Observations on concentration levels and trends over 25 years
Air pollution is an issue that concerns many communities.
The need for air quality measurements has many drivers including: measuring the exposure of the general population to a variety of toxic compounds; informing policy development, assessing compliance with legislative target values. The measurement infrastructure must be able to readily respond to rapidly changing requirements such as the specification of new pollutants, or assessment of episodes e.g. trans-boundary pollution events. In particular, the determination of the total concentrations of metals in ambient air is of great importance within this framework.
Observations on concentration levels and trends over 25 years
|London (Cromwell Road)||Roadside|
|London (Horseferry Road)||Urban Background|
|Walsall 2||Industrial Background|
Figure 1. The location and designation of the sites on the UK Heavy Metals Monitoring Network
Until relatively recently, the presence of metals in ambient air had not been a primary topic of concern to air quality research. This has changed over the last couple of decades as the toxic effect of metals on human health and on the environment has become more completely understood. The total concentration levels of Pb, Ni, As, Cd and Hg, allowable in the PM10 fraction of ambient air (particles with an aerodynamic diameter of 10 µm or less), are now the subject of European legislation1,2. In order to enable the enforcement of legislation the total concentration levels of ambient metals need to be measured at a network of sites. This is especially true in the European Union (EU) where member states are required to demonstrate compliance with the Air Quality Framework Directive3 and its Daughter Directives (DDs)4.
The effect of metallic compounds on ambient air quality has a long history in the UK. The first anthropogenic metal-bearing particulates may well have been produced by lime kilns, producing iron, in the Middle Ages. By the seventeenth century links had been made between compounds in the air and human health. The introduction of other combustion processes, such as coal burning during the industrial revolution, added to an increasing burden of metallic compounds emitted into ambient air until well into the 20th century.
Air quality policies, such as the Clean Air Act of 19565, have since reduced the contributions from these sources, especially in cities, but the more modern sources, such as industry, metals refining, and lead in petrol have taken over as the major contributors of the metallic components of ambient particulates. In the early 1970s ambient lead concentrations as high as 4 µg.m-3 were not unusual in London air. Since then, abatement strategies and legislation (for example the EEC Motor Fuel (Lead Content of Petrol) Regulations in 1976) have reduced the measured levels of metals in particulate matter dramatically.
However, as the traditionally high levels of some metals in ambient air have fallen, and they have therefore become less important, the environmental profile of others has increased. This has been because either they are linked with more severe health effects than previously thought (such as iron) or they are emerging pollutants (such as mercury from crematoria, or precious metals from catalytic converters – such as platinum) that require increased monitoring.
In 1996, the EU published the Air Quality Framework Directive 96/62/EC3 followed by a series of Daughter Directives that set limits for the concentrations of a range of pollutants in ambient air across EU Member States. The 1st DD sets a limits value for the concentration of Pb in the PM10 fraction of particulate matter, whilst the 4th DD sets target values for the concentrations of Ni, As, and Cd in the PM10 fraction of particulate matter, and the concentration of total gaseous mercury (TGM) in ambient air (referred to here as vapour phase mercury). The concentration of particulate phase mercury is not explicitly covered by the 4th DD.
The UK Government’s Department for the Environment, Food and Rural Affairs (Defra) runs a series of nationwide networks to assess the levels of pollutants in ambient air to assess compliance against European and UK air quality objectives6, and to determine ecological and environmental impacts. Nine important trace elements (Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn) have been measured in the UK since 1976. The present metals network of 17 sites, is operated on behalf of Defra by the National Physical Laboratory (NPL) and is referred to as the UK Heavy Metals Monitoring Network (UKHMMN). The current UKHMMN is based on sites that were set up to measure:
- Lead emissions from road vehicles
- Emissions from specific industrial processes
- ‘Trace and multi-elements sites’ monitoring in urban background locations for important trace elements
Network operation – overview
The UKHMMN currently measures: Pb as required by the 1st DD; As, Cd and Ni as required by the 4th DD; and also for Cr, Cu, Fe, Hg, Mn, Pt, V and Zn, all in the PM10 fraction of ambient air. 13 of these sites also measure total gaseous mercury in order to comply with the requirements of the 4th DD (mercury is the only metal to have significant concentrations in gaseous form). The current UKHMMN sites have a variety of ‘designations’ which provides an indication of the type of processes and locations each site is likely to be monitoring. A map displaying the locations of the current and designations of UKHMMN sites is shown Figure 1. Finalised data is sent to the UK Air Quality Archive for publication7. Results are expressed as volume concentrations in terms of ng (of elemental metal) per m3 (of ambient air).
