The Hygroscopicity of Functionalized Insoluble Aerosol Surfaces

CHUN-NING MAO, Kanishk Gohil, Akua Asa-Awuku, University of Maryland

     Abstract Number: 43
     Working Group: Aerosol Chemistry

Abstract
Insoluble particles can provide a surface for water molecules and can act as cloud condensation nuclei(CCN) under supersaturated conditions. The water-uptake process (hygroscopicity) depends on the bulk chemistry of the nuclei and the chemical and physical surface properties. However, the impact of molecular surface chemistry for CCN activity has remained elusive; the porous structure and the non-sphericity of particles have made aerosol measurement and CCN prediction challenging. To understand the role of the surface chemistry in the water uptake, we study the CCN activity of non-porous, spherical polystyrene latex (PSL) particles with (PSL-NH₂ and PSL-COOH) and without (plain PSL) surface modification. Three thermodynamic models, traditional Köhler theory, the Flory-Huggins Köhler theory and the Frenkel-Halsey-Hill adsorption theory (FHH-AT) predict the activation behavior. Among all, the Frenkel-Halsey-Hill adsorption theory, with two parametrizations, A_FHH and B_FHH, is the only model capable of agreeing with measurement and predicting the slight differences between the different molecular level functions on PSL surfaces. PSL-NH₂ has the highest CCN activity, while the plain type PSL shows the lowest. The B_FHH parameter represents the interaction between the bulk nuclei and the water molecules and when B_FHH is constrained to unity, the A_FHH for PSL-NH₂, PSL-COOH and plain PSL is 0.23, 0.21, and 0.18 respectively. The results of the constrained fit imply that the CCN activity of the functionalized surface correlates to the polarity of functionalized groups. Lastly, we develop a single parameter hygroscopicity representation for insoluble aerosol derived from principles of the FHH-AT model. We demonstrate that the hygroscopicity based on FHH-AT model can predict the decreasing trend with the dry particle size of insoluble particles.