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Atmospheric Environment 42 (2008) 8819–8826

Contents lists ava

Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Short communication

Treatment of losses of ultrafine aerosol particles in long samplingtubes during ambient measurements

Prashant Kumar a,b,*, Paul Fennell c, Jonathan Symonds d, Rex Britter a

a Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UKb School of Engineering, University of Surrey, Guildford GU2 7XH, UKc Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UKd Cambustion Limited, J6 The Paddocks, Cambridge CB1 8DH, UK

a r t i c l e i n f o

Article history:Received 19 June 2008Received in revised form 20 August 2008Accepted 3 September 2008

Keywords:Aerosol depositionParticle diffusion lossesParticle number distributionStreet canyonUltrafine particles

* Corresponding author. Department of EngineCambridge, Cambridge CB2 1PZ, UK. Tel.: þ44 1221223 332662.

E-mail addresses: pp286@cam.ac.uk, pp286@redifrb11@cam.ac.uk (R. Britter).

1352-2310/$ – see front matter � 2008 Elsevier Ltddoi:10.1016/j.atmosenv.2008.09.003

a b s t r a c t

Long sampling tubes are often required for particle measurements in street canyons. Thismay lead to significant losses of the number of ultrafine (those below 100 nm) particleswithin the sampling tubes. Inappropriate treatment of these losses may significantly changethe measured particle number distributions (PND), because most of the ambient particles,by number, exist in the ultrafine size range. Based on the Reynolds number (Re) in thesampling tubes, most studies treat the particle losses using the Gormley and Kennedylaminar flow model (Gormley, P.G., Kennedy, M., 1949. Diffusion from a stream followingthrough a cylinderical tube. Proceedings of Royal Irish Academy 52, 163–169.) or the Wellsand Chamberlain turbulent flow model (Wells, A.C., Chamberlain, A.C., 1967. Transport ofsmall particles to vertical surfaces. British Journal of Applied Physics 18, 1793–1799.). Ourexperiments used a particle spectrometer with various lengths (1.00, 5.47, 5.55, 8.90 and13.40 m) of sampling tube to measure the PNDs in the 5–2738 nm range. Experiments wereperformed under different operating conditions to measure the particle losses throughsilicone rubber tubes of circular cross-section (7.85 mm internal diameter). Sources ofparticles included emissions from an idling diesel engine car in a street canyon, emissionsfrom a burning candle and those from the generation of salt aerosols using a nebuliser in thelaboratory. Results showed that losses for particles below z20 nm were important and werelargest for the smallest size range (5–10 nm), but were modest for particles above z20 nm.In our experiments the laminar flow model did not reflect the observations for small Re. Thismay be due to the sampling tubes not being kept straight or other complications. In situcalibration or comparison appears to be required.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

In many studies of ambient aerosols, experimentalconditions force the use of long sampling tubes whencommonly deployed instruments such as the ScanningMobility Particle Sizers (SMPS), the Electrical Low Pressure

ering, University of3 332681; fax: þ44

fmail.com (P. Kumar),

. All rights reserved.

Impactor (ELPI), the Aerodynamic Particle Sizer (APS) or theDifferential Mobility Spectrometer (DMS500) are used formeasuring particle number distributions (PND) and/orparticle number concentrations (PNC). These studiesinclude a recent experimental campaign (Kumar et al.,2008a–d) where a fast response differential particle spec-trometer (DMS500) measured the PNDs at the roadside.Losses of ultrafine (those below 100 nm) particles insidethe sampling tubes were observed, with smaller particlessuffering greater losses. Correction for these losses (bynumber but not by mass) is critical since most of theambient particles, by number, exist in the size range (below

