UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C. 20460 

 

OFFICE OF THE ADMINISTRATOR 

 SCIENCE ADVISORY BOARD 

November 24, 2010 

EPA-CASAC-11-001 

The Honorable Lisa P. Jackson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Subject: Review of the “Near-road Guidance Document – Outline” and “Nearroad Monitoring Pilot Study Objectives and Approach” 

Dear Administrator Jackson: 

The Clean Air Scientific Advisory Committee (CASAC) Ambient Air Monitoring 

and Methods Subcommittee (AAMMS) met on September 29-30, 2010 to review EPA’s 

Near-road Guidance Outline and Near-road Monitoring Pilot Study Objectives. The 

attached CASAC report was approved at a public teleconference on November 8, 2010. 

The impetus for the development of near-road monitoring guidance comes from the 

recent revisions to the primary National Ambient Air Quality Standard (NAAQS) for 

NO2 . In its revisions to the primary NO2 NAAQS issued on February 9, 2010, EPA set 

out requirements for an NO2 monitoring network that includes monitors at locations 

where maximum NO2 concentrations are expected to occur, including within 50 meters of 

major roadways. 

Although the requirement for near-road monitoring comes from the revised NO2

rule, EPA’s Office of Air Quality Planning and Standards (OAQPS) is developing 

guidance for an expanded monitoring capability to be used for a variety of pollutants, 

pursuant to the Agency’s commitment to multi-pollutant monitoring approaches. 

OAQPS sought CASAC’s advice in developing a guidance document for state and local 

monitoring agencies as they implement the NO2 near-road network that is required to be 

operational by January 1, 2013. OAQPS also sought CASAC’s advice on the objectives 

and design of a near-road monitoring pilot study that is intended to inform EPA, state and 

local air monitoring agencies on siting issues. 

CASAC recognizes the importance for public health of better characterizing nearroad pollutant concentrations. In implementing near-road monitoring in the context of 

the NO2 NAAQS, EPA unavoidably faces major issues that are described in detail in the 

responses to the charge questions and highlighted in this letter: 

i 
• While NO2 historically has been used as the indicator for ambient oxides of 

nitrogen (NOx), it is one of multiple gases comprising NOx,which also includes 

nitric oxide (NO), nitrous acid (HONO), and other species. CASAC notes that the 

ratio of NO2 to NOx varies substantially, and NO2 concentrations can 

significantly under represent NOx levels in the near-road environment; 

• The epidemiological evidence on NO2 is largely based in the population-oriented 

monitor data. If these monitors are not maintained in sufficient number, 

continuity of data will be lost, both for health research purposes and for tracking 

trends of NO2 concentration;

• In moving to implement near-road multi-pollutant monitoring, EPA will need to 

find a way to optimize siting for measuring concentrations of the various 

pollutants included in the platform. For example, the optimal siting for NO2 may 

not be the same as for CO. 

CASAC was asked to comment on two short documents: “Near-road Guidance 

Document – Outline” (a 1-page document) and “Near-road Monitoring Pilot Study 

Objectives and Approach” (a 4-page document). 

EPA’s draft Near-road Guidance Document – Outline provided a very preliminary 

list of the general factors that should be considered in developing a near road monitoring 

network, including those factors that should be considered in siting monitoring locations. 

However, 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. 

EPA’s Near-road Monitoring Pilot Study Objectives was also a very brief 

overview of the objectives for a real-world pilot study in near-road monitoring 

implementation. With respect to the factors EPA is considering for siting monitors, we 

note that the primary focus of the monitor site selection process is on annual average 

daily traffic (AADT). However, the approach may place too much weight on these data. 

Other factors to be considered include the physical characteristics and the patterns of 

vehicle use at the site (e.g., fleet mix, roadway design, congestion patterns, terrain, etc.), 

modeling information, and preliminary monitoring studies (e.g., results of 

multiscale/saturation studies). CASAC suggests that EPA should allow states and local 

agencies to bring as much information to bear on the site selection process as is 

appropriate, with prioritization of installation of sites based on review of all relevant data 

in discussions between monitoring agencies and EPA offices. 

We encourage evaluation of the use of a “true” NO2 monitor and inclusion of this 

instrument in the near-road monitoring program. We endorse the views expressed in the 

letter of August 19, 2010 from the National Association of Clean Air Agencies to 

Assistant Administrator Paul Anastas encouraging ORD “to increase its focus on the 

development and advancement of ambient air monitoring reference, equivalent and other 

sampling and analytical methods.” Measurement methods germane to near-road 

monitoring mentioned in that letter include those for PM2.5, PM10-2.5, and ultrafine 

particles. We also urge EPA to address known biases in measurements made with the 

NO2 FRM. 

ii 
As a general matter, CASAC is deeply concerned about the timing proposed for 

the current network deployment, as well as for the Pilot Study. The revised NO2

NAAQS, issued on February 9, 2010, mandates that state and local air monitoring 

agencies deploy the near-road network by January 1, 2013. This ambitious schedule may 

make it difficult to absorb lessons learned from EPA’s Pilot Study to evaluate and 

improve the siting and monitoring process. If possible, given this mandated date of 

deployment, EPA might consider deploying the network in stages over time, e.g., 10-20 

sites the first year, 20-40 the next and the rest in the final year. As part of this staged 

approach, EPA should consider using near-road sites appropriately paired with sites from 

the National Core (NCore) monitoring network that is set to come online January 1, 2011 

in the first round deployment. Consideration should be given to areas with larger 

populations. Such a staged approach would be consistent with the recommendations 

from the CASAC Oxides of Nitrogen Primary National Ambient Air Quality Standards 

(NAAQS) Review Panel (see Samet, Sept. 9, 2009). In this way, the network can evolve 

based on lessons learned from the Pilot Study as well as from the operation of the initial 

sites. 

CASAC is concerned that there could be a decrease in the number of populationoriented NO2 monitors in the new network. Many of the health studies that were 

considered in the latest NAAQS NO2 review were based on the population-oriented 

monitors of the current network. CASAC strongly recommends that a great majority of 

these monitors be maintained, particularly those that have been used in past healthfocused studies. By maintaining these monitors, air quality trends could be tracked 

without discontinuity. 

Just as we recommended a staged approach to the deployment of the near-road 

monitoring network, CASAC also recommends a tiered approach to the design of the 

near-road monitoring sites. A few sites should be comprehensively equipped such that 

they can provide comprehensive information about the composition of mobile source 

emissions and how pollutant concentrations and mixtures change over time with changes 

in sources and control measures. The bulk of the sites could be more modestly equipped. 

For example, the modestly equipped sites would [also] include optical black carbon (as a 

surrogate for elemental carbon), carbon monoxide (CO), meteorology and ultra-fine 

particulate matter (PM) monitoring capabilities responsive to the needs for assessing 

attainment with the applicable standards and the extent of near-road pollution exposure, 

as well as for use in health studies. We provide more detail in our responses to the 

associated charge questions. 

State and local resource constraints are another cause for concern, particularly in 

view of this schedule. For all of the criteria pollutants as well as for NO2 specifically, 

CASAC encourages EPA to commit the resources necessary to focus on the development 

and advancement of ambient air monitoring methods, with specific attention paid to 

assessment and possible modification of the Federal Reference and Equivalent Methods. 

We are concerned that the current time frame for the NO2 near-road network may not 

allow adequate time to appropriately plan and execute the Pilot Study and then to 

iii 
interpret and use the resulting findings in designing the near-road network. The decisions 

that will be made have broad implications related not only to NO2, but to other criteria 

pollutants and the characterization of multiple-pollutant exposures from roadway sources. 

CASAC appreciates the opportunity to provide input to EPA at this early stage in 

the process. The CASAC and AAMMS membership is listed in Enclosure A. CASAC’s 

consensus responses to the Agency’s charge questions are presented in Enclosure B. 

Individual review comments from the AAMMS are compiled in Enclosure C. 

Sincerely, 

/Signed/ /Signed/ 

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

CASAC Ambient Air Monitoring & Clean Air Scientific Advisory 

Methods Committee Committee 

 

Enclosures 

iv 
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 of 

commercial products does not constitute a recommendation for use. CASAC reports are 

posted on the EPA website at http://www.epa.gov/CASAC. 

v 
Enclosure A

U.S. Environmental Protection Agency 

Clean Air Scientific Advisory Committee 

Ambient Air Monitoring and Methods Subcommittee for the 

Review of Near-Road Monitoring to 

Support Measurement of Multiple National 

Ambient Air Quality Standard (NAAQS) Pollutants 

CHAIR 

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

Engineering, Georgia Institute of Technology, Atlanta, GA 

MEMBERS 

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

Management (NESCAUM), Boston, MA 

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

Laboratory, University of Nevada, Reno, NV 

Mr. Bart Croes, Chief, Research Division, California Air Resources Board, Sacramento, 

CA 

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

Center, State University of New York, Albany, NY 

Dr. Delbert Eatough, Professor of Chemistry, Department of Chemistry and 

Biochemistry , Brigham Young University, Provo, UT 

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

and Biomolecular Engineering, Clarkson University, Potsdam, NY 

Dr. Kazuhiko Ito, Assistant Professor, Department of Environmental Medicine, School 

of Medicine, New York University, Tuxedo, NY 

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

of Minnesota, Minneapolis, MN 

Mr. Richard L. Poirot, Environmental Analyst, Air Pollution Control Division, 

vi 
Department of Environmental Conservation, Vermont Agency of Natural Resources, 

Waterbury, VT 

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

Box 1180 , Washington University , St Louis, MO 

Dr. Warren H. White, Research Professor, Crocker Nuclear Laboratory, University of 

California, Davis, CA 

Dr. Yousheng Zeng, Air Quality Services Director, Providence Engineering & 

Environmental Group LLC, Baton Rouge, LA 

SCIENCE ADVISORY BOARD STAFF 

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

Agency, Science Advisory Board, Washington, DC 

vii 
viii 

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 Allen, Senior Scientist, NESCAUM (Northeast States for Coordinated Air 

Use Management), 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, Science Advisory Board Staff Office, 

Washington, D.C. 
Enclosure B 

Consensus Response to Charge Questions on Near-Road Monitoring 

to Support Measurement of Multiple NAAQS Pollutants 

Questions on Near-road Guidance Outline 

1. The accompanying draft guidance document outline provides an initial 

thought of the major topics required in the near-road monitoring guidance 

that will aid state monitoring agencies in the identification and 

implementation of NO2 near road monitoring sites from a multi-pollutant 

perspective. Please comment on the overall content of the recommended 

topics in the draft outline. Please provide suggestions on any missing 

subjects that should be included in the guidance document and any 

unnecessary topics that are currently listed in the attached draft, if 

applicable. 

It is not clear that EPA has identified the reasons (scientific and regulatory objectives) for 

the multi-pollutant monitoring. We recommend that EPA clearly identify the monitoring 

and associated scientific objectives for the near-road monitoring program, and then build 

the document around these objectives. A possible set of objectives focused on NO2, for 

example, might be: 

• Identification of hourly averaged NO2 by the most artifact free method available. 

• Identification of hourly averaged concentrations of other NAAQS pollutants 

which accompany the observed NO2 concentrations. 

• Identification of the atmospheric processes which contribute to the observed 

concentrations of NO2. 

• Identification of the sources which contribute to both measured concentrations of 

NO2 and the other measured NAAQS pollutants 

The document should then discuss fully the objectives and the scientific and regulatory 

bases for these objectives in the Introduction. This will, in turn, support the selection of 

the recommended pollutants to be monitored in the program and the protocols to be 

followed. 

In addition, we suggest the following more minor areas where the Guidance Document 

needs strengthening. 

• General. The success of the near-road pilot project is critical to the future 

deployment of the 126 near-road monitoring sites by the end of 2012. Thus 

timing (and resources) are tight. EPA needs to focus on defining the objectives 

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for the program and identify those key elements of the program essential to 

meeting those objectives. EPA needs to identify how to leverage existing sites, 

how the sites will be operated, how the data analysis will be accomplished, and 

how many sites are needed to meet the objectives of the program. Finally we 

encourage EPA to evaluate the near-road excess (or deficiency) for key indicators, 

e.g. NO2, CO, black carbon, ultrafine particles. This will necessitate background 

measurements (ideally made at an appropriate existing sites) which allow an 

estimation of the gradient away from the road. 

• Background. The background should document the scientific and regulatory 

bases for the multi-pollutant objectives, as outlined in the bullets above. The 

literature review needs to be more comprehensive than at present. Some guidance 

on possible literature to site is included in individual responses. A conceptual 

model should be formed in this section that includes dispersion, deposition, 

chemical conversion and physical conversion. Special attention should be given 

to NO2 formation by the titration of O3 and NO2 depletion by photochemistry. 

The Background section should acknowledge that finding a site where NO2 and 

the various measured other pollutants all have the highest concentrations is not 

likely and compromises will be necessary and acceptable. 

• Identifying Candidate Near-road Site Areas. We believe the criteria outlined for 

this section are generally adequate for the identification of a site where near 

maximum NO2 concentrations near a given near-road site may be determined. 

However, there is an apparent assumption that AADT is the primary siting 

criteria. The importance of meteorological and terrain variables should be 

emphasized. Further, since NO2 is a pollutant affected by both emissions from 

the roadway and nearby elevated pollutants (VOC, ozone, etc.), these factors 

should be considered in the site identification process. One critical element 

missing from the outline is the importance of the specific distance from the 

roadway chosen for study. The effect of differences in the gradient from the 

roadway of the multi pollutants studied needs to be acknowledged and discussed. 

 

• Modeling. This section should include a discussion of the types of models 

available and databases that can be used to inform the siting process. The models 

discussed should include both emissions and air quality models. 

• Monitoring. This section should provide a summary of methods used in past 

roadside measurement studies, passive and active monitors that can be efficiently 

deployed at many locations and methods to interpret the data acquired. The 

emphasis at this level should be on the identification of high “true” NO2 hourly 

average concentrations. Care needs to be taken to insure that the use of mobile 

monitoring methods give results that do reflect the diurnal and seasonal locations 

of peak concentrations. 

• Near-road Site Selection. The items outlined here seem reasonable. This section 

might draw from some of the existing guidance for sampler siting. 

x 
• Recommended Near-road Site Documentation. The adequacy of this section will 

depend on the EPA objectives for the Pilot Study in addressing the appropriate 

multi-pollutant monitoring objectives. This will be better defined as the 

objectives of the near-road monitoring program are better defined. 

2. EPA and NACAA envision the near-road guidance document to be written 

from a multi-pollutant perspective. What pollutants and sub-species does 

the subcommittee believe should be included for consideration and 

discussion in the near-road monitoring guidance? Some potential species for 

consideration include NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black 

carbon, air toxics (e.g., benzene, toluene, xylene, formaldehyde, acrolein, or 

1, 3, butadiene), and ammonia. Please prioritize the recommended 

pollutants and provide the rationale for their ranking, including how this 

pollutant measurement will contribute to scientific and regulatory 

knowledge of near-road air quality and adverse human health effects. 

The list and ranking of pollutants varied across the AAMMS members. Some questioned 

the usefulness of these indicators for the compliance purpose. The following ranking is 

based on the votes for the pollutants that had at least five votes from thirteen members of 

the AAMMS who submitted votes, taking into consideration the average ranking and its 

variability. The AAMMS believes meteorological parameters (wind speed and direction) 

should be one of the highest tier measurements considered as part of this network. 

Several members considered traffic counts as potentially important information. Other 

pollutants mentioned by some members include “true NO2” measured by techniques such 

as photolytic conversion difference, hourly metals by automated on-line XRF, and 

nephelometer measurement. 

CASAC recognizes that not all of the pollutants on the list can be measured at all the sites 

because of the level of the cost and operational logistics. Therefore, the AAMMS 

recommends a tiered approach, in which a few of the sites would collect a more 

comprehensive set of pollutants, while a majority of them would measure only the first 

tier of pollutants in the list. We recommend that all of the data be obtained on an hourly 

averaged basis to allow intercomparison. We recognize this ranking could change 

depending on EPA’s objectives for the monitoring network and the evolution of 

priorities. 

Pollutant group Rationale and comments 

NO2, NO, NOx • Same instrument 

Black carbon • Diesel tracer 

• Potential health relevance 

• Continuous methods available 

• Not a direct measure of carbon 

CO • Gasoline vehicle tracer 

• Dilution factors 

• Potential health relevance 

xi 
• Continuous methods available 

• Pending new NAAQS 

Ultra-fine particles / particle number 

concentration 

• Strong roadside gradient 

• Potential health relevance 

• Continuous methods available 

Particle-size distribution • For a limited number of sites 

• Potential health relevance 

• More comprehensive health relevance than 

just particle number 

PM10-2.5 • Potential health relevance 

• Re-suspended roadside particles 

• Speciation for metals 

• Important in future PM NAAQS reviews 

PM2.5 • Continuous methods available 

• Potential health relevance 

• Speciation for metals, major components and 

organic marker compounds 

• Possibly include semi-volatiles 

EC/OC • Potential health relevance 

• Direct measure of carbon 

CO2 • Normalization of pollutants to CO2 allows 

estimation of fuel-based emission factors 

• Could be important if signal large enough 

Ozone • To understand photo-chemical processes 

• Not to be used for health effects analysis 

NOy • Total nitrogen oxides and mass closure 

SO2 • To verify fuel content compliance 

• Pending new NAAQS 

BTEX (benzene, toluene, 

ethylbenzene, and xylenes) and 1,3 

butadiene 

• Potential health relevance 

• Continuous methods available 

• Provides unique information as to how 

changing fuel characteristics and control 

systems are impacting emissions 

• Can provide additional info on which class of 

vehicle is impacting monitor 

3. Identifying Candidate Near-road Site Areas 

a. AADT & Fleet Mix – To consider fleet mix with regard to NO2, an 

idea is to encourage states that have fleet mix information to take an 

approach that uses average, fleet-wide grams per mile emissions 

estimates (one for light duty vehicles and one for heavy duty vehicles), 

combined with AADT information to further weight which road 

segments in an area may be more conducive to produce peak 

pollutant concentrations. EPA would use the latest emission factor 

information to aid such a calculation. Given the variability in 

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emission rates from on-road vehicles based on vehicle technology, 

fuel, speed, environmental conditions, etc., does the subcommittee 

believe this approach is an appropriate way to “consider” fleet mix in 

near-road site selection or is a more refined inventory and modeling 

analysis required? 

b. AADT & Fleet Mix – Further, should the suggested approach above 

in question 4a to consider fleet mix via the use of average, fleet-wide 

emission factors, or the use of inventory and modeling analysis, take 

into account mobile source controls that are “on the books” but have 

not yet been fully realized due to fleet turnover? If so, how far out into 

the future should states consider their effects? 

c. Roadway Design – Studies suggest and support the concept that 

roadway design influences pollutant dispersion near the road. The 

EPA suggests establishing sites at-grade with the road, without any 

nearby obstructions to air flow; however, the Agency recognizes that 

this might not always be feasible. Does the subcommittee agree with 

this recommendation for locating sites at-grade with no obstructions? 

What priority should be placed on this factor within the guidance, 

given the need for flexibility in identifying appropriate site locations? 

d. Congestion Patterns – The congestion of a roadway can be estimated 

by the metric “Level of Service” (LOS). LOS uses a letter grade from 

A to F to identify a roadway’s performance, with “A” the best 

conditions where traffic flows at or above the posted speed limit and 

all motorists have complete mobility between lanes to “F” the worst 

congestion where travel time cannot be predicted and generally traffic 

demand exceeds the facility’s capacity. Since motor vehicles generally 

emit more pollutants during congestion operations (although noting 

that NOx and select other pollutant emissions can also increase with 

increasing speed), how important a parameter should LOS be in the 

determination of appropriate near-road monitoring sites? Does the 

subcommittee have a view on how reliable LOS estimates are across 

the country? 

e. Terrain– State and local air agencies are required to consider terrain 

in the near-road monitoring site selection process, which in some cases 

may be inherently part of the roadway design. However, EPA 

recognizes that some states and local air agencies may have to make 

selections from amongst similar candidate sites that differ only by 

terrain, e.g. cut section versus open terrain, with or without 

vegetation, etc. Does the subcommittee agree that terrain and 

vegetation should be a consideration in the siting process? What 

priority should this parameter have in the overall process? 

f. Meteorology – EPA took comment on, but did not finalize the 

requirement for near-road monitoring sites to be climatologically 

downwind of the target road segment. Reasons were because the 

additional limitations this would introduce in finding candidate sites 

would be in exchange for what may be a small increase in the 

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opportunity to monitor peak NO2 concentrations. Further, with sites 

being within 50 meters of target road segments, the phenomenon of 

upwind meandering (pollutant transport upwind due to vehicle 

induced turbulence) further reduces that absolute need to be 

climatologically downwind. Finally, EPA recognized that, logically, 

the potential for peak NO2 concentration may very well occur when 

winds are calm or parallel (or nearly parallel) to the target road, 

allowing for pollutant build-up, as opposed to when winds are normal 

to the road. Although there is no requirement to be downwind, in the 

preamble to final NO2 NAAQS rule, EPA encouraged it when 

possible. EPA and NACAA intend to do the same in the guidance 

document. Does the subcommittee agree with this approach? 

The specific factors identified in this charge question can all usefully inform the selection 

of near-road monitoring locations. An additional factor that may exhibit characteristic 

gradients in larger metropolitan areas is the middle-scale oxidant or odd-oxygen 

concentration, [O3]+[NO2]. A factor that needs emphasis is the role of freeways with 

high truck traffic volumes. As NOx and VOC emissions are continuously reduced over 

time, ambient oxidant levels will also go down, which means that titration of NO by 

ozone to form NO2 will become less important in comparison to direct NO2 emissions 

from new and retrofitted diesel trucks. NO2 emissions are highest during cruise mode, 

not congestion conditions for these trucks. CASAC recommends against giving too 

much emphasis to AADT, noting for example that peak NO2 concentrations can occur in 

areas where widespread congestion limits AADT. To account for such interactions 

between different determinants of NO2 , the Agency should consider developing a 

conceptual or screening model to guide their integration. This model would be intended 

as a tool to help rank candidate locations, and would not require quantitative 

concentration predictions. Recognizing the near-road NO2 measurement’s intended focus 

on NAAQS compliance rather than population exposure, CASAC stresses the importance 

of exposure in the overall balance of siting considerations. 

In response to specific elements of this charge question, CASAC’s recommendations are 

as follows. 

a. States and cities vary considerably in the resolution of information available 

for fleet characteristics and fleet mix. States can be encouraged to use all 

available data in their planning, including local features such as truck and bus 

corridors. 

b. Ambient monitoring is conducted in the ‘here and now’. Fleet turnover is too 

slow to require consideration of its future effects in the initial planning. 

However, the transition to ‘cleaner’ diesels may have the effect of increasing 

the ratio of NO2 to NOx in primary tailpipe emissions of NOx. 

c-e. CASAC agrees that roadway design, terrain, and congestion patterns all merit 

consideration. Information from loop monitors should also be considered. As 

noted above, it recommends that the interactions and tradeoffs among their 

effects be accounted for in a screening tool developed by the Agency for this 

purpose. 

xiv 
f. CASAC agrees that it is undesirable to site a near-road monitor 

climatologically upwind. 

4. Modeling is another tool that may be useful in the identification of candidate 

near-road sites. In particular, the use of mobile source emissions modeling 

with MOVES and local-scale dispersion modeling with AERMOD, can be 

presented as part of the guidance document. Please comment on the 

available modeling tools, and their pros and cons, that the subcommittee 

believes may be appropriate to discuss and/or recommend for use in the 

near-road monitoring guidance document. 

Like the various considerations discussed in Charge Question 3, modeling tools can also 

be used to inform the likely location of the maximum NO2 levels in an area. Taking a 

centralized (e.g., EPA led) approach, emissions and air quality models could be used to 

develop screening tools or screening criteria that incorporate many of the factors 

discussed in Charge Question 3. Alternatively, the models could directly be used by state 

and local agencies to inform the siting process. MOVES, along with other emissions 

inventory modeling tools, can provide valuable information about the spatial intensity of 

traffic-related NOx emissions, as well as NOx emissions from other sources. MOVES is a 

link-based model which has significant advantages for this application over the historical 

emissions models that were based on trip-average cycles. Air quality models, such as the 

CMAQ and CAMx chemical transport models and the AERMOD and CALINE 

dispersion models, can also be used to help identify candidate geographic areas within the 

metropolitan area and to compare and contrast candidate specific candidate monitoring 

sites, respectively. It is appreciated, however, that there significant uncertainties 

associated with the use of models for simulating the location of an extreme concentration 

statistic. The approaches used by AERMOD to simulate the conversion of NO to NO2

are relatively crude, and guidance should be given as to which approach should be used. 

State and local agencies should be encouraged to use spatially-resolved emissions 

estimates, and air quality model results if readily available or available with relatively 

modest effort. 

5. In regard to the process of identifying candidate near-road monitoring sites, 

beyond the evaluation of factors noted above in question 3, and the potential 

use of modeling, the use of saturation monitoring and on-road monitoring 

are also possible tools that state and local air agencies may choose to utilize 

in the near-road site selection process. 

“Multi-scale” monitoring is a more specific term than saturation monitoring. Before the 

pilot study or site selection, a literature review should be conducted that summarizes past 

roadside measurement and modeling studies, passive and active monitors that can be 

efficiently deployed at many locations, and methods to interpret the data acquired. 

Roadside monitoring for site selection and long-term monitoring would benefit from 

small, portable sensors that don’t require a large infrastructure (i.e., shelter, air 

conditioning, etc.) to operate. A pilot study plan should be assembled to clearly define the 

objectives of locating the sites (i.e., maximum hourly NO2 concentration), rationale for 

xv 
the selection of the measurements, measurement principles, concentration range, 

minimum detection limits, and potential interferences and biases. It should also elaborate 

on method intercomparison and modeling approaches. 

Sampling sites should be located at the prevailing downwind location in an array (e.g., to 

estimate perpendicular vs. effective distance from the road; Barzyk et al., 2009). The 

number of sampling sites would depend on the downwind distance in the micro- (10 – 

100 m), middle- (100 – 500 m) and neighborhood- (500 m – 4 km) scales. 

Portable active or passive monitors that can be easily and inexpensively deployed would 

provide a good indication of where concentrations might be highest. Levels are likely to 

vary by season and often show an exponential decrease with distance from the curbside. 

Passive monitors using NO2-absorbing filters have some potential biases, but they have 

also been shown to be comparable and correlated with continuous measurements for 

integration times on the order of weeks. Passive samplers are a cost-effective and 

practical technology for mapping average spatial gradients as a prelude to sampler siting. 

However, the validity and sensitivity of the portable active or passive NO2 sampling 

systems for hourly measurements needs to be verified. To get better precision, collocated 

continuous monitors with duplicate passive samplers are needed at some of the sites. Past 

studies show an average coefficient of variation for duplicate passive NO2 measurements 

of 5 – 30% 

a. If a state were inclined to use saturation monitoring to aid in the selection 

of a near-road monitoring site, and considering that the NO2 standard is 

a 1-hour daily maximum standard, what are the pros and cons to using 

passive devices to saturate an area to gather data? 

Pros are low expense and operating cost. The major con is the much longer than onehour averaging time, unless the passive device is modified to obtain active hourly data. 

The key is to avoid the need to relate long-duration passive sampler NO2 measurements 

to the one-hour NO2 NAAQS, which is defined as the maximum allowable concentration 

in an area (primarily near major roadways), expressed as a three-year average of the 98th

percentile of the annual distribution of daily maximum one-hour concentrations. One 

option is to collect samples from sub-daytime periods (e.g., morning and evening rush 

hours) that are integrated over several days. A timer-based sampler may be used for this 

application. If many of the passive samplers are used for multi-scale monitoring it is 

important to position the sampling inlet at the same height (2 - 7 m above ground) at all 

sites 

b. Likewise, what are the pros and cons to using non-passive devices, such 

as near real-time or continuous devices including, but not limited to 

portable, non-FEM chemiluminescence methods for NO2 or Gas Sensitive 

Semiconductors (GSSs) for NO2 and other pollutants of interest? 

