Impact of COVID-19 policy changes on tropospheric NO2 in the Yangtze River Basin: insights from GF-5 02 EMI-II observations

This study evaluates the performance of the EMI-II (Environmental trace gases Monitoring Instrument-II) sensor in retrieving tropospheric NO2 vertical column densities (VCDs) and investigates NO2 concentration trends in the Yangtze River Basin following significant COVID-19 policy adjustments. Using comparative analyses with TROPOMI (TROPospheric Ozone Monitoring Instrument) and GEMS (Geostationary Environment Monitoring Spectrometer) products, EMI-II demonstrated robust capabilities in capturing NO2 spatial distributions over key regions such as the Yangtze River Delta (YRD), North China Plain, and Sichuan-Chongqing urban clusters, with spatial correlation coefficients (r) reaching 0.94 and root mean square error (RMSE) as low as 1.98x10(15) molecules/cm(2) in monthly comparisons, indicating strong agreement with established datasets. Following COVID-19 restrictions' relaxation in December 2022 and the Spring Festival in January 2023, NO2 levels in the Yangtze River Basin decreased by 26.94%, with specific declines of 41.90% in the YRD, 28.44% in central urban clusters, and 21.16% in the Sichuan-Chongqing region. These changes were driven by reduced vehicular emissions, industrial activity, and population mobility during the holiday period. The results underscore the interplay between anthropogenic activity, policy measures, and regional air quality, affirming EMI-II's potential as a reliable tool for monitoring atmospheric pollutants and analyzing environmental responses to societal changes.