Tracking Ammonia Emission and Chemistry in Fresh Traffic Derived Plumes Utilizing Nitrogen Stable Isotopes

Atmospheric ammonia (NH₃) is an important trace gas that serves as the primary inorganic base in the atmosphere. NH₃ plays a key role in the neutralization of acidic gases, leading to the formation of ammonium nitrate (NH₄NO₃) and ammonium sulfate ((NH₄)₂SO₄) that represent a significant component of PM_2.5. Therefore, it is critical to understand NH_x (NH₃ + NH₄⁺) dynamics including NH₃ emission sources and gas-to-particle phase conversions, particularly in urban areas in which its human health impacts are magnified. However, quantifying the influence of NH₃ emission sources and sink processes 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. Here, we present novel results involving the quantification of NH_x dynamics via concentration and N isotopic composition (δ¹⁵N) of speciated NH_x measurements from several different traffic-derived plumes utilizing a denuder-filter pack sampling system that has been laboratory verified for its δ¹⁵N accuracy and precision. Summer and winter stationary measurements were conducted at a near-highway (Providence, RI, USA along I-95) air monitoring station to evaluate the seasonal impact on NH_x dynamics in fresh plumes. Mobile highway measurements were conducted in the northeast corridor from Boston, Massachusetts to Washington, D.C., USA during the winter to investigate the spatial patterns involving NH_x dynamics. Additionally, stationary measurements were conducted in a tunnel located in Shenyang, a megacity in northeast China. Our results indicate that traffic derived NH₃ emissions play a key role in inorganic PM_2.5 formation, particularly ammonium nitrate (NH₄NO₃) during the winter, which has important implications for urban wintertime air quality. At all three monitoring locations, vehicle derived NH₃ tends to have a consistent δ¹⁵N that is distinct relative to other major NH₃ emission sources. Thus, δ¹⁵N(NH_x) has the potential to be used to quantitatively track NH₃ emission sources, which has important implications for our understanding of NH_x dynamics in complex urban environments and may help guide future NH₃ emission regulations.