Global, continental, and national variation in PM, O, and NO concentrations during the early 2020 COVID-19 lockdown.

Lockdowns implemented in response to COVID-19 have caused an unprecedented reduction in global economic and transport activity. In this study, variation in the concentration of health-threatening air pollutants (PM, NO, and O) pre- and post-lockdown was investigated at global, continental, and national scales. We analyzed ground-based data from >10,000 monitoring stations in 380 cities across the globe. Global-scale results during lockdown (March to May 2020) showed that concentrations of PM and NO decreased by 16.1% and 45.8%, respectively, compared to the baseline period (2015-2019). However, O concentration increased by 5.4%. At the continental scale, concentrations of PM and NO substantially dropped in 2020 across all continents during lockdown compared to the baseline, with a maximum reduction of 20.4% for PM in East Asia and 42.5% for NO in Europe. The maximum reduction in O was observed in North America (7.8%), followed by Asia (0.7%), while small increases were found in other continents. At the national scale, PM and NO concentrations decreased significantly during lockdown, but O concentration showed varying patterns among countries. We found maximum reductions of 50.8% for PM in India and 103.5% for NO in Spain. The maximum reduction in O (22.5%) was found in India. Improvements in air quality were temporary as pollution levels increased in cities since lockdowns were lifted. We posit that these unprecedented changes in air pollutants were mainly attributable to reductions in traffic and industrial activities. Column reductions could also be explained by meteorological variability and a decline in emissions caused by environmental policy regulations. Our results have implications for the continued implementation of strict air quality policies and emission control strategies to improve environmental and human health.