## 2 Aviation Fuel Emissions
### Aviation \(So_{2}\) Emissions
Short-term exposure to sulfur dioxide is positively correlated with respiratory system mortality and  
Figure 3: Overall emissions of six pollutants in China–foreign routes from 2014 to 2021 [81]. The orange, purple, yellow, pink, green and grey icons represent CO2, CO, UHC, NOx, PM2.5 and SO2 emissions, respectively. all-cause mortality rates [96]. Exposures to SO\({}_{2}\) exceeding 500 ug/m\({}^{3}\) for 10 min can stimulate the respiratory tract of susceptible individuals, leading to symptoms such as acute bronchial asthma, pulmonary edema, and respiratory distress [97]. Predictably, the sulfur content in fuel correlates directly with the emissions of SO\({}_{2}\) after combustion. In the study by [PERSON] et al. [98], JP-8 and synthetic jet (synjet) fuel were blended at different volume ratios of 0%, 25%, 50%, and 75%. The mass fraction of sulfur elements in the fuel was 0.06, 0.05, 0.04, and 0.02, respectively. Combustion experiments with the T63 engine showed that the SO\({}_{2}\) emissions from the four blending ratios of JP-8/synjet fuel were approximately 34, 23, 15, and 7 ppm, respectively, with equivalence ratios of 1.00, 1.05, and 1.10 exhibiting similar emission quantities and trends. In the Alternative Aviation Fuel Experiment (AAFEX), researchers employed a sulfate particulate analyzer (SPA) in conjunction with a trace-level pulsed fluorescence SO\({}_{2}\) analyzer to measure SO\({}_{2}\) gas emissions from a DC-8 aircraft equipped with a CFM56 engine. The results indicate that, under APU load, the SO\({}_{2}\) emission indices (EI-SO\({}_{2}\)) for JP-8 were 1.5-1.8 g kg\({}^{-1}\), while those for FT-2 were 0.13-0.15, suggesting a 92% reduction in aviation SO\({}_{2}\) emissions with SAF use, albeit with minimal differences in CO and NOx emissions, as shown in Figure 4a. The SO\({}_{2}\) emissions from a 50:50 volume blend of FT-1 (Shell) and FT-2 (Sasol) fuels were approximately half that of pure JP-8, attributed to the high FSC of JP-8 at around 1200 ppm compared with negligible sulfur in neat FT fuels. Additionally, compared with JP-8, FT-2 exhibited nearly a 100% reduction in EI-particle-bound sulfate emissions and a five-fold reduction in EI-particle-bound organics. As shown in Figure 4b,c, EI-SO\({}_{2}\) remained relatively constant with increases in exhaust gas temperature (EGT) and power setting, while black carbon (BC) emissions decreased with rising EGT, with JP-8 exhibiting a greater reduction than FT-2. At varying EGT, the EI-SO\({}_{2}\) for FT-2 fuel ranges from 0.12-0.14 g/kg fuel, while for JP-8, it ranges from 1.51-1.58 g/kg fuel. EI-SO\({}_{2}\) shows a slight increase with increasing power. The EI-SO\({}_{2}\) for blended fuels falls between that of the JP-8 and FT fuels. Figure 4a-c correspond with [34; 99; 100], respectively. [PERSON] et al. [101] incorporated fatty acids ethyl esters (FAEE) derived from rapeseed oil (RO) as bio-additives into Jet A-1 fuel at proportions of 10% and 20%. The sulfur contents in Jet A-1 and modified FAEE were 0.025% and 0.0085%, respectively. Traditional aviation fuel and blends containing 10% and 20% modified FAEE generated respective SO\({}_{2}\) pollution gases of 0.0367, 0.0158, and 0.0281 kg per landing-take-off (LTO) cycle, indicating that a 10% blend of modified FAEE effectively reduced SO\({}_{2}\) emissions by nearly 57% compared with pure Jet A-1. [PERSON] et al. [102] examined the combustion parameters and emission characteristics of Jet A and Jet A/biodiesel (BD) blends in a JETCAT P80 microengine, simulating idle, cruise, and max stages by setting throttle positions at 18.7%, 30%, and 94%, respectively (BD belongs to biofuel rather than SAF, and is here for reference). SO\({}_{2}\) was detected using an MRU analyzer with a measurement range of 0-2000 ppm and an accuracy within \(\pm\)10 ppm or 5% reading.