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–88268820

100 nm) where these losses are also the largest (Kumaret al., 2008b). Based on the Reynolds number in thesampling line, several studies (Agus et al., 2007; Kumaret al. 2008a,c; Lingard et al., 2006) treat particle losses instraight tubes using either the Gormley and Kennedy(1949) laminar model or the Wells and Chamberlain’s(1967) turbulent model. Unfortunately, only a few studiesdiscuss the importance of such losses (Noble et al., 2005;Symonds et al., 2007a; Wang et al., 2002). Noble et al.(2005) measured particles in a continuous field measure-ment for laminar flow conditions in sampling tubes usinga SMPS and an APS through a 3 m long aluminium tube(2.54 cm internal diameter) that was vertically orientedwith slight bends for connections to the sampling instru-ments. They reported that the penetration efficiency wassubstantially smaller than the theoretical penetration effi-ciency for both ultrafine and fine particles. Similarly,Symonds et al. (2007a) reported that the penetration effi-ciency of particles in the 5–100 nm range for samplingtubes up to 25 m long, made of silicone rubber, was muchcloser to the turbulent flow model even though the Rey-nolds number indicated laminar flow. Wang et al. (2002)reported the effect of bends and elbows on diffusion ofmono disperse particles (5–15 nm) for a range of Reynoldsnumbers (80–950). They concluded that in a flow passagewith four elbows, each having 90� bends, the penetrationefficiency was up to 44% smaller than for a straight tube ofsame length. However, particle losses and their appropriatetreatment for different length of sampling tubes, as wouldtypically be used in ambient aerosol studies, need furtherattention.

In this study, a DMS500, with a four-way switchingsystem, measured the PNDs in a broad (5–2738 nm) sizerange, pseudo-simultaneously. Various lengths of samplingtubes, between 1 and 14 m, made of silicone rubber, having7.85 mm internal diameter (i.d.), were used to measure theparticle penetration efficiencies. Three different andcontinuous sources of particles were used: an idling diesel-engined car in a street canyon, a burning candle and saltaerosols from a nebuliser (PARI LCþ) in the laboratory.

The aim of the study was to investigate particles lossesin sampling tubes of various lengths under different oper-ating conditions of the DMS500. The experimental resultsare compared with particle loss models for laminar andturbulent flow and it is shown that such losses areextremely important for the ultrafine particles that aredominant (by number) in the urban environment (Kumaret al., 2007, 2008a–d; Longley et al., 2003).

2. Methodology

2.1. Theories of particle losses in sampling line

There are five main mechanisms which may lead toparticle losses on to the surface of a sampling tube; these aresedimentation (gravitational), thermophoresis, electro-static, inertial impaction and diffusion (Friedlander, 2000;Hinds, 1999; Ketzel and Berkowicz, 2004). Of all potentiallosses, those due to diffusion and inertial impaction aremost important for ambient particle measurements(Hinds, 1999). The second of these is only important under

turbulent flow conditions and for particles larger than100 nm (Lee and Gieseke, 1994). Gormley and Kennedy(1949) first derived the equation for diffusional losses ina fully developed laminar flow (Reynolds number,Re< 2300) through a tube of circular cross section (diameterdt) with a uniform inlet PNC, as a function of a dimensionlessdeposition parameter b ¼ 4DL=pd2

t U, where D is thediffusion coefficient (see supplementary Section S.1 fordetails), L is the tube length and U is the average flowvelocity through the tube. A simplified version of a morecomplicated and more accurate expression (Hinds, 1999),gives penetration efficiency P, which is the fraction ofentering particles (Nin) that exit (Nout) through a tube, withan accuracy of 1% for all values of b to be:

P¼Nout=Nin¼0:819e�11:5bþ0:0975e�70:1b for b�0:009 (1a)

P ¼ Nout=Nin ¼ 1�5:50b2=3þ3:77b for b< 0:009 (1b)

For straight sampling tubes, Ramamurthi et al. (1990)confirmed the accuracy of the above equations during theirstudy that used radioactive 218PoOx aerosol clustersthrough a 2.2 cm i.d. tube of various lengths (i.e., 88, 205and 317 mm). Their work was supported by Alonso et al.(1997) for particles down to 2 nm diameter by analysis ofthe penetration efficiencies of nanometre-sized aerosolparticles through a plastic tube of 10 mm i.d.

Under turbulent pipe flow conditions (Re> 2300), thedeposition onto tube surfaces is more complicated andequations describing it cannot be solved explicitly.Assuming that turbulent flow provides a constantconcentration everywhere beyond a thin boundary layernext to the surface of the tube walls where the flow islaminar and the concentration decreases linearly fromconstant to zero at the surface walls, Wells and Chamber-lain (1967) gave an expression for diffusive depositionvelocity ðVdÞ through the laminar sub-layer for turbulentflow in a tube as:

Vd ¼0:04U

Re1=4

�rgD

h

�2=3

(2)

where rg and h are the density and the viscosity of fluidpassing through the tube. Using the deposition velocity (asgiven in Eq. (2)), Lee and Gieseke (1994) presented anempirical equation for the overall penetration througha tube of length L subjected to losses to the walls bydiffusion and inertial impaction from turbulent flow as:

P ¼ Nout

Nin¼ exp

��4VdL

dtU

�(3)

Eq. (3) also includes inertial deposition but this is onlysignificant for particles larger than around 100 nm.

