Comment on egusphere-2025-4824

Abstract. Atmospheric observations of the widely used hydrofluoroolefins (HFOs) HFO-1234yf (2,3,3,3-tetrafluoroprop-1-ene), and HFO-1234ze(E) (E-1,3,3,3-tetrafluoroprop-1-ene), and the hydrochlorofluoroolefin (HCFO) HCFO-1233zd(E) (E-1-chloro-3,3,3-trifluoroprop-1-ene) are reported from the Advanced Global Atmospheric Gases Experiment (AGAGE) network. Since 2011, pollution events have grown in magnitude and frequency at sites which are influenced by regional emissions, while remote stations show first appearances of these substances. For HFO-1234yf and HFO-1234ze(E) winter peak mole fractions in background northern hemisphere air have grown from 0.03 ppt (picomol mol-1, parts-per-trillion in dry air) in the mid-2010s to 0.25 ppt in 2024, while the atmospherically more stable HCFO-1233zd(E) showed an increase from 0.06 ppt to 0.45 ppt. This suggests increasing usage of these haloolefins to replace hydrofluorocarbons (HFCs), which are regulated for phase-down over the next decades. Using European observations and the inverse modeling frameworks InTEM, ELRIS, and RHIME we determine emission trends and regional distribution. For Northwest Europe, emissions of HFO-1234yf increased steadily and rapidly from