Pros are short-duration samples, on the order of an hour or less. Cons are instrument 

procurement and operating expense, potentially higher than desired detection limits, and 

xvi 
reliability of new technologies. There are several currently available or emerging 

technologies for microsensors. In addition to NO2, NO can often be obtained from these 

same sensors. Miniature sampling systems have been used for remote monitoring, 

emission sampling, and unmanned aerial vehicles, and these might also be used for multiscale monitoring These portable instruments need to be evaluated with regard to their 

sensitivity, stability, and accuracy. 

c. Finally, what would be the pros and cons, to a state or local agency 

attempting to use a specially outfitted vehicle to collect mobile 

measurements to assist in the near-road site selection process for NO2

specifically as well as other pollutants of interest? 

Several mobile emissions systems or instrumented mobile vans have been applied to 

characterizing on-road and roadside concentrations. Pros are that these systems are 

moveable and obtain many different pollutant measurements. Unless state or local 

agencies are already equipped with a mobile sampling van, the disadvantages are the high 

cost of assembling or contracting these laboratories and the snapshot nature of their 

measurements. They often need to be attended and can be parked for only a short time 

period. However, mobile monitoring systems are useful for site selection if they can be 

parked at the same location and perform sampling for a few days to one week. The state 

or local agencies may also consider using a fully-equipped trailer rather than a mobile 

van for siting. Eventually, the trailer can be converted to a permanent monitoring 

location. 

6. EPA recognizes that CO concentrations are primarily influenced by gasoline 

vehicles as opposed to NO2 and PM2.5 concentrations, which are currently 

more heavily influenced by heavy-duty (diesel) vehicle emissions. If EPA 

were to propose a new set of minimum monitoring requirements for CO 

near roads, the near-road monitoring stations created under the 

implementation of the NO2 monitoring requirements may be an 

advantageous infrastructure for state and local air agencies to leverage. 

However, EPA believes there are two issues not specifically considered in the 

near-road NO2 monitoring language that might influence where near-road 

CO monitors may be most appropriately placed. The two issues are 1) the 

consideration of where light duty vehicles are operating under ‘cold-start’ 

conditions, which may often not be on the larger arterials or highways in an 

area, and 2) the impacts of light duty vehicle congestion and idling in areas 

such as urban street canyons and/or urban cores. 

a. Does the subcommittee believe that the light duty cold start and 

congestion factors will significantly influence the location of peak CO 

concentrations in an area? What priority should these factors be 

given when compared with the factors (AADT, Fleet Mix, Roadway 

Design, Congestion Patterns, Terrain, and Meteorology) already being 

considered for peak NO2? 

xvii 
The spatial distribution of cold start vehicles associated with urban commuting is in 

general broad and short term and their contribution to emissions associated with major 

highways adjacent to residential neighborhoods is likely small. To the extent cold start 

emissions contribute, they would mainly be in terms of CO and not NO2 concentrations 

and would not likely influence peak CO concentrations. That being said, at least one 

exception comes to mind. The departure of motor vehicles from major events (e.g. 

stadiums) where 20-30K vehicles may be simultaneously started and caught in 

congestion for 10s of minutes to an hour or more. The cold start contribution, here again, 

is limited in time but could contribute significantly as an emissions hot-spot impacting 

commuter exposes and concentrations in nearby neighborhoods. The prioritization of 

congestion and cold start factors relative to AADT, fleet mix, roadway design, terrain and 

meteorology should consider sensitivity analyses using line source models as outlined by 

the FHWA’s procedures for assessing traffic impacts for CO. 

b. Does the subcommittee have an opinion on whether, and possibly 

how, these two issues of vehicles operating under cold start conditions 

and light duty vehicle congestion and idling in urban street canyons 

and/or urban cores be considered in a future, nationally applicable, 

CO monitoring proposal? Are there other factors that may affect 

peak CO concentrations and not affect peak NO2 concentrations that 

should also be considered for any future CO monitoring proposal? 

Factors that affect near-road CO and NOx concentrations will be the distribution of 

gasoline and diesel vehicles. The primary NO2 fraction of NOx exhaust emissions will 

vary with engine type (gasoline spark ignition or diesel) and control equipment, e.g., 

three way catalyst and diesel particle filter trap technologies. CO emissions come 

principally from the gasoline engine exhaust and their peak concentrations will occur in 

the immediate vicinity of the emission source. While NO2 peak concentrations from 

primary emissions will behave similarly to CO, NO2 peak concentrations from secondary 

reactions will lag behind as NO emissions react with entrained ozone to form NO2. The 

entrainment and transformation time afford the opportunity for the exhaust plume to 

transport and diffuse resulting in peak NO2 concentrations that are displaced in time and 

space as compared to CO. 

Idling in urban street canyons is problematic and should be discouraged. If, as in most 

cases, it is due to major traffic congestion it will result in high exposures involving 

commuters, pedestrians and local residence. Decisions to monitor at such locations must 

consider these exposures relative to other near-road exposure environments associated 

with high density population regions. 

7. Does the committee believe that siting considerations for identifying the 

location of peak NO2 concentrations will likely address all of the high 

priority siting considerations for PM (particularly PM2.5) as well? If not, 

what other factors should be considered and what are the advantages in 

considering these factors for identifying the location of maximum PM 

concentration? 

xviii 
The majority of panelists acknowledged that there is an increment to PM2.5 in the near 

road environment. They did not suggest that the near road environment will represent the 

areas of highest PM2.5 concentration. This is due to the variety of sources of PM2.5

including the combination of primary emissions and secondary formation processes. 

The committee was also generally in agreement that mass based measurements and 

specifically the PM2.5 FRM is not appropriate for use at the near road NO2 sites. The 

PM2.5 FRM measurement has poor capture efficiency for the highly volatile emissions 

from mobile sources and is not suitable for the collection of hourly averaged data. The 

use of this method in the near road network would underestimate the significance of 

mobile sources and under predict the risk associated with this source of PM2.5. 

In general, the locations where maximum PM2.5 concentration are likely to be found 

include areas that are subject to regional transport and local stationary and area sources, 

are primarily urban, and are away from sinks of PM including ventilated roadways, heavy 

vegetation and water bodies. 

The Panel also discussed other fractions of PM in the near road environment including 

ultrafine particle count and size distribution and coarse PM (PMc). The siting for 

measurements of these size fractions are more suited to the near road environment. 

However, because the processes that affect downwind concentrations of NO2, UFP and 

PMc differ (Karner et al. 2010*), optimal downwind sites for those pollutants may also be 

different. 

8. In addition to PM2.5 mass, what other PM-related measurements are 

desirable at near-road monitoring stations (e.g., UFP number, black carbon, 

EC/OC, PM coarse, etc.)? 

Other PM measurements that should be considered include sub 100 nm ultrafine particles 

(UFP), BC, and speciated coarse particles. 

BC is an important vehicular emission that can be measured routinely in sampling 

networks. We recommend that it be measured. 

Measurements of UFP could be made either with a condensation particle counter (CPC) 

or an aerosol mobility spectrometer. A CPC measures the total concentration larger than 

the CPC’s minimum detectable size but provides no information about size. Mobility 

spectrometers provide information about particle concentration and size, which would be 

valuable for understating new particle formation and assessing health effects. Given 

current resource constraints, state and local agencies might be unable to collect and 

analyze data from mobility spectrometers. However, agencies might work collaboratively 

with interested universities. This would likely lead to a nested network design, with most 

 

*

 Karner, A. A., Eisinger, D. S. and Niemeier, D. A. (2010). Near-Roadway Air Quality: 

Synthesizing the Findings from Real-World Data. Environ. Sci. Technol. 44:5334–5344. 

xix 
stations using CPCs and a more limited number aerosol mobility spectrometers. Mobility 

distributions are currently being measured routinely in sampling networks worldwide. 

Vehicles emit coarse particles and resuspend coarse road dust that may be coated with 

toxic contaminants. We recommend measurements of the coarse particle metal content at 

the well instrumented sites. 

Questions regarding the monitor siting criteria for microscale CO, microscale PM2.5, 

and the new near-road NO2 siting criteria 

9. To allow for near-road monitoring infrastructure to be multi-pollutant, and 

in reflection of the recently promulgated near-road NO2 siting criteria, 

reconsideration of the existing microscale CO siting criteria presented in 

sections 2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E may be 

warranted. Does the subcommittee believe that reconsideration of 

microscale CO siting criteria is appropriate? Specifically, would an 

adjustment of CO siting criteria to match those of microscale PM2.5 and 

microscale near-road NO2 sites be logical and appropriate? 

CASAC recommends that sampling criteria for CO and other monitors at sites installed to 

monitor near-road NO2 match those for NO2. The sampling configurations of existing 

micro-scale CO monitors should be assessed in terms of their own sampling objectives, 

and need not necessarily conform to those of near-road NO2 monitors. 

10. Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and 

table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5

and microscale near-road NO2 is appropriate and proposed, should there be 

consideration to maintain the requirement on how urban street canyon or 

urban core microscale CO sites should be sited? 

The Panel did not feel it had enough data to make a recommendation with respect to this 

charge question. 

11. Does the subcommittee have an opinion on how “urban street canyons” or 

“urban core” might be defined, perhaps quantitatively, and with regard to 

use in potential rule language? 

To define “urban street canyons” in relevant regulations, CASAC offers the following 

factors for EPA’s consideration. EPA might also consult experts in other relevant 

disciplines to further refine this definition. 

• Traffic information such as AADT, fleet mix, posted speed limit or/and actual 

speed, traffic light cycle 

• Street geometry 

xx 
o Ratio of the height of street side buildings to the width of the street (H/W 

ratio). Some approaches need to be developed to treat the following 

scenarios for H/W ratio calculation 

 different heights of buildings on the two sides of the street – use 

the lower one or use the average. Using the lower one may be 

better. 

 tiered buildings – possibly use the average of all tiers, or use a 

graduated factor based on the angle between the vertical line and 

the line drawn along the tiered building – the larger the angle 

(more opening on the top of the street canyon), the lower the 

effective height (He) will be used in the H/W calculation. 

o One-way vs. two-way street (more plug flow in a one-way street and more 

turbulent flow in a two-way street). 

o Is the street lined with trees on the sidewalk? Tree canopy may have an 

effect of an umbrella and trap portion of pollutants at the street level. 

o Slope of the street – higher vehicle emission on steeper streets unless it is 

a one-way street and the traffic direction is downhill. 

o Some way to normalize the H/W ratio with respect to number of traffic 

lanes on the street. One approach would be modifying the H/W ratio to 

H*L/W, where L = number of lanes. 

• Meteorological factors: Frequency of calm conditions and/or drainage flow may 

influence concentrations. 

• Terrain: The angle between the street and prevailing wind direction (higher 

concentrations are expected if the angle is 90 degree). 

Although a set of cut-off values reflecting the above mentioned factors could be used to 

define urban street canyons, it is not advisable to set some clear-cut criteria (insufficient 

information exists to set clear-cut thresholds). It may be more appropriate to consider a 

street an urban street canyon if more than a certain number of these conditions are met. 

This will be a qualitative approach to define urban street canyons. 

For the definition of “urban core”, the CASAC has some suggestions for EPA to 

consider. EPA may use the U.S. Census Bureau definition of an “urban area” as a starting 

point. Per U.S. Census Bureau, an urban area is defined as "Core census block groups or 

blocks that have a population density of at least 1,000 people per square mile and 

surrounding census blocks that have an overall density of at least 500 people per square 

mile." Land features such as a river or a ridge may divide a CBSA into multiple urban 

cores. EPA may use population density to rank urban cores. EPA may further enhance 

this definition by factoring in “traffic density”, which could be calculated as the sum of 

AADT for every unit of road length (e.g., a length comparable to street block) in an area 

under consideration divided by the size of the area. 

Questions regarding the near-road monitoring pilot study 

12. EPA and NACAA will select the locations for permanent sites that are part 

of the near-road pilot study based on which state or locals volunteer to 

xxi 
participate and can process grant funds in a timely manner to deploy 

equipment. From this pool of volunteers, selection should be made on 

certain attributes that provide the best potential to fulfill pilot study 

objectives. In the attached draft white paper, EPA and NACAA have 

proposed some potential criteria for consideration in selecting where the 

fixed, permanent stations should be located. These considerations include 

choosing a large and a relatively small urban area based on population, an 

area with varied or complex terrain, an urban area with an operational NOX

analyzer representative of neighborhood or larger spatial scales for 

comparison to the near-road NOX analyzer, and an urban area with a 

cooperative (or non-cooperative) Department of Transportation. Does the 

Subcommittee agree with these considerations? Further, are there other 

considerations that should be evaluated in selecting pilot cities to house 

permanent near-road monitoring stations as part of the pilot study? 

Available funding constrains this near-road pilot to only 2 or 3 sites unless EPA leverages 

existing sites and infrastructure that are near-road-ish, possibly considering cities that are 

already conducting multi-pollutant assessment at multiple locations (Atlanta and New 

York City for example). Areas with existing "urban background" sites that have relevant 

near-road pollutant measurements in place (that may include ozone for a measure of total 

urban oxidants) are desirable to assess the near-road excess for key pollutants. The range 

of variables in this charge question cannot be fully evaluated with only a few sites. In 

reality, given the severe constraints of funding and timing for the pilot, the siting decision 

may be driven largely by which S/L agencies have the resources to support the pilot work 

and where they can find and deploy a reasonable site quickly. With only 2-3 sites, it may 

be appropriate to choose "generic" sites (avoiding extremes of topography, etc.) that are 

most likely to represent a large fraction of the final network and are near the middle of 

the 0-50 meter distance from the road (e.g., ~ 20-30 meters) and have HDD as a 

significant fraction of traffic. Reliance on AADT or an MSA's population are useful 

inputs but often may not be good indicators of the location of the maximum 1-hour NO2. 

The pilot fixed sites are not likely to inform how a wide range of siting characteristics 

would affect 1-hour NO2 concentrations. We might come closer to that goal by focusing 

the pilot on NO2 saturation studies with less emphasis (e.g. funding) on the fixed sites, 

but that is not a practical solution to the other goals of this pilot. There is some evidence 

suggesting that core urban zones not at large roadways may have the highest 1-hour NO2 

values for some urban locations; saturation studies are ideal for assessing these sites. 

Cooperation of the local DOT may be useful for local traffic pattern characterizations. It 

may be worth encouraging academic or private sector groups to add in-kind supplemental 

measurements if that does not create multi-organization logistical issues. Finally, EPA, 

California Air Resources Board and several research groups have mobile monitors that 

could be deployed to quickly assess potential locations of highest 1-hour NO2

concentrations. 

13. EPA and NACAA have proposed that at least two urban areas should have 

permanent near-road monitoring stations (that would fulfill NO2 near-road 

xxii 
monitoring requirements) implemented for the pilot study. Please comment 

on the minimum equipment/pollutant measurement complement that should 

be deployed at each site and also the ideal equipment complement that each 

site should or could have, respectively. Specifically, what pollutants (e.g., 

NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics 

(such as benzene, toluene, xylene, formaldehyde, acrolein, or 1,3, butadiene) 

and ammonia) and other information should the pilot study measure or 

gather at the fixed, permanent monitoring stations, and by what methods? 

This list should be in priority order, as feasible, and can include any NAAQS 

or non-NAAQS pollutant by any method (FRM/FEM and/or non-reference 

or equivalent methods), any particular type of other equipment for 

gathering supporting data such as meteorology or traffic counts. 

The majority of AAMMS supports the deployment of at least two pilot study sites with a 

minimum measurement suite of NO2/NO/NOx , black carbon, particle number 

concentrations (UFP), CO, meteorological parameters, and traffic counts; and ideally the 

pollutant measurements tabulated in the response to Charge Question #2. Measurements 

for vehicle class and speed distributions are also encouraged, especially if a screening 

tool will be developed because this information can be used in its evaluation. If an FEMapproved photolytic NO2 monitor is available, its deployment along with an FRM NO2

monitor at each site is strongly encouraged. Other AAMMS members support the 

deployment of a single site with more extensive measurements to provide insights into 

key science questions about NO2. This would require a longer time commitment than 

currently programmed for the pilot study, but would ultimately better inform the 

regulatory process. Additional pilot study objectives could include the evaluation of 

passive sampling methods to be used in the saturation studies. 

14. EPA and NACAA have proposed four to five urban areas to have saturation 

monitoring, using either passive devices and/or continuous/semi-continuous 

saturation type multi-pollutant monitoring packages (i.e., several types of 

monitors in one mountable or deployable “package”). Please provide 

comment on: 

a. The pollutants that should be measured with the saturation devices at 

each saturation site. 

b. The number of saturation devices per pollutant, both passive and/or 

continuous/semi-continuous, that may be deployed in each pilot city. 

c. Whether placing saturation monitoring devices near certain road 

segments should include, at a minimum: 1) the highest AADT segment 

in an area, 2) the road segment with the highest number of heavy-duty 

truck/bus counts, 3) at a road segment with more unique roadway 

design, congestion pattern, or terrain in the area, and 4) if feasible, at 

a lower AADT segment with a similar fleet mix, roadway design, 

xxiii 
congestion, terrain, and meteorology as the top AADT road segment 

in the area. 

The Subcommittee members expressed a wide range of views on the feasibility, ideal 

configuration(s) and potential usefulness of including a saturation monitoring component 

in the proposed pilot study. The pollutants sampled, number of sites per city, the number 

of cities and the kinds of sites for sampling need to be considered collectively, and in 

light of the limited budget and very short time frame available if the results are going to 

be of any use to the states in the establishment of permanent near-road sites. The kinds of 

sampling locations recommended by EPA and NACAA are reasonable, but may be 

overly prescriptive, considering uncertainties in the kinds of sampling approaches that 

could actually be implemented. 

The simplest possible approach might be limited exclusively to the use of passive 

samplers and focused on NO2-only using Ogawa-type passive devices. Disadvantages of 

this approach include the single pollutant focus and longer-term cumulative nature of the 

resultant data. Assuming there are reasonable correlations between peak hourly 

concentrations and long term averages (as there have been at near-road sites in the UK), 

the sample aggregation of passive samplers may not be a major problem. Advantages of 

this simple approach include the very minimal siting constraints, the low (sampler, labor 

and analytical) cost per sample, which would allow deployment at a much larger array of 

locations, and the current availability of units with well characterized performance 

specifications. To support this simplified approach, it would be helpful if EPA would 

analyze data from existing NO2 sites that are located reasonably near major roads to 

determine how well weekly mean NO2 from multiple sites predicts the daily 1-hour 

maximum NO2 concentrations for the same time period. 

The more complex approach suggested by EPA would attempt to develop a portable 

compact “package” of active, continuous samplers for multiple pollutants of interest, 

including NO2, CO, PM2.5. Advantages include the ability to characterize and compare 

short-term hourly peak concentrations for different pollutants at different kinds of sites. 

Disadvantages include anticipated high cost per unit (fewer cities and sites), more 

constraining siting requirements (power, security), unproven track records for data 

quality, and anticipated time delays for equipment procurement, testing and field 

deployment. 

A third "intermediate" approach that CASAC recommends considering would be to 

combine passive samplers with timed, battery operated pumps which would draw a fixed 

flow rate of air through a small chamber housing containing passive NO2 samplers (and 

possibly other passive samplers for NOx, BC, O3) during specific time periods such as 

the morning rush hour(s) and afternoon hours of expected maximum secondary 

formation. The sample pump would improve the sensitivity and reduce the sample 

variability compared to purely passive devices, and would retain the advantages of 

relatively low cost and flexible siting locations of the passive samplers. However, 

xxiv 
xxv 

this approach is much more complex than a simple passive sampler, and thus would 

require substantial resources to properly develop the method and evaluate its performance 

before it could be used. 

Without a significant amount of EPA technical support in method development and 

evaluation, the two more highly time-resolved saturation pilot study approaches 

described here are not practical; state and local air agencies do not have the resources to 

support this level of effort. We recommend that EPA provide appropriate resources to 

allow deployment of time-resolved saturation samplers such that these spatial studies can 

better inform the final network design. 

One possible alternative to use of saturation samplers at fixed locations would be to 

employ mobile sampling platforms to explore spatial and temporal patterns for multiple 

pollutants. While the high costs of such units precludes their development with available 

pilot study funds, it is possible that some states may already have such mobile units 

available, and the committee recommends that their use be considered as an alternative to 

fixed site sampling in locations where such units may be available.
Enclosure C 

Individual Comments from 

Ambient Air Monitoring and Methods Subcommittee 

Mr. George Allen................................................................................................................ 2

Dr. Judith Chow................................................................................................................ 10

Mr. Bart Croes .................................................................................................................. 29

Dr. Kenneth Demerjian..................................................................................................... 38

Dr. Delbert Eatough.......................................................................................................... 45

Mr. Dirk Felton ................................................................................................................. 53

Dr. Kazuhiko Ito ............................................................................................................... 61

Mr. Rich Poirot ................................................................................................................. 68

Dr. Jay Turner ................................................................................................................... 80

Dr. Warren H. White......................................................................................................... 84

Dr. Yousheng Zeng........................................................................................................... 90

1 
Mr. George Allen 

There are two major topics covered by this Advisory: 

1. How to best determine the likely location of the annual maximum value for urban-area 

1-hour NO2 concentration to assist in the siting of the both the pilot and 126 site network 

monitors. 

2. In addition to NO2, what other near-road (near-road) relevant pollutants should be 

measured for both the pilot fixed-sites and the full network. 

EPA should be acknowledged for moving forward with a multi-pollutant near-road 

network, an important initial step towards better understanding the exposure issues 

behind the observed near-road health effects. Although the initial focus of this network is 

NO2, measurements of other non-NAAQS pollutants are critical for characterization of 

near-road zones of influence. 

The Charge Questions are broken down into several categories, some of which are not 

well defined or have substantial content overlap: 

Guidance document development for the 126-site network 

CO and PM network issues related to near-road monitoring 

Harmonization of siting criteria for near-road multi-pollutant monitoring (including probe 

height) 

near-road pilot studies -- saturation and fixed site designs: what to sample and where 

EPA assumes that these NO2 sites will all be near major roads, but existing data suggest 

that may not meet the network goals. Thus, the term near-road (NR) as used here also 

includes congested urban core areas that may be locations of maximum 1-hour NO2 

concentrations. 

The NR pilot project is critical to informing the deployment of 126 NR NO2 monitoring 

sites by the end of 2012 (just over 2 years from now with siting plans due summer 2012). 

However, there is insufficient time to get and analyze all the data (saturation and fixed 

sites) from a pilot network; pilot studies must be done during both winter and summer 

seasons to account for potentially large seasonal NO2 variability; primary sources 

dominate in winter, with secondary sources a factor only during summer mid-day and 

afternoon when ozone may be present (not during morning rush-hour). EPA meeting 

materials (introduction to Charge Question 12) state that the saturation study should be 

performed before (to inform) deploying the fixed pilot sites, further extending the time 

needed to complete a proper pilot project. 

I am very concerned about the level of available funding for this pilot, which includes 

mulitple multi-pollutant sites, several saturation studies, urban background monitoring 

for pollutants of interest, and assumes substantial in-kind support from S/L agencies. A 

major challenge will be how to get useful information from this pilot effort with limited 

available resources ($800k) and a very short time-frame to adequately inform the larger 

NR network deployment. EPA needs to address who will do the data analysis for this 

2 
pilot (presumably a contractor), develop a plan for the analysis, and get external input on 

that plan. Because of these time constraints, if at all possible I recommend that the full 

network implementation be delayed or staggered or (ideally) both. If staggered, the first 

round of sites (10-15 in the largest urban areas?) could include additional (more 

intensive) measurements to better inform later phases. 

As EPA notes in their background material, there are many factors to consider for NR 

monitor siting; all of them can not adequately be addressed without a relatively largescale pilot program. Thus, it may be useful to leverage existing sites that meet some of 

the NR siting requirements in the NO2 rule, and enhancing those existing sites, rather 

than deploying new sites. This would allow for more pilot sites and speed completion of 

the pilot study, but limit the “ideal” pilot siting design. I’d expect the saturation studies 

to be of more value re: informing the siting process, and the fixed sites to support the 

saturation studies and vet new methods. To support consideration of this approach, it 

would be very helpful if EPA could supply a list of existing NO2 sites that could at least 

loosely be considered NR (e.g., micro-scale siting), along with additional site meta-data 

such as AADT, vehicle type mix, other pollutants currently measured, and available 

matching urban background measurements. 

Finally, I strongly encourage EPA to evaluate the “NR” excess for key indicators (at a 

minimum: NO2, UFP and BC) as part of this pilot study. This requires a “matched set” 

of indicator measurements at the NR site and an appropriate “urban background” site - 

perhaps an NCore site if not too distant from the NR site. The “NR excess” metric allows 

an estimation of the gradient away from the road. Without the background 

measurements, the NR indicator data (BC, UFP) have no useful context. 

Charge Question 1. Content/Topics of Guidance Outline for full network siting and 

implementation. 

Since this is a brief outline, there is not a lot to comment on. 

3b: AADT is over-emphasized as an initial step in site selection. It should be listed with 

the considerations in 3c. See other comments on AADT in response to charge question 3. 

4. If modeling is going to be done by S/L agencies (that is my understanding), not EPA, 

it may have limited application. There is a wide range of expertise and resources across 

S/L air agencies for this kind of work. 

5. Saturation monitoring may be the most effective approach for site identification, 

although the need for simplicity and low cost limits the use to multi-day samples rather 

than one to a few hours duration. 

6. 6d seems to duplicate 3c. 

3 
Charge Question 2. “What pollutants and sub-species does the subcommittee believe 

should be included for consideration and discussion in the near-road monitoring 

guidance? Some potential species for consideration include NO2, NOX, NO, CO, PM 

(Ultrafine, 2.5, and 10), black carbon, air toxics (e.g., benzene, toluene, xylene, 

formaldehyde, acrolein, or 1, 3, butadiene), and ammonia. Please prioritize the 

recommended pollutants and provide the rationale for their ranking, including how this 

pollutant measurement will contribute to scientific and regulatory knowledge of nearroad air quality and adverse human health effects.” 

For the full network, NO/NO2,UFP (CPC), BC, and CO, and meteorological 

measurements are the most important. NO2 and CO are NR-relevant NAAQS; no further 

discussion needed. UFP (particle # concentration by CPC) and BC are robust indicators 

of NR pollution influence and critical (along with NOx) to determine the “NR excess” 

pollution influence. PM2.5 should also be done, although it is generally not a good 

“indicator” of NR pollutants since the urban background is already elevated. The air 

toxics listed here are very useful but are a second-tier group. Ammonia is not of much 

interest despite the SCRT NOx controls now used for HDD. The list can be broken into 

tiers as follows: 

Tier 1 - must do:

For the 126 network site sites, NO/NO2, UFP (CPC), BC, CO, and meteorological 

measurements are the most important. NO2 and CO are NR-relevant NAAQS; no further 

discussion needed. UFP (particle # concentration by CPC) and BC are robust indicators 

of NR pollution influence and critical (along with NOx) to determine the "NR excess" 

pollution influence; # conc [UFP] has not been shown by itself to be of use for health 

effects yet. BC does show up as a significant predictor of a wide range of health 

outcomes, although it may [or may not] be acting as an indicator for other NR pollutants. 

Met (primarily wind) is critical to understanding temporal variations in NR pollutants. I 

would not recommend 3-d sonic for all sites [reliability issues at NR sites], despite the 

additional value those data have. 

Guidance for NO2 must include data acquisition of large negative values and address 

issues related to NO-NOx channel balance (matched gain) to avoid degradation of hourly 

NO2 data when NO is high and rapidly varying. Guidance for CPC/UFP must address 

diffusional losses of particles < ~25 nm in the sample inlet train. 

