## 2 Aviation Fuel Emissions
### Aviation PM Emissions
It is estimated that in 2017, approximately 67,000 deaths occurred in China due to environmental PM\({}_{2.5}\) and O\({}_{3}\) emissions, with the densely populated coastal regions of eastern China experiencing the greatest impact due to intensive aviation activities [115]. Compared with standard aviation fuel with a fuel sulfur content (FSC) of around 600 ppm, the use of ultra-low sulfur jet fuel (ULSJ) with an FSC of 15 ppm effectively reduces surface PM\({}_{2.5}\) concentrations, preventing approximately 620 deaths annually. Additionally, it increases the global average net radiative effect (RE) by reducing the aerosol cloud albedo effect (aCAE) by +7.0 mW m\({}^{-2}\)[87]. [PERSON] et al. investigated the emission indices of nvPM mass and nvPM number from a CFM56-7B engine using a blend of Jet A-1 and 32% HEFA-SPK fuel. They found that blending fuels reduced the geometric mean diameter (GMD) and geometric standard deviation (GSD) of emissions at every point during ground idle and take-off, particularly with lower thrust levels, resulting in a relatively larger reduction in emissions. For instance, during ground idle, the SAF blend reduced nvPM number by 60%, while during take-off, the reduction was 10%. SAF blended fuel reduced the nvPM mass emission by 20% and the nvPM number emission by 25% over a standardized LTO cycle [116]. [PERSON] et al. conducted emissions tests on Jet A-1, a catalytic hydrothermolysis jet fuel (Readijet), and an ATJ fuel using a CFM56-5C4 engine. The hydrogen mass fractions in the three fuels were in the order of ATJ (15.5%) > Jet A-1 (14.1%) > Readijet (13.7%), while the aromatic content by volume was Readijet (20.9%) > Jet A-1 (15.6%) > ATJ (below 1%). ATJ fuel primarily consists of various branched aliphatic compounds such as iso-C\({}_{12}\)H\({}_{26}\) and iso-C\({}_{16}\)H\({}_{34}\). Aliphatic hydrocarbons, as opposed to aromatic compounds with the basic structure required for PM precursors, exhibit a lower tendency for PM formation. The PM formation tendency of hydrocarbons can be roughly ordered as follows: aromatics > cycloalkanes > branched alkanes > straight-chain alkanes. The results of this study confirm the above conclusions, showing a 70% reduction in particle mass for ATJ fuel compared with Jet A-1. Readijet fuel, with higher aromatic content and lower fuel hydrogen content, exhibited a significant increase in PM emissions [78]. [PERSON] et al. [117] conducted a study on the PM emissions levels of the CF34-3A1 turbofan engine and the TPE331-6-252B turboprop engine. They found that, at different power settings, the EI-PM of the former was generally lower than that of the latter. However, both engines exhibited a minimum EI-PM at moderate power conditions, typically between 40-60% thrust. At high power settings, nvPM emissions such as black carbon or soot predominated, while with decreasing engine power, the mass fraction of vPM containing organic compounds and sulfur exceeded 50%. [PERSON] et al. [118] discovered that a 50:50 blend of ATJ/Jet A-1 fuel, under high engine load, could reduce total particle number and mass by approximately 18% and 53%, respectively, compared with Jet A-1. [PERSON] et al. [119] found that, under all engine loads of the GE CF-700-2D-2 engine, Jet A-1 exhibited the highest EI-PM, with a moderate reduction of 7-25% for 100% CH-SKA fuel across the engine load range. For 50% HEFA-SPK and 100%FT-SPK fuels, EI-PM reductions were approximately 40-60% and 70-95%, respectively. The emission trends of BC were consistent across all fuels, but compared with Jet A-1, emissions from 100% CH-SKA, 50% HEEA-SPK, and 100% FT-SPK were reduced by 38-50%, 58-82%, and 70-95%, respectively. The EI-BC of Jet A-1 and three types of SAFs showed a strong correlation with changes in aromatic compounds and the H/C ratio in the fuels. As widely recognized, the sulfur content in fuel has the greatest impact on the volatile aerosol fraction. [PERSON] et al.