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

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.