Atmospheric fates and global warming potential of HFO-1234ze(E) and its degradation product trifluoroacetaldehyde (CF3CHO)

Abstract

Hydrofluoroolefins (HFOs) are replacing high-GWP hydrofluorocarbons (HFCs) across multiple applications including foam blowing, refrigeration, and aerosols, but their atmospheric degradation and climate consequences remain uncertain. We use the GEOS-Chem 3-D chemical transport model, supported by AtChem2 box-model simulations, to develop a complete representation of the atmospheric chemistry and fate of HFO-1234ze(E) and its key intermediate product, CF₃CHO. We focus on HFO-1234ze(E) as it is the dominant isomer in commercial use. The model includes newly measured CF₃CHO photolysis quantum yields to form fluoroform (HFC-23), the recently identified chemical pathways of HFO-1234ze(E) ozonolysis and CF₃CHO reversible reaction with HO₂, and explicit wet and dry deposition parameterisations. Using observationally constrained global HFO-1234ze(E) emissions of 15 Gg yr⁻¹, simulated HFO-1234ze(E) surface mixing ratios agree well with 2020-2024 observations at 8 Advanced Global Atmospheric Gases Experiment (AGAGE) network sites. We find that 99.6% of HFO-1234ze(E) is removed by reaction with OH, with the remaining 0.4% lost to ozonolysis. Sensitivity tests for effective Henry's law constants (K_H) spanning 10 - 10⁶ M atm⁻¹ show sensitivity of CF₃CHO fate to K_H up to 10⁴ M atm⁻¹ and saturation at higher K_H^*. Using an upper bound of 10⁵ M atm⁻¹, deposition accounts for ≈51% of total CF₃CHO loss in GEOS-Chem (20% dry, 31% wet), with photolysis contributing ≈33% and OH reaction ≈15%. The reversible reaction with HO₂ contributes around 1% to net CF₃CHO loss due to rapid conversion of the reaction products back to reactants. We calculate a total (direct + indirect) GWP₁₀₀ for HFO-1234ze(E) of around 15, with CF₃CHO photolysis to HFC-23 contributing 8.2. We also estimate a maximum potential formation of 4.5 Gg yr⁻¹ of trifluoroacetic acid (TFA) under current emissions assuming complete conversion of wet-deposited CF₃CHO from HFO-1234ze(E), suggesting a potential unrecognised TFA source from all CF₃CHO sources.