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(2.5) and NO(x) showed an overall reduction across the basin. O(3) concentrations decreased in the western part of the basin and generally increased in the downwind areas. The NO(x) 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(2) 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(3) suggests additional mitigation will be necessary to comply with air quality standards.