Pimchanok Su-Ungkavatin

Ligia Tiruta-Barna

Lorie Hamelin

Methodological framework for Life Cycle Assessment of Sustainable Aviation (SA) systems

Keywords: Battery, Biofuels, Electrofuels, Energy consumption, Hydrogen, Life Cycle Assessment (LCA), Power-to-X, Sustainable aviation fuels (SAF), Tailpipe emissions, Highlights

A comprehensive framework is proposed for life cycle assessment (LCA) in the field of commercial aviation (passengers and cargo), capable to ensure transparency and comparability when evaluating the overall environmental performances of four emerging aviation systems, i.e., biofuels, electrofuels, electric, and hydrogen. The projected global revenue passenger kilometer (RPK) and is suggested as the functional unit for two timeframes representing near-term (2035) and long-term (2045), and for two segments, namely domestic and international. To solve the difficulty of comparing liquid fuels and electric aviation, the framework proposes a methodology to translate projected RPK into energy requirements for each of the studied sustainable aviation systems. Generic system boundaries are defined with their key activities for all four systems, with the biofuel system being subdivided into two categories to distinguish whether it stems from residual or land-dependent biomass. The activities are grouped in seven categories: (i) conventional (fossil-based) kerosene activity, (ii) conversion processes from feedstock supply (to fuel or energy production for aircraft operation), (iii) counterfactual uses of constrained resources and displacement effects associated to co-products management, (iv) aircraft manufacture, (v) aircraft operation, (vi) additional infrastructure needed, and (vii) end-of-life management (aircraft and batteries). Considering applying regulations, the framework also includes a methodology to handle: (i) hybridization (the use of more than one source of energy/propulsion system to power an aircraft), (ii) the mass penalty affecting the number of carried passengers in some of the systems, and (iii) impacts stemming from non-CO2 tailpipe emissions -aspects that are currently neglected in most LCA studies. The proposed framework builds upon the most recent knowledge in the field; however, some choices are dependent on upcoming scientific advances concerning e.g., tailpipe emissions at high altitude and their environmental impacts, new aircraft configuration, etc., and are subject to significant uncertainties. Overall, this framework provides a guideline for LCA practitioners addressing new energy sources for future aviation.

EF

g pollutant kg -1 fuel Emission factors, commonly represented in gram ofpollutant per kg fuel supplied, with the exception of aviation induced cloudiness expressed in microgram (µg) per kg fuel supplied (Table 6) F

Introduction

The last decade have seen the emergence of growing concern, from researchers [START_REF] Baumeister | Mitigating the Climate Change Impacts of Aviation through Behavioural Change[END_REF][START_REF] Hasan | Climate change mitigation pathways for the aviation sector[END_REF], industry (Boeing 2020;Airbus 2021a;Qatar Airways 2021), governments (European Commission, 2021;[START_REF] Hirst | Aviation, decarbonisation and climate change[END_REF][START_REF] Soone | Sustainable Aviation Fuels Guide[END_REF], and the society [START_REF] Mkono | Eco-anxiety and the flight shaming movement: implications for tourism[END_REF]University of Sheffield, 2019), with respect to reducing the climate impact of aviation. To address this challenge, considerable effort has been devoted to the development of sustainable aviation (SA) technologies and fuels [START_REF] Su-Ungkavatin | Biofuels, electrofuels, electric or hydrogen?: A review of current and emerging sustainable aviation systems[END_REF]. Such developments include alternative liquid hydrocarbon fuels (biofuel and electrofuel), as well as carbon-free (electric battery and hydrogen) approaches. However, it remains unclear which approach should prevail from an environmental standpoint. While life cycle assessment (LCA) is an acknowledged methodology to quantify and compare the environmental impacts of such emerging alternatives [START_REF] Sala | The evolution of life cycle assessment in European policies over three decades[END_REF], the particular case of SA poses several methodological challenges that need to be addressed to allow meaningful comparisons via LCA.

A variety of LCA studies for SA alternatives have been performed, as shown in Table 1. While valuable, these studies have essentially focused on biofuel approaches, particularly the hydroprocessed esters and fatty acids (HEFA) route from oil feedstock (whether cultivated or from a residual lipid source). Accordingly, the most commonly used functional unit to compare scenarios is megajoules (MJ) of fuel produced or consumed on-board, which poses challenges when comparing alternatives that are not liquid hydrocarbon drop-in fuels, e.g. those involving batteries or fuel cells. Moreover, these liquid hydrocarbon pathways do not, unlike carbon-free alternatives, involve the need to consider additional infrastructure, e.g., charging infrastructure for batteries or storage tanks for hydrogen (whether on-ground or on-board), which remain somewhat overlooked in existing SA LCAs. In addition, many existing LCA studies on SA systems are not in compliance with the International Organization for Standardization (ISO) standards (ISO 14040:2006 andISO 14044:2006), accounting for a single impact only, namely, climate change (Table 1). More importantly, we found no studies in which biofuels, electrofuels, electric (battery), and hydrogen aviation were simultaneously compared. One study [START_REF] Johanning | Comparison of the potential environmental impact improvements of future aircraft concepts using life cycle assessment[END_REF], however, did simultaneously compare biofuels, batteries, and hydrogen, shedding light on important technical issues shaping the so-called 'mass penalty' for hydrogen and batteries systems. These are further detailed in [START_REF] Su-Ungkavatin | Biofuels, electrofuels, electric or hydrogen?: A review of current and emerging sustainable aviation systems[END_REF] and include the degree of energy hybridization (HE, the ratio of the installed propulsion power of electric motors to the total installed propulsion power at the propeller shaft) and, the gravimetric index (GI, the ratio of the fuel mass to the mass of the full fuel system including the tank), which are in turn interdependent with aircraft technical specifications, that is, the fundamental aircraft design. Other crucial issues remain disregarded in LCAs of SA systems. This includes, among others, the consideration of non-CO2 tailpipe emissions, e.g., water vapor and aviation soot (Winther and Rypdal 2019a). The importance of the latter issue has been stressed in a variety of recent studies (e.g., [START_REF] Lund | Emission metrics for quantifying regional climate impacts of aviation[END_REF]; European Union Aviation Safety Agency 2020; [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF], including the recent Assessment Report 6 (AR6) of the Intergovernmental Panel on Climate Change (IPCC) [START_REF] Masson-Delmotte | Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]. In addition, the need for blending alternative liquid fuels (either with fossil-or another alternative to kerosene) is often disregarded.

To bridge these gaps and challenges and to fairly compare and quantify the environmental implications of carbonbased and carbon-free emerging SA systems, the present study proposes a guideline for LCAs of SA, addressing each of the previously identified issues. This work is organized into 6 sections. Section 2 starts by describing the SA systems and their particularities. The selection of a functional unit is discussed in section 3, while generic system boundaries are proposed in section 4 for each SA system. Section 4 also encompasses several technical aspects related to the calculation of the reference flows, such as the translation of the selected functional units into energy requirements, the inclusions of technical constraints (e.g., hybridization and mass penalty) along with the additional infrastructure, and end-of-life (EoL) management. Additionally, non-CO2 tailpipe emissions calculation procedures are discussed in section 5. Finally, the general conclusions are presented in section 6.

Table 1. Non-exhaustive list of publications presenting an environmental assessment of sustainable aviation (SA) systems.

Reference Goal and scope (Main contributions)

Technologies included Data sources (foreground and background)

Functional unit

System boundaries (SBs) and multifunctionality (MF) b LCIA method, impact categories, and LUC Alternative hydrocarbon liquid fuels [START_REF] Han | Life-cycle analysis of bio-based aviation fuels[END_REF] • Investigate the GHG emissions of liquid biobased fuel and discuss different allocation methods for multifunctionality

• HEFA from oilseed crops • FT from maize stover, co-feeding with coal • Pyrolysis from maize stover • GREET database • Literature reviews 1 MJ fuel combusted SB: Cradle-to-Gate from feedstock cultivation to fuel combustion in aircraft engine; MF: Both SE and Alloc. (mass and energy) GWP (presumably GWP100, but not clearly stated; IPCC AR4), LUC emissions explicitly excluded [START_REF] Li | Life cycle assessment of camelina oil derived biodiesel and jet fuel in the Canadian Prairies[END_REF] • Analyze the overall environmental impacts of biodiesel (road) and jet HEFA applying system expansion to handle multifunctionality [START_REF] Pierobon | Life cycle assessment of residual lignocellulosic biomass-based jet fuel with activated carbon and lignosulfonate as co-products[END_REF] • Environmental assessment of one specific biofuel pathway for aviation with both allocation (mass) and system expansion methods

Sustainable Aviation (SA) systems

We propose clustering SA systems for commercial aircraft into two main categories: (i) carbon-based, including biofuels and electrofuels, and (ii) carbon-free, including electric and hydrogen. These alternatives represent the predominant systems envisaged for future aviation. They are summarized here but are comprehensively detailed in [START_REF] Su-Ungkavatin | Biofuels, electrofuels, electric or hydrogen?: A review of current and emerging sustainable aviation systems[END_REF].

Carbon-based systems

Carbon-based approaches, more specifically, hydrocarbon fuels, are compatible with existing infrastructure having similar properties to conventional (fossil-based) kerosene-type fuels, commonly referred to as Jet A-1 or Jet A.

Biofuels (alternative kerosene)

Biofuels can be produced from land-dependent biomass, e.g., palm and soybean oil (so-called 1 st generation), residual biomass, e.g., primary forestry residues (so-called 2 nd generation), or arable land-free biomass, e.g., algae (3 rd generation). At the time of writing, six certified biofuel pathways have been certifiedby the American Society for Testing and Materials (ASTM) International (ASTM International Standard, 2022), namely Fischer-Tropsch (FT), HEFA, direct sugar to hydrocarbon, and alcohol intermediate pathways, including syngas-and sugarfermentation to ethanol or more commonly to isobutanol for the latter, both requiring further upgrading to jet fuels [START_REF] Su-Ungkavatin | Biofuels, electrofuels, electric or hydrogen?: A review of current and emerging sustainable aviation systems[END_REF]. Other pathways including a variation of the certified production pathways are also awaiting certification, these include: hydrodeoxygenation synthetic aromatic kerosene (HDO-SAK), high freeze point HEFA (HFP-HEFA), integrated hydropyrolysis (IH 2 ) and alcohol-to-jet synthetic kerosene with aromatics (ATJ-SKA) [START_REF] Humphris-Bach | Targeted aviation advanced biofuels demonstration competition -feasibility study: Final report[END_REF]. The detailed process flows of these pathways are presented in (Suungkavatin et al., 2023).

Electrofuels (alternative kerosene)

Electrofuels are also liquid hydrocarbons; however, unlike biofuels, they rely on separate carbon (C) and hydrogen (H2) sourcing. These fuels are also known as power-to-liquid, synthetic fuel, or E-fuels. The carbon source essentially stems from CO2 that is either captured from point sources, e.g., fossil or biogenic CO2 from power plants or industrial production sites (e.g., power generation plants, cement factories, steel industries) [START_REF] Lanzatech | A Hybrid Catalytic Route to Fuels from Biomass Syngas: Final Report[END_REF]Last Schmick MT, 2011;[START_REF] Macdowell | An overview of CO2 capture technologies[END_REF], or non-point sources such as the atmosphere (so-called Direct Air Capture; DAC [START_REF] Brynolf | Electrofuels for the transport sector: A review of production costs[END_REF]Deutz and Bardow, 2021;[START_REF] Ishimoto | Putting Costs of Direct Air Capture in Context[END_REF][START_REF] Keith | A Process for Capturing CO2 from the Atmosphere[END_REF]).

