Nation-wide studies of organic aerosols were conducted on a molecular level in 15 Chinese cities. The results showed strikingly high levels of organic compounds (e.g., annual concentrations of polycyclic aromatic hydrocarbons, phthalates, sugars and diacids are 110, 370, 400 and 830 ng m-3, respectively), especially in the mid-west region during winter (up to 125 ¿g m-3 organic carbon). Fossil fuel combustion and/or biomass burning products are 3-30 times more abundant in winter than in summer. In contrast, significant quantity of phthalates (168-2200 ng m-3) was detected in summer. Concentrations of the pollutants are generally 1-3 orders of magnitude higher than those in developed countries. Their source strengths are characterized in winter by fossil fuel combustion, followed by secondary oxidation, plant wax emissions and biomass burning, whereas in summer by secondary oxidation, followed by fossil fuel combustion and plastic emissions.