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This study investigates the droplet formation characteristics ofsecondary organic aerosol (SOA) formed during the ozonolysis ofsesquiterpene β-caryophyllene (with and without hydroxylradicals present). Emphasis is placed on understanding the role ofsemi-volatile material on Cloud Condensation Nucleus (CCN) activityand droplet growth kinetics. Aging of β-caryophyllene SOAsignificantly affects all CCN-relevant properties measuredthroughout the experiments. Using a thermodenuder and two CCNinstruments, we find that CCN activity is a strong function oftemperature (activation diameter at ~0.6% supersaturation: 100±10 nm at20°C and 130±10 nm at 35°C), suggesting that thehygroscopic fraction of the SOA is volatile. The water-solubleorganic carbon (WSOC) is extracted from the SOA and characterizedwith Köhler Theory Analysis (KTA); the results suggest that the WSOCis composed of low molecular weight (<200 g mol<sup>−1</sup>) slightlysurface-active material that constitute 5–15% of the SOA mass.These properties are similar to the water-soluble fraction ofmonoterpene SOA, suggesting that predictive understanding of SOA CCNactivity requires knowledge of the WSOC fraction but not its exactspeciation. Droplet growth kinetics of the CCN are found to bestrongly anticorrelated with WSOC fraction, suggesting that theinsoluble material in the SOA forms a kinetic barrier that delaysdroplet growth. Overall, volatilization effects can increase activationdiameters by 30%, and depress droplet growth rate by a factor of two;these results may have important implications for the droplet formation characteristics of SOA,and the atmospheric relevance of CCN measurements carried out at temperatures differentfrom ambient

Relating CCN activity, volatility, and droplet growth kinetics of β-caryophyllene secondary organic aerosol