Tier 2 - could do:

4 
PM2.5 should also be done despite issues with the FRM/FEM methods; although it is 

generally not a good "indicator" of NR pollutants since the urban background is already 

elevated, it is a NAAQS that may often be highest at NR sites. The air toxics listed here 

are very useful for health impact, but are probably too expensive; acrolein and HCHO 

can not be done at hourly time-scales using practical methods. PM-coarse, and metals are 

doable at 1-h time-scales, but are resource intensive with health implications unclear. 

Paired indicator measurements at urban background site (BC, UFP) would be very useful 

for assessment of NR excess and estimation of the shape of the gradient away from the 

road, but maybe not everywhere. 

Tier 3- could do at a small subset of sites [~10-15] to serve research needs:

Hourly EC/OC, BTEX and other air toxics, SMPS or similar size-resolved UFP; hourly 

metals [automated on-line XRF]; true [photolytic] NO2, O3 [maybe NO 

chemiluminescent method to avoid measurement artifacts at low concentrations], CO2 

[up and downwind], traffic counts or remote sensing of traffic. Paired indicator 

measurements at urban background site (BC, UFP). Dry nephelometers with pm-1.0 inlet 

cuts for fast-response PM-fine indicator. 3-D met at 3 and 10 meters. NOy paired with 

“true” photolytic NO2 may be useful for determination of total reactive nitrogen and 

NOz. 

Don't do:

Ammonia is not of much interest despite the SCRT NOx controls now used for HDD, and 

not practical at 1-hour resolution. Nitrate (no reason to expect it to be a NR issue, and 

little to no value for health effects); if NOy is done, ammonium nitrate is included in the 

NOz species. 

There was some discussion of commercial availability of a suitably robust UFP 

instrument (CPC) that could be successfully used in a large routine monitoring network. 

As of May 2010, a commercial water-based CPC is now available that appears to meet 

these needs; a brief evaluation of the shipping version (not pre-production) of this 

instrument is at: 

http://home.comcast.net/~g_allen/TSI-3783-CPC_Allen.pdf

Charge Question 3. Guidance for Identifying Candidate NR Sites for the full network 

AADT and fleet mix are two criteria of uncertain value. For NO2, HDD is the driving 

on-road primary source; highest NO2 might be found where there is a lot of HDD, 

significant congestion, and poor dispersion. If local ozone titration (summer only, 

secondary NO2) is a substantial driver of elevated NR NO2, the fleet mix becomes less 

important. AADT without related congestion data can be misleading; a free-flowing 

highway with high AADT is not likely to contribute to high NO2 levels. 

Terrain is a major factor; see work by Wang and Zhang, “Modeling Near-Road Air 

Quality Using a Computational Fluid Dynamics Model, CFD-VIT-RIT”, EST, 2009: 43 

(7778–7783). Highest NR impact might be expected where the roadway is somewhat 

depressed relative to curbside terrain. Sound barriers and trees/vegetation (Baldauf-EPA 

5 
work) may reduce NR impact for some parameters. 

Meteorology would ideally be measured at 10 meters to avoid the worst of road-induced 

turbulence (although at the time-scale of 1-hour, larger scale wind patterns would still 

likely dominate the wind data). It is not practical to require 3-d wind at all sites, although 

this could be useful. Sigma-Theta wind data may be useful as an indication of 

turbulence, as well as the different between 1-hour average scalar and resultant wind 

speed. Wind data should be sampled at 1-second intervals for these turbulence related 

metrics, and 5-minute averages may be very useful in a detailed assessment of periods of 

high concentration. 

For Up/Down wind siting, I recommend avoiding sites that are upwind relative to 

prevailing wind direction. The highest impact may occur where prevailing winds are 

parallel to the roadway, at which point there is no real up or down-wind side of the road. 

Charge Question 4. Use of modeling for NR Guidance Document 

This is not my area of expertise, although the concerns stated in Q1 part 4 remain, as well 

as the demonstrated ability of the models under consideration to reasonably predict 

locations of 1-hour maximum NO2 concentrations. Mobile source oriented models do 

not take micro-scale ozone titration into effect, and thus are useful only where primary 

NO2 would be expected to be the driver of 1-hour maximum NO2. 

Charge Question 5. Saturation and On-road Monitoring (Guidance Document) 

Saturation monitoring is a very useful tool to screen potential sites and learn more about 

the characteristics of sites with likely maximum NR impact. Ideally, a saturation study 

does not have to be constrained to NO2; with modest firmware modifications to existing 

personal monitors, BC can now be monitored for in these studies. While the BC (optical 

method) is highly time-resolved, the most practical method for NO2 remains the TEAbased passive samplers; these have been well characterized. They could be adapted for 

use in this work to measure only morning rush hour over a 5-weekday period by making 

the flow “active”: e.g., a timer, a suitable sampler “housing”, and a small (~ 200 ccm) 

pump. Some initial effort in modification and characterization of existing passive NO2 

samplers would be needed. The passive samplers should be run at a fixed monitoring NR 

pilot site (with NO2 and BC) to validate field performance. The variability inherent in 

passive NO2 methods will be substantially reduced, since wind speed effects on 

“effective sampling rate” are essentially eliminated. These passive samplers typically 

need 50 ppb-hour of NO2 to provide stable data. With active sampling, this number 

drops, perhaps by 2 times. For a 15-hour (3h x 5days) sample period, once might expect 

useful data down to a few ppb. Ozone could also be added to any passive sampler-based 

approach. With NO and NO2, perhaps assisted by addition of BC and O3, the influence 

of primary vs. secondary NO2 can be assessed with morning rush-hour samples and 

afternoon (~ 2-5pm local time) samples by looking at the ratios of these pollutants. This 

could answer one of the more complex questions for NO2 -- do primary or secondary 

process drive the maximum 1-h concentrations for a given site/season? For winter and 

morning rush-hour periods (minimal ozone), it is reasonable to assume that primary 

6 
sources will dominate. For summer afternoons, that assumption can not be made. Taken 

together, this suggests that secondary processes are a potential driver of high NR NO2 

only for summer mid-day and afternoon periods. 

Finally, in addition to official NWS data, reasonably local wind data must be collected 

for any saturation study. These data may be available from NOAA-MADIS meso-net 

sites; these sites have automated QC (done by MADIS) that can be used to assess data 

quality, and highly time-resolved (sub-hourly) historical data are available on the web. 

Other than for BC personal samplers using optical filter techniques, I do not recommend 

“on-line” methods for the saturation study. Cost, complicated siting logistics, and data 

quality are the primary concerns here. 

Having suggested this saturation study approach, the reality is that without significant 

EPA support, the best a S/L agency is likely to be able to do is a simple passive NO2 

study, perhaps limited to weekday sampling rather than a full week. The Ogawa sampler 

can do NO2 and NOx in a single sampler, providing an indirect measurement of NO, 

which would be very useful. Since these samples would not be constrained to periods of 

likely NO2 maxima, existing NR-ish hourly data should be analyzed to determine how 

well a 5 or 7-day NO2 mean is correlated with the 1-h maximum for that period. This is 

an analysis EPA could easily do. 

I do not recommend on-road monitoring for this pilot project, in part because it is very 

resource intensive to do in a useful manner, and with the limited resources available a 

saturation study has more value with regard to the relevant siting questions especially 

when seasonality is considered. An exception could be if a local organization has a 

suitable mobile monitoring platform and the resources to deploy it. A recent example of 

mobile van spatial characterization of NR pollution is “Short-term variation in nearhighway air pollutant gradients on a winter morning” (Durant et al., ACP, 2010; 

http://www.atmos-chem-phys.net /10/8341/2010/

Charge Question 6. CO -- fleet mix, cold starts and urban canyons 

On-road sources of CO are different than NO2; there is essentially no CO from HDD. 

Thus, the areas of highest CO impact may be very different. Cold starts, idling, and fleet 

mix in urban canyons (and existing data) all suggest that a site for highest NO2 impact in 

an urban area may not be the location of highest CO impact. Finally, urban canyon siting 

is generally going to be very close to the curb (horizontally at least) -- typically < 5-10 

meters. But this pilot can not afford to address the urban canyon CO issue. 

Charge Question 7. NR PM2.5 and PM10 

Existing data has shown only a modest increment in NR PM2.5 in urban areas, primarily 

since the urban background is already elevated. PM2.5 is not generally a useful indicator 

of NR pollution excess gradients. Still, in urban areas without dominant industrial 

sources, the highest PM2.5 would normally be found near areas with substantial local 

traffic. PM10 would be expected to be somewhat higher at NR sites because of dust 

7 
reintrainment; minimal hourly NR PM-coarse data exist, so it is difficult to assess this 

parameter at this time scale. 

Charge Question 8. Other PM-related measurements desirable at near-road monitoring 

stations 

See Q 2 above. It must be noted that for UFP (CPC particle # concentration), the inlet 

can not be at the height of NO2 and similar pollutants unless a carefully designed aerosol 

manifold is used. Diffusional losses of particles < ~20-30 nm can be large unless 

appropriate sample inlet trains are used, and it is common to see # concentration mode 

peaks in this size range at NR sites. 

Charge Question 9. Would an adjustment of CO siting criteria to match those of 

microscale PM2.5 and microscale near-road NO2 sites be logical and appropriate? 

Yes. 

Charge Question 10. Should there be consideration to maintain the requirement on how 

urban street canyon or urban core microscale CO sites should be sited? 

Yes. 

Charge Question 11. ...how “urban street canyons” or “urban core” might be defined, 

perhaps quantitatively, and with regard to use in potential rule language? 

These terms may be difficult to define for rule language across all urban areas. Right 

now, the approach seems to be “you know it when you see it.” 

Charge Question 12. ...potential criteria for consideration in selecting where the fixed, 

permanent [NR pilot] stations should be located. 

There will not be more than 2 or 3 of these sites in this NR pilot, as noted in the charge 

questions. Thus, the range of variables in this charge question can not be fully evaluated. 

In reality, giving the constraints of funding and timing noted above, the siting decision 

may be driven largely by what S/L agencies have the resources to support the pilot work 

and where they can find and deploy a reasonable site quickly. With only 2-3 sites, it may 

be best to choose “generic” sites that are most likely to represent a large fraction of the 

final network. We are not going to learn how a wide range of siting characteristics would 

effect results from this pilot. We might come closer to that goal by constraining the pilot 

to saturation studies, with no fixed sites. But that is not a practical solution to the broader 

goals of this pilot. Cooperation of local the DOT may be useful, but the kind of data that 

needs to be collected for this pilot might be real-time remote traffic sensing that can 

estimate % large truck traffic -- data not usually available from a DOT. 

Charge Question 13. ...minimum equipment/pollutant measurement complement that 

should be deployed at each site and also the ideal equipment complement that each site 

8 
should or could have... 

See Q 2 above, and my background comments about the importance of having matched 

urban-scale monitors for evaluation of the “NR excess” for key indicator measurements. 

It may also be useful to have real-time remote sensing of traffic count as used in the Las 

Vegas NR MSAT study -- this can also give information on large truck vs. other vehicle 

traffic. 

Charge Question 14.

EPA is proposing that saturation studies for NO2 would be conducted in 4 or 5 urban 

areas. This is a reasonable goal. 

A) “The pollutants that should be measured with the saturation devices at each saturation 

site.” 

NO2 and maybe BC if resources allow. See Q 5 above. Met should be collected at one 

site in the area, possibly relying on existing MADIS meso-net data. 

B) “The number of saturation devices per pollutant, both passive and/or continuous / 

semi-continuous, that may be deployed in each pilot city.” 

I assume this Q is “how many sites?”. This is budget driven, but I suggest at least 6, with 

one of those being at a fixed site with robust NO2 and BC measurements (as a validation 

site). For those cities with a NR pilot fixed site, the collocation would be done there. For 

other cities, the most NR-ish site would be used for collocation of saturation study 

monitors. 

C) “Whether placing saturation monitoring devices near certain road segments should 

include, at a minimum: 1) the highest AADT segment in an area, 2) the road segment 

with the highest number of heavy-duty truck/bus counts, 3) at a road segment with more 

unique roadway design, congestion pattern, or terrain in the area, and 4) if feasible, at a 

lower AADT segment with a similar fleet mix, roadway design, congestion, terrain, and 

meteorology as the top AADT road segment in the area.” 

All except #4 above, as well as one or more urban canyon or similar non-highway site 

and a collocation site with regulatory monitors for QC use. 

9 
Dr. Judith Chow 

Subject: Preliminary Response to Charge Questions on NO2 Near Road Monitoring to 

Support Measurement of Multiple NAAQS Pollutants 

Date: September 20, 2010 

1. Comment on the overall content of the recommended topics in the draft outline. 

Provide suggestions on any missing subjects that should be included in the 

guidance document and any unnecessary topics that are currently listed in the 

attached draft, if applicable. 

The Section 2 literature review needs to be more comprehensive than indicated. It should 

include a discussion of the relationships between the different pollutants at emission and 

the likely changes that they will experience with downwind transport from the roadway. 

It should emphasize the multipollutant (Chow et al., 2010a; Greenbaum and Shaikh, 

2010; Hidy and Pennell, 2010; Mauderly et al., 2010) nature of near road exposure. A 

few reviews and meta-analyses of near-road concentrations have been published 

(Smichowski et al., 2008; Seigneur, 2009; Karner et al., 2010; Zhou and Levy, 2007) that 

can be used as starting points, although these are not specific to NO2 concentrations. A 

conceptual model should be formed in this section that includes dispersion, deposition, 

chemical conversion and physical conversion. Special attention should be given to NO2

formation by NO titration of O3 and NO2 depletion by photochemistry. 

Section 3 should contain a definition of source zones of influence and receptor zones of 

representation, defining middle-, neighborhood-, and urban scales (Chow et al., 2002). 

Compromises necessary to obtain multipollutant characterization should be defined. The 

list of variables seems complete, but there is an apparent assumption that AADT is the 

primary siting criteria. Meteorological and terrain variables also probably have important 

effects. One might find high levels in street canyons than on open roadways with good 

ventilation. 

Section 4 needs elaboration on the models to be considered and how their reported 

performance. There are several models that compare dispersion models with 

measurements, use new approaches such as computerized fluid dynamics (CFD) models 

to evaluate vehicle-induced turbulence and the effects of roadside obstructions, and that 

attempt to simulate chemical and physical transformations (Baik et al., 2007; Baker et al., 

2004; Berkowicz et al., 2008; Buccolieri et al., 2009; Chakrabarty et al., 2000; Chan et 

al., 1995; Chang et al., 2009; Cheng et al., 2008; Cheng et al., 2009; Chu et al., 2005; 

Clarke et al., 2004; Di Sabatino et al., 2008; Dixon et al., 2006; Gidhagen et al., 2004b; 

Gidhagen et al., 2004a; Gokhale et al., 2005; Grawe et al., 2007; Gromke et al., 2008; 

Kang et al., 2008; Kondo et al., 2006; Kondo and Tomizuka, 2009; Kumar et al., 2009; Li 

et al., 2006; Liu and Leung, 2008; McNabola et al., 2009; Moussiopoulos et al., 2008; 

Murena et al., 2008; Murena et al., 2009; Ning et al., 2005; Oettl et al., 2006; Pohjola et 

al., 2003; Rodden et al., 1982; Sahlodin et al., 2007; Santiago and Martin, 2008; Solazzo 

et al., 2007; Tay et al., 2010; Tsai and Chen, 2004; Vardoulakis et al., 2002; Vardoulakis 

et al., 2003; Venkatram et al., 2007; Wang et al., 2006; Wang and Zhang, 2009; Xie et 

10 
al., 2006; Yassin et al., 2008; Yassin et al., 2009; Yim et al., 2009; Zhou and Levy, 2008; 

Zhu and Hinds, 2005). 

In Section 5, “multi-scale” monitoring is a more specific term than saturation monitoring. 

This section should provide a summary of roadside measurement studies, passive and 

active monitors that can be efficiently deployed at many locations, and methods to 

interpret that data acquired. The conclusion might be that roadside monitoring for site 

selection and long-term monitoring needs small, portable sensors that don’t require a 

large infrastructure (i.e., shelter, air conditioning, etc.) to operate. 

Section 6 might draw from some of the existing guidance for sampler siting (U.S.EPA, 

1997; U.S.EPA, 1998). Site documentation in Section 7 should include coordinates, 

photographs of the siting probe, and pictures of the surroundings. 

2. What pollutants and sub-species should be included for consideration and 

discussion in the near-road monitoring guidance and what should be the priority 

of measurement? 

See response to question 13. 

3. What external variables should be used to identify candidate near-road 

monitoring sites? 

a. Given the variability in emission rates from on-road vehicles based on 

vehicle technology, fuel, speed, environmental conditions, is the fleet mix 

in near-road site selection or is a more refined inventory and modeling 

analysis required? 

Fleet mix is a good starting point, but this is likely to vary by time of day and the diurnal 

breakdown is unlikely to be available. Fleet mix should only be one variable considered 

in site selection. 

b. Should the suggested approach consider fleet mix via the use of average, 

fleet-wide emission factors, or the use of inventory and modeling analysis, 

take into account mobile source controls that are “on the books” but have 

not yet been fully realized due to fleet turnover? If so, how far out into the 

future should states consider their effects? 

Real-world emissions are likely to be quite different from certification-type emissions. It 

will take a long time for fleet evolution. Better to establish monitoring sites soon so that 

improvements can be tracked through long-term trends. It may be that the emission 

reduction measures that are “on the books” are not as effective as originally thought. 

c. The EPA suggests establishing sites at-grade with the road, without any 

nearby obstructions to air flow; however, the Agency recognizes that this 

might not always be feasible. Does the subcommittee agree with this 

recommendation for locating sites at-grade with no obstructions? What 

priority should be placed on this factor within the guidance, given the need 

for flexibility in identifying appropriate site locations? 

Higher concentrations will probably be found in more confined areas (e.g., street 

canyons) than near open roads with no obstructions. Obstructions between the vehicles 

and the monitors should be minimized. 

11 
d. How important a parameter should LOS be in the determination of 

appropriate near-road monitoring sites? Does the subcommittee have a 

view on how reliable LOS estimates are across the country? 

Congestion varies throughout the day and on weekends vs. weekdays. LOS may provide 

a first-cut on roads that have congestion, but there is no evidence on how accurate it is as 

a congestion indicator. 

e. Should terrain and vegetation should be a consideration in the siting 

process? What priority should this parameter have in the overall process? 

Terrain is very important. Even small roadway dips can accumulate pollutants in 

hotspots (Bowen et al., 1993; Bowen and Egami, 1994). 

f. Although there is no requirement to be downwind, in the preamble to final 

NO2 NAAQS rule, EPA encouraged it when possible. EPA and NACAA 

intend to do the same in the guidance document. Does the subcommittee 

agree with this approach? 

No. Sampling should take place downwind of the prevailing wind. Even under stagnant 

conditions the ram effect of the vehicles will create flows parallel to the roadway. See 

Figure 6 pollution rose from Oettl et al. (2006), as reproduced below. Nothing is detected 

when the sampling location is upwind of the roadway. 

(Oettl et al., 2006) 

Comment on the available modeling tools, and their pros and cons. 

See comments under Question 1. Roadway models must consider more than just linear 

dispersion of inert pollutants. 

How might saturation and on-road monitoring be used for near-road site selection? 

Portable active or passive monitors that can be easily and inexpensively deployed would 

provide a good indication of where concentrations might be highest. Levels are likely to 

vary by season, as noted in Figure 1 of Zou et al. (2006), and show an exponential 

decrease with distance from the curbside. Passive monitors using NO2-absorbing filters 

have some potential biases, but have also been shown to be comparable with continuous 

12 
measurements for integration times on the order of weeks (Ayers et al., 1998; Beckerman 

et al., 2008; Berkowicz et al., 2008; Crouse et al., 2009; De Fouquet et al., 2007; Douglas 

and Beaulieu, 1983; Faus-Kessler et al., 2008; Gilbert et al., 2003; Gonzales et al., 2005; 

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

et al., 2007; Krochmal and Gorski, 1991; Mukerjee et al., 2004; Nash and Leith, 2010; 

Norris and Larson, 1999; Ozden and Dogeroglu, 2008; Parra et al., 2009; PiechockiMinguy et al., 2006; Plaisance et al., 2004; Rava et al., 2007; Sekine et al., 2008; Shooter 

et al., 1997; Van Reeuwijk et al., 1998; Vardoulakis et al., 2009). This is probably a costeffective and practical technology for mapping average spatial gradients as a prelude to 

sampler siting. 

(Zou et al., 2006) Reprinted from Atmospheric Environment, 40, Zou et al., “Shifted power-law 

relationship between NO2 concentration and the distance from a highway: A new dispersion model based 

on the wind profile model, Copyright (2006) with permission from Elsevier. 

If a state were inclined to use saturation monitoring to aid in the selection of a near-road 

monitoring site, and considering that the NO2 standard is a 1-hour daily maximum 

standard, what are the pros and cons to using passive devices to saturate an area to gather 

data? 

Pros are low expense and operating cost. The major con is the long averaging time, much 

longer than 1 hour. 

Likewise, what are the pros and cons to using non-passive devices, such as near real-time 

or continuous devices including, but not limited to portable, non-FEM 

chemiluminescence methods for NO2 or Gas Sensitive Semiconductors (GSSs) for NO2

and other pollutants of interest? 

Pros are short-duration samples, on the order of an hour or less. Cons are instrument 

procurement and operating expense, potentially higher than desired detection limits, and 

reliability of new technologies. 

Finally, what would be the pros and cons, to a state or local agency attempting to use a 

specially outfitted vehicle to collect mobile measurements to assist in the near-road site 

selection process for NO2 specifically as well as other pollutants of interest? 

13 
Several mobile emissions systems have been applied to characterizing on-road and 

roadside concentrations (Bukowiechi et al., 2002; Bukowiecki et al., 2003; Cocker et al., 

2004a; Cocker et al., 2004b; Durbin et al., 2007; Herndon et al., 2005; Isakov et al., 2007; 

Kittelson et al., 2004; Kittelson et al., 2006; Morawska et al., 2007; Nussbaum et al., 

2009; Pirjola et al., 2004). Pros are that these systems are moveable and obtain many 

different pollutant measurements. Cons are the large cost of assembling or contracting 

these laboratories and the snapshot nature of their measurements, as they usually need to 

be attended and can be parked for only a short time period. 

To what extent will light duty cold start and congestion factors will significantly 

influence the location of peak CO concentrations in an area? 

The cold start segment has been found to affect emissions for many pollutants, including 

CO (Cadle et al., 2001; Chan and Zhu, 1999; Chase et al., 2000; Cook et al., 2007; Cotte 

et al., 2001; Gullett et al., 2006; Huai et al., 2004; Joumard and Andre, 1990; Joumard et 

al., 2000; Kittelson et al., 2006; Korin et al., 1999; Lenaers, 1996; Lough et al., 2005; 

Ludykar et al., 1999; Maricq et al., 1999; Mathis et al., 2005; Pornet et al., 1995; 

Ristimaki et al., 2005; Schauer et al., 2008; Singer et al., 1999; Weilenmann et al., 2005; 

Weilenmann et al., 2009; Westerholm et al., 1996). This is of fairly short duration 

(minutes) and would most likely affect emissions in garages, driveways, parking lots and 

side streets rather on the heavily-travelled thoroughfares. If it is desired to characterize 

cold starts, sampling systems should be located near where cars turn onto major arteries 

from nearby neighborhoods. Even so, only those living most closely to the intersection 

will exhibit cold start emissions. 

What priority should these factors be given when compared with the factors (AADT, 

Fleet Mix, Roadway Design, Congestion Patterns, Terrain, and Meteorology) already 

being considered for peak NO2? 

Priority should be low. 

Do these two issues of vehicles operating under cold start conditions and light duty 

vehicle congestion and idling in urban street canyons and/or urban cores be considered in 

a future, nationally applicable, CO monitoring proposal? 

This is probably better treated as an emission standard that would minimize cold start 

emissions through technological means. 

Are there other factors that may affect peak CO concentrations and not affect peak NO2

concentrations that should also be considered for any future CO monitoring proposal? 

CO is relatively inert and is often used to normalize other pollutants for dispersion 

downwind of a roadway (Zhang and Wexler, 2004). It is expected that NO2 emissions 

will disperse in a similar manner, although they still experience transformation processes 

that differ from those of CO. 

Will siting considerations for identifying the location of peak NO2 concentrations address 

all of the high priority siting considerations for PM (particularly PM2.5) as well? If not, 

what other factors should be considered and what are the advantages in considering these 

factors for identifying the location of maximum PM concentration? 

No. PM2.5 is a combination of primary and secondary particles from a wide variety of 

emission sources. Roadside sampling is useful for characterizing the motor vehicle 

contribution, but it may bias the urban- and regional-scale PM2.5 compositions and 

exposures. These monitors would be considered Special Purpose Monitors according to 

the PM siting criteria (U.S.EPA, 1997). 

14 
In addition to PM2.5 mass, what other PM-related measurements are desirable at nearroad monitoring stations (e.g., UFP number, black carbon, EC/OC, PM coarse, etc.)? 

Particle size distribution and particle number by continuous methods. PM speciation on 

filters, including elements, ions, OC/EC, and organic markers, would be useful to 

develop source profiles for emission inventory speciation and receptor modeling. 

To allow for near-road monitoring infrastructure to be multi-pollutant, and in reflection 

of the recently promulgated near-road NO2 siting criteria, reconsideration of the existing 

microscale CO siting criteria presented in sections 2, 6.2, and table E-4 in 40 CFR Part 58 

Appendix E may be warranted. Does the subcommittee believe that reconsideration of 

microscale CO siting criteria is appropriate? Specifically, would an adjustment of CO 

siting criteria to match those of microscale PM2.5 and microscale near-road NO2 sites be 

logical and appropriate? 

40CFRPart58 Appendix E calls for CO roadside monitors to be located at 2 to 10 m from 

the nearest traffic lane for open roads. In a street canyon, the monitor is to be at least 10 

m from an intersection. An NO2 monitor might register higher concentrations near the 10 

m downwind location owing to NO2 formation by reaction of the NO2 with O3. It seems 

that a reasonable compromise on the setback could be derived that would serve both 

purposes. A more detailed examination of NO2, NO, CO, and O3 data is needed in the 

pilot study to better determine the optimum distance from the roadside. 

Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and table E-4 in 

40 CFR Part 58 Appendix E to match that of microscale PM2.5 and microscale near-road 

NO2 is appropriate and proposed, should there be consideration to maintain the 

requirement on how urban street canyon or urban core microscale CO sites should be 

sited? 

The NO2 and CO siting criteria should be the same. There is more to be gained from the 

multipollutant measurements than is lost by slight differences in maximum hourly 

concentrations. 

Does the subcommittee have an opinion on how “urban street canyons” or “urban core” 

might be defined, perhaps quantitatively, and with regard to use in potential rule 

language? 

There is a reasonable literature on measurements in street canyons (Baik and Kim, 2002; 

Bakeas and Siskos, 2003; Boddy et al., 2005a; Boddy et al., 2005b; Buccolieri et al., 

2010; Cai et al., 2008; Caton et al., 2003; Chan et al., 2003; Cheng et al., 2009; Chu et 

al., 2005; Di Sabatino et al., 2008; Dobre et al., 2005; Eliasson et al., 2006; Genikhovich 

et al., 2005; Gromke et al., 2008; Hang et al., 2009; Kassomenos et al., 2004; Kim and 

Baik, 2004; Kumar et al., 2008; Kumar et al., 2009; Lam et al., 2008; Li et al., 2005; Li et 

al., 2009; Longley et al., 2003; Longley, 2004; Longley et al., 2004; McNabola et al., 

2009; Molina, 1996; Moussiopoulos et al., 2008; Murena and Vorraro, 2003; Murena et 

al., 2008; Prajapati et al., 2009; Santiago and Martin, 2008; Scaperdas and Colvile, 1999; 

So et al., 2005; Stein and Toselli, 1996; Tay et al., 2010; Tsai et al., 2005; Venegas and 

Mazzeo, 2000; Voigtlander et al., 2006; Xie et al., 2003; Xie et al., 2005; Xie et al., 2006; 

Xie et al., 2007; Yassin et al., 2009), supplementing the street canyon modeling literature 

cited in the response to Question 1. These measurement and modeling studies need to be 

critically evaluated to answer this question. A quick survey suggests that there are 

various degrees of roadside obstructions that will have large effects on concentrations. 

