Atmospheric Environment 36 (2002) 4323–4335

Study of ultrafine particles near a major highway with

heavy-duty diesel traffic

Yifang Zhua, William C. Hindsa,*, Seongheon Kimb, Si Shenc,

Constantinos Sioutasc

aDepartment of Environmental Health Sciences, University of California Los Angeles, 650 Charles E. Young Drive South, Los Angeles,

CA 90095, USA bSchool of Earth and Environmental Sciences (BK21), Seoul National University, Seoul, South Korea c

Department of Civil and Environmental Engineering, University of Southern California, 3620 South Vermont Avenue, Los Angeles,

CA 90089, USA

Received 6 March 2002; accepted 20 May 2002

Abstract

Motor vehicle emissions usually constitute the most significant source of ultrafine particles (diameter o0.1 mm) in an

urban environment. Zhu et al. (J. Air Waste Manage. Assoc., 2002, accepted for publication) conducted systematic

measurements of the concentration and size distribution of ultrafine particles in the vicinity of a highway dominated by

gasoline vehicle. The present study compares these previous measurements with those made on Interstate 710 freeway in

Los Angeles. The 710 freeway was selected because more than 25% of the vehicles are heavy-duty diesel trucks. Particle

number concentration and size distribution in the size range from 6 to 220 nm were measured by a condensation particle

counter and a scanning mobility particle sizer, respectively. Measurements were taken at 17, 20, 30, 90, 150, and 300 m

downwind and 200 m upwind from the center of the freeway. At each sampling location, concentrations of carbon

monoxide (CO) and black carbon (BC) were also measured by a Dasibi CO monitor and an Aethalometer, respectively.

The range of average concentration of CO, BC and total particle number concentration at 17 m was 1.9–2.6 ppm, 20.3–

24.8 mg/m3

, 1.8  105–3.5  105/cm3, respectively. Relative concentration of CO, BC and particle number decreased

exponentially and tracked each other well as one moves away from the freeway. Both atmospheric dispersion and

coagulation appears to contribute to the rapid decrease in particle number concentration and change in particle size

distribution with increasing distance from the freeway. Average traffic flow during the sampling periods was 12,180

vehicles/h with more than 25% of vehicles being heavy-duty diesel trucks. Ultrafine particle number concentration

measured at 300 m downwind from the freeway was indistinguishable from upwind background concentration. These

data may be used to estimate exposure to ultrafine particles in the vicinity of major highways.

r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Ultrafine particles; Freeways; Diesel; Carbon monoxide; Black carbon

1. Introduction

Epidemiological data from air pollution studies have

shown a consistent relationship between increases in

particulate matter (PM) exposure and contemporary

increases in mortality and morbidity (Schwartz, 1991;

Dockery et al., 1993; Pope et al., 1995; Vedal, 1997).

However, the underlying biological causes of the health

effects of PM exposure and the correct measurement

metric are unclear. For example, it is not clear whether

the mass concentration (Osunsanya et al., 2001) or the

number concentration (Peters et al., 1997; Penttinen

et al., 2001) is most important in causing these adverse

*Corresponding author.

E-mail address: whinds@ucla.edu (W.C. Hinds).

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

PII: S 1352-2310(02)00354-0
PM health effects. Currently, there are several hypotheses used to explain the association of PM and observed

adverse health effect. One argues that particle surface

contaminants, such as transition metals, contribute

towards ill health (Fubini et al., 1995; Gilmour et al.,

1996), wherein the ultrafine particles are thought to act

as vehicles for those contaminants, initiating local lung

damage when the particles deposit on the epithelial

surfaces. Another hypothesis is that the physical

characteristics (e.g. number, size, shape, aggregation

properties) are important in producing health effects

(Berub ! e et al., 1999). Particle shape and size are critical !

factor controlling where the inhaled particles deposit in

the various regions of human respiratory system by the

complex action of aerosol deposition mechanisms

(Hinds, 1999).

Recent toxicological studies have concluded that

ultrafine particles (diameter o100 nm) are more toxic

than larger particles with the same chemical composition

and at the same mass concentration (Ferin et al., 1990;

Oberdorster, 1996, 2001; Donaldson et al., 1998, 2001; .

