The formation of atmospheric cloud droplets due to secondary organic aerosol (SOA) particles is important for quantifying the Earth’s radiative balance under future, possibly warmer, climates, yet is only poorly understood. While cloud activation may be parametrized using the surface tension depression that coincides with surfactant partitioning to the gas–droplet interface, the extent to which cloud activation is influenced by both the chemical structure and reactivity of the individual molecules comprising this surfactant pool is largely unknown. We report herein considerable differences in the surface tension depression of aqueous pendant droplets that contain synthetically prepared ozonolysis products derived from α-pinene and β-caryophyllene, the most abundant of the monoterpenes and sesquiterpenes, respectively, that are emitted over the planet’s vast forest ecosystems. Oxidation products derived from β-caryophyllene were found to exhibit significantly higher surface activity than those prepared from α-pinene, with the critical supersaturation requiredfor cloud droplet activation reduced by 50% for β-caryophyllenealdehyde at 1 mM. These considerable reductions in the critical supersaturationwere found to coincide with free energies of adsorption that exceed∼25 kJ/mol, or just one hydrogen bond equivalent, dependingon the ammonium sulfate and oxidation product concentration in thesolution. Additional experiments showed that aldehyde-containing oxidationproducts exist in equilibrium with hydrated forms in aqueous solution,which may modulate their bulk solubility and surface activity. Equilibrationtime scales on the order of 10–5 to 10–4 s calculated for micrometer-sized aerosol particles indicate instantaneoussurface tension depression in the activation processes leading tocloud formation in the atmosphere. Our findings highlight the underlyingimportance of molecular structure and reactivity when consideringcloud condensation activity in the presence of SOA particles.
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