Mechanisms of Reactions of HO2 and RO2 Radicals

Abstract

The peroxy radicals are an important link in the reaction chain that develops ozone in the atmosphere through their reactions with NO. This chapter explores the kinetics and mechanisms of these RO2 reactions. In Chapters III and IV, the kinetics and mechanisms of the reactions of organic compounds with the major atmospheric oxidants [HO, NO3, and O3] were discussed. The organic radicals formed in these reactions add O2 to form organic peroxy radicals (RO2). The rate coefficient for these reaction is typically of the order of (10−12–10−11) cm3 molecule−1 s−1 under tropospheric conditions. One atmosphere (1 atm) of air contains 5 × 1018 molecule cm−3 of O2, and the lifetime of organic radicals with respect to addition of O2 to give peroxy radicals is 10–100 nanoseconds. Addition of O2 is essentially the sole atmospheric fate of the organic radicals formed during the oxidation of organic compounds. As examples, consider the HO-initiated oxidation of ethane and acetone (M is a third body, such as N2, which collisionally deactivates the nascent peroxy radical): . . . HO + CH3CH3 → CH3CH2 + H2O . . . . . . CH3CH2 + O2 + M → CH3CH2O2 + M . . . . . . HO + CH3C(O)CH3 → CH3C(O)CH2 + H2O . . . . . . CH3C(O)CH2 + O2 + M → CH3C(O)CH2O2 + M . . . Because of the rapidity and exclusivity of the O2 addition to alkyl radicals, the organic peroxy radicals (CH3CH2O2 and CH3C(O)CH2O2) can be thought of as the primary products of the initial oxidation step. HO2 radicals are formed in reactions of O2 with alkoxy radicals (e.g., CH3O) and by the association reaction of H atoms with O2: . . . CH3O + O2 → CH2O + HO2 . . . . . . H + O2 + M → HO2 + M . . . Peroxy radicals (HO2 and RO2) have a rich atmospheric chemistry and undergo reactions with NO, NO2, HO2, and other peroxy radicals (R′O2). Unimolecular isomerization is also an important fate for larger organic peroxy radicals where the peroxy radical can abstract a hydrogen atom from another part of the organic moiety (the peroxy radical bites its own tail). Reactions of peroxy radicals with NO3 radicals at night, and ClO and BrO radicals in maritime environments, can also be of importance on local scales.