A single-parameter hygroscopicity model for functionalized insoluble aerosol surfaces

Abstract

Abstract. The impact of molecular level surface chemistry for aerosol water-uptake and droplet growth is not well understood. In this work, spherical, nonporous, monodisperse polystyrene latex (PSL) 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 with experimental data. The experimentally determined single hygroscopicity parameter, κ, was found within 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 that 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 two empirical parameters (AFHH and BFHH). The FHH-AT prediction agrees with the experimental data and can differentiate the water uptake behavior of the particles owing to surface modification of PSL surface. PSL-NH2 exhibits slightly higher hygroscopicity than the PSL-COOH, whereas 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.

Hybrid water adsorption and solubility partitioning for aerosol hygroscopicity and droplet growth

“…It can be inferred that A FHH follows the trends of solubility and is most likely controlled by functional groups, and B FHH drives overall droplet growth across different compositions and molar volumes. The results here are consistent with Mao et al (2022), who showed that the A FHH values correlated with functionalized surfaces of aerosol with the same core (polystyrene latex, PSL). This suggests that the A FHH values may play a more important role with compounds of similar molar volume and highlights the importance of functionalized groups and isomeric structures in determining overall droplet growth.…”

“…For the FHH-AT, a similar κ parameterization as KT can be developed by combining the water activity with the FHH isotherm using Eq. ( 7) (Mao et al, 2022),…”

“…Additionally, an important assumption in FHH-AT and other similar adsorption models is that the aerosols are treated as completely water-insoluble. Only recently, in a companion paper (Mao et al, 2022), has FHH-AT been shown to work for insoluble particles with watersoluble and molecular-level functionalized surfaces. Thus, there now exists a transitional regime from a soluble to waterinsoluble models to correctly describe droplet growth.…”

“…Specifically, a new CCN activity model is developed by combining the components of the solubilitymodified KT with the FHH isotherm. This work is a companion and extension to the single-parameter framework developed in Mao et al (2022). Throughout this paper, this model is referred to as the Hybrid Activity Model (HAM).…”

Electric Field-Induced Water Condensation Visualized by Vapor-Phase Transmission Electron Microscopy

“…This method could prove useful for investigating water condensation on water insoluble and low hygroscopicity aerosol particles that only condense water under supersaturated conditions. 38 Water droplet growth kinetics observed by VPTEM were drastically slower than expected under conventional atmospheric conditions due to decreased water vapor diffusion rates and radiolytic hydrogen production. Taken together, these results are expected to provide a baseline understanding that will enable delineating electron beam−sample interactions from the aerosol phenomena of interest and correcting for electron beam effects on physical parameters like temperature and humidity in more complex aerosol-phase systems, such as water condensation on atmospheric aerosols.…”