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AIR QUALITY EXPERT GROUP
Particulate Matter inthe United KingdomSummary
Prepared for:Department for Environment, Food and Rural Affairs;Scottish Executive; Welsh Assembly Government; and Department of the Environment in Northern Ireland
This is a summary of the second report produced by the Air Quality Expert Group.
United Kingdom air quality information received from the automatic monitoringsites and forecasts may be accessed via the following media:Freephone telephone helpline 0800 556677TELETEXT page 156Website: http://www.airquality.co.uk
http://www.defra.gov.uk/environment/airquality/
Department for the Environment, Food and Rural AffairsNobel House17 Smith SquareLondon SW1P 3JRTelephone: 020 7238 6000Website: www.defra.gov.uk
© Crown copyright 2005
Copyright in the typographical arrangement and design rests with the Crown.
This publication may be reproduced free of charge in any format or mediumprovided that it is reproduced accurately and not used in a misleading context. The material must be acknowledged as Crown copyright with the title andsource of the publication specified.
This document is also available on the AQEG website at:http://www.defra.gov.uk/environment/airquality/aqeg
Published by the Department for the Environment, Food and Rural Affairs. Printed in April 2005 on material containing 80% post-consumer waste and20% elemental chlorine-free pulp.
Product code PB10580a ISBN 0-85521-144-X
AQEG is Professor Mike Pilling (Chairman); Professor Helen ApSimon; Dr DavidCarruthers; Mr David Carslaw; Dr Roy Colvile; Professor Richard Derwent OBE; Dr Stephen Dorling; Professor Bernard Fisher; Professor Roy Harrison OBE; Dr Mathew Heal; Professor Duncan Laxen; Dr Sarah Lindley; Dr Ian McCrae; Mr John Stedman; Professor Mike Ashmore; Dr Mike Jenkin; Dr Peter Woods; Mr Stephen Moorcroft; Dr Tim Murrells; Dr Paul Quincey; and Mr Ken Stevenson.
When referencing this document, please use the following phrasing:AQEG (2005) Particulate Matter in the UK: Summary. Defra, London.
IntroductionThe Air Quality Expert Group (AQEG) wasset up in 2002, following the publication ofthe ‘Air Quality Strategy for England,Scotland, Wales and Northern Ireland’,which describes the plans drawn up by theGovernment and Devolved Administrationsto protect and improve air quality in theUnited Kingdom (UK).
AQEG aims to help the UK Governmenttackle air pollution by examining wherepollutants come from and what theirconcentrations are in the UK.
This is a Summary of AQEG’s second report,Particulate Matter in the UnitedKingdom. We have included a glossary atthe back to explain certain terms (these areshown in bold the first time they occur inthe document).
What is particulate matter?Airborne particulate matter is made up of a collection of solid and/or liquid materialsof various sizes that range from a fewnanometres in diameter (about the size ofa virus) to around 100 micrometres (100µm, about the thickness of a human hair).It consists of both primary components,which are released directly from the sourceinto the atmosphere, and secondarycomponents, which are formed in theatmosphere by chemical reactions. Particulate matter comes from both human-made and natural sources. It contains arange of chemical compounds and theidentity of these compounds provides clues to its origin (see Tables 1 and 2).
Measurements of the concentration ofparticulate matter in air are made byrecording the mass of particulate matter in one cubic metre of air, using the unitsmicrograms per cubic metre, µg m–3.
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Particulate matter is classified according to its size and this classification is used in concentration measurements. For example,PM10 is – to a good approximation – the
concentration of particles that are less thanor equal to 10 µm in diameter; similarly PM2.5describes the concentration of particles thatare less than or equal to 2.5 µm in diameter.
Table 1. Components of particulate matter.
Primary components Sources
Sodium chloride Sea salt.
Elemental carbon Black carbon (soot) is formed during high temperature combustion of fossil fuels such as coal, natural gas and oil (diesel and petrol) and biomass fuels such as wood chips.
Trace metals These metals are present at very low concentrationsand include lead, cadmium,nickel, chromium, zinc and manganese. They are generated by metallurgicalprocesses, such as steel making, or by impurities found in or additives mixed into fuels used by industry. Metals in particles are also derived from mechanicalabrasion processes, e.g. during vehicle motion and break and tyre wear.
Mineral components These minerals are found in coarse dusts from quarrying, construction anddemolition work and from wind-driven dusts. They include aluminium, silicon,iron and calcium.
Secondary components Sources
Sulphate Formed by the oxidation of sulphur dioxide (SO2) in the atmosphere to formsulphuric acid, which can react with ammonia (NH3) to give ammonium sulphate.
