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How does an airport affect local air quality? |
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Aviation air quality concerns are principally related to the areas on and around airports. Further, for most airports the most significant air quality related emissions presently come from ground transport (cars, buses, trains etc). However, because of factors such as growth in demand, more public transport access to airports, and the long service life of aircraft, it is widely expected that aircraft will eventually become the dominant air quality related pollution source for many airports. The significance of aviation’s impact on air quality will vary depending on many other factors such as, background pollution levels, other sources of pollution, weather and proximity of residential areas. Around many airports some large emission sources already exist (power stations, factories) that are not related to the airport at all. Also local roads and motorways, even roads associated with an airport, may be heavily used by non-airport traffic.
Aircraft engines produce emissions that are similar to other emissions resulting from any oil based fuel combustion. These, like any exhaust emissions, can effect local air quality at ground level. It is emissions from aircraft below 1,000 ft above the ground (typically around 3 kilometres from departure or, for arrivals, around 6 kilometres from touchdown) that are chiefly involved in influencing local air quality. These emissions disperse with the wind and blend with emissions from other sources such as domestic heating emissions, factory emissions and transport pollution.
The chief local air quality relevant emissions attributed to aircraft operations at airports are as follows :
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- Oxides of Nitrogen (NOx);
- Carbon Monoxide (CO);
- Unburnt hydrocarbons (CH4 and VOCs);
- Sulphur Dioxide (SO2);
- Fine Particulate Matter (PM10 and PM2.5);
- Odour.
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These are produced by aircraft engines, auxiliary power units, apron vehicles, de-icing, and apron spillages of fuel and chemicals. Local factor influence the significance of individual emissions species for each airport, but often NOx is by far the most abundant and is often considered the most significant pollutant from an air quality standpoint.
It should also be borne in mind that, to a greater or lesser degree, these emissions also have climate change implications as does carbon dioxide (CO2) that is also emitted near the ground. Water vapour emitted at low altitude is not generally considered as significant for climate change.
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How can air quality affect aviation? |
As our scientific understanding of Air Quality improves, it is being taken increasingly seriously by policy makers. The EC has produced legislation setting limits for different pollutants together with a requirement for Member States to quantify and publish Air Quality impact and to operate plans and controls. As the demand for air transport steadily increases and other industries reduce their emissions, the proportion of aviation’s contribution to air pollution levels around airports is increasing. There are also international standards (ICAO) for aircraft engines which are aimed principally at reducing green house gas emissions. There is a trade-off however, because unless a step change in technology can be found, we have reached the stage where attempts to reduce one gas species for fuel economy and climate change purposes (e.g. CO2) can increase the emission of an air quality species (NOx).
Already some airports have some degree of operational or development constraint based on air quality concerns.
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How can airport atmospheric emissions be quantified? |
This is a complicated topic mainly because emissions to air disperse and mix with emissions from other sources. Presently there are no international standards for air quality quantification methods. Two overall types of quantification are required:
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This involves sampling the local air and analysing for NOx, particulates and other important pollution species. The sampling is often done on a 24 hour continuous basis. Locating the measurement equipment is important because of prevailing weather patterns, the position of emission sources linked to the airport and the proximity of residential areas. Various analysis equipment exists including mechanical-chemical sampling equipment and real-time spectrum analysis monitors. Sometimes, mobile monitoring stations or a combination of different methods are used to allow greatest flexibility. Measurement should be seen in the context of all significant contributing emitters that can affect ambient conditions. Measured pollution levels set the context for the airport, help to understand modelled emission levels (see next item) and help to validate modelled values.
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This involves creating an inventory for all significant emitters linked to the airport such as aircraft, ground vehicles (airside and landside), fixed plant such as boilers and fugitive emission sources such as maintenance facilities. The characteristics of the emitters are also determined (operating patterns and emission levels). This data is combined with typical weather patterns in a sophisticated model to predict with reasonable accuracy, the degree of contribution of the airport to local pollution levels and what the dispersion patterns are. When combined with measurement, and estimates of airport growth and the anticipated changes to third party emissions, an estimate of current and future emissions levels can be determined.
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It is important to realise that emissions vary for different operation types for the same equipment. For example an aircraft engine on high thrust will emit proportionally less particulates than when on lower thrust, but the converse is true for NOx emissions which increase with thrust. This type of emission pattern should be taken into account when associating emission values with equipment. ICAO has developed a database of standard emission factors, for various operation types for a range of common civil aircraft engine types.
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How can we manage airport air quality related emissions? |
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Emissions to atmosphere can be considered as waste products. Subject to trade-offs mentioned earlier, any measure to improve the more efficient use of combustion equipment associated with an airport, will generally reduce pollution. Thus energy efficiency in buildings, increased use of public transport, the use of less polluting energy sources and of course more efficient aircraft operations will all help. These measures may produce a benefit in terms of impact mitigation or it may allow an increase in the sustainability benefits arising from an airport's success, by freeing up “environmental capacity” for more aircraft movements. Thus, because an airport is seen as a single emission driver, reducing emissions in one area can free up capacity in another.
Before seeking specifically to control emissions for air quality reasons it’s important to understand the extent of existing problems (and get base-line performance) thoroughly. However, in principle, reducing emissions (even when an urgent air quality problem does not exist) is usually good for business as well as being good for the environment. It is also important to ensure that the full potential range of environmental benefits is taken into account in the business case for an emission reduction initiative:
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- air quality;
- energy resource conservation and financial savings;
- land demand;
- climate change; and potentially,
- noise.
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Some guidance on mitigation exists already such as the ICAO Circular 303, AN 176:
“Operational opportunities to minimize fuel use and emissions” and the IATA “Guidance Material and Best Practices for Fuel and Environmental Management” published Dec 2004.
The following measures are commonly used at airports to address emission below 1000 ft:
- low fuel/emission aircraft departure procedures;
- Continuous Descent Approach and Low Power - Low Drag techniques;
- avoiding aircraft queuing on the ground;
- avoiding unnecessary use of aircraft Auxiliary Power Units;
- taxiing management (e.g. towing and single engine taxi);
- increasing the use of public transport, cycling and pedestrian access to an airport (probably the major potential source of benefit);
- supporting and encouraging staff to “car share” or to use more sustainable transport access;
- the use of electric vehicles or less polluting fuels (liquid and natural gas);
- use less polluting fuels in airport buildings;
- ensure adequate vehicle maintenance;
- avoiding combustion equipment running when not required;
- energy management in buildings and for airfield systems (very often the most cost effective opportunity);
- fugitive emission controls.
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All of these can contribute to reducing air quality related emissions, whilst at the same time delivering other economy and climate change benefits. For operational measures however, there may also be trade-offs with capacity and noise, and a full assessment should be made before adoption.
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Last validation: 11/07/2007
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