Atmospheric Environment 38 (2004) 3625–3635

ARTICLE IN PRESS

*Correspond

9-191-8318.

E-mail addr

1352-2310/$ - se

doi:10.1016/j.at

‘‘Sniffer’’—a novel tool for chasing vehicles and measuringtraffic pollutants

L. Pirjolaa,b,*, H. Parviainena, T. Husseinb,c, A. Vallia, K. Hamerib,c, P. Aaaltob,A. Virtanend, J. Keskinend, T.A. Pakkanene, T. Makelae, R.E. Hillamoe

aDepartment of Technology, Helsinki Polytechnic, P. O. Box 4020, Helsinki FIN-00099, FinlandbDepartment of Physical Sciences, University of Helsinki, P. O. Box 64, FIN-00014, Finland

cDepartment of Physics, Finnish Institute of Occupational Health, Topeliuksenkatu 41, Helsinki FIN-00250, Finlandd Institute of Physics, Tampere University of Technology, P. O. Box 692, Tampere FIN-33101, Finland

eFinnish Meteorological Institute, Air Quality Research, Sahaajankatu 20 E, Helsinki FIN-00810, Finland

Received 16 October 2003; received in revised form 8 March 2004; accepted 15 March 2004

Abstract

To measure traffic pollutants with high temporal and spatial resolution under real conditions a mobile laboratory

was designed and built in Helsinki Polytechnic in close co-operation with the University of Helsinki. The equipment of

the van provides gas phase measurements of CO and NOx, number size distribution measurements of fine and ultrafine

particles by an electrical low pressure impactor, an ultrafine condensation particle counter and a scanning mobility

particle sizer. Two inlet systems, one above the windshield and the other above the bumper, enable chasing of different

type of vehicles. Also, meteorological and geographical parameters are recorded. This paper introduces the construction

and technical details of the van, and presents data from the measurements performed during an LIPIKA campaign on

the highway in Helsinki. Approximately 90% of the total particle number concentration was due to particles smaller

than 50 nm on the highway in Helsinki. The peak concentrations exceeded often 200,000 particles cm�3 and reached

sometimes a value of 106 cm�3. Typical size distribution of fine particles possessed bimodal structure with the modal

mean diameters of 15–20 nm and B150 nm. Atmospheric dispersion of traffic pollutions were measured by moving

away from the highway along the wind direction. At a distance of 120–140m from the source the concentrations were

diluted to one-tenth from the values at 9m from the source.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Urban aerosol; Size distribution; Exhaust gases; Mobile measurements

1. Introduction

Urban air quality is mostly dominated by traffic

emissions (e.g. Schauer et al., 1996; Harrison et al., 1999;

Karppinen et al., 2000). The emissions are characterised

by large number of sources with low source heights.

Long-range transport, secondary aerosol formation, and

ing author. Tel.: +358-9-191-8718; fax: +358-

ess: liisa.pirjola@helsinki.fi (L. Pirjola).

e front matter r 2004 Elsevier Ltd. All rights reserve

mosenv.2004.03.047

resuspension from the road surface as well as meteor-

ological conditions are also of importance. Automotive

engines are major sources of fine particles. Due to the

emission regulations and advanced technology, the

specific mass emissions (in g km�1) have been decreased

markedly, about one order of magnitude since the 1970s,

both for passenger cars and heavy-duty engines (Ahlvik

et al., 1998). Although particulate mass emissions from

spark ignition engines with new technology are typically

10–100 times lower than those of diesel engines,

considerable increase in the number concentrations of

d.

ARTICLE IN PRESSL. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–36353626

nanoparticles from spark ignition engine has been

recently reported (Baumgard and Johnson, 1996;

Kittelson, 1998). Particularly fine and ultrafine particles

have recently become of great interest due to their

adverse health effects (e.g. Dockery and Pope, 1994;

Donaldson et al., 1998; Pope et al., 2002).