Nitrate Formed by the oxidation of nitrogen oxides (NOx – which consists of nitric oxide(nitrogen monoxide, NO) and nitrogen dioxide (NO2) in the atmosphere to formnitric acid, which can react with NH3 to give ammonium nitrate. Also present as sodium nitrate.
Water Some components of the aerosol form of particulate matter, such as ammoniumsulphates and ammonium nitrates, take up water from the atmosphere.
Primary and secondary Sourcescomponents
Organic carbon Primary organic carbon comes rom traffic or industrial combustion sources.Secondary organic carbon comes from the oxidation of volatile organiccompounds (VOCs). There may be several hundred individual components.Some of these trace organic compounds, such as certain polycyclic aromatichydrocarbons, are highly toxic.
Note that abbreviations such as SO2 and NOx are often used instead of the names of the chemicals in full. We will follow this practice in this Summary.
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Table 2. Precursors of secondary particulate matter.
Primary components Sources
SO2 SO2 is formed by the combustion of sulphur-containing fuels such as coal. Ship fuels such as heavy fuel oil are also a source. However, SO2 emissions from coal-burning power stations have been much reduced by the use of a technology called flue gas desulphurisation and automotive fuels are now low in sulphur.
NOx NOx is formed by the combustion of fuels used in power generation, domesticheating and traffic. See the AQEG report Nitrogen Dioxide in the UnitedKingdom for more details.
NH3 Ammonia (NH3) is emitted mainly from agricultural sources, particularly livestock waste.
VOCs Aromatic compounds such as benzene and toluene are generated by traffic and solvents. Monoterpenes come from vegetation, especially conifers and heathers.
Where does it come from?Primary particles are released into theatmosphere from a number of stationaryand mobile sources. The major mobile sourceis road transport, which produces primaryparticles when fuels are burned or lubricantsused up in the engine, when tyres and brakeswear down and from road dust. The mainstationary sources are the burning of fuelsfor industrial, commercial and domesticpurposes. Emissions of dust can alsogenerate high concentrations of particulatematter close to quarries and constructionsites. Primary particles can also be producedfrom natural sources, for example sea sprayand dust from the Saharan desert travellingvast distances. Secondary particulate matteris formed from chemical reactions of thegases NH3, SO2 and NOx released into theatmosphere (see Table 1). Secondaryparticulate matter is also formed fromorganic compounds by reactions that occurin the atmosphere. These organic compoundsare released when fuel is burned or whenfuel or solvents evaporate; they are alsoemitted naturally by vegetation.
The sources of particulate matter are shownin Figure 1. To the right of the figure areurban background and roadside sites (theseterms are defined later in this summary, on page 9) where people live and work.Furthest to the left of the figure are thesources of secondary particulate matter,VOCs, NOx and SO2. The formation ofsecondary particulate matter takes time.During the hours or even days over whichthis happens, the air containing thepollution can travel long distances and sothese gases may have been emitted onmainland Europe. The sources of primaryparticulate matter are shown in the centreof the figure.
What are the health effectsof particulate matter?
Particulate matter can affect our health.The available evidence suggests that it isthe fine components of PM10, which have a diameter of 2.5 µm or less and are formedby combustion, that are the main cause ofthe harmful effects of particulate matter.These fine particles consist of carbon, trace
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metals (such as copper and zinc) and organiccompounds. There is less evidence to connectsecondary inorganic particulate matter (suchas sulphates and nitrates) or larger particleswith adverse health effects, although theycannot be ruled out.
Particles cause the most serious healthproblems among those susceptible groupswith pre-existing lung or heart diseaseand/or the elderly and children. There isevidence that short- and long-term exposureto particulate matter cause respiratory andcardiovascular illness and even death. It islikely that the most severe effects on healthare caused by exposure to particles over longperiods of time. However, UK estimatesindicate that short-term exposure to thelevels of PM10 that we experienced in 2002led to 6,500 deaths and 6,400 hospitaladmissions being brought forward that year,although it is not possible to know by whatlength of time those deaths were broughtforward. Work published by the Committeeon Medical Effects of Air Pollution(COMEAP) in 2001 also indicates that for
each 1 µg m–3 decrease in PM2.5 over thelifetime of the current population ofEngland and Wales, between 0.2 and 0.5million years of life will be gained. This isequivalent, on average, to 1.5 – 3.5 daysfor every individual in England and Wales.The effect is unlikely to be evenly spreadacross the population, however, and somepeople will gain much more.
