## 2 Comparative Method
### Selected Metrics for Comparison  
The assessment of different OIs and their comparison focuses on the climate mitigation potential, as well as resulting impacts on the stakeholders of the air transportation system. Hence, suitable KPIs need to be identified. For the climate impact assessment, the corresponding climate metric, time horizon and emission scenario are determined [52]. Possible climate metrics include the quantity of emissions, which causes changes in atmospheric concentrations of greenhouse gases (GHG), resulting RF, global warming potential (GWP), global temperature potential (GTP), and average temperature response (ATR) [2; 52; 53; 54; 55]. While emission quantities deal as a first indicator directly related to fuel consumption, RF indicates radiation changes caused by concentration changes in the atmosphere and is influenced by background emissions as well as reference time. GWP and GTP link to a chosen time horizon (e.g., 20, 50, 100 years) and summarize the impact on radiation or temperature changes, respectively. ATR describes the mean future temperature development over a period up to the chosen time horizon [52]. Relevant advantages and disadvantages of different climate metrics are summarized in Table 2[56; 57]. In the following, the comparison of climate effects is focused on ATR over a time horizon of 100 years (ATR100) incorporating a future-emission background scenario (F-ATR) as defined by the business-as-usual scenario (BAU) in [PERSON] et al. [8]. This metric, widespread in literature, e.g., [14; 15; 55], reduces the dependency on the time horizon and is directly related to climate impact, in other words, near-surface temperature change. The 100 year time horizon is typically used in policy making and enables a proper consideration of long-lived species such as CO\({}_{2}\)[57]. The climate mitigation potential of an OI is defined in the following as the reduction in ATR100 caused by the implementation of an OI with respect to the reference case without implementation of the OI. Non-climate KPIs describe the different measures' effects on operations and the air transportation system. These parameters can be further differentiated into technical, operational, economic, and acceptance KPIs. Technical KPIs are directly related to the efficiency and performance of the aircraft and can primarily be described by fuel consumption or fuel efficiency. Operational KPIs relate to the tactical and strategic operations of the involved parties, for instance, described by aircraft utilization or number of landing-take-off (LTO) cycles per time period. Airline networks can further be characterized by their available seat kilometers (ASK). Consequences for air navigation service providers (ANSP) can be described by the number of aircraft movements in an airspace sector, the routing efficiency, i.e., the divergence from the optimum route for a flight, and the added flight distance or time. Economic effects of implementing in-flight OIs can be analyzed by changes in direct operating cost (DOC) combining variable and fixed operating costs and summarizing changes in fuel consumption, flight time, flight distance, and LTO cycles performed on a monetary basis [58; 59; 60; 61]. Changing airline networks may also lead to profit and revenue changes. Moreover, impact on airports can be described by changes in the number of LTO cycles. When implementing changes to the ground processes, fuel and energy consumption are relevant as they are directly related to operational costs. Furthermore, the implementation of OIs typically requires a substantial initial investment. In addition, acceptance of measures can be qualitatively analyzed by investigating community, market, and socio-political acceptance [62].