Advised by Spyros N. Pandis
The ability of secondary organic aerosol (SOA) particles formed during the ozonolysis of alpha-pinene and other monoterpenes to act as cloud condensation nuclei (CCN) was investigated using a static CCN counter and a cylindrical continuous-flow streamwise thermal gradient CCN counter developed by Droplet Measurement Technologies (DMT). Secondary organic aerosol (SOA) was produced from the reaction of alpha-pinene and monoterpene mixtures (alpha-pinene, beta-pinene, limonene and 3-carene) with ozone in a 12 m3, temperature-controlled smog chamber. The initial monoterpene concentrations were in the 10-30 ppb range and an excess of ozone was used. The CCN concentration, activation diameter and droplet growth kinetic information were monitored as a function of supersaturation for several hours and their changes with age were quantified. Both fresh and aged monoterpene SOA are quite active as CCN. The initial concentrations of ozone and monoterpene precursor do not appear to affect the activity of the resulting SOA. However, reactions of the hydroxyl radicals produced during the monoterpene ozonolysis lead to further oxidation of the SOA material and an improvement of their CCN properties with time. The DMT CCN counter measured a decrease in CCN activation diameter for alpha-pinene SOA of approximately 3 nm hr-1 at 0.33% supersaturation. The particles were inferred to have a surface tension of 0.62 mN m-1, approximately a 70% soluble fraction, and no kinetic limitation to droplet growth. The activation diameters of alpha-pinene and mixed monoterpene SOA were consistent (within 10% or so) with the predictions of classical Kohler theory assuming that all the material was soluble in water.