On street observations of particulate matter movement and dispersion due to traffic on an urban road

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doi:10.1016/j.at

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Atmospheric Environment ] (]]]]) ]]]–]]]

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On street observations of particulate matter movement anddispersion due to traffic on an urban road

Aditya Patraa,�,1, Roy Colvilea, Samantha Arnolda,2, Emma Bowenb,Dudley Shallcrossc, Damien Martinc, Catheryn Pricec, James Tated,

Helen ApSimona, Alan Robinse

aCentre for Environmental Policy, Imperial College London, South Kensington, London SW7 2AZ, UKbDepartment of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK

cSchool of Chemistry, University of Bristol, Bristol BS8 1TS, UKdInstitute of Transport Studies, University of Leeds, Leeds LS2 9JT, UK

eSchool of Mechanical Engineering, University of Surrey, Surrey GU2 7XH, UK

Received 13 July 2006; received in revised form 23 October 2006; accepted 31 October 2006

Abstract

Empirical models for particulate matter emissions from paved road surfaces have been criticised for their lack of realism

and accuracy. To support the development of a less empirical model, a study was conducted in a busy street at the

DAPPLE site in Central London to understand the processes and to identify important parameters that influence emission

from paved roads. Ordinary road gritting salt was applied to the road and the particulate matter entering the air at near-

road surface level was monitored using optical particle counters. The grit acted as a tracer. The grit moved rapidly along

the road in the direction of traffic flow. Build-up of material at the kerb indicated material being thrown across the road by

the traffic. Coarser particles were resuspended faster than the finer ones. A clear decay profile was seen in the case of

particles larger than 2mm; particles smaller than 2mm did not show any decay pattern during the experiment duration.

Grinding of material appears to control the reservoir of fine particles on the road surface. The amount of material

resuspended by traffic is about 30% less than those removed along the road and a factor of 6 higher than the amount

removed across the road. Resuspension accounts for 40% of the total material removed from a road segment and 70% of

the material removed together along and across the road. On average a single vehicle pass removes 0.08% of material

present on a road segment at that instant. The calculation scheme is obtained from a short-duration study and therefore

further studies of long duration involving varying road geometry and different traffic and meteorological condition need to

be carried out before applying parameter estimates presented in this paper.

Keywords: Particles; Particulate matter fluxes; Dispersion; Resuspension; Traffic-induced surface emission

mosenv.2006.10.070

ing author. Tel.: +91326 220 3010;

0 2429.

ess: adityakpatra@yahoo.com (A. Patra).

ntral Mining Research Institute, Barwa Road,

1, India.

lder Associates (UK) Ltd, Nottingham, NG12

s article as: Patra, A., et al., On street observations of p

tmospheric Environment (2007), doi:10.1016/j.atmosen

1. Introduction

Several studies have demonstrated that road dust,entrained into the air by the movement of vehicles,is a major source of ambient PM10 in cities

articulate matter movement and dispersion due to traffic on an

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worldwide (Abu-Allaban et al., 2003b; AQEG,2004; Barrowcliffe et al., 2002; Goodwin et al.,1999; Harrison et al., 1997a,b; Jaecker-Voirol andPelt, 2000; Kim et al., 1990; Manoli et al., 2002;Rogge et al., 1993). Earlier studies have alsoreported the importance of injection of road dustparticles, especially the coarse fractions of roaddust, into the atmosphere through resuspension bywind and vehicle-induced turbulence (Braaten et al.,1990; Hall, 1989; Kulmala et al., 1986). With theintroduction of cleaner fuels and better technology,exhaust emissions have decreased appreciably de-spite an overall increase in traffic (DEFRA, 2000,2003; Goodwin et al., 1999). This makes roadsurface emissions, which are either a comparable orgreater source than exhaust emission (Abu-Allabanet al., 2003b; APEG, 1999; AQEG, 2004; Ball andCaswell, 1983; DEFRA, 2000; During et al., 2002a;Jaecker-Voirol and Pelt, 2000; Johansson et al.,2004; Schulze, 2001), potentially a more significantsource that needs to be controlled in order toachieve future air quality standards.

In recent times, a substantial amount of work onvehicle related emission has been carried out in theUK (NAEI, 2003) and by the European Environ-ment Agency (EEA, 1999). Studies in Germanyhave tried to estimate, separately, the contributionof exhaust and non-exhaust emission (During et al.,2002a,b). Extensive studies in this area have alsobeen carried out by the US Environmental Protec-tion Agency (USEPA, 1995) and documented asAP-42 emission factor, widely used in subsequentstudies.