15 
To what extent are the pilot study site selection criteria of a large and a relatively small 

urban area based on population, an area with varied or complex terrain, an urban area 

with an operational NOX analyzer representative of neighborhood or larger spatial scales 

for comparison to the near-road NOX analyzer, and an urban area with a cooperative (or 

non-cooperative) Department of Transportation complete and adequate?. 

Additional criteria should include periods of morning stagnation and low inversion, 

differing morning O3 levels that might enhance NO2 through NO titration, cold as well as 

warm environments that might experience different emission levels owing to cold starts. 

Comment on the minimum equipment/pollutant measurement complement that should be 

deployed at each site and also the ideal equipment complement that each site should or 

could have, respectively. Specifically, what pollutants (e.g., NO2, NOX, NO, CO, PM 

(Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene, xylene, 

formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other information should 

the pilot study measure or gather at the fixed, permanent monitoring stations, and by what 

methods? This list should be in priority order, as feasible, and can include any NAAQS 

or non-NAAQS pollutant by any method (FRM/FEM and/or non-reference or equivalent 

methods), any particular type of other equipment for gathering supporting data such as 

meteorology or traffic counts. 

As implied in the charge questions, a specialized multi-pollutant monitoring package 

should be assembled and applied in these studies. There are several examples of such 

packages that have been assembled for neighborhood-scale studies, on-board emissions 

sampling, and unmanned aerial vehicles that have contain potentially applicable sensors, 

but these would need to be evaluated with respect to their sensitivity, stability, and 

accuracy. Data should be acquired over 1 min averages or less so that individual plumes 

can be detected. 

Nitrogen dioxide (NO2). This is top priority because it is the focus of the study. There 

are several currently available or emerging technologies for microsensors (Brunet et al., 

2008; Currie et al., 1999; Egashira et al., 1996; Forleo et al., 2005; Gurlo et al., 1998; Oto 

et al., 2001; Sitnikov et al., 2005; Talazac et al., 2001). NO can often be obtained from 

these same sensors. 

Carbon dioxide (CO2): Normalizing other pollutants to CO2 allows fuel-based emission 

factors to be developed (Kean et al., 2000; Sawyer et al., 2000). Commercially available 

microchip IR sensors are available for CO2 measurements (Chow et al., 2010b). 

Black Carbon (BC): On a short-duration minute basis, this would allow cold starts and 

diesel exhaust to be separated from others and related to the NO2 emissions from 

individual vehicle plumes. A portable aethalometer (Hansen and Mocnik, 2010) is 

available for filter transmission measurements of BC, and more portable photoacoustic 

measurement systems (Kok and Baumgardner, 2010) are emerging. 

Carbon Monoxide (CO): CO is a priority pollutant and is an indicator of gasoline engine 

contributions, especially for cold starts and poorly maintained engines. Several small 

detectors are available (Do and Chen, 2007; Oto et al., 2001). 

Ozone (O3): This would be important for estimating NO titration to NO2. Several 

microsensors are available or are emerging technologies (Do and Chen, 2007; Gurlo et 

al., 1998; Ulanovsky et al., 2001; Vallejos et al., 2007). 

Particle number: This would indicate a potential adverse health effect. Portable CPC 

counters are available. 

16 
 PM10 and PM2.5: Coarse particles (PM10-2;5) may be affected by road dust while PM2.5 is 

largely from vehicle exhaust. Optical particle counters can provide a real-time surrogate 

for these components (Wang et al., 2009; Cheng, 2008; Heim et al., 2008; Linnainmaa et 

al., 2008). 

EPA and NACAA have proposed four to five urban areas to have saturation monitoring, 

using either passive devices and/or continuous/semi-continuous saturation type multipollutant monitoring packages (i.e., several types of monitors in one mountable or 

deployable “package”). Please provide comment on: 

The pollutants that should be measured with the saturation devices at each saturation site. 

See answers to Question 13. 

The number of saturation devices per pollutant, both passive and/or continuous/semicontinuous, that may be deployed in each pilot city. 

Four sampling systems should be located downwind of the roadway at various distances. 

One should be located in a neighborhood near the road and one should be located upwind 

of the urban area. 

Whether placing saturation monitoring devices near certain road segments should 

include, at a minimum: 1) the highest AADT segment in an area, 2) the road segment 

with the highest number of heavy-duty truck/bus counts, 3) at a road segment with more 

unique roadway design, congestion pattern, or terrain in the area, and 4) if feasible, at a 

lower AADT segment with a similar fleet mix, roadway design, congestion, terrain, and 

meteorology as the top AADT road segment in the area. 

These are good suggestions. Experiments should be designed to determine which 

variables most affect the ambient concentrations. 

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28 
Mr. Bart Croes 

It’s not clear what EPA is trying to accomplish with its proposed near road monitoring 

program. If it’s to determine compliance with the new 1-hour national ambient air quality 

standard for nitrogen dioxide (NO2), then it seems that the primary focus should be on 

monitoring population centers rather than siting monitors near roadways. For example, 

how will nonattainment boundaries be established for these microscale environments? 

California has had a one-hour standard for NO2 for several decades and studied peak 

levels and trends throughout the State. Although future NO2 levels will be increasingly 

driven by direct emissions from heavy-duty diesel vehicles, current peak locations in 

California are in intermediate downwind areas (e.g., eastern border of Los Angeles 

County) where photochemical conversion (and not just immediate ozone titration) has 

taken place, not in the source areas. 

If the purpose is to conduct multi-pollutant monitoring to help inform exposure and 

health studies, then linkages with these types of research studies appears to be missing 

from the documents. 

The timeframe for establishing a national network (by January 1, 2013) seems much too 

rushed to allow for full availability and analysis of results from the near road monitoring 

pilot study. 

Charge Questions: 

1. The accompanying draft guidance document outline provides an initial thought of 

the major topics required in the near-road monitoring guidance that will aid state 

monitoring agencies in the identification and implementation of NO2 near road 

monitoring sites from a multi-pollutant perspective. Please comment on the 

overall content of the recommended topics in the draft outline. Please provide 

suggestions on any missing subjects that should be included in the guidance 

document and any unnecessary topics that are currently listed in the attached 

draft, if applicable.

It’s not clear what EPA is trying to accomplish with its proposed near road 

monitoring program. The goals should be clearly delineated in order to comment on 

this brief outline. It would also be helpful to see the target number of pages for each 

section and the extent of the literature review. As the outline points out, the available 

literature is extensive (see the Health Effects Institute special report on TrafficRelated Air Pollution: A Critical Review of the Literature on Emissions, Exposure, 

and Health Effects) and a conducting a synthesis of existing near road and saturation 

monitoring datasets would be a useful addition. 

2. EPA and NACAA envision the near-road guidance document to be written from a 

multi-pollutant perspective. What pollutants and sub-species does the 

subcommittee believe should be included for consideration and discussion in the 

near-road monitoring guidance? Some potential species for consideration include 

NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics (e.g., 

29 
benzene, toluene, xylene, formaldehyde, acrolein, or 1,3-butadiene), and 

ammonia. Please prioritize the recommended pollutants and provide the rationale 

for their ranking, including how this pollutant measurement will contribute to 

scientific and regulatory knowledge of near-road air quality and adverse human 

health effects. 

Again, some clarity is needed on what EPA is trying to accomplish with its proposed 

near road monitoring program. If it’s to determine compliance with existing air 

quality standards, then only NO2 and CO are all that is necessary, as the roadway 

environment as not the peak location for PM2.5 and PM10 (because secondary PM is 

so important in determining peak locations). If the purpose is to follow motor vehicle 

criteria pollutant and air toxic emission trends, or to conduct multi-pollutant 

monitoring to help inform exposure and health studies, then NOX, PM, black carbon, 

and the air toxics should also be included. 

3. Identifying Candidate Near-road Site Areas 

a. Annual Average Daily Traffic (AADT) & Fleet Mix – To consider fleet mix with 

regard to NO2, an idea is to encourage states that have fleet mix information to 

take an approach that uses average, fleet-wide grams per mile emissions 

estimates (one for light duty vehicles and one for heavy duty vehicles), combined 

with AADT information to further weight which road segments in an area may be 

more conducive to produce peak pollutant concentrations. EPA would use the 

latest emission factor information to aid such a calculation. Given the variability 

in emission rates from on-road vehicles based on vehicle technology, fuel, speed, 

environmental conditions, etc., does the subcommittee believe this approach is an 

appropriate way to “consider” fleet mix in near-road site selection or is a more 

refined inventory and modeling analysis required? 

The highest CO sites in California are the ones with the highest AADT for lightduty vehicles and oldest fleet, and the peak one-hour levels occur during winter. 

In the past, the highest NO2 levels were the same sites and season because of the 

reaction 2 NO + O2 Æ 2 NO2, which is important at low temperatures and NOX > 

1 ppm. But future NO2 levels will be dominated by heavy-duty diesel vehicles 

(HDDV) and near-source mixing with ozone. Thus, a key factor in roadside NO2

levels is the fraction of heavy-duty diesel vehicles (HDDV) that are equipped 

(2007-2009 model year) or retrofitted with a catalyzed diesel particulate filter. In 

California, while most diesel vehicles emit 5% NO2 (from Caldecott Tunnel data), 

this can increase to 17% for retrofits and 45% for 2007-2009 model year 

(although absolute NOX, and consequently NO2 is lowered by 50%). In other 

states that do not have California’s NO2 limit for retrofits (or just went into place 

in 2009), the NO2 fraction can be much higher, although admittedly the number of 

retrofits is a much lower in the rest of the U.S. Since existing EPA emission 

models predict NOX and not NO2, this capability would need to be added. 

b. AADT & Fleet Mix – Further, should the suggested approach above in question 

3a to consider fleet mix via the use of average, fleet-wide emission factors, or the 

30 
NOX and NO2 emissions will continue to decrease as the 2010 model year 

HDDVs (with 90% NOX control over pre-2007 vehicles) become a bigger fraction 

of the fleet. Since these decreases will take place somewhat uniformly across the 

nation, the purpose of this question isn’t clear. Is the intent to forecast emission 

trends for States to get out of monitoring requirements? 

c. Roadway Design – Studies suggest and support the concept that roadway design 

influences pollutant dispersion near the road. The EPA suggests establishing sites 

at-grade with the road, without any nearby obstructions to air flow; however, the 

Agency recognizes that this might not always be feasible. Does the subcommittee 

agree with this recommendation for locating sites at-grade with no obstructions? 

What priority should be placed on this factor within the guidance, given the need 

for flexibility in identifying appropriate site locations? 

Obstructions between the roadways and the monitors should be minimized, but 

shouldn’t proximity to where people live be a more important consideration? A 

more practical consideration in site selection is the cost of leases. For an ARB 

freeway study in Lodi, rent started out at $750/month for both sites. Once the 

landowners realized that their proximity to the freeway was worthwhile to ARB 

they increased the rent to $1500/month. 

d. Congestion Patterns – The congestion of a roadway can be estimated by the 

metric “Level of Service” (LOS). LOS uses a letter grade from A to F to identify a 

roadway’s performance, with “A” the best conditions where traffic flows at or 

above the posted speed limit and all motorists have complete mobility between 

lanes to “F” the worst congestion where travel time cannot be predicted and 

generally traffic demand exceeds the facility’s capacity. Since motor vehicles 

generally emit more pollutants during congestion operations (although noting 

that NOX and select other pollutant emissions can also increase with increasing 

speed), how important a parameter should LOS be in the determination of 

appropriate near-road monitoring sites? Does the subcommittee have a view on 

how reliable LOS estimates are across the country? 

As NOX and VOC emissions are continuously reduced over time, ambient oxidant 

levels will also go down, which means that titration of NO by ozone to form NO2

will become less important in comparison to direct NO2 emissions from new and 

retrofitted diesel trucks. NO2 emissions are highest during cruise mode, not 

congestion conditions for these trucks, and truckers avoid congestion. Experts at 

Departments of Transportation should be consulted on the reliability of the 

congestion metric. 

e. Terrain– State and local air agencies are required to consider terrain in the near31 
road monitoring site selection process, which in some cases may be inherently 

part of the roadway design. However, EPA recognizes that some states and local 

air agencies may have to make selections from amongst similar candidate sites 

that differ only by terrain, e.g. cut section versus open terrain, with or without 

vegetation, etc. Does the subcommittee agree that terrain and vegetation should 

be a consideration in the siting process? What priority should this parameter 

have in the overall process? 

NOX (and likely NO2) emissions increase with grade, but these are unlikely to be 

areas of heavy congestion and truck traffic. Vegetation is a sink for particles and 

ozone, and heavily vegetated areas should be avoided. 

f. Meteorology – EPA took comment on, but did not finalize the requirement for 

near-road monitoring sites to be climatologically downwind of the target road 

segment. Reasons were because the additional limitations this would introduce in 

finding candidate sites would be in exchange for what may be a small increase in 

the opportunity to monitor peak NO2 concentrations. Further, with sites being 

within 50 meters of target road segments, the phenomenon of upwind meandering 

(pollutant transport upwind due to vehicle induced turbulence) further reduces 

that absolute need to be climatologically downwind. Finally, EPA recognized 

that, logically, the potential for peak NO2 concentration may very well occur 

when winds are calm or parallel (or nearly parallel) to the target road, allowing 

for pollutant build-up, as opposed to when winds are normal to the road. 

Although there is no requirement to be downwind, in the preamble to final NO2 

NAAQS rule, EPA encouraged it when possible. EPA and NACAA intend to do the 

same in the guidance document. Does the subcommittee agree with this 

approach? 

An analysis of existing roadside CO data would be informative regarding this 

issue. In the peak Los Angles site, the highest levels were recorded during 

stagnant winter-time conditions with low-speed meandering winds, meaning that 

there was no consistent upwind or downwind direction. 

4. Modeling is another tool that may be useful in the identification of candidate 

near-road sites. In particular, the use of mobile source emissions modeling with 

MOVES and local-scale dispersion modeling with AERMOD, can be presented as 

part of the guidance document. Please comment on the available modeling tools, 

and their pros and cons, that the subcommittee believes may be appropriate to 

discuss and/or recommend for use in the near-road monitoring guidance 

document. 

Modeling requires hour-by-hour local-scale data on the fleet mix (not just HDDV 

counts but also the proportions of retrofitted and MY 2007-2009 and 2010+ vehicles), 

driving conditions (average speeds), background air quality (NO2, ozone), and 

meteorology (wind speed and direction, stability) that is unlikely to be available. I 

think a screening approach using the factors identified in my response to Charge 

32 
Question #3 (with appropriate weightings developed using a literature review and 

analysis of existing data) should be just as useful and much easier to apply. 

5. In regard to the process of identifying candidate near-road monitoring sites, 

beyond the evaluation of factors noted above in question 3, and the potential use 

of modeling, the use of saturation monitoring and on-road monitoring are also 

possible tools that state and local air agencies may choose to utilize in the nearroad site selection process. 

a. If a state were inclined to use saturation monitoring to aid in the selection of a 

near-road monitoring site, and considering that the NO2 standard is a 1-hour 

daily maximum standard, what are the pros and cons to using passive devices to 

saturate an area to gather data? 

Given their relatively long sampling times, passive samplers would only be useful 

to determine likely peak 1-hour locations. In studies of pollution levels in Los 

Angeles communities near freeways and ports, both the passive samplers and 

mobile monitoring platform documented the sharp gradients in the vicinity of 

freeways. The Ogawa saturation monitors for NO2 had a precision of ~1.5 ppb 

(5%) for 1-week integrated samples, and comparison with collocated FRM 

analyzers indicated good accuracy (m=1.0380 and r2

=0.9905). 

b. Likewise, what are the pros and cons to using non-passive devices, such as near 

real-time or continuous devices including, but not limited to portable, non-FEM 

chemiluminescence methods for NO2 or Gas Sensitive Semiconductors (GSSs) for 

NO2 and other pollutants of interest? 

No comment, not my area of expertise. 

c. Finally, what would be the pros and cons, to a state or local agency attempting to 

use a specially outfitted vehicle to collect mobile measurements to assist in the 

near-road site selection process for NO2 specifically as well as other pollutants of 

interest? 

The ARB has a mobile monitor that has proven to be a very useful tool to identify 

near-source pollutant gradients. We use an electric vehicle to avoid self pollution 

issues and to have a large enough battery to run all the equipment. The vehicle 

and equipment cost are high, on the order of $250,000, and it takes expert staff to 

set up and operate the mobile monitor. 

6. EPA recognizes that CO concentrations are primarily influenced by gasoline 

vehicles as opposed to NO2 and PM2.5 concentrations, which are currently more 

heavily influenced by heavy-duty (diesel) vehicle emissions. If EPA were to 

propose a new set of minimum monitoring requirements for CO near roads, the 

near-road monitoring stations created under the implementation of the NO2

monitoring requirements may be an advantageous infrastructure for state and 

local air agencies to leverage. However, EPA believes there are two issues not 

33 
specifically considered in the near-road NO2 monitoring language that might 

influence where near-road CO monitors may be most appropriately placed. The 

two issues are 1) the consideration of where light duty vehicles are operating 

under ‘cold-start’ conditions, which may often not be on the larger arterials or 

highways in an area, and 2) the impacts of light duty vehicle congestion and 

idling in areas such as urban street canyons and/or urban cores. 

a. Does the subcommittee believe that the light duty cold start and congestion 

factors will significantly influence the location of peak CO concentrations in an 

area? What priority should these factors be given when compared with the factors 

(AADT, Fleet Mix, Roadway Design, Congestion Patterns, Terrain, and 

Meteorology) already being considered for peak NO2? 

b. Does the subcommittee have an opinion on whether, and possibly how, these two 

issues of vehicles operating under cold start conditions and light duty vehicle 

congestion and idling in urban street canyons and/or urban cores be considered 

in a future, nationally applicable, CO monitoring proposal? Are there other 

factors that may affect peak CO concentrations and not affect peak NO2

concentrations that should also be considered for any future CO monitoring 

proposal? 

The major factors in the location of peak CO levels are high light-duty vehicle 

volumes, high fractions of older and poorly maintained vehicle (e.g., high 

emitters), stagnant meteorology, and containment by nearby buildings (e.g., urban 

street canyons). Parking garages can be a hot spot because of cold starts and 

pollutant containment by the building, but the duration of high CO periods can be 

short, and population exposure is generally low, so they should not be a priority 

for a future national CO monitoring proposal. Rather a focus on high emitters in 

populated areas (i.e., environmental justice communities) would be a useful 

complement to the consideration of factors for high NO2 levels. 

7. Does the committee believe that siting considerations for identifying the location 

of peak NO2 concentrations will likely address all of the high priority siting 

considerations for PM (particularly PM2.5) as well? If not, what other factors 

should be considered and what are the advantages in considering these factors 

for identifying the location of maximum PM concentration? 

The near roadway data for California from multiple studies shows that PM2.5, even 

in a coastal source area, is primarily from secondary formation. There was 

surprisingly little variation throughout the communities monitored in special studies. 

Several studies using ultrafine PM networks across a community showed large spatial 

and temporal variations associated with proximity to fresh combustion products. 

8. In addition to PM2.5 mass, what other PM-related measurements are desirable at 

near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM 

coarse, etc.)? 

34 
In California special studies, we found that different BC and PM measurement 

methods have significant differences. They tend to agree qualitatively but not 

quantitatively. CPC ultrafine PM measurements are sensitive to the effective size cuts 

of the instruments and some instruments needed frequent calibrations. The different 

methods for measuring EC and OC also yield different concentrations. If near-road 

monitoring is going to be used for assessing compliance with air quality standards, 

the methods need to be comparable (equivalent) to the reference method. If near-road 

measurements are only to better understand the atmospheric processes and pollutant 

relationships, then only a good QA program is required to be useful. The most critical 

part of any near-road monitoring program will be the siting criteria for the 

monitors/samplers as a small change in distance (or possibly height), orientation to 

the high traffic volume direction during the light/calm winds in the morning 

commute, and number of vehicles during the period of typically stable air (surface 

inversion and light/calm wind) could all have a strong impact on the concentrations 

measured. 

9. To allow for near-road monitoring infrastructure to be multi-pollutant, and in 

reflection of the recently promulgated near-road NO2 siting criteria, 

reconsideration of the existing microscale CO siting criteria presented in sections 

2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E may be warranted. Does the 

subcommittee believe that reconsideration of microscale CO siting criteria is 

appropriate? Specifically, would an adjustment of CO siting criteria to match 

those of microscale PM2.5 and microscale near-road NO2 sites be logical and 

appropriate? 

Yes, consistency seems warranted. 

10. Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and 

table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5 and 

microscale near-road NO2 is appropriate and proposed, should there be 

consideration to maintain the requirement on how urban street canyon or urban 

core microscale CO sites should be sited? 

No comment, not my area of expertise. 

11. Does the subcommittee have an opinion on how “urban street canyons” or 

“urban core” might be defined, perhaps quantitatively, and with regard to use in 

potential rule language? 

No comment, not my area of expertise. 

12. EPA and NACAA will select the locations for permanent sites that are part of the 

near-road pilot study based on which state or locals volunteer to participate and 

can process grant funds in a timely manner to deploy equipment. From this pool 

of volunteers, selection should be made on certain attributes that provide the best 

potential to fulfill pilot study objectives. In the attached draft white paper, EPA 

35 
and NACAA have proposed some potential criteria for consideration in selecting 

where the fixed, permanent stations should be located. These considerations 

include choosing a large and a relatively small urban area based on population, 

an area with varied or complex terrain, an urban area with an operational NOX

analyzer representative of neighborhood or larger spatial scales for comparison 

to the near-road NOX analyzer, and an urban area with a cooperative (or noncooperative) Department of Transportation. Does the Subcommittee agree with 

these considerations? Further, are there other considerations that should be 

evaluated in selecting pilot cities to house permanent near-road monitoring 

stations as part of the pilot study? 

Because of the potential influence of high NO2 emissions from retrofitted or post2007 HDDV on roadside levels, a freeway with high heavy-duty truck traffic volumes 

should be the primary consideration in site selection. It is also important that an ozone 

measurement be co-located with the neighborhood scale NOX analyzer so that total 

oxidant levels can be determined. USEPA, ARB, and several research groups have 

mobile monitors that could be deployed to quickly find the location of highest NO2. 

13. EPA and NACAA have proposed that at least two urban areas should have 

permanent near-road monitoring stations (that would fulfill NO2 near-road 

monitoring requirements) implemented for the pilot study. Please comment on the 

minimum equipment/pollutant measurement complement that should be deployed 

at each site and also the ideal equipment complement that each site should or 

could have, respectively. Specifically, what pollutants (e.g., NO2, NOX, NO, CO, 

PM (Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene, 

xylene, formaldehyde, acrolein, or 1,3-butadiene) and ammonia) and other 

information should the pilot study measure or gather at the fixed, permanent 

monitoring stations, and by what methods? This list should be in priority order, 

as feasible, and can include any NAAQS or non-NAAQS pollutant by any method 

(FRM/FEM and/or non-reference or equivalent methods), any particular type of 

other equipment for gathering supporting data such as meteorology or traffic 

counts. 

The list of pollutants is appropriate and in priority order, although air toxics could be 

dropped to conserve resources. Meteorological parameters (wind speed, wind direct, 

temperature) and light- and heavy-duty truck counts would also be useful. 

14. EPA and NACAA have proposed four to five urban areas to have saturation 

monitoring, using either passive devices and/or continuous/semi-continuous 

saturation type multi-pollutant monitoring packages (i.e., several types of 

monitors in one mountable or deployable “package”). Please provide comment 

on: 

a. The pollutants that should be measured with the saturation devices at each 

saturation site. 

b. The number of saturation devices per pollutant, both passive and/or 

continuous/semi-continuous, that may be deployed in each pilot city. 

36 
c. Whether placing saturation monitoring devices near certain road segments 

should include, at a minimum: 1) the highest AADT segment in an area, 2) the 

road segment with the highest number of heavy-duty truck/bus counts, 3) at a 

road segment with more unique roadway design, congestion pattern, or terrain in 

the area, and 4) if feasible, at a lower AADT segment with a similar fleet mix, 

roadway design, congestion, terrain, and meteorology as the top AADT road 

segment in the area. 

I think mobile monitoring measurements are much more useful, and more relevant 

to the averaging time of the NO2 standard. EPA staff can look at the details of 

ARB’s year-long saturation monitoring study in the communities downwind of 

the Ports of Los Angeles and Long Beach for information on methods, quality 

assurance, data analysis techniques, and results that may be useful in designing 

the pilot study. (http://www.arb.ca.gov/research/mobile/hcm/sat-mon/satmon.htm) 

37 
Dr. Kenneth Demerjian 

Charge Questions

Questions regarding the near-road monitoring guidance document 

1. The accompanying draft guidance document outline provides an initial thought of 

the major topics required in the near-road monitoring guidance that will aid state 

monitoring agencies in the identification and implementation of NO2 near road 

monitoring sites from a multi-pollutant perspective. Please comment on the 

overall content of the recommended topics in the draft outline. Please provide 

suggestions on any missing subjects that should be included in the guidance 

document and any unnecessary topics that are currently listed in the attached 

draft, if applicable. 

2. EPA and NACAA envision the near-road guidance document to be written from 

a multi-pollutant perspective. What pollutants and sub-species does the 

subcommittee believe should be included for consideration and discussion in the 

near-road monitoring guidance? Some potential species for consideration include 

NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics (e.g., 

benzene, toluene, xylene, formaldehyde, acrolein, or 1, 3, butadiene), and 

ammonia. Please prioritize the recommended pollutants and provide the rationale 

for their ranking, including how this pollutant measurement will contribute to 

scientific and regulatory knowledge of near-road air quality and adverse human 

health effects. Tier I - NO2, NOX, NO, CO, CO2, SO2, EC/OC, BTEX aerosol size 

distribution and total number concentration for routine near-road monitoring. Tier 

II - PM organics (HOA, OOA), NH3, HONO, H2CO and 1,3 - butadiene. 

