Kinetics and product yields of the acetyl peroxy + HO_2 radical reaction studied by photoionization mass spectrometry

The acetyl peroxy radical (CH_3C(O)O_2) is a key intermediate in the oxidn. of carbonyl-contg. hydrocarbons in the troposphere. Reaction of acetyl peroxy radicals with HO_2 has been suggested as a source of OH radicals in low-NO_x conditions. Previous work on this reaction obsd. only two product channels forming (1) peracetic acid and (2) acetic acid. Recent expts. have shown that there is a third channel that generates the radicals OH and acetoxy: CH_3C(O)O_2 + HO_2 → (1) CH_3C(O)OOH + O_2 (2) CH_3C(O)OH + O_3 (3) CH_3C(O)O + O_2 + OH This last pathway to OH formation would then contribute to the apparent isoprene OH recycling suggested by discrepancies between atm. models and field observations of OH. There have, however, been significant disagreements among expts. on the yield of OH in this reaction. We report our preliminary studies of acetyl peroxy reaction with itself and HO_2. Expts. were conducted at the Advanced Light Source synchrotron at the Lawerence Berkeley National Lab. using tunable VUV ionizing radiation coupled to the Sandia National Lab. pulsed-laser-photolysis multiplexed photoionization mass spectrometer to detect time-resolved photofragmets of reactants, intermediates, and products. From these results, we report new values for the branching fractions of the three product channels in the acetyl peroxy + HO_2 radical reaction.