REVIEW PAPER

Near-road air quality monitoring: Factors affecting network

design and interpretation of data

R. Baldauf & N. Watkins & D. Heist & C. Bailey &

P. Rowley & R. Shores

Received: 4 September 2008 /Accepted: 9 February 2009 / Published online: 12 March 2009

# The Author(s) 2009. This article is published with open access at Springerlink.com

Abstract The growing number of health studies identifying

adverse health effects for populations spending significant

amounts of time near large roadways has increased the interest

in monitoring air quality in this microenvironment. Designing

near-road air monitoring networks or interpreting previously

collected near-road monitoring data is essential for transportation system planning, environmental impact assessments,

and exposure assessments in health studies. For these

applications, care must be taken in determining the pollutants

of interest for both air quality and health assessments. In

addition, planners and data analysts need to evaluate and

understand the potential influence of the roadway type,

design, and presence of roadside structures on the potential

transport and dispersion of traffic-emitted pollutants on these

air quality and health evaluations. This paper summarizes key

factors related to the collection and interpretation of near-road

air quality data from the perspective of the pollutants of

interest and the location of the monitoring sites.

Keywords Air quality . Health studies. Monitoring network

design . Near-road . Traffic emissions

Introduction

In recent years, a large number of epidemiological studies

have examined associations between living near major

roads and adverse health endpoints. These studies indicate

that populations living, working, or going to school near

major roads may be subjected to an increased risk for a

number of adverse health effects, including respiratory,

cardiovascular, premature mortality, birth and developmental effects, and cancer (e.g., Pearson et al. 2000; Wilhelm

and Ritz 2003; Finkelstein et al. 2004; Gauderman et al.

2005; McConnell et al. 2006; Adar and Kaufman 2007;

Samet 2007; Samal et al. 2008). Air quality monitoring

studies have measured elevated concentrations of pollutants

emitted directly by motor vehicles near large roadways—

relative to overall urban background concentrations—

although these studies have been for short durations (e.g.,

Zhu et al 2002; Harrison et al. 2003; Reponen et al. 2003;

Kim et al. 2004; Baldauf et al. 2008a). These elevated

concentrations generally occur within a few hundred meters

of the road; however, this distance may vary depending on

the traffic patterns, environmental conditions, topography,

and the presence of roadside structures.

The referenced health studies examining near-road

populations generally employed non-specific metrics to

Air Qual Atmos Health (2009) 2:1–9

DOI 10.1007/s11869-009-0028-0

R. Baldauf : C. Bailey : P. Rowley

Office of Air and Radiation, Office of Transportation and Air

Quality, National Vehicle and Fuel Emissions Laboratory,

U.S. Environmental Protection Agency,

Ann Arbor, MI, USA

R. Baldauf : R. Shores

Office of Research and Development, National Risk Management

Research Laboratory, U.S. Environmental Protection Agency,

Research Triangle Park, NC, USA

N. Watkins

Office of Air and Radiation, Office of Air Quality Planning

and Standards, U.S. Environmental Protection Agency,

Research Triangle Park, NC, USA

D. Heist

Office of Research and Development, National Exposure

Research Laboratory, U.S. Environmental Protection Agency,

Research Triangle Park, NC, USA

R. Baldauf (*)

U.S. Environmental Protection Agency,

109 T.W. Alexander Dr., NC 27711, USA

e-mail: Baldauf.Richard@epa.gov
represent exposures to traffic-generated pollutants. Typical

exposure metrics include distance to the nearest road, traffic

volume on the nearest roads [usually as the average annual

daily traffic (AADT) count], or a summary measure of total

traffic density within a specified area around the residence/

school/workplace. The desire to better understand the

relationship between exposure and health effects necessitates establishing longer term near-road air quality monitoring capabilities, as well as interpreting historical

monitoring data for air quality and health assessments.

