Nucleation and the Volatility Basis Set

New particle formation occurs broadly via two processes. First, small clusters can be stabilized by a reaction between constituents, such as proton transfer in acid-base clusters like sulfuric acid and ammonia. Second, constituents can simply be sufficiently sticky to remain clustered until more constituents arrive to make the cluster grow. This is classical nucleation and it depends on high vapor saturation ratios (supersaturation). It can occur for argon under the right conditions, but in the atmosphere nucleation involving organics is most interesting. Organics add the feature that they are an incredibly rich mixture of constituents, generally highly oxygenated, each with many oxygenated functional groups. We can describe this rich mixture in terms of volatility using the volatility basis set (VBS), and it has been established that nucleation appears to be second order with respect to the concentration of the least volatile class in the VBS, the so-called Ultra Low Volatility Organic Compounds (ULVOCs). Here we present an analysis of the overall volatility distribution to determine the fraction of ULVOCs that govern nucleation in both neutral and ion-induced nucleation. This depends on overall saturation ratios as well as the overall volatility distribution. The framework successfully describes the temperature and concentration dependence of both neutral and ion-induced nucleation for the canonical alpha-pinene + ozone system measured at the CERN CLOUD experiment between 223 and 298 K. Ultimately there are two competing effects: volatility drops as temperature drops, increasing saturation ratios, but the peroxy radical autoxidation chemistry that creates the highly functionalized ULVOCs accelerates as temperature increases, increasing saturation ratios in the opposite sense. Both theory and observations show a minimum in nucleation rates between 263 and 278 K, with higher rates to either side.