Recent development in measurement technology has

enabled observations of particle size distribution with a

high temporal resolution by the scanning mobility

particle sizer (SMPS) and the electronic low pressure

impactor (ELPI). The number size distribution of fine

particles from diesel engine has been shown to follow a

bimodal structure; the nucleation and Aitken mode

particles in the 5–50 nm range dominate the number

concentration with a peak concentration at 20–30 nm,

the other peak is at around 100 nm. Nucleated particles

are formed from semi-volatile organic and sulphur

compounds during exhaust dilution and cooling (Kit-

telson et al., 2000; Khalek et al., 2000). Accumulation

mode particles are primarily composed of carbonaceous

agglomerates and adsorbed materials whereas the coarse

mode particles consist of asphalt aggregates and traction

sand (Pakkanen et al., 2001). The fraction of black and

organic carbons in particulate emissions depends on the

engine type (e.g. Kirchstetter et al., 1999).

Besides a high temporal resolution also a high spatial

resolution is needed to study traffic pollutions and

particle size distributions in real driving conditions.

Mobile laboratories have been shown to be useful tools

as recently reported by e.g. Kittelson et al. (2000),

Bukowiecki et al. (2002), Vogt et al. (2003), and Seakins

et al. (2002). A Finnish national project LIPIKA (Fine

particles from traffic and their relation to laboratory test

measurements) funded by the National Technology

Agency (TEKES) and the Ministry of Transport and

Communications Finland was initiated in September

2002 with the following objectives: (1) to investigate how

well the dynamometer measurements carried out in

laboratory conditions correlate with the real measure-

ments in street canyons and on roads, (2) to study the

effects of traffic emissions on urban air, especially to

investigate how necessary it is to measure number

concentrations of nucleation mode particles instead of

measuring only soot particles under traffic conditions,

(3) to study how sensitive the results are against

environmental conditions, different driving speeds and

different fuels, and (4) to build a mobile laboratory

(‘‘sniffer’’) for sampling on roads and highways, and

chasing in tunnels and underground sites. The partners

are Tampere University of Technology, Finnish Meteor-

ological Institute, University of Helsinki and Finnish

Institute of Occupational Health, Helsinki Polytechnic,

Technical Research Centre of Finland, and Helsinki

Metropolitan Area Council.

The aim of this work is to introduce the technical

solutions and the equipment of a novel mobile

laboratory, and present the first results of the LIPIKA

campaign performed at the highway in Helsinki. In this

paper, we focus only on aerosol measurements.

2. Mobile laboratory

2.1. Vehicle design

A mobile laboratory for the measurements of traffic

pollutants, called ‘‘sniffer’’, was designed and built by

the Laboratory of Automotive Engineering of Helsinki

Polytechnic. Construction of the sniffer is part of a

Finnish national project LIPIKA (September 2002–May

2005).

Unlike stationary measuring sites, the mobile labora-

tory enables measurements under real city traffic

conditions, chasing in tunnels and other underground

sites as well as surveys on roads and highways. It is also

useful for static measurements when the instruments

should be rapidly moved from site to site as for example,

in dispersion studies. A diesel vehicle, Volkswagen LT35

(length 5585mm, width 1933mm, height 2570mm, max

total weight 3550 kg) was selected as the platform of the

instrumentation. The walls in the boot space were

thermally insulated and overlaid, also the floor was

overlaid. One seat with a seat belt, a table and

dismountable racks for the analysers were installed in

the boot space. To decrease vibration and shock stresses

which might damage the analysers while driving,

vibration dampening by rubber was used. For different

meteorological conditions the van was equipped with

heating and air conditioning systems.

The power for the instruments (max 2.5 kW) is

provided by four 12V/230Ah batteries. Connecting

two of them in series gave 24V, then coupling two such

pairs in parallel produced a battery system whose

capacity 460Ah is enough for our instrumentation.

The DC voltage is inverted to 230V/50Hz AC by an

inverter Phoenix Multi (Victron Energy). The inverter

also recharges the batteries from an external 230V/

50Hz power network—this is also used for stationary

measurements. Full capacity of the batteries permits the

sniffer to operate for 5 h. To be able to measure

continuously while driving an extra 24V charger was

installed into the van engine system.

