Air Pollution

There is increasing concern over the environmental impacts arising from pollutants emitted by aircraft engine exhausts.  These emissions can:

• affect local or regional air quality;
• lead to ozone production or destruction (depending on the altitude of the emissions);
• affect the earth’s climate directly (e.g. carbon dioxide emissions) or indirectly through modification of processes involved in the climate system (e.g. cloud formation).

Kerosene is the main component of aviation fuel which is used to provide propulsion for current commercial aircraft.  Like other fossil fuels, kerosene produces carbon dioxide and water vapour as the products of complete combustion.  As the combustion process is not completely efficient, partial oxidation products such as carbon monoxide and oxygenated organic compounds are produced and emitted.  Depending on the load on the engine and the mixing processes in the combustor, soot can be formed and emitted together with unburnt hydrocarbons.  The emission profiles vary with the phase of the flight.  Although the sulphur content of the fuel is reduced during the refining process, aviation fuel still contains a small amount of sulphur and this is converted to sulphur oxides during the combustion process.  The temperatures reached in the jet engine combustor result in the formation of nitrogen oxides (NOx) through high temperature reactions of the nitrogen and oxygen present in the combustion air.

The impact of the aviation industry on local air quality, especially in the vicinity of airports, has long been recognised.  The pollutants of concern are the emissions of nitrogen oxides, carbon monoxide, hydrocarbons and soot.  Emissions limits from aircraft engine exhausts have been defined for these pollutants by the International Civil Aviation Organisation (ICAO), although the limits only apply during the Landing Take-Off (LTO) cycle.  In addition to aircraft emissions, local air quality is further compromised by pollution from motor vehicles along extensive road networks that provide access to airports.

The other environmental concern for the aviation industry is the contribution of aircraft exhaust emissions to climate change.  The most important emissions are considered to be those of carbon dioxide and nitrogen oxides.  Subsonic aviation currently contributes between 2-3% of the carbon dioxide emitted from all fossil-fuel combustion.  The emissions of nitrogen oxides lead to the production of ozone in the upper troposphere.  There is also increasing scientific concern about the sulphur oxide and soot emissions.  The impact of aerosol and cloud formation from these species could potentially have a large but currently uncertain impact on climate change.

ERCD is able to give advice on the latest research and requirements for compliance with regulatory guidelines.