Churg et al., 1999; Brown et al., 2000). Currently,

however, only the mass concentration of PM o10 mm in

aerodynamic diameter (PM10) and o2.5 mm (PM2.5) are

regulated. Information about ultrafine particles is

usually not available. In fact, even though ultrafine

particles represent over 80% of particles in terms of

number concentration in an urban environment (Morawska et al., 1998a, b), the less numerous but much

heavier particles of the accumulation (0.1–2 mm) and

coarse (2.5–10 mm) modes dominate mass concentration

measurements. Thus, number concentration, together

with the size distribution of ultrafine particles, is needed

to better assess ambient air quality and its potential

health effects.

Emission inventories suggest that motor vehicles are

the primary direct emission sources of fine and ultrafine

particles to the atmosphere in urban areas (Schauer

et al., 1996; Shi et al., 1999; Hitchins et al., 2000).

Although traffic-related air pollution in urban environments has been of increasing concern, most studies have

focused on gaseous pollutants, total mass concentration,

or chemical composition of particulate pollutants

(Kuhler et al., 1994; Clairborn et al., 1995; Williams

and McCrae, 1995; Janssen et al., 1997; Roorda-Knape

et al., 1998a, b; Wrobel et al., 2000). Booker (1997)

found that particle number concentration was strongly

correlated with vehicle traffic while PM10 was essentially

uncorrelated with traffic. Since the majority of particle

number from vehicle exhaust are in the size range 20–

130 nm for diesel engines (Morawska et al., 1998a, b)

and 20–60 nm for gasoline engines (Ristovski et al.,

1998), it is important and necessary to quantify ultrafine

particle emission levels, and to determine ultrafine particle behavior after emission as they are transported away

from the emission source—busy roads and freeways.

Morawska et al. (1999) measured the horizontal and

vertical profiles of submicrometer particulates (16–

626 nm) near a major arterial route in the urban area

of Brisbane, Australia. They found, with the exception

of measurements in close proximity to the road (about

15 m), that the horizontal ground-level profile measurements did not show statistically significant differences in

fine particle number concentration for up to 200 m

distances away from the road. Hitchins et al. (2000)

examined the particle size distribution and concentration in the size range from 15 nm to 20 mm at distances

from a road ranging from 15 to 375 m at two sites in

Australia. They conducted measurements under different wind conditions and found that when the wind is

blowing directly from the road, the concentration of the

fine and ultrafine particles decayed to about half of their

maximum at a distance of 100–150 m from the road. Shi

et al. (1999) measured ultrafine particle number concentrations and size distributions at a busy roadside and

at nearby urban background sites in Birmingham,

United Kingdom. They observed a faster decline of

particle number concentration than mass concentration.

In a recent study, Shi et al. (2001) reported that the

fraction of particles o10 nm represents more than about

40% of the total particle number concentrations at 4

and 25 m from the roadside curb.

While there have been recent studies of ultrafine

particles from traffic in other countries, except for Zhu

et al. (2002), no comparable work has been done in the

Los Angeles basin, a home to more than 15 million

individuals and 10 million vehicles contributing to daily

traffic. Previous studies have shown that meteorological

conditions may affect substantially the characteristics of

PM emitted from vehicles. Kittelson et al. (2001) found

in their on-road PM measurements that the concentration of particles in the nuclei mode increases by nearly a

factor of 10 as the (air) temperature is reduced from

251C to 151C. This observation suggests that there could

be significant differences in the tendency to form semivolatile nanoparticles between, for example, northern

Europe and Southern California.

Zhu et al. (2002) conducted a systematic ultrafine

particle study near one of the busiest freeways in the Los

Angeles basin, Interstate 405. Traffic on that freeway

was dominated by gasoline-powered cars and light

trucks, with o5% of vehicles being heavy-duty diesel

trucks. In the US, spark ignition vehicles usually

account for most of the vehicles operating on highways.

