Characterizing the spatiotemporal nitrogen stable isotopic composition of ammonia in vehicle plumes

Vehicle emissions have been identified as an important urban source of ammonia (inline-formulaNH₃). However, there are large uncertainties regarding the contribution of vehicle emissions to urban inline-formulaNH₃ budgets, as well as the role of inline-formulaNH₃ in spatiotemporal fine particulate matter (inline-formulaPM_2.5) formation and nitrogen (N) deposition patterns. The N stable isotopic composition (inline-formulaδ¹⁵N) may be a useful observational constraint to track inline-formulaNH₃ emission sources and chemical processing, but previously reported vehicle inline-formulaδ¹⁵N(NH₃) emission signatures have reported a wide range of values, indicating the need for further refinement. Here we have characterized inline-formulaδ¹⁵N(NH₃) spatiotemporal variabilities from vehicle plumes in stationary and on-road measurements in the USA and China using an active inline-formulaNH₃ collection technique demonstrated to accurately characterize inline-formulaδ¹⁵N(NH₃) on the order of hourly time resolution. Significant spatial and temporal inline-formulaδ¹⁵N(NH₃) variabilities were observed and suggested to be driven by vehicle fleet composition and influences from inline-formulaNH₃ dry deposition on tunnel surfaces. Overall, a consistent inline-formulaδ¹⁵N(NH₃) signature of inline-formula6.6±2.1 ‰ (inline-formula $M15inlinescrollmathml\stackrel{normal\; ‾}{x}\pm \mathrm{normal\; 1}\mathrm{italic\; \sigma }$ 33pt11ptsvg-formulamathimge41cf8a8db9b73596012c373bf3f0c65 acp-20-11551-2020-ie00001.svg33pt11ptacp-20-11551-2020-ie00001.png ; inline-formulan=80) was found in fresh vehicle plumes with fleet compositions typical of urban regions. Our recommended vehicle inline-formulaδ¹⁵N(NH₃) signature is significantly different from previous reports. This difference is due to a large and consistent inline-formulaδ¹⁵N(NH₃) bias of approximately inline-formula−15.5 ‰ between commonly employed passive inline-formulaNH₃ collection techniques and the laboratory-tested active inline-formulaNH₃ collection technique. This work constrains the inline-formulaδ¹⁵N(NH₃) urban traffic plume signature, which has important implications for tracking vehicle inline-formulaNH₃ in urban-affected areas and highlights the importance of utilizing verified collection methods for accurately characterizing inline-formulaδ¹⁵N(NH₃) values.