## 2 Aviation Fuel Emissions
### Aviation UHC Emissions
High VOC concentrations can also affect the central nervous system, leading to headaches, dizziness, fatigue, and, in severe cases, impairing cognitive functions and behavior [145; 146; 147]. The comparison of UHC emissions between SAFs and traditional aviation fuels is controversial. Some scholars and engine manufacturers assert that both have similar UHC emission indices (EI-UHC), while others believe SAFs exhibit either lower or higher EI-UHC. The divergence in opinions arises from the interplay between engine conditions and fuel properties influencing UHC emissions. [PERSON] et al. utilized a GTCP85 aircraft APU combustion platform and measured UHC emissions using the Signal 3000 HMFID hydrocarbon analyzer. They discovered no significant trend in CO, UHC, and NOx emissions across 18 different blends of HEFA and Jet A-1 fuels, although there was an overall decline in UHC emissions compared with Jet A-1. The engine operating conditions yielding increasing UHC emissions were, in order: main engine start (MES), environmental control system (ECS), and no load (NL). In MES mode, the engine operates at minimum capacity with an air to fuel ratio (AFR) of approximately 50. The low pressures and combustor temperatures in MES mode contribute to lower UHC emissions [68]. [PERSON] et al. found that, in the premixed pre-evaporated Bunsen burner (PPBB) tests, blends of hydrothermal-condensation-hydrotreating jet (HCHJ) biofuel with RP-3 significantly reduced PM\({}_{2.5}\) and UHC emissions. In tests with the ZF850 jet engine, varying thrust from 48% to 97% showed no clear trend in EI-UHC for 0%, 5%, and 10% HCHJ blends with RP-3. Increasing AFR at low power reduced EI-UHC but prolonged combustion time due to lower temperatures. At high power, decreasing AFR increased EI-UHC due to higher fuel consumption, though higher combustion temperatures accelerated reactions. EI-UHC emissions were lowest in the 55-90% thrust range, with emission peaks at 48% and 95% thrust [148]. [PERSON] et al. conducted studies on the SR-30 turbojet workbench using Jet-A, a synthetic gas to liquid (GTL) fuel, and a blend of 30% Jatropha methyl ester (JME) with 70% Jet-A. Similar to CO emissions, EI-UHC decreased with increasing angular speed. At 50,000-60,000 rpm, EI-UHC ranked from highest to lowest as GTL fuel, Jet-A, and the 30% JME blend. Above 70,000 rpm, the order shifted to Jet-A, GTL fuel, and the 30% JME blend, with differences in EI-UHC among the fuels diminishing as speed increased [149]. In addition to engine operating conditions, EI-UHC is also related to the properties of aviation fuel itself. [PERSON] et al. 's findings indicate that increasing the concentration of Camelina in blended fuels reduces EI-UHC, primarily due to the increase in primary zone temperature with higher Camelina concentration. Conversely, EI-UHC increases with higher concentrations of Jatropha in the blend, as Jatropha contains more carbon compounds and aromatics, leading to higher UHC emissions at low temperatures. Changes in the viscosity and density of fuel blends can alter spray characteristics, resulting in improper mixing in the combustion zone and subsequent UHC emissions. Moreover, EI-UHC is influenced by factors such as fuel residence time, evaporation time, and adiabatic flame temperature [77]. [PERSON] et al. 's tests on the Olympus E-start HP turbojet engine show that for a 50% throttle valve position, 50% cotton methyl ester (CTME) blend fuel and 50% corn methyl ester (CRME) blend fuel reduce EI-UHC by 36.5% and 30.8%, respectively, compared with JetA-1. Additionally, EI-UHC decreases with increasing blend ratios of biodiesel, explained by the higher oxygen content in biofuels. The higher EI-UHC of JetA-1 may be due to its higher distillation points, leading to incomplete evaporation and combustion of the final fuel fraction [150]. [PERSON] et al. conducted emission tests on diesel and two different Jatropha biodiesel blends (15%, 25%) in an IS/60 Rovers gas turbine.