## 2 Aviation Fuel Emissions
### Aviation PM Emissions
[120] found that reducing FSC to near zero decreased the EI-aerosol number by 10-100 times, especially when the fuel flow rate was low (below 0.4 kg s\({}^{-1}\)), resulting in significant reductions in EI-total number and EI-non-volatile particulate matter (EI-nvPM), albeit with diminished benefits at high thrust settings. [PERSON] et al. [103] conducted particle number emissions tests using a GE CF700-2D2 engine and observed that the EI-volatile particulate matter for Jet A1 fuel was higher than that for JP-5, while the trend reversed for EI-nvPM. This discrepancy stemmed from Jet A1's higher FSC and JP-5's higher aromatic content. The ATJ-SPK blend, with aromatic and sulfur contents of 8% vol and 0 ppm, respectively, significantly reduced total particle number emissions by up to 97% compared with Jet A-1. [PERSON] et al. 's study indicates that particulate matter can modulate the quantity of cirrus ice crystals at current emission levels. They utilized an Airbus A320 equipped with two IAE-V2527-A5 engines to compare the emission performance of neat Jet A1 and semisynthetic jet fuel blends (FT-SPK or HEEA-SPK with Jet A-1). Semisynthetic jet fuels with high hydrogen content and low aromatic hydrocarbon content can lead to a 50% to 70% reduction in both soot and ice number concentrations, along with an increase in ice crystal size. The reduction in ice crystal number results in decreased energy deposition in the atmosphere, thereby reducing warming. Dicyclic naphthalene is a more effective precursor of soot than monocyclic aromatic or aliphatic hydrocarbon structures [121]. [PERSON] et al. [122] investigated aircraft engine emissions using neat FT fuel, used cooking oil (UCO)-derived HEEA alternative fuel, and blends of these with traditional aviation fuel, revealing significant reductions in EI-nvPM and sulfur oxides after blending with SAF. [PERSON] et al. [123] conducted pollutant emission tests on a DC-8 aircraft equipped with four CFM56-2-C1 engines using three types of fuel: medium-sulfur-content Jet A fuel, low-sulfur-content Jet A fuel, and a blend of 50:50 HEFA and low-sulfur-content Jet A fuel. Compared with medium-sulfur-content Jet A fuel, low-sulfur-content Jet A fuel and the blend fuel reduced volatile fine particle number by approximately 9% and 52%, respectively; the blend fuel decreased non-volatile fine particle number by approximately 45%, while, unexpectedly, low-sulfur-content Jet A fuel increased non-volatile fine particle number by 30%. The application of the blend fuel reduced volatile particle volume, non-volatile particle volume, and total particle volume by approximately 50%. Additionally, blend fuel exhibited varying degrees of reductions in BC, CO and NOx emissions, as shown in Figure 5. The NOx emissions of three fuels exhibit minimal variation, consistent with previous findings. However, low-sulfur-content Jet A fuel and the blend fuel respectively reduce CO emissions by 33% and 22%, which can potentially be attributed to combustion heat. Table 2 illustrates the aromatics content, burning platform/methods, and instrumentation of standard specification SAF and related fuels. As per ASTM regulations, Jet A or Jet A-1, representing conventional aviation fuels, require an aromatics content below 25/26.5 vol%. Among the seven ASTM-certified SAFs listed, five have maximum aromatics content specifications of 0.5 mass%, while FT-SPK/A and CHJ fuels have an aromatics content limited to below 20 and 21.2 mass%, respectively. Aromatics content in neat SAFs and their blends is notably lower than traditional aviation fuels, leading to reduced PM emissions in engine testing, thus affirming the sustainability and environmental friendliness of SAFs.