During the COVID-19 lockdown (24 Jan to 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 analysed the NO emission reductions calculated with an inverse algorithm applied to daily NO 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 to 50% for cities, about 40% for power plants and 15 to 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 to 50%) around the Spring festival and a slow recovery from mid-February to mid-March.