The release of organic compounds from both anthropogenic and biogenic sources is known to be a key factor in complex atmospheric processes. Brown carbon (BrC) aerosols are primarily formed from atmospheric organic compounds and have only recently been identified as important climate forcers. The light-absorbing nitroaromatic chromophores embedded in the aerosol particulate are particularly responsible for its solar absorptivity from the visible to the UV, releasing reactive photofragments as a result. In this study we investigate the UV dissociation dynamics of two BrC chromophores – ortho-nitrophenol and nitroresorcinol – to nitric oxide (NO) detected using 1 + 1 resonance enhanced multiphoton ionization (REMPI) combined with velocity map imaging (VMI). Ion images collected from 226 and 355 nm photolysis of both parent compounds reveal that dissociation is slow compared to their period of rotation. The corresponding TKER distributions are broad and featureless, indicating modest energy partitioning to translation and subsequent high cofragment internal energy. Furthermore, product state distributions at both photolysis wavelengths report on contrasting dissociation mechanisms unique to ortho-nitrophenol and nitroresorcinol. Complimentary high-level calculations are carried out to obtain a molecular-level description of the fragmentation mechanisms. By gaining a better understanding of the dissociation dynamics of BrC chromophores, a more accurate modeling of their photochemical outcomes from solar radiation absorption will be obtained.

UV photodissociation dynamics of brown carbon chromophores: ortho-Nitrophenol and nitroresorcinol

2019 Spring National Meeting

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