Responses to Significant Comments on the

2009 Proposed Rule on the

Primary National Ambient Air Quality Standards

for Nitrogen Dioxide

(July 15, 2009; 74 FR 34404)

Docket Number OAR-2006-0922

U.S. Environmental Protection Agency

January 2010
Table of Contents

I. Introduction........................................................................................................................... 1

II. Responses to Significant Comments on the Scientific Evidence and Exposure/Risk 

Information.................................................................................................................................... 2

A. General comments on the scientific evidence .................................................................. 2

B. Comments on the epidemiologic evidence....................................................................... 2

C. Comments on the controlled human exposure evidence................................................ 10

D. Comments on air quality, exposure, and risk analyses .................................................. 13

III. Responses to Significant Comments on the Adequacy of the Current Standard ......... 35

IV. Comments on A New Short-Term NO2 Primary Standard ............................................ 37

A. Indicator ......................................................................................................................... 37

B. Averaging time............................................................................................................... 38

C. Form ............................................................................................................................... 39

D. Approach and level......................................................................................................... 40

1. Comments on the approach to setting the standard .................................................... 40

2. Comments on standard level....................................................................................... 45

3. Comments on the annual standard.............................................................................. 51

V. Technical Issues with Monitoring Requirements............................................................. 53

A. Comments on near-road monitor siting requirements.................................................... 53

B. Comments on area-wide monitor requirements............................................................. 59

C. Comments on the need for a research monitoring network ........................................... 61

D. Justification of meteorological measurements............................................................... 65

E. Monitoring technology................................................................................................... 65

F. Timing of monitor deployment.......................................................................................... 68

G. Other monitoring issues ................................................................................................. 69

VI. Air Quality Index ................................................................................................................ 72

VII. Comments on the Process for Reviewing the NO2 Primary NAAQS............................. 74

VIII. Interpretation of the Clean Air Act............................................................................ 76

IX. Comments on Implementation........................................................................................... 78

X. References............................................................................................................................ 86
Frequently Cited Documents

The following documents are frequently cited throughout EPA=s response to comments, often by

means of the short names listed below:

Integrated Science Assessment (ISA):

EPA. (2008a). ISA for Oxides of Nitrogen - Health Criteria. National Center for 

Environmental Assessment, Research Triangle Park, NC. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=194645.

Preamble to the final rule (final notice):

Preamble to the Final Rule on the Review of the Primary National Ambient Air Quality

Standards for Nitrogen Dioxide; to be published in the Federal Register in January or 

February of 2010.

Proposal notice:

Primary National Ambient Air Quality Standard for Nitrogen Dioxide: Proposed Rule. 

74 FR 34404, July 15, 2009.

Risk and Exposure Assessment (REA):

EPA. (2008b). Risk and Exposure Assessment to Support the Review of the NO2 

Primary National Ambient Air Quality Standard. Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. Available at: 

http://www.epa.gov/fedrgstr/EPA-AIR/2009/July/Day-15/a15944.pdf
Acronyms and Abbreviations

AADT Annual average daily traffic

AAM Alliance of Auto Manufacturers

AAQM Ambient Air Quality Monitoring

AASHTO American Association of State Highway and Transportation Officials

ACC American Chemistry Council

ACSBPP Annapolis Center for Science-Based Public Policy

AGCA Association General Contractors of America 

AHR Airways hyperresponsiveness

AQRL AirQuality Research and Logistics

ALA American Lung Association

ANPR Advance Notice Of Proposed Rulemaking

API American Petroleum Institute

AQI Air quality index

ATS American Thoracic Society

CAA Clean Air Act

CAAPCOA California Air Pollution Control Officer's Association

CAC Clean Air Council

CASAC Clean Air Scientific Advisory Committee

CBSA Core-based statistical area

CE Consumers Energy

CO Carbon monoxide

COPA Colorado Petroleum Association

CSE Coalition for a Safe Environment

Dow Dow Chemical Company

EDF Environmental Defense Fund

EEI Edison Electric Institute

EJ EarthJustice

EMA Engine Manufacturers Association

EPA U.S. Environmental Protection Agency

Exxon ExxonMobil Refining and Supply Company

FEM Federal Equivalent Method

FEV Forced Expiratory Volume

FRM Federal Reference Method

GASP Group Against Smog and Pollution

HCPHES Harris County Public Health and Environmental Services
HEI Health Effects Institute

HLI Healthy Lungs Initiative

IADNR Iowa Department of Natural Resources

INDEM Indiana Department of Environmental Management

INGAA Interstate Natural Gas Association of America

IPAMS Independent Petroleum Association of Mountain States

ISA Integrated Science Assessment

MET Meteorological

MODNR Missouri Department of Natural Resources Air Pollution Control Program

NAAQS National Ambient Air Quality Standards

NACAA National Association of Clean Air Agencies

NAM National Association of Manufacturers

NCDENR North Carolina Department of Environment and Natural Resources

NESCAUM Northeast States for Coordinated Air Use Management

NIOSH National Institute of Occupational Safety and Health

NMA National Mining Association

NMED New Mexico Environment Department Air Quality Bureau

NO2 Nitrogen Dioxide

NOX Nitrogen Oxides

NPRA National Petrochemical and Refiners Association

NRDC National Resource Defense Council 

NSR New Source Review

NTAA National Tribal Air Association

NYDOH New York Department of Health

NYSDEC New York State Department of Environmental Conservation

OIPA Oklahoma Independent Petroleum Association

OLM Ozone Limiting Method

OSHA Occupational Safety and Health Association

PAW Petroleum Association of Wyoming

PEL Permissible Exposure Level

PM Particulate Matter

ppb Parts Per Billion

ppbv Parts Per Billion by Volume

ppmv Parts Per Million by Volume

PSD Prevention of Significant Deterioration

PVMRM Plume Volume Molar Ratio Method

QC Quality control
REA Risk and Exposure Assessment

RHAMC Respiratory Health Association of Metropolitan Chicago

RIA Regulatory Impact Analysis

SCAQMD South Coast Air Quality Management District

SDDENR South Dakota Department of Environment and Natural Resources

SIL Significant Impact Level

SIP State Implementation Plan

STEL Short-Term Exposure Level

TFI The Fertilizer Institute

TXCEQ Texas Commission on Environmental Quality

UPA Utah Petroleum Association

UARG Utility Air Regulatory Group

VADOT Virginia Department of Transportation
I. Introduction

This document, together with the preamble to the final rule on the review of the primary

national ambient air quality standards (NAAQS) for nitrogen dioxide (NO2), presents the 

responses of the Environmental Protection Agency (EPA) to the public comments received on 

the 2009 NO2 NAAQS proposal notice (74 FR 34404). All significant issues raised in timely

public comments have been addressed. Where comments were submitted after the close of the 

public comment period, EPA has responded to the extent feasible.

Due to the number of comments that addressed similar issues, this response-to-comments 

document does not generally cross-reference each response to the commenter(s) who raised the 

particular issue involved, although commenters are identified in some cases where they provided 

particularly detailed comments that were used to frame the overall response on an issue. 

The responses presented in this document are intended to augment the responses to 

comments that appear in the preamble to the final rule or to address comments not discussed in 

the preamble to the final rule. Although portions of the preamble to the final rule are 

paraphrased in this document where useful to add clarity to responses, to the extent any

ambiguity is introduced by this paraphrasing, the preamble itself remains the definitive statement 

of the rationale for the revisions to the standards adopted in the final rule.

In many instances, particular responses presented in this document include cross 

references to responses on related issues that are located either in the preamble to the NO2

primary NAAQS final rule, or in this Response to Comments document. In other instances the 

comment is appropriately addressed by the Agency’s discussion in other parts of the record. All 

issues on which the Administrator is taking final action in the NO2 primary NAAQS final rule 

are addressed in the NO2 NAAQS rulemaking record. 

 Accordingly, this Response to Comments document, together with the preamble to the NO2

primary NAAQS final rule and the information contained in the Integrated Science Assessment 

(ISA) (EPA, 2008a), the Risk and Exposure Assessment (REA) (EPA, 2008b), and the Notice of 

Proposed Rulemaking should be considered collectively as EPA’s response to all of the 

significant comments submitted on EPA’s 2009 NO2 primary NAAQS proposed rule. This 

document incorporates directly or by reference the significant public comments addressed in the 

preamble to the NO2 NAAQS final rule as well as other significant public comments that were 

submitted on the proposed rule.

Consistent with the final decisions presented in the notice of final rulemaking, comments 

on the following topics are addressed in this document: the scientific evidence and exposure/risk 

information (section II); the adequacy of the current NO2 standard to protect public health 

(section III); revisions to the current standard in terms of indicator (section IV.A), averaging time 

(section IV.B), form (section IV.C), and level (section IV.D); revisions to the NO2 monitoring 

network (section V); the air quality index (VI); the process for reviewing the standard (section 

VII); interpretation of the Clean Air Act (section VIII); and implementation of the standard

(section IX);. 
II. Responses to Significant Comments on the Scientific Evidence

and Exposure/Risk Information

A. General comments on the scientific evidence

(1) Comment: Some commenters (e.g., AAM, ACC) stated that it is not clear what was 

meant in the ISA by a “likely causal” relationship, stating that this classification is 

“somewhat ambiguous” and “at odds with” frameworks developed by IARC and 

NAS/IOM (ACC, p. 2) These groups concluded that it would have been preferable not to 

include this characterization of the evidence in the ISA. 

Response: EPA’s causality framework is not at odds with other frameworks. EPA’s use 

of a five-level hierarchy in the ISA is consistent with EPA’s Guidelines for Carcinogen 

Risk Assessment (EPA, 2005). Excerpts from those guidelines are included in Annex A 

(AX1 pp. 1-6 to 1-9); the second of five descriptors is “Likely to be Carcinogenic to 

Humans.” IARC’s classification of carcinogens is also excerpted in Annex A (AX1 pp. 1-

14 to 1-20), and it includes Group 1 (carcinogenic), Group 2A (probably carcinogenic), 

Group 2B (possibly carcinogenic), Group 3 (not classifiable) and Group 4 (probably not 

carcinogenic). In contrast with commenters’ assertion that IARC’s system differs from 

EPA’s, these five groups and subgroups are clearly analogous to the five tiers in EPA’s 

framework. Commenters also refer to the NAS/IOM framework (IOM, 2007) that was 

used as a resource for EPA’s causal framework and excerpted in Annex A (AX1 pp. 1-9 

to 1-12). The four categories used in that system – “sufficient”, “equipoise and above”, 

“below equipoise” and “against” – are different from those used in most other 

classifications. However, the IOM report included detailed discussion of the approach to 

evaluating evidence from across scientific disciplines that served as an important 

resource for EPA’s weight of evidence determination. In developing its framework 

drawing causal inferences, EPA carefully considered and drew upon the work of previous 

assessments to build a framework for determination of causality that is consistent with 

EPA’s cancer risk assessment guidelines as well as those of other expert organizations. 

The development and implementation of this framework has been lauded by the CASAC 

in recent ISA reviews such as that in the PM NAAQS review (Samet, 2009a). 

B. Comments on the epidemiologic evidence

(1) Comment: Several industry groups (e.g., API, AAM, ACC) commented that the 

epidemiologic studies did not support a causal relationship between NO2 and health 

effects due to uncertainties and issues related to model specification. These commenters 

state that EPA has ignored or understated issues such as selection of degrees of 

smoothing appropriate to adjust for weather and time trends and selection of lag period.

Response: EPA has not ignored issues related to model specification for epidemiologic 

studies, such as selection of models and approaches to adjust for meteorological and 

temporal variables. EPA agrees with commenters that these are important issues. In this 

and previous NAAQS reviews, EPA has carefully evaluated such issues, including 
sponsorship of workshops and special analyses of epidemiologic data, such as the Health 

Effects Institute (HEI) report (2003) cited by several commenters (e.g., ACC, AAM). As 

observed in the NOx ISA (p. 3-1), extensive discussions of the issues surrounding model 

selection and model specification have been presented in the PM AQCD (EPA, 2004) and 

the Ozone AQCD (EPA, 2006) and are thus not reiterated at length in this ISA. The NOx 

ISA makes clear, however, that these issues were were carefully considered in selecting 

studies for inclusion in the ISA and interpreting the results of the body of epidemiologic 

evidence. 

Commenters specifically refer to the Health Effects Institute report on the reanalysis of a 

series of time-series epidemiologic studies to evaluate alternative modeling strategies, 

and include a brief quote from the Health Review Committee Panel (HEI, 2003): 

“Neither the appropriate degree of control for time, nor the appropriate specification of 

the effects of weather, has been determined for time-series analysis.” (ACC, p. 7, quoting 

HEI, 2003, p. 269). Taken in full context, it can be seen not as condemnation of all 

epidemiologic time-series studies but rather as a call for further investigation. The 

primary conclusion drawn by the HEI panel was, rather: “The overall impact of revising 

these studies include: While the number of studies showing an association of PM with 

mortality was slightly smaller, the PM association persisted in the majority of studies. In 

some of the large number of studies in which the PM association persisted, the estimates 

of PM effect were substantially smaller. In the few studies in which investigators 

performed further sensitivity analyses, some showed marked sensitivity of the PM effect 

estimate to the degree of smoothing and/or the specification of weather” (HEI, 2003, p. 

269). The results of this extensive set of reanalyses was thoroughly discussed in the PM 

AQCD (EPA, 2004, Section 8.4.2). EPA has not ignored or downplayed these issues, in 

fact, EPA has scrutinized epidemiologic studies and has funded independent reanalyses 

of epidemiologic studies.

EPA observes that the reanalyses discussed above and cited by commenters are primarily 

on PM health effects studies. Few reanalyses have specifically focused on NOx; however 

EPA has recognized that model specification issues are generally similar for both PM and 

the gaseous criteria air pollutants. EPA agrees that model specification and control of 

time-varying factors such as weather, remains important and has considered the available 

evidence in its evaluation of epidemiologic studies. The EPA also agrees with 

commenters that season-specific analyses have not been widely used in epidemiologic 

studies for NOx,1however, EPA observes that a number of studies included in the ISA 

did report results from seasonal models. The EPA disagrees that the available studies can 

support a conclusion that air pollution effects are consistently and strongly modified by 

season. In summary, EPA has not ignored the potential influence of model selection and 

specification in reviewing epidemiologic studies; rather, EPA has scrutinized such issues 

in previous assessments as well as the NOx ISA. These issues were fully considered in 

 

1

 It should be noted that use of smaller data subsets will markedly reduce statistical power to 

detect associations and also reduce the precision on any findings when the data are stratified by

season.
drawing the conclusion that there was a likely causal relationship between short-term 

exposure to NOx and respiratory morbidity.

(2) Comment: AAM challenges ISA conclusions on several health outcome categories on 

pp. 31-44. The commenter contends that “EPA overstates the strength and consistency of 

epidemiological evidence regarding various potential health effects”.

Response: The EPA disagrees with these commenters approach to assessing health 

effects evidence as well as their conclusion regarding the lack of a scientific basis to 

support the continuation of NAAQS to protect against the health effects of NO2. The 

EPA contrasts these commenters’ narrow focus on counting the numbers of 

epidemiologic studies that report results with statistical significance, without regard to 

other considerations that are important to consider in a comprehensive appraisal of the 

evidence, with the approach taken by EPA in the ISA. Specifically, EPA recognizes the 

distinction between evaluation of the relative scientific quality of individual study results, 

and evaluation of the pattern of results in a body of evidence. The EPA has done both. 

The more detailed characterizations of individual studies include assessment of the 

quality of the study, based on criteria for assessment of the epidemiologic studies that are 

described in Section AX 1.1.2 (pg. 1-3) and Figure AX1.1-1 (pg. 1-2) of the Annexes to 

the ISA for Oxides of Nitrogen. Statistical significance is an indicator of the precision of 

a study’s results, which is influenced by the size of the study, as well as exposure and 

measurement error, model specifications, and other such factors.

In developing an integrated assessment of the health effects evidence for NO2, EPA has 

emphasized the importance of examining the pattern of results across various studies, and 

not focusing solely on statistical significance as a criterion. It is important not to focus 

on results of statistical tests to the exclusion of other information. As observed by 

Rothman (1998): 

Many data analysts appear to remain oblivious to the qualitative 

nature of significance testing. Although calculations based on 

mountains of valuable quantitative information may go in to it, 

statistical significance is itself only a dichotomous indicator. As it 

has only two values, significant or not significant, it cannot convey

much useful information....Nevertheless, P-values still confound 

effect size with study size, the two components of estimation that 

we believe need to be reported separately. Therefore, we prefer 

that P-values be omitted altogether, provided that point and 

interval estimates, or some equivalent, are available. (Rothman, 

1998, p. 334)

The concepts underlying EPA’s approach to integrated assessment of statistical 

associations reported for the health effects on NO2 have been discussed in numerous 

publications, including a recent report by the U.S. Surgeon General on the health 

consequences of smoking (Centers for Disease Control and Prevention, 2004). This 
report also cautions against over-reliance on statistical significance in evaluating the 

overall evidence for an exposure-response relationship.

