UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

 WASHINGTON D.C. 20460

OFFICE OF THE ADMINISTRATOR

SCIENCE ADVISORY BOARD

February 23, 2012

EPA-CASAC-12-003

The Honorable Lisa P. Jackson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Subject: Review of the Draft Near-Road Nitrogen Dioxide Monitoring Technical Assistance 

Document 

Dear Administrator Jackson:

The Clean Air Scientific Advisory Committee (CASAC) has reviewed EPA’s “Near-Road NO2

Monitoring Technical Assistance Document (TAD) – Draft August 11, 2011.” This letter summarizes 

CASAC’s views on monitoring issues pertaining to the draft EPA document, and encloses CASAC’s 

responses to the EPA charge questions (Enclosure A) and a compilation of individual panel member 

comments (Enclosure B). 

In February 2010, EPA promulgated new minimum monitoring requirements for the nitrogen dioxide 

(NO2) monitoring network in support of a newly revised 1-hour NO2 National Ambient Air Quality 

Standard (NAAQS). In the new monitoring requirements, state and local air monitoring agencies are 

required to install near-road NO2 monitoring stations in larger urban areas at locations where maximum 

NO2 concentrations are expected to occur, including within 50 meters of major roadways. 

In August 2010, EPA’s Office of Air Quality Planning and Standards (OAQPS) requested that CASAC 

review the initial phase of EPA’s Near-Road project. In November 2010, CASAC issued a final report 

(EPA-CASAC-11-001) for its review of the outline for the TAD entitled “Near-road Guidance 

Document – Outline” and “Near-road Monitoring Pilot Study Objectives and Approach.” OAQPS 

considered CASAC’s recommendations and drafted the TAD to provide guidance to state and local air 

monitoring agencies on how to successfully implement near-road NO2 monitoring, and to assist 

stakeholders in making decisions associated with siting monitors as required by the recently 

promulgated primary NAAQS for NO2. 

CASAC generally found that the TAD was well written and provided information that would assist state, 

local and tribal agencies with siting of NO2 monitors. The document adequately deals with the EPA’s 

approach for using Annual Average Daily Traffic (AADT) to prioritize monitoring locations, and many 

of the details and issues associated with monitor siting. CASAC recognizes that EPA has released an 

updated, 12/21/11 version of the TAD on its website1

 1

that further refines and advances the guidance and 

http://www.epa.gov/ttn/amtic/nearroad.html
2

addresses issues raised by CASAC’s Air Monitoring and Methods Subcommittee (AMMS) during its 

two public teleconference calls on September 29 and November 17, 2011, though CASAC is not 

reviewing those revisions as part of this report. CASAC provides the following priority 

recommendations to strengthen and improve EPA’s Near-Road program. Additional details on these and 

other recommendations are enclosed. We have three major recommendations.

First, the TAD must clearly define the objectives of the Near-Road NO2 network in the context of EPA’s 

Policy Assessments for the review of NAAQS. In the November 2010 report, CASAC noted that “the 

objectives of the network are not well defined in the current outline. High priority should be given to 

developing clear objectives and providing a rationale for each.” This advice was not carried through into 

the TAD. A revised draft TAD should clearly state the objectives of the Near-Road NO2 network, along 

with the rationale for each objective. In the absence of clearly stated network objectives, the

Subcommittee had difficulty assessing how well the information provided in the TAD would lead to 

appropriate siting decisions. Furthermore, it is difficult for state, local and tribal agencies to choose 

suitable monitoring locations without a clear understanding of the network objectives. For example, the 

TAD should define the specific population exposures that are to be characterized by the monitors, 

specifically with regard to population exposures and on-road vehicle occupants. The EPA asserted that 

the NO2 standard applies to those in vehicles on the roadway in addition to those in areas adjacent to the 

roadway. Applying the NO2 standard to populations on the roadway has significant ramifications for 

choosing appropriate site locations because the near-road monitoring program required in the 

regulation may not capture on-road concentrations. While the highest exposures to roadway emissions 

may be on the roadway itself, routine monitoring on the roadway is not practical for safety and other 

reasons. Furthermore, the gradients away from the roadway are steep and the concentrations at a 

receptor can be very different from the measured values at a specific distance. Thus, it will be difficult to 

use single-site monitoring data to assess exposures of populations near roadways.

Second, the TAD should more specifically state how siting decisions should be made given the range of 

available information that can be used, identify what would or would not be allowed in terms of 

monitoring site placement, and describe the real limits on making such selections. For example, should 

results from monitoring, modeling or the use of the AADT take precedence? CASAC continues to view 

that too much emphasis is placed on the AADT. Another example is that the current guidelines appear 

to allow monitoring in a roadway tunnel. CASAC strongly recommends against monitoring in tunnels or 

other unusual places to enforce a NAAQS.

Third, the current document does not provide guidance specific to siting of the second monitor in those 

areas requiring two near-road NO2 monitors. CASAC suggests that separate guidance be developed and 

that the objectives of the second site may be differentiated from the first site. For example, the second 

monitor could be located to test hypotheses as to the sources of NO2 impacting population exposures or 

to sample other pollutants as part of the multipollutant network. 

In laying out the objectives of the network, EPA also should address the need to continue to characterize 

exposures to the broader populations in urban areas. CASAC continues to be concerned with the 

potential decrease in the number of monitors sited to characterize population-wide exposures; these 

monitors have provided the exposure data that is critical for epidemiologic analyses. CASAC strongly 

recommends that key population-oriented monitors be maintained, particularly those that have been used 

in past health-focused and accountability studies. Population-oriented monitors allow exploration of 

pollutant gradients, and can be used for comparison to near-road exposures. CASAC also strongly 

encourages a dialog with the epidemiology community and other stakeholders to minimize the loss of 

critical data.
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CASAC appreciates the opportunity to provide input to the EPA on this issue. We look forward to 

receiving the Agency’s response. 

Sincerely,

/Signed/ /Signed/

Dr. Armistead (Ted) Russell, Chair Dr. Jonathan M. Samet, Chair 

CASAC Air Monitoring and Methods Clean Air Scientific Advisory Committee 

Subcommittee

Enclosures
i

NOTICE

This report has been written as part of the activities of the EPA's Clean Air Scientific Advisory 

Committee (CASAC), a federal advisory committee independently chartered to provide extramural 

scientific information and advice to the Administrator and other officials of the EPA. CASAC provides 

balanced, expert assessment of scientific matters related to issues and problems facing the Agency. This 

report has not been reviewed for approval by the Agency and, hence, the contents of this report do not 

necessarily represent the views and policies of the EPA, nor of other agencies within the Executive 

Branch of the federal government. In addition, any mention of trade names or commercial products does 

not constitute a recommendation for use. CASAC reports are posted on the EPA website at: 

http://www.epa.gov/casac.
ii

U.S. Environmental Protection Agency

Clean Air Scientific Advisory Committee

Air Monitoring and Methods Subcommittee 

CHAIR

Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering, Georgia 

Institute of Technology, Atlanta, GA

MEMBERS 

Dr. David T. Allen, Professor, Department of Chemical Engineering, University of Texas, Austin, TX

Mr. George A. Allen, Senior Scientist, Northeast States for Coordinated Air Use Management 

(NESCAUM), Boston, MA

Dr. Linda Bonanno, Research Scientist, Office of Science/Division of Air Quality, New Jersey 

Department of Environmental Protection, Trenton, NJ 

Dr. Doug Burns, Research Hydrologist, U.S. Geological Survey, Troy, NY

Dr. Judith Chow, Research Professor, Desert Research Institute, Air Resources Laboratory, University 

of Nevada, Reno, NV

Dr. Kenneth Demerjian, Professor and Director, Atmospheric Sciences Research Center, State 

University of New York, Albany, NY

Mr. Eric Edgerton, President, Atmospheric Research & Analysis, Inc., Cary, NC

Mr. Henry (Dirk) Felton, Research Scientist, Division of Air Resources, Bureau of Air Quality 

Surveillance, New York State Department of Environmental Conservation, Albany, NY

Dr. Philip Fine, Atmospheric Measurements Manager, South Coast Air Quality Management District, 

Diamond Bar, CA

Dr. Philip Hopke, Bayard D. Clarkson Distinguished Professor, Department of Chemical and 

Biomolecular Engineering, Clarkson University, Potsdam, NY

Dr. Rudolf Husar, Professor, Mechanical Engineering, Engineering and Applied Science, Washington 

University, St. Louis, MO

Dr. Daniel Jacob, Professor, Atmospheric Sciences, School of Engineering and Applied Sciences, 

Harvard University, Cambridge, MA
iii

Dr. Peter H. McMurry, Professor, Department of Mechanical Engineering, University of Minnesota, 

Minneapolis, MN

Dr. Allen Robinson, Professor, Department of Engineering and Public Policy, Carnegie Mellon 

University, Pittsburgh, PA

Dr. James Jay Schauer, Professor, Department of Civil and Environmental Engineering, College of 

Engineering, University of Wisconsin - Madison, Madison, WI

Dr. Jay Turner, Associate Professor, Environmental & Chemical Engineering, Campus Box 1180, 

Washington University, St Louis, MO

Dr. Yousheng Zeng, Managing Partner, Providence Engineering & Environmental Group LLC, Baton 

Rouge, LA

SCIENCE ADVISORY BOARD STAFF

Mr. Edward Hanlon, Designated Federal Officer, U.S. Environmental Protection Agency, Washington, 

DC
iv

U.S. Environmental Protection Agency 

Clean Air Scientific Advisory Committee (CASAC) 

CHAIR 

Dr. Jonathan M. Samet, Professor and Flora L. Thornton Chair, Department of Preventive Medicine, 

University of Southern California, Los Angeles, CA 

MEMBERS 

Mr. George A. Allen, Senior Scientist, Northeast States for Coordinated Air Use Management 

(NESCAUM), Boston, MA 

Dr. Joseph D. Brain, Cecil K. and Philip Drinker Professor of Environmental Physiology, Department 

of Environmental Health, Harvard School of Public Health, Harvard University, Boston, MA 

Dr. H. Christopher Frey, Professor, Department of Civil, Construction and Environmental 

Engineering, College of Engineering, North Carolina State University, Raleigh, NC 

Dr. Armistead (Ted) Russell, Professor, Department of Civil and Environmental Engineering, Georgia 

Institute of Technology, Atlanta, GA 

Dr. Helen Suh, Senior Lecturer on Environmental Chemistry and Exposure Assessment, Department of 

Environmental Health, School of Public Health, Harvard University, Boston, MA 

Dr. Kathleen Weathers, Senior Scientist, Cary Institute of Ecosystem Studies, Millbrook, NY 

SCIENCE ADVISORY BOARD STAFF 

Dr. Holly Stallworth, Designated Federal Officer, U.S. Environmental Protection Agency, Washington, 

DC 
v

Abbreviations and Acronyms 

AADT Annual Average Daily Traffic

AMMS Air Monitoring and Methods Subcommittee 

CAA Clean Air Act 

CASAC Clean Air Scientific Advisory Committee

CBSA Core Based Statistical Areas 

CFR Code of Federal Regulations

CO Carbon Monoxide

DOT U.S. Department of Transportation

EPA U.S. Environmental Protection Agency

HD Heavy-Duty Trucks

LD Light-Duty Passenger Vehicles 

NAAQS National Ambient Air Quality Standards 

NOx Oxides of Nitrogen 

NR Near-road

OAQPS EPA Office of Air Quality Planning and Standards

ORD EPA Office of Research and Development

O3 Ozone 

PM Particulate Matter

QA/QC Quality Assurance/Quality Control 

SAB EPA Science Advisory Board

SOx Sulfur Oxides 

TAD Near-Road NO2 Monitoring Technical Assistance Document 

VOCs Volatile Organic Compounds 
1

Enclosure A

CASAC Air Monitoring and Methods Subcommittee (AMMS)

Consensus Responses to Charge Questions

The EPA’s Office of Air Quality Planning and Standards (OAQPS) drafted the Near-road NO2

Monitoring Technical Assistance Document (TAD) to provide state and local air monitoring agencies 

with recommendations on how to successfully implement required near-road NO2 monitors. In 

developing the TAD, OAQPS collaborated with multiple state and local air monitoring agencies and 

federal and state departments of transportation. At an early phase of development of the TAD, OAQPS 

requested that the Clean Air Scientific Advisory Committee (CASAC) provide advice through its review 

of EPA’s Near-road Guidance Outline and Near-road Monitoring Pilot Study Objectives. In November 

2010, CASAC issued a final report for its Review of the “Near-road Guidance Document – Outline” and 

“Near-road Monitoring Pilot Study Objectives and Approach” (available at 

Background

On February 9, 2010, new minimum monitoring requirements for the nitrogen dioxide (NO2) monitoring 

network were promulgated (75 FR 6474) in support of a revised Oxides of Nitrogen National Ambient 

Air Quality Standard (NAAQS). The NO2 NAAQS was revised to protect against peak 1-hour exposures 

that may occur anywhere in an area, and included a 1-hour level of 100 parts per billion (ppb), 98th

percentile form, averaged over three years, while retaining the annually averaged NAAQS of 53 ppb. 

In the preamble to the final NO2 rulemaking, the EPA recognized that roadway-associated exposures 

account for a majority of ambient exposures to peak NO2 concentrations. In particular, the EPA 

recognized that increased vehicle miles traveled (VMT) can result in an increased potential for exposure 

and associated risks to human health and welfare. In the final NO2 rulemaking, the Agency required that 

ambient monitoring be conducted at the locations where peak, ambient 1-hour NO2 concentrations can 

be expected to occur in an area, with a focus on characterizing those maximum NO2 concentrations 

attributable to mobile source emissions near major roads. In addition, the EPA required that ambient air 

monitoring agencies submit their plans for any required near-road (NR) NO2 stations by July 1, 2012, 

and that the near-road NO2 monitoring network be implemented and operational by January 1, 2013. 

http://yosemite.epa.gov/sab/sabproduct.nsf/ACD1BD26412312DC852577E500591B37/$File/EPACASAC-11-001-unsigned.pdf). OAQPS considered CASAC’s November 2010 advice, drafted the TAD, 

and requested that CASAC’s Air Monitoring and Methods Subcommittee (AMMS) provide advice and 

ideas on how to improve the draft TAD. CASAC’s AMMS held two public teleconference calls on 

September 29 and November 17, 2011 to review EPA's draft Near-road NO2 Monitoring TAD. The 

chartered CASAC reviewed the AMMS Review Report on the Near-road NO2 Monitoring TAD during 

the January 27, 2012 public teleconference, and approved the Report after the teleconference. 

CASAC focused on the following charge questions as part of its review, and provides the following 

responses to these charge questions.
2

Monitoring Objectives

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives?

