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)

Experimental Study of the Reactions of Limonene with OH and OD Radicals: Kinetics and Products

The kinetics of the reactions of limonene with OH and OD radicals has been studied using a low-pressure flow tube reactor coupled with a quadrupole mass spectrometer: OH + C₁₀H₁₆ → products (1), OD + C₁₀H₁₆ → products (2). The rate constants of the title reactions were determined using four different approaches: either monitoring the kinetics of OH (OD) radicals or limonene consumption in excess of limonene or of the radicals, respectively (absolute method), and by the relative rate method using either the reaction OH (OD) + Br₂ or OH (OD) + DMDS (dimethyl disulfide) as the reference one and following HOBr (DOBr) formation or DMDS and limonene consumption, respectively. As a result of the absolute and relative measurements, the overall rate coefficients, <i>k</i>₁ = (3.0 ± 0.5) × 10^–11 exp((515 ± 50)/<i>T</i>) and <i>k</i>₂ = (2.5 ± 0.6) × 10^–11 exp((575 ± 60)/<i>T</i>) cm³ molecule^–1 s^–1, were determined at a pressure of 1 Torr of helium over the temperature ranges 220–360 and 233–353 K, respectively. <i>k</i>₁ was found to be pressure independent over the range 0.5–5 Torr. There are two possible pathways for the reaction between OH (OD) and limonene: addition of the radical to one of the limonene double bonds (reactions and ) and abstraction of a hydrogen atom (reactions and ), resulting in the formation of H₂O (HOD). Measurements of the HOD yield as a function of temperature led to the following branching ratio of the H atom abstraction channel: <i>k</i>_2b/<i>k</i>₂ = (0.07 ± 0.03) × exp((460 ± 140)/<i>T</i>) for <i>T</i> = (253–355) K