## 2 Aviation Fuel Emissions
### LCA of CO\({}_{2}\) Emissions
The lifecycle inventory can be divided into five categories: Firstly, raw material acquisition and processing, which involves recording and quantifying the energy use, water consumption, land use changes, and related GHG emissions associated with the cultivation/collection and preprocessing of raw materials (e.g., plant oils, algae, waste biomass) used in SAF production. Secondly, there is fuel production, which includes analysis of the direct and indirect energy requirements, chemical inputs, and resultant emissions (including but not limited to CO2, NOx, SOx, and particulate matter) associated with conversion technologies such as hydrotreating and Fischer-Tropsch synthesis/Thirdly, transportation and storage considers the logistics chain from SAF production sites to airports, including the energy consumption and emissions during transportation. Fourthly, there is the usage phase. Although the combustion emissions of SAF in aircraft are typically similar to conventional jet fuel, the net lifecycle emissions can be significantly reduced due to differences in feedstock and production methods. This phase requires the assessment of direct emissions from flight operations. Finally, there is waste management and recycling, which involves analysis of the treatment methods and environmental impacts of byproducts or waste generated after SAF use [38; 39]. Figure 1: Boundary analysis of LCA systems for SAFs and Jet A-1 fuel. The lifecycle inventory can be divided into five categories: raw material acquisition and processing, fuel production, transportation and storage, usage phase, and waste management and recycling. WTWa = well-to-wake. Based on LCA findings, [PERSON] et al. project that, by 2050, employing AJF could potentially reduce the life cycle greenhouse gases emission index (EI-GHGs) of aviation by up to 68.1%, with annual investments in AJF production capacity amounting to only about one-fifth of the global investment in pre-existing oil refining [40]. Moreover, research by [PERSON] et al. indicates that EI-GHGs of hydroprocessed renewable jet (HRJ), pyrolysis jet, and FTJ from corn stover could decrease by 41-63%, 68-76%, and 89%, respectively, compared with traditional aviation fuels. The GHG emissions of HRJ fuel in the WTWa phase are primarily influenced by fuel production activities, reflected in the consumption of H\({}_{2}\) and natural gas (NG), with the second source being related to the choice and utilization rate of fertilizers [41]. [PERSON] et al. have analyzed the GHG emission savings of different SAFs compared with traditional aviation fuels, ranging from highest to lowest, as follows: FT (86-104%), HTL (77-80%), pyrolysis (54-75%), UCO-based HEFA (68%), and sugarcane-(71-75%) and corn stover-based ATJ (60-75%) [42]. FT process jet fuel could achieve 50-90 wt% GHG reduction in life cycle and the potential reduction on PM [43]. Furthermore, selecting suitable raw materials and conversion technologies, utilizing sustainable hydrogen sources, and implementing carbon capture and storage technologies can further reduce the greenhouse gas emissions of SAFs throughout the life cycle [44; 45]. [PERSON] et al. have conducted LCA analyses on biojet fuels derived from microalgae, pongamia, and sugarcane molasses. While the fossil energy ratio (FER) and GHG reduction effect of microalgae-based biofuel (FER = 1.0, 53% lower than aviation kerosene) are inferior to sugarcane molasses (FER = 1.7, 73% lower than aviation kerosene), the former demonstrates advantages in nutrient avoidance, reduced water usage, and lower land utilization rates, highlighting the superiority of third-generation biofuel feedstocks [46]. Figure 2 summarizes the lifecycle GHG emissions of various aviation fuels, with traditional aviation fuel emitting 90 g CO\({}_{2}\)-eq/MJ. The CO\({}_{2}\)-eq emissions of SAFs produced via different pathways vary.