The aviation White Paper, issued in December, supported a substantial increase in the UK's airport capacity (ENDS Report 347, pp 42-43 ). The Government backed the aviation industry's desire for further expansion of Heathrow - provided compliance with EU air quality limits could be ensured.
The main problem is an EU limit value for annual mean concentrations of NO2 of 40µg/m3, which must be met from 2010. The White Paper made clear that exceedences around Heathrow "would not be acceptable and would be against the law" - but expressed confidence that a "concerted effort" could ensure compliance "within the 2015-2020 timescale", the time a new runway would be needed.
However, a report for the DfT - carried out to inform the White Paper but not released until mid-February - paints a much less optimistic picture of the prospects for compliance.
The report estimates the number of people who will be exposed to excessive levels of NO2 in 18 different scenarios. The consultants used data similar to those used by BAA last year after the company complained that the DfT's 2002 modelling - which predicted that up to 35,000 people would be exposed to levels above the EU limit - relied on overly pessimistic assumptions (ENDS Report 341, pp 38-39 ).
The results still appear to spell trouble for the proposed new runway. For example, one of the new scenarios, which takes account of reduced thrust on take-off, a 20% reduction in "airport related landside vehicle emissions", and a 40% cut in mean aircraft NOx emissions, still predicts that more than 7,000 people would be exposed to excessive levels of NO2.
Adding a £20 congestion charge for road access to the airport and a 50% reduction in "airside emissions" to this mix would still leave over 1,500 people exposed.
Only in the most extreme scenario do exceedences fall to a minimal level, with just 16 people exposed. However, this scenario would involve putting the M4 motorway near Heathrow and its spur road into tunnels fitted with equipment to capture and destroy NO2 emissions, as well as a capping flights at 550,000 a year, 100,000 below capacity.
Campaigners against the Heathrow expansion seized on the report. "The Government should stop chasing its impossible dream and accept the fact that a third runway cannot be built without breaching EU limits," says John Stewart, chair of HACAN Clearskies.
BAA pointed out that the new modelling did show a significant reduction in the number of people exposed compared to the DfT's 2002 work. The company says this shows that a third runway cannot be ruled out - and is challenging many of the assumptions underlying the latest model.
"The modelling is a massive stride forward, but there are still a number of steps it can go," says Graham Earl, environment strategy manager at BAA Gatwick. "It's not a problem with the modelling per se, but with the modelling inputs. [For one], we need to look at how well models perform in relation to data on the ground."
Dr Earl says more work needs to be done on the chemistry of NO2, especially how it disperses under different conditions, and into transport movements around Heathrow. He says that much of the problem is a result of non-airport related traffic - and suggests that as such, it is a problem for local authorities rather than BAA.
BAA was unable to give anything more than vague answers about the scope for mitigation measures beyond those covered in the DfT's latest scenarios. It suggested that public transport provision and increased car park charges need to be looked at in more depth. In April, the company will introduce differential landing charges for more polluting planes.
BAA figures suggest that in 2002 the airport contributed less than 10% of NOx emissions in areas over a mile away from the airport, rising to around 30% in "near-airport" locations and 60% by the airport. However, these figures do not take account of traffic to and from the airport.
The DfT has now confirmed that it plans to carry out "two to three years" more work on modelling NO2 levels around Heathrow.
Meanwhile, Hillingdon and Hounslow councils, the local authorities nearest to the airport, are currently preparing consultation documents to help develop air quality action plans. Both say "nothing is off the agenda", including placing parts of the road network in tunnels - although it seems clear that such an option would not pass a cost-benefit analysis.
The White Paper makes clear that if the environmental constraints at Heathrow cannot be satisfied, a new runway at Gatwick will be the favoured option. The Government has been constrained from promoting this option because a planning agreement between BAA and the local authority rules out expansion before 2019.
One other factor in the mix is a growing debate about the basis for the troublesome EU limit value. In a recent communication with the European Commission, DEFRA questioned the cost-benefit case of reducing levels of NO2 in "hot spots" (ENDS Report 347, pp 47 ).