This paper summarizes important factors to be considered in designing near-road air quality monitoring networks

or interpreting previously collected data. These factors are

becoming increasingly important in designing an ambient

air monitoring strategy, especially in the USA, as federal

and state, local, and tribally run networks change to adapt

to new National Ambient Air Quality Standards (NAAQS)

requirements and evolving monitoring objectives. This

paper presents options and considerations for specific

pollutants and monitoring techniques, addresses the evaluation of near-road air pollutant impacts (including siting

logistics and concerns), evaluates the effects of roadside

structures on pollutant transport and dispersion, and

describes issues related to the spatial extent of monitoring

air pollutants near roads.

Measurements

Collecting and interpreting near-road air quality measurements requires information on source characteristics as well

as on the resulting air pollutant concentrations. The air

pollutants measured should be chosen based on the

emission characteristics of motor vehicles as well as

potential public health concerns. The following sections

discuss potential parameters and pollutants of interest for

near-road monitoring.

Traffic activity

Important parameters for traffic activity that can be readily

obtained for near-road monitoring assessments include the

number of vehicles, the fleet mix, and vehicle speeds. Each

of these parameters has an effect on the concentration and

characteristics of near-road pollutants. Traffic volume

provides information on the overall source input. The fleet

mix indicates the role of vehicle type on near-road air

quality; for example, emissions can be quite different for

locations where light-duty passenger vehicles primarily

burn gasoline than where heavy-duty trucks primarily burn

diesel. Speed provides an indication of the type of operation

on the highway when compared to traffic volumes; for

example, low speeds with higher traffic volumes often

indicate road congestion, while high speeds with lower

volumes suggest relatively little congestion.

Around the world, many national and state transportation

agencies maintain a network of traffic surveillance equipment ranging from external devices, such as video cameras

and radar, to instruments embedded in the road pavement,

including weigh-in-motion sensors. Prior to selecting the

site for a near-road monitoring study, the researcher should

contact the appropriate transportation agencies to identify

the locations of their monitoring systems in order to

leverage the existing traffic monitoring infrastructure and

maximize the amount of data for use in the study. Air

quality monitoring close to an existing traffic surveillance

site will provide valuable information for interpreting the

relationship between the air quality measurements and

emissions from traffic operations. If a traffic monitoring

site is not available, the installation of traffic surveillance

equipment should be considered, if feasible.

Meteorology

Meteorological measurements, namely wind speed and

direction, temperature, humidity, and atmospheric stability,

are needed for interpreting near-road air quality data.

According to Venkatram et al. (2007), the most important

factors driving the dispersion of traffic-generated emissions

are wind speed and wind direction; in particular, the

standard deviation of the vertical fluctuation in wind. An

important factor in near-road air quality measurements is

the influence of moving traffic on wind-flow fields near the

road. This “vehicle-induced turbulence” can extend tens of

meters from the road (Kalthoff et al. 2005). As a result,

conducting meteorological measurements at the air quality

monitoring site should be strongly considered. Since

vehicle-induced turbulence has a horizontal and vertical

effect, measurements collected at multiple heights and in

the horizontal and vertical components are most useful for

comparing general air mass flow from the road with

vehicle-induced turbulence. In addition to these local wind

flow and turbulence measurements, data on other parameters, such as temperature and relative humidity, will still be

important since these parameters affect the generation,

transformation, and transport of pollutants emitted by motor

vehicles.

Air quality

Air quality measurement considerations include both the

types of pollutants monitored as well as the location of

these measurements. This section focuses on the types of

pollutants of interest for near-road monitoring, including

criteria pollutants (those with NAAQS), air toxics, and

select particulate matter (PM) components. When new

2 Air Qual Atmos Health (2009) 2:1–9
monitoring networks are being designed, the ultimate

choice of pollutants to be measured depends on the overall

objective of the monitoring site(s). The following sections

discuss potential compounds of interest, with the aim of

facilitating the selection process, and the surrogate measurements that can assist in interpreting traffic emission

impacts on near-road air quality and human health effects.