The AP-42 emission factor is based on anempirical approach that relates vehicle weight andsilt loading (‘‘silt’’ is particulate matter less than75mm in size; ‘‘loading’’ is amount of silt on theroad surface per unit area of road surface) toemission rate of particulate matter. The approach iscriticised by many researchers (Countess et al.,2001; Kantamaneni et al., 1996; Zimmer et al., 1992)after their field studies showed widely varyingresults. When these are applied in the UK, theyresult in improbably high emissions (Buckinghamet al., 1997). The Airborne Particles Expert Group(APEG, 1999) concludes that it is not applicable inthe UK environment. Doubts have been raised onthe scientific basis of importance of silt loadingwithout inclusion of the processes contributing to it(Gamez et al., 2001; Venkatram, 2000, 2001).Therefore, studies having a more mechanisticapproach have been emphasised (Countess et al.,

Please cite this article as: Patra, A., et al., On street observations of p

urban road. Atmospheric Environment (2007), doi:10.1016/j.atmosen

2001; Kuhns et al., 2001; Nicholson and Branson,1990; Nicholson, 2001; Venkatram, 2000, 2001).

Here we describe an experiment designed toobserve and quantify, as far as possible, the disper-sion of a bulk sample of dust along a traffickedsection of road in an urban environment. Theprincipal aim of the study is to understand thephysical processes that cause movement of particulatepollutants in the urban road environment, identifyand quantify those processes, and parameterise themfor a model that will be reported separately. Thispaper presents experimental design, instrumentation,main observations of this measurement study andparameterisation of material removal processes.

2. Experimental method

2.1. Site description

The experiment was carried out at Gloucester Place,south of the intersection with Marylebone Road inCentral London (Fig. 1). Gloucester Place is a pavedroad, aligned approximately North-South, joiningOxford Street to the south and Park Road to thenorth, intersecting Marylebone Road in between.Marylebone Road is a dual carriageway approxi-mately 26m wide and Gloucester Place is a three-laneroad approximately 13m wide. Arnold et al. (2004)have described more details of the study site. Themeasurements described here were confined to alength of Gloucester Place between Crawford Streetto the south and Marylebone Road to the north,about 180m in length. Buildings, height varyingbetween 14 and 23m, on both sides make GloucesterPlace a street canyon with roads intersecting itperpendicularly at intervals. The main criteria for siteselection are several fold. First, the site includes a busypublic road at the centre of London that isrepresentative of major urban roads in large cities.Second, unidirectional flow of vehicles from south tonorth simplifies the determination of the rate oftraffic-induced movement/dispersion of particulatematter. Finally, there was considerable existinginfrastructure for the field experiment from an on-going multidisciplinary research campaign ‘‘Disper-sion of Air Pollution and Penetration into the LocalEnvironment’’ (DAPPLE, http://www.dapple.org.uk).

2.2. Application of grit

The on-street measurements were made on 26May 2004 between 10:30 and 16:30 (All times in this

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Marylebone Road

Crawford Street west

(Diagram not to scale)

Crawford Street east

York Street west York Street east

Bickenhall StreetSalisbury Place

Route of gritting lorry

Site 4: Location of Grimm 4 (G4)

Site 5: Location of ultrasonicanemometer 2 (A2)

P-Trak 4 (P4) and

Site 1: Location of Grimm 1 (G1),

anemometer 1 (A1)

Build-up of grit at kerb at time t1(Width = ef = 20 mm,

Depth = 2 mm)

Site 2: Location of Grimm 2 (G2)

SF6 sampling pointsS1…S6

Traffic signal lightsL

Predominant street-level winddirection

Predominant roof-level winddirection

Direction of traffic

SF6 release pointsX1, X2

Grit location at time t1ABCD

Initially gritted section

(Length = ad = 120 m,

Width = ab = 13 m)

WestminsterCouncil House

BickenhallMansions

BickenhallHotel

MontaguMansions

CO2 monitor (C1)

Site 3: Location of Grimm 3 (G3),

P-Trak 2 (P2) and ultrasonic

Fig. 1. Schematic diagram of the experiment site showing gritted road surface and sampling locations.

A. Patra et al. / Atmospheric Environment ] (]]]]) ]]]–]]] 3

paper are in a 24 h framework and in BritishSummer Time). Ordinary road surface gritting salt,of the type used to grit roads in winter, was appliedto a section of the carriageway at the experimentalsite. The grit used was rock salt, supplied by OnyxEnvironmental Group plc (OEG, 2005). The gritwas deposited by an OEG gritting vehicle, cali-brated to deposit a known volume via a horizontallyspinning wheel underneath the rear of the vehicle.This had the effect of spraying the grit onto the roadfrom a height of approximately 1m. Grittingcommenced at 11:15 and took approximately5min to complete. The grit acted as a tracer. Itsmovement, interpreted from data captured in road-side instruments, represents the movement of solidmaterial on a paved road surface and the aim of the

experiment was to observe its fate during theminutes to hours after application.

