Spatiotemporal impacts of COVID-19 on air pollution in California, USA

Highlights: • The impacts of COVID-19-related interventional policies on air pollution are spatiotemporally analyzed over California. • The lockdown policy reduced the overall emissions of air pollutants in California. • During the lockdown, NO₂ decreased near major power plants and increased over major transportation hubs. • After reopening, state-mean air pollution metrics returned to normal trends in California. Various recent studies have shown that societal efforts to mitigate (e.g. "lockdown") the outbreak of the 2019 coronavirus disease (COVID-19) caused non-negligible impacts on the environment, especially air quality. To examine if interventional policies due to COVID-19 have had a similar impact in the US state of California, this paper investigates the spatiotemporal patterns and changes in air pollution before, during and after the lockdown of the state, comparing the air quality measurements in 2020 with historical averages from 2015 to 2019. Through time series analysis, a sudden drop and uptick of air pollution are found around the dates when shutdown and reopening were ordered, respectively. The spatial patterns of nitrogen dioxide (NO₂) tropospheric vertical column density (TVCD) show a decreasing trend over the locations of major powerplants and an increasing trend over residential areas near interactions of national highways. Ground-based observations around California show a 38%, 49%, and 31% drop in the concentration of NO₂, carbon monoxide (CO) and particulate matter 2.5 (PM_2.5) during the lockdown (March 19–May 7) compared to before (January 26–March 18) in 2020. These are 16%, 25% and 19% sharper than the means of the previous five years in the same periods, respectively. Our study offers evidence of the environmental impact introduced by COVID-19, and insight into related economic influences.