The major source of oxides of nitrogen (NOx) that produce ground-level ozone (O3) come from mobile sources. Model calculations and ambient measurements both suggest that major California cities are currently in a “NOx-limited” regime where decreasing NOx concentrations lead to higher O3 concentrations, making current emissions control programs counter-productive in the short term. Shifting traffic patterns associated with COVID-19 may have reduced NOx emissions from mobile sources by more than ~50% in densely populated urban areas in California. This “natural experiment” provides an opportunity to (i) test the ability of models to simulate O3 response to deep cuts to ambient NOx concentrations, (ii) more accurately predict the amount of NOx reduction needed to achieve O3 benefits, and (iii) improve confidence in the long-term benefits of emissions control plans. This project will collect air pollution measurements using a modular transportable smog chamber in urban locations adjacent to major freeways in the City of Sacramento and the City of Redlands both during and after COVID-19 stay-at-home orders. The project team will then use chemical transport models (CTMs) to predict O3 concentrations during the time period when COVID-19 shelter-in-place mandates have greatly reduced NOx emissions from mobile sources. Predictions will be compared to the actual air pollution measurements collected. The ability of the modeling systems to accurately predict ambient ozone concentrations in the presence of these large emissions perturbations will verify the completeness of the model chemical mechanism, the accuracy of the model emissions inventory, and the effectiveness of emissions control programs that seek to reduce O3 concentrations by reducing NOx emissions. The evaluated modeling systems will be used to predict how O3 concentrations respond to a range of NOx and volatile organic compounds emissions controls and predict how much further NOx emissions need to decrease in order to achieve O3 benefits and in what year those O3 benefits will start to appear.