The area gritted covered a stretch of carriagewayapproximately 120m in length, between CrawfordStreet and Bickenhall Street, and 13m in width(same as width of the Gloucester Place). Grittingstarted from a–b and ended at c–d (Fig. 1). In orderto obtain a visible coating of grit, the quantityapplied to the road was 20 gm�2, representative ofWestminster City Council’s (WCC) ‘‘snow precau-tionary gritting’’ regime, making the approximatemass of grit deposited 31.2 kg, out of which thePM10 fractions were about 0.50% by mass i.e.,0.156 kg. Particle size distribution was obtained byusing graduated sieves, of a 100 g sample of gritcollected from the gritting lorry.

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2.3. Roadside measurements

Particle concentrations in the air at near road-surface level were measured in eight particle sizechannels from 0.75 to 415mm every minute usingfour Grimm Model 1.105 light scattering particlecounters, G1–G4 located on the east and westpavements of Gloucester Place, at Sites 1–4,respectively.

To assist the interpretation of these measure-ments of the dust tracer after it became airborne, acombination of gas tracer measurements were madesimultaneously in the same area using knownreleases of sulphur hexafluoride (SF6) as well asthe continuous emissions of fine particles andcarbon dioxide (CO2) from the vehicle exhaust.Two independent releases of SF6 (Claiborn et al.,1995; Kantamaneni et al., 1996) were performed toquantify the average dispersion relation overdistances 0–100m. Each involved the constant flowrelease of SF6 at point X1 or X2. A number ofpaired receptors, S1–S6, were placed at distancesaway from the source and time integrated sampleswere taken at street level using Tedlar bags (Fig. 1).Samples were analysed in triplicate on site using aHewlett Packard 6890 Gas Chromatograph withElectron Capture Detection. A dilution ratio at eachof the receptor points was calculated and thecrossroad gradients were determined.

Two P-Trak Model 8525 butanol supersaturationultrafine particle counters (NYSERDA, 2002; TSI,2004), P2 and P4, deployed at Sites 1 and 3,respectively, measuring at 1Hz, captured time-varying levels of particulate matter in the size range0.02 to 1 mm which is expected to be dominated byexhaust emissions. Concentrations of CO2 abovethe local background, also attributable to vehicleexhaust emissions, were measured every 2 s at Site 3by infra-red absorption using a battery-poweredsensor with 1

8s response time and �2% precision,

constructed in-house from components supplied byEdinburgh Instruments. Changes in the ratio of dustto ultrafine particles and CO2 in roadside airthroughout the experiment may therefore be attrib-uted to the rise and decay times of dust levels on theroad after the deposition of the grit.

Ultrasonic anemometers (Gill Scientific Instru-ments, Research Grade 3-axis type) recorded street-level (at Site 1) and rooftop (on WestminsterCouncil House at Site 5) wind speed and directionat a frequency of 20.83Hz. Traffic flow onGloucester Place was measured every 96 s by

collecting data from the SCOOT (Split Cycle andOffset Optimisation Technique) induction loopcounters (Arnold et al., 2004; Hunt et al., 1982;Tate and Bell, 2002), calibrated by local manualcounts during the days prior to the dust releaseexperiment, and supplemented by occasional ap-proximate manual counts of numbers of vehiclespassing per traffic signal cycle on the day.

The Grimm and P-Trak monitors were factorycalibrated before deployment. In addition, the fourGrimms were run side-by-side in Gloucester Placeimmediately at the end of the measurement periodto ensure identical particle counts within the limit ofcounting statistics. All datalogging clocks weresynchronised using GPS or radio controlled clocks.Further details of the DAPPLE quality controlprocedures are outlined by Arnold et al. (2004).

3. Results

3.1. General condition

At the start of experiment, the sky was slightlyovercast. With brief sunny spells, the weather stayeddry until shortly after the experiment was finished.Fig. 2 shows wind directions at the Gloucester Placeduring the period of study. At the roof level, thepredominant wind direction was from NE to SE(across the Gloucester Place from the East to theWest) with the mean wind direction of 174�. Winddirections at street level, measured close to thefacade of WCC, were largely from SSE-SSW (meanwind direction of 91�), a northernly channelled flowalong Gloucester Place in the direction of trafficflow with a significant cross-street component. Themean, mode and 90th percentile of the roof-levelwind speed were 1.8, 1.4 and 3:2m s�1, respectively.The corresponding values for street-level wind speedwere 1.6, 1.6 and 2:6m s�1 (Fig. 3a). Wind speed atstreet-level was intermittent and was characterisedby periods of low speed, especially when the trafficwas not moving. On average street-level windspeed was 70% of the roof-level wind speed.Traffic on Gloucester Place consisted of a combina-tion of motor cycles, cars, taxis, light goods vehicles(LGVs), heavy goods vehicles (HGVs), busesand coaches. The average traffic flow was 4800vehicles per hour (1600 per lane) (Fig. 3b), buses/coaches and HGVs having a share of 10% of thetotal flow. Traffic flow was not continuous becauseof two traffic lights that influenced the flow oftraffic within the experimental site. The average

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90 8070

290280

270260250

120110

290280270

260250

120110

(a) (b)

Fig. 2. Wind directions at Gloucester Place: (a) wind direction at roof level; and (b) wind direction at street level (arrow indicates mean

wind direction. See Fig. 1 for orientation of u along 0� and v along 90�).

durations of the traffic lights were 42 s of greenphase and 47 s of red phase.

