Fine particulate matter(e.g.,PM2.5)can incorporate with toxic trace metals and other chemicals,posing high risks to human health.Due to rapid economic development and urbanization in China in the last few decades,PM2.5 pollution has become a serious public health concern.In the present study,the spatial variation patterns of PM2.5 and associated trace metal concentrations during a winter haze episode were investigated in megacities Beijing,Xian,Xiamen,Guangzhou,and Hong Kong.The volume concentrations of trace metals and Pb isotopic compositions revealed distinct patterns of PM2.5 pollution among these cities.Extremely high PM2.5 pollution with high volume concentrations of trace metals(especially As,Cd,Co,Pb,and Zn)was found in northern China.With such high PM2.5 pollution,the PM2.5 loading in haze days would account for approximately half or more than half of the contribution to the whole-year PM2.5 loading in northern China.In contrast,lower PM2.5 loading and lower volume concentrations of most trace elements were observed in southern China.The Pb isotopic compositions in PM2.5 from northern China had lower 206Pb/207Pb ratios than those from southern China,suggesting more input of Pb from coal combustion in the northern region.Winter dusts from northwest deserts should also be a significant source of PM2.5 and trace metals in northern China,as indicated by back trajectory analysis of air mass transport.The carcinogenic and non-carcinogenic risks of PM2.5-associated trace metals via inhalation were also assessed in the present study.The results provide quantitative information on the health risks of PM2.5-associated heavy metal pollution in China.