2.2. Inlets and chasing

Two different inlet systems opening towards the

driving direction were constructed, one above the

windshield at a height of 2.4m (main inlet) and

the other above the bumper at a height of 0.7m (chasing

inlet), shown in Fig. 1. Since exhaust tubes of different

type of cars lie in different positions, more or less in the

left or right side, to better catch exhaust gas the chasing

ARTICLE IN PRESS

Wind sensor RH,

Maininlet

Chasing inlet

Video camera

Pumpfor SMPS

Gas

Batteries

Inverter

Tools

SMPSELPI

NOX -analyser

CO -analyser

Flowmeter

GPS

Computers

CPC

Fig. 1. Phantom figure of the equipment of the mobile laboratory ‘‘sniffer’’.

L. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–3635 3627

inlet was branching horizontally and centred into four

inlet tubes with intervals of 25, 60, and 25 cm. The total

length of the main inlet is 3.65m.

The particle inlet was manufactured of stainless steel

AISI 316. To avoid losses of larger particles by

impaction the tubes were bent with big radii. The

aerosol inlet dimensions were designed so that for

our present instrumentation flow is laminar in the tube.

The flow rate is 11.5 lpm and the tube diameter inside

the van is 27mm leading to the flow velocity of

0.32m s�1 and Reynolds number of 576 (o2000). The

mouth diameter of the main inlet is reduced to 5.3mm

so sampling at the inlet mouth is isokinetic at a relative

speed of 30 kmh�1 (driving speed with respect to air).

However, to require isokinetic sampling is not very

relevant for the LIPIKA campaign since we are more

interesting in smaller particles, especially nucleation

mode particles. The inner diameter of each chasing inlet

is 3mm and below the bumper the four inlets were

merged together.

The gas inlets were made of Teflon to minimise losses

in the inlet tube (inner diameter 4mm). The gas inlets

are located next to the particle inlets to get sampling

from the same exhaust plume.

A series of qualitative tests were performed under

controlled driving conditions to discern the optimum

inlet for various sources and conditions. The sniffer was

chasing an Euro 2 category bus (Volvo B10B LE) and

two different passenger cars (Mercedes Benz 200D and

Chrysler Voyager) in a special test lane that was closed

for other traffic. During the test day wind speed was low,

below 2m s�1. The exhaust plume was coloured with

orange and gray smoke (Brandax-O and Brandax-VS,

Bjornax). The smoke cartridges were fixed around the

exhaust tube of the chased vehicle, and after ignition

they burned for 4min. The distance of the sniffer and the

vehicle chased was kept constant by continuous record-

ings by a laser instrument (PD 28), the values between 5

and 15m were used. The tests were also performed with

different driving velocities (20–80 kmh�1), and the

situations were recorded by the digital video camera. It

was found that the most important factor determining

the dispersion of the exhaust plume was the type of the

car, especially the design of its rear, whereas the speed

ARTICLE IN PRESS

(a)

(b)

(c)

Fig. 2. Tests for the exhaust plume to reach the main inlet or

the chasing inlet. Chasing a bus with an exhaust tube in the

middle of the rear (a), a passenger car with an exhaust tube in

the left side of the rear (b) and a mini van with an exhaust tube

in the left side of the rear (c).

L. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–36353628

and distance had only minor effect. The tests showed

that the chasing inlets are suitable for chasing passenger

cars, and the main inlets for chasing buses and mini

vans. As an example, Fig. 2 illustrates three still photos

of the dispersion of the exhaust plume when the sniffer

was chasing a bus with the exhaust tube in the middle of

the rear (a), a passenger car (b) and a mini van (c). The

passenger car and the mini van had the exhaust tubes in

the left side of the rear.