Particulate matter and theUK Air Quality Strategy
To protect our health, Defra and theDevolved Administrations have set two airquality objectives for PM10 in their AirQuality Strategy:
1. The 24-hourly objective, which is theconcentration of PM10 in the air averagedover 24 hours, is designed to make surethat we are not exposed to highconcentrations of PM10 for short periodsof time. High concentrations can ariseduring pollution episodes, which areshort periods of high levels of pollution
VOCs
SO2
NOx
NaCl Dust
VOCs
Soot
Transboundary transport
Oxidation
Dispersion
Urban background
location
Roadside location
NH3
Figure 1. Sources of particulate matter.
that are usually associated withparticular weather conditions.
2. The annual objective, which is theconcentration of PM10 in the air averagedover one year, aims to protect us frombeing exposed to PM10 over a long time period.
The European Union (EU) has alsodeveloped legislation to limit our exposureto air pollutants through limit values– concentrations not to be exceeded. Table 3shows the objectives and limit values thatneed to be achieved by the UK for PM10concentrations. In addition, emissions ofparticulate matter are controlled bylegislation at national and European levels(for example, by limiting engine emissionsfrom road vehicles).
The EU has also proposed stricter limitvalues to be achieved by 1 January 2010.These involve a reduction of the number ofallowed exceedences of the 24-hourly limit
value from 35 to 7 and a decrease in theannual limit value from 40 µg m–3 to 20 µgm–3. These are called indicative limitvalues, because they currently have nolegal standing. They will be subject tofurther assessment and discussion byexperts and policy-makers before a decisionis made on whether to adopt them. In itsAddendum to the Air Quality Strategy, theUK Government and DevolvedAdministrations adopted these stricterindicative limit values as provisionalobjectives to be achieved in the UK by 31December 2010.
The provisional annual objective is slightlystricter still for Scotland and slightly lessstrict for London. This reflects the lowerconcentrations in Scotland and the highervalues found in London. Although thesetighter objectives have no legal standing inEngland, Wales and Northern Ireland theyhave been put into regulations in Scotlandfor the purposes of Local Air QualityManagement.
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How are emissions ofparticulate matterchanging?
Figure 2 shows the total emissions ofprimary PM10 in the UK from 1970 to2001. The fall in total emissions has beendominated by reductions in emissions fromdomestic heating, energy production andfrom industrial combustion largely as a resultof a decrease in the use of coal as a fuel. The reductions in particulate matteremissions in the UK are similar to or greaterthan those achieved in mainland Europe.During 1990 to 2001, emissions of primaryPM10 in the UK fell by 42% compared with
23% in Germany, 13% in France and 39%in Sweden.
We expect emissions to fall by a further28% by 2010, mainly from reductions inpower station and road transportemissions, but to level off between 2010and 2020. However, the reduction may notbe as great as this if the recent growth inthe numbers of diesel cars on the roadcontinues. The Department for Transportnow expects that more diesel cars will besold in the UK than it had previouslyforecast. Emissions of particulate matterfrom diesel cars are greater than those frompetrol cars.
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Table 3. EU limit values and the UK Air Quality Strategy objectives for particulate matter(measured as PM10).
Legislation 24-hourly limit Annual limit Achieve byvalues and values andobjectives objectives
EU First Daughter 50 µg m–3 with up to 40 µg m–3 1 January 2005Directive (99/30/EC) 35 exceedences a year
UK Air Quality 50 µg m–3 with up to 40 µg m–3 31 December 2004Strategy (2000) 35 exceedences a year
For the 24-hourly objective, an agreed number of exceedences is permitted. This allows for situations, such as Bonfire Night,
where it is not possible or practical to expect 24-hourly average concentrations to remain within the agreed limit (see page 14).
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Figure 2. PM10 emissions in kilotonnes in the UK by source from 1970 to 2001.
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Production processes
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Road transportIndustrial combustion
Other
The air quality models used in the AQEGreport to predict future concentrations ofparticulate matter used the older, lowerestimates of the numbers of diesel cars. Asa result, it is likely that future emissions will besomewhat higher than originally predicted.
There have also been reductions in theemissions of those pollutants that lead tothe formation of secondary particulate matter.Since 1990, SO2 emissions, which lead tosulphate formation, have fallen by over 83%in the UK. This decrease is attributable tothe switch from coal to gas and the use offlue gas desulphurisation at power stationsand other industrial plants to trap SO2before it reaches the atmosphere. In addition,NOx emissions, which lead to nitrateformation, have fallen by 39% since 1990 tomeet increasingly strict European standards.These reductions in NOx emissions are
largely due to improvements in enginedesign and to the fitting of three-waycatalysts to petrol cars. Reductions inemissions from power stations have alsobeen important.
