1-s2.0-S1352231002004673-main

Atmospheric Environment 36 (2002) 49074918

Vertical and horizontal proles of airborne particulate matternear major roads in Macao, China

Ye Wua, Jiming Haoa,*, Lixin Fua, Zhishi Wangb, Uwa Tangb

aDepartment of Environmental Science and Engineering, Tsinghua University, Beijing 100084, Peoples Republic of ChinabFaculty of Science and Technology, University of Macao, Macao, Peoples Republic of China

Received 5 April 2002; accepted 28 June 2002

Abstract

Vertical proles, horizontal proles and size distribution of airborne particulate matter were measured near major

roads in Macao using DustTrak and TEOM monitors. A signicant decrease in the concentrations of PM10, PM2.5 and

PM1, as the height above the ground increases from 2 to 79m, was found. At the height of 79m, the concentrations of

PM10, PM2.5 and PM1, decrease to about 60%, 62% and 80% of the maximum occurring at 2m above the ground,

respectively. However, the horizontal proles near another major road revealed there was no signicant trend of

decrease in concentrations of particulate matter as the distance from the road increases. Over the total measured

distance (0228m), the maximum decreases of PM10, PM2.5 and PM1 are only 7%, 9% and 10%, of the maximum

occurring at 2m from the road, respectively. The daytime averaged PM2.5 and PM1 contribute 6667% and 5160%,

respectively, of the total PM10 mass after the particle readings by DustTrak were recalibrated by TEOM. It showed that

ne particles and submicrometer particles contributed a major part of PM10 at the roadside in Macao, which is most

likely attributed to the combinations of local sources including exhausted particulate matter from vehicles and

resuspended ne dust, and secondary particles (sulfate, nitrate and ammonium) of regional scales.

r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: PM10; PM2.5; PM1; Vertical prole; Horizontal prole; Particle size distribution; Trafc

1. Introduction

Throughout the last 30 years, an increasing body of

research has consistently shown statistically signicant

positive associations between ambient PM10 and daily

mortality counts and various indices of morbidity, and

these associations appear stronger for ne particles,

which are generally marked as PM2.5 (US EPA, 1996;

Vedal, 1997; IIASA, 2000a, b). Based on the totality of

evidence, many countries and international organiza-

tions issued air quality standards for airborne particu-

late matter to protect public health. PM10 and PM2.5were introduced as a standard in the US in 1987 and

1997, respectively. The new EU directive 1999/30/EC of

22 April 1999 also gave limit values for PM10. Since

1996, there have been amendatory National Ambient

Air Quality Standards (NAAQS) in China for TSP and

PM10, and the second class of NAAQS, which pertain to

urban areas, do not permit ambient PM10 concentra-

tions to exceed 100mgm3 for an annual arithmeticaverage and 150 mgm3 for a 24-hour average.Particulate matter pollution in those districts near

major roads was often found to be more severe than

urban background since these districts are affected

directly by various important primary sources, which

are frequently grouped into tailpipe exhaust from motor

vehicles, brake/tire wear, and resuspended road dust

(Lamoree and Turner, 1999). Resuspended road dust is

generally considered to be the most important source of

PM10, especially for the fraction of coarse mode

*Corresponding author. Tel.: +86-10-6278-2195; fax: +86-

10-6277-3650.

E-mail address: [email protected] (J. Hao).

1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 4 6 7 - 3

(between 2.5 and 10 mm). Motor vehicle emissions,however, usually constitute the signicant source of ne

and ultrane particles, such as PM2.5 and PM1. There-

fore, it is necessary to quantify the particle emission

levels with different size fraction (PM10, PM2.5 and/or

PM1), and also to determine particle behavior after

emissions, as they are transported away from the road.

There have been several studies conducted on

behavior of mass concentrations and elemental compo-

sition of airborne particulate matter near the road.

Janssen et al. (1997) compared the mass concentration

and elemental composition of airborne particulate

matter at street and urban background locations in

Netherlands. PM10 and PM2.5 concentrations were

found to be only 1.3 times higher near the road

compared with the background sample, however, black

smoke (elemental carbon) concentrations were 2.6 times

higher. Furthermore, the levels of Si and Fe (abundant

species in road dust) were signicantly higher in PM10,

and to a lesser extent in PM2.5, which suggested that

reentrained road dust, and not vehicle tailpipe exhaust,

was the dominant source for PM10 and PM2.5 in vicinity

of major road. Another study examined street-level

concentrations of PM10, PM2.5 and NO2 in Hong Kong

was conducted by Lam et al.(1999). Although the

correlation between the PM10, PM2.5 and NO2 concen-

trations and annual average daily trafc volume is

relatively weak, a noticeable trend showing higher

concentrations for higher trafc volumes can be still

found.

Proles of mass concentrations of PM10, PM2.5, NO2,

black smoke and benzene at increasing distances

from a major motorway were conducted by Roorda-

Knape et al. (1998). Monitoring sites were set up

at approximately 50, 100, 150, and 300m from a major

motorway at two different locations in the study.