2.2. Instrumentation

A four-way solenoid switching system was used witha fast response differential mobility spectrometer (Cam-bustion DMS500) to measure the particle number and sizedistributions in 5–2738 nm size range through differentlengths of tube pseudo-simultaneously. Cylindrical sampling

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–8826 8821

tubes of 7.85 mm i.d. and various lengths, Lbase (1.00 m), L1

(5.47 m), L2 (5.55 m), L3 (8.90 m) and L4 (13.40 m), were usedto analyse the particle losses inside the electrically conduc-tive sampling tubes that were made of silicone rubber. Asseen in Fig. 1, there were two parts to the switching system,namely a stainless steel manifold having one inlet and fouroutlets, and a four-way solenoid switching system. A steelfunnel was fixed at the head of steel manifold inlet. Theoutlets of the manifold were connected to cyclones by smallpieces of rubber sampling tubes. The cyclones were thenconnected to the four-way switching system by anotherpiece of silicone rubber sampling tube; the length of thesesmall connecting sections is included in above-mentionedtube lengths Lbase, L1, L2, L3 and L4. Finally, the four-wayswitching system was connected to the DMS500, as seen inFig. 1. The cyclones prevent large particles entering thesampling tube and the instrument. All the sampling tubeswere laid horizontally on the ground but small bends werepresent due to the difference in height between the instru-ment and the emission sources of particles.

The DMS500 used is capable of measurements over twosize ranges (i.e., 5–1000 nm and 5–2738 nm). Each of theseranges requires different set-points for the instrument’sinternal flows, voltages and pressure. The use of these tworanges in these experiments enabled two different sampleflow rates and pressures to be examined (discussed in detailin Section 3.2). To measure the PNDs in the 5–1000 nmrange, steel restrictors with holes of z1.00 mm i.d. wereplaced upstream of the cyclones to maintain a flow rate of 8standard litres/min (slpm) and a pressure of 0.25 bar (thesame as the instrument’s classification column) inside thesampling tube. However, the instrument could also be usedto measure PNDs in the 5–2738 nm range, when restrictorsof 0.5 mm i.d. were substituted for the 1.00 mm ones,maintaining a flow rate of 2.5 slpm and pressure 0.16 barinside the sampling tube (the classifier operates at lowerpressure to achieve the increased electrical mobility sizerange). The orifice plates (a) reduce the pressure inside thesampling tubes hence reducing the likelihood of particleagglomeration, (b) set the sample flow rate to that required

Diesel Car Salt aerosols Candle

L4 L2Lbase/L1

L3

Cyclone with Restrictors

Stainless steel mani

7.85 mm internal diaconductive rubber samp

Manifold Inlet

Fig. 1. Schematic diagram of experimental setup. Lbase/L1, L2, L3 and L4 are total lenbetween the four-way switching system and the DMS500 was 0.30 m.

by the instrument, and (c) improve the time response of theinstrument.

2.3. Measurements

Three different sources for nearly steady-state emis-sions were selected to measure the particle losses throughthe different lengths of sampling tubes. Exhaust emissionsfrom a stationary diesel engine car at idle (Model: Rover 25TD) in a street canyon (500 mm from the tail pipe)mimicked real field (i.e., operational) conditions. To verifythese results, further experiments were replicated in thelaboratory (i.e., controlled) conditions using particle emis-sions from a burning tea light candle and salt aerosolsgenerated by a nebuliser (Pari LCþ) at two different pres-sures (0.5 and 2.0 bar) to change the aerosol concentration.The details of the use of this nebuliser to generate saltaerosols have been described elsewhere (Fennell et al.,2007). The tea light candle consisted circular plugs ofparaffin wax (37 mm diameter and 15 mm high) in a thickaluminium casing (Daeid and Thain, 2002). The DMS500measures particles based on electrical mobility equivalentdiameter (hereafter called as Dp) that implicitly takes intoaccount the characteristics of aerosols (i.e., shape, surfacearea, charge etc.). The different types of particle sourceswere found not to influence the particle losses in this study,as is discussed later in Section 3.2 (Figs. 3–5) where experi-mental data from all sources show similar penetration effi-ciency for a particular particle diameter. This is because theequations governing losses of particles (see Section S.1 insupplementary material) are based on the aerodynamicmobility diameter, which is very close to Dp, so that differ-ences between different aerosols are explicit in the models.Measurements were taken at a sampling frequency of 0.5 Hz,rather than the maximal frequency of 10 Hz to improve thesignal-to-noise ratio, continuously for 1-min in each tubelength. These were repeated for four complete cycles (i.e.4�1-min measurements for each tube length taken on fourdifferent occasions). Further details of the experiment andthe operating conditions are shown in Table 1.

