## 2 Aviation Fuel Emissions  
### LCA of CO\({}_{2}\) Emissions  
In 2019, the aviation industry contributed approximately 3% of global CO\({}_{2}\) emissions. Without swift technological advancements and enhanced policies, CO\({}_{2}\) emissions from aviation could increase 2.5\(\times\) from 2015 to 2050 [24]. However, projections suggest that, by mid-century, this figure could increase to 4.6-20.2%, due to a projected annual growth rate of 5% in aviation activities. Aviation and road transportation emit 560-867 g and 45-102 g of CO\({}_{2}\)/ton-km, respectively, with the former being approximately ten times higher than the latter [25]. CO\({}_{2}\) emitted from aircraft engines during complete fuel combustion is released directly into the upper atmosphere, playing a significant role in climate change [26; 27]. CO\({}_{2}\), as the most prominent greenhouse gas (GHG), possesses the ability to absorb and re-emit thermal radiation (infrared radiation) and has a long atmospheric lifetime [28]. The increasing concentrations of CO\({}_{2}\) and other GHGs in the atmosphere lead to the absorption and re-emission of more infrared radiation emitted from the Earth's surface, resulting in an overall increase in the retention of heat in the Earth's atmosphere [29; 30]. This phenomenon contributes to global warming and associated climate change, such as sea level rise, extreme weather events, and disruption of ecosystems. In fact, both CO\({}_{2}\) and non-CO\({}_{2}\) emissions from the aviation industry collectively contribute to its impact on the global climate system, with their combined effects causing surface warming. Emissions of CO\({}_{2}\), water vapor, NOx, and soot, along with contrail cirrus formation, result in net positive radiative forcing (RF), thereby inducing warming, whereas sulfate aerosol production results in negative RF, thus inducing cooling. Persistent contrail cirrus clouds interfere with the Earth's incoming solar radiation and outgoing infrared radiation, with the net effect being climate warming. NOx emissions cause warming through short-term ozone increase and cooling through long-term reductions in ozone, methane, and stratospheric water vapor [31]. [PERSON] et al. have found that the net effective radiative forcing (ERF) from aviation in 2018 was +100.9 mW/m\({}^{2}\), with major contributions from contrail cirrus, CO\({}_{2}\), and NOx at 57.4, 34.3, and 17.5 mW/m\({}^{2}\), respectively. The CO\({}_{2}\) term accounts for a net positive (warming) ERF, representing less than half (34%) of the aviation net ERF in 2018 [32]. In terms of human health, elevated levels of CO\({}_{2}\) near the airport can lead to respiratory issues such as shortness of breath, exacerbation of asthma symptoms, and decreased cognitive function. Prolonged exposure to high concentrations of exhaust emissions may also contribute to cardiovascular problems and increased susceptibility to respiratory infections [33]. As described in the introduction, SAF stands as one of the pivotal methods for reducing carbon emissions in the aviation industry, albeit typically not achieved by altering fuel composition to lower CO2 generation during combustion. For instance, under comparable conditions, the CO2 emission indices (EI-CO2) for FT fuels derived from natural gas, coal, and JP-8 fuel are 3090, 3110, and 3160 g/kg, respectively, with marginal differences [34]. Conventionally, the comparison of CO2 emissions between SAFs and traditional aviation fuels is based on life cycle analysis (LCA), a method assessing the environmental impacts on human and ecological health caused by a system [35; 36]. The system boundary of LCA is depicted in Figure 1, illustrating all inputs and outputs of various fuels (AT), HEFA, SIP, Jet A-1) throughout each process or stage in the well-to-wake (WTWa) phase. LCA aims to scientifically and systematically quantify the resource consumption, energy usage, emissions output, and potential ecological impacts of SAFs throughout its entire lifecycle [37].