Impacts of Traffic Reductions Associated With COVID-19 on Southern California Air Quality.

On 19 March 2020, California put in place Stay-At-Home orders to reduce the spread of SARS-CoV-2. As a result, decreases up to 50% in traffic occurred across the South Coast Air Basin (SoCAB). We report that, compared to the 19 March to 30 June period of the last 5 years, the 2020 concentrations of PM and NO showed an overall reduction across the basin. O concentrations decreased in the western part of the basin and generally increased in the downwind areas. The NO decline in 2020 (approximately 27% basin-wide) is in addition to ongoing declines over the last two decades (on average 4% less than the -6.8% per year afternoon NO concentration decrease) and provides insight into how air quality may respond over the next few years of continued vehicular reductions. The modest changes in O suggests additional mitigation will be necessary to comply with air quality standards.