Sections 1 and 2 could be combined and the discussion of objectives and rationale could be 

strengthened. The primary operational goal of the network is clearly stated, i.e., to capture peak NO2

concentrations near roadway. In the TAD, “near roadway” is defined as ideally less than 20 meters from 

the active traffic lane, although up to 50 meters is allowed by the rule. However, the rationale for that 

siting and the objective of the network design, in terms of what populations are expected to be protected 

by the network, is not described. It is very important that the TAD provide a clear description of what 

the network is intended to accomplish in terms of health protection and environmental goals and how 

that drives the monitoring network design and probe siting. This would include a clear description of the 

populations subject to elevated vehicle exhaust exposure (such as those living or recreating near the road 

or those in vehicles on the road); the near-road network objectives as stated in the NO2 final rule 

preamble should be included in this section. It is unclear if siting criteria should take into account the 

presence of residential exposures -- does it matter if no one lives near the near-road site? If the network 

is intended to be able to characterize on-road concentrations, even 20 meters might be too far from the 

road since gradients going away from the road in the first 20 meters are steep and highly variable.

A secondary (longer-term) goal of creating a near-road (NR) monitoring network infrastructure and 

providing data for a range of NR-relevant pollutants or indicators to support health assessments also 

should be stated. This section should note the importance of NR siting relative to nearby communityscale air monitoring stations to allow evaluation of the “NR excess” and estimation of gradients from the 

road for pollutants of interest. References to studies on health effects for the exposures of interest would 

be helpful. It would be useful to specify NR network requirements as stated in the NO2 final rule and 

characterize to what extent options are available to agencies in order to adhere to regulatory 

requirements. This section also should include language to clarify the options agencies would have if 

they are in noncompliance due at least in part to local traffic sources. It would be helpful to include the 

reasoning behind the monitoring network, a brief scientific overview of pollutant interactions (dynamics 

of ozone titration), and a discussion on the limitations of the data that would be generated by the 

network.

Site-Selection Factors

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors (AADT, fleet mix, congestion 

patterns, roadway design, terrain, and meteorology) required to be considered as part of the near-road 

NO2 site selection process?

Among the six factors that are discussed in the current draft of the TAD, there is too much emphasis on 

Annual Average Daily Traffic (AADT). More attention should be given to congestion patterns, 

background concentrations, NO–O3 chemistry, physical mixing, terrain, and meteorology, especially as 

it influences flow patterns and NO2 concentrations in the 0–50 m closest to roadways. 

Additional factors that should be considered include the potential of sites to characterize hot spots, the 

potential of sites to characterize human exposure, public accessibility, safety, the potential influence of 

other nearby NO2 sources (e.g., cumulative effects of nearby roadways), the availability of ancillary 
3

measurements at the site (e.g., traffic counts with high temporal resolution or other measures of 

congestion), and the availability of ancillary nearby measurements (background NO2, ozone and 

meteorological data). In addition, the TAD should more explicitly and clearly address the issue of 

location of the site within a distance of 0-50 m of the roadway (e.g., consider 20 m or less with a vertical 

height dependent on distance from roadway and terrain features). As documented in individual panel 

member comments, there is extensive scientific literature on the significant concentration changes for 

NO2 that occur within 50 m of roadways. The TAD should provide guidance on which parts of this 

distribution are of greatest interest and how to weight the six factors and incorporate these data into the 

roadway process. For example, the TAD should specify whether the goal is to measure maximum 

average concentration, concentrations at locations that are likely to experience the highest value for the 

98th percentile concentration, or the concentrations most likely to be associated with human exposures.

Fleet Equivalent AADT

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5? 

The Fleet Equivalent (FE) AADT metric is an acceptable starting point for initially ranking road 

segments based on combined Light-Duty Passenger Vehicle (LD) and Heavy-Duty (HD) Truck traffic 

counts. The accuracy, however, of a national default value must be determined. Although the FE-AADT 

is a reasonable starting point to identify hot spot roadway segments for potential NO2 monitoring, it is 

unlikely that this approach will capture the details of local on-road/near-road hot spot pollutant 

exposures impacting commuters and near-road neighborhood environments (e.g., see HEI 2010).

Before consideration of the FE-AADT as a screening tool, further characterization of uncertainties in the 

default values and assumptions in the approach should be provided. These include:

• Identification of the variation in HD emissions with the age of the vehicle fleets and the variation 

in vehicle fleet age across Core Based Statistical Areas (CBSAs) and regions.

• An assessment of the contribution of HD gross emitters on total HD emissions and the likely 

locations of these outlier sources.

• An estimate of the introduction and penetration of clean diesel technologies (filter trap and 

Selective Catalytic Reduction, or SCR) and their impact on the direct primary emissions of NO2.

• Applying available local emission inventory data to refine the choice of a default ratio of 10 for 

heavy-duty to light-duty NOx emissions (HDm) for specific road segments. 

• The distribution of traffic congestion should be considered whenever possible utilizing locationspecific temporal transportation data. Incorporation of such data should improve upon the current 

congestion ranking as applied in the TAD, which has limited power and does not provide any 

specificity to distinguish within the severe ranking category.

The EPA should review existing research study results and, to the extent possible, develop statistics on 

HD multiplier values based on roadside monitoring across different CBSAs. This information will 

provide monitoring agencies with a reasonable estimate of FE accuracy to help determine how precisely 

they should follow the rankings produced by the FE metric and allow for the consideration of other 

available factors affecting site selection. 

 
4

The Health Effects Institute (HEI) Special Report 17. 2010. Traffic-Related Air Pollution: A 

Critical Review of the Literature on Emissions, Exposure, and Health Effects. 

http://pubs.healtheffects.org/view.php?id=334.

Roadway Pollutant Dispersion

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Overall, CASAC concurs that this section of the TAD does a good job of describing the physical 

considerations in identifying a near-road monitoring site. Subcommittee members suggested a few 

additional considerations that should be included in this section, and were critical of one of the figures in 

the section. Highway interchanges where multiple roads intersect were discussed as being likely 

locations of the highest near-road NO2 concentrations, and the members felt this could be more 

explicitly stated in this section (possibly as a sub-section of 6.1 Roadway Design). Alternatively, a brief 

discussion of interchanges might be more appropriate in Section 5 as part of the road segment ranking 

process. Local highway density combined with congestion and slowed speeds at interchanges increase 

the likelihood of high NO2 concentrations; in particular, the effects of road density should be discussed 

more clearly in this draft TAD.

CASAC recommends that EPA explicitly mention two types of locations to avoid when considering 

monitoring sites because they could impact dispersion: (1) locations near stands of trees, and (2) those 

near significant surface waters such as lakes, rivers and coastal waters (because these waters may affect 

NO2 concentrations). Finally, CASAC recommends that Fig. 6-2 be deleted because the figure does not 

represent the downwind concentration profiles of ultrafine particles that have been reported for some 

other locations, nor does it represent the behavior of NO2 near roadways. Due to their temperaturedependent volatility and high coagulation rates, 20 nanometer (nm) size particles are expected to behave 

differently than NO2 or conserved pollutants such as CO in the near-road environment. Presentation of 

similar data on the near-road behavior of a conserved gas such as CO is encouraged in a revised Fig. 6-

2.

Siting Requirements for Near-Road Monitors

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures?

Section 7 is well-written and clearly presents minimum requirements for siting near-road monitors. 

However, there are several areas where additional information could be provided. The presentation 

could be strengthened by including results from photochemical models with high space-time resolution 

to illustrate NO2 formation and photolysis as a function of downwind distance from road segments. 

Detailed modeling also could be performed to provide bounding estimates of both primary and 

secondary NO2 as a function of downwind distance. Findings from two recent modeling studies (Wang 

and Zhang 2009; Wang et al. 2011) might provide useful insights for site selection. It should be noted 

that AERMOD and other regulatory models generally lack the detailed chemistry and/or space-time 

resolution to deal with problems of this nature at such proximity to a complex and dynamic source.
5

Another concern involves the horizontal placement of monitors with respect to walls, parapets, etc. The 

guidance allows a separation of as little as 1 meter horizontal distance between an inlet and adjacent 

(supporting) structure. Depending on the nature of the structure, and other variables, there likely will be 

a perturbation of air flow near the structure. This, in turn, will enhance or depress NO2 concentrations. 

Table 7 should state explicitly that the probe should be on the side of the supporting structure facing the 

traffic or above the supporting structure.

One topic that may need further discussion is the relationship between the sample inlet distance from the 

road and the inlet height. The TAD separately discusses inlet distance and height, and should discuss 

the effect of inlet height at different distances from the road. Inlet height should be in the range of 2-7 

meters, taking into account likely exposures in the vicinity of the site. Traffic speed, wind speed and the 

location of barriers all will affect downwind concentrations. It would be useful to study some of these 

effects using Computational Fluid Dynamics (CFD) models, as described in various recent studies (see 

example references in comments from J. Chow). Such information will provide monitoring agencies 

with a better picture of the distribution of near-road pollutant concentrations to aid their decisions 

regarding sample inlet location.

Finally, there was no mention of residence time in inlet lines or materials of construction for inlet lines. 

In some cases, it might be necessary or desirable to locate an NO2 inlet at some distance (10-20 meters) 

from an equipment shelter, rather than adjacent to it. If so, then some effort should be made to minimize 

residence time and to maintain clean, inert sample lines.

Wang, Y.J.; DenBleyker, A.; McDonald-Buller, E.; Allen, D.; and Zhang, K.M. 2011. Modeling 

the chemical evolution of nitrogen oxides near roadways. Atmospheric Environment 45(1):43-52. 

Wang, Y.J.; and Zhang, K.M. 2009. Modeling near-road air quality using a computational fluid 

dynamics model, CFD-VIT-RIT. Environmental Science & Technology 43(20):7778-7783.

Exploratory Monitoring

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process?

Yes, although some improvements can be made. Section 8 offers several reasonable exploratory 

monitoring options: saturation designs; focused monitoring campaigns; and mobile monitoring. 

However, further guidance is needed to help the user to specify the purpose(s) of the exploratory 

monitoring. There are many possible objectives; e.g., finding the maximum near-road NO2; 

characterizing the spatio-temporal gradients; assessing the relationship between the multiple pollutants, 

to permit choosing the monitoring approach that will best provide the information needed to achieve the 

defined objectives. The combination of these methods also should be discussed. In many cases, it will be 

desirable to use near-road models to integrate and to interpret the results of the exploratory monitoring. 

Thus, there needs to be integration between this monitoring guidance and the modeling material 

provided in Section 9. 
6

Dispersion Modeling

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process?

The TAD should stress the importance of using models as a companion to exploratory monitoring. It is 

not clear that AERMOD is the best tool for near-road plume modeling applications and it would be 

helpful for the TAD to offer other options. There are line source models available that could be better 

adapted or adopted, such as CALINE4, or AERLINE when it becomes available. Properly accounting 

for background NO2 is expected to be a major issue in modeling near-road NO2 in urban areas. 

Background NO2 should be assessed by accounting for multiple background sources. The TAD 

discusses the issue of dealing with background point sources of NO2 in AERMOD but the network of 

upwind roadways may be more relevant. An example of AERMOD application to near-road NO2

simulation, along with description of the user interface, would be a useful addition to the TAD.

It is essential that the plume dispersion model account for NO- NO2- O3 chemistry and the TAD should 

provide clearer guidance on this topic. Tier 1 and Tier 2 of AERMOD, where NO2 is treated as inert or 

as a fixed fraction of NOx, are unacceptable. Tier 3, accounting for NO- NO2- O3 photochemistry, 

should be used if AERMOD is the model chosen. The maximum near-road NO2 conditions are not likely 

to be associated with directly emitted NO2 but rather with emitted NO that is oxidized to NO2 by ozone. 

The time scale for oxidation is ~1 min under high-ozone conditions but may be much longer under 

stable conditions when ozone is titrated. Thus near-road NO2 is not necessarily highest under the most 

stagnant conditions, nor is it necessarily highest close to the roadway. The chemistry involved is simple 

but its coupling to meteorological conditions has some subtleties that are highly relevant to the 

conditions and locations where maximum NO2 is to be expected. A short tutorial on NO- NO2- O3

chemistry, its coupling to plume dispersion, and the implications for near-road NO2 would be a very 

useful addition to the TAD.

If a multi-pollutant sampling strategy is adopted that involves measurements of other non-conserved 

species such as ultrafine particles, models also will be required to interpret measurements of their 

downwind concentrations.

Characterizing Candidate Near-Road Sites

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included.

The list of items in Section 10 is quite comprehensive (and repetitive with the rest of the document). 

Some items that are missing or underemphasized include:

• representativeness of site (e.g., avoid unique situations like toll booths, tunnels, and 

acceleration ramps);

• other NO2 sources (e.g., nearby roads, other sources);

• existing monitoring sites (both near-road and background sites);

• roadway grade; and 

• surrounding land use (and population density).
7

CASAC commends the Agency for recommending the use of new digital resources to facilitate field 

reconnaissance.

Collaboration with Transportation Agencies

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section?

Section 11 of the TAD covers the intended topic very thoroughly and might be edited to remove 

excessive detail. The considerable discussion of “air rights” in Section 11 is confusing since it would 

seem that an “easement” (right to use the land) is just as important as “air rights”. While the discussion

of safety issues here is critical, a bulleted list of key safety elements to consider would be helpful to 

highlight the most relevant information. This section assumes that a Department of Transportation 

(DOT) would be involved only if the site is on a DOT right of way, which is not always the case; DOTs 

often own land that is near the highway but completely off the Right of Way (ROW) or the safety zone. 

This circumstance is more likely for a site within 50 meters of the active traffic lane than for one within 

20 meters. 

There are some elements of this section that do not warrant more than a brief mention: 

• A Near-Road site should never require access from within the ROW; this limitation should rule 

out a site from further consideration. 

• The need to enhance roadway safety infrastructure for a Near-Road site as discussed on page 11-

8 is very unlikely to happen due to funding limitations.

Site Comparison Matrix

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13?

CASAC agrees that there are some advantages to developing the matrix and suggests adding some 

additional information to make the matrix more useful. A category for the distance to interchanges, 

intersections and other road segments should be incorporated. There also should be a qualifier for each 

element to assist in determining the appropriate weight for each of the categories. The direct element-toelement comparison of candidate sites will help to justify the ultimate site selected and the locations not 

selected should be considered as potential replacement sites. Since monitoring sites in urban areas often 

have to be relocated due to frequent and sometimes unexpected construction, repaving and repair 

projects, backup locations should be developed for each site in case it needs to be changed quickly. 

 

The site selection matrix is one of many tools that can be used in the near-road site selection process. 

However, use of the matrix may not be necessary and could be burdensome in CBSAs where there are 

few choices or where a suitable candidate site already is available. The one element that could not be 

quantified, but that likely will play into the site selection process, is the application of “local 

knowledge.” Road segments likely will have different rankings on days when traffic flow is disrupted 

due to accidents, road and bridge closures or mass transit shut downs. These types of issues are difficult 

to objectively quantify and present in a matrix format for comparison with other site attributes. 
8

Priorities for Pollutant Monitoring 

Charge Question 11a: Does the AMMS concur with the order of presentation of each pollutant or metric 

of interest in the near-road environment, as was suggested by the previous AMMS panel, within Section 14?