It could also stem from biomass, e.g., gasified to syngas (LanzaTech Inc., 2017). Hydrogen can stem from conventional methane cracking from biomass (e.g., [START_REF] Lepage | Biomass-to-hydrogen: A review of main routes production, processes evaluation and techno-economical assessment[END_REF] or can be produced from water either through electrolysis [START_REF] Buttler | Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review[END_REF][START_REF] Carmo | A comprehensive review on PEM water electrolysis[END_REF][START_REF] Thomas | Large scale PEM electrolysis : technology status and upscaling strategies[END_REF], thermochemistry [START_REF] Abanades | Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides[END_REF][START_REF] Ozbilen | A comparative life cycle analysis of hydrogen production via thermochemical water splitting using a Cu-Cl cycle[END_REF], or emerging techniques such as photo-electrochemistry [START_REF] Joy | Nanomaterials for photoelectrochemical water splitting -review[END_REF][START_REF] Sathre | Life-cycle net energy assessment of large-scale hydrogen production via photoelectrochemical water splitting[END_REF] and bio-photolysis [START_REF] Ghiasian | Biophotolysis-Based Hydrogen Production by Cyanobacteria[END_REF]. Water splitting represents the dominant vision of future renewable energy and low fossil carbon systems, electrolysis being the most discussed pathway [START_REF] Pinaud | Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry[END_REF][START_REF] Ambel | Electrofuels what role in EU transport decarbonisation?[END_REF]. Carbon and hydrogen are converted to aviation electrofuels through gas-to-liquid pathways either via FT or methanol conversion, the latter still awaiting certification [START_REF] Schmidt | Power-to-Liquids: Potentials and Perspectives for the Future Supply of Renewable Aviation Fuel[END_REF].

Carbon-free systems

Carbon-free alternatives encompass electric-and hydrogen-based energy sources.

Electric (battery)

Electric aviation typically refers to aviation concepts involving electric motor(s), whether completely electric (also known as all-electric; here the electricity is either stored in batteries, or generated via fuel cells), or turboelectric (where there is an electric motor but no battery) configurations. Here, we refer to electric aviation as systems involving batteries only, rather than all systems involving an electric engine; the term electric (battery) will be used henceforth.

Hydrogen (H2)

Hydrogen has a superior gravimetric energy density (120 MJ kg -1 ; [START_REF] Bruce | Opportunities for hydrogen in commercial aviation[END_REF], whether in gaseous or cryogenic form, in comparison to conventional (fossil-based) kerosene (43 MJ kg -1 ; de Klerk 2010). Hydrogen can propel planes via direct combustion, electricity supplied via a fuel cell, or a combination of the two. In the former case, additional developments regarding engines along with changes in specific components, e.g., highpressure pump, injector, and cryogenic heat exchanger [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF][START_REF] Thomson | Focus Roland Berger Hydrogen: A future fuel for aviation[END_REF], are required. For the latter case (combination of fuel cells and combustion), high-power fuel cells are mandatory for the purpose of propulsion; an increase from 0.75 kW kg -1 to 2 kW kg -1 is expected to make fuel cells feasible as an energy source on-board commercial aircraft (Baroutaji et al. 2019a;[START_REF] Bruce | Opportunities for hydrogen in commercial aviation[END_REF][START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF]. Another issue is the mass of hydrogen storage systems, particularly for aircraft, because H2 has a volumetric energy density four times lower than conventional kerosene. Of the hydrogen-storage methods, cryogenic tanks are the most attractive, given their reasonable mass and volume, more so than gaseous and hydride storage methods [START_REF] Baroutaji | Comprehensive investigation on hydrogen and fuel cell technology in the aviation and aerospace sectors[END_REF]. Therefore, they are considered in this analysis [START_REF] Van Zon | Liquid hydrogen powered commercial aircraft: Analysis of the technical feasibility of sustainable liquid hydrogen powered commercial aircraft in[END_REF]. A hydrogen-storage tank needs to be insulated to ensure the desired form of hydrogen, either gaseous or liquid, and the necessary amount for the flight. Developments of H2 GIs approaching that of a 0.30-0.63 kg H2 kg -1 fuel system are expected [START_REF] Baroutaji | Comprehensive investigation on hydrogen and fuel cell technology in the aviation and aerospace sectors[END_REF][START_REF] Thomson | Focus Roland Berger Hydrogen: A future fuel for aviation[END_REF], making hydrogen systems competitive with current conventional (fossil-based) kerosene systems (ca. 0.75 kg kerosene kg -1 fuel system; Winnefeld et al. 2018a). Cryogenic tanks can be positioned inside (integral) or outside (non-integral) the fuselage [START_REF] Bruce | Opportunities for hydrogen in commercial aviation[END_REF]. The former option is an efficient method to reduce drag, independent of the fuselage design, making it a realistic and feasible approach [START_REF] Bruce | Opportunities for hydrogen in commercial aviation[END_REF][START_REF] Rondinelli | Challenges and Benefits offered by Liquid Hydrogen Fuels in Commercial Aviation Aviation[END_REF]. However, this feature may affect the available carrying volume in the passenger cabin (van Zon, 2018) and may only be applicable to short-to-medium-range flights. Meanwhile, the overall propulsion efficiency may be negatively affected in the latter case [START_REF] Baroutaji | Comprehensive investigation on hydrogen and fuel cell technology in the aviation and aerospace sectors[END_REF][START_REF] Rondinelli | Challenges and Benefits offered by Liquid Hydrogen Fuels in Commercial Aviation Aviation[END_REF]. Alternatively, a blended wing body (BWB) may better accommodate hydrogen tanks in addition to significantly improving the lift-to-drag ratio (Airbus, 2020;Rompokos et al., 2020).

Need for blending/hybridization

Carbon-based SA systems have, up to now, been applied in a hybrid fashion with conventional (fossil-based) aviation fuels. This is because their use is currently limited to that of a drop-in fuel (from 10% up to 50%, depending on the type) on the basis of existing standards applied to aviation fuels, namely, ASTM D1655 and DEF STAN 91-091. Even though aviation fuels can be clustered into kerosene (i.e., Jet A, Jet A-1, TS-1, Jet Fuel No.

3, and JP-5), gasoline (known as aviation gasoline; Avgas), and a mixture of the two (i.e., Jet B and JP-4) (Chevron Products company, 2007), we propose considering the Jet A and Jet A-1 kerosene fuel types as reference systems because they are predominantly deployed for commercial aviation (Chevron Products company, 2007;[START_REF] Gokulakrishnan | Autoignition of aviation fuels: Experimental and modeling study[END_REF]. Furthermore, the blending limit is set based on a variety of parameters, including the minimum aromatics content (8% by volume) in the final fuel, set to avoid engine seal shrinkage. Yet, a variety of existing carbon-based SA fuels have negligible aromatics contents, including the widely produced HEFA-synthetic paraffinic kerosene (SPK) [START_REF] Canteenwalla | Testing of Synthesized Aromatic Kerosene (SAK) Aviation Fuel Blends at Simulated Altitudes[END_REF]Virent Inc., 2015). In addition, the SPK maximum blending quota set by ASTM (or even beyond that) is based on the characteristics of fossil-based kerosene, as detailed in an analysis by [START_REF] Buttler | Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review[END_REF]. However, the technical and safety aspects of using 100% alternative carbon-based SA fuels are being intensively investigated to overcome this limitation. Recently, aircraft and engine equipment manufacturers tested 100% HEFA-SPK on one engine for comparison with other engines run on standard jet fuel in commercial aircraft, e.g., Airbus (2021b); Boeing (2021); Rolls-Royce ( 2021). No difference was observed in the engine behavior. As a result, unblended alternative fuels have been announced as being compatible with state-of-the-art engines by Airbus (2021b), [START_REF] Boeing | Boeing Commits to Deliver Commercial Airplanes Ready to Fly on 100% Sustainable Fuels[END_REF], and Deutsche Aircraft (Deutsche Aircraft, 2022).

Based on current technological developments and demonstrations, electric (battery) aviation can already be used for short flights (limited to approximately 1,000 km) with a limited number of passengers (carrying less than 20 passengers), as in the case of the Eviation Alice (Eviation, 2022) and Heart Aerospace ES-19 (Heart Aerospace, 2022), without the need of hybridization with fossil kerosene or alternative SA fuels [START_REF] Baumeister | The emission reduction potentials of First Generation Electric Aircraft (FGEA) in Finland[END_REF][START_REF] Ribeiro | Environmental assessment of hybrid-electric propulsion in conceptual aircraft design[END_REF]. However, insufficient energy density and mass penalties are vital challenges for flying with electric (battery) aviation, in particular for commercial aircraft (e.g., A320, Boeing 747) [START_REF] Ribeiro | Environmental assessment of hybrid-electric propulsion in conceptual aircraft design[END_REF][START_REF] Thomson | Think : Act nevigating complexity Aircraft Electrical Propulsion -Onwards and Upwards, It is not a question of if, but when[END_REF]. Accordingly, this SA energy source is, in the near-term, applicable to long-range flights, e.g., >1,000 km for the A320 model [START_REF] Ribeiro | Environmental assessment of hybrid-electric propulsion in conceptual aircraft design[END_REF], only in hybridization with either conventional (fossilbased) kerosene (near-term), hydrocarbon-based (medium-term), or hydrogen-based fuel cell or combustion (longer-term) SA systems.

From a theoretical point of view, a hydrogen-based SA system does not require hybridization. In other words, there are no significant technical constraints that would require its hybridization with another SA system for longerrange flights with reasonable carrying capacities. Even though announcements of 100% hydrogen-powered planes have been made for the near-term future, e.g., ZeroAvia announced planes carrying 100-200 passengers with a flight range of up to approximately 4,000 km in 2030 [START_REF] Zeroavia | Evaluating the regional impact of aircraft emissions on climate and the capabilities of simplified climate model[END_REF], no 100% hydrogen demonstration flights have yet been made.

Clustering into near-and long-term perspectives

To synthesize, figure 1 presents an overview of the SA systems that ought to facilitate comprehensive and complete LCAs for future aviation systems. Additional considerations between near-and long-term perspectives are further addressed (some are not shown in Fig. 1), namely (i) improvement of overall propulsion efficiency (OPEi), implying less fuel consumption in long-term scope, depending on the aircraft model (Table 3); (ii) different types of heat sources may react to a demand change in the near-and long-term future and (iii) the use of the novel blended wing body (BWB) configuration, considered only in the long-term international segment at the light of theongoing ZEROe project (Airbus, 2021c). For heat, low (LT) and high (HT) temperatures are defined with a border of 100°C. Natural gas is the heat source considered in the near-term for both LT and HT while renewable electricity is considered for the long-term, with heat pumps supplying LT and electric furnace/boiler supplying HT.

Fig. 1. Proposed framework for LCA of SA systems with two-time horizons: near-and long-term futures. This framework is further detailed in the SM2.

Notes.

a Hybridization refers to the requirement of fossil kerosene in addition to alternative energy sources, for aircraft operation. This column also includes the mass penalties derived from using different energy sources. A "X" means that the mass penalty is not relevant, while a "√" means it should be considered.

b The ASTM-certified pathways are retrieved from [START_REF] Humphris-Bach | Targeted aviation advanced biofuels demonstration competition -feasibility study: Final report[END_REF]. Fuel variations from the same pathways are included, namely HEFA-SPK and Hydroprocessed hydrocarbon HEFA-SPK (HC HEFA-SPK), both considered as derived from the HEFA process.

c The distance is considered based on the different types of batteries (in the different near-and long-term scopes) (see details SM2), while the number of passengers is assumed to be unchanged.

Functional unit selection

Table 1 highlights the fact that existing LCAs of SA have essentially used the production and combustion of a fuel (in energy units) as a basis to define their functional units. While this may be useful when comparing liquid fuels for similar engines, it becomes limiting when introducing SA alternatives with adapted infrastructures and aircraft engines.

The key service supplied by commercial aviation is the transport of a certain number of passengers over a certain distance in a given timeframe. Some types of SA for commercial aviation may involve planes able to carry fewer passengers and/or be limited in the distance they can fly, e.g., electric (battery). This implies that a distinction between short-and long-range flights is necessary, often denoted as domestic and international travel, respectively (ICAO 2015; International Air Transport Association (IATA) 2021). Because the 'domestic' and 'international' flights are not defined based on the distance [START_REF] Susan | ICAO: Glossary[END_REF], all aircraft capable of operating for short-and long-distance are considered in the scope of this framework (see SM1 and SM2).

Here, we propose using the revenue passenger kilometer (RPK), a transportation industry metric widely used by airline industries, aviation organizations (e.g., ICAO and IATA), and life cycle inventory (LCI) databases (e.g., ecoinvent and GREET), as a basis to set functional units for SA LCAs. In essence, this unit is expressed in boarded passenger kilometers and reflects the number of passengers carried and the number of kilometers traveled in a given flight for a given period during which a certain number of flights is taking place (ICAO 2015).

