## 4 Discussion  
The overall results of this study confirm that the use of SAFs can be a promising solution for reducing the environmental impacts of the aviation sector. Although the GHG emissions associated with flight operations were almost identical due to fuel combustion, NO\({}_{\text{x}}\) and particulate matter emissions were reduced significantly in this phase. Additionally, large amounts of CO\({}_{2}\) were sequestered in the upstream chains of fuel production, reducing GHG emissions compared with fossil kerosene in a life cycle perspective. Based on current production conditions and the currently available electricity mix, biokerosene especially is a promising alternative to fossil kerosene in terms of its environmental impacts. However, the cultivation of the feedstock poses potential challenges in terms of agricultural land use if it competes with the food industry. In addition, synthetic kerosene based on hydrogen produced by SMR or electrolysis is largely disadvantageous compared with fossil kerosene. With regard to synthetic kerosene (SMR), this is due to the use of natural gas, which is required as feedstock for SMR. In terms of synthetic kerosene (PEM and SOEC), the high energy demand in hydrogen production in combination with the current composition of the electricity mix are responsible. However, concerning the electricity mix, it can be assumed that renewable sources will be increasingly used for electricity generation in the future. This will have a positive influence on the production of SAFs as well. This assumption is supported by a scenario analysis carried out. An energy mix for the year 2050 was modeled based on the work of [PERSON] et al. [75], which was based 100% on RE. In particular, the environmental impacts from energy-intensive hydrogen production could be significantly reduced, which results in synthetic kerosene (PEM-RE and SOEC-RE) being associated with the lowest overall impacts in terms of climate change. While the results also show that there was still improvement potential from an economic perspective, currently, the use of SAFs is not yet competitive with fossil kerosene, mainly due to the significantly higher production costs. These are up to six times higher than for fossil kerosene, thus lacking incentives for their use in flight operations. This applies to both scenarios with the current electricity mix and an electricity mix completely based on RE. At this point, political measures are necessary (e.g., substitution, subsidy programs, or tax measures for non-SAFs) to allow for the economical production of SAFs on a large industrial scale and increase their competitiveness. At the same time, the question remains whether an expansion of the production capacities of SAFs is possible with the current capacities of electricity generation. In particular, the high energy demand in hydrogen production using electrolysis could lead to overloads in the electricity grid, resulting in production restrictions. On the other hand, the energy-intensive production of SAFs can positively contribute to balancing the fluctuating supply of electricity from renewable resources. Further uncertainties exist concerning the fuels analyzed and their production. For the study carried out, the focus was on second-generation biokerosene and hydrogen-based synthetic kerosene. For hydrogen production, SMR, as well as PEM and SOEC, were considered. However, there is research that has been conducted addressing third-generation biokerosene [76] as well as other hydrogen production pathways (e.g., methane pyrolysis) [77]. In this article, FTS was assumed to be the synthesis process due to its market maturity. Still, other synthesis processes (e.g., methanol synthesis and methanation) can be considered in future analyses. Against this background, the analysis of the synthesis of ammonia-based fuels using Haber-Bosch processes would be interesting. Since they are carbon-free, no CO\({}_{2}\) is emitted during combustion.