<p>Surface-level ozone (O<sub>3</sub>) is a secondary air pollutant that has adverse effects on human health. In the troposphere, O<sub>3</sub> is produced in complex cycles of photochemical reactions involving nitrogen oxides (NO<sub>x</sub>) and volatile organic compounds (VOCs). Determining if O<sub>3</sub> production will be decreased by lowering NO<sub>x</sub> emissions (“NO<sub>x</sub>-sensitive”), VOC emissions (“VOC-sensitive”), or both (“the transition zone”) can be done by using the formaldehyde (HCHO; a VOC species) to nitrogen dioxide (NO<sub>2</sub>; a component of NO<sub>x</sub>) concentration ratio ([HCHO]/[NO<sub>2</sub>]; “FNR”). Generally, lower FNR values indicate VOC sensitivity while higher values indicate NO<sub>x</sub> sensitivity. In this study, we use FNRs calculated from 2019–2021 TROPOspheric Monitoring Instrument (TROPOMI) satellite data and 2019 Photochemical Assessment Monitoring Station (PAMS) ground-based data to investigate the ozone–NO<sub>x</sub>–VOC sensitivity of the Lake Michigan region, an area that regularly exceeds the United States Environmental Protection Agency’s regulatory standards for O<sub>3</sub>. We find that TROPOMI FNRs are always greater than PAMS FNRs, indicating that they must be interpreted with different threshold values to infer O<sub>3</sub> chemistry sensitivities. Further analysis of TROPOMI FNRs reveals that during both typical O<sub>3</sub> season days and Chicago, Illinois, O<sub>3</sub> exceedance days, the average O<sub>3</sub> chemistry sensitivity is: (1) VOC-sensitive in the Chicago metropolitan area (CMA), (2) transitional in the areas surrounding the CMA and up the western Lake Michigan coastline to Milwaukee, Wisconsin, and (3) NO<sub>x</sub>-sensitive in the rest of the domain. However, the magnitude of FNR values change during exceedance days, indicating that areas that are NO<sub>x</sub>-sensitive (VOC-sensitive) during typical O<sub>3</sub> season days increase in NO<sub>x</sub>-sensitivity (VOC-sensitivity). Additionally, the transition zone area decreases by 25 % on exceedance days. Comparing weekends to weekdays, O<sub>3</sub> chemistry in the Chicago metropolitan area becomes more NO<sub>x</sub>-sensitive on weekends due lower NO<sub>x</sub> emissions. Finally, analysed 10-meter wind data shows that the lake breeze circulation, which transports high O<sub>3</sub> levels from over Lake Michigan to onshore coastal areas, is stronger during O<sub>3</sub> exceedance days compared to typical O<sub>3</sub> season days, and there are no major weekday-weekend differences in the properties of the 10-meter wind field.</p>