Atmospheric Formation of Trifluoroacetic Acid (TFA) from Fluorinated Refrigerants: Current Evidence and Remaining Uncertainties

The phase-down of hydrofluorocarbons (HFCs) is driving a shift to hydrofluoroolefins (HFOs), hydrochlorofluoroolefins (HCFOs) and their blends. HFOs and HCFOs have a low global warming potential (GWP) and are short-lived, but several of them are oxidised in the atmosphere to trifluoroacetic acid (TFA), which is removed by wet and dry deposition and persists in water and soil. This short review examines these fluorinated gases as atmospheric precursors of TFA and the implications for their use. Their conversion to TFA depends on the carbonyl formed: HFO-1234yf, the main refrigerant in mobile air conditioning, gives trifluoroacetyl fluoride and hence TFA almost quantitatively, whereas HFO-1234ze(E) and HCFO-1233zd(E) form trifluoroacetaldehyde, whose uncertain fate leads to lower conversion and to some formation of the potent greenhouse gas HFC-23. Because these precursors react within days to weeks, TFA formation shifts from a global to a continental scale, with deposition concentrated in and downwind of emission regions. Recent modelling suggests that HFO-1234yf may already exceed HFC-134a as a source of TFA over Europe. Open questions concern the fate of trifluoroacetaldehyde, source attribution and the monitoring of TFA in air.