AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Condensational Kinetics of Viscous Amorphous Organic Aerosol
NICHOLAS ROTHFUSS, Aleksandra Marsh, Grazia Rovelli, Markus Petters, Jonathan P. Reid, North Carolina State University
Abstract Number: 344 Working Group: Aerosol Chemistry
Abstract Accurate representation of aerosol hygroscopic growth is relevant to model the global radiation budget, precipitation processes, and visibility. As viscosity modulates bulk diffusion, it has been suggested that at high viscosities the penetration of water molecules into the particle bulk will be retarded, resulting in kinetic limitations to hygroscopic growth. Condensational kinetics for a series of micron-sized sugar solution droplets (sucrose, glucose, raffinose, trehalose) were investigated at temperatures between -7.5 °C and 20 °C using a comparative kinetics measurement applied in a cylindrical electrodynamic balance (EDB). Near-instantaneous relative humidity (RH) switching was achieved by switching gas flows (~20-80% RH) between the top and the bottom of the EDB chamber. During the RH switching the confined particle viscosity changed from a value characteristic of high equilibrium solution viscosity (104 Pa∙s to ≥ 1012 Pa∙s) to an RH characteristic of low equilibrium solution viscosity (≤ 100 Pa∙s). Similar experiments were performed for aqueous sodium nitrate and tetraethylene glycol, which are less viscous than the sugar solutions (≤100 Pa∙s at 20% RH). Condensational timescales were derived from fitting the collected time-dependent measurements of droplet radius to a Kohlrausch-Williams-Watts-style equation. Typical timescales for the sugars were ~1 order of magnitude slower than timescales predicted using a diffusional growth model that accounts only for mass and heat flux to and from the particle surface, suggesting that low particle viscosity retarded the hygroscopic growth. However, these timescales were also fast enough relative to atmospheric timescales to argue that at high RH moderately hygroscopic particles will plasticize from water uptake sufficiently quickly such that kinetic limitations arising from high viscosity do not affect warm-cloud microphysical processes.