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Discussion papers
https://doi.org/10.5194/acp-2019-900
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-900
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Nov 2019

Submitted as: research article | 07 Nov 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Predicting Secondary Organic Aerosol Phase State and Viscosity and its Effect on Multiphase Chemistry in a Regional Scale Air Quality Model

Ryan Schmedding1,*, Quazi Z. Rasool1,*, Yue Zhang1,4, Havala O. T. Pye1,2, Haofei Zhang3, Yuzhi Chen1, Jason D. Surratt1, Ben H. Lee5, Claudia Mohr5,a, Felipe D. Lopez-Hilfiker5,b, Joel A. Thornton5, Allen H. Goldstein6,7, and William Vizuete1 Ryan Schmedding et al.
  • 1Department of Environmental Science and Engineering, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27516
  • 2National Exposure Research Laboratory, Office of Research and Development, Environmental Protection Agency, Research Triangle Park, Durham, North Carolina, 27709
  • 3Department of Chemistry, University of California at Riverside, Riverside, California, 92521
  • 4Aerodyne Research, Inc., Billerica, Massachusetts, 01821
  • 5Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195
  • 6Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720
  • 7Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720
  • apresent address: Department of Environmental Science and Analytical Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
  • bpresent address: Tofwerk AG, CH-3600 Thun, Switzerland
  • *These authors contributed equally to this work.

Abstract. 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 surrounding an aqueous core. This phase separation can decrease the partitioning of semi-volatile and low-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 their: glass transition temperature (Tg), Oxygen to Carbon (O : C) ratio, concentrations relative to 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 formed at the ground/surface level was predicted to be phase separated with core-shell morphology i.e. aqueous inorganic core surrounded by organic coating, 68.5 % of the time for continental United States. The phase states of organic coatings switched between semi-solid and liquid states, depending on the environmental conditions. The semi-solid shell occurring with lower aerosol liquid water content (western United States and at higher altitudes) has a viscosity that was predicted to be 102–1012 Pa ⋅ s which resulted in organic mass being decreased due to diffusion limitation. The organic phase was primarily liquid where aerosol liquid water was dominant (eastern United States and at surface), with a viscosity < 102 Pa ⋅ s. 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 viscous organic shell. The implementation of phase separation parameters in CMAQ led to a reduction of fine particulate matter (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 isoprene epoxydiol (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.

Ryan Schmedding et al.
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Short summary
Accurate model predictions of aerosol concentrations is a known challenge. It is assumed in many modeling systems that aerosols are in a homogenously mixed phase state. It has been observed that aerosols do phase separate and can form a highly viscous organic shell with an aqueous core impacting the formation processes of aerosols. This work is a model implementation to determine an aerosol's phase state using glass transition temperature and aerosol composition.
Accurate model predictions of aerosol concentrations is a known challenge. It is assumed in many...
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