How is particulate mattermeasured?
The aim of the measurement techniques is to determine the concentration ofparticulate matter in outdoor air. Since theair quality objectives are framed in terms ofthe concentration of PM10, it is necessary toexclude particles greater than 10 µm fromthe analysis. This is achieved by drawing the air through a size-selective inlet.Measurements are also made of PM2.5 forwhich a different size-selective inlet is required.
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One way of determining particulate matterconcentrations is the filter-basedgravimetric method. With this method acertain volume of air is drawn through asize-selective inlet and then through a filter.The particulate matter is deposited on thefilter, which is then weighed to determinethe mass of particulate matter that was inthat volume of air.
A widely used alternative method uses the TEOM (Tapered Element OscillatingMicrobalance). The filter in a TEOM ismounted on a tapered glass tube thatvibrates continually. As particles collect onthe filter, the vibration gets slower. Thechange in vibration can be measured veryprecisely without stopping sampling, givinga continuous measurement of the amountof particulate matter being collected.
The EU defines the filter-based gravimetricmethod as its reference method.
Other methods, used by member countriesto establish compliance with EU limitvalues, must be related back to thisreference method. One disadvantage of thegravimetric method is that it takes about 24hours to make a measurement, and theresults are reported only after several days.In the UK, the TEOM analyser is widelyused because it gives results that can bemade available immediately to the public.However, this method requires that the airpassing into the TEOM analyser is heatedand this leads to the loss of semi-volatilematerial such as some organic compounds,ammonium nitrate and water. The filter-based gravimetric method does not use aheated inlet so it is necessary to adjust thedata collected from TEOMs using a scalingfactor to allow the direct comparison ofTEOM measurements with measurementsmade with the EU’s gravimetric referencemethod and therefore the EU limit values.
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Particulate mattermonitoring networks
Networks of monitoring sites for particulatematter have been established throughoutthe UK. The majority of sites use a TEOManalyser, but a few use the gravimetric andother methods. Most sites monitor PM10,but a few also measure PM2.5. The mainnetworks are:
• national networks, including theAutomatic Urban and Rural Network(AURN) – which is funded by Defra andthe Devolved Administrations but whichalso includes some local authority sites,including some from the London AirQuality Network (LAQN);
• regional networks – which consist of theLAQN and other local authority sites; and
• other monitoring sites – which areoperated by local authorities and groupssuch as the Highways Agency and theelectricity-generating companies.
Figure 3 shows the locations of the 240PM10 sites that were operational in 2003 in the UK and that were used by AQEG toproduce this report. The locations of sitesmeasuring PM2.5 are also shown. Themonitoring sites are classified into differenttypes depending on their location, forexample, kerbside, roadside and urbanbackground. This classification is brieflydescribed in Table 4.
Table 4. Definitions of types of monitoring site.
Type Definition of site
Kerbside Sites with sample inlets within one metre of the edge of a busy road.
Roadside Sites with sample inlets between one metre of the edge of a busy road and the back of the pavement (usually five metres from the roadside).
Urban centre Sites away from roads in city and town centres (for example, in pedestrianprecincts and shopping areas).
Urban background Sites in urban locations (for example parks and urban residential areas) awayfrom specific emission sources. These locations broadly represent city-wide background concentrations.
Urban industrial Sites where industrial emissions can make a significant contribution to measuredpollution concentrations.
Suburban Sites typical of residential areas on the outskirts of a town or city.
Rural Sites in the open country away from roads and industrial and residential areas.
Remote Sites in the open country in isolated rural areas that only have regionalbackground pollution concentrations for most of the time.
There are also 123 sites nationally thatmonitor black smoke. Black smoke is darkparticulate matter – essentially soot – whichis measured from the degree of staining of a filter when a certain volume of air is drawnthrough it. The main advantage of the blacksmoke monitoring sites is that some of themhave been operating for many years, giving
information on trends in black smokeconcentrations over several decades. Themethod is also a good indicator of aparticular source of particulate matter – elemental carbon from stationary andmobile combustion sources. Thedisadvantage of black smoke measurementsis that they cannot be related to the EU
Figure 3. Location of (a) PM10 and (b) PM2.5 monitoring sites.
