UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

 WASHINGTON D.C. 20460 

OFFICE OF THE ADMINISTRATOR 

SCIENCE ADVISORY BOARD 

September 09, 2009 

EPA-CASAC-09-014 

The Honorable Lisa P. Jackson 

Administrator 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Subject: Comments and Recommendations Concerning EPA's Proposed Rule for the Revision of 

the National Ambient Air Quality Standards (NAAQS) for Nitrogen Dioxide 

Dear Administrator Jackson: 

The Clean Air Scientific Advisory Committee (CASAC) Oxides of Nitrogen Primary National 

Ambient Air Quality Standards (NAAQS) Review Panel held a public teleconference on August 10, 2009 

to discuss the Environmental Protection Agency’s (EPA) Proposed Rule for the Revision of the NAAQS 

for Nitrogen Dioxide (NO2) published in the Federal Register on July 19, 2009 (74 FR 34404-34466). In 

that notice, EPA proposed to retain the current annual standard and institute a new one-hour standard 

based on a two-tier (area-wide and near-road) monitoring network. The proposal also described an 

alternative approach to setting a new one–hour standard, which would be based on area-wide monitors 

only. 

Prior to reviewing the proposed rule, CASAC had provided advice to EPA on its Integrated 

Science Assessment (ISA) for Oxides of Nitrogen – Health Criteria and on EPA's draft Risk and Exposure 

Assessment (REA) to Support the Review of the NO2 Primary National Ambient Air Quality Standard. 

The REA synthesized available scientific information discussed in the ISA to characterize NO2 exposures 

and the associated risks with different profiles of exposure, but did not discuss the alternative monitoring 

scenarios described in EPA's July proposed rule. In reviewing the REA, CASAC supported a short-term 

standard for NO2 and in reviewing the proposal, CASAC supports the proposed one-hour averaging time 

in EPA's proposed rule. 

Since EPA's REA did not address NO2 monitoring issues, the CASAC panel discussed 

monitoring issues, EPA's proposed monitoring approach, and proposed alternative approach extensively 

during the August teleconference. There was a split view on the two approaches among both CASAC and 

CASAC panel members with a majority of each favoring the Agency's proposed two-tiered monitoring 

network because they thought this approach would be more effective in limiting near-roadway exposures 

that may reach levels in the range at which some individuals with asthma may be adversely affected. 

Other members acknowledged the need for research and development of near-road monitoring data for 

criteria pollutants in general but favored retention of EPA's current area-wide monitoring for NO2

regulatory purposes, due to the lack of epidemiological data based on near-roadway exposure 

measurements and issues related to implementing a near-road monitoring system for NO2. CASAC panel 

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members agreed to present these different views in this letter, as there was not consensus. Enclosure A 

provides further comments about approaches to near-road monitoring. 

Assuming that the Agency proceeds with the proposed two-tier monitoring approach, CASAC 

recommends the following: 

1) retention of the current annual NO2 standard. 

2) establishment of a one-hour standard with the following characteristics: 

• use of NO2 as the appropriate indicator. 

• a form based on a three-year average. The 98th percentile is preferred by CASAC 

for the form, given the likely instability of measurements at the upper range and the 

absence of data from the proposed two-tier approach. 

• the level of the one-hour NO2 standard should be within the range of 80-100 ppb and 

not above 100 ppb. In its letter of December 2, 2008, CASAC strongly voiced a 

consensus view that the upper end of the range should not exceed 100 ppb, based on 

evidence of risk at that concentration. The lower limit of 80 ppb was viewed as 

reasonable by CASAC; selection of a value lower than 80ppb would represent a 

policy judgment based on uncertainty and the degree of public health protection 

sought, given the limited health-based evidence at concentrations below 100 ppb. 

If the alternative monitoring approach is selected, CASAC recommends that a one-hour NO2

standard be set in the range of 50 to 75 ppb, if protection is to be approximately equivalent to the range of 

80-100 ppb with the two-tier approach. 

CASAC members recognize that the Agency is moving quickly to promulgate the new NAAQS 

for NO2 on a court-ordered schedule. However, the proposed rule involves a substantially new approach 

to monitoring that might have benefited from a more in-depth review by CASAC than was possible with 

the schedule for comments. In the enclosure, CASAC comments on the need for a comprehensive 

approach to developing a near-roadway network that would involve other pollutants, in addition to NO2. 

This letter summarizes the main recommendations from the CASAC, while the enclosed 

comments (Enclosure A) provide further detail. Enclosure B provides a panel roster. We hope that our 

comments will prove useful as you evaluate the current NAAQS for NO2. If you have questions about 

our advice, please do not hesitate to contact me. 

