Chemical reactions in atmosphere occur in gas phase, as well as in and on aerosol droplets. It becomes widely accepted that the aerosols serve as chemical processing units for absorbed and adsorbed species. However, computational modeling of chemical and photochemical reactions in condensed phase and on surfaces is challenging due to high computational cost and immense complexity of underlying mechanisms. In this work QM/EFP methods are applied to characterize UV/Vis spectroscopy of molecules absorbed and adsorbed to water droplets. The Effective Fragment Potential (EFP) method is a fragmentation technique that can be applied to describe the 'surrounding' effects of a given molecule using a QM/EFP scheme. The EFP energy consists of electrostatics, polarization, dispersion, exchange-repulsion and charge transfer terms. EFP utilizes an excellent description of electrostatic and polarization terms, with distributed multipoles up to octupoles and anisotropic polarizability tensors, ensuring rigorous description of interactions between the solute's electronic states and the environment.Polycyclic aromatic hydrocarbons (PAHs) are air pollutants infamously known for their mutagenic and carcinogenic properties. The excitation spectrum of a PAH anthracene has been modeled in bulk water and at the air-water interface, in order to understand how different water environments affect electronic bands in terms of average shift, intensity, and broadening. A variety of excited state methods, EOM-CCSD, TD-DFT, and CIS(D) are used to describe electronic excited states of anthracene, while water molecules are represented by EFP fragments. A similar QM/EFP approach is applied to analyze excitation spectrum of OH radical in and on water droplets. Developed QM/EFP techniques and user-friendly scripts open up a possibility of rigorous black-box modeling of photochemistry in atmosphere.

QM/EFP for atmospheric chemistry: Modeling electronic spectroscopy in and on aerosol particles

248th National Meeting (2014)

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