Hydrofluorocarbon
Hydrofluorocarbons (HFCs) are a class of synthetic organic compounds composed solely of carbon, hydrogen, and fluorine atoms, developed as alternatives to ozone-depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) for applications including refrigeration, air conditioning, foam blowing, aerosol propellants, solvents, and fire suppression.[1][2][3] Exhibiting negligible ozone depletion potential due to the absence of chlorine or bromine, HFCs nonetheless function as potent greenhouse gases with global warming potentials typically hundreds to thousands of times greater than carbon dioxide over a 100-year horizon, enabling even trace atmospheric concentrations to exert substantial radiative forcing.[2][3][4] The phase-out of ozone-depleting substances under the 1987 Montreal Protocol accelerated HFC adoption, with global emissions rising sharply thereafter and contributing an estimated 1-2% to anthropogenic radiative forcing by the early 21st century, though this share is projected to peak and decline under regulatory interventions.[3][4] The 2016 Kigali Amendment to the Montreal Protocol established a framework for phasing down HFC production and consumption by 80-85% relative to baseline levels by 2047, with developed nations initiating reductions earlier and developing nations following staggered timelines, potentially averting up to 0.4°C of warming by 2100 if fully implemented.[5][6] While HFCs addressed stratospheric ozone risks effectively, their unintended climate trade-offs have prompted scrutiny, including evidence of minor direct ozone depletion from certain variants under high-altitude photolysis conditions, underscoring the challenges of sequential environmental substitutions without comprehensive lifecycle assessments.[7][3]Chemical Structure and Properties
Molecular Composition
Hydrofluorocarbons (HFCs) are synthetic organic compounds composed solely of carbon, hydrogen, and fluorine atoms.[8] These elements form molecules where carbon serves as the backbone, with hydrogen and fluorine atoms bonded to it, typically resulting in stable, non-reactive structures under normal conditions.[1] Unlike chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs), HFCs lack chlorine or other halogens beyond fluorine, defining their elemental composition.[8] The molecular architecture of HFCs generally features saturated or partially unsaturated hydrocarbon chains with partial fluorination, expressed empirically as C_xH_yF_z, where x ranges from 1 to 6, y and z vary to achieve specific functionalities, and the degree of fluorination influences volatility and stability.[9] Commercial HFCs are predominantly acyclic alkanes with 1–4 carbon atoms, where fluorine substitution replaces one or more hydrogen atoms on the carbon skeleton, enhancing thermodynamic properties for applications like refrigeration.[10] Bonding consists of strong carbon-fluorine (C–F) and carbon-hydrogen (C–H) single bonds, with carbon-carbon (C–C) linkages forming the chain; the high electronegativity of fluorine contributes to the polarity and low reactivity of these molecules.[9] Common HFCs illustrate this composition, as shown in the following examples:| Designation | Systematic Name | Molecular Formula |
|---|---|---|
| HFC-23 | Trifluoromethane | CHF₃ |
| HFC-32 | Difluoromethane | CH₂F₂ |
| HFC-134a | 1,1,1,2-Tetrafluoroethane | CH₂FCF₃ |
| HFC-125 | Pentafluoroethane | CHF₂CF₃ |
Physical and Thermodynamic Properties
Hydrofluorocarbons (HFCs) are colorless gases or low-boiling liquids at ambient conditions, characterized by high chemical stability, low solubility in water, and generally non-polar nature due to strong C-F bonds.[12] Their physical properties, including moderate molecular weights and densities, enable efficient phase changes in thermodynamic cycles. Most common HFCs are non-flammable and exhibit low toxicity, though variants like HFC-32 possess mild flammability.[13] Key thermodynamic properties such as boiling points, critical temperatures, and liquid densities vary by compound but support applications requiring latent heat absorption near 200-250 kJ/kg and specific heat capacities around 1-1.5 kJ/kg·K in liquid phase.[14] Vapor pressures are high, facilitating compression in refrigeration systems, with equations of state like perturbed-hard-sphere-chain models accurately predicting PVT behavior.[15] International standards, developed from experimental data by NIST and others, provide precise enthalpy, entropy, and sound speed values for design purposes.[16] The following table summarizes physical and thermodynamic properties for selected common HFCs:| HFC Designation | Chemical Formula | Molecular Weight (g/mol) | Boiling Point at 1 atm (°C) | Critical Temperature (°C) | Liquid Density at 25 °C (kg/m³) |
|---|---|---|---|---|---|
| HFC-32 | CH₂F₂ | 52.02 | -51.6 | 78.1 | - |
| HFC-125 | CHF₂CF₃ | 120.02 | -48.5 | 66.0 | 1245 |
| HFC-134a | CH₂FCF₃ | 102.03 | -26.1 | 101.1 | 1206 |
History and Development
Origins as CFC Alternatives
Hydrofluorocarbons (HFCs) emerged in the 1980s as engineered substitutes for chlorofluorocarbons (CFCs), driven by mounting evidence of stratospheric ozone depletion attributed to chlorine released from CFCs in the presence of ultraviolet radiation.[21] Scientific assessments, including the 1974 hypothesis by Mario Molina and F. Sherwood Rowland on CFC-induced ozone loss, prompted regulatory action, culminating in the 1987 Montreal Protocol, which required signatory nations to phase out CFC production and consumption by 1996 in developed countries.[22] HFCs, lacking chlorine atoms, were selected for their chemical stability and inability to catalyze ozone breakdown via the same radical chain reactions as CFCs, while retaining similar thermodynamic properties suitable for refrigeration cycles.[23] Chemical industry leaders, particularly DuPont, invested in HFC research anticipating CFC restrictions, with development focusing on compounds like 1,1,1,2-tetrafluoroethane (HFC-134a) as direct analogs to CFC-12 (dichlorodifluoromethane). DuPont secured a patent for an HFC-134a manufacturing process in 1982, involving hydrofluorination of precursor hydrocarbons, which enabled scalable production without ozone-depleting halogens.[24] Although HFC-134a had been synthesized earlier—appearing in chemical literature as early as 1936 and patented in 1959 for potential refrigerant use—its commercial viability as a CFC replacement was not pursued until the ozone crisis, with pilot-scale testing in the mid-1980s confirming low toxicity and non-flammability.[23] By the early 1990s, HFC-134a entered commercial production, first adopted in new automotive air conditioning systems in Europe and the United States around 1992–1994 to comply with accelerating CFC phase-outs under the Montreal Protocol's London Amendments (1990).[25] Other HFCs, such as HFC-32 and HFC-125, followed in blends like R-410A for stationary air conditioning, expanding the class's role beyond mobile refrigeration. This transition prioritized ozone safety over initial concerns about HFCs' infrared absorption properties, which later revealed their potent greenhouse gas effects.[22]Commercial Adoption and Expansion
Hydrofluorocarbons (HFCs) achieved widespread commercial adoption starting in the early 1990s, driven by the phase-out of ozone-depleting chlorofluorocarbons (CFCs) under the 1987 Montreal Protocol and its subsequent amendments. DuPont pioneered the commercialization of HFC-134a (1,1,1,2-tetrafluoroethane) in 1991 as a direct CFC substitute in refrigeration, air conditioning, and aerosol propellants, leveraging its zero ozone depletion potential while maintaining comparable thermodynamic performance.[26] This marked the shift to HFCs in developed markets, with initial production scaled by major chemical firms including DuPont and ICI (now Ineos Fluor), focusing on high-volume applications like domestic refrigerators and automotive systems.[23] By 1993, HFC-134a entered the automotive sector, replacing CFC-12 (R-12) in new vehicle air conditioning systems in the United States and Europe, with full regulatory mandates accelerating retrofits and original equipment adoption by the mid-1990s.[27] Concurrently, HFCs expanded into stationary refrigeration and foam-blowing agents, where blends like R-404A and R-507A gained traction for commercial and industrial uses due to their efficiency in low- and medium-temperature systems.[28] Global production capacity grew rapidly, supported by investments from Honeywell and Arkema, as HFCs filled the void left by CFC bans; by the late 1990s, they dominated new equipment installations in North America and Europe.[23] Market expansion intensified into the 2000s, particularly in developing countries exempt from early CFC phase-outs under the Montreal Protocol, leading to a surge in HFC consumption for emerging air conditioning demands in Asia and Latin America.[28] The total global refrigerant market, inclusive of HFCs, increased from approximately 300,000 metric tons in 1990 to over 520,000 metric tons by 2000—a 73% rise—with HFCs capturing a growing share as hydrochlorofluorocarbon (HCFC) transitions began under accelerated protocols.[29] This period saw HFC-410A emerge as a popular high-pressure blend for residential air conditioners, further broadening applications amid rising global cooling needs. Emissions data reflect this uptake, with U.S. HFC releases from substitution rising steadily post-1990, underscoring commercial proliferation despite emerging concerns over global warming potentials.[30] By the 2010s, HFC refrigerants underpinned an industry serving billions in cooling capacity, though subsequent Kigali Amendment negotiations in 2016 signaled limits to unchecked expansion.[31]Applications and Industrial Uses
Refrigeration and Air Conditioning
Hydrofluorocarbons (HFCs) function as the working fluids in vapor-compression refrigeration and air conditioning systems, where they undergo phase changes to absorb heat from enclosed spaces and reject it to the environment, enabling efficient cooling.[32] These systems, including domestic refrigerators, commercial chillers, and room air conditioners, rely on HFCs for their favorable thermodynamic properties, such as appropriate boiling points, high latent heat of vaporization, and compatibility with system components like compressors and heat exchangers.[33] Unlike earlier chlorofluorocarbons (CFCs), HFCs exhibit zero ozone depletion potential, making them suitable replacements mandated by the Montreal Protocol's phase-out of ozone-depleting substances starting in 1987, with widespread adoption accelerating in the 1990s.[34][35] Key HFC refrigerants include R-134a (1,1,1,2-tetrafluoroethane), which became the standard for new mobile vehicle air conditioning (MVAC) systems in 1994 and is also used in household refrigerators and medium-temperature commercial refrigeration due to its non-flammability, low toxicity, and stability.[36] R-410A, a near-azeotropic blend of 50% R-32 (difluoromethane) and 50% R-125 (pentafluoroethane), dominates residential split-system air conditioners and heat pumps installed since the early 2000s, offering higher volumetric capacity and efficiency than the phased-out HCFC-22 (R-22) while maintaining similar safety profiles.[37][38] Other blends like R-407C (a mix of R-32, R-125, and R-134a) serve as retrofits for R-22 systems in commercial air conditioning, providing comparable performance with minimal equipment modifications.[37] In low-temperature applications, such as industrial freezers, HFCs like R-404A (R-125, R-143a, and R-134a) deliver high cooling capacity despite higher energy demands.[39] HFCs' prevalence in refrigeration and air conditioning stems from their energy efficiency in heat transfer—often outperforming alternatives in system design—and operational safety, with most exhibiting ASHRAE A1 classification (non-toxic, non-flammable).[40] Globally, the refrigeration and air conditioning sector consumes the majority of HFCs, with estimates indicating that by 2023, HFC-based systems accounted for over 80% of new equipment installations in developing regions like Asia-Pacific, where demand for cooling has surged due to urbanization and rising incomes.[41] In commercial settings, such as supermarkets and data centers, HFC cascades or blends enable precise temperature control across wide ranges, from -40°C in freezers to ambient cooling in server rooms.[42] Production volumes reflect this dominance: for instance, R-32 demand grew to support mini-split air conditioners, contributing to HFC refrigerants' market value exceeding $3.7 billion in 2024, though constrained by emerging phase-down regulations.[43]Foam Blowing and Other Applications
