Photolysis study of fluorinated ketones under natural sunlight conditions

Abstract

UV-visible absorption cross-sections are reported for CF₃C(O)CH₃, CF₃C(O)CH₂CH₃, and CH₃CH₂C(O)CH(CH₃)₂. The photolysis rate constants of CF₃C(O)CH₃, CF₃C(O)CH₂CH₃, and CF₃CF₂C(O)CF(CF₃)₂ were measured from smog-chamber experiments carried out in a 400 L Teflon-bag reactor under sunlight irradiation. Actinic radiation profiles from the “Tropospheric Ultraviolet and Visible Radiation Model” were used to obtain quantum efficiencies of photolysis: 0.34 ± 0.08, 0.24 ± 0.06, and (4.4 ± 0.6) × 10⁻² for CF₃C(O)CH₃, CF₃C(O)CH₂CH₃, and CF₃CF₂C(O)CF(CF₃)₂, respectively. These values correspond to wavelength ranges of 295–345 nm (for CF₃C(O)CH₃ and CF₃C(O)CH₂CH₃) and 295–360 nm (for CF₃CF₂C(O)CF(CF₃)₂). The photolysis rate constants change significantly with the seasons, with the yearly averages being (2.3 ± 0.7) × 10⁻⁶, (1.8 ± 0.6) × 10⁻⁶, and (2.1 ± 0.8) × 10⁻⁶ s⁻¹ for CF₃C(O)CH₃, CF₃C(O)CH₂CH₃, and CF₃CF₂C(O)CF(CF₃)₂, respectively. Photolysis processes are fast and responsible for the short gas-phase lifetimes of the studied ketones, which are 5.1 ± 2.2, 6.5 ± 2.5 and 5.5 ± 1.5 days. The radiative forcing efficiencies are provided to assess the contribution of emissions of these gases to climate change. As a result of the short atmospheric lifetimes, their global warming potentials are negligible. Theoretical calculations involving ground and excited states justify the higher photolysis quantum efficiencies of CF₃C(O)CH₃ and CF₃C(O)CH₂CH₃ compared to CF₃CF₂C(O)CF(CF₃)₂, which shows increased photolysis rate constants in the absence of O₂.