The role of temperature in cloud droplet activation

 

Authors

Sara Irene Christensen — North Carolina State University
Markus D Petters — North Carolina State University
Paul Ziemann — University of California
Sonia Kreidenweis — Colorado State University

Category

Aerosol Properties

Description

Organic aerosols in the atmosphere are composed of a wide variety of species, reflecting the multitude of sources and growth processes of these particles. Especially challenging is predicting how these particles may act as cloud condensation nuclei (CCN). Köhler theory relates the particle’s dry diameter to its critical supersaturation. A hygroscopicity parameter, kappa, parameterizes this relationship in terms of the particle’s chemical composition. Previous studies have characterized kappa values for pure and mixed compounds at laboratory temperatures, but did not account for a potential dependence of the relationship between critical supersaturation and particle dry diameter on temperature. Here we characterize the temperature dependence of kappa derived from CCN measurements for a variety of organic compounds in the laboratory. Single-compound organic aerosol and secondary organic aerosol from the reaction of alpha-pinene and O3 were generated in the laboratory, and activation properties were analyzed using size-resolved CCN analysis. The CCN instrument was placed inside a temperature-controlled enclosure. During the experiment the temperature inside the enclosure was ramped down from approximately 40° to 0°C over a period of approximately six hours. Size-selected sample aerosol was brought into the enclosure and was allowed to thermally equilibrate before entering the CCN instrument. Between the CCN instrument inlet and the point of activation, the sample was warmed again by ~3–4°C, which was necessary to establish the supersaturation inside the instrument. Activation data were collected by alternating diameter scans between organic aerosol and ammonium sulfate aerosol for calibration. Three possible outcomes were observed as temperature decreased: activation diameters of the organic compounds increased, remained constant, or decreased relative to the activation diameters of ammonium sulfate. The latter two observations can be explained by a weak dependence of water activity on temperature and a moderate dependence of solubility in water on temperature. We anticipate that our results will help to guide input and parameterization choices in models that rely on theory to extrapolate laboratory and ambient data to temperatures that are different than those during data collection. The objective is to better constrain droplet activation in mid- and high-level clouds and to ultimately decrease the uncertainty surrounding the aerosol indirect effect.