https://www.sciencedaily.com/releases/2020/07/200727194737.htm

The study, by scientists from the University of Leeds, UK and the Southern University of Science and Technology, China, analysed air pollutant concentrations from China's national network of around 1,300 monitoring stations to quantify the response of air pollution across China during the COVID-19 lockdown.

They found that the falls in some air pollutants like nitrogen dioxide (NO2) were substantial, whereas other pollutants like particulate matter (PM) and ozone pollution were only slightly reduced or barely affected.