total annual N and S deposition in throughfall in 1996 were 18.8 and 2.9 kg·ha-1, respectively, at Camp Paivika (CP) and 2.9 and 0.4 kg·ha-1, respectively, at Barton Flats (BF). Deposition of N in throughfall in the four transects at CP ranged from 12.1 to 31.7 kg·ha-1·year-1. Spatial variability was high because of heterogenous canopy cover and varying exposure to air pollution. Annual estimated stand-level deposition of N and S in fog were 10.9 and 1.9 kg·ha-1, respectively, at CP, and 0.6 and 0.2 kg·ha-1, respectively, at BF. We estimated that N deposition in fog contributed 35% of the total annual N deposition at CP and 13% at BF. Analogous values for S were 39% at CP and 26% at BF. Ammonium deposition in throughfall and fog at CP were 72 and 122% as high as NO3- deposition because of elevated NH3 emissions from dairy farms in the Chino-Norco area 34 km southwest of CP.
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