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TopAeronautical Emissions: An Introduction
As per (Deuber, 2004, p. 26) aviation emissions can be sub divided into ground level,landing take off, climb and descent and cruise phase. A pictorial illustration of flight phases is appended in Figure 1.
During cruising, as well as at the end of the climb phase and at the beginning of the descent phase, an aircraft in international traffic flies predominantly at an altitude of between 9 and 12 km above sea level. Only supersonic aircraft fly at a higher altitude of around 17 to 20 km. Emissions from subsonic aircraft affect, for the most part, the upper troposphere, those of supersonic aircraft the lower stratosphere. The boundary layer between both spheres, the tropopause, is also greatly affected. Ozone chemistry, in particular, reacts more sensitively to anthropogenic emissions in these spheres than at ground level. This is attributable to slower blending processes, lower temperatures and more limited background contamination. The altitude of the tropopause depends on geographical latitude, time of the year and current weather conditions. On the equator, it lies at an altitude of around 16 km, at the poles at around 8 km. The chemical composition of the spheres differs. Since this has a decisive influence on the effect of aviation emissions, the point of emission plays an important role with reference to their greenhouse gas effect. Differentiation according to flight altitude and geographic latitude is of fundamental importance for effect analysis.
As per (Federal Aviation Authority, 2005, pp. 1-2), aircraft produces emissions like automobiles. The jet engine like all other types of engines produces Carbon dioxide (CO2), Water Vapor (H2O), Nitrogen oxides (NOx), Carbon mono oxide (CO), oxides of Sulphur (SOx), unburned hydrocarbons (Known as Volatile Organic Compounds (VOC)), and other compounds. These particulates and VOC are also known as Hazardous Air Pollutants (HAPs). The exhaust of a jet engine comprise mainly of CO2 (approximately 70%), H2O (Under 30% approximately) and under 1% of other combustion by products. These byprodcuts produced are genrally considered as Green House Gases. As per the estimates about 10% of all types of aircraft emissions except CO and VOC. The bulk of aircraft emissions approximately 90% occur at high altitude. Aircrafts are not the only source of emission the ancillary ground support equipment (like Ground Power Units etc) and other associated airport transportation are also a source of emissions. The impacts aviation emissions on environment are iluustrated in Figure 2.
Figure 2. Impact of aviation emissions on environment (adapted from IPPR, 2001; IPCC, 1999)
The recent growth of air travel is also one of the eye raiser of environmental aspects of aviation sector. As per (United States Department of Transportation, 2002) 21.5% increase in population, 32% increase in labor force and 90% increase in GDP are the driving factors for growth in demand of air travel. In strategic terms the demand of air travel will be on the rise in preceding years which also means rise in aeronautical emissions.
As per (Federal Aviation Authority, 2005, pp. 3-4) aviation sector has developed significantly during last few decades. The shift from heavy piston engines to powerful jet engines and then evolution in fuel efficiency has been signifcant. As per (Wickrama, 2003), by 2020 around 70% of in service fleet of aircraft would retire or replaced by modern technologically advances aircraft. Figure 3 illustrates the estimated number of aircraft.
Figure 3. Illustration of aircraft replacement by 2020 Source; ICAO report