Partitioning of semivolatile surface-active compounds between bulk, surface and gas phase

We present a model study demonstrating that surface partitioning of volatile surfactants enhances their uptake by submicron liquid droplets. In submicron-sized droplets, surface partitioning of a surface-active volatile species may significantly decrease its equilibrium partial pressure, thus increasing the total flux of the surfactant from gas phase to aqueous phase. Such uptake of volatile organic species into aqueous aerosols can be followed by aqueous-phase chemistry to form low-volatility secondary organic aerosol material, leading to increased aerosol mass. In the study, we used an air parcel model that includes simplified aqueous- and gas-phase chemistry, condensation/evaporation, and a model of aqueous-phase thermodynamics that takes into account the partitioning of surfactants between the bulk and surface phases. We modeled the uptake and aqueous-phase chemical reactions of methylglyoxal, as it is a moderate surfactant that forms less volatile secondary organic material via aqueous-phase chemical reactions with the hydroxyl radical as well as hydronium and ammonium ions. Our model simulations show an order of magnitude higher uptake of methylglyoxal in aqueous aerosols of cloud condensation nuclei sizes (less than 200 nm in radius) when surface partitioning is taken into account, compared to when surface partitioning is neglected. As a consequence, the production of SOA through the aqueous-phase chemical processing of methylglyoxal is also enhanced, but to a lesser degree, because condensation of the hydroxyl radical from gas phase limits the production.