he effect of hydrophobic glassy organic material on the cloud condensation nuclei activity of internally mixed particles with different particle morphologies

Description

Particles composed of organic and inorganic components can assume core-shell morphologies. Previous studies have shown that hydrophobic organic shells can slow water vapor diffusion into the particle core. The kinetic limitation of water uptake may increase the critical supersaturation required to activate such particles into cloud droplets. Here we test this hypothesis through laboratory experiments. Polyethylene was used as proxy for a hydrophobic glassy organic material. Temperature dependent viscosity and cloud condensation nuclei (CCN) activity of the polyethylene were measured. Results show that the viscosity of 50 nm polyethylene particles is 5×106 Pa·s at ~61°C. Extrapolation of the temperature dependent viscosity indicates that the particles are glassy at room temperature. Polyethylene particles were inactive as CCN at diameters less than 300 nm at 1% water supersaturation. Ammonium sulfate was used as proxy for CCN active inorganic material. Internally mixed particles were generated using coagulation of oppositely charged particles; charge-neutral polyethylene-ammonium sulfate dimers were then isolated for online observation. Morphology of these dimers was then varied by partially or completely melting the polyethylene such that the liquefied polyethylene partially or completely engulfed the ammonium sulfate. Critical supersaturation was measured as a function of dry particle volume, particle morphology, and organic volume fraction. Our results show that kinetic limitations do not change the critical supersaturation for particles composed of equal volumes of polyethylene and ammonium sulfate. A slight increase in critical supersaturation is observed for particles composed predominately of polyethylene when shifting from the dimer morphology to the core-shell morphology. Water transfer will be less hindered in hydrophilic glassy organic materials due to the plasticizing effect of water on dissolving organic compounds. Based on extensive evidence from literature data, hydrophobic glassy organic materials are likely not prevalent in the atmosphere. Furthermore, timescales of humidification are shorter in CCN instruments than in atmospheric updrafts. Therefore, our experiments suggest that, near laboratory temperatures, mass transfer limitation by glassy organic shells is unlikely to affect cloud droplet activation.