AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Does Aerosol Chemistry Affect Droplet Kinetics?
EMMANUEL FOFIE, Ashley Vizenor, Akua Asa-Awuku, University of California, Riverside
Abstract Number: 256 Working Group: Aerosols, Clouds, and Climate
Abstract Cloud droplet size influences particle scattering, the optical depth, and global cloud radiative forcing. In this study, we explore the droplet growth of simple inorganic salts (sodium chloride and ammonium sulfate) and organic acids such as succinic, malonic and oxalic acid. We also explore the droplet growth of activated complex aerosol from secondary organic aerosol (SOA) generated from seeded and un-seeded chamber experiments.
A DMT CCN counter with an enhanced resolution optical particle counter measures droplet sizes. The bin sizes are decreased two-fold and calibrated. The UC Riverside environmental chamber generated secondary organic aerosol from the ozonolysis of tricyclic sesquiterpenes and internally mixed, organic coated aerosols; dicarboxylic acids condense onto an (NH$_4)$_2 SO$_4 core.
Aerosol activate, form cloud condensation nuclei (CCN) and their droplet growth is characterized according to their mass accommodation coefficient and mixing states. A coupled analysis of laboratory CCN experiments and droplet growth modelling, Continuous Flow Streamwise Thermal Gradient CCN (CFSTGC) model, explores the effects of chemical composition on the droplet sizes when characterized by the hygroscopicity and the mass accommodation of water.
Results indicate that the final droplet sizes of activated inorganic salts and organic acids have different thermodynamic water uptake (hygroscopicity) but similar droplet kinetics. The hygroscopicities of pure and mixed compounds range from 0.1 to 1.2 and estimated mass accommodation coefficients can range over an order of magnitude (0.1 to 1.) Hence the observed kinetics of these compounds may not be divergent enough to result in a statistically significant difference in final droplet sizes. The more complex SOA ozonolysis show quantifiable differences in activated droplet sizes. The average hygroscopicity is ~0.1 and the estimated accommodation coefficient increases from 0.05 to 0.15.
We discuss reasons why the differences in aerosol systems can have such a vast influence on droplet sizes.