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

Authors

Mark E. Fenn, USDA Forest ServiceFollow
Andrzej Bytnerowicz, USDA Forest Service
Susan L. Schilling, USDA Forest Service
Dena M. Vallano, University of California, Santa Cruz
Erika S. Zavaleta, University of California, Santa Cruz
Stuart B. Weiss, Creekside Center for Earth Observations
Connor Morozumi, University of California, Santa Cruz
Linda H. Geiser, U.S. Forest Service
Kenneth Hanks, USDA Forest Service

Document Type

Article

Date of this Version

2018

Citation

Science of the Total Environment 625 (2018), pp. 909–919.

Comments

U.S. government work.

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 hot spots 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.