Impacts of Covid-19 Restrictions 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, significant decreases in traffic and mobility occurred, up to a 50% reduction, across the South Coast Air Basin (SoCAB). Concurrently, the SoCAB experienced a month-long period of unusually clean air between March and April. Both the Safer-at-home orders and the stretch of clean air were also, however, coincident with anomalously rainy weather and frequent storm systems in the Basin. Here we show that the low concentrations of secondary pollutants (ozone and particulate matter) in late March and early April was mainly due to weather and the atypical amount of rain. Nevertheless, the decreases in traffic and industry due to Covid-19 restrictions did lead to a significant drop in atmospheric NO_x concentrations (~25% across the SoCAB). The decreases in NO_x from Covid-19 restrictions take place in the context of significant ongoing declines over the last two decades (~6.5% per year lower afternoon NO₂). The 2020 reductions in NO₂ are not consistent across the Basin. In Pasadena, concentrations were ~16% lower than the values expected by the consistent 6.5% per year decrease observed between 2000 and 2019 while levels in San Bernardino, on the eastern edge of the Basin, concentrations were only ~5% lower. The substantial additional NO_x decline in 2020 provides insight into how air quality will be affected by the next few years of vehicular NO_x reductions and suggests further considerations are needed to ensure air quality compliance.