Primary and secondary glyoxal formation from aromatics: Experimental evidence for the bicycloalkyl-radical pathway from benzene, toluene, and p-xylene

A new approach is presented to study the ring-cleavage process of benzene, toluene, and p-xylene (BTX). DOAS (differential optical absorption spectros copy) was used for the simultaneous measurement of the respective ring-reta ining products as well as glyoxal (a ring-cleavage product) in a series of experiments at the EUPHORE outdoor simulation chamber, Valencia/Spain. The good time resolution of the DOAS measurements (1-2 min) allowed the primary formation of glyoxal to be separated from any further contributions throug h additional pathways via reactions of stable intermediate compounds (secon dary glyoxal formation). The ring-retaining products and glyoxal were ident ified as primary products. The primary glyoxal yield was found to be essent ially identical to the overall yield of glyoxal formed over the time scale of the experiments. The negligible contribution from secondary glyoxal form ation pathways was quantitatively understood for the experimental condition s of this study and was found to be representative for the troposphere. The yield of glyoxal was determined to be 35% +/- 10% for benzene and about 5% higher for toluene and p-xylene. For benzene, the yield of hexadienedial w as estimated to be less than or equal to 8%. It is concluded that ring-clea vage pathways involving the bicycloalkyl radical are major pathways in the oxidation of monocyclic aromatic hydrocarbons, i.e., BTX The branching rati o for the bicycloalkyl radical intermediate, proposed to form from the reac tion of the aromatic-OH adduct with atmospheric oxygen, could be directly i dentified with the primary glyoxal yield for the benzene system and as a lo wer limit in the case of toluene and p-xylene. Implications for the chemica l behavior of aromatic hydrocarbons in the atmosphere are discussed.