## 2 Aviation Fuel Emissions
### Aviation \(NO_{x}\) Emissions  
The \(NO_{x}\) emissions in the aviation industry typically originate from three reaction mechanisms: thermal-NO\({}_{x}\), prompt-NO\({}_{x}\), and fuel-NO\({}_{x}\)[48; 49]. Thermal-NO\({}_{x}\) is primarily generated in high-temperature regions above 1800 K, influenced primarily by temperature, excess air ratio, and residence time in the high-temperature zone [50; 51]. Prompt-NO\({}_{x}\) is minimally influenced by temperature. Its formation mechanism involves the decomposition of hydrocarbons during combustion, where CH radicals break the molecular bonds of nitrogen gas, generating the significant intermediate product HCN. Subsequently, HCN reacts with the radicals O and OH produced within the flame [52; 53]. Fuel-NO\({}_{x}\) is produced through the combustion of nitrogen elements present in the fuel [54]. NO\({}_{x}\) emissions from aviation, along with CO\({}_{2}\) emissions and aviation-induced cloudiness, are major contributors to RF [55]. The climatic impact of NO\({}_{x}\) takes complex forms. On one hand, it contributes to the production of ozone (O\({}_{3}\)), a positive climate forcing agent that leads to warming [56; 57]. The O\({}_{3}\) production efficiency from aviation NO\({}_{x}\) emissions is approximately four times higher than that from surface NO\({}_{x}\) emissions. On the other hand, O\({}_{3}\) photolysis generates excited oxygen atoms that react with water vapor to produce OH radicals, which are the primary sink for methane (CH\({}_{4}\)) in the troposphere, thereby reducing methane concentration, a negative climate forcing agent that leads to cooling. The decrease in methane concentration, as a major precursor to O\({}_{3}\) formation, results in a reduction in O\({}_{3}\) concentration and stratospheric water vapor (SWV) levels. Despite the intricate interplay of reactions leading to uncertainties, it is generally believed that the net NO\({}_{x}\) response from aircraft leads to a positive, warming RF [58]. According to [PERSON] et al. 's predictions, aircraft NO\({}_{x}\) emissions in 2050 are expected to reach 2.17 Tg(N) year\({}^{-1}\) under scenarios of low air traffic growth and optimistic technological development. Conversely, under high air traffic growth and low technological development scenarios, emissions could reach 5.59 Tg(N) year\({}^{-1}\). This represents a significant increase from the 0.71 Tg(N) year\({}^{-1}\) recorded in 2006. They also suggest that the net NO\({}_{x}\) RF of the aviation industry will vary greatly depending on background conditions. Reducing surface emissions and having a cleaner atmospheric background by 2050 could mitigate NO\({}_{x}\) RF to some extent. For example, if anthropogenic surface emissions remain at current levels, the net NO\({}_{x}\) RF under high air traffic growth in 2050 would be 17.5 mW m\({}^{2}\). This is 48% higher than under the 2050 representative concentration pathway (RCP) 2.6 scenario, which represents the cleanest background, and 28% higher than under the 2050 RCP 8.5 background conditions [59]. NO\({}_{x}\) emissions in the aviation industry exhibit strong spatial variability, with aircraft emissions of NO\({}_{x}\) at cruising altitudes above 2000 ft increasing total RF by approximately \(2\pm 1\) mW m\({}^{-2}\) through gas-phase photochemical reactions [60]. Short-lived NO\({}_{x}\) gases cause sensitive changes in RF in the local regions of emission, but long-lived methane and carbon dioxide counterbalance regionally, resulting in globally averaged RF [61]. Besides CO\({}_{2}\), NO\({}_{x}\) emissions from aviation dominate the pollutants' emission quantities and environmental impacts, with increasing influence primarily concentrated around emission-dense airports, such as those in economic zones or tourist destinations [62]. The majority of NO\({}_{x}\) emissions arise from thermal processes, such as engine testing typically controls power output, resulting in similar combustion temperatures.