This report is concerned with identifying the sources of airborne particulate matter and in predicting future changes in concentrations arising from controls applied to the various source categories

ApSimon, H

Clarke, A G

Conlan, B

Coster, S

Derwent, RG

Fisher, B

Hall, I

Harrison, R M

Hickman, J

Mark, D

Maynard, R L

McAughey, J

McMahon, N

Murrells, T

Pooley, F

Prosser, H

Richards, R

Stedman, J

Vawda, Y

Williams, M

CEDA Repository

Source Apportionment of Airborne Particulate Matter in the United Kingdom

Centre for Environmental Data Analysis Digital Repository

Measurements used in this Report3Airborne Particles Expert Group2In this section of the report, a review is presentedof the sampling equipment and methodology thatwas employed to make the particle concentrationmeasurements discussed in this report. Theconcentration parameter commonly used is themass concentration of the PM10 fraction, which issimilar to those particles which penetrate thelarynx and enter the thoracic region of therespiratory system. It is the particulate mattercapable of passing an inlet of definedcharacteristics with a 50% sampling efficiency at10 µm aerodynamic diameter – to a fairapproximation PM10 is particulate matter smallerthan 10 µm aerodynamic diameter. Otherconcentration metrics used are the massconcentrations of the PM2.5 or PM1 fractions.Some quoted data has been obtained using olderinstruments sampling “total suspendedparticulate” (TSP).  Unfortunately these data are oflimited use, as TSP is an old specification forairborne particles, and is not necessarily relevant tothose particles that can enter the human respiratorysystem.  Its definition is specific to a given type ofinstrument, the performance of which varies withwindspeed and particle size distribution.Sampling instruments generally fall into twocomplementary categories: a) direct-readinginstruments which provide continuousmeasurements of particle concentration, normallylogged as 1 hr averages; and b) simple gravimetricfilter samplers, which collect particles on a filter fora given length of time (normally 24 hrs) from whichthe mass concentration is obtained some days laterafter the filter has been weighed. The collectedsample can also be analysed chemically todetermine the composition of the sampled particles.In order to give some structure to this review, it isconvenient to consider four main types of samplingsituation:1)  Measurements of particle levels in theambient atmosphere as part of national  andother sampling networks.2)  Ad-hoc measurements of particle levels in theambient atmosphere to study specific topics.3)  Regulated measurements of emissions frompower stations and other industrial processeswhere the emission is controlled through achimney or exhaust duct.4) Laboratory-based measurements of theemission of particles from vehicle exhausts.The aim is to review the reliability of the methodsin terms of the mass concentration measurementsobtained, and to discuss how the characteristics ofthe sampling methods may affect the conclusionsdrawn.2.1 MEASUREMENTS FROM NATIONAL NETWORKS IN THE UKThe Department of the Environment, Transportand the Regions undertake PM10 measurementsautomatically and continuously at 47 urban andthree rural sites.  In addition, PM2.5 are measuredat four sites and particle numbers at three sites.The mass concentration of the PM10 fraction ofambient airborne particles is determinedcontinuously using a Tapered Element OscillatingMicrobalance (TEOM). This device uses theprinciple that the frequency of mechanicaloscillation of an element, such as a tapered glasstube, is directly proportional to the mass of thetube. Changes in the effective mass of the tube (dueto the deposition of particles on a small filter fixedto the free end of the tube) is reflected in a changein the resonant frequency. In the environmentalversion of the instrument (Model 1400A), air at16.7 l/min is sampled through a SA 246b PM inlet,and transported through an insulated metal pipe toa sample divider, where a 3 l/min sample is passedthrough the 16 mm PTFE coated quartz fibre filter(Pallflex). The inlet and sensing arrangement areheld at a temperature of 50°C to minimise errorsfrom the evaporation and condensation of wateron the filter.  