Hill made a point of commenting on the value, or lack thereof, of statistical testing in the 

determination of cause: “No formal tests of significance can answer those [causal] 

questions. Such tests can, and should, remind us of the effects the play of chance can 

create, and they will instruct us in the likely magnitude of those effects. Beyond that, 

they contribute nothing to the ‘proof’ of our hypothesis” (Hill 1965, p. 299).

Hill’s warning was in some ways prescient, as the reliance on statistically significant 

testing as a substitute for judgment in a causal inference remains today (Savitz et al., 

1994; Holman et al., 2001; Poole 2001). To understand the basis for this warning, it is 

critical to recognize the difference between inductive inferences about the truth of 

underlying hypotheses, and deductive statistical calculations that are relevant to those 

inferences, but that are not inductive statements themselves. The latter include p values, 

confidence intervals, and hypothesis tests (Greenland 1998; Goodman 1999). The 

dominant approach to statistical inference today, which employs those statistical 

measures, obscures this important distinction between deductive and inductive inferences 

(Royall 1997), and has produced the mistaken view that inferences flow directly and 

inevitably from data. There is no mathematic formula that can transform data into a 

probabilistic statement about the truth of an association without introducing some formal 

quantification of external knowledge, such as in Bayesian approaches to inference 

(Goodman 1993; Howson and Urbach 1993). Significance testing and the 

complementary estimation of confidence intervals remain useful for characterizing the 

role of chance in producing the association in hand (CDC, 2003, pp. 23-24).

Accordingly, the statistical significance of individual study findings has played an 

important role in EPA’s evaluation of the study’s results and EPA has placed greater 

emphasis on studies reporting statistically significant results. However, in the broader 

evaluation of the evidence from many epidemiologic studies, EPA has also emphasized 

the pattern of results for drawing conclusions on the relationship between NO2 and health 

outcomes, as well as consideration of the integration of epidemiologic evidence with 

findings from laboratory studies.

The EPA considered the results of studies conducted in many different countries to draw 

conclusions about the likelihood of a causal relationship between NO2 and health 

outcomes. Because EPA places greater weight on US and Canadian studies in 

determining standard levels, the ISA, REA and proposal notice present graphical results 

from epidemiologic studies in these two countries, standardized to a common increment 

of NO2, and based on similar analytic strategies (i.e. single-pollutant results). EPA 

believes that the examination of multipollutant model results and the inherent instability

that often occurs in effect estimates for correlated pollutants in such studies justifies the 

use of single pollutant model results, in addition to multi-pollutant model results, for 

comparing effect estimates for NO2 and health outcomes. 
The comparisons across studies in the ISA begins with an evaluation of the overall 

pattern of excess risk results – whether generally positive or centered around zero - the 

consistency in size of effect estimates, the precision of the studies evidenced in the width 

of the confidence intervals, with special attention to comparisons of similar effect 

categories. 

Finally, it is important to reiterate that the EPA’s evaluation of the scientific evidence 

used in the current NOX NAAQS review was the subject of exhaustive and detailed 

review by the CASAC and the public. Two drafts of the ISA were released for CASAC 

and public review at public meetings. Evidence related to the substantive issues raised by 

submitted comments were evaluated in the ISA drafts and discussed at length in public 

CASAC meetings. This process ensured that overemphasis on any study or group of 

studies was addressed. Following the final meeting with CASAC on the ISA, the 

CASAC panel found the coverage of the literature in the ISA to be appropriate. 

(3) Comment: Several industry groups (e.g., API, AAM, ACC, ACSBPP, CE, COPA, Dow, 

EEI, EMA, Exxon, NMA, TFI, UARG) and an individual (Roger McClellan) commented

that, given the presence of numerous copollutants in the air, epidemiologic studies do not 

support the contention that NO2 itself is causing health effects. These groups concluded 

that EPA has overstated the strength and consistency of the epidemiologic evidence. For 

example, API commented that “These [epidemiologic] studies do not provide support for 

a causal association between short-term NO2 exposure and respiratory effects and, thus, 

do not provide appropriate support for a 1-hour daily maximum NO2 standard.” API also 

stated that “Most [epidemiologic studies] only report statistically significant findings in 

single-pollutant, but not multi-pollutant, models, and no statistically significant finding is 

large, robust, or consistent.” Several industry commenters (e.g., API, ACC, ACSBPP) 

further noted that the ISA relies primarily on single-pollutant models when interpreting 

epidemiologic studies, rather than evaluating the results within the context of the full 

suite of air pollutants. These commenters noted that this can lead to overestimating NO2

effects and to double or triple counting by attributing effects to NO2 in the current review 

that are attributed to different pollutants in other NAAQS reviews. Similar conclusions 

were also expressed by other commenters. 

Response: These comments and EPA’s responses are discussed in section II.E.2.a of the 

final rule. The Administrator’s consideration of the epidemiologic evidence specifically 

as it relates to decisions on the adequacy of the current standard, the averaging time of a 

new short-term standard, and the level of the short-term standard are discussed in sections 

II.E and II.F of the final rule. 

EPA has not focused solely on the results of single pollutant models, but has also 

carefully examined the implications on multiple pollutant results. The greatest weight of 

evidence for multi-pollutant results has been placed on two-pollutant models (NO2 plus 

one additional pollutant), as the inclusion of each additional pollutant in the model can 

decrease model stability. This decrease in model stability is often reflected in wider 

confidence intervals, making it less likely for a statistically significant result to be 
observed. Thus, when a statistically significant effect estimate observed in a single 

pollutant model is no longer statistically significant in a copollutant model (even though 

the magnitude and direction of the effect estimate has not changed substantially), this 

may be an artifact of model instability. Results in Figures 3.1-5, 3.1-7, 3.1-10 and 3.1-

11, indicate that the association of NO2 with respiratory morbidity is robust to the 

addition of copollutants. While some individual studies may report specific findings that 

are more influenced by copollutants, Figures 3.1-5, 3.1-7, 3.1-10 and 3.1-11 clearly 

demonstrate EPA’s conclusion of robustness. 

The EPA strongly disagrees that the agency is “double or triple counting by attributing 

effects to NO2 in the current review that are attributed to different pollutants in other 

NAAQS reviews”. The EPA consistently recognizes that other pollutants are also 

associated with health outcomes, as is reflected in the fact that EPA has established 

regulations to limit emissions of the particulate criteria pollutants as well as other gaseous 

criteria pollutants. In its assessment of the health evidence regarding NO2, EPA has 

carefully evaluated the potential for confounding, effect measure modification or other 

interactions between NO2 and other criteria pollutants, and concluded that the results 

attributable to NO2 are robust.

(4) Comment: Some industry groups (e.g., API, Exxon Mobil (Exxon)) commented that 

reliance on central monitors in epidemiologic studies leads to a high degree of exposure 

misclassification. Specifically, Exxon commented that “there is a lack of correlation 

between measurements from ambient monitors for NO2 and those from personal monitors 

for NO2. This makes the observational epidemiology data invalid for assessing the 

connection between exposure and health effects for NO2.” API commented that “All 

[epidemiologic] studies used measurements from central monitors, which likely led to a 

high degree of exposure misclassification.” 

Response: As discussed in the ISA (section 5.2.2), EPA agrees that there is variability in 

the extent to which NO2 concentrations measured by ambient monitors correlate with 

personal exposures. However, EPA disagrees with the conclusion of the commenter that 

“This makes the observational epidemiology data invalid for assessing the connection 

between exposure and health effects for NO2.” Rather, as noted in the conclusions of the 

ISA (section 5.2.2, p. 5-3), “The errors and uncertainties associated with the use of 

ambient NO2 concentrations as a surrogate for personal exposure to ambient NO2

generally tend to reduce rather than increase effect estimates, and therefore are not 

expected to change the principal conclusions from NO2 epidemiologic studies.” 

Therefore, EPA continues to judge that it is appropriate to consider the NO2

epidemiologic evidence in reviewing the NAAQS.

(5) Comment: UARG also stated the following with regard to the epidemiologic evidence: 

Another compelling source of uncertainty regarding the 

epidemiologic studies on which EPA relies is the discrepancy
between the levels at which health effects were observed in these 

studies versus human clinical studies. The epidemiologic studies 

on which EPA relies in the Proposed Rule report effects at NO2

concentrations much lower than the doses used in human clinical 

studies.

Response: As discussed in detail in the final notice (sections II.A and II.F.4), controlled 

human exposure studies reported effects down to the lowest concentration evaluated; no 

studies have been conducted at lower concentrations. While these concentrations are 

higher than the mean concentrations reported in epidemiologic studies, it is important to 

note that epidemiologic studies conducted in the United States have reported associations 

between ambient NO2 concentrations measured at area-wide monitors in the current 

network and increased respiratory symptoms, emergency department visits, and hospital 

admissions. Area-wide monitors in the urban areas in which these epidemiologic studies 

were conducted are not sited in locations where localized peak concentrations are likely 

to occur. Thus, they do not measure the highest ambient NO2 concentrations across the 

area. Rather, the area-wide NO2 concentrations measured by these monitors are used as 

surrogates for the distribution of ambient NO2 concentrations across the area, a 

distribution that includes NO2 concentrations that are higher than the area-wide 

concentrations measured in study locations. 

As noted in the ISA (section 5.3.2.1 and Figure 2.4-13), 1-hour NO2 concentrations 

measured at area-wide monitors in the United States have been observed in the range of 

100 to 200 ppb. In addition, even higher 1-hour NO2 concentrations could occur on 

and/or around major roads, where NO2 concentrations could be 30-100% higher than 

measured by existing area-wide monitors (section II.A.2 of the final notice). Therefore, 

EPA does not agree that the NO2 concentrations that occurred in the locations of 

epidemiologic studies did not include concentrations that overlapped with those reported 

to cause respiratory effects in controlled human exposure studies. 

(6) Comment: AAM contends that “the ISA uses the results of respiratory symptom studies 

to claim coherence with the hospital and ED admission results. However the authors of 

the Mortimer et al. and Schildcrout et al. multi-city studies that the Agency relies on do 

not implicate NO2, per se, but summer time air pollution and fine PM, respectively”, and 

claims that the characterization of the results in studies by Mortimer et al. (2002), and 

Schildcrout et al. (2006) is not consistent with the authors’ conclusions. 

Response: The commenter is wrong in the characterization of these studies. They have 

inaccurately characterized the authors’ conclusions from these studies. In fact, Mortimer 

et al. (2002) concluded that “Nitrogen dioxide, sulphur dioxide, and particles with a 50% 

cut-off aerodynamic diameter of 10 μm were associated with increases in symptoms, with 

nitrogen dioxide exhibiting the strongest influence”. Similarly in the Schildcrout et al. 

(2006) study, the authors conclude that “Among the pollutants studies, carbon monoxide 

and nitrogen dioxide appeared to capture the most relevant health information. Sulfur 

dioxide showed less evidence for a relation with asthma exacerbations unless it was 
considered in combination with carbon monoxide or nitrogen dioxide. There was no 

evidence of a warm-season effect of ozone or of a year-round effect of PM10.” In fact, 

fine PM (PM2.5) was not even included in the study by Schildcrout et al. (2006), though 

the commenter contends that the authors implicate "fine PM" as the agent causing the 

respiratory morbidity.

(7) Comment: Several industry groups (e.g., ACC, API, NAM) commented on the reliance 

on the epidemiologic study by Delfino et al. (2002). Specifically, as part of a Request for

Correction (RFC) submitted under EPA’s Information Quality Guidelines2, NAM stated 

the following: 

In the Final REA, EPA relied on a purported association between 

short-term NO2 exposure and asthma from a study that was not 

properly reviewed in the Final ISA to support selection of a lower 

bound for potential short-term NO2 standards. This use of a study 

that has not been fully reviewed by EPA scientists violates EPA 

Guidelines requiring use of the “best available science.” The study 

in question did not find any association between asthma symptoms 

and NO2 exposure after controlling for the effect of particulates. 

EPA must include a proper review of this study in the Final ISA, 

and must explain why it believes the study would provide any

support for selection of a standard for NO2.

Response: These comments and EPA’s responses are discussed in detail in section 

II.E.2.a of the final rule. 

(8) Comment: As part of that RFC, NAM also stated the following: 

EPA assessments of several studies in the Final ISA differ 

materially from analyses of these same studies in EPA documents 

for prior NAAQS. Differing scientific evaluations by EPA which 

appear to depend on the regulatory purpose for which data are 

being evaluated violate EPA Guidelines requiring “objectivity.” 

 

2 NAM submitted a Request for Correction under the Information Quality Act in regards to the 

ISA for NOx (and seeking conforming changes to the final REA for the primary NO2 NAAQS). 

http://www.epa.gov/QUALITY/informationguidelines/documents/09002.pdf. Consistent with 

EPA's Information Quality Guidelines, EPA responded to this request by deferring to the notice 

and comment process, despite NAM's failure to submit a comment during the initial comment 

period relating to these documents. 

http://www.epa.gov/QUALITY/informationguidelines/documents/09002-response.pdf. EPA has 

addressed NAM's concerns in the final notice and this RTC. EPA has fully satisfied the 

requirements of the Information Quality Act, and has acted consistently with EPA's Information 

Quality Guidelines. 
EPA must either correct its current analyses to be consistent with 

its prior conclusions or explain why it believes those prior 

conclusions were incorrect.

Response: These comments and EPA’s responses are discussed in detail in section 

II.E.2.a of the final rule. 

C. Comments on the controlled human exposure evidence

(1) Comment: A number of industry commenters disagreed with EPA’s reliance on a metaanalysis of controlled human exposure studies, which was included in the final ISA. API 

stated that “Evidence from controlled human exposure studies of airway

hyperresponsiveness in sensitive individuals does not support the need for a short term 

standard at the levels on which US EPA is taking comment.” ACC described a number 

of shortcomings of the ISA meta-analysis. These commenters generally referenced a 

recently published meta-analysis (Goodman et al., 2009) and recommended that EPA rely 

on that meta-analysis rather than the analysis included in the final ISA. Roger McClellan 

made similar comments and also referenced a recently published article in support of his 

views (Hesterberg et al., 2009). 

Response: EPA generally disagrees with the commenters’ characterization of the “metaanalysis” of NO2-induced airway responsiveness. Furthermore, EPA has considered the 

Goodman et al. (2009) study and has provided a detailed response to these comments in 

section II.E.2.b of the final rule. 

(2) Comment: NAM provided the following comment as part of their RFC. 

If EPA elects to retain the meta-analysis in the Final ISA, EPA must 

correct the Final ISA to resolve inconsistencies between the conclusions 

in the section on Airway Hyperresponsiveness and in the summary

chapter.

Response: EPA disagrees with this comment and finds there is no inconsistency to be 

resolved between the discourse presented in the Final ISA Section 3.1.3 (Airway

Hyperresponsiveness) and associated text presented in the Final ISA Section 5.3.2.1. 

(Respiratory Effects Related to Short-Term Exposure). The section in Chapter 3 of the 

Final ISA to which the commenter refers was clearly labeled as a summary of the 

previous section. Specifically, section 3.1.3.3. was entitled “Summary of Short-Term 

Exposure on Airway Responsiveness.” Summaries are intended to provide brief 

statement of main points or substance of a matter. However, just as an abstract of a paper, 

summaries do not present all substantive or pertinent matter contained in the larger body

of a paper or, in this case, the previous section on airways responsiveness. Furthermore, 

in Section 3.1.7 (Summary and Integration–Respiratory Health Effects with Short-term 

Exposure) it is specifically stated on pg 3-42 that, “Nonspecific responsiveness was also 

increased following 30-min exposures of resting asthmatic subjects to 0.2- to 0.3-ppm 

NO2 and following 1-h exposures to 0.1-ppm NO2.” In Section 5.3.2.1 it is stated on pg 
5-10 and 5-11 that, “...increases in nonspecific hyperresponsiveness were observed ... 

between 0.2 and 0.3 ppm NO2 for 30-min exposures and at 0.1 ppm NO2 for 60-min 

exposures in asthmatics.” Thus, there is clearly no merit to the commenters’ assertion of 

inconsistency in presentation of levels to which asthmatics experience NO2-induced 

airway hyperresponsiveness between Chapters 3 and 5.