However, since diesel vehicles emit more PM on a fleet

averaged, gram-per-vehicle mile mass basis (Kittelson

et al., 2001), and that diesel engine exhaust has been

proposed as carcinogen in animals and probably

carcinogenic for humans (IARC, 1989), it is necessary

and timely to conduct a comprehensive study of

ultrafine particles in the vicinity of a diesel vehicle

dominated freeway. Thus, the aim of the present paper is

4324 Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335
to systematically evaluate ultrafine particles in the

vicinity of the 710 freeway in the Los Angeles basin, a

freeway where more than 25% of vehicles are heavyduty diesel trucks. Particle number concentration and

size distribution in the size range from 6 to 220 nm are

measured along with CO and black carbon (BC) as a

function of distances upwind and downwind the 710

freeway. The results from the current study are

compared to these by Zhu et al. (2002) which were

obtained near the 405 freeway.

2. Experimental

2.1. Description of sampling site

This study was conducted in the City of Downey

along Southern Avenue between 30 August and 27

October 2001. The location was chosen for its proximity

to the freeway and the lack of other nearby ultrafine

particle emission sources. Southern Avenue is located

perpendicular to Interstate 710 Freeway and Garfield

Avenue near the Los Amigos Country Club. Freeway

710 runs generally north and south near the sampling

site and parallels the Los Angeles River.

This location is ideal for this study for several reasons.

First, there are no other major roadways near the

sampling sites along Southern Avenue. Second, businesses along Southern Avenue generally have large open

land areas with little activities during the day. Thus,

there is minimal local traffic influence at the sampling

locations. Third, the freeway is at the same elevation as

Southern Avenue. The only separation between the

freeway and Southern Avenue is a metal chain link fence

along the freeway. This allowed measurements as close

as 3 m from the edge of the freeway. Fourth, a nearby

residential area approximately 200 m upwind from the

freeway was easily accessible for sampling.

During the sampling period, a fairly consistent

eastward wind developed each day starting at approximately 11:00 AM. This wind carried the freeway

vehicular emissions directly to the sampling location.

The 710 freeway has eight lanes, four north bound and

four south bound. It is approximately 26 m wide

including a 1-m-wide median strip. Measurement site

locations for this study were designated by their distance

from the center of the median strip. Thus, the distance

from each sampling location to the nearest traffic lane is

13 m less than the indicated distance.

Freeway 710 is a major truck route in Southern

California with a large percent of the traffic consisting of

heavy-duty diesel trucks. During the sampling period,

traffic density ranged from 180 to 230 vehicles/min

passing the sampling site, total for both directions, with

approximately 25% of the vehicles being heavy diesel

trucks.

2.2. Sampling and instrumentation

Wind speed and direction were measured at a fixed

site 6 m above the ground level 20 m downwind of 710

freeway, which also served as a particle number

concentration control site. Wind data were averaged

over 1 min intervals and logged into a computerized

weather station (Wizard III, Weather Systems Company, San Jose, CA). Throughout each measurement

period, the traffic strength on the freeway, defined as

number of vehicles passing per minute, was continuously

monitored by a video recorder (camcorder), which

captures all eight lanes of the freeway. After each

sampling session, the videotapes were replayed and

traffic density counted manually. Three 1-min samples

were randomly selected from each 10-min interval. Cars,

light trucks, and heavy-duty trucks were counted

separately to estimate the traffic density by type of

vehicle.

Particle number concentration and size distribution in

the size range from 6 to 220 nm were measured by a

condensation particle counter (CPC 3022A; TSI Inc., St.

Paul, MN) and a scanning mobility particle sizer (SMPS

3936, TSI Inc., St. Paul, MN). The sampling flow rate of

the SMPS was adjusted to 1.5 lpm in order to measure

particles as low as 6 nm as well as to minimize the

diffusion losses of ultrafine particles during sampling.

Flexible, conductive tubing (Part 3001940, TSI Inc., St.

Paul, MN) was used for sampling to avoid particle losses

due to electrostatic forces. The sizing accuracy of the

SMPS was verified in the laboratory by means of

monodisperse polystyrene latex spheres (PSL, Polysciences Inc., Warrington, PA). Data reduction and

analysis of the SMPS output was done by the Aerosol

Instrument Manager software (version 4.0, TSI Inc., St.

