## 2 Aviation Fuel Emissions
### Aviation UHC Emissions  
The formation mechanisms of UHC in jet fuel combustion within aircraft engines typically involve the following aspects. Firstly, incomplete combustion, where, under certain engine operating conditions, such as low-speed flight, takeoff, landing, or when there is insufficient oxygen supply in the combustion chamber, fuel may not fully mix with oxygen or complete the oxidation reaction within the limited time, resulting in the partial combustion of hydrocarbons and the formation of UHC. In real combustion processes, rich fuel conditions can lead to the production of CO, aromatics, UHC, and soot, while lean conditions can release oxygen-rich substances like ketones, peroxides, and NO\({}_{\text{x}}\)[124]. Secondly, local low-temperature zones, where the combustion chamber may have regions of lower temperature, especially at the flame front edges or where fuel spray contacts the cold wall surface. Low temperatures can inhibit combustion reactions, causing some fuel molecules to exit with the exhaust before oxidizing. Thirdly, there is poor fuel atomization, where, if there are issues with the fuel nozzle or improper fuel pressure, the fuel may not atomize adequately into fine droplets, with larger droplets less likely to evaporate quickly and mix with air, increasing the likelihood of incomplete combustion. Finally, there is short residence time. Modern jet engines aim for high efficiency with compact combustion chambers, resulting in very short fuel residence times. This can prevent complete combustion of some fuel before it is expelled [125; 126; 127; 128; 129; 130]. UHCs are a subset of volatile organic compounds (VOCs). A significant portion of VOC emissions from aircraft engines consists of UHCs [131; 132]. These compounds undergo photochemical reactions with nitrogen oxides under sunlight, producing ozone and other secondary pollutants such as peroxyacetyl nitrates (PANs), which are primary contributors to photochemical smog and significantly impact air quality [133; 134]. Although most UHCs have a short atmospheric lifespan, their involvement in various chemical reactions can lead to the formation of GHGs, like methane and carbon dioxide, indirectly contributing to global warming [124; 135]. Over 8 million people fly on commercial airplanes daily, with approximately 5% suffering from respiratory diseases. Therefore, it is necessary to maintain high air quality onboard to protect the health of both crew and passengers. Formaldehyde, benzene, tetrachloroethylene, trichloromethane, 1,2-dichloroethane, and naphthalene are the six primary VOCs in the current cabin environment, posing risks to crew members that exceed the acceptable levels recommended by the US EPA [136]. Xylene in the cabin is significantly associated with irritation of the eyes, nose, and throat. Aldehydes, potential oxidation products of ozone, can also irritate human senses. Common symptoms among crew members and pilots include dry eyes and fatigue [137]. These VOCs may impact perceived indoor air quality and lead to passenger complaints. [138]. In air monitoring studies at Zurich Airport, a significant presence of reactive C2-C3 olefins and isoprene was found in engine exhaust. Isoprene was not detected in refueling emissions, and the benzene to toluene ratio in exhaust (1.7) was also higher than in kerosene refueling emissions (well below 1) [139]. Exposure to UHCs and VOCs poses health risks to humans. Inhalation of UHCs can irritate the eyes, nose, and throat, cause coughing, wheezing, and other respiratory issues, and increase the risk of chronic bronchitis and emphysema [140; 141]. Long-term exposure to high VOC concentrations may raise the risk of cardiovascular diseases, including heart disease and hypertension. Polycyclic aromatic hydrocarbons (PAHs), which are highly toxic and potentially carcinogenic, can increase the risk of cancers such as skin and lung cancer through inhalation or skin contact [142; 143; 144].