Recently, a different type of direct-reading monitor has been providing PM10 dataonto the national Automatic Urban Network4(AUN) network from the affiliated site at BelfastEast. This is a MetOne BAM 1020 b -attenuationmass monitor and uses the attenuation of b -particles by airborne particles collected on a filtertape to give mass concentration measurements. Itsminimum sampling time is one hour, and it has theadvantage of retaining the particle samples forfuture chemical analysis.A direct-reading instrument has many advantagesover simple gravimetric filter samplers, associatedwith its ability to give real-time, massconcentration measurements. Once set up, themanpower involved in obtaining hourly, daily andweekly mass concentration data is limited to a two-weekly visit to check and if necessary to change thefilter in the TEOM. It can be used to giveinformation on diurnal trends, to track the passageof pollution episodes. In addition, it has sufficientsensitivity to provide hourly measurements of thePM2.5 and PM1 fractions. The main drawback withthe TEOM instrument is associated with therequirement to keep the filter at a constanttemperature of 50°C. This has led to differencesbetween mass concentrations given by the TEOMand co-located gravimetric filter samplers, forwhich the collection filters are unheated and aretherefore at ambient temperatures. The effect ofthis difference is variable, being dependent uponthe location, the environmental conditions and themagnitude of the concentration. Generally, theTEOM gives lower particle concentration valuesthan the gravimetric samplers.  Some comparativedata have been collected for the TEOM with co-located gravimetric samplers (Partisol). This allowsthe preliminary examination of the relationshipbetween gravimetric and TEOM monitors,  shownin Table 2.1, and Figures 2.1 and 2.2 (DETR et al.,1999).  The data from Ribble Valley (Table 2.1 andFigure 2.1), which are subject to some influence ofindustrial sources, show a reasonably good fit to alinear relationship with a minimal intercept.  In thissituation the Partisol data are consistently about15% higher that the TEOM mass measurements.The data from Cornwall (Table 2.1 and Figure 2.2)illustrate a different situation in which the data arebest fitted by a curvilinear relationship.  Attemptsto fit by linear regression suggest a substantialintercept which is not borne out by examination ofthe data.  Thus, both the slope and interceptssuggested by linear regression of these data are verymisleading.  At a Partisol measured concentrationof 50 µg m-3 the TEOM is reading about 38 µg m-3,indicating Partisol results about 32% higher thanthe TEOM broadly consistent with the relationshipfound in South Yorkshire.  Thus, at concentrationsaround the air quality standard, the extent ofunder-reading by the TEOM relative to the Partisolindicated by the data in Table 2.1 is around 15-30%.  Further longer term studies have beenestablished by the Department of the Environment,Table 2.1  Regression equations for Partisol and TEOM from the First Phase Air Quality Review and AssessmentStudies (DETR et al., 1999)Local Authority Correlation Coefficient, rRibble Valley Partisol =  1.15 x TEOM - 0.05 0.92South Yorkshire Partisol =  1.3 x TEOM 0.89Cornwall Partisol =  1.55 x TEOM - 6.09 0.94London Kerbside Partisol =  1.2 x TEOM - 3.1 0.92Figure 2.1  Comparison of PM10 concentrations (µg m-3)measured by TEOM and Partisol instruments at RibbleValley, April to September, 1997.5Airborne Particles Expert GroupTransport and the Regions to provide a rigorousanalysis of the relationship between concentrationsfrom these two sampler types.  It is most probablethat this difference is associated with theevaporation of semi-volatile species such asammonium nitrate and some organic compounds.Therefore, in source apportionment studies usingdata obtained from TEOMs,  care must be takenwhen predicting the secondary particlecontribution to the total mass concentration. It isuseful to bear in mind that the UK air qualitystandard for PM10 of 50 µg m-3 as a maximum 24hour running average was recommended by theExpert Panel on Air Quality Standards using theresults of an epidemiological study which used datafrom a TEOM instrument (EPAQS, 1995).  Thereis therefore a consistency between the monitoringmethod and the standard, and hence anyunderestimation of PM10 by the TEOM relative toother sampling methods does not imply arelaxation in the protection of public health.The Black Smoke and SO2 monitoring networkshave been in operation for over 30 years and havesuccessfully monitored the massive decline in theconcentrations of these pollutants resulting fromthe implementation of the Clean Air Acts. Despitethe introduction of  PM10 measurements into thenational network, the measurement of BlackSmoke is still carried out at 226 sites. Theinstrument comprises a downward-facing funnelinto which air is drawn at about 1.5 l/min. Thesampled particles are transmitted some 2 m downglass and plastic tubing to be collected on aWhatman  No 1 filter paper. The quantity of dustis assessed by means of light reflectance, with theblack particles produced from incompletecombustion of fossil fuels contributing most to themeasurement. The equivalent mass concentrationis then estimated by use of a Standard SmokeCurve.   