Atmospheric oxidation of hydrofluoroolefins and hydrochlorofluoroolefins by ozone produces HFC-23, PFC-14, and CFC-13

Abstract

Hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs) are fluorinated compounds developed to replace refrigerant and propellant gases known to be ozone-depleting substances and/or potent greenhouse gases (GHGs). Their short atmospheric lifetimes result in low direct global warming potentials, but the environmental impacts of their degradation products remain poorly understood. We show that gas-phase ozonolysis of four HFOs produces the long-lived GHGs trifluoromethane (HFC-23) or carbon tetrafluoride (PFC-14), while HCFO-1233xf produces the ozone-depleting chlorotrifluoromethane (CFC-13). At 298 K and 1 bar, the HFC-23 yield is + (7.9 0.2)% 0.4 from HFO-1234ze(E) ozonolysis, and the PFC-14 yield is + (1.04 0.05)% 0.07 from HFO-1225ye(E), + (1.02 0.05)% 0.05 from HFO-1225ye(Z), and + (0.12 0.01)% 0.04 from HFO-1234yf, while HCFO-1233xf ozonolysis produces CFC-13 in + (0.034 0.006)% 0.009 yield. Global model integrations quantify the atmospheric impacts of these breakdown products. Mechanistic computational studies link the HFO and HCFO molecular structures to the formation of these persistent species, identifying structural features that favor their production. These results highlight the importance of considering not only the direct environmental influence of replacement compounds but also the consequences of their atmospheric degradation. The new insights that emerge will guide efforts to design compounds with lower long-term environmental impacts.