Volatility and viscosity have substantial impacts on gas-particle partitioning, formation and evolution of aerosol and hence the predictions of aerosol-related air quality and climate effects. Here aerosol volatility and viscosity at a rural site (Gucheng) and an urban site (Beijing) in the North China Plain (NCP) in summer and winter were investigated by using a thermodenuder coupled with a high-resolution aerosol mass spectrometer. The effective saturation concentration (C.sup.*) of organic aerosol (OA) in summer was smaller than that in winter (0.55 Âµg m.sup.-3 vs. 0.71-0.75 Âµg m.sup.-3 ), indicating that OA in winter in the NCP is more volatile due to enhanced primary emissions from coal combustion and biomass burning. The volatility distributions varied and were largely different among different OA factors. In particular, we found that hydrocarbon-like OA (HOA) contained more nonvolatile compounds compared to coal-combustion-related OA. The more oxidized oxygenated OA (MO-OOA) showed overall lower volatility than less oxidized OOA (LO-OOA) in both summer and winter, yet the volatility of MO-OOA was found to be relative humidity (RH) dependent showing more volatile properties at higher RH. Our results demonstrated the different composition and chemical formation pathways of MO-OOA under different RH levels. The glass transition temperature (T.sub.g) and viscosity of OA in summer and winter are estimated using the recently developed parameterization formula. Our results showed that the T.sub.g of OA in summer in Beijing (291.5 K) was higher than that in winter (289.7-290.0 K), while it varied greatly among different OA factors. The viscosity suggested that OA existed mainly as solid in winter in Beijing (RH = 29 Â± 17 %), but as semisolids in Beijing in summer (RH = 48 Â± 25 %) and Gucheng in winter (RH = 68 Â± 24 %). These results have the important implication that kinetically limited gas-particle partitioning may need to be considered when simulating secondary OA formation in the NCP.