Atmospheric Oxidation of Toluene in a Large-Volume Outdoor Photoreactor:  In Situ Determination of Ring-Retaining Product Yields

Abstract

Experiments on the photooxidation of toluene/NO x /air mixtures were performed in the European Photoreactor (EUPHORE), a large-scale outdoor reaction chamber located in Valencia/Spain. The objective of the study was the in situ determination of the yields of ring-retaining products by differential optical absorption spectroscopy (DOAS) and the elucidation of their formation pathways. The experiments were performed with toluene concentrations between 0.68 and 3.85 ppm and initial NO x concentrations ranging from 3 to 300 ppb, i.e., down to the range actually observed in the lower atmosphere. The ring-retaining product yields were found to be 5.8 ( 0.8%, 12.0 ( 1.4%, 2.7 ( 0.7%, and 3.2 ( 0.6% for benzaldehyde, o-cresol, m-cresol, and p-cresol, respectively. Under the experimental conditions, no dependency of the yields on the NO x concentration or the toluene/NO x ratio could be found. The formation kinetics of the cresols are in line with a "prompt" formation mechanism, i.e., abstraction of a hydrogen atom from the toluene-OH adduct (toluene-OH + O 2 f cresols + HO 2 ). In addition, substantial evidence was found that reaction with NO 3 radicals represents an important sink for cresols in smog chamber studies conducted under conditions of NO x concentrations above the range observed in the troposphere, possibly also under tropospheric conditions.

Formation of Peroxy Radicals from OH−Toluene Adducts and O2

“…21 Since no data for toluene-d 8 were available, the vapor pressures of toluene and toluene-d 8 were measured with capacitance manometers (Baratron, MKS) in a thermostated vacuum cell at about 279 K. The vapor pressure of toluene-d 8 was found to be higher by a factor of (1.04 ( 0.01) while the absolute value for toluene was within 5% of the literature data. 21 The ratio of 1.04 was then used to calculate toluene-d 8 concentrations at the actual temperature of the saturator.…”

“…However, at higher temperatures (T g 400 K), where the abstraction Reaction 1b dominates, significant differences were observed. 28,33 Tully et al 28 derived Arrhenius expressions for Reaction 1b for toluene and toluene-d 8 . By extrapolation of these expressions, a benzyl radical yield of 2% can be estimated for toluene-d 8 at room temperature, compared with 7% in the case of toluene.…”

“…By extrapolation of these expressions, a benzyl radical yield of 2% can be estimated for toluene-d 8 at room temperature, compared with 7% in the case of toluene. 3 Atkinson 2 has given an empirical expression of the kinetic isotope effect of Reaction 1 at elevated temperatures which, extrapolated to room temperature, results in a similar yield of benzyl radicals for toluene-d 8 . Thus, the slightly lower k 1 determined for toluene-d 8 (Table 1) can be explained by a lower rate constant of Reaction 1b.…”

“…3 Atkinson 2 has given an empirical expression of the kinetic isotope effect of Reaction 1 at elevated temperatures which, extrapolated to room temperature, results in a similar yield of benzyl radicals for toluene-d 8 . Thus, the slightly lower k 1 determined for toluene-d 8 (Table 1) can be explained by a lower rate constant of Reaction 1b. However, the difference is not significant within the error limits of the rate constants which are also dependent on a correct vapor pressure determination.…”

“…The purity of the buffer gases was 99.999% (N 2 ) and 99.995% (O 2 ) (Messer Griesheim). Toluene and toluene-d 8 were introduced by purging the liquids in a thermostated gas saturator 20 at T ) (279.9 ( 0.1) K. Toluene (Aldrich, 99+) and toluene-d 8 (Aldrich, 99+ at. % D) were used as purchased.…”