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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 27 May 2019

Submitted as: research article | 27 May 2019

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

Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

Xiaoxiao Li1, Shaojie Song2, Wei Zhou1, Jiming Hao1, Douglas R. Worsnop3,4, and Jingkun Jiang1 Xiaoxiao Li et al.
  • 1State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China
  • 2School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
  • 3Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Finland
  • 4Aerodyne Research Inc., Billerica, Massachusetts 01821, USA

Abstract. Aerosol liquid water (ALW) is ubiquitous in ambient aerosol and plays an important role in the formation of both aerosol organics and inorganics. To investigate the interactions between ALW and aerosol organics during haze formation and evolution, ALW was modelled based on long-term measurements of submicron aerosol compositions from different seasons in Beijing. ALW contributed by aerosol inorganics (ALWinorg) was modelled by ISORROPIA-II, and ALW contributed by organics (ALWorg) was estimated with κ-Köhler theory, where a real-time hygroscopicity parameter of the organics (κorg) calculated from the real-time organic oxygen-to-carbon (O/C) was used. Particle overall hygroscopicity (κtotal) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALWorg, which is often neglected in traditional ALW modelling, contributes a significant fraction (18–32 %) to the total ALW in Beijing. The highest ALWorg fraction appears in the cleanest days when both the organic fraction and κorg are relatively high. The distinct variation of O/C, from 0.2 to 1.3, indicates the wide variety of organic components. This emphasizes the necessity of using real-time κorg, instead of using a fixed κorg, to calculate ALWorg in Beijing. The significant variation of κorg which was calculated from O/C, together with the highly variable organic or inorganic volume fractions, leads to a wide range of κtotal (between 0.20 and 0.45), which exerts great impacts on the water uptake. The variation of organic O/C, or derived κorg, was found to be influenced by T, ALW, and aerosol mass concentrations. Among which, T and ALW both have promoting effects on O/C. During high-ALW haze episodes, although the organic fraction decreases rapidly, O/C, or derived κorg, increases with the increase in ALW, suggesting the formation of more soluble organics via aqueous/heterogeneous-phase process. A positive feedback loop is thus formed: during high-ALW episodes, the ever-increasing κorg, together with the decreasing particle organic fraction (or increasing particle inorganic fraction), increases κtotal, thus further promotes the ability of particles to uptake water.

Xiaoxiao Li et al.
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Xiaoxiao Li et al.
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Short summary
Aerosol liquid water is ubiquitous in ambient aerosol. Using long-term aerosol chemical composition to model the aerosol water in Beijing, we found that water absorbed by organics contributes a significant fraction to the total aerosol water. We emphasized the hygroscopicity of organics is highly variable and should be taken into consideration in modelling. A positive feedback loop between organic hygroscopicity and aerosol water was found as one of the driven factors of severe haze in Beijing.
Aerosol liquid water is ubiquitous in ambient aerosol. Using long-term aerosol chemical...