24 Jun 2022
24 Jun 2022
Status: this preprint is currently under review for the journal ACP.

Quantifying the Importance of Vehicle Ammonia Emissions in an Urban Area of the Northeastern US Utilizing Nitrogen Isotopes

Wendell W. Walters1,2, Madeline Karod1,3, Emma Willcocks4, Bok H. Baek5, Danielle E. Blum2,6, and Meredith G. Hastings1,2 Wendell W. Walters et al.
  • 1Department of Earth, Environmental, and Planetary Sciences, Brown University; Providence, RI 02912, USA
  • 2Institute at Brown for Environment and Society, Brown University; Providence, RI 02912, USA
  • 3Chemistry and Physics Department, Simmons University; Boston MA 02215, USA
  • 4Program in Biology, Division of Biology and Medicine, Brown University; Providence, RI 02912, USA
  • 5Center for Spatial Information, Sciences, and Systems, George Mason University; Fairfax, VA 22030, USA
  • 6Department of Chemistry, Brown University; Providence, RI 02916, USA

Abstract. Atmospheric ammonia (NH3) is a critical component of our atmosphere that contributes to air quality degradation and reactive nitrogen deposition; however, our knowledge of NH3 in urban environments remains limited. Year-long ambient NH3 and related species were measured for concentrations and the nitrogen isotopic compositions (δ15N) of NH3 and particulate ammonium (pNH4+) to understand the temporal sources and chemistry of NH3 in a northeastern US urban environment. We found that urban NH3 and pNH4+ concentrations were elevated compared to regional rural background monitoring stations, with seasonally significant variations. Local and transported sources of NHx (NH3 + pNH4+) were identified using polar bivariate and statistical back trajectory analysis, which suggested the importance of vehicles, volatilization, industry, fuel combustion, and biomass burning emissions. Utilizing a uniquely positive δ15N(NH3) emission source signature from vehicles, a Bayesian stable isotope mixing model indicates that vehicles contribute 30.7±11.6 % (mean±1σ) to the annual background level of urban NHx, with a strong seasonal pattern with higher relative contribution during winter (45.8±13.0 %) compared to summer (20.8±9.7 %). The decrease in the relative importance of vehicle emissions during the summer was suggested to be driven by temperature-dependent NH3 emissions from volatilization sources based on wind direction, back trajectory, and NH3 emission inventory analysis. This work highlights that reducing vehicle NH3 emissions should be considered to improve wintertime air quality in this region.

Wendell W. Walters et al.

Status: open (until 05 Aug 2022)

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Wendell W. Walters et al.

Wendell W. Walters et al.


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Short summary
Atmospheric ammonia and its products are a significant source of urban haze and nitrogen deposition. We have investigated the seasonal source contributions to a mid-sized city in the northeastern US megapolis utilizing geospatial statistical analysis and novel isotopic constraints, which indicate that vehicle emissions were significant components of the urban reduced nitrogen budget. Reducing vehicle ammonia emissions should be considered to improve ecosystems and human health.