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Abstract: The Yangtze River Delta (YRD) region is one of the most economically developed regions of China and characterized by high level of urbanization, which comes with the consequence of severe deterioration of its atmospheric environment in a large regional scale. In order to characterize the fine particle pollution features representative for YRD, especially for the abundant and complex carbonaceous component, we deployed advanced on-line measurement at a regional site in Jiaxing in the central YRD area during June 29–July 15 (the summer campaign) and December 11–23 (the winter campaign), with a high resolution aerosol mass spectrometer (AMS) and a single particle soot photometer (SP2). The mean PM1 mass concentration was observed to be 32.9 μg m−3 in summer, with organic matter (OM) and black carbon (BC) contributing 32.1 and 9.1%, respectively, and 41.9 μg m−3 in winter, with OM and BC contributing 30.3 and 16.9%, respectively. The O/C ratio of OM, an indicator for the oxidation level of organic aerosol, was calculated to be 0.28 in summer and 0.33 in winter, and the number fraction of internally mixed BC particles (NIB), an indicator for the aging level of BC, was calculated to be 0.51 in summer and 0.53 in winter. Two popular independent methods were used to estimate the amount of secondary organic aerosol (SOA) in Jiaxing: one is the positive matrix factorization method utilizing the high resolution aerosol mass spectra (the AMS-PMF method), and the other is the simple OC/EC ratio method (the OC/EC method). The AMS-PMF method estimated that SOA averagely accounted for 68.3 and 30.2% of OM in summer and winter, respectively. The SOA amount estimated by the OC/EC method was found to agree well with that by the AMS-PMF method in summer, but strong biomass burning events in winter made the OC/EC method over-estimate SOA largely. The seasonal variation of SOA indicates that SOA production in YRD is more determined by ambient temperatures than by precursors. [Copyright &y& Elsevier]