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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.

Organic aerosol formation during the atmospheric oxidation of toluene was investigated using smog chamber systems. Toluene oxidation was initiated by the UV irradiation of either toluene/air/NOx or toluene/air/CH3ONO/NO mixtures. Aerosol formation was monitored using scanning mobility particle sizers and toluene loss was monitored by in-situ FTIR spectroscopy or GC-FID techniques. The experimental results show that the reaction of OH radicals, NO3 radicals and/or ozone with the first generation products of toluene oxidation are sources of organic aerosol during the atmospheric oxidation of toluene. The aerosol results fall into two groups, aerosol formed in the absence and presence of ozone. An analytical expression for aerosol formation is developed and values are obtained for the yield of the aerosol species. In the absence of ozone the aerosol yield, defined as aerosol formed per unit toluene consumed once a threshold for aerosol formation has been exceeded, is 0.075 +/- 0.004. In the presence of ozone the aerosol yield is 0.108 +/- 0.004. This work provides experimental evidence and a simple theory confirming the formation of aerosol from secondary reactions.