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Cycling of OH and HO(2) by NOx in the tropospheric oxidation of aromatics

The tropospheric oxidation of aromatic compounds, mainly benzene and toluene, initiated by reaction with hydroxyl (OH) radicals, was investigated in laboratory experiments by 5 collaborating groups at Hannover (Germany), Lille (France), Risø (Denmark), Patras (Greece), and Wuppertal (Germany). The first 3 groups employed pulsed OH production experiments covering time-scales from microseconds to about one second, the latter 2 groups employed smog chamber experiments with continuous OH sources.
Reaction of the benzene-OH adduct with O(2) was investigated at Lille and Risø and 3 possible product channels of the benzene-OH + NO(2) reaction were excluded by pulse radiolysis/UV or IR-TDL absorption. Cycling experiments in the presence of O(2) and NO at Hannover, where OH is recovered from HO(2) by reaction HO(2) + NO, showed that HO(2) is promptly formed, ie probably without a preceeding RO(2) + NO reaction. Furthermore, similar experiments with benzene-d(6) showed the same high yield of HO(2) ie no DO(2), ruling out roughly half of the previously proposed mechanisms. Among the remaining mechanisms is one proposed at Wuppertal in which benzene oxide is believed to be a product of the adduct + O(2) reaction. Benzene oxide was synthesized and characterized at Wuppertal. Its reaction with OH forms similar products as aromatics do. On photolysis, competitive with OH reactions in the atmosphere, it forms phenol which is also observed as a product from benzene degradation.
Smog chamber experiments with toluene at Patras showed that ring-retaining products are unimportant, especially that cresol yields tend to zero at atmospheric conditions. This was also observed at Wuppertal, where phenolic type products were excluded also from p-xylene. In runs on benzene a 15% yield of phenol was observed at Patras.

Reported by

Fraunhofer Gesellschaft zur Förderungder Angewandten Forschung eV
Nikolai-Fuchs-Strasse 1
30625 Hannover
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