Isotopic Characterization of Reactive Nitrogen Species in Two US Metropolitan Areas in the Summertime

Despite significant reductions of precursor emissions, mitigation of surface ozone (O₃) and particulate matter (PM) pollution remains challenging. Many US megacity regions still frequently experience summertime surface O₃ concentrations >70 ppbv with severe human health effects. As major O₃ precursors, nitrogen oxides (NO_x) and nitrous acid (HONO) as well as their oxidation products, nitric acid and particulate nitrate, play important roles in modulating O₃ and PM. However, there are significant gaps in quantitative understanding of their sources, transport, chemical transformation and sinks, especially in urban areas influenced by complex mixutures of emissions and varying meteorological conditions. Stable isotopic compositions (¹⁵N/¹⁴N, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O) and concentration measurements can provide multi-dimensional constraints on the sources and transformations of these reactive nitrogen (RN) species.

In the past three summers (2021 – 2023), we performed ground measurements in Southeast Michigan during the Michigan-Ontario Ozone Source Experiment (MOOSE) campaign, as well as in the New York Metropolitan Area during the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaign. We used N isotopic compositions to characterize the sources of RN, and combined N and O isotopic compositions to decipher the chemical transformations (i.e., RN cycling). We will compare and contrast the RN cycling and influences on oxidation chemistry, constrained by the isotopic results, for the two metropolitan areas on the Great Lakes versus Atlantic coastlines and investigate their implications for simulating urban air quality.