Phase state and viscosity of secondary organic aerosols over China simulated by WRF-Chem

Abstract. Secondary organic aerosols (SOA) can exist in liquid, semi-solid or amorphous solid states, which are rarely accounted for in current chemical transport models (CTMs). Missing the information of SOA phase state and viscosity in CTMs impedes accurate representation of SOA formation and evolution, affecting the predictions of aerosol effects on air quality and climate. We have previously developed a method to estimate the glass transition temperature (T_g) of an organic compound based on volatility. In this study, we apply this method to predict the phase state and viscosity of SOA particles over China in summer of 2018 using the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem). This is the first time that spatial distributions of the SOA phase state over China are investigated by a regional CTM. Simulations show that T_g values of dry SOA range from ~287 K to 305 K, with higher values in the northwestern China where SOA particles have larger mass fractions of low volatility compounds. Considering water uptake by SOA particles, the SOA viscosity also shows a prominent geospatial gradient that highly viscous or solid SOA particles are mainly found in the northwestern China. The lowest and highest SOA viscosity values both occur over the Qinghai-Tibet Plateau that the solid phase state is predicted over dry and high-altitude areas and the liquid phase state is predicted mainly in the south of the plateau with high relative humidity during the summer monsoon season. The characteristic mixing timescale of organic molecules in 200 nm SOA particles is calculated based on the simulated particle viscosity and the bulk diffusion coefficient of organic molecules. Calculations show that during the simulated period the percent time of the mixing timescale longer than 1 h is > 70 % at the surface and at 500 hPa in most areas of the northern China, indicating that kinetic partitioning considering the bulk diffusion in viscous particles may be required for more accurate prediction of SOA mass concentrations and size distributions over these areas. Sensitivity simulations show that including the formation of extremely low-volatile organic compounds, the percent time that a SOA particle is in the liquid phase state decreases by up to 12 % in the southeastern China during the simulated period. With an assumption that the organic and inorganic compounds are always internally mixed in one phase, we show that the water absorbed by inorganic species can significantly lower the simulated viscosity over the southeastern China. This indicates that constraining the uncertainties in simulated SOA volatility distributions and accurately predicting the occurrence of phase separation would improve prediction of viscosity in multicomponent particles in southeastern China.