Local air quality

Local air quality issues are becoming more of a concern and are often cited as the limiting factor in the growth of airports.

Unlike noise, which is usually dominated by aircraft operations around busy airports, there are a number of sources of emissions, and the contribution of each individual source to local air quality, is often not well known, or understood.

Of principle concern around airports are the concentrations of nitrogen dioxide (NO2) - one of the components of NOx; and particulates, especially those with mean aerodynamic diameters less than 10 µm (PM10), and less than 2.5 µm (PM2.5), the maximum concentrations of which are regulated by EU Directive and UK law. In this respect, the combustion of kerosene by aircraft engines is extremely efficient, and results predominantly in the emission of carbon dioxide and water, plus trace amounts of other gasses and particles, and although aircraft have a contribution to local air quality, road vehicles – especially Diesel powered ones, are a significant source of these emissions.

Unlike noise, where microphones can measure discrete noise events such as aircraft departures, this is not possible with continuous emissions monitoring as all NOx emissions are essentially identical, regardless of where they are generated. As a result modelling has been widely used to try to identify the contributions from different sources, instead.

British Airways is involved in a number of initiatives to help improve the understanding of aircraft emissions and the way they are modelled. We are involved with the Project for the Sustainable Development of Heathrow (PSDH) set up by the UK Dft and also are helping the International Civil Aviation Organisation (ICAO) to write guidance for the modelling of aircraft emissions for local air quality assessments. British Airways has also carried out its own modelling exercise, using a conservative evaluation of recorded information about aircraft operations, power levels and times, and other aspects of aircraft operations such as ground running of engines and Auxiliary Power Unit (APU) emissions. Results from our modelling suggested that less than half of the NO2 emissions at areas outside the airport, where aircraft had the greatest effect, were actually due to aircraft.

To substantiate findings and provide data for verifying the modelling work, we have commissioned Netcen to carry out measurements using diffusion tubes along a transect from the airport up to the M4 motorway, approximately 1.9 km north of the airport. After three whole years of monitoring, the results appear to suggest that there is a rapid reduction in NO2 concentrations away from the airport boundary, to the north, in line with our earlier modelling study.

Diffusion tube monitors at Terminal 4

We have shared the results of our monitoring with a number of other interested groups, and along with monitored levels carried out by a number of bodies concentrated around Heathrow airport, have made them publicly available on the 'Heathrowairwatch' website. This is the initiative of a body comprising the four Local Authorities around Heathrow airport, the airport operator, BAA Heathrow, and British Airways.

In a similar way to the way noise is managed through the “Balanced Approach”, we believe that a similar approach, though necessarily more complex, could be applied to managing local air quality around airports. We have been working with the Association of European Airlines, within ICAO to promote this idea.

Although all sources would have to be part of any process, applied to aircraft operations, this could consist of the same four main elements:

Aircraft emissions arise from operation of the main engines which dominate the aircraft sources. Auxiliary Power Units (APU’s), are often necessary to provide a source of on board electrical, pneumatic and hydraulic power for the aircraft, and it’s air conditioning systems - they also produce the same emissions, but to a lesser extent.

Emissions standards for new aircraft types, are established by ICAO, and published in Annex 16 volume 2, one of the technical annexes to the Convention on International Civil Aviation (Chicago, 1944). Emissions limits are set for four emissions species

The NOx standard has been reviewed and tightened three times. The latest, CAEP/6 standards will come into force for newly certificated types at the beginning of 2008.

NOx certification levels for British Airways types

The application of less than full power for take-off is the most efficient way of reducing NOx emissions from aircraft operations, as long as safety considerations are met. British Airways uses this technique universally apart from take-off of Boeing 747 aircraft at night at Heathrow and Gatwick, when full power is always used to meet the noise level limits.

Continuous Descent Approach (CDA) is also an excellent method for reducing fuel burn and low level emissions, and again is always used where, and when it is possible to do so.

Land use planning covers a wide range of measures aimed at reducing the impact of aircraft emissions around airports. The most effective long-term options are similar to those for noise, but as aircraft impacts are close to the airport, would be much smaller in size.

Nitrogen dioxide can be measured in a number of ways, but the two most often used are by continuous 'chemi-luminescent' monitor and diffusion tubes.

Chemi-luminescent monitors: these are accurate monitors used to measure a number of gasses, including NO2, on a continuous basis. Their major drawbacks are that they are relatively bulky and expensive, and require a power supply. They make use of the quirk that when NO reacts with ozone to produce NO2, it also emits a small quantity of light. This is measured in a photomultiplier tube, or solid state device and is proportional to the concentration of NO in the gas sample.

This technique may be used to measure total NOx concentrations by first using a catalyst to reduce all the NOx to NO. The difference between the concentration of NOx and NO, then gives the concentration of NO₂.

Diffusion tubes: a much cheaper and more feasible solution to monitoring NO₂ concentrations, at a number of locations. They consist of a small tube (test-tube size), one end containing a pad of NO₂ absorbing material, the other end is opened for a set exposure time. The amount of NO₂ absorbed is determined by adding a reagent to form a coloured 'azo' dye, the optical density of which is measured in a laboratory using a spectrophotometer, to determine the average NO₂ concentration.

The laboratory used to carry out the analysis can significantly affect the results from NO₂ diffusion tubes. For this reason, best practise is to use three tubes at each monitoring point, and to co-locate one set with an existing continuous chemi-luminescent monitor. This way any bias can be corrected by referring the results back to the continuous monitor, and the three tubes will identify any anomaly.

"BTEX" diffusion tubes are used to measure concentrations of the organic compounds: benzene, toluene, ethyl-benzene and xylenes. In attempting to identify the source of the NO₂ emissions, monitoring the concentration of the hydrocarbon, benzene, can sometimes be used as an indicator, as petrol used in automobile engines contains benzene but diesel and the kerosene used in aircraft engines do not. By analysing the ratio of benzene to NO₂, it may be possible to indicate the relative NO₂ source.

In the British Airways monitoring study, one set of NO₂ tubes has been co-located with the existing continuous chemi-luminescent monitor (LHR 2 monitor) just within the airport boundary, to identify any bias with the diffusion tubes. In addition, at most sites north of the airport the NO₂ diffusion tubes have been supplemented with BTEX tubes. The results from these, in conjunction with those from the NO₂ tubes, has been used to indicate where petrol fuelled road traffic has had an effect on the local NO₂ concentrations.