Sincerely, 

/Signed/ 

Dr. Jonathan Samet, Chair 

Clean Air Scientific Advisory Committee 

Enclosures 

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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. 

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Enclosure A 

General Comments on Proposed NO2 Monitoring Requirements 

 A number of issues were identified by CASAC that are specific to roadside monitoring 

and the use of data from near-roadway monitors to determine attainment. 

1. Support for EPA's two-tiered monitoring network for NO2 regulatory monitoring

 A majority of CASAC panelists supported the Agency's proposed two-tiered monitoring network 

because they thought this approach would be more effective in limiting near-roadway exposures that may 

reach levels in the range at which some people with asthma may be adversely affected, as indicated in 

clinical and controlled human exposure studies. Members also indicated support for near-road monitoring 

if EPA took steps to address concerns outlined in Section 2 of this enclosure. 

 Panel members also supported the proposed two-tiered approach because basing 

regulations on area-wide monitoring alone was problematic. Such an approach would require 

EPA to embed uncertainties and assumptions about the relationship between area-wide and roadside monitoring into the area-wide standard. Panel members supported the two-tiered approach 

as a means to start gathering specific information about roadside exposures that will be used for 

public health protection. 

 The area-wide monitoring utilizing existing sites should continue as proposed. The areawide monitoring would provide a link to historically observed concentrations. It would thus 

support showing progress in reducing NOx for NO2, ozone and PM control programs and would 

also continue to support health analyses. In contrast, new roadside measurements of NO2 alone 

would provide minimal information on near-road multi-pollutant exposures and may have 

limited value in supporting future health studies. 

2. Support for alternative area-wide monitoring only for NO2 regulatory monitoring [EPA 

solicited comment on a lower range of levels (>50 to <75 ppb) to provide a similar degree of 

public health protection to that intended by the proposed approach and proposed range of 

levels (in conjunction with the same forms as those proposed]

 As noted in the Federal Register (July 15, 2009, pg. 34436):"In making judgments 

regarding the weight to place on the scientific evidence and exposure/risk information, the 

Administrator has considered the results of epidemiologic studies, controlled human exposure 

studies, and exposure/risk analyses as well as the uncertainties associated with this evidence and 

these analyses. Specifically, she notes the following: "The ISA concluded that epidemiologic 

studies provide the strongest support for the relationship between short-term exposure to NO2 

and respiratory morbidity." While it is recognized that the epidemiologic studies likely included 

people exposed along roadways among those affected populations, those epidemiologic studies 

that are relied upon by the Administrator (e.g., those displayed in Figures 4 and 5 of the Federal 

Register) did not generally use near-roadway exposure data, but instead usually relied upon 

conventional area-wide monitoring in developing their reported NO2-health effects associations. 

As such, in the face of a lack of sufficient near-roadway health effects studies to develop direct 

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exposure-health effects relationships, it would be better to set the standard on the same area-wide 

monitoring basis as employed in the epidemiologic studies upon which it now relies. 

 Several CASAC panel members noted the difficulty of designing a roadside monitoring 

network for NO2 at this time. Roadside monitors will be more likely to experience excursions of 

concentrations to very high values than area-wide monitors because they are nearer to sources 

and more susceptible to periods of high episodic emissions, low dispersion and other events that 

could lead to high peak monitored levels that are not reflective of more widespread levels (e.g., 

heavy congestion near a monitor, and trucks idling near the monitor). In the absence of more 

detailed roadside data collection, it will be difficult to identify the causes of extreme near-road 

NO2 concentrations or to understand how such concentration values are reflected in exposures of 

nearby urban populations. Further data collection and analyses are needed to determine the 

causes and frequency of extreme concentrations monitored by near-road monitors and to 

understand how such peak concentrations relate to those measured in the current, populationoriented network. 

 The concentrations monitored will be very sensitive to the specific location of near-road 

monitors, including exact distance from the roadway, height of monitor inlet, orientation with 

respect to prevailing wind directions (during peak traffic periods), and other likely factors. 

Differences in the microscale environments of different roadside monitors may result in one area 

being out of attainment even though the area actually has similar, and possibly lower, maximum 

NO2 levels than other areas. This possibility also exists for area-representative monitoring, but 

the likelihood of such non-representative values is reduced because the monitoring approach is 

not intended to capture the extreme upper end of the distribution. Various logistical 

considerations are likely to further constrain the options for establishing new roadside 

monitoring sites. Practical siting considerations could mean that selected locations will not be 

reflective of the maximum concentrations in the target area. Unless siting criteria are very 

tightly constrained, data may not be directly comparable across different cities or sites within 

cities. 

