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 5years, the 2020 concentrations of PM2.5 and NOx 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 NOx 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 NO2 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. Plain Language Summary On 19 March 2020, California put in place Stay-At-Home orders to reduce the spread of SARS-CoV-2. As a result, there was much less traffic in Southern California. Reduced traffic along with a month-long stretch of unusually rainy weather at the beginning of the lockdown led to significant reductions in PM2.5 and NOx levels across the basin. Concentrations of O-3, on the other hand, showed inconsistent changes across the basin. The response of O-3 to these large changes in nitrogen oxide concentrations suggests mitigation efforts beyond those associated with continuing vehicle emission reductions will be important to meet clean air goals. Key Points SoCAB maximum 1-hr NOx and 24-hr PM2.5 concentrations decreased 27% and 29%, respectively, between 19 March and 30 June of 2015-2019 and 2020 The 8-hr daily maximum O-3 showed inconsistent changes across the basin during the COVID-19 associated decrease of atmospheric NOx concentrations During a shift to a NOx-limited regime, a better understanding of VOC emission sources is needed to improve air quality in the SoCAB