Theoretical study on the mechanisms, kinetics and risk assessment of OH radicals and Cl atom initiated transformation of HCFC-235fa in the atmosphere

Abstract

Hydrochlorofluorocarbons (HCFCs) are important greenhouse gases and ozone-depleting substances. Thus, a thorough understanding of their atmospheric fate is essential for preventing and controlling atmospheric pollution. Herein, the atmospheric transformation mechanism of CF₃CH₂CClF₂ (HCFC-235fa) by the OH radical and the Cl atom was carried out at the dual-level of CCSD(T)/aug-cc-pVTZ//M06-2X/6-311+G(d,p). The reaction rate coefficients were calculated using the multistructural canonical variational transition state theory with small curvature tunneling (MS-CVT/SCT) at 200–1000 K. The k^MS-CVT/SCT(CF₃CH₂CClF₂ + OH) and k^MS-CVT/SCT(CF₃CH₂CClF₂ + Cl) values are 9.05 × 10⁻¹⁵ and 1.95 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ at 297 K, respectively. The results show that the role of OH is more important than Cl in the degradation of CF₃CH₂CClF₂. The atmospheric lifetimes (83 days–77.93 years), ozone destruction potential (0.001–0.023), and global warming potentials (GWP₁₀₀ = 21.06–5157.35) of CF₃CH₂CClF₂ were assessed, and these results indicate that CF₃CH₂CClF₂ is atmospherically persistent and environmentally unfriendly. The evolution mechanisms of CF₃C·HCClF₂, CF₃C(OO˙)HCClF₂, and CF₃C(O˙)HCClF₂ in the presence of O₂, HO₂˙, and NO were investigated and discussed. The resulting products of CF₃CH₂CClF₂ are mostly highly oxidized multi-functional compounds in the forms of aldehydes, ketones, and organic nitrates. A computational assessment of acute and chronic toxicities was performed at three levels of nutrition in order to improve the understanding of the potential toxicity of CF₃CH₂CClF₂ and its degradation products to the aquatic environment. The acidification potential of CF₃CH₂CClF₂ was calculated to be 1.141 and presumed to contribute to the formation of acid rain. The results may contribute to describing HCFCs' atmospheric fate, persistence, and environmental risks.