AbstractAmbient ozone (O₃) concentrations in Southeast Michigan (SEMI) can exceed the U.S. National Ambient Air Quality Standard. Despite past efforts to measure O₃ 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 O₃ exceedances. In this study, we examine the chemical drivers of O₃ 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 (NO_x)‐limited O₃ 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 O₃ production in SEMI is limited by VOC emissions. This finding implies that imposing stricter regulations on VOC emissions should be prioritized for the SEMI O₃ nonattainment area. This study, through its use of ground‐based HCHO/NO₂ ratios and box modeling to assess O₃‐VOC‐NO_x sensitivities, has significant implications for air quality policy and the design of effective O₃ pollution control strategies, especially in O₃ nonattainment areas.