Formation of phenol and carbonyls from the atmospheric reaction of OH radicals with benzene

Abstract

The gas-phase reaction of OH radicals with benzene has been studied in a flow tube operated at 295 ± 2 K and 950 mbar of synthetic air or O₂. Ozonolysis of tetramethylethylene (dark reaction) with a measured OH radical yield of 0.92 ± 0.08 or photolysis of methyl nitrite in the presence of NO served as the OH sources. For investigations in the presence of NO_x, the conditions were chosen so that more than 95% of the OH/benzene adduct reacted with O₂ even for the highest NO₂ concentration occurring in the experiment. In the absence of NO_x, a phenol yield from the reaction of OH radicals with benzene of 0.61 ± 0.07 was measured by means of long-path FT-IR and UV spectroscopy over a wide range of experimental conditions. This yield was confirmed by measurements performed in the presence of NO_x. Detected carbonyls were glyoxal, cis-butenedial and trans-butenedial with formation yields of 0.29 ± 0.10, 0.08 ± 0.03 and 0.023 ± 0.007, respectively, measured in synthetic air and in the presence of NO_x. There was no significant difference in the product yields applying both experimental approaches for OH generation (dark reaction or photolysis). Nitrobenzene and o-nitrophenol were detected in traces. The yield of nitrobenzene increased with increasing NO_x resulting in a maximum formation yield of 0.007. The detected products in the presence of NO_x account for ∼78% of the reacted carbon. Butenedial yields from benzene degradation are reported for the first time. In the absence of NO_x, glyoxal, cis-butenedial and trans-butenedial were also detected, but with distinctly lower yields compared to the experiments with NO_x.