AURN, Automatic Urban and Rural Network; DA, Devolved Administrations; LAQN, London Air Quality Network; KAQN, Kent AirQuality Network; HBAPMN, Hertfordshire and Bedfordshire Air Pollution Monitoring Network; SAQSG, Sussex Air Quality SteeringGroup; Birmingham CC, Birmingham Calibration Club, run by the local authority but with data handling managed by a single company;Calibration Club, run by local authorities but with data handling managed by a single company; Highways Agency – TRL, run by theHighways Agency but with data handling managed by TRL; Transport for London – TRL, run by Transport for London but with datahandling managed by TRL; Heathrow, run by Heathrow Airport Ltd.; JEP Innogy and JEP Powergen, run by power generators.
(a)
(b)
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reference method and so have beenreplaced by measurements of PM10 andPM2.5 in the monitoring networks.
Finally, if we are to understand the sourcesof particulate matter, it is essential that we
also measure and assess its composition. To this end, limited measurements are alsomade of metals, the major ions (such assulphate, nitrate and chloride), elementalcarbon (soot) and organic carbon (carbon in the form of organic compounds).
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What are theconcentrations ofparticulate matter andhow are they changing?
It is important to assess whether thereductions that have taken place in theemissions of particulate matter to air arereflected in the concentrations we measure.
The longest continuous particulate mattermeasurements that have been recorded arethose of black smoke. Figure 4 illustratesthe substantial reductions in black smokeconcentrations achieved in the LondonBorough of Lambeth between 1961 and1998; the decrease reflects the substantialdecline in coal use. Several severe wintersmog events occurred, particularly inLondon, during the last century. The worstepisode was in December 1952. The numberof deaths brought forward, in excess ofthose that would normally have beenexpected, during the first three weeks ofDecember was between 3,500 and 4,000.Particulate matter was a major contributorto these health effects and also led to verypoor visibility conditions, sometimes downto only a metre or so.
Domestic coal burning was a majorcontributor to these smogs. The air quality objectives set by Governmentin response to EU legislation are given in terms of PM10 concentrations.Measurements of PM10 started in 1992 andso have been made over a much shortertime period compared to measurements of black smoke. Figure 5 shows the annualaverage PM10 concentrations recorded at a set of long-running urban backgroundPM10 monitoring sites in the UK. The falls in PM10 levels recorded at all sites duringthe 1990s have stopped in recent years andat some sites the concentrations have evenincreased. The reasons for these changesare unclear but they may have been relatedto the atmospheric conditions, particularlyduring 2003, when the weather was unusual– especially over the summer when record-breaking temperatures were recorded. We need to continue to examine themonitoring data as they become availableto see whether the expected reductions inemissions are seen in the concentrations. It is clear, though, that the decreases inemissions of primary particulate matter arenot fully reflected in the PM10 concentrationsrecorded by the monitors.
Figure 4. Annual average black smoke concentrations in the London Borough of Lambethbetween 1961 and 1998.
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A comparison between the different typesof monitoring site reveals interestinginformation which helps us to understandwhy this is the case. Figure 6 shows theannual mean PM10 concentrations recordedat 196 monitoring sites in 2003. These sitesinclude roadside, urban background andrural sites. They use TEOM measurements,and the scaling factor discussed earlier hasnot been applied. The pink, green and bluelines show the average PM10 concentrationsrecorded across each monitoring site type.Although, as expected, the roadside average
is greater than the urban backgroundaverage, which in turn is greater than therural average, the differences are not large.If the same analysis is carried out foranother pollutant – NOx – a much biggerdifference is seen. NOx is emitted exclusivelyfrom combustion sources, especially roadtraffic. It is clear that, compared to NOx,PM10 concentrations are much more evenlydistributed across the UK. This observationreflects the wide range of PM10 sourcesand, in particular, the contribution ofsecondary particulate matter.
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Figure 5. Annual mean PM10 concentrations (TEOM, not scaled) at long-running urbanbackground monitoring sites in the UK between 1992 and 2003.
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Figure 6. Roadside, urban background and rural annual average PM10 concentrations (TEOM,not scaled) at 196 sites in 2003. The horizontal lines show averages across site types:roadside (blue), urban background (green) and rural (pink).
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Looking at the way in which PM10concentrations change with time and theirrelationship to concentrations of otherpollutants in air can help us understandwhat the sources are. For example, thestrongest link between PM10 and otherpollutants generated by traffic, such as NOxand carbon monoxide, is found at roadsidemonitoring sites. On Marylebone Road inLondon, the concentrations of particulatematter recorded during the week are higherthan those found on Sundays, despite therebeing little difference in the overall volumeof traffic. A probable explanation for this is that the amount of heavy-duty traffic(such as lorries) is reduced on Sundays.Heavy-duty vehicles emit more particulatematter than other traffic.