In Table 2, we propose an additional level of granularity for the RPK functional unit by distinguishing the type of segment (domestic or international) and the time scope (near-term or long-term). This is done by considering projected RPKs worldwide (ICAO 2018). Accordingly, on the basis of prospective data availability, the year 2035 was considered as a representative market for the "near-term", and the year 2045 as a market for the "long-term". However, a different geographical scope could be considered (e.g., flights departing from a specific country or flights from a given airline), provided data are available. Several airlines (e.g., Air France-KLM, Lufthansa Group, Qatar Airways, and Singapore Airlines) publish such information, albeit typically for historical data, ninvolving the need to extrapolate these past trends into the future. Based on Table 2, an adequate functional unit to compare the broadly available SA types could be, for the near-term time scope, "Ensuring the annual global supply of 6 trillion RPK of domestic flights and 9 trillion RPK of international flights". Moreover, in addition to carrying passengers, commercial airlines typically simultaneously carry merchandise, i.e., freight [START_REF] Nice | Passenger Airline Freight -How Airline Freight Works | HowStuffWorks[END_REF][START_REF] Page | How pilots operate cargo flights on passenger aircraft[END_REF]. The mass/volume of cargo transported is typically associated with the type of aircraft and the segment traveled.

To facilitate a comparison of different technologies, we propose translating the functional unit in terms of the energy flow, as further detailed in Sections 4.3 and 4.4. The freight service (if any) is then incorporated at this stage, representing an additional mass being carried.

Life cycle modeling issues 4.1 System boundaries

In LCA, the system boundaries define which activities are included in the analysis. We propose, as prescribed by the ISO standards for LCA (ISO 14040 2006 andISO 14044 2006;ISO 2006aISO , 2006b), an approach using system expansion with substitution (consequential approach), i.e., an attempt to reflect the uses of all possible co-products generated to supply the selected functional unit, and their environmental consequences. We further propose distinguishing five types of SA systems (three carbon-based and two carbon-free), namely, (a) biofuel from residual biomass; (b) biofuel from land-dependent biomass; (c) electrofuel; (d) electric (battery) aviation; and (e) hydrogen aviation. The system boundaries involved for these five systems are represented in Figure 2.

If compared with one another, existing and emerging SA systems involve a large number of differences. For LCA of one or another system, all processes included in the product system must be considered, as LCA by its nature encompasses all cradle-to-grave processes. However, as LCA is comparative, it is also an accepted simplification practice to disregard processes that are unaffected among the reference and compared alternatives. As investments would be performed from the current state-of-the-art, i.e. from a system based on kerosene, we propose to take this as a starting point. That is, any activity that requires additional investments in comparison to today's kerosene system must be considered, while activities/infrastructure involving no changes (e.g., same fuel tanks whether it is kerosene or biofuel) can be disregarded in the perspective of comparative analyses.

We identified seven categories of activities in the five SA systems illustrated in Figure 2 (represented by a color code).

The first is the "conventional (fossil-based) kerosene system" (in yellow). This encompasses all processes related to the need for blending with kerosene, as detailed in Section 2.3 (from the extraction of the resource to its production). According to the SA system, the amount of fossil kerosene involved will differ and may even be zero depending on the time horizon and/or the flight segment considered in the definition of the functional unit.

The second category is the "feedstock system"; this covers all processes related to the production of the feedstock involved prior to the operation of the aircraft (in green). This includes the conversion of the biomass or carbon plus hydrogen into bio-based kerosene in the biofuel systems as well as the harvest and/or collection of the residual biomass or the cultivation of the land-dependent biomass. It also includes the capture of carbon (C) in the electrofuel system and the hydrogen production in the electrofuel and hydrogen systems. Finally, it covers all processes related to battery manufacture in the electric aviation system.

The third category, labeled "counterfactual and displacement" (in blue), reflects several effects. The latter, i.e., displacement effect, refers to the consequences of the uses of the different co-products generated in the production chain. These co-products are sold to the most competitive market segment and, accordingly, will supply a service that, in turn, will no longer need to be supplied in the way it was in a world without the additional production of the SA under study. This chain of displacement-replacement events is typically referred to as substitution [START_REF] Brandao | Consequential Life Cycle Assessment: What, How, and Why? Encyclopedia of Sustainable Technologies[END_REF][START_REF] Ekvall | Attributional and Consequential Life Cycle Assessment[END_REF][START_REF] Weidema | Has ISO 14040/44 Failed Its Role as a Standard for Life Cycle Assessment[END_REF]. The other effect of this category reflects the fact that some feedstock-related resources are constrained, whether as a result of the economic system, i.e., the production of residual biomass will not increase as a result of a demand change for bio-based kerosene, or because of a biophysical constraint, e.g., land. This effect is often referred to as the "counterfactual" (e.g., [START_REF] Lodato | Towards sustainable methane supply from local bioresources: Anaerobic digestion, gasification, and gas upgrading[END_REF][START_REF] Shapiro-Bengtsen | Should residual biomass be used for fuels, power and heat, or materials? Assessing costs and environmental impacts for China in 2035[END_REF][START_REF] Tonini | GHG emission factors for bioelectricity, biomethane, and bioethanol quantified for 24 biomass substrates with consequential life-cycle assessment[END_REF] and reflects the foregone use of a limited resource, here induced by using it for aviation biofuel. It should be highlighted that the mineral resources needed for batteries are not considered as being biophysically constrained resources because their long-term availability is determined primarily according to geopolitical and economic constraints rather than biophysical ones [START_REF] Blengini | Study on the EU's list of Critical Raw Materials[END_REF].

The fourth category, "aircraft manufacture" (in grey), relates to the very production of the aircraft needed to supply the services defined by the functional unit. This is included as a category on its own to reflect two main points: (i) that not all SA systems will require the same number of aircraft and, (ii) that the different SA systems may involve different aircraft designs, e.g., BWB for H2 long-haul flights (Airbus, 2020).

The fifth category, here called "operation" (in red), represents the energy consumption during the aircraft use to supply the aviation service, considering the different lifetimes of the different types of aircraft involved. While some authors argue that the differences are negligible when comparing conventional (fossil-based) kerosene to biofuels use (e.g., [START_REF] De Jong | Life-cycle analysis of greenhouse gas emissions from renewable jet fuel production[END_REF][START_REF] Gawron | Combustion and emissions characteristics of the turbine engine fueled with HeFA blends from different feedstocks[END_REF][START_REF] Rahmes | Sustainable Bio-Derived Synthetic Paraffinic Kerosene (Bio-SPK) Jet Fuel Flights and Engine Tests Program Results[END_REF] or even electrofuels [START_REF] De Jong | Life-cycle analysis of greenhouse gas emissions from renewable jet fuel production[END_REF], other studies claim that there is a difference in terms of emissions like soot and particulate matter [START_REF] Chan | Benchmarking data from the experience gained in engine performance and emissions testing on alternative fuels for aviation[END_REF][START_REF] Chan | Characterization of Emissions from the Use of Alternative Aviation Fuels[END_REF], as discussed in Section 5. However, the operation of the battery and hydrogen systems differ significantly because they do not involve the combustion of hydrocarbons, as further detailed in Section 5. This also has implications during the life cycle impact assessment phase. Therefore, here we propose having the aircraft operation as a category in its own right.

The sixth category, "infrastructure" (white box) reflects the required infrastructure and operation of the airport, which will depend on the type of energy source. For example, an electric system will require charging infrastructure that is not needed in other systems, while hydrogen and electrofuels will imply a certain storage infrastructure for hydrogen, as further detailed in Section 4.5.

The "end-of-life" (black box) management of the aircraft involved in the different SA systems will also vary as a result of the differences between the aircraft themselves and the fact that the electric SA system may involve the recycling of batteries, as discussed in Section 4.5. end-of-life management (whether for batteries or aircraft). Hybridization is shown here with conventional (fossil-based) kerosene only, representing the near-term horizon in the case of biofuels and electrofuels.

Translating RPK demands into energy flows

Functional units based on the global RPK projections shown in Table 2 (according to time scope and segment type) can be translated into propelling energy requirements [EJ] over a certain period of interest on the basis of a few parameters, as detailed in Eq. (1). Because these projections are based on fossil kerosene, they are translated here into energy terms (ERPK,ij) for a given couple segment (j)-aircraft (i) considering fossil kerosene (or more specially, Jet A/Jet A-1) as a propellant.

ERPK,ij = ECref,i × (1 + (1 -OPEi)) × MTK,ij × 10 -12 (1) ECref,i = EF,ref,i Dmax,ref,i × MPL,max,ref,i (1.1) EF,ref,i = EDref × MF,max,ref,i × 10 3 (1.2) MTK,ij = [RPKj × c] + [(RPKj × c) × Fi] (1.3) MF,max,ref,i = MTF,max,ref,i (Trip fuel) + MRF,max,ref,i (Reserve fuel) (1.4)
In other words, ECref,i, the reference energy consumption rate, is based on conventional (fossil-based) kerosene, and is the energy consumed per payload mass per kilometer for a given aircraft (i), with the payload mass being the mass of the passengers plus its share in terms of the freight and baggage mass (MTK,ij). ECref,i, in turn, depends on EF,ref,i (Eqs. (1.1 and 1.2)), i.e., the energy corresponding to the kerosene fuel needed to supply the maximal distance a given aircraft can ensure at full capacity (MF,max,ref,i). ECref,i also depends on the total payload mass of a given aircraft (MPL,max,ref,i), including the overall passenger, baggage, and freight mass, and on the maximal achievable distance of an aircraft (i) with the reference fuel (Dmax,ref,i). The eventual freight service is, in this way, included in the functional unit. EF,ref,i is calculated based on the reference kerosene gravimetric energy density EDref., and the maximum total mass of the fuel (MF,max,ref,i) or the mass of the battery pack. These figures are often directly provided by aircraft manufacturers. It should be noted that Eq. (1.1) slightly underestimates ECref,i, In fact, at maximum payload, the maximual distance cannot be achieved. This trade-off is aircraft-specific and can be uncovered through a so-called payload-range analysis [START_REF] Ackert | ZEROe Towards the world's first zero-emission commercial aircraft[END_REF] if data are available. Else, we propose the simplification in Eq. (1.1) as an alternative.

MTK,ij stems from the original functional unit (the RPK projection) for a given segment (j) and connects the passengers to the payload mass of the aircraft. The constant (c) defines the mass of the passengers and their luggage; this is herein invariably taken to 100 kg (ICAO 2009). The variable Fi represents the proportion of freight mass out of c (passenger plus luggage mass) for a given aircraft (i), which varies from 7% to 70% (Supplementary Material 2 (SM2), Excel spread sheet). Finally, MF,max,ref,i (used to calculate ECref,i) includes both the trip fuel needed for the maximal achievable distance (MTF,max,ref,i), which is aircraft model dependent, and the reserve fuel (MRF,max,ref,i), which is determined as a share of MTF,max,ref,i; further details are given in SM2 (also for (battery) electric planes). MTF,max,ref,i corresponds to the difference between the aircraft takeoff mass and its landing mass. The calculation of ERPK,ij also involves OPEi, the overall propulsion efficiency of a given aircraft (i), defined as the achievable engine performance (National Academies of Science Engineering and Medicine 2016; IATA 2017).

Table 3 summarizes the specific values considered for each parameter and presents a translation of the ICAO projections (International Civil Aviation Organization (ICAO), 2018) (a basis for defining the functional unit) from the original RPK to EJ y -1 (defined as ERPK,ij) based on the reference fossil kerosene considered (Jet A/Jet A-1). Notes. For tube and wing (TW) models, A320 and A350 XWB were determined to be representative planes for domestic and international services, respectively. Heart Aerospace ES-19 is considered to be a representative all-electric aircraft for domestic ranges carrying up to 20 passengers per flight mission. The Boeing X-48B is considered for a blended wing body (BWB) aircraft.

Calculation details and data sources are available in SM2.

Hybridization

For LCAs of commercial aircraft, the need for hybridization, needs to be considered both as a function of the studied SA technology and in light of the studied time scope and type of transport segment to be covered, as defined by the functional unit. This implies a variety of possibilities regarding hybridization that are summarized in Table 4. In terms of system boundary activities, it is considered in the "Aircraft operation" box, as shown in Fig. 2. Notes: The definitions for domestic and international routes shown in Supplementary Material 1 (SM1) are derived from the ICAO Glossary [START_REF] Susan | ICAO: Glossary[END_REF]. It is assumed that the expected ranges, whether domestic or international, are unchanged from one-time scope to the other, meaning that the amounts of energy required are unchanged. *Combustion or fuel cell mode.