It showed that black smoke and NO2 concentrations

declined with increased distance from the roadside,

however, no gradient was found for PM10, PM2.5and benzene. A pilot study examined horizontal and

vertical proles of number concentrations of submic-

rometer particulates in relation to a busy road in

Brisbane, Australia (Morawska et al., 1999). It con-

cluded that the horizontal prole measurements did not

provide any evidence of a statistically signicant

difference in ne particle number concentration with

respect to distance at ground level up to a distance of

200m with the exception of measurements in close

proximity to the freeway (about 15m). The vertical

prole measurements also revealed no signicant corre-

lation between particle number concentration and

height. Further study conducted by Hitchins et al.

(2000) measured horizontal prole of concentrations of

ne and ultrane particles from vehicle emissions near a

major road in Brisbane. There was a clear decrease in

ne and ultrane particle number concentration (in the

range 0.0150.697 mm) as distance from the roadincreased, as well as the larger particles measured

separately (in the range 0.520mm). PM2.5 levels alsodecreased with distance to around 75% for wind from

the road and to 65% for wind parallel to the road, at a

distance of 375m in the study.

Macao, one of the Special Administrative Regions in

China (another is Hong Kong), is located in South

China. Macao has three main island components with a

total area of only 19 km2; however, its vehicle popula-

tion has exceeded 110 thousand in 1999, which resulted

in one of the highest trafc densities in the world. Motor

vehicle emissions are considered to be the main source of

air pollutants in Macao since it is not directly inuenced

by industrial emissions (Hao et al., 2000).

Previous major studies at the Department of Envir-

onmental Science and Engineering (DESE) of Tsinghua

University (TU) focused on gaseous vehicle emissions

and determined their behavior as they are transported

away from the road in Macao. These studies presented

information on the real-world trafc activities, emission

inventory of motor vehicles and patterns of dispersion

and transportation of the exhaust air pollutants in

street canyon of Macao (He et al., 1998, 1999; Hao

et al., 2000).

As part of a program to assess emission characteristics

from motor vehicles in Macao, the purpose of this

research was to measure vertical and horizontal proles

of airborne coarse particles (PM10) and ne and

submicrometer particles (PM2.5 and PM1) near major

roads. Also, the study attempts to acquire particle size

distribution, including the contributions of PM2.5 and

PM1 to total PM10 mass, separately, at the roadway.

Due to funding and labor constrains, long-term

monitoring was not an option; instead, the daytime

period from 3 to 12 December in 2001 was chosen for

this monitoring.

2. Experimental techniques and procedures

Monitoring was conducted at four sites near major

roads in Macao. Site 1 at the junction of Avenida de

Horta e Costa Road and Rua de Francisco Xavier

Pereira Road was chosen for vertical prole measure-

ment, site 2 near Avenida da Amizade Road for

horizontal prole measurement, and site 3 and site 4

for single point measurement, at the roadside of

Avenida da Amizade Road and Rua da Ribeira do

Patane Road, respectively. Totally 6 DustTrak monitors

with PM10, PM2.5 and PM1 inlets, respectively, were

used at all of four sites to provide an assessment of the

relationship between particle mass concentration and

vertical and horizontal distance in relation to major

roads. Also, a TEOM 1400a was used at sites 3 and 4 to

recalibrate the readings of the DustTrak, and to acquire

Y. Wu et al. / Atmospheric Environment 36 (2002) 490749184908

the PM2.5 and PM1 contributions to PM10 near major

roads.

2.1. Instrumentation

2.1.1. DustTrak

The DustTrak Aerosol Monitor (model #8520, TSI

Inc., St. Paul, MN) is a rugged, portable instrument that

uses particulate light scattering to infer PM concentra-

tions where the amount of scattered light is proportional

to the volume concentration of the aerosol. For particles

smaller than one-third the wavelength of the laser

(780 nm), the scattered light decreases as a function of

the sixth power of the diameter, thus limiting the

smallest detectable particles to approximately 0.1 mm.The instrument is calibrated using Arizona Test Dust to

relate light scattering intensity to aerosol mass concen-

trations, and would need to be recalibrated at the

roadside in the study. Different size-selective impactors

are available for the inlet of the DustTrak allowing

measurements of PM10, PM2.5, and PM1. The ow rate

of the instrument is 1.7 lmin1.

DustTrak data can be logged at user-dened intervals

and gives an average reading of mass concentration over

the specied interval. For this study, readings were

logged every minute and averaged over 1 h. Totally

6 DustTraks were used in the study, and labeled as

D1D6, respectively.

2.1.2. Tapered element oscillating microbalance

(TEOM)

The concentrations of PM10 and PM2.5 were mon-

itored continuously by a TEOM 1400a ambient parti-

culate monitor (Rupprecht & Patashnick Co., Inc.,

Albany, NY) at sites 3 and 4 to recalibrate the readings

of DustTrak in this study. The US EPA has designated

the TEOM 1400a PM10 Monitor as an equivalent

method for the determination of PM10 concentrations

in ambient air (Designation no. EQPM-1090-079). The

TEOM 1400a monitor collected particles continuously

on a Teons-coated borosilicate glass ber lter

mounted on the tip of a glass element which oscillates

in an applied electric eld. The resonant frequency of the

element decreases as mass accumulations on the lter,

directly measuring inertial mass. Temperatures are

maintained at a constant value, typically 501C, in thisstudy, to minimize thermal expansion of the tapered

element. At the exit of the PM10 or PM2.5 inlet the

design 16.7 lmin1 ow is isokinetically split into a

3.0 lmin1 sample stream that is sent to the instruments

mass transducer and a 13.7 lmin1 exhaust stream.

