Abstract. Secondary organic aerosol(SOA) is an important component of fine particular matter(PM2.5). Most air quality models use an equilibrium partitioning method along with the saturation vapor pressure(SVP) of semivolatile organic compounds(SVOCs) to predict SOA formation. However, the models typically assume that the organic particulate matter(OPM) is an ideal mixture and ignore the partitioning of water vapor to OPM. In this study, the Community Multiscale Air Quality model(CMAQ) is updated to investigate the impacts of water vapor partitioning and nonideality of the organicwater mixture on SOA formation during winter (January) and summer (July) of2013 over eastern China. The updated model treats the partitioning of water vapor molecules into OPM and uses the universal functional activity coefficient (UNIFAC) model to estimate the activity coefficients of species in the organicwater mixture. The modified model can generally capture the observed surface organic carbon(OC) with a correlation coefficientR of0.7 and the surfaceorganic aerosol (OA) with the mean fractional bias(MFB) and mean fractional error(MFE) of0.28 and0.54, respectively. SOA concentration shows significant seasonal and spatial variations, with high concentrations in the North China Plain(NCP), central China, and the Sichuan Basin(SCB) regions during winter (up to 25gm3) and in the Yangtze River Delta(YRD) during summer (up to 16gm3). In winter, SOA decreases slightly in the updated model, with a monthly averaged relative change of 10%20% in the highly concentrated areas, mainly due to organicwater interactions. The monthly averaged concentration of SOA increases greatly in summer, by 20%50% at the surface and 30%60% in the whole column. The increase in SOA is mainly due to the increase in biogenic SOA in inland areas and anthropogenic SOA in coastal areas. As a result, the averaged aerosol optical depth(AOD) is increased by up to 10%, and the cooling effect of aerosol radiative forcing(ARF) is enhanced by up to 15% over the YRD in summer. The aerosol liquid water content associated with OPM(ALWorg) at the surface is relatively high in inland areas in winter and over the ocean in summer, with a monthly averaged concentration of 0.53.0and 57gm3, respectively. The hygroscopicity parameter ofOA based on the Khler theory is determined using the modeled ALWorg. The correlation of with the O:C ratio varies significantly across different cities and seasons. Analysis of two representative cities, Jinan (in the NCP) and Nanjing (in the YRD), shows that the impacts of water partitioning and nonideality of the organicwater mixture on SOA are sensitive to temperature, relative humidity(RH), and the SVP of SVOCs. The two processes exhibit opposite impacts on SOA in eastern China. Water uptake increases SOA by up to 80% in the organic phase, while including nonunity activity coefficients decreases SOA by up to 50%. Our results indicate that both water partitioning into OPM and the activity coefficients of the condensed organics should be considered in simulating SOA formation from gasparticle partitioning, especially in hot and humid environments.