Jiaxing is a medium-sized city in the Yangtze River Delta (YRD), which showed complex local and surrounding pollution sources. To study the COVID-19 impact on the ambient PM2.5 in Jiaxing, we collected the PM2.5 samples from January 2 to April 25, 2020 and analyzed their chemical compositions (including carbon components, water-soluble ions (WSIs), and inorganic elements). The concentration of PM2.5 was 83.13 ± 30.93 μg/m3 before COVID-19 pandemic, and then remarkably decreased with COVID-19 outbreak...