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Volume 16, issue 14
Atmos. Chem. Phys., 16, 8817-8830, 2016
https://doi.org/10.5194/acp-16-8817-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 8817-8830, 2016
https://doi.org/10.5194/acp-16-8817-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 19 Jul 2016

Research article | 19 Jul 2016

Relative humidity-dependent viscosity of secondary organic material from toluene photo-oxidation and possible implications for organic particulate matter over megacities

Mijung Song1,2, Pengfei F. Liu3, Sarah J. Hanna1, Rahul A. Zaveri4, Katie Potter5, Yuan You1, Scot T. Martin3,6, and Allan K. Bertram1 Mijung Song et al.
  • 1Department of Chemistry, University of British Columbia, Vancouver, BC, Canada
  • 2Department of Earth and Environmental Sciences, Chonbuk National University, Jeollabuk-do, Republic of Korea
  • 3John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 4Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
  • 5School of Chemistry, University of Bristol, Bristol, UK
  • 6Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA

Abstract. To improve predictions of air quality, visibility, and climate change, knowledge of the viscosities and diffusion rates within organic particulate matter consisting of secondary organic material (SOM) is required. Most qualitative and quantitative measurements of viscosity and diffusion rates within organic particulate matter have focused on SOM particles generated from biogenic volatile organic compounds (VOCs) such as α-pinene and isoprene. In this study, we quantify the relative humidity (RH)-dependent viscosities at 295±1K of SOM produced by photo-oxidation of toluene, an anthropogenic VOC. The viscosities of toluene-derived SOM were 2 × 10−1 to  ∼ 6 × 106Pas from 30 to 90%RH, and greater than  ∼ 2 × 108Pas (similar to or greater than the viscosity of tar pitch) for RH ≤ 17%. These viscosities correspond to Stokes–Einstein-equivalent diffusion coefficients for large organic molecules of  ∼ 2 × 10−15cm2s−1 for 30%RH, and lower than  ∼ 3 × 10−17cm2s−1 for RH ≤ 17%. Based on these estimated diffusion coefficients, the mixing time of large organic molecules within 200nm toluene-derived SOM particles is 0.1–5h for 30%RH, and higher than  ∼ 100h for RH ≤ 17%. As a starting point for understanding the mixing times of large organic molecules in organic particulate matter over cities, we applied the mixing times determined for toluene-derived SOM particles to the world's top 15 most populous megacities. If the organic particulate matter in these megacities is similar to the toluene-derived SOM in this study, in Istanbul, Tokyo, Shanghai, and São Paulo, mixing times in organic particulate matter during certain periods of the year may be very short, and the particles may be well-mixed. On the other hand, the mixing times of large organic molecules in organic particulate matter in Beijing, Mexico City, Cairo, and Karachi may be long and the particles may not be well-mixed in the afternoon (15:00–17:00LT) during certain times of the year.

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