Measurements of photo-oxidation products from the reaction of a series of alkyl-benzenes with hydroxyl radicals during EXACT using comprehensive gas chromatography

Abstract

Photo-oxidation products from the reaction of aseries of alkyl-benzenes, (benzene, toluene, p-xylene and 1,3,5-trimethyl-benzene) with hydroxyl radicals in the presence of NOx have been investigated using comprehensive gas chromatography (GC×GC). A GC×GC system has been developed which utilises valve modulation and independentseparations as a function of both volatility and polarity. A number of carbonyl-type compounds were identified during a series of reactions carried out at the European Photoreactor (EUPHORE), a large volume outdoor reaction chamber in Valencia, Spain. Experiments were carried as part of the EXACT project (Effects of the oXidation of Aromatic Compounds in the Troposphere). Two litre chamber air samples were cryo-focused, with a sampling frequency of 30 minutes, allowing the evolution of species to be followed over oxidation periods of 3-6 hours. To facilitate product identification, several carbonyl compounds, which were possible products of the photo-oxidation, were synthesised and used as reference standards. For toluene reactions, observed oxygenated intermediates found included the co-eluting pair-angelicalactone/4- oxo-2-pentenal, maleic anhydride, citraconic anhydride, benzaldehydeand p-methyl benzoquinone. In the p-xylene experiment, the products identified were E/Z-hex-3-en-2,5- dione and citraconic anhydride. For 1,3,5-TMB reactions, the products identified were 3,5-dimethylbenzaldehyde, 3,5-dimethyl-3H-furan-2-one and 3-methyl-5-methylene-5Hfuran- 2-one. Preliminary quantification was carried out on identified compounds using liquid standards. Comparison ofFTIR and GC×GC for the measurement of the parent aro matics generally showed good agreement. Comparison of the concentrations observed by GC×GC to concentrationtime profiles simulated using the Master Chemical Mechanism, MCMv3, demonstrates that this mechanism significantly over-predicts the concentrations of many product compounds and highlights the uncertainties which exist in our understanding of the atmospheric oxidation of aromatics. © 2003 European Geosciences Union.