Experimental studies have been conducted to determine the rate coefficient and mechanism of the reaction between methylglyoxal (CH3COCHO, MGLY) and the OH radical over a wide range of temperatures (233-500 K) and pressures (5-300 Tort). The rate coefficient is pressure independent with the following temperature dependence: k(3)(T)= (1.83 +/-0.48) x 10(-12) exp((560 +/- 70)/T) cm(3) molecule(-1) s(-1) (95 % uncertainties). Addition Of 02 to the system leads to recycling of OH. The mechanism was investigated by varying the experimental conditions ([O-2], [MGLY], temperature and pressure), and by modelling based on a G3X potential energy surface, rovibrational prior distribution calculations and master equation RRKM calculations. The mechanism can be described as follows:
MGLY + OH -> H2O + CH3COCO* (R3)
CH3COCO* -> CH3CO + CO (R4a)
CH3COCO* -> CO+CH3CO* -> CH3 + CO (R4b)
CH3CO + O-2 -> OH +other products (R5a)
CH3CO + O-2 + M -> CH3CO(O-2) + M (R5b)
Addition of oxygen to the system shows that process (4) is fast and that CH3COCO completely dissociates. The acetyl radical formed from reaction (4) reacts with oxygen to regenerate OH radicals (5a). However, a significant fraction of acetyl radical formed by reaction (R4) is sufficiently energised to dissociate further to CH3 + CO (R4b). Little or no pressure quenching of reaction (R4b) was observed. The rate coefficient for OD + MGLY was measured as k(9)(T) = (9.4 +/- 2.4) x 10(-13) exp((780 +/- 70)/T) cm(3) molecule(-1) s(-1) over the temperature range 233-500 K. The reaction shows a noticeable inverse (k(H)/k(D) < 1) kinetic isotope effect below room temperature and a slight normal kinetic isotope effect (k(H)/k(D) > 1) at high temperature. The potential atmospheric implications of this work are discussed.