(3) Comment: Some commenters (e.g., API, COPA) concluded that AHR is not an 

appropriate endpoint to inform decisions on standard level. Because exposure to other 

agents in combination with exposure to NO2 is required to trigger AHR, the commenters 

said EPA would need to demonstrate the frequency and extent of exposure to these other 

agents and whether these exposures would occur at sufficient concentrations to actually

trigger AHR. And if AHR is occurring subsequent to ambient NO2 exposure, the 

commenters argued that the frequency of NO2-induced AHR versus background rates of 

AHR has not been documented; therefore, the significance to public health of a NAAQS 

based on this endpoint cannot be determined (COPA, API). Specifically, API stated the 

following:

EPA has not documented the frequency of NO2-induced AHR, if in 

fact it is occurring, subsequent to exposure to NO2 in ambient air. 

Nor has the Agency compared the estimated frequency of AHR 

induced through combined exposure to ambient NO2 and 

provocative agents in the environment to background rates of AHR 

produced by other causes. Thus, it is not possible to judge the 

signficance to public health of a NAAQS based on reducing the 

estimated reduction in the incidence or magnitude of AHR, or 

whether reducing ambient NO2 will have any measurable impact 

on the overall incidence of AHR. 

Response: EPA disagrees with these commenters’ views on NO2-induced airway

responsiveness. EPA believes that its interpretation of airway hyperresponsiveness 

following short-term exposure to NO2 is appropriate and supported by the available 

scientific information. Human clinical studies provided evidence for airway

hyperresponsiveness, i.e., a heightened bronchoconstrictive response to a challenge agent, 

following short-term exposure to NO2 (see sections 3.1.3 and 3.1.7 of Final ISA). Since 

these experimental human studies evaluated the airway responsiveness to challenge 

agents following exposure to both NO2 and clean air as a control, they allowed 

determination of the independent contribution of NO2 on airway responsiveness. An 

evaluation of all other agents or factors that might affect airway responsiveness, as 

suggested by the commenters, would not better define the independent effect of NO2 on 

airway responsiveness. Table 3.1-3 in the Final ISA specifically provided the fraction (or 

frequency) of NO2-exposed asthmatics experiencing non-specific airway

hyperresponsiveness. The ISA concluded that “[t]ransient increases in airway

responsiveness following NO2 exposure have the potential to increase symptoms and 

worsen asthma control” (ISA, section 5.4). Given this, combined with the large size of 

the asthmatic population in the U.S., the REA concluded that it is appropriate to consider 

NO2-induced airway hyperresponsiveness in characterizing NO2-associated health risks 
(REA, section 10.3.2). CASAC endorsed this conclusion in their letters to the 

Administrator on the final REA and on the proposal (Samet, 2008b; Samet, 2009b). 

(4) Comment: Comment: API expressed the concern that EPA is not using a standard 

approach to establishing NAAQS. As an example they note that, while EPA has 

characterized Forced Expiratory Volume (FEV) decrements in the past as adverse health 

effects, the NO2 Gradient meta-analysis (Goodman et al, (2009) showed decrements in 

FEV of only 1.6 percent. In revising the ozone NAAQS, a decrement of 3 percent was 

characterized by EPA as “relatively small” and seemingly not adverse. 

Response: EPA disagrees with the commenters’ interpretation of NO2-induced changes 

in airway responsiveness and their approach taken to discern the magnitude of this 

response. Due to differences in study protocols in the NO2-airway response literature 

(ISA, section 3.1.3), EPA judged it appropriate in the ISA meta-analysis to assess only

the fraction of asthmatics experiencing increased or decreased airway responsiveness 

following NO2 exposure. Examples of differences in the study protocols include the NO2

exposure method (i.e., mouthpiece versus chamber), subject activity level (viz., rest 

versus exercise) during NO2 exposure, choice of airway challenge agent, and 

physiological endpoint used to quantify airway responses. Therefore, EPA judged it 

inappropriate in the ISA meta-analysis to try to assess the magnitude of the NO2-induced 

change in airway responsiveness. EPA further notes that Goodman et al. (2009) also 

recognized heterogeneity among studies as a limitation in their analyses. 

EPA further disagrees with the commenter’s comparison and interpretation of the effect 

of ozone on FEV1 versus the effect of NO2 on airway responsiveness to challenge which 

was subsequently assessed by FEV1 in some studies and by other physiological endpoints 

such as specific airway resistance in other studies. Ozone itself causes transient 

decrements in FEV1, whereas, NO2 does not. Rather, in individuals with asthma, NO2-

induces an increase in the responsiveness of the airways to subsequent challenge. FEV1 is 

only one of several physiological endpoints used to discern that a change in airway

responsiveness has occurred. Based on studies that evaluated these different endpoints, 

the ISA concluded that “[t]ransient increases in airway responsiveness following NO2

exposure have the potential to increase symptoms and worsen asthma control” (ISA, 

section 5.4). Given this, combined with the large size of the asthmatic population in the 

U.S., the REA concluded that it is appropriate to consider NO2-induced airway

hyperresponsiveness in characterizing NO2-associated health risks (REA, section 10.3.2). 

CASAC endorsed this conclusion in their letters to the Administrator on the final REA 

and on the proposal (Samet, 2008b; Samet, 2009b). In summary, ozone causes transient 

changes in FEV1, whereas, NO2 causes transient changes in airway responsiveness. 

These are different health endpoints and EPA disagrees with the commenter’s 

comparison and interpretation.

(5) Comment: AAM concludes that “[r]eliance on the Orehek et al. 1976 study in an 

unpublished meta-analysis to claim an effect at 0.10 ppm is scientifically unsound.” 
Response: Orehek et al. (1976) was peer-reviewed and published in a scientific journal. 

EPA specifically recognized criticism of the statistical approach used by Orehek in 

section 15.3.1 of the 1993 NOx CD. However, the statistical significance of findings 

presented by Orehek et al. (1976) were not material to inclusion of data into the metaanalysis provided in the 1993 NOx CD or the Final ISA. EPA rejects the assertion that 

inclusion of the Orehek et al. (1976) study in its meta-analysis was scientifically

unsound. EPA further notes that the Orehek et al. (1976) study data were also included in 

the analysis by Goodman et al. (2009).

(6) Comment: AAM also concludes that “[t]he effects of NO2 from controlled studies have 

not changed materially since the last review.” Given this, they question EPA’s 

conclusion that exposure to NO2 concentrations at or above 100 ppb could increase 

airway responsiveness in asthmatics. 

Response: As shown in Table 15-10 of the 1993 NOx CD, data from four studies at 100 

ppb and a study at 140 ppb were analyzed as a group and showed increased airway

responsiveness in 65% of resting asthmatics (p < 0.01). As shown in Table 3.1-3 of the 

Final ISA, even when this analysis is limited to three studies of changes in nonspecific 

airway responsiveness following exposure to 0.1 ppm NO2, 66% of resting asthmatics (p 

≤ 0.05) experienced an increase in airway responsiveness. Therefore, as discussed in 

section II.E.2.b of the final rule, EPA concludes that scientific evidence reviewed in both 

the 1993 NOx CD and Final ISA support that exposure of asthmatic individuals to levels 

of 0.1 ppm NO2 and greater results in an increase in their airway responsiveness. In 

addition, since the 1993 NOx CD, there is a substantial new body of epidemiological 

evidence showing that short-term NO2 exposure is associated with a broad range of 

respiratory morbidity effects (Final ISA, Section 3.1). This new epidemiological evidence 

is consistent with and supported by controlled human exposure studies showing effects of 

NO2 on airway responsiveness.

(7) Comment: AAM concluded that, because clinical studies of NO2-induced airway

hyperresponsiveness have not reported increased respiratory symptoms or medicine use 

in exposed subjects, the NO2 effect on airway responsiveness is not adverse. 

Response: The intent of the clinical studies to which the commenter refers was to 

evaluate the effect of NO2 exposure on airway responsiveness, not to evaluate the effect 

of airway hyperresponsiveness on respiratory symptoms or medicine use. EPA disagrees 

with the commenters’ characterization of these studies.

D. Comments on air quality, exposure, and risk analyses 

(1) Comment: Several commenters discussed the analyses of NO2-associated exposures and 

health risks presented in the REA. As in past reviews (EPA 2005a, 2007a, 2007b), EPA 

has estimated allowable risks associated with the current standard and potential 

alternative standards to inform judgments on the public health risks that could exist under 

different standard options. A few industry commenters (e.g., API, NMA, UARG)

concluded that the Administrator should consider modeled exposures and risks associated
with actual NO2 air quality rather than with NO2 concentrations adjusted to simulate just 

meeting the current annual standard or potential alternative 1-hour standards. These 

commenters noted that such simulations require large adjustments to air quality and are 

highly uncertain and that NAAQS are intended to address actual, rather than highly 

improbable, risks to health. 

Response: These comments and EPA responses are discussed in detail in section II.E.2.c 

of the final rule. 

(2) Comment: API/AECOM (December 1, 2008 memo) specifically commented on the EPA 

approach used to assess air quality that would just meet the current and alternative 

standards. The three major comments were focused results in the EPA memo by Rizzo 

(2008) that investigated trends in NO2 air quality concentrations over time. The 

commenters charged that (1) the high concentration to low concentration air quality

comparisons are not linear and not proportional, (2) even with “a good fit to the data, the 

peak-to-mean ratio depends on the extremes of the distribution”, and added that the good 

statistical fits Rizzo (2008) provided are the result of the “middle percentile values, e.g., 

from 10% to 90%”, and (3) the ambient concentration adjustment “does not consider the 

atmospheric chemistry that is involved in the formation of NO2 from NOX emissions”.

Response: The current and alternative standard air quality scenarios are hypothetical 

scenarios, with the adjustments to the ambient concentrations determined by an analysis 

of the historical trends in air quality data at each monitor. EPA noted in the REA (section 

7.4.5) that “there is uncertainty in adjusting concentrations” considering uncertainty in 

“future source emission scenarios and how these would relate to observed trends in 

current and historical air quality.” However, given the demonstrated strong relationship 

of these concentrations over time by Rizzo (2008), EPA believed a proportional approach 

would best represent the modeled air quality scenarios. EPA agrees that demonstration of 

a linear relationship between the high and low concentration years is not the same as a 

proportional relationship. EPA acknowledged the presence of features that deviate from 

proportionality in the air quality comparisons (i.e., presence of positive and negative 

regression intercepts, curvilinear relationships) as potentially contributing to uncertainty 

in the estimated concentrations used in the alternative air quality standard scenarios (REA 

page 131). 

 

The Rizzo (2008) analysis of air quality concentration trends not only provided graphical 

comparisons and general goodness of fit statistics but also provided statistics on the 

individual points that deviated significantly from linearity for up to three high-year to 

low-year comparisons per monitor. “Studentized residuals were computed and compared 

to a students t distribution at a significance level of 10%” (Rizzo (2008) page 5). The 

identified points are provided in a series of tables for each ambient monitor in the six 

cities analyzed (Tables 1 to 6). On average, 71 percent of the points that significantly

deviated from linearity occurred at the absolute extreme ends of the air quality

distribution (i.e. the minimum (0) and maximum (100th) percentile values). When 
considering deviations that occurred at the upper percentiles (i.e.,≥95th percentile),3

an 

even greater percentage (74%) occurred at the absolute maximum value. This indicates 

that the linear relationship extends well beyond the 90th percentile value for the majority 

of the comparisons; that is on average, the upper bound of the linear range extends 

through the 99th percentile of the distribution. 

EPA acknowledged that, given the scenario modeled and the deviations from 

proportionality, the selected approach may contribute to both over and under-prediction 

in ambient concentrations (Table 7-31, REA). As far as the impact of deviations from 

proportionality, Rizzo (2008) added that “[i]n the majority of the monitor cases, the yintercept of the best fit line is positive” and that this “indicates a larger decline between 

high and low year concentrations in the upper end of the distribution than in the lower 

percentiles”. Further Rizzo (2008) stated “[m]ost although not all of the 98th and higher 

percentile points that deviate from their best fit lines do fall below the line”, meaning 

“that percentile point has declined even more strongly than the middle of the 

distribution”. For these cases (where a positive intercept exists and upper percentile 

points are below the linear regression line), when adjusting concentrations upwards based 

on the annual average concentration, the adjusted upper percentile concentrations would 

be lower than that observed during a comparable high concentration year. Most of the 

alternative standard scenarios required an upward adjustment (see comment below). 

Therefore, it is possible that most of the extreme upper percentile concentrations used in 

the alternative standard scenarios were under-estimated using the proportional adjustment 

approach rather than over-estimated. At the limited number of monitor-years noted by 

Rizzo (2008) where a negative regression intercept was present and upper percentile 

points are above the linear regression line, it is possible that the extreme upper percentile 

concentrations used in the alternative standard scenarios were over-estimated. 

EPA agrees with the commenter that the primary transformation reaction for NO2 is 

through reaction with atmospheric oxidants, primarily ozone. However, this 

transformation reaction and the potential amount of ozone needed for the reaction to 

occur is not needed to address the hypothetical alternative air quality scenarios . One 

focus of the NAAQS review is to determine the level of the pollutant that is protective of 

public health. In this instance, the integrated review plan poses a series of questions 

relevant to the NOX review including “Do exposure estimates suggest that exposures of 

concern for NOX-induced health effects will occur with current ambient levels of NO2 or 

with levels that just meet current, or potential alternative, standards?” (US EPA, 2007a).

As stated, the objective is to determine what level of NO2 is considered protective against 

health effects associated with NO2 exposures, not what level of NO2 is plausible under 

certain atmospheric conditions. It can also be stated that future strategies that control 

ambient ozone concentrations such that the ozone NAAQS would be met are to protect 

against the health effects associated with ambient ozone (and other atmospheric oxidants 

ozone is serving as a surrogate for), not ambient NO2. 

 

3

There were no upper percentile values less than the 95th percentile determined as statistically

significant.
(3) Comment: Comments from the American Chemistry Council (ACC; May 30, 2008 

memo) were also directed towards analyses of the current and alternative air quality

scenarios, however these commenters primarily questioned the relationship between 

analyses conducted by McCurdy (1994) used in the prior 1995 NAAQS review and the 

current analyses. These commenters suggest that conclusions drawn during this current 

NAAQS review regarding the relationship between concentrations and benchmark

exceedances differ from conclusions drawn in the previous NAAQS review. The 

commenters also charge that “for each monitor in the city, EPA determined the annual 

average concentration and scaled all hourly concentrations by the ratio of the current 

standard (53 ppb) and the annual average concentration.” A series of comments were 

received in three technical memos dated by the ACC on May 30, 2008 (regarding the 1st

draft REA), September 26, 2008 (regarding the 2nd draft REA without chapter 8), October 

22, 2008 (regarding the 2nd draft REA chapter 8), and summarized in their September 14, 

2009 memo. 

Response: The conclusions drawn in the current NAAQS review regarding the 

relationship between annual average NO2 concentrations and frequency of 1-hour 

benchmark exceedances are not necessarily contrary to conclusions made in the prior 

NO2 primary NAAQS review, as the ACC suggests. In fact, section A-6 of Appendix A 

(US EPA, 2008c) documents the investigation of both linear and non-linear relationships 

between number of benchmark level exceedances and the annual average NO2

concentrations using recent ambient monitoring data (i.e., years 1995-2006). As 

described in the Scope and Methods Plan (US EPA, 2007), EPA planned to perform a 

similar non-linear regression modeling approach first described by McCurdy (1994). The 

non-linear regression approach used by McCurdy (1994) to support the previous 1995 

NAAQS review was developed to estimate the number of exceedances associated with 

varying annual average concentrations, including the current standard of 0.053 ppm.

As part of the current review, EPA expanded the regression approach used by McCurdy 

(1994) and evaluated four different regression models (either using normal or Poisson 

distributions by linear and exponential links) and using either all data combined or data 

stratified by location (see Appendix A-6, US EPA, 2008c). The current regression 

analysis included construction of a model similar to that used by McCurdy (1994) (i.e., a 

normal distribution, an exponential link, and stratified by location). The performance of 

all regression models constructed was evaluated using R-square and log-likelihood 

statistics. While a model employing a Poisson distribution and using an exponential link 

function demonstrated the best performance of the models currently investigated (i.e., it 

was still a non-linear relationship though different from that derived by McCurdy

(1994)), the overall predictive capabilities of such a regression approach was generally 

poor, particularly for evaluating annual average NO2 concentrations just meeting the 

current standard. For many locations, there were extremely wide 95% prediction 

intervals estimated using the regression models, a direct result of data limitations (i.e., 

there were very few exceedances of benchmark concentrations > 150 ppb using recent 

ambient concentrations and most locations had annual average NO2 concentrations well 

below the current standard). These results presented in the REA are entirely consistent 
with the statement made in the 1995 staff paper that was cited by the ACC as a different 

conclusion:

However, because all areas of the country reporting NO2 air 

quality data are attaining the existing standard and because of the 

nonlinear relationship of 1-hour peaks and annual averages, it is 

not possible to estimate with any degree of confidence what the 

frequency and magnitude of 1-hour peaks would be if the standard 

was selected from the upper end of the suggested range.