Paul, MN). Measurements were taken at 17, 20, 30, 90,

150, and 300 m downwind and 200 m upwind from the

center of the freeway 710. At each location, three size

distribution samples were taken in sequence with the

SMPS. Scanning time for each was 180 s.

In addition to size distribution and the total number

concentration, the concentrations of BC and carbon

monoxide (CO), were monitored simultaneously at each

sampling location. Before each measurement session, all

instruments were time synchronized. Data were averaged after collection over the time periods corresponding to the scanning intervals of the SMPS. A Dual Beam

Aethalometer (Model AE-20, Andersen Model RTAA900, Andersen Instruments Inc., Smyrna, GA) was used

to measure the BC concentrations every 5 min. Concentrations of CO were measured by a near-continuous

CO monitor (Dasibi Model 3008, Environmental Corp.,

Glendale, CA) every minute. The CO monitor was

calibrated by means of standard CO gas (RAE systems

Inc., Sunnyvale, CA) in the laboratory and automatically zeroed each time the power was turned on.

Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335 4325
Electric power for the control site CPC and Weather

Station was obtained by an extension cord to a nearby

office. Electric power for other sampling instruments at

the sampling locations was supplied by a 1.2 kW

gasoline-powered portable power generator (Model

EU 1000i, Honda Motor Co., LTD., Tokyo, Japan).

The generator was placed approximately 50 m downwind of each sampling location. Both total particle

number and CO concentrations were measured at the

control site with the generator turned on and with it

turned off. No detectable difference was observed.

Table 1 gives the sampling dates and times and

summarizes the instruments that were used on each date.

The weather station and control CPC were placed at the

20 m downwind control site and sampled throughout the

sampling period each day. All other applicable instruments were moved together and sampled simultaneously

at each sampling location. It takes about 10 min to

complete sampling at each location and 120 min to

complete a set, all six locations. Three to four sets were

performed on each sampling date.

3. Results and discussion

The results presented below include measurements of

total particle number concentrations by a control CPC,

wind velocity by a Weather Wizard III, both positioned

at a fixed location 20 m downwind of the freeway; and

CO, BC concentration, and ultrafine particles size

distributions upwind and at six downwind distances

from freeway 710.

3.1. Wind effects

Changes in wind conditions have been reported to

modify dramatically the pattern of total particle number

concentration versus distances from a major road

(Hitchins et al., 2000). Consistency in wind speed and

direction allows data from different days to be averaged

together (Zhu et al., 2002). Wind speed and direction

were measured, averaged and logged over every 1-min

interval throughout each sampling period. One hundred

wind data points were randomly selected out of more

than 5000 observations from all the sampling dates and

plotted in Fig. 1. The orientation of freeway 710 and the

sampling road, Southern Avenue, are also shown in the

Fig. 1. The Weather Wizard III instrument recorded

wind direction at a 22.51 interval (e.g. 11.251 on either

side of N, NNE, etc.) and wind speed at 0.4 or 0.5 m/s

intervals. In the figure, duplicate observations were

spread out slightly in both directions to better illustrate

how strong the wind was and how often the wind came

from certain directions. Based on all 5000 observations,

the percent of sampling time that the wind came from

each 22.51 segment is also shown in Fig. 1. As shown in

Fig. 1, about 80% of the time, the wind was coming

directly from the freeway towards the sampling road

with a speed o3 m/s. The consistency of observed wind

direction and speed is a result of a generally low synoptic

wind velocities and a consistent sea breeze in the

sampling area.

In this study, we found that not only wind direction,

but also wind speed, played an important role in

determining the characteristics of ultrafine particles near

the 710 freeway, similar to the observations made by

Zhu et al. (2002) near the 405 freeway. However the

pattern of total particle number concentrations as a

function of wind speed is somewhat different for the two

studies. Fig. 2 shows total particle number concentrations measured by the control CPC, located 20 m

downwind of the 710 freeway versus wind speed.

Averaged data for the 405 freeway from Zhu et al.

