The objectives of this study were to measure the atmospheric reaction rates of dihydroxy(methyl)benzene and (methyl)benzoquinone compounds with the OH radical and to elucidate the atmospheric reaction mechanisms for phenol and the cresol isomers with both OH and NO3 radicals. The study has yielded the following results:
Relative rate constants have been measured for the reactions of OH radicals with 1,2-dihydroxybenzene, 1,2-dihydroxy-3-methylbenzene, 1,2-dihydroxy-4-methylbenzene, 1,4-benzoquinone and methyl-1,4-benzoquinone. The rate constants (in units of cm3 molecule-1 s-1) were found to be (10.4 ± 2.1) x 10-11 for 1,2-dihydroxybenzene; (20.5 ± 4.3) x 10-11 and (15.6 ± 3.3) x 10-11 for 1,2-dihydroxy-3-methylbenzene and 1,2-dihydroxy-4-methylbenzene, respectively; (0.46 ± 0.09) x 10-11 and (2.35 ± 0.47) x 10-11 for 1,4-benzoquinone and methyl-1,4-benzoquinone, respectively. The present kinetic data will improve the kinetic data base required to model the degradation mechanisms of aromatic compounds and to develop structure reactivity relationships for OH radicals with VOCs especially for oxygenated aromatic compounds. Using the kinetic data obtained in this study, in combination with an average tropospheric OH radical concentration of [OH] = 1.6 x 106 cm-3 estimated atmospheric residence times of the compounds due to reaction with OH radicals have been determined. From the results it can be concluded that the 1,2-dihydroxybenzenes have very short atmospheric lifetimes and probably will only influence the photochemical oxidant formation on a local scale, whereas the benzoquinones with significantly longer lifetimes will have a more regional influence. Product analyses of the OH and NO3 radical initiated oxidation of phenol and the cresol isomers under simulated atmospheric conditions were performed in a 1080 l quartz glass reactor at Wuppertal University and in the EUPHORE outdoor smog chamber facility in Valencia/Spain. In the case of the OH initiated oxidation of phenol (or cresols), besides the already known nitrophenol products, new ring-retaining products were experimentally determined for the first time, namely, 1,2-dihydroxybenzene and 1,4-benzoquinone from phenol, 1,2-dihydroxy-3-methylbenzene and methyl-1,4-benzoquinone from ortho-cresol, 1,2-dihydroxy-3-methylbenzene, 1,2- dihydroxy-4-methylbenzene, methyl-1,4-benzoquinone and 3-methyl-4-nitrophenol from meta-cresol and 1,2-dihydroxy-4-methylbenzene from para-cresol. From the results of the present study it can be concluded that 1,2-dihydroxy(methyl)benzenes isomers are the major gas-phase reaction products of the OH radical initiated oxidation of phenol and the cresol isomers. Interaction of the OH - phenol and OH - cresol adducts with O2 will mainly lead to the formation of 1,2-dihydroxybenezenes and HO2. In addition, the large yields of 1,2-dihydroxybenzenes support that addition at the position ortho to the OH substituent dominates. The results also support that formation of (methyl)benzoquinone compounds, mainly via the OH addition channel. The close similarity of the nitrophenol formation yields from phenol and cresol isomers with the fraction of overall OH radical reaction estimated to proceed by H-atom abstraction from the OH group implies that the nitrophenol formation from phenolic compounds arises from the abstraction channel. Based on the results of the product studies, general degradation mechanisms have been constructed for the OH initiated oxidation of phenol and the cresol isomers.
Studies of the NO3 radical initiated oxidation of phenol and the cresol isomers have been carried out in two chamber systems: 1080 l quartz glass reactor at Wuppertal University and in the EUPHORE outdoor smog chamber facility in Valencia/Spain. The observed oxidation products and their distribution can be used to improve the general degradation mechanism of phenol and cresol isomers towards NO3 radicals. The products detected during this study were as follows: 2-nitrophenol and 4-nitrophenol from phenol; 6-methyl-2-nitrophenol and 1,4-benzoquinone from ortho-cresol; 3-methyl-2-nitrophenol, 3-methyl-4-nitrophenol, 5-methyl-2-nitrophenol and methyl-1,4-benzoquinone from meta-cresol; 4-methyl-2-nitrophenol from para-cresol. HNO3 has also been identified as important co-product. The experimental results support that the degradation reactions initiated by NO3 radicals of phenol and the cresol isomers form large yields of HNO3. Formation of methyl-1,4-benzoquinone in low yield has been identified for the first time as an oxidation product in the NO3 radical initiated oxidation of ortho- and meta-cresol. The results from this study show that major products from the NO3 radical initiated reaction of phenol and cresol isomers are mixtures of nitrophenol isomers and with low yield of methyl-1,4-benzoquinone. The gas-phase reactions of the NO3 radical with phenolic compounds have been postulated to proceed via an overall H-atom abstraction mechanism which occurs through an NO3 - aromatic adduct which can decompose back to reactants or to form methyl phenoxy radicals and HNO3. The methyl phenoxy radicals thus formed will react with NO2 to form nitrophenols isomers. The results of the present study show that the abstraction reaction pathway leading to the phenoxy radical accounts for about 75 - 95% of the overall reaction of phenolic compounds with NO3 radicals. Consequently reaction pathways leading to ring cleavage products, if indeed taking place, will be of minor as importance.
The data obtained within the present work, concerning the products of the OH and NO3 radical initiated oxidation of phenol and the cresol isomers, can be used to improve the knowledge on the atmospheric degradation mechanisms of aromatic hydrocarbons. The mechanisms constructed for the degradation of phenol and cresol isomers can be incorporated into atmospheric models to obtain more accurate estimates of production of O3, other photo-oxidants and HNO3 through the photo-oxidation of aromatic compounds.