Based on the performance results of the regression analyses (and documented in 

Appendix A-6), an alternative approach was selected to evaluate the number of 

benchmark exceedances associated with annual average and other time-averaged NO2

concentrations. Rather than using a regression model constructed from the as is air 

quality to extrapolate to ambient levels approaching the current or alternative standards, 

EPA adjusted the ambient NO2 concentrations such that they simulate levels of NO2 that 

just meet a standard. These adjusted ambient data “allow comparisons of the level of 

public health protection that could be associated with just meeting the current and 

potential alternative standards” (REA, page 59).

A proportional approach was used to adjust the as is air quality to simulate air quality just 

meeting the current and alternative standards (see REA section 6.3.1 and Appendix A-7). 

Because all locations currently have ambient NO2 concentrations below the current 

standard, an approach was needed to, at a minimum, adjust concentrations upwards to 

reflect concentrations that just meet the current annual average standard in the location. 

The approach also needed to be appropriate for adjusting air quality that just meets 

several potential alternative standards, standards of varying form (98th and 99th

percentile), concentration level (50, 100, 150, 200 ppb), and averaging time (1-hour). A 

proportional adjustment approach was selected by EPA and justified by the results of two 

analyses: the evaluation of the stability in hourly and annual average concentration 

variability over the span of the data used in the analysis (Appendix A-7) and the 

comparisons of concentration distributions of historical and recent air quality (Rizzo, 

2008). While annual and hourly NO2 concentrations have steadily declined over time, 

the concentration variability in each of these measures has generally remained constant 

over the same time period (Figure A-100). Concentration trends within monitoring sites 

in six cities were evaluated to determine how the distribution of air quality changed with 

time at each monitor. In comparing high concentration (historical air quality) to low 

concentration years (recent air quality) at the same ambient monitor, the majority of 

monitors demonstrated features of proportionality, that is the proportional change in 

concentration at each percentile of the air quality distribution was similar. The results of 

both of these analyses indicated that a proportional approach can be used to adjust 

ambient concentrations (upwards or downwards) in representing alternative air quality

conditions. 

Ambient measurements at a single monitor were used within each of the 17 named 

locations and with the two aggregate locations to estimate the specific adjustment factors 
needed to simulate air quality just meeting the current and alternative standards. This 

single design monitor in each location was selected by having the highest annual average 

NO2 concentration or the highest 98th or 99th 1-hour daily maximum NO2 concentration 

to estimate the respective adjustment factors. Because the concentration adjustment was 

considered proportional, all 1-hour concentrations were multiplied by the same specific 

factor developed for each air quality scenario and in each location. The adjustment 

factors developed from the single monitor in each location was applied to concentrations 

at all monitors within each location, such that only one monitor had concentrations that 

just met the current or alternative standard (i.e., the design monitor), while all other 

monitors in that location had ambient concentrations below that of the current or 

alternative standards (page 61, REA). Following the proportional adjustment of ambient 

NO2 concentrations (as well as when using the as is air quality), the number of 

exceedances of each benchmark were counted. This approach does not assume there is a 

linear relationship between the annual average NO2 concentration and the number of 

benchmark exceedances. The commenter is confusing the proportional approach used to 

adjust the ambient concentrations (where comparisons of high concentration with low 

concentrations demonstrate features of proportionality – see Rizzo, 2008) with the 

regression models that related the number of exceedances with the annual average 

concentrations (which is not linear, as demonstrated by the conclusions in the prior 1995 

review, is discussed above, and is documented in REA Appendix A-6). The two 

approaches are not comparable in this regard, though the objective of each analysis is the 

same; to estimate of the number of 1-hour benchmark exceedances when considering 

alternative air quality scenarios.

There is uncertainty associated with such a concentration adjustment and has been 

qualitatively described in REA section 7.4.5. However, EPA notes that there is likely 

less uncertainty in the approach and results generated when using adjusted concentrations 

that are closest to existing as is air quality. For most locations, this was the either a 98th

or 99th percentile alternative standard at a 1-hour daily maximum concentration of 50 ppb 

(see Table A-107).4 On average, to have air quality just meet the alternative 1-hour daily 

maximum standard level of 50 ppb, the minimum concentration adjustment was less than 

10% and ranged from a low of zero percent (i.e., the as is air quality for a given location 

had either the 98th or 99th 1-daily maximum equal to 50 ppb) to only as high as 28 

percent. To simulate just meeting the 50 ppb level of either the 98th or 99th percentile 1-

hour daily maximum standard, a downward adjustment of air quality was applied in most 

locations and year-group of data considered.5 To just meet the 100 ppb level (either form 

of the 1-hour daily maximum standard), all locations required an upward adjustment. At 

this 100 ppb level, the 99th percentile daily maximum standard required the lowest 

concentration adjustment at all locations; on average, the upward concentration 

adjustment was 70% (min 1%, max 131%). Taken together, this means that most 

 

4 Only Los Angeles (2001-2003), Other CMSA/MSA (2001-2003), and Provo (2004-2006) had 

as is air quality more similar with a 1-hour 99th percentile 100 ppb standard level out of a total of 

38 location/year-group combinations. 

5 When considering the two forms (98th and 99th), the 19 locations, and two year-groups (2001-

2003 and 2004-2006), 61/76=80% needed a downward adjustment to meet the 50 ppb level. 
locations have as is concentrations within the range of the two alternative standard levels 

of 50 to 100 ppb and within the range of the newly proposed standard. As the data for 

simulating just meeting these standard levels required the least concentration adjustment, 

there is less of an impact of the uncertainty associated with the air quality adjustment 

procedure on the estimated number of daily benchmark exceedances associated with 

these air quality scenarios. 

(4) Comment: AAM (September 14, 2009 memo) claimed that the approaches used in the 

REA to characterize health risks from ambient NO2 lead to significant overestimation of 

actual risk. Specifically, they argued that the first approach (comparing monitoring data 

with potential health benchmarks) is known to overestimate the distribution of actual 

human exposures because monitored concentrations do not necessarily equal human 

exposures. 

Response: EPA believes that the ambient air quality has been appropriately characterized 

in chapter 7 of the REA. The REA also provides ample context for how the ambient air 

quality data can serve as a useful indicator of human exposure (sections 2.3.2, 6.1, 9.6, 

and 10.3.2). EPA recognizes that population personal exposure is not directly equivalent 

to ambient air quality measurements however there is no justification for discounting the 

ambient monitoring data simply because they are perceived by AAM to “overestimate 

personal exposures.” The data represent actual measurements of NO2 levels that a 

population may be exposed to, provided the population encounters the given 

concentration in both space and time.

EPA notes that the available evidence indicates that at-risk populations can be exposed to 

ambient NO2 concentrations greater than that represented by an ambient monitor. EPA 

has clearly stated in the REA and NPRM that the current ambient monitoring network 

does not have a sufficient number or appropriate siting of monitors to reflect NO2

concentrations occurring on or near major roads in most, if not all, urban areas of the U.S. 

The existing monitoring network is not designed to capture the spatial gradient in NO2

concentrations surrounding roadways in any location, which is why modeling approaches 

were selected by EPA staff to estimate on- and near-road NO2 concentrations.

(5) Comment: AAM (September 14, 2009 memo) commented that “there are no reliable data 

indicating either on-road or near-road exposure in recent years that exceed the 0.20 or 

0.30 benchmarks” and referenced several studies to support their claim.

Response: EPA notes first that the benchmark values have an averaging time of 1-hour. 

None of the studies cited by the AAM reported 1-hour NO2 concentrations. For example, 

AAM cites Beckerman et al. (2008) and Singer et al. (2004), which were both studies 

where weekly average concentrations were measured. A paper by Roorda-Knape et al. 

(1998) is also mentioned, where two-week average concentrations were measured. Given 

the expected diurnal and day-of-week variation of NO2 concentrations (see ISA, section 
2.4.4) it is not a surprise that the average concentrations reported in these studies do not 

approach a peak 1-hour NO2 concentration at or above 200 ppb. 

There were only two studies cited by AAM that measured NO2 concentrations inside 

vehicles: a study by Westerdahl et al. (2005) conducted in Los Angeles, CA and one by

Riediker et al. (2003) conducted in Raleigh, NC. EPA notes that the researchers in the 

Los Angeles study did measure NO2 concentrations across a two-hour averaging period, 

which is the closest any measurement study came to a 1-hour averaging period. 

However, based on the study design, the primary purpose of the study is to collect data 

more relevant for PM exposure than for NO2 exposure. The time-of-year the data were 

collected may not correspond to when NO2 concentrations are highest. The NOx ISA 

reported that “in Los Angeles and Riverside, CA, monthly maxima tend to occur from 

autumn through early winter, with minima occurring from spring through early summer”. 

EPA notes that the Westerdahl et al. (2005) measured NO2 on February 14th and 20th and 

April 7th and 16th (see Table 2 of Westerdahl et al., 2005). In addition, the study drive 

times when measurements were collected did not necessarily correspond to when NO2

peak concentrations are expected to occur. According to the NOx ISA, “NO2 typically

exhibits daily maxima during the morning rush hours”. Drive times selected for 

evaluation in the Westerdahl et al. (2005) study were as follows: 2-6PM, 11AM-1PM, 

9AM-12PM, and 11AM-2PM, times-of-day that do not correspond to the morning rush 

hours. 

In the second study cited by AAM that reported NO2 concentrations on-roads and inside 

vehicles, Riediker et al (2003) reported that sample averaging times were about 9-hours 

in duration and all occurred “during late-shift patrols (3 p.m. to midnight)”. As discussed 

above by EPA, these collection times are not consistent with the averaging time of 

interest (i.e., 1-hour) or the time-of-day when maximum concentrations are expected to 

occur (i.e., morning rush hours). Second, the monitoring events in the Riediker et al. 

(2003) study did not always occur on- or near major roadways. Often times the 

automobile was parked in a lot, potentially further limiting the measurement of any

maximum NO2 concentrations. Riediker et al. (2003) states “[o]n average, troopers spent 

35% of their shift away from the car, either in the office, in jail, in hospitals, for dinner”, 

etc. The authors of the study also note that “pollutant concentrations in the occupied cars 

were significantly higher compared to the parked cars (Mann-Whitney U-test)” (Riediker 

et al., 2003). In their comments, AAM stated that the maximum concentration of 0.548 

ppm reported by Riedeker et al. (2003) was “flawed” and “invalid”. Nowhere in the 

Riediker et al. (2003) paper was it stated that the 9-hour average NO2 measurement was 

flawed or invalid, only mentioned as an “outlier”. Thus, this maximum reported NO2

concentration remains a valid measurement. In further review of this paper, EPA notes 

also that the reported maximum 8-hour average roadside NO2 concentration did in fact 

exceed 200 ppb (212.1 ppb; see Table 1 of Riedeker et al. (2003)), contradicting the 

above AAM comment. Given the diurnal profile of NO2, this suggests that on this day

there was likely more than one hour where roadside concentrations exceeded 200 ppb. 

(6) Comment: AAM (September 14, 2009 memo) commented that the distribution of ratios 

used to represent the relationship between on-road and ambient concentrations in the air 
quality characterization (chapter 7 of the REA) resulted in overestimates of on-road NO2

concentrations. AAM refer to a statement made by Westerdahl et al. (2005) regarding the 

relationship between on-road and ambient concentrations that “roadway concentrations of 

[CO and] NO2 were usually no more than about twice the ambient concentrations”. They 

further comment that higher ratios developed using lower background concentrations will 

not be appropriate in urban situations having a high background and will “substantially

over-estimate the on-road increment”.

Response: EPA notes that this statement made by Westerdahl et al. (2005) supports an 

upper bound for the ratio between on-road and away from road concentrations of at least 

two. In other words, on-road NO2 concentrations may be at least 100% higher than away

from road concentrations. This percent difference is consistent with statements made in 

the REA about the on-road concentrations that “simulated on-road annual average NO2

concentrations are, on average, 80% higher than the respective ambient levels at distances 

≥100 m from a road” (section 7.3.2, page 97) and statements made in the ISA regarding 

the relationship between the two concentrations (section 2.5.4, page 2-36). In addition, 

the use of “usually” by Westerdahl et al. (2005) in their description indicates that most of 

the time the ratio was a factor of two or less, but also suggests that there were some 

instances that the factor difference was greater than two. This statement also supports the 

ratios EPA used to adjust ambient NO2 concentrations in estimating on-road 

concentrations. Table 7-10 (REA) indicates that most of the ratios (64% and 79%, for the 

summer and not summer ratios, respectively) used by EPA in estimating on-road NO2

concentrations were less than a factor of two.6 EPA also note that in addition to the 

several papers EPA used to develop the on-road ratios used in Chapter 7 of the REA, an 

average factor of two enhancement on- or near roads concentrations is also reported by

Beckerman et al. (2008) using the normalized NO2 concentrations provided in that study

(see Figure 4 of Beckerman et al., 2008).

EPA believes that the ratio approach developed from measurement data available in nearroad studies and used to simulate the on-road NO2 concentrations in the REA chapter 7 

was reasonable. EPA acknowledged in the REA that there is uncertainty in the estimated 

on-road NO2 concentrations, particularly when using ambient monitoring concentrations 

that may have been influenced by non-road sources (e.g., section 7.4.6 REA, page 135). 

We agree with AAM that this particular uncertainty in the approach used to simulate onroad concentrations would tend to lead to overestimation of on-road NO2 concentrations. 

However, EPA notes that there are other sources of uncertainty indentified in the 

approach used that could also contribute to underestimation of on-road concentrations. 

Localized areas such as those within urban street canyons, tunnels, and major 

intersections may have instances where on-road NO2 concentrations are greater than a 

factor of two times the ambient monitor concentration. For example, Vardoulakis et al. 

(2004) reported mean NO2 concentrations at a major intersection were about 2.1 times 

greater than on-road NO2 concentrations measured a few hundred meters distance away. 

 

6 Note that to generate the ratio, the m factor has a value of 1 added to it. See REA equation (7-

2).
The ISA (section 2.5.4) reported that NOX concentrations can be 7 times greater at a 

tunnel exit when compared with NOX concentrations measured at the tunnel entrance. 

Note that the maximum ratio used to simulate on-road concentrations in the air quality

characterization was 3.7 (REA, Table 7-10).

In addition, EPA also acknowledged that there could be uncertainty in the application of 

ratios developed from rural locations to urban locations (see REA page 137). However, 

given the limited information on the characterization of the study areas used to develop 

the data, the limited number of studies containing relevant data, and that there was no 

apparent difference observed in the distribution of developed ratios when considering the 

study locations as having a rural or urban designation, EPA elected to stratify and apply 

the ratios based on observed seasonal differences in the distributions. 

(7) Comment: AAM commented (September 14, 2009 memo) that AERMOD was 

developed and tested primarily for stationary source applications. AAM referenced a 

recent EPA review of 21 air-quality modeling tools that simulate line-type sources to use 

in near roadways applications (EPA/600/R-09/001) including an evaluation of AERMOD 

and several other roadway dispersion models. AAM states that this “EPA review 

indicates that AERMOD has not been compared rigorously for line source applications 

and that it contains a very simplistic algorithm for line sources.”

Response: First, the highways in Atlanta were simulated not as line sources, but as a set 

of area sources, a common use of this regulatory model.

Second, the referenced EPA review (US EPA, 2008) explicitly cites numerous 

comparisons of AERMOD in various configurations to the performance of some of the 

other available models, with AERMOD consistently providing reasonable concentration 

predictions. For example, “AERMOD was evaluated with respect to other models such 

as ADMS-Roads, ISCST3, and CTDMPLUS. When considering only the highest 

predicted and observed concentrations, it was found that ISCST3 overpredicts by a factor 

of seven, on average, whereas ADMS-Roads and AERMOD underpredicted, on average, 

by about 20%. It also was determined that ADMS-Roads performance is slightly better 

than AERMOD (Hanna et al., 1999). In complex terrain, AERMOD consistently

produced lower regulatory design concentrations than ISCST3, not an unexpected result 

because ISCST3 uses algorithms from a screening model (COMPLEX1) in its 

calculations. In comparisons with CTDMPLUS and observed data, AERMOD 

consistently performed better than CTDMPLUS, a model approved by EPA for 

regulatory applications in complex terrain. The model has not been compared rigorously

for line-source applications. Because AERMOD is used most commonly for dispersion 

analyses of stationary point sources, area sources, and volume sources, there is no 

accommodation for different roadway geometries (e.g., bridges and deep roadway cuts).”

