Abstract View
Temperature Dependent Entropy Driven Water Uptake in Phase Separated Aerosol Particles
MARIA LBADAOUI-DARVAS, Satoshi Takahama, Athanasios Nenes, LAPI, EPFL (Switzerland)
Abstract Number: 354
Working Group: Aerosols, Clouds and Climate
Abstract
Cloud droplets activation and growth are related to water uptake by aerosol particles. Interfacial mass transfer— that governs water uptake if mean free path is comparable to particle size — is quantified by the mass accommodation coefficient (α), probability that a molecule hitting the surface of a particle is absorbed. Mass accommodation coefficients below 0.1 lead to increased supersaturation development and larger droplet number in clouds – with important implications for aerosol-cloud interactions. Reduced α values are observed if a hydrophobic film covers the particle; and are unlikely to be representative of global values. In ambient particles, especially at high humidity, surface tension reductions can still have a significant impact on droplet formation. Liquid-liquid phase separation (LLPS), which occurs in organic aerosol if the O:C ratio is below ~0.8, yields particles having complex organic coating. LLPS is suspected to contribute to the discrepancies between measured a modeled CCN activities, but its exact role is to date unclear.
We perform steered molecular dynamics simulations to reconstruct the free energy profile of water uptake across a vapor/hydroxy cis-pinonic acid/water double interface at 300 K and 200 K. Free energy profiles are used to estimate a range of effective accommodation coefficients. Near-unity α values are observed for the lower temperature, whereas at room temperature 0.05<α<0.38, which can impart a mild direct effect on supersaturation and cloud droplet development. The free energy minimum near the organic/water interface — which originates from locally increased conformational entropy from local mixing—at 300 K results in a water concentration gradient in the organic phase, which helps to maintain LLPS and low surface tension at very high RHs. The effect diminishes at 200 K. These results highlight the need to consider a temperature dependent parametrization of α in parcel model simulations.