3.2. Qualitative observations of grit movement

The initial movement of dust was much fasterthan expected, followed by a slower decay and finalaccumulation of mostly coarser material along theEast side of the carriageway. A visible plume of dustfollowed the vehicle. After about an hour, clearbuild-up of particulate at the edge of the road anddeposition of grit on the pavements were seen. Afterone and half hours, no appreciable grit on roadsurface was seen, indicating majority of the resus-pendible grit being removed from the road surfaceby that time. This was also corroborated by the factthat no visual plume was now seen after any lightvehicle pass. However when the buses and coachespassed over the gritted segment, visible plumes werestill observed. This may be the grit locked within theinterstices on the road surfaces which were difficultto entrain into the air due to small turbulencecreated by the movement of light vehicles, but gotentrained into the air by the movement of vehicleswith more turbulent vehicle wake coupled withgreater disturbance of the road surface by theweight of the vehicle. Similar observations havebeen reported from earlier studies (AQEG, 2004;Nicholson and Branson, 1990; Sehmel, 1973, 1976).Larger vehicles also had tyre tracks closer to the

kerb where accumulation of dust was most persis-tent and therefore, suspended this more frequently.After about 3 h, the road surface appeared clean,except for a small sand dune like feature along thewindward facing kerb indicating a process thatcaused flow of grit across the road.

3.3. Roadside observation of exhaust and non-

exhaust emissions

Fig. 3 shows the variability in wind speed andtraffic flow compared with variability in roadsideCO2 and fine particle concentrations that areexpected to be associated with exhaust emissions.Only the particles 0:75 to 1 mm measured in thesmallest size channel of the Grimm (Fig. 3d) show aweak response to the application of grit at 11:15.Fig. 4 shows in more detail a period of time whenthe P-Trak, Grimm and CO2 monitors were allworking simultaneously, indicating how it is diffi-cult to relate individual peaks to specific wind speedor traffic flow changes, but that the generallyhomogeneous noise in the data are all similar whenthe different averaging times of the various mea-surements are taken into account. Short-termemissions peaks associated with individual vehiclesare discussed in more detail by Kaur et al. (2006),while the slower variation in emissions caused bythe cyclic variation in traffic movement under trafficsignal control is considered by Wang et al. (2005)

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-1) G1(0.75-1.0)

G2(0.75-1.0)

Fig. 3. (a) Wind speed; (b) traffic flow rate; (c) CO2 concentration; and (d) concentration of PM0:75�1.

A. Patra et al. / Atmospheric Environment ] (]]]]) ]]]–]]]6

who note that changes in wind direction in the streetcanyons have a similar timescale to the changes inemissions. We therefore will not consider thesecomplex flow and emissions variations here. Moresignificantly, quantitative comparison of the SF6

tracer release with the traffic flow and CO2

measurements, taking averages over the duration

of the tracer sampling, is broadly consistent withexpected CO2 emissions factors for the GloucesterPlace vehicle fleet over an appropriate drive cycle.

Fig. 5 shows that the progressively coarserparticle size fractions counted in the higher channelsof the Grimm monitor show a very differentpattern, with increasing response to the gritting

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200000

400000

600000

800000

fine p

counts

Ultrafines Traffic

PM0.75-1 Traffic

CO2 Traffic

Fig. 4. (a) Traffic and CO2; (b) traffic and ultrafine particles; and (c) traffic and PM0:75�1.

event with increasing particle size. There was also arise in background levels of mainly PM2:5 fraction inthe latter part of the experiment that is largelyobscured by the local exhaust contribution to thefinest particles (Fig. 3d) but seen most clearly in the1 to 3:5 mm range (Figs. 5a and b), consistent withdata from two nearby London urban backgroundmonitoring sites. But most significantly, immedi-ately after gritting, the levels of particles in air atSite 2 increased as follows: 0.75–1mm by a factor of1.5, 1–2mm by a factor of 1.5, 2–3:5mm by a factorof 2.5, 3.5–5mm by a factor of 6.5, 5–7:5 mm by a

factor of 9, 7.5–10 mm by a factor of 13, 10–15mmby a factor of 15 and 415mm by a factor of 30.After some time, it was observed that grit hadmoved along the road in the direction of traffic. Theleading edge c–d had moved to a new position C–D,relatively more distance than the movement oftrailing edge, which moved from a–b to A–B(Fig. 1). Irrespective of particle sizes, the particulatematter level at Site 2 had come down to itspregritting level within the period of experiment.However, varying rates of decay for differentparticle sizes are observed. Particulate matter larger

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180010:5

Time10:5

70011:25

90011:25

Increase due to rise

in background level

40011:25

25011:25

(a) (b)

(d)(c)

(e) (f)

Fig. 5. Decay rates of particulate matter level as a function of particle size.

than 2mm follows an exponential decay pattern(Figs. 5b–f) of the following form:

mt ¼ m0e�t=T0 , (1)

where, mt is the amount of material on road at time t

(kg), m0 is the amount of material on road at the endof the gritting (kg) and T0 is the time taken for thematerial on the road to decay by a factor of e�1 (s).