Although AERMOD does not currently accommodate complex roadway geometries, 

enhanced turbulence, plume rise, and other characteristics of near road dispersion can be 

accounted for by selection of appropriate input parameters, such as initial sigma-y, initial 

sigma-z, and emission release heights (each of which are documented in the REA).
Furthermore, according to Appendix W (70 FR 68218), “AERMOD is a best state-of-thepractice Gaussian plume dispersion model whose formulation is based on planetary

boundary layer principles” and isthe current preferred model for near-field and urbanscale air dispersion modeling applications. While Appendix W lists CALINE-3 as the 

preferred model for roadways, this model is not generally applicable for modeling long 

periods in very large, complex modeling domains such as that considered here. In fact, 

CALINE- 3 and CAL3QHCR rely on older dispersion algorithms from ISCST2 (Bailey, 

2009), further justifying the use of AERMOD rather than CALINE-3 or CAL3QHCR. 

We note also that currently available versions of the MOVES emissions model will 

predict NOX emissions 30-50 percent higher than the MOBILE model used in the present 

analysis (Beardsley, 2009), suggesting that the newer emissions model would have 

predicted even higher NOX concentrations and therefore higher NO2 concentrations. 

Moreover, following analysis of this dispersion model review report (US EPA, 2008) by

Isakov (2009), AERMOD was chosen as the best platform for further model development 

to address roadway scenarios. Given the diversity of source emissions, complexity of the 

scenario to be modeled in an urban environment, and requisite spatial and temporal 

resolution for detailed analyses used in estimating exposures, AERMOD was also judged 

here to be the most appropriate model for this application.

(8) Comment: AAM (September 14, 2009 memo) believes that the second approach used to 

characterize health risk (estimating risk based on detailed exposure modeling), for a 

variety of reasons, overestimates the risk from NO2. One reason identified by AAM was 

that since AERMOD is not a photochemical model, it does not include the key reactions 

converting NO to NO2 in urban areas. In particular, concern was expressed by the 

commenter regarding EPA assumptions made for the photochemical conversion rate as 

an air pacel moves downwind, and questioning whether peak NO2 concentrations are 

likely to occur on or next to the roadway.

API/AECOM (September 14, 2009 memo) had similar comments regarding the OLM and 

PVMRM methods used by AERMOD, that is “[b]oth methods are simple and 

conservative because they assume that only nitric oxide titration by ozone (Equation 2) 

governs NO2 formation and that all of the ambient ozone can be consumed”. Later they

commented “consideration should be given to using a model platform that includes plume 

transport time so that if time dependant chemical reactions are needed, they can be 

included as a function of transport time”. 

Response: As discussed in the REA, the AERMOD model was selected to provide the 

high spatial resolution required for this exposure assessment. Although the chemical 

parameterizations of the AERMOD model are far less detailed than the chemistry

algorithms in photochemical models, in the near field, no photochemical model would 

have provided the necessary spatial resolution for this study. Furthermore, most 

photochemical models are optimized for predictions of O3, and may not provide adequate 

performance in predicting NO2, even at the coarser spatial scales at which they are 
applied. Given these considerations, AERMOD was judged to be the most appropriate 

model for this application. 

EPA recognizes that numerous photochemical reactions take place in the troposphere that 

influence the relative concentrations of NO and NO2 from NOX emissions. The complex 

reactions can be commonly summarized as:

VOC + NOX + h O3 + PAN + HNO3... + Particles, etc. (1)

(Finlayson-Pitts and Pitts, 1999). However, to a first order, the characterization of the 

ratio of concentration of NO to NO2 can be represented by the photostationary state 

equation:

 

[ ]

[ ]

4

3 2

3

k NO

j NO

O  (2)

where j3 results from the following 

NO2 + h NO + O (3)

and k4 from the following

O3 + NO NO2 + O2. (4)

(e.g., Atkinson et al., 1988). 

This describes a photostationary state where NO and O3 cannot coexist in significant 

concentrations and the concentration of NO2 is proportional to the concentration of O3. 

The commenter(s) stated that the mechanisms in AERMOD do not appropriately

parameterize the role of organics in the processing of NO to NO2 via equation (2). 

However, although NO is converted to NO2 in a reaction sequence initiated by the 

hydroxyl radical in an attack on organic compounds and involving numerous free 

radicals, much of the OH and HO2 radicals involved are themselves created by photoreactions involving O3. Thus, to the first approximation, equation (2) is a reasonable 

approximation of the balance of NO2 and NOX, particularly during the daytime when 

NO2 concentrations are highest. 

While the OLM/ARM workgroup (1998) noted that “oxidation by ozone is typically the 

main reaction for NO2 formation”, and that “the reaction rate is essentially

instantaneous”, they also noted that “the total amount of NO2 conversion is limited by

how quickly the plume entrains surrounding air”. Therefore, the OLM algorithms may

somewhat overstate the on-road and near-road NO2 concentrations (all other factors being 

equal) by not accounting for the possibility of limited entrainment of O3-laden air in 

locations with limited mixing, such as street canyons. However, given the rapid 

formation of NO2 in the presence of ozone, we feel that incorporating transport time into 
a model formulation would be unlikely to substantially affect model predictions in this 

application. 

The location of the peak NO2 concentration in this study were derived from objective 

modeling predictions, not a priori assumptions. OLM’s characterization of NO2/NOX

ratios as a function of downwind distance is documented in a series of sensitivity tests by

MACTEC (2004). The concentration of NO2 is a function of both the photostationary

balance of equation (2) within the plume and the decreasing concentrations of the 

dispersing plume.

Despite certain limitations in the OLM approach identified above, the model predictions 

show good agreement with measured values off the roadway. And as discussed below, 

the onroad model predictions compare favorably with reported measurement values at 

other locations. 

(9) Comment: API/AECOM commented that “there are many other sources of NO that are 

already competing for ambient ozone”... “unless the ambient monitor is located in an 

area similar to the source, it is unrealistic to assume that all of the monitored ozone is 

available to convert NO to NO2” at the source location.

Response: We agree that the modeling methodologies could be strengthened by

enhanced resolution of O3 concentrations. However, the scale over which the reactions 

occur is critical. Monitors should represent the average concentrations to which the 

plumes are exposed in traveling from a source to the receptor, not the O3 concentrations

immediately adjacent to source, where the noted titration will reduce the O3

concentrations below that available elsewhere in the domain. 

The exception would be where the receptor is immediately adjacent to the source, e.g., 

the on-road and near-road microenvironments. In these cases, the OLM algorithms may

somewhat overstate the on-road and near-road NO2 concentrations (all other factors being 

equal) by not accounting for the possibility of limited entrainment of O3-laden air in 

locations with limited mixing, such as street canyons. However, for this study the overall 

model performance appears to be reasonable, both when compared to literature values 

and when compared to regional observations (see comments responses 8 through 10 

below).

(10) Comment: AAM (September 14, 2009 memo) commented that, “In addition to mischaracterizing the spatial distribution of NO2, AERMOD mis-characterizes the temporal 

distribution, as shown in Figure 8-7 of the REA where the modeled peak NO2 from the 

morning rush hour occurs about three hours earlier than the peak in the monitored 

concentrations in Atlanta.” They further comment “maximum concentrations occur 

under conditions of minimum dispersion where the impact of turbulence and heat 

generated by traffic will be greatest. Since the EPA review acknowledges that AERMOD 

has not been rigorously evaluated for line source applications and the algorithm is 
simplistic compared to other line source models that account for turbulence and other 

traffic effects in greater detail, its predictions of maximum roadway impacts are suspect.”

API/AECOM had similar comments on the diurnal concentration profile stating 

“AERMOD is somewhat successful in simulating the afternoon dip in NO2

concentrations associated with increased atmospheric ventilation, but the model 

substantially overestimates (with about 50% to 70% over prediction) and prematurely 

times the early morning and evening peaks. The overestimates are probably associated 

with an overestimation of ambient ozone and the overly conservative treatment of 

atmospheric reactions that limit peak NO2 concentrations.”

Response: The discrepancy between modeled and observed peak concentration when 

considering the diurnal profile is less likely due to the model parameterizations employed 

than to limitations in the underlying hourly vehicle activity data from TDM modeling 

outputs. The activity data were based on TDM modeling outputs from the regional 

planning organization and considered the best available information, although somewhat 

limited in specification of the temporal profile. Furthermore, the OLM mechanism 

should not be related to any morning overpredictions since the ambient ozone 

concentrations are generally low at that time. The uncertainty regarding this input 

variable and magnitude of impact on estimated concentrations was acknowledged by

EPA in section 8.12.1.4 of the REA.

We agree that with equivalent emissions, maximum concentrations will occur under 

conditions of minimal dispersion, such as low wind speeds and mixing heights. During 

these times, typically early morning, effects of vehicle induced turbulence could be 

significant. However, in the present study, over-predictions occur primarily during the 

afternoon traffic peak, when ambient turbulence should dominate any traffic induced 

values and ambient temperatures are high, minimizing buoyancy effects. Thus any model 

over-predictions at these times are less likely to be due to the parameterized traffic 

impacts on dispersion.

As discussed above, the cited EPA review notes that AERMOD is commonly used for 

area source applications, which was the way roadway links were characterized for this 

study; and input parameters were selected to account for roadway effects. 

(11) Comment: AAM (September 14, 2009 memo) commented that the data in the “REA 

shows in Figures 8-6 and 8-7 that the AERMOD-estimated concentrations substantially

overestimate measured NO2 concentrations in Atlanta particularly at the upper percentiles 

of the distribution. Figure 8-8 also indicates that AERMOD overestimates the maximum 

on-road concentrations compared to the ratio method used in Chapter 7, which as shown 

above, itself overestimates maximum on-road exposures.”

A few other commenters made similar comments. For example, ACSBPP and API

concluded that AERMOD may overestimate NO2 concentrations, particularly in the 

upper percentiles of the distribution. These commenters generally concluded that, by 
relying on the AERMOD dispersion model, EPA may be overestimating the risks from 

current ambient concentrations and the contributions to risk from on-roadway and nearroadway exposures.

Response: Figure 8-8 compares on-road to non-road receptor concentration ratios 

generated by using the concentrations estimated by AERMOD with concentration ratios 

generated using the measurement data extracted from the extant literature (i.e., the 

empirical method). This figure does not compare on-road concentration predictions. 

Figure 8-8 shows that AERMOD generally predicts both higher and lower (i.e., more 

variable) concentration ratios than the empirical method. The accuracy of neither method 

could be demonstrated with direct observations, due to the lack of available on-road, or 

even near-road, measurement data in the modeling domain. But as noted in the REA, 

given the greater number of receptors modeled by AERMOD, the AERMOD approach 

may better represent the variability in NO2 concentration ratios than the empirical 

method. 

Table 8-7 presents a comparison of on-road NO2 concentration predictions between the two 

methods. The comparison suggests that the AERMOD on-road concentration predictions are 

higher than those of the empirical method by about 10 to 15 ppb at each percentile level. EPA 

notes however that the accuracy of neither method could be demonstrated with direct 

observations, due to the lack of available on-road measurement data in the modeling 

domain.

Figures 8-6 and 8-7 show that in the off-road environment, the high end of the 

concentration distribution is overestimated at the exact location of one monitor (ID 

130890002), but shows very good agreement at the exact locations of the other two (IDs 

130893001 and 131210048). For example, Table B-37, which presents the data used to 

construct Figure 8-6, shows that for the latter two monitors, the discrepancies between 

the AERMOD predictions and the measured values for percentiles 90 through 99 is less 

than 10 ppb. In addition, discrepancies between observed and predicted concentrations 

are even less when the envelope of all receptors within 4 km of the monitoring location is 

considered. 

(12) Comment: AAM (September 14, 2009 memo) comment that “measured concentrations in 

on-road and near-road studies documented in the ISA and summarized in the previous 

section demonstrate that there are no valid measurements of NO2 exposures as high as the 

upper percentiles of exposure predicted by AERMOD. The REA refers to the 0.548 ppm 

maximum NO2 concentration in Riedecker et al. (2003) to support the upper end of the 

AERMOD predictions, but, as Riedecker et al. admits, it is not a valid measurement. 

There is additional evidence in the literature that microscale monitoring will not identify

unmonitored “hot spots” of exposure to motor vehicle pollutants. The South Coast Air 

Quality Management District has carried out two studies that compared motor vehicle air 

toxic exposures at microscale sites in Los Angeles suspected of being unmonitored “hot 

spots” with exposures at current monitoring sites. In both cases, the exposures at the 

anticipated hot spots were similar to the exposures at the fixed neighborhood-scale 

monitoring sites.”
Response: As noted above, when comparing predicted concentrations, either from 

AERMOD or the empirical method, to any measurement data, it is important to consider 

differences in averaging times. For example, Riediker et al. (2003) measured NO2 inside 

North Carolina Highway Patrol cars on duty in Raleigh and determined mean 

concentrations of about 42 ppb with a range of 2 – 548 ppb for measurements of varying 

durations, with an average sampling duration of about 9 hours. As described above, the 

authors did not state the maximum concentration of 548 ppb was “invalid” but as an 

outlier. According to Riediker et. al. (2003), invalid measurements resulting from 

“laboratory or handling problems were excluded from the analysis.” Note that the 

roadside measurements from the same study have a mean concentration of about 50 ppb 

with a range of 13 – 212 ppb (about an 8 hour averaging time). The present study in 

Atlanta estimated hourly average on-road concentrations of 43 ± 25 ppb (mean and 

standard deviation) and a maximum value of 556 ppb (REA Table 8-7). Even without 

considering study differences (e.g., averaging times, times-of-year, times-of-day

included), the mean and maximum AERMOD on-road concentration predictions are 

comparable with the mean and maximum measured roadside concentrations reported by 

Riediker et al (2003).

In another study CARB (2003) measured NO2 concentrations inside school buses on 

urban and suburban/rural routes in Los Angeles with average commute time of 

approximately 85 minutes and found commute average concentrations of about 70 ppb 

(urban; range 34 – 120 ppb) and 45 ppb (Rural/suburban; range 23 – 68 ppb). The mean 

and standard deviation of the on-road concentrations from the Atlanta exposure 

assessment are also consistent with this measurement study.

EPA disagrees with the commenters about the relevance of the MATES studies (II and 

III) referred to by the commenter to the exposure assessment presented in the REA. First, 

the MATES studies were designed estimate risk associated with longer-term exposures to 

air toxics, not short-term 1-hour exposures to nitrogen dioxide. In fact, NO2 was not 

measured in either of these studies. The ambient monitoring was conducted in 

neighborhoods potentially downwind of important emissions sources; no measurements 

were made on-roadways. Comparisons were made between concentrations measured at 

neighborhood microscale monitors to fixed site monitors located several miles away. 

The studies do not compare on-road, near-road, and away from road concentrations along 

a specific transect line. The study monitoring objectives were to not only assess potential 

mobile source influence to neighborhood air quality, but also considered influence from 

other localized sources. For example, in the MATESII study (SCAQMD, 2000) only 3 of 

14 microscale monitoring sites used for their “hot-spot” analysis were selected “because 

of influence and proximity to major mobile sources.

Conclusions reported by SCAQMD (2000) are consistent with the AAM comments in 

that “the monitoring at each of the 14 microscale sites did not register significantly higher 

levels of any toxic air contaminants.” SCAQMD also added that “it cannot be concluded 

that ‘hot spots’ do not exist at other locations.” The latter statement is made because of 

extreme limitations noted in the microscale monitoring program and to caution readers 
against drawing particular conclusions based on the data. SCAQMD notes that “the 

intent to investigate a number of different sites, with available resources, limited the 

power of the microscale study to detect localized disparities in air toxic levels. The 

microscale study should therefore be regarded as more of a “pilot study” than as a study 

to definitively address possible differences in community air pollutant exposures within 

the South Coast Air Basin. These factors should be taken into consideration to avoid 

possible over-interpretation of the results.” EPA notes that in the summary statements 

made for one of the microscale monitors placed to capture potential mobile source 

influences, “measured concentrations indicate more on-road activity at Montclair than at 

Fontana” (SCAQMD, 2000).

The MATESIII study does not refer to the monitoring of “hot spots” at all. The selection 

of monitoring sites “was done to ensure sufficient resolution to monitor representative 

concentrations of varying land use types and characterize spatial gradients in the Basin” 

(SCAQMD, 2008). The authors also note “[a]s in MATES II, due to the limited number 

of mobile monitoring platforms, each microscale site study lasted for a shorter duration 

than the overall study for approximately two to several months.” This suggests the same 

caution should be applied regarding the over-interpretation of the results as recommended 

for the MATESII study. It was not apparent to EPA how many of the five microscale 

sites used in this study were placed in areas expected to receive significant mobile source 

influence, however SCAQMD found statistically significantly higher concentrations of 

mobile-source related pollutants at two of their microscale monitors compared with 

corresponding measurements at paired fixed site monitors. For example, 24-hour average 

concentrations of benzene, ethylbenzene, toluene, and xylene were 5 to 6 times as higher 

at the Santa Ana site than at the Anaheim site (SCAQMD 2008, page 5-8). The other 

three microscale monitors may have been sited to capture localized emissions other than 

mobile sources, given high concentrations noted for hexavalent chromium, 1,3-butadiene, 

and manganese composition in PM10. While limited in direct relevance to the REA, in 

general, the overall trend in both MATES studies support the importance of mobile 

source influence to ambient concentrations. 

