## 3 Results
### Detailed Analysis of the Environmental Impacts  
In the following, the environmental impact scores of fossil kerosene and the SAFs are investigated further. For this purpose, the total impacts are split into the shares associated with the fuel supply chain and flight operation, and the key drivers of the impacts are analyzed. Figure 4 indicates that the fuel supply chain was mainly responsible for a majority of the environmental impacts. In terms of SAFs, these were the main driver of environmental impacts for six of the impact categories analyzed (POF, PMF, HT, FRD, ALO, and MRD), accounting for 83-98% of the total impacts; however, the causes varied. Regarding biderosene, between 83% (HT) and 98% (ALO) of the environmental impacts resulted directly from the fuel supply chain, while for ALO, this was mainly due to the cultivation of miscanthus (responsible for 94%), and the large environmental impact for PMF, POF, and FRD was mainly due to FTS. During the FTS process, a large amount of emissions, including sulfur dioxide, volatile organic compounds, and particulate matter smaller than 10 \(\upmu\)m, was released, accounting for 92% (PMF) and 90% (POF) of the impacts. Regarding FRD, FTS was responsible for 46% of the total impact. In addition, 43% of the results were from the upstream chain of the required hydrogen, which included the production of biogas as well as the refining to biomethane. This also applied to HT and MRD, where the upstream chain of hydrogen was responsible for 80% and 55% of the impacts, respectively. The main reason for this was the energy demand during these production steps. For synthetic kerosene (SMR), the fuel production using FTS was predominantly responsible for 86% (HT) up to 97% (PMF and POF) of the environmental impacts. For ALO, HT, and MRD, this was mainly due to the chemicals required for gas purification and their upstream chains, which accounted for 56-76% of the total impacts. In addition, production of the required hydrogen was generally responsible for 7-26% of the environmental impacts of ALO, HT, and MRD. The situation was different when it came to FRD, where 76% of the environmental impact could be attributed to the required natural gas for hydrogen production. Its extraction and processing were responsible for 71% of the total impact. Concerning the impact categories of PMF and POF, the process of FTS itself was responsible for 81-88% of the total impacts. This was due to the emissions resulting from the FTS, such as sulfur dioxide, volatile organic compounds, and particulate matter smaller than 10 \(\upmu\)m.  
Regarding synthetic kerosene (PEM and SOEC), which performed the worst in all impact categories except ALO, the high energy demand during production was always responsible for the negative environmental impacts. Thus, between 36% (POF) and 96% (HT) of the total impacts could be attributed to the energy demand in hydrogen production. This was mainly due to the composition of the electricity mix, which at the current time still consists of almost 50% fossil and non-renewable sources [74]. Thus, the extraction, processing, and final conversion of lignite, hard coal, and wood chips into electricity is a significant driver of the environmental impacts. An exception to this was the impact category CC where, in addition to the fuel supply chain, flight operation had a significant impact. For biokerosene and synthetic kerosene (SMR), it was responsible for 93% and 76% of the total impact, respectively. This was due to the GHGs resulting from combustion, such as CO\({}_{2}\), of which about 2.5 kg were emitted per liter of fuel burned. In the case of synthetic kerosene (PEM and SOEC), the absolute amount of GHGs emitted during the combustion of the fuels was almost identical. Yet, only 20-26% was attributable to flight operation. As discussed previously for other impact categories, this was due to the high energy demand in hydrogen production and the composition of the electricity mix.