(2002) are also plotted for comparison. The CPC was

programmed to archive averaged total particle number

concentrations at 1-min interval in synchronization with

the averaging time of the meteorological data. Only

wind data within 722.51 of normal to the freeway was

used in this figure which accounts for more than 60% of

the total observations. The difference between the

absolute value of total particle number concentration

is due in part to the difference in the sampling distance.

The control CPC was located 20 m downwind from the

710 freeway but 30 m from the 405 freeway. Assuming

the fitted exponential decay characteristics of ultrafine

particles holds right to the edge of the freeway, it is thus

Table 1

Sampling dates, time and instruments used

Date Time Weather Wizard III Control CPC SMPS CO monitor Aethalometer

08/30/01 10:00–15:30    

09/05/01 10:30–16:00  

09/21/01 10:00–15:00    

09/25/01 10:30–16:00    

10/05/01 10:30–16:00    

10/24/01 10:00–15:30   

10/30/01 10:00–15:30  

4326 Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335
not surprising, as discussed below, that the CPC will

read a greater total particle number concentration at

20 m in the present study than at 30 m in that by Zhu

et al. (2002), given similar traffic load on both freeways.

However, the relative particle number concentration as

function of wind speed are somewhat different in these

two studies. The relative particle number concentration

decreased as the wind speed increased near the 405

freeway. In contrast, particle number concentration in

the 710 freeway first increases, reaches a maximum

around 1.5 m/s, and then decreases. There is no obvious

explanation for the observed difference. In both studies,

data showed large error bars, and the data of low wind

speed (o1 m/s) were very limited. In addition, the 405

Fig. 1. Wind direction and speed at sampling site.

Fig. 2. Total particle number concentration measured by CPC located at 20 m downwind from freeway 710 versus wind speed. Bars

indicate one standard deviation.

Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335 4327
freeway is elevated approximately 4.5 m above the

surrounding terrain, while, the 710 freeway is at ground

level, the same as the sampling location. Lower speed

wind would be expected to cause less atmospheric

dilution, and thus lead to greater particle number

concentrations, as Zhu et al. (2002) reported. However,

at extremely low wind speeds, it would take a

considerably longer time for the wind to carry particles

to the sampling port of the CPC, which gives ultrafine

particles more time to coagulate with either themselves

or with larger particles, a phenomenon that would

decrease the total particle number concentration. This

may partially explain the observed ‘‘n’’ shape curve in

the current study.

3.2. Traffic effects

The portion of freeway 710 passing through the City

of Downey is a major truck shipping route. The average

traffic volume per hour during the measurement period

was: 8730 cars, 870 light trucks, 2580 heavy trucks, and

12180 total vehicles. It is apparent from these numbers

that diesel emission vehicles on the 710 freeway

represent about 30% of vehicles while on the 405

freeway they represent o5% (Zhu et al., 2002). Fig. 3

compares the traffic volume on both the 405 and the 710

freeways. Error bars represent one standard deviation. It

is seen that the 710 freeway has about 7 times as many

diesel vehicles and 70% of gasoline vehicles as the 405

freeway. The total vehicle numbers on both freeways are

quite similar 12,180 versus 13,900/h for the 405 freeway.

Zhu et al. (2002) reported that a traffic slowdown on

freeway 405 was associated with a drop in total particle

number concentration indicating that fewer ultrafine

particles are emitted during such events. In this study,

the traffic speed on the 710 freeway stayed constant

through out the sampling period. No traffic slow down

was observed. The difference in the variability of traffic

volume on both freeways is indicated by the error bars in

Fig. 3.