It takes about an hour since the end of gritting forparticles with size 415mm to come down to itspregritting level. This time increases with decreasingparticle size until particles with size 2 to 3:5mmtaking about 1 h and 30min (Fig. 6a). Therefore, therate of decay increases with increasing size ofparticulate matter; T0 decreases from 15min for 2to 3:5mm particles to 9min for 415mm particles

(Fig. 6b). Some of these coarse particles might haveescaped to kerbs. For very fine particles up to 2mm,no clear decay pattern is observed at Site 2.However, for particles of this size range, differencesin patterns between Sites 1 and 2 are observed.While at Site 2, the particulate matter level camedown to pregritting level within 2.5 h, at Site 1, theywere at a level higher than the pregritting level by afactor of 2 to 2.5 until the end of the experiment at15:40 (Figs. 3d and 5a). This indicates grinding ofmaterial by traffic. With increase of particle size, theratio of first peak particulate matter levels immedi-ately after gritting at Sites 1 and 2 decreases from41 to o1. This again can be attributed to grindingof material on the road while they travel along theroad.

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These observations are from G1 and G2, follow-ing instrument failure of G3 and G4, although themeasurements of ultrafine particles at Sites 1 and 3suggest concentration of G3 and G4 may be a factorof 2 to 5 higher than G1 and G2, respectively.

3.4. Movement of material along the road

Fig. 7 shows that, irrespective of particle sizes, thepattern of material movement along the road,during the first 30min after gritting, was similar.Although gritting was over by 11:20, particulatematter levels recorded by both G2 and G1 were onthe decline or at same general level until 11:24,suggesting no effect of gritting on particulate matterlevel at road side air until that time. Most of thisdelay is expected since the gritting was carried outduring a point in the traffic signal sequence where a

2 - 3.5 3.5 - 5 5 - 7.5 7.5 - 1

1000.0

Particle size (μm

Fig. 6. (a) Decay of particulate matter level as a function of

few vehicle were moving on the Gloucester Place. At11:24, G2 recorded a sharp rise of particulate matterlevel while particulate matter level at Site 1 was yetto be influenced by gritting. This indicates that ittook less than 4min for the grit to become airborneand reach the pavement right across the gritted area.It is likely that air emissions of grit commencedearlier but wind at that point of time was blowingaway from the receptor causing some of this delayto happen. With a time delay of about 1min, at11:25, the effect of gritting was seen at Site 1 and atthe same time the particulate matter level at Site 2reached its maximum. Then, for the next 15minsince the first arrival of grit at Site 1, there followeda period of rapid material transfer along the roadfrom gritted section (abcd) to an initially ungrittedsection (dcCD), between Salisbury Place andMarylebone Road, along the direction of traffic

0 10 - 15 >15

2-3.5 μm

3.5-5 μm

5-7.5 μm

7.5-10 μm

10-15 μm

>15 μm

time; and (b) decay time as a function of particle size.

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11:2011:24 11:38

11:2511:29

(c) (d)

Fig. 7. Arrival times of airborne particulate matter downstream of initially gritted section of road.

movement. At about 11:38, the particulate matterlevel of G1 exceeded the general particulate matterlevel of G2, indicating much of the material beingmoved from the initially gritted section to makematerial on the new road segment (dcCD) morethan that of gritted section.

4. Quantitative parameterisation of the movement of

particulate matter

4.1. Flux along the road

Visual observation indicates involvement of twoprocesses for material movement along the road.While the vehicle wake may be a dominant processfor some movement of suspended particles, materialsticking to the outer surface of the wheels and thuscarried along the road is a more likely mechanismfor movement of material that is not alreadyairborne. From the time measurements describedin Section 3.4, different speeds with which material

moves along the road are estimated as follows:

uxmax ¼x1� 2

txmin, ð2Þ

ux avg ¼x1� 2

tx avg, ð3Þ

u0x ¼ux avg

nv, ð4Þ

where, uxmax is the maximum speed of materialmovement along the road by the whole fleet ofvehicles (m s�1), ux avg is the average speed ofmaterial movement along the road by the wholefleet of vehicles (m s�1), u0x is the average distance ofmaterial movement along the road by one vehicle(mv�1), x1� 2 is the distance between Sites 1 and 2(m), txmin is the minimum time elapsed between endof gritting and first arrival of grit at Site 1 (indicatedby first peak of Grimm 1) (s), tx avg is the timeelapsed between end of gritting and time whengeneral level of grit in air at Site 1 exceeded that atSite 2 (indicated by the peaks of Grimm 1

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consistently higher than the peaks of Grimm 2) (s)and nv is the traffic flow rate (v s�1).

