During the COVID-19 lockdown in China low air pollution levels were reported as a consequence of the reduced economic and social activities. Quantification of the pollution reduction is not straightforward due to effects of transport, meteorology, and chemistry. Here we have analysed the NO emission reductions calculated with an inverse algorithm applied to daily NO observations from the TROPOMI instrument onboard the Copernicus Sentinel-5P satellite. This method allows 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 around the Spring festival and a slow recovery from mid-February to mid-March.