Readings of mass concentrations were logged every

10min and averaged over 1 h in this study.

2.2. Sites and procedures

Four sites were chosen for this monitoring study. All

of the sites were near major roads: (1) site 1, for vertical

prole measurement, is located at the junction of

Avenida de Horta e Costa Road and Rua de Francisco

Xavier Pereira Road; (2) site 2, for horizontal prole

measurement, is in the vicinity of Avenida da Amizade

Road; (3) site 3, for single point measurement, is located

at the roadside of Avenida da Amizade Road, which is

about 500m east from site 2; (4) site 4, for single point

measurement also, is located at the roadside of Rua da

Ribeira do Patane Road. Schematic representations of

the four sites selected for measurements are shown in

Figs. 13, respectively.

Hourly meteorological data during the measurement

were obtained from the Macao Meteorological Service.

The meteorological station is located on Taipa Granda

Avenida de Horta e Costa Road

Building

Up

1 (D6) 2 (D5)

3 (D1)4 (D3)

5 (D4)

6 (D2)

Site 12m8m

19m

30m

59m

79m

N

Fig. 1. Site 1 for vertical prole measurement.

Y. Wu et al. / Atmospheric Environment 36 (2002) 49074918 4909

Mount, which is about 34 km south from the monitor-

ing sites. Hourly data of wind speed, wind direction,

temperature, relative humidity and precipitation were

used for this work. Wind directions followed a similar

pattern during most of the sampling period (about 5

days), which were characterized by northerly wind.

However, about 3 days of sampling followed another

directions of southeasterly wind. The sampling roads

were chosen that the winds were perpendicular or nearly

perpendicular to them during most of the sampling

period.

The trafc ow was acquired by counting the number

of cars, light-duty trucks, heavy-duty vehicles (including

buses) and motorcycles for 30min of each hour from

8:00 a.m. to 20:00 p.m. The average trafc ow per hour

during the measurements for all of the four sites were:

(1) Avenida de Horta e Costa Road: 795 cars, 453 light-

duty vehicles, 237 heavy-duty vehicles, 1346 motor-

cycles, total vehicles 2831; (2) Rua de Francisco Xavier

Pereira Road: cars 365, light-duty vehicles 129, heavy-

duty vehicles 124, motorcycles 1108, total vehicles 1726;

(3) Avenida da Amizade Road: cars 1029, light-duty

vehicles 145, heavy-duty vehicles 107, motorcycles 352,

total vehicles 1633; (4) Rua da Ribeira do Patane Road:

cars 782, light-duty vehicles 251, heavy-duty vehicles

395, motorcycles 1856, total vehicles 3284.

Measurements at site 1 were taken at different heights:

2, 8, 19, 30, 59 and 79m. Totally 6 DustTrak monitors

(labeled as D1D6, separately) with PM10, PM2.5 and

PM1 inlets, respectively, were used here during the

sampling period from 8:00 a.m. to 20:00 p.m. on 46

December 2001. Due to constrains of DustTrak

monitors, only one particle size fraction can be

measured each day, i.e., 4 December for PM10, 5

December for PM2.5 and 6 December for PM1,

respectively. Sampling tubes were extended vertically

from the window at least 0.5m for each DustTrak.

Measurements at site 2 were made at distances of 2,

42, 72, 120, 170 and 228m from the road. Because the

strongest decrease in concentration was expected to

occur in the rst 100150m (Leu, 1992), four monitor

locations were situated within 150m in the study. There

are two small roads crossing the monitoring site,

however, had a negligible amount of trafc (less than

10 vehicles per hour). Six DustTrak monitors were used

here during the sampling period from 8:00 a.m. to 20:00

p.m. on 46 December 2001. Again, each day only one

particle size fraction was measured, i.e., 10th of

December for PM1, 11th of December for PM10 and

the 12th of December for PM2.5, respectively. The

monitoring height of each tube was about 1.52.0m

above the ground.

As mentioned above, different particle size fraction of

vertical and horizontal proles were measured on

different days due to constraint of DustTraks. It may

cause bias because the characteristics of particles may

be different during variant sampling days. However, as

the measurement were all conducted at workdays, the

pattern of hourly vehicle trafc ow is relatively steady,

which means the emission characterizations of local

sources on the same road, e.g., exhausted particulate

matter from vehicles, could be considered as relatively

steady at the same hour during different sampling days.

Therefore, we assumed that the particle characteristics

were relatively homogeneous on the same road during

the sampling days in the study.

Site 3 is located at the roadside, for single point

measurements. On each side of the site three DustTrak

monitors were installed with 10, 2.5 and 1 mm inlets,respectively, during the daytime from 8:00 a.m. to 20:00

p.m. on 3 December. A TEOM 1400a was installed at

Avenida da Amizade Road

N Site 2 Building

2m

42m

72m

120m

170m

228m

1 (D1)

2 (D2)

4 (D4)

3 (D6)

6 (D5)

5 (D3)

Fig. 2. Site 2 for horizontal prole measurement.

