Influence of Anthropogenic Emission Reduction on O3 Chemical Formation Regimes in California during the COVID-19 Pandemic

Abstract:

Parts of the world have experienced increased O₃ levels during the COVID-19 pandemic despite significant reduction in anthropogenic air pollutant emissions. However, O₃ chemistry is non-linear, and the variation of ambient O₃ levels depends on the relative abundance of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) in the atmosphere. California experienced significant reduction in human activities following the Governor’s Executive Order N-33-20 (i.e., shelter-in-place order) on March 19, 2020, which perceivably shifted the O₃ chemical formation regimes in the State. Understanding such shift during the COVID-19 pandemic can provide California with the knowledge to optimize future environmental policies and regulations aimed at achieving the 8-hour O₃ National Ambient Air Quality Standards (NAAQS).

This study presents the ambient maximum daily 8-hour average O₃ levels (MDA8) at 26 O₃ monitoring sites in the South Coast Air Basin of California. In addition, spatially comprehensive satellite TROPOspheric Monitoring Instrument (TROPOMI) HCHO and NO₂ data and their ratios (HCHO/NO₂) were used to investigate the directionality of O₃ sensitivity to changing levels of precursor gases in the basin. Interannual monthly comparisons suggested that TROPOMI NO₂ levels decreased by 34% as a result of the shelter-in-place order, while TROPOMI HCHO levels decreased by 4%. The relatively larger decrease in TROPOMI NO₂ drove the basin-average TROPOMI (HCHO/NO₂) to be larger by 46% compared to the previous year, suggesting that the atmosphere became more sensitive to NO_x particularly in areas that were VOC-limited prior to the shelter-in-place order. Despite this basin-average shift, the spatial variation of TROPOMI (HCHO/NO₂) indicated areas that can significantly benefit from additional NO_x control to cross the O₃ ridgeline and further decrease O₃ in the long run (e.g., corridor from downtown LA to Riverside). Such locations could benefit from VOC reduction in the short-term, although supporting analyses demonstrated that NO_x mitigation strategy is still the most viable pathway in achieving 8-hour O₃ NAAQS.