Predicting Secondary Organic Aerosol Phase State and Viscosity and its Effect on Multiphase Chemistry in a Regional Scale Air Quality Model
Schmedding, R., Q. Rasool, Y. Zhang, H. Pye, H. Zhang, Y. Chen, J. Surratt, B. Lee, C. Mohr, F. Lopez-Hilfiker, J. Thornton, A. Goldstein, AND W. Vizuete. Predicting Secondary Organic Aerosol Phase State and Viscosity and its Effect on Multiphase Chemistry in a Regional Scale Air Quality Model. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 20(12):8201-8225, (2020). https://doi.org/10.5194/acp-20-8201-2020
Fine particles in the atmosphere exhibit complex phase states including the possibility of multiple liquid and/or heterogeneous phases. Most models, including the CMAQ model, consider fine particles to consist of a homogeneous, uniform population of particles. This work examines how phase separation and viscosity limitations affect the formation of PM2.5 mass.
Atmospheric aerosols are a significant public health hazard and have substantial impacts on the climate. Secondary organic aerosols (SOA) have been shown to phase separate into a highly viscous organic outer layer and an aqueous core. This phase separation can decrease the partitioning of semi-volatile species to the organic phase and alter the extent of acid-catalyzed reactions in the aqueous core. A new algorithm that can determine SOA phase separation based on the glass transition temperature (Tg), O:C ratio, sulfate concentrations, and meteorological conditions were implemented into the Community Multiscale Air Quality Modeling (CMAQ) System version 5.2.1 and was used to simulate the conditions in the continental United States for the summer of 2013. SOA was predicted to be phase separated at the surface level 68.5% of the time. The viscosities of the organic phase at the surface layer were primarily liquid, with a viscosity < 102 Pa•s. The phase states of organic coatings switched between semi-solid and liquid states, depending on the environmental conditions. The semi-solid shell has a viscosity that was predicted to be 102-1012 Pa•s which resulted in organic mass being decreased due to diffusion limitation. Phase separation while in a liquid phase state, i.e. Liquid-Liquid Phase Separation (LLPS), also reduces reactive uptake rates relative to homogenous internally mixed liquid morphology, but was lower than aerosols with thick and highly viscous organic shell. The implementation of phase separation parameters in CMAQ led to a reduction of PM2.5 organic mass, with a marginal change in bias and error (< 0.1 μg/m3) compared to field data collected during the 2013 Southern Oxidant and Aerosol Study. Sensitivity simulations assuming higher dissolution rate of IEPOX into the particle phase and the treatment of aerosol water content mitigated this worsening in model performance, pointing out the need to better constrain the parameters that govern phase state and morphology of SOA.
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