Hydrofluorocarbons (HFCs) function as blowing agents in the manufacture of closed-cell foams, where they expand under heat and pressure to create insulating structures with low thermal conductivity. Rigid polyurethane and polyisocyanurate foams, commonly produced using HFCs such as HFC-245fa and HFC-365mfc, provide thermal insulation in residential and commercial buildings, refrigeration appliances, and vehicle panels.[44] [45] These agents were adopted as replacements for hydrochlorofluorocarbons (HCFCs) like HCFC-141b, phased out under the Montreal Protocol due to ozone depletion, offering zero ozone-depleting potential while maintaining foam density and dimensional stability.[46] Global HFC consumption in building and construction foams equated to approximately 38 million metric tons of CO2 equivalent in 2010, reflecting their role in energy-efficient insulation that reduces long-term heating and cooling demands.[47] In polyurethane foam production for appliances and spray-in-place applications, HFC-134a and HFC-152a are utilized for their compatibility with polyols and isocyanates, yielding foams with closed-cell contents exceeding 90% for superior moisture resistance.[48] Extruded polystyrene (XPS) boardstock employs blends of HFC-134a with other agents to achieve uniform cell structure and high compressive strength, essential for under-slab and perimeter insulation.[44] These applications leverage HFCs' thermodynamic properties, including boiling points around -26°C for HFC-134a and 15°C for HFC-245fa, which facilitate controlled expansion without excessive flammability risks compared to hydrocarbons.[45] Beyond foam blowing, HFCs serve in fire suppression systems, where HFC-227ea (also known as FM-200) interrupts chemical reactions in flames without residue, protecting sensitive electronics in data centers and aviation.[49] This agent, with a boiling point of -16.3°C, disperses rapidly in total flooding systems, achieving extinguishment concentrations of 7-9% by volume.[31] HFC-134a acts as a propellant in metered-dose inhalers for pharmaceutical delivery, ensuring consistent aerosolization of medications like bronchodilators since its approval by regulatory bodies in the 1990s as a CFC alternative.[50] Minor uses include precision cleaning solvents in semiconductor manufacturing, where HFCs dissolve oils and fluxes without damaging substrates, and as sterilants in medical equipment processing.[32] These non-foam applications represent a smaller fraction of total HFC demand, with fire suppression and aerosols comprising less than 5% of U.S. consumption as of 2018.[2]Environmental Impacts
Atmospheric Lifetime and Ozone Interaction
Hydrofluorocarbons (HFCs) exhibit atmospheric lifetimes that vary significantly by specific compound, generally ranging from several years to over two centuries, determined primarily by their reactivity with hydroxyl (OH) radicals in the troposphere.[51][52] Common HFCs used in refrigeration, such as HFC-134a (CH₂FCF₃), have lifetimes of about 14 years, reflecting efficient breakdown via hydrogen abstraction by OH radicals.[3] In contrast, more stable variants like HFC-23 (CHF₃) persist for approximately 222 years due to fewer reactive sites.[53] This variability stems from molecular structure: the presence of C-H bonds enables tropospheric degradation for most HFCs, preventing substantial accumulation in the stratosphere where ozone resides.[54] Unlike chlorofluorocarbons (CFCs), which release chlorine atoms that catalytically destroy ozone upon stratospheric photolysis, HFCs lack chlorine or bromine and thus exhibit negligible direct ozone depletion potential (ODP), conventionally assigned a value of zero.[54][55] Their hydrogen content promotes rapid tropospheric removal, with only trace fractions reaching altitudes above 15 km, minimizing interactions with ozone.