Dependence between the Photochemical Age of Light Aromatic Hydrocarbons and the Carbon Isotope Ratios of Atmospheric Nitrophenols

Concepts were developed to establish relationships between the stable carbon isotope ratios of nitrophenols in the atmosphere and photochemical processing of their precursors, light aromatic volatile organic compounds. The concepts are based on the assumption that nitrophenols are formed dominantly from the photo-oxidation of aromatic VOC. A mass balance model as well as various scenarios based on the proposed mechanism of nitrophenol formation were formulated and applied to derive the time integrated exposure of the precursors to processing by OH-radicals (∫[OH]dt) from ambient observations taken between 2009 and 2012 in Toronto, Canada. The mechanistic model included the possibility of isotopic fractionation during intermediate steps, rather than during the initial reaction step alone. This model included knowledge of kinetic isotope effects of the precursor VOC with the hydroxyl radical and their respective rate constants, as well as isotope ratio source signatures. While many of these values are known, there were some, such as the kinetic isotope effects of reactions of the intermediate compounds, which were unknown. These values were predicted based on basic principles and published laboratory measurements of normal kinetic carbon isotope effects and were applied to the mechanistic models. Due to uncertainty of the estimates based on general principles three scenarios were used with different values for isotope effect that were not known from laboratory studies. Comparison of the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt with published results of laboratory studies and ambient observations was used to narrow the range of plausible scenarios of the mechanistic model and to eliminate the mass balance based model as useful formulation of a the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt.