Ambient ozone (O₃) concentrations in southeast Michigan (SEMI) often exceed the U.S. National Ambient Air Quality Standards (NAAQS), forcing local authorities to seek O₃attainment strategies in this region. Despite the efforts in measurement of O₃ precursors and mechanism elucidation from previous studies, the changing emission patterns and composition require new measurements and updated mechanism to understand the causes for the frequently observed O₃exceedances in SEMI. In this study, we employed an observationally-constrained zero-dimensional (0-D) box model and backward trajectory analysis to explore the potential causes of O₃ exceedances in SEMI, based on the Phase I MOOSE (Michigan-Ontario Ozone Source Experiment) field study performed during May – June 2021. We conducted a series of model sensitivity simulations to investigate O₃ production regimes in responses to both varied NO_x and VOC concentrations, respectively, with other model input constrained by MOOSE field measurements. Our sensitivity simulations revealed that the formaldehyde (HCHO) to nitrogen dioxide (NO₂) ratios for the transitional regimes of O₃chemical production were 0.18-4.1. Given that the averaged daytime HCHO to NO₂ ratio during MOOSE Phase I was 1.13 ± 0.83, it suggested that O₃ production in SEMI was likely limited by both volatile organic compounds (VOC) and nitrogen oxides (NO_x) emissions. This finding implied that imposing stricter regulations on VOC and NO_x emissions should be prioritized for the SEMI O₃ nonattainment area. In addition, we performed the 36-hr backward trajectory and cluster analyses revealed that O₃ exceedances in SEMI were also impacted by the long-range transport of air pollutants likely originating from Ontario during periods of northeastern flows, although southwestern flows from other U.S. states (e.g., Illinois, Indiana, and Ohio) also contributed to the elevated O₃ in SEMI. As such, performing a joint O₃ attainment strategy in the US-Canadian border region is imperative to minimize impacts of transboundary transport on both Michigan and Ontario.

Francesca Majluf

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