Office of Research and Development

National Exposure Research Laboratory, Atmospheric Modeling Division

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June 10, 2009

Wind Tunnel Simulation - Six Lane Roadway

Noise

Barriers

Flow

Vlad Isakov

2009 R/S/L Modelers Workshop

Near-Road Modeling Research
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Near road impact assessment 

priority research area for ORD

–Over 36 million people in this country live within 100 meters of a fourlane or larger highway. A number of health studies have identified 

increased risks of adverse health effects for populations spending 

significant time near major roads 

–Emission inventories reveal that mobile sources significantly contribute 

to local and national concentrations of a number of air pollutants. 

–Air quality monitoring studies have measured elevated concentrations 

of pollutants emitted directly by motor vehicles near large roadways, 

adding to the public health concern for adverse health effects resulting 

from exposures to motor vehicle emissions.

–EPA’s Clear Air Research multi-year plan increases the emphasis on 

research to better understand the linkages between sources and 

health outcomes. 

–A Near-Road Action Plan has been developed to integrate all of 

EPA/ORD’s ongoing and planned activities addressing near-road 

issues. 
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Environmental Issue

► Need to further understand the atmospheric transport and 

dispersion of emissions within the first few hundred meters of the 

roadway, a region often characterized by complex flow (e.g. sound 

barriers, road cuts, buildings and vegetation). 

► Therefore, new studies of near-road emissions, and field and 

laboratory measurements of concentration distributions are 

necessary. 

► To simulate 

potential exposures, 

an improved 

algorithm is needed 

for modeling 

roadway dispersion 
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Research Objectives

The ORD near-road research program’s overall goal is to understand the 

source-to-health outcome continuum for roadway pollutants. The subobjectives related to AMAD’s work include the following:

► Characterization of the spatial variability of traffic-related pollutants 

near roadways using wind tunnel, computational fluid dynamics (CFD) 

and tracer and year-long field studies of roadway dispersion. The 

resulting data bases will be used for the development and evaluation of 

applied modeling tools.

► Assessment of how roadway design, meteorology, topography, and 

surrounding structures and vegetation affect near-road air quality.

► Assessment of inadequacies in applied dispersion models and 

development of improved modeling tools for linking traffic emissions to 

population exposures for use in regulatory decision-making, for 

evaluation of mitigation techniques and for transportation planning.
4

Research Approach

The overall approach to understanding the spatial distribution of 

traffic-related pollutants and for improving our tools for simulating the 

effects of roadside structures includes:

1) Examining existing tools - reviewing and evaluating existing models 

and data bases related to near road dispersion;

2) Developing measurement methods - developing methods for 

measuring near-road concentration distributions in both the laboratory 

and in actual urban situations;

3) Performing new studies - collecting and analyzing field, tracer, and 

wind tunnel measurements, as well as CFD results to understand the 

basic structure of flow and dispersion;

4) Improving algorithms and modeling methodologies - developing 

and evaluating improved algorithms and incorporating them into a 

selected near-field modeling platform (e.g. AERMOD) and as a subgrid 

algorithm within CMAQ.
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1) Examining existing tools

An exhaustive review of 21 available dispersion models that simulate line-type 

sources was completed (Report EPA/600/R-09/001). Based on the findings and 

recommendations of this report, AERMOD, was chosen as the best platform for 

further development for roadway scenarios. AERMOD is the current near-field 

and urban scale model recommended by the EPA for regulatory assessments. 

A methodology has been developed as part of this program 

to use mobile platform measurements over an urban area 

to characterize air-toxic concentration distributions (Isakov, 

et al. 2007). Successfully demonstrated on fine particulate 

and formaldehyde measurements in Wilmington, DE, the 

use of a mobile platform provides several advantages over 

fixed monitoring networks by providing measurements on a 

quick response basis over a large part of an urban area. 

This methodology was adopted by EPA and is currently 

being applied with the GMAP mobile sampling vehicle in a

series of intensive field studies examining the 

impact of roadside barriers on local air quality and 

their potential use for mitigation purposes.

2) Developing meas. Methods
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3) Performing new studies

An important tool for examining near road dispersion is the EPA’s 

meteorological wind tunnel where the flow and dispersion around 

roadway scenarios (e.g. barriers, road cuts) have been examined.
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Complementing the wind tunnel studies, in particular to examine 

atmospheric stability, is a tracer study of line-source emissions upwind of a 

single 6 meter high barrier simulating a typical road-side noise barrier.

3) Performing new studies (continued)
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Field measurements have been conducted along an eight-lane expressway in 

Raleigh, NC, examining the concentration gradient in a clearing and behind a noise 

barrier. The barrier influence is substantial. These data are critical for model 

development and evaluation.

