Aircraft emissions: gaseous and particulate
Commercial aircraft cruise at altitudes between 8 and 13 km, with a tendency over time for higher average cruise altitudes. These gaseous and particulate emissions at high altitude change the concentration of atmospheric greenhouse gases, including CO2, ozone, and methane. Oxides of nitrogen emissions are generated in the highest temperature regions of the combustor, usually in the primary combustion zone, before the products are diluted. Unburned hydrocarbons, like carbon monoxide, are associated with combustion inefficiency. Contrails, or condensation trails, are clouds produced by aircraft engine exhaust. Of all aircraft emissions at altitude, those of interest for their effect on contrail formation are water vapor, sulfur gases, and fine particles. All sulfur emissions in the engine exhaust are caused by the combustion of sulfur introduced into the fuel. Alternative fuels are being considered as an alternative method for reducing greenhouse gas emissions and are being studied as replacements for current conventional-petroleum-derived aircraft engine fuels.