AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Droplet Growth Kinetics from Scanning Flow CCN Analysis Data Using an Instrument Model
Tomi Raatikainen, Terry Lathem, Jack Lin, Richard Moore, ATHANASIOS NENES, Georgia Institute of Technology
Abstract Number: 200 Working Group: Instrumentation and Methods
Abstract Cloud droplet activation, and especially subsequent droplet growth, are kinetically limited by the transport of water vapor; first to the droplet surface, and then to the bulk solution. Typical inorganic salts dissociate rapidly and have a negligible effect on the vapor-liquid interface, thus droplet growth depends mainly on bulk hygroscopicity and ambient water vapor concentration. Organics species however may further slow down water vapor condensation by forming compressed surface films or exhibiting slow dissolution kinetics (promoted perhaps by glassy or highly viscous organic states). These effects are often described by an effective water vapor uptake coefficient. The information about ambient water vapor uptake coefficients is highly limited, but it has been suggested that kinetic limitations may exist in dry and cool environments.
Measurements of the water vapor uptake coefficient can be inferred from Cloud Condensation Nuclei (CCN) counters that also measure the size of activated droplets; however, the technique has proven to be challenging and requires a fully coupled instrument model. The size of activated droplets is not only dependent upon the water vapor uptake coefficient, but also the instrument operation conditions, which include column temperature, particle hygroscopicity and size distribution, and water vapor depletion effects. The influence of each of these parameters on droplet size must be carefully understood with a CCN droplet growth model in order to accurately infer the water uptake coefficient. We have developed and tested a droplet growth model for the Droplet Measurement Technologies CCN counter operated in constant flow mode (Raatikainen et al., ACPD, 2012). The model is being updated to include growth kinetics for Scanning Flow CCN Analysis (SFCA) (Moore and Nenes, Aerosol Sci. Technol., 2009), where supersaturations are scanned rapidly by changing flow rates. The updated model can also be used to predict instrument and supersaturation responses to changes in operation parameters.