PM2.5,defined as particles with aerodynamic size less than or equal to 2.5 μm,is more harmful than larger particles.The greater specific surface area of PM2.5is beneficial to the sorption of toxic metals and organic pollutants.Research showed that heavy metals such as Cd,Pb,Ni which were mostly carcinogenic,mutagenic or genotoxic,were mostly existed in the fine particles in the atmosphere.These toxic contaminants in PM2.5 may reach deep into the lungs or even into the blood and consequently cause serious threat to human health.Studies about heavy metals in airborne particles in Chongqing are very few and mostly carried out in the central urban area.There is nearly no research about the heavy metals in PM2.5 in Wanzhou,the second largest city in Chongqing.Therefore,the main purpose of this study is to characterize the pollution of heavy metals in PM2.5 in typical seasons in Wanzhou.PM2.5 samples were collected in July and December 2013 from an urban site in Wanzhou,Chongqing and analyzed for the content of five toxic heavy metals(Pb,Cu,Ni,Cr,Cd)using graphite furnace-atomic absorption spectrophotometer.Based on these data,the concentration levels,temporal distributions and sources were investigated and compared in summer and winter.The average concentrations of Pb,Cu,Ni,Cr and Cd were 0.223,0.027,0.039,0.024 and 0.013 μg m-3,respectively.Compared with those in other cities in China,the concentrations in present study were a little higher than in coastal cities such as Shenzhen and Guangzhou,except for Cu which was higher in Guangzhou.This might because the weather in Wanzhou is unfavorable for the dispersion of pollutants.But the concentrations in present study were much lower than those in Tianjin,mainly due to the coal burning for warmth in Tianjin.Moreover,the levels in Wanzhou were lower than in the central urban areas in Chongqing except for Ni.The concentrations of studied heavy metals were mostly higher in winter than summer except for Ni.This might mainly due to the more abundant rainfall in summer.The temporal distribution of these metals also indicated that rainfall has played an important role in decrease the levels of these metals in the atmosphere.Backward trajectory analysis was used to investigate the potential sources for the elevated concentrations in some days.The result showed that air masses from southwest China pass Yunnan-Guizhou plateau were dominant in summer but air masses from the direction of Siberia and Mongolia were dominant in winter.Sources of these heavy metals in PM2.5 in Wanzhou were investigated by calculating the enrichment factor of studied metals.The results showed that the studied five metals with EFs ranging from 59 to 1910742 were seriously contaminated by human activities,especially for Cd and Pb with EFs up to 6311 and 1910742.The contamination was more seriously in winter than in summer.Furthermore,by analyzing the correlations between metals and investigating the surrounding environment,Ni and Cr were mainly from nearby metallurgical manufacture and processing area while Pb and Cd were mostly affected by traffic and coal burning in Wanzhou.