Atmospheric chemistry of (CF3)2CCH2: OH radicals, Cl atoms and O3 rate coefficients, oxidation end-products and IR spectra

Abstract

The rate coefficients for the gas phase reactions of OH radicals, k₁, Cl atoms, k₂, and O₃, k₃, with 3,3,3-trifluoro-2(trifluoromethyl)-1-propene ((CF₃)₂CCH₂, hexafluoroisobutylene, HFIB) were determined at room temperature and atmospheric pressure employing the relative rate method and using two atmospheric simulation chambers and a static photochemical reactor. OH and Cl rate coefficients obtained by both techniques were indistinguishable, within experimental precision, and the average values were k₁ = (7.82 ± 0.55) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ and k₂ = (3.45 ± 0.24) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, respectively. The quoted uncertainties are at 95% level of confidence and include the estimated systematic uncertainties. An upper limit for the O₃ rate coefficient was determined to be k₃ < 9.0 × 10⁻²² cm³ molecule⁻¹ s⁻¹. In global warming potential (GWP) calculations, radiative efficiency (RE) was determined from the measured IR absorption cross-sections and treating HFIB both as long (LLC) and short (SLC) lived compounds, including estimated lifetime dependent factors in the SLC case. The HFIB lifetime was estimated from kinetic measurements considering merely the OH reaction, τ_OH = 14.8 days and including both OH and Cl chemistry, τ_eff = 10.3 days. Therefore, GWP(HFIB,OH) and GWP(HFIB,eff) were estimated to be 4.1 (LLC) and 0.6 (SLC), as well as 2.8 (LLC) and 0.3 (SLC) for a hundred year time horizon. Moreover, the estimated photochemical ozone creation potential (ε^POCP) of HFIB was calculated to be 4.60. Finally, HCHO and (CF₃)₂C(O) were identified as final oxidation products in both OH- and Cl-initiated oxidation, while HC(O)Cl was additionally observed in the Cl-initiated oxidation.