Achieving the EU limit values for the protection of human health has proved difficult, particularly for NO2 and to a lesser extent PM10, by Dr Claire Holman, Chair, Air Panel, Chartered Institute of Water and Environmental Management, based on the Panel’s Policy Position Statement on nitrogen dioxide and its evidence to the Environmental Audit Commission investigation on Air Quality.
Air pollution in major cities across Europe is failing to meet the EU limit values set for the protection of human health. While there has been a significant improvement in air quality in recent decades, air pollution is still expected to reduce life expectancy of every person in the United Kingdom by, on average, six months, at an estimated annual cost of up to £17 billion.
A 1999 EU Directive set the limit value for fine airborne particles (PM10) and nitrogen dioxide (NO2) to be achieved by 2005 and 2010 respectively. In recognition of the difficulty in meeting these limits in urban areas, a 2008 ambient air quality directive was agreed by Member States, which allows them to apply for time extensions to meet those limits provided they can demonstrate to the European Commission that the limits can be met by the extended deadlines of 2011 (PM10) and 2015 (NO2).
The health effects of PM10 are well quantified. It has been shown that exposure can exacerbate symptoms of cardiovascular and respiratory disease, causing premature death. According to the World Health Organization there is no safe level of exposure to PM10. It has been calculated that there were more than 50,000 premature deaths in the UK due to PM10 in 2005. To put this into context there were fewer than 3,500 deaths in road accidents in 2005.
The short term health effects of nitrogen dioxide (NO2) are also well established. At higher concentrations it can cause irritation of the lungs and can exacerbate existing respiratory conditions including asthma.
In the UK it is expected that the PM10 limit values will be achieved by 2011. Of more concern are the numerous exceedences of the annual mean limit value predicted for nitrogen dioxide (NO2). Despite the large reduction in emissions of nitrogen oxides over the last 20 years – UK emissions have reduced by about 50% since 1990 – high levels are seen in all urban areas of any significant size. But it is not only large cities that are adversely affected; small market towns where traffic has to go through narrow historic streets have also been shown to exceed the limit value.
NO2 is formed during combustion of fossil fuels and by various chemical processes in the atmosphere. Combustion of fuels such as petrol and diesel produce a mixture of nitrogen oxides, principally nitric oxide (NO) and a smaller amount of NO2 known as primary NO2. Once emitted into the atmosphere, nitric oxide reacts with ozone and other oxidants in the air to form NO2. Further reactions can create and destroy NO2 but it is the initial reaction of nitric oxide with ozone and the primary emissions of NO2 that are the most important contribution to total NO2 concentrations in urban areas.
Air quality management in the UK
The UK has a long history of managing air quality going back to when coal was first used as a fuel. Since the Clean Air Act enacted more than 60 years ago, the winter smogs (smoke + fog = smog) caused by burning coal gradually disappeared, so that by the late 1970s this type of smog was a thing of the past.
Since then the dominant source of poor air quality has been, in most areas, traffic emissions. This is despite air emissions from motor vehicles being controlled in Europe since the early 1970s.
UK legislation and the Air Quality Strategy require local authorities to regularly review air quality in their area and assess whether they will meet the air quality objectives. These objectives are similar to the EU Limit Values, but can have different compliance dates. In addition, the objectives only apply where there is relevant exposure over the averaging period of the limit value. The limit values are applicable at all locations except where 1) members of the public do not have access and there is no fixed habitation; 2) on factory premises or at industrial installations to which all relevant provisions concerning health and safety at work apply, and 3) on the carriageway of roads and on the central reservations of roads except where there is normally pedestrian access. In addition, the limit values are mandatory whereas there is no legal obligation to meet the objectives.
Where there is a risk of exceeding a UK air quality objective the local authority must declare an Air Quality Management Area (AQMA), and most then need to prepare an Air Quality Action Plan. A large number of local authorities (more than 230) have declared one or more AQMAs for exceedences of the NO2 objective.
This system of Local Air Quality Management has been very successful in increasing the understanding of air quality across the country, and it has resulted in a huge increase in the amount of monitoring data. What has been less successful, however, has been the Air Quality Action Plans. In most local authorities there has been little improvement.
Air Quality Management Areas are often declared due to emissions from the major trunk roads that come under the responsibility of the Highways Agency. Although the Agency has responsibility for air quality in theory, there is little evidence of measures being implemented that have had a significant impact on air quality. Often the Agency’s contribution is limited to giving directions to local planning authorities where new development is predicted to increase traffic. While it is recognised that it is difficult to reduce emissions from Agency roads without a very significant reduction in emissions/traffic the Agency has been reluctant to even consider measures that might improve the situation, such as implementing lower speed limits. Where lower speeds have been introduced the primary objective has been to relieve congestion, not to improve air quality.
