Dependence between the photochemical age of light aromatic hydrocarbons and the carbon isotope ratios of atmospheric nitrophenols

Abstract

Concepts were developed to establish relationships between the stable carbon isotope ratios of nitrophenols in the atmosphere and the photochemical processing of their precursors, light aromatic volatile organic compounds. These concepts were based on the assumption that nitrophenols are formed dominantly from the photo-oxidation of aromatic volatile organic compounds (VOCs). 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 made between 2009 and 2012 in Toronto, Canada. The mechanistic model included the possibility of isotopic fractionation during intermediate steps, rather than only during the initial reaction step. This model takes kinetic isotope effects for the reaction of the precursor VOC with the hydroxyl radical and their respective rate constants into account, as well as carbon isotope ratio source signatures. While many of these values are known, there are some, such as the kinetic isotope effects of reactions of first- and second-generation products, which are unknown. These values were predicted in this study based on basic principles and published laboratory measurements of kinetic carbon isotope effects and were applied to the mechanistic model. Due to the uncertainty of the estimates based on general principles, three scenarios were used with different values for isotope effects 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 for the mechanistic model. The results also suggests that mass-balance-based models do not adequately describe the dependence between nitrophenol carbon isotope ratios and ∫[OH]dt.