3. Identifying Candidate Near-road Site Areas 

a. AADT & Fleet Mix – To consider fleet mix with regard to NO2, an idea is 

to encourage states that have fleet mix information to take an approach 

that uses average, fleet-wide grams per mile emissions estimates (one for 

light duty vehicles and one for heavy duty vehicles), combined with 

AADT information to further weight which road segments in an area may 

be more conducive to produce peak pollutant concentrations. EPA would 

use the latest emission factor information to aid such a calculation. Given 

the variability in emission rates from on-road vehicles based on vehicle 

technology, fuel, speed, environmental conditions, etc., does the 

subcommittee believe this approach is an appropriate way to “consider” 

fleet mix in near-road site selection or is a more refined inventory and 

modeling analysis required? First cut at identifying potential near-road 

monitoring sites should be to consider the application of GIS methods for 

traffic exposure. These and other methods were recently reviewed by HEI, 

Traffic-Related Air Pollution: A Critical Review of the Literature on 

Emissions, Exposure, and Health Effects, Special Report 17, January, 

2010. 

b. AADT & Fleet Mix – Further, should the suggested approach above in 

question 4a to consider fleet mix via the use of average, fleet-wide 

emission factors, or the use of inventory and modeling analysis, take into 

account mobile source controls that are “on the books” but have not yet 

38 
been fully realized due to fleet turnover? If so, how far out into the future 

should states consider their effects? 

c. Roadway Design – Studies suggest and support the concept that roadway 

design influences pollutant dispersion near the road. The EPA suggests 

establishing sites at-grade with the road, without any nearby obstructions 

to air flow; however, the Agency recognizes that this might not always be 

feasible. Does the subcommittee agree with this recommendation for 

locating sites at-grade with no obstructions? What priority should be 

placed on this factor within the guidance, given the need for flexibility in 

identifying appropriate site locations? The choice of sites with minimal 

roadway design influences affect pollutant dispersion should be a priority, 

but not an absolute requirement. Microenvironments with high pollutant 

exposures near local neighborhoods should be considered and will likely 

reflect some combination of terrain, road grade, traffic volume and 

congestion influences. 

d. Congestion Patterns – The congestion of a roadway can be estimated by 

the metric “Level of Service” (LOS). LOS uses a letter grade from A to F 

to identify a roadway’s performance, with “A” the best conditions where 

traffic flows at or above the posted speed limit and all motorists have 

complete mobility between lanes to “F” the worst congestion where travel 

time cannot be predicted and generally traffic demand exceeds the 

facility’s capacity. Since motor vehicles generally emit more pollutants 

during congestion operations (although noting that NOx and select other 

pollutant emissions can also increase with increasing speed), how 

important a parameter should LOS be in the determination of appropriate 

near-road monitoring sites? Does the subcommittee have a view on how 

reliable LOS estimates are across the country? I have no firsthand 

knowledge on estimating LOS, but it seems that there are likely many 

innovative approaches that could be offered up by DOT and the traffic 

engineering community who deal with congestion mitigation issues on a 

daily basis. Discussions with this community regarding estimating LOS 

should be EPA’s first priority. 

e. Terrain– State and local air agencies are required to consider terrain in the 

near-road monitoring site selection process, which in some cases may be 

inherently part of the roadway design. However, EPA recognizes that 

some states and local air agencies may have to make selections from 

amongst similar candidate sites that differ only by terrain, e.g. cut section 

versus open terrain, with or without vegetation, etc. Does the 

subcommittee agree that terrain and vegetation should be a consideration 

in the siting process? What priority should this parameter have in the 

overall process? Terrain and vegetation should be considered and 

documented as part of the siting process, but should not be a major priority 

factor in the selection process. 

f. Meteorology – EPA took comment on, but did not finalize the requirement 

for near-road monitoring sites to be climatologically downwind of the 

target road segment. Reasons were because the additional limitations this 

39 
would introduce in finding candidate sites would be in exchange for what 

may be a small increase in the opportunity to monitor peak NO2

concentrations. Further, with sites being within 50 meters of target road 

segments, the phenomenon of upwind meandering (pollutant transport 

upwind due to vehicle induced turbulence) further reduces that absolute 

need to be climatologically downwind. Finally, EPA recognized that, 

logically, the potential for peak NO2 concentration may very well occur 

when winds are calm or parallel (or nearly parallel) to the target road, 

allowing for pollutant build-up, as opposed to when winds are normal to 

the road. Although there is no requirement to be downwind, in the 

preamble to final NO2 NAAQS rule, EPA encouraged it when possible. 

EPA and NACAA intend to do the same in the guidance document. Does 

the subcommittee agree with this approach? The siting of monitors in local 

neighborhoods in the proximity of major roadways (i.e., <500 meters) is 

more important than its placement at a location that is climatologically 

downwind. That being said, if EPA wants to address the influence of 

climatology on monitor siting, the application of traditional climatological 

line source models would be the starting point. 

4. Modeling is another tool that may be useful in the identification of candidate 

near-road sites. In particular, the use of mobile source emissions modeling with 

MOVES and local-scale dispersion modeling with AERMOD, can be presented as 

part of the guidance document. Please comment on the available modeling tools, 

and their pros and cons, that the subcommittee believes may be appropriate to 

discuss and/or recommend for use in the near-road monitoring guidance 

document. As mentioned above the application of traditional line source models to 

address siting issues with respect to climatology is fairly straight forward. The 

application of more sophisticated emissions and exposure models does not seem 

necessary to address the climatology issue. 

5. In regard to the process of identifying candidate near-road monitoring sites, 

beyond the evaluation of factors noted above in question 3, and the potential use 

of modeling, the use of saturation monitoring and on-road monitoring are also 

possible tools that state and local air agencies may choose to utilize in the nearroad site selection process. 

a. If a state were inclined to use saturation monitoring to aid in the selection 

of a near-road monitoring site, and considering that the NO2 standard is a 

1-hour daily maximum standard, what are the pros and cons to using 

passive devices to saturate an area to gather data? Saturation monitoring 

for NO2 is temporally limited to 24 hr averages. The diurnal pattern of 

NO2 varies with season and max 1 hr averages can occur at mid-morning 

and mid-afternoon depending on season. Spatial mapping of NO2 (and 

other) concentrations with fast response monitoring technologies (e.g. 

QCL multipath IR spectroscopy) can provide significant insights to near 

road exposures in local neighborhoods. 

b. Likewise, what are the pros and cons to using non-passive devices, such as 

near real-time or continuous devices including, but not limited to portable, 

non-FEM chemiluminescence methods for NO2 or Gas Sensitive 

40 
Semiconductors (GSSs) for NO2 and other pollutants of interest? QCL 

multipath IR spectroscopy have been demonstrated for NO2, HONO, 

H2CO, CO, and 1,3–butadiene and have been operated from mobile 

platforms providing spatial mapping or gradient measurement associated 

with fixed site monitoring. 

c. Finally, what would be the pros and cons, to a state or local agency 

attempting to use a specially outfitted vehicle to collect mobile 

measurements to assist in the near-road site selection process for NO2

specifically as well as other pollutants of interest? This is the method of 

choice, but the availability of specially outfitted mobile measurement 

platforms is limited, as are the dollars to support such measurements. 

Questions regarding the CO monitoring network and near-road monitoring 

6. EPA recognizes that CO concentrations are primarily influenced by gasoline 

vehicles as opposed to NO2 and PM2.5 concentrations, which are currently more 

heavily influenced by heavy-duty (diesel) vehicle emissions. If EPA were to 

propose a new set of minimum monitoring requirements for CO near roads, the 

near-road monitoring stations created under the implementation of the NO2

monitoring requirements may be an advantageous infrastructure for state and local 

air agencies to leverage. However, EPA believes there are two issues not 

specifically considered in the near-road NO2 monitoring language that might 

influence where near-road CO monitors may be most appropriately placed. The 

two issues are 1) the consideration of where light duty vehicles are operating 

under ‘cold-start’ conditions, which may often not be on the larger arterials or 

highways in an area, and 2) the impacts of light duty vehicle congestion and idling 

in areas such as urban street canyons and/or urban cores. 

a. Does the subcommittee believe that the light duty cold start and 

congestion factors will significantly influence the location of peak CO 

concentrations in an area? What priority should these factors be given 

when compared with the factors (AADT, Fleet Mix, Roadway Design, 

Congestion Patterns, Terrain, and Meteorology) already being considered 

for peak NO2? The spatial distribution of cold start vehicles associated 

with urban commuting is in general broad and short term and their 

contribution to emissions associated with major highways adjacent to 

residential neighborhoods is likely small. That being said, at least one 

exception comes to mind. The departure of motor vehicles from major 

entertainment events (e.g. a football or baseball stadium) where 20-30K 

vehicles may be simultaneously started and caught in congestion for 10s 

of minutes to an hour or more. The cold start contribution, is again limited 

in time but could contribute significantly as an emissions hot-spot 

impacting commuter exposes and concentrations in nearby neighborhoods. 

b. Does the subcommittee have an opinion on whether, and possibly how, 

these two issues of vehicles operating under cold start conditions and light 

duty vehicle congestion and idling in urban street canyons and/or urban 

cores be considered in a future, nationally applicable, CO monitoring 

proposal? Are there other factors that may affect peak CO concentrations 

41 
and not affect peak NO2 concentrations that should also be considered for 

any future CO monitoring proposal? The near-road NO2 concentrations 

are closely tied to secondary reactions with urban ozone concentrations 

and entrainment processes into highway line source NOx plumes. This in 

part, contributes to seasonal differences in near-road NO2 concentrations 

and its fractional contribution to NOx. Other factors affecting near-road 

NO2 monitoring is the distribution of gasoline and diesel vehicles. 

Example data analyses depicting the effects of these factors on NO2

measurements are available upon request. 

Questions regarding the PM monitoring network and near-road monitoring 

7. Does the committee believe that siting considerations for identifying the location 

of peak NO2 concentrations will likely address all of the high priority siting 

considerations for PM (particularly PM2.5) as well? If not, what other factors 

should be considered and what are the advantages in considering these factors for 

identifying the location of maximum PM concentration? Monitoring of number 

concentration of ultrafine particles has spatial and temporal characteristics that do 

not sync all that well with that of NO2. But it remains to be seen if the health 

community can make the case for health outcomes for particles <100nm. 

8. In addition to PM2.5 mass, what other PM-related measurements are desirable at 

near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM 

coarse, etc.)? Mobile measurement platforms are capable of performing fast 

response measurements (<1 minute) of key primary emission components (in 

addition to NO2) of interest to health effects community. These include aerosol 

size distribution, EC, PM organics (HOA, OOA), NH3, HONO, H2CO, CO, CO2

and 1,3 - butadiene. 

Questions regarding the monitor siting criteria for microscale CO, microscale PM2.5, 

and the new near-road NO2 siting criteria 

9. To allow for near-road monitoring infrastructure to be multi-pollutant, and in 

reflection of the recently promulgated near-road NO2 siting criteria, 

reconsideration of the existing microscale CO siting criteria presented in sections 

2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E may be warranted. Does the 

subcommittee believe that reconsideration of microscale CO siting criteria is 

appropriate? Specifically, would an adjustment of CO siting criteria to match 

those of microscale PM2.5 and microscale near-road NO2 sites be logical and 

appropriate? The CO siting criteria should be adjusted to match those of 

microscale PM2.5 and near-road NO2 so there is consistency in the near-road 

multi-pollutant monitoring infrastructure. 

10. Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and 

table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5 and 

microscale near-road NO2 is appropriate and proposed, should there be 

consideration to maintain the requirement on how urban street canyon or urban 

core microscale CO sites should be sited? Carry over of the urban street canyon or 

urban core microscale CO siting requirements should be done, keeping in mind 

the multi-pollutant consistency requirements mentioned in question (9). 

11. Does the subcommittee have an opinion on how “urban street canyons” or “urban 

core” might be defined, perhaps quantitatively, and with regard to use in potential 

42 
rule language? Consider emission density and spatial volume defining the urban 

street canyon and in the case of the urban core consider emission density and 

temporal persistence. 

12. EPA and NACAA will select the locations for permanent sites that are part of the 

near-road pilot study based on which state or locals volunteer to participate and 

can process grant funds in a timely manner to deploy equipment. From this pool 

of volunteers, selection should be made on certain attributes that provide the best 

potential to fulfill pilot study objectives. In the attached draft white paper, EPA 

and NACAA have proposed some potential criteria for consideration in selecting 

where the fixed, permanent stations should be located. These considerations 

include choosing a large and a relatively small urban area based on population, an 

area with varied or complex terrain, an urban area with an operational NOX

analyzer representative of neighborhood or larger spatial scales for comparison to 

the near-road NOX analyzer, and an urban area with a cooperative (or noncooperative) Department of Transportation. Does the Subcommittee agree with 

these considerations? Further, are there other considerations that should be 

evaluated in selecting pilot cities to house permanent near-road monitoring 

stations as part of the pilot study? EPA should competitively fund several 

extramurally pilot studies in conjunction with matching state environmental 

monitoring funds to address this question. 

13. EPA and NACAA have proposed that at least two urban areas should have 

permanent near-road monitoring stations (that would fulfill NO2 near-road 

monitoring requirements) implemented for the pilot study. Please comment on the 

minimum equipment/pollutant measurement complement that should be deployed 

at each site and also the ideal equipment complement that each site should or 

could have, respectively. Specifically, what pollutants (e.g., NO2, NOX, NO, CO, 

PM (Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene, 

xylene, formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other 

information should the pilot study measure or gather at the fixed, permanent 

monitoring stations, and by what methods? This list should be in priority order, as 

feasible, and can include any NAAQS or non-NAAQS pollutant by any method 

(FRM/FEM and/or non-reference or equivalent methods), any particular type of 

other equipment for gathering supporting data such as meteorology or traffic 

counts. Optimal deployment would consider one permanent near-road monitor 

station with a mobile measurement platform. The minimum complement of 

measurement parameters at the fixed site would include NO2, NOX, NO, CO, CO2, 

aerosol size distribution and total number concentration, PM2.5 and PM10 mass, 

EC/OC, BTEX and ammonia. The minimum complement of measurement 

parameters for the mobile platform include aerosol size distribution, EC, PM 

organics (HOA, OOA), BTEX, NH3, HONO, H2CO, CO, CO2 and 1,3 - 

butadiene. 

14. EPA and NACAA have proposed four to five urban areas to have saturation 

monitoring, using either passive devices and/or continuous/semi-continuous 

saturation type multi-pollutant monitoring packages (i.e., several types of 

monitors in one mountable or deployable “package”). Please provide comment 

on: I am not convinced this is a viable method. The only passive devices deployed 

43 
systematically for saturation monitoring have been Ogawa badges for NO2. Jury is 

still out on saturation monitoring using battery operated PM samplers. With some 

R&D investment in wireless unmanned saturation sensors/samplers, routine 

saturation monitoring could be in the future. The proposal to use of current 

passive measurement devices is less than inspiring. 

a. The pollutants that should be measured with the saturation devices at each 

saturation site. 

b. The number of saturation devices per pollutant, both passive and/or 

continuous/semi-continuous, that may be deployed in each pilot city. 

c. Whether placing saturation monitoring devices near certain road segments 

should include, at a minimum: 1) the highest AADT segment in an area, 2) 

the road segment with the highest number of heavy-duty truck/bus counts, 

3) at a road segment with more unique roadway design, congestion 

pattern, or terrain in the area, and 4) if feasible, at a lower AADT segment 

with a similar fleet mix, roadway design, congestion, terrain, and 

meteorology as the top AADT road segment in the area. 

44 
Dr. Delbert Eatough 

The nature of the advice which AAMMS might give to EPA on the Charge Questions 

outlined in the material provided to the committee will be dependent on the objectives of 

the multiple pollutants studies to be conducted as part of the near road monitoring 

program put in place in response to monitoring requirements outlined in the new NAAQS 

for NO2 released in February of this year (EPA CFR Parts 50 and 58, 2010). 

As stated by EPA in the Charge Questions document provided to AAMMS: 

Purpose of the Advisory

EPA is seeking CASAC advice on the concepts and information that should be 

included in the forthcoming near-road monitoring guidance document, advice on 

how future near-road monitoring requirements, for pollutants such as Carbon 

Monoxide (CO) and Particulate Matter (PM), may be drafted in a way to mesh with 

the existing Nitrogen Dioxide (NO2) requirements and foster a multi-pollutant 

monitoring infrastructure, and the objectives, approach, and execution of the nearroad monitoring pilot study. 

This is a bold new direction being taken by EPA which will move the concept of multipollutant monitoring in support of Clean Air objectives forward in a significant way. 

While reasonable detail is given in the charge questions related to the approach to be used 

and execution of the near-road monitoring pilot study, little detail is given on the 

scientific objectives of both the pilot study and the near-road monitoring program which 

is required under the NO2 NAAQS. I will start these comments by framing some of my 

thoughts on the possible objectives of this program, with the belief that the nature of the 

advice which might be given in very dependent on the identified objectives. I should 

emphasis that the literature cited in my comments is illustrative only and not intended to 

be a complete review of what is currently know. 

I. Objectives of the Near-Road Monitoring Program.

A. NO2 Monitoring Time Scale. 

As outlined by EPA, the advice must consider the near-road monitoring requirements of 

the NAAQS for NO2, which is to have ambient monitoring conducted at the location of 

maximum NO2 concentrations in an area, which at a minimum is directly attributable to 

mobile source emissions. While not explicitly stated in the charge questions, I assume 

that this means that the monitoring to be conducted will be focused on the 1-hour time 

period requirement of the NAAQS. This is an important point because the information 

one can gain from the near-road monitoring program is dependent on the time period 

chosen for study. For example, 1-hour average monitoring allows the identification of 

the effects of many diurnal variations which the identification of such details as diurnal 

changes in sources and atmospheric processes (Eatough, 2008). However, there are 

45 
important processes which occur on a much shorter time period which will not be as well 

identified in the program (Zhu 2002a, b). My comments here are limited to 

considerations for a 1-hour average monitoring program. 

B. NO2 Monitoring Objectives Other than Identification of Maximum 

Concentrations.

If we were discussing the identification of the maximum concentration of TSP Pb, the 

sampling objectives would be relatively straightforward because particulate Pb is a 

species which is expected to be relatively conserved in the atmosphere after emission. 

However, the story is much more complex with respect to NO2 if one wants to know, in 

addition to identification of maximum concentrations, the atmospheric factors which 

contributed to the maximum concentrations observed and the effect of the NO2 formation 

pathways on other NAAQS pollutants such a ozone, or the effect of ambient ozone on the 

formation of NO2. 

The great majority of primary nitrogen oxides are currently emitted from combustion 

sources in the form of NO (g) (Finlayson-Pitts, 2000). In the presence of ozone, HO2ڄ or 

RO2ڄ, NO is oxidized to NO2, 

 NO (g) + O3 (g) → NO2 (g) + O2 (g) (1)

NO (g) + HO2ڄ ሺg) → NO2 (g) + OHڄ) g) (2) 

 NO (g) + RO2 ڄ) g) → NO2 (g) + ROڄ) g) (3) 

Ozone, in turn is formed from the photolysis of NO2, OHڄ from the photolysis of O3 and 

46 
HO2ڄ and RO2ڄ from the reaction of OHڄ) and at night NO3ڄ (radicals with gas phase 

organic compounds. Thus, the relative concentrations of NOX and gas phase organic 

compounds control the concentration of ozone in a complex manner described by ozone 

isopleths (Finlayson-Pitts 2000), and, in turn, these processes control the concentrations 

of NO2. In general, at low NOX concentrations the concentration of ozone is little 

effected by the concentration of VOC and the system is NOX limited. However, at low 

concentrations of VOC, the concentrations of ozone can decrease with increasing NOX

concentrations as NO reacts with ozone and NO2 competes with VOC for the OHڄ radical 

by the irreversible formation of nitric acid, 

 NO2 (g) + OHڄ) g) → HNO3 (g) (4) 

Concentrations of NO2 observed at a site will be effected by this complex chemistry. 

Complete understanding of the etiology of NO2 concentrations identified in a near-road 

monitoring program will require the identification of each of these factors (Kuprov 

2010). I have assumed that understanding these chemical contributions to the observed 

NO2 concentrations will be one of the monitoring objectives. 

C. Multi-Pollutant Monitoring Objectives. 

The multi-pollutant monitoring portion of the near-road monitoring program plan being 

developed for the NO2 requirements could have several objectives: 

$ Identification of concentrations of other NAAQS pollutants which accompany the 

observed NO2 concentrations. 

$ Identification of the contribution these and other key pollutants make to the 

observed NO2 concentrations (see my comments in B.) 

$ Identification of the atmospheric processes which contribute to the observed 

concentrations of NO2 and the other monitored pollutants (e.g. Kuprov, 2010; 

Wilson 1977). 

$ Identification of the sources which contribute to both measured concentrations of 

NO2 and the other measured NAAQS pollutants (e.g. Eatough 2008). 

My comments assume that meeting all of these objectives is important in the design of 

the program. Another possible objective of the monitoring program is the identification 

47 
of toxic compounds to inform health studies. While this is an important objective, it is 

somewhat less directly related to the above and I have not given this objective high 

priority. 

I have not attempted to frame an individual response here to all charge questions as there 

are some where I am not an expert. The charge questions are given in the consensus 

report. 

II. Response to the Charge Questions. 

Charge Question 1. 

I suggest the following are areas where the Guidance Document may need strengthening 

above what I think is intended in the outline: 

• Introduction: Based on the material in the Pilot Study draft, it is not clear 

that EPA has yet identified the reasons (scientific objectives) for the multipollutant monitoring. I have discussed this issue in I.C. I recommend that EPA 

decide which of the scientific objectives outlined there are included in its vision (I 

have indicated I think all should be) and discuss fully these objectives and the 

scientific basis for these objectives in the Introduction. This will, in turn, support 

the selection of the recommended pollutants to be monitored in the program. 

• Background. The background should also contain the scientific basis for 

the multi-pollutant objectives, as outlined in the bullet above. 

• Identifying Candidate Near-road Site Areas. I believe the criteria outlined 

for this section are adequate for the identification of a site where maximum NO2

concentrations near a given near-road site may be determined. However, since 

NO2 is a secondary pollutant and its concentrations will be effected by both 

emissions from the roadway and from any other nearby elevated sources (VOC, 

ozone, etc.). These factors should be considered in the site identification process. 

• Modeling. I am not an expert in this area. However, modeling should 

take into account the factors I have discussed above. 

• Monitoring. One potential problem with saturation sampling is that if all 

data are not collected under identical conditions, certainly with respect to time 

and traffic flow, the comparison of results for the various saturation samplers may 

not be meaningful. I am also concerned about this issue in connection with the 

use of mobile monitoring. How will assurance be obtained that a comparison of 

measurements at two locations at different times gives the same result as a 

comparison of measurements at two different sites at the same time. How will 

diurnal variability be taken into account? These issues are discussed in the 

consensus report. 

48 
• Near-road Site Selection. The items outlined here seem reasonable. 

• Recommended Near-road Site Documentation. The adequacy of this 

section will be dependent on the adequacy of the EPA objectives for the Pilot 

Study in addressing the appropriate multi-pollutant monitoring objectives. This 

will be an area discussed under other Charge Questions. 

Charge Question 2. 

I have suggested in I.C. objectives that should be part of the multi-pollutant monitoring 

scheme. My thoughts on species which should be included to meet each of these 

scientific objectives (the objectives are repeated here) are given below. Again, all these 

measurements need to be made on a one-hour time basis. It is also recognized that most 

of these measurements will not be made at all sites but only at the limited number of 

advanced sites as discussed in the consensus document. 

• Identification of concentrations of other NAAQS pollutants which accompany the 

observed NO2 concentrations. (CO, PM10, PM2.5, Ozone and {probably for some, 

but not all sites} sulfur dioxide) 

• Identification of the contribution these and other key pollutants make to the 

observed NO2 concentrations (see my comments in B.) (VOC related to ozone 

formation, NOX, NOY {including a minimum of gas and particulate nitrate in 

addition to NOX}).

• Identification of the atmospheric processes which contribute to the observed 

concentrations of NO2 and the other monitored pollutants (e.g. Kuprov, 2010, 

Wilson 1977). (The species listed in the two proceeding bullets.)

• Identification of the sources which contribute to both measured concentrations of 

NO2 and the other measured NAAQS pollutants (e.g. Eatough 2008). (Fine 

particulate OC and EC, BC and UV C. In addition techniques are now becoming 

available for the hourly measurement of fine particulate elements and organic 

markers on an hourly basis. These last two measurements would be lower 

priority, but where they can be measured would greatly add to meeting this 

objective).

I have not listed any of the toxic gases included in the charge question, but they are 

relevant to health objectives and might be added if EPA wants to add an objective for this 

specific purpose. That is a little different than the atmospheric chemistry objectives on 

which I have focused. 

Charge Question 5. 

Saturation monitoring can aid greatly in the identification of a suitable near-road 

monitoring site. My only concern with respect to this Charge Question is that it be made 

49 
clear that the saturation monitoring must meet two key objectives: 

• The data must be available on a one-hour average basis, consistent with NAQQS 

requirement for NO2. 

• All saturation data must be obtained at all locations on the same time basis so the 

results are not significantly confounded by the diurnal and seasonal variations in 

NO2 emissions and formation chemistry. 

Because of the inherent problems in meeting the items in the above two bullets, care 

needs to be taken in the use of an outfitted vehicle to assist in the road-site selection 

process. 

Charge Question 7. 

While peak concentrations of ultrafine particles will frequently be associated with 

emissions from vehicles, the concentrations of PM2.5 will not. In almost all urban studies 

I am aware of, the maximum concentration of PM2.5 are not dominated by primary 

emissions but the secondary formation of nitrate and organic material, and in the east by 

regional sulfate. None of these contributions can be elucidated from near-road 

monitoring. However, the total pollutants suggested here to be monitored in the program 

can inform the secondary formation processes which lead to these elevated PM 

concentrations. 

Charge Question 8. 

I have outlined my thoughts on this charge question in the response to Charge Question 2. 

Charge Question 13.

I have listed my thoughts on equipment needed as outlined in this Charge question in my 

response to Charge Question 2, with an indication of contributions to be expected for 

each measurement. The relative priority which might be assigned depends on whether or 

not EPA agrees with my outline of objectives in Section I. It is recognized that these 

measurements will be made at a limited number of sites. My priority order and suggested 

measurements are: 

1. Top priority (measurement of NO2 and NAAQS pollutants): 

• NO2, hourly averaged data by an artifact free measurement as well as an 

FRM or FEM technique. 

• Ozone, hourly averaged data by an FRM or FEM technique. 

• PM10 and PM2.5 by a dichot FDMS TEOM method (to avoid the loss of 

volatile material). 

2. Second priority (measurement of species which will inform NO2 chemistry. 

50 
• VOC, hourly averaged data. 

• NOX, hourly averaged data by an FRM or FEM technique. 

• Ozone hourly averaged data by an FRM or FEM technique (also listed in 

1.). 

• NOY hourly averaged data. 

• Nitric acid and particulate phase nitrate, hourly averaged by an IC 

technique (e.g., the URG AIM) where nitrate is known to be high, e.g. LA 

or western mountain valleys. 

3. Third priority (data to aid in source apportionment, including separation of 

gasoline and diesel vehicle contributions). 

• Hourly average EC and OC, preferable by a Sunset dual oven instrument. 

• BC and UV hourly average Aethalometer data. 

• Hourly averaged fine particulate elemental and trace organic marker data. 

Charge Question 14. 

 

First a general comment. The saturation studies are intended to aid in the 

identification of near-road sites which will give maximum NO2 concentrations. 

These then will become the site(s) which are used to meet the NO2 near-road 

monitoring requirements. To meet this requirement the key data each saturation 

study must provide are hourly average NO2 concentrations which define at least a 

couple of weeks diurnal variation in the NO2 concentrations. Less than hourly and 

less than complete diurnal coverage will not truly inform on maximum 

concentrations. I am not certain that a passive device can meet this need, so I assume 

a semi-continuous device or a modified passive sampler (see the consensus report) 

would be used. 

a. NO2 

.

b. I would think 4 to 6 is a reasonable number, but I defer to others who have 

conducted saturation studies. 

c. Of the criteria listed in the charge question, 1) and 2) seem most important. I 

would also pick a site where impact from VOCs nearby is important. 

REFERENCES 

Eatough, D.J., Grover, B.D., Woolwine, W.R., Eatough, N.L., Long, R., and Farber, 

R. (2008). “Source Apportionment of 1-hr Semi-Continuous Data from Riverside 

During SOAR 2005 Using Positive Matrix Factorization.” Atmos. Environ. 42: 2706-

2719. 

EPA (2010). 40 CFR Parts 50 and 58, Primary National Ambient Air Quality 

51 
Standards for Nitrogen Dioxide; Final Rule, Federal Register, February 9, 2010, pp 

6474-6537. 

Finlayson-Pitts B.J. and Pitts, J.N. Jr. (2000) Chemistry of the Upper and Lower 

Atmosphere: Theory, Experiments and Applications, Academic Press. 

Kuprov R., Eatough D.J., Hansen J.C., Cruickshank T. And Olson N. (2010)

Composition and Secondary Formation of Fine particulate Material in the Salt Lake 

Valley: Winter 2009, J. Air & Waste manage. Assoc., in press. 

