On describing particle nucleation within the Volatility Basis Set

We describe atmospheric particle nucleation within the Volatility Basis Set (VBS) by identifying nucleating vapors (“nucleators”) with sufficiently high saturation ratios to drive nucleation under either neutral (termed nLVOC) or ion induced (cLVOC) conditions. These vapors are a subset of Ultra Low Volatility Organic Compounds (ULVOCs, with a saturation mass concentration below 3 × 10 −9 µg m −3 ), which mainly arise from the oxidation of monoterpenes and other volatile hydrocarbons in the atmosphere. We determine the effective nucleator concentrations via nucleation efficiencies based on critical saturation ratios for neutral and charged processes, and then apply these efficiencies to the overall volatility (concentration) distribution. The nucleator concentrations thus depend on the overall yield and volatility distribution of ULVOC species, as well as ambient temperature. Using organic vapor volatility distributions for α -pinene ozonolysis measured in the CERN CLOUD chamber, we can reproduce the experimental neutral and ion-induced nucleation rates between 223 and 298 K, over a wide range of ULVOC concentrations and nucleation rates, spanning typical atmospheric values. For this system of oxygenated organic molecules from α -pinene, two competing effects prevail. As temperature drops from 298 K, the slowing rate of autoxidation lowers the ULVOC yield and so initially reduces the nucleation rates. However, at about 263 K, the colder temperatures reduce the volatilities sufficiently for nucleation rates to reverse course and start to increase with further decrease in temperature. This effect is most pronounced for neutral nucleation. The CLOUD measurements show this behavior and it is faithfully reproduced in the VBS nucleation model.