OH Radical Initiated Reactions of Unsaturated Ketones: Kinetics and Atmospheric Implications

When volatile organic compounds undergo atmospheric oxidation, they can form various oxygenated organic compounds, including unsaturated ketones. Unsaturated ketones in the atmosphere can impact air quality and human health. The kinetics for the oxidation of 5-hexen-2-one (3-buten-1-yl methyl ketone, 3BMK) and 4-hexen-3-one (ethyl-1-propenyl ketone, E1PK) with OH radicals were investigated experimentally and computationally. The experiments were carried out using pulsed laser photolysis - laser-induced fluorescence technique over 263-358 K and over 26-30 Torr. At 298 K, the rate coefficients for the reactions of 3BMK and E1PK were (4.72 ± 0.12) × 10^-11 and (8.45 ± 0.13) × 10^-11 cm³ molecule^-1 s^-1, respectively. The temperature-dependent rate coefficients were expressed using the Arrhenius equations to be k_3BMK+OH^T=(263-358)K = (2.45 ± 0.68) × 10^-12 exp{-(-948 ± 85)/T} and k_E1PK+OH^T=(263-358)K = (1.16 ± 0.81) × 10^-12 exp{-(-1241 ± 212)/T} cm³ molecule^-1 s^-1. The purpose of the computational study was to improve the comprehension of the kinetics and thermochemistry. The atmospheric lifetime for both compounds were calculated to be ∼4 and ∼2 days for 3BMK and E1PK, respectively. Gas chromatography-mass spectrometry (GC-MS) was used to examine the secondary organic products that resulted from the reactions at 298 K and 760 Torr. 2-Hydroxyacetaldehyde and acetaldehyde were detected as the products of the degradation of 3BMK and E1PK.