Criteria pollutants

For this paper, criteria pollutants include compounds that

are covered by the NAAQS in the USA and the air quality

guidelines of the World Health Organization (WHO). Nearroad monitoring can be carried out to assess the impacts of

transportation systems on local air quality pollutant concentrations, evaluate motor vehicle emission control programs, and elucidate the (negative) impacts of

transportation sources on local human health and/or the

environment.

Carbon monoxide Motor vehicles significantly contribute

to ambient carbon monoxide (CO) concentrations. Although CO emissions from motor vehicles have declined

through emission control technologies and regulations in

many countries, motor vehicles remain the primary source

of this pollutant in most locations. All motor vehicles emit

CO, but the majority of CO emitted from this source occurs

from light-duty, gasoline-powered vehicles. In addition to

health concerns from CO exposures, CO may be a useful

indicator of the transport and dispersion of inert, primary

combustion emissions from traffic sources since CO does

not react in the near-road environment.

Oxides of nitrogen Motor vehicles also significantly contribute to ambient nitrogen oxides (NOx) emissions in most

countries. Although all motor vehicles emit NOx, the

majority of on-road emissions occur from diesel vehicles.

In terms of primary emissions, the majority of NOx exhaust

is in the form of nitrogen oxide (NO). Nitrogen dioxide

(NO2), which is the focus of concern in terms of adverse

health effects, quickly forms by a photochemical reaction in

the ambient air in the presence of NO and ozone (O3).

However, primary NOx emissions from more technologically advanced heavy-duty diesel engines with aftertreatment devices may contain a much greater percentage

of NO2 in exhaust emissions. Thus, NO and NO2 will be

present in varying concentrations in the near-road microenvironment.

Sulfur dioxide Although motor vehicles emit sulfur dioxide

(SO2) and other sulfur-containing compounds, traffic

sources typically make only small contributions to ambient

concentrations. In addition, the introduction of low-sulfur

fuels in the USA and other countries to allow for the use of

after-treatment devices in diesel vehicles should further

reduce SO2 emissions. Therefore, this pollutant is not

commonly monitored in near-road applications. However,

the monitoring of SO2 may be useful near other types of

transportation facilities, such as airports, railway yards,

and seaports, as sources at these locations may still burn

high sulfur fuels. In addition, it may be useful to measure

this pollutant (or other sulfur-containing compounds) in

specific situations to identify the influence of other

sources at the monitoring site, such as power generation

or ship emissions.

Ozone Ozone is not directly emitted from motor vehicles,

and O3 measurements are not typically collected for nearroad applications. However, the presence of elevated NO

concentrations in the near-road microenvironment may lead

to lower concentrations due to “ozone scavenging” as part

of the formation of NO2 from NO and O3. Thus, O3

measurements may be useful under select circumstances as

support for health studies investigating the role of O3 and

other co-pollutants on adversely affecting public health

given the potentially lower concentrations of this pollutant

relative to other pollutants in this microenvironment.

Lead Before the introduction of unleaded gasoline throughout much of the world, motor vehicle lead (Pb) emissions

were a major public health concern. While Pb is no longer

added to gasoline in many countries, motor vehicle fuels

still contain trace amounts of Pb from crude oil. Lead is

also present in trace amounts in lubricating oil. Other

sources that may contribute to ambient Pb concentrations in

the near-road environment include brake wear, tire wear,

and the degradation of wheel weights used for aftermarket

tire balancing. Re-suspended road dust may also contain Pb

from historically deposited industrial or mobile source

emissions.

Particulate matter Motor vehicles emit significant amounts

of PM through combustion, brake wear, and tire wear, and

they may also contribute to elevated near-road PM

concentrations by re-suspending dust present on the road

surface. In the USA, the NAAQS regulates ambient

concentrations of PM <10 μm in diameter (PM10) and

PM <2.5 μm in diameter (PM2.5). Both of these PM size

fractions are emitted from motor vehicles. In general, PM

emitted by combustion will be in the PM2.5 size fraction.

