Products of the gas-phase reactions of aromatic hydrocarbons: Effect of NO2 concentration

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_x and in the presence of varying concentrations of NO and NO₂. The formation yield of o-cresol from toluene increased from 0.123 ± 0.022 in the absence of NO_x to 0.160 ± 0.008 for an average NO₂ concentration of 1.7 × 10¹⁴ molecule cm³. The formation yield of 2,3-butanedione from o-xylene was 0.092 ± 0.013 in the absence of NO_x, and in the presence of NO_x decreased from 0.16 at an average NO₂ concentration of (7–8) × 10¹² molecule cm^?3 to 0.09 at an average NO₂ concentration of ca. 7 × 10¹³ molecule cm^?3. The formation yield of 2,3-butanedione from 1,2,3-trimethylbenzene increased from 0.18 in the absence of NO_x to 0.444 ± 0.053 in the presence of ca. (0.16–3.6) × 10¹³ molecule cm^?3 of NO₂. 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 O₂ and NO₂ and with the NO₂ reaction rate constants being ca. 10⁵ higher than the O₂ reaction rate constants at room temperature. Under typical tropospheric conditions the reactions of the hydroxycyclohexadienyl radicals with O₂ will dominate over their reactions with NO₂. © 1994 John Wiley & Sons, Inc.