Abstract
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 the role of NH3 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 USA and China using an active NH3 collection technique demonstrated to accurately characterize 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. Overall, a consistent δ15N(NH3) signature of 6.6±2.1 ‰ (x‾±1σ; n=80) was found in fresh vehicle plumes with fleet compositions typical of urban regions. Our recommended vehicle δ15N(NH3) signature is significantly different from previous reports. This difference is due to a large and consistent δ15N(NH3) bias of approximately −15.5 ‰ between commonly employed passive NH3 collection techniques and the laboratory-tested active NH3 collection technique. This work constrains the δ15N(NH3) urban traffic plume signature, which has important implications for tracking vehicle NH3 in urban-affected areas and highlights the importance of utilizing verified collection methods for accurately characterizing δ15N(NH3) values.
Highlights
Atmospheric ammonia (NH3) is a critical component of the atmosphere and the global nitrogen (N) cycle (Behera et al, 2013; Galloway et al, 2004)
Our analysis indicated that δ15N(NH3) variability was influenced by fleet composition and NH3 dry deposition in aged vehicle plumes measured in a tunnel
Our measurements indicate a δ15N(NH3) range of −3.0 ‰ to 10.1 ‰ from vehicle-derived plumes representing a variety of driving conditions and fleet compositions that included stationary measurements conducted in Providence, RI, USA, and Shenyang, Liaoning, China, and mobile on-road measurements performed in the northeastern USA
Summary
Atmospheric ammonia (NH3) is a critical component of the atmosphere and the global nitrogen (N) cycle (Behera et al, 2013; Galloway et al, 2004). As the primary atmospheric alkaline molecule, NH3 plays an essential role in the neutralization of sulfuric acid (H2SO4) and nitric acid (HNO3), leading to the formation of ammonium nitrate (NH4NO3), ammonium bisulfate (NH4HSO4), and ammonium sulfate ((NH4)2SO4) (Behera and Sharma, 2012) These compounds are the most abundant secondary components of inorganic fine particulate matter (PM2.5), which has important implications for air quality, human health, visibility, and global climate change (Behera and Sharma, 2010; Updyke et al, 2012; Wang et al, 2015). At the nearhighway monitoring site, wind sector analysis found no statistical difference in δ15N(NH3) when sorted by wind direction for either summer or winter (Fig. 2) This indicates that transport from local NH3 point sources other than vehicle emissions played a minor role in the seasonal δ15N(NH3) difference. When the tunnel was closed, [NHx] averaged 79.4 ± 14.4 ppbv (Table 3), which is elevated compared to urban background [NH3] measurements previously reported from a megacity in China (Beijing) during winter of 5.22 ± 3.75 μg m−3
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