Ratio of PM2.5 to PM10 Mass Concentrations in Beijing and Relationships with Pollution from the North China Plain

Five years of PM2.5/PM10 ratios and their potential relationships with wind and relative humidity (RH) were analyzed for three areas in Beijing (northwestern mountainous area, urban area and southern suburbs), and these values were compared with those of five other cities, including Tianjin, Dalian, Hangzhou, Shanghai and Guangzhou. In the past five years, both PM2.5 and PM10 mass concentrations have decreased over the three Beijing regions, particularly during winters. The PM2.5/PM10 ratios have distinct seasonal characteristics, with more frequent high ratios in winter than in other seasons. The high ratio frequency decrease is most evident in winter and in the southern Beijing suburbs. This fine particle proportion decrease is related to air pollution control policies, including the national project ‘Coal to Gas’. High PM2.5/PM10 ratios are linked to heavy pollution levels and low wind speeds, indicating the importance of PM2.5 accumulation during pollution events in Beijing. The higher PM2.5/PM10 ratios in Beijing are also closely related to southerly winds and high humidity, indicating the contribution of anthropogenic pollution transported from the south. Due to similar geographic environments, Tianjin is similar to Beijing in terms of the frequency distribution characteristics of the PM2.5/PM10 ratios. The coastal city of Dalian is further north and not similar to Beijing or Tianjin, owing to a sea breeze influence. Different from Beijing and Tianjin, the southern cities of Hangzhou, Shanghai and Guangzhou show almost no change in the PM2.5/PM10 ratio frequency distribution with increases in wind speed and relative humidity.