During the COVID-19 lockdown (24 January-20 March) in China low air pollution levels were reported in the media as a consequence of reduced economic and social activities. Quantification of the pollution reduction is not straightforward due to effects of transport, meteorology, and chemistry. We have analyzed the NO(x)emission reductions calculated with an inverse algorithm applied to daily NO(2)observations from TROPOMI onboard the Copernicus Sentinel-5P satellite. This method allows the quantification of emission reductions per city and the analysis of emissions of maritime transport and of the energy sector separately. The reductions we found are 20-50% for cities, about 40% for power plants, and 15-40% for maritime transport depending on the region. The reduction in both emissions and concentrations shows a similar timeline consisting of a sharp reduction (34-50%) around the Spring festival and a slow recovery from mid-February to mid-March. Plain Language Summary During the COVID-19 lockdown in China, air quality had strongly improved. Here we study what sources were reduced and how much the reduction per city was. We used TROPOMI observations of the Sentinel-5P satellite, which monitors the Earth's atmosphere daily. We focused on observations of the pollutant "nitrogen dioxide," an important precursor of air pollution in the atmosphere. With our novel methodology we are able to calculate the pollution back to the sources of the emissions, whether these are big cities, industrial regions, power plants, or busy shipping lanes. We applied this method to East China, where the 36 biggest Chinese cities are located. Almost all those cities showed strong emission reductions of 20-50% during the lockdown in February 2020. Besides urban China, we found an average emission reduction of 40% over coal power plants and a reduction in maritime transport by 15-40% depending on the region. The period of reduced emissions lasted until around the end of February, and the emissions slowly returned to normal during the month March 2020. Exception is the region Wuhan, the center of the COVID-19 crisis, where emissions started to rebound since 8 April, the end of their lockdown period.
