Abstract View
Assessing the Role of Surface Tension Depression in Sesquiterpene Organic Aerosol CCN Activity
BENJAMIN SCHULZE, Ryan Ward, Yuanlong Huang, Andreas Zuend, John Seinfeld, California Institute of Technology
Abstract Number: 299
Working Group: Aerosols, Clouds and Climate
Abstract
Numerous studies suggest that low-solubility, surface-active organic species can depress the surface tension of activating cloud droplets. Detailed understanding of this process is needed to accurately represent CCN activation, as particle critical supersaturation depends on surface tension variation during droplet growth. Sesquiterpene oxidation products, which constitute a particularly hydrophobic class of biogenic SOA, may reduce surface tension when present in activated particles. However, the existence of such surface tension effects, as well as their dependence on factors such as dry particle size, chemical composition, and the presence/absence of inorganic solutes, is largely unknown. Here, we evaluate experimentally the CCN activity of mixed β-caryophyllene SOA-ammonium sulfate (SOA-AS) particles of varying size, organic mass fraction, and oxidative age generated in the Caltech Photooxidation Flow Tube reactor (CPOT). We use multiple droplet activation models, including the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model and its gas–particle equilibrium variants to interpret the observations. These analyses permit evaluation of the extent to which simplified parameterizations of CCN activity, such as the κ-Kohler model, reasonably describe sesquiterpene SOA activation across the broad range of particle sizes, oxidative ages, and organic:inorganic mass ratios found in the atmosphere. Simultaneous measurements of aerosol composition using an Aerodyne AMS and filter-based thermal-desorption CIMS allow linkage of CCN properties to changes in the abundance of specific organic species. Using an aerosol cloud parcel model, we evaluate changes in cloud droplet number concentrations produced as a result of different assumptions regarding the SOA-AS CCN activation properties. Our results provide insight into the physicochemical detail needed to accurately represent biogenic SOA CCN activation in regional and global models.