Atmospheric oxidation of toluene in a large-volume outdoor photoreactor: In situ determination of ring-retaining product yields

Experiments on the photooxidation of toluene/NOx/air mixtures were performe d in the European Photoreactor (EUPHORE), a large-scale outdoor reaction ch amber located in Valencia/Spain. The objective of the study was the in situ determination of the yields of ring-retaining products by differential opt ical absorption spectroscopy (DOAS) and the elucidation of their formation pathways. The experiments were performed with toluene concentrations betwee n 0.68 and 3.85 ppm and initial NOx concentrations ranging from 3 to 300 pp b, i.e., down to the range actually observed in the lower atmosphere. The r ing-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 t he yields on the NOx concentration or the toluene/NOx ratio could be found. The formation kinetics of the cresols are in line with a "prompt" formatio n mechanism, i.e., abstraction of a hydrogen atom from the toluene-OH adduc t (toluene-OH + O-2 --> cresols + HO2). In addition, substantial evidence w as found that reaction with NO3 radicals represents an important sink for c resols in smog chamber studies conducted under conditions of NOx concentrat ions above the range observed in the troposphere, possibly also under tropo spheric conditions.