The results obtained using this method willbe biased strongly towards black particles, andonly particles < 4 µm will reach the filter and henceare sampled efficiently. However, as a means ofmeasuring the contribution to particles in theatmosphere, these attributes (darkness and size) arebeneficial as only the black particles are detectedand the majority of smoke particles are below 1µm. The conversion from reflectance to massconcentration is the weak link if the concentrationof all airborne particles is required, as the whiteparticles and those larger than 4 µm will beappreciably underestimated. The National Acid Deposition Network includes 8sites as part of the European Monitoring andEvaluation Programme (EMEP) where SO2concentrations and particulate sulphate levels aremeasured on a daily basis. The inlet and particletransmission to the Whatman 40 filter are verysimilar to the Black Smoke method. Consequently,it is doubtful whether particles larger than 4 µmwill reach the filter for analysis.  This is, however,sufficient to give high collection efficiency forsecondary sulphates, of which the vast proportionfalls below this size cut.Measurements of particulate ammonium have justbeen established by the Department of theEnvironment, Transport and the Regions.2.2 AD-HOC MEASUREMENTSA wide range of instruments have been used instudies in the UK to investigate specific topics, theresults from which are used in this report. Apartfrom the use of TEOMs described above, they areFigure 2.2  Comparison of PM10 concentrations (µg m-3)measured by TEOM and Partisol instruments atCornwall, April to October, 1997.6generally gravimetric devices in which particles arecollected onto filters for  periods of 24 hours to 1week, after which the filters are returned to thelaboratory for weighing. The results give theaverage mass concentration of particles over thesampling period.   Some of the instruments used togather data referred to in this report include:– R & P Partisol: which samples ambient air at16.7 l/min through a PM10, PM2.5, or PM1sampling head, with particles being collectedon an unheated  47 mm filter.– Graseby Andersen Hi-Vol PM10: whichsamples at 1200 l/min through a PM10sampling head onto an unheated  25 cm by20 cm glass fibre filter.– EnviroTechnology MiniVol: this can operateon battery power to sample at 5 l/minthrough an unvalidated sampling headdesigned to select PM10 particles forcollection on an unheated 47 mm filter.– Graseby Andersen Lo-Vol dichotomoussampler: samples at 16.7 l/min through a PM10sampling head and then uses a virtual impactorarrangement to provide two separate sampleson 37 mm filters - one with particles ofaerodynamic diameter less than 2.5 µm and theother with  particles in the 2.5 - 10 µm range.– Andersen cascade impactor: separates particlesinto eight size bands between 0.43 and 9 µm,and uses a preseparator at the entry to selectparticles less than 10 µm at the flow rate of28.3 l/min. Particles are collected on quartzfibre filters for weighing and chemical analysis.– R & P ACCU: this is a set of eight unheated47 mm filter holders which uses the 13.7l/min of sampled air that the TEOMdischarges.  Unfortunately, the connectionsbetween the sampling inlet and the filters arevia plastic pipes of variable length, and soinevitably particles of all sizes will be lost tothe internal walls leaving the samplescollected on the filters to be of an uncertainsize fraction.These samplers provide mass concentrationmeasurements and also provide samples forcompositional analysis. Although none of theinstruments has a heated line, there is still thepossibility of losing some volatile particles,especially ammonium nitrate, if the filters are notchanged regularly and the conditions of transport,storage and analysis are not carefully controlled.This has the potential to produce an uncontrollederror into the determination of semi-volatilespecies, dependent upon the history of the sample.Other aerosol samplers include the Rotheroe andMitchell Acid Sampler used since the early 1970s(Eggleton & Atkins, 1972) to collect one hourlysamples of particles for the determination of