Photooxidation of 2-Pentanone in the Gas Phase: Photo-Products, Reaction Mechanism, OH Reaction Kinetics, and Atmospheric Implication

Abstract

Photooxidation of 2-pentanone (2PN, C3H7C(O)CH3), an atmospherically abundant volatile linear carbonyl compound and a proposed second-generation fuel, was studied in a laboratory reactor under simulated tropospheric conditions under the exposure of ultraviolet (UV) light of wavelength of 311 nm, which is abundant in the troposphere. Formic acid (HC(O)OH), acetic acid (CH3C(O)OH), formaldehyde (HCHO), methanol (CH3OH), and carbon dioxide (CO2) were identified as the photooxidation products using FTIR spectroscopy as the detection method. Photodissociation of 2PN resulted in the formation of acetone (CH3C(O)CH3) and ethylene (C2H4) by Norrish type-2 reaction, while carbon monoxide (CO) was found to be formed by both photodissociation and photooxidation. The yields of CH3C(O)CH3, C2H4, CO, CH3OH, HC(O)OH, and HCHO were determined to be (54.0 ± 2.7), (61.7 ± 3.4), (47.7 ± 4.7), (8.1 ± 0.7), (4.4 ± 0.3), and (2.7 ± 0.7) %, respectively. The yield of CH3C(O)OH cannot be measured due to its low absorbance and spectral interferences by other species in the probe FTIR spectra. A reaction mechanism involving photodissociation and subsequent reactions of the produced radicals is presented for the formation of different photooxidation products. A reaction modeling study was performed for further corroboration of the proposed mechanism of photooxidation reaction. Reinvestigated here also was the reaction kinetics of 2PN with OH by the relative rate method using FTIR detection, and a rate constant of (5.1 ± 0.8) × 10–12 cm3 molecule–1 s–1 at room temperature was estimated, which agrees well with previous reports. The estimated tropospheric lifetime of 2PN by a UV photooxidation channel (∼3.2–7.2 days) is comparable with that for reaction with the OH (∼2.3 days). Carboxylic acids formed in the photooxidation of 2PN can be significant for atmospheric acidity and secondary organic aerosol (SOA) formation.