Relationships of relative humidity with PM 2.5 and PM 10 in the Yangtze River Delta, China

Severe particulate matter (PM, including PM and PM ) pollution frequently impacts many cities in the Yangtze River Delta (YRD) in China, which has aroused growing concern. In this study, we examined the associations between relative humidity (RH) and PM pollution using the equal step-size statistical method. Our results revealed that RH had an inverted U-shaped relationship with PM concentrations (peaking at RH = 45-70%), and an inverted V-shaped relationship (peaking at RH = 40 ± 5%) with PM , SO , and NO . The trends of polluted-day number significantly changed at RH = 70%. The very-dry (RH