Impact of control measures and typhoon weather on characteristics and formation of PM2.5 during the 2016 G20 summit in China

Abstract The implementation of strict emission control during the G20 summit in China and the occurrence of Typhoon Malarkas in September 2016 provided a valuable opportunity to examine the role of control measures and special weather condition in the formation of PM2.5. Water-soluble inorganic ions (WSII) were measured hourly in addition to PM2.5 mass concentration and black carbon (BC) and gaseous pollutants in Ningbo located in the Yangtze River Delta (YRD) region of China. Three distinct cases, i.e., a control case, a normal case, and a typhoon case, were investigated during the study period. PM2.5 was higher in the control case (37.5 μg m−3) than in the normal case (29.8 μg m−3), whereas the lowest PM2.5 (14.2 μg m−3) was observed in the typhoon case. The analyses of meteorology and backward trajectory suggested that stable weather and regional transport from inland regions accounted for the high PM2.5 under strict control. Only the concentrations of Ca2+ and NO3− decreased in the control case, while those of all water-soluble inorganic components (except Na+ and Mg2+) decreased substantially in the typhoon case. SO42− dominated the WSII, with the highest contribution, 62%, in the control case. This result was attributed to a stagnant atmosphere with a high relative humidity (RH), which was beneficial for the transformation of SO2 to SO42−. The control and typhoon cases both resulted in extremely low NO2, but the contribution of NO3− to PM2.5 was reduced in the control case and increased in the typhoon case. The close correlation of the NOR (nitrate oxidation ratio) with Na+ and Mg2+ in the typhoon case was indicative of sea salt associated with the typhoon which provided a surface for the heterogeneous formation of NO3−. The presented results facilitate a better understanding of the characteristics and formation of PM2.5 under the influence of artificial control and natural intervention.