Wilson, W.E., Spiller, L.L., Ellestad, T.G., Lamothe, P.J., Dzubay, T.D., Stevens, 

R.K., Macias, E.S., Fletcher, R.A., Husar, J.D., Husar, R.B., Whitbe, K.T., Kittelson, 

D.B. and Cantrell, B.K. (1977) “General Motors sulfate Dispersion Experiment: 

Summary of EPA Measurements.” J. Air Pollut. Cont. Assoc. 27: 46-51.

Zhu, Y., Hinds, W.C., Kim, S., and Sioutas C. (2002a) “Concentration and Size 

Distribution of Ultrafine Particles Near a Major Highway.” J. Air & Waste Manage. 

Assoc. 52: 1032-1042. 

Zhu, Y., Hinds, W.C., Kim, S., Shen S., and Sioutas C. (2002b) “Study of Ultrafine 

Particles Near a Major Highway with Heavy-duty Diesel Traffic.” At. Environ. 36: 

4323-4335. 

52 
Mr. Dirk Felton 

Questions regarding the near-road monitoring guidance document 

1. Please comment on the overall content of the recommended topics in the 

draft outline. Please provide suggestions on any missing subjects that should 

be included in the guidance document and any unnecessary topics that are 

currently listed in the attached draft, if applicable. 

The background section should include language suggesting that finding one near-road 

location where all pollutants of interest have the highest concentrations is not likely and 

compromises will be necessary and acceptable. For example, NO2 concentrations may 

well be higher in dense urban neighborhoods away from well ventilated busy highways. 

This section should also include a discussion of the limitations of the various monitoring 

methods in the near-road environment. The PM-2.5 FRM has demonstrated poor capture 

efficiency for volatile fresh emissions from mobile sources and CO monitoring for a 

health based NAAQS near roadways may not be warranted. 

Modeling and saturation or mobile monitoring should not be required in the site selection 

process. This type of work is beyond the capacity of many monitoring agencies, and 

since these methods have not been uniformly demonstrated or well documented, they are 

not likely to provide much assistance. If monitoring agencies have existing information 

that could provide this type of information, they certainly should consider the information 

and make it available to their Regional EPA office during the site approval process. 

The last section on site documentation should not be burdensome to the monitoring 

agencies. Much of the “NCore type” of site characterization documentation was 

designed so that EPA staff would have a convenient way to review monitor siting. If this 

type of information is important to the EPA then they can collect the data when they visit 

the sites. 

2. EPA and NACAA envision the near-road guidance document to be written 

from a multi-pollutant perspective. What pollutants and sub-species does 

the subcommittee believe should be included for consideration and 

discussion in the near-road monitoring guidance? Some potential species for 

consideration include NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black 

carbon, air toxics (e.g., benzene, toluene, xylene, formaldehyde, acrolein, or 

1, 3, butadiene), and ammonia. Please prioritize the recommended 

pollutants and provide the rationale for their ranking, including how this 

pollutant measurement will contribute to scientific and regulatory 

knowledge of near-road air quality and adverse human health effects. 

The discussion of potential species for consideration in the near road monitoring 

guidance is complicated by the currently available methods for some of these pollutants. 

PM and specifically PM-2.5 would certainly be considered to be a candidate but the EPA 

does not have an acceptable method for measuring PM-2.5 in the near-road environment. 

The EPA should consider the development of a suitable method for PM-2.5 to be of the 

utmost necessity. 

CO monitoring is recommended and the siting requirements should be modified to be 

identical to the near road NO2 requirements. CO may or may not be found at high 

53 
enough concentrations to be a health issue, but it will be useful as a tracer of primary 

emissions. 

Ultrafine monitoring is recommended but an appropriate minimum and maximum size 

range must be considered. Ultrafine particles have been found to affect health and data 

from near road sites will assist in the understanding of how mobile sources interact with 

ambient ultrafine levels. 

BC is a combustion tracer so it should only be contemplated for installation at a near road 

site if there is another BC instrument within the same CBSA. Since the NATTs program 

requires BC, it makes sense to only install these instruments in the near road sites that 

also have a NATTs site. This limited deployment should provide some information on 

the gradient of BC between the near road environment and a central monitor. 

This program should include air toxics that have been identified as known human 

carcinogens. This would include benzene, formaldehyde and 1, 3 - butadiene. Air toxics 

monitoring at the near road air sites should be encouraged in the CBSAs that also have a 

NATTs site. This would allow for comparisons between the near road and community 

scale monitors. Over time, the changes in vehicle technology and fuels will continue to 

impact concentrations of these air toxics which large segments of the population are 

exposed to daily. Currently, concentrations of these specific air toxics already exceed 

acceptable concentrations in many areas of the country. A critical evaluation of the 

impacts of these changes on the concentration of these air toxics in urban areas across the 

U.S. is extremely important.

3. Identifying Candidate Near-road Site Areas 

a. AADT & Fleet Mix – does the subcommittee believe this approach is 

an appropriate way to “consider” fleet mix in near-road site selection 

or is a more refined inventory and modeling analysis required? 

Monitoring agencies should be permitted to use the best available information in order to 

help with site selection. There are too many disadvantages to specifying a one size fits 

all standard approach to site selection. Each CBSA has its own set of variables such as 

roadway restrictions on vehicle type or vehicle type at certain hours of the day, tolls that 

vary as a function of vehicle type, bridge restrictions, weather conditions such as cold 

weather or high winds and differences in required vehicle emission controls. 

Additionally, congestion in dense urban areas with numerous roadways in close 

proximity to one another can lead to higher NO2 concentrations than AADT and fleet mix 

would indicate. 

Some monitoring agencies also have very good inventories and existing modeling work 

done for specific CBSAs and not as much information for others. The guidance 

document should encourage the monitoring agencies to utilize all available tools and 

sources of information to help select the most appropriate monitoring locations.

b. AADT & Fleet Mix – Further, should the suggested approach above 

in question 4a to consider fleet mix via the use of average, fleet-wide 

emission factors, or the use of inventory and modeling analysis, take 

into account mobile source controls that are “on the books” but have 

not yet been fully realized due to fleet turnover? If so, how far out into 

the future should states consider their effects? 

Ambient monitoring is concerned with the current exposure to the population from 

sources of pollutants. Fleet turnover in most cases is a long term process especially for 

54 
the largest trucks unless there is a specific program in place to help replace these vehicles 

more quickly. The EPA has the ability to review monitor siting once a year when the 

monitoring agencies submit their annual network plan. This should provide ample 

opportunity for the EPA to suggest changes to the monitoring network in light of changes 

in AADT and fleet turnover. 

c. Roadway Design – Does the subcommittee agree with this 

recommendation for locating sites at-grade with no obstructions? 

What priority should be placed on this factor within the guidance, 

given the need for flexibility in identifying appropriate site locations? 

The design of the roadway is extremely significant and it makes sense to recommend that 

the preferred installation be at grade with no obstructions. That said, the guidance 

document should expand on why this is important and where low or high concentrations 

are likely to be found. Low concentrations may be found adjacent to roadways elevated 

on piers, near coastlines or large water bodies. High concentrations may be found 

adjacent to roadways situated in dense urban areas surrounded by tall buildings, next to 

below grade roadways or near roadways that are also influenced by tunnel ventilation 

systems. These high concentration locations may be the preferred locations for some 

CBSAs particularly if they are also significant for population exposure. 

d. Congestion Patterns – how important a parameter should LOS be in 

the determination of appropriate near-road monitoring sites? Does 

the subcommittee have a view on how reliable LOS estimates are 

across the country? 

It is likely that on stagnant days the roadways with poor level of service, including 

congested slow moving traffic will cause levels of pollutants to accumulate to higher than 

expected levels. LOS information should be used to qualify the AADT data on a CBSA 

specific basis. 

e. Terrain – Does the subcommittee agree that terrain and vegetation 

should be a consideration in the siting process? What priority should 

this parameter have in the overall process?

Terrain and vegetation are very important and should rank near the top of the site 

selection criteria. Both of these factors drastically affect path length, population 

exposure, effective probe height and an agency’s ability to site a monitor. Vegetation can 

be a sink of many pollutants as well as a screen for the efficient transport of pollutants 

between the source and the monitor. 

f. Meteorology - Although there is no requirement to be downwind, in 

the preamble to final NO2 NAAQS rule, EPA encouraged it when 

possible. EPA and NACAA intend to do the same in the guidance 

document. Does the subcommittee agree with this approach? 

The importance of other factors such as AADT, terrain and population exposure should 

take priority over the prevailing wind direction. If the monitoring agency has candidate 

locations that are otherwise equal it is preferred to select the one that is predominantly 

downwind from the near road emissions. 

4. Modeling is another tool that may be useful in the identification of candidate 

near-road sites. In particular, the use of mobile source emissions modeling 

with MOVES and local-scale dispersion modeling with AERMOD, can be 

presented as part of the guidance document. Please comment on the 

55 
available modeling tools, and their pros and cons, that the subcommittee 

believes may be appropriate to discuss and/or recommend for use in the 

near-road monitoring guidance document. 

5. In regard to the process of identifying candidate near-road monitoring sites, 

beyond the evaluation of factors noted above in question 3, and the potential 

use of modeling, the use of saturation monitoring and on-road monitoring 

are also possible tools that state and local air agencies may choose to utilize 

in the near-road site selection process. 

General Comment: All of these tools are resource intensive and are likely to be beyond 

the scope of many monitoring agencies. The use of these tools should not be required. 

a. what are the pros and cons to using passive devices to saturate an area 

to gather data? 

The NO2 standard is a 3-Yr average of 1-Hr maximum values, so a typical passive 

sampler which is exposed for days to weeks will provide better information if the traffic 

signature is relatively consistent throughout the course of a day. This is likely to be the 

case in the largest CBSAs. Passive samplers will not provide as much information in 

smaller CBSAs with variable traffic congestion and the affects of other significant 

sources of pollutants. 

b. Likewise, what are the pros and cons to using non-passive devices, 

such as near real-time or continuous devices including, but not limited 

to portable, non-FEM chemiluminescence methods for NO2 or Gas 

Sensitive Semiconductors (GSSs) for NO2 and other pollutants of 

interest?

The concept is good but, of course, the real issue with many of these instruments is the 

comparability to data from the FRM. If a monitoring agency is able to use several of 

these instruments to look at potential sites in relation to each other, not to a nearby FRM, 

then these non-regulatory instruments could be useful. 

c. Finally, what would be the pros and cons, to a state or local agency 

attempting to use a specially outfitted vehicle to collect mobile 

measurements to assist in the near-road site selection process for NO2

specifically as well as other pollutants of interest?

This could be a very expensive component of the site selection process and would only 

be likely to occur if the monitoring agency already had access to this type of vehicle. The 

advantage is that candidate areas can be identified relatively quickly compared to other 

screening methods. The disadvantages to these types of measurements are that they are a 

snapshot in time that could have little relationship to the periods when the concentrations 

are expected to be the highest. 

Questions regarding the CO monitoring network and near-road monitoring 

6. The two issues are 1) the consideration of where light duty vehicles are 

operating under ‘cold-start’ conditions, which may often not be on the 

larger arterials or highways in an area, and 2) the impacts of light duty 

vehicle congestion and idling in areas such as urban street canyons and/or 

urban cores. Questions regarding the PM monitoring network and near-road 

monitoring 

56 
7. Does the committee believe that siting considerations for identifying the 

location of peak NO2 concentrations will likely address all of the high 

priority siting considerations for PM (particularly PM2.5) as well? If not, 

what other factors should be considered and what are the advantages in 

considering these factors for identifying the location of maximum PM 

concentration? 

The answer to this question is complicated by the fact that the NO2 regulation incorrectly 

identifies near-road environments as the areas where NO2 concentrations will always be 

highest. Secondly, the traditional method of measuring PM-2.5, the FRM is not well 

suited to capturing the highly volatile emissions emitted from mobile sources. Thirdly, a 

significant portion of PM-2.5 in most areas is either due to transport or due to secondary 

particle formation, neither of which have anything to do with micro-scale NO2 siting 

criteria. 

A PM-2.5 network design that only includes locations that are near-road micro-scale sites 

will only provide information about that source. An adequate PM-2.5 network must 

include sites that also provide information that are relevant for upwind assessment, 

population exposure, seasonal differences that can be evaluated in terms of particle 

formation and lastly micro-scale source attribution. 

PM-2.5 is more likely to be highest in dense urban neighborhoods away from the well 

ventilated roadways. These areas are subject to emissions from transport, domestic 

heating and cooking, stationary sources and mobile sources.

8. In addition to PM2.5 mass, what other PM-related measurements are 

desirable at near-road monitoring stations (e.g., UFP number, black carbon, 

EC/OC, PM coarse, etc.)? 

The EPA must define how it intends to collect PM-2.5 mass. The FRM will miss a 

significant fraction of the volatile component of the mobile source emissions. This will 

create the situation where the apparent risk from these sources or from living in one of 

these areas is under estimated. In general, it is not advisable to err on the side of 

underestimating a health risk when establishing a NAAQS oriented monitoring program. 

It is preferable to delay a requirement to monitor PM-2.5 at these locations until the EPA 

develops a suitable method that reliably includes a majority of the volatile component of 

PM-2.5. One suggestion is for the EPA to encourage the manufacturers of continuous 

PM-2.5 monitors to develop instruments without the FEM algorithms that reduce PM 

concentrations in an attempt to emulate the filter based FRM. This will provide a more 

realistic indication of the actual PM concentrations that the populations in these areas are 

exposed to everyday. 

UFP number monitoring is on its way to becoming an acceptable monitoring technique 

but it is not quite ready for routine use. The EPA should invest in limited deployment 

monitoring demonstrations to assist the vendors as they develop better more reliable 

instrumentation. Some of the issues that need to be resolved include, how small do we 

need to go, which bins are appropriate from a health perspective and how do we QA 

these instruments. UFP data could provide valuable information for the health 

community as they investigate air quality related health effects. 

The EPA has a BC monitoring requirement in place at the NATTs sites but this data is 

rarely used. The data is also subject to artifacts due to filter changes and interference 

from co-pollutants on the filter substrate. The EPA should really determine if there is a 

57 
need for this data before the requirement to monitor for a non-criteria pollutant is added 

to this network. 

EC/OC data is more interesting because it provides information that can help regulators 

understand the losses from the PM-2.5 FRM and to develop potential pollution control 

programs. This information may only be necessary in the MSAs where the PM NAAQS 

could be exceeded and control strategies are required. 

PMcoarse is of limited value because the quality of the PM-2.5 data is poor and because 

of the small scale that the resultant data represents. 

Questions regarding the monitor siting criteria for microscale CO, microscale PM2.5, 

and the new near-road NO2 siting criteria 

9. Does the subcommittee believe that reconsideration of microscale CO siting 

criteria is appropriate? Specifically, would an adjustment of CO siting 

criteria to match those of microscale PM2.5 and microscale near-road NO2

sites be logical and appropriate? 

The 3 questions (9,10 and 11) assume that there are high enough CO concentrations at 

typical near-road locations to justify including health related NAAQS CO monitors at 

these sites. The EPA must determine if this is the case before establishing a new, 

expensive and potentially un-necessary monitoring requirement. 

The siting criteria for CO does need to be updated if CO is included at a near-road 

monitoring location. The more restrictive height requirement is not necessary for sites 

where turbulence is expected to create well mixed conditions at these monitors. The 

criteria used for micro-scale PM and NO2 is acceptable. 

10. should there be consideration to maintain the requirement on how urban 

street canyon or urban core microscale CO sites should be sited? 

The micro-scale PM and NO2 siting requirements are adequate for street canyon CO 

monitoring. The existing CO canyon monitoring guidance included a wind direction 

provision that is un-necessary and made it more difficult to find suitable locations. 

11. Does the subcommittee have an opinion on how “urban street canyons” or 

“urban core” might be defined, perhaps quantitatively, and with regard to 

use in potential rule language? 

It is useful to be able to refer to an area as a street canyon or as an urban core. Street 

canyons are relatively easy to define because they are defined by the structures in their 

immediate vicinity. Urban Cores have to be defined on a larger MSA basis. 

Street Canyons have the following attributes. They are relatively narrow in comparison 

to the height of the structures on either side of the street. It should be relatively easy to 

model an optimum or minimum width to height ratio that would define a street canyon. 

A ratio of (1) width to (1 or 1.5) height might be a good starting point. 

Urban Cores can be defined by a number of parameters such as the reasonable geographic 

center of an MSA, the approximate centroid of emissions sources, area of densest 

population or highest congestion. Very large MSAs could easily have multiple urban 

cores due to geography and neighborhood layout such as in NYC where boroughs in 

some ways can emulate small cities.

58 
Questions regarding the near-road monitoring pilot study 

The EPA ORD has been operating near-road monitoring locations for more than a year in 

a couple of locations including Las Vegas. The results from these campaigns should be 

used to determine if the NO2 and other pollutant concentrations were high enough to 

warrant further development of near-road monitoring efforts. The data should also be 

evaluated to determine if the selected monitoring methods were appropriate for these 

locations. 

12. Does the Subcommittee agree with these considerations? Further, are there 

other considerations that should be evaluated in selecting pilot cities to 

house permanent near-road monitoring stations as part of the pilot study?

The reliance on a CBSA’s population is too simplistic. The population of a CBSA does 

not specifically provide information about how roadways are used within a CBSA. For 

example, this ranking does not include the number of people who commute from outside 

of the CBSA every day or use car pools or mass transit including busses, trains, subways 

and ferries. 

13. Please comment on the minimum equipment/pollutant measurement 

complement that should be deployed at each site and also the ideal

equipment complement that each site should or could have, respectively. 

Specifically, what pollutants (e.g., NO2, NOX, NO, CO, PM (Ultrafine, 2.5, 

and 10), black carbon, air toxics (such as benzene, toluene, xylene, 

formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other 

information should the pilot study measure or gather at the fixed, 

permanent monitoring stations, and by what methods? This list should be in 

priority order, as feasible, and can include any NAAQS or non-NAAQS 

pollutant by any method (FRM/FEM and/or non-reference or equivalent 

methods), any particular type of other equipment for gathering supporting 

data such as meteorology or traffic counts. 

It is acceptable to deploy more monitoring parameters at the pilot sites than at the routine 

sites, however, the EPA must be aware of the resources necessary to operate these sites. 

The pilot sites should have NO2, CO, ultrafine and BC monitors. PM-2.5 should be 

included only if PM-2.5 FEMs are operated without the algorithms that reduce the mass 

to match the FRM. These instruments could utilize the EPA parameter description 88500 

which cannot be compared to the PM2.5 NAAQS and is defined as total atmospheric 

PM-2.5. 

The pilot sites should be selected based on the availability of NCore and NATTs sites 

within the same CBSA to permit data comparisons with the near-road site. 

14. EPA and NACAA have proposed four to five urban areas to have saturation 

monitoring, using either passive devices and/or continuous/semi-continuous 

saturation type multi-pollutant monitoring packages (i.e., several types of 

monitors in one mountable or deployable “package”). Please provide 

comment on: 

The deployable semi-continuous package that EPA is considering is not practical for use 

by monitoring agencies. The unit is expensive, may or may not provide data that is 

comparable to criteria monitors and would require power and security wherever it is 

deployed. 

59 
a. The pollutants that should be measured with the saturation devices at 

each saturation site. 

The saturation devices only have to be able to monitor NO2. Once the high 

NO2 site is found, other parameters can be included when the monitoring site 

is fully established. 

b. The number of saturation devices per pollutant, both passive and/or 

continuous/semi-continuous, that may be deployed in each pilot city. 

The most effective use of saturation samplers is to help rank the top 2-4 sites 

that have been identified through the rest of the site selection process. It 

would be too expensive and labor intensive to use saturation monitoring to 

select sites across wide areas in a CBSA. 

c. Whether placing saturation monitoring devices near certain road 

segments should include, at a minimum: 1) the highest AADT segment 

in an area, 2) the road segment with the highest number of heavy-duty 

truck/bus counts, 3) at a road segment with more unique roadway 

design, congestion pattern, or terrain in the area, and 4) if feasible, at 

a lower AADT segment with a similar fleet mix, roadway design, 

congestion, terrain, and meteorology as the top AADT road segment 

in the area. 

It is sensible to use saturation monitors to compare sites that cannot be ranked 

in another way. The monitoring agency should include the feasibility of 

establishing a monitor in a specific area in the initial consideration of potential 

sites. Saturation samplers can help the agency differentiate a feasible site on 

one type of roadway to another feasible site on another type of roadway. 

60 
Dr. Kazuhiko Ito 

Note: The charge questions below are truncated to save space. 

General Comment: 

I understand the importance of measuring NO2 near roadways, but I am very concerned 

about a possible reduction in the number of community-wide monitors that can result 

from the new network plan. According to EPA, there are currently about 400 NO2

monitors nationwide, many of which are community-wide monitors, and very few (3?) 

are micro-scale monitors. The new network plan will create 126 near-road monitors, but 

only 53 community-wide monitors will be required to operate. The epidemiological 

studies that reported short-term associations between NO2 and respiratory morbidity used 

the community-wide monitors. Because the near-road monitors may not adequately 

reflect the entire city’s residents’ exposures, we may be reducing the number of NO2

monitors that can be used for community based epidemiological studies. I have more 

comments on this as part of response to Charge Question 2. 

Questions regarding the near-road monitoring guidance document 

Charge Question 1: Please comment on the overall content of the recommended topics 

in the draft outline (of the near-road monitoring guidance document). Please provide 

suggestions on any missing subjects that should be included in the guidance document 

and any unnecessary topics that are currently listed in the attached draft, if applicable. 

Comment: 

The overall content and the topics listed in the outline look generally adequate. I 

have a few comments below. 

In addition to the literature review mentioned as part of Background, EPA can 

conduct analysis of the existing data (though the monitors are not sited in the required 

near-road scale) to describe the relationship between NO2 and other pollutants (e.g., CO, 

EC, etc.). 

In identifying candidate near-road areas, it may not be just AADT but the density 

of high AADT areas that are important, particularly in cities with high population density 

areas. 

 

Charge Question 2: What pollutants and sub-species does the subcommittee believe 

should be included for consideration and discussion in the near-road monitoring 

guidance? Please prioritize the recommended pollutants and provide the rationale for 

their ranking, including how this pollutant measurement will contribute to scientific and 

regulatory knowledge of near-road air quality and adverse human health effects. 

61 
Comment: 

I need to first comment on my general rationale of measuring co-pollutants at the 

proposed near-road monitoring location before discussing the prioritizing co-pollutants. I 

feel that there is a gap between the recognition of the short-term associations between 

NO2 and respiratory morbidity in the observational epidemiological studies, the results 

from human laboratory studies, and the suggestion to measure hourly NO2 values at nearroad locations and set a standard level at such locations with the 1-hr averaging time. I 

understand that this has been already decided, but I think we should keep the options to 

evaluate the implication of this decision in case we may need to reconsider this strategy. 

The issues I need to raise are the following: 

• The observational studies that reported short-term associations between NO2 and 

respiratory morbidity mentioned in the FR (40 CFR Parts 50 and 58) used NO2 data from 

monitors that were not near-road monitors. Unless the associations were due to 

exacerbations of respiratory conditions in the sub-populations who lived near roads, it is 

possible that the data to be collected at near-road monitors may not correlate well with 

the measurement at the existing NO2 monitors, and they may not even correlate well with 

the citywide respiratory morbidity time-series because the near-road measurements may 

be highly influence by local sources that are not relevant to the rest of the city. 

• Correlations between NO2 and co-pollutants at near-road monitors may be different 

from those at non-near-road monitors (i.e., those used in the observational 

epidemiological studies). Therefore, it is possible that these co-pollutants measured at 

near-road monitors have different impacts on the health effects models compared to those 

reported in the past epidemiological studies. 

 Because of these issues, I think it is essential to retain at least some of the existing 

non-NR NO2 monitors until the issues are resolved. Otherwise, it is possible that we end 

up with discontinuation of data that are useful for epidemiological studies. 

The review of epidemiological studies in the NO2 ISA concluded that the 

associations between NO2 and respiratory morbidity were robust to the inclusion of other 

co-pollutants (i.e., no strong indication of confounding) in the health effects models, but 

these co-pollutants were mostly other criteria pollutants (PM10, PM2.5, ozone, CO, SO2) 

with a few exceptions in studies outside US that examined non-criteria pollutants (e.g., 

benzene, coarse particles, ultra-fine particles). Determining confounding by co-pollutants 

that come from the same source(s) as NO2 is methodologically difficult, but it would be 

beneficial to measure, at least in a pilot study, the pollutants that may also have the health 

effects (including effect modification or synergism) or may be useful as markers to 

distinguish among the sources that emit NO2. When the EPA mentions “a multi-pollutant 

perspective”, I assume that there are several rationales for the idea. Ranking/prioritizing 

62 
individual pollutants is difficult because there are uncertainties about the importance of 

specific pollutants, but they can be grouped in terms of the rationale for measuring them, 

and the rationale may be ranked. 

Pollutant group Rationale (objective and feasibility) and 

comment 

NO2, NO, NOx • Same instrument 

CO • Marker of gasoline vehicles 

• Multi-pollutant assessment using the 

instruments already used for NAAQS 

• Potential health relevance 

Black carbon • Diesel traffic marker 

• Potential health effects 

Ultra-fine particles / particle number 

concentrations 

• Potential health relevance 

• Expected to be high near-roadways 

PM10-2.5, and their chemical 

constituents such as Br, Zn, Cu, Sb 

Traffic: re-suspended dust, tire ware, brake 

ware. 

PM2.5 and its chemical constituents 

such as EC, OC 

• To determine “excess PM2.5” beyond urban 

background 

• Potential health effects 

SO2 • Sources other than traffic that produce NO2. 

Charge Question 3 (Identifying Candidate Near-road Site Areas): 

Comment: All of these items (a) through (f) (and population density, which was 

mentioned in the outline but not here) seem important to consider in identifying candidate 

near-road sites. However, without actually determining the relationships between these 

factors and the NO2 levels, presumably through the pilot project, it would be difficult to 

evaluate the adequacy of siting a monitor based on the information alone. EPA should 

look for studies that attempted to investigate these issues. The data from the New York 

City Community Air Survey (NYCCAS) may be useful. They have been conducting 2-

week sampling measurements of NO2, SO2 (winter only), ozone, PM2.5 and its chemical 

63 
constituents at 150 locations within New York City. They are conducting land-use 

regression of the measured pollutants including NO2 as a function of a number of geocoded emission data including traffic volume and other local combustion sources (e.g., 

residual oil burning). The information from the analysis of the NYCCAS data may be 

useful in evaluating the limitation of AADT data. 

Charge Question 4: Please comment on the available modeling tools (e.g., MOVES, 

AERMOD, etc.), and their pros and cons, that the subcommittee believes may be 

appropriate to discuss and/or recommend for use in the near-road monitoring guidance 

document. 

Comment: I have not used these models and have not seen the model validation of these 

models as applied to NO2 and other traffic air pollution. Therefore, I cannot comment on 

this. 

Charge Question 5: The use of saturation monitoring and on-road monitoring are also 

possible tools that state and local air agencies may choose to utilize in the near-road site 

selection process. 

5 (a): What are the pros and cons to using passive devices to saturate an area to gather 

data? 

Comment: The obvious pros include the low cost and small dimension. The obvious 

cons include the long sampling period required for the detection limit of the passive 

sampler. However, the spatial distribution of NO2 constructed from such sampling would 

be still useful in determining the siting of a sampler. The relationship between the 1-hr 

peak NO2 data and the data from passive samplers can be determined from a pilot study. 

The NYCCAS data mentioned above may be useful to do this, since the study already 

collected 2-years of data (to identify the high NO2 area) and the study is still going on (to 

measure hourly data at the high NO2 areas to compare the two-week vs. hourly data). 

5 (b): What are the pros and cons to using non-passive devices, such as near real-time or 

continuous devices including, but not limited to portable, non-FEM chemiluminescence 

methods for NO2 or Gas Sensitive Semiconductors (GSSs) for NO2 and other pollutants 

of interest? 

