From Cluster Properties to Concentrations and from Concentrations to Cluster Properties

A large fraction of atmospheric aerosol particles are formed from condensable vapors in the air. This particle formation process has been observed to correlate in many locations with the sulfuric acid concentration, but the very first steps of cluster formation have remained beyond the reach of experimental investigation until recently. Charged clusters can now be detected and characterized starting from the smallest sizes and even neutral clusters consisting of only a few molecules can be detected, although their composition cannot be fully characterized. However, measuring the concentrations of different cluster types does not tell the full story of how the clusters were formed, and detailed simulations are needed in order to get a full understanding of the cluster formation pathways. Cluster formation is described by a set of nonlinear differential equations that cannot be solved analytically in any realistic situation. The best way to understand the complex behavior of cluster populations is by cluster kinetics simulations. The focus of this Thesis is on developing tools for simulating cluster formation, and using the simulation results to improve the detailed understanding of atmospheric aerosol particle formation. As sulfuric acid has been identified as the main driving force of cluster formation in many locations, it is also the main compound in the simulations of this Thesis. It cannot explain the observed atmospheric particle formation rates alone, and other possible participating species considered in this Thesis are ammonia, dimethylamine and water. In the first two papers of the Thesis, theoretical values are used for the collision and evaporation rates, and simulated cluster concentrations and formation rates are compared to experimental observations. The simulation results agree well with experimental findings from two very different studies. The third and fourth paper asses existing methods for interpreting cluster measurements and point out details that should be taken into account: the effect of dipole moments on chemical ionization of neutral molecules and clusters, and the conditions for the widely used nucleation theorem to be valid. The last paper introduces a new method for extracting cluster evaporation rates from measured cluster distributions.