Biofuels/Electrofuels

Here, we introduce the degree of energy hybridization (HE,alt) to represent the energy distribution between the alternatives (whether biofuels or electrofuels) and the reference fuels (here conventional fossil-based kerosene), as illustrated in Eq.

(2). HE,alt,ij depends on the allowable (maximum) blending ratio (Bmax,ij, expressed in terms of the volume) for a given segment (j) and type of aircraft i and the alternative fuel characteristics, including the fuel density (ρ alt ) and the gravimetric energy density expressed in units of kg L -1 and MJ kg -1 , respectively; details are given in SM2.

HE,alt,ij = E F,alt,ij E F,alt,ij + E F,ref,ij (2)
E F,alt,ij = Bmax,ij × ρ alt × ED alt (2.1)
Here, EF,alt,ij and EF,ref,ij express the energy consumption for alternatives and conventional (fossil-based) kerosene, respectively, depending on the type of aircraft and segment.

Alternative biofuels (including electrofuels) are currently considered as blending components with conventional kerosene, with varying quotas (Bmax,ij) from 10% to 50%, as illustrated in SM2. We here consider the maximum blending ratio of each certified fuel details are provided in SM2. This blending necessary is considered for near-term horizon.

In fact, unblended SPK has been announced for a new generation of engines, as explained in Section 2.3. It is therefore considered that 100% SPK (Bmax,ij) can be applied in the long-term.

Electric (battery) SA

The degree of energy hybridization (HE) is introduced to represent the energy distribution between the battery and the liquid fuel, either kerosene or an alternative liquid fuel (providing technical issues such as the lack of aromatics are resolved in the latter case), as shown in Eq. ( 3).

H E,bat,ij = E bat, ij E bat, ij + E F,ref.,ij (3)
The battery (electrical) and reference fuel, denoted as conventional kerosene energy, are represented by Ebat,ij and EF,ref,ij, respectively [START_REF] Hoelzen | Conceptual design of operation strategies for hybrid electric aircraft[END_REF]. EF could also be alternative kerosene (biofuels or electrofuels; EF,alt,ij).

HE,bat,i ranges between 0 and 1, where HE,bat = 1 would represent an aircraft solely supplied by battery energy. Several HE values have been examined; detailed calculations are provided in SM2. The choice of HE,bat is also based on the accepted mass penalty, as described in Section 4.4. Thus, the known value of HE allows to calculate the Ebat value.

Although no hybridization for all-electric is considered, the limited distances are addressed in respect of the different types of batteries (see SM2).

Mass penalty

Both electric (battery) and hydrogen SA systems involve an overall mass that is greater than that of the reference conventional kerosene system for specific aircraft and segments. To overcome this, the number of seats available on-board can be reduced (which affects the number of passengers transported per aircraft and therefore MPL,ref).

Here, we hypothesize that the seat numbers should be reduced by a maximum of 40% of full capacity (i.e., a passenger load factor above 60%) to ensure a certain level of economic activity. This assumption is approximate and can be changed; yet, it corresponds to the drop that was observed in aircraft worldwide between 2019 and 2020 as a result of the COVID-19 pandemic (Statista, 2021).

The payload mass of the SA system, either electric (battery) or hydrogen, can be determined based on the maximum landing mass (ML,max), information that is generally provided by aircraft manufacturers (SM2). Then, taking the mass of the reserve fuel to be MRF,max,ref, (liquid hydrogen (LH2) or kerosene), the overall additional mass related to the batteries (e.g., the mass of the batteries and the electrically driven components) or the LH2 storage system including the fuel cell when in fuel cell mode) to be Msystem, and the operating empty mass of the aircraft to be MOE, the number of allowable payloads can be calculated for a given aircraft (i) as shown in Eq. ( 4):

MPL,max,system = ML,max -MRF,max,ref -Msystem -MOE (4)

Nseat,max,system = M PL,max,system × (1 -Fi) c

(5)

The number of maximum seats available (Nseat,max,system) depends on MPL,max,system.

Accordingly, the seat changing capacity (SCseat,system,i) can be calculated as

SCseat,system [%] = N seat,max,ref -N seat,max,system N seat,max,ref , (6)
where Nseat,max,ref is the maximum number of seats available depending on the aircraft (i) model, as illustrated in SM2. Finally, the total number of aircraft needed (Naircraft, RPK,ij) to supply the functional unit can be calculated as:

N aircraft,RPK,ij = RPKj D max,lifespan,i × N seat,max,system ×PL (7)
where Dmax,lifespan,i refers to the maximum distance for the whole lifespan of aircraft, depending upon the model obtained from [START_REF] Cox | Life cycle assessment of air transportation and the Swiss commercial air transport fleet[END_REF]. For simplification, Eq. ( 7) assumes passenger loading facor of 100%, namely that, all available seats are occupied by passengers.

In terms of system boundary activities, the mass penalty is included within "aircraft manufacture"), and allows to reflect that more aircraft are needed to serve the projected number of passengers during the studied time scope calculations are detailed in SM2.

The maximum distance traveled (Dmax,system) in the case of an all-electric aircraft in which different types of Li-based batteries are considered, can be calculated according to the Breguet range equation [START_REF] Staack | The potential of full-electric aircraft for civil transportation: from the Breguet range equation to operational aspects[END_REF].

Electric (battery) SA

The key parameter defining the mass penalty of the electric (battery)/kerosene hybrid system is the mass of the batteries required to supply Ebat for a given aircraft (i) model, as defined in Eq. ( 3). This parameter is referred to as MF,bat and is defined as

M F,bat = E bat e bat × n eff , ( 8
)
where ebat is the specific energy density of the battery [Wh kg -1 ] and neff is its efficiency [%], that is, out of the total battery capacity, the amount of useful energy that is supplied. The latter parameter is fixed at 95% based on Redondo-Iglesias et al. ( 2019), while a compilation of data from the literature concerning the energy density of a variety of Li-based batteries is available in SM2.

Mbat (Eq. ( 9)) only relates to the mass of the battery prior to its insertion in the aircraft. The mass of other components including all the accompanying required infrastructure, e.g., the battery management and cooling systems, is referred to here as Mbat, others. According to [START_REF] Ellingsen | Life Cycle Assessment of a Lithium-Ion Battery Vehicle Pack[END_REF], approximately 60% of the overall battery system mass (MF, bat) is due to Mbat,i, the remaining 40% being here attributed to Mbat,others.

MF,bat = Mbat + Mbat,others

Theoretically, improvements regarding kerosene savings or batteries that are more performant (greater than neff × ebat) could lead to a greater proportion of Ebat (or HE,bat) than calculated here with limited reductions in the carrying passenger capacity, as shown in the study of [START_REF] Ribeiro | Environmental assessment of hybrid-electric propulsion in conceptual aircraft design[END_REF]. The present framework can therefore be regarded as conservative.

Hydrogen SA

The mass penalty for the hydrogen SA system relates to the mass of the required cryogenic storage tank infrastructure (Mtank). To represent this, we used the concept of the gravimetric index (GI; sometimes known as the system gravimetric capacity) defined by [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF][START_REF] Winnefeld | Modelling and Designing Cryogenic Hydrogen Tanks for Future Aircraft Applications[END_REF], expressing the mass of LH2 (MF, LH2) with respect to the system's mass (MF, LH2 + Mtank)

GI = M F,LH2 M F,LH2 + M tank ( 10
)
GI values range from 0 to 1 (Winnefeld et al. 2018a;[START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF]. Here, we propose, on the basis of the aircraft (i) models presented in [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF], a GI value of 0.4-0.5 for the near-term horizon ( 2035), which can be increased for longer-term ( 2050), considering technical improvements that will likely allow a decrease in Mtank. For batteries, the total energy required is assumed to correspond to EF,ref (Eq. (1.2)), which defines, based on a gravimetric energy density of LH2 of 120 MJ kg -1 , the value of MF,LH2.

In addition, MF,LH2 is composed of useable LH2 and vented H2, both associated only with the flight stage.

MF,LH2 = MF,useable LH2 + MF, vented H2 ( 11)

The recent study of [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF] suggests considering a ratio of 90% useable LH2 (Χuseable LH2) and 10% vented H2.

4.5 Required infrastructure and end-of-life (EoL)

Needs of novel fueling/charging infrastructure and operation

The transition period from fossil-based kerosene, or alternative liquid fuels, to emerging energy systems, needs to be handled gradually because all aircraft will not change overnight; this is considered in the sixth category of the proposed inventory system. Substantial changes to existing airport infrastructure and investments would need to adapt to a rampup/down implementation corresponding to the demand shares of these future systems.

In the case of electric-based avitation, the battery plug-in charging (BPC) infrastructures are considered as the crucial ensuring turnaround time requirements depending on several parameters such as the power level, charging efficiency, charge-point/aircraft plugs and sockets [START_REF] Collin | Advanced electric vehicle fast-charging technologies[END_REF]. While these could be phased-out using the battery-swapping stations (BSS) [START_REF] Sarker | Optimal Operation and Services Scheduling for an Electric Vehicle Battery Swapping Station[END_REF]. However, short-time battery exchange systems and high investment costs are major challenges [START_REF] Li | Planning of Electric Public Transport System under Battery Swap Mode[END_REF], particularly for fixed scheduled flights. At the time of writing, the BPC, which presently facilitates terrestrial transport modes, is the most practical approach at an airport via the application of flexible cables and chargers for different models. Whatever BPC or BSS, in a nutshell, providing a high-power electricity supply would require substantial changes to existing airport infrastructure (International Air Transport Association (IATA), 2017;[START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF].

Hydrogen aviation concepts can be adopted via either combustion engines or fuel cells. LH2 has a higher physical energy density (70.8 kg m -3 at 1 bar, 20.3 K; [START_REF] Andersson | Large-scale storage of hydrogen[END_REF] relative to the compressed gaseous form (39 kg m -3 at 700 bar, 293 K; (Derking et al., 2019a), both being possible in aviation and aerospace applications [START_REF] Baroutaji | Comprehensive investigation on hydrogen and fuel cell technology in the aviation and aerospace sectors[END_REF][START_REF] Bruce | Opportunities for hydrogen in commercial aviation[END_REF]. LH2 storage applications and fueling infrastructures (whether in on-site or off-site production) have been considered corresponding to several parematers, namely, the insulation facilities (to minimize liquid evaporation loss (boil-off); (Derking et al., 2019b;[START_REF] Tietze | Bulk Storage Vessels for Compressed and Liquid Hydrogen[END_REF]), the availability of resources (e.g., water, electricity for on-site production) [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF], space limitations at airports, and the overall forecasted fuel demand related to the airport profile [START_REF] Stiller | Airport liquid hydrogen infrastructure for aircraft auxiliary power units[END_REF]. LH2 delivery to an aircraft can be achieved via insulated refueling vehicles [START_REF] Van Zon | Liquid hydrogen powered commercial aircraft: Analysis of the technical feasibility of sustainable liquid hydrogen powered commercial aircraft in[END_REF] or an alternative method such as a hydrant system [START_REF] Bruce | Opportunities for hydrogen in commercial aviation[END_REF][START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF]. The former approach appears to be the most flexible and viable fueling technology for the near-term horizon [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF][START_REF] Stiller | Airport liquid hydrogen infrastructure for aircraft auxiliary power units[END_REF] because a cryogenic hydrant refueling system is technically and economically infeasible, with costs five times higher than that of conventional hydrant systems [START_REF] Mckinsey | Hydrogen-powered aviation A fact-based study of hydrogen technology, economics, and climate impact by[END_REF]. Additional details of configuration variants are available in the SM1.

EoL management for used airframes and batteries

EoL management approaches for airframes and battery packs are summarized in Table 5.

Table 5. End-of-life management.