The composition of the particles alsoprovides information on their origins, asshown in Table 1. Iron, for example, isrepresentative of road dust, chloride ofmarine particles and sulphate of secondaryparticles formed from SO2. Measurementsof composition are, therefore, essential toidentifying the emissions sources of bothprimary and secondary particulate matter.
Are there conditions that can lead to highconcentrations ofparticulate matter?
Pollution episodes are short periods of highlevels of pollution, such as when 24-hourlyaverage particulate matter concentrationshigher than 50 µg m–3 are recorded. Thereare several types of episode.
• In winter, pollution episodes areparticularly severe during weatherconditions known as inversions. Whenthe ground is cold and there is little wind,emissions become trapped near to theground and pollutant concentrations
from sources such as traffic and domesticheating can reach high levels.
• Particulate matter concentrations rise on the nights around November 5th,when Bonfire Night is celebrated. Theconcentrations can be particularly highunder inversion conditions but are not so high under windy conditions.
• Summer episodes take place when theweather is hot and sunny and there islittle wind. In the presence of sunlight,emissions of VOCs and NOx lead to theproduction of high concentrations ofsecondary particulate matter. A severeepisode occurred in August 2003 inSoutheastern England during a heatwavethat affected much of Europe. Thesesummer smog episodes are also associatedwith high concentrations of ozone.Secondary particle concentrations canalso increase at other times of the year,as happened in Spring 2003.
• Gales can cause between one and fivesea salt episodes each year from seaspray. Even sites well inland canexperience increased concentrations ofparticulate matter from this source.
• One or two episodes happen each yearwhen we experience high concentrationsof dust, blown from the Sahara desert.
• In September 2002, hourly PM10concentrations over 100 µg m–3 wereobserved at several locations in the UKas a result of forest fires in Western Russia.
• Local pollution episodes can occur as aresult of, for example, construction works.
By and large, the number of exceedencesof a 24-hourly mean of 50 µg m-3 hasdeclined over the last 10 years, althoughthere were more exceedences in 2003 thanin any year since 1997.
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Do current concentrationsexceed the EU limit values?
PM10 results for 2002 and 2003 give a goodindication of what concentrations will belike in 2005, when the objectives and limitvalues are to be achieved. Table 4 showsthe number and percentage of sites thatexceeded the annual mean and 24-hourlylimit values in each year. The percentageexceedences of the stricter indicative limitvalues for 2010 are also shown in Table 4.The sites are divided into two types:roadside and kerbside – where PM10concentrations tend to be higher becauseof traffic-related emissions – and other sites.The results show that there were a largenumbers of exceedences during 2002 and2003 and demonstrate the differences thatcan occur from one year to the next. Thehigh PM10 concentrations experienced during2003 were discussed previously on page 14.
What are theconcentrations in placeswithout monitoring sites?
We use computer models to work out theconcentrations of PM10 in those parts ofthe UK where there are no monitoring sites.The types of modelling methods used wereoutlined in the Summary of AQEG’s firstreport Nitrogen Dioxide in the UnitedKingdom. The models for PM10 are morecomplicated than those used for NO2 becauseof the complex nature of particulate matterand the many components and sources,both primary and secondary, that must betaken into account. The models are built onthe data we get from UK particulate mattermonitoring sites and also consider howpollutants disperse in the air. Figure 7 showsthe estimated annual average backgroundconcentrations of PM10 during 2002. Themap shows that higher concentrations ofparticulate matter are found in urban areas.
Below 1212 – 14
14 – 16
16 – 1818 – 20
20 – 2222 – 24Above 24
Figure 7. Estimated UK annual average background PM10concentrations (µg m–3) during 2002.
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How are particulate matterconcentrations predicted to change in the future?
Models are also used to predict futureconcentrations of pollutants and have totake into account changes in future emissionsarising, for example, from the use of
improved technology such as particle traps on diesel vehicles. Figure 8 shows thecalculated annual average concentrationsof particulate matter in London that werepredicted for 2004 and that are predictedfor 2010 and shows clearly the reductionsin concentrations of this pollutant that arepredicted over that period.
Table 4. Numbers and percentages of roadside and kerbside and other PM10 monitoringsites exceeding the 2005 and 2010 EU limit values in 2002 and 2003.
