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Discussion papers
https://doi.org/10.5194/acp-2019-819
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-819
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Oct 2019

Submitted as: research article | 28 Oct 2019

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

Treatment of non-ideality in the multiphase model SPACCIM-Part2: Impacts on the multiphase chemical processing in deliquesced aerosol particles

Ahmad J. Rusumdar1,a, Andreas Tilgner1, Ralf Wolke1, and Hartmut Herrmann1 Ahmad J. Rusumdar et al.
  • 1Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, 04318, Germany
  • anow at: FERCHAU Engineering, Niederlassung Karlsruhe, Karlsruhe 76185, Germany

Abstract. Tropospheric deliquesced particles are characterised by concentrated non-ideal solutions (aerosol liquid water or ALW) that can affect the occurring multiphase chemistry. However, such non-ideal solution effects have generally not yet been considered in and investigated by current complex multiphase chemistry models in an adequate way. Therefore, the present study aims at accessing the impact of non-ideality on multiphase chemical processing in concentrated aqueous aerosols. Simulations with the multiphase chemistry model (SPACCIM-SpactMod) are performed in different environmental and microphysical conditions with and without a treatment of non-ideal solutions in order to assess its impact on aqueous-phase chemical processing.

The present study shows that activity coefficients of inorganic ions are often below unity under 90 % RH-deliquesced aerosol conditions, and that most uncharged organic compounds exhibit activity coefficient values of around or even above unity. Due to this behaviour, model studies have revealed that the inclusion of non-ideality considerably affects the multiphase chemical processing of transition metal ions (TMIs), oxidants, and related chemical subsystems such as organic chemistry. In detail, both the chemical formation and oxidation fluxes of Fe(II) are substantially lowered by a factor of 2.8 in the non-ideal base case compared to the ideal case. The reduced Fe(II) processing in the non-ideal base case, including lowered chemical fluxes of the Fenton reaction (−70 %), leads to a reduced processing of HOx/HOy. under deliquesced aerosol conditions. Consequently, higher multiphase H2O2 concentrations (larger by a factor of 3.1) and lower aqueous-phase OH concentrations (lower by a factor of ≈ 4) are modelled during non-cloud periods. For H2O2, a comparison of the chemical reaction fluxes reveals that the most important sink, the reaction with HSO3, contributes with a 40 % higher flux in the non-ideal base case than in the ideal case, leading to more efficient sulfate formation. On the other hand, the chemical fluxes of the OH radical are about 50 % lower in the non-ideal base case than in the ideal case, including lower degradation fluxes of organic aerosol components. Thus, considering non-ideality influences the chemical processing and the concentrations of organic compounds under deliquesced particle conditions in a compound-specific manner. For example, the reduced oxidation budget under deliquesced particle conditions leads to both increased and decreased concentration levels, e.g. of important C2/C3 carboxylic acids. For oxalic acid, the present study demonstrates that the non-ideality treatment enables more realistic predictions of high oxalate concentrations than observed under ambient highly polluted conditions. Furthermore, the simulations implicate that lower humidity conditions, i.e. more concentrated solutions, might promote higher oxalic acid concentration levels in aqueous aerosols due to differently affected formation and degradation processes.

Ahmad J. Rusumdar et al.
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Status: open (until 23 Dec 2019)
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Short summary
In the present study, simulations with the multiphase chemistry model SPACCIM-SpactMod are performed. The investigations aim at assessing the impact of a detailed treatment of non-ideality in multiphase models dealing with aqueous aerosol chemistry. The model studies demonstrate that the inclusion of non-ideality considerably affects the multiphase chemical processing of transition metal ions, oxidants, and related chemical subsystems such as organic chemistry in aqueous aerosols.
In the present study, simulations with the multiphase chemistry model SPACCIM-SpactMod are...
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