High resolution mapping of nighttime light and air pollutants during the COVID-19 lockdown in Wuhan

The novel coronavirus (COVID-19) has induced unprecedented improvements of air quality due to drastic shrinking of human activities during the pandemic lockdown in 2020. While declines of most air pollutants have been globally evidenced in most cities worldwide, there is few detailed spatial knowledge at local scale. Therefore, we present here a high resolution mapping of the 2018–2020 evolution of human activities and air pollutants in Wuhan. Human activities were assessed by nighttime light radiance. We measured the air quality index (AQI) as the maximum value among air quality sub-indices of SO₂, NO₂, CO, O₃ and particulate matter. We also compared mean monthly pollutant concentration during January–April in 2018, 2019 and 2020. Mapping results show that variations of nighttime light radiance were heterogenous at local scale, showing both rises and declines in the same district. The radiance decreased in eight districts located mostly in the city center, as a result of lower human activity, but the radiance increased in the five surrounding districts, as a consequence of people staying at home. AQI was low during lockdown, averaging at 57, but showed strong daily variations with a slight pollution around February 5 with AQI rising to 126. During this pollution event, particulate matter, SO₂, NO₂ and CO levels were positively correlated, suggesting common sources, but were not correlated with ozone; and particulate matter, SO₂, NO₂ and CO decreased with relative humidity, suggesting removal by precipitation. Comparison of 2020 data with previous years shows that particulate matter and NO₂ were highly reduced, CO was less reduced due to ongoing power industries, SO₂ first declined then increased to exceed 2018–19 values due to coal combustion, and ozone levels was more abundant due both to less NO_x pollution and the weekend effect.