Air Quality Monitoring AWE International Issue 20 Sep 2009 © AWE International 2009

This article considers air quality monitoring in the UK, in particular its role in meeting the requirements of the Convention on Long range Transboundary Air Pollution. It will describe the direct monitoring of air pollutants, and also how modelling is used to give a picture for the UK. Finally it will consider another key aspect of air quality – that of monitoring for effects.


Since the days of ‘acid rain’ in the 1980s, which brought public attention to the hidden nature of air pollution, networks have been operating across the UK, as in many other European states to measure air quality and the chemical composition of rain. The smoke and smog of industrialisation were all too obvious in their visible effects on buildings and on human health, but in Britain the Clean Air Acts of the late 1950s removed much of the obvious air pollution. The demise of smoke was monitored by a network of smoke and sulphur dioxide monitors, mostly in urban areas. This monitoring network has now been radically curtailed, because sulphur dioxide concentrations are now too small to be measured with the techniques used. Half a century later, although less visibly obvious, human health and environmental damage are still important drivers for further controls on pollutant emissions. Air Quality targets now exist for a range of pollutants, and are enforced in law by EU Directives.

The original European air quality directive (96/62/EC) became valid in 1996. Since then there have been various daughter directives which have added pollutants or changed limits for particular components. Because of these additions, a new directive (2008/50/EC) was adopted in May 2008. Its aim is to merge the existing legislation into a single directive (with the exception of the 4th daughter directive). The limits would not change with this new directive except for particulate matter with diameters less than 2.5?m (PM2.5). The member states were given two years to implement the new directive, but exceptions could be granted.

The emphasis of air quality and measurement has worked. Controls on emissions have seen remarkable reductions in the traditional air pollutants of smoke and sulphur dioxide – with less than 10% of the sulphur dioxide emitted now compared to the peak of emissions in the 1970s. However, concerns about particulate matter (PM) and its influence on human health have led to air quality limits for PM10 (particles with diameters less than 10 ?m) and PM2.5 which can penetrate the lungs. These limits are still exceeded in some parts of some cities, as are limits for exposure to nitrogen dioxide, emitted by chimneys and vehicle exhausts. Environmental concern has moved from ‘acid rain’ to nitrogen deposition, which is threatening nature reserves and areas that cannot tolerate too much nitrogen. Here a major culprit is intensive agriculture, responsible for wide scale emissions of ammonia into the atmosphere, which is returned to the earth’s surface from the atmosphere as ammonia gas, after conversion to ammonium particles, or as ammonium ions in rain.

It was discovered many years ago that air pollutants are no respecters of international boundaries, so control measures, and studies to follow the movements of pollutants across borders, are now organised internationally. Within Europe, the UN Economic Commission for Europe (UNECE) organises several Task Forces under the Convention on Long-Range Transboundary Air Pollution (LRTAP) which was established in 1979 and came into force in 1983. The Task Forces each report to one of three working groups within the Convention: The European Monitoring and Evaluation Programme (EMEP), the Working Group of Effects (WGE) and the Working Group on Strategies and review (WGSR). Since the Convention came into force, it has been extended by eight protocols that commit participating countries to controlling their emissions of air pollutants. The three most recent Protocols, on “Heavy Metals”, “Persistent Organic Pollutants” and “Acidification, Eutrophication, and Ground Level Ozone”, came into force in 2003, 2003 and 2005, respectively.

The EMEP programme provides scientific support to the Convention on atmospheric monitoring and modelling, emission inventories and projections and integrated assessment modelling. Member states contribute to the work of EMEP by providing data that can be used to estimate emissions, transport and transformation of air pollutants, including routine monitoring of air quality. Part of the work of EMEP is to consider environmental aspects of new chemical substances that might require monitoring. In the 2004-2009 programme of EMEP it was a priority to enforce the measurements of Persistent Organic Pollutants (POPs) and Heavy Metals. The EMEP has to have consideration of geographic range as well as particular pollutants. Indeed for its 2010 strategy, currently under discussion, a priority will be to extend the programme over the Mediterranean, Eastern Europe and central Asia regions. Its aim is to establish a network of 20-30 level 2 monitoring sites. This is a monitoring site which does more than the basic (level 1) monitoring and includes fine time resolution, continuous measurements of nitrogen oxides and air concentrations of cadmium and lead. In the Convention it is expected all countries larger than 50,000km2 should have at least one level 2 site. Sites where level 2 monitoring is in place for particular topics are known as ‘supersites’. It is also important to realise that the measurements are used to underpin modelling activities, which are a key input from EMEP into the LRTAP.