For air quality issues on local roads there can be a conflict between the air quality responsibility of the local authority and the transport authority. Whether both responsibilities are within a single, or different authorities, the transport function often has other priorities for traffic than environmental improvements, and their strategies at times have been counterproductive.
This conflict is mirrored at central government level. The monitoring and management of air quality is undertaken in the UK by the Department for the Environment, Food and Rural Affairs (Defra), whereas the main cause – road traffic – is the responsibility of the Department for Transport, which has historically been more concerned with traffic congestion and infrastructure provision than environmental protection.
Local Air Quality Management was first implemented in 1997, yet new air quality management areas continue to be declared, as monitoring programmes have been extended and non-compliance with the objectives identified. Few have been revoked. We now know more about air quality than at any time in history, yet air quality management has failed to deliver clean air.
Exceedence of the NO2 limit value
Nitrogen dioxide is proving the most difficult pollutant to meet the limit value. This is largely because despite a large reduction in emissions of nitrogen oxides, the ambient concentrations of NO2 in many locations in urban areas have declined at a slower rate than predicted, and in some cases have increased.
UK government computer modelling has predicted that in 2010 exceedences of the annual mean limit value will occur along nearly 2,500 km of roads, and the one hour objective along about 330 km, and by 2015 that exceedences will still occur along 850 km and 25 km of roads respectively. Even by 2020 exceedences are predicted to continue in some places.
Their analysis of the reasons for future exceedences of the EU limit values suggests that different approaches will be needed in different locations to ensure compliance. The main causes of the exceedences are emissions from heavy goods vehicles and buses, and high background concentrations. It is clear that new measures, at both national and local levels, will be required. The high levels of NO2 measured in small towns must be adequately taken into account when drawing up these new measures.
The UK government modelling does not properly account for congested traffic and street canyon effects, only includes the major road network, and has consistently forecast improvements since the early 1990s.
Modelling air pollution is difficult, and the models themselves contain a number of uncertainties. However, of key importance to the accuracy of the models is the emission data used; that is, the quantities of pollutants emitted from each source and the fact the models first calculate total nitrogen oxides concentrations, and then convert this to NO2 concentrations using various approaches.
The emission data is based on tests carried out on more than a thousand vehicles, but the majority of tests are on older vehicle types that are becoming less common in the current vehicle fleet. Some emission data for recent vehicles have been obtained from very few tests. The government models have only recently used the most recent emission factors available.
For vehicles meeting future emission standards it is assumed that emissions will drop in direct proportion to the improvement in the emission limits, which has historically proven to be an over simplistic approach. It is therefore hard to have confidence when projecting forward into the future.
There is evidence from monitoring programmes that the nitrogen oxides concentrations are declining, but this is not matched by a similar decline in NO2 concentrations.
Drawing on the results of remote sensing emissions from a large number of in-use vehicles undertaken by the University of Leeds and IVL in Sweden, nitrogen oxides emissions from petrol cars seem to have declined by about 98% since before the introduction of three way catalysts at the beginning of the 1990s known as Euro 1. There is some evidence of catalyst degradation on Euro 1 and Euro 2 vehicles, but emissions from modern petrol cars are well controlled (new cars meet Euro 4/5 standards).
However, for diesel cars, which cannot be fitted with a similar three way catalyst, the successive vehicle emissions standards have not been so successful in reducing nitrogen oxides emissions.
For new heavy goods vehicles (trucks) emissions appear to have remained unchanged over the last decade, and for public service vehicles (buses) there is evidence that the emissions have actually increased despite more stringent EU emission limits being introduced in 2000/01 (Euro 3), 2005/06 (Euro 4) and 2008/09 (Euro 5). This reduction in the limit values for nitrogen oxides emissions from Euro 4 to Euro 5 was 50% over the regulatory test cycles.
There are several methods for the reduction of nitrogen oxides emissions. Many European engine manufacturers opted for selective catalytic reduction using urea, because it allowed them to optimise their engine-out emissions for fuel consumption and particle emissions and reduce the nitrogen oxides emissions using the exhaust after-treatment. The result was an increase in fuel efficiency, one of the selling points for long distance trucks. However, the Dutch government and others have identified a problem with vehicles using this technology in urban area – the emissions are largely uncontrolled and can be higher than in engines built to the Euro 1 standard.
One area where there is little information is the quantity of NO2 directly emitted from vehicles (known as primary NO2). As noted above, most nitrogen oxides are emitted in the form of nitric oxide and this is converted in the atmosphere to NO2. It is generally estimated that about 5 to 10% of the nitrogen oxides are emitted in the form of NO2. Indeed, there is good evidence that this was the case in the 1990s to early 2000s. However, there is increasing evidence that the proportion of primary NO2 is increasing on more modern vehicles, particularly diesel vehicles, and this will have profound impacts on our estimates of future concentrations.