Substituting measured values into Eqs. (2)–(4)gives an estimate of 11m s�1, 0:06m s�1 and0:05mv�1 for uxmax, ux avg and u0x, respectively.Using the traffic flow variability along the Glouce-ster Place (Fig. 3b), u0x has a range of0.03–0:09mv�1.

Therefore, material removal along the road isestimated as follows:

f x ¼f 0x

nvSlw¼

ux avg

nv l¼

u0xl, (5)

where, f x is the fraction of material removed from aroad segment along the road by one vehicle (v�1), f 0xis the amount of material removed from a roadsegment along the road by one vehicle (g v�1), F x isthe amount of material removed from a roadsegment along the road in unit time by the wholefleet of vehicles (g s�1), ux avg is the average speed ofmaterial movement along the road by the wholefleet of vehicles (m s�1), u0x is the average distance ofmaterial movement along the road by one vehicle(m v�1), nv is the traffic flow rate (v s�1), S is thesurface material loading on the road segment(gm�2), l is the length of the road segment (m)and w is the width of the road segment (m).

Substituting measured values into Eq. (5) gives anestimate of 3:9� 10�4 v�1 for f x. Using the time-series of traffic flow along the Gloucester Place, f x

has a range of 2:6� 10�4–7:1� 10�4 v�1.

4.2. Flux across the road

After about an hour of the gritting event, therewas a clear build-up of material along the kerb(Section 3.2). It stretched through the entire lengthof the gritted road segment. The build-up at thedownwind kerb was sufficient enough to make anapproximate visual estimate of dimensions as20mm wide and 2mm deep. Without cross-streetwind, build-up would have been seen on both sidesof the road, but in the presence of cross-street windwe observed all the material on the downwind side.Therefore, material removal across the road hasbeen estimated as follows:

f y ¼f 0y

nvSlw¼

tynvSw, (6)

where, f y is the fraction of material removed from aroad segment across the road by one vehicle (v�1),f 0y is the amount of material removed from a road

segment across the road by one vehicle (g v�1), F y isthe amount of material removed from a roadsegment across the road in unit time by the wholefleet of vehicles (g s�1), r is the particle density(gm�3), S is the surface material loading on theroad segment (gm�2), l is length of the materialbuild-up along the kerb (same as the length ofgritted segment) (m), w is the width of the roadsegment (m), wy is the width of the material build-up along the kerb (m), dy is the depth of thematerial build-up along the kerb (m), nv is the trafficflow rate (v s�1) and ty is the time taken for the kerb-side build-up to take place (s).

Substituting measured values into Eq. (6) gives anestimate of 5:0� 10�5 v�1 for f y. Using the time-series of traffic flow along the Gloucester Place, f y

has a range of 3:3� 10�5–9:1� 10�5 v�1. Largercross-road fluxes may arise where there is a strongercross-road component of the street-level wind.

4.3. Flux to air

A fraction of the material is lost from the roadsurface due to emission to air. The varying levels ofair concentration with time was recorded by the on-site Grimms and the dispersion coefficient isobtained from the tracer study. Therefore, emis-sions to air can be estimated as follows:

f z ¼f 0z

nvSlw¼

C � Clb

wnvS, (7)

where, f z is the fraction of material removed from aroad segment to air by one vehicle (v�1), f 0z is theamount of material removed from a road segmentto air by one vehicle (g v�1), F z is the amount ofmaterial removed from a road segment to air in unittime by the whole fleet of vehicles (g s�1), R is theresuspension flux (i.e., mass emitted from unit areaof road in unit time) (gm�2 s�1), S is the surfacematerial loading on the road segment (gm�2), nv isthe traffic flow rate (v s�1), C is the concentration ofparticulate matter at the receptor (gm�3), Clb is thelocal background concentration of particulatematter at the Gloucester Place (gm�3) (Clb is thelowest pre-gritting airborne particulate matter levelrecorded by the roadside Grimms on the day ofexperiment), w is the dispersion coefficient (sm�1), l

is the length of the road segment (m) and w is thewidth of the road segment (m).

Substituting measured values into Eq. (7) gives anestimate of 3:1� 10�4 v�1 for f z. Using the time-series of traffic flow along the Gloucester Place, f z

