Constraining Ammonia Emissions in Vehicle Plumes Utilizing Nitrogen Stable Isotopes

Abstract. Vehicle emissions have been identified as an important urban source of ammonia (NH3). However, there are large uncertainties regarding the contribution of vehicle emissions to urban NH3 budgets, as well as its role in spatiotemporal fine particulate matter (PM2.5) formation and nitrogen (N) deposition patterns. The N stable isotopic composition (δ15N) may be a useful observational constraint to track NH3 emission sources and chemical processing, but previously reported vehicle δ15N(NH3) emission signatures have reported a wide range of values, indicating the need for further refinement. Here we have characterized δ15N(NH3) spatiotemporal variabilities from vehicle plumes in stationary and on-road measurements in the US and China using a laboratory- and field-verified NH3 collection technique shown to be accurate for characterizing δ15N(NH3) on the order of hourly time resolution. Significant spatial and temporal δ15N(NH3) variabilities were observed and suggested to be driven by vehicle fleet composition and influences from NH3 dry deposition on tunnel surfaces. The reactive NH3 sink associated with particulate ammonium (pNH4+) formation was found to have a minimal impact on the vehicle plume δ15N(NH3) measurements due to the vast majority of NHx (= NH3 + pNH4+) residing as NH3. Overall, a consistent δ15N(NH3) signature of 6.6 ± 2.1 ‰ ( x  ± 1σ; n = 80) was found in vehicle plumes with fleet compositions typical of urban regions. Overall, these measurements constrain the δ15N(NH3) urban traffic plume signature, which has important implications for tracking vehicle NH3 in urban-affected areas.