Temperature dependent rate coefficients of the OH reaction with methacrolein and of the 1,4 H-shift reaction of the organic peroxy radical from methacrolein

We present the measurement of the temperature dependent rate coefficient of the OH reaction with methacrolein between 280 and 340 K using an OH reactivity instrument that allows to accurately determine the loss rate of OH reactants. Experiments were performed in synthetic air (presence of oxygen). In the case of methacrolein, OH radicals are regenerated by the 1,4 H-shift reaction of the aldehyde group of the MACR-1-OH-O₂ radical which is the main peroxy radicals formed its OH reaction. Therefore, the observed OH decay deviates from a single-exponential decay expected from a pseudo-first order loss reaction. This allows to determine the rate coefficient of the peroxy radical isomerization reaction using model calculations, in which reaction rates are optimized to best describe the observed OH radical decay. This method is one of few direct measurements of atmospherically relevant peroxy radical isomerization reaction coefficients that have been reported in literature so far. Experimentally derived values are compared to quantum-chemical calculations of the 1,4 H-shift reaction rate and to rate coefficients reported in literature for the isomerization reaction and the OH reactions of methacrolein.