(13) Comment: AAM (September 14, 2009 memo) commented on the approach used to 

estimate in-vehicle and near-road concentrations used to estimate exposures. Specifically 

they state that based on the AERMOD on-road to non-road receptor concentrations, 

“multiplicative factors from 1 to as high as 10 to 30 inappropriately were being used to 

estimate near-road exposures from the estimates of ambient concentrations.” The 

commenter argued that “multiplicative factors as high as 30 are clearly suspect compared 

to the data in the literature on in-vehicle NO2 exposures” They also question the use of 

the same APEX proximity factor for both in-vehicle and near-road microenvironments 

given the sharp reduction in NO2 concentrations with distance from a roadway.

Response: EPA compared the on-road/non-road ratio distributions derived from the 

AERMOD hourly receptor concentrations with those of the ratios derived from onroad/near-road measurement study data (REA Figure 8-8, Appendix B Table B-39). For 

this comparison, the AERMOD concentrations were first time-averaged across seven 

days to approximate the time-averaging of the literature-derived measurement data (i.e., 
most of these observations were time-averaged over 7 to 14 days). The seasonal patterns 

of the ratios was consistent (summer ratios > not summer ratios), though the AERMOD 

generated ratios exhibited greater variability and had consistently higher values at the 

upper percentiles of the distribution. The maximum ratio for the not-summer season 

based on the empirical data was equal to the value of the 76th percentile AERMOD 

derived ratio. The maximum ratio for the summer season based on the empirical data 

was equal to the value of the 85th percentile AERMOD derived ratio. Note that the 

AERMOD ratios were generated using concentrations from 5,570 on-road receptors 

simulated in the modeling domain and the corresponding nearest non-road receptor at a 

distance ≥ 100 meters from the road. Given the greater diversity of the AERMOD 

modeled receptor locations compared with the literature derived data (a total of 11 studies 

using 36 roadway sites), we expected there would be greater variability and hence a 

greater range of ratios in the AERMOD derived ratios.

The ratios used for the APEX modeling were calculated using the AERMOD predicted 

hourly concentrations. Ratios having a 1-hour averaging time are most appropriate for 

this APEX application since the PROX factor is applied to a 1-hour ambient 

concentration. EPA expected that there would be greater variability in the 1-hour ratios, 

likely generating a distribution of PROX factors having a greater range of values than 

observed with the longer averaging time. EPA notes that there are no data reported in the 

extant literature that have on-road/non-road concentrations or ratios of 1-hour averaging 

time. In addition to the seasonal influences observed (summer/not-summer), EPA also 

stratified the ratios by the time-of-day based on analysis of this as a potential influential 

factor. Three time ranges were selected to further stratify the ratios. Lognormal 

distributions were then fit to each of the six stratifications, with lower and upper bounds 

of the distributions approximated by using the 5th and 95th percentiles (Appendix B Table 

B-42). In probabilistic exposure analyses, setting bounds on a fitted distribution is a 

commonly used method to control against exceptional or potentially unrealistic values. 

Note in REA Table 7-34, the minimum and maximum observed on-road ratios were at 

the 2.3-7.8th and 94.4-97.8th percentiles, respectively of a lognormal fitted distribution, 

supporting the selection of the 5th and 95th percentiles as reasonable lower and upper 

bounds.

In evaluating the ratio distributions, clearly the daytime (6AM-7PM) ratio distributions 

are distinct and higher than the nightime ratio distributions (7PM-6AM), confirming that 

stratification by at least two time-of-day ranges was appropriate. Again, seasonal 

differences were consistent with that of the ratios derived from the empirical data, 

confirming that seasonal stratification was appropriate. EPA acknowledges that it is 

possible that there are other potential influential factors that could be considered in 

further stratifying and applying the PROX factors, but given the limited time and 

resources allocated to perform the assessment, additional analyses were not performed.

When considering (1) the absence of observed on-road concentrations of 1-hour 

averaging times, (2) having no on-road concentration measurements within the modeling 

domain, (3) the reasonable agreement in the AERMOD concentration predictions with 

available measurement data, and (4) the reasonable agreement in the AERMOD on-road 
concentration predictions with simulated on-road concentrations using the empirical 

ratios, EPA believes that the PROX factor distributions generated are an adequate 

representation of the relationship between on-road and non-road concentrations, even 

with upper bounds of the distributions that extend to values of 9 to 30. Again, EPA is not 

aware of any 1-hour measurement data derived from a study conducted in a similar urban 

environment across an entire year that could confirm or refute the level of the upper 

bound used for each of the ratio distributions.

EPA used the same on-road PROX factor distributions to calculate the near-road 

microenvironmental concentrations. The near-road microenvironment generally refers to 

locations in close proximity to vehicle emissions such as sidewalks, bus stops, and 

parking garages, where concentrations would likely be similar to those on roadways. Not 

accounting for decay/dispersion of the on-road concentration with such short distances 

from a roadway was considered by EPA as an appropriate assumption. EPA notes that 

the maximum concentration does not always occur on the roadway, it can occur at a 

distance from the road (e.g., Beckerman et al., 2008). Therefore, there may be instances 

where the near-road concentrations are either greater than or less than that observed on 

the road. In the absence of robust measurement data to inform the development of a 

potential adjustment factor to approximate on-road exposure concentrations that deviate 

above or below nearby on-road concentrations, EPA assumed a relationship of unity.

EPA recognizes that when using these ratio distributions to estimate in-vehicle or nearroad exposures, there may be instances where these ratios are unsuitably applied to an 

exposure event, given other potential influential factors mentioned above that are not 

considered in the exposure calculation. This is an important uncertainty that could 

contribute to both over- and under-estimates in exposure concentrations. We 

acknowledged that accurately modeling the upper percentiles of the distribution is of 

concern given that the exposure metric of interest is exposures above selected benchmark 

levels. EPA qualitatively summarized the overall impact of these and other input data 

uncertainties in the REA (section 10.3.2.1) indicating that there is the “possibility that we 

are over-predicting upper percentile NO2 exposures” and therefore over-predicting the 

number of days with exceedances, particularly the highest benchmark levels. Given the 

duration of the exposure assessment (i.e., an entire year), this uncertainty is likely to have 

a greater impact on the number of days per year an individual was estimated to 

experience an exposure above a benchmark rather than the estimated number/percent of 

persons exposed above a particular benchmark level, though the magnitude of which 

remains uncertain for either exposure metric.

(14) Comment: AAM (September 14, 2009 memo) commented that a study by Chock (1977) 

“demonstrated that the turbulence and heat generated by the traffic had a significant 

effect on the on-road and near-road wind and concentration fields”. They further add 

“[t]hese effects limit concentrations that can build up on and near roadways under 

adverse ambient meteorology and are not included in AERMOD.” 

Response: We agree that road-induced turbulence and buoyancy effects will dominate in 

the early morning hours where stability is likely high and mixing heights and 
temperatures are low. In these cases, the local mechanical turbulence will overwhelm the 

ambient turbulence and lead to the effects explicitly designed to be sampled by Chock. 

As noted above, on-road vehicle induced turbulence and plume rise were addressed in the 

selection of input parameters for initial horizontal and vertical dispersion and emission 

release heights for on-road, as documented in the REA (section 8.4.3.3). Additionally, to 

mitigate morning over-predictions, the mechanical mixing height was raised in cases 

where AERMOD-defined mixing heights were judged to be unreasonably low, as 

discussed in the REA.

(15) Comment: API/AECOM (September 14, 2009 memo) commented on the importance of 

developing a database that “provides information on NO2 to NOx emission ratios” 

suggesting that “the estimation of the NO2 to NOX ratio is critical in modeling peak 

hourly NO2 concentrations.

Response: We agree that knowledge of the initial NO2/NOX ratio is limited and that the 

ratio can be variable. Although we note that EPA’s new mobile source emissions model, 

MOVES, can provide separate estimates of NO, NO2, and NOX emissions. 

We also note that the importance of the accuracy of the initial ratio depends on the degree 

of conversion of NO to NO2 predicted by the model. That is, in cases where more 

complete conversion is predicted, the sensitivity of the final NO2/NOX ratio to its initial 

value is minimized. In the present exposure assessment where the dispersion model 

predicts a high rate of conversion, this effect of this on the output concentration will be 

small.

Furthermore, Carslaw (2005) attributes much of the observed increase in NO2/ NOX

emissions ratios to the increased use of diesel particulate filters (DPF) in London, UK. 

Even if this supposition is correct, it is unlikely to be relevant to the present modeling 

results. For comparison, diesel vehicles make up over fifty percent of new light duty 

vehicles in the EU (Dieselnet, 2008) whereas the North American diesel passenger 

vehicle market share is less than 1 percent (Haight, 2003). Regardless, we agree that, 

generally, improved characterization of this ratio would be advantageous in future 

modeling efforts.

(16) Comment: One commenter drew attention to a potential error that causes 

underestimation of NO2 impacts using the modeling software AERMOD (CAAPCOA). 

They provided the following description this error and other modeling shortcomings that 

will affect the models used to inform the selection of a short-term NO2 standard:

“The use of AERMOD with a more refined Conversion Tier (e.g., OLM [Ozone 

Limiting Method] or PVMRM [Plume Volume Molar Ratio Method]) produces 

lower 1-hour NO2 impacts than less refined methods do. However, California air 

district modelers have discovered that an error exists in the multi-source, 

combined-plume application of AERMOD (the OLMGROUP keyword) that 
biases the results low. This means that 1-hour NO2 levels calculated using the 

more refined methods may be underestimating the impact. Furthermore, the 

AERMOD model requires newer MET [meteorological] data sets that are more

complicated and require data inputs that may not be readily available for all areas 

in California. Consequently, not all air districts have converted to the AERMOD 

model.”

The commenter urges EPA to address this potential model error prior to establishing a 

1-hr NO2 standard.

Response: EPA was aware of this error while conducting the NO2 exposure assessment. 

The version of AERMOD that was used in our analysis had been corrected for this error. 

(17) Comment: In their October 22, 2008 memo, the ACC commented that the exposure 

assessment results for Philadelphia (1st draft REA) were very different from results 

presented for Atlanta (Final REA). They specifically noted “there are substantially

higher numbers of exceedances and numbers of individuals exposed to repeated 

exceedances for Atlanta compared to Philadelphia”. The commenter requested 

discussion or comparisons of the differences between the two locations.

Response: EPA considered performing the exposure modeling in five locations described 

in the Scope and Methods Plan (i.e., Atlanta, Detroit, Philadelphia, Phoenix, and Los 

Angeles) however there were limited data, time, and resources available to perform 

exposure modeling in all five locations. Following the CASAC review of the 1st draft 

REA and considering comments made on that document, EPA judged the model 

evaluations to be an important factor in selecting a location for an improved exposure 

assessment. Given the availability of personal exposure measurements within a similar 

time frame as the exposure modeling and a complementary quantitative risk assessment 

to be performed in the same location, EPA elected to focus the exposure modeling effort 

on Atlanta, GA rather than continue with the modeling that was initiated for the 1st draft 

REA (i.e., Philadelphia County).

The exposure modeling approach used for Philadelphia in the 1st draft REA (and 

documented in Appendix B-3 of the final REA) is not directly comparable to the 

exposure modeling performed for Atlanta (REA, Chapter 8). Therefore, a direct 

comparison of the exposure results obtained for each location would be unreasonable. As 

stated in REA Appendix B, section B-1, “due to differences in the approach used in the 

Philadelphia analysis, the results are not directly comparable to the Atlanta case-study.” 

For example, one of the most significant differences between the two assessments was 

how the emissions from minor roadways were addressed. In Atlanta minor roadways 

were modeled as area sources using AERMOD, considered by EPA an important 

improvement to the modeling approach used for this assessment. In Philadelphia minor 

roads were not modeled as emission sources, but considered as part of the unaccounted 

concentration when comparing the modeled concentrations considering all other emission 

sources at the ambient monitor locations. It is possible that these differing model 

approaches used were related to another important difference: the derived distributions of 
in-vehicle and near-road proximity factors. In Atlanta, the proximity distributions were 

stratified by season and hour-of-day, having a time-weighted geometric mean of about 

2.7 (REA Appendix B, Table B-42). In Philadelphia, the proximity distributions were 

stratified only by hour-of-day, having a time-weighted geometric mean of about 1.4 

(REA Appendix B, Figure B-11). These examples were just two of several important 

differences designed to improve the Atlanta assessment for the 2nd draft and final REA, 

building upon what was learned from the 1st draft approach used for Philadelphia. No 

improvements or adjustments were made to the exposure modeling approach used for 

Philadelphia, thus results for that assessment should only be considered as preliminary.

(18) Comment: Some commenters (e.g., API) noted that indoor NO2 concentrations can be 

higher than the levels that EPA is considering for an hourly NO2 NAAQS due to indoor 

emission sources. These commenters conclude that, given that people spend the majority

of their time indoors, the Agency should consider NO2 exposures indoors when 

evaluating whether the proposed NAAQS would have a meaningful effect on risk from 

NO2. 

Response: We agree that indoor concentrations of NO2 can be elevated relative to 

ambient concentrations, particularly in indoor environments containing sources of NO2

such as gas stoves (ISA, section 2.5.5). Indoor sources were considered as part of the 

exposure assessment presented in the REA. Even when indoor sources were included in 

modeling, the majority of exposures to peak NO2 concentrations (i.e., 1-hour 

concentrations at or above 100 ppb) were attributable to roadway-associated sources 

(REA, Figures 8-17 and 8-18). 

(15)
III. Responses to Significant Comments on the Adequacy of the 

Current Standard

(1) Comment: CASAC agreed that, based on the available information, the current NO2

standard is not requisite to protect public health with an adequate margin of safety and 

that revisions to the standard are appropriate. Their letter to the Administrator on the 

final REA (Samet, 2008) stated that “CASAC concurs with EPA’s judgment that the 

current NAAQS does not protect the public’s health and that it should be revised.” In 

supporting adoption of a more stringent NAAQS for NO2, CASAC generally relied on 

the assessment of the scientific evidence presented in the ISA, the results of assessments 

presented in the REA, and the conclusions of the policy assessment chapter of the REA. 

A number of public commenters also called for revising the current standard. This 

included environmental groups (e.g., CAC, EJ, EDF, NRDC, GASP); medical/public 

health organizations (e.g., AACPR, ALA, AMA, ATS, NAMDRC, NACPR, ACCP); 

State, local, and tribal agencies and organizations (e.g, NACAA, NESCAUM, agencies in 

CA, IA, IL, MI, MO, NC, NM, NY, TX, VA, WI, and tribes including NTAA, Fon du 

Lac); and a number of individual commenters. These commenters generally concluded 

that the current NO2 standard needs to be revised and that a more stringent standard is 

needed to protect the health of sensitive population groups. In supporting this 

conclusion, these commenters typically relied upon the evidence and information 

presented in the proposal and on CASAC’s recommendation. 

Response: We generally agree with these commenters’ conclusions regarding the 

adequacy of the current standard. The Administrator’s conclusions regarding adequacy

are discussed in more detail in section II.E.3 of the final rule. 

(2) Comment: Some industry commenters (e.g., AAM, API, Dow, INGAA, UARG), one 

State commenter (INDEM), and Roger McClellan expressed support for retaining the 

current annual standard alone. In supporting this view, these commenters generally 

concluded that available evidence supports a judgment that the current standard provides 

adequate protection of public health. They also typically concluded that the available 

evidence and information is not sufficient to support revision of the standard. These 

commenters generally relied upon their judgments regarding the scientific evidence and 

exposure/risk information and on the uncertainties associated with that evidence and 

information, as discussed above (section II) in more detail. For example, the UARG 

stated that “EPA has failed to demonstrate that the present NO2 NAAQS is no longer at 

the level requisite to protect public health with an adequate margin of safety.” The 

INGAA stated that “...EPA should be compelled to retain the current standard and defer 

a decision on a new short-term standard until the science is more clearly defined.” Roger 

McClellan commented that “there is a substantial body of scientific information to undergird a policy judgment to re-affirm the present annual standard set at 0.053 ppm NO2

measured at area-wide monitoring sites.” 
Response: These comments and EPA’s responses are discussed in detail in section II.E.2 

of the final rule. The Administrator’s conclusions on the adequacy of the current 

standard are described in section II.E.3 of the final rule. 