Parameters Lifespan

EoL approach, current and announced

Airframes

Average 25 years for passenger-carrying aircraft [START_REF] Elsayed | Best Practices and Standards in Aircraft End-of-Life and Recycling[END_REF][START_REF] Wong | Composites recycling solutions for the aviation industry[END_REF] • Conversion from passenger-carrying aircraft to air freighter, expanding the in-service time by 10-20 years [START_REF] Elsayed | Best Practices and Standards in Aircraft End-of-Life and Recycling[END_REF][START_REF] Forsberg | World Fleet Forecast 2017-2036[END_REF][START_REF] Wong | Composites recycling solutions for the aviation industry[END_REF] Once an aircraft is no longer being operated, either as a passenger craft or a freighter, and is disassembled, three significant EoL options are possible. i) Reuse of components, e.g., engines and door handles, in aircraft still being operated, under the supervision of the authorized/certified sector (Dornfeld and Linke 2012; [START_REF] Elsayed | Best Practices and Standards in Aircraft End-of-Life and Recycling[END_REF][START_REF] Wong | Composites recycling solutions for the aviation industry[END_REF] ii) Recycling applications for recyclable materials • Aerospace applications for aircraft creation [START_REF] Elsayed | Best Practices and Standards in Aircraft End-of-Life and Recycling[END_REF]van Heerden and Curran, 2010) • Non-aerospace applications of recycled composites, e.g., in automotive or sporting goods [START_REF] Vieira | Strategy and management for the recycling of carbon fiber-reinforced polymers (CFRPs) in the aircraft industry: a critical review[END_REF][START_REF] Yang | Recycling of composite materials[END_REF] iii) Land-filling as the last option for non-recyclable components (Dornfeld and Linke 2012) Battery pack a Battery degradation rates are roughly estimated in terms of calendar ageing (years) and/or cycle ageing (charge-discharge cycles) by manufacturers. This data gap is usually solved via assumptions based on the reported cycles, state-of-charge, and estimated years for environmental impact analyses.

• Life expectancies of batteries depend on numerous factors, including environmental conditions, such as outranged temperature [START_REF] Keil | Aging of lithium-ion batteries in electric vehicles: Impact of regenerative braking[END_REF][START_REF] Narayan | Estimating battery lifetimes in Solar Home System design using a practical modelling methodology[END_REF][START_REF] Pipistrel | Qatar Airways Group highlights sustainability initiatives for World Environment Day[END_REF], battery utilization [START_REF] Ellingsen | Life Cycle Assessment of a Lithium-Ion Battery Vehicle Pack[END_REF][START_REF] Narayan | Estimating battery lifetimes in Solar Home System design using a practical modelling methodology[END_REF][START_REF] Onori | A new life estimation method for lithium-ion batteries in plug-in hybrid electric vehicles applications[END_REF], and charging speed [START_REF] Collin | Advanced electric vehicle fast-charging technologies[END_REF]. Lifetime of batteries significantly on the battery composition [START_REF] Collin | Advanced electric vehicle fast-charging technologies[END_REF].

• When the batteries reach 70%-80% of the initial battery capacity (defined as the first-life EoL) and are certified for second-use standard b [START_REF] Narayan | Estimating battery lifetimes in Solar Home System design using a practical modelling methodology[END_REF][START_REF] Richa | Environmental trade-offs across cascading lithium-ion battery life cycles[END_REF][START_REF] Zhou | A calcium looping process for simultaneous CO2 capture and peak shaving in a coal-fired power plant[END_REF], batteries can be: Directly reused (repacking c might be performed) for automotive purposes in electric vehicles [START_REF] Bobba | Sustainability Assessment of Second Life Application of Automotive Batteries[END_REF][START_REF] Martinez-Laserna | Battery second life: Hype, hope or reality? A critical review of the state of the art[END_REF] Re-purposed (repacking c is required) as stationary energy storage, e.g., in households, industrial sites, or electricity production sites [START_REF] Bobba | Sustainability Assessment of Second Life Application of Automotive Batteries[END_REF][START_REF] Olsson | Circular business models for extended ev battery life[END_REF][START_REF] Richa | Environmental trade-offs across cascading lithium-ion battery life cycles[END_REF] • When the end-of-second use retained capacity is reduced to 60% of the initial capacity, recovery of valuable metallic components such as cobalt nickel [START_REF] Beaudet | Key challenges and opportunities for recycling electric vehicle battery materials[END_REF][START_REF] Bobba | Sustainability Assessment of Second Life Application of Automotive Batteries[END_REF][START_REF] Boyden | The Environmental Impacts of Recycling Portable Lithium-Ion Batteries[END_REF] for uses either in battery manufacturing or other industries is possible.

Notes: a The battery management system, the cooling system, and the battery packaging are not considered; b The certification process involves dismantling at the pack or module level and inspecting the hardware of the modules [START_REF] Bobba | Sustainability Assessment of Second Life Application of Automotive Batteries[END_REF]); c The repacking process is the stage following certification and involves inserting new materials and/or components [START_REF] Bobba | Sustainability Assessment of Second Life Application of Automotive Batteries[END_REF].

Life cycle inventory and impact assessment for tailpipe emissions

Overall approach

We propose dividing the tailpipe aircraft pollutant emissions into two well-acknowledged flight phases, namely, taxi, takeoff, and landing (LTO) and climb, cruise, and descent (CCD) for flight activities above 915 m altitude [START_REF] Winther | EMEP/EEA air pollutant emission inventory guidebook[END_REF] in order to account for the difference in their emissions with respect to the flight phases. Herein, CCD emissions are equivalent to the term "cruise emissions" presented in the studies by [START_REF] Cox | How to include non-CO 2 climate change contributions of air travel at ETH Zurich[END_REF][START_REF] Jungbluth | Recommendations for calculation of the global warming potential of aviation including the radiative forcing index[END_REF][START_REF] Klöwer | Quantifying aviation's contribution to global warming[END_REF][START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF], where the majority of flights take place. The scope considered in this framework of subsonic aircraft with cruise altitudes of 8-12 km [START_REF] Klöwer | Quantifying aviation's contribution to global warming[END_REF][START_REF] Olsen | Comparison of model estimates of the effects of aviation emissions on atmospheric ozone and methane: AVIATION EFFECTS ON OZONE AND METHANE[END_REF]Zhang, 2017).

CO2 drives the climate impact of aviation, particularly important for fossil-based or alternative fuels (bio/electro-fuels). Non-CO2 emissions (see SM1), e.g., water vapor and NOx from the combustion of hydrocarbon-based fuels or H2 in air, also generate a significant climate impact [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF], on top of affecting other impacts (e.g. photochemical ozone formation for NOx). However, currently, these emissions are not addressed in official targets, e.g., European Union Emission Trading Scheme, the CORSIA initiative [START_REF] Niklaß | Integration of Non-CO2 Effects of Aviation in the EU ETS and under CORSIA[END_REF], the Kyoto Protocol [START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF], because they can vary depending on the background atmospheric conditions, flight range, and cruise altitude and latitude [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF][START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF]; International Renewable Energy Agency (IRENA), 2019). Nevertheless, it is claimed that these emissions contribute to one-up to two-thirds of the climate impact of aviation [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF][START_REF] Forster | It is premature to include non-CO2 effects of aviation in emission trading schemes[END_REF][START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF][START_REF] Niklaß | Integration of Non-CO2 Effects of Aviation in the EU ETS and under CORSIA[END_REF]. This section will accordingly focus on the climate impact, where there is the most debate, but it should be remembered that other impacts are also affected by the different tailpipe pollutants.

In general, the tailpipe emissions of a given aircraft (i) model and segment (j) can be obtained using the following equation:

TPEpollutant,phase = ∑(EFpollutant,phase × MTF, phase) (

Here, TPEpollutant,phase indicates the tailpipe emission for a given pollutant for the phase LTO or CCD. The emission factor of the pollutant in phase (EFpollutant,phase) depends on the energy source. Unless specific data are available, the emissions from the reserve fuel (RF) can be assumed to be negligible because such fuel is not consumed during the flight.

The maximum trip fuel consumption can be estimated by:

M TF,max,phase = 1-Χ RF,ref × E RPK,ij × Χ TF,phase ED ref . (12.1)
The energy proportion of the RF for conventional kerosene is defined as ΧRF,ref depending on the aircraft model (i); details are given in SM2. XTF,phase is the proportion of fuel consumption in the phase (LTO or CCD), and EDF is the gravimetric energy density of the energy sourcing, and can be whether the reference fuel (conventional kerosene) or alternative kerosene (biofuels/electrofuels) (see SM2).

In this framework, the overall tailpipe emissions are estimated according to the SA type, except for all-electric (battery) aviation for which it is considered that no emissions are generated during the flight. The climate impact is further expressed in terms of CO2 equivalent by multiplying TPEpollutant,phase by the characterization factor, CFpollutant,phase, including global warming potential (GWP) and the global temperature change potential (GTP). In addition, the climate impact could be declined according to the flight phase (LTO and CCD).

Climate impact, CO2-equivalent= ∑ TPE pollutant, LTO ×CF pollutant,LTO + pollutant

∑ (TPE pollutant,CCD ×CF pollutant,CCD ) pollutant ( 13
)
where the amounts of emitted pollutants can be calculated using Eq. ( 12), and CF is the characterization factor used to convert the TPEpollutant,phase value of interest to its CO2 equivalent in mass units.

Estimating tailpipe emissions per SA type

In this section, the EFpollutant,phase of each emitted pollutant by phase is defined according to the type of fuel. They are considered to apply for the pollutant's amount computation (TPEpollutant,phase) (yellow in Fig. 3).

Default EFpollutant,phase values are proposed in Table 6. These are based on IPCC's Emission factor database (EFDB) when available, and on a compilation of literature otherwise (details in SM3).

Even though biofuels/electrofuels (alternative kerosene) follow similar trends to fossil-based kerosene in terms of hydrocarbons, the differences in their chemical compositions, such as the aromatic constituents, the hydrogen-to-carbon (H/C) ratios, result in differences in the combustion properties [START_REF] Braun-Unkhoff | About the emissions of alternative jet fuels[END_REF][START_REF] Chan | Benchmarking data from the experience gained in engine performance and emissions testing on alternative fuels for aviation[END_REF][START_REF] Timko | Combustion products of petroleum jet fuel, a fischer-tropsch synthetic fuel, and a biomass fatty acid methyl ester fuel for a gas turbine engine[END_REF]. For example, the emission factors for CO2 in the case of lower carbon-content fuels (mainly in the SPK fuel category), are quantitatively reported as being lower Wahl et al. 2013;[START_REF] Braun-Unkhoff | About the emissions of alternative jet fuels[END_REF][START_REF] Gawron | Combustion and emissions characteristics of the turbine engine fueled with HeFA blends from different feedstocks[END_REF].

Similarly, soot emissions are reduced for fuels with a lower level of aromatic content [START_REF] Braun-Unkhoff | About the emissions of alternative jet fuels[END_REF][START_REF] Chan | Benchmarking data from the experience gained in engine performance and emissions testing on alternative fuels for aviation[END_REF][START_REF] Williams | Impact of alternative fuels on emissions characteristics of a gas turbine engine -Part 2: Volatile and semivolatile particulate matter emissions[END_REF]. The reduction of NOx emissions has also been investigated [START_REF] Chan | Characterization of Emissions from the Use of Alternative Aviation Fuels[END_REF]Wahl et al., 2013). However, this phenomenon cannot be interpreted from the chemical composition of the fuels only, because it also depends on the operating conditions [START_REF] Braun-Unkhoff | About the emissions of alternative jet fuels[END_REF][START_REF] Timko | Combustion products of petroleum jet fuel, a fischer-tropsch synthetic fuel, and a biomass fatty acid methyl ester fuel for a gas turbine engine[END_REF][START_REF] Yang | An overview on performance characteristics of bio-jet fuels[END_REF].

Because some EFpollutant,phase are unavailable for alternative kerosene (biofuels and electrofuels), the EFpollutant, phase values of fossil-based kerosene are proposed to be used as a proxy. Similarly, if the EFpollutant,phase of each flight phase, such as H2O and soot, are not available, the available value is used as a proxy for both phases.

In the case of hydrogen, two main climate forcers are considered owing to the available data: water vapor and NOx emissions. The latter results from the combustion with air (di-nitrogen) as a result of high-temperature flame generation [START_REF] Baroutaji | Comprehensive investigation on hydrogen and fuel cell technology in the aviation and aerospace sectors[END_REF][START_REF] Khandelwal | Progress in Aerospace Sciences Hydrogen powered aircraft : The future of air transport[END_REF]. Hydrogen fuel leads to higher water emissions per energy content of the fuel, e.g., approximately 2.6 fold more water than hydrocarbon fuels.

