Atmospheric oxidation of α , β -unsaturated ketones: kinetics and mechanism of the OH radical reaction

Abstract. The OH-radical-initiated oxidation of 3-methyl-3-penten-2-one and 4-methyl-3-penten-2-one was investigated in two atmospheric simulation chambers at 298±3  K and 990±15  mbar using long-path FTIR spectroscopy. The rate coefficients of the reactions of 3-methyl-3-penten-2-one and 4-methyl-3-penten-2-one with OH radicals were determined to be ( 6.5 ± 1.2 ) × 10 - 11 and ( 8.1 ± 1.3 ) × 10 - 11 cm 3 molecule - 1 s - 1 , respectively. To enlarge the kinetics data pool the rate coefficients of the target species with Cl atoms were determined to be ( 2.8 ± 0.4 ) × 10 - 10 and ( 3.1 ± 0.4 ) × 10 - 10 cm 3 molecule - 1 s - 1 , respectively. The mechanistic investigation of the OH-initiated oxidation focuses on the RO2+NO reaction. The quantified products were acetoin, acetaldehyde, biacetyl, CO2 and peroxyacetyl nitrate (PAN) for the reaction of 3-methyl-3-penten-2-one with OH radicals and acetone, methyl glyoxal, 2-hydroxy-2-methylpropanal, CO2 and peroxyacetyl nitrate (PAN) for the reaction of 4-methyl-3-penten-2-one with OH, respectively. Based on the calculated product yields an upper limit of 0.15 was determined for the yield of RONO2 derived from the OH reaction of 4-methyl-3-penten-2-one. By contrast, no RONO2 formation was observed for the OH reaction of 3-methyl-3-penten-2-one. Additionally, a simple model is presented to correct product yields for secondary processes.