v.2006.10.070

ARTICLE IN PRESS

Table 1

Summary of flux estimates

Parameters Average value Range

f x 3:9� 10�4 v�1 2:6� 10�4 v�1–7:1� 10�4 v�1

f y 5:0� 10�5 v�1 3:3� 10�5 v�1–9:1� 10�5 v�1

f z 3:1� 10�4 v�1 2:1:� 10�4 v�1–5:4� 10�4 v�1

f 0x 1:2� 101 g v�1 8:1 g v�1–2:2� 101 g v�1

f 0y 1:6 g v�1 1:0 g v�1–2:8 g v�1

f 0z 9:6 g v�1 6:5 g v�1–1:7� 101 g v�1

Fx 1:6� 101 g s�1 –

Fy 2:0 g s�1 –

Fz 1:3� 101 g s�1 –

f x, f y and f z: fractions of material removed by one vehicle along

the road, across the road and to air; f 0x, f 0y and f 0z: amount of

material removed by one vehicle along the road, across the road

and to air; and Fx, Fy and Fz: amount of material removed by

the whole fleet of vehicles along the road, across the road and to

has a range of 2:1� 10�4–5:4� 10�4 v�1. Theresuspension flux R varies between 4 � 10�4 and10 � 10�4 gm�2 s�1 except immediately after grit-ting when it goes up by a factor of 102 of this range.The flux to air represents the ultimate removal ofthe material from the road surface to air; previouscycles of resuspension and deposition of material onroad surface have therefore been included in the fluxalong and across the road.

4.4. Summary of flux estimates

Estimates of fluxes (Table 1) show that with asingle vehicle pass the average amount of materiallost from a road segment in different directions areas follows: 0.04% along the road, 0.005% across theroad and 0.03% into the air. Therefore, the totalamount lost from a road segment after a singlevehicle pass is 0.075% of the material available onthe road surface at that instant.

Eqs. (8)–(11) summarise the relationship betweenmaterial emission and loss along and across theroad.

k1 ¼f z

f x þ f y þ f z

, ð8Þ

k2 ¼f y

, ð9Þ

k3 ¼f z

, ð10Þ

k4 ¼f z

f x þ f y

. ð11Þ

Substituting values for f x, f y and f z from Table 1 inEqs. (8)–(11), the average values of k1, k2, k3 and k4

are estimated as 0.4, 0.13, 0.8 and 0.7. The estimateindicates 40% of the material removed from theroad surface are due to resuspension. Materialremoved across the road is about 13% of thatremoved along the road. Amount of materialremoved through resuspension is 20% less thanthe amount removed along the road. The amount ofmaterial removed through resuspension is about70% that removed together through along-streetand across-street movement.

5. Discussion and conclusions

When a vehicle passes over the gritted surface, theparticulate matter level recorded by Grimm in-creases by a factor between 5 and 30 for differentparticle sizes, indicating the process of resuspension.

Just after gritting very fast movement of materialalong the road is observed. Rate of movement slowsdown with time as the material available on theroad decreases. Grit thrown to the edge of the roadindicates a process of across-street movement ofmaterial. Coarse material becomes resuspendedmore easily than finer material. This agrees withobservation from earlier studies (Abu-Allaban etal., 2003a,b; AQEG, 2004; Barrowcliffe et al., 2002;Corn and Stein, 1965; Garland, 1983; Harrisonet al., 1997a; Lin et al., 1999; Manoli et al., 2002;Nicholson and Branson, 1990; Nicholson, 1993).Although strong dependence of resuspension onparticle size has largely been reported from studiesinvolving resuspension due to wind (Corn and Stein,1965; Garland, 1983; Harrison et al., 1997a),Nicholson and Branson (1990) have observed fromfield investigation that they are equally applicable totraffic-induced resuspension. Hinds (1982) attri-butes this to drag force increasing faster thanadhesive force with increasing particle diameterand Corn and Stein (1965) suggest protrusion oflarge particles further into the turbulent air streamas the reason. The exponential decay rates of coarsefractions with time are in contrast to the inverserelationship between resuspension and time pro-posed from theoretical (Reeks et al., 1988) as well asexperimental works (Garland, 1979; Nicholson,1993) on resuspension by wind. Traffic-inducedturbulence is generally stronger than wind-inducedturbulence in an urban street, except at times whenstrong prevailing and in-street air flows are present.

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Therefore it is not surprising that the exponentialdecay rates of material, consistent with observationsreported from some earlier works on wind-inducedresuspension (Anspaugh et al., 1975; Linsey, 1978),are seen in this case. Fine fractions up to 2mmremained on road for a longer time, indicated byGrimm data that showed their level above the localbackground until the end of the experiment. Onereason for this is that these were not resuspendedeasily, as already discussed. The other reason isgrinding of coarser materials under wheels of trafficthat replenishes the fine material reservoir andtherefore, prohibits a distinct material decay patternto emerge. Other plausible evidence of grinding isdiscussed in Section 3.3.

The rate of movement of material along the roadin the direction of traffic flow was estimated byobserving the difference in arrival time of elevatedconcentrations of micrometer-sized particles inroadside air adjacent to and a short distancedownstream of a section of three-lane, one-waybuilding-lined street onto which rock salt wasapplied. Initial movement, dominated by particlesseveral micrometres in size and above, was0:11m s�1. After several minutes, slower movementof finer particulate matter was observed, at a speedof 0:06m s�1. Vehicle speeds were fairly typical forCentral London, moving at 25–50 kmh�1 during thegreen phase of the traffic lights, with queues formingwhen the lights were red. Assuming 20 kmh�1

(equivalent to 5:6m s�1) as the average speed ofvehicle along the Gloucester Place, the averagespeed of the material movement along the road isabout 1.1% of the vehicle speed and 3.8% of theaverage street-level wind speed. These speeds wereobserved at an average traffic flow of 4800 vehiclesh�1 (1600 vehicles per lane), and it is reasonable toassume the speed would be proportional to trafficflow on a similar road with fewer or a larger numberof vehicles per lane.

