Preprints
https://doi.org/10.5194/acp-2022-331
https://doi.org/10.5194/acp-2022-331
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10 Jun 2022
10 Jun 2022
Status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Not All Types of Secondary Organic Aerosol Mix: Two Phases Observed When Mixing Different Secondary Organic Aerosol Types

Fabian Mahrt1,2, Long Peng1,3, Julia Zaks1, Yuanzhou Huang1,a, Paul E. Ohno4,5, Natalie R. Smith6, Florence K. A. Gregson1, Yiming Qin4,b, Celia L. Faiola6, Scot T. Martin4,7, Sergey A. Nizkorodov6, Markus Ammann2, and Allan K. Bertram1 Fabian Mahrt et al.
  • 1Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T1Z1 Canada
  • 2Laboratory of Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
  • 3Institute for Environmental and Climate Research, Jinan University, Guangzhou 511443, China
  • 4John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
  • 5Center for the Environment, Harvard University, Cambridge, MA 02138, USA
  • 6Department of Chemistry, University of California, Irvine, Irvine, CA 92697, USA
  • 7Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
  • anow at: Anton Paar Canada Inc., 4920 Place Olivia, H4R 2Z8 Saint Laurent, Canada
  • bnow at: Department of Chemistry, University of California, Irvine, CA 92697-2025, USA

Abstract. Secondary organic aerosol (SOA) constitutes a large fraction of atmospheric aerosol. To assess its impacts on cli-mate and air pollution, knowledge of the number of phases in internal mixtures of different SOA types is required. Atmospheric models often assumed that different SOA types form a single phase when mixed. Here, we present visual observations of the number of phases formed after mixing different anthropogenic and biogenic SOA types. Mixing SOA types generated in environmental chambers with oxygen-to-carbon (O / C) ratios between 0.34 to 1.05, we found six out of fifteen mixtures of two SOA types to result in two phase particles. We demonstrate that the number of phases depends on the difference in the average O / C ratio between the two SOA types (Δ(O / C)). Using a threshold Δ(O / C) of 0.47, we can predict the phase behavior of over 90 % of our mixtures, with one- and two-phase particles predicted for Δ(O / C) < 0.47 and Δ(O / C) ≥ 0.47, respectively. This Δ(O / C) threshold further allows to predict if mixtures of fresh and aged SOA form one- or two-phase particles in the atmos-phere. In addition, we show that phase separated SOA particles form when mixtures of volatile organic compounds emitted from real trees are oxidized.

Fabian Mahrt et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-331', Anonymous Referee #1, 03 Aug 2022
  • RC2: 'Comment on acp-2022-331', Anonymous Referee #2, 03 Aug 2022
  • EC1: 'Editor's Comment on acp-2022-331', James Allan, 17 Aug 2022
  • AC1: 'Comment on acp-2022-331', Fabian Mahrt, 02 Sep 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-331', Anonymous Referee #1, 03 Aug 2022
  • RC2: 'Comment on acp-2022-331', Anonymous Referee #2, 03 Aug 2022
  • EC1: 'Editor's Comment on acp-2022-331', James Allan, 17 Aug 2022
  • AC1: 'Comment on acp-2022-331', Fabian Mahrt, 02 Sep 2022

Fabian Mahrt et al.

Fabian Mahrt et al.

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Executive editor
Organic aerosol remain one of the more complex and hard to predict when studying atmospheric aerosols and their influences on air quality, meteorology and climate. Among its many complexities is the phase and viscosity of the organic matter, which dictates how it interacts with other particulate components and the gas phase, in turn affecting growth rates and cloud activation. There have been a number of previous works studying phase separation, where the organic matter becomes immiscible with an aqueous component (containing inorganic salts), but this new letter presents compelling visual evidence that different organic phases are also capable of separation. Different secondary organic aerosol (SOA) mixtures were created and some mixtures exhibited separation, with a factor being the oxygen-to-carbon ratio of the material, likely a surrogate for polarity. If this behaviour is found to be important in atmospheric aerosols this represents a new direction in how these may need to be represented in models.
Short summary
The number of condensed phases in mixtures of different secondary organic aerosol (SOA) types determines their impact on air quality and climate. Here we observe the number of phases in individual particles that contain mixtures of two different types of SOA. We find that SOA mixtures can form one- or two-phase particles, depending on the difference in the average oxygen-to-carbon (O / C) ratios of the two SOA types mixed.
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