No, the specific requirements for near-road NO2 monitoring for different state and local agencies might 

vary, and it is difficult to follow the exact ranking order. However, CASAC suggests that rather than an 

ordered list, pollutants should be grouped by priority (e.g., primary, secondary, tertiary) to allow 

flexibility in near-road monitoring. Following are the recommendations for priority grouping:

• Primary Group: NO/NO2 and CO, O3, and meteorology

• Secondary Group: Air toxics (BTEX), black carbon (BC), particle size concentration 

(preferable) or particle number concentration, and traffic counters/ DOT cameras

• Tertiary Group: CO2, PM2.5 and PM10-2.5, and organic and elemental carbon (OC and EC, 

respectively)

The Primary Group contains NO/NO2 and CO, which are required as part of current EPA regulations for 

near-road monitoring. The NO2/NOx ratio will be highly related to O3, so O3 is important for 

understanding O3-NO chemistry and to distinguish primary versus secondary contributors to near-road 

NO2. Meteorology data (especially high-resolution [1–5 s] wind speed and wind direction) is needed to 

understand the turbulence induced by moving vehicles that will disperse emissions and the importance 

of nearby structures (e.g., barriers, surface roughness).

The Secondary Group contains air toxics from combustion and other sources which are of great health 

concern, especially for those residents in close proximity to a busy roadway. BC is a good indicator of 

primary emissions from high emitters and/or heavy-duty diesel vehicles. Particle size distribution 

provides information on potential impacts, dynamics and sources. Particle number concentration is an 

emerging health indicator, with its measurements to be used for future European (and possibly U.S.) 

engine certification. Traffic counters/DOT cameras will allow for accurate representation of road use. 

The Tertiary Group includes an inexpensive, fast-response CO2 monitor, which would allow fuel-based 

emission factor distributions to be estimated. An optical particle counter for PM may be more useful 

than a compliance PM2.5 or PM10 sampler. Expenses associated with operations and maintenance for 

continuous OC/EC analyzers also should be recognized.

Furthermore, for priority/criteria pollutants on the list, the EPA should clarify whether agencies are 

restricted to using EPA methods for the analysis of the pollutants, or whether use of other methods 

would be acceptable.

Charge Question 11b: Does the AMMS concur with the description of each pollutant or other metric 

discussed in Section 14, including its impact on human health (as appropriate), the reason for interest in the 

near-road environment, and the description or suggestions for measurement? 

Yes, there is an adequate description of each pollutant of interest.

Charge Question 11c: Does the AMMS believe that a pollutant or other metric should be removed from the 

list within Section 14, or that an unlisted item should be included within this section? 
9

Yes, sulfur dioxide and lead should be removed from the list. Traffic counters or the use of DOT 

cameras should be added to the list as part of the Secondary Group, since they are easy to install and 

operate and will allow the examination of congestion patterns as elevated NO2 concentrations are 

recorded.
10

Enclosure B

Comments from Individual Members of the CASAC Air Monitoring and Methods 

Subcommittee (AMMS)

MR. GEORGE ALLEN ..........................................................................................................................................................11

DR. LINDA BONANNO .........................................................................................................................................................13

DR. DOUG BURNS .................................................................................................................................................................14

DR. JUDITH CHOW ..............................................................................................................................................................15

DR. KENNETH DEMERJIAN...............................................................................................................................................28

DR. ERIC EDGERTON..........................................................................................................................................................31

MR. DIRK FELTON...............................................................................................................................................................34

DR. PHIL FINE .......................................................................................................................................................................38

DR. RUDOLF HUSAR............................................................................................................................................................39

DR. DANIEL JACOB..............................................................................................................................................................42

DR. PETER H. MCMURRY ..................................................................................................................................................45

DR. ALLEN ROBINSON........................................................................................................................................................48

DR. JAMIE SCHAUER ..........................................................................................................................................................53

DR. JAY TURNER..................................................................................................................................................................55

DR. YOUSHENG ZENG ........................................................................................................................................................59
11

Comments from Mr. George Allen

These comments are a broad overview of the draft document, with some specific comments on section 

11.

The TAD is a very comprehensive review of all possible issues that might be in play for a Near Road

site. There are parts that could benefit from editing to make them shorter, clearer, and more concise. If 

the key information in the TAD is not readily accessible, it is less likely to be used. This document 

would be much more readable if the essential components were condensed into a 10-minute read, with 

additional supplemental material referenced in appendices. The first 25 sites will be deployed in large 

cities which generally have experienced monitoring staff that do not need the level of detail currently in 

this TAD.

It must be realized and clearly acknowledged that in the longer run (< 10 years?], Near Road sites are 

not likely to be about compliance monitoring. With the current CO NAAQS, there are almost no 

attainment issues. Over time (the next several years), as HDD NOx engine controls already in effect 

penetrate the fleet, NO2 will likely also become a minimal issue with regard to determining attainment 

with the NAAQS. Thus, the longer term siting and network design must take into consideration the 

underlying goal of the Near Road network: providing data to support health effect assessment. This 

means much more than just adding other key measurements like BC and particle # concentration. Site 

selection must consider the availability of relevant “background” sites that would allow estimation of the 

Near Road “excess” of a wide range of relevant pollutants. This information, along with detailed wind 

and traffic data, would allow modelers to develop estimates of multi-pollutant surfaces near roadways 

(including estimation of exposure gradients away from the road) with relatively similar terrain as the 

single Near Road site. This surface modeling is an essential component of any attempt to do near-road 

health assessments. A single site without a relevant “background” site pair has little to no value in a nonNAAQS context - it can not be used for health effect assessments since it represents only a single microscale environment.

Along with an appropriate background site, highly detailed traffic data [including vehicle class 

information], is very important. In large urban areas, the DOTs usually have several “permanent” hiresolution traffic counters. These two siting factors [background site and detailed traffic data] are critical 

in making the Near Road data useful in any context beyond demonstration of attainment with the 

NAAQS. These factors should be the primary drivers of any site selection process after AADT, terrain, 

and congestion are considered.

BC has been a useful marker for HDD, but like HDD NOx, it will continue to trend down over the next 

several years, eventually becoming lost in the urban and transport background. Thus, to the extent that 

“clean” diesel emissions are of interest (VOCs may not be removed by current control technologies), 

effort might be needed to establish a different HDD “marker” that is relatively easy to deploy and 

operate and be highly time-resolved. This is likely to be a difficult task. Along these lines, other air-toxic 

combustion products such as acrolein and 1-3 buta-d may become substantially more important than 

CO, NO2/NOx or even BC. We may eventually end up with a Near Road network with UFP and air 

toxics as the key measurements. These longer term aspects should be considered during network design.

One key component to Near Road site selection is that staff involved in the process must already be very 

familiar with the urban area under consideration. It is nearly impossible [considering limited agency 

staff resources] to do this without an understanding of the area[s] under consideration.
12

Along with the site comparison matrix in ch. 12, a decision flow chart, such as used in section 3, pg. 3-2, 

would be useful for site selection. The most important elements would be at the top -- those elements 

that would eliminate a potential site from further consideration early on in the decision process. This 

would help focus attention on the more subtle and fuzzy components of site selection. This would of 

course require at least some degree of prioritization of matrix components, but this has already been 

done in the TAD.

Although not required by the NO2 or CO NAAQS, on-site met monitoring is essential; it is critical to 

interpretation of the data. It may not have to be sonic 3-d, but some wind measurement must be made 

on-site or very near [<100 meters] from the site. Ideally wind would be measured on-site with very high 

time resolution [1 to 5-seconds] to provide information on turbulence conditions. Although EPA did 

receive some negative comments about sonic wind systems during the NO2 NAAQS comment period, 

that technology continues to improve, especially with regard to reliability and resistance to 

contamination. Sonic systems still need ongoing QC to verify proper operation, and this is more difficult 

to do than with mechanical systems. One approach is to establish an initial relationship between the 

Near Road wind sensor and the nearest wind data source. This relationship can be reviewed over time, 

and if the relationship pattern changes, something has probably failed. If Near Road met data are 

submitted to CWOP, this QC is automatically done by MADIS is and readily available.

The TAD makes no mention of consideration of siting near environmental justice neighborhoods. This 

should be part of the matrix, since lower SES neighborhoods are usually considered to a more 

susceptible and vulnerable population.

Section 11.

There is considerable discussion of “air rights” in section 11 of the TAD. This is confusing to me, since 

it would seem that “easement” [right to use the land] is just as important if not more so. This section also 

assumes that a DOT would be involved only if the site was on DOT right of way. This is not always the 

case -- DOTs sometimes own land that is near the highway but completely off the right of way or the 

safety zone. Examples of this case from Boston will be provided in my final comments.

Page 11-3, last line: I would expect that any Near Road site would never require access from within the 

ROW. This should be a show-stopper, not a discussion point. Likewise, the need to enhance roadway 

safety infrastructure for a Near Road site as discussed on page 11-8 is a non-starter; funds are not likely 

to be available for this kind of effort.

While the discussion of safety is critical, a bulleted list of key elements would be helpful. The discussion 

in 11.3 is excessively detailed, and thus not easy to access.

Section 14.

The list developed by the Nov. 2010 CASAC report is useful guidance, and the first 4 or 5 pollutants are 

indeed essential. Of the pollutants discussed in this section, CO2 and SO2 have very minimal value. 

Ozone, which did not rank very high in the CASAC list, might be very useful to sort out the primary vs. 

secondary contributions to Near Road NO2. With a matched background site and Near Road NO, NO2, 

O3, and wind data, much of this could be sorted out. To the extent that Near Road NO2 is from local 

titration, that argues for siting to be further from the road [> 50 meters]. This is unfortunately in conflict 

with nearly all other Near Road network objectives however.
13

Comments from Dr. Linda Bonanno

1) Shouldn't the TAD have a glossary section?

2) Throughout document, important to note which are requirements and which things are not

3) page 4-7: There is a big space before the word data, and it happens in a cople of places on that page, 

not sure what it means

4) page 10-5: The term Jersey Barrier is used on page 10-5 and then defined on page 11-6, should be 

vice versa,

5) pg 14-4 4th sentence down......NOy species present in are dominated... present in what?

6) Also on pg 14-4 at bottom, EPA plans to continue to work with academia, should have a contact at 

EPA

7) Section 14.4 black (elemental) carbon section, need to define what portion of EC or BC can be said to 

represent diesel. Not all EC or BC in ambient environment is from diesel...
14

Comments from Dr. Doug Burns

General comments pertinent to Sections 6 and 10:

Response: In general, I believe that this TAD does a good job of providing guidance to monitoring 

personnel as to how to locate sites and the criteria to use in site selection. My biggest concern is whether 

adequate guidance or necessary priority has been given to the issue of background NOx emissions and 

other potential local sources beyond the immediate roadway. It seems that this issue is critical in linking 

NO2 concentrations to the immediately adjacent roadway. The issue of background and local sources is 

discussed at various places in the document, but for example, is not listed in Section 6 under “Physical 

Considerations”. Instead, this issue gets raised in Section 10 (10-10 Surrounding Land Use). In my view, 

the background and other source issues are deserving of mention as a primary site consideration factor in 

Section 6. I am also uncertain whether consulting emissions inventories will be adequate for this task; 

number one because these inventories are somewhat out-of-date and number two because there are many 

sources of NOx such as landfills, wastewater treatment plants, and wetlands that may not be included in 

inventory data. Additionally, there is no real guidance as to what is meant by “nearby” sources in 

Section 10-10. What distance or radius should be considered? For example, there is evidence from the 

literature that overall road density in addition to near road sources provides significant NO2. Shouldn’t a 

measure such as road density also be part of an assessment of monitoring site adequacy? 

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Response: I found that this Section generally does a good job of discussing the key issues and 

providing helpful guidance on matters such as barriers, topography, and meteorological conditions. I 

wonder if guidance should also be offered to avoid (if possible) roadside locations with a high density of 

mature trees given the evidence shown in Fig. 6-2. I note that the current discussion focuses primarily on 

noise barriers, but the data in Fig. 6-2 seems to suggest that the presence of mature vegetation along the 

roadway likely has an even greater effect than does noise barriers.

A second point is whether you might also include in the guidance that where possible, a roadway is 

selected that is near-perpendicular to the prevailing wind direction. The section already mentions that 

the downwind side of the road is preferred, but this could vary quite a bit depending on the angle of the 

road with respect to the dominant wind direction. This would be criterion to use when deciding among 

several road segments that are fairly close regarding the other criteria.
15

Comments from Dr. Judith Chow

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives? 

Response: Yes, the objectives are adequately stated for NO2 near-road monitoring and site selection. 

However, the TAD should clarify if the goal is to measure maximum NO2 concentrations at near-road or 

to capture maximum human exposure in near-road environments.

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors (AADT, fleet mix, congestion 

patterns, roadway design, terrain, and meteorology) required to be considered as part of the near-road 

NO2 site selection process? 

Response: No. There is too much emphasis on annual average daily traffic (AADT) with fleet mix and 

congestion modifications and an insufficient discussion of and guidance for the other factors. Notably 

missing are discussions of human exposure, background NO2 contributions, and NO2 transformation 

potential.

• AADT is discussed on pp. 3-1 to 3-5, pp. 4-2 to 4-4, and pp. 5-9 to 5-14: It seems that >250,000 

vehicles/day is overly restrictive. The examples in Table 1 of the TAD (p. 5-12) don’t meet this 

criterion. With this limit, monitoring would be confined to 8 or more lane superhighways that are 

often elevated or depressed, have buffer zones around them, and have sound barriers in 

neighborhoods. Limiting monitoring to these roads would emphasize exposure of other drivers on 

the road rather than people near the road.

• Fleet mix is discussed on pp. 4-4 to 4-5 and pp. 5-13 to 5-21. The NO2/NOx ratio for gasoline vs. old 

diesel vs. new diesel should be considered in the Fleet Equivalent AADTs. Gasoline engines 

typically have a ratio of ~5%, while old diesels may have ratios >10%, and new diesels with ureabased SCRs may have ratios as high as 70% (but with much lower total NOx emissions) (Alvarez et 

al., 2008; Grice et al., 2009).

• Congestion patterns are discussed on pp. 4-5 to 4-8 and pp. 5-22 to 5-25. The conjecture that 

congestion is a secondary factor needs to be supported by evidence. One could argue that congested 

traffic during the rush hours with calm meteorology would minimize turbulence caused by traffic 

flow, thereby allowing more NO2 to accumulate at the roadside.

