The invention of aircrafts enabled people to travel to any part in the world within hours. The number of aircrafts manufactured have progressively increased in the last 50 years, especially for commercial aircrafts. Due to the massive number of aircrafts flying in the sky, air traffic has become significant. As a result more people are concerned about the pollution produced by the aircrafts to the atmosphere at different altitudes. Public concerns regarding the emissions around airports first surfaced during the mid to late 60s. The initial regulatory activity was in the USA where the Environmental Protection Agency (EPA), which had been set up under the 1970 U.S. Clean Air Act, published draft proposals by setting limits for carbon monoxide (CO), hydrocarbon (HC) and oxide of nitrogen (NOx) emissions over a standard Landing and Take-off (LTO) cycle. The aircraft engine industry has been trying to improve the technology of reducing noise and pollutants produced by engines.

The objective is to investigate aircraft pollution and see how we could minimize it. Each aircraft and engine combination has its own paticular emission profile. It could be worked out according to the LTO cycle and by employing specific emission factors, i.e. grams of pollutants per kilogram of fuel. The list below shows the time spent and engine power requirement during each mode. 

Generally the emission profile for commercial aircrafts is around:

19 minutes for idling and taxiing 
42 seconds for take-off 
2.2 minutes for climb-out. 

The rated engine power setting for each of the modes is different, as listed below: 

7% of capacity when idling 
100% for take-off 
85% for climb-out 
30% for approach. 

The chart shows the amount of pollutant per kilogram of fuel produced by the aircraft engines during various phases of flight. We can see that the engines produced more NOx during SLTO and climb-out. High engine power requirement at these two modes means more fuel consumption, hence more pollutants emitted. 

Subsonic Aircraft

The subsonic aircrafts cruise at lower altitude (8~15 km), around the tropopause, and their effects can be significantly different to those of the supersonic aircraft. The main emissions produced are CO2, H2O, nitrogen oxides, unburned hydrocarbons, soot and sulphate particles.

Effects

Water vapour can form contrails behind the aircraft consisting of thin ice clouds which have an effect on global warming as they reflect incoming solar radiation and lead to a reduction in the amount of UV radiation reaching the surface. CO2 and water vapour could increase the greenhouse effect. Sulphate particles and soot could have a cooling effect.