(3) Comment: The Oklahoma Independent Petroleum Association (OIPA) pointed out that 

“since 1990, NO2 emissions have decreased 35% despite the 63% increase in the gross 

domestic product, a 45% increase in the vehicle miles traveled, a 21% increase in 

population, and a 20% increase in energy consumption. Between 2001 and 2007, NO2

emissions decreased by 20%. In addition, EPA anticipates that nitrogen oxide (NOx) 

emissions will decrease substantially over the next 20 years as a result of the ongoing 

implementation of mobile source emissions standards.” As a result of these and similar 

statements by commenters, several groups concluded that a more stringent NO2 standard 

is unnecessary because the current standard is sufficient to protect against both long- and 

short-term exposures (e.g., API, Dow, INGAA, VADOT, MODNR, INDEM).

Response: As noted by the commenters, NOX emissions have decreased and are expected 

to continue to decrease as a result of ongoing implementation of mobile source emissions 

standards. However, the Clean Air Act (section 109) requires that primary NAAQS 

“shall be ambient air quality standards the attainment and maintenance of which in the 

judgment of the Administrator, based on such criteria and allowing an adequate margin of 

safety, are requisite to protect the public health.” Given this requirement, the primary 

NAAQS are set such that, in the Administrator’s judgment, public health is protected 

with an adequate margin of safety in locations that meet the standards. As discussed 

more fully in the final rule (section II.E.3), the scientific evidence and the exposure/risk 

information support the Administrator’s conclusion that important NO2-related public 

health risks are present in some locations with ambient NO2 concentrations below those 

allowed by the current annual standard and that a new standard is needed to provide the 

requisite degree of protection for public health. 
IV. Comments on A New Short-Term NO2 Primary Standard

This section discusses comments received on EPA’s proposed 1-hour standard. Some 

commenters provided comments on the cost or economic impact of monitoring, implementation, 

or compliance associated with the proposed NO2 NAAQS. As noted in section I.B of the 

preamble, the Clean Air Act bars consideration of costs in setting the NAAQS, and accordingly

EPA has not considered costs, including the costs or economic impact of monitoring, 

implementation or compliance, in revising the NO2 NAAQS.

A. Indicator

CASAC and some public commenters addressed the issue of the indicator for the 

standard (AAM, API, Dow, and MODNR). All of these commenters endorsed the 

proposal to continue to use NO2 as the indicator for ambient NOx, though some 

commenters expressed the need for special considerations when using NO2 as a surrogate 

for NOx. 

(1) Comment: API concluded that the proposed regulatory language would change the 

denomination of the standard from one regulating NO2 to one regulating oxides of 

nitrogen, with compliance to be determined by measurement of NO2 in the ambient air. 

API recommends that EPA clearly state that this language is not a substantive change and 

merely reflects the continuation of the Agency’s regulation of NO2 (and only NO2) as an 

indicator for all oxides of nitrogen.

Response: We agree that no substantive change is being made with regard to the 

indicator and that NO2 is the indicator for the oxides of nitrogen. This is indicated in the 

regulatory text for 50.11 in the final rule. 

(2) Comment: Some commenters (e.g., AAM) asserted that the tendency to overestimate NO2

concentrations with the Federal Reference Method (FRM) should be considered with 

regard to margin of safety, and that EPA should develop an FRM not prone to positive 

interference. 

Response: EPA is required to set the NAAQS at a level requisite to protect public health 

“allowing a margin of safety”. Thus, EPA takes “margin of safety” into consideration in 

setting the NAAQS. EPA separately considers the accuracy and precision of 

measurement methods in determining FRM and FEM requirements. The Administrator 

believes that the continued use of the chemiluminescence FRM is appropriate for 

comparison to the NAAQS. The issue of interference in FRM measurements and the 

discussion of the development of alternative methods that could be used in determining 

NO2 concentrations are discussed in section III.A.1 of the preamble to the final rule. 

Although the FRM can be subject to positive artifacts resulting in varying degrees of 

overestimation of NO2 concentrations, this overestimation should be minimized for 

monitors sited in urban locations, particularly near NOX sources such as roads. 

Additionally, the ISA concluded that for monitors sited in urban locations near roadways 

or other sources of NOX emissions, the overestimation of NO2 concentrations is typically 

less than 10% (ISA, section 2.3). 
B. Averaging time

(1) Comment: CASAC endorsed the establishment of a new standard with a 1-hour 

averaging time. CASAC stated the following in their comments on the proposal (Samet, 

2009b): 

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. 

The rationale offered by CASAC in support of a new 1-hour standard was generally the 

same as that put forward in the final REA and the proposal. Specifically, that rationale 

considered the available scientific evidence, which supports a link between 1-hour NO2

concentrations and adverse respiratory effects, and air quality information presented in 

the REA, which suggests that a 1-hour standard can protect against effects linked to 

short-term NO2 exposures while an annual standard would not be an effective or efficient 

approach to protecting against these effects. 

A number of public commenters also endorsed the establishment of a new standard with a 

1-hour averaging time. These included a number of State agencies and organizations 

(e.g., NACAA, NESCAUM and agencies in CA, IL, NM, TX, VA); environmental, 

medical, and public health organizations (ACCP, ALA, AMA, ATS, CAC, EDF, EJ, 

GASP, NACPR, NAMDRC, NRDC); and a number of individual commenters. The 

supporting rationale offered by these commenters often acknowledged the 

recommendations of CASAC and the Administrator’s rationale as discussed in the 

proposal.

Response: We agree with these comments on the need for a new 1-hour standard. 

Comments on averaging time are discussed in section II.F.2.b of the final rule. The 

Administrator’s final decision on averaging time is discussed in section II.F.2.c of the 

final rule. 

(2) Comment: Though many industry commenters recommended not revising the current 

annual standard (see above), several of these groups did conclude that if a short-term 

standard were to be set, a 1-hour averaging time would be appropriate (e.g., CPA, Dow, 

NAM, PAW, UPA). 

Response: As discussed above, industry commenters who disagreed with setting a new 

1-hour standard generally based this conclusion on their interpretation of the scientific 

evidence and their conclusion that this evidence does not support the need to revise the 

current annual standard. These comments are discussed in detail in section II.E.2 of the 

final notice. The Administrator’s conclusions regarding averaging time are discussed in 

section II.F.2.c of the final notice. 
(3) Comment: Two state commenters recommended that further studies be conducted to 

determine whether the 24-hr standard is as protective of human health as the 1-hr 

standard (e.g., NYDOH, SDDENR). One of these commenters specifically noted that 

epidemiological studies do not provide sufficient evidence that distinguishes effects 

between 1-hour and 24-hour exposure periods (SDDENR).

Response: The Administrator’s conclusion that available scientific evidence supports 

setting an NO2 standard with a 1-hour averaging time is described in detail in section 

II.F.2.c of the final notice. In addition, EPA notes that the court-ordered schedule for this 

review requires a notice of final rulemaking to be signed by January 22, 2010.

C. Form

(1) Comment: Many commenters emphasized that, for a standard reflecting the maximum 

allowable NO2 concentration anywhere in an area (i.e., the approach adopted in the final 

rule, section II.F.4.d), a form should be chosen that provides regulatory stability. To this 

end, CASAC favors a 3-year average of the 98th percentile (or 7th or 8th highest) of the 

distribution of annual 1-hour daily maximum concentrations. Specifically, CASAC 

commented that “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.” 

A number of other commenters also recommended a form based on the 3-year average of 

the 98th percentile (or 7th or 8th highest) of the distribution of annual 1-hour daily

maximum concentrations (e.g., Dow, SDDENR, NCDENR, API, PAW, VADOT, ACC, 

ExxonMobil, INDEM, NESCAUM, AQRL, and IPAMS). These commenters typically

argue that the 98th percentile form will provide a more stable statistic and, therefore, will 

better protect against unusual events and the possibility that concentrations measured by 

near-road monitors will be highly variable and responsive to small changes in monitor 

placement.

Response: EPA agrees with the comments that a 98th percentile form is appropriate. Her 

consideration of comments on form and her rationale for her final decision are discussed 

in detail in sections II.F.3.b and II.F.3.c, respectively. 

(2) Comment: Several commenters recommended either a 99th percentile form or a more 

stringent form. For example, some commenters recommended a “no exceedance” form, 

claiming that such a form would provide “more coherence in the protection for a standard 

that targets peak exposures than a form based in the 98th or 99th percentile” (ALA, EJ, 

EDF, NRDC). These same commenters noted that if EPA is unwilling to accept a “noexceedence” form, they strongly favor a 99th percentile standard over a 98th percentile 

standard. They pointed out that a 98th percentile form would allow for as many as 21 

exceedances in a three-year period, and concluded that this would be unacceptable. 
Alternatively, GASP recommends using a form allowing one exceedence per year over a 

three-year period, arguing that this will prevent several of the top measurements that 

contribute to adverse health effects from being dismissed. Several commenters prefer the 

3-year average of the 99th percentile (or 4th highest) of the distribution of 1-hour daily

maximum concentrations (e.g., TXCEQ, HLI, RHAMC, MODNR, NMED, NESCAUM).

Response: The Administrator recognizes that the public health protection provided by 

the 1-hour NO2 standard is based on the entire standard including the level of the 

standard (see below), in conjunction with the averaging time and form of the standard. In

light of her decision to set a new 1-hour standard that reflects the maximum allowable 

NO2 concentration anywhere in an area (see section II.F.4.d of the final rule), she 

concludes (consistent with the advice of CASAC) that an appropriate consideration with 

regard to form is the extent to which specific statistics could be unstable in locations 

where maximum NO2 concentrations are expected, such as near major roads. When 

considering alternative forms for the standard, the Administrator notes that an unstable 

form could result in areas shifting in and out of attainment, potentially disrupting ongoing 

air quality planning without achieving public health goals. Given the limited available 

information on the variability in peak NO2 concentrations near important sources of NO2

such as major roadways, and given the recommendation from CASAC that the potential 

for instability in the 99th percentile concentration is cause for supporting a 98th percentile 

form, the Administrator judges it appropriate to set the form based on the 3-year average 

of the 98

th percentile of the annual distribution of 1-hour daily maximum NO2

concentrations. This decision is discussed in section II.F.3.c of the final rule. 

(3) Comment: Some commenters stated a preference for expressing the form in terms of the 

nth highest concentration rather than in terms of a percentile. These commenters believe 

that using the nth highest form would increase transparency (e.g., Dow, AQRL). AQRL

noted that “it can provide an unambiguous determination of a ‘design value’ in the face 

of missing data if concentrations are high. The percentile form does not provide that 

certainty if data are missing.”

Response: We disagree that a percentile form does not provide certainty regarding the 

determination of the design value. As discussed above, and in the final rule (section 

II.F.3), the Administrator has determined it appropriate to set a form based on the 3-year 

average of the 98

th percentile of the annual distribution of 1-hour daily maximum NO2

concentrations. The interpretation of this standard, including requirements for when data 

are missing, is described in Appendix S to 40 CFR part 50 and in section IV of the 

preamble to the final rule. 

D. Approach and level

1. Comments on the approach to setting the standard 

(1) Comment: We received a number of comments on the most appropriate approach to 

setting the 1-hour NO2 standard. In their comments on the proposal, CASAC was split 
regarding the most appropriate approach to setting the 1-hour standard. In their letter to 

the Administrator on the proposal (Samet, 2009b), CASAC stated the following:

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 nearroadway 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.

As indicated in their letter, the majority of CASAC Panel members favored the proposed 

approach of setting a 1-hour standard that reflects the maximum allowable NO2

concentration anywhere in an area and linking such a standard with a 2-tiered monitoring 

network that would include both near-road and area-wide monitors. The 

recommendation of these CASAC Panel members was based on their conclusion that the 

proposed approach would be more effective than the alternative at limiting near-roadway

exposures to NO2 concentrations that could adversely affect asthmatics. 

In contrast, the minority of CASAC Panel members expressed support for the alternative 

approach of setting a 1-hour standard that reflects the allowable area-wide NO2

concentration. These CASAC Panel members concluded that there would be important 

uncertainties associated with the proposed approach. Specifically, they noted that the key

U.S. NO2 epidemiologic studies relied upon area-wide NO2 concentrations and that this 

introduces uncertainty into the use of these studies to inform a standard that reflects the 

maximum allowable NO2 concentration anywhere in an area. As a result of this 

uncertainty, CASAC Panel members who favored the alternative approach noted that “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” (Samet, 2009b). These CASAC 

Panel members also noted uncertainties associated with identifying appropriate 

monitoring sites near roads (see below and section III.B.2 of the final notice for more 

discussion of monitoring comments). 

Consistent with the views expressed by the majority of CASAC members, a number of 

commenters concluded that the most appropriate approach would be to set a 1-hour 

standard that reflects the maximum allowable NO2 concentration anywhere in an area and 

to couple that standard with a requirement that monitors be placed in locations where 

maximum concentrations are expected, including near major roads. This view was 

expressed by some State and local agencies (e.g., in CA, IA, NY, TX, WA, WI), by the 

majority of environmental organizations (e.g., CAC, EDF, EJ, GASP, NRDC), by the 

ALA, and by a number of individual commenters. Several additional medical and public 
health organizations (ACCP, AMA, ATS, NADRC, NACPR) did not explicitly express a 

recommendation regarding the approach though these organizations did recommend that, 

in setting a 1-hour standard, particular attention should be paid to NOX concentrations 

around major roadways. In support of their recommendation to adopt the proposed 

approach and to focus monitoring around major roads, these commenters generally 

concluded that a primary consideration should be the extent to which the NO2 NAAQS 

protects at-risk populations that live and/or attend school near important sources of NO2

such as major roads. As such, these comments were typically consistent with the 

rationale discussed by the Administrator in the proposal in support of setting a 1-hour 

standard that reflects the maximum allowable NO2 concentration anywhere in an area. 

Consistent with the view of the minority of CASAC Panel members, several industry

groups and an individual commenter (Roger McClellan) noted uncertainties associated 

with using epidemiologic studies to inform decisions on a standard reflecting the 

maximum allowable NO2 concentration anywhere in an area. Specifically, Roger 

McClellan stated the following: 

In considering the results of the six epidemiological studies that are at the 

core of the EPA assessment, it is important to recognize all of them used 

NO2 concentration data from central area monitors as a surrogate for 

personal exposure. These data are not applicable to establishing standards 

for near roadway monitors. 

Similar concerns with the proposed approach were expressed by several industry groups. 

Response: As described in detail in section II.F.4.c of the final notice, the Administrator 

agrees with the majority of CASAC Panel members and other commenters who 

concluded that the most appropriate approach to setting the 1-hour NO2 standard is to set 

a standard reflecting the maximum allowable NO2 concentration anywhere in an area. 

Her consideration of the uncertainties associated with this approach, as stated by the 

minority of CASAC Panel members and industry commenters, is also discussed in the 

final notice (sections II.F.4.b and II.F.4.c). 

(2) Comment: Some industry groups concluded that EPA is inappropriately combining 

decisions on the standard with decisions on the monitoring network. Specifically, AAM 

states the following with regard to the proposed approach 

[T]he effective stringency of a NAAQS depends on more than 

simply the numerical level selected. It also depends on the 

sensitivity of the monitoring system, and the ability of that system 

to determine accurately the ambient levels of the pollutant in a 

consistent manner at monitoring stations across the country. By

affixing the stringency of a primary NAAQS to a monitoring 

system that does not now exist and will be subject to varying 

interpretation by regional, state and local officials, EPA is in effect 

delegating the final decision on the NAAQS standard to those 
officials, contrary to statutory requirements that judgments 

concerning such standards reside with the Administrator. 

Response: We disagree with this characterization of the standard. As discussed in detail 

in the final notice (section II.F.4.c), the Administrator is setting a 1-hour standard 

reflecting the maximum allowable NO2 concentration anywhere in an area. It follows 

from this standard that monitors should be placed in locations where maximum NO2

concentrations are expected to occur. As discussed in the final notice (section II.A.2 and 

II.F.4.c), available information supports the conclusion that these maximum 

concentrations will occur around major roads in many areas. Therefore, the 

Administrator is requiring monitors within 50 m of major roads (see section III of the 

final notice). In situations where maximum NO2 concentrations are expected to occur in 

locations other than near major roads (e.g., near multiple smaller roads and/or stationary

sources), the Regional Administrator can also require monitors in these locations. EPA 

establishes criteria for the method of monitoring and for quality assurance/quality control 

(QA/QC). Any hourly data from monitors meeting FRM/FEM and QA/QC requirements 

will be considered in making designations. 

(3) Comment: Several industry and state commenters recommended that EPA select its 

alternate monitoring proposal of using area-wide monitors to be used in conjunction with 

a new short-term NO2 standard (e.g., EMA, SCCC, NMED). In their recommendation of 

the alternate monitoring network, NMED noted that the use of area-wide monitors is 

more applicable for measuring ambient air concentrations than near-road monitors since 

few people live or work as close to major roadways as near-road monitoring stations 

would be required to be located. Other commenters (e.g., Exxon) also concluded that 

monitor siting requirements should allow for consideration of population. 