Rua da Ribeira do Patane Road

N

Site 4 Building2 (D3, D6, D2)

Building

1 (D1, D5, D4, TEOM1400A)

Avenida da Amizade Road

N

Site 3 Building

1 (D1, D5, D4, TEOM1400A)

2 (D3, D6, D2)

Building

Fig. 3. Sites 3 and 4 for single point measurement.


south of the road (Location 1, see Fig. 3) with PM10 and

PM2.5 inlets, separately, used to recalibrate the readings

of DustTrak monitors. Concentrations were measured

simultaneously with these monitors. The sampling

height of DustTraks and TEOM 1400a are about 2.5

and 3m above the ground level, respectively. Site 4 is

also used for single point measurement, and the same

experimental procedures as those used in site 3 were

applied. Site 4 is located at the roadside of another

major road, and the monitoring period was from 8:00

a.m. to 19:00 p.m. on 7 December.

2.3. Quality assurance and quality control

Parallel measurements were conducted with these 6

DustTrak monitors at the same site with 10, 2.5 and

1mm inlets, respectively, on 8 and 9 December. Theratios of averaged readings of other DustTraks (D1,

D3D6) divided by the readings of D2 that was used as a

reference monitor and their linear correlation coef-

cients are summarized in Table 1. Excellent correlations

(0.951.00) were found for the readings of all of the

DustTraks. There were slight differences of averaged

readings between each other DustTrak and D2 with

10mm inlets, as well as the 2.5 and 1mm inlets, which wasprobably due to the small shift in the cut-point of these

inlets or other systematic bias from the monitors. The

largest difference found was about 10% for D4 and D2

with 1 mm inlets. In the study, the hourly averagedconcentrations of each DustTrak monitor were normal-

ized to eliminate the systematic bias.

3. Results and discussion

The measurements were conducted between 3 and 12

December 2001. The detailed sampling periods and

meteorological conditions are listed in Appendix A.

3.1. Vertical profiles at site 1

Fig. 4ac present the hourly vertical PM10, PM2.5and PM1 concentration proles in daytime at the site 1,

respectively, and the vertical proles of the daytime

averaged particulate matter concentrations are summar-

ized in Fig. 5. The concentrations of particulate

matter in Figs. 4 and 5 are measured by DustTraks,

and not recalibrated yet. Although the concentrations

by DustTraks are not real mass concentrations, the

vertical proles measured by DustTraks could be

considered as similar as those measured by other

monitors such as TEOM if we assume that particle

characteristics are relatively homogeneous on the same

road. In this section and followed section on horizontal

proles, the decreasing trends of particle concentrations

are of concern, instead of the detailed concentration

values.

It is clear that there is a signicant decrease in the

concentrations of PM10, PM2.5 and PM1, as the height

above the ground increases.

Those samples collected on 4 December show a strong

decreasing trend in concentrations of PM10 as the height

increases. At the height of 8m (location 2), the daytime

averaged PM10 concentration decays to 74% of the

maximum occurring at 2m above the ground (the closest

measurement point above the ground). However, as the

height increases, the attenuation of PM10 concentrations

slows down signicantly. At the height of 79m (location

6), the concentration of PM10 decreases to about 60% of

the maximum. There is only 14% attenuation over the

measured height from 8 to 79m. Wind direction

changed during the measuring period (see Appendix

A). It was characterized by northerly winds at about 201(from the road towards the measuring site) prevailing in

the morning and changing direction to southeasterly

winds at about 1501 (nearly parallel to the road) at noonand maintained this direction throughout the afternoon.

However, the result demonstrates that there is no

Table 1

Results of parallel measurement with 6 DustTrak monitors at the same site

DustTrak PM10 PM2.5 PM1

Ratioa R2b nc Ratio R2 n Ratio R2 n

D1 0.945 0.992 14 0.953 0.997 8 0.972 0.999 5

D3 1.030 0.960 6 0.977 0.989 8 0.972 1.000 8

D4 0.928 0.946 6 0.918 0.994 8 0.896 1.000 8

D5 0.968 0.999 8 0.924 0.989 8 0.943 1.000 6

D6 0.984 0.950 6 0.973 0.996 8 0.994 1.000 8

aRatio=the averaged readings of other DustTraks divided by readings of D2.bR2=correlation coefcient.cn=number of hourly averaged samples.


statistically signicant difference between the vertical

proles in the morning and those in the afternoon (see

Fig. 4a).

Those samples collected on 5 December also show a

signicant decreasing trend of in concentrations of

PM2.5 as the height increases, although the attenuation

did not follow the same pattern as PM10. At the height

of 8m, the daytime averaged PM2.5 concentration

decayed to only 87% of the maximum at the ground

level, not as strong a decrease as for PM10. However, the

concentrations of PM2.5 still decreased signicantly until

the height reached 19m (location 3), where the

concentration decays to 73% of the maximum. Then,

the decrease of PM2.5 also slows down as the height

increases. At the height of 79m, the concentration of

PM2.5 decays to 62% of the maximum.