Site type Total number Annual limit value 24-hourly limit valueand year of sites 2005a 2010b 2005c 2010d
Roadside and kerbside (2002) 67 5 (7%) 66 (99%) 18 (27%) 56 (84%)
Other (2002) 108 1 (1%) 87 (81%) 8 (7%) 58 (55%)
Roadside and kerbside (2003) 76 13 (17%) 74 (97%) 45 (59%) 75 (99%)
Other (2003) 111 2 (2%) 100 (90%) 20 (18%) 104 (94%)
aNumber (and percentage) of sites with an annual mean greater than 40 µg m–3. bNumber (and percentage) of sites with an annualmean greater than 20 µg m–3. cNumber (and percentage) of sites with a 24-hourly mean greater than 50 µg m–3 for more than 35 daysper year. dNumber (and percentage) of sites with a 24-hourly mean greater than 50 µg m–3 for more than 7 days per year.
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Figure 8. Annual average particulate matter (µg m–3) concentrations in London calculatedfor (a) 2004 and (b) 2010.
(a)
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0 10 20 Kilometers
19202122232425262728>29
Annual averagePM10 (µg/m3)
0 10 20 Kilometers
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Annual averagePM10 (µg/m3)
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Analysis of the data from maps of this sortallows us to predict likely future exceedencesof the air quality objectives. The results showthat the 24-hourly objective was moredifficult to achieve than the annual objectivein 2004 and that there will be exceedencesalong major roads, mainly in London. Thesemaps also show that the stricter air qualityobjectives suggested for 2010 will lead tomany more exceedences at roadside andeven at urban background locations.However, it is important to remember thatthese 2010 objectives are provisional andcurrently have no legal standing in England,Wales and Northern Ireland. In Scotland the2010 objectives have been put intoregulations for the purpose of Local AirQuality Management.
It is difficult to make accurate predictionsabout future air quality for a number ofreasons.
• The number of pollution episodes variesfrom year to year.
• The complex nature of particulate matterand the large number of possiblesources make modelling – and thereforepredictions – difficult. AQEG argues
in its report that Defra and the DevolvedAdministrations should increase thenumber of monitoring sites at whichmeasurements are made of thecomposition of particulate matter. Thiswould make it easier for us to work outthe origins of the particulate matter,which in turn would improve theaccuracy of our models. For example, wedo not have a detailed understanding ofthe contributions that nitrate andsulphate make to regional backgroundconcentrations of particulate matter.
• Secondary organic compounds alsocontribute to regional background levels of particulate matter, especially in summertime. By increasing the numberof sites monitoring the composition ofparticulate matter across the UK, weshould be better able to understand howmuch is secondary particulate matter,how much comes from natural sourcessuch as sea spray and vegetation andhow much is from human-made sourcessuch as traffic. Ultimately this informationwill help the Government to betterassess what can be done to furtherprotect human health.
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The futureIt is clear that, although road traffic emissionsare a major source of particulate matternear to roads, the regional contribution toparticulate matter is substantial. Controllingbackground particulate matter must,therefore, be a central part of any UK strategyto control exposure to particulate matter. In addition, because there is no known safelevel for exposure to particulate matter, it isnot appropriate to rely solely on the use ofair quality objectives. They focus attentionon ‘hotspots’ – places where the pollutantconcentration is high, for example close tobusy roads, but where relatively few peopletend to live.
To maximise the benefits of our controls onparticulate matter, we must aim to reduceexposure more widely. AQEG recommendsthat forms of regulation be considered thatare based on reducing the averageexposure to particulate matter experiencedby the UK population, which wouldcomplement the 24-hourly and annualobjectives.
Particulate matter has many sources andshows links to several different pollutants. Traffic contributions lead to highconcentrations of both particulate matterand NO2 near to roads. During summerpollution episodes, regional backgroundconcentrations of ozone and of particulatematter can both be high because they areproduced in the atmosphere by relatedchemical reactions. The sources of thesesecondary pollutants are widespread andinclude mainland Europe. The effects ofpollutants are not only limited to healtheffects. Acid deposition originates from theemissions of NOx, NH3 and SO2, which also form secondary particulate matter.Particulate matter and ozone also influenceclimate change.
There is a tendency to treat pollutants, theireffects and their control independently,which neglects these relationships. AQEGrecommends a more flexible and holisticapproach, which will deliver more cost-effective solutions to air quality management,including the improvement of particulatematter air quality.
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Glossary
Air quality objective Standards are concentrations of pollutants in the air thathave been set to achieve a certain level of air quality.Objectives are the policy target dates by which thestandards should be achieved, either without exception or with a permitted number of exceedences in a year.
Air Quality Strategy Report published by the UK Government describing plans to improve and protect air quality in the UK. A copy can be found at:http:/www.defra.gov.uk/environment.airquality/strategy/index/htm.
