Consecutive reactions of aromatic-OH adducts with NO, NO2 and O2: benzene, naphthalene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline R. Koch1,2, R. Knispel1, M. Elend1, M. Siese1,2, and C. Zetzsch1,2 1Fraunhofer-Institute of Toxicology and Experimental Medicine, Hannover, Germany 2Atmospheric Chemistry Research Laboratory, University of Bayreuth, Germany
Abstract. Consecutive reactions of adducts, resulting from OH radicals and aromatics,
with the tropospheric scavenger molecules O2, NO and NO2 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 O2 or NO, and
triexponential decays were obtained in the presence of NO2. 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 O2 concentrations from
atmospheric composition down to well below 100 ppm have been carried out for
benzene, toluene and p-xylene. The drop of the effective rate constant of
removal by OH occurs at reasonable O2 levels, given the FP/RF results.
Comparison of the adduct reactivities shows for all aromatics of this study
that the reaction with O2 predominates over that with NO2 under
all tropospheric conditions, and that a reaction with NO may only occur
after the reaction with O2.
Citation: Koch, R., Knispel, R., Elend, M., Siese, M., and Zetzsch, C.: Consecutive reactions of aromatic-OH adducts with NO, NO2 and O2: benzene, naphthalene, toluene, m- and p-xylene, hexamethylbenzene, phenol, m-cresol and aniline, Atmos. Chem. Phys., 7, 2057-2071, doi:10.5194/acp-7-2057-2007, 2007.