Sampling and analytical methodology:
Particulate samples are taken at all sites in the Network using Partisol 2000 instruments, fitted with PM10 size selective heads, operating at a calibrated flow rate of approximately 1m3.h-1. Samples are collected for a period of one week onto 0.8 µm pore size GN Metricel membrane filters. After sampling the filters are cut in half accurately, and each portion digested at temperatures up to 220°C using a microwave digester. The digestion mixtures used are: 5 ml of nitric acid and 5 ml hydrochloric acid for Pt and Hg analysis; and 8 ml of nitric acid and 2 ml hydrogen peroxide for all other metals. Analysis for particulate- phase metals follows the standard procedure detailed in European Standard EN 14902.
Sampling for vapour-phase mercury takes place at 13 of the 17 Network sites, using low volume pumps to draw air through ‘Amasil’ (gold-coated silica) tubes at a rate of 100 ml.min-1 for either one week or four weeks, depending on the specific site and the expected ambient concentration. The short sampling line is preceded by a 0.8 µm pore size filter to remove particulates. Analysis of vapour-phase mercury samples takes place using a calibrated PS Analytical Sir Galahad II analyser by atomic fluorescence.
Measured ambient metals concentrations
The long-term data sets provided by nationwide metals measurement in the UK over the last 25 years provide an excellent opportunity to elucidate long terms trends in ambient concentrations of a variety of metallic elements. The limit and target values set by the 1st and 4th DDs are displayed in Table 1.
The 1st and 4th DDs also specify data quality objectives such as time coverage, data capture rates, and maximum allowable measurement uncertainties at concentrations corresponding to the limit or target values of the relevant target pollutants. The UK’s performance in 2006 against these limit and target values, as assessed by the average concentration measured by the UKHMMN, is displayed in Figure 3.
Trends in ambient lead concentration
The annual average lead concentration over all sites measured by the UKHMMN from 1980 to 2006 is shown in Figure 4.
Lead concentrations remained relatively high in the early 1980s and showed an increase in the mid- to late-1980s. This was principally because of the inclusion in the UKHMMN of industrial and roadside site recording very high lead levels. In the 1990s, the lead emitted from road vehicles was dramatically reduced as a result of the EEC Motor Fuel (Lead Content of Petrol) Regulations in 1976, and because industrial processes emitting lead have become cleaner and sparser. As a result, measured lead concentrations decreased dramatically, falling below the requirements of the 1st DD in 1990 and below the UK Air Quality Objective for 2008, in 1995. In recent years, further reductions in industry output have caused lead levels to decrease further.
Airborne sources of lead currently include the combustion of solid waste, coal, and oils, emissions from iron and steel production and lead smelters, and tobacco smoke; however, these levels are only a fraction of those measured a quarter of a century ago, and the current UK annual average concentration is less than 4% of the limit value imposed by the 1st DD.
Figure 3. A summary of the annual mean concentrations of the metals relevant to the 1st and 4th DDs measured by the UKHMMN in 2006 as a percentage of the relevant air quality limit and target values. The solid bars indicate the annual mean of all sites; the lines indicate the annual means at the site with the highest concentrations. The mercury objective is taken from a threshold value quoted in a draft of the 4th DD. Mercury refers to the sum of the vapour phase and particulate phase concentrations
Trends in ambient concentrations of the 4th DD metals
The annual average nickel and cadmium concentrations over all sites measured by the UKHMMN from 1980 to 2006 are shown in Figure 5.