 To identify the locations with maximum NO2 levels will be difficult, and the proposed 

approach may miss the locations where the maxima occur. The proposed approach to 

identification of monitor sites may be unsatisfactory in many locations, because it is based 

primarily on traffic count. Diesel trucks emit more NOx than cars, and it would be expected that 

the actual maximum levels would be found in areas of high diesel activity (including rail yards 

and shipping) with reduced dispersion (e.g., street canyons). Open freeways that have high 

usage by light duty vehicles would not necessarily have the highest levels. In the design of a 

network to identify the maximum NO2 levels in an area, we would support a more broad 

assessment to predict the locations having the likely highest NO2 levels based upon investigation 

of model results [e.g., Community Multiscale Air Quality (CMAQ) and Land Use Regression 

(LUR) modeling, and Gaussian plume models, if available] and also emissions inventories. For 

example, CMAQ results, if available at a 4 or 12 km resolution, could be used to help inform 

analyses directed at identifying locations with likely higher area-wide NO2 levels, although such 

analyses would be less informative on locations with the highest NO2 levels in a city. The 

Agency could also use diesel truck activity. We note that the traffic data bases include activities 

by various classes of vehicles. Thus, we strongly support further analysis of the current NO2

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data, diesel truck activity data, model results, and emissions inventories, as well as vehicle 

activity data, to identify where area-representative sites might be placed to capture broad areas 

with higher NO2 levels. 

The use of diesel particle filters (DPFs) adds complexities to siting and to data 

interpretation. DPFs can lead to significant increases in the fraction of NO2 in NOx emissions. 

This phenomenon has been observed in London, England, where there have been NO2 increases 

in areas where DPFs are being used, and it may increase the potential to have local NO2 “hot 

spots” in locations with more presumably “cleaner” diesel vehicles in use. This phenomenon 

also will impact how near-road observations are related to more general population exposures. 

CASAC is further concerned by the suggestion in the proposal that building tops may be 

used for monitor placement. This placement should be done with caution as building tops may 

not be informative of much higher NO2 concentrations at lower heights where dispersion is 

limited. 

The expected high spatial variability of roadside NO2 concentrations also raises questions 

about the spatial representativeness of roadside monitor results. How large a non-attainment area 

is defined by a roadside violation? What population (if any) is assumed to be affected by the 

roadside concentrations? What is the public health message in presenting the results? Will the 

daily AQI for the county be driven by the relatively highest measurement from among 

neighborhood-scale (8-hr) ozone, (24-hour) PM2.5 monitors and roadside 1-hour NO2 – the latter 

of which by design does not represent the populations covered by the PM2.5 and O3 monitors? 

Would an effective control strategy be to reroute traffic from the high-speed urban freeway (with 

the monitor) onto low speed urban residential streets (without monitors)? 

 If the Administrator decides to adopt the alternative approach, monitoring issues remain. 

1. Preferably the NO/NO2 monitoring would be done in locations that include CO, ozone 

and PM species monitoring as well. 

2. EPA should follow the CASAC recommendation in regard to the level of the standard. 

3. Having the state/local air quality managers assess how the current network satisfies the 

goal of providing data representative of area-wide (not maximum) concentrations is 

appropriate. As noted above, the population threshold may be too low (it is not apparent 

that all cities with a population above 350,000 will have NO2 levels approaching the 

potential revised standards), and such a choice needs further support and review. One 

could consider requiring that removal of NO/ NO2 monitors not be allowed, and 

repositioning would need to have approval by the Regional Administrator. Further, each 

state could be required to have at least one NO2 monitor sited to represent areas with 

higher NO2 exposures. Giving the Regional Administrator discretion to require 

additional monitoring to address community impacts is justified based on appropriate 

analyses (e.g., using modeling and emissions data). 

4. The definition of “area-wide” is not very specific, though it reflects the realities of 

monitoring. It would be useful to specify a minimum distance that a monitor could be 

placed from a major facility (including freeways and highways), e.g., that a monitor that 

is representative of area-wide NO/ NO2 should not be placed within 100 m of a major 

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facility (with a further definition of major facility in terms of expected NOx emissions; 

for roads this could be in terms of NOx per 100m). 

3. Characterizing near-road exposures for multiple pollutants

If EPA undertakes near-road monitoring, the Agency should proceed with care and seek 

advice and review of the design of the network by technical experts, such as those in the CASAC 

Ambient Air Quality Monitoring Subcommittee. 

