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Discussion papers
https://doi.org/10.5194/acp-2018-899
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-899
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Sep 2018

Research article | 05 Sep 2018

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

Viscosities, diffusion coefficients, and mixing times of intrinsic fluorescent organic molecules in brown limonene secondary organic aerosol and tests of the Stokes-Einstein equation

Dagny A. Ullmann1, Mallory L. Hinks2, Adrian Maclean1, Christopher Butenhoff3, James Grayson1, Kelley Barsanti5, Jose L. Jimenez4, Sergey A. Nizkorodov2, Saeid Kamal1, and Allan K. Bertram1 Dagny A. Ullmann et al.
  • 1Department of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
  • 2Department of Chemistry, University of California, Irvine, CA 92697, USA
  • 3Department of Physics, Portland State University, Portland, Oregon, USA
  • 4Cooperative Institute for Research in the Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
  • 5Department of Chemical and Environmental Engineering and Center for Environmental Research and Technology, University of California, Riverside, CA, USA

Abstract. Viscosities and diffusion rates of organics within secondary organic aerosol (SOA) remain uncertain. Using the bead-mobility technique, we measured the viscosities as a function of water activity (aw) of SOA generated by the ozonolysis of limonene followed by browning by exposure to NH3 (referred to as brown limonene SOA or brown LSOA). These measurements together with viscosity measurements reported in the literature show that the viscosity of brown LSOA increases by 3–5 orders of magnitude as the aw decreases from 0.9 to approximately 0.05. In addition, we measured diffusion coefficients of intrinsic fluorescent organic molecules within brown LSOA matrices using rectangular area fluorescence recovery after photobleaching. Based on the diffusion measurements, as the aw decreases from 0.9 to 0.33, the average diffusion coefficient of the intrinsic fluorescent organic molecules decreases from 5.5∙10-9cm2s-1 to 7.1∙10-13cm2s-1 and the mixing times of intrinsic fluorescent organic molecules within 200nm brown LSOA particles increases from 0.002s to 14s. These results suggest that the mixing times of large organics in the brown LSOA studied here are short (<1hr) for aw and temperatures often found in the PBL. Since the diffusion coefficients and mixing times reported here correspond to SOA generated using a high mass loading (~1,000µgm-3), biogenic SOA particles found in the atmosphere with mass loadings ≤10µgm-3 are likely to have higher viscosities and longer mixing times. These new measurements of viscosity and diffusion were used to test the accuracy of the Stokes-Einstein relation for predicting diffusion rates of organics within brown LSOA matrices. The results show that the Stokes-Einstein equation gives accurate predictions of diffusion coefficients of large organics within brown LSOA matrices when the viscosity of the matrix is as high as 102 to 104Pas. These results have important implications for predicting diffusion and mixing with SOA particles in the atmosphere.

Dagny A. Ullmann et al.
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Short summary
We measured the viscosity and diffusion of organic molecules in secondary organic aerosol (SOA) generated from the ozonolysis of limonene. The results suggest that the mixing times of large organics in the SOA studied are short (< 1 hr) for conditions found in the planetary boundary layer. The results also show that the Stokes-Einstein equation gives accurate predictions of diffusion coefficients of large organics within the studied SOA up to a viscosity of 102 to 104 Pa s.
We measured the viscosity and diffusion of organic molecules in secondary organic aerosol (SOA)...
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