Subsonic aircraft operate primarily in the upper troposphere and lower stratosphere.
Engine exhaust emissions of gases and particles from fleets of these aircraft are potentially in sufficient amounts to affect atmospheric ozone and climate, particularly with the projected growth in air traffic. The impact of these emissions depends on the relative change they induce in the background atmosphere and on their role in atmospheric photochemical, dynamical, and radiative processes. Photochemical processing of these emissions can affect ozone and other important species. Formation of contrails and changes in clouds caused by aircraft emissions can also affect climate. For engines operating at cruise conditions, a growing body of in-flight and engine chamber test data is available for gaseous and particle emissions.