Water interactions with condensed carboxylic acids: adsorption and desorption of water on valeric acid surfaces

Abstract

Organic aerosol particles undergo phase transitions through water uptake and release, which directly influence their physicochemical properties and impact aerosol–cloud interactions, climate, and air quality. Here, we combine environmental molecular beam (EMB) experiments with molecular dynamics (MD) simulations to investigate water interactions with valeric acid (VA) as a model organic aerosol system. Water molecules colliding with VA surfaces are predominantly trapped, with only a minor inelastic scattering channel observed. Most trapped molecules are weakly bound and desorb rapidly (69–83%), while a smaller fraction occupies more strongly bound surface states, leading to desorption on millisecond timescales (7–16%) or longer-term accommodation (5–20%). The water sticking coefficient shows little temperature dependence over 160–260 K, but depends strongly on film thickness, i.e., molecularly thin VA coatings exhibit higher sticking probabilities than micrometer-thick layers. These results suggest that molecularly thin VA coatings may exhibit differences in molecular arrangement that could contribute to differences in hygroscopic behavior compared to bulk-like surfaces.