Browsing by Subject "computational study of atmospheric molecular clusters"

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  • Kubečka, Jakub (Helsingin yliopisto, 2021)
    A suspension of fine solid particles and liquid droplets in the air is called an aerosol. Atmospheric aerosols play an important role in climate and also affect human health. Some of these aerosols are formed in the atmosphere by collisions of gas molecules with favorable interactions. The agglomerations of molecules formed in this process are referred to as molecular clusters. Unstable molecular clusters usually break apart quickly. In contrast, stable molecular clusters may become the nuclei of subsequent growth by condensation of other vapor molecules, and eventually form new atmospheric fine particles (this process is called new particle formation = NPF). This process is typically accompanied by a nucleation barrier, which has to be surmounted to form the new particle. It is essential to understand and accurately describe the molecular mechanism behind this process as our current understand- ing of NPF is incomplete, leading to significant uncertainties when it comes to forecasting NPF-related phenomena (e.g., mists, clouds). I utilize computational quantum chemistry (QC) to evaluate the stability of molecular clusters, which determines their decomposition rates. The surmounting of the (free) energy nucleation barrier is about a probabilistic competition between cluster evaporation and cluster growth due to the collision with other condensable molecules in the air. The collision rate can be approximately calculated from kinetic gas theory. The evaporation rate can then be calculated using the detailed balance equation, which, however, requires thermodynamic calculations using computationally demanding QC methods. Moreover, to calculate thermodynamic properties of a molecular cluster, the cluster structure has to be known beforehand. The main focus of this thesis is studying molecular cluster structures/configurations and searching for those configurations that can be most probably found in the atmospheric air. The process of searching for various configurations is known as configurational sampling. I discuss methods of configurational sampling and suggest an approach for configurational sampling of atmospherically relevant molecular clusters. The core of the research results shown in this work are applications of the configurational sampling protocol, and the Jammy Key for Configurational Sampling (JKCS) program, which was developed over the course of my Ph.D. studies.