2.3. Instrumentation

The mobile laboratory provides measurements of

particles, some gaseous pollutants, meteorological and

geographical parameters with high spatial and time

resolutions. Particle size distribution is measured by the

electrical low pressure impactor (ELPI, Dekati Ltd.)

with flow rate of 10 lpm (Keskinen et al., 1992). The

ELPI with the electrical filter stage enables real time

particle size distribution in the size range of 7 nm–10 mm(aerodynamic diameter) with 12 channels. The particles

are charged, size classified by inertial impaction and

electrically detected. The offset currents were always

checked before and after the measurement. Especially,

the nucleation mode particle size distribution with the

high size resolution is measured by the Hauke-type

SMPS, where particles are first neutralised, then

classified by a DMA based on their electrical mobility,

and counted by a CPC 3025 (TSI, Inc.). Measurement

size range is 3–50 nm (mobility diameter), the number of

channels is 20 and the scan-up time mostly 30 s. The

sampling flow rate is 1.5 lpm. Sheat air and aerosol flows

were checked daily. Additionally, the total number

concentration of particles larger than 3 nm is detected by

an ultrafine condensation particle counter CPC 3025

(TSI, Inc.). Due to high traffic particle emissions a

passive clean air dilution system was installed. The

sample is divided into two flows, one flowing directly

and the other through a filter. After merging a dilution

ratio of the sample is 1:3 which has been experimentally

verified. All instruments were calibrated before the

campaign and as mentioned the flow rates were checked

daily.

Since the total length of the main inlet is 3.65m and

the flow velocity 0.32m s�1, the time delay of the

sampling air before ELPI is 11–12 s. Since the tube

length before the branching of SMPS and CPC is 2.20m

and after that the tube size is only 4mm, the air sample

residence time is somewhat shorter, about 7–8 s. Particle

deposition by diffusion when flowing through the tube

might be an important loss mechanism for ultrafine

particles. According to Hinds (1999) we calculated the

penetration, i.e. fraction of entering particles that exit,

for 5, 10 and 15 nm particles and the values were 86%,

94% and 97%. Consequently, the losses are considered

while analysing the results of SMPS.

The present instrumentation is applicable to observing

concentrations of carbon monoxide (CO), nitrogen

monoxide (NO) as well as nitrogen oxides (NOx)

ARTICLE IN PRESSL. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–3635 3629

(=NO+NO2). The concentration of CO is measured by

a CO gas monitor (Model CO12M, Environnement

S.A.), based on the ability of a CO molecule to absorb

infrared radiation. Detection range is 0.05–20 ppm with

a time resolution of 30 s. In the NOx analyser (Model

APNA 360, Horiba) the measuring cycle is composed of

three different parts: NO-, NOx- and zero level

measurements. NO is measured by first oxidising the

sample gas by ozone, and then measuring the subse-

quent luminescent light emitted by NO2 molecules left in

an excited state after the reaction. The light intensity is

proportional to the concentration of NO molecules.

NOx is measured directing the sample to the reaction

chamber through a deoxidation converter, in which NO2

is first converted to NO. The NOx detection limit is

about 0.5 ppb and the measurement range reaches

up to 4 ppm with a time resolution of 20 s. Zero and

span checks were performed a couple of times per

day for both analysers. Both analysers were calibrated

with 5 different certified standard gas concentrations.

However, in this work we concern only particle

concentrations.

Since the sampling flow rate of both analysers is

1.0 lpm and since the diameter of the sampling tube is

4mm, the residence time inside the tube before the

analysers is only 1.4 s.

The weather station at the roof of the van provides the

meteorological parameters such as relative wind speed

and direction by an ultrasonic wind sensor (Model

WAS425AH, Vaisala) as well as temperature and

relative humidity by humidity and temperature probes

(Model HMP45A, Vaisala). Additionally, a global

position system (GPS V, Garmin) saves the van speed

and the driving route and a video camera in the cab

records the traffic situations useful for data analysis.

Always at the start of the measurements the clocks of all

instruments are checked according to the satellite time

given by GPS.

The analogical outputs of the meteorological sensors

and the gas analysers are connected to a data acquisition

card (PCMCIA, National Instruments Ltd.) via a

connector block. Data acquisition was performed by

an LABVIEW programme running on a PC. The same

computer also acquires GPS and video output. A second

PC runs the SMPS and CPC systems storing the total

number concentration and size distribution data, and a

third computer controls the ELPI. The data are always

stored every second but 1-min averages are calculated.

