Kinetics and near-infrared spectroscopy of chlorine-substituted peroxy radicals

Abstract

Peroxy radicals form in the atm. oxidn. of nearly every volatile org. compd. (VOC). Their competing fates propagate or terminate free radical chem. in the troposphere. Atomic chlorine is a minor tropospheric oxidant, found in marine and coastal regions, which reacts with volatile alkenes to yield chlorine-substituted peroxy radicals. In this work, we present the near-IR absorption spectra of peroxy radicals formed in the chorine-initiated oxidn. of a variety of VOCs including isoprene, the most important nonmethane hydrocarbon emitted into the troposphere. We use cavity ringdown spectroscopy to observe the weak A-X electronic transition of peroxy radicals in the near-IR region. By monitoring the temporal decay of absorption features under varying radical environments, we report rate consts. for the self reaction of chlorine-substituted peroxy radicals as well as their dominant atm. fates: reaction with HO₂ and NO. Reaction with HO₂ is important in pristine environments while reaction with NO dominates in polluted regions. We compare our results to those available for hydroxyl-substituted and alkyl peroxy radicals to elucidate the effect of chlorine substitution on peroxy radical reactivity. To our knowledge, these results report the first near-IR absorption spectrum of a peroxy radical derived from isoprene oxidn. The real-time kinetic measurements of the reaction with HO₂ and NO provide insight into the highly atmospherically relevant isoprene system.

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