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

• Published in: Atmospheric chemistry and physics Vol. 19; no. 5; pp. 3325 - 3339
• Main Authors: Tandon, Ankit, Rothfuss, Nicholas E, Petters, Markus D
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 14-03-2019; Copernicus Publications, EGU; Copernicus Publications
• Subjects: Ammonium; Ammonium compounds; Ammonium salts; Ammonium sulfate; Analysis; Animal morphology; Atmospheric chemistry; Charged particles; Cloud condensation nuclei; Cloud condensation nuclei activity; Cloud droplets; Clouds; Clouds (Meteorology); Coagulation; Condensates; Condensation; Condensation nuclei; Dimers; Droplets; Environmental aspects; ENVIRONMENTAL SCIENCES; Environmental Sciences & Ecology; Heating; Hydrophobicity; Laboratories; Laboratory experiments; Mass transfer; Meteorology & Atmospheric Sciences; Morphology; Nucleus; Polyethylene; Polyethylenes; Properties; Proxy; Sulfates; Supersaturation; Temperature; Temperature dependence; Uptake; Viscosity; Water uptake
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-19-3325-2019

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.