Since combustion-emitted particles are typically present at

<0.1 μm in diameter, these emissions tend to contribute

little to ambient PM2.5 mass concentrations, but they do

contribute significantly to PM number concentrations and

may impact the chemical composition of the PM2.5 mass

collected near the road relative to urban background

Air Qual Atmos Health (2009) 2:1–9 3
conditions. Accumulation mode PM formed from motor

vehicle combustion emissions may also impact PM2.5 mass

concentrations further downwind from the road. Particulate

matter emitted through mechanical processes (brake wear,

tire wear, re-suspended road dust) will tend to be in the

PM10 size fraction and can lead to elevated mass concentrations. As a result, both PM10 and PM2.5 mass measurements may be useful for near-road applications with the

expectation of higher impacts on PM10 mass as opposed to

PM2.5 mass.

Most PM10 and PM2.5 mass measurements use filterbased, gravimetric analyses over a 24-h sample collection

period. However, diurnal variations in traffic and meteorology can have a tremendous impact on near-road air quality

that may not be identifiable in 24-h average measurements.

Therefore, continuous PM measurements provide useful

information for near-road applications, although care must

be taken in choosing a sampling method. Optical PM mass

samplers typically cannot detect particles less than approximately 0.2–0.5 μm in diameter and, consequently, these

measurement devices may not capture a significant amount

of the PM mass related to primary motor vehicle combustion emissions. In addition, some continuous PM samplers

heat the inlet air prior to analysis. Since motor vehicle PM

emissions contain a significant amount of semi-volatile

organic compounds, these samplers can underestimate the

PM mass in the near-road environment by volatilizing the

organic PM prior to collection in the sampler.

Unregulated pollutants

In addition to criteria pollutants, motor vehicles emit a large

number of other compounds which can cause adverse

health effects. These are often characterized as air toxics or

hazardous air pollutants. A discussion and listing of

potential air toxics of concern for near-road monitoring

can be found in the U.S. Environmental Protection

Agency’s (EPA) Mobile Source Air Toxics (MSAT) Rule

(U.S. EPA 2007). These pollutants include volatile organic

compounds (VOCs), aldehydes, and organic and inorganic

PM constituents. The reasons for monitoring these pollutants in a near-road program include concerns over adverse

human health effects, ecological effects, and the evaluation

of the effectiveness of mobile source control programs.

This section discusses potential monitoring activities for a

number of classes of air toxic compounds, but a discussion

of all potential air toxic compounds identified in the MSAT

rule is beyond the scope of this document.

Volatile organic compounds These air toxics are found in

the gas phase in ambient air. Typical VOCs of concern for

near-road monitoring include, but are not limited to,

benzene, toluene, ethylbenzene, xylenes, and styrene. A

more detailed listing of potential VOCs of health concern is

included in the MSAT Rule. Volatile organic compounds

are typically measured by the collection of ambient air

using evacuated canister sampling and subsequent analysis

on a gas chromatograph (GC). Another class of VOCs,

carbonyls, is typically collected in another way, as

described in the next section. For evacuated canister

sampling, the sample collection time can vary from

instantaneous grab sample to averaging times of more than

24 h depending on the collection orifice used. As discussed

for PM sampling, shorter averaging times can be important

to discern the impacts of varying vehicle and environmental

conditions on near-road air quality. Auto-GCs can be used

to measure select VOC pollutant concentrations semicontinuously at a monitoring site. A number of manufacturers also advertise semi-continuous analyzers for one or

more VOCs of interest using various GC technologies.

Aldehydes Aldehydes emitted from motor vehicles include,

but are not limited to, formaldehyde, acetaldehyde, and

acrolein. A more detailed listing of aldehydes with potential

health concerns is included in the MSAT Rule. Aldehydes

are typically measured using cartridges containing dinitrophenyl hydrazine (DNPH). However, other methods,

including evacuated canisters and cartridges with dansylhydrazine (DNSH), have been used to measure ambient

concentrations of some of these compounds. Sample

collection periods of 24 h or more are typically required

for assessing ambient aldehyde concentrations although a

few manufacturers advertise semi-continuous analyzers for

select compounds.

