## 2 HFC Phase Down  
The impact of F-gases on the health and well-being of societies and on ecosystems is now well recognized by the scientific community, governments, international entities, and the general public. International agreements, with transposition to national legislations, have been signed in the last few decades aiming to establish policy regimes at global and local levels to reduce F-gas emissions and to limit their contribution to climate change. Initially, the Vienna Convention for the Protection of the Ozone Layer (1985) and the Montreal Protocol (1987) determined the phasing out of ozone-depleting substances, such as chlorosulfoocarbons (CFCs). Then, the Kyoto Protocol (1997) and the Paris Agreement (2015) determined the need to limit the atmospheric levels of GHGs at safe values. However, these agreements did not solve the F-gas emission problem. In fact, the phasing out of ozone-depleting substances under the Montreal Protocol and the UE Regulation (EC) No 1005/2009 led to a drastic increase in the utilization of F-gases, such as HFCs, as substitutes for those substances [21]. Then, the Kigali Amendment to the Montreal Protocol, covered by the United Nations Framework Convention on Climate Change (UNFCCC), was signed in 2016. This amendment, which entered into force on 1 January 2019, established the first international regulation to decrease F-gas emissions and replace them by more environmentally friendly alternative refrigerants, aiming at avoiding up to a 0.5 \({}^{\circ}\)C increase in global temperature by the end of the century [22]. The European Union imposed strong F-gas regulations to achieve the goals of the Paris Agreement and of the Kigali Amendment, and the EU's objectives included limiting the rise in the temperature due to climate change at 2 \({}^{\circ}\)C and cutting the overall GHG emissions by 20% in 2020, by at least 55% in 2030, and up to 95% in 2050, compared to 1990 levels [23]. The first EU F-gas Regulation (Regulation (EC) No 842/2006) established measures to improve the management and recovery of F-gases and to restrict the use and marketing of F-gases in cases in which more environmentally friendly and cost-effectivealternatives exist [24]. This regulation was replaced by the Regulation (EC) No 517/2014, which enforced new mechanisms to ensure an effective reduction in HFC consumption by 79% and the F-gas emissions by two thirds of the average of 2009-2012 levels by 2030 [23]. This regulation establishes the phase-down of F-gases, bans the use of F-gases in new equipment for which alternative gases are available, establishes annual quotas of produced and imported HFCs, regulates the equipment maintenance to prevent leaks, and regulates the recovery, reclaim or destruction of F-gases at EoL equipment [23]. On one hand, the EU HFC phase-down is defined on carbon dioxide equivalents (CO\({}_{2}\)eq) and therefore it has a greater impact on high GWP refrigerants, such as R-404A (GWP 3922), R-410A (GWP 2088), R-407A (GWP 2107), and R-134a (GWP 1430) [23]. On the other hand, it does not apply to new alternatives such as unsaturated HFCs, which are only subjected to reporting obligations [23]. This instigated industries to make the transition into refrigerants with a lower GWP (using, for example, unsaturated HFCs, as discussed in Section 2.1). The European Commission allocates quotas in tonnes (CO\({}_{2}\)eq) to importers and producers of bulk gases and mandates the registry of HFCs for all companies covered by the quota system, including gas importers and producers and equipment importers. According to EU data, the demand for HFCs has been decreasing since the EU F-gas Regulation was implemented, as a consequence of the increased demand for low-GWP alternatives. In 2019, total HFCs placed in the market were 2% below the quota limit for that year and the amount of reserve quotas is increasing each year [25]. Total imports of F-gases have decreased from 2017, as in 2019 the amounts were 20% (or 39% as CO\({}_{2}\)eq) below those of 2017 (Figure 1).