LRTAP also monitors the effects of air quality via the Working Group on Effects. It has a number of Task Forces that provide information on the degree and geographic extent of the impacts of air pollutants on human health and the environment. Together, the Task Forces monitor, measure and model the impact of air quality on crops, natural vegetation, forest trees, rivers, lakes and streams, ecosystems, materials and cultural heritage, and human health.

The third working group, the WGSR, is the main negotiating body for the Convention and focuses on policy-oriented activities. These include assessing scientific knowledge and negotiations in relation to preparing and revising Protocols, promoting the exchange of technological knowledge, and preparing proposals for the further development and extension of the Convention.

Monitoring in the UK

Air Quality Monitoring AWE International Issue 20 Sep 2009 © AWE International 2009

The EU Directives on Air Quality require member states to monitor compliance with air quality limits. Consequently, a wide range of pollutants is monitored routinely in the UK, overseen and partly funded by Defra (the Department of the Environment, Food and Rural Affairs) under its Air Quality and Industrial Pollution Programme (AQIP). As part of its contribution to EMEP, Defra funds the operation of two ‘supersites’ in the UK, one remote rural site at Auchencorth, in southern Scotland, and the other at Harwell, in Oxfordshire. Both sites have been used for air monitoring for many years, but supersite status has required major upgrades in the frequency and scope of measurements. The Centre for Ecology & Hydrology is the current contractor for this work, with the site at Harwell operated by AEA Technology plc. The range of gases, particles and rainfall components measured at Auchencorth, which became a supersite in 2006, is given in Table 1; a similar capacity is being developed at Harwell, which was established as a supersite in 2009. One of the key objectives of these supersites is to provide a broad range of measurements at hourly intervals, so that computer models of pollutant emissions and transport can be evaluated against data over short time scales.

Automatic analyzers for the major trace gases (ozone, nitrogen oxides and sulphur dioxide) and for particulate matter (PM) are commercially available, and many air quality monitoring sites have been established across the UK to demonstrate compliance with air quality objectives.

Air Quality Monitoring AWE International Issue 20 Sep 2009 © AWE International 2009

The Automatic Urban & Rural Network (AURN) includes many sites operated by local authorities; further details can be found at However, a comprehensive suite of air quality measurements at the supersites has required the routine operation of novel analytical instrumentation, capable of making automatic measurements of trace gases and particles on an hourly basis. An example of the type of data now routinely available is provided by ‘MARGA’ (Monitoring instrument for Aerosols and Gases). This analyzer samples air through a rotating glass double-walled cylinder coated with a thin film of water. The water-soluble gases, such as ammonia, nitric and nitrous acids, hydrogen chloride and sulphur dioxide, dissolve in the water film, while particles are swept through the cylinder without being captured. The particles then encounter a jet of steam, which turns them into droplets that can be captured. The water film containing the dissolved gases, and the captured droplets, are then separately analysed on-line using ion chromatography, to identify and quantify the various dissolved components. An example from 2008 is shown in Figure 1; the different gases all show different behaviour, reflecting their different sources.

The particles over the same period (Figure 2) show a different pattern; ammonium, nitrate and sulphate show similar behaviour, because these particles have usually travelled long distances from the original source of the pollutant emissions, and their concentrations at the measuring site depend on where the wind has come from. The chloride data come from the remains of sea-spray, and so reflect wind directions from the coast. The link between the first group and pollution is confirmed by the data on black carbon (‘soot’) which shows the same temporal pattern (Figure 3).

Air Quality Monitoring AWE International Issue 20 Sep 2009 © AWE International 2009

The advantages of a supersite can be clearly seen in these figures. By having so many different measurements all made at one site there is no doubt as to the comparability of measurements.

