AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
The Role of Temperature in Cloud Droplet Activation
Sara Christensen, MARKUS PETTERS, Paul Ziemann, Sonia Kreidenweis, North Carolina State University
Abstract Number: 409 Working Group: Aerosols, Clouds, and Climate
Abstract 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 measure its potential dependence 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 ozone 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 degree to 0 degree C over a period of approximately six hours. Activation data were collected by alternating diameter scans between organic aerosol and ammonium sulfate aerosol for calibration. The organic aerosol kappa values remained constant or decreased as temperature decreased. These observations can be explained by a weak dependence of water activity on temperature and a moderate dependence of solubility in water on temperature. As a case study, we demonstrate that Köhler theory valid for sparingly soluble compounds can quantitatively explain the temperature dependent activation properties of adipic acid aerosol. 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.