Zhu et al. (2002) reported that both wind speed and

traffic density affected the characteristics of ultrafine

particles near the 405 freeway, and the control CPC

responded to these effects reasonably well. Thus,

subsequent data for ultrafine particle analysis at

increasing distances from the freeway were all normalized to the control CPC’s reading. An average CPC

reading, CN; was obtained based on all the measurements. In Figs. 4–6, number concentration and size

distribution data were scaled to CN by dividing each

measurement by the ratio of CPC reading for the period

of measurement to CN:

3.3. Change in ultrafine particle size distribution with

increasing distance

Fig. 4 depicts ultrafine particle size distributions at 17,

20, 30, 90, 150 and 300 m downwind and 300 m upwind

of freeway 710. The size distributions are smoothed and

shown together with common scales for both axes. The

horizontal axis represents particle size on a logarithmic

scale, while the vertical axis represents normalized

particle number concentration in the size range of 6–

220 nm as measured by the SMPS. Data were averaged

for all applicable sampling dates for each distance from

the freeway. As shown in Fig. 4, ultrafine particle size

Fig. 3. Traffic volume comparison for the 405 and 710 freeway. Bars indicate one standard deviation.

4328 Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335
distribution changed markedly and its number concentration dropped dramatically with increasing distance.

At the nearest sampling location, 17 m downwind from

the center of the freeway, the dominant mode was

around 10 nm with a modal concentration of more than

3.2  105

/cm3. This mode remained at 10 nm for the

second sampling location, 20 m downwind from the

freeway, but its concentration dropped to 2.4  105

/cm3.

It shifted to larger size range and its concentration kept

decreasing for farther sampling locations. This mode

was not observed at distance >150 m downwind from

the freeway. The dramatic decrease of particle number

concentration in the size range around 10 nm was likely

due to atmospheric dilution and several atmospheric

aerosol particle loss mechanisms that favor small

particles, diffusion to surfaces, evaporation, and coagulation. The smaller the particle, the greater its diffusion

coefficient and its Brownian motion. Particles of 10 nm

diffuse about 80 times faster than particles of 100 nm

(Hinds, 1999). As particle size gets smaller, the Kelvin

effect becomes more important, making it easier for

molecules to leave the particle’s surface by evaporation.

In addition, when two small particles collide due to their

Brownian motion (coagulate), they form a bigger

particle. Thus, coagulation reduces number concentrations and shifts the size distribution to larger sizes.

Fig. 5. Normalized particle number concentration for different size ranges as a function of distance from the 710 freeway.

Fig. 4. Ultrafine particle size distribution at different sampling locations near the 710 freeway.

Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335 4329
In Fig. 4, the second mode at 17 m downwind from

the freeway was around 20 nm with a concentration of

1.5  105

/cm3. This mode remained at similar size range

and concentration for the next sampling location, 20 m,

but shifted to 30 nm at 30 m downwind from the

freeway. It is of particular note that, while the

concentration for the primary mode, 10 nm mode,

decreased about 60% of its maximum value from 17

to 30 m with a slight shift in its mode, the 20 nm mode

concentration did not change significantly but the modal

size shifted noticeably. This second mode continued to

shift to larger sizes with increasing distance from

the freeway. In general number concentrations for

smaller particles, dpo50 nm, dropped significantly

with increasing distances from the freeway, but for

larger ones, dp > 100 nm, number concentrations

decreased only slightly. These results are in excellent

agreement with what Zhu et al. (2002) reported for

freeways impacted mostly by gasoline vehicles, which

suggests that coagulation is more important than

atmospheric dilution for the smallest ultrafine particles

and vice versa for large particles. Ultrafine particle

concentrations measured at 150 and 300 m downwind of

the 710 freeway were statistically within the variation

of the 300 m upwind background concentration.

The maximum number concentration that was observed next to the freeway was about 30 times

greater than that for the background location. This

suggests that people who live or work within 100 m

downwind of major traffic sources, or spend a substantial amount of time commuting on such highways,

will have a much higher ultrafine particle exposure

than those who do not. This result can be used in

epidemiological studies to estimate exposure to

ultrafine particles.

Based on Fig. 4, it is clear that vehicle-emitted

ultrafine particles of different size ranges behave quite

differently in the atmosphere. Zhu et al. (2002) showed

the decay of ultrafine particle number concentrations in

four size ranges 6–25, 25–50, 50–100 and 100–220 nm.

They found coagulation played a significant role in

modifying the particle size distribution of vehicleemitted ultrafine particle downwind of a freeway.

Fig. 5 was prepared in the same ways as Zhu et al.