A smaller roadside network with more comprehensive measurements to better 

characterize near-road exposures to and gradients for a complex mixture of mobile source 

pollutants may provide more information than a new roadside monitoring network with hundreds 

of sites having only NOx monitors and meteorological instrumentation. In addition to 

characterizing NO2 and NO and meteorology, such monitoring should include CO, continuous 

PM, with speciation as possible (e.g., including elemental carbon or black carbon, continuous 

nitrate and sulfate, as possible, and possibly CO2 and gaseous VOCs). Such an alternative 

network, as discussed below, would not be designed to have all monitors necessarily placed in 

locations with the highest NO2 concentrations, but would also include locations that are impacted 

by heavy duty vehicle emissions, light duty vehicle emissions and suspended road dust 

Consideration would need to be given to the extent to which the network is designed with 

orientation towards areas with minimal dispersion (street canyons). Another siting criterion 

should be whether there is a location of interest near the roadway (e.g., a school, hospital, park, 

etc.) that may be attended or inhabited by persons in susceptible or vulnerable subpopulations. 

The need to have a more comprehensive set of measurements is highlighted by the likelihood of 

more extensive use of DPFs. If only NO2 observations were available, it might appear that air 

quality was deteriorating, when in actuality significant reductions in particulate matter are 

occurring. This would affect how such observations might be used in assessing the impact of 

controls and associating pollutant concentration changes to health outcomes. 

CASAC is very strongly supportive of a special-purpose monitoring network oriented 

towards roadside monitoring that is not used for attainment purposes at this point but for 

research. It would be designed to characterize pollutants in areas with high exposure to trafficderived pollutants of all types, including heavy duty and light-duty vehicle emissions, tire and 

brake wear and road dust, and would include pollutants beyond NO and NO2 as discussed above. 

Detailed meteorological measurements would be well justified at these more intensive sites, as 

would automatic traffic counters. 

Further, such a special network would specifically include variability in terms of site 

location (different distances to the facility, different highway characteristics) and type of 

monitoring to provide the type of data that would better inform our understanding of pollutant 

dynamics, control strategy effectiveness, and how a future road-side network should be designed 

for use in attainment decisions and standard setting. Such a network should go in quickly to 

capture the changes in emissions that are occurring in response to various controls. Funding for 

such special purpose monitoring may have to come predominantly from EPA. One approach for 

establishing the monitoring would be through competitive grants to state and local agencies. The 

network would likely be limited in scope, possibly on the order of 20 to 50 sites and it could 

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evolve over time as information is gained. A single city might have multiple near-road specialpurpose monitors to address issues in siting near-road monitors and to understand relationships 

between roadside concentrations and larger population exposures. Cities with various 

characteristics should be chosen for assessing near-road monitoring issues. This approach 

appears to fit well with a recent EPA document, “Ambient Air Monitoring Strategy for State, 

Local, and Tribal Air Agencies,” which states: 

“With this background, EPA’s Strategy currently recognizes (1) the importance of near 

roadway exposures, and (2) the need for further exploration of the meaning of these 

exposures to both NAAQS-oriented monitoring networks and air toxics networks. 

Monitoring near roadways has, to date, been limited to research-level monitoring. As 

monitoring networks evolve, it is vital that monitoring near roadways be further 

investigated and eventually integrated into the monitoring networks. Currently, EPA and 

others continue to evaluate strategies for incorporating this monitoring into the other 

components of the monitoring Strategy primarily as a means of determining health risks 

and impacts on urban attainment. EPA intends to consult with SLTs and other 

stakeholders about the eventuality of developing the near-roadway component of ambient 

monitoring. The primary consideration would be to operate a small number of sites 

spaced in varying geographical areas of the country in an initial attempt to address nearroadway issues. Outcomes from EPA’s Office of Research and Development’s (ORD) 

near-roadway studies that began in 2006 and extend through 2009 or 2010 would heavily 

influence where, how, and when a near-roadway monitoring pilot would occur.”

 If there is a mechanism to begin more comprehensive roadside monitoring but not 

include such data in attainment determination, then a two-tier monitoring approach is potentially 

attractive for support of health studies that could be used in future NAAQS reviews. However, 

as discussed above, such monitoring should not be limited to NO and NO2 only, but also include 

CO, continuous PM mass, and continuous total carbon/elemental carbon (or black carbon). 

Without this additional data, the results would be significantly less informative. If, as part of the 

current CO and PM reviews, a network of roadside monitoring of all of these species was 

proposed and implemented, the data could potentially be used in supporting a future NAAQS 

revision for those species. The data would be more readily used in a multi-pollutant air quality 

control program and for the determination of the effectiveness of control measures. Monitoring 

of NO/NO2 alone would not be as informative, particularly given the changing make-up of both 

light and heavy duty fleets, applied controls, and the change in traffic volumes. 