The rate of suspension of particulate matter intothe roadside air was estimated from the roadsideconcentrations in air. Point source tracer releasemeasurements and emissions factor ratio methodsproduce broadly consistent emissions estimates,giving grounds for confidence that the measure-ment, although approximate, is robust. The mostimportant new information provided by this experi-ment however, is that the flux of material to air isapproximately 40% of the total outgoing fluxes inall directions and 70% of the sum of the fluxes alongand across the road. The fluxes to air in Section 4.3

refer to PM10. Particles larger than PM10 may alsobecome suspended into the air, but these thenredeposit to the road surface. These have thereforebeen included in our calculations of along-street andacross-street movements, but not in PM10 emissionsto air.

The fluxes presented in Section 4.4 compare withthe studies by Sehmel (1973, 1976) that hadfollowed almost identical technique as ours andhave reported resuspension fractions of 10�5–10�2

(10�3%–1%) per vehicle on the same day of tracerapplication. Using ZnS tracer on an asphalt road,he estimated resuspension due to traffic from thedownwind concentration of tracer and the amountof loss of material from road. However thedifference is that while Sehmel (1973, 1976) attrib-uted the entire loss to resuspension, this studyidentifies them as a sum of losses along threedirections and suggests that amount resuspended isabout three-fourth of the material lost along theroad, and is several times higher than the amountlost across the road to the kerbs. Material lostalong the road is more than the combined loss dueto resuspension and material removed across theroad. Therefore this study gives a first estimate ofthe contributions of different processes that causeroad dust redistribution due to traffic on an urbanstreet.

The experiment was a pilot experiment of shortduration which we did not repeat due to the publicnuisance it created. The interpretation of resultsare subjected to the uncertainties involved in thistype of study, such as (a) uncertainty of trafficemission factors; (b) uncertainty associated withmeteorological data and (c) inherent uncertaintiesassociated with dispersion process. A long-termexperiment should be conducted with detailedmodelling to estimate and verify these parameters.More field measurements should be carried out toestimate the range of the parameters under differentstreet and meteorological condition, particularly atan air-flow perpendicular to the road or against theflow of traffic. In our case it was almost achannelled flow in the same direction as the trafficand therefore we anticipate our along-street move-ment of material may have been overestimatedwhich can be checked from an experiment withpredominant wind direction perpendicular to road.The across-street parameter could be different if theexperiment was conducted on a road without theeffect of street canyon. In that case sy could bemore. There is also uncertainty on how much has

v.2006.10.070

precisely gone to the kerb and in what time, becausethe deposits on the pavement may have partiallycome from the across-street component. Material ofdifferent mineralogical composition could allowcollection of on-road samples to improve theaccuracy of our initial estimates. Further studiesof this type are needed to develop a database ofrange of parameter values applicable for pavedroads with varying road geometry, traffic flow andcomposition, road condition and meteorologicalcondition.

Similar features can be observed with ordinarywinter gritting, but the importance of this controlledexperiment is the short length of road to whichgritting was applied, which allows the time-constants of decay of grit concentration to be usedto parameterise the movement of the particulatematter. The experiment showed that a model ofparticle emissions from paved road surfaces needsto be two-dimensional and dynamic, allowing forthe movement of material along and across theroad, and the characteristic times for build-upand removal of material. It is proposed that thisnot only will increase the physical basis of the modelbut also will reduce the gap between observationand model prediction, two main criticisms themodels that parameterise the emission only asinstantaneous silt loading and traffic parametersat the point of emission face (Countess et al., 2001;Fitz and Bufalino, 2002; Kantamaneni et al., 1996;Nicholson, 2001; Venkatram and Fitz, 1998;Venkatram et al., 1999; Venkatram, 2000, 2001;Zimmer et al., 1992). The development and evalua-tion of a simple mass-balance model to predictnon-exhaust paved road emissions based onthis approach is in progress and will be reportedlater.

Acknowledgements

The authors would like to thank the GreaterLondon Authority for funding this study as part ofthe DAPPLE, an EPSRC funded project (GrantReference: GR/R78183/01). We also thank theDAPPLE consortium, which is comprised of multi-disciplinary research groups from six U.K. univer-sities, for its contribution in field study and dataanalysis. We are grateful to Westminster CityCouncil Department of Highways Managementand Cleansing Management for the gritting event.A.K. Patra acknowledges receipt of a Common-wealth Scholarship.

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