The overall rate constant for the reaction OH + CH 3 COOH f 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 f 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.

A high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer was used to investigate the minor channel (1b) producing nitric acid, HNO3, in the HO2 + NO reaction for which only one channel (1a) is known so far: HO2 + NO --> OH + NO2 (1a), HO2 + NO --> HNO3 (1b). The reaction has been investigated in the temperature range 223-298 K at a pressure of 200 Torr of N2 carrier gas. The influence of water vapor has been studied at 298 K. The branching ratio, k1b/k1a, was found to increase from (0.18(+0.04/-0.06))% at 298 K to (0.87(+0.05/-0.08))% at 223 K, corresponding to k1b = (1.6 +/- 0.5) x 10(-14) and (10.4 +/- 1.7) x 10(-14) cm3 molecule(-1) s(-1), respectively at 298 and 223 K. The data could be fitted by the Arrhenius expression k1b = 6.4 x 10(-17) exp((1644 +/- 76)/T) cm3 molecule(-1) s(-1) at T = 223-298 K. The yield of HNO3 was found to increase in the presence of water vapor (by 90% at about 3 Torr of H2O). Implications of the obtained results for atmospheric radicals chemistry and chemical amplifiers used to measure peroxy radicals are discussed. The results show in particular that reaction 1b can be a significant loss process for the HO(x) (OH, HO2) radicals in the upper troposphere.

Reactions of OH radicals with dimethyl sulfoxide (CH3)2SO (DMSO) (reaction 1) and methane sulfinic acid CH3S(O)OH (MSIA) (reaction 2) have been studied at 298 K and 200 and 400 Torr of N2 using a newly constructed high-pressure turbulent flow reactor coupled to an ion molecule reaction mass spectrometer. The experimental setup is discussed in detail. The reactions of OH with DMSO and MSIA were found to proceed with predominant formation of MSIA and SO2, respectively. The yields of MSIA in reaction 1 and of SO2 in reaction 2 were estimated to be 0.9 ± 0.2. The reaction rate constants k 1 = (9 ± 2) × 10-11 cm3 molecule-1 s-1 and k 2 = (9 ± 3) × 10-11 cm3 molecule-1 s-1 were obtained. These results indicate that the OH-addition route of the gas-phase atmospheric oxidation of dimethyl sulfide, CH3SCH3 (DMS), which produces DMSO as a primary intermediate, would result in high yields of SO2, which is a precursor of H2SO4. The results then suggest that the other major end product of DMS oxidation, methane sulfonic acid CH3SO3H (MSA), would not be produced by gas-phase reactions involving MSIA as suggested so far, but rather by liquid-phase reactions.