Final Report Summary - MOCAPAF (Role of Molecular Clusters in Atmospheric Particle Formation) Atmospheric aerosol particles are known to affect the global climate both directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei. Significant fractions of the aerosol particles, also those of them that act as cloud condensation nuclei, originate from clustering of gaseous molecules. We have developed a comprehensive model for clustering of condensable trace gases in the atmosphere. The model is based on first principles quantum mechanical description of the clusters. The model reproduces recent experimental mass spectrometric data well. Mass spectrometry techniques can detect only charged clusters, and our model can be used to link measured data to properties of undetectable neutral clusters. We have used the model to analyze experimental data and understand the operation of chemical ionization mass spectrometers used to measure trace gas concentrations. We have assessed the feasibility of suggested atmospheric particle formation processes involving sulphuric acid, ammonia, dimethylamine, water and various oxidized organic molecules. Fundamental tests performed using our model point out that some of the concepts widely used in atmospheric particle formation, such as the critical cluster size, may be irrelevant or misleading. We have performed novel first principles molecular dynamic simulations required to understand the detailed dynamics of the collisions leading to cluster formation and growth.