The samples collected on 6 December exhibit a decay

of the PM1 concentrations that is less signicant than

the PM10 and PM2.5s as the height increases. At the

height of 8m, the daytime averaged PM1 concentration

decays to about 86% of the maximum at the ground

level. The curve of PM1 (see Fig. 5) shows a dip at the

height of 19m, where the concentration of PM1 decays

to 77% of the maximum. Then, the concentrations of

PM10 (12/4/2001)

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

TIME

CON

C. (m

gm-3 )

1(D6)2(D5)3(D1)4(D3)5(D4)6(D2)

PM2.5 (12/5/2001)

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

TIME

CON

C. (m

gm-3 )

1(D6)2(D5)3(D1)4(D3)5(D4)6(D2)

PM1 (12/6/2001)

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

TIME

1(D6)2(D5)3(D1)4(D3)5(D4)6(D2)

CON

C. (m

gm-3 )

(a) (b)

(c)Fig. 4. Site 1 hourly vertical PM concentration proles in daytime.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0 10 2 0 30 4 0 50 60 7 0 80 9 0 100Height above the ground (m)

CON

C. (m

g m-3 )

PM10 (12/4/2001)PM2.5 (12/5/2001)PM1 (12/6/2001)

Fig. 5. Site 1 vertical proles of the daytime averaged PM

concentrations.


PM1 increase slightly, as the height increases from 19 to

79m. At the height of 79m, the total decrease of PM1concentration is about 20%. The reasons for the dip

have not been investigated, but more likely relate to

other source interference at those heights, such as

residential cooking.

The vertical proles of site 1 at the roadside suggest

that PM10, PM2.5 and PM1 concentrations are affected

signicantly by trafc sources at ground level, e.g.,

resuspended road dust for PM10, tailpipe exhaust from

motor vehicles for PM2.5 and PM1, etc., which resulted

in a signicant decrease in the concentration of PM10,

PM2.5 and PM1, as the height above the ground

increases. And, since the particles in the coarse

mode have the much higher settling velocity than those

ne and ultrane particles, attenuation of the concen-

trations of PM10 is more signicant than the PM2.5and PM1 at a low height level, e.g., from the location 1

(2m above the ground) to location 2 (8m above the

ground).

Furthermore, site 1 is located at the junction of two

major roads, both of which are narrow roads with width

less than 15m, surrounded by high buildings with height

more than 30m at both sides of the roads, which

undoubtedly results in signicant street canyon. Special

ow circulation will be generated, and the wind

characteristics including wind speed and wind direction

are height-dependent in general (Hoydysh, 1988; De-

paul, 1985). Therefore, the change of wind will

inevitably affect the trend of vertical prole of particle

concentrations. As the aspect ratio (building height to

street width) of site 1 is high, the ow at the bottom of

street canyon is more stable than upper part, which

makes air pollutant accumulate at the bottom (Fu et al.,

2000). However, due to lack of the meteorological

monitors, the wind characteristics in different height

were not measured, and detailed analysis of the inuence

of wind to particle concentrations is not available.

3.2. Horizontal profiles at site 2

Fig. 6ac present the hourly horizontal PM10, PM2.5and PM1 concentration proles at the site 2, respec-

tively, and the horizontal proles of the daytime

averaged particulate matter concentrations are summar-

ized in Fig. 7. Similarly, the concentrations of particu-

late matter in Figs. 6 and 7 are measured by DustTraks,

and not recalibrated yet.

The samples collected on 10 December show a weak

trend of decrease in concentrations of PM1 as the

distance from the road increases. The measurement did

not begin until 14:00 p.m. because it rained in the early

morning (see Appendix A and Fig. 6c). At the distance

of 170m (location 5), the daytime averaged PM1concentration decays to only 90% of the maximum

occurring at 2m from the road (the closest measurement

point from the road), which is the maximum decrease in

concentrations of PM1 over the total measured distance.

The samples collected on 11 December shows that the

vertical prole of PM10 follows a similar pattern as

demonstrated by PM1. Over the total measured distance,

the maximum decrease of PM10 is only 7%, which

occurred at the distance of 228m.

The samples collected on 12 December also show a

weak trend of decrease in concentrations of PM2.5 as

the distance from the road increases. The curve of PM2.5(see Fig. 7) shows a slightly dip at the distance

of 72m, where the daytime averaged concentration of

PM2.5 decays to 91% of the maximum occurring at 2m

from the road. Wind direction changed during the

measurement period on this day (see Appendix A);

it was characterized by northerly winds at about 201(from the road towards the measuring site) prevailing in

the morning and changing direction to southeasterly

winds at about 1251 (from the measuring site towardsthe road) at noon and maintained this direction

throughout the afternoon. The total concentration of

PM2.5 from all of the 6 DustTraks decreased signi-

cantly after the wind direction changed. And, the results

in the afternoon show that the concentrations of PM2.5at other locations have no statistically signicant

difference except at location 1 (the closest point from

the road); the average concentration of PM2.5 at location

1 is about 12% higher than at other locations in the

afternoon.