[56] Modeling studies indicate that even under projected emissions scenarios, HFCs contribute at most a 0.035% reduction in total column ozone by 2050, far below the impacts of ozone-depleting substances phased out under the Montreal Protocol.[7] Indirect effects, such as altered stratospheric dynamics from HFC-induced warming, may amplify minor depletions in specific regions, yielding ODPs on the order of 10⁻⁴ for some compounds like HFC-134a.[56] However, empirical observations and assessments confirm HFCs' overall ozone neutrality, validating their role as transitional substitutes for CFCs despite primary concerns over radiative forcing.[54][57]Global Warming Potential and Climate Contributions
Hydrofluorocarbons (HFCs) possess global warming potentials (GWPs) that vary widely depending on the specific compound, typically measured over a 100-year time horizon relative to carbon dioxide (CO₂), which has a GWP of 1 by definition. These values reflect the integrated radiative forcing from emission of 1 kg of the gas compared to 1 kg of CO₂, accounting for both direct infrared absorption and atmospheric lifetime. Common HFCs exhibit GWPs ranging from approximately 130 for HFC-152a to over 14,000 for HFC-23, with many falling between 1,000 and 4,000; for instance, HFC-134a has a 100-year GWP of 1,530, HFC-125 around 3,500, and HFC-32 about 677 according to updated IPCC AR6 estimates incorporating carbon cycle feedbacks.[58][59]| HFC Compound | Chemical Formula | 100-Year GWP (AR6) |
|---|---|---|
| HFC-23 | CHF₃ | 11,700 |
| HFC-32 | CH₂F₂ | 677 |
| HFC-125 | CHF₂CF₃ | 3,350 |
| HFC-134a | CH₂FCF₃ | 1,530 |
| HFC-143a | CH₃CF₃ | 5,900 |
Emissions Sources and Mitigation Realities
Hydrofluorocarbons (HFCs) are emitted primarily through leaks from refrigeration and air conditioning (RAC) equipment during manufacturing, installation, operation, servicing, and end-of-life disposal, with RAC accounting for the dominant share of global emissions.[31][67] In the United Kingdom, RAC contributed 79.5% of total HFC emissions in 2023, reflecting patterns observed globally where operational leaks from commercial and residential systems predominate due to higher system pressures and usage in warmer conditions.[67][68] Additional sources include foam blowing agents in insulation and manufacturing processes, as well as fugitive releases from aerosols and fire suppressants, though these represent smaller fractions compared to RAC.[69] Global HFC emissions reached approximately 0.88 gigatonnes of CO2 equivalent (CO2-eq) per year as of recent atmospheric observations, comprising about 2% of total anthropogenic greenhouse gases but amplified by global warming potentials hundreds to thousands of times that of CO2.[70][31] A significant portion of HFC emissions stems from "banked" stocks—HFCs already installed in existing equipment worldwide, estimated to release gases over decades through gradual leaks and improper disposal.[71] Servicing and decommissioning activities often involve venting, which accounts for substantial releases, particularly in regions with lax enforcement, as equipment from the 1990s CFC phase-out era continues to operate with HFC retrofits.[72] In the United States, for instance, stationary RAC systems are a key vector, with emissions exacerbated by seasonal demand spikes increasing leak rates.[73] Mitigation efforts focus on containment to curb leaks, recovery and recycling of used HFCs, and transitions to lower-global-warming-potential alternatives under frameworks like the Kigali Amendment to the Montreal Protocol.[74] Containment strategies, including mandatory leak detection and repair thresholds (e.g., 15 pounds or more of charge in U.S. EPA rules finalized in 2024), prove most effective in the short term by minimizing operational emissions from existing banks, potentially avoiding releases equivalent to millions of metric tons of CO2-eq annually.[74][75] The Kigali Amendment, effective since 2019, has reduced projected HFC emissions by about 20% through phasedowns in production and consumption, though full implementation lags in developing nations.[31] Despite these measures, mitigation realities reveal persistent challenges: emissions from banked HFCs exhibit time-lagged releases spanning equipment lifecycles, with abatement potential limited by incomplete recovery rates and regional noncompliance.