Atmospheric Chemistry of c‑C5HF7 and c‑C5F8: TemperatureDependent OH...

Spectra, I., G. W. Potentials, T. Gierczak, F. Bernard, D. K. Papanastasiou, and J. Burkholder (2021), Atmospheric Chemistry of c‑C5HF7 and c‑C5F8: TemperatureDependent OH Reaction Rate Coefficients, Degradation Products,, J. Phys. Chem. A, 125, 1050-1061, doi:10.1021/acs.jpca.0c10561.

c-C5HF7 (1H-heptafluorocyclopentene) and c-C5F8 (perfluorocyclopentene) are potent greenhouse gases presently used as replacement compounds in Si etching. A thorough understanding of their potential impact on climate and air quality necessitates studies of their atmospheric reactivity, radiative properties, and atmospheric degradation pathways. The predominant atmospheric removal process for these compounds is expected to be via reaction with the OH radical. In this study, rate coefficients, k, for the gas-phase reaction of the OH radical with c-C5HF7 and c-C5F8 were measured over a range of temperatures (242−370 K) and pressures (50−100 Torr, He) using a pulsed laser photolysis−laser-induced fluorescence technique. In addition, a complementary relative rate technique, employing multiple reference compounds, was used to study the reactions between 273 and 372 K at 100 Torr (He) total pressure. Reaction rate coefficients were found to be independent of pressure over this range of conditions with k1(296 K) = (4.59 ± 0.10) × 10−14 cm3 molecule−1 s−1 and k1(T) = (4.00 ± 0.40) × 10−13 exp(−(631 ± 30)/T) cm3 molecule−1 s−1 for c-C5HF7 and k2(296 K) = (4.90 ± 0.14) × 10−14 cm3 molecule−1 s−1 and k2(T) = (3.59 ± 0.4) × 10−13 exp(−(591 ± 25)/T) cm3 molecule−1 s−1 for c-C5F8. Stable end-products were measured following the OH radical-initiated degradation of c-C5HF7 and c-C5F8 in the presence of O2. F(O)CCF2CF2CF2CH(O), CF2O, and CO2 were observed as the major end-products in the oxidation of c-C5HF7 with molar yields of 0.64, 1.27, and 0.53, respectively. For c-C5F8, F(O)CCF2CF2CF2CF(O), CF2O, and CO2 were observed with molar yields of 0.66, 0.63, and 0.43, respectively. The total carbon mass balance in both systems was 1.0 ± 0.15. The high yield of a C5-dicarbonyl endproduct is consistent with a ring opening at the carbon−carbon double bond site for both c-C5HF7 and c-C5F8. A comparison of the present kinetic and degradation product results with previously published studies is presented. A rate coefficient upper limit for the gas-phase reaction of O3 with c-C5HF7 and c-C5F8 of 1 × 10−21 cm3 molecule−1 s−1 was measured as part of this work. Atmospheric lifetimes for c-C5HF7 and c-C5F8 are estimated to be 252 and 236 days, respectively. Infrared absorption spectra of c-C5HF7 and cC5F8 were also measured and found to agree, to within 5%, with results from previous studies. The well-mixed and lifetime adjusted radiative efficiencies (RE, W m−2 ppb−1) and 100 year time horizon global warming potential (GWP) for c-C5HF7 are 0.35, 0.24, and 46.7 and for c-C5F8 are 0.38, 0.25, and 46.2, respectively.

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