The impact of the lockdown during the COVID-19 pandemic outbreak on NOx pollution in China, as derived from TROPOMI and variational atmospheric inverse modelling

In 2020, China’s response to the COVID-19 breakdown included strict regulations on mobility in several provinces. Multiple studies showed that these measures caused a decrease in the emissions of nitrogen oxides NOx (= NO + NO2). In this study, we exploit the high spatial resolution and coverage of the TROPOMI nitrogen dioxide (NO2) observations, over Eastern China, in order to provide an estimate of this decrease down to the level of provinces. We assimilate these observations in NOx atmospheric inversions for the years 2019 through 2021, 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 for the North Chinese Plain region, and of  its adjoint (both including the MELCHIOR-2 chemistry scheme). This framework allows to control the emissions at 0.5° resolution, and then to target emissions at province scale, but also to account for a full chemistry scheme in the atmospheric process. The prior estimate of the anthropogenic emissions for this Bayesian inversion framework is based on a combination of the Carbon-Monitor and CEDS inventories, accounting for the day-to-day variations of these emissions. The corrections of the prior anthropogenic and natural emissions allows to decrease the misfits to the TROPOMI NO2 observation by up to 50%, so that the inverted emissions are highly consistent with these satellite data. Furthermore, the satellite coverage of the domain is good, with more than 60% of the model domain observed 95% of the days. Our results show a decrease in NOx emissions observed in most of Eastern China, during January, February, and March 2020, reaching -40% in February 2020 as compared to 2019. In some Chinese provinces, such as Shanghai, Qinghai, Jiangsu, Hubei and Henan, the reduction in NOx emissions accounted for -38%, -29%, -31%, -36%, and -24% respectively. In North Eastern China, however, our results show an increase in the NOx emissions in three major provinces: Jilin (+11.35% in January 2020), and Liaoning (+16.33% in March 2020). The yearly total emissions of NOx in Eastern China were slightly lower in 2020 than those in 2019, with emissions of 15.58 and 15.76 TgNO2/year, respectively. While in 2021, the total emissions of NOx accounted to16.42 TgNO2/year. We compared the emissions in 2021 to those in 2019, and we found that the levels are higher in most of China, especially in February reaching +45% in the North East, for instance. We show that our results are consistent with other studies that focused on the change in NOx emissions in China, during the COVID-19 lockdown period.