Viscosities, Diffusion Coefficients, and Mixing Times of Intrinsic Fluorescent Organic Molecules in Brown Limonene Secondary Organic Aerosol and Tests of the Stokes–Einstein Equation

Authors

Dagny A. Ullmann, University of British Columbia
Mallory L. Hinks, University of California, Irvine
Adrian M. Maclean, University of British Columbia
Christopher Butenhoff, Portland State UniversityFollow
James W. Grayson, University of British Columbia
Kelley Barsanti, Portland State UniversityFollow
Jose L. Jimenez, University of Colorado, BoulderFollow
Sergey A. Nizkorodov, University of California, Irvine
Saeid Kamal, University of British Columbia
Allan K. Bertram, University of Colorado, BoulderFollow

Sponsor

This work was funded by the Natural Science and Engineering Research Council of Canada. Jose L. Jimenez was supported by DOE (BER/ASR) DE-SC0016559. Support from the MJ Murdock Charitable Trust (grant 2012183) for computing infrastructure at Portland State University is acknowledged.

Published In

Atmospheric Chemistry and Physics

Document Type

Article

Publication Date

2-2019

Subjects

Atmospheric chemistry, Viscosity, Diffusion, Environmental chemistry, Volatile organic compounds

Abstract

Viscosities and diffusion rates of organics within secondary organic aerosol (SOA) remain uncertain. Using the bead-mobility technique, we measured viscosities as a function of water activity (a_w) 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 a_w decreases from 0.9 to 0.33, the average diffusion coefficient of the intrinsic fluorescent organic molecules decreases from 5:5 x 10^-9 to 7:1 x 10^-13 cm² s^-1 and the mixing times of intrinsic fluorescent organic molecules within 200 nm brown LSOA particles increases from 0.002 to 14 s. These results suggest that the mixing times of large organics in the brown LSOA studied here are short (<1 >h) for a_w and temperatures often found in the planetary boundary layer (PBL). Since the diffusion coefficients and mixing times reported here correspond to SOA generated using a high mass loading (~1000 μg m^-3), biogenic SOA particles found in the atmosphere with mass loadings ~10 μg m^-3 are likely to have higher viscosities and longer mixing times (possibly 3 orders of magnitude longer). 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 10⁴ Pa s. These results have important implications for predicting diffusion and mixing within SOA particles in the atmosphere.

Description

Originally appeared in Atmospheric Chemistry and Physics, volume 19, number 3, published by the European Geosciences Union. May be accessed at https://doi.org/10.5194/acp-19-1491-2019.

The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-1491-2019-supplement.

© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License.

DOI

10.5194/acp-19-1491-2019

Persistent Identifier

https://archives.pdx.edu/ds/psu/27929

Citation Details

Ullmann, D. A., Hinks, M. L., Maclean, A. M., Butenhoff, C. L., Grayson, J. W., Barsanti, K., ... & Bertram, A. K. (2019). Viscosities, diffusion coefficients, and mixing times of intrinsic fluorescent organic molecules in brown limonene secondary organic aerosol and tests of the Stokes–Einstein equation. Atmospheric Chemistry and Physics, 19(3), 1491-1503.