4. Additional issues

 The CASAC panel determined that the proposed data quality objectives are appropriate. 

 The chemiluminescent approach is appropriate. It would be good to improve and extend 

the technique to give both true NO2 as well as NOy. However, that would not be viable within 

the time frame of this rulemaking. 

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 The justification for the meteorological measurements is unclear in regard to assessing 

attainment. The need for and uses of such data should be better supported. We agree that such 

monitoring would be very useful at comprehensive monitoring sites. 
Enclosure B 

ROSTER 

U.S. Environmental Agency 

Clean Air Scientific Advisory Committee 

Oxides of Nitrogen Primary National Ambient Air Quality Standards Panel 

CHAIR 

Dr. Jonathan M. Samet, Professor and Chair, Department of Preventive Medicine, University 

of Southern California, Los Angeles, CA 

CASAC MEMBERS 

Dr. Joseph Brain, Philip Drinker Professor of Environmental Physiology, Department of 

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

Dr. Ellis B. Cowling, University Distinguished Professor At-Large Emeritus, Colleges of 

Natural Resources and Agriculture and Life Sciences, North Carolina State University, Raleigh, 

NC 

Dr. James Crapo, Professor of Medicine, Department of Medicine, National Jewish Medical 

and Research Center, Denver, CO 

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

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

Dr. Donna Kenski, Data Analysis Director, Lake Michigan Air Directors Consortium, 

Rosemont, IL* 

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

Georgia Institute of Technology, Atlanta, GA 

 

CONSULTANTS 

Prof. Ed Avol, Professor, Preventive Medicine, Keck School of Medicine, University of 

Southern California, Los Angeles, CA 

Dr. John R. Balmes, Professor, Department of Medicine, Division of Occupational and 

Environmental Medicine, University of California, San Francisco, CA* 

Dr. Terry Gordon, Professor, Environmental Medicine, New York University School of 

Medicine, Tuxedo, NY 

Dr. Dale Hattis, Research Professor, Center for Technology, Environment, and Development, 

George Perkins Marsh Institute, Clark University, Worcester, MA 

*Unable to participate in the August 10, 2009 CASAC Public Teleconference 

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Dr. Rogene Henderson, Senior Scientist Emeritus, Lovelace Respiratory Research Institute, 

Albuquerque, NM 

Dr. Patrick Kinney, Associate Professor, Department of Environmental Health Sciences, 

Mailman School of Public Health , Columbia University, New York, NY 

Dr. Steven Kleeberger, Professor and Lab Chief, Laboratory of Respiratory Biology, National 

Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle 

Park, NC 

Dr. Timothy V. Larson, Professor, Department of Civil and Environmental Engineering, 

University of Washington, Seattle, WA 

Dr. Kent Pinkerton, Professor, Regents of the University of California, Center for Health and 

the Environment, University of California, Davis, CA* 

Dr. Edward Postlethwait, Professor and Chair, Department of Environmental Health Sciences, 

School of Public Health, University of Alabama at Birmingham, Birmingham, AL 

Dr. Richard Schlesinger, Associate Dean, Department of Biology, Dyson College, Pace 

University, New York, NY 

Dr. Christian Seigneur, Director, Centre d'enseignement et de recherche en environnement 

atmosphérique (CEREA), Ecole nationale des ponts et chaussées (ENPC), Université Paris-Est, 

CEREA - ENPC, Marne la Vallée, , France 

Dr. Elizabeth A. (Lianne) Sheppard, Professor, Biostatistics and Environmental & 

Occupational Health Sciences, School of Public Health, University of Washington, Seattle, WA 

Dr. Frank Speizer, Edward Kass Professor of Medicine, Channing Laboratory, Harvard 

Medical School, Boston, MA 

Dr. George Thurston, Professor, Environmental Medicine, NYU School of Medicine, New 

York University, Tuxedo, NY 

Dr. James Ultman, Professor, Chemical Engineering, Bioengineering Program, Pennsylvania 

State University, University Park, PA 

Dr. Ronald Wyzga, Technical Executive, Air Quality Health and Risk, Electric Power 

Research Institute, Palo Alto, CA 

SCIENCE ADVISORY BOARD STAFF 

Dr. Angela Nugent, Designated Federal Officer, 1200 Pennsylvania Avenue, NW 

1400F, Washington, DC, Phone: 202-343-9981, Fax: 202-233-0643, (nugent.angela@epa.gov) 

*Unable to participate in the August 10, 2009 CASAC Public Teleconference