The horizontal proles of site 2 at the roadside reveal

there is no signicant trend of decrease in concentrations

of PM10, PM2.5 and PM1 as the distance from the

road increases. The reasons maybe relate to (1) the

sampling heights, which is low above the ground (only

1.52.0m) resulting in signicant interference from

wind-blown dust near the sampling locations since the

wind speed is high (more than 3m s1 in average) during

the sampling period; and (2) the relatively low vehicle

volume at site 2, which is less than 40% of total trafc

volume at site 1, resulting in relatively low particulate

matter emissions from vehicle exhaust and resuspended

road dust. The latter reason revealed that background

particulate matter, e.g., secondary particles and sea salt,

which are relatively steady as the distance from the road

increases, maybe a very important contributor to total

particle mass concentrations near road in Macao. The

ndings from this study can be compared with other

literature data on horizontal proles of particulate

matter near major road. For example, Roorda-Knape

et al (1998) found there was no clear decline for PM10and PM2.5 concentrations at an increasing distance from

a major motorway; however, Hitchins et al. (2000)

concluded that PM2.5 levels decreased with distance to

around 75% when the wind blew from the road and to

65% for wind parallel to the road, at a distance of 375m

in their study.


3.3. Single point measurements at sites 3 and 4

3.3.1. Particle size distribution near major roads

Table 2 lists the roadside particle size distribution at

sites 3 and 4. The sample arithmetic mean ratios of

PM2.5/PM10 by DustTraks at both sites are between 0.95

and 0.97, and the ratios of PM1/PM10 are also as high as

0.740.87. Since DustTrak is calibrated using Arizona

Test Dust to relate light scattering intensity to aerosol

mass concentrations, its readings were not real mass

concentrations near roads, and need to be recalibrated

by TEOM to acquire the real particle size distribution

and particle concentrations near roads in this study. The

comparisons of hourly PM10 and PM2.5 DustTrak

measurements to TEOM are illustrated in Fig. 8. Good

PM10 (12/11/2001)

0.000

0.050

0.100

0.150

0.200

0.250

0.300

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

TIME

CON

C. (m

gm-3 )

1(D1)2(D2)3(D6)4(D4)5(D3)6(D5)

PM2.5 (12/12/2001)

0.000

0.050

0.100

0.150

0.200

0.250

0.300

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

1(D1)2(D2)3(D6)4(D4)5(D3)6(D5)

PM1 (12/10/2001)

0.000

0.050

0.100

0.150

0.200

0.250

0.300

8:00

9:00

10:0

011

:00

12:0

013

:00

14:0

015

:00

16:0

017

:00

18:0

019

:00

1(D1)2(D2)3(D6)4(D4)5(D3)6(D5)

TIME

CON

C. (m

gm-3 )

TIME

CON

C. (m

gm-3 )

(a) (b)

(c)Fig. 6. Site 2 hourly horizontal PM concentration proles in daytime.

0.0000.0200.040

0.0600.0800.1000.1200.140

0.1600.1800.200

0 50 100 150 200 250Distance from the road (m)

CON

C. (m

g m-3 )

PM10 (12/11/2001)PM25 (12/12/2001)PM1 (12/10/2001)

Fig. 7. Site 2 horizontal proles of the daytime averaged PM

concentrations.


correlations were found for the readings of DustTrak

monitors and TEOM. It is seen that the slope of the

linear regression is 0.002 and the correlation coefcient

is 0.82 for PM10; and for PM2.5, the slope is 0.0029 with

a correlation coefcient of 0.91. Then, the sample

arithmetic mean ratios of PM2.5/PM10 at both sites

decrease to be between 0.66 and 0.67 after the readings

of PM2.5 and PM10 by DustTrak were recalibrated by

TEOM. If we assume that PM1 could be recalibrated the

same ratio as PM2.5 (0.0029, see Fig. 8b), the sample

arithmetic mean ratios of PM1/PM10 are between 0.51

and 0.60. Because the sample chamber and inlet air are

heated to 501C to measurement in the TEOM system, itundoubtedly aggravates the particle volatilization bias

for ammonium nitrate and organic compounds, most of

which are ne particles. The ratios of PM2.5/PM10 and

PM1/PM10 will be higher if the volatilization bias

(negative sampling artifact) of TEOM is considered. It

suggests that ne particles and submicrometer particles

contributed major part of PM10 concentrations at

roadside in Macao. The high ratios of ne and

submicrometer particles in total PM10 mass is most

likely attributed to the contribution of local sources

including vehicle tailpipe exhaust and resuspended ne

dust, and, the secondary particles (sulfate, nitrate and

ammonium) of regional scales may be important

contributors to ne particles near roads in Macao too.

3.3.2. Particle concentrations near major roads

Results of the daytime averaged concentrations of

PM10, PM2.5 and PM1 by DustTraks (not recalibrated

by TEOM yet) at sites 3 and 4 are shown in Table 3. The

mean concentrations of PM10, PM2.5 and PM1 at

location 2 are all statistically signicantly lower than

those at location 1 at site 3, but the differences are small

(less than or equal to 10%). The mean concentration of

PM10 at location 2 is statistically signicantly higher

than that at location 1 at site 4, however, the statistically

signicant difference of PM1 between these two loca-

tions is not found. Since all of the locations are close to

the road, where are signicantly inuenced by the

sources, e.g., reentrained road dust and vehicle exhausts,

the differences of particle concentrations between

location 1 and location 2 of the roads may be weakened.

