The formation of the adducts of OH with benzene, toluene and phenol and consecutive reactions of the adducts with NO, NO2 and O2 are investigated in the temperature range from ∼298–374 K employing the flash photolysis/resonance fluorescence technique in Ar at 133 mbar. Biexponential decays of OH can be observed over at least 2 orders of magnitude in the presence of the aromatics, and the three reaction channels: abstraction of H by OH, addition of OH and unimolecular decomposition of the adduct are distinguished in overlapping temperature ranges. Rate constants and preliminary Arrhenius parameters are obtained for these channels for benzene, toluene, and phenol. The reactions of the adducts with NO are very slow and can be neglected (<10−13 cm3 s−1) in all three cases. Values of ∼3·10−11 cm3 s−1 are obtained for the rate constants of the reactions of the adducts with NO2, almost independent of reactant and temperature. Preliminary determinations of the adduct reactivities against O2 ∼ 2 · 10−16 (benzene‐OH) and ∼5 · 10−16 cm3 s−1 (toluene‐OH) near room temperature indicate that this path predominates under tropospheric conditions. The results for O2 are confirmed by direct observations of the adduct benzene‐OH by the cw UV‐laser longpath absorption technique.

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 .

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