29 Jun 2022
29 Jun 2022
Status: this preprint is currently under review for the journal ACP.

A Single Parameter Hygroscopicity Model for Functionalized and Insoluble Aerosol Surfaces

Chun-Ning Mao1, Kanishk Gohil1, and Akua Asa-Awuku1,2 Chun-Ning Mao et al.
  • 1Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD 20742 USA
  • 2Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742 USA

Abstract. The impact of molecular level surface chemistry for aerosol water-uptake and droplet growth is not well understood. In this work, spherical, non-porous, monodisperse Polystyrene Latex particles treated with different surface functional groups are exploited to isolate the effects of aerosol surface chemistry for droplet activation. PSL is effectively water-insoluble and changes in the particle surface may be considered a critical factor in the initial water-uptake of water insoluble material. The droplet growth of two surface modified types of PSL (PSL-NH2 and PSL-COOH) along with plain PSL was measured in a supersaturated environment with a Cloud Condensation Nuclei Counter (CCNC). Three droplet growth models - traditional Köhler (TK), Flory-Huggins Köhler (FHK) and the Frenkel-Halsey-Hill adsorption theory (FHH-AT) were compared to experimental data. The experimentally determined single hygroscopicity parameter, κ, was found in the range from 0.002 to 0.04. The traditional Köhler prediction assumes Raoult’s law solute dissolution and underestimates the water-uptake ability of the PSL particles. FHK can be applied to polymeric aerosol; however, FHK assumes the polymer is soluble and hydrophilic. Thus, the FHK model generates a negative result for hydrophobic PSL and predicts non-activation behavior that disagrees with the experimental observation. The FHH-AT model assumes that a particle is water-insoluble and can be fit with 2 empirical parameters (AFHH and BFHH). The FHH-AT prediction agrees with the experimental data and can differentiate the water uptake behavior of the particles due to surface modification of PSL surface. PSL-NH2 exhibits slightly higher hygroscopicity than the PSL-COOH, while plain PSL is the least hygroscopic among the three. This result is consistent with the polarity of surface functional groups and their affinity to water molecules. Thus, changes in AFHH and BFHH can be quantified when surface modification is isolated for the study of water-uptake. The fitted AFHH for PSL-NH2, PSL-COOH and plain PSL is 0.23, 0.21 and 0.18 when BFHH is unity. To simplify the use of FHH-AT for use in cloud activation models, we also present and test a new single parameter framework for insoluble compounds, κFHH. κFHH is within 5 % agreement of the experimental data and can be applied to describe a single-parameter hygroscopicity for water-insoluble aerosol with surface modified properties.

Chun-Ning Mao et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-339', Anonymous Referee #1, 19 Jul 2022
  • RC2: 'Comment on acp-2022-339', Anonymous Referee #3, 20 Jul 2022

Chun-Ning Mao et al.

Chun-Ning Mao et al.


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Short summary
The impact of molecular level surface chemistry for aerosol water-uptake and droplet growth is not well understood. In this work we show changes in molecular level surface chemistry can be measured and quantified. In addition, we develop a single-parameter model, representing changes in aerosol chemistry that can be used in global climate models to reduce the uncertainty in aerosol-cloud predictions.