Moreover, both site 3 and site 4 are narrow roads (less

than 15m of road width) surrounded by buildings with

Table 2

Size distribution of particulate matter concentrations near major roads

Sampling sites Site 3 (Avenida da Amizade Road) Site 4 (Rua da Ribeira do Patane Road)

Location 1 Location 2 Location 1 Location 2

PM2:5=PM10 PM1=PM10 PM2:5=PM10 PM1=PM10 PM2:5=PM10 PM1=PM10 PM2:5=PM10 PM1=PM10

Arithmetic mean %Ra 0.952 0.833 0.970 0.796 0.965 0.866 NAd 0.738

Recalibrated mean %Rrb 0.656 0.575c 0.669 0.549c 0.665 0.597c NAd 0.509c

Standard deviation sr 0.009 0.013 0.011 0.011 0.053 0.014 NAd 0.049

Number of samples n 11 11 10 12 9 10 NAd 10

a %R=size fraction by DustTraks only, not recalibrated by TEOM yet.b %Rr=readings of DustTraks were recalibrated by TEOM (see Fig. 8).cAssumes that PM1 could be recalibrated the same ratio as PM2.5 (0.0029, see Fig. 8b).dNA=not available because of the power malfunction of D6.

y = 0.002xR2 = 0.8182

0.000

0.050

0.100

0.150

0.200

0.250

0.0 20.0 40.0 60.0 80.0 100.0TEOM CONC. (ug m-3)

DU

STTR

AK

CO

NC.

(mg m

-3 ) Avenida da Amizade Road (site 3)

PM10

y = 0.0029xR2 = 0.9114

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.0 20.0 40.0 60.0 80.0 100.0

DU

STTR

AK

CO

NC.

(mg m

-3 )

Ruada Ribeira do Patane Road (site 4) Avenida da Amizade Road (site 3)

PM2.5

TEOM CONC. (ug m-3)

(a)

(b)Fig. 8. Comparison of hourly PM DustTrak measurements to

TEOM.


height more than 20m at both roadsides, which

undoubtedly results in remarkable street canyon. Special

ow circulation will be generated, and result in higher

air pollutant concentrations at leeward side than wind-

ward side (Hoydysh, 1988; Depaul, 1985). The arith-

metic mean concentrations of PM10, PM2.5 and PM1 at

leeward side of sites 3 and 4 are all higher than those at

windward side, which revealed that the effect of street

canyon exactly occurred during the sampling period.

The concentrations of particulate matter in Table 3

are the readings by DustTraks, which are not recali-

brated yet. The real mass concentrations will decrease

signicantly after the DustTrak readings are recalibrated

by TEOM (see Fig. 8). The recalibrated daytime

Table 3

Daytime averaged particle concentrations near major roads

Sampling sites Site 3 (Avenida da Amizade Road) Site 4 (Rua da Ribeira do Patane Road)

Location 1 Location 2 Location 1 Location 2

PM10 (mgm3)a 0.116 0.109n 0.204 0.258nn

sb/nc 0.017/11 0.015/11 0.047/9 0.045/9

PM2.5 (mgm3)a 0.111 0.106nn 0.193 NAd

sb/nc 0.015/11 0.014/11 0.050/10 NAd

PM1 (mgm3)a 0.097 0.086n 0.186 0.195nnn

sb/nc 0.013/11 0.012/11 0.049/10 0.025/10

aArithmetic mean concentration by DustTrak, not recalibrated by TEOM.bs=the standard deviation.cn=number of samples.dNA=not available because of the power malfunction of D6.nStatistically signicant difference compared with location 1 (paired t-test; po0:01).nnStatistically signicant difference compared with location 1 (paired t-test; po0:05).nnnNo statistically signicant difference compared with location 1 (paired t-test; po0:05).

PM10

y = 5E-05x + 0.0388R2 = 0.6066

0.0000.0500.1000.1500.2000.2500.3000.3500.4000.450

0 1000 2000 3000 4000 5000Hourly traffic flow

CON

C. (m

g m-

3 )


PM2.5

y = 6E-05x + 0.0072R2 = 0.6055

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0 1000 2000 3000 4000 5000


PM1

y = 5E-05x + 0.0146R2 = 0.6695

0.0000.0500.1000.1500.2000.2500.3000.3500.4000.450

0 1000 2000 3000 4000 5000


Hourly traffic flow

CON

C. (m

g m-

3 )

CON

C. (m

g m-

3 )

Hourly traffic flow

(a) (b)

(c)Fig. 9. Correlations of particle concentrations and hourly trafc ow in daytime.


averaged mass concentrations of PM10 and PM2.5 at site

3 are 54.558.0 and 36.638.8 mgm3, respectively. Atsite 4, the real mass concentrations of PM10 and PM2.5are 102.0129.0 and 66.6 mgm3, respectively. Theparticle concentrations at site 4 are much higher than

those at site 3, which is most likely attributed to

the much higher trafc volume at site 4. As mentioned

above, the real mass concentrations of particulate

matter in the study may be underestimated, due to

the thermal loss of volatile material in the TEOM

system.

