Seasonal Variations in the Viscosity of Secondary Organic Aerosols over China and Their Impacts on N2O5 Uptake

Secondary organic aerosol (SOA) phase state significantly influences its environmental impacts, yet current chemical transport models typically assume SOA particles are homogeneous liquids. Here we coupled a volatility-based viscosity parameterization online into the WRF-Chem to simulate the spatiotemporal variations in SOA phase state over China for 2018. Using the simulated viscosity, we further quantified the effect of dynamic phase states on the heterogeneous uptake of N2O5 . Results reveal pronounced spatial, seasonal, and vertical contrasts. The dry glass transition temperature ranges from ~280 to 305 K over most of China, peaking in summer and reaching a minimum in winter. Surface SOA remains semi-solid or solid (η > 10^7 Pa s) year-round in the arid northwest but is predominantly liquid in the humid southeast. Vertically, viscosity increases with altitude, with the transition from semi-solid to solid occurring at ~2 km in winter and above 3 km in other seasons. Consequently, the N2O5 bulk diffusion coefficient Db varies by around five orders of magnitude with SOA viscosity. A constant Db (10^-5 cm^2 s^-1 ), as commonly adopted in models, overestimates γ(N2O5 ) by 1-3 orders of magnitude relative to viscosity-dependent calculations. These findings underscore the necessity of incorporating dynamic phase states and viscosity-dependent Db parameterizations into atmospheric models to improve the accuracy of heterogeneous chemistry and regional nitrate simulations.