• Roadway design and structures are discussed on pp. 6-2 to 6-7: Figure 6-1 in the TAD (p. 6-5) is a 

good illustration (the caption needs to describe the wind direction and speed), but more evidence is 

needed on the effect of road design and structures. It would be useful to study some of these effects 

using Computational Fluid Dynamics (CFD) models, as illustrated in Figure 1 below and in other 

studies (Belalcazar et al., 2010; Gidhagen et al., 2004; Hahn et al., 2009; Karim and Nolan, 2011; 

Kondo et al., 2006; Kondo and Tomizuka, 2009; Kumar et al., 2009; Sahlodin et al., 2007; Wang et 

al., 2011; Wang and Zhang, 2009). Street canyons surrounded by tall buildings have been shown to 

concentrate and recirculate pollutants that might result in higher concentrations than those measured 

downwind of a heavily-travelled roadway (Benson et al., 2008; Buccolieri et al., 2011; Cai et al., 

2008; Dixon et al., 2006; Eliasson et al., 2006; Gousseau et al., 2011; Grawe et al., 2007; Gromke et 

al., 2008; Hanna et al., 2006; Lam et al., 2008; Li et al., 2006; Murena et al., 2009; Salmond et al., 

2010; Solazzo et al., 2007; Tay et al., 2010; Yassin et al., 2009; Yim et al., 2009; Zhou and Levy, 

2008)
16

Figure 1. Computational Fluid Dynamics (CFD) Modeling of dispersion downwind of a roadside 

sound barrier (Hagler et al., 2011). The plume is elevated by the barrier and dispersed on the 

downwind side.

• Terrain is discussed on p. 6-8: This topic seems highly related to roadway structures. 

• Meteorology is discussed on pp. 6-8 to 6-9. Only wind direction is discussed. More needs to be 

added on the turbulence that would disperse the emissions and the importance of nearby structures 

(e.g., surface roughness) and moving vehicles in inducing that turbulence.

• Human exposure potential is discussed on pp. 12-2 to 12-3. This should be one of the prime 

considerations and should be moved to Sections 5 or 6. Why can’t the “number of ways” to consider 
17

human exposure be “listed here?” It might be that measurements near a bus-stop or transit center on 

a busy street would yield higher exposures than superhighway emissions, owing to the proximity of 

the people to the emission sources (e.g., bus exhaust pipes). 

• Background concentrations and chemical transformations. The roadside NO2 will be an increment 

over the neighborhood- (0.5–4 km) and urban-(4–100 km) scale NO2 levels (Chow et al., 2002). It 

may be that a road with lower AADT shows higher levels owing to its proximity to other well-used 

roads in an urban area. Figures 2 and 3 below are examples of some analyses that would be useful to 

examine the relationships among the different variables.

The relationship from Marylebone Road in London (Figure 2; Carslaw and Beevers, 2005) used hourly 

NO2, NOx, and O3 data to estimate primary NO2 fractions from vehicle exhaust and NO2 formed through 

reactions of NO with O3. Reacted NO2 increases rapidly for NOx<100 ppb until roadside O3 is depleted. 

Background levels were determined from urban-scale monitors. In Figure 3, Wang et al. (2011) showed 

that a NO2/NOx ratio of 5% may not be suitable for most roadways, especially those with a high fraction 

of heavy-duty truck traffic. High O3 concentrations and peak NO2 concentrations occurred within 20–50 

m downwind of the road due to the high initial NO2/NOx conversion rates near roadways.

The TAD would be more useful if it contained an example that illustrates the different steps in the 

analysis, along the lines of network design guidance for PM2.5 and PM10 in U.S. EPA (1997). It starts 

with a fairly detailed description of AADT and its modifications, with illustrative tables, for the Tampa 

area, then it becomes less specific for the following steps. The political and population statistical 

boundaries may be adequate in the eastern U.S., but this is not how air quality management regions are 

defined in the western U.S. with large counties containing relatively small populated areas surrounded 

by terrain (Clark County Department of Air Quality and Environmental Management, 2004; Seitz, 

2000), or that consist of portions of several counties (SCAQMD, 2011).

Figure 2. Estimation of background, primary emissions, and reacted emissions of NO2 as a 

function of NOx levels along Marylebone Rd. in London (Carslaw and Beevers, 2005). Reacted 

NO2 increases rapidly for NOx<100 ppb until roadside O3 is depleted. Background levels are 

determined from urban-scale monitors. 
18

Figure 3. Higher NO2 may be measured further downwind when O3 is high, as shown by roadside 

Computational Fluid Dynamics (CFD) Modeling (Wang et al., 2011). 

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet 

Equivalent AADT metric introduced and discussed within Section 5?

Response: Yes. See the recommendation under Question 2 to consider the NO2/NOx ratio from 

gasoline- vs. old and new diesel-powered engines. 

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion 

and suggested how those effects can be considered in the near-road site selection process? 

Response: No. Many statements are made without sufficient support. Figure 6-1 of the TAD (p. 6-5) is 

useful, but a broader weight of evidence is needed.

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures? 

Response: There are several well-established roadside monitoring sites to address human exposure in 

other countries (e.g., Hong Kong, India) that might provide some insight regarding data that can be 

acquired and analyzed. Existing air quality monitoring sites should be examined first. Are there already 

existing roadside sites that are likely to represent human exposure? Some analysis of the existing data in 

the airshed, especially the high exposure sites (e.g., bus stop–transit centers in Manhattan, NY) should 

be performed to determine how well existing monitors represent the desired spatial scales.
19

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site 

selection process? 

Response: Middle- (100–500 m) or neighborhood-scale studies would be a better term than “saturation 

study.” A table outlining some of the instrumentation, accuracy, precision, averaging times, and 

detection limits with appropriate citations would be useful. Passive NO2 filter adsorption has been 

widely studied and its advantages and disadvantages have been investigated (Ayers et al., 1998; 

Beckerman et al., 2008; Crouse et al., 2009; De Fouquet et al., 2007; Douglas and Beaulieu, 1983; 

Gilbert et al., 2003; Hauser et al., 2009; Heal et al., 1999; Heal et al., 2000; Heal and Cape, 1997; 

Henderson et al., 2007; Jimenez et al., 2011; Kirchner et al., 2005; Krochmal and Gorski, 1991; 

McConnaughey et al., 1985; Mukerjee et al., 2004; Mukerjee et al., 2009; Nash and Leith, 2010; 

Nishikawa et al., 2009; Norris and Larson, 1999; Ozden and Dogeroglu, 2008; Piechocki-Minguy et al., 

2006; Plaisance et al., 2004; Sather et al., 2006; Sekine et al., 2008; Shooter et al., 1997; Sickles, II and 

Michie, 1987; Van Reeuwijk et al., 1998; Vardoulakis et al., 2009). Several microsensors are available 

that might be more useful for evaluating where and when high NO2 levels might occur. There are also 

several examples of mobile-lab and in-plume monitors that might be useful for determining real-world 

emission rates and NO2/NOx ratios for different engine types (Beckerman et al., 2008; Bukowiechi et al., 

2002; Herndon et al., 2004; Johnson et al., 2008; Johnson et al., 2009a; Kittelson et al., 2004; 

Maciejczyk et al., 2004; Morawska et al., 2007; Nussbaum et al., 2009; Pirjola et al., 2004; Pirjola et al., 

2006; Pirjola et al., 2009; Shorter et al., 2005; Wang et al., 2009a; Yli-Tuomi et al., 2005; Zavala et al., 

2006; Zhu et al., 2009).

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process? 

Response: A more concrete example would be useful. Other models and data analysis methods might be 

more accurate than AERMOD for the middle-scale, as suggested under Question 2.

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included. 

Response: This seems repetitive of Sections 4, 5, and 6 except for the safety issues. A checklist or 

outline for site documentation might be more useful.

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there 

opportunities to improve upon the material presented within this section? 

Response: No comment.
20

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13? 

Response: Table 8 of the TAD (p 12-5) should include all of the considerations listed under Question 2. 

The focus is too much on the roadway while it is a combination of variables that influences 

concentrations and exposures.

Charge Question 11: Does the AMMS: 

a. Concur with the order of presentation of each pollutant or metric of interest in the near-road 

environment, as was suggested by the previous AMMS panel, within Section 14? 

Response: The priority monitoring should be: 1) NO2/NO; 2) CO; 3) O3; 4) wind speed and direction; 5) 

BC; 6) particle number; 7) CO2; 8) PM2.5 or PM10 mass (or surrogate); 9) toxics; 10) lead; 11) SO2; and 

12) OC. Priorities 1 to 3 are obvious. CO is less of an issue with modern engine technology, but it may 

result from high emitters (Bishop and Stedman, 2008). In addition, it is an EPA requirement to collocate 

CO at near-road monitoring sites. The NO2/NO ratio will be highly related to O3, so this is the next 

priority. On-site meteorological data are needed to understand NO-O3 chemistry. BC is a good indicator 

of both primary emissions and high emitters and is relatively easy to measure and analyze data with an 

aethalometer (Hansen and Mocnik, 2010). Particle number is an emerging health indicator with a 

variation of its measurement to be used for future European (and possibly U.S.) engine certification 

(Dwyer et al., 2010a; Dwyer et al., 2010b; Giechaskiel et al., 2008; Johnson et al., 2009b; Wang et al., 

2010). An inexpensive (e.g., $5,300 for a LI-COR sensor) fast-response CO2 monitor would allow fuelbased emission factor distributions to be estimated from the other short-duration measurements (Sawyer 

et al., 2000; Sawyer, 2010). An optical particle counter (Peters et al., 2006; Wang et al., 2009b) for PM 

would be more useful than a filter compliance sampler to estimate the distribution of fuel-based 

emission factors. Toxics, lead, SO2, and OC would not probably be worth the expense for modern fuels, 

unless there is a specific need by the local agency.

b. Concur with the description of each pollutant or other metric discussed in Section 14, including its 

impact on human health (as appropriate), the reason for interest in the near-road environment, and the 

description or suggestions for measurement? 

Response: See answer above for importance of each measurement. There is a lack of balance in the 

measurement descriptions, with NOx receiving more emphasis than others. An update of U.S. EPA 

(1998a; 1998b) guidance might be useful and incorporated by reference.

c. Believe that a pollutant or other metric should be removed from the list within Section 14, or that an 

unlisted item should be included within this section? 

Response: No, the TAD should address the utility of the collected data, who would examine these data, 

and how they would be used to enhance understanding of the measured concentrations.

References

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real-world NOx emissions of modern light vehicles - derived from a reliable online measuring 

method. Atmos. Environ., 42(19):4699-4707. 

Ayers, G.P.; Keywood, M.D.; Gillett, R.; Manins, P.C.; Malfroy, H.; Bardsley, T. (1998). Validation of 

passive diffusion samplers for SO2 and NO2. Atmos. Environ., 32(20):3586-3592. 
21

Beckerman, B.; Jerrett, M.; Brook, J.R.; Verma, D.K.; Arain, M.A.; Finkelstein, M.M. (2008). 

Correlation of nitrogen dioxide with other traffic pollutants near a major expressway. Atmos. 

Environ., 42(2):275-290. 

Belalcazar, L.C.; Clappier, A.; Blond, N.; Flassak, T.; Eichhorn, J. (2010). An evaluation of the 

estimation of road traffic emission factors from tracer studies. Atmos. Environ., 44(31):3814-

3822. 

Benson, J.; Ziehn, T.; Dixon, N.S.; Tomlin, A.S. (2008). Global sensitivity analysis of a 3-dimensional 

street canyon model - Part II: Application and physical insight using sensitivity analysis. Atmos. 

Environ., 42(8):1874-1891. 

Bishop, G.A.; Stedman, D.H. (2008). A decade of on-road emissions measurements. Environ. Sci. 

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Comments from Dr. Kenneth Demerjian

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives?

In reviewing the TAD in its entirety, it falls short of addressing the principal goal of deploying near-road 

NO2 monitoring which is to identify neighborhood populations at risk to high exposures of NO2 

concentrations due to their proximity to major roadways. This is different from the over-arching 

objective identified in the TAD “...placing monitor probes as near as practical to highly trafficked roads 

where peak NO2 concentrations are expected to occur....” The primary objective of roadside 

monitoring needs to be clarified, as compliance monitoring and NO2 population exposures have 

different site demands for monitors.

The intersection of AADT and CBSA is a rather crude filter that is unlikely to identify high pollutant 

exposure risk neighborhoods in proximity to major roadway segments. The subject of traffic related 

exposures is discussed extensively in the HEI Special Report 17 “Traffic-Related Air Pollution: A 

Critical Review of the Literature on Emissions, Exposure, and Health Effects,” a reference 

conspicuously missing in this document. 

HEI Special Report 17, (2010). Traffic-Related Air Pollution: A Critical Review of the Literature on 

Emissions, Exposure, and Health Effects http://pubs.healtheffects.org/view.php?id=334.

The required consideration of the six factors in the site selection process: AADT, fleet mix, congestion 

patterns, roadway design, terrain, and meteorology are necessary but not sufficient. EPA must provide 

guidance on the uncertainties associated with each of these parameters and how they are affected by 

seasonal and regional factors. Also typical daily and weekday, weekend variances should be provided. 

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2

site selection process? 

More emphasis has to be placed on the quantification of traffic congestions and its contribution to 

roadway and near neighborhood hot spots. 

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet 

Equivalent AADT metric introduced and discussed within Section 5? 

The Fleet Equivalent AADT metric described in Section 5 of the TAD is a reasonable first step for 

triaging hot spot roadway segments for potential NO2 monitoring, but is unlikely to identify specific 

local hot spot pollutant exposures at the roadway/neighborhood intersection. Before implementation of 

the FE-AADT as a screening tool, several uncertainties in the approach should be addressed.

1) HD emissions will vary with the age of the vehicle fleet which likely varies within CBSA and by 

region. Emissions from HD fleets are not routinely monitor like the LD fleet and aged HD 

vehicles are likely more affected by gross emitters. Estimates of the uncertainties in HD 

emissions and their potential impact on local hot spot exposures should be documented across 

typical CBSAs under consideration. 

2) A quantitative treatment of traffic congestion must be developed as it is a critically important 

component to exposure assessment. The LOS ranking A-F as applied in section 5.3 has no power 
29

and does not provided any specificity to distinguish within the F ranking. Dismissing the 

quantitative treatment of traffic congestion based on these results is not acceptable and more 

effort needs to be made to address the congestion metric. 

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

The section requires more emphasis on the effects of the subject factors (i.e., roadway design/structures, 

terrain, ...) on population exposures in adjacent/nearby neighborhoods. .

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures?

The sting requirements as described pertain to compliance monitoring. If this is the appropriate objective 

and not population exposure to near-road NO2 pollution then it is adequate. Factors not taken into 

consideration that will affect the vertical distribution of NO2 on or near road include vehicle wake 

effects, tail pipe exhaust placement, traffic congestion, thermal convection and stable cold air drainage 

flow.

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process?

The section does identify the typical approaches that can be applied for exploratory monitoring to help 

with the near-road site selection process. The discussion should be more critical providing pros and cons 

of the various approaches and an assessment regarding which fair better in characterizing the spatial 

distribution of NO2 concentrations and their relative utility in addressing compliance vs. exposures 

objectives. 

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process?

A much broader array of dispersion line source models needs to be considered. 

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to

appropriately characterize individual candidate road sites is complete and adequately described? If the 

list is considered incomplete, please provide a list of the missing characteristics that should be included.

The list provided is comprehensive and identifies the key elements that should be considered in 

characterizing individual candidate road sites for potential on-road, near road monitoring. 

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities 

to improve upon the material presented within this section?

