Atmospheric Chemistry of the Phenoxy Radical, C 6 H 5 O(•): UV Spectrum and Kinetics of Its Reaction with NO, NO 2 , and O 2 J. Platz and O. J. Nielsen † Atmospheric Chemistry, Plant Biology and Biogeochemistry Department, Risø National Laboratory, DK-4000, Roskilde, Denmark T. J. Wallington,* ,‡ J. C. Ball, M. D. Hurley, A. M. Straccia, and W. F. Schneider Ford Motor Company, 20000 Rotunda DriVe, Mail Drop SRL-3083, Dearborn, Michigan 48121-2053 J. Sehested § Haldor Topsoe A/S, NymoelleVej 55, DK-2800 Lyngby, Denmark ReceiVed: May 12, 1998; In Final Form: June 29, 1998 Pulse radiolysis and FT-IR smog chamber experiments were used to investigate the atmospheric fate of C 6 H 5 O(•) radicals. Pulse radiolysis experiments gave σ(C 6 H 5 O(•)) 235 nm ) (3.82 ( 0.48) × 10 -17 cm 2 molecule -1 , k(C 6 H 5 O(•) + NO) ) (1.88 ( 0.16) × 10 -12 , and k(C 6 H 5 O(•) + NO 2 ) ) (2.08 ( 0.15) × 10 -12 cm 3 molecule -1 s -1 at 296 K in 1000 mbar of SF 6 diluent. No discernible reaction of C 6 H 5 O(•) radicals with O 2 was observed in smog chamber experiments, and we derive an upper limit of k(C 6 H 5 O(•) + O 2 ) < 5 × 10 -21 cm 3 molecule -1 s -1 at 296 K. These results imply that the atmospheric fate of phenoxy radicals in urban air masses is reaction with NO x . Density functional calculations and gas chromatography-mass spectrometry are used to identify 4-phenoxyphenol as the major product of the self-reaction of C 6 H 5 O(•) radicals. As part of this study, relative rate techniques were used to measure rate constants for reaction of Cl atoms with phenol [k(Cl + C 6 H 5 OH) ) (1.93 ( 0.36) × 10 -10 ], several chlorophenols [k(Cl + 2-chlorophenol) ) (7.32 ( 1.30) × 10 -12 , k(Cl + 3-chlorophenol) ) (1.56 ( 0.21) × 10 -10 , and k(Cl + 4-chlorophenol) ) (2.37 ( 0.30) × 10 -10 ], and benzoquinone [k(Cl + benzoquinone) ) (1.94 ( 0.35) × 10 -10 ], all in units of cm 3 molecule -1 s -1 . A reaction between molecular chlorine and C 6 H 5 OH to produce 2- and 4-chlorophenol in yields of (28 ( 3)% and (75 ( 4)% was observed. This reaction is probably heterogeneous in nature, and an upper limit of k(Cl 2 + C 6 H 5 OH) e 1.9 × 10 -20 cm 3 molecule -1 s -1 was established for the homogeneous component. These results are discussed with respect to the previous literature data and to the atmospheric chemistry of aromatic compounds. 1. Introduction Aromatic compounds such as toluene, ethyl benzene, and the xylenes are important constituents of automotive gasoline. Typical gasoline blends currently sold in the United States have an aromatic content of 20-30% by volume. 1 It is well- established that aromatic species are important components of automobile tailpipe exhaust and evaporative emissions and contribute to formation of ozone 2 and secondary organic aerosol 3 in urban air. Unfortunately, our understanding of the atmo- spheric chemistry of aromatic compounds is incomplete, and assessments of the environmental impact of the atmospheric release of such species are uncertain. One experimental problem associated with the study of the atmospheric chemistry of aromatic compounds is the scarcity of sources for the intermediate radical species that are formed in the sequence of oxidation reactions. Fortunately, this limitation has been lifted partially by recent reports of a convenient source for the phenoxy radical, C 6 H 5 O(•), namely, reaction of Cl atoms with C 6 H 5 OH. 4-6 Berho and Lesclaux 5 used reaction 1 as a source of phenoxy radicals in their flash photolysis study of the kinetics of reactions 2 and 3. No reaction between phenoxy radicals and O 2 was observed by Berho and Lesclaux, 5 and an upper limit of k 2 < 2 × 10 -18 † E-mail: ole.john.nielsen@risoe.dk. ‡ E-mail: twalling@ford.com. § E-mail: jss@topsoe.dk. 7964 J. Phys. Chem. A 1998, 102, 7964-7974 S1089-5639(98)02221-X CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998