AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Size-dependence of Aerosol Hygroscopicity Parameters at Sub- and Super-saturation
ANDREAS ZUEND, Natasha Hodas, John Seinfeld, McGill University
Abstract Number: 180 Working Group: Aerosol Chemistry
Abstract The equilibrium response in the size change of a hygroscopic aerosol particle to a change in ambient relative humidity is determined by the chemical composition of the particle and the thermodynamics of gas-particle partitioning and condensed phase mixing. At high relative humidity, the diameter growth factor is usually dominated by the uptake/loss of water, which is commonly represented in atmospheric models by a simplified hygroscopicity parameterization, such as the κ-Köhler model. The single-parameter κ-Köhler model is appropriate in many cases and experimental values for κ are frequently reported for single components as well as for the characterization of mixtures from laboratory and field studies. However, in the case of aerosol mixtures containing sparingly water-soluble organics, which only contribute to the water uptake at or near water-supersaturated conditions, κ values determined at lower relative humidities will be inappropriate for the description of particle size at supersaturation and the prediction of cloud condensation nuclei activity. This is a key contributor to an observed contrast in κ parameters determined at sub-saturation vs. CCN activation conditions for certain aerosol systems.
In this presentation, we show how state-of-the-art thermodynamic equilibrium models provide insight into the behavior of diameter growth factors and related hygroscopicity parameters at sub- and supersaturated conditions. Furthermore, in applications of κ-Köhler theory for CCN activation, it is oftentimes overlooked that κCCN hygroscopicity parameters are size-dependent quantities, particularly so for ultrafine aerosol. We present a case study of this size dependence based on thermodynamic model predictions accounting for size and non-ideal mixing effects. In principle, such model predictions allow for an appropriate choice of κCCN values in applications of κ-Köhler theory.