Preprints
https://doi.org/10.5194/egusphere-2023-336
https://doi.org/10.5194/egusphere-2023-336
28 Feb 2023
 | 28 Feb 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

A thermodynamic framework for bulk–surface partitioning in finite-volume mixed organic–inorganic aerosol particles and cloud droplets

Ryan Schmedding and Andreas Zuend

Abstract. Atmospheric aerosol particles and their interactions with clouds are among the largest sources of uncertainty in global climate modeling. Aerosol particles in the ultrafine size range with diameters less than 100 nm have very high surface area to volume ratios, with a substantial fraction of molecules occupying the air–droplet interface. The partitioning of surface-active species between the interior bulk of a droplet and the interface with the surrounding air plays a large role in the physicochemical properties of a particle and in the activation of ultrafine particles, especially those of less than 50 nm diameter, into cloud droplets. In this work, a novel and thermodynamically rigorous treatment of bulk–surface equilibrium partitioning is developed through the use of a framework based on the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model in combination with a finite-depth Guggenheim interface region on spherical, finite-volume droplets. We outline our numerical implementation of the resulting modified Butler equation, including accounting for challenging extreme cases when certain compounds have very limited solubility in either the surface or bulk phase. This model, which uses a single, physically constrained interface thickness parameter, is capable of predicting the size-dependent surface tension of complex multicomponent solutions containing organic and inorganic species. We explore the impacts of coupled surface tension changes and changes in bulk–surface partitioning coefficients for aerosol particles ranging in diameters from several µm to as small as 10 nm and across atmospherically relevant relative humidity ranges. The treatment of bulk–surface equilibrium leads to deviations from classical cloud droplet activation behavior as modeled by simplified treatments of the Köhler equation that do not account for bulk–surface partitioning. The treatments for bulk–surface partitioning laid out in this work, when applied to the Köhler equation, are in agreement with measured critical supersaturations of a range of different systems. However, we also find that challenges remain in accurately modeling the growth behavior of certain systems containing small dicarboxylic acids, especially in a predictive manner. Furthermore, it was determined that the thickness of the interfacial phase is sensitive parameter in this treatment; however, constraining it to a meaningful range allow for predictive modeling of aerosol particle activation into cloud droplets, including cases with consideration of co-condensation of semivolatile organics.

Ryan Schmedding and Andreas Zuend

Status: open (until 11 Apr 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2023-336', Alison Bain, 03 Mar 2023 reply
    • AC1: 'Reply on CC1', Ryan Schmedding, 21 Mar 2023 reply
  • RC1: 'Comment on egusphere-2023-336', Anonymous Referee #1, 21 Mar 2023 reply

Ryan Schmedding and Andreas Zuend

Ryan Schmedding and Andreas Zuend

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Short summary
Aerosol particles below 100 nm in diameter have high surface area to volume ratios. The enrichment of compounds in the surface of an aerosol particle may lead to depletion of that species in the interior bulk of the particle. We present a framework for modeling the equilibrium bulk-surface partitioning of mixed organic-inorganic particles including cases of co-condensation of semi-volatile organic compounds and species with extremely limited solubility in the bulk or surface of a particle.