Branching fractions for H2O forming channels of the reaction of OH radicals with acetaldehyde

Branching fractions for two hydrogen abstraction channels of the OH + CH3CHO reaction producing H2O + CH3CO (a) and H2O + CH2CHO (b) were determined using chemical ionization mass spectrometry. Reaction took place in a high-pressure turbulent flow reactor at 200 Torr of carrier gas N-2. The branching fraction for the abstraction from the methyl group, (5.1(+2.4)/(-1.7)) %, was determined at room temperature by direct detection of the vinoxy radicals (CH2CHO) using proton-transfer ionization. The total H2O yield of (97.7 +/- 4.7) % was obtained at T = 298 and 248 K by comparison with CH + C6H12 reaction, confirming that in this temperature range reaction proceeds by H-atom abstraction. Secondary reactions of the acetyl and vinoxy radicals in the presence of O-2 and NO2 were studied by monitoring the stable products CH2O, (HCO)(2), and CH3OH using proton-transfer ionization. It was found that formaldehyde is the major coproduct of OH in the CH3CO + O-2 reaction.