[76] In eastern China and Japan, HFC emissions accelerated sharply from 2016 to 2018, outpacing global phase-down expectations due to expanded RAC deployment amid rising demand.[77] End-of-life recovery, while effective near-term (e.g., reducing U.S. emissions via state mandates like California's 40% cut by 2030), diminishes over time without parallel transitions to non-HFC refrigerants, and destruction technologies for byproducts like HFC-23 offer up to 84% reduction potential but require scaled infrastructure.[78][79][80] Overall, while regulatory tools like the U.S. AIM Act's 85% phasedown by 2036 address leaks and reclamation, empirical data indicate that without rigorous enforcement and innovation in low-leak systems, banked emissions could offset gains, underscoring the causal primacy of physical containment over mere production caps.[74][75]Regulatory Measures
International Frameworks like Montreal and Kigali
The Montreal Protocol on Substances that Deplete the Ozone Layer, signed on 16 September 1987 by 24 countries and entering into force on 1 January 1989, created a binding international regime to eliminate production and consumption of ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs) and later hydrochlorofluorocarbons (HCFCs).[81] By permitting hydrofluorocarbons (HFCs)—which possess zero ozone-depleting potential—as substitutes for phased-out ODS, the Protocol facilitated the widespread commercial adoption of HFCs in refrigeration, air conditioning, and other sectors without initially imposing controls on them.[34] This transition averted further ozone layer damage but shifted environmental concerns toward HFCs' potent greenhouse gas effects, with global warming potentials thousands of times higher than carbon dioxide over a 100-year horizon.[82] Recognizing HFCs' climate contributions—estimated to account for up to 0.5°C of potential warming by century's end without intervention—Parties to the Montreal Protocol adopted the Kigali Amendment on 15 October 2016 in Kigali, Rwanda, extending the treaty's scope to mandate a global phase-down of HFC production and consumption. The Amendment entered into force on 1 January 2019 after ratification by 20 instruments, including major producers, and has since achieved near-universal ratification by over 150 parties as of 2025.[83] It establishes differentiated baselines and reduction milestones: developed countries (non-Article 5 parties) use a 2011–2013 average as baseline, freezing consumption in 2019 and reducing to 80% by 2036, 69% by 2040, and 15% by 2047; developing countries (Article 5 parties) are split into two groups, with most (Group 1) using a 2020–2022 baseline, freezing in 2028 and tapering to 20% by 2047, while a subset of hotter-climate nations (Group 2) freezes in 2024 and reaches 20% by 2045.[82] Exemptions apply for essential uses, production for export, and limited feedstock applications, with compliance monitored through mandatory reporting to the Ozone Secretariat.[84] The frameworks' integration leverages the Montreal Protocol's proven enforcement mechanisms—such as the Multilateral Fund, which has disbursed over $3.9 billion since 1991 to support phase-outs in developing countries—potentially avoiding 135–210 gigatons of CO2-equivalent emissions through HFC reductions, equivalent to removing nearly half of current fossil fuel emissions over the century. However, implementation challenges persist, including illegal trade in HFCs and varying national capacities, underscoring the Protocol's reliance on technical assistance and technology transfer for equitable adherence.[82]National and Regional Policies