The PCs are connected with each other via local

network.

3. Results and discussion

The first campaign of the LIPIKA project was

carried out on a busy highway Itavayla in Helsinki area

on 10–26 February 2003. This suburban area with high

industrial capacity lies 5 km east from downtown and is

a passing-through area to the eastern centre of Helsinki.

In the morning, traffic is busy towards west 2290

vehicles h�1 (Hannu Seppala, person. comm.). The

sniffer measured traffic pollutants during rush hours

by driving on the highway and standing at bus stops at

the highway (Section 3.1). The sampling was taken by

the main inlet with the driving speed of 40 kmh�1. The

sniffer was not chasing any single vehicle, just sampling

particles from busy traffic. To investigate the dispersion

of traffic plumes along the wind direction also the

stationary measurements by the sniffer were performed

at different sites on the way of 140m away the highway

(Section 3.2). At the same location, there were two

stationary measurement sites, placed 9 and 65m from

the highway. The schematic map of the sampling sites is

presented in Fig. 3. Also shown is the background area

at the Saunalahti bay about 600m northwest of the

highway. During the selected days (17–20 February) the

prevailing wind direction was northwest and the wind

speed was mostly about 2m s�1 and always below

4m s�1. The mild winter temperature varied between

1�C and 4�C in daytime and the relative humidity

between 60% and 90%. Note that all times are local

Finnish winter times.

3.1. The morning rush time on Itavayla

The typical traffic volume during morning rush hours

on weekdays on Itavayla on both lanes is approximately

3065 vehicles h�1 (Hannu Seppala, person. comm.).

However, we counted for 16 vehiclesmin�1 in the lane

to east and 30 vehiclesmin�1 in the lane to west in

average, but should be noted that the campaign week

happened to be a winter holiday week of students at

schools and therefore many parents also had vacation

that week.

Fig. 4(a) depicts the total number concentration of

particles (Ntot) measured by ELPI and CPC during the

morning rush hours at 7:30–10:55. All values are

averages over 1min. The peak concentrations often

exceeded 105 cm�3 that is the maximum limit when CPC

is operated without dilution as was the case that day.

For comparison, also plotted is the particle number

concentration of the nucleation mode (3–50 nm) mea-

sured by SMPS and the number concentration of

particles larger then 56 nm measured by ELPI. The

latter values are at least one order of magnitude smaller

the total number concentration, and thus approximately

90% of the total particle concentration belong to the

nucleation mode. The filter stage of ELPI that records

particles in the size range of 7–30 nm seems to over-

estimate that concentration. The concentration of the

second stage (30–56 nm) compared quite well with the

concentration of SMPS in the size range of 30–50 nm.

ARTICLE IN PRESS

Urban back-ground, BG

N

S

EW

XXX X X

X

Bus stop, BS1

5 km to downtown 100 m

Dispersion studies

Sau

nal

ahti

Bus stop, BS2

Itävä

ylä

HERTTONIEMI

Fig. 3. The map of the measurement sites.

L. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–36353630

Based on ELPI measurements, the concentration of the

accumulation mode particles (200 nm–1 mm) stayed

almost constant at around 103 cm�3.

The background concentration, BG in Fig. 4(a),

measured at the bay varies from 20,000–35,000

particles cm�3, covering the typical daytime range

during the campaign; at nighttime the concentration

used to reduce to a value of 10,000 cm�3, sometimes to a

20% lower value. The peak value in the background

concentration at 9:45 (the second BG range in Fig. 4(a))

is due to a snow plow, most of the particles staying in

the nucleation mode.

The peak concentrations detected while driving (DR)

or standing at the bus stops (BS1 to northeast, BS2 to

southwest) are due to individual vehicles or clusters of

vehicles; the highest peaks mainly came from heavy duty

vehicles, as recorded manually or can be seen from the

video file. For example, all peak concentrations in the

range of 200,000–500,000 cm�3 can be explained by a

bus or a truck accelerating in front of the sniffer. As a

curiosity, the maximum peak value of 1.3� 106 cm�3 at

8:51 was due to a very old motor home with a very

poorly tuned engine. Although the traffic density is

higher to the west direction the concentrations measured

at the bus stop on the lane to west (BS2) are lower than

at the bus stop to east (BS1). This might be due to the

wind direction from northwest that carries the traffic

emissions away from BS2 towards BS1 (Fig. 3).

