Impacts of water partitioning and polarity of organic compounds on secondary organic aerosol over Eastern China

Secondary organic aerosol (SOA) is an important component of fine particular matter (PM_2.5) in China. Most air quality models use an equilibrium partitioning method along with estimated saturation vapor pressure of semi-volatile organic compounds (SVOCs) to predict SOA formation. However, this method ignores partitioning of water vapor to the organic aerosols and the organic phase non-ideality, both of which affect the partitioning of SVOCs. In this study, the Community Multi-scale Air Quality model (CMAQv5.0.1) was used to investigate the above impacts on SOA formation during winter (January) and summer (July) of 2013 over eastern China. The organic aerosol module was updated by incorporating water partitioning into the organic particulate matter (OPM) and considering non-ideality of organic-water mixture. The modified model can generally capture the observed organic carbon (OC), the total organic aerosol (OA) and diurnal variation of PM_2.5 at ground sites. SOA concentration shows significant seasonal and spatial variations, with high concentration levels in North China Plain (NCP), Central China and Sichuan basin (SCB) areas during winter (up to 25 μg m^-3) and in Yangtze River Delta (YRD) during summer (up to 12 μg m^-3). When water partitioning is included in winter, SOA concentrations increase slightly, with the monthly-averaged daily maximum relative difference of 10-20% at the surface and 10-30% for the whole column, mostly due to the increase in anthropogenic SOA. The increase in SOA is more significant in summer, by 20-90% at the surface and 30-70% for the whole column. The increase of SOA over the land is mostly due to biogenic SOA while the increase of SOA over the coastal regions is related with that of anthropogenic origin. Further analysis of two representative cities, Jinan and Nanjing, shows that changes of SOA are favored under hot and humid conditions. The increases in SOA cause a 12% elevation in the aerosol optical depth (AOD) and 15% enhancement in the cooling effects of aerosol radiative forcing (ARF) over YRD in summer. The aerosol liquid water content associated with OPM (ALW_org) at the surface is relatively high over the land in winter and over the ocean in summer, with the monthly-averaged daily maximum of 2-9 and 5-12 μg m^-3, respectively. By using the -Köhler theory, we calculated the hygroscopicity of OA with modeled ALW_org, finding that the correlation with O:C ratio varies significantly across different cities and seasons. Water partitioning into OPM only promotes SOA formation, while non-ideality of organic-water mixture only leads to decreases in SOA in most regions of eastern China. Water partitioning into OPM should be considered in air quality models in simulating SOA, especially in hot and humid environments.