Aircraft emissions: gaseous and particulate

Aircraft emit gases and particles directly into the atmosphere, and these become unique pollutant sources at cruise altitude, the upper troposphere, and the lower stratosphere. The gaseous emissions of aircraft engines contain mostly greenhouse gases, such as carbon dioxide (CO2), and water (H₂O) vapor. They also contain species that occur in smaller amounts but have particular health and environmental concerns. These species include nitric oxide (NO) and nitrogen dioxide (NO₂) (together termed nitrogen oxides, NO_x), sulfur oxides (SO_x), carbon monoxide (CO), and unburned hydrocarbons (UHCs) of many species lumped together. Aircraft also emit soot particles and some gaseous species that may act as precursors of fine particles. Many of these emissions affect human health when they are inhaled at the ground level. They also impact the air quality near airports; for example, NO_x contributes to photochemical smog.