NO_x emissions changes from 2019 to 2021 in Eastern China as estimated through variational inversions and TROPOMI satellite data

Abstract. China is one of the largest emitters of nitrogen oxides NO_x (= NO + NO₂) worldwide, and up-to-date estimates are crucial as the country faces rising pressure to curb emissions. We estimate NO_x emissions over Eastern China (101.75–132.25° E; 17.75–50.25° N) from 2019 to 2021, focusing on the impacts of COVID-19 and the Chinese Lunar New Year (LNY). Using high-resolution NO₂ observations from TROPOMI, onboard the Sentinel-5 Precursor satellite, our estimates are at the regional, national, and provincial scales. They are produced using the Community Inversion Framework (CIF), coupled to the CHIMERE regional chemistry transport model at 0.5° resolution. Our results show a drop in NO_x emissions of -35 % during February 2020, as compared to 2019. The drop in NO_x emissions is driven mostly by lockdown-related mobility restrictions and LNY festivities. Provincial reductions amounted to -15 %, -16 %, -17 % in Hubei, Shanghai and Jiangsu respectively. Anthropogenic NO_x emissions over Eastern China fell by 0.4 TgNO₂/year in 2020 vs. 2019, and by 0.2 TgNO₂/year in 2021 compared to 2020. The yearly budget of NO_x emissions for 2019, 2020 and 2021 are 18.3, 17.9, and 17.7 TgNO₂ respectively. Our estimates of recent past years offer insights to guide future strategies and policies to reduce NO_x emissions in China and its provinces. These results highlight the advantages of combining high dimensional variational inversion methods with high-resolution satellite data, to strengthen air quality monitoring and support more effective regulations.