From Boston to Washington, DC, levels of nitrogen dioxide, one form of air pollution, are down by roughly 30% compared with the previous five years, according to NASA satellite data.

Worldwide, skies have been clearer this year. Weather plays a part, but the shutdown of most of society from COVID-19 is a major factor.

https://www.voanews.com/usa/covid-19-clean-air-gains-wont-last-experts-say