3000

• Absorption of longwave radiation (+) 21-22 • Influence the cirrus cloud formation (+) 21-22 CCD 3150 1 2984 2-3,5-6 • Absorption of longwave radiation (+) 21-22 • Influence the cirrus cloud formation (+) 21-22 H 2 O LTO 1199.50 9-12 1199.50 1199.50 9-12 1199.50 8575.00 9-10,13 8575.00

• Absorption of shortwave radiation (+) 21-23 • Removed through the hydrological cycle within a few weeks 21-23 • Contribution to contrail formation (as condensation nuclei) (+) 21-23 CCD 1199.50 9-12 1199.50 9-12 8575.00 9-10,13 •Absorption of shortwave radiation (+) 21-23 • Persist up to 0.5-1 year in the stratosphere 21-23 • Contribution to contrail formation (as condensation nuclei) (+) 21-23 Soot LTO 0.03 13 0.03 0.02 4-13 0.02

• Particulate emissions (e.g., respiratory morbidity) 20 • Absorption of longwave radiation (+) 24-25 • Precursor for contrail and cirrus cloud formations (+) 24- 25 CCD 0.03 13 0.02 4-13 • Absorption of longwave radiation (+) 24-25 • Precursor for contrail and cirrus cloud formations (+) 24- 25 SO 2 LTO 1.20 14 1.20 1.20 14 1.20

• Contribution to photochemical smog formation 16,17,23 • Particulate emission (e.g., respiratory morbidity) 20 • Reflection of solar radiation (-) 16,17,23 CCD 1.20 14 1.20 14 • Reflection of solar radiation (-) 16,17,,23 No data available Notes: a Emissions of NOx are given in grams of NO2, including the conversion of NO to NO2 [START_REF] Niklaß | Integration of Non-CO2 Effects of Aviation in the EU ETS and under CORSIA[END_REF] [START_REF] Rypdal | Background papers: IPCC Expert meetings on good practice guidance and uncertainty management in National Greenhouse Gas Inventories[END_REF] are used as calculation baseline. e For the future scenario as applied in this framework, the CO2 EF is applied as a proxy for AIC quantification when the actually traveled distances are used [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF]. To ensure tractability, numbers are presented with a maximum of 2 significant digits after the point. This, however, is not to be seen as an indication of precision.

Acronyms: AIC (Aviation induced cloudiness); CCD (Climb, cruise, and descent flight phase); LTO (Landing and take-off flight phase); NMVOC (Non-methane volatile organic compounds) [START_REF] Barrett | Guidance on the use of AEDT Gridded Aircraft Emissions in Atmospheric Models[END_REF]; 12 [START_REF] Winther | EMEP/EEA air pollutant emission inventory guidebook[END_REF]; 13 (Tveitan, 2020); 14 [START_REF] Miller | Reduction of Sulphur Limits in Aviation Fuel Standard (SULPHUR)[END_REF]; 15 [START_REF] Li | Long-term declining in carbon monoxide (CO) at a rural site of Beijing during 2006-2018 implies the improved combustion efficiency and effective emission control[END_REF]; 16 [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF]; 17 [START_REF] Myhre | Anthropogenic and Natural Radiative Forcing Supplementary Material[END_REF]; 18 [START_REF] Szopa | Short-Lived Climate Forcers[END_REF]; 19 [START_REF] Forster | The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity[END_REF]; 20 [START_REF] Andrae | Progress in Life Cycle Impact Assessment: Water Vapor Emissions and Respiratory Inorganics[END_REF]; 21 [START_REF] Skowron | The impact of emissions of nitrogen oxides from aviation on tropospheric chemistry -the counterbalancing roles of ozone and[END_REF] ; 22 [START_REF] Lee | Aviation and global climate change in the 21st century[END_REF] ; 23 [START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF] ; 24 [START_REF] Lee | Transport impacts on atmosphere and climate: Aviation[END_REF]; 25 [START_REF] Jungbluth | Recommendations for calculation of the global warming potential of aviation including the radiative forcing index[END_REF]; 26 [START_REF] Sheng | Assessment of stratospheric fuel burn by civil commercial aviation[END_REF].

Assessment of the climate impact of tailpipe emissions

In the conventional LCA approach, climate characterization factors (CFs) are required in addition to emission factors (EFpollutant, phase; in section 5.2) to calculate the impact (Eq. 13).

The most commonly used metric for global warming impact in LCA is the GWP which has primarily been defined and applied to well-mixed (long-lived and short-lived) GHG (e.g. CO2, N2O, CH4). Extension of GWP to other short-lived, indirect climate forcers, or high altitude emissions from aviation, is a matter of current investigations (Atmosfair gGmbH, 2016). The impacts of these non-CO2 emissions have not been integrated into the available LCA literature, as illustrated in Table 1, because of the high uncertainties in the reported CF values; these uncertainties arise from numerous factors, such as the atmospheric conditions, latitudinal variations, and the distance traveled [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF][START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF]. Moreover, these CFs, particularly for the aviation sector, were not presented in the most recent IPCC report (AR6) [START_REF] Forster | The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity[END_REF][START_REF] Szopa | Short-Lived Climate Forcers[END_REF].

As an alternative to GWP, CFs were derived for the GTP metric, as shown in Table S8 [START_REF] Boucher | Aviation science and research needs[END_REF][START_REF] Boucher | Climate trade-off between black carbon and carbon dioxide emissions[END_REF][START_REF] Shine | Comparing Climate Impacts of Emissions of Greenhouse Gases[END_REF]. With the GTP metric, the additional inclusion of climate sensitivity, i.e., the change in the equilibrium surface temperature per unit RF, is required which introduces more uncertainty in the climate impact evaluation [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF][START_REF] Shine | Comparing Climate Impacts of Emissions of Greenhouse Gases[END_REF].

In order to alleviate the inconsistency of using GWP or GTP metrics for short-lived climate forcers, new metrics were recently proposed like GWP*/GTP* (primarily demonstrated for CH4; [START_REF] Allen | A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation[END_REF][START_REF] Allen | New use of global warming potentials to compare cumulative and short-lived climate pollutants[END_REF], and CGTP [START_REF] Collins | Stable climate metrics for emissions of short and long-lived species-combining steps and pulses[END_REF]. Sustained (step) emission rates are used instead of pulse emission to define such metrics. These new metrics, applied to changes in the emission rate (in a unit of mass per annum) rather than changes in the emission amount (in a unit of mass), were demonstrated as more relevant to account for time-dependent emissions of e.g. a country [START_REF] Collins | Stable climate metrics for emissions of short and long-lived species-combining steps and pulses[END_REF].

However, at present, there are no CFs available for these new metrics. In addition, the emission rate (instead of a quantity) is not always available/compatible with conventional (static) LCA. For these reasons, the conventional GWP/GTP is proposed in our framework.

In addition to using CFs, another approach to reveal the overall tailpipe emissions in CO2-equivalent is using the CO2 emission weighting factor (EWF). This factor is the ratio of warming effects (radiative forcing; RF; e.g., in [START_REF] Fahey | The use of non-CO 2 multipliers for the climate impact of aviation : The scientific basis[END_REF][START_REF] Jungbluth | Recommendations for calculation of the global warming potential of aviation including the radiative forcing index[END_REF][START_REF] Sausen | Aviation radiative forcing in 2000: An update on IPCC[END_REF]) or GWP in, e.g., [START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF][START_REF] Jungbluth | Recommendations for calculation of the global warming potential of aviation including the radiative forcing index[END_REF])) of the overall emissions including CO2 and non-CO2) and those of CO2 emission alone. The EWF can be directly multiplied with the known CO2 emissions at the CCD phase to account for overall climate change effects (CO2 and non-CO2). This simple approach has been proposed to estimate non-CO2 impacts in e.g., European Union Emission Trading Scheme (EU ETS) [START_REF] Derking | Liquid hydrogen storage: status and future perspectives[END_REF][START_REF] Niklaß | Integration of Non-CO2 Effects of Aviation in the EU ETS and under CORSIA[END_REF]. However, the EWF values are variously reported without gathering a global agreement at the time this work was performed. On top of that, this factor is not applicable to the carbon-free approach (e.g., H2). Further details are available in SM1 and elsewhere [START_REF] Cox | How to include non-CO 2 climate change contributions of air travel at ETH Zurich[END_REF][START_REF] Fahey | The use of non-CO 2 multipliers for the climate impact of aviation : The scientific basis[END_REF][START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF][START_REF] Lee | Aviation and global climate change in the 21st century[END_REF][START_REF] Sausen | Aviation radiative forcing in 2000: An update on IPCC[END_REF]. The existing CFs for aviation (e.g., in [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF], [START_REF] Fuglestvedt | Transport impacts on atmosphere and climate: Metrics[END_REF]) are specific for high altitudes, implying that the use is adapted to the CCD phase. The LTO emissions should also be taken into account, especially for short flights (<1,500 km) because the proportion of fuel consumption is relatively important (for example ecoinvent 3.8 indicates approx. 18-40% CO2 emitted at low altitudes). Nevertheless, the CFs for LTO phase emissions have not been established, and we propose to use the common CFs. In the LCA literature, it is not always clear to which phase the inventory and impact calculation is applied [START_REF] Jungbluth | Recommendations for calculation of the global warming potential of aviation including the radiative forcing index[END_REF].

The available CFs for aviation emissions are summarized in Tables S7 andS8. There is no distinction between the tailpipe emissions of CO2, CH4, N2O, and NMVOC and the surface (on-ground) emissions of these pollutants, so the common CFs apply to them.

To better understand the implications of using the different state-of-the-art approaches for calculating the global warming impact in LCA, an application case is presented hereafter for the four SA systems. The emission factors and impact calculation methods exposed in the previous sections are used. For the comparison of the methods, three key parameters are considered: i) the impact calculation method, ii) the emission factors (EFs), and iii) the share of fuel consumption between the LTO and CCD phases. In addition, 16 scenarios (fossil kerosene and biofuels/electrofuels) and 8 scenarios (H2) are further analyzed. The different cases calculated are described in table 7. The detailed calculation can be accessed in SM3. The CFs for well-mixed GHGs, CO, and NMVOC are those established for the surface emissions, no specific values are available for aviation (high altitude). All mentioned CFs are available in the IPCC assessment report [START_REF] Forster | The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity[END_REF][START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF][START_REF] Myhre | Anthropogenic and Natural Radiative Forcing Supplementary Material[END_REF] The CFs for aviation forcers are taken from [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF] and are applied for both LTO and CCD phases. This is due to the lack of CFs for LTO emissions from aviation.

Method 2 (M2): Similar to M1, the CFs for well-mixed GHGs, CO, and NMVOC are the same whether LTO or CCD, as no specific values are addressed for aviation (high altitude). Whereas the other remaining emissions are considered depending on the phase.

For the LTO emissions, the CFs for unspecified (any anthropogenic activities) NOx, soot (as a mixture of organic carbons and black carbons; [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF]), and SO2 are retrieved from the IPCC assessment report [START_REF] Myhre | Anthropogenic and Natural Radiative Forcing Supplementary Material[END_REF]. The impacts derived from water vapor is excluded because it is considered that H2O vapor is removed through the hydrological cycle in the troposphere [START_REF] Niklaß | Integration of Non-CO2 Effects of Aviation in the EU ETS and under CORSIA[END_REF]. The AIC impact is considered negligible as it is typically generated at high altitudes (EPA, 2000).

For the other aviation emissions at the CCD phase, the CFs of [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF] are used, similar to the M1.

Method 3 (M3): Only the CFs of well-mixed GHGs, CO, and NMVOC are considered. This corresponds to the method applied in the conventional LCA approach.

It is noted that for hydrogen SA, only NOx, and water vapor are considered in the tailpipe emissions because of the lack of information in this field. Besides, in conventional LCA there are no CFs for these substances.

Emission weighting factor (EWF)

Method 4 (M4): The EWF-based GWP100 with the factor of "1.7" is selected to harmonize with the CFs GWP100 obtained from [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF]. This factor is multiplied by the known CO2 emissions at the CCD phase to obtain the overall CCD emissions. This figure is further added to the CO2 emissions in the LTO phase (Atmosfair gGmbH, 2016;[START_REF] Cox | How to include non-CO 2 climate change contributions of air travel at ETH Zurich[END_REF].