3) Performing new studies (continued)
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March 2008: Pilot study along I-85 in 

Durham, NC

Fall 2008: Roadside barrier study 

at 3 sites in the Triangle Region

3) Performing new studies (continued)
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• Cook et al. (2008) describe an approach for 

developing detailed highway emission inventories based 

on emission factors and traffic inventories for individual 

road links. This approach was applied with a hybrid 

model in New Haven, CT. Substantial spatial gradients 

were observed near roadways.

• Applying a line-source model of Venkatram et al. 

(2007), the eight-lane expressway in the Raleigh 2006 

field study was simulated for an area with no terrain 

changes or obstructions to the flow. The concentration 

gradient for benzene was matched well.

• Results of EPA wind tunnel studies provide initial 

indications that for distances beyond the cavity zone of 

the barrier, the effects of noise barriers can be simulated 

with a no-barrier model by adjusting the effective source 

location upwind by a distance of 5 to 10 barrier heights 

(Heist et al., 2007).

4) Improving algorithms 

and modeling 

methodologies
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Local topography, roadway design, and roadside features affect the 

transport and dispersion of traffic emitted pollutants away from the 

road. These features may be used by urban and transportation 

planners to mitigate population exposures near roads.

4) Improving algorithms and 

modeling methodologies (continued)
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Depressed 

Roadway

Algorithm*

*preliminary work based on 

surface-coal-mine algorithm 

currently in AERMOD

Based on previous wind 

tunnel studies (Perry et al., 

1994), an algorithm has 

been developed for 

inclusion into AERMOD for 

simulating the influence of 

depressed roadways. The 

figure at right compares the 

modeled concentration 

gradient with newlycollected independent wind 

tunnel data. 

4) Improving algorithms and 

modeling methodologies (continued)
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Next Steps

► Continued improvement of line-source type algorithms for the 

AERMOD for regulatory applications. 

► Further wind tunnel studies and possible tracer studies to examine 

additional roadway configurations, wind direction and atmospheric 

stability influences. 

► Collaboration with other EPA laboratories and the FHWA on field 

campaigns in Las Vegas, Detroit, and Raleigh to better understand the 

relationship between traffic emissions and roadway-related air pollution 

concentration gradients. 

► Computational fluid dynamics modeling of near-road dispersion to 

complement wind tunnel and tracer studies.

► Use of the improved near-road dispersion models to support planned 

environmental health studies in Atlanta during 2009. This is an integral 

part of the "Air Pollution Exposure and Health Cooperative Agreement" 

between EPA/NERL and Emory University. 
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EPA-Emory/GA Tech Cooperative Agreement Hypotheses:

– Accounting for finer spatial resolution in ambient concentrations in 

epidemiologic analyses will yield estimates of pollutant exposures that 

differ substantially from estimates based on ambient monitoring data alone

– The use of these refined estimates will provide greater power to detect 

epidemiologic associations of interest, particularly for heterogeneous, 

traffic-related pollutants

Requirements of Air Quality Models: It’s all about exposure

– Regulatory models, e.g., for permitting, applied to predict the peak of the 

concentration distribution, unpaired in time and space, for comparison to 

AQ standards (i.e., maximum exposed individual)

– Air quality models used for exposure and risk assessments require skill at 

predicting concentration distributions paired in time and space (i.e., 

population exposure/risk)

– Growing need for integrated exposure and risk-based approaches to 

health and environmental impact assessments places higher demands on 

photochemical and dispersion model skill

Using improved near-road dispersion models 

to support exposure and health studies
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Research is building block for 

successful exposure/risk studies

–Need continued emphasis on improving our atmospheric models 

through support of our basic research & development

–Rigorous testing and evaluation are critical for necessary 

improvement in model inputs and science, e.g., 

• Challenges with meteorology at fine scales

• Complex urban environments

• Improvements in local scale emissions inventories

• Need more resolved local emissions 

• Modeling science to improve chemistry and physical processes 

at fine scales 

–Better understanding and characterization of model 

uncertainty/variability at fine scales

–Evaluation/comparison of techniques across projects, e.g., 

• Atlanta, NYC, Cleveland, Detroit, Houston are possibilities
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Research and Application Roles: 

NERL & OAQPS
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Contributors/Collaborators

EPA/NERL: Steve Perry, David Heist, Joe Touma, Tom 

Pierce, Val Garcia, Alan Vette

EPA/NRMRL: Gayle Hagler, Richard Baldauf

EPA/OTAQ: Chad Bailey, Richard Cook

EPA OAQPS: Roger Brode, Tyler Fox

NOAA Field Research Division: Kirk Clawson, 

Rick Eckman

University of California – Riverside: Akula Venkatram