Rate Constant and Mechanism of the Reaction of OH Radicals with Acetic Acid in the Temperature Range of 229−300 K

The overall rate constant for the reaction OH + CH 3 COOH → products in the temperature range of 229−300 K was determined using a chemical ionization mass spectrometer coupled to a high-pressure turbulent flow reactor (∼200 Torr of carrier gas N 2 ). A strong negative temperature dependence of the rate constant was found in this range which can be expressed in Arrhenius form as k 1 ( T ) = ((2.2 ± 0.2) × 10 -14 ) exp((1012 ± 80)/ T ) cm 3 molecule -1 s -1 with k 1 = 6.6 × 10 -13 cm 3 molecule -1 s -1 at 298 K. When these results are combined with previous measurements in the range of 298−446 K, 8 a three-parameter expression can be derived: k 1 ( T ) = (2.45 × 10 -16 )( T /298) 5.24±0.68 exp((2358 ± 189)/ T ) cm 3 molecule -1 s -1, describing the curvature of the Arrhenius plot observed at T > 300 K. A branching fraction of (64 ± 17)% was determined between 300 and 249 K for the H-atom abstraction from the carboxyl group OH + CH 3 COOH → CH 3 + CO 2 + H 2 O. This latter parameter was measured as the yield of CO 2, formed as a result of the fast decomposition of the primary CH 3 C(O)O radical. Atmospheric implications of the obtained results are discussed. The obtained k 1 value provides a lifetime of CH 3 COOH in the upper troposphere (UT) that is a factor of 2 lower than that calculated so far from existing recommendations. The data also show that acetic acid could be as significant as methane in influencing the oxidative capacity of the UT considering that concentrations of CH 3 COOH from hundreds of pptv to a few ppbv have been measured during several campaigns.