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

Unlabelled: We provide updated spatial distribution and inventory data for on-road NH emissions for the continental United States (U.S.) On-road NH emissions were determined from on-road CO emissions data and empirical NH:CO vehicle emissions ratios. Emissions of NH from on-road sources in urbanized regions are typically 0.1-1.3tkmyr while NH emissions in agricultural regions generally range from 0.4-5.5tkmyr, with a few hotspots as high as 5.5-11.2tkmyr. Counties with higher vehicle NH emissions than from agriculture include 40% of the U.S.

Population: The amount of wet inorganic N deposition as NH 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 NH based on data from 2014 to 2016, illustrating the near-universal elevated proportions of NH in deposition across the U.S. Case studies of on-road NH emissions in relation to N deposition include four urban sites in Oregon and Washington where the average NH-N:NO-N ratio in bulk deposition was 2.3. At urban sites in the greater Los Angeles Basin, bulk deposition of NH-N and NO-N were equivalent, while NH-N:NO-N in throughfall under shrubs ranged from 0.6 to 1.7. The NH-N:NO-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-N was strongly correlated with summertime NH concentrations. On-road emissions of NH should not be ignored as an important source of atmospheric NH, as a major contributor to particulate air pollution, and as a driver of N deposition in urban and urban-affected regions.