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) globally. With ongoing efforts and global pressure to reduce its emissions, up-to-date estimates of NO_x emissions remain essential. In this study we focus on the evolution of NO_x emissions over Eastern China (101.75–132.25° E; 17.75–50.25° N) between 2019 and 2021, including the analysis of the impact of the COVID-19 outbreak and the Chinese Lunar New Year national break (LNY) on NO_x emissions. We exploit the high spatial resolution and coverage of the TROPOMI Sentinel-5 Precursor nitrogen dioxide (NO₂) observations, and provide an estimate of this evolution down to the level of Chinese provinces. We assimilate TROPOMI NO₂ observations in NO_x atmospheric inversions based on the variational inversion drivers of the Community Inversion Framework (CIF), coupled to a 0.5° resolution configuration of the CHIMERE regional chemistry transport model.

Our results show a decrease in NO_x anthropogenic emissions in most of Eastern China during early 2020 compared to 2019. This decrease approaches -40 % in February 2020 as compared to 2019 due to i) the lockdowns and strict regulations on mobility associated with the COVID-19 outbreak (lockdowns extended from 23 January to 8 April in Wuhan, and from late January or early February 2020 in other parts of China, extending till March 2020) and ii) the celebrations of the LNY that took place during February 2020, before the lockdowns were imposed. In some Chinese provinces, such as Shanghai, Qinghai, Jiangsu, Hubei, Henan and Beijing, the reductions in NO_x emissions were -38 %, -29 %, -31 %, -36 %, -24 %, and -16 % respectively during February 2020 relative to 2019. The annual total (anthropogenic + biogenic) emissions of NO_x in Eastern China decreased by 0.2 TgNO₂/year in 2020, compared to 2019. However, in 2021, our estimates show an annual increase of +4 % in NO_x emissions as compared to 2019, amounting to 16.7 TgNO₂/year in 2021.

This study advances emissions quantification by delivering NO_x emissions down to the scale of Chinese provinces, a refinement that is not always addressed in regional inversions. Our approach, therefore, makes it possible to evaluate targeted policy interventions by resolving sub-national emissions. This shows how crucial it is to use high-resolution data when working to improve air quality and address climate change.