4-Way(Solenoid)SwitchingSystem

DMS500

COMPUTER(Data

Logging)

fold system

meterling tubes

(Figure not to Scale)

gth of sampling tubes between the manifold and DMS500; the tube length

Table 1Summary of experimental and operating conditions

Cases Aerosol source Place of sampling Length of samplingtubes used

Sampling conditions

Car1 Diesel car 50 cm away from exhaust tail pipe Lbase Particle size range 5–2738 nm,Sample line pressure 160 mbAmbient temperature 8.2 �CSample flow rate 2.5 slpmReynolds number (Re)¼ 461

Car2 Diesel car 50 cm away from exhaust tail pipe L1, L2, L3 and L4

Car3 Diesel car At the exit (i.e., 10 cm awayfrom exhaust tail pipe)

Lbase, L2, L3 and L4

Cand Candle Candle flame 15 cm belowthe manifold inlet

Lbase, L2, L3 and L4 Particle size range 5–2738 nmSample line pressure 160 mbIndoor temperature 26 �CSample flow rate 2.5 slpmRe¼ 461

Salt1 NebulisedNaCl(aq)

Generated at 2 bar pressure Lbase, L2, L3 and L4

Salt2 NebulisedNaCl(aq)

Generated at 2 bar pressure Lbase, L2, L3 and L4 Particle size range 5–1000 nmSample line pressure 250 mbIndoor temperature 26 �CSample flow rate 8 slpmRe¼ 1409

Salt3 NebulisedNaCl(aq)

Generated at 0.5 bar pressure Lbase, L2, L3 and L4

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–88268822

3. Results and discussion

3.1. Effect of tube length on particle number distributions

To derive the losses in the sampling tubes, it is assumedthat the losses in Lbase are equivalent to the losses in thefirst metre of each of other four tubes, and subsequentlosses are determined in each tube relative to their ‘‘cor-rected’’ length (i.e., their actual length minus 1 m). The sizedependent penetration efficiencies for the effective lengthof different tubes were then defined as the numberconcentration through the effective lengths L1, L2, L3 and L4

of tube divided by the number concentration penetratingLbase. Fig. 2 shows the effect of various length of samplingtubes on particulate emissions from a diesel-engined car.As the tube lengths increase from Lbase to L4, the magnitudeof the PNDs decreased. The PNDs in smaller size range(below z20 nm) showed larger changes, indicating rela-tively greater losses of particles with increased length ofsampling tubes. However, the PNDs above z20 nm showednegligible changes, indicating modest particle loss. The

1 10 100 1000

Lbase

L1

L2

L3

L4

Dp (nm)

X 108

0

2

4

6

8

dN/d

logD

p (c

m-3

)

Fig. 2. Effect of various lengths of sampling tubes on a typical diesel carexhaust emissions; figure represents cases Car1 and Car2.

PNCs, which were obtained by integrating the PND profilesseen in Fig. 2, decreased about 3, 7, 28 and 32% for L1, L2, L3

and L4, respectively, with reference to Lbase. It should benoted that the majority of the decrease in PNC was forparticles below z20 nm where the maximum losses arealso expected due to the higher diffusivity of smallerparticles. For example, penetration efficiencies for L4 wereobserved to be z10% for 5 nm diameter, z40% for 10 nmdiameter, and well above 60% for particles in the 10–20 nmsize range. Other sources also showed similar trend forpenetration efficiencies, as can be seen from Figs. 3–5.

