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The COVID‐19 epidemic has substantially limited human activities and affected anthropogenic emissions. In this work, daily NOx emissions are inferred using a regional data assimilation system and hourly surface NO2 measurement over China. The results show that because of the coronavirus outbreak, NOx 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, NOx emissions fell by more than 60% and ~30% in many large cities and most small to medium cities, respectively. Overall, NOx 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 NOx emissions in China. Plain Language Summary: In this study, we quantitatively estimate the impact of the COVID‐19 lockdown on NOx emissions over China based on nationwide surface hourly NO2 monitoring data. We find that NOx emissions dropped sharply after 31 January and began to gradually recover after 20 February across the country; NOx emissions fell by more than 60% in many large cities and decreased by ~30% in most small to medium cities. Across the whole mainland China, NOx emissions were reduced by 36% due to the COVID‐19 lockdown. Key Points: The spatiotemporal variations of NOx emissions across mainland China during COVID‐19 well inferred from surface NO2 observationsNOx emissions fell by more than 60% in many large cities and by 36% across the whole mainland China due to the lockdownNOx emissions dropped sharply after 31 January and began to gradually recover after 20 February across the country [ABSTRACT FROM AUTHOR]

The COVID-19 epidemic has substantially limited human activities and affected anthropogenic emissions. In this work, daily NO x emissions are inferred using a regional data assimilation system and hourly surface NO 2 measurement over China. The results show that because of the coronavirus outbreak, NO x 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 x emissions fell by more than 60% and ~30% in many large cities and most small to medium cities, respectively. Overall, NO x 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 x emissions in China., (©2020. The Authors.)