Jet aircraft engines emit tiny particles that serve as condensation nuclei. High-altitude water vapor collects on these particles, crystallizes, in turn creating streaks of frozen water vapor, otherwise known as contrails, from airplanes operating at high altitudes. Some contrails join with other contrails and expand into huge, natural-looking clouds of cirrus characteristics that can cover large areas of the sky. A 1999 report issued by the Intergovernmental Panel on Climate Change, entitled, Aviation and the Global Atmosphere, discusses contrail formation and its effects in more detail. The influence of contrails on cirrus clouds is noted as a key area of scientific uncertainty that limits the ability to project aviation impacts on climate and ozone.
Further work is required to reduce scientific and other uncertainties of aviation impacts.
In regard to air quality impacts, although jet aircraft contribute much less air pollution than that from motor vehicles, their overall emissions are increasing every year as air travel becomes more popular. It provides an estimation of the contribution of aircraft to air quality emissions in ten urban areas. Before an engine type can be sold or installed on an aircraft, the manufacturer must test its emissions on the ground by simulating in-use conditions. EPA sets the emission standards for the engines, and the FAA monitors the certification process and enforces the standards. Emission standards apply to essentially all commercial aircraft, comprising scheduled and freight airlines.