Comment: The pros: Ability to measure hourly data: The cons: Need to validate the 

correspondence with the FRM/FEM measurements. A pilot study is required for this. 

5 (c): What would be the pros and cons, to a state or local agency attempting to use a 

specially outfitted vehicle to collect mobile measurements? 

64 
Comment: I am not sure what the pros would be unless all the other information leaves a 

kind of uncertainty that can be resolved by the mobile measurements. The cons include 

the cost and resources required to conduct the measurements. 

Questions regarding the CO monitoring network and near-road monitoring 

Charge Question 6 (a): Does the subcommittee believe that the light duty cold start and 

congestion factors will significantly influence the location of peak CO concentrations in 

an area? 

Charge Question 6 (b): Does the subcommittee have an opinion on whether, and 

possibly how, these two issues of vehicles operating under cold start conditions and light 

duty vehicle congestion and idling in urban street canyons and/or urban cores be 

considered in a future, nationally applicable, CO monitoring proposal? 

Comment: I don’t know the data regarding the impact of cold start conditions on CO 

peaks and therefore cannot comment. 

Questions regarding the PM monitoring network and near-road monitoring 

Charge Question 7: Does the committee believe that siting considerations for identifying 

the location of peak NO2 concentrations will likely address all of the high priority siting 

considerations for PM (particularly PM2.5) as well? If not, what other factors should be 

considered and what are the advantages in considering these factors for identifying the 

location of maximum PM concentration? 

Comment: It depends on the region of US, but PM (particularly PM2.5) may be 

dominated by regional background PM levels, so the impact of the near-road pollution on 

the monitor will need to take into consideration (subtract) the data from non-NR PM 

monitor. I guess the NCore sites will be sufficient for this purpose where they exist. 

Charge Question 8: In addition to PM2.5 mass, what other PM-related measurements are 

desirable at near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM 

coarse, etc.)? 

Comment: All of these would be “desirable”, but are funds available to measure these in 

the new near-road monitors? 

Questions regarding the monitor siting criteria for microscale CO, microscale PM2.5, 

and the new near-road NO2 siting criteria 

Charge Question 9: Does the subcommittee believe that reconsideration of microscale 

CO siting criteria is appropriate? Specifically, would an adjustment of CO siting criteria 

to match those of microscale PM2.5 and microscale near-road NO2 sites be logical and 

appropriate? 

65 
Comment: I appreciate the historical background and the original reasons for the siting 

criteria for different NAAQS pollutants. None of these criteria of data collection were 

originally meant for epidemiological studies. However, since the researchers used these 

data from the regulatory monitors for observational epidemiological studies, and because 

the findings from these studies are in part influencing the process of setting NAAQS, it is 

inevitable that the siting criteria will need to accommodate the need to use the data for 

epidemiological investigation. These studies often use multi-pollutant regression models 

to examine potential confounding effects, which tacitly assumes that pollution variables 

equally represent the population exposures. The reported short-term associations 

between CO and mortality and cardiovascular morbidity raise a concern that these 

associations are observed despite the potential inadequacy of the exposure metric to 

represent population exposure (i.e., potential attenuation of associations). For this reason, 

I think it is appropriate to adjust CO siting criteria to match those of microscale PM2.5 and 

microscale near-road NO2 sites. 

Charge Question 10: Even if the adjustment of microscale CO siting criteria in sections 

2, 6.2, and table E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5

and microscale near-road NO2 is appropriate and proposed, should there be consideration 

to maintain the requirement on how urban street canyon or urban core microscale CO 

sites should be sited? 

Comment: If we are assuming that there can be CO monitors for different purposes (and 

there is sufficient funds), yes, I think there should be consideration to maintain CO 

monitors that will measure the maximum impact that is separate from the population 

epidemiology. 

Charge Question 11: Does the subcommittee have an opinion on how “urban street 

canyons” or “urban core” might be defined, perhaps quantitatively, and with regard to use 

in potential rule language? 

Comment: Establishing such definitions would require some analysis of available data 

to characterize the relationship between the pollution levels and emission/environment 

conditions (AADT, building density, etc.). 

Questions regarding the near-road monitoring pilot study 

 

Charge Question 12: EPA and NACAA will select the locations for permanent sites ... 

these considerations include choosing a large and a relatively small urban area based on 

population, an area with varied or complex terrain, an urban area with an operational 

NOX analyzer ... Does the Subcommittee agree with these considerations? Further, are 

there other considerations that should be evaluated in selecting pilot cities to house 

permanent near-road monitoring stations as part of the pilot study? 

66 
Comment: Given the limited budget, EPA should consider the cities that already have 

infrastructure to conduct a pilot study or the cities that are already conducting multipollutant assessment at multiple locations. Atlanta and NYC come to my mind. 

Charge Question 13: EPA and NACAA have proposed that at least two urban areas 

should have permanent near-road monitoring stations (that would fulfill NO2 near-road 

monitoring requirements) implemented for the pilot study... Specifically, what pollutants 

and other information should the pilot study measure or gather at the fixed, permanent 

monitoring stations, and by what methods? 

Comment: See my comment on Charge Question 2 for the list of pollutants. In terms of 

other information any geo-coded information related to traffic and other emission sources 

would be useful. The reports from the NYCCAS project for such information may be 

useful (available from http://www.nyc.gov/html/doh/html/eode/nyccas.shtml). 

Charge Question 14: EPA and NACAA have proposed four to five urban areas to have 

saturation monitoring ...Please provide comment on: 

a. The pollutants that should be measured with the saturation devices at each saturation 

site. 

b. The number of saturation devices per pollutant, both passive and/or continuous/semicontinuous, that may be deployed in each pilot city. 

c. Whether placing saturation monitoring devices near certain road segments should 

include, at a minimum: 1) the highest AADT segment in an area, 2) the road segment 

with the highest number of heavy-duty truck/bus counts, 3) at a road segment with more 

unique roadway design, congestion pattern, or terrain in the area, and 4) if feasible, at a 

lower AADT segment with a similar fleet mix, roadway design, congestion, terrain, and 

meteorology as the top AADT road segment in the area. 

67 
Mr. Rich Poirot 

Comments on Near Road Monitoring 

1. The accompanying draft guidance document outline provides an initial 

thought of the major topics required in the near-road monitoring guidance that will 

aid state monitoring agencies in the identification and implementation of NO2 near 

road monitoring sites from a multi-pollutant perspective. Please comment on the 

overall content of the recommended topics in the draft outline. Please provide 

suggestions on any missing subjects that should be included in the guidance 

document and any unnecessary topics that are currently listed in the attached draft, 

if applicable. 

I think the draft outline for the guidance document seems reasonably complete and 

contains no unnecessary topics. One critical topic which seems missing from the outline 

is the importance of the specific distance from the roadway. I think “within 50 meters” is 

too broad a range, and that most 

of the near-road influence falls 

off within, rather than beyond, 

that distance. Extensive, longterm experience sampling nearroad NO2 in the UK (where 

“roadside” monitors are within 

5 meters from the road and 

where additional “kerbside” 

monitoring is conducted within 

1 meter – both at heights 

between 2 and 3 meters) 

indicates that the “roadside 

increment” declines, 

predictably, with the log of 

distance from the road, as illustrated in the attached figure. 

From: NO2 Concentrations and Distance from Roads (2008) Air Quality Consultants Ltd. 

http://www.airquality.co.uk/laqm/documents/FallOffWithDistanceReptJuly08.pdf

This effect of distance is considered sufficiently predictable that a nomograph is available 

that estimates concentrations at any distance from measurements at any other distance 

(within 50 m). 

http://www.airquality.co.uk/laqm/tools/NO2withDistancefromRoadsCalculatorIssue2.xls

While these estimates are derived from annual average concentrations, a similar 

relationship will occur for hourly near-road concentrations (of NO2 and other mobile 

source pollutants like BC, ultrafines, CO, etc.). Location changes within the 50 meter 

distance could easily result in changing the incremental roadway contributions by a factor 

of 2 or 3. I think there is a need to further constrain this distance range in the guidance, or 

perhaps the standard could be expressed in terms normalized to a specific distance (say 

68 
10 or 20 m). For ultrafine particles, especially those in the < 25 nm size range, the 

roadside gradient is likely to be even steeper. See for example: 

Zhu et al. 2004: “Aerosol Science & Technology: Seasonal Trends of Concentration and 

Size Distribution of Ultrafine Particles Near Major Highways in Los Angeles.” 38(suppl 

1):5–13; Sioutas et al., 2005 Exposure Assessment for Atmospheric Ultrafine Particles 

(UFPs) and Implications in Epidemiologic Research. Environ Health Perspect 113(8): 

doi:10.1289/ehp.7939; Durant et al., (2010) Short-term variation in near-highway air 

pollutant gradients on a winter morning, Atmos. Chem. Phys. Discuss., 10, 5599–5626. 

I also think the emphasis on AADT as the primary focus for site selection is overstated 

relative to “other near-road considerations” which are likely more important (in addition 

to the specific distance) Note for example the following Table A3.6 from the 2007 Air 

Quality Expert Group report on Trends in Primary Nitrogen Dioxide in the UK, that 

maximum hourly (98th percentile) NO2 correlates poorly (R2

 = 0.24) with counts of total 

vehicles, but shows a much higher correlation (R2

 = 0.66) with counts of (diesel) bus 

traffic, based on measurements at 53 roadside sites in the UK. 

From: UK air quality modelling for annual reporting 2007 on ambient air quality 

assessment under Council Directives 96/62/EC, 1999/30/EC and 2000/69/EC 

http://www.airquality.co.uk/rep

orts/cat09/0905061048_dd1200

7mapsrep_v8.pdf 

Long-term experience with 

roadside NO2 monitoring and 

modeling in the UK has also 

indicated that the enhanced 

dispersion effects of vehicle speed 

more than offset slight increases in 

NOx emission rates, leading to 

69 
70 

eed 

near-road NO2 impacts 

 the UK. 

 Primary Nitrogen Dioxide in the UK (2007) Air Quality Expert Group 

gov.uk/environment/quality/air/airquality/publications/primaryno2-

decreasing per-vehicle NO2 impacts with increasing AADT, especially on high sp

“motorways” (analogous to US Interstates). Note the attached Figure 3.8 “adjustment 

factors” used to reduce per/vehicle NO2 emissions for modeling 

in

From: Trends in

Report, Annex 

http://www.defra.

trends/index.htm
Other minor suggestions include: 

• The relative age of the fleet may be an important component of the fleet mix, 

which may vary among cities and within urban neighborhoods. (This may be 

somewhat less important for NO2 than for other near-road pollutants). 

• The “load” on vehicles during rush hour(s) could be an important factor. For 

example, diesel vehicles in stop-and-go traffic and/or on an uphill grade will lead 

to high NO2 regardless of AADT. 

• The “expandability” of a site – i.e. the ability to accommodate additional samplers 

for various other mobile source pollutants – should also be a site selection 

consideration. 

• The availability (or establishment) of a “representative” urban background site 

(for NO2 and ideally for other MV pollutants) should also be an important 

consideration. Identification of “roadside increments” for the multiple pollutants 

is critical, as is the ability to project measurements from specific microscale sites 

to larger population exposures. 

• The measured or expected neighborhood-scale background may also be 

important. Other things being equal, a high traffic road in the midst of other high 

traffic areas is likely to experience higher concentrations than a similar roadway 

on the edge of the urban area. 

2. EPA and NACAA envision the near-road guidance document to be written 

from a multi-pollutant perspective. What pollutants and sub-species does the 

subcommittee believe should be included for consideration and discussion in the 

near-road monitoring guidance? Some potential species for consideration include 

NO2, NOX, NO, CO, PM (Ultrafine, 2.5, and 10), black carbon, air toxics (e.g., 

benzene, toluene, xylene, formaldehyde, acrolein, or 1, 3, butadiene), and ammonia. 

Please prioritize the recommended pollutants and provide the rationale for their 

ranking, including how this pollutant measurement will contribute to scientific and 

regulatory knowledge of near-road air quality and adverse human health effects. 

Since NO2 is the focus of the revised NAAQS and new monitoring requirements, it (and 

the NO & NOx which typically come along with it) has to be given top priority. It seems 

possible however that of the many, various mobile source pollutants, NO2 may be one of 

the least health-relevant, and a new, large monitoring network measuring just NO2 would 

be a waste of scarce resources. All of the other pollutants listed above would also be of 

interest, but would be prohibitively expensive to add at all sites. I also wonder whether 

the near-road influence of these many mobile source pollutants (or even of just NO2

alone) is something that can be or should be addressed in a large network approach. 

Rather than prioritizing the above list and seeing a few species measured at a large 

number of (similar) sites, I would prefer to see a nested network, within which 

many/most of the above species could be added at a smaller subset of sites. Instrument 

costs and the availability of reliable, continuous samplers should also be an important 

consideration. 

I think black carbon (preferably multi-wavelength) should be given high priority, given 

its relevance to health (& climate & visibility) effects, its strong influence – like NO2 – 

from diesel emission sources, and the availability of reliable continuous instruments. 

Continuous OC (OC/EC) data would also be useful at some sites to help assess effects of 

71
fleet mix, SOA formation, condensation, destruction, etc. If reliable instruments are 

available and affordable, particle number count (including ultrafine particle sizes) would 

also be an important measurement, and likely to increase rapidly near roadways (well 

within 50 meters). I would give BC and number count a higher priority than PM2.5 mass, 

and it seems likely that FRM/FEM PM2.5 samplers will substantially understate the semivolatile fraction near roadways (although FDMS TEOMS have not generally performed 

well in the field). There is likely a steep PM coarse gradient near roadways, but PM10

(subtraction) measurements would be a poor way to characterize this. Collection of PM in 

different size fractions in large (aggregated) sample volumes – to support molecular level 

organic analysis and bioassay work - could also be useful at a few sites. CO 

measurements would be especially useful for contrasting pollutant mixes at sites (or 

times) with different diesel vs. spark engine fleet mixes. The “toxic” species listed above 

would likely show strong roadway increases, but are also likely to be prohibitively costly 

(or too labor intensive) to add at most sites. I wonder if there’s any possibility of moving 

or establishing one of the NAATS sites to a near-road location? Our (very small) VT 

state agency has recently had reasonable success operating a continuous BTEX 

instrument from Synspec – for which I believe a 1,3 butadiene option is available. Results 

from the Las Vegas MSAT near-road toxics study could be quite relevant here, and some 

consideration might be given to modifying planned future phases of that study to make it 

more relevant to the objectives of the new near-road NO2, CO NAAQS requirements and 

related multi-pollutant monitoring plans. 

I don’t know the availability, reliability or costs of continuous NH3 instruments, but 

better characterization of MV NH3 emissions would be desirable at a few sites at least. 

Possibly some periodic UC Davis DRUM sampling would be a useful complement at a 

few sites – if equipped with a streaker or somesuch to add time resolution to the 

ultrafines. I haven’t actually seen that configuration in action, and don’t know about 

current analytical capabilities at the DELTA Group. Ozone measurements might be 

useful at selected sites (including the urban background sites), as the contribution of 

secondary NO2 formation, even in near road environments, isn’t necessarily trivial, and 

interesting changes may occur with efforts to attain new ozone and NO2 standards. 

In addition to the above pollutant species list, and meteorological measurements, other 

measurements that should be considered include traffic counters (which can separate light 

& heavy duty MV) and or possibly cameras (which can be especially useful for 

evaluating extreme events). It should also be noted that the important objective of 

characterizing incremental roadway contributions for any of the above pollutants would 

benefit (as for NO2) from a measurements at a paired urban background site. Some 

(nearby) remote sensing (FEAT) could also be useful, or perhaps establishing near-road 

sites near locations where FEAT-type measurements have recently conducted (and may 

be periodically repeated). 

http://www.feat.biochem.du.edu/assets/databases/Cal/Tricity_NH3_SO2_NO2_2008_Re

port_ARB.pdf 

3. Identifying Candidate Near-road Site Areas 

a. AADT & Fleet Mix 

As indicated above, I think AADT alone is a poor indicator. 

b. AADT & Fleet Mix 

72
A metric which ‘diesel-weighted’ the AADT would be preferable to AADT alone, but 

again, count is not really the key issue, especially on high speed highways. Two trucks 

passing the monitor at 60 mph will not cause twice the impact of 1 truck at 30 mph... 

Also, given the 1-hour standard, the traffic and fleet mix on weekday morning rush hours 

are likely to be most important. 

c. Roadway Design 

Assuming that you mean “no obstructions” between the road and the monitor, this seems 

reasonable, and it seems unlikely that that no suitable sites without such obstructions will 

be available. Barriers beyond the monitoring site that constrain the further dispersion of 

roadway pollutants should not be avoided and (in urban street canyons) may well lead to 

some of the highest population exposures. Conceivably, adding barriers - sound barriers, 

trees, etc. - might be considered as an exposure mitigation strategy. It might also be noted 

that large fractions of the population spend time within a 5 or so meters of congested 

urban streets, but population proximity to the edges of high-speed interstates with 

maximum AADTs is typically more distant. 

d. Congestion Patterns 

Conceptually, “level of service” sounds like an important indicator, although I don’t 

know how reliable such data is on a national scale. As indicated earlier, I think NO2

emission increases with speed are relatively small and offset by increased dispersion. I 

would expect higher concentrations during times/places of highest congestion, rather than 

during high speed driving conditions. 

e. Terrain 

Terrain could be an important, especially during the winter in mountain/valley locations, 

in urban street canyons, or near roadway dips which are below grade. 

e. Meteorology 

I think the Agency’s proposed approach – strongly encouraging but not formally 

requiring “downwind” location is reasonable for all the reasons given. Ideally the 

“downwind” location would concurrently reflect the periods of highest traffic congestion 

and lowest wind speeds and mixing heights. In case of doubt, saturation sampling could 

help determine locations of maximum expected impact. Established sites which met 

measurements indicate are persistently upwind during rush hour should be replaced.

4. Modeling is another tool that may be useful in the identification of candidate 

near-road sites. In particular, the use of mobile source emissions modeling with 

MOVES and local-scale dispersion modeling with AERMOD, can be presented 

as part of the guidance document. Please comment on the available modeling 

tools, and their pros and cons, that the subcommittee believes may be 

appropriate to discuss and/or recommend for use in the near-road monitoring 

guidance document. 

Modeling may be a useful tool, but unless site-specific meteorology and vehicle mix, 

volume and congestion data are available, I’m not sure it would lead to a better site 

selection than a “common sense” approach. AERMOD also often performs poorly in 

complex terrain. 

73
5. 

a. If a state were inclined to use saturation monitoring to aid in the selection of a 

near-road monitoring site, and considering the NO2 standard is a 1-hour daily 

maximum standard, what are the pros and cons to using passive devices to 

saturate an area to gather data? 

Unless the time periods for 

saturation sampling turn out to be 

atypical, the longer (than 1 hour) 

aggregation times for passive 

samplers may not be that big a 

problem. Based on the long-term, 

multi-site data from roadside sites in 

the UK, the peak hourly and annual 

average concentrations are well 

correlated across space, as indicated 

in Figure 3.3 pasted here. Note also 

the high correlation (R2 =0.98) 

between annual average NO2 98th

percentile hourly values from 53 

UK roadside sites in Table A3.6 

above. 

From: 

http://www.airquality.co.uk/reports/

cat09/0905061048_dd12007mapsre

p_v8.pdf

A disadvantage of passive samplers is that while reasonably accurate units are available 

for NO2, NO, NOx & BC, there may not be comparably accurate units for CO. This limits 

the ability to explore different NO2/CO ratios in a saturation approach. An advantage of 

passive samplers is their low cost and subsequent ability to deploy many units 

inexpensively. If passive sampling were limited to NO2 & NO, a relatively dense 

exploratory saturation sampling program could be considered, including innovative 

mounting of passive samplers on (and/or in) busses, commuter cars, taxis ect. This will 

open the can of worms regarding whether there’s intent to protect people in their cars at 

rush hour, but that issue probably needs to be addressed at some point. See also George 

Allen’s recommendations combining passive samplers with active, timed inlets. 

b. Likewise, what are the pros and cons to using non-passive devices, such as near 

real-time or continuous devices including, but not limited to portable, non-FEM 

chemiluminescence methods for NO2 or Gas Sensitive Semiconductors (GSSs) 

for NO2 and other pollutants of interest? 

 

Potential advantages include the ability to collect data with higher time resolution and for 

more species than passive samplers allow. I don’t know the current instruments, and 

defer to others on the committee. 

c. Finally, what would be the pros and cons, to a state or local agency attempting to 

use a specially outfitted vehicle to collect mobile measurements to assist in the 

74
near-road site selection process for NO2 specifically as well as other pollutants of 

interest? 

Other than the prohibitive cost, this could be an excellent way to help select sites and 

characterize the space/time patterns of exposures to multiple roadway pollutants. I 

wouldn’t automatically rule this out, as it may turn out that some states or research 

groups may have access to such mobile sampling equipment and would be willing to 

operate it at reasonable costs. In my view the pilot studies should be conducted in fewer, 

rather than more locations, and this might be a good way to produce some useful data in a 

few (1 or 2) study areas. If only EPA had an adequately funded Office of Research and 

Development... There may also be some useful low-tech ways to combine “ordinary” 

mobile sampling vehicles (buses, taxis, commuter vans etc.) with passive samplers that 

could provide some useful information. 

6. 

a. Does the subcommittee believe that the light duty cold start and congestion 

factors will significantly influence the location of peak CO concentrations in an 

area? What priority should these factors be given when compared with the 

factors (AADT, Fleet Mix, Roadway Design, Congestion Patterns, Terrain, and 

Meteorology) already being considered for peak NO2? 

Yes, these are important considerations. I don’t believe however that maximum NO2 and 

CO will necessarily occur at vastly different kinds of locations, and that some kinds of 

sites would be suitable for quantifying near-road influences from both pollutants. A 

relatively high fraction of diesel vehicles does not necessarily mean that emissions from 

spark engine vehicles will not be high as well. Congested sites where vehicle mixes 

change by time of day and day of week will be especially informative. Having 

concurrent, collocated data for multiple species, while searching for single pollutant “hot 

spots” is not likely to improve understanding of population exposures, help discern 

effects of co-varying pollutants, nor lead to development of effective abatement 

strategies. 

b. Does the subcommittee have an opinion on whether, and possibly how, these two 

issues of vehicles operating under cold start conditions and light duty vehicle 

congestion and idling in urban street canyons and/or urban cores be considered 

in a future, nationally applicable, CO monitoring proposal? Are there other 

factors that may affect peak CO concentrations and not affect peak NO2 

concentrations that should also be considered for any future CO monitoring 

proposal? 

CO will also be influenced by residential wood combustion and other space heating 

emissions and so northern mountain valley locations with high traffic counts and 

congestion plus limited dispersion on cold winter mornings (when secondary NO2

formation is minimal) may see relaticely higher CO concentrations. As with NO2, I’m 

not convinced that CO is the most (or second most) health-relevant component of 

roadway emissions, and would hope that suitable near-road sites could be identified to 

75
address both pollutants, with a smaller number of sites added to address specific COspecific concerns when the CO NAAQS revision is final. For both pollutants, I think the 

objective should be to characterize near-road population exposures to mix of trafficrelated emissions, and not just to witch-hunt for the worst-case locations of maximum 

single-pollutant concentrations. 

7. Does the committee believe that siting considerations for identifying the location of 

peak NO2 concentrations will likely address all of the high priority siting 

considerations for PM (particularly PM2.5) as well? If not, what other factors 

should be considered and what are the advantages in considering these factors for 

identifying the location of maximum PM concentration? 

While there is likely a significant near-road enhancement of local PM2.5 concentrations, I 

think this roadway enhancement is proportionally much smaller for PM2.5 mass - compared 

to the roadway enhancement of NO2, BC, ultrafines, etc., and that PM2.5 should not be a 

priority consideration in siting. Also, since diesel emissions are major contributors to 

roadway NO2 and PM2.5 there should not be much conflict in siting objectives. 

8. In addition to PM2.5 mass, what other PM-related measurements are desirable at 

near-road monitoring stations (e.g., UFP number, black carbon, EC/OC, PM coarse, 

etc.)? 

As indicated above, I would give all of the above a higher priority than PM2.5 mass 

measurements, and would push more for continuous instruments that would better 

characterize the entire particle size distribution. Roadway fine particle concentrations are also 

likely to include a substantial semi-volatile component, which is not well characterized by 

PM2.5 FRM (or FEM) instruments. The roadway increment in coarse particle concentrations 

is likely to be proportionately greater than for fine particles, and coarse-only sampling should 

be given a higher priority at some of these sites. Past consideration of setting an “urban” 

coarse particle NAAQS, was based on an assumption of greater inherent toxicity in urban 

areas. But this (logical) assumption was not supported by much measurement data. The 

carbon species (BC and/or EC/OC) and particle number information will be more useful than 

PM2.5 mass for health effects studies and source attribution, especially given the longer 

averaging times – 24-hour and annual – for the PM2.5 NAAQS and currently stated intent to 

keep the annual standard “controlling”. 

 

9. To allow for near-road monitoring infrastructure to be multi-pollutant, and in 

reflection of the recently promulgated near-road NO2 siting criteria, 

reconsideration of the existing microscale CO siting criteria presented in sections 2, 

6.2, and table E-4 in 40 CFR Part 58 Appendix E may be warranted. Does the 

subcommittee believe that reconsideration of microscale CO siting criteria is 

appropriate? Specifically, would an adjustment of CO siting criteria to match those 

of microscale PM2.5 and microscale near-road NO2 sites be logical and 

appropriate? 

76
As previously indicated, I think the 50 meter range proposed for “near-road” NO2 is too large 

and should be tightened prior to attempting to harmonize the various microscale criteria for 

various pollutants. Conceptually, near-road measurements for multiple pollutants might 

“standardized” to expected concentrations at a single fixed distance, or perhaps a (closer than 

50 m) maximum and a minimum sampling distance could be specified, with a smaller 

number of “research” sites encouraged that could collect useful data very close to roadsides 

(inside the minimum distance) that would help characterize the roadway contribution without 

being used for compliance determination. 

10. Even if the adjustment of microscale CO siting criteria in sections 2, 6.2, and table 

E-4 in 40 CFR Part 58 Appendix E to match that of microscale PM2.5 and 

microscale near-road NO2 is appropriate and proposed, should there be 

consideration to maintain the requirement on how urban street canyon or urban 

core microscale CO sites should be sited? 

Off hand, I don’t see why near-road distances for CO and NO2 should be different, or why 

CO should be relaxed to NO2 distances. 

11. Does the subcommittee have an opinion on how “urban street canyons” or “urban 

core” might be defined, perhaps quantitatively, and with regard to use in potential 

rule language? 

No opinion. 

12. EPA and NACAA will select the locations for permanent sites that are part of the 

near-road pilot study based on which state or locals volunteer to participate and can 

process grant funds in a timely manner to deploy equipment. From this pool of 

volunteers, selection should be made on certain attributes that provide the best 

potential to fulfill pilot study objectives. In the attached draft white paper, EPA and 

NACAA have proposed some potential criteria for consideration in selecting where 

the fixed, permanent stations should be located. These considerations include 

choosing a large and a relatively small urban area based on population, an area with 

varied or complex terrain, an urban area with an operational NOX analyzer 

representative of neighborhood or larger spatial scales for comparison to the nearroad NOX analyzer, and an urban area with a cooperative (or non-cooperative) 

Department of Transportation. Does the Subcommittee agree with these 

considerations? Further, are there other considerations that should be evaluated in 

selecting pilot cities to house permanent near-road monitoring stations as part of the 

pilot study? 

