Low-Pressure Photolysis of 2,3-Pentanedione in Air: Quantum Yields and Reaction Mechanism

Dicarbonyls in the atmosphere mainly arise from secondary sources as reaction products in the degradation of a large number of volatile organic compounds (VOC). Because of their sensitivity to solar radiation, photodissociation of dicarbonyls can dominate the fate of these VOC and impact the atmospheric radical budget. The photolysis of 2,3-pentanedione (PTD) has been investigated for the first time as a function of pressure in a static reactor equipped with continuous wave cavity ring-down spectroscopy to measure the HO₂ radical photostationary concentrations along with stable species. We showed that (i) Stern–Volmer plots are consistent with low OH-radical formation yields in RCO + O₂ reactions, (ii) the decrease of the photodissociation rate due to pressure increase from 26 to 1000 mbar is of about 30%, (iii) similarly to other dicarbonyls, the Stern–Volmer analysis shows a curvature at the lower pressure investigated, which may be assigned to the existence of excited singlet and triplet PTD states, (iv) PTD photolysis at 66 mbar leads to CO₂, CH₂O and CO with yields of (1.16 ± 0.04), (0.33 ± 0.02) and (0.070 ± 0.005), respectively, with CH₂O yield independent of pressure up to 132 mbar and CO yield in agreement with that obtained at atmospheric pressure by Bouzidi et al. (2014), and (v) the PTD photolysis mechanism remains unchanged between atmospheric pressure and 66 mbar. As a part of this work, the O₂ broadening coefficient for the absorption line of HO₂ radicals at 6638.21 cm^–1 has been determined (γ_O2 = 0.0289 cm^–1 atm^–1)

Gas-Phase Reaction of Hydroxyl Radical with Hexamethylbenzene

Aromatic hydrocarbons are important components of polluted ambient air. The reaction of OH radicals with hexamethylbenzene (HMB) is a prototype system to study ipso addition leading eventually to dealkylation. We have investigated the OH + HMB and OD + HMB reactions between 323 and 433 K using a discharge fast-flow reactor coupled to a time-of-flight mass spectrometer with single-photon VUV photoionization (10.54 eV). The H atom abstraction channel has been found to be equal to (13.7 ± 4.4) % at 330 K leading to (11.1 ± 3.6) % at 298 K, higher than predicted by commonly used structure–reactivity relationships. The back dissociation rate constant has also been measured and has been found to be smaller than the rate of other aromatic hydrocarbons, in good agreement with density functional theoretical calculations. The dealkylation channel, leading to pentamethylphenol (PMP) + CH₃, is always found to be the minor channel, estimated inferior to 2% at 298 K