## 2 Aviation Fuel Emissions
### Aviation PM Emissions  
Compared with other sources of pollution in urban environments, aircraft engine PM emissions constitute a distinct pollution source [105]. Aviation PM emissions can be categorized into primary and secondary PM. Primary PM refers to solid and liquid particles emitted directly from the engine without undergoing chemical transformations in the atmosphere. Non-volatile particulate matter (nvPM) and volatile particulate matter (vPM) constitute the emissions of primary particulate matter (PM) in aviation activities [106]. BC, the primary component of nvPM, forms within aircraft engines under temperature and pressure conditions, where incompletely oxidized carbon atoms aggregate and condense into solid particles carried by airflow [107]. Additionally, metallic oxides (e.g., iron, copper, etc., originating from engine wear materials) and sulfates (combustion byproducts of sulfur-containing fuels) contribute directly to atmospheric particulate matter concentrations as they undergo minimal physical or chemical alterations in the atmosphere. Besides combustion processes, nvPM formation is influenced by factors such as engine design, combustion conditions, and aerodynamic effects. vPM comprises organic compound vapors, partial sulfides, and nitrides, which may initially exist in gaseous form during emissions but condense into solid or liquid particles during cooling, becoming part of particulate matter. vPM emissions occur at the engine exit, where hot combustion exhaust cools, sulfuric acid droplets act as representative condensation nuclei, and further condensation of water and organic substances occurs on the surface. During post-emission transport, the quantity and quality of particles undergo significant changes through processes like aggregation, condensation, and evaporation, forming vPM [108]. Secondary PM is not directly emitted but formed through complex chemical reactions in the atmosphere. When gaseous pollutants emitted by aircraft (such as nitrogen oxides (NO\({}_{\text{x}}\)), volatile organic compounds (VOCs), etc.) react with other components in the atmosphere (such as oxygen, water vapor), new solid or liquid particles are formed. These reactions can generate sulfate, nitrate, organic aerosols, etc., which constitute the primary components of secondary PM, significantly impacting long-distance transport and long-term air quality. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  
The physicochemical composition of nanoparticles emitted by aircraft engines resembles diesel exhaust particles, which are classified as mutagenic and carcinogenic agents [109]. Studies investigating the impact of LTO emissions from 99 airports on public health have shown that premature mortality increased from an estimated 75 cases per year in 2005 to 460 cases per year by 2025, with a 6.1-fold increase, of which 2.1 times were attributed to PM\({}_{2.5}\)-related emissions [110]. Emissions of sulfur dioxide and sulfate aerosols contribute to an increase in the concentration of fine particulate matter with a dry diameter of less than 2.5 \(\upmu\)m (PM\({}_{2.5}\)). Prolonged exposure to high concentrations of PM\({}_{2.5}\) increases the likelihood of respiratory and cardiovascular diseases, lung cancer, chronic physiological changes, and mortality [111; 112; 113; 114]. PM\({}_{2.5}\) emissions during aircraft takeoff and landing significantly impact human health around airports, causing approximately 3600 premature deaths annually with standard aviation fuel containing approximately 600 ppm of sulfur. This accounts for 0.1% of global premature deaths, with the highest mortality rates attributed to aviation in Europe, eastern North America, and eastern China. Aviation-related PM\({}_{2.5}\) contributes approximately 1% to the total premature mortality attributed to PM\({}_{2.5}\) exposure.