Abstract. Knowledge of aerosol chemistry in densely populated regions is critical for effective reduction of air pollution, while such studies have not been conducted in Changzhou, an important manufacturing base and populated city in the Yangtze River Delta (YRD), China. This work, for the first time, performed a thorough chemical characterization on the fine particulate matter (PM 2.5 ) samples, collected during July 2015 to April 2016 across four seasons in this city. A suite of analytical techniques was employed to measure the organic carbon (OC), elemental carbon (EC), watersoluble organic carbon (WSOC), water-soluble inorganic ions (WSIIs), trace elements, and polycyclic aromatic hydrocarbons (PAHs) in PM 2.5 ; in particular, an Aerodyne soot particle aerosol mass spectrometer (SP-AMS) was deployed to probe the chemical properties of water-soluble organic aerosol (WSOA). The average PM 2.5 concentration was found to be 108.3 µg m −3 , and all identified species were able to reconstruct ∼ 80 % of the PM 2.5 mass. The WSIIs occupied about half of the PM 2.5 mass (∼ 52.1 %), with SO 2− 4 , NO − 3 , and NH + 4 as the major ions. On average, nitrate concentrations dominated over sulfate (mass ratio of 1.21), indicating that traffic emissions were more important than stationary sources. OC and EC correlated well with each other and the highest OC / EC ratio (5.16) occurred in winter, suggesting complex OC sources likely including both secondary and primary ones. Concentrations of eight trace elements (Mn, Zn, Al, B, Cr, Cu, Fe, Pb) can contribute up to ∼ 5.0 % of PM 2.5 during winter. PAH concentrations were also high in winter (140.25 ng m −3 ), which were predominated by median/high molecular weight PAHs with five and six rings. The organic matter including both watersoluble and water-insoluble species occupied ∼ 21.5 % of the PM 2.5 mass. SP-AMS determined that the WSOA had average atomic oxygen-to-carbon (O / C), hydrogen-to-carbon (H / C), nitrogen-to-carbon (N / C), and organic matter-toorganic carbon (OM / OC) ratios of 0.54, 1.69, 0.11, and 1.99, respectively. Source apportionment of WSOA further identified two secondary OA (SOA) factors (a less oxidized and a more oxidized oxygenated OA) and two primary OA (POA) factors (a nitrogen-enriched hydrocarbon-like traffic OA and a local primary OA likely including species from cooking, coal combustion, etc.). On average, the POA contribution outweighed SOA (55 % vs. 45 %), indicating the important role of local anthropogenic emissions in the aerosol pollution in Changzhou. Our measurement also shows the abundance of organic nitrogen species in WSOA, and the source analyses suggest these species are likely associated with traffic emissions, which warrants more investigations on PM samples from other locations.