Background concentration Refers to the concentration of a pollutant in theatmosphere in a region away from pollutant sources.
Cardiovascular illness Disease relating to or involving the heart and the blood vessels.
Committee on Medical An Advisory Committee of independent experts that Effects of Air Pollution provides advice to Government Departments and Agencies (COMEAP) on all matters concerning the potential toxicity and
effects upon health of air pollutants.
Concentration The amount of a pollutant in a given volume of air.
Dispersion The process by which a pollutant spreads from its source in the absence of a wind.
Emission Direct release of a pollutant into the atmosphere.
Episode Short period of high concentrations of pollution.
Exceedence A period of time when the concentration of a pollutant is higher than the air quality objective.
Indicative limit value A concentration not to be exceeded after a certain datethat may become legally binding if adopted in legislation.
Kilotonne One-thousand tonnes.
Limit value A concentration not to be exceeded that becomes legallybinding after a certain date.
Microgram (µg) One-millionth of a gram.
Micrometre (µm) One-millionth of a metre.
Model In this document, the term refers to a mathematical way of converting knowledge of air pollutant emissions intoconcentrations in the atmosphere.
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µg m–3 Micrograms of a substance in each cubic metre of air. A unit for describing the concentration of air pollutants inthe atmosphere. A concentration of 1 µg m–3 means thatone cubic metre of air contains one microgram of pollutant.
Nanometre (nm) One-billionth (one thousand millionth) of a metre.
Ozone Toxic unstable gas made up of three oxygen atoms; thechemical formula is O3. At ground level ozone is formed by sunlight-driven chemical reactions of NOx with VOCs.
Particulate filter Device for reducing emissions of particulate matter fromdiesel vehicles.
Primary pollutant A pollutant that is released straight into the atmosphere(contrast with secondary pollutant).
Reference method The method that the EU uses to define its air quality limitvalues. If member countries use another method formonitoring, they need to demonstrate that their methodgives the same results as the reference method.
Respiratory illness Disease relating to or involving the lungs and breathing.
Sample inlet The point, usually at the end of a sampling tube, fromwhich air is taken in to be measured by an air pollutionmonitoring instrument.
Secondary pollutant A pollutant that is formed in the atmosphere by chemicalreactions (contrast with primary pollutant).
Semi-volatile compounds Compounds such as ammonium nitrate that willevaporate on mild heating. Volatile compounds evaporateat even lower temperatures.
Size-selective inlet A sample inlet that ensures that only particles of adiameter equal to or less than a specified value are passedto the monitoring instrument.
Three-way catalyst Device fitted to the exhaust of petrol cars to reduceemissions of carbon monoxide, NOx and VOCs.
VOCs (volatile organic Chemicals containing carbon that are gases, such as ethane, compounds) or that easily evaporate at room temperature, such as
benzene. VOCs are released into the atmosphere fromthe burning of fossil fuels and from other sources. SomeVOCs, such as isoprene, are emitted by vegetation.
25Further informationAdditional copies of this publication may be ordered from:
AQEG SecretariatAir and Environment Quality Division4/E14 Ashdown House, 123 Victoria Street,London SW1E 6DETelephone 020 7082 8421Fax 020 7082 8385Email [email protected]
Current and forecast pollutant concentrationsFreephone telephone helpline 0800 556677TELETEXT Page 156Internet http://www.airquality.co.uk
ContactsEngland Air and Environment Quality Division add defra
4/E14 Ashdown House, 123 Victoria Street,London SW1E 6DETelephone 020 7082 8421Fax 020 7082 8385Email [email protected]://www.defra.gov.uk/environment/airquality/
Northern Air and Environmental Quality add DoEIreland 1st Floor, Commonwealth House,
35 Castle Street, Belfast BT1 1GU Telephone 028 9054 6425 Fax 028 9054 6424 Email [email protected] http://www.ehsni.gov.uk/environment/air/air.shtml
Scotland The Scottish ExecutiveAir, Noise and Nuisance Team, 1-G North, Victoria Quay,Edinburgh EH6 6QQTelephone 0131 244 7250 Fax 0131 244 0211Email [email protected]://www.scotland.gov.uk/about/ERADEN/ACEUAQT/00016215/
Wales The National Assembly for WalesDepartment of Environment, Planning & CountrysideCathays Park, Cardiff CF1 3NQTelephone 029 2082 3499Fax 029 2082 3658Email [email protected]://www.countryside.wales.gov.uk/
PB10580a
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