3.3.3. Relation to vehicular traffic

Since both sites 3 and 4 were located at the roadside,

the local trafc conditions would affect directly the

measured concentration levels of particulate matter. The

correlations of hourly particle concentrations and

hourly trafc ow in daytime at sites 3 and 4 are

presented in Fig. 9ac for PM10, PM2.5 and PM1,

respectively. The obtained correlation coefcients are

equal to 0.61, 0.61 and 0.67, respectively. The results

reveal that correlations between the particulate matter

and the trafc volumes are signicant, showing a clear

trend of higher concentrations for higher trafc

volumes.

4. Summary and conclusions

Vertical and horizontal proles of airborne particulate

matter in daytime were measured near major roads. It is

clear that there is a signicant decrease in the

concentrations of PM10, PM2.5 and PM1, as the height

above the ground increases from 2 to 79m at site 1. At

the height of 79m, the concentrations of PM10, PM2.5and PM1, decrease to about 60%, 62% and 80% of the

maximum occurring at 2m above the ground, respec-

tively. The vertical proles at the roadside suggest that

particle concentrations are affected signicantly by those

sources at ground level from trafc district, e.g.,

resuspended road dust and tailpipe exhaust from motor

vehicles.

Table 4

Sampling periods and meteorological conditions used in the study

Sampling site Date WS (m s1)a WDb P (mm)c RH (%)d

Site 1 (PM10) 12/4/2001 2.9 (1.73.9) 08:0013:00: Northerly wind at about

201 (from the road towards themeasuring site)

0 61 (4879)

13:0020:00: Southeasterly wind at

about 1501 (nearly parallel to the road)Site 1 (PM2.5) 12/5/2001 4.1 (2.85.4) Southeasterly wind at about 1301 (nearly

parallel to the road)

0 69 (6474)

Site 1 (PM1) 12/6/2001 6.9 (5.77.9) Northerly wind at about 101 (from theroad towards the measuring site)

0 64 (6071)


0 79 (7781)

Site 2 (PM2.5) 12/12/2001 2.9 (1.45.6) 08:0013:00: Northerly wind at about

201 (from the road towards themeasuring site)

0 72 (6281)


about 1251 (from the measuring sitetowards the road)


1.6 (08:00) 81 (7590)

0.2 (11:00)

Site 3 12/3/2001 3.0 (2.04.1) 08:0017:00: Northeasterly and easterly

winds at about 45901 (nearly parallel tothe road)

0 72 (6479)


about 1251 (from location 1 towardslocation 2)

Site 4 12/7/2001 5.2 (3.18.5) Northerly wind at about 251 (fromlocation 2 towards location 1)

0 61 (5371)

aWS=wind speed; arithmetic mean (minimummaximum).bWD=wind direction.cP=precipitation.dRH=relative humidity; arithmetic mean (minimummaximum).


The horizontal proles of site 2 at the roadside reveal

there is no signicant decreasing trend in concentrations

of particulate matter as the distance from the road

increases. Over the total measured distance, the max-

imum decrease of PM10, PM2.5 and PM1 are only 7%,

9% and 10%, of the maximum values occurring at 2m

from the road, respectively. The reasons may be related

to low sampling heights, resulting in interference from

other ground-level sources near the sampling locations,

and the relatively low vehicle volume at site 2.

Also, the particle size distribution at the roadside was

acquired in this study. The daytime averaged PM2.5 and

PM1 contributed 6667% and 5160%, respectively, of

the total PM10 mass after the particle readings by

DustTrak were recalibrated by TEOM at both sites 3

and 4 (we assume that PM1 could be recalibrated using

the same ratio as PM2.5). It suggested that ne particles

and submicrometer particles contributed the major part

of PM10 at the roadside in Macao, which may be

attributed to the combinations of sources including

exhaust particulate matter from vehicles and reentrained

ne dust, and secondary particles (sulfate, nitrate and

ammonium) of regional scales.

Acknowledgements

The research described in this paper is funded by the

National Natural Science Foundation of China &

Macao Foundation (Grant No. 40045015). The authors

thank Jingnan Hu and Litao Wang (DESE, TU) for

their contribution to the collection of the samples, and

Mr. Freed, C.N. of US EPA for his helpful advice to

improve our paper.

Appendix A

The detailed sampling periods and meteorological

conditions are listed in Table 4.

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Vertical and horizontal profiles of airborne particulate matter near major roads in Macao, ChinaIntroductionExperimental techniques and proceduresInstrumentationDustTrakTapered element oscillating microbalance (TEOM)

Sites and proceduresQuality assurance and quality control

Results and discussionVertical profiles at site 1Horizontal profiles at site 2Single point measurements at sites 3 and 4Particle size distribution near major roadsParticle concentrations near major roadsRelation to vehicular traffic

Summary and conclusionsAcknowledgementsReferences

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