Particulate matter constituents The PM present near roadways contains a number of organic and inorganic constituents that may pose a public health risk. Organic PM

samples are most often collected on filters backed by a

cartridge to collect the gas-phase constituents. Sample

collection methods typically consist of high-volume samplers to maximize the amount of PM mass obtained for

detailed chemical and physical analysis; thus, collection

times can be from 24 h to over a week to collect an ample

amount of mass. Inorganic PM samples are also usually

collected on filters using high-volume samplers and longer

sampling times in order to collect sufficient mass for

elemental analyses. A detailed listing of organic and

inorganic PM compounds of health concern is included in

the MSAT Rule. In addition, detailed speciation of organic

and elemental PM compounds present in near-road samples

can be useful in conducting or evaluating source apportionment studies to estimate the impacts of traffic emissions on

near-road PM concentrations, although the long sample

averaging times required for this analysis may limit the

4 Air Qual Atmos Health (2009) 2:1–9
ability to discern differences in vehicle activity on PM

organic and inorganic air quality impacts.

Surrogate measurements

A number of surrogate measurements can also be considered to assist in interpreting traffic emission impacts on

near-road air quality and to determine possible causes of

adverse health effects. One common surrogate has been the

use of CO to represent the impacts of other non-reactive gas

emissions that are more difficult to measure from motor

vehicles. While studies do show that CO and other nonreactive VOC concentrations tend to correlate in the nearroad environment, the magnitude of VOC concentrations

relative to CO concentrations may be difficult to discern

because of varying impacts from control strategies and

emission sources. Regulations that have led to reductions in

CO emissions may not equally affect VOC emission rates.

In addition, CO is emitted by fuel combustion, whereas

VOCs are emitted from both combustion and evaporation

processes. Carbon monoxide may also not be a consistent

surrogate for roadways dominated by heavy-duty diesel

vehicle traffic due to low emission rates of this pollutant

from these vehicles.

Other surrogate measurements focus on PM constituents

that are primarily emitted from motor vehicles and which

may pose a public health concern. These surrogate

measurements are discussed in the following sections.

Particulate matter number concentration

As previously discussed, PM emitted through the combustion process occurs primarily in the ultrafine size range (i.e.,

<0.1 μm in diameter); thus, the impact on PM mass may be

negligible. However, emissions of these small particles

occur in extremely large quantities; therefore, PM number

concentration measurements often provide a good indication of primary PM exhaust emissions from motor vehicles.

In addition, several health studies suggest that ultrafine

particles may lead to adverse health effects, as identified in

the near-road literature. A number of devices exist to

measure PM number concentrations, ranging from inexpensive industrial hygiene monitors to research-grade

ambient air monitors. Most of these devices can provide

number concentration measurements in near real-time,

although the range of particle sizes and concentrations

detected do vary. When measurements from different

devices are compared, any differences in particle size

ranges detected must be noted.

Black (or elemental) carbon Black carbon (BC), often

referred to as “soot,” is a common constituent emitted from

motor vehicles. Another measurement is elemental carbon

(EC), which is detected using different techniques. Both BC

and EC are operationally defined and represent the

graphitic-containing portion of PM. Although BC and EC

are often associated with emissions from heavy-duty diesel

engines, a portion of all motor vehicle combustion

emissions contains these constituents. Other sources of

BC or EC exist in urban areas, but emissions from motor

vehicles usually dominate these sources in terms of nearroad air quality, thus making BC or EC measurements a

useful parameter for identifying impacts from motor vehicle

emissions.

