Relationships of relative humidity with PM2.5 and PM10 in the Yangtze River Delta, China

Severe particulate matter (PM, including PM2.5 and PM10) 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 PM2.5 concentrations (peaking at RH = 45-70%), and an inverted V-shaped relationship (peaking at RH = 40 +/- 5%) with PM10, SO2, and NO2. The trends of polluted-day number significantly changed at RH = 70%. The very-dry (RH < 45%), dry (RH = 45-60%) and low-humidity (RH = 60-70%) conditions positively affected PM2.5 and exerted an accumulation effect, while the mid-humidity (RH = 70-80%), high-humidity (RH = 80-90%), and extreme-humidity (RH = 90-100%) conditions played a significant role in reducing particle concentrations. For PM10, the accumulation and reduction effects of RH were split at RH = 45%. Moreover, an upward slope in the PM2.5/PM10 ratio indicated that the accumulation effects from increasing RH were more intense on PM2.5 than on PM10, while the opposite was noticed for the reduction effects. Secondary transformations from SO2 and NO2 to sulfate and nitrate were mainly responsible for PM2.5 pollution, and thus, controlling these precursors is effective in mitigating the PM pollution in the YRD, especially during winter. The conclusions in this study will be helpful for regional air-quality management.