Branching Fractions for H₂O Forming Channels of the Reaction of OH Radicals with Acetaldehyde

Branching fractions for two hydrogen abstraction channels of the OH + CH 3 CHO reaction producing H 2 O + CH 3 CO (a) and H 2 O + CH 2 CHO (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 (CH 2 CHO) using proton-transfer ionization. The total H 2 O yield of (97.7 ± 4.7) % was obtained at T = 298 and 248 K by comparison with OH + C 6 H 12 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 NO 2 were studied by monitoring the stable products CH 2 O, (HCO) 2, and CH 3 OH using proton-transfer ionization. It was found that formaldehyde is the major coproduct of OH in the CH 3 CO + O 2 reaction.