The best aircraft emissions model has uncertainty of up to 30%. It leads to more sites being shown with underestimated than overestimated air pollution levels.
The background air quality data has uncertainty of up to 10%.
An anomalous year (like 2003) could easily lead to much higher NOx emissions than those predicted using the ‘normal’ base year 2002.
The model has not included the effects of climate change, which is likely to increase the importance of aircraft sources compared with road transport.
Use of average values / certification tests could significantly underestimate future aircraft emissions. Political and commercial pressures help to dictate the technology capability of engines offered to the airlines.
Actual ground-level temperature and pressure may be quite different from those assumed. NOx emissions are highly sensitive to these factors.
Temporal variations in the use of different types of aircraft have not been modelled. It is unclear whether and how this affects confidence.
Vehicles’ actual NOx emissions can be much greater than the applicable European standard; the full reduction in NOx due to improvements in vehicle engines may thus not be achieved.
Other new sources of regional NOx have not been modelled e.g. power station at Rochester, waste incinerator at Belvedere, growth in railway sources after 2010 etc. These could all increase the assumed baseline levels of NOx.
Assuming a ‘no construction’ scenario for the roads near Heathrow is probably unrealistic, given the development pressures on the area. Such a scenario would lead to an unrealistically low baseline of NOx levels.
Depending on how much the population around the airport grows after 2002, the model could significantly underestimate the number of people affected by air pollution from the airport
So although the models make an active effort to take into account many uncertainties, and generally use a relatively precautionary approach, the future NOx levels that they predict may well be a significant underestimate – certainly the uncertainties are large enough that Government cannot be ‘confident’ that EU NOx standards can be achieved.