Modelling the monitoring measurements

The main use of all these supersite measurements is to evaluate UK and European scale computer models of the transport of pollutants through the atmosphere. It is not possible to measure every component of the atmosphere everywhere, so models have been developed which take the reported emissions of pollutants from national inventories, and feed them into meteorological models of weather conditions, along with calculations of the chemical reactions that take place in the atmosphere. The models predict the movement and concentrations of the various pollutants and their reaction products (e.g. nitric acid from nitrogen oxide emissions) across the whole of the model domain (i.e. UK or Europe). One such model (EMEP4UK), derived from the European EMEP model but at a finer spatial scale over the UK, has been developed by the University of Edinburgh in collaboration with CEH and funded by Defra. It can be used to predict concentrations at the supersite, and to set these into a context across the whole country. An example is shown in Figure 4 (courtesy of Dr M Vieno, University of Edinburgh). The blue line shows the model predictions of sulphate concentrations in air through the month of June, 2006. The red line shows the measured data from MARGA at Auchencorth, and the green data are results from a highly sensitive aerosol measurement instrument which was running during the month at the nearby CEH laboratories. The agreement is very good on the whole, with the major peaks and troughs clearly reproduced. The inset maps show the modelled pattern of sulphate concentrations for three periods: during the high peak, showing that air with high sulphate concentrations was crossing southern Scotland at the time, a period where the supersite was on the edge of an area with high concentrations, and the final period with very low concentrations reflecting very clean air across the whole of the UK.

Air Quality Monitoring AWE International Issue 20 Sep 2009 © AWE International 2009

Monitoring data are also used to produce maps of the concentration and deposition of pollutants across the country, and to different types of vegetation. These maps are then used to estimate the degree to which natural habitats are at risk from eutrophication (deposition of excess nutrients) or acidification. Wet deposition is collected in simple rain gauges, and analysed for the major ionic components to provide an average annual concentration of material in rain across the country. This can then be combined with the much better known amounts of rainfall to provide the total wet deposition. The gases and particles are measured using a low-cost technique developed by CEH, known as DELTA (Denuder for Long Term Atmospheric sampling). Air is sampled through a series of chemically-coated glass tubes which selectively absorb the trace gases; particles pass through to be trapped on filter papers. At the end of each month, tubes and filters are sent to the laboratory for chemical analysis. Maps are currently available for nitric acid, sulphur dioxide, hydrogen chloride and ammonia (e.g. Figure 5 shows the map for nitric acid. Circles indicate the monitoring sites). A full suite of maps is available at the website

Ammonia concentrations are much more variable spatially compared to nitric acid, so many more sites are included in the network to monitor ammonia concentrations, using both the DELTA samplers, and another type of sampler developed by CEH, the ALPHA (Adapted Low cost Passive High Absorption) sampler. This device requires no power supply and relies on the controlled diffusion of ammonia gas into the sampler, where it is chemically trapped for subsequent analysis. Maps of ammonia concentration show the importance of agricultural emissions (Figure 6).

Air Quality Monitoring AWE International Issue 20 Sep 2009 © AWE International 2009

The variability of ammonia concentrations across the country is even greater than suggested by the map, where the concentrations have been estimated between the individual sampling sites as if there were a gradual change from one to the next. To get a better idea of the spatial variability CEH has developed a statistical transport model that uses the emissions across the country to predict air concentrations (Figure 7).

Effects of air quality

As described above a key component of Air Quality science is determining the effect of adverse air quality. There are several Task Forces under the WGE which do this. The Task Force on the impacts of air pollutants on crops and (semi-) natural vegetation is led by the UK and CEH has responsibility for coordinating the activities. The programme focuses on two air pollution problems: impacts of ozone pollution on vegetation1 and the atmospheric deposition of heavy metals to vegetation2. In addition, the ICP Vegetation Task Force takes into consideration impacts of pollutant mixtures (e.g. ozone and nitrogen) and atmospheric nitrogen deposition, the consequences for biodiversity and the modifying influence of climate change on the impacts of air pollutants on vegetation. Over 200 scientists representing 33 European countries plus the USA contribute effects data or modelling expertise to the ICP Vegetation.

The ozone effects activities of the ICP Vegetation are overviewed here to provide an example of the type of contributions the programme makes to the LRTAP Convention. Participants monitor the impacts of Air Quality Monitoring ambient ozone pollution on vegetation by conducting experiments with species that are known to be ozone-sensitive (e.g. white clover) and by surveying fields and natural vegetation for injury symptoms. Through this collaborative work, it has been found that visible ozone damage has been recorded on over 30 crop species and 80 species of natural vegetation, with effects noted in 18 European countries.


Air quality monitoring in the UK is driven at the European level by the EC Directives and the need to comply with the LRTAP Convention. The UK has well established monitoring networks which have resulted in appropriate well managed datasets. It also has demonstrated modelling capabilities which will allow confidence in the future predictions thereby allowing policy to be driven with evidence.


  1. ‘Evidence of Widespread Ozone damage to vegetation in Europ (1990-2006)’, ICP Vegetation Working Group, 2007
  2. ‘Spatial and Temporal Trends in Heavy metal Accumulation in Mossess in Europe (1990-2005)’, ICP Vegetation Working Group 2008