Atmospheric nucleation and initial steps of particle growth: Numerical comparison of different theories and hypotheses

New particle formation in boreal forest environment in Hyytiälä, Finland, was studied in an aerosol dynamical model. Basing on the concepts of activation or kinetic collision of two pre-existing clusters at 1.5 nm, several semi-empirical nucleation rate formulae were parameterized. The mechanisms had linear or squared dependence on the concentration of sulphuric acid, or a low volatile organic vapor, or both, and they all showed good agreement with field measurements. A new method for examining the power dependence of apparent formation rate (at 2 nm or 3 nm) on sulphuric acid concentration was developed. The new method produced exponents 1.6-2.1 for cluster activation, and exponents 2.4-3.1 for kinetic collision, which suggests that the activation scenarios are in better agreement with experimental observations that imply exponents around 1-2. However, it was found that if low volatile organic vapors with concentrations exceeding that of sulphuric acid are present, they have a major role in shaping the temporal behavior of the apparent formation rates, causing error in the exponent analyses. Finally, a sensitivity study showed that the analyzed exponents grew even further, if the size of the critical cluster was assumed smaller that 1.5 nm.