Confirming the minimal role of in-atmosphere production on the global HFC-23 budget using a 3D chemical and transport model

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 (HFCs) and hydrofluoroolefins (HFOs) contributes to this gap, but only a conservative upper limit has been estimated for the magnitude of this source. This uncertainty is due, in part, to limited experimental measurements of the photolysis quantum yield of trifluoroacetaldehyde (CF3CHO), a key degradation product that forms HFC-23 via photolysis. The parameters governing CF3CHO deposition are also poorly understood. Here, we use a 3D chemistry and transport model, STOCHEM-CRI, to further constrain the magnitude of in-atmosphere HFC-23 production, using recent estimates of source gas emissions and explicitly parametrised photolysis and deposition. Furthermore, we perform an ensemble of simulations to account for 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, substantially lower than previous estimates. This accounts for < 0.5 % of the discrepancy between reported and measurement-derived emissions, suggesting that this source makes a negligible contribution to the overall HFC-23 budget and that unreported direct emissions are likely responsible for the vast majority of the discrepancy. 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.