The section seems reasonable, but I would defer to the opinions of my colleagues from state agencies.
30

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13?

I would suggest that AQS metadata (AB site street information) consider including a congestion 

indicator (quantitative or qualitative index)

Charge Question 11: Does the AMMS:

a. Concur with the order of presentation of each pollutant or metric of interest in the nearroad environment, as was suggested by the previous AMMS panel, within Section 14?

I do not concur with the order. Propose replacing with the following order: NO2, (& NO), BC, 

meteorology, CO, traffic counters/cameras, CO2, OC, O3, PM mass, PM number, SO2, air toxics and 

Pb. 

b. Concur with the description of each pollutant or other metric discussed in Section 14, 

including its impact on human health (as appropriate), the reason for interest in the 

near-road environment, and the description or suggestions for measurement?

The rationale for the selection of pollutants to be considered for multi-pollutant monitoring is 

acceptable. 

c. Believe that a pollutant or other metric should be removed from the list within Section 14, 

or that an unlisted item should be included within this section?

No – The list is acceptable.
31

Comments from Dr. Eric Edgerton 

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives? 

Response: Yes. However, it might be worthwhile changing the title of the document to something like 

“Near-Road NO2 Monitoring Technical Assistance Document for Site Selection”, since the vast 

majority of space is devoted to site selection methodology. There is little or nothing in the document 

concerning data management and quality control. Given the near-road environment, it would be useful 

to have some guidance on data aggregation and summary statistics. For example, reporting sub-hourly 

averages, minima, maxima and standard deviations could provide valuable information on artifacts that 

occur with the approved method for NO2 (i.e., difference between NOx and NO).

In addition, Sections 1 and/or 2 should lay out the main options for site selection (road segment 

characterization, exploratory monitoring and modeling. The document places great emphasis on road 

segment characterization, such that the monotiroing and modeling options almost come as a surprise to 

the reader. Which options satisfy ALL requirements set forth in the CFR?

Finally, it is not entirely clear (at least to me) which NO2 metric(s) and which population or subpopulation is driving the site selection process.

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2 

site selection process? 

Response: No. Relative to the others, there is way too much emphasis on AADT. Physical mixing, 

chemistry, roadway orientation and the overall density of traffic segments needs more discussion.

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5? 

Response: This seems like a static calculation that doesn’t take into account changes in emissions due to 

mandated controls and new technologies. Also, it might be useful to consider statistics beyond annual 

averages, such as maxima or standard deviations.

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion.

Response: This is a pretty good start. In terms of meteorology, I didn’t see anything about roadway 

orientation and predominant wind direction. One might want to site a monitor such that there is a long 

fetch more or less aligned with the road. In terms of grade, wouldn’t EPA want to encourage upslope 

versus downslope?

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures? 

Response: Yes.
32

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process? 

Response: Information is needed on comparability of methods and how to deal with different averaging 

times (e.g., hourly continuous data versus multi-day passive sampling).

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process? 

Response: The decription of the models is adequate, but it might be more useful if EPA developed a 

user interface tool for application of these models.

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? 

Response: Section 10 is titled Field Reconnaissance, but most of the items listed do not require field 

reconnaissance; rather they serve as background information leading up to field reconnaissance. Such 

information should be gathered and assimilated, then used to identify specific candidate sites for Near 

Road monitoring. Field reconnaissance comes into play for the purpose of ground-truthing and refining 

priorities of candidate site locations.

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section? 

Response: No Comment. 

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13? 

Response: Further description of how to use the matrix in the decision-making process. More 

quantitative measure(s) of potential population exposure and relevant non-road sources. Why even 

consider upwind locations?

Charge Question 11: Does the AMMS: 

a. Concur with the order of presentation of each pollutant or metric of interest in the near-road 

environment, as was suggested by the previous AMMS panel, within Section 14? 

Response: No. I would move ozone up in the list and replace Pb with Trace Elements, to the extent they 

are not covered under Air Toxics. On average, ozone concentrations should be relatively low at nearroad sites. Concentrations will often be zero, and when they are zero, incremental NO2 can be used to 

estimate primary NO2 emissions. I would delete CO2.

b. Concur with the description of each pollutant or other metric discussed in Section 14, including its 

impact on human health (as appropriate), the reason for interest in the near-road environment, and the 

description or suggestions for measurement? 

Response: Yes.
33

c. Believe that a pollutant or other metric should be removed from the list within Section 14, or that an 

unlisted item should be included within this section? 

Response: See a.
34

Comments from Mr. Dirk Felton

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives? 

Response: The objective of finding a suitable location for a near road monitor is well covered. 

The TAD is missing all of the reasoning behind the network design, the scientific overview of 

pollutant interactions and a discussion on the limitations of the data resulting from this network. 

The most important issue in near road monitoring is understanding the gradient of NO2 and other 

mobile source pollutants in relation to distance from the edge of the road. There should be 

recommendations to make inlet distance from the road and height above the road as equivalent as 

possible particularly for sites within the same CBSA. If these distances are not equivalent, the 

data will be less useful for comparisons between sites. There should also be a discussion of the 

importance of having a nearby non near-road population exposure monitor so that the 

significance of the near road concentrations can be determined. 

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2 

site selection process? 

Response: The TAD only looks at these factors because the regulation is too focused on the 

emissions from segments of individual roadways. The effects of multiple roadways in dense 

urban areas can often lead to higher NO2 concentrations that can be observed near the edge of a 

single heavily trafficked road. The on-going New York Community Air Survey, which is 

referenced in Section 8.1.1, utilizes passive samplers to clearly show that NO2 levels are higher 

in the center of Manhattan than near the edge of the roadways with higher AADT. 

The regulation and the supporting TAD should be flexible enough to permit and encourage 

monitor siting at the locations where the NO2 levels are expected to have the greatest impact on 

human health. The EPA should add a seventh factor that accounts for the cumulative effect of 

multiple road segments in larger urban areas. The design of the seventh factor could be a 

weighted sum of the expected emissions impact from road segments and stationary sources 

within a kilometer or so of the candidate site. 

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5? 

Response: The FE metric is an acceptable approach for initially ranking road segments based on 

combined LD and HD traffic counts. The accuracy, however, of a national default value must be 

determined. Different regions have older or newer LD and HD fleets, more or less extensive 

clean diesel campaigns, road segments with low or steep grades and different degrees of 

congestion on road segments. All of these factors will affect the FE value for a particular CBSA. 

Some States and other research programs have performed remote sensing campaigns on multiple 

road segments within a CBSA. The data from these segments can be compared to the expected 

emissions based on known LD and HD traffic counts during the monitoring campaign in order to 

provide an estimate of the accuracy of the HD multiplier.
35

The EPA should review existing research study results and consolidate the HD multiplier values 

determined by roadside monitoring from as many different CBSAs as possible. Having this 

information available will provide monitoring agencies with a reasonable estimate of FE 

accuracy. Monitoring agencies can use the FE accuracy to determine how precisely they should 

follow the rankings produced by the FE metric. This will allow for the importance of FE ranking 

to be matched to other available factors affecting site selection. 

The FE metric is going to be more useful in the larger CBSAs where the LD and HD traffic are 

often segregated to some extent based on tolls, weight restrictions or outright prohibitions for 

HD vehicles such as on New York City’s Parkways. In smaller CBSAs where there are fewer 

transportation routes, it is likely that the FE will not significantly impact NO2 near-road site 

selection.

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process? 

Response: In the terrain and meteorology sections, the proximity to water bodies is not covered 

adequately. Urban areas and major roads are often built alongside the rivers and seashores that 

initially encouraged development in these areas. Monitoring in these river valleys and along the 

shores of large water bodies will tend to reduce the concentrations of locally emitted pollutants 

in comparison to sites away from the influence of these water bodies.

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures? 

Response: The siting requirements are in fact quite minimal. Unusual sites such as locations 

within tunnels could meet these siting requirements but would provide little relevant information 

for populations living near roadways. This section should encourage the selection of sites that 

have an open fetch between the road and the monitor inlet as well as between the inlet and 

nearby residents. If there is a barrier between the affected population and the monitor inlet, the 

site’s only purpose would be emission characterization and the resulting data would have little 

value for population exposure. 

The allowance for wall mounted inlets is puzzling. The wall, even at 1 meter spacing will still 

represent a barrier to air movement and will trap pollutants between the inlet and the roadway. 

The use of a site with this type of inlet will also preclude the use of this site for other pollutants 

such as PM ≥ 2.5 which require vertical inlets. These wall mounted inlets should only be 

permitted in areas where no other sites are possible and where only limited supplemental 

measurements are anticipated. 
36

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process? 

Response: The TAD omits how the information from exploratory monitoring should be 

weighted in relation to the other factors. Existing data from passive samplers such as from the 

New York Community Air Survey, which is referenced in Section 8.1.1, indicate that the highest 

1-hr NO2 levels in New York City are likely to be found in mid-town away from the highways 

with the highest AADT. The design of the NO2 near road network should be flexible enough to 

accommodate the combined NO2 contributions of multiple road segments within dense urban 

communities. The use of a seventh site factor as outlined in my response to charge question 2 

could help to incorporate the data from exploratory monitors in the site selection process. 

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included. 

Response: The section barely touches on surrounding land use. This category should be 

expanded to include population exposure, stationary sources as well as how wide an area a 

specific site represents. Sites that can be said to represent a significant length of a roadway, 

similar nearby roadways and larger neighborhoods should be considered to be more valuable 

than a monitor that only represents a single road segment.

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section? 

Response: The legal requirement to install a near road monitoring site should be explicitly 

included in the TAD. Does this requirement extend to the DOT if a location within their right of 

way is the most suitable location for a near road monitor? This is information that will be needed 

by the monitoring agencies as well as by the EPA Regional offices.

Monitoring agencies should consider installing traffic cameras at each near-road site or asking 

the local DOTs to include these sites in their system. Having this information readily available 

will assist with the validation of outlier 1-Hr NO2 data after these sites begin collecting data. In 

dense urban areas, accidents, vehicle fires, road maintenance, snow removal activities and 

mowing can all have significant short-term impacts on NO2 concentrations. 

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13? 

Response: The site selection matrix is an idealized version of the way sites are likely to be 

selected by today’s under staffed, over worked and underfunded monitoring agencies. Candidate 

sites that run into a road block such as an access, lease cost or a safety issue will be dropped 

from consideration and it would be a waste of time to continue to research the site/segment 

parameters necessary to complete the matrix. 
37

The site comparison matrix would be more relevant if it was expanded to include a seventh 

factor that incorporates the impacts from additional road segments as suggested in my response 

to charge question 2.

It is preferable to collect the site segment information necessary to complete the matrix only for 

an Agency’s most promising 2-4 locations within each CBSA. The effort to collect this 

information is only warranted if the sites are feasible and are likely to be approved by the local 

DOT and EPA. The advantage to having several fully evaluated candidate sites in each CBSA is 

that the sites that are not ultimately selected are essentially pre-approved as back up monitor 

locations. Urban monitoring locations are often impacted by road and bridge construction, 

building construction and other urban planning initiatives. Many of the installed near road sites 

will have to be relocated within the next 5-10 years and it would be sensible to maintain a short 

list of acceptable replacement sites.

Charge Question 11: Does the AMMS: 

a. Concur with the order of presentation of each pollutant or metric of interest in the near-road 

environment, as was suggested by the previous AMMS panel, within Section 14? 

Response: Air Toxics should be moved up in order of relevance. These compounds are likely to 

be much more valuable to the monitoring and health communities.

b. Concur with the description of each pollutant or other metric discussed in Section 14, including its 

impact on human health (as appropriate), the reason for interest in the near-road environment, and the 

description or suggestions for measurement? 

Response: The discussion of PM number concentration should include a discussion of size 

distribution, inlet configuration and inlet uniformity from site to site.

The PM section should clearly indicate that the short comings of both the PM2.5 FRM and the 

PM2.5 FEMs will be more pronounced in the near road environment. 

The OC section should include a caveat that discusses the limitation of the CSN carbon sampler. 

This sampler is optimized for use in rural areas as part of the visibility program. It is not as 

useful for capturing the higher proportion of semi-volatile OC expected to be prevalent in the 

near-road environment.

c. Believe that a pollutant or other metric should be removed from the list within Section 14, or that an 

unlisted item should be included within this section? 

Response: PM2.5 should be removed from the list until a method is approved that is better able 

to handle semi-volatile PM. SO2 should be removed from the list because the fuel reformulations 

have already occurred and the concentrations in the near road environment are expected to be 

low. CO2 should be removed from the list because only a few monitors are necessary nationwide 

for objectives related to characterizing green house gas emissions. 
38

Comments from Dr. Phil Fine

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors (AADT, fleet mix, congestion 

patterns, roadway design, terrain, and meteorology) required to be considered as part of the near-road 

NO2 site selection process? 

Response: In general, yes. In practice, AADT and fleet mix should be the driving factors for choosing 

candidate sites (i.e. total NOX emissions), and then logistics will be the driving factor in making final 

selections.

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5? 

Response: The factor of 10 is a reasonable default, but many areas should have mobile source NOx 

inventories that should provide a more accurate factor. 

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section? 

Response: The discussion in this area is understandably brief, since requirements in different states will 

be different. While this section may help states get the approval process started, it may not help when 

state-specific requirements and restrictions are encountered. What would help is a national and regional 

EPA outreach effort to FHWA and state DOTs to highlight the importance of the program and prepare 

them for the access requests. Another option that should be mentioned is private or publicly owned land 

within 50 meters that is not DOT controlled. Permissions and approvals may be much easier in these 

locations. 

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13? 

Response: I believe this question refers to section 12.3. I suggest inclusion or the availability of a 

sample matrix rather than just a description of the parameters to put in the matrix.
39

Comments from Dr. Rudolf Husar

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives?

Response: The TAD contains a set of well-structured technical instructions and guidelines for the 

location selection of the roadway monitoring sites. However, the TAD does not offer a well-defined 

criteria for the ‘optimal’ site, nor for the optimization in general.

Buried in a paragraph on page 4-6: the objective of the monitoring effort is to characterize the peak 

NO2 concentrations that are occurring in the area.. Is finding and ‘characterizing’ the peak hourly 

near-road NO2 the siting optimization criteria? 

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2 

site selection process? 

Response: Why is the distance to the source not a major factor? It is an exponential factor. The 

recommended 20-50 m distance and 2-7m in elevation covers a wide range of constrictions near the road 

and introduces considerable ambiguity in attaching meaning to the measurement 

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5? 

Response: He formula makes sense. The data for the fleet mix is the problem. The diurnal, weekly, 

seasonal cycles of the mix, particularly for the HD vehicles is hard to get. So, little info is available for 

the source of the NO2 near the road.

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Response: It is made way too complicated in TAD. As if there were no regularities but randomness 

everywhere. Here are two charts from R Poirot’s comments in the Nov 2010 review: 
40

Left Chart: The concentration of roadway emissions (normalized to 20m distance) declines 

exponentially. The closer you get to the source, the higher the concentration. Is this law of dispersion 

different in the US? 

