Investigation of the oxidation of methyl vinyl ketone (MVK) by OH radicals in the atmospheric simulation chamber SAPHIR

The photooxidation of methyl vinyl ketone (MVK) was investigated in the atmospheric simulation chamber SAPHIR for conditions at which organic peroxy radicals (RO 2 ) mainly reacted with NO ( high NO case) and for conditions at which other reaction channels could compete ( low NO case). Measurements of trace gas concentrations were compared to calculated concentration time series applying the Master Chemical Mechanism (MCM version 3.3.1). Product yields of methylglyoxal and glycolaldehyde were determined from measurements. For the high NO case, the methylglyoxal yield was (19 ± 3) % and the glycolaldehyde yield was (65 ± 14) %, consistent with recent literature studies. For the low NO case, the methylglyoxal yield reduced to (5 ± 2) % because other RO 2 reaction channels that do not form methylglyoxal became important. Consistent with literature data, the glycolaldehyde yield of (37 ± 9) % determined in the experiment was not reduced as much as implemented in the MCM, suggesting additional reaction channels producing glycolaldehyde. At the same time, direct quantification of OH radicals in the experiments shows the need for an enhanced OH radical production at low NO conditions similar to previous studies investigating the oxidation of the parent VOC isoprene and methacrolein, the second major oxidation product of isoprene. For MVK the model–measurement discrepancy was up to a factor of 2. Product yields and OH observations were consistent with assumptions of additional RO 2 plus HO 2 reaction channels as proposed in literature for the major RO 2 species formed from the reaction of MVK with OH. However, this study shows that also HO 2 radical concentrations are underestimated by the model, suggesting that additional OH is not directly produced from RO 2 radical reactions, but indirectly via increased HO 2 . Quantum chemical calculations show that HO 2 could be produced from a fast 1,4-H shift of the second most important MVK derived RO 2 species (reaction rate constant 0.003 s −1 ). However, additional HO 2 from this reaction was not sufficiently large to bring modelled HO 2 radical concentrations into agreement with measurements due to the small yield of this RO 2 species. An additional reaction channel of the major RO 2 species with a reaction rate constant of (0.006 ± 0.004) s −1 would be required that produces concurrently HO 2 radicals and glycolaldehyde to achieve model–measurement agreement. A unimolecular reaction similar to the 1,5-H shift reaction that was proposed in literature for RO 2 radicals from MVK would not explain product yields for conditions of experiments in this study. A set of H-migration reactions for the main RO 2 radicals were investigated by quantum chemical and theoretical kinetic methodologies, but did not reveal a contributing route to HO 2 radicals or glycolaldehyde.