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

Atmospheric ammonia (NH3) is a critical component of our atmosphere that contributes to air quality degradation and reactive nitrogen deposition. NH3 plays a key role in the neutralization of acidic gases, leading to the formation of ammonium nitrate and ammonium sulfate that represent a significant component of PM2.5. Therefore, it is critical to understand NH3 emission sources, particularly in urban areas in which its human health impacts are magnified. However, quantifying the influence of NH3 emission sources is often a challenging task due to the coexistence of many locally produced sources including emissions from local traffic, fuel combustion, industrial processes, humans, and long-range transport from agricultural livestock waste emissions and fertilizer volatilization. Here we present results on quantifying the role of vehicle emissions to Providence, RI. Year-long ambient NH3 and related species were measured for concentrations and nitrogen isotope 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 localized and transported 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.