Recent research shows that 30-50% of the emissions from modern diesel cars are in the form of primary NO2. For light goods vehicles (vans) the primary NO2 emissions might be even higher. In the UK in October 2010 almost 55% of new cars were diesel fuelled, compared with about 20% in 2003.
Recent research by King’s College in London on behalf of the UK government shows that NO2 concentrations close to roads in London have not been declining in recent years, in common with many other major European cities. Although there is significant variation between monitoring sites, on average the rate of improvement appears to have reduced from around 2003. At background monitoring sites – that is at some distance from the immediate effect of local traffic emissions – and representative of many residential areas, annual mean concentrations declined by almost 2 µg/m3 each year on average outside London, and almost 3 µg/m3 inside London until 2003. Since then this improvement has slowed down to almost nothing. For roadside sites the average improvements were similar outside London, but within inner London concentrations were slowly increasing prior to 2003 but now are increasing by about 1 µg/m3. The King’s College analysis suggests that the primary NO2 emissions have tripled over a ten year period. In London they are now more than 20% and about 15% elsewhere. London, and in particular central London, has a higher proportion of diesel light duty vehicles due to a combination of fewer private cars driving within the congestion charge area and the large number of diesel taxis.
NO2 concentrations in smaller towns
Most air quality studies have concentrated on areas where it was expected that air quality objectives would be exceeded e.g. where there are high levels of traffic in our major towns and cities.
However, there are numerous locations in smaller towns with relatively low traffic volumes where the measured NO2 concentrations are much higher than expected and often well above the objective level. It is hard to identify the reasons this is occurring. Possible causes are the type of monitoring being carried out, emission factors not accounting for local operating conditions and the change in primary NO2 emissions in the local area. Often small market towns have trucks travelling along narrow historic streets with two or three storey houses on each side which inhibit the dispersion of the exhaust emissions. Given the very high concentrations being observed it is essential that further research is carried out to fully understand why this is occurring and to devise mitigation methods to reduce concentrations.
Achieving the EU limit values for the protection of human health has proved difficult, particularly for NO2 and to a lesser extent PM10. Air pollution will continue to be an important public health issue for many years to come, as it takes time to introduce and fully implement new emission controls.
The main source of the problem is road traffic. While European vehicle emissions standards controlling nitrogen oxide emissions were first introduced several decades ago, they have not all been equally effective in reducing emissions of nitrogen oxides from road traffic. In particular, controls on emissions from light duty diesel vehicles and heavy duty diesel engines fitted with selective catalytic reduction appear to have been significantly less effective in real-world operation than those on light duty petrol vehicles. In addition, primary NO2 emissions from light duty diesel vehicles have increased in recent years and many now be up to 50% of the nitrogen oxides emissions. These factors, combined with the increasing market penetration of diesel vehicles, are likely to be a major cause of the lack of improvement in ambient NO2 concentrations at many locations, particularly in central London.
Modelling future UK compliance with ambient air quality standards has failed to fully take these factors into account. In addition, it has focused on the trunk road network and has not investigated the high concentrations seen in many small towns across the country.
These issues are not unique to the UK, and similar exceedences of the NO2 limit value are seen across Europe. It is unlikely that the NO2 limit value will be achieved, even in the postponed compliance year of 2015. To achieve the limit values by this date would require radical measures to control traffic, as well as reduce emissions from vehicles. These are unlikely to be politically acceptable, certainly in the short term.
1 Office for National Statistics http://www.statistics.gov.uk/cci/nugget.asp?id=1208 Author
Dr Claire Holman has a first degree in molecular science, a PhD in air pollution and 30 years’ experience in the sector working extensively on air quality, emission control, emission forecasting, dispersion modelling, health effects, carbon management and climate change. Frequently called as an expert witness in litigation over air quality, her experience has included numerous air quality assessments for waste management facilities and energy plants, mineral extraction schemes, land development and highway projects.
She has contributed air quality and climate change chapters to a large number of environmental statements, and successfully negotiated with pollution regulators on many projects to secure consents. She has also advised several national governments in Europe and Asia, as well as the European Commission, on strategic air quality and carbon issues and has sat on a number of Environment Agency, Defra and DfT Committees.
Dr Holman also helped to formulate the Institute of Air Quality Management’s 2009 guidance on significance criteria, and has contributed to the 2010 Environmental Protection UK’s air quality and planning guidance. She is a Fellow of the Chartered Institution of Water and Environmental Management where she chairs the air panel. She is also a Fellow of the Institution of Environmental Sciences, the Royal Society of Arts and the Institute of Air Quality Management.
Dr Holman is currently a Director at Peter Brett Associates and joins international environmental consultancy ENVIRON as a Principal in early 2011.
Published: 10th Dec 2010 in AWE International