The formation yields of selected products of the OH radical-initiated reactions of toluene, o-xylene, and 1,2,3,-trimethylbenzene have been measured in the absence of NO, and in the presence of varying concentrations of NO and NOz. The formation yield of o-cresol from toluene increased from 0.123 2 0.022 in the absence of NO, to 0.160 2 0.008 for an average NO2 concentration of 1.7 X 1014 molecule cm3. The formation yield of 2,3-butanedione from o-xylene was 0.092 2 0.013 in the absence of NO,, and in the presence of NO, decreased from 0.16 at an average NO2 concentration of (7-8) X lo1' molecule cm-3 to 0.09 at an average NO2 concentration of ca. 7 X 1013 molecule ~m -~.
The formation yield of 2,3-butanedione from 1,2,3-trimethylbenzene increased from 0.18 in the absence of NO, to 0.444 2 0.053 in the presence of ca. (0.16-3.6) x 1013 molecule cm-3 of NOz. These product data are consistent with literature kinetic data showing that the hydroxycyclohexadienyl radicals formed by OH radical addition to the aromatic ring react with both 0 2 and NO2 and with the NO2 reaction rate constants being ca. lo5 higher than the 0 2 reaction rate constants at room temperature. Under typical tropospheric conditions the reactions of the hydroxycyclohexadienyl radicals with 0 2 will dominate over their reactions with NOz.