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Discussion papers
https://doi.org/10.5194/acp-2019-191
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-191
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 01 Mar 2019

Research article | 01 Mar 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).

Predictions of diffusion rates of organic molecules in secondary organic aerosols using the Stokes-Einstein and fractional Stokes-Einstein relations

Erin Evoy1, Adrian M. Maclean1, Grazia Rovelli2,a, Ying Li3, Alexandra P. Tsimpidi4,5, Vlassis A. Karydis4,6, Saeid Kamal1, Jos Lelieveld4,7, Manabu Shiraiwa3, Jonathan P. Reid2, and Allan K. Bertram1 Erin Evoy et al.
  • 1Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T 1Z1, Canada
  • 2School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK
  • 3Department of Chemistry, University of California, Irvine, California, 92697-2025, USA
  • 4Atmospheric Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 5National Observatory of Athens, Institute for Environmental Research & Sustainable Development, 15236 Palea Penteli, Greece
  • 6Forschungszentrum Jülich, Institute of Energy & Climate Research, IEK-8, 52425 Jülich, Germany
  • 7The Cyprus Institute, Nicosia 1645, Cyprus
  • anow at: Chemical Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94611, USA

Abstract. Information on the rate of diffusion of organic molecules within secondary organic aerosol (SOA) is needed to accurately predict the effects of SOA on climate and air quality. Often, researchers have predicted diffusion rates of organic molecules within SOA using measurements of viscosity and the Stokes-Einstein relation (D ∝ 1/η where D is the diffusion coefficient and η is viscosity). However, the accuracy of this relation for predicting diffusion in SOA remains uncertain. We measured diffusion coefficients over eight orders in magnitude in proxies of SOA including citric acid, sorbitol, and a sucrose-citric acid mixture. These results were combined with literature data to evaluate the Stokes-Einstein relation for predicting diffusion of organic molecules in SOA. Although almost all the data agrees with the Stokes-Einstein relation within a factor of ten, a fractional Stokes-Einstein relation (D ∝ C/ηt) with t = 0.93 and C = 1.66 is a better model for predicting diffusion of organic molecules in the SOA proxies studied. In addition, based on the output from a chemical transport model, the Stokes-Einstein relation can over predict mixing times of organic molecules within SOA by as much as one order of magnitude at an altitude ~ 3 km, compared to the fractional Stokes-Einstein relation with t = 0.93 and C = 1.66. These differences can be important for predicting growth, evaporation, and reaction rates of SOA in the middle and upper part of the troposphere. These results also have implications for other areas where diffusion of organic molecules within organic-water matrices is important.

Erin Evoy et al.
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Short summary
We measured diffusion rates of organic molecules in a number of proxies for secondary organic aerosol (SOA). We compared measured diffusion with predictions from two relations: the Stokes-Einstein relation and a fractional Stokes-Einstein relation. We found that the fractional relation does a better job of predicting diffusion rates in this case. Output from an atmospheric model shows that mixing times predicted using the two relations differ by up to one order of magnitude at an altitude ~ 3 km.
We measured diffusion rates of organic molecules in a number of proxies for secondary organic...
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