## 2 Aviation Fuel Emissions
### Aviation CO Emissions  
In certain circumstances, such as insufficiently high temperatures, relative oxygen deficiency, and uneven fuel/air mixture, combustion of aviation fuel can produce the toxic gas CO [73]. CO exhibits approximately 250 times greater affinity to hemoglobin in the blood than oxygen (O2), hence, upon inhalation, the colorless and odorless CO tightly binds with the iron in hemoglobin, leading to oxygen displacement. Blood carrying CO circulates throughout the body, depositing in the myocardium, skeletal muscles, nervous system, and brain, with myoglobin demonstrating similarly elevated affinity to CO compared with O2 [74]. The most common symptoms following acute CO exposure include nausea, headache, dizziness, vomiting, altered consciousness, and subjective weakness. Severe poisoning symptoms encompass mental confusion, seizures, myocardial infarction, coma, and even death [75]. Auxiliary power units (APUs) and the main engines providing thrust are the primary sources of CO production on aircraft, with APUs predominantly utilized on the ground to supply the power needed to start the main engines, electrical systems, air conditioning, and compressed air. Elevated CO concentrations near airports or within cabins can lead to aerotoxic syndrome in individuals. Under comparable conditions, SAFs exhibit only minor differences in CO emissions compared with traditional aviation fuels, far less than the disparities observed in SO2 and PM emissions between the two. According to [PERSON] et al. 's review, various conventional aviation fuels exhibit minor changes or even reductions (within 10%) in CO emissions when blended with different SAFs [76]. Discrepancies in the emission index of CO (EI-CO) across different fuels may stem from factors such as viscosity, carbon content, Sauter mean diameter (SMD), and final boiling point. Higher viscosity may result in a greater cone angle and premature flame extinguishment near the combustion chamber walls, leading to incomplete combustion and increased CO generation, while larger hydrocarbons can contribute to an increase in EI-CO. Lower SMD and final boiling point facilitate combustion on one hand but, on the other, prematurely remove the fuel from the primary zone by increasing the cone angle and reducing residence time, resulting in reaction quenching [77]. [PERSON] conducted emissions tests on neat Jet A-1, Readijet, and ATJ using a CFM56-5C4 engine, ensuring a stable environmental condition with minimal variability in parameters affected by environmental factors. There were no significant differences in CO and NO\({}_{x}\) emissions among the three tested fuels. NO\({}_{x}\) emissions from all fuels showed nearly linear growth with combustion chamber inlet temperature and thrust setting, whereas carbon monoxide emissions predominantly occurred at low power settings [78]. [PERSON] et al. [65] investigated the correlation between CO emissions and brake power of Jet A-1 blended with MBJ fuels, finding that the difference in EI-CO between blended fuels and Jet A-1 was negligible. The similarity in CO emissions could be attributed to the nearly identical viscosity of MJB and Jet A-1, with a viscosity difference of approximately 0.8% for J20 (MBJ volume blending ratio at 20%) and Jet A-1. In [PERSON] et al. 's study [79], during ground idle and 7% idle periods, neat natural gas-derived FT fuels exhibited around a 20% reduction in EI-CO compared with Jet A/Jet A-1, while 50% FT fuel fell within the range of Jet A/Jet A-1. Blends of fatty acid methyl ester (FAME) with jet fuel at 20:80 and 40:60 ratios showed a 5-10% decrease in EI-CO compared with Jet A/Jet A-1. [PERSON] et al. [63] found that the application of neat SAFs and blend fuels resulted in a reduction in EI-CO of 0-20% compared with JP-8/Jet A-1, regardless of engine power. They have suggested that the theoretically lower carbon content in neat SAFs could contribute to lowering EI-CO, although the actual reduction is limited.