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To control the spread of COVID-19,rigorous restrictions have been implemented in China,resulting in a great reduction in pollutant emissions.In this study,we evaluated the air quality in the Yangtze River Delta during the COVID-19 lockdown period using satellite and ground-based data,including particle matter(PM),trace gases,water-soluble ions(WSIs)and black carbon(BC).We found that the impacts of lockdown policy on air quality cannot be accurately assessed using MODIS aerosol optical depth(AOD)data,whereas the tropo-spheric nitrogen dioxide(NO2)vertical column density can well reflect the influences of these restrictions on human activities.Compared to the pre-COVID period,the PM2.5,PM10,NO2,carbon monoxide(CO),BC and WSIs during the lockdown in Suzhou were observed to decrease by 37.2%,38.3%,64.5%,26.1%,53.3%and 58.6%,respectively,while the sulfur dioxide(SO2)and ozone(O3)increased by 1.5%and 104.7%.The WSIs ranked in the order of NO3->NH4+>SO42->Cl->Ca2+>K+>Mg2+>Na+during the lockdown period.By comparisons with the ion concentrations during the pre-COVID period,we found that the ions NO3-,NH4+,SO42-,Cl-,Ca2+,K+and Na+decreased by 66.3%,48.8%,52.9%,56.9%,57.9%and 76.3%,respectively,during the lockdown,in contrast to Mg2+,which increased by 30.2%.The lockdown policy was found to have great impacts on the diurnal variations of Cl-,SO42-,Na+and Ca2+.