Prompt HO _2 formation following the reaction of OH with aromatic compounds under atmospheric conditions

摘要

The secondary formation of HO _2 radicals following OH + aromatic hydrocarbon reactions in synthetic air under normal pressure and temperature was investigated in the absence of NO after pulsed production of OH radicals. OH and HO _x (=OH + HO _2) decay curves were recorded using laser-induced fluorescence after gas-expansion. The prompt HO _2 yields (HO _2 formed without preceding NO reactions) were determined by comparison to results obtained with CO as a reference compound. This approach was recently introduced and applied to the OH + benzene reaction and was extended here for a number of monocyclic aromatic hydrocarbons. The measured HO _2 formation yields are as follows: toluene, 0.42 ± 0.11; ethylbenzene, 0.53 ± 0.10; o-xylene, 0.41 ± 0.08; m-xylene, 0.27 ± 0.06; p-xylene, 0.40 ± 0.09; 1,2,3-trimethylbenzene, 0.31 ± 0.06; 1,2,4-trimethylbenzene, 0.37 ± 0.09; 1,3,5- trimethylbenzene, 0.29 ± 0.08; hexamethylbenzene, 0.32 ± 0.08; phenol, 0.89 ± 0.29; o-cresol, 0.87 ± 0.29; 2,5-dimethylphenol, 0.72 ± 0.12; 2,4,6-trimethylphenol, 0.45 ± 0.13. For the alkylbenzenes HO _2 is the proposed coproduct of phenols, epoxides, and possibly oxepins formed in secondary reactions with O _2. In most product studies the only quantified coproducts were phenols whereas only a few studies reported yields of epoxides. Oxepins have not been observed so far. Together with the yields of phenols from other studies, the HO _2 yields determined in this work set an upper limit to the combined yields of epoxides and oxepins that was found to be significant (≥0.3) for all investigated alkylbenzenes except m-xylene. For the hydroxybenzenes the currently proposed HO _2 coproducts are dihydroxybenzenes. For phenol and o-cresol the determined HO _2 yields are matching the previously reported dihydroxybenzene yields, indicating that these are the only HO _2 forming reaction channels. For 2,5-dimethylphenol and 2,4,6-trimethylphenol no complementary product studies are available.