CHF3 (HFC-23) and CF3CHO Quantum Yields in the Pulsed Laser Photolysis of CF3CHO at 248, 266, 281, and 308 nm

Abstract

Ultraviolet (UV) photolysis of trifluoroacetaldehyde (CF₃CHO), an atmospheric degradation product of anthropogenically emitted compounds, is a potential source of fluoroform (CHF₃, HFC-23), which is a potent greenhouse gas. In this study, CF₃CHO photolysis quantum yields, ϕ_CF3CHO(λ, P), and CHF₃ product and quantum yields were measured following the pulsed laser photolysis of CF₃CHO at 248, 266, 281, and 308 nm at total pressures between 100 and 678 Torr (N₂/NO bath gas mixtures). Loss of CF₃CHO was measured using Fourier transform infrared spectroscopy (FTIR), and the formation of CHF₃ was measured by FTIR (248, 266, and 281 nm) and off-line gas chromatography–mass spectrometry (GC/MS) (248, 266, and 308 nm). ϕ_CF3CHO(248 nm, 650–678 Torr) and ϕ_CF3CHO(266 nm, 650 Torr) were measured to be 1.03 ± 0.06 and 0.94 ± 0.06, respectively, i.e., near unity, using CBrF₂CBrF₂ (Halon-2402) and CF₃I as the actinometry references, respectively. The CHF₃ molar product yields at 248 and 266 nm were measured to be 0.348 ± 0.02 and 0.438 ± 0.015, respectively. CF₃CHO quantum yields measured for 281 and 308 nm photolysis were found to be pressure-dependent over the 100 to 650 Torr (N₂/NO) pressure range. A Stern–Volmer analysis of the CF₃CHO quantum yield results yielded k_Q/k_D values of (3.25 ± 0.20) × 10^–20 cm³ molecule^–1 at 281 nm and (1.91 ± 0.02) × 10^–19 cm³ molecule^–1 at 308 nm, i.e., ϕ_CF3CHO(281 nm, 650 Torr) = 0.54 ± 0.06 and (308 nm, 650 Torr) = 0.204 ± 0.028. The CHF₃ molar product yields obtained at 281 and 308 nm showed a weak decrease with increasing pressure with values of 0.105 ± 0.002 and (1.71 ± 0.70) × 10^–3 obtained at 650 Torr, respectively. All quoted measurement uncertainties are 2σ standard deviation. An evaluation of this photochemical source of CHF₃ following the atmospheric degradation of hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), hydrofluoroolefins (HFOs), and hydrochlorofluoroolefins (HCFOs) that are observed in the present-day atmosphere is presented and the atmospheric implications discussed.