In Fig. 4(b) average particle size distributions for BS1

and BS2 at two different time periods as well as for the

background concentration (BG at 9:48–9:58) are shown.

For this figure (as well as for Fig. 6(b) and (c)) the SMPS

data have been corrected by taking into account the

diffusion losses. As seen from Fig 4b the fine particle size

distributions are bimodal with the geometric mean

diameters located at 15–20 nm and around 150 nm.

Similar results that are typical for diesel aerosol

sampling have been reported by Kittelson et al. (2000),

Bukowiecki et al. (2002), Vogt et al. (2003), and Hussein

et al. (2004). The modal mean values are almost the

same for the background size distribution but the

concentrations of particles smaller than 300 nm are

clearly lower.

As stated by Bukowiecki et al. (2002) it is difficult to

estimate the emissions of single vehicles. Since the local

background concentration is expected to be rather high

in traffic-rich regions the measured concentrations do

not necessarily represent the emissions of a single

vehicle. Bukowiecki et al. determined the local back-

ground concentrations to be not the minimum value

over 1min concentration values but the 5% percentile

value (the mean value of the lowest values covering 5%

of all values). Their calculations showed that the

background values reach the range of 30,000–

50,000 cm�3 on busy main road, and consequently the

estimated traffic emissions caused the concentration to

reach up to 400,000 cm�3. In spite of much higher local

background concentrations in Fig. 5(a), also calculated

as 5% percentile, our results for peak concentrations

from traffic emissions plotted in Fig. 5(b) are in good

agreement with the results by Bukowiecki et al. (2002).

The highest peak 6.7� 105 cm�3 comes from the old

motor home and the next highest (2.8�4)� 105 cm�3

from buses.

ARTICLE IN PRESS

Fig. 4. (a) The total particle number concentration measured by ELPI (7 nm–10mm) and CPC (3 nm–1mm) on the highway Itavayla in

Helsinki, Finland, on 17 February 2003. Also shown is the number concentrations of nanoparticles (3 nm–50nm) measured by SMPS

and larger particles in the range of 56 nm–1mm measured by ELPI. All concentration values are averages over 1min. (b) The average

size distributions of particles at the bus stops and at the background site. BG=background site, DR=driving on Itavayla, BS1=bus

stop to northeast at Itavayla, BS2=bus stop to southwest at Itavayla.

L. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–3635 3631

3.2. Dispersion

The dispersion of freshly emitted traffic particles to

southeast from Itavayla was measured on 19 February

2003. The mobile laboratory stood at 6–9 different sites

from 5m up to 140m away from the highway (Fig. 3)

downwind (nowthwest), and measured the particle size

distributions for 5min at each site. The whole measure-

ment cycle took about an hour, and two cycles were

conducted during the afternoon rush hour.

Fig. 6(a) reveals the average values of the total

particle number concentration along with the standard

deviation error bars at each sampling site measured by

ELPI and SMPS. Here the total particle concentration is

the sum of the SMPS concentration (particle diameter

o50 nm) and the total concentration of the stages 3–12

by ELPI (particle diameter >50 nm). During dispersion

the total number concentration reduces roughly one

order of magnitude, 83–85% as well as particles smaller

than 50 nm whereas particles smaller than 321 nm (50%

cut size) decrease about 50–60%. At the first site, 9m

from the highway, 92–96% of total particles were

smaller than 50 nm.

The evolution of the particle size distributions during

the measurement cycles are presented in Fig. 6(b).

Each curve represents the average size distribution

over 5min at the sites mentioned in the legend. Also

shown is the averaged background size distribution

ARTICLE IN PRESS

Fig. 5. (a) The ELPI raw data (1 s intervals) and 1min 5%-

percentile of the total number concentration. See details in the

text. (b) The instantaneous peak concentrations from the

vehicles emissions.

L. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–36353632

observed at the bay at 16:03–16:08. In addition, the size

distributions measured at the stationary site located

65m from the highway are shown in Fig. 6(c). The

average standard deviation of the size distribution is

approximately 30% over the whole dispersion measure-

ment interval. This means that during the measurement

time the traffic situation and dispersion conditions can

be considered to remain reasonably stable. Again,

fine particles remain bimodal size distribution with a

slightly stronger accumulation mode than in Fig. 4(c).

By comparing the cases in Fig. 6(b) higher concentra-

tions of especially particles smaller than 300 nm can be

observed during the later rush time. This must be

due to a greater average traffic density that increased

from about 44 vehiclesmin�1 at 14:50–15:45 up to

58 vehiclesmin�1 with the more busy rush time at

16:22–17:20.

During the way of 9–125m away from traffic the

size distributions become lower and approach the

background size distribution. The main mechanism

for the rapid drop in the number concentrations of

size distribution while moving away from the source

region is dilution with background air as shown by

model calculations (e.g. Shi et al., 1999; Pohjola et al.,

2003).

Table 1 summarises the total number concentrations

as well as the concentrations in different size classes

measured by SMPS and ELPI at different sites. This

might be an interesting table for modelling purposes, to

verify dispersion models and to compare the modelled

results with the observations.

4. Conclusions

The mobile laboratory designed and built in Helsinki

Polytechnic has performed very well in the winter

campaign of the LIPIKA project in Helsinki. It

showed to be a useful tool for measurements of traffic

pollution with high temporal and spatial resolution

under real traffic conditions. However, some thoughts

of improvements have already risen. Chasing experi-

ments of single vehicles are not studied in this paper,

only some qualitative tests are shown. While chasing, a

CO2 analyser or some other instrument with a fast

response time should be installed into the van to

indicate staying in the rapidly varying plume. To be

flexible for different type of measurements, an extra

inlet system for variable number of instruments is

needed in future. Since the ground wind is very

turbulent, the wind sensor should be lifted to 8m to

get a better indication of the direction of the air mass

sampled.

The fine particle concentrations measured by ELPI,

CPC and SMPS compared rather well, even though in

some cases ELPI overestimated the number concentra-

tion of the smallest particles. Typical size distribution of

fine particles on the highway in Helsinki possessed

bimodal structure with the modal mean diameters of

15–20 nm and B150 nm. Approximately 90% of the

particles were smaller than 50 nm. The peak concentra-

tions exceeded often 200,000 particles cm�3 and reached

sometimes a value of 106 cm�3.

Atmospheric dispersion of traffic pollutions were

measured by moving away from the highway along

the wind direction. Combined with the stationary site,

the mobile laboratory allows very rapid distribution

measurement with confidence on the stability of

the source and dispersion conditions. At a distance of

120–140m from the source the concentrations were

diluted to one-tenth from the values measured at 9m

from the source. This kind of dispersion data that

is not easily found in the literature might be applied

ARTICLE IN PRESS

Fig. 6. Two dispersion studies of traffic emissions measured by moving away from the source on Itavayla in Helsinki, Finland, on 19

February 2003. Shown are the total particle number concentration measured by ELPI and SMPS (a), and the average particle size

distributions as a function of the distance from the source in the afternoon rush hours (b). The stationary SMPS data at 65m away

from the highway is presented in (c).

L. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–3635 3633

for modelling purposes, especially for comparison of

modelled results with observations and for help of the

evaluation of dispersion models. Future work is to

couple a road network dispersion model CAR-FMI

(Harkonen et al., 1995, 1996) and a monodisperse

aerosol dynamics model MONO32 (Pirjola and Kulma-

la, 2000; Pirjola et al., 2003) and compare the presented

data with simulations.

