Cloud condensation nuclei activity and droplet formation of primary and secondary organic aerosol mixtures

Understanding the mixing behavior of anthropogenic primary and biogenic secondary organic aerosol (POA and SOA) is important for characterizing their interactions with water vapor. The following work expands upon previous studies and investigates cloud condensation nuclei (CCN) activity and droplet kinetics of alpha-pinene SOA formed in an environmental chamber and mixed with diesel or motor oil-diesel fuel POA. The changes in the aerosol mixing are similar to previously published work but this study provides new CCN activity and droplet information. The CCN activity of the unmixed aerosol systems are measured separately; kappa = 0.15, 0.11, 0.022 for alpha-pinene SOA, diesel POA and motor oil-diesel fuel POA, respectively. In the alpha-pinene SOA + diesel POA mixture, the CCN activity, characterized by kappa-hygroscopicity, decreases from kappa = 0.15 to 0.06 after an initial injection of the POA but increases to kappa = 0.12. The increase in CCN activity occurs after particle collision (coagulation and wall-loss) rates dominate aerosol processes in the chamber. The alpha-pinene SOA + motor oil-fuel POA does not readily mix and the CCN activity of the complex system increases with time (from kappa = 0.022 to 0.10). An empirical equation using unit mass resolution (UMR) AMS data of two different ion fragments reasonably predicts CCN activity of the POA and SOA mixtures. CCN measurement may be a promising tool to gain additional insight into the complex mixtures of organic aerosol and subsequent interactions with water vapor.