Consecutive reactions of aromatic-OH adducts with NO, NO<sub>2</sub> and O<sub>2</sub>: benzene, naphthalene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline

Abstract

Abstract. Consecutive reactions of adducts, resulting from OH radicals and aromatics, with the tropospheric scavenger molecules O 2 , NO and NO 2 have been studied for benzene, naphthalene, toluene, m-and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline by observing decays of OH at temperatures where the thermal back-decomposition to OH is faster than 3 s −1 , typically between 300 and 340 K. The experimental technique was resonance fluorescence with flash photolysis of water as source of OH. Biexponential decays were observed in the presence of either O 2 or NO, and triexponential decays were obtained in the presence of NO 2 . The kinetic analysis was performed by fitting the relevant rate constants of the reaction mechanism to whole sets of decays obtained at various concentrations of aromatic and scavenger. In the case of hexamethylbenzene, the biexponential decays suggest the existence of the ipso-adduct, and the slightly higher necessary temperatures show that it is even more stable.In addition, smog chamber experiments at O 2 concentrations from atmospheric composition down to well below 100 ppm have been carried out for benzene, toluene and pxylene. The drop of the effective rate constant of removal by OH occurs at reasonable O 2 levels, given the FP/RF results.Comparison of the adduct reactivities shows for all aromatics of this study that the reaction with O 2 predominates over that with NO 2 under all tropospheric conditions, and that a reaction with NO may only occur after the reaction with O 2 .

Secondary organic aerosol formation from biomass burning intermediates: phenol and methoxyphenols

“…Thus, ipso addition of OH in the gas-phase may be more relevant for more highly-substituted and/or more complex substitutents on aromatics. Koch et al (2007) also pointed out several liquid-phase studies that find evidence of ether bond cleavage. Again, ipso addition of hydroxyl radical is cited as being responsible for ether bond cleavage (removal of an oxy substitutent) and C-C cleavage (removal of a methyl substituent) in some cases.…”

“…This is the case for alkylbenzenes, in which Noda et al (2009) observed phenol formation from OH-initiated photooxidation of toluene and cresol formation from xylenes. Prior, Koch et al (2007) provided evidence that the ipso addition of OH is relevant for the case of hexamethylbenzene. The extent by which these processes occur in the gas-phase is still unclear, as Aschmann et al (2010) does not observe the dealkylation as in Noda et al (2009).…”

Global modeling of secondary organic aerosol formation from aromatic hydrocarbons: high- vs. low-yield pathways

“…As explained in detail in Ng et al (2007), the alternative reaction of the aromatic-OH adducts with NO 2 is not expected to play a significant role under chamber conditions (Koch et al, 2007). It has been shown that high (>100 ppb) levels of NO x affect the fate of the aromatic-OH-O 2 peroxy radicals Klotz et al, 2002;Volkamer et al, 2002b) (Zhao et al, 2005;Fan and Zhang, 2006), though formation of the phenol product was not considered.…”

“…Atmospheric reaction of these aromatics with the hydroxyl radical (OH) initiates a complex series of gas-phase reactions (Calvert et al, 2002;Koch et al, 2007). As noted above, a crucial factor governing the nature of the gas-phase chemistry and subsequent aerosol formation is the NO x level.…”

“…The mechanisms for atmospheric oxidation of both biogenic and anthropogenic hydrocarbons are complex and not fully understood; it does appear, however, that the observed low-and high-NO x behavior of SOA formation (Song et al, 2005;Ng et al, 2007) hinges on the competitive reactions of the bicyclic peroxy radicals (RO 2 ) that result from initial attack of the aromatic species by OH radicals followed quickly by O 2 addition and cyclization (Johnson et al, 2005;Koch et al, 2007), written here simply as…”