## 2 Aviation Fuel Emissions
### Aviation \(So_{2}\) Emissions  
The combustion of aviation fuels is linked to emissions in a complicated manner, with aircraft engine manufacturers consistently striving to develop machinery with high combustion efficiency and low pollution levels [80]. According to statistics, emissions from the aviation industry in 2019, including CO\({}_{2}\), CO, HC, NO\({}_{x}\), PM\({}_{2.5}\), and SO\({}_{2}\), amounted to 56,960,300.60 tons, 400,395.30 tons, 37,998.96 tons, 388,677.70 tons, 5,393,817.46 tons, and 68,719.92 tons, respectively, comprising a respective 98.42%, 0.7%, 0.07%, 0.68%, 0.01%, and 0.12% of the total emissions. Moreover, the average SO\({}_{2}\) emissions from 2017 to 2019 increased by 12.98% compared with 2014-2016, ranking second among all pollutants emitted, following only NO\({}_{x}\)[23]. Figure 3 depicts the comprehensive emission trends of the six pollutants on China-foreign routes from 2014 to 2021 [81]. The figure illustrates a notable increase in the total emissions of six pollutants on China-foreign routes from 2014 to 2016, with the SO\({}_{2}\) emissions rising from 34,505 tons to 56,301 tons. The introduction of emission reduction measures, such as the CNG2020 strategy, led to a gradual decrease in the growth rates of the six pollutants, some even turning negative. From 2017 to 2019, the average growth rates of CO\({}_{2}\), HC, NO\({}_{x}\), SO\({}_{2}\), PM\({}_{2.5}\), and CO were 1.19%, \(-\)1.43%, 2.51%, 1.19%, 4.34%, and \(-\)0.23%, respectively. The outbreak and spread of the COVID-19 pandemic in 2020-2021 resulted in a sharp reduction in the number of China-foreign routes, bringing the SO2 emissions to a low level. Most research on emissions has focused on CO2, NOx, and CO, with less attention given to SO2 emissions [82]. However, SO2 emissions from the aviation industry pose significant risks to human health and the natural environment. The sulfur element in SAF raw materials typically originates from the preprocessing process rather than the raw materials themselves [83]. Negligible sulfur elements and aromatic hydrocarbon content determine that the SO2 emissions from SAF differ significantly from petroleum-based fuels [84,85]. The distinctive aspect of aviation pollutants lies in their accumulation directly within the upper troposphere and lower stratosphere during cruising altitudes (8-12 km), exerting complex and potent influences on Earth's ecosystem [86]. While sulfur in aviation fuel can mitigate fuel pump wear to some extent, emissions of sulfur oxides are directly associated with this sulfur [87]. Approximately 98% of sulfur in fuel converts to SO2, with the remainder appearing as SO3 in exhaust gases [88,89]. Lower combustion temperatures favor the generation of SO3 through reaction equilibrium, while metallic substances in the fuel or fuel system expedite the oxidation of SO2 to SO3 [90]. Oxides of sulfur combine with water to form sulfuric acid, which is absorbed on the surface of soot particles, creating volatile sulfate aerosol particles [91]. Under ice supersaturation conditions, these particles may serve as nuclei for ice crystals, leading to the formation of contrail cirrus clouds. Contrails enhance the uptake of sulfur oxides and subsequent conversion to sulfate aerosols, resulting in soot wet deposition and an eventual contribution to acid rain during deposition [92,93]. Contrail cirrus clouds alter the radiative properties and water budget of the upper troposphere, thereby affecting climate. Concurrently, acid rain inflicts severe damage on water sources, soil, vegetation, and other ecosystems. Exposure to SO2 in the atmosphere increases the risk of respiratory system diseases among individuals, including asthma attacks, bronchiolitis, upper airway infections, and pneumonia [94]. Even in environments with low concentrations of SO2 (<200 ppb), asthmatic individuals exhibit sensitive and rapid airway constriction symptoms [95].