## 1 Introduction  
Aviation is a strong driver for modern economies, promoting social and regional cohesion. In recent years, the aviation sector has contributed to increased tourism, and stimulated economies. Global air traffic increased by more than two-thirds in the last 15 years to reach about 39 million flights in 2019. During the same time, the number of passengers more than doubled and the worldwide air freight traffic increased by half. At European level, in 2019, over 1.2 billion passengers travelled across the 500 airports of the region in 9.3 million flights. In 2019, commercial aviation emitted over 0.9 \(\mathrm{G}\mathrm{t}\mathrm{C}\mathrm{O}_{\mathrm{2 eq}}\) globally, representing about 2% of global anthropogenic greenhouse gas (GHG) emissions and about 12% of the emissions from the transport sector. In Europe, the departing flights from Europe accounted for 5.2% of total GHG emissions in 2019 and 18.3% of transport sector emissions [1, 2]. With respect to the consolidated trends of the last decades, the sector faced a disruptive phase, due to the strong impact of the COVID-19 pandemic. In 2020, flights decreased to just 4 million and emissions decreased by more than 50%. Following the recovery from the pandemic, it is expected that air traffic will gradually recover to its pre-crisis levels and the numbers of flights are foreseen to grow slowly until 2050 [2]. Regardless of the short-term effects of such a crisis, a growing concern exists for the increasing environmental impact of the sector [3]. In 2020, the European Commission (EC) adopted the Sustainable and Smart Mobility Strategy, which sets the objective to boost the uptake of sustainable fuels for aviation [4]. More recently, the EC launched the ReFuel EU aviation proposal, aimed at supporting the EU aviation market to ensure a gradually increasing share of sustainable aviation fuels (SAFs) in Europe, limiting the detrimental effects on competitiveness [5]. The European Commission has proposed, in its ReFuelEU Aviation initiative, a blending mandate for the fuel suppliers to European Union (EU) airports, with minimum share of SAFs increasing to 5% in 2030 and to 63% in 2050. To achieve this mandate, approximately 2.3 million tonnes of SAF would be required by 2030 and 28.6 million tonnes by 2050. This mandate includes a sub-mandate for synthetic aviation fuels that should increase from 0.7% in 2030 to 28% in 2050 [2]. The Renewable Energy Directive 2018/2021 (RED II) set minimum targets in the EU for the use of 14% renewable energy for 2030. The proposed revision for the RED II (COM(2021) 557 final) as part of the Fit for 55 package calls for a 13% reduction of carbon intensity (CI) of the transport sector. To be eligible, biofuels used in transport must comply with the sustainability and GHG emissions-saving criteria laid down in RED II. The carbon intensity of a fuel is defined as the lifecycle of GHG emissions per unit of energy, i.e., \(\text{gCO}_{\text{2 eq}}/\text{MJ}\). According to RED II, in order to qualify as renewable energy source for aviation, biofuels must achieve an at least 65% reduction in their carbon intensity against the fossil fuel baseline of 94 gCO2 eq/MJ. At international level, in 2016 the assembly of the United Nation's International Civil Aviation Organization (ICAO) agreed on the adoption of a global market-based scheme to limit international aviation CO2-equivalent (CO2 eq) greenhouse gas emissions: the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) [6]. CORSIA requires airlines to offset \(\text{CO}_{\text{2e}}\) emissions that exceed 2019 levels. On the basis of impact assessments and available scientific knowledge, CORSIA has been framed to allow offsetting either through credits or through the use of CORSIA-eligible fuels (CEFs). CEF is a category comprising both sustainable alternative fuels (SAFs) and lower carbon aviation fuels (LCAFs).