|Relevant Legislation||Element||Limit/Target Value||Upper Assessment Threshold (UAT) h||Lower Assessment Threshold (LAT)|
|Ni||20.0ng.m-3||14.0 ng.m-3||10.0 ng.m-3|
|4th DD||As||6.0 ng.m-3||3.6 ng.m-3||2.4 ng.m-3|
|Cd||5.0 ng.m-3||3.0 ng.m-3||2.0 ng.m-3|
|1st DD||Pb||0.50 µg.m-3||0.35 µg.m-3||0.25 µg.m-3|
|UK Strategy||Pb||0.25 µg.m-3||–||–|
Table 1. The limit and target values set by the 1st and 4th DDs, and by the UK Air Quality Strategy. The ‘threshold level’ listed for mercury is from a previous draft of the 4th DD – no limit or target value is specified in the final version. The UK Air Quality Strategy target for lead is to be achieved by the end of 2008. A previous target of 0.50 µg.m-3 was to be achieved by 2005. The Upper and Lower Assessment thresholds listed in the First and 4th DD indicate the levels that trigger the requirement for certain quantities of fixed monitoring 2
Figure 4. The annual average lead concentration over all sites measured by the UKHMMN from 1980 to 2006. The relevant 1st DD and UK Air Quality objectives are shown as broken lines
The presence of nickel in ambient air is a result of releases from oil and coal combustion, nickel refining, the steel industry, and sewage sludge incineration. Average nickel concentrations, which were relatively constant during the 1980s, fell during the 1990s, mainly owing to a decrease in manufacturing activity and the introduction of abatement strategies. They are now less than a quarter of the target values specified by the 4th DD. Cadmium levels have decreased relatively steadily over the last 25 years as the burning of fossil fuels such as coal or oil has reduced, and current average levels are well below the target value specified in the 4th DD.
However cadmium levels still remain relatively high, but are not greater than the 4th DD target value, around a small number of metals smelting plants in the UK. Generally, it is the measured concentrations at these plants that dominate the annual average concentration over all sites.
Arsenic and mercury have only been measured by the UKHMMN since 2003. Arsenic has some natural sources, such as volcanoes and the erosion of arsenic-containing minerals, as well as anthropogenic sources, such as the burning of arsenic-based wood preservatives, and as a general (rather than specific) by-product of some industrial processes.
Since arsenic measurement began in the UK average recorded levels have remained low, and are currently only one sixth of the target value specified in the 4th DD at 1.04 ng.m-3. As a result of arsenic having few specific sources its annual average concentration over all sites is not dominated by a few high values, and is relatively constant across the country, even at sites with different designations (e.g. industrial, urban background etc).
Major anthropogenic sources of atmospheric mercury include, emissions from small-scale fuel burning, electric lamp breakage, laboratory use, dental preparation, landfills, crematoria, and sludge application, as well as certain manufacturing industries (such as chlor-alkali processes), coal and oil burning, and the incineration of solid waste. During 2003, 2004, 2005 and 2006 the annual average particulate mercury concentration over all sites was very low across the UK having a value of 0.4 ng.m-3 in 2006.
However, the annual average concentrations of vapour phase mercury over all sites were higher, with an average mercury vapour to particulate phase mercury concentration ratio over this period of approximately 10 (with a range of between 25 and 2 at the individual sites). Average vapour phase mercury concentrations were 4.6 ng.m-3 in 2006.
Trends in ambient concentrations of other metals measured by the UKHMMN
Iron is of increasing interest as a constituent of particulate matter since it has recently been reinvestigated with relation to its health effects , and it is by far the most abundant metal measured by the UKHMMN. All other metals, with the exception of zinc, were at least 20 times less abundant than iron in particulate matter in 2006. Iron concentrations were approximately five times those of zinc. The proportion of particulate matter that is made up by iron is also large: it comprises up to 1% of the particulate matter mass. The major sources of iron in ambient air, apart from the steel and metals refining industries, are from re-suspended ‘road-dust’. Measured concentrations of iron have fallen steadily over the last 25 years, and are now less than half their 1980 value.
Chromium, Copper, Manganese, Vanadium and Zinc
The UKHMMN also measures several other industrially and toxicologically important metals over the past 25 years. The main sources of copper and zinc in ambient air are from metals smelters and from combustion processes, especially those involving fossil fuels. Copper levels have been relatively low in the UK, with few very active industrial sources contributing to the annual average over all sites.
