Tropospheric NO2 in China during the COVID-19 lock-down is largely decreased due to the controlled human mobility

Tropospheric NO₂, one of the major atmospheric pollutants, is harmful to human health and plays an important role in atmospheric chemistry. Tropospheric NO₂ concentration is controlled by anthropogenic emission and meteorological conditions, such as solar radiation and humidity. In early 2020, unprecedented lockdowns and travel bans were implemented in China to fight COVID-19, leading to a large decrease of NO₂ compared with preceding years. To isolate the effects of anthropogenic emission and weather, we applied a linear regression model between satellite observed NO₂ column density and meteorological variables. Compared with 2017, it is found that atmospheric NO₂ in 2020 changed -37.8 ± 16.3 %, with the contribution of weather +8.1 ± 14.2 % and anthropogenic emission -49.3 ± 23.5 %. Similar results were also found in 2018 and 2019, revealing that the effect of reduced emission on atmospheric NO₂ is counteracted by the weather, and is actually larger than the observation. The simulations by GEOS-Chem model also supported the results in typical regions. The reduction of NO₂ induced by anthropogenic emission can be well explained by human mobility, showing significantly negative correlations to migration indices provided by Baidu Location-Based-Service, particularly that inside the city. The intra-city migration index can explain 40.4 ± 17.7 % variance of the emission-induced reduction of NO₂ in 29 megacities, each of which has a population of over 8 million in China.