NOx reductions during COVID-19: Has ozone chemistry in the Los Angeles area become NOx-limited?

For the past decade, California's statewide strategies for reducing emissions of O3 precursors have largely focused on reducing NOx emissions from mobile sources. This will continue into the next decade as recent regulations related to next-gen and zero emission vehicles are phased in.

Because tropospheric O3 is formed via complex non-linear photochemical reactions, quantifying the expected air quality benefits from projected NOx reductions is challenging. This work integrates observations from multiple perspectives, including regulatory monitoring networks, satellites, and photochemical models to explore the effectiveness of future NOx controls in the South Coast air basin (SoCAB), which includes the Los Angeles metropolitan area. This analysis takes advantage of two `natural experiments', the weekday-weekend effect and COVID-19, to observe the air quality response to real-world changes in NOx emissions.

Historically, O3 in a large portion of the SoCAB has been higher on weekends than on weekdays. This phenomenon arises due to reduced truck traffic on weekends, indicating a NOx-saturated photochemical regime whereby NOx reductions lead to increased O3 (also called VOC-limited). Historical observations of satellite (OMI) HCHO/NO2 ratios support these findings, with low HCHO/NO2 ratios indicating a NOx-saturated environment for urban areas of the basin during the O3 season. In recent years, the weekend/weekday O3 ratio has been decreasing towards 1 while the HCHO/NO2 ratio has been increasing with stable HCHO levels and decreasing NO2 levels. Combined, these indicators (the weekend/weekday O3 ratio and HCHO/NO2) suggest that NOx controls have pushed the SoCAB towards the transition point to NOx-limited. During late spring and early Summer of 2020, we observed weekend/weekday O3 ratios less than 1, and higher-than-expected HCHO/NO2 ratios (due to large decreases in observed column NO2), which indicates that the additional NOx reductions brought about by COVID-19 may have been sufficient to push the SoCAB into a NOx-limited regime for the first time during O3 season. These results provide useful benchmarks for photochemical modeling and guidance as to the magnitude of NOx controls required to meet ambient air quality standards.