Assessment of COVID-19 lockdown measures on air quality over the Sichuan Basin, China: insights from multiple observations

The COVID-19 lockdown measures have resulted in significant changes in human activities and thereby providing a natural experiment for understanding the underlying mechanism of air pollutants’ response to emission changes. Here, we use surface observations, MODIS AOD, and Tropospheric Monitoring Instrument (TROPOMI) NO₂ columns to investigate the spatial and temporal changes of criteria air pollutants during various stages of lockdown across the Sichuan Basin (SCB) situated in southwestern China. Basin-wide declines of CO (-13.8%), NO₂ (-50.9%), and SO₂ (-16.1%) concentrations were observed during the lockdown period compared with the pre-lockdown period, which is mainly attributed to substantial reductions in anthropogenic emissions associated with traffic and industrial emissions. In spite of the significant reductions in anthropogenic emissions, elevated O₃ levels (up to 50.2%) and PM_2.5 exceedances were widely depicted due to unfavorable meteorological conditions. TROPOMI NO₂ columns show good agreement with reductions reported in the MEIC emission inventory, which further confirm the critical impacts of the COVID-19 lockdown on anthropogenic emissions. This work clearly shows the magnitude of changes and varied patterns of air pollutants in response to reduced anthropogenic emissions, demonstrating the crucial role of meteorological fields in causing air pollution even under considerable emission reductions in the SCB.