Acknowledgements

This work was funded by the National Techno-

logy Agency of Finland (TEKES), the FINE techno-

logy program LIPIKA (contract 629/31/02). We

wish to acknowledge Mr. Turo Tiilila, Mr. Kalle

Kalaja, Mr. Petri Ketonen, Mr. Juha Seppala and

Mr. Timo Lehtonen of Helsinki Polytechnic for their

ARTIC

LEIN

PRES

S

Table 1

The average total particle number concentration as well as the number concentrations of each stage and background site (BG) measured by ELPI at each site during the dispersion

cases

Site Ntot cm�3 3–20 nm

SMPS

20–50 nm

SMPS

73nm

ELPI

122 nm

ELPI

205 nm

ELPI

321 nm

ELPI

491nm

ELPI

772 nm

ELPI

1.25mmELPI

1.98mmELPI

3.13mmELPI

6U37mmELPI

19 February, 14:50–15:45

9m 1.19E+05 7.77E+04 3.68E+04 2.20E+03 1.61E+03 8.36E+02 2.43E+02 3.16E+01 5.49E+00 1.34E+00 2.39E�01 8.53E�02 7.63E�02

37m 9.89E+04 5.42E+04 3.85E+04 2.88E+03 2.01E+03 1.03E+03 2.68E+02 3.60E+01 6.10E+00 9.93E�01 1.52E�01 4.35E�02 4.14E�02

65m 6.30E+04 3.57E+04 2.26E+04 2.16E+03 1.49E+03 7.78E+02 1.96E+02 3.10E+01 5.15E+00 1.13E+00 1.44E�01 2.80E�02 3.21E�02

95m 5.00E+04 2.59E+04 2.03E+04 1.81E+03 1.19E+03 6.17E+02 1.55E+02 2.94E+01 4.65E+00 1.20E+00 1.88E�01 3.06E�02 2.10E�02

125m 2.60E+04 1.47E+04 0.88E+04 1.13E+03 8.28E+02 4.62E+02 1.07E+02 2.74E+01 4.23E+00 1.43E+00 2.55E�01 1.14E�02 3.02E�03

BG 2.02E+04 0.95E+04 0.82E+04 1.07E+03 7.98E+02 4.65E+02 9.88E+01 3.16E+01 5.63E+00 2.25E+00 6.41E�01 1.24E�01 2.11E�02

19 February, 16:22–17:20

9m 1.99E+05 1.04E+05 8.00E+04 7.16E+03 4.68E+03 2.25E+03 4.95E+02 5.37E+01 1.20E+01 2.14E+00 3.87E�01 1.08E�01 1.10E�01

24m 1.58E+05 1.06E+05 4.37E+04 3.59E+03 2.67E+03 1.37E+03 3.19E+02 4.22E+01 8.20E+00 1.63E+00 2.61E�01 4.55E�02 4.53E�02

39m 5.85E+04 3.06E+04 2.29E+04 2.31E+03 1.63E+03 8.37E+02 1.77E+02 3.46E+01 6.41E+00 1.86E+00 4.17E�01 4.86E�02 4.29E�02

65m 6.18E+04 3.56E+04 2.12E+04 2.29E+03 1.62E+03 8.28E+02 1.70E+02 3.33E+01 6.72E+00 2.14E+00 4.31E�01 1.94E�02 1.97E�02

125m 3.91E+04 1.61E+04 1.70E+04 2.73E+03 1.99E+03 1.02E+03 2.14E+02 3.55E+01 7.30E+00 2.87E+00 9.19E�01 2.06E�01 1.56E�01

BG 1.99E+04 0.91E+04 0.83E+04 1.07E+03 7.98E+02 4.65E+02 9.88E+01 3.16E+01 5.63E+00 2.25E+00 6.41E�01 1.24E�01 2.11E�02

The geometric mean diameters of the stages are shown. The concentrations of the two lowest stages of ELPI (7–56 nm) are replaced by the nanoparticle concentrations measured by

SMPS.

L.

Pirjo

laet

al.

/A

tmo

sph

ericE

nviro

nm

ent

38

(2

00

4)

36

25

–3

63

53634

ARTICLE IN PRESSL. Pirjola et al. / Atmospheric Environment 38 (2004) 3625–3635 3635

contribution while designing and constructing the

mobile laboratory.

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