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

Abstract

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 (γO2 = 0.0289 cm(-1) atm(-1)).