(2002). The measured particle number concentrations in

each SMPS size bin were combined in the corresponding

size range, and the result was normalize to averaged

wind speed. The general trends of sub-grouped ultrafine

particle decay curves are quite comparable to those

given by Zhu et al. (2002), Figs. 7a and b. Total particle

number concentration in the size range of 6 to 25 nm

accounted for about 70% of total ultrafine particle

number concentration and dropped sharply, by about

80%, at 100 m, and leveled off after 150 m. Overall, it

decayed exponentially through out the whole measured

distance. Number concentrations in the next two size

ranges 25–50 and 50–100 nm, all experienced a shoulder

between 17 and 150 m. These results are in excellent

agreement with what Zhu et al. (2002) observed and can

be explained by particles, in smaller size ranges,

coagulating with these particles to increase their size.

Fig. 6 compares the ultrafine particle size distributions

at 30 m downwind from the 710 and the 405 freeways.

Three-mode lognormal fitting was used for 405 freeway.

Raw data were smoothed by averaging for 710 freeway.

Heavy-duty diesel trucks on the 710 freeway represent

more than 25% of traffic while on the 405 freeway they

represent o5% (Zhu et al., 2002). Average PM emission

rate for heavy-duty diesel trucks is about 0.4 g/mi

(California ARB, 2000) while for passenger cars is

Fig. 6. Comparison of ultrafine particle number concentration at 30 m downwind from 405 and 710 freeway.

4330 Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335
Fig. 7. Decay curves of: (a) CO, (b) BC and (c) particle number concentration near the 405 and 710 freeway.

Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335 4331
about 0.08 g/mi (EPA, 2000). Thus, on the 710 freeway,

about 60% of PM emission is due to heavy-duty diesel

trucks ((0.25  0.4)/(0.25  0.4+0.75  0.08)=62.5%).

In Fig. 6, both size distributions have three distinct

modes. The concentration for the first mode, between 10

to 20 nm, is slightly higher near the 405 freeway. This

mode is likely to arise from homogeneous nucleation of

semi-volatile materials and is similar to that previously

reported for direct laboratory measurement of gasoline

vehicle emissions (Ristovski et al., 1998). The concentration for the second mode, around 30 nm, is about

30% higher near the 710 freeway than that near the 405

freeway. This mode probably comprises mainly of BC

and is likely due to the much higher diesel emissions on

the 710 freeway. The last mode, around 70 nm,

represents an insignificant contribution to number

concentrations for these two freeways and in both cases

are comparable to the background concentrations.

3.4. Decay of carbon monoxide, black carbon and particle

number concentration

To make this freeway study more comprehensive, the

concentrations of CO, BC, and particle number were

also measured at increasing distance from the freeway

on selected dates, as shown in Table 1. CO and BC were

intentionally selected because their ambient concentrations are closely related to vehicular emissions. Averaged concentration and range of values at different

distances from the freeway of each measured property

are summarized in Table 2. CO and BC concentrations

decreased noticeably when moving away from the traffic

sources, similar to the findings of the study by Zhu et al.

(2002).

Figs. 7a–c were prepared by comparing the decay

characteristic of CO, BC and particle number concentrations near the 405, gasoline vehicle dominated, and

the 710, diesel vehicle dominated, freeways. Exponential

decay was found to be a good estimator for predicting

total particle number concentrations at different locations (Zhu et al., 2002). Each data point in the figure

represents an averaged value for all measurements with

similar wind directions. The solid line was the best fitting

exponential decay curve, determined using SigmaPlot

2000 nonlinear curve fitting procedure. The best fitting

exponential decay equations and R2 values are also

given in the figure. It can be seen, in general, all three

pollutants decay at a similar rate near both freeways.

