Further constraining the role of in-atmosphere production on the global HFC-23 budget

Abstract. A large discrepancy of at least 10 Gg yr^-1 exists between reported emissions of the potent greenhouse gas HFC-23 (CHF₃, trifluoromethane) and emissions derived from atmospheric measurements. In-atmosphere production of HFC-23 from the breakdown of fluorinated source gases such as hydrofluorocarbons and hydrofluoroolefins contributes to this gap, but the magnitude of this source is weakly constrained. This uncertainty is due, in part, to limited experimental measurements of the photolysis quantum yield of trifluoroacetaldehyde (CF₃CHO), a key degradation product which forms HFC-23 via photolysis. The parameters governing CF₃CHO deposition are also poorly understood. Previous work reported an upper limit of the contribution of the in-atmosphere source to the global HFC-23 burden. Here, we use a 3D chemistry and transport model to further constrain this contribution, using recent estimates of source gas emissions, kinetic rate constants, photolysis rates and deposition parameters, as well as considering the uncertainties in these values. We find that in-atmosphere production of HFC-23 is in the range 0.013–0.035 Gg yr⁻¹, significantly lower than previous estimates. This accounts for <0.5 % of the discrepancy between reported emissions and those derived from atmospheric observations, suggesting that this source makes a negligible contribution to the overall HFC-23 budget. As part of this work, we also calculate indirect global warming potentials for the HFC-23 source gases HFO-1234ze(E), HFO-1336mzz(Z) and HCFO-1233zd(E) and find that their impact on climate is up to ten times higher than previously reported.