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In-Silico Investigation of Electron Ionization Mass Spectra of Nitro-Heterocyclic Chromophores in Brown Carbon Aerosols
MEGAN WOODS, Kunpeng Chen, Nilofar Raeofy, Roya Bahreini, Ying-Hsuan Lin, University of California, Riverside
Abstract Number: 514
Working Group: Carbonaceous Aerosol
Abstract
Brown Carbon (BrC) aerosol’s ability to absorb solar radiation has emerged as an uncertain factor in climate forcing. Heterocycles are abundant components in biomass burning emissions; however, the optical properties of their resultant secondary organic aerosol remain unclear. These uncertainties stem from an incomplete characterization of BrC chromophores. Mass spectrometry-based analytical measurements have been widely used to speciate BrC aerosol constituents. Nevertheless, due to the large variety and chemical complexity of BrC aerosols, there is a lack of authentic standards readily available to aid in the identification of these compounds. Utilizing Quantum Chemical Electron Ionization Mass Spectra (QCEIMS) to simulate a theoretical EI mass spectrum may facilitate overcoming this barrier. In this study, we examined the performance of varying Density Functional Theory (DFT) computational methods (e.g., PBE and B97D) as well as the new standalone XTB-2 method in QCIEMS to determine a method of high accuracy and computational efficiency. We assessed the performance of these computational methods on nitro-heterocyclic compounds, including 2-nitrofuran, 2-nitrothiophene, and 2-nitropyrrole. In preliminary work, the accuracy of the computational chemistry functional methods was evaluated using Stein/Scott matching method to compare the theoretical spectra to National Institute of Standards and Technology (NIST) EI standards. The gradual gradient approximation (GGA) DFT methods PBE, B97D, and the XTB-2 method on 2-nitrofuran showed matching scores of 679, 591, and 681, respectively, while their required computational times were 192, 36, and 2 hours, respectively. Our results indicate that XTB-2 method may be used to speciate molecular structures of BrC chromophores when authentic standards are not readily available. Together with experimental characterization of BrC constituents obtained from aerosol mass spectrometry (AMS), this technique may be used in conjunction with positive matrix factorization (PMF) to develop comprehensive source profiles of complex aerosols.