I’m not sure sufficient funds are available to address so many different kinds of locations. It 

will be necessary to take maximum advantage of existing sites (including urban background 

sites and those operated by research groups) to the extent possible. The 

availability/participation of academic or private sector groups to add supplemental 

measurements should also be encouraged. With such limited funds, is it necessary to 

intentionally include a small city or an area with a non-cooperative DOT? If small cities are 

77
included (or not), selected sites should be adjacent to roadways with high volumes of diesel 

traffic and frequent rush hour congestion. Given the rapid decline of roadway influence with 

distance, priority should be given to sites which are substantially closer than 50 meters from 

the road (10 to 20 meters max). Possibly the effects of complex terrain could be more 

effectively studied in a winter saturation study than with permanent monitors, although the 

exaggerated diurnal stagnation patterns and extent to which these correspond to rush hour 

periods would be useful to characterize with continuous instruments. It would also be useful 

to consider urban areas which have different kinds of public transportation systems, bus and 

taxi fleets and associated fuels, etc. 

13. EPA and NACAA have proposed that at least two urban areas should have 

permanent near-road monitoring stations (that would fulfill NO2 near-road 

monitoring requirements) implemented for the pilot study. Please comment on the 

minimum equipment/pollutant measurement complement that should be deployed at 

each site and also the ideal equipment complement that each site should or could 

have, respectively. Specifically, what pollutants (e.g., NO2, NOX, NO, CO, PM 

(Ultrafine, 2.5, and 10), black carbon, air toxics (such as benzene, toluene, xylene, 

formaldehyde, acrolein, or 1,3, butadiene) and ammonia) and other information 

should the pilot study measure or gather at the fixed, permanent monitoring 

stations, and by what methods? This list should be in priority order, as feasible, and 

can include any NAAQS or non-NAAQS pollutant by any method (FRM/FEM 

and/or non-reference or equivalent methods), any particular type of other 

equipment for gathering supporting data such as meteorology or traffic counts. 

The objectives seem to be somewhat mixed here. To a large extent the proposed pilot study 

seems to be focused on gaining insights into the process of citing near-road monitors (for 

NO2 and to a lesser extent for CO NAAQS compliance determination. The emphasis is on 

understanding the relative importance of various traffic and roadway indicators (of varying 

and often unknown quality) to guide NO2 site selection, the logistical and institutional 

difficulties associated with establishing new sites in challenging environments, etc. From this 

perspective, the subsequent use of any resulting measurement data (other to confirm whether 

NO2 and/or CO are exceeding or close to NAAQS) is almost irrelevant. Retaining several of 

the fixed location sites, and building them into much more comprehensive sites where the 

objective is to actually learn something about near-road multi-pollutant exposures is an 

entirely different (but no less desirable) objective. 

In selecting these few comprehensive sites, I would try to assure that they are close enough to 

roadways to capture the extreme gradients for pollutants like NO2 and ultrafines, and also 

make sure there’s a relatively nearby representative urban site with similar measurements to 

help quantify the roadway increment. Because roadway emissions, and to a large extent 

population exposures in near-road locations tend to have large diurnal variability, I would 

generally limit the measurements to species that can be quantified continuously. Beyond that 

I defer to others on the committee to prioritize the species. 

14. EPA and NACAA have proposed four to five urban areas to have saturation 

monitoring, using either passive devices and/or continuous/semi-continuous 

78
saturation type multi-pollutant monitoring packages (i.e., several types of monitors 

in one mountable or deployable “package”). Please provide comment on: 

a. The pollutants that should be measured with the saturation devices at each 

saturation site. 

b. The number of saturation devices per pollutant, both passive and/or 

continuous/semi-continuous, that may be deployed in each pilot city. 

I don’t have much expertise here, but think that (especially given the very limited 

budget), the selection of species and number of sites are inter-related and depend on 

available methods and costs. Ideally, the minimum species for saturation sampling would 

include at least NO2, NOx, CO, BC, but I don’t believe there are sufficiently reliable 

passive samplers for CO. Possibly a nested approach could be applied with larger 

numbers of passive NO2 samplers where applicable and smaller numbers of portable 

continuous devices for other key species. Note also George Allen’s suggestion to 

combine passive samplers with timed pump inlets – which might improve both pollutant 

sensitivity and temporal resolution. See also previous comments 

c. Whether placing saturation monitoring devices near certain road segments 

should include, at a minimum: 1) the highest AADT segment in an area, 2) the 

road segment with the highest number of heavy-duty truck/bus counts, 3) at a 

road segment with more unique roadway design, congestion pattern, or terrain 

in the area, and 4) if feasible, at a lower AADT segment with a similar fleet mix, 

roadway design, congestion, terrain, and meteorology as the top AADT road 

segment in the area. 

All of the above seem like reasonable (but somewhat idealized) kinds of locations. I 

question whether it will really be possible to identify “a lower AADT segment with a 

similar fleet mix, roadway design, congestion, terrain, and meteorology as the top AADT 

road segment in the area” or if in doing so it could realistically be assumed that differing 

AADTs were the sole cause of any differences in concentrations. The effect of differing 

AADTs might better be explored by sampling during different time periods along a single 

road segment. To the extent possible, it would be useful if these sites were located at 

similar distances from, and at rush hour downwind directions from the associated 

roadways. Assuming this may not be possible, lines of additional passive NO2 sensors 

might be added at each site, perpendicular to roadways and in upwind and downwind 

directions. Meteorological measurements may also be needed at some or all of these 

sites, and similar kinds of species measurements should be added a non-road 

representative urban site to help define the roadway increments from the different 

saturation sites. 

79
Dr. Jay Turner 

1) Please provide suggestions on any missing subjects that should be included in the 

guidance document and any unnecessary topics that are currently listed in the 

attached draft, if applicable. 

The outline for the guidance document is fine. 

2) Please prioritize the recommended pollutants and provide the rationale for their 

ranking, including how this pollutant measurement will contribute to scientific and 

regulatory knowledge of near-road air quality and adverse human health effects.

In general, the priority should be to monitor for indicator compounds for motor 

vehicle exhaust. In descending order of priority: (i) NO/NOx, opportunistic because 

the NO2 measurement method will likely by NO2 by difference (NOx minus NO); (ii) 

BC, as a second indicator for diesel emissions and thus sharpen the interpretation of 

the NO2 data; (iii) CO, a stronger indicator for gasoline-fueled vehicle emissions, to 

compare and contrast with the indicators for diesel emissions; (iv) air toxics, an 

indicator for vehicle emissions; and (v) PM as an indicator for diesel emissions, albeit 

with confounding by road dust (depending on the PM site that is monitored). For 

many of these pollutants there is the potential for confounding by high upwind 

concentrations – that is, the measurement (absolute concentration and/or 

concentration variations) may not necessarily be a dominated by vehicle emissions 

from the roadway. 

3) Identifying Candidate Near-road Site Areas...

Is the objective to monitor at the highest NO2 site within 50m of a roadway, or at the 

site with highest NO2 attributed to the proximate roadway? That is, should there be 

consideration of aggregate upwind effects, e.g. from a dense roadway network, that 

leads to high background NO2 at the roadway to be monitored. Or, is the interest in 

selecting a roadway with high NO2 difference across the roadway? If it is the former 

case then an expanded list of considerations for identifying candidate sites is needed. 

In general, I support the development of a screening tool to guide the site 

identification process. Screening tools have been used for hot spot analyses and a 

similar approach could be used in the site selection process. One approach is to use 

dispersion modeling to create look-up tables to semi-quantitatively) relate roadway 

(and other sites) characteristics to potential impacts. A more refined analysis could 

subsequently be taken to prioritize the sites identified from the screening process. 

a) The proposed approach to consider AADT and fleet mix is reasonable as long as 

there is also a consideration of vehicle speed (e.g. through the LOS, below) to 

capture the speed dependence of emission rates. Emission rate estimates based on 

more sophisticated and/or site-specific inputs could be used if the area has such 

information readily available, but is not a high priority. 

b) The consideration of on-the-books vehicle emission controls is a low priority in 

the site selection process. Perhaps more important out-year considerations would 

be any programmed or planned changes to the roadway corridor. 

c) A preference to sites at grade with no obstructions is a reasonable objective. 

There should be some flexibility, however, especially if the impact of other 

80
designs or obstructions is such that a specific candidate site is still expected to a 

highNO2 concentration zone. 

d) The consideration of LOS is best handled through a screening tool that is 

grounded in dispersion modeling (see my preamble this question, above). The 

key is to capture the speed dependence of emissions. 

e) I am less concerned for vegetation if the focus is on NO2 as long as the vegetation 

does not lead to a significant airflow obstruction. This might be important factor, 

however, for other candidate pollutants. 

f) Upwind pollutant meandering is certainly observed especially when winds are 

light and variable (in which case, the meandering is driven wind direction 

variations) and/or the vehicle-induced turbulence dominates over the prevailing 

air flow. That said, preference should be given to site locations that are nominally 

downwind for the meteorological conditions leading to highest impacts. Perhaps 

a screening tool could be used to identify the conditions. A key aspect is 

overlaying the prevailing diurnal wind patterns with the diurnal traffic patterns to 

estimate conditions – and thus near-field locations – for maximum impacts. 

4) Please comment on the available modeling tools, and their pros and cons, that the 

subcommittee believes may be appropriate to discuss and/or recommend for use in 

the near-road monitoring guidance document.

The use of MOVES and AERMOD might be overly burdensome for the site selection 

process. As previously described, I advocate a screening tool be developed that is 

grounded in dispersion modeling that provides semi-quantitative estimates of impacts. 

I agencies have MOVES outputs, which are link-based rather than trip-average and 

thus likely more relevant to the specific roadway environment, then this information 

could be used as input to the screening model. However, for many agencies it might 

be too burdensome to generate MOVES output for the near-roadway site selection 

process. 

5) The use of saturation monitoring and on-road monitoring are also possible tools that 

state and local air agencies may choose to utilize in the near-road site selection 

process...

Saturation monitoring could be helpful in prioritizing candidate sites. There is 

substantial literature on saturation studies with passive monitors, but in these cases 

the integration times are typically long. The crux for this application is a saturation 

monitoring strategy that has sufficiently high time resolution to be relevant to 1-hour 

conditions (this does not mean that 1-hour resolution is needed). Given there is 

typically diurnal structure to both traffic patterns and dispersion conditions, one 

strategy is to collect samples for sub-daily time periods but integrated over several 

days (e.g. a battery of timer-based saturation samplers). 

6) If EPA were to propose a new set of minimum monitoring requirements for CO near 

roads, the near-road monitoring stations created under the implementation of the 

NO2 monitoring requirements may be an advantageous infrastructure for state and 

local air agencies to leverage. However, [...]. 

I have no preliminary comments on this matter. 

81
7) Does the committee believe that siting considerations for identifying the location of 

peak NO2 concentrations will likely address all of the high priority siting 

considerations for PM (particularly PM2.5) as well? 

It might be adequate if the emphasis of the PM monitoring is on that component 

related to diesel exhaust emissions, and the emphasis in all of the monitoring is on 

roadway-specific impacts and not cumulative impacts which include consideration of 

upwind sources. With improved vehicle emissions control technology, the relative 

contribution of road dust, tire wear, and brake wear to the traffic-induced PM 

becomes more important. If the goal is to capture these impacts in the PM 

monitoring, this could lead to diverging siting considerations for NO2 and PM. 

9) In addition to PM2.5 mass, what other PM‐related measurements are desirable at near‐road

monitoring stations (e.g., UFP number, black carbon, EC/OC, PM coarse, etc.)?

While in the ideal case it might be desirable to monitor for various PM components, 

practical considerations likely make UFP number and black carbon the most 

reasonable candidates. UFP number can be highly variable and confounded by other 

atmospheric dynamics events. Thus, its measurement is most useful as site pairs 

across the roadway. Thus might be impractical. Black carbon would be of interest in 

its own right and to compare and contrast to NO2. 

10) Does the subcommittee believe that reconsideration of microscale CO siting criteria 

is appropriate? Specifically, would an adjustment of CO siting criteria to match 

those of microscale PM2.5 and microscale near-road NO2 sites be logical and 

appropriate?

I have no preliminary comments on this matter. 

10) Should there be consideration to maintain the requirement on how urban street 

canyon or urban core microscale CO sites should be sited? 

I have no preliminary comments on this matter. 

11) Does the subcommittee have an opinion on how “urban street canyons” or “urban 

core” might be defined, perhaps quantitatively, and with regard to use in potential 

rule language? 

I have no preliminary comments on this matter. 

12) Does the Subcommittee agree with the stated considerations for selecting sites for the 

pilot study? 

The stated considerations are reasonable, although there relative weighting needs to 

be defined because some considerations are more important than others. Assuming 

resources will be limited, preference should be given to large urban areas with 

sufficient exiting monitoring infrastructure to place the road-side measurements in 

context. 

13) Please comment on the minimum equipment/pollutant measurement complement that 

should be deployed at each pilot study site and also the ideal equipment complement 

that each site should or could have, respectively. 

The minimum additional measurements should include NO/NOx, CO, black carbon, 

meteorology (perhaps at a setback or other nearby representative location), and traffic 

characterization (vehicle count, class, and speed). CO2 could also be useful. It would 

be ideal to have each of these parameters (but certainly NO2) measured at a 

representative “background” site (background from the perspective of the roadway). 

The measurement matrix should take into consideration whether the data would be 

used to evaluate any screening tool(s) developed to aid in site selection. Beyond this 

82
minimum list of measurements, others could be added to fulfill specific study 

objectives. It is possible that research groups would be interested in adding 

measurement which would leveraging the investment in site infrastructure, and it 

would be great to accommodate this to the extent practicable. 

14) Please comment on the saturation study design details. 

The details are reasonable but it might be worthwhile to refine the saturation study 

objectives and then revisit the study design details. For example, is it desired to 

harmonize certain details within an urban area (or between areas, if possible) to more 

clearly evaluate the impact of other details? In one design the saturation monitors 

could be placed about the same distance from roadways, while in another design they 

could be placed at different distances from the roadways. If distance from roadway is 

a key parameter, then this could impact the data and its interpretation. 

Inconsistencies in upwind/downwind siting could strongly influence the interpretation 

of data from short-term saturation monitoring studies. If a screening tool was 

developed, one objective might be the evaluation of this tool through careful design 

of the saturation studies. 

83
Dr. Warren H. White 

AADT, fleet mix, roadway design, congestion, terrain, and meteorology all affect 

ambient concentrations, in complicated and interdependent ways. However, the 

combined near-road effect of all these influences can be described with just a few degrees 

of freedom. The framework outlined below is hardly new (e.g. White, 1977), but seems 

worth revisiting in light of the new rule. 

The key to a simple description is that not much chemistry has a chance to occur 

in the short time air spends near the road. A cross-road wind component of only 1 m/s, 

for example, carries air from 150 m on one side to 150 m on the other in just 5 minutes. 

On such a time scale the complex chemistry of smog formation can be considered 

determined by the surrounding air, independent of the fresh emissions. More precisely, 

the only reactions needing consideration are the rapid scavenging of O3 by NO 

 [1] O3 + NO Æ O2 + NO2, 

and the rapid photolysis of NO2 to yield 

 [2] O2 + NO2 Æ O3 + NO 

after additional steps. These reactions leave unchanged the concentrations of odd oxygen 

[Ox] = [O3] + [NO2] and nitrogen oxides [NOx] = [NO] + [NO2], and their relative rates 

establish a photostationary state that is generally fairly well approximated in the 

atmosphere: 

[3] [O3][NO]/[NO2] ≈ k2/k1. 

 

Since Ox and NOx are chemically conserved near the road, their concentrations 

respond only to physical dilution and mixing. They can be modeled as the sum of a 

variable contribution from roadway vehicle exhaust and a uniform background supplied 

by the surrounding air. For given concentrations [Ox]0 and [NOx]0 at the monitor, the 

reactive species can be expressed in terms of NO2: 

 [NO] = [NOx]0 – [NO2] and [O3] = [Ox]0 – [NO2]. 

Substituted into the photostationary equilibrium [3], these identities yield a quadratic 

equation in [NO2] that can be solved for [NO2] in terms of [Ox]0, [NOx]0, and k2/k1. The 

following plots illustrate some features of the relationship. 

The conservation of odd oxygen limits microscale NO2 maxima to the sum of 

directly-emitted primary NO2 plus the reservoir of odd oxygen available in the 

surrounding air. An important siting consideration is therefore the middle-scale ozone 

background, which I did not see mentioned in the Study Approach or Charge Questions. 

This background bounds the NO2 produced from primary NO emissions, contrary to the 

impression one might get from statements such as this (FR v74, n134, 7/15/2009, 

p34441): “However, since the rate of conversion of mobile source NO to NO2 ... is a 

generally rapid process, (i.e., on the order of a minute (ISA Section 2.2.2)), NO2 behaves 

like a primary pollutant in the near-road environment, exhibiting peak concentrations on 

or closely adjacent to roads.” 

84
I will be happy to supply the spreadsheet used to generate Figures 1-3 to anyone 

else who might like to play with it. 

Reference:

White, Warren H. (1977) NOx-O3 photochemistry in power plant plumes: comparison of 

theory with observation. Environmental Science & Technology 11, 995-1000. 

Figure 1. If oxidant background is 50 ppb (~ 50 ppb O3 PRB + <1 ppb NO2 PRB), then 

even 25% NO2 in the fleet exhaust and 200 ppb near-road NOx is not enough to make 100 

ppb NO2. 

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Figure 2. The most favorable condition for NO2 at a typical background oxidant level (75 

ppb O3 + 25 ppb NO2) is a dark sky (small photostationary ratio) to minimize NO2

photolysis. 

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Figure 3. In the absence of elevated exhaust NO2/NOx ratios, background oxidant is 

needed to convert the primary NO emissions. 

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Figure 4. Concentrations of conserved primary emissions like CO (“tracer”) fall off more 

rapidly with distance from the roadway than those of NO2 do. 

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Questions for EPA: 

I am not convinced that a substantial near-road monitoring program for NO2 and other 

traffic-related species is a good use of Agency resources. I think it will be hard to 

implement in a meaningful way, and I don’t see great potential value in the data it will 

produce. I recognize that the decision has been made already, and that I am not required 

to understand the reasons behind it. I could better focus on our charge questions, 

however, if I had answers for the following questions of my own. 

1. What is meant by hourly NO2 concentrations – should they be equivalent to 

actual arithmetic averages of instantaneous concentrations? Exhaust concentrations at a 

near-road sampling inlet can vary greatly within a few seconds. In a given setting 

(background oxidant levels and meteorology), NO2 concentrations depend nonlinearly on 

exhaust concentrations. Under these conditions an instrument’s time response – and the 

nature of any ‘internal averaging’ – requires careful characterization. The reliance on a 

difference method (NOx-NO) further heightens the challenge for measurements near the 

road, where the signal/noise ratio is least. Is the goal, as it was with the PM2.5 FRM, to 

replicate the undefined and uncontrolled shortcomings of historical data that underlie 

existing epidemiological analyses? Or is it to make an accurate measurement of NO2? 

2. How are concentrations from microscale locations to be linked to available 

public health statistics for epidemiologic analyses? Data from neighborhood- or urbanscale monitors have demonstrated utility for epidemiology because they are indicative of 

typical exposures for identifiable populations large enough to generate routine public 

health statistics. The numbers of residences near microscale monitors will be small, and 

the vehicle occupants driving by them will be anonymous. Will site-specific panel 

studies be required to connect the near-road data to health effects? 

3. How large a slice of the monitoring pie is ultimately contemplated for near-road 

monitoring? The Agency deserves great credit for recognizing the need “to support 

measurement of multiple NAAQS pollutants” in calling this meeting. “Maximum 

expected hourly concentrations” are likely to occur at different locations (with different 

vehicle mixes and road characteristics) for different candidate species (e.g. NOx, CO, 

black carbon, PM0.1, and PM10). And health researchers will view the consequently 

different pollutant mixes as an important environmental signals for epidemiological 

analyses. Measuring different species at different sites would clearly be of little value for 

anything more than a NAAQS-compliance determination. Are we looking at NCORE on 

steroids, something like 75 x (number of traffic-related species) for the total number of 

sites? 

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Dr. Yousheng Zeng 

General Comments 

Near-road monitoring requirements: The purpose of near-road monitoring is to protect 

the health of residents living near roadways. There should be a screening criterion: In a 

particular CBSA, if there are no residents living within the 50-m corridor, near-road 

monitoring should be exempted. Following a similar line of thinking, if there is only one 

community within the 50-m corridor, the near-road monitor should be sited at this 

community, and not necessarily at a location where the impact is highest. In this case, 

other siting analysis is unnecessary. 

The end-point of near-road monitoring: Normally when an ambient monitor shows 

exceedance of NAAQS, state/local authorities are required to develop a State 

Implementation Plan (SIP) to bring the area into attainment with NAAQS. The State 

Implementation Plan will include some control measures to achieve attainment. If a nearroad NO2 monitor shows exceedance of NAAQS, how will a non-attainment area be 

delineated and what does EPA expect the state/local authority to do? Due to the nature of 

significant concentration gradient along the roadways, the area with high NO2

concentrations could be extremely small. What will be the basis for designating an area 

as non-attainment area? The non-attainment is basically caused by mobile sources. In 

some areas, it is largely attributable to vehicles passing through the area on the interstate 

highways. What can the state/local authority do to achieve attainment? If the state/local 

authority cannot do anything, what is the point of requiring this type of near-road 

monitoring? EPA could conduct some studies and achieve attainment through regulations 

on vehicle emission standards. 

Charge Question 3.c 

In urban areas, the road segments that have high AADT are commonly elevated 

roadways. Requiring monitoring sites at-grade will either miss the plume from the 

roadways or significantly limit the choices for the monitoring sites. As far as the vertical 

location is concerned, the guidance document should consider the two factors – (1) the 

monitor’s probe intake should be in the general vertical area of the plume coming from 

the roadways; and (2) the residence time for sample to travel from the probe intake to the 

analyzer will meet the criteria (20 sec.), i.e., no extremely tall probe from the ground that 

cause a long residence time. As long as these two criteria are met, there is no need to 

specify whether the monitor needs to be at-grade. 

Charge Question 4 

In many traffic related air quality impact analyses (e.g., air quality analyses as part of 

required NEPA process for highway projects), the CALINE3 and CAL3QHC models are 

used. They are still listed as preferred models on the EPA SCRAM webpage. EPA should 

evaluate these models along with AERMOD and provide guidelines on which model 

should be used for siting near-road monitors. 

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In this guidance document, EPA should explain if and how Ambient Ratio Method 

(ARM) and Ozone Limiting Method (OLM) can be used in conjunction with AERMOD 

model to convert freshly emitted NO to NO2. On June 28, 2010, EPA issued a memo 

addressing these issues for more general NO2 modeling 

(http://www.epa.gov/nsr/documents/20100629no2guidance.pdf). There should be some 

consistence between these modeling policy memos and this guidance document to be 

developed. If the guidance document identifies CALINE3 or CAL3QHC as allowed 

model, it also should explain if and how ARM and OLM can be used. 

Also see my response to Charge Question 6 on modeling. 

Charge Question 5 

A trailer-based transportable monitor will be very useful and practical for near-road 

monitoring. It will be self contained (a generator, analyzer, zero air, calibrator, retractable 

met tower, wireless modem, etc.) in a relatively small trailer. It can be pulled by a pickup truck to a candidate site for a day, a week, or a longer period of monitoring. It will be 

moved to another candidate site. Once the candidate site screening is completed, the 

trailer can be stationed in the chosen permanent site, blocked up and tied down to serve 

as the permanent near-road monitor in that CBSA. The data generated by such a system 

will have the same quality as fixed monitoring station. Compared to a motor vehicle 

based monitor, the trailer-based unit offers comparable mobility at much lower cost, and 

it can used as a fixed monitor at a permanent site for years. In terms of data quality and 

comparability, the data generated by a FRM or FEM analyzer in the trailer-based monitor 

has a higher quality and confidence level than the data generated by other screening 

instruments (passive devices and portable instruments). Presumably there will be no 

meteorological (met) instruments collocated with passive or portable devices. An analysis 

of the data gathered by these devices will rely on met data from nearby met stations. For 

near-road monitoring, the wind conditions will be relevant and extremely localized. The 

analysis based on met data from some distance could be misleading. 

Charge Question 6 

Peak CO concentrations are expected in urban street canyons and/or urban cores, 

especially at intersections where cars are idling in front of traffic light and the impact is 

coming from more than one street. I am not familiar with typical NO2 concentrations in 

this type of situation as compared to NO2 concentrations near major highway with heavy 

traffic. I am sure this type of data is available. If NO2 concentrations in urban street 

canyons are comparable to the NO2 concentrations near major highway, using one site to 

serve the monitoring need for both CO and NO2 should be encouraged in the guidance 

document. Otherwise, it would be infeasible to make a compromise between the two 

needs and the monitoring for CO and NO2 should be addressed separately. 

The CAL3QHC model is design to predict CO concentrations near road intersections. If 

EPA has validated the model, should the modeling be sufficient for determination of 

compliance with CO NAAQS and therefore no monitoring is required? In the recent SO2

NAAQS rule, EPA is changing its long-standing position of using monitoring data for 

NAAQS attainment determination, and will use modeling for NAAQS attainment 

determination. For the same rationale, using a modeling analysis to determine NO2 and 

CO NAAQS attainment seems reasonable. Similar to (actually even worse than) the case 

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of SO2, ambient CO and NO2 concentrations have an extremely high spatial variability 

near road. One monitor showing compliance with NAAQS at one street corner or road 

segment does not mean that the NAAQS is attained at different street corner or road 

segment. Modeling can cover a much larger space at a much lower cost. Even at the same 

street corner or road segment, moving the monitor by one meter could make the 

difference of attaining or not attaining NAAQS. 

Charge Question 11 

Before responding to Charge Question 11, I would like to ask if monitoring these 

pollutants with extremely high spatial variability in a micro-scale is a good idea. See my 

response to Charge Question 6. If the answer is no, there is no need to spending resources 

to develop definition of “urban street canyons” and “urban core” and associated guidance 

for monitoring. 

In case EPA wants to pursue monitoring at street locations with high traffic volume and 

high spatial concentration variability, the following elements should be considered in 

defining urban street canyons: 

• Traffic information similar to the one for near-road monitoring (e.g., AADT, 

posted speed limit, traffic light cycle) 

• Street geometry 

o Ratio of street side building height to the width of the street (H/W ratio). 

Need to develop an approach to the treatment of (1) different heights of 

buildings on the two sides of the street and (2) tiered buildings. 

o One-way vs. two-way street (more plug flow in one-way and more 

turbulent in two-way street). 

o Is the street lined with trees on the sidewalk? Tree canopy may have an 

effect of umbrella and trap portion of pollutants at the street level. 

o Some way to normalize the H/W ratio with respect to number of traffic 

lanes on the street. This factor may not be important because the effect 

may have been incorporated by the combination of H/W ratio and the 

traffic volume (e.g., AADT). 

• Meteorological factors: the angle between the street and prevailing wind direction 

(higher concentrations are expected if the angle is 90 degree). 

In the context of ambient air quality monitoring rule, perhaps a set of cut-off values 

reflecting the above mentioned elements can be used to define urban street canyons. 

Charge Question 14.c. 

The impact of mobile sources to ambient air quality is governed by two types of factors, 

vehicle emissions and dispersion conditions. For compliance monitoring, the monitors 

should be placed near the highest impact area, which means both emissions and 

dispersion conditions are equally important. For the pilot study, however, factors 

associated with dispersion (e.g., terrain, roadway design, extremely micro-scale 

meteorological conditions) should be given more attention than factors related to the level 

of vehicle emissions (e.g., AADT, fleet mix) because the emission rates can be 

characterized well using current tools (e.g., MOVES), AND the emission rates are the 

only parameter that impact ambient concentration in a linear or near linear fashion. If the 

pilot study can provide better understanding of the dispersion, the impact of a higher 

emission scenario can be anticipated or predicted by simply substituting the emission 

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rates. The information derived from a more dispersion focused pilot study will be more 

useful than an emission focused pilot study.