Site location considerations

The complexity of urban land use makes the process of

selecting appropriate monitoring sites an extremely important consideration for near-road assessments. This complexity is important when the objective is to understand

historical monitoring data. Factors such as the type of road,

traffic activity patterns, roadway design features, and the

presence of nearby structures and barriers will influence the

transport and dispersion of pollutants emitted by nearby

traffic. In addition, the number and location of samplers

will affect the type of information obtained and the manner

is which the data can be applied to decision-making

regarding public health issues. The following sections

provide more details on site location issues and logistical

considerations for establishing near-road air quality monitoring programs or interpreting near-road monitoring data.

Roadway type

Most health and air quality studies implicating roadways as

a public health concern have generally focused on highways supporting large volumes of traffic. These transportation facilities typically support a large number of

passenger cars and large trucks operating at higher speeds

during most times of the day. However, most urban

highways experience regular periods of congestion during

morning and afternoon rush hours at which time vehicles

tend to operate at low speeds and/or even idle. On these

high-speed highways, motor vehicle emissions occur

primarily from fuel combustion, fluid evaporation, brake

and tire wear, and re-suspended road dust.

Most urban areas also contain arterial roadways with

large volumes of both passenger cars and large trucks,

although these roads typically support much lower traffic

volumes than highways. Some health studies suggest that

residential proximity to arterial traffic emissions may relate

to a higher public health risk than highway traffic emissions

Air Qual Atmos Health (2009) 2:1–9 5
(Reponen et al. 2003). Several factors lead to differences in

traffic-generated pollutant emissions between an arterial

road and a highway. Vehicles on arterial roads may

experience more stop-and-go operations from traffic signals

and turning vehicles, which may lead to higher combustion

emissions from acceleration and long idling times and more

brake and tire wear from acceleration and deceleration. In

addition, some vehicles on arterial roads may be operating

under cold start conditions, where combustion emissions

increase because the vehicle has not been operating for a long

enough time to fully heat the catalytic converter (Baldauf et

al 2005). Under these conditions, vehicle emissions tend to

be higher, and the types and ratios of VOC and PM

compounds emitted may be different from higher volume

roadways. Thus, near-road monitoring around differing

road types yield differing air quality measurements.

Traffic volumes and modes of operation

There is no known threshold for total traffic volume, or the

percentage of large trucks, below which near-road populations are protected from adverse health effects or pollutant

concentrations are maintained below NAAQS levels.

Traffic emissions depend on a number of factors, including

traffic activity and traffic volume, and both of these factors

must be considered when data are interpreted or near-road

monitoring sites are established.

Roadway design

Roadway design can influence the amount of emissions

generated from motor vehicles and the transport and dispersion of the pollutants away from the road. Road grades, ramps,

intersections, and traffic merge locations can all impact on the

type and amount of vehicle emissions. Road grades create an

increased load on vehicles ascending the grade, leading to

increased exhaust emissions and potential tire wear, while

vehicles descending the grade likely experience increased

brake emissions. The presence of ramps, intersections, and

lane merge locations can also lead to increased brake wear

emissions and idling vehicle conditions due to increased

congestion. The type of pavement on the road may also affect

the amount of wear emissions and re-suspended road dust, as

asphalt pavements also emit small levels of VOCs.

The topography around the roadway influences pollutant

transport and dispersion away from the road. At-grade

roadways will experience the least amount of resistance to

pollutant dispersion if no other structures exist near the road.

However, cut section roads, where the roadway is below

ground-level and bordered by either vertical or sloped cut

walls, increase the number of vortices created by wind flow

into and along the cut section roadway, thereby increasing

pollutant dispersion. In addition, as winds flow up and out of

the cut section, the plume off the roadway may be more

elevated than under an at-grade condition. Elevated roadways

may concentrate pollutants near the road or enhance pollutant

dispersion away from the road depending on design. The

amount of mixing will vary if the road is elevated by a bridge

or solid fill material. Figure 1 shows an example from a wind

tunnel study comparing roadway design configurations and

changes in near-road air pollutant concentrations to illustrate

these effects.