3.2. Effect of operating conditions on the size-dependentpenetration of particles in various tube lengths

Size-dependent penetration efficiencies of particles inL2, L3 and L4 for all cases described in Table 1 are shown inFigs. 3–5, respectively. The penetration efficiencies shownin these figures are estimated based on the PNCs whichwere averaged over the time period of four complete cyclesas mentioned in Section 2.2. Particle emissions fromvarious sources were nearly in steady-state conditionssince the geometric mean diameter with standard devia-tion over the averaged periods were 13.7� 0.7, 12.5� 0.5and 10.2� 0.8 nm for car emissions, salt aerosols andcandle emissions, respectively. Tube length L1 is notconsidered for further analysis since L1 z L2. The results forCar2 and Car3 are generally similar with the penetrationefficiencies being similar to the theoretical ‘‘turbulent’’ flowmodel except for Car3 with the longest sampling tube andfor the larger particles. The candle results for all threelengths of sampling tubes also produce a similar result tothe Car2 and Car3.

Sensitivity analysis of Eqs. (1)–(3) suggest that apartfrom the length of the sampling tubes, their diameter,sample flow rate or mean velocity (since both are linked),sample line pressure and temperature are variablesaffecting the particle losses in the sample line. The effectof different tube diameters on penetration efficienciescould not be seen because our experiments used onlya fixed diameter tube. The effect of sample line tempera-ture (assumed to be equal to ambient temperature) on

0

20

40

60

80

100

10

Lam (Car2, Car3)Turb (Car2, Car3)

Car2Car3

0

20

40

60

80

100

1 10 100

Lam (Salt2, Salt3)

Turb (Salt2, Salt3)

Salt2

Salt3

a

c

Lam (Cand, Salt1)Turb (Cand, Salt1)

CandSalt1

b

1 10 100

Lam (avearge)

Turb (average)

Average(Car2, Car3, Cand, Salt1)

d

Dp (nm)

Pen

etra

tion

(

)

Fig. 3. Size dependent measured and modelled penetration in sampling tube L2 for (a) Car2 and Car3 (P¼ 160 mb, Ta¼ 8.2 �C, Q¼ 2.5 slpm, Re¼ 461), (b) Candand Salt1 (P¼ 160 mb, Ta¼ 26 �C, Q¼ 2.5 slpm, Re¼ 461), (c) Salt2 and Salt3 (P¼ 250 mb, Ta¼ 26 �C, Q¼ 8 slpm, Re¼ 1409), and (d) average of (a) and(b). Acronyms P, Ta, Q and Re represent the sample line pressure, ambient temperature, sample flow rate and Reynolds number, respectively. Further descriptionsof all cases plotted in Figs. a–d are described in Table 1.

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–8826 8823

penetration efficiencies seem to be modest for ourconditions. This can be seen by the comparison of Figs. 3a,4a and 5a (for 8.2 �C) with Figs. 3b, 4b and 5c (for 26 �C),respectively, where all other conditions were similarexcept temperature (see Table 1). For instance, the averageof the modelled penetration efficiency in the 5–100 nmsize range for L2 showed fractional changes; z89% for8.2 �C as compared with z88.8% for 26 �C, as seen inFig. 3a and b. However, this difference was larger forexperimental results where the average penetration effi-ciencies were z78% for 8.2 �C as compared with z73% for26 �C.

When using the DMS500, the sample line pressure andthe sample flow rate are not independent variables, due tothe requirement for 8 slpm when 0.25 bars is used, or2.5 slpm with 0.16 bars. Considering our experimentalconditions (as explained in Table 1 for different cases) thesample flow rate was identical (2.5 slpm) for the first fourcases, and these cases are averaged and shown in part (d) ofeach of Figs. 3–5 whereas this was 8 slpm for the last twocases, as seen in Table 1 and in part (c) of Figs. 3–5.Comparison of Figs. 3c, 4c and 5c with Figs. 3d, 4d and 5d,respectively, indicate the effect of change in the sampleflow rate on the measured and modelled results. Asexpected from the sensitivity analysis, the penetration

efficiencies for modelled results for all lengths of samplingtubes were consistently larger for 8 slpm sample flow rate(Figs. 3c, 4c and 5c) than for 2.5 slpm sample flow rate (Figs.3d, 4d and 5d) since the diffusive deposition velocity, asseen in Eq. (2), increases considerably with increasedsample flow rate. For example, the average of the modelledpenetration efficiency in the 5–100 nm size range for L2

during laminar flow conditions was z93% for 8 slpm ascompared with z88% for 2.5 slpm, as seen in Fig. 3c and d.Surprisingly, this effect was not clearly distinguishable onthe measured penetration efficiencies which were muchsmaller (and nearly similar for both sample flow rates) thanthose for modelled penetration efficiencies for laminar flowconditions; z74% for 8 slpm as compared with z73% for2.5 slpm. Moreover, the average of the measured penetra-tion efficiencies in the 5–100 nm size range was found to becloser to those for the turbulent flow model, where thesewere z60% for 8 slpm than z54% for 2.5 slpm. The reasonsfor this are discussed in Section 3.3. Similar observationswere found for other lengths (L3 and L4) of the samplingtubes, as can be seen from Figs. 4 and 5.

