Contrails are clouds formed when water vapour condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapour comes from the air around the plane; and, some is added by the exhaust of the aircraft. The exhaust of an aircraft contains both gas (vapour) and solid particles. Both of these are important in the formation of contrails. Some elements of the exhaust gasses are not involved in contrail formation but do constitute air pollution. Emissions include carbon dioxide, water vapour, nitrogen oxides (NOx), carbon monoxide, hydrocarbons such as methane, sulphates (SOx), and soot and metal particles.
This post is part of the Skywatch Friday meme