Right Chart: The annual average and the 98% hourly data correlate well. The slope may vary some, but 

it provides a useful guide. 

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures?

Response: As seen on the above chart (Left), between 20 and 50 m distance from the ‘kerb’, the 

concentration declines by a factor of two. At 2 meters from the kerb (truck) the NO2 concentration is 

higher by a factor of two. In other words, a person in the car on the same lane as the trucks is exposed to 

a concentration four times that of the sampler at 50m. So which is the relevant concentration, at the 

location of the drivers or the arbitrary location of the sampler?

Also, how would one establish an exceedance? Normalize all the data to the 20m distance? Can such a 

procedure withstand legal scrutiny?

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process?

Response: The section discusses several exploratory monitoring options: saturation study; focused 

monitoring campaign and mobile monitoring. A ‘Saturation study” or a more sophisticated “Multiscale” monitoring study would be helpful if dispersion from roadway emissions roadway emissions was 

a new topic. I would recommend reading and analyzing the existing studies. 

Focused monitoring program may be helpful to verify physical and/or empirical dispersion models (e.g. 

the above chart). Mobile monitoring over a specific candidate road segment would be terrific if 

combined with model(s) and the planned monitoring site. Driving up-end down the road segment could 

establish the relationship between the ambient concentration over the roadway and the chosen 

monitoring site. 

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process?

Response: Models should be companions to the monitoring and complement the observations. Clearly, 

chemical kinetics is superfluous; even the aerosol size distribution is frozen right after the tailpipe 

(coagulation is a second order process). EPA should recommend a specific simple modeling procedure 

similar to this TAD. 

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included.

Response: The Agency is to be commended for recommending the use of new digital recourses for 

evaluating and characterizing candidate monitoring sites. One-foot resolution satellite images along with 

mapping tools like Google Earth provide a simple and relevant view of the roadways, 3D terrain, 

surrounding environment, etc. At the resolution of these maps, distance measurements can also be 

performed. Furthermore, virtually all US urban areas are now documented with thousands of georeferenced photographs that are shared through Paronamio, Flickr and other photo-sharing websites that 

can be displayed on maps. The combination of these resources can resolve many of the questions related 
41

to:

Road Segment Identification

Road Segment Type

Road Segment End Points

Interchanges

Roadway Design

Terrain

Roadside Structures

With these electronic resources, the burden placed on the air managers and the DOT offices can be 

considerably reduced.
42

Comments from Dr. Daniel Jacob 

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives?

Response: Yes.

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2 

site selection process?

Response: I’m surprised that little weight is given to background NO2. This background could be of 

great importance considering that the 1-h NAAQS is 100 ppb but urban NO2 concentrations upwind of 

the roadway can easily be tens of ppb. The TAD recognizes the importance of background NO2 as 

provided by point sources upwind, but this may be less relevant than the network of other roadways in 

the urban area. An isolated roadway with high AADT may have lower roadside NO2 than a downtown 

roadway with lower AADT.

I don’t understand why below-grade highways would cause less near-road NO2 than at-grade highways. 

Under stable conditions, elevated NO2 could pool in the depressed roadway bed and eventually spill in 

the surrounding area, causing higher concentrations than an at-grade highway would. 

Meteorology is not important for transport alone. NO-NO2 chemistry is coupled to meteorology through 

availability of ozone, solar radiation, and NO to NO2 conversion time (translating into distance from 

roadway), it seems to me that some work is needed using a plume dispersion model with NO-NO2 

chemistry (such as AERMOD) to identify the worst meteorological conditions for NO2 and provide 

general guidance to local agencies on this matter. The worst meteorological conditions for an inert 

pollutant may not be necessarily be the worst for NO2.

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5?

Response: Looks good to me, I’m no expert.

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Response: I think this could be improved. See my response to Charge Question 2.
43

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures?

Response: I think that plume dispersion modeling including NO-NO2 chemistry would be very 

beneficial in identifying the expected location of peak NO2 concentrations for different meteorological 

conditions. This could also be done using NO2 measurement transects near roadways for a range of 

meteorological scenarios (morning and evening rush hours, different seasons, different wind speeds, 

etc.). The general recommendation of the TAD is to place the site as close to the roadway as possible 

and as low-altitude as possible (2 m), but this may not be where NO2 concentrations are highest because 

of the time lag for NO conversion to NO2.

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process?

Response: I think that it’s important to emphasize the need for exploratory monitoring over a range of 

meteorological conditions expected to cause high NO2 (see response to Charge Question 5). PSDs seem

useless for this purpose because of the multi-day integration time (as opposed to the 1-h metric of the 

NAAQS) and this could be better recognized. It seems to me that the best approach is with a mobile unit 

doing transects parallel to and normal to the highway under traffic and meteorological conditions where 

maximum NO2 is expected. If it is difficult to make NO2 measurements from a mobile unit with high 

temporal resolution, the aerosol number concentration could be used as a tracer instead (although that 

would not factor in the time lag for NO-to-NO2 conversion, see response to Charge Question 5).

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process?

Response: Tier 3 AERMOD modeling including NO-NO2 chemistry seems essential. Treating NO2 as 

inert or assuming a fixed NO2/NOx ratio is inadequate – that ratio is expected to greatly vary downwind 

of highways. The contribution of nearby highway sources to the upwind background should also be 

recognized.

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included.

Response: I have no expertise on this.
44

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section?

Response: I have no expertise on this.

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13?

Response: There may be the need to better consider the role of nearby roads in contributing to the NO2 

background. This could be very important. See my response to Charge Question 2.

Charge Question 11: Does the AMMS: 

a. Concur with the order of presentation of each pollutant or metric of interest in the near-road 

environment, as was suggested by the previous AMMS panel, within Section 14? 

Response: I suggest giving a higher priority to ozone because of its value of interpreting NO2 in terms 

of the effect of NO-NO2 titration (higher ozone leading to higher NO2). I would also suggest including 

NO if possible, for the same reason and with even more importance (NOx = NO+NO2 could be viewed 

as a conserved tracer on the time scales of interest).

b. Concur with the description of each pollutant or other metric discussed in Section 14, including its 

impact on human health (as appropriate), the reason for interest in the near-road environment, and the 

description or suggestions for measurement? 

Response: I think that the (marginal) interest of SO2 is that it can provide a signature on point source 

background influences on the site. This could be stated.

c. Believe that a pollutant or other metric should be removed from the list within Section 14, or that an 

unlisted item should be included within this section? 

Response: I would remove Pb (this is not a roadway pollutant anymore). I would add NO (see response 

to a).
45

Comments from Dr. Peter H. McMurry

General Comments/Overview: 

Overall, I found the document to be well written. I feel that it provides very clear guidance to state, local 

and tribal agencies for factors that need to be considered as they proceed towards implementation of 

near-road NO2 monitoring stations before January 1, 2013. 

The goal is to measure exposure hot-spots in the vicinity of roadways. This will be accomplished by 

sampling NO2 at fixed locations. In addition to measuring NO2, agencies are encouraged to consider a 

multi-pollutant sampling strategy that would include other species that are emitted by vehicles.

I have two observations that might bear consideration:

•The document does not address exposures of vehicle passengers. While those exposures would likely 

be for short periods, the short-term exposures could be significantly higher than for residence living 

downwind of roadways. For example, concentrations of NO2 within tunnels or above below-grade 

highways might be considerably greater than concentrations 20 to 50 m downwind of highways. It might 

be a good idea to state explicitly that the document does not apply to exposures of vehicle passengers.

•The document provides guidance on factors that should be avoided when selecting sampling sites (e.g., 

try not to sample on a side of the roadway that is predominantly upwind; do not sample downwind of 

elevated roadways on pilings, etc.) I think, however, agencies would also benefit from a clear statement 

of measurement objectives. For example, is the objective to locate the sampler at a site that would 

measure the maximum concentrations to which residents might be exposed, or is it to find a site that 

meets the guidelines specified in the TAD yet leads to the minimum value of the measured concentration 

(i.e. the fewest exceedences)? 

For example, on p. 6-8 the document states "Another example might be considering roads through 

valleys, where, due to the increased potential for inversion conditions within the valley, higher near-road 

NO2 concentrations may be found than what is found along alignments on the tops of hills, along 

hillsides, or in open terrain." Would it be good or bad to locate a sampling site in such a location?

Specific Comments:

Section 6. Physical considerations for candidate near-road monitoring sites.

Figures 6-1 and 6-2 illustrate the impact of roadway design on downwind concentration profiles. These 

observations show clearly that measured concentrations are strongly dependent on downwind distance 

within about the first 100 m from the road's edge. The guidelines are for sampling sites to be located a 

distance of preferably within 20 m and not more than 50 m from the road's edge. 

Published data for pollutants that might reasonably be assumed to be conserved near the roadway (e.g., 

CO) show that, concentrations might be expected to decrease by roughly a factor of two as the sampling 

location moves from 20 to 50 m (see, e.g., Figure 2 in Zhang et al. (2005 ). Given the tolerances 

specified in NAAQSs, a factor of two is significant. Agencies should be given some guidance: should 

they preferentially site sampling locations 50 m from the road to avoid exceedences, or should they put 

them 20 m or less from the road (if possible) to ensure that maximum concentrations are obtained within 

the constraints provided by this TAD?

I do not feel that Figure 6-2 is consistent with actual measurements of "20 nm particles" downwind of 

roadways, and I recommend that it be replaced or deleted. First, in the atmosphere one would not carry 

out studies with monodisperse (e.g., 20 nm) particles. Instead, one would measure distribution functions 

and report concentrations of particles in a specified size interval (e.g., 15 to 25 nm). Also, the results 
46

shown in this figure are not representative of particle decay rates downwind of roadways for particles in 

the 20 nm size range. For example, Zhu et al. (2002) show that concentrations of 6 to 25 nm particles 

decay much more rapidly with distance than concentrations of particles in other size ranges (25 to 50 

nm; 50 to 100 nm; 100 to 220 nm). This observation has been extensively discussed (Zhang et al. 2004; 

Jacobson et al. 2005; Zhu et al. 2009). It is the consensus of these researchers that concentrations of the 

smallest particles decrease more rapidly than concentrations of conserved pollutants due to (i) 

evaporation, and (ii) enhanced multimodal coagulation rates of the small particles as their size decreases 

by evaporation. Figure 6-2 does not discuss what is known about the behavior of 20 nm particles 

downwind of roadways, and might imply that they can reasonable be regarded as conserved. This would 

be inappropriate for this document.

In particular, I have no idea what is meant by the "concentration of 20 nm particles." To obtain 

meaningful results for concentration from DMA-CPC data, it is necessary to integrate between two 

sizes. The upper and lower size limit should have been specified. It is possible that for these 

measurements the DMA was set to classify 20 nm and then 75 nm particles, and that size distributions 

were not measured. In this case, the data cannot be interpreted unless the DMA flowrates are also 

mentioned (i.e., two different DMAs operated at different flowrates but both set to select 20 nm particles 

would report different results for 20 nm concentrations.) I feel that when authors report data, the data 

should be described in sufficient detail to allow for unambiguous interpretation. It would have been easy 

for the authors to do this. Secondly, while I have no doubt that this plot reflects something that was 

measured during this study, it is not necessarily representative of what has been observed for sub 25 nm 

particles downwind of freeways, and therefore I think it should probably be discussed in the context of 

those previous studies. The document provides no clue that sub 25 nm particles downwind of freeways 

are not conserved.

Also for Figures 6-1 and 6-2, information on wind direction relative to the roadway should be 

mentioned.

References are not given. I assume Baldauf et al. (2009) is an EPA report. Baldauf and coworkers have 

also written some peer reviewed journal articles (Hagler et al. 2009). I would encourage you to also refer 

to the papers by Zhu and coworkers. Their size-resolved measurements provide a better understanding 

of size-dependent concentration profiles downwind of freeways.

Section 7. Siting Criteria

My principle concern with this section is summarized above. The agencies need to be provide clear 

guidance: what is the measurement goal? The TAD leaves quite a bit of room for interpretation, and the 

resulting outcome may vary significantly given the strong dependence of concentrations on the sampling 

site chosen.

Details:

Section 8.2 requires careful editing. 

References Cited:

Hagler, G. S. W., R. W. Baldauf, E. D. Thoma, T. R. Long, R. F. Snow, J. S. Kinsey, L. Oudejans and 

B. K. Gullett (2009). "Ultrafine particles near a major roadway in Rayleigh, North Carolina: Downwind 

attenuation and correlation with traffic-related pollutants." Atmospheric Environment 43: 1229-1234.
47

Jacobson, M. Z., D. B. Kittelson and W. F. Watts (2005). "Enhanced coagulation due to evaporation and 

its effect on nanoparticle evolution." Environmental Science & Technology 39: 9486-9492.

Zhang, K. M., A. S. Wexler, D. A. Niemeier, Y. F. Zhu, W. C. Hinds and C. Sioutas (2005). "Evolution 

of particle number distribution near roadways. Part III: Traffic, analysis and on-road size resolved 

particulate emission factors." Atmospheric Environment 39(22): 4155-4166.

Zhang, K. M., A. S. Wexler, Y. F. Zhu, W. C. Hinds and C. Sioutas (2004). "Evolution of particle 

number distribution near roadways. Part II: the 'road-to-ambient' process." Atmospheric Environment 

38(38): 6655-6665.

Zhu, Y., J. Pudota, D. Collins, D. Allen, A. Clements, A. DenBleyker, M. Fraser, Y. Jia, E. McDonaldBuller and E. Michel (2009). "Air pollutant concentrations near three Texas roadways, Part I: Ultrafine 

particles." Atmos. Environ. 43: 4513-4522.

Zhu, Y. F., W. C. Hinds, S. Kim, S. Shen and C. Sioutas (2002). "Study of ultrafine particles near a 

major highway with heavy-duty diesel traffic." Atmospheric Environment 36(27): 4323-4335.
48

Comments from Dr. Allen Robinson

General comment 

This is a very complex problem. Near road way pollutant concentrations vary strongly in space and time 

based on a large number of factors, many of which are discussed in the TAD. NO2 concentrations will 

also depend on photochemistry (in addition to pollutant dispersion). In light of these facts, trying to 

select one or two sites that are representative of near road exposures to NO2 in a CBDA is a very 

daunting task. Given the sensitivity of NO2 with location presumably whether or not a CBDA will be in 

or out of attainment will depend very strongly on specific site locations. For example, I could envision a 

CBDA with (in reality) worse NO2 problems than some other CBDA but being ranked as being better 

(even in attainment) based on where the one or two monitors are sited in the different CBDA. Therefore, 

site selection is going to be critical. This document is a good start at describing the site selection process 

but I think that given the very limited number of sites (one or two) per CBDA I think that alot more 

thought and specificity needs to go into the site selection criteria in order to make fair and consistent 

attainment designations among different areas. Right now the TAD leaves alot of room for states to 

interpret the siting requirements. 

