Abstract:Fine particulate matter was sampled in Shanghai urban area during two air pollution events, from October 10th to 21st, 2009. Based on the analysis of the pollution episode, characteristics of PM2.5 and its chemical composition were analyzed to discuss the pollution sources and formation mechanism. The haze event, which occurred on October 14th, was mainly caused by the secondary transformation of local air pollutants. The ratio of PM2.5 to PM10 exceeded 60% and the concentrations of water soluble ions in PM2.5 were very high. During the floating dust event from October 17th to 19th, concentration of coarse particulate matter rose and the proportion of PM2.5 decreased. The secondary ions transferred from fine particles to coarse particles and concentration of Ca2+ was high. Meanwhile, concentrations of K+, EC and OC in PM2.5 were high due to the straw burning in Yangtze Delta. Therefore, together with local emission control, the control of secondary fine particulate matter and its precursors should be also emphasized.