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

Submitted as: research article 26 Apr 2019

Submitted as: research article | 26 Apr 2019

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

Liquid-liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

Mijung Song1,2, Adrian M. Maclean2, Yuanzhou Huang2, Natalie R. Smith3, Sandra L. Blair3, Julia Laskin4, Alexander Laskin4, Wing-Sy Wong DeRieux3, Ying Li3, Manabu Shiraiwa3, Sergey A. Nizkorodov3, and Allan K. Bertram2 Mijung Song et al.
  • 1Department of Earth and Environmental Sciences, Chonbuk National University, Jeollabuk-do, 54896, Republic of Korea
  • 2Department of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
  • 3Department of Chemistry, University of California Irvine, Irvine, CA 92697, USA
  • 4Department of Chemistry, Purdue University, Wes Lafayette, IN 47907, USA

Abstract. Information on liquid-liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ~ 70 % to ~ 100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the organic-rich outer phase can lower the kinetic barrier for activation to a cloud droplet. At ≤ 10 % RH, the viscosity was in the range of ≥ 1 × 108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38–50 % RH the viscosity was in the range of 1 × 108–3 × 105 Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O : C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes–Einstein relation, at ≤ 10 % RH diffusion coefficients of organics within diesel fuel SOA is ≤ 5.4 × 10−17cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) is ≳ 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low RH conditions and on time scales of minutes to hours. At 38–50 % RH, the calculated organic diffusion coefficients are in the range of 5.4 × 10−17 to 1.8 × 10−13 cm2 s−1 and calculated τmixing values are in the range of ~ 0.01 h to ~ 50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.

Mijung Song et al.
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Mijung Song et al.
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