In the end, I am skeptical that one can deploy only one or two sites in a very large area to robustly 

characterize near road way exposures that inherently have very strong spatial variations. A better 

approach would be to implement more sites (likely impossible given fiscal climate). One may also be 

able to use models (e.g. to calculate some standard spatial distributions around model road ways) which 

are then used to interpret data from different sites in a more consistent way. If done correctly, this could 

potential allow for more consistent attainment designations. However, the modeling is not 

straightforward so some real thought would be needed.

A few more comprehensive case studies are needed that describes how EPA would weight all the factors 

to ultimately arrive at a site. The TAD has a partial case study for Tampa Bay, which uses the AADT 

and fleet composition data. However, what is shown (AADT) is the easy part because those are 

quantitative data that can be easily combined to create overall ranking. I fear that this will lead to states 

just basing site selections on AADT analysis and not really considering other factors (which may be as 

important). It would be good to continue that case study and show how to consider all of the relevant 

factors discussed in the TAD to ultimately come up with a recommended site or two in Tampa Bay. 

Doing complete case studies for several locations would likely be very helpful. Especially if the case 

studies were selected to illustrate important issues that would be commonly encountered.

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives?

Response: The objective of the TAD is clearly stated. It is to provide guidance for site selection by 

which agencies might implement near-road NO2 monitoring stations. However, the document does not 

provide much rationale for behind this objective specifically the scientific motivations and potential 

limitations of site selection. What is goal of NO2 measurement? What is it supposed to represent for 

NAAQS determination. For example, the TAD discussing many factors that are states should consider to 

select a site but it does not state what the ultimate objective of what is guiding the site selection. Is the 

site supposed to have the highest NO2 concentrations, is it supposed to be representative of some 

population weighted exposure of near road population exposure to NO2, etc.?

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 
49

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2 

site selection process?

Response: No. The TAD does a reasonable job of discussing many factors that need to be considered 

but it is not clear how to weight them because the TAD does ultimately specify what the overall 

scientific and other objective is for site selection (highest NO2, etc. see response to question 1). If this 

overall objective for site selection was clearly stated then it would provide states importance on how to 

weight these different factors. I think this important because while some factors (AADT and fleet mix) 

can easily combined to create rationale ranking other important factors (e.g. terrain, metereology, safety, 

etc) cannot be so easily combined into a numerical rank. Therefore, states will need to do the ultimate 

weighing themselves so it is important they know the ultimate objectives driving site selection.

One factor in particular that I don’t think is emphasized enough in the TAD is the distance from road 

way. The data in the TAD (and other data in literature) show that there is a very steep gradients in 

pollutant concentrations near road way. Therefore the exact distance a site located from the roadway will 

likely have a significant effect on measured NO2 concentrations and ultimately attainment designations. 

This gradient depends on site specific factors so it is impossible to a priori know where peak 

concentrations would be (and even for a given physical location it will change with metereology). 

Therefore it seems like scoping studies are important. However, 

One note on Figure 6-1, it was not clear if this was for an inert pollutant, e.g. CO, or for NO2. NO2 

concentrations will depend on photochemistry so that needs to be accounted for the results to provide 

guidance for site selection (dispersion of inert pollutant is not the same). Similarly Figure 6-2 shows 

results for 20 nm particles. 20 nm particles is a very dynamic pollutant whose concentrations are rapidly 

changing due to the coupling of dilution and microphysics (evaporation) as one moves away from road. 

The physics that controlling 20 nm particle is different than NO2 so it is not clear that 20 nm provides 

guidance for NO2 selecting NO2 sites.

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5?

Response: Fleet equivalent measure seems like a rational, first step to try to incorporate the differences 

in emission factors between LDV and HDV. One factor that might be considered would be that the 

emissions of diesel fleet are going to change dramatically over the next decade or two as aftertreatment 

technologies are implemented. For just DPF equipped vehicles NO2 emissions can increase 

significantly. For DPF and SCR equipped vehicles they should go down. The effectiveness of these 

technologies will then change as vehicles age (this aging is not well understood given our limited 

experience). It seems like these issues need to be considered because it will influence the truck to car 

ratio of NOx emissions and this ratio will change with time (presumably decreasing over time).

Another issue that the FE AADT should more formally account is the effect of congestion. This can 

have large effect on NOs emission, but is currently not part of FE AADT. One approach might be to 

make the HDm value a function of the congestion ratings.

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Response: This section provides a reasonable, largely qualitative discussion the effects of these 

parameters on near-roadway pollutant dispersion. However, it does not really address the question of 
50

how these factors should be considered in selecting sites. For example, it states that valleys coupled with 

inversions could lead to high levels. Is that a good thing or a bad thing in terms of site selection? Table 

6 provides guidance on how to weight some of these issues but is not comprehensive. More effort needs 

to be made to make Table 6 more comprehensive. Essentially, the document describes the effect of some 

structure but does then provides little guidance to the states of how to weight that information. 

Ultimately, I believe this shortcoming is tied to the document not clearly stating the scientific criteria 

that is driving site selection. 

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures?

Response: Is 1 m offset adequate for probes installed on walls, etc? I am skeptical.

As in other statements, distance from roadway seems like a key parameter that needs to be better 

constrained.

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process?

Response: Given the sensitivity of NO2 concentrations to distance from roadway and a myriad of other 

factors, I think that exploratory studies of a short list of sites would be very important. It would be very 

useful for EPA to provide more guidance of how this data would be used. For example, can the EPA 

provide a couple of case studies based on actual data collected at a couple of sites (analogous to the 

Tampa Bay AADT example) that illustrate how states would interpret and use pilot data to select a site. 

One approach that is described in the TAD is saturation sampling with passive monitors. While these are 

relatively cheap to deploy in a distributed network they measure long term average concentrations. 

However, the regulation is for 1 hr peak NO2. The documentation mentions this issue but it seems to me 

it is a major problem. It is not clear that the long-term data will provide much guidance for site selection 

and therefore I question the overall utility of the approach in terms of aiding the selection to identify 1 hr 

peak NO2. 

It is not clear over what spatial area the exploratory studies should be performed. Just within the first 50 

m of roadway. What about over a longer spatial scale which may incorporate more people. Upwind and 

downwind? Many near road way studies have been performed. EPA should describe one or two of those 

in the TAD as an example for good practices for study design.

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process?

Response: Given the complexity of near road way pollutant dispersion, it seems like coupled modeling 

and exploratory studies will be important to select robust sites. One issue that was not discussed in 

modeling was potential effects of “background NO2.” The document has a very strong road segment 

focus however these road segments are generally part of a complex urban environment with many 

sources. These other sources (other nearby roads, industrial sources, etc.) could strongly influence the 
51

NO2 levels around a particular road segment. There needs to be more guidance on how to use the 

models to incorporate this complexity into the analysis. 

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included.

Response: The list is pretty comprehensive (and repetitive with earlier sections). A table or checklist of 

items might be useful in organizing material. 

A few factors not on list.

The document has a strong road segment focus. The potential for other NOx sources (and background 

NO2 in general) should be considered.

What about using existing sites?

Surrounding land use seems to be a key issue that is not discussed in this section. In particular 

population seems like a critical parameter to consider. Also sites with large populations that are 

influenced by “multiple” road segments may be more valuable.

Road way grade seems like an important issue.

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section?

Response: I am not an expert in this area, but it seemed reasonable. This section should provide air 

agency enough information to effectively interact with DOT.

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13?

Response: No response.

Charge Question 11: Does the AMMS: 

a. Concur with the order of presentation of each pollutant or metric of interest in the near-road 

environment, as was suggested by the previous AMMS panel, within Section 14? 

Response: O3 should be higher given role in NO to NO2 conversion. Maybe one could combine NOx 

data with O3 to make simple estimate of NO2 levels using a model?

PM number – I would move down. Given the regulatory focus on PM mass it seems that mass and major 

mass constituents from motor vehicles (OC, EC, road dust) should be higher on list. Number is 

interesting, but number size distributions would be much more useful given the transformations that 

occur near road way

Air toxics should be moved up list. Right after PM mas and major mass components.
52

My list

NO2

Meteorology

EC

CO

O3

PM mass

OC

Air toxics

Pb

PM number distributions

SO2 seems like least important (remove?) on list.

b. Concur with the description of each pollutant or other metric discussed in Section 14, including its 

impact on human health (as appropriate), the reason for interest in the near-road environment, and the 

description or suggestions for measurement? 

c. Believe that a pollutant or other metric should be removed from the list within Section 14, or that an 

unlisted item should be included within this section? 

Response: I would remove SO2.

It seems like NO would be important given the relationship with NO2.
53

Comments from Dr. Jamie Schauer 

Charge Question 1: Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives? 

Response: Section 1 seems like a good place to briefly note the health studies that have shown health

risk of residing and traveling near roadways. This provides a strong motivation to approach near-road 

exposures from a multi-pollutant perspective. 

Charge Question 2: Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors (AADT, fleet mix, congestion 

patterns, roadway design, terrain, and meteorology) required to be considered as part of the near-road 

NO2 site selection process? 

Response: The weight of factors seems appropriate in the context of TAD

Charge Question 3: Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5? 

Response: I would recommend providing more information on calculating the HDm. The current write 

up seems to suggest that the national default value is good enough and does not really provide incentive 

or motivation to have a more site specific value. I would recommend site specific values where possible. 

Charge Question 4: Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Response: Given the target audience, I think this section is appropriate in terms of the scope and level 

of detail. 

Charge Question 5: Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures? 

Response: Given the target audience, I think this section is appropriate in terms of the scope and level 

of detail. 

Charge Question 6: Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process? 

Response: This section may be more useful if a summary of the pros and cons of the different 

exploratory monitoring approaches were explicitly stated and summaries. 
54

Charge Question 7: Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process? 

Response: None

Charge Question 8: Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included. 

Response: In this section and in the report in general, it seems that some characterization of the 

roadway grade (i.e. incline or decline) needs to be considered. Clearly, this will have a significant 

impact on HDD emissions. 

Charge Question 9: From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section? 

Response: No suggestions to improve

Charge Question 10: Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13? 

Response: No suggestions to improve

Charge Question 11: Does the AMMS: 

a. Concur with the order of presentation of each pollutant or metric of interest in the near-road 

environment, as was suggested by the previous AMMS panel, within Section 14? 

b. Concur with the description of each pollutant or other metric discussed in Section 14, including its 

impact on human health (as appropriate), the reason for interest in the near-road environment, and the 

description or suggestions for measurement? 

c. Believe that a pollutant or other metric should be removed from the list within Section 14, or that an 

unlisted item should be included within this section? 

Response: In section 14.7 concerning PM, it may be useful to explain the relative roadway sources that 

impact PM2.5 versus Coarse PM (PM10-PM2.5). They key point is that brake wear, tire wear and 

resuspended road dust will impact will largely be Coarse PM and tailpipe emissions will largely be 

submicron PM. 
55

Comments from Dr. Jay Turner

Charge Question 1. Does the TAD, particularly based upon the information provided in Sections 1 and 

2, provide clear objectives of the document and give appropriate rationale for the objectives?

Response: The objectives are generally clear. It might be helpful to directly include excerpts from 

the final rule such as section 4.3 from Appendix D of Part 58 and the revisions to Appendix E of Part 

58. These sections could be appended to the TAD or in some cases excerpts could be added as text 

boxes to the main body to reinforce the requirements and constraints to the monitor siting approach 

that are imposed by the regulation. These issues are discussed throughout the TAD with reference to 

the final rule, but in some cases it would be helpful to have the formal language handy. 

Charge Question 2. Does the AMMS believe that the suggested approach in the TAD places an 

appropriate amount of weight and consideration on all six factors required to be considered (AADT, 

fleet mix, congestion patterns, roadway design, terrain, and meteorology) as part of the near-road NO2 

site selection process?

Response: It is understandable that the required approach starts with AADT and builds in the 

additional factors because AADT data should be readily available for all affected areas. On the other 

hand, many areas have time-resolved traffic data and the TAD provides no clear vision for how these 

data could be most effectively incorporated into the ranking process. Congestion metrics such as 

level of service (LOS) partially capture the within day dynamics but this is a rather coarse grained 

metric and cannot be used to refine the prioritization of roadway segments with the same LOS. 

Indeed, the Tampa example demonstrates that the vast majority of road segments ranked in the top 

30 have an F ranking for the LOS. In light of this issue, if hourly data are available then an 

additional useful metric may be the daily maximum hourly traffic volume (better yet, the daily 

maximum hourly fleet equivalent hourly traffic volume). 

Can the EPA offer any guidance on how to cluster the ranked results? For example, the Tampa 

example provides a ranked list. Can approaches be taken subsequently group the segments into 

highest priority, moderate priority, and lowest priority? Clearly there are no bright lines for making 

such distinctions but on the other hand the rank ordering of road segments should not be overly 

interpreted given the subjective linkage between the ranking criteria and maximum hourly NO2 

concentrations.

The TAD clearly describes the criteria for determining whether a second monitor is required (e.g. 

Figure 3-1). However, the objectives for the second monitor should be discussed in more detail. The 

final rule states “Where one CBSA is required to have two near-road NO2 monitoring stations, the 

sites shall be differentiated from each other by one or more of the following factors: fleet mix; 

congestion patterns; terrain; geographic area within the CBSA; or different route, interstate, or 

freeway designation.” At a minimum this description should be provided with additional guidance 

offered if possible. 

It would be helpful to provide additional guidance on whether and how background concentrations 

should be considered. Background concentrations are mentioned throughout the document but it 

would be helpful to discuss its role in the site selection process in more detail given many of the high 

traffic roadways may be in proximity to other roadways such as a road segments within an urban 

core transportation network. The emphasis clearly is on impacts from the adjacent roadway. The 
56

TAD discusses impacts from nearby point sources and such but it would be helpful to step back and 

reflect upon the role of background concentrations when assessing candidate monitoring sites. 

Charge Question 3. Does the AMMS see opportunities to improve the usefulness of the Fleet Equivalent 

AADT metric introduced and discussed within Section 5?

Response: Overall, I like the approach as a screening tool. Of course the use of road-segment 

specific fleet mix would be ideal but it is respected these data are not available in all affected areas. 

In the absence of such data, the approach will likely need to be more thoughtful than the “county by 

county characterization” mentioned on page 5-13 as one possible approach to fleet mix 

categorization. Additional guidance should be provided on approaches that could be taken to assign 

fleet mixes to road segments in the absence of segment-specific data. 

The nomenclature could be improved to provide clarity. For example, the term “Fleet Equivalent 

(FE) AADT” is vague. It is actually a light duty (LD) vehicle equivalent measure because the heavy 

duty vehicle counts are being scaled to the number of equivalent light duty vehicles in terms of 

emissions. Second, the HD-to-LD emission ratio is represented by HDm and the HD annual average 

daily traffic count is HDc. The notation for these two variables is too similar. Consider representing 

HDm as (EFHD)/(EFLD) where EFi is a representative emission factor for vehicle class i. 

Charge Question 4. Within Section 6, does the AMMS believe we have adequately described the effects 

of roadway design, roadway structures, terrain, and meteorology on roadway pollutant dispersion and 

suggested how those effects can be considered in the near-road site selection process?

