The atmospheric chemistry of alkoxy radicals determines the first-generation oxidation products of organic compounds in the atmosphere. There are three competing fates for alkoxy radicals: reaction with molecular oxygen (O2), isomerization, and decomposition (Atkinson and Arey, 2003b; Devolder, 2003; Orlando et al., 2003b; Calvert et al., 2008). Reaction with O2 preserves the carbon chain of the parent alkane and results in the production of a carbonyl compound and HO2. Unimolecular decomposition usually results in the formation of an alkyl radical and a carbonyl compound with a shortening of the carbon chain. Unimolecular isomerization usually leads to multifunctional oxidation products (e.g., 1,4-hydroxycarbonyls and 1,4-hydroxynitrates) and a preservation of the carbon chain. These potentially competing pathways are illustrated in Figure VI-A-1 for the 2-pentoxy radical: Absolute rate coefficients for these processes have been obtained for only a few of the smaller alkoxy radicals. For example, rate coefficients have been firmly established only over a range of temperatures for reaction of a subset of the C1–C6 alkoxy radicals with O2; dissociation rate coefficients have only been directly measured for ethoxy, 2-propoxy, 2-butoxy, and tert-butoxy radicals (Balla et al., 1985; Blitz et al., 1999; Caralp et al., 1999; Devolder et al., 1999; Fittschen et al., 1999, 2000; Falgayrac et al., 2004); and no direct measurement of isomerization rates have been reported to date. A large portion of the database describing the atmospheric behavior of alkoxy radicals has been built up primarily from two sources: (1) environmental chamber experiments, where end-product distributions observed under atmospheric conditions have been used to infer relative rates of competing alkoxy radical reactions (e.g., Carter et al., 1976; Cox et al., 1981; Niki et al., 1981a; Eberhard et al., 1995; Aschmann et al., 1997; Orlando et al., 2000a; Cassanelli et al., 2006); and (2) from theoretical methodologies that lend themselves well to the study of unimolecular processes (e.g., Somnitz and Zellner, 2000a, 2000b, 2000c; Mereau et al., 2000a, 2000b; Fittschen et al., 2000; Lin and Ho, 2002; Mereau et al., 2003; Davis and Francisco, 2011). An overview of these three classes of competing alkoxy radical reactions (reaction with O2, unimolecular decomposition, and isomerization) is given in this section.
The Department of Energy's Atmospheric Chemistry Program: A critical review