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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-715
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 24 Jul 2018

Research article | 24 Jul 2018

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

Cloud droplet activation of secondary organic aerosol is mainly controlled by molecular weight, not water solubility

Jian Wang1, John E. Shilling2, Jiumeng Liu2,a, Alla Zelenyuk3, David M. Bell3,b, Markus Petters4, Ryan Thalman1,c, Fan Mei2, Rahul A. Zaveri2, and Guangjie Zheng1 Jian Wang et al.
  • 1Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, New York 11973, USA
  • 2Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA
  • 3Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA
  • 4Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University at Raleigh, Raleigh, North Carolina 27695, USA
  • apresent address: School of Environment, Harbin Institute of Technology, Harbin, China 150001
  • bpresent address: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland
  • cpresent address: Department of Chemistry, Snow College, Richfield, Utah 84627, USA

Abstract. Aerosol particles strongly influence global climate by modifying the properties of clouds. An accurate assessment of the aerosol impact on climate requires knowledge of the concentration of cloud condensation nuclei (CCN), a subset of aerosol particles that can activate and form cloud droplets in the atmosphere. Atmospheric particles typically consist of a myriad of organic species, which frequently dominate the particle composition. As a result, CCN concentration is often a strong function of the hygroscopicity of organics in the particles. Earlier studies showed organic hygroscopicity increases nearly linearly with oxidation level. Such increase of hygroscopicity is conventionally attributed to higher water solubility for more oxidized organics. By systematically varying the water content of activating droplets, we show that for the majority of secondary organic aerosols (SOA), essentially all organics are dissolved at the point of droplet activation. Therefore, the organic hygroscopicity is not limited by solubility, but is dictated mainly by the molecular weight of organic species. Instead of increased water solubility as previously thought, the increase of the organic hygroscopicity with oxidation level is largely because (1) SOA formed from smaller precursor molecules tend to be more oxidized and have lower average molecular weight and (2) during oxidation, fragmentation reactions reduce average organic molecule weight, leading to increased hygroscopicity. A simple model of organic hygroscopicity based on molecular weight, oxidation level, and volatility is developed, and it successfully reproduces the variation of SOA hygroscopicity with oxidation level observed in the laboratory and field studies.

Jian Wang et al.
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Earlier studies showed organic hygroscopicity increases with oxidation level. Such increase is conventionally attributed to higher water solubility for more oxidized organics. By systematically varying the water content of activating droplets, we show that for secondary organic aerosols, essentially all organics are dissolved at the point of droplet activation. Therefore, the organic hygroscopicity is not limited by solubility, but is dictated mainly by the molecular weight of organic species.
Earlier studies showed organic hygroscopicity increases with oxidation level. Such increase is...
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