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 NO_x (= 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 NO_x 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 NO_x 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 NO_x emissions accounted for -38%, -29%, -31%, -36%, and -24% respectively. In North Eastern China, however, our results show an increase in the NO_x emissions in three major provinces: Jilin (+11.35% in January 2020), and Liaoning (+16.33% in March 2020). The yearly total emissions of NO_x 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 NO_x 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 NO_x emissions in China, during the COVID-19 lockdown period.