Figure 5. The annual average cadmium (circles) and nickel (squares) concentrations over all sites measured by the UKHMMN from 1980 to 2006. The relevant 4th DD objectives are shown as broken lines.
However these levels have not decreased very much over 25 years and some high episodes were observed during the later 1980s and early 1990s. Annual average zinc concentrations over all sites have also decreased over the last 25 years, but zinc remains the second most abundant metal measured by the UKHMMN, and high levels at one site (commissioned in 2003) dominate the concentrations averaged over all sites, which partially explains an increase in levels measured during the last four years.
The most important industrial sources of chromium in the atmosphere are those related to ferrochrome production. Additionally, ore refining, cement-producing plants, automobile brake lining and catalytic converters for automobiles also contribute to the atmospheric concentrations of chromium. Manganese can also be released into the air by iron and steel production plants, power plants, and coke ovens, whilst anthropogenic vanadium sources include the combustion of fossil fuels, particularly residual fuel oils, which constitute the single largest overall sources of vanadium to the atmosphere. As the level of these activities has steadily decreased in the UK, the concentration levels of both elements have fallen dramatically over the last quarter of a century, currently well under half their 1980 values.
Platinum is an increasingly important metallic pollutant arising from the proliferation of catalytic converters in automobiles. Platinum can also be emitted to the atmosphere from industrial filters used in the oxidation of ammonia. Pt has been measured by the UKHMMN since 2003. However, its measured concentration is, and has remained, extremely low over this period with recorded levels of: <0.01, <0.01, 0.02 and 0.02 ng.m-3 in 2003, 2004, 2005 and 2006 respectively.
The infrastructure to measure certain metal concentrations in ambient particulates on a nationwide basis in the UK has existed for 25 years. In general, the measured ambient concentrations of all the metals measured by the Network, and its predecessors, have fallen steadily during this time.
These downward trends have been significant, especially after 1991 when the faster decreases are observed. This has been the result of a combination of abatement strategies, legislation and changing fuel usage. With the exception of a few point source sites, the annual average concentrations of all metals at individual sites around the UK is now very low, and appears to be continuing to decrease. The annual average concentrations of Pb, Ni, As, Cd and Hg in UK ambient air are well below the limit and target values required by the European Air Quality Directive’s First and Fourth Daughter Directives. Network data is available on the Air Quality Archive website7.
Whilst the use of modelling to predict ambient concentrations from estimated emission data is increasingly considered to be an alternative to fixed measurements, especially where concentrations are low, it may be some time until monitoring networks for assessing ambient air quality are replaced.
The current challenge for the environmental monitoring community is to find more accurate and more efficient ways of performing these measurements, and proving equivalence with standard methods, if necessary. The development of new analytical techniques will play a large part in improving the accuracy of these measurements, and the strategic planning of air quality networks, including the measuring multiple pollutants at each site together with integrated strategies for standard method development, should improve cost efficiency in the long term.
It is also increasingly recognised that the bio-availability, and therefore the toxicity, of a metal is dependent on its actual chemical form. Consequently, in the future, the continued development of measurement strategies, and analytical techniques for the speciation analysis of metals in environmental samples, will increase in importance as legislation moves towards the requirement for total chemical compositional analysis for metallic compounds in particulate matter.
- Council Directive 1999/30/EC of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air, Official Journal of the European Union 1999, L163, 41.
- Council Directive 2004/107/EC of the European Parliament and of the Council of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air, Official Journal of the European Union 2005, L023, 3.
- Council Directive 96/62/EC of 27 Sept 1996 on ambient air quality assessment and management, Official Journal of the European Union 1996, L296, 55.
- http://europa.eu.int/comm/environment/air/ambient.htm (accessed July 2007).
- Clean Air Act, 1956, Ministry of Housing and Local Government and Department of Health for Scotland, HMSO, London, 1956.
- The Air Quality Strategy for England, Scotland, Wales and Northern Ireland: Working Together for Clean Air, ISBN 0 10 145482-1, TSO, London, 2000.
- www.airquality.co.uk (accessed July 2007).
Published: 10th Jan 2007 in AWE International