Further investigation of the measured results showedthat particle losses are very significant between 5 and10 nm for all cases (Figs. 3–5); these are as high as z90% for5 nm particles and z60% for 10 nm diameter. There is

1 10 100

Lam (avearge)

Turb (average)Average (Car2, Car3, Cand, Salt1)

0

20

40

60

80

100Lam (Car2, Car3)Turb (Car2, Car3)Car2Car3

Dp (nm)

a

d

0

20

40

60

80

100

1 10 100

Lam (Salt2, Salt3)Turb (Salt2, Salt3)Salt2Salt3

c

Lam (Cand, Salt1)Turb (Cand, Salt1)CandSalt1

b

Pen

etra

tion

(

)

Fig. 4. Size dependent measured and modelled penetration in sampling tube L3 for (a) Car2 and Car3 (P¼ 160 mb, Ta¼ 8.2 �C, Q¼ 2.5 slpm, Re¼ 461), (b) Candand Salt1 (P¼ 160 mb, Ta¼ 26 �C, Q¼ 2.5 slpm, Re¼ 461), (c) Salt2 and Salt3 (P¼ 250 mb, Ta¼ 26 �C, Q¼ 8 slpm, Re¼ 1409), and (d) average of (a) and (b). Furtherdescriptions of all cases plotted in Figs. a–d are described in Table 1.

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–88268824

a sharp rise in penetration efficiency between 10 nm and20 nm diameter. Above 20 nm diameter particle losses aremodest.

3.3. Comparison of experimental results with laminar andturbulent flow diffusion models

Particle losses for various cases are compared belowwith diffusion models for laminar (Hinds, 1999) andturbulent (Wells and Chamberlain, 1967) flow, which arerepresented by Eqs. (1) and (3), respectively. As shown inTable 1, flow conditions in the sampling tubes were laminar(Re¼ 461 and 1409) during all the experiments. For allcases, particle losses were substantially larger for theexperimental results than for modelled results calculatedfor laminar flow, as seen in Figs. 3–5. Interestingly, all thecases presented in Figs. 3–5 for L2, L3 and L4, respectively,show that measured penetration efficiencies for particlesbelow z20 nm diameters are generally closer to themodelled diffusion losses for turbulent flow. Conversely,the measured penetration efficiencies for particles abovez20 nm indicated modest particle losses, but part (d) ofFigs. 3–5, interestingly, shows that the measured penetra-tion efficiencies are closer to the modelled penetrationefficiencies for the laminar flow; no clear explanation forthis similarity was found. Particle losses below z20 nmonly are discussed further in this article.

As discussed in Section 2.1, studies for short and straighttubes (Alonso et al., 1997; Ramamurthi et al., 1990)confirmed the laminar flow model under conditionscommonly found within aerosol instruments. Conversely,when relatively longer sampling tubes are used, particlelosses where laminar flow is expected have been found tobe considerably larger than the modelled losses for laminarflow conditions (Wang et al., 2002; Noble et al., 2005;Symonds et al., 2007b), as is confirmed by our brief study.The present study tested these findings for various oper-ating conditions and for different lengths of sampling tubesranging from 1 to 14 m, as described in Table 1. It might beexpected that losses greater than those which are predictedby the model could result from other mechanisms such asin the connections to the switching system. However, thesecan be neglected here since they are common to allsampling tubes.

