Can Organic Coatings Limit Water Uptake by Aerosol Particles?

Dynamic water uptake by aerosol is a key factor in particle growth and cloud droplet formation. Organic particles with an oxygen-to-carbon ratio not exceeding 0.8 readily undergo liquid-liquid phase separation forming particles consisting of an organic-rich outer and an inorganic rich inner phase with either a partially engulfed or a core shell morphology (Song et. al.,Faraday Discuss.,165, 289-316, 2013). The latter is observed prevalently if the organic phase consists of small molecules such as α-pinene oxidation products (O'Brien et. al.,Environ. Sci.Tech., 49, 4995-5002, 2015). Reductions in droplet surface tension due to an organic-rich outer layer can facilitate cloud activation (Ovadnevaite et al.,Nature, 546, 637-641, 2017), but mass transfer limitations of water vapor over timescales of cloud droplet formation can be a limiting factor in kinetic growth of particles (Chuang et al.,Nature, 390, 594-596, 1997). To explore the latter issue, we estimate viscosity and diffusion coefficients for binary mixtures of water and α-pinene oxidation products in a concentration range representative of the outer phase using molecular dynamics simulation. Even though calculated viscosities indicate that our selected systems always exhibit liquid-phase character, the magnitude of bulk-phase diffusion resistance can be comparable or even exceed resistance to interfacial mass transfer when organic-rich molecules break the hydrogen bonding network of water. We extend our analysis by estimating the thickness at which the organic layer becomes an important factor, and relate obtained resistance values to refined molecular structure and oxygen-to-carbon ratios.