Regardless of the roadway design, the activity of vehicles

on the road induce turbulence at the point of pollutant

emission, leading to enhanced pollutant mixing. In addition,

the elevated temperature of exhaust emissions enhance

thermal buoyancy in the plume, providing an initial mixing

zone for vehicle-emitted pollutants that depends on the

number, type, and speed of vehicles on the road. The more

turbulence present, the more initial dilution of pollutants will

occur. The initial mixing and turbulence effects will impact on

decisions regarding monitor siting, as discussed in more detail

in a later section of this paper.

Roadside structures

In addition to topographic feature changes near the road,

structures may be present that impact pollutant transport

and dispersion. These structures include noise barriers,

vegetation, and buildings. Physical barriers affect pollutant

concentrations around the structure by blocking initial

dispersion and increasing turbulence and initial mixing of

the emitted pollutants. Figure 2 illustrates the potential

effects of roadside barriers from field measurements taken

from an open field, from behind a noise barrier, and from

X/H

Ground level concentration

0 10 20 30 40 0

5

10

15

20 Flat terrain

Noise barrier, upwind only

Noise barrier, upwind and downwind

Depressed roadway (6m), vertical walls

Depressed roadway (6m), sloped walls

Elevated roadway (6m), slopedwalls

Depressed roadway (6m), sloped, with barriers

Fig. 1 Wind tunnel study results comparing downwind air pollutant

concentrations from a road at-grade with no obstructions to airflow

with roadway configurations containing varying topography and

roadside structures. The distance downwind is expressed in multiples

of the height of the noise barrier studied (6 m for this study). The

ground-level concentrations have been non-dimensionalized as CULx/

Qy where C is the measured concentration, U is the wind speed at a

height of 30 m, Lx is the width of the road, and Qy is the pollutant

emission rate per unit length of roadway. These results are for winds

perpendicular to the road (Heist et al. 2007)

6 Air Qual Atmos Health (2009) 2:1–9
behind a noise barrier and vegetation stands. A description

of the role of common barriers and structures in urban

environments follows.

Noise barriers Noise barriers reduce noise levels from traffic

by blocking and deflecting sound waves. These barriers also

affect air pollutant dispersion, leading to increased vertical

mixing due to the upward deflection of air flow caused by the

structure. Studies suggest that this upward deflection of air

creates a recirculation cavity downwind of the barrier

containing a well-mixed, and potentially lower, zone of

pollution concentrations. However, noise barriers adjacent to

a roadway may also inhibit air movements off the road,

leading to elevated on-road pollutant concentrations (Bowker

et al. 2007; Baldauf et al. 2008b).

Vegetation Vegetation stands will also affect near-road

pollutant concentrations. The complex and porous structure

of tree limbs and leaves increase air turbulence and promote

mixing and dispersion. Trees and other vegetation also

reduce pollutant concentrations by enhancing the deposition

of certain pollutants, notably PM components. Variables

such as the type of vegetation, plant height, and vegetation

thickness will likely influence the extent of mixing and

deposition experienced at the site, although the specific

relationships of these factors have not been identified

(Heichel and Hankin 1976; Munch 1993; Bussotti et al.

1995; Heath et al. 1999; Beckett et al. 2000; Bowker et al.

2007; Baldauf et al. 2008b).

Buildings Nearby buildings and other related structures

also affect air transport and dispersion. The presence of

large, solid structures impact on air flow, creating eddies

and wake regions similar to those described for noise

barriers. The effect on air pollutant transport and dispersion

will depend on the number and type of structures present.

Isolated, detached structures will likely impact only on

pollutant concentrations in close proximity to the structure.

However, large numbers of buildings may create funneling

effects that can actually concentrate pollutants at certain

locations. This phenomenon is often referred to as the

“street canyon” effect. Buildings also provide surface areas

for particle deposition.

Monitor siting considerations

When monitoring sites are being assessed for their

suitability to determine the impact of traffic emissions on

near-road air quality, a number of factors need to be

considered, including distance from the road, upwind/

background monitoring, horizontal spacing, and the need

for vertical measurements.