Prediction of particle penetration efficiencies is notstraightforward since they depend on the complex flowfield inside the tubes. It is probable that non-straightness(or small bends) in the sampling tubes will producesecondary flows, and intermittent enhanced movement ofthe particles towards the sampling tube walls. These flowconditions can also occur due to any roughness in theinner-walls of the sampling line; however, this effect willbe relatively small and can be ignored (Pope, 2003). Theevidence of the presence of secondary flow can be

1 10 100

Pene

trat

ion

(%)

Lam (avearge)

Turb (average)Average (Car2, Car3, Cand, Salt1)

Lam (Cand, Salt1)Turb (Cand, Salt1)CandSalt1

0

20

40

60

80

100

0

20

40

60

80

100

Lam (Car2, Car3)Turb (Car2, Car3)Car2Car3

Dp (nm)

a

d

1 10 100

Lam (Salt2, Salt3)Turb (Salt2, Salt3)Salt2Salt3

c

b

Pen

etra

tion

(

)

Fig. 5. Size dependent measured and modelled penetration in sampling tube L4 for (a) Car2 and Car3 (P¼ 160 mb, Ta¼ 8.2 �C, Q¼ 2.5 slpm, Re¼ 461), (b) Candand Salt1 (P¼ 160 mb, Ta¼ 26 �C, Q¼ 2.5 slpm, Re¼ 461), (c) Salt2 and Salt3 (P¼ 250 mb, Ta¼ 26 �C, Q¼ 8 slpm, Re¼ 1409), and (d) average of (a) and (b). Furtherdescriptions of all cases plotted in Figs. a–d are described in Table 1.

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–8826 8825

supported by arguments based on the Dean number (De).The strength of secondary flow produced by flow througha smooth bend of radius R can be represented by De¼ Re(dt/2R)0.5; where dtis the tube diameter (Pui et al., 1987). Anincrease in De will increase the strength of the secondaryflow (Tsai and Pui, 1990), suggesting that for a fixed tubediameter an increase in angle of bend decreases the bendradius and increases De. For example increasing the angleof bend from 45� to 90� reduces the radius of bend by z31%and increases De by z13%. This suggests that an increase inangle of bend could result in much smaller particle pene-tration efficiencies than expected during laminar flowconditions. Furthermore, change in diameter of thesampling tubes is equally important for changing the De.For example, an increase in tube diameter from 7.85 mmto 10 mm (z22%) decreases Re by z22%, resulting ina decrease of z11% in De; however our experiments werelimited to sampling tubes of fixed diameter.

The presence of secondary flow in sampling tubes isfurther substantiated by the fact that the measured pene-tration efficiencies are closer to the modelled penetrationefficiencies from Wells and Chamberlain’s (1967) expres-sion (Eq. (3)) that takes in to account the diffusion ofparticles under turbulent flow conditions.

The above arguments suggest that an individual lay-outand diameter of a longer sampling tube are important

aspects for changing the flow conditions. Consequently, insitu calibration of the particle penetration is likely to beessential for the appropriate treatment of particle losses,though our results show that this can be conducted in thelaboratory, prior to any field campaign, since the sizedependent penetration is similar for all aerosols investi-gated, in accordance with theory.

4. Summary and conclusions

Experiments were made under different operatingconditions of the DMS500 particle spectrometer. Irre-spective of any measured size range and different operatingconditions used in various set of experiments, it wasfound that losses for particles smaller than z20 nm wereimportant and needed to be taken into account when usinglong sampling tubes (>1 m). Maximal losses were found forparticles between 5 and 10 nm, whereas losses weremodest for particles larger than z20 nm. It can beconcluded that ignoring these losses, especially below20 nm (where a substantial number of the particles inambient aerosols are to be found) may appreciably changethe measured PNDs in the ambient environment. It wasalso apparent that even when the Reynolds number indi-cated that the flow was laminar, the turbulent penetrationmodel of Hinds described particle losses best. Of course, it

P. Kumar et al. / Atmospheric Environment 42 (2008) 8819–88268826

is most prudent to determine the losses of particles for anyparticular experimental setup directly.

This study presents preliminary results, and it would beinteresting to perform similar experiments for differentdiameter of tubes and for various Reynolds numbers. Moreimportantly, a study producing a thumb rule for angle ofbends at which particle losses are minimum and/or areclose to laminar flow model, will greatly benefit ambientaerosol studies.

Acknowledgements

Prashant Kumar acknowledges receipt of the Cam-bridge-Nehru Scholarship and the Overseas ResearchScholarship Award for sponsoring his Ph.D. The authorsthank Prof. A.N. Hayhurst and Dr. J.S. Dennis for the loan ofthe DMS500.

Appendix A. Supplementary data

Supplementary data associated with this article can befound in the online version, at doi:10.1016/j.atmosenv.2008.09.003

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