As no carbon emissions are considered, the impacts from H2 combustion are not compatible with this approach, therefore the hydrogen SA is excluded from the comparison.

ii) Emission factors (EFs) E1: The reported EFs for LTO and CCD phases regarding the fuel types are used independently of the flight phase. They are referred to as the values with the headline "overall" in Table 6.

E2:

The EFs depending on the flight phase, are employed as stated with the headline "per phase" in Table 6.

iii) Fuel consumption F1: The fuel consumption at LTO is assumed to account for 10% (by mass or energy) of the total fuel demand (IPCC, 2006;[START_REF] Rypdal | Background papers: IPCC Expert meetings on good practice guidance and uncertainty management in National Greenhouse Gas Inventories[END_REF].

F2: Approximately 3% (by mass or energy) of fuel consumption is considered applicable during the LTO flight phase. This figure is retrieved from ecoinvent 3.8: transport, passenger aircraft, long-haul (>4,000 km) {GLO}. In addition, this is retrieved from the shares of CO2 emissions at low and high altitudes (the direct relationship to the amount of fuel consumption is assumed; [START_REF] Cox | How to include non-CO 2 climate change contributions of air travel at ETH Zurich[END_REF][START_REF] Mulder | On the reduction of NOx-emission levels by performing low NOx flights[END_REF]. Notes.

1 GWP100 of well-mixed GHGs, including CO2, CH4, and N2O, is extracted from the IPCC assessment reports [START_REF] Forster | The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity[END_REF].

2 GWP100 for CO and non-methane volatile organic compounds (NMVOC) are retrieved from IPCC [START_REF] Myhre | Anthropogenic and Natural Radiative Forcing Supplementary Material[END_REF] and [START_REF] Forster | Changes in Atmospheric Constituents and in Radiative Forcing[END_REF], respectively.

3 GWP100 for aviation climate forcers, NOx, soot, H2O, SO2, and AIC, are obtained from [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF]. 4 In method 2 (M2), GWP100 of NOx, soot, and SO2 for unspecified (anthropogenic activities) applied for LTO emissions are available in the IPCC assessment report [START_REF] Myhre | Anthropogenic and Natural Radiative Forcing Supplementary Material[END_REF].

5 Emission weighting factor (EWF are computed based on the GWP100 metric, with the factor "1.7," [START_REF] Lee | The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018[END_REF] in order to avoid the bias from the other sources.

6 LTO fuel consumption (% by mass) of 10% is assumed based on (IPCC, 2006;[START_REF] Rypdal | Background papers: IPCC Expert meetings on good practice guidance and uncertainty management in National Greenhouse Gas Inventories[END_REF] 7 LTO fuel consumption (% by mass) of 3% is obtained from the CO2 emissions shared between low and high altitudes (when the amounts of the CO2 emissions are directly related to the amount of fuel consumed; [START_REF] Cox | How to include non-CO 2 climate change contributions of air travel at ETH Zurich[END_REF][START_REF] Mulder | On the reduction of NOx-emission levels by performing low NOx flights[END_REF]). Data are available in ecoinvent 3.8: transport, passenger aircraft, long-haul (>4,000 km) {GLO}. 8 The relative changes (∆; %) have been calculated with respect to fossil-kerosene as reference.

In the graph: From Fig. 4, it is evident that M1, M2, and M4, with the inclusion of other aviation forcers along with CO2 provide significantly higher impacts than M3, which considers only CO2 (conventional approach). The other aviation climate forcers represent up to 40% of the total impacts (details in SM3). The small variations between the results of M1, M2, and M4 are due to the choice of applied EFs as some pollutants are reported in different amounts in a particular phase, e.g., CH4 and NMVOC are higher in the LTO phase.

When compared to the fossil-kerosene, biofuels/electrofuels and hydrogen systems demonstrate a better performance, of up to 12% and 87%, respectively, and this is for both M1 and M2 methods. This ranking is quantitatively conserved when variations in the fuel share between phases are considered (F1, F2) or when the EFs are global or detailed by phases (E1, E2).

Regarding the tailpipe emissions and impact integration in the LCA framework, M1 and M2 are highlighted as the most complete approaches for conventional (static) LCA studies because these methods include all climate forcers and flight phases. These approaches should be completed when specific CFs according to the altitude and latitude will be developed.

Unlike the EWF, this approach is not limited by energy sources in which the total emissions are not computed relative to CO2.

It should be noted that for a complete assessment of environmental performance in the aviation sector, not only climate impact but also other environmental impacts must be considered. For example, in Table 6, soot, NOx, SO2, and CO are all particulate pollutants associated with respiratory health hazards, acidification or eutrophication, mainly affecting low altitude areas [START_REF] Andrae | Progress in Life Cycle Impact Assessment: Water Vapor Emissions and Respiratory Inorganics[END_REF][START_REF] Andrae | Progress in Life Cycle Impact Assessment: Water Vapor Emissions and Respiratory Inorganics[END_REF][START_REF] Eastham | Aviation-attributable ozone as a driver for changes in mortality related to air quality and skin cancer[END_REF][START_REF] Gilmore | Temporal and spatial variability in the aviation NOx-related O3 impact[END_REF][START_REF] Li | Long-term declining in carbon monoxide (CO) at a rural site of Beijing during 2006-2018 implies the improved combustion efficiency and effective emission control[END_REF][START_REF] Yim | Global, regional and local health impacts of civil aviation emissions[END_REF]. Some methods (e.g., on European Platform on Life Cycle Assessment; (European Commission, 2022)), provide the CF for the upper troposphere and lower stratosphere (UTLS) sub-compartment for particulate matter, which should be applied if available. Those CFs are already provided by the LCIA methods, e.g., Environmental Footprint (EF; adopted by European Commission).

Noise impact should also be integrated into LCIA, especially for the airports' locations and their surroundings.

The CFs for environmental noise from transportation have already been integrated into some LCIA methods, including the Swiss eco-factors [START_REF] Frischknecht | Swiss Eco-Factors 2013 according to the Ecological Scarcity Method: Methodological fundamentals and their application in Switzerland[END_REF] and the Environmental Priority Strategies (EPS) (Steen, 1999a[START_REF] Steen | A systematic approach to environmental priority strategies in product development (EPS): version 2000 -General system characteristics[END_REF]. Additional CFs have been intensively studied (e.g., [START_REF] Meyer | Characterization of damages from road traffic noise in life cycle impact assessment: A method based on emission and propagation models[END_REF], [START_REF] Cucurachi | Characterisation factors for life cycle impact assessment of sound emissions[END_REF]). They should further be expanded to the other LCIA and LCI database (e.g., ecoinvent).

Conclusions

A comprehensive and complete framework is proposed for ISO-compliant LCA, built on the consequential approach, for the field of commercial aviation for passenger transport, capable to ensure transparent and robust comparability when evaluating the overall environmental performances of emerging energy-sourcing for aviation, i.e., biofuels, electrofuels, electric, and hydrogen. Overall environmental performace implies all quantifyable environmental impacts of state-of-the-art LCIA methodologies, and not a single impact. However, our methodology does not develop additional indicator, e.g. noise, that are relevant to aviation.

The aviation service unit is defined as the projected global RPK and is selected as the functional unit for two timeframes, i.e., 2035 (short-term) and 2045 (long-term), and for two segments namely domestic and international.

The system boundaries are defined with the key activities for five (three carbon-based and two carbon-free) systems: (i) biofuel from residual biomass, (ii) biofuel from land-dependent biomass, (iii) electrofuel, (iv) electric (battery) aviation, and (v) hydrogen aviation. The activities are grouped into seven categories: (i) conventional (fossil-based) kerosene activity, (ii) feedstock with the conversion process, (iii) counterfactual and displacement, (iv) aircraft manufacture, (v) aircraft operation, (vi) additional infrastructure needed, and (vii) end-of-life management (applied to aircraft or batteries).

A model is proposed to translate the functional unit into energy requirements in each timeframe and for each service segment, which allows to compare all type of aviation systems. The model also includes (i) hybridization (the use of more than one source of energy/propulsion system to power an aircraft), and (ii) the mass penalty affecting the number of carried passengers,. All these aspects are currently neglected in most LCA studies.

This work also provides a complete review of the tailpipe emissions by energy-sourcing category, including details by flight phase. A deep analysis of the existing methods for climate impact calculation and their utilization with LCI is provided. Whichever approaches (here called M1, M2, and M4) are employed, their results are not very different (up to 6% difference between M1, M2, M4) relative to excluding non-CO2 emissions (M3). Therefore, despite the uncertainties, efforts to address non-CO2 climate impacts are recommended, rather than considering them negligible.

Overall, this framework provides a guideline for LCA practitioners when considering new energy sources in the future aviation sector.

Fig. 2 .

 2 Fig. 2. System boundaries covered in the proposed framework: (a) biofuel from residual biomass; (b) biofuel from landdependent biomass; (c) electrofuel (CO2 from the atmosphere); (d) electric (battery) aviation; and (e) hydrogen aviation.Categories include the following: fossil kerosene system; feedstock system, any processes prior to aircraft operation; counterfactual and displacement; aircraft manufacture; aircraft operation; airportrelated infrastructures and operations; and end-of-life management (whether for batteries or aircraft). Hybridization is shown here with conventional (fossil-based) kerosene only, representing the near-term horizon in the case of biofuels and electrofuels.

Fig. 3 .

 3 Fig. 3. Overall tailpipe emissions calculation based on the fuel consumption data applied in this framework. The energy requirement can be calculated with two different formulas (as expressed in a) and b)) regarding the data availability (as shown in formular's required parameters).

Fig. 4 .

 4 Fig. 4. Comparative CO2 equivalent calculation approaches from the combustions of fossil kerosene, alternative kerosene (biofuels/electrofuels), and H2. The M1/M2/M3/M4-E1-F1 scenarios are presented in a bar graph with their net values. The net results of other scenarios are displayed in the scatter plots.

( a )

 a The combustion of hydrogen is considered, in which NOx and H2O are the main climate forcers clustered as non-CO2 in the displayed graph. (b) The non-CO2 emissions in method 3 (M3) are referred to a CO, CH4, N2O, and NMVOC. Not considered in that flight phase (as used in M2) Acronyms: CCD (Climb, cruise, and descent flight phase); EWF (Emission weighting factor); LTO (Landing and take-off flight phase)

  Non-CO2 emissions from aviation fuel combustion are stated as being excluded in the study.b The displacement accounts for the system expansion method.c The "and combustion" portion is not expressively phrased by the authors of the original study, but the results shown were included in the functional unit Acronyms for: (I) Production technologies: ATJ: Alcohol-to-Jet; DSHC: Direct sugar to hydrocarbon; FT: Fischer-Tropsch process; HEFA: Hydroprocessed esters and fatty acids;