Response: It seems that intersections/interchanges could be hot spots for NO2 concentrations. The 

ranking methodology focuses on roadway segments and gives too little attention to the confluence of 

roadway segments as being an important consideration. It is merely mentioned as a “desirable 

attribute” in Section 6 and one of the field characteristics that should be documented (Section 10.4) 

but should be given much more weight in the prioritization. Using the Tampa example, do two of the 

ranked segments represent a crossing of some type? If so, they should be collectively made a higher 

priority. 

The guidance states that air channeling by terrain should be considered. The emphasis is on the 

macro-scale rather than the micro-scale such as air channeling by along cut-section roadways. It 

might be useful to clarify that the distinction between seeking to capture high near-roadway 

concentrations versus high on-roadway concentrations. 

Charge Question 5. Within Section 7, does the AMMS believe we have adequately discussed the siting 

requirements and provided appropriate suggestions for how to properly site monitor probes while 

considering the design of the target road and/or roadside structures?

Response: The discussion is generally fine. The criteria are necessarily subjective in the absence of 

very detailed air flow modeling. The last sentence of Section 7 is not clear – what is meant by the 

agencies should “consider more than one linear pathway between the target road segment and the 

monitor probe”? This issue is discussed earlier in the section but this summary sentence does not 

bring the discussion together.
57

Charge Question 6. Does the AMMS believe that Section 8 has adequately discussed and explained the 

varied approaches on the optional use of exploratory monitoring as part of the near-road site selection 

process?

Response: This section provides some discussion of the context for conducting exploratory 

monitoring and could be more fully developed. It is stated that exploratory monitoring may be useful 

to compare and contrast sites that are ranked as high priority locations. Within this context, another 

example would be the case of intersections or interchanges between two road segments that were 

each ranked moderately high to determine whether their additive effects significantly increase their 

ranking. This would best be done by exploratory monitoring or modeling of the highest-ranks sites 

and these cases of intersections/interchanges, with the former needed for to provide context for the 

interpreting the latter. 

Charge Question 7. Within Section 9, does the AMMS see opportunities to improve the description of 

how the (optional) use of AERMOD and MOVES can be used to conduct dispersion modeling in the 

near-road site selection process?

Response: I will address this charge question in more detail in my final comments. 

One area that could be refined is the section on urban/rural classification in Section C.6.3. In the 

discussion of Figure 6-1 it is stated that “urban and rural concentrations are nearly equal at short 

distances but as distance from the source increases, the urban concentrations become much less than 

the rural concentrations.” This figure and the level of detail in the subsequent text are certainly 

important if sources in addition to the road segment are being modeled. However, if only the road 

segment is being modeled then, as stated, the differences are insignificant over the spatial scales of 

interest for siting the monitor (within 50 m). This conclusion needs to be highlighted. 

Charge Question 8. Within Section 10, does the AMMS believe the list of items needed to appropriately 

characterize individual candidate road sites is complete and adequately described? If the list is 

considered incomplete, please provide a list of the missing characteristics that should be included.

Response: To be addressed in my final comments.

Charge Question 9. From an air agency perspective, does the AMMS find that the definitions and 

explanation of transportation agency policies and expectations are adequate? Are there opportunities to 

improve upon the material presented within this section?

Response: Section 11 discusses the monitor site logistics including the need to coordinate with 

appropriate transportation agencies. In each of the affected areas there already exists a forum for 

exchanging information – transportation and air quality planning coordination required through the 

designated Metropolitan Planning Organization. As discussed in Section 11 the coordination will 

often go far beyond the transportation and air quality management planning level because 

instruments such as access agreements and permits may be involved. The bullet lists in Section 11.4 

provide a reasonable overview of the key considerations. 
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Charge Question 10. Does the AMMS have ideas for improvement with respect to the organization and 

usefulness of the suggested site comparison matrix discussed within Section 13?

Response: The comparison matrix provides a nice framework. While this is not a strictly prioritized 

list, I believe the presence of interchanges/intersections should be moved up the table (e.g., 

immediately follow Congestion Information) and specifically call out cases where the crossing road 

segment is highly ranked. 

For the meteorology parameter, the description should include a qualitative indicator of the likely 

representativeness of the data used. For example, if the data are from the area’s airport and the site 

characteristics would lead one to believe such data may not be representative for the specific road 

segment, this should be qualified. In particular, attributes that increase the frequency of calm 

conditions should be mentioned.

Charge Question 11. Does the AMMS: (a) Concur with the order of presentation of each pollutant or 

metric of interest in the near-road environment, as was suggested by the previous AMMS panel, within 

Section 14? (b) Concur with the description of each pollutant or other metric discussed in Section 14, 

including its impact on human health (as appropriate), the reason for interest in the near-road 

environment, and the description or suggestions for measurement? (c) Believe that a pollutant or other 

metric should be removed from the list within Section 14, or that an unlisted item should be included 

within this section? 

Response: I am comfortable with the ranked list with the following exception. Is Section 14.8 

intended to be Organic Carbon in general – including both gaseous and PM species – or exclusively 

PM? The title and first sentence suggest the former but the remainder of the section focuses on 

particulate matter OC with key gaseous species addressed in Section 14.12 (Air Toxics). Also, 

mention of the HR-AMS seems inappropriate because it is strictly a research grade instrument. If 

anything, the ACSM would be a better instrument to mention in this context.

Section 14 should start with a list or table of the pollutants and metrics. My final comments will 

include suggestions for refining some of these pollutant-specific summaries. For example, the 

second paragraph of Section 14.7 (PM Mass) can be tightened up and I will provide specific 

suggestions.

My final comments will include suggestions for revising the presentation and wording in a various 

places throughout the entire document to improve clarity. 

 
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Comments from Dr. Yousheng Zeng 

Charge Question 1 – Objectives and rationale

Response: The draft TAD does provide clear objectives of the document itself, i.e., how to implement 

near-road NO2 monitoring. The draft TAD also identifies objectives of the near-road monitoring 

network and describes rationale to support these objectives. With multiple objectives, it is necessary to 

clarify and prioritize the objectives. The following recommendations are made: 

1. To clearly and explicitly state the minimum regulatory requirements and/or provide a summary 

of requirements by regulations (not just cite the regulation).

2. To clarify and prioritize the objectives of the near-road monitoring network: 

attainment/nonattainment designation, health effect study, modeling study, multi-pollutant study, 

etc. Another way to address this is to explain how EPA intends to use the monitoring data 

generated by this near-road monitoring network.

To clarify if the goal of setting up the monitors is to capture/measure (1) the highest NO2 concentrations 

on a short-term (hourly) basis without consideration of population exposure, or (2) the highest NO2 

concentrations with consideration of potential exposure by near-road residents.

Charge Question 2 – Weight and considerations on six factors in the site selection process 

Response: I believe that the TAD has given appropriate consideration on all six factors. However, the 

weight given to FE AADT may be too high. In addition, EPA should add some factors for consideration 

in the site selection process. These factors are not specified in 40 CFR 58 App. D, but they may 

influence the monitoring results or the use of the results. These factors may include:

1. Cumulative effect of multiple roadways. The site selection process discussed in the draft TAD 

focuses on individual roadways. They are treated as isolated roadways. In actual environment, if 

there are multiple roadways in very close proximity (e.g., a freeway segment in a city that is 

parallel to two major streets (not frontage roads), one on each side 100 meters away from the 

freeway. When evaluated individually, each of them may not be ranked high. However, the 

combined effect of these roadways can be significantly higher. 

2. Nearby NO2 stationary point or area sources. This issue is related to the monitoring objectives. 

Should we select a site that has the highest NO2 concentration (whether it is caused by traffic or 

traffic plus nearby stationary sources)? Should we avoid influence of stationary NO2 sources 

(i.e., only vehicle contributions) or are we concerned about cumulative effect of all sources?

3. Public accessibility. As stated in the TAD, the near-road monitor should be placed within 50 

meters from the outer lane of the roadway. If there is no public access to the 50-m zone for a 

roadway segment (e.g., barbwire/fence along the roadway, natural terrain, etc., or combination of 

these), this segment should not be considered for monitoring even when the AADT is very high. 

The test of public accessibility for definition of “ambient air” has been a long-standing policy in 

the EPA PSD permit program. The same policy should be applicable to near-road NO2 

monitoring. 

The TAD should discuss the nature and effect of these factors; provide guidance on how to treat them in 

the site selection process and how to document them in the monitoring plans. 

Charge Question 3 – FE AADT metric 

Response: Use of Fleet Equivalent AADT to normalize fleet mix (i.e., converting HD vehicle traffic to 

equivalent LD vehicle traffic) and compare road segments on the normalized, FE AADT, basis is a 

significant improvement over the method based on AADT. There might be ways to further classify 
60

vehicle types beyond the two classes (i.e., HD and LD). However, I think the method proposed in the 

draft TAD is most practical and adequate for most cases. For certain road segments (e.g., highway in 

Central Business Districts) where no HD vehicles are allowed, the monitoring agency can simply make 

AADT=FE AADT. 

The draft TAD provides step-by-step procedures. However, the issue of multiple roadways in close 

proximity (see discussion and example above in response to Charge Question 2) and issue of major 

intersections is not addressed in the FE AADT based ranking scheme described in the draft AADT. 

Some high NO2 areas may be missed.

There are cases where the median between divided highways is very wide. Should each direction be 

treated as a separate roadway in the ranking? If so, how wide does the median have to be in order for the 

two directions to be treated as separate roadways?

Charge Question 4 – Roadway pollutant dispersion 

Response: The opening part of Section 6 and Table 6 provide a summary of the three factors that affect 

pollutant dispersion. Although the impact of these factors to dispersion is adequately described, the 

guidance on how to factor in the impact is not very clear. Should a monitoring site be selected so that the 

highest NO2 concentration (the worst dispersion) is detected or avoided? It appears that conflicting 

message is given: sometimes the idea is to detect the highest near road pollutant concentrations; and 

other times the guidance is to avoid the worst dispersion conditions (e.g., presence of sound walls). In 

Table 6, some attributes are considered desirable because they cause better dispersion or lower 

concentrations, such as “at grade with surrounding terrain”, “low barriers present”, and “flat or gentle 

terrain”. However, other attributes are also considered desirable while they cause opposite effect (i.e., 

higher concentrations). The examples are “near ramps, intersections, lane merge locations”, “within a 

valley”, and “relative downwind locations”. Similar confusion exists under the heading of “Less 

Desirable Attributes”. Does TAD instruct monitoring agencies to select a site that is expected to have 

the highest NO2 concentrations? It would be very helpful that the principle used in dealing with these 

dispersion factors is clearly explained in the opening part of Section 6.

Charge Question 5 – Siting requirements and monitoring probe 

Response: Section 7, specifically Table 7, provides a good summary of the regulatory requirements. 

Recommendations are specific and easy to follow. One area that may need further discussion is the 

relationship between the probe horizontal and vertical placement. The horizontal placement and vertical 

placement are discussed separately. Should there be a discussion on the interplay between the two? 

There is a range in both dimensions: horizontally from “as near as practicable” to 50 meters; vertically 

from 2 meters to 7 meters. When the horizontal distance is very close to the traffic, should the vertical 

distance be in the lower range, closer to 2 meters, rather than the higher range, closer to 7 meters? Under 

a condition of perpendicular wind, the plume coming out of the vehicle tailpipe will be closer to the 

ground and will gradually disperse as distance increases. Therefore at a very short horizontal distance 

from traffic, the plume may be closer to the ground and a probe intake position near 7 meters may be too 

high to intercept with the plume. If the probe is placed further away from traffic (further distance 

downwind from the traffic), the plume will be better dispersed, and a higher probe position may not be a 

significant issue.

It may also be a good idea to discuss the sample line length. If the probe intake is 7-meter high and the

analyzer is at the ground level, there may be a long sample line running from the probe intake to the 

analyzer. A long sample line may cause issues in response time or loss of target compounds. The TAD 

should provide some guidance on either sample line length or sample residence time in the line.
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Charge Question 6 – Exploratory monitoring in near-road site selection process 

Response: The TAD provides some options and guidance on how to use exploratory monitoring in site 

selection process. It would be more helpful to provide some guidance on how to use the data collected 

from the exploratory monitoring to assist the site selection. 

It is important to document traffic information (e.g., vehicle count, fleet mix, level of congestion, etc.) 

and other conditions (e.g., day of the week, time of the day, wind condition, etc.) that may affect the 

monitoring results. Some general recommendations on duration for each exploratory monitoring are also 

helpful. 

Charge Question 7 – Use of AERMOD and MOVES modeling 

Response: The TAD references well established modeling guidance documents. However, the 

application of the models for this purpose is different as compared to typical modeling analysis in air 

permitting processes. In a typical regulatory modeling analysis, the finest receptor grid has a spacing of 

25 meters or larger. This spacing may not be sufficient if the model is used for the first and second 

purposes described in the opening paragraph of Section 9. A very significant concentration gradient is 

expected in the 50-m zone near road. A much finer spatial resolution is needed for this type of modeling 

analysis. 

The EPA modeling guidance memos (e.g., the March 2011 memo on 1-hour NO2 modeling) are 

intended for typical permit modeling analysis. It focuses on traditional industrial sources and the 

modeling domain is large enough to use certain treatment to address the chemical reaction of NO/NO2. 

The techniques discussed in the draft TAD, e.g., NO2/NOx in stack ratio, Ozone Limiting Method 

(OLM), and Plume Volume Molar Ratio Method (PVMRM), may not be applicable/suitable to the nearroad condition, or some adjustment may be needed. Simply referencing the existing modeling guidance 

memo seems inappropriate or inadequate. 

Charge Question 11 – Multi-pollutant monitoring 

Response: Near road sites for NO2 monitoring will be treated as ambient air. Monitoring of other 

criteria pollutants (e.g., CO, PM2.5, ozone) at these sites is technically feasible but will be problematic 

in terms of their regulatory implications. Exceedance of NAAQS in ambient air will cause designation 

of the area as nonattainment and there will be many serious consequences (development of SIP, 

Nonattainment New Source Review (NNSR) permitting, transportation and general conformity 

determination, attainment schedule, penalties, etc.). The regulatory framework for ozone, CO, and 

PM2.5 nonattainment is designed without special consideration of near road conditions. The criteria for 

ambient monitoring network for the purpose of attainment/nonattainment determination are different. It 

is one thing to establish a multi-pollutant monitoring network for research purposes, and a much more 

complex issue for regulatory purposes. An exceedance of NAAQS for ozone, PM2.5, or CO at a nearroad site will bring a suite of regulatory issues that deserve much more careful considerations outside of 

the monitoring community. When suggesting multi-pollutant monitoring, the TAD should include 

cautionary comments and/or discuss the possible consequences and the way to manage them. If EPA is 

not prepared to provide corresponding policy guidance on these issues, the TAD should discourage 

multi-pollutant monitoring that include criteria pollutants, and provide guidance on how to approach the 

issue for pure research purposes.