Examining the Summertime Ozone Formation Regime in Southeast Michigan Using MOOSE Ground-Based HCHO/NO₂ Measurements and F0AM Box Model

Ambient ozone (O-3) concentrations in Southeast Michigan (SEMI) can exceed the U.S. National Ambient Air Quality Standard. Despite past efforts to measure O-3 precursors and elucidate reaction mechanisms, changing emission patterns and atmospheric composition in SEMI warrant new measurements and updated mechanisms to understand the causes of observed O-3 exceedances. In this study, we examine the chemical drivers of O-3 exceedances in SEMI, based on the Phase I MOOSE (Michigan-Ontario Ozone Source Experiment) field study performed during May to June 2021. A zero-dimensional (0-D) box model is constrained with measurement data of meteorology and trace gas concentrations. Box model sensitivity simulations suggest that the formaldehyde to nitrogen dioxide ratio (HCHO/NO2) for the transition between the volatile organic compounds (VOCs)- and nitrogen oxides (NOx)-limited O-3 production regimes is 3.0 +/- 0.3 in SEMI. The midday (12:00-16:00) averaged HCHO/NO2 ratio during the MOOSE Phase I study is 1.62 +/- 1.03, suggesting that O-3 production in SEMI is limited by VOC emissions. This finding implies that imposing stricter regulations on VOC emissions should be prioritized for the SEMI O-3 nonattainment area. This study, through its use of ground-based HCHO/NO2 ratios and box modeling to assess O-3-VOC-NOx sensitivities, has significant implications for air quality policy and the design of effective O-3 pollution control strategies, especially in O-3 nonattainment areas.