			• ATJ from residual • Li-ion batteries	• ecoinvent	1 GJ fuel	SB: Cradle-to-Gate	• TRACI 2.1 LUC emissions
			woody biomass along applied in the Ce-Liner	database	combusted	from residual	• ACD, EUT, not mentioned
			with the generation of electric-aircraft project	• NREL Life		collection to fuel	SMOG, RES, CC,
			lignosulfonate	Cycle Inventory		combustion in	carcinogens,
				data for		aircraft engine; MF:	noncarcinogens,
				lignocellulosic		SE, Alloc. (mass)	and ecotoxicity;
				biomass			LUC emissions not
				conversion into			mentioned
	HTL: Hydrothermal liquefaction;		ethanol
	• Literature (II) Impact categories: ACD: Acidification potential; CC: Climate change; EUT: Eutrophication potential; HUT: Human toxicity; GWP: Global warming potential;
	reviews GWP100: Global warming potential on a 100-year horizon; MED: Metal depletion; OZD: Ozone depletion; PMF: Particle matter formation;
	(Capaz et al., POF: Photochemical oxidant formation; • Analyze the	• HEFA, FT, and ATJ	• ecoinvent,	1 MJ fuel	SB: Cradle-to-Gate	ReCiPe
	2020) RESP: Respiratory effects; SMOG: Smog formation; environmental impacts of with different	USLCI, and	combusted	from feedstock	midpoint (H)
	1G/2G feedstock in Brazil (III) Others: AR: Assessment Report; CCLUB: Carbon Calculator for Land Use Change; GHG: Greenhouse gases; feedstock generations GREET databases	cultivation to fuel	method v.1.13;
	for aviation biofuels using GREET: Greenhouse gases, Regulated Emissions, and Energy use in Transportation; • Literature reviews		combustion in	LUC emissions
	the economic allocation GTAP: Global Trade Analysis Project; USLCI: U.S. Life Cycle Inventory; IPCC: Intergovernmental Panel on Climate Change; LCIA: Life Cycle Impact Assessment; aircraft engine; MF: explicitly
	method LUC: Land use change; NREL: National Renewable Energy Laboratory;			Alloc. (market and	excluded
	(IV) Multi-functionality (MF) methods: SE: System expansion; Alloc: Allocation.		energy)
	(Liu et al.,	• LCA (GHG emissions	• Solvent-based DAC	• DAC pilot plant	1 gCO2	SB: Cradle-to-Gate	GWP100 a ; IPCC
	2020)	only) for electrofuels	and water electrolysis	data from Carbon	captured and 1	from raw material	AR5
		(aviation and road)	for hydrogen	Engineering Ltd.	MJ fuel	supply to fuel
		production with C from	production, and FT for	• Literature reviews	combusted	combustion in
		direct air capture (DAC)	fuel production			aircraft engine; MF:
						SE, Alloc. (mass
						and emission)
	(Moretti et	• Analyze carbon	• ABE fermentation of	• ecoinvent	1 MJ fuel	SB: Cradle-to-Gate	GWP100; IPCC
	al., 2021)	footprint and investigate	potato by-product and	database	produced	from feedstock	AR5
		the economic	sugar beet, followed by	• Experimental data		cultivation to fuel
		performance of sugar	alcohol condensation			distribution; MF:
		fermentation through an	and hydrotreatment, to			Alloc. (market,
		acetone-butanol-ethanol	jet fuels			energy, and mass)
		(ABE) route to jet fuels,
		derived from agricultural
		by-products from food
		processing
	Elgowainy et	• Environmental	• Three conversion	• GREET database	MJ fuel	SB: Cradle-to-Gate	GWP100
	al. (2012)	assessment of different	technologies: FT with		combusted, kg	from aviation fuel	(presumably
		alternative technologies	natural gas, coal, and		of payload for	productions to fuel	AR5); LUC
		with various (passenger	biomass; fast pyrolysis		each km of	combustion in	emissions not
		and freight) aircraft	of cellulosic biomass;		great-circle	aircraft; MF: SE,	mentioned
		classes	and HEFA from		distance, and	Alloc. (mass)
			vegetable and algal oils		number of
					passengers for
					each km of
					great-circle
					distance
	Electric aviation (battery-based)
	(Ribeiro et	• Environmental	• Lithium-based	• ecoinvent	• Flight	SB: Cradle-to-grave	• ReCiPe
	al., 2020)	assessment of hybrid-	technologies including	database	missions over	from raw material	midpoint (H)
		electric propulsion	Li-sulfur and Li-air	• Literature reviews	battery lifetime	extraction,	method v.1.11
					(charging	including battery	• CC, HUT,
					number =	system production,	ACD, EUT,
					number of	electricity	SMOG, PMF,
					missions)	consumption, and	OZD
						hybridization in an
						aircraft, to the end-
						of-life of the
						batteries; MF: SE
	Hydrogen-powered aviation
	(Tveitan,	• Conceptual design for	• Solid oxide cell water	• ecoinvent	MJ energy	SB: Cradle-to-Gate	• ReCiPe
	2020)	hydrogen-powered	electrolysis and solid	database	consumed per	from hydrogen	midpoint (E)
		aircraft	oxide fuel cells (H2	• Literature reviews	km traveled	production to the	method V1.13
		• Environmental	production and	• Piano-X software		use phase with fuel	• GWP, OZD,
		assessment of a hydrogen-	electricity generation	for conceptual		cells in the aircraft;	HTP, PMF,
		electric aircraft in	on-board, respectively)	aircraft design		MF: the multi-	POF, MED
		comparison with a				functionality
		traditional aircraft				method is not
						defined.
	Alternative hydrocarbon liquid fuel, electric (battery), and hydrogen
	(Johanning,	• Conceptual aircraft	• Alternative liquid	• Literature reviews	• Passenger	SB: Cradle-to-grave	ReCiPe method
	2015)	design	fuels produced via	• PrOPerA software	kilometer	from energy	(perspective
		• Environmental	vegetable oil hydration	for conceptual		sourcing production	used, whether
		assessment of different	from algae	aircraft design		to the use phase in	individualist,
		alternatives, including	• Natural gas steam			the aircraft; MF: the	hierarchical, or
		alternative liquid fuels,	reforming for hydrogen			multi-functionality	egalitarian, is
		electric (battery), and	production			method is not	not specified.);
		hydrogen				defined.

a

Table 2 .

 2 Global revenue passenger kilometers (RPK) by segment, for 2035 and 2045, as forecasted by the International Civil Aviation Organization (ICAO).

	Projected demand in trillion a RPK
	Service segment b	2035	2045
	Domestic	6	8
	International	9	14
	Total	15	22
	a 10 12 ; b As defined in ICAO (2018). ICAO's official definitions for domestic and international flights are provided
	in Supplementary Material 2 (SM2).

Table 3 .

 3 Translating global RPK demands into energy flows [EJ y -1 ] (ERPK,ij) based on fossil kerosene or all electric system.

	Service	Plane	Fi	MF,max,ref,i	Maximum	Share of	OPEi			ERPK,ij
	segment j	configurations i		[tonne]	distance	MTF,max,ref,i to			[EJ y -1 demand]
					(Dmax,ref,i)	calculate
					[km]	MRF, ref,i	2035	2045	2035	2045
	Domestic	TW (A320 neo)	7%	14.70	6300	21%	58%	65%	4.41	5.61
	route	TW (ES-19)	-	3.00	452	-	78%	87%	2.31	2.84
	International	TW (A350)	69%	88.65	15550	14%	58%	65%	7.12	10.57
	route
		BWB (X-48B)	54%	134.74	15742	9%	73%	81%		16.71

Table 4 .

 4 Hybridization requirements foreseen for commercial aircraft for the 4 SA approaches studied, per segment and time scope.

		Near-term horizon (2035)	Long-term horizon (2045)
	Type of SA	Domestic route	International	Domestic route	International route
			route
	Biofuels	Hybridization	Hybridization	No hybridization	No hybridization
				(Unblended biofuels)	(Unblended biofuels)
	Electrofuels	Hybridization	Hybridization	No hybridization	No hybridization
				(Unblended electrofuels)	(Unblended electrofuels)
	Electric (battery)	Hybridization	Hybridization	Hybridization	Hybridization
	All-electric	No hybridization	No hybridization	No hybridization	No hybridization
	Hydrogen (H2) *	No hybridization	No hybridization	No hybridization	No hybridization

Table 6 .

 6 Emission factors(EFpollutant,phase) proposed in the proposed framework depending on the type of energy source (in g pollutant kg -1 fuel used). Detailed data (with the 475 min/max ranges) can be found in SM3. 476

	Pollutants a	Phase	Fossil kerosene per phase Overall b	Alternative kerosene (Biofuels/Electrofuels) per phase Overall b	Hydrogen (H 2 ) per phase Overall b	Notable impact on the environment (Climate change impact: + warming/-cooling)
	Well-mixed greenhouse gases (GHGs)
		LTO	3155 1		3016 2-5			• Greenhouse gas (+) 20
	CO 2	CCD	3150 1	3152.50	2984 2-3,5-6	3000		• Greenhouse gas (+) 20 • Stratospheric CO2 from CH4 oxidation 26
		LTO	0.48 1		0.48 1			• Greenhouse gas (+) 20 • Precursor for photochemical O 3 formation (+) 17
	CH 4			0.24		0.24		• Greenhouse gas (+) 20
		CCD	0.00 1		0.00 1			• Precursor for photochemical O 3 formation (+) 17
								• Enhancement of stratospheric H 2 O (+) 17
		LTO	0.10 1		0.10 1			• Greenhouse gas (+) 20
	N 2 O	CCD	0.10 2	0.10	0.10 1	0.10		• Greenhouse gas (+) 20 • Depletion of stratospheric O 3	17
	Other aviation forcers
								• Harmful gas for living organisms (e.g., respiratory
								problems) 16-17
	CO	LTO	14.77 1	10.38	11.67 7	10.34		• Acceleration/contribution to the photochemical smog
								formation (+) 17
								• Precursor for photochemical O 3 formation (+) 18-19
		CCD	6.00 1		9.00 7			• Precursor for photochemical O3 formation (+) 18-19
		LTO	10.35 1		10.35 1			• Contribution to tropospheric O 3 formation (+) 17 • Destruction of ambient methane concentration (-) 17,19
	NMVOC			6.03		6.03
		CCD	1.70 1		1.70 1			• Contribution to tropospheric O 3 formation (+) 17 • Destruction of ambient methane concentration (-) 17,19

Table 6 .

 6 (Cont') Emission factors(EFpollutant,phase) proposed in the proposed framework depending on the type of energy source (in g pollutant kg -1 fuel used). Detailed data (with 477 the min/max ranges) can be found in SM3. 478

	Pollutants a	Phase	Fossil kerosene per phase Overall b	Alternative kerosene (Biofuels/Electrofuels) per phase Overall b	Hydrogen (H 2 ) per phase Overal b	Notable impacts on the environment (Climate change impact: + warming/-cooling)
	Other aviation forcers (Cont')
							• Contribution to photochemical smog formation 18,21
			LTO	14.20 1	1.71 3-4	7.93 8	• Particulate emissions (e.g., respiratory morbidity) 20 • Induce the tropospheric O 3 formation (+) 16,18, 21
							• Destruction of ambient methane concentration (-) 16, 18,21
	NO x	c		14.10	2.10	7.87	• Destruction of ambient methane concentration (-)
							(associated with the reduction of stratospheric H 2 O (-))
			CCD	14.00 1	2.50 2,5	7.81 8	16,18
							• Increase stratospheric O3 formation as a protective layer
							26
			LTO	3155 1	3016 2,5
	AIC d,e		3152.50

  ). b Overall AV. refers to the average EFpollutant derived from LTO and CCD phases. c Contrail and cirrus cloud formations are included in AIC. d Only NOx emissions are considered when hydrogen is combusted (van Zon, 2018). The EFNOx is computed from 80% NOx emissions reduction relative to fossil kerosene (van Zon, 2018). The NOx emissions from fossil kerosene retrieved from (Intergovernmental Panel on Climate Change (IPCC), 2006;

  Refs.: 1 (Intergovernmental Panel on Climate Change (IPCC), 2006); 2 (Chan et al., 2017); 3 (Chan et al., 2016); 4 (Chan et al., 2015); 5 (Gawron et al., 2020); 6 (Gawron and Białecki, 2018); 7 (Wahl et al., 2013); 8 (van Zon, 2018); 9 (Guynn et al., 2004); 10 (Klug and Faass, 2001); 11

Table 7

 7 Defined parameters for comparative approaches in tailpipe emission calculation (with the calculation basis 545 of GWP100 metric). Further detailed calculations can be retrieved in SM3. 546 This method uses CFs for well-mixed GHGs and other aviation forcers independently from the phase of flight. It means the emissions from LTO or CCD are used with the same CFs:

	Parameters	Description

Acknowledgements

This study was conducted within the framework of the research project Cambioscop (https://cambioscop.cnrs.fr), partly financed by the French National Research Agency, Programme Investissement d'Avenir (ANR-17-MGPA-0006), and the Region Occitanie (18015981). Pimchanok Su-ungkavatin was additionally funded by the French Ministry for Europe and Foreign Affairs (Doctoral grant).

CRediT authorship contribution statement

Pimchanok Su-ungkavatin: Conceptualization, Visualization, Data curation, Modelling, Formal analysis, Investigation, Methodology, Original draft preparation, Review & editing. Ligia Tiruta-Barna: Conceptualization, Supervision, Investigation, Writing -review & editing. Lorie Hamelin: Conceptualization, Funding acquisition, Investigation, Resources, Supervision, Writing -Review & editing.

Credits

The icons presented in the manuscript have been taken from https://thenounproject.com (creators: Gorreá; Oleksandr Panasovskyl; Made; JuliAya; Ataur Rahman; art shop; Graphic Nehar; rivercon; Icongeek26; Lomaxy; Viral faisalovers).