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

[摘要] Photo-oxidation products from the reaction of a series of alkyl-benzenes, (benzene,toluene, p-xylene and 1,3,5-trimethyl-benzene) with hydroxyl radicals in the presenceof NO_x have been investigated using comprehensive gas chromatography(GCxGC). A GCxGC system has been developed which utilises valve modulation andindependent separations as a function of both volatility and polarity. A number ofcarbonyl-type compounds were identified during a series of reactions carried out atthe European Photoreactor (EUPHORE), a large volume outdoor reaction chamber inValencia, Spain. Experiments were carried as part of the EXACT project (Effects ofthe oXidation of Aromatic Compounds in the Troposphere).Two litre chamber airsamples were cryo-focused, with a sampling frequency of 30 minutes, allowing theevolution of species to be followed over oxidation periods of 3-6 hours.To facilitateproduct identification, several carbonyl compounds, which were possible products ofthe photo-oxidation, were synthesised and used as reference standards.

For toluene reactions, observed oxygenated intermediates found included theco-eluting pair a-angelicalactone/4-oxo-2-pentenal, maleic anhydride, citraconicanhydride, benzaldehyde and p-methyl benzoquinone.In the p-xylene experiment,the products identified were E/Z-hex-3-en-2,5-dione and citraconic anhydride.For1,3,5-TMB reactions, the products identified were 3,5-dimethylbenzaldehyde,3,5-dimethyl-3H-furan-2-one and 3-methyl-5-methylene-5H-furan-2-one. Preliminaryquantification was carried out on identified compounds using liquid standards.Comparison of FTIR and GCxGC for the measurement of the parent aromatics generally showed good agreement.Comparison of the concentrations observed byGCxGC to concentration-time profiles simulated using the Master Chemical Mechanism, MCMv3, demonstrates that this mechanism significantly over-predictsthe concentrations of many product compounds and highlights the uncertainties whichexist in our understanding of the atmospheric oxidation of aromatics.

[学科分类] 大气科学