Continuous measurements of meteorological parameters, gaseous pollutants, particulate matters, and the major chemical species in PM 2.5 were conducted in urban Hangzhou from 1 September to 30 November 2013 to study the potential sources and formations of PM 2.5 pollution. The average PM 2.5 concentration was 69 µg¨m´3,~97% higher than the annual concentration limit in the national ambient air quality standards (NAAQS) of China. Relative humidity (RH) and wind speed (WS) were two important factors responsible for the increase of PM 2.5 concentration, with the highest value observed under RH of 70%-90%. PM 2.5 was in good correlation with both NO 2 and CO, but not with SO 2 , and the potential source contribution function (PSCF) results displayed that local emissions were important potential sources contributing to the elevated PM 2.5 and NO 2 in Hangzhou. Thus, local vehicle emission was suggested as a major contribution to the PM 2.5 pollution. Concentrations of NO 2 and CO significantly increased in pollution episodes, while the SO 2 concentration even decreased, implying local emission rather than region transport was the major source contributing to the formation of pollution episodes. The sum of SO 4 2´, NO 3´, and NH 4 + accounted for~50% of PM 2.5 in mass in pollution episodes and the NO 3´/ EC ratios were significantly elevated, revealing that the formation of secondary inorganic species, particularly NO 3´, was an important contributor to the PM 2.5 pollution in Hangzhou. This study highlights that controlling local pollution emissions was essential to reduce the PM 2.5 pollution in Hangzhou, and the control of vehicle emission in particular should be further promoted in the future.