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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  07 Jul 2020

07 Jul 2020

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This preprint is currently under review for the journal ACP.

Mass Accommodation and Gas-Particle Partitioning in Secondary Organic Aerosols: Dependence on Diffusivity, Volatility, Particle-phase Reactions, and Penetration Depth

Manabu Shiraiwa1 and Ulrich Pöschl2 Manabu Shiraiwa and Ulrich Pöschl
  • 1Department of Chemistry, University of California, Irvine, CA92625, USA
  • 2Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany

Abstract. Mass accommodation is an essential process for gas-particle partitioning of organic compounds in secondary organic aerosols (SOA). The mass accommodation coefficient is commonly described as the probability of a gas molecule colliding with the surface to enter the particle phase. It is often applied, however, without specifying if and how deep a molecule has to penetrate beneath the surface to be regarded as incorporated into the condensed phase (adsorption vs. absorption). While this aspect is usually not critical for liquid particles with rapid surface-bulk exchange, it can be important for viscous semisolid or glassy solid particles to distinguish and resolve the kinetics of accommodation at the surface, transfer across the gas-particle interface, and further transport into the particle bulk.

For this purpose, we introduce a novel parameter: an effective mass accommodation coefficient αeff that depends on penetration depth and is a function of surface accommodation coefficient, volatility, bulk diffusivity, and particle-phase reaction rate coefficient. Application of αeff in the traditional Fuchs-Sutugin approximation of mass-transport kinetics at the gas-particle interface yields SOA partitioning results that are consistent with a detailed kinetic multilayer model (KM-GAP, Shiraiwa et al., 2012) and two-film model solutions (MOSAIC, Zaveri et al., 2014) but deviate substantially from earlier modeling approaches not considering the influence of penetration depth and related parameters.

For highly viscous or semisolid particles, we show that the effective mass accommodation coefficient remains similar to the surface accommodation coefficient in case of low-volatile compounds, whereas it can decrease by several orders of magnitude in case of semi-volatile compounds. Such effects can explain apparent inconsistencies between earlier studies deriving mass accommodation coefficients from experimental data or from molecular dynamics simulations.

Our findings challenge the approach of traditional SOA models using the Fuchs-Sutugin approximation of mass transfer kinetics with a fixed mass accommodation coefficient regardless of particle phase state and penetration depth. The effective mass accommodation coefficient introduced in this study provides an efficient new way of accounting for the influence of volatility, diffusivity, and particle-phase reactions on SOA partitioning in process models as well as in regional and global air quality models.

Manabu Shiraiwa and Ulrich Pöschl

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Manabu Shiraiwa and Ulrich Pöschl

Manabu Shiraiwa and Ulrich Pöschl


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Short summary
Mass accommodation is a crucial process in secondary organic aerosol partitioning that depends on volatility, diffusivity, reactivity, and particle penetration depth of the chemical species involved. For efficient kinetic modeling, we introduce an effective mass accommodation coefficient that accounts for the above influencing factors, can be applied in the common Fuchs-Sutugin approximation, and helps to resolve inconsistencies and shortcomings of earlier experimental and model investigations.
Mass accommodation is a crucial process in secondary organic aerosol partitioning that depends...