Abstract

The amount of traffic on California’s roadways decreased by approximately fifty percent during the COVID-19 pandemic lockdown in March and April of 2020. Conventional wisdom led to the expectation that reduced traffic would result in reduced ozone (O3) concentrations—ozone being a main component of smog—yet ozone concentrations increased during this period. Internal combustion vehicles emit oxides of nitrogen (NOx) and volatile organic compounds (VOCs). These emissions are precursors for ozone formation, but the relationship between these precursor emissions and the final ozone concentration is complex. The ratio of NOx/VOCs determines if the ozone formation will be “NOx-limited” or “NOx-rich”. Major NOx reductions are required to reduce ozone concentrations when the atmosphere is NOx-rich. Small NOx reductions in a NOx-rich atmosphere can actually increase ozone concentrations.

To inform ongoing efforts to reduce ambient O3 concentrations, this brief highlights findings from research that collected and analyzed air pollution measurements 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 results provide an updated estimate for how many more years of NOx control will be required before O3 benefits are realized.