The effect of hydrophobic glassy organic material on the cloud condensation nuclei activity of particles with different morphologies

Abstract

Abstract. Particles composed of organic and inorganic components can assume core-shell morphologies. The kinetic limitation of water uptake due to the presence of a hydrophobic viscous outer shell may increase the critical supersaturation required to activate such particles into cloud droplets. Here we test this hypothesis through laboratory experiments. Results show that the viscosity of polyethylene particles is 5×106 Pa s at 60 ∘C. Extrapolation of temperature dependent viscosity measurements suggests that the particles are glassy at room temperature. Cloud condensation nuclei (CCN) activity measurements demonstrate that pure polyethylene particles are CCN inactive at diameters less than 741 nm and 2.5 % water supersaturation. Thus, polyethylene is used as proxy for hydrophobic glassy organic material. Ammonium sulfate is used as proxy for hygroscopic CCN active inorganic material. Mixed particles were generated using coagulation of oppositely charged particles; charge-neutral polyethylene–ammonium sulfate dimer particles were then isolated for online observation. Morphology of these dimer particles was varied by heating, such that 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. The data show that kinetic limitations do not change the critical supersaturation of 50 nm ammonium sulfate cores coated with polyethylene and polyethylene volume fractions up to 97 %. Based on these results, and a synthesis of literature data, it is suggested that mass transfer limitations by glassy organic shells are unlikely to affect cloud droplet activation near laboratory temperatures.

Predicting the influence of particle size on the glass transition temperature and viscosity of secondary organic material

“…When the organic phase is viscous, gas-particle equilibration time scales of semi-volatile organic compounds can become long ( ), thus necessitating kinetic instead of thermodynamic frameworks to model secondary organic aerosol formation 6 – 9 . Measured equilibration time scales for water in viscous and glassy solids are fast at warmer temperatures ( ) 10 – 13 . In turn, viscosity exerts little influence on atmospheric processes involving water uptake.…”

“…The relationships between viscosity and diffusion-coefficients, to first order predicted by the Stokes–Einstein equation, have been clarified. The Stokes–Einstein equation holds for predicting the diffusion of larger organic molecules, but errs for small molecules such as water 2 , 9 – 11 , 13 , 47 , 48 . First estimates of the prevalence of gassy organic states in the atmosphere have been made 19 , 32 , 49 .…”

Toward closure between predicted and observed particle viscosity over a wide range temperature and relative humidity

“…The DCIC technique has been described extensively in prior publications (Champion et al, 2019;Marsh et al, 2018;Petters, 2016, 2017a;Rothfuss et al, 2019;Petters, 2018;Tandon et al, 2019;Petters et al, 2019). The basic concept is briefly introduced and then modifications made to enable low temperature measurements will be described.…”

“…The shape parameter was converted to viscosity using the Frenkel sintering theory (Pokluda et al, 1997;Rothfuss and Petters, 2016). Example logistic curves including data for sucrose and citric acid aerosol are given in prior work Petters, 2016, 2017a;Marsh et al, 2018;Rothfuss et al, 2019;Tandon et al, 2019;Petters et al, 2019) and the supporting information.…”

“…Dimer particles are composed of a polyethylene monomer and either a sucrose or citric acid monomer particle. Polyethylene particles were generated using an evaporation-condensation system that is described in Tandon et al (2019) and Rothfuss et al (2019). Citric acid (≥99.5%, Sigma Aldrich, St. Louis, MO, USA) and sucrose (≥99.5%, Sigma Aldrich, St. Louis, MO, USA) were dissolved in HPLC grade water (5 g of solute in 100 ml of HPLC Grade submicron filtered water, Fischer Scientific, USA) and atomized using a constant output atomizer (TSI 3076).…”