In the United States, the American Innovation and Manufacturing (AIM) Act of 2020 mandates an 85% phase-down of hydrofluorocarbon (HFC) production and consumption from baseline levels by 2036, implemented through an allowance allocation system managed by the Environmental Protection Agency (EPA).[85] The EPA has established stepwise reductions, reaching 90% of baseline by 2022, 15% by 2036, alongside rules for technology transitions restricting high-global warming potential (GWP) HFCs in sectors like refrigeration and air conditioning starting in 2024, and requirements for leak repair and reclamation to minimize emissions.[2] In October 2025, the incoming Trump administration proposed reconsidering certain EPA restrictions on HFC use in specific equipment to potentially ease sectoral compliance burdens.[86] Some states, such as Washington, have enacted supplementary bans on high-GWP HFC refrigerants for new equipment sales since July 2021.[87] The European Union enforces HFC reductions via the F-Gas Regulation (EU) 2024/573, which introduces the world's first total HFC phase-out by 2050 and an 80% consumption cut by 2030 relative to 2015 levels, building on a quota system initiated in 2015.[88] [89] Key provisions include bans on virgin HFCs in new stationary refrigeration systems with GWP over 150 from 2025, expanded leak prevention mandates, and incentives for low-GWP alternatives, while exempting reclaimed or recycled HFCs from quotas.[90] China, the largest HFC producer and consumer, ratified the Kigali Amendment in 2021 and aligns its phase-down with the Montreal Protocol schedule, freezing consumption in 2024 and achieving 80-85% reductions by 2045-2047, supported by import/export licensing and a national plan prohibiting HFC use in new refrigerators and freezers from January 1, 2026.[91] [92] India, having ratified the Kigali Amendment in September 2021, commences its HFC phase-down in 2032 with reductions of 10% by 2032, 20% by 2037, 30% by 2042, and 85% by 2047 from its 2024-2028 baseline, emphasizing technology transfers and domestic manufacturing of alternatives to support compliance in its rapidly expanding cooling sector.[93]Phase-Out Transitions
Global Phase-Down Schedules
The Kigali Amendment to the Montreal Protocol, adopted on 15 October 2016, mandates a stepwise global phase-down of hydrofluorocarbon (HFC) production and consumption to curb their contribution to radiative forcing, with targets exceeding 80% reductions from baselines over approximately three decades.[82][34] Baselines are defined as the average annual HFC consumption (in CO₂-equivalent metric tons) over specific historical periods: 2011–2013 for developed countries (non-Article 5 Parties) and 2020–2022 for developing countries (Article 5 Parties).[34] The amendment differentiates schedules by country group to account for varying economic capacities and prior HCFC phase-out timelines, with developed nations starting reductions earlier and achieving deeper cuts sooner.[34][94] Production phase-down mirrors consumption, though parties may trade quotas under specified conditions.[34] For developed countries, the phase-down begins with a 10% reduction below baseline by 2019, escalating through interim quotas to an 85% reduction (15% of baseline allowed) by 2036.[95][82] Stepwise quotas include approximately 63% of baseline allowed by 2024, 44% by 2028, 21% by 2032, and 15% thereafter, enforced via national allocation mechanisms like those in the EU and US.[96][97] Developing countries under Article 5 are split into Group 1 (e.g., China, India, Brazil, covering most by population and HFC use) and Group 2 (e.g., certain Gulf states and small islands with delayed HCFC phase-outs). Group 1 freezes consumption at baseline levels from 2024, initiating reductions in 2029 to reach an 80% cut (20% of baseline) by 2047; Group 2 freezes in 2028, with reductions from 2032 toward the same endpoint.[34][98][99]| Phase-Down Step | Developed Countries (% of Baseline Allowed) | Group 1 Developing (% of Baseline Allowed) | Group 2 Developing (% of Baseline Allowed) |
|---|---|---|---|
| Pre-Phase (Baseline Reference) | 100% (2011–2013 avg.) | 100% (2020–2022 avg.) | 100% (2020–2022 avg.) |
| Initial Reduction/Freeze | 90% (by 2019) | 100% freeze (2024–2028) | 100% freeze (2028–2031) |
| Mid-Term Reductions | 63% (by 2024); 44% (by 2028); 21% (by 2032) | 90% (2029–2033); ~65% (2034–2039) | 90% (2032–2035); stepwise to ~65% (mid-2030s) |
| Final Targets | 15% (by 2036 onward; 85% reduction) | 20% (by 2047; 80% reduction) | 20% (by 2047; 80% reduction) |