This implies that atmospheric dilution plays a comparable role in both studies. As discussed previously, the

average wind speed for these two studies are all close to

1.5 m/s. The discrepancies of the curves were mainly due

to the different traffic fleet compositions on these two

freeways. The 710 freeway has more than 25% heavy

diesel trucks while the 405 freeway has o5%. It is well

known that diesel engines emit less CO and more BC

comparing to spark ignition engines (Kittelson et al.,

2001). Fig. 7a shows that the concentration of CO near

the 710 freeway is generally half of that near the 405

freeway. By comparison, Fig. 7b shows the BC concentration near a diesel vehicle dominated freeway is more

than three times greater than that near a gasoline vehicle

dominated freeway. As shown in Fig. 7c, the total

particle number concentration close to the 405 freeway

is somewhat higher than that near the 710 freeway, but

drops faster with downwind distance. Since the rate of

coagulation increases with decreasing particle size down

to 20 nm (Hinds, 1999), the observed result suggests

more of the smallest ultrafine particles, mostly in nanosize range, were emitted from the 405 freeway. This may

be explained by a total of 20% more vehicles on the 405

freeway. It was previously reported that number

emission rates from the spark-ignition vehicles were

much lower than from the diesel vehicles under most

operating conditions, but were similar under high-speed

highway cruise conditions (Rickeard et al., 1996;

Kittelson, 1998). It should also be noted that the

exponential decay characteristic appears to extend to

about 3 m downwind from the edge of the freeway for all

three pollutants. Based on our results we conclude that

atmospheric dilution is so rapid that average concentration decays continuously after leaving the tailpipe.

Table 2

Measured averaged concentrations at increasing distances from the freewaya

Measurement Upwind (m) Downwind distance (m)

200 17 20 30 90 150 300

CO 0.1 2.3 2.0 1.7 0.5 0.4 0.2

(ppm) (0.0–0.2) (1.9–2.6) (1.5–2.4) (1.1–1.9) (0.2–0.7) (0.1–0.5) (0.1–0.3)

Black carbon 4.6 21.7 19.4 17.1 7.8 6.5 5.5

(mg/m3

) (3.1–5.9) (20.3–24.8) (16.5–21.6) (12.6–19.3) (4.5–9.3) (3.9–9.2) (3.5–7.7)

Number concentration 0.48 2.0 1.8 1.6 0.72 0.61 0.49

(  105/cm3) (0.36–0.57) (1.8–2.5) (1.5–2.5) (1.2–1.9) (0.42–1.1) (0.35–0.98) (0.30–0.59)

aRange given in parenthesis.

4332 Y. Zhu et al. / Atmospheric Environment 36 (2002) 4323–4335
Fig. 8 shows the decay curves for relative concentrations of CO, BC and total particle number. The curves

are normalized and extended to reach 1.0 at the

downwind edge of the 710 freeway. Background

concentrations are also shown in the figure. It is seen

that CO, BC and particle number concentration

decreased about 60–80% in the first 100 m and then

leveled off somewhat after 150 m, similar to what Zhu

et al. (2002) reported. Background CO has a much lower

relative concentration while background BC and particle

number concentrations are comparable. Thus, CO was

diluted more quickly and significantly than BC and

particle number concentration. In general, CO, BC and

particle number concentrations tracked each other very

well. These results confirm the common assumption that

vehicular exhaust is the major source for CO, BC and

ultrafine particles near a busy freeway. They also

support the conclusion made by Zhu et al. (2002) that

for the conditions of these measurements the decreasing

characteristics of any of these three pollutants could be

used interchangeably to estimate the relative concentration of the other two pollutants near freeways.

4. Conclusions and summary

Wind speed and direction are important in determining the characteristic of ultrafine particles near freeways.

The average concentrations of CO, BC and particle

number concentration at 17 m was 1.9–2.6 ppm, 20.3–

24.8 mg/m3

, 1.8  105–3.5  105/cm3, respectively. Relative concentration of CO, BC and particle number

tracked each other well as one moves away from the

freeway. Exponential decay was found to be a good

estimator for the decrease of these three pollutants’

concentration with distance along the wind direction

starting from the edge of the freeway. Measurements

show that both atmospheric dilution and coagulation

play important roles in the rapid decrease of particle

number concentration and the change in particle size

distribution with distance away from a freeway.

Acknowledgements

This work was supported by the Southern California

Particulate Center and Supersite: US Environmental

Protection Agency under grant number R82735201, and

California Air Resources Board under contract number

98-316. The authors also would like to thank Mr.

Yuqing Zhang, for his assistance with the field

measurement.

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