thesulphate, nitrate and the ammonium content. Thisinstrument uses a roll of Whatman 41 filter paper tocollect sequential hourly samples at 250 l/min.Ambient air is sampled through a mushroom shapedinlet designed to prevent the entry of particles largerthan 100 µm, although there are no data on itssampling performance as a function of windspeedand particle size. The filter tape is interleaved withMelinex film to avoid cross-contamination of thesamples. However, unless the filter tape is kept cooland returned to the laboratory for analysis veryquickly, there is again the potential to lose some ofthe semi-volatile nitrate species.2.3 REGULATED MEASUREMENTS OFSTATIONARY EMISSION SOURCESThe measurement of particulate emissions fromstationary sources, such as power stations andother major industries, is specified by BritishStandard BS 3405, with improved standardspublished by the International StandardsOrganisation (IS 9096). This method involves theuse of an isokinetic sampler positioned inside thechimney with a long transmission pipe connectingto a filter holder positioned outside the stack. Thishas the benefit of allowing the collected sample tobe maintained at a temperature lower than thatwithin the stack, thereby minimising the loss ofcollected material by volatilisation. However, themethod suffers from considerable particle losses onthe internal walls of the transmission pipe betweenprobe entry and filter. The internal walls of the pipe7Airborne Particles Expert Groupmust be washed and the deposited particlescombined with those on the filter. The accuracy ofthe method is ±25 %. Very rarely is the PM10fraction measured - it is normally the total emittedparticulate concentration that is measured with theisokinetic system. Estimation of the PM10 fractionis then determined from knowledge of the particlesize distribution. This estimate is reasonableprovided that the process does not change.However, considerable uncontrolled errors couldarise if the size distribution of the emitted particlesdoes change. A situation where the PM10 fraction is measured isin the testing of various types of solid fuel.  Here, asingle cyclone is positioned at the entry of thesampling probe to select the 10 µm particles to givean estimate of the PM10 fraction. 2.4 MEASUREMENTS OF THE EMISSION OFPARTICLES FROM VEHICLE EXHAUSTSMeasurements of the emissions of particles fromvehicle exhausts are generally carried out in thelaboratory using Constant Volume Sampling (CVS)dilution tunnels to condition the exhaust and makeit suitable for sampling. Isokinetic sampling isnominally carried out, although the systemrequirements specified in the relevant EC Directive(70/220/EEC) can lead to significant errors for thelarge particles, with virtually no particles ofaerodynamic diameter larger than 5 µm beingmeasured. However, as most particles emitted fromvehicle exhausts are considerably less than 1 µmdiameter, this inadequacy may not be important,apart from during the removal of largeagglomerated particles from the internal walls ofthe exhaust system during start up. The mainuncontrolled variable in the measurements is thelength of flexible pipe connecting the tail pipe tothe dilution tunnel. This should be kept as short aspossible, but the existence of both front and rearwheel drive vehicles poses considerable problemsto the test houses.  Finally the filters are required tobe kept at a temperature of below 52°C so as tolimit the losses of semi-volatile materials from thefilter during collection.ReferencesDETR, et al.,  (1999)  The First Phase Air QualityReview and Assessment Studies - a summary.Department of the Environment, Transport and theRegions, Welsh Office, Scottish Office, Departmentof Environment for Northern Ireland, in press.Eggleton, A.E.J and Atkins, D.H. (1972) Results ofthe Tees-side Investigation. Report AERE-R6983.AEA Technology.EPAQS (1995)  Particles. Department of theEnvironment Expert Panel of Air QualityStandards, HMSO, London.Note to ReadersThis Chapter seeks only to indicate some of theequipment choices available to those wishing tomeasure airborne particulate matter.  It makes noattempt to be exhaustive, and some commercialinstruments have not been included.  Omission of aninstrument should not be inferred as conveying anyopinion by the Expert Group and equally, inclusiondoes not constitute an endorsement by AirborneParticles Expert Group.

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Source Apportionment of Airborne Particulate Matter in the United Kingdom

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