NO Emission Changes Over China During the COVID-19 Epidemic Inferred From Surface NO Observations.

The COVID-19 epidemic has substantially limited human activities and affected anthropogenic emissions. In this work, daily NO emissions are inferred using a regional data assimilation system and hourly surface NO measurement over China. The results show that because of the coronavirus outbreak, NO emissions across the whole mainland China dropped sharply after 31 January, began to rise slightly in certain areas after 10 February, and gradually recover across the country after 20 February. Compared with the emissions before the outbreak, NO emissions fell by more than 60% and ~30% in many large cities and most small to medium cities, respectively. Overall, NO emissions were reduced by 36% over China, which were mainly contributed by transportation. Evaluations show that the inverted changes over eastern China are credible, whereas those in western China might be underestimated. These findings are of great significance for exploring the reduction potential of NO emissions in China.