Distance from the road Results from previous near-road

monitoring studies show that air pollutant concentration

gradients are quite steep near the road. Many of these

studies suggest that traffic-generated pollutants decrease

exponentially with distance from the road, with most

showing that background pollutant levels are reached

within 200–500 m from the road. However, the magnitude

and shape of this curve depends on source characteristics,

meteorology, and roadway design features, as previously

discussed. Figures 1 and 2 demonstrate that the changing

concentrations with distance tend to be exponential;

however, the shape and magnitude of these curves can

change, especially with changes in roadway configuration

and the presence of roadside structures.

To account for this decrease in pollutant concentrations

in the near-road microenvironment, many studies utilize

multiple monitoring stations at varying distances from the

road, with the highest density of monitors within the first

100 m from the road. If resource limitations prevent the

establishment of multiple monitoring sites, a distance most

representative of population exposures may be more

appropriate. A minimum distance of 10–20 m from the

road should be considered in order to minimize the

influence of vehicle-induced turbulence on the concentration variability of pollutant measurements.

Upwind/background monitoring Collecting upwind and

background air quality measurements may be useful in

assessing traffic emission impacts from a roadway. Paired

monitors on either side of a road can limit the influence of

changing wind directions when interpreting near-road air

20 nm size particles

0

100

200

300

400

500

600

700

800

900

20406080

100120140160180200220240260280300

Distance from I-440 (m)

PM conc (number/cm3)

Open Field

Noise Barrier Only

Noise Barrier & Vegetation

Fig. 2 Mobile monitoring measurements of the number of 20-nm

sized particulate matter (PM) present at varying distances from a

125,000 average annual daily traffic (AADT) highway for an at-grade

road: no obstructions (open field), behind only a noise barrier (noise

barrier only), behind a noise barrier with mature vegetation stands

(noise barrier & vegetation). Bars 95% Confidence intervals for each

distance (Baldauf et al. 2008b).

Air Qual Atmos Health (2009) 2:1–9 7
quality data. When winds shift in direction, an upwind and

downwind pair of monitors will still remain.

When upwind monitoring locations are being considered,

care must be taken in ensuring that this site is an appropriate

distance from the road. Vehicle-induced turbulence may allow

traffic-generated pollutants to “meander“ upwind, resulting in

elevated pollutant concentrations at distances as far as 50–100

m upwind of the road (Venkatram et al. 2007).

Horizontal spacing Since topographic features influence

pollutant concentrations near roads, multiple samplers at

equal distances from the road, but under varying topographic or roadside structure conditions, can be used to

confirm the effect of these varying conditions. In addition

to fixed-site sampling at multiple locations, temporary or

mobile monitoring using instrumented vehicles may be

considered for short-term monitoring comparisons.

Vertical measurements As previously discussed, topographic features and roadside structures also enhance vertical

mixing of the plume emitted by traffic sources. Concentration measurements at multiple elevations can assess the

variability in plume height with changing traffic, meteorology, and roadway designs. In addition, vertical measurements may be useful for identifying pollutant concentrations

at air inlet heights for nearby buildings, such as schools or

residences.

Summary

This paper describes factors to consider when implementing

a near-road air quality monitoring program or interpreting

data collected at existing monitoring sites. Care must be

taken in determining the pollutants of interest for both air

quality and health assessments. Planners and data analysts

need to evaluate the influence of the roadway type, design,

and presence of near-road structures on the potential

transport and dispersion of traffic-emitted pollutants on air

quality and health evaluations. Further near-road studies

will increase our understanding of roadways as emission

sources impacting on local- to regional-scale pollutant

concentrations. This increased understanding will, in turn,

allow near-road monitoring, as a component of ambient air

monitoring strategy, to be more appropriately included in

overarching air quality management practices.

Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any

noncommercial use, distribution, and reproduction in any medium,

provided the original author(s) and source are credited.

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