Response: As noted in the proposal and the final notice (section II.F.4.c), millions of 

people in the United States live, work and/or attend school near important sources of 

NO2, including major roads. In addition, people commuting on major roads can be 

exposed to elevated NO2 concentrations. As described in the preamble, the Administrator 

concludes that short-term elevated NO2 concentrations, including those occurring on or 

near roads, pose public health risks. The Administrator further concludes it is appropriate 

to locate monitors where maximum concentrations are expected to occur, and that 

generally it is appropriate to locate monitors near major roads. States are to consider 

population in the site selection process if a state identifies multiple acceptable candidate 

sites where maximum hourly NO2 concentrations are expected to occur.

 

(4) Comment: AQRL commented that EPA’s alternative monitoring approach should not be

chosen and implemented if one of the principal reasons for the proposed 1-hr NO2

standard is to develop the near-road monitoring network.

Response: The rationale supporting a 1-hour standard reflecting the maximum allowable 

NO2 concentration anywhere in an area is discussed in detail in section II.F.4.d. This 

rationale does not include the development of a near-road monitoring network. 
(5) Comment: Several commenters (e.g., AAM, API) concluded that it is not appropriate to 

use the information discussed in the proposal on the NO2 concentration gradient around 

roads to inform decisions on the 1-hour standard. AAM concluded that the focus of the 

proposed approach on NO2 concentrations around major roadways is not justified (and 

would result in a standard that is more stringent than necessary) because the REA and the 

proposal overstate the extent to which NO2 concentrations near roads are higher than NO2

concentrations farther away from the road. AAM used data from the existing NO2

monitoring network as the basis for their conclusion that “roadside monitors are not 

measuring high NO2 concentrations.” Specifically, they claim that their analyses suggest 

the following: 

 Many of the presently located urban monitoring sites are already located within 50

meters of a major roadway. 

 These monitors are not measuring the high concentrations that are of concern to EPA. 

 Consequently, there does not appear to be a need to initiate a massive deployment of

new NO2 monitors within 50 m of major roadways.

Other commenters also noted that the highest concentrations of NO2 may be in urban 

canyons, near stationary sources, or near airports (e.g., NESCAUM). 

Response: With regard to the general point made by several commenters that maximum 

NO2 concentrations may not always occur near major roads, we note that, although the 

Administrator concluded in the proposal that maximum NO2 concentrations in many

areas are likely to occur around major roads, she also allowed for situations where this is 

not the case. Specifically, she proposed to set a 1-hour NO2 standard that reflects the 

maximum allowable NO2 concentration anywhere in an area, regardless of where that 

maximum concentration occurs.7 Therefore, the proposed approach to setting the 

standard would be expected to limit the maximum NO2 concentrations anywhere in an 

area even if in some areas, as is contended by AAM, those maximum NO2 concentrations 

do not occur near roads.

With regard to the analyses submitted by AAM, we agree that there is uncertainty

associated with estimates of roadway-associated NO2 concentrations (see REA, sections 

7.4.6 and 8.4.8.3 for detailed discussion of these uncertainties) and in identifying 

locations where maximum concentrations are expected to occur. However, we note that 

the Administrator’s conclusions regarding the relationship between NO2 concentrations 

near roads and those away from roads rely on multiple lines of scientific evidence and 

information. Specifically, the administrator relied in the proposal on the following in 

drawing conclusions regarding the distribution of NO2 concentrations across areas:

 

7

To measure maximum concentrations, the Administrator proposed monitoring provisions that 

would require monitors within 50 meters of major roads and to allow the Regional Administrator 

to require additional monitors in situations where maximum concentrations would be expected to 

occur in locations other than near major roads (e.g., due to the influence of multiple smaller 

roads and/or stationary sources). 
 Monitoring studies discussed in the ISA and REA that were designed to characterize 

the NO2 concentration gradient around roads, which indicated that NO2

concentrations near roads are elevated compared to concentrations in the same area 

but away from the road 

 Air quality and exposure analyses presented in the REA which estimate that NO2

concentrations on roads could be 80% higher than away from the road, on average 

across locations, and that roadway-associated exposures account for the majority of 

exposures to NO2 concentrations at or above 100 ppb

In contrast, to support their conclusions about the NO2 concentration gradient, AAM 

relies largely on an analysis of existing NO2 monitors that focused on 6 locations with a 

total of 42 monitors. While this analysis does provide information on NO2 concentrations 

at different distances from roads, the existing NO2 monitoring network was not designed 

to characterize the spatial gradients in NO2 concentrations surrounding roadways. 

Rather, concentrations of NO2 measured by existing monitors may likely reflect 

contributions from a combination of mobile and stationary sources, with one or the other 

dominating depending on the proximity of the monitors to these sources. 

Specifically, one example used by the commenters to support their conclusion that NO2

concentrations near roads are not elevated is that of St. Louis, where the monitors closer 

to a road measured lower NO2 concentrations than a monitor farther away from the road. 

However, a closer look at the monitor classified as being away from the road provides a 

potential explanation for these observations and illustrates the limitations of relying 

exclusively on the existing monitoring network to characterize the NO2 concentration 

gradient around roads. The monitor used (ID 295100086) is characterized by EPA’s Air 

Quality System (AQS) as having a “high concentration” monitoring objective, is 133 m 

from a major road, and is surrounded by 35 NOX emission sources within 10 km having 

median emissions of about 17 tons per year (see REA, Appendix A, Tables A-7 and A-8). 

Thus, the single site used in St. Louis for the ≥ 100 m bin may not necessarily reflect 

concentrations at a site distant from a roadway, is likely not exclusively impacted by

roadway emissions, and would likely not be useful in determining the relationship 

between near-road NO2 concentrations and those away from the road. The REA 

identified and discussed situations such as this as an important uncertainty when 

estimating on-road concentrations (REA, section 7.4.6, page 135).

Given the considerations above, we conclude that the analysis submitted by AAM, which 

does not consider relevant lines of evidence and information other than their analysis of 

existing monitors, does not appropriately characterize the relationship between NO2

concentrations near roads and those away from roads.

2. Comments on standard level 

(1) Comment: In commenting on the proposal, CASAC discussed both the proposed range 

of standard levels (i.e., 80-100 ppb) and the alternative range of standard levels (i.e., 50-

75 ppb). Though, as discussed above, they were split on which approach should be 

adopted for setting the 1-hour standard, CASAC did express the consensus conclusion 
that, if the Agency finalizes a 1-hour standard in accordance with the proposed approach 

(i.e., standard level reflects the maximum allowable NO2 concentration anywhere in an 

area and a monitoring network is established that includes monitors near major roads), 

then it is appropriate to consider the proposed range of standard levels from 80 to 100 

ppb. Specifically, the CASAC letter to the Administrator on the proposal (Samet, 2009b) 

stated the following with regard to the proposed approach: 

[T]he 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. 

A number of State and local agencies and organizations also expressed support for setting 

the level of the 1-hour NO2 standard within the proposed range of 80 to 100 ppb, though 

only a few of these State and local agencies (e.g., in CA, IA, MI, NY, TX) made this 

recommendation in conjunction with a recommendation to focus monitoring near major 

roads and other important sources of NO2. 

Response: As is discussed in section II.F.4.d of the final rule, the Administrator has 

judged it appropriate to set a standard level of 100 ppb. Her rationale, which incorporates

her consideration of the above comments, is discussed in detail in this section. 

(2) Comment: A number of environmental organizations (e.g., CAC, CSE, EDF, EJ, GASP, 

NRDC) and medical/public health organizations (e.g., ACCP, ALA, AMA, ATS, 

NACPR, NAMDRC) supported setting a standard level below 80 ppb for a standard that 

reflects the maximum allowable NO2 concentration anywhere in an area. Several of these 

groups recommended a standard level of 50 ppb. This recommendation was typically 

based on the commenters’ interpretation of the epidemiologic and controlled human 

exposure evidence as well as the NO2 exposure and risk information. 

Some of these commenters noted that the 98th percentile area-wide NO2 concentration 

was below 80 ppb in the location of a single key U.S. epidemiologic study (i.e., 50 ppb in 

study by Delfino). Given this, commenters concluded that the standard level should be 

set at 50 ppb. Their comments on the monitoring network generally favored a 

requirement to place monitors near major roads and, therefore, these commenters 

appeared to favor a standard level as low as 50 ppb and to recommend that such a 

standard level reflect the maximum allowable NO2 concentration anywhere in an area. In 

their comments, the ALA, EDF, EF, and NRDC stated the following: 
Considering the Delfino study alone on EPA’s terms, that is, 

focusing on the 98th percentile of the 1-hour daily maximum 

concentrations, EPA reports a concentration of 50 ppb where 

asthma symptoms were observed. Based primarily on this study, 

EPA concluded in the REA that it was appropriate to set the lower 

end of the range at 50 ppb, which corresponded to the lowestobserved effects level of airway hyperresponsiveness in 

asthmatics. To provide the strongest public health protection, we 

therefore urge the level of the standard be set at 50 ppb. 

In some cases, the same commenters also appeared to recommend setting a standard level 

below 50 ppb because mean area-wide NO2 concentrations reported in locations of key

U.S. epidemiologic studies are below this concentration. Specifically, with regard to the 

key U.S. epidemiologic studies, these commenters (e.g., ALA, EDF, EJ, NRDC) stated 

the following:

These studies clearly identify adverse health effects such as 

emergency room visits and hospital admissions for respiratory

causes at concentrations currently occurring in the U.S. Mean 

concentrations for all but two of these studies are about or below 

50 ppb, suggesting that the standard must be set below this level to 

allow for a margin of safety. 

Some of these commenters also concluded that the proposed range of standard 

levels fails to provide a margin of safety for effects reported in controlled human 

exposure studies. 

Response: The Administrator’s decision to set a standard level of 100 ppb is discussed in 

detail in section II.F.4.d of the final rule. The rationale for this decision includes 

consideration of the above comments. Specific comments on standard level are discussed 

in section II.F.4.c of the final rule. 

(3) Comment: Several environmental and public health groups (e.g., ALA, EDF, EJ, NRDC) 

concluded that the REA analyses of NO2 exposures and risks suggest that “only an hourly

standard of no more than 50 ppb would protect against harm from peak exposures.” 

Response: We disagree with this characterization. As discussed in sections II.C and 

II.F.4.d of the final rule, the analyses of different potential alternative standards in the 

REA are based on the current monitoring network, which contains primarily area-wide 

monitors. The Administrator concludes that the results of exposure and risk analyses 

provide support for limiting area-wide NO2 concentrations to no higher than 100 ppb. 

The Administrator is setting a new 1-hour standard that reflects the maximum allowable 

NO2 concentration anywhere in an area. Therefore, when considering the REA analyses 

in the context of such a standard, the Administrator considered available information on 

the relationship between the higher NO2 concentrations around roads and area-wide NO2

concentrations. Specifically, as described in detail in section II.F.4.d, she noted that a 
standard level of 100 ppb reflecting the maximum allowable NO2 concentration 

anywhere in an area would be expected to limit area-wide NO2 concentrations to 

approximately 50 to 75 ppb. Therefore, she concluded that a standard level of 100 ppb, 

for a standard that reflects the maximum allowable NO2 concentration anywhere in an 

area, is consistent with conclusions based on the NO2 exposure and risk information. 

(3) Comment: Several industry groups (e.g., AAM, Dow, NAM, NPRA) and Roger 

McClellan concluded that, if EPA does choose to set a new 1-hour standard, the level of 

that standard should be above 100 ppb. As a basis for this recommendation, these groups 

typically emphasized uncertainties in the scientific evidence. Specifically, these 

commenters typically concluded that available epidemiologic studies do not support the 

conclusion that NO2 causes reported health effects. This was based on their assertion that 

the presence of co-pollutants in the ambient air precludes the identification of a specific 

NO2 contribution to reported effects. Some industry groups also concluded that 

epidemiologic studies should not be used to inform decisions on the standard level 

because these studies were based on area-wide, rather than near-road, NO2

concentrations. As a result, these commenters recommended that a 1-hour standard 

should be based on the controlled human exposure evidence and that, in considering that 

evidence, EPA should rely on the meta-analysis of NO2 airway responsiveness studies 

conducted by Goodman et al., (2009) rather than the meta-analysis included in the final 

ISA. They concluded that in relying on the ISA meta-analysis, EPA has inappropriately 

relied on a new unpublished meta-analysis that has not been peer-reviewed, was not 

reviewed by CASAC, and was not conducted in a transparent manner. 

Response: EPA recognizes the uncertainties in the scientific evidence that are discussed 

by these industry commenters; however, we strongly disagree with their conclusions 

regarding the implications of these uncertainties for decisions on the NO2 NAAQS. 

These comments, and EPA’s responses, are discussed in detail in section II.E.2 of the 

final rule and above (section II) in this Response to Comments Document. The 

Administrator’s conclusions on standard level are described in section II.F.4.d of the final 

rule. 

(4) Comment: Some commenters argued that EPA should not adjust the standard level to 

account for postulated near-road concentrations because, due to differences in 

concentration gradient, it would establish a level that is arbitrary and variable across 

locations (e.g., NPRA, NYSDEC, AAM). AAM further argued that EPA’s information 

about the concentration gradient was not sufficiently precise or “factual” to inform EPA’s 

decision making. 

Response: As described in section II.F.4.d of the final rule, EPA has not adjusted the 

standard level to account for near-road concentrations. Rather, the Administrator has 

considered the extent to which specific standard levels would be expected to protect 

against exposure to the distribution of NO2 concentrations across an area and, therefore, 

against the array of respiratory effects that have been linked to short-term NO2 exposures. 

These considerations are described in detail in section II.F.4.d of the final rule. EPA 
disagrees with AAM’s suggestion that information about the concentration gradient 

should be disregarded in reviewing the standard.

(5) Comment: One commenter (NYDOH) concluded that too much weight has been placed 

on clinical studies and that more emphasis should be placed on epidemiologic studies in 

setting the standard level. 

Response: As discussed in detail in section II.F.4.d of the final rule, the Administrator 

has considered both the controlled human exposure and epidemiologic evidence in setting 

the standard level. Both lines of evidence provide information on the extent to which 

specific NO2 standards would be expected to protect against the range of respiratory

effects that have been linked to short-term exposures to NO2. 

(6) Comment: INGAA pointed out that the related health-based standards (e.g., Short-term 

Exposure Level (STEL) or ceiling Permissible Exposure Level (PEL) from Occupational 

Safety and Health Association (OSHA) or the National Institute of Occupational Safety 

and Health (NIOSH)) are higher than the proposed NO2 NAAQS. They claimed that 

personnel have not been negatively affected by NO2 exposures at their facility. The 

commenter also observed that “when comparing the relative margin of OSHA or NIOSH 

standards to the NAAQS for other gases, the proposed NO2 NAAQS is more stringent. 

For example, the OSHA NO2 PEL (ceiling) is 5 ppmv [parts per million by volume] 

(i.e., 5000 ppbv [parts per billion by volume]) and the NIOSH STEL is 1 ppmv (1000 

ppbv). These levels are one to two orders of magnitude higher than ranges being 

considered by EPA for the 1-hour NO2 NAAQS. By comparison, the CO [carbon 

monoxide] NAAQS 1-hour average standard is 35 ppmv, compared to a NIOSH STEL of 

200 ppmv, a factor of 5.7. This appears to indicate a more conservative approach for the 

proposed NO2 NAAQS.”

Response: The CAA requires that air quality criteria and NAAQS be reviewed 

periodically. After consideration of the latest scientific knowledge, the NAAQS must be 

set at levels that are, in the Administrator’s judgment, requisite to protect public health, 

including the health of sensitive subpopulations, with an adequate margin of safety. The 

Administrator has considered this requirement in setting a new 1-hour NO2 standard as 

described in section II.F.4. In contrast, occupational standards, such as those mentioned 

by the commenter, are established based on different legal requirements and may not be 

updated in the same manner as the NAAQS. Therefore, there is no expectation that the 

ratio of the NAAQS to an occupational standard would be a basis for selecting a 

NAAQS. 

(7) Comment: NESCAUM argues that “while ozone rapidly oxidizes NO to NO2, there is 

usually insufficient ozone to produce substantially elevated levels of NO2 from this 

reaction in near-roadway exposure scenarios during morning rush hours, nighttime, and 

at least half of the year (during the non-ozone season).”

Response: While EPA acknowledges that the extent to which NO2 concentrations near 

roads are elevated will vary with ozone concentrations, we disagree with the conclusion 
that NO2 concentrations near roads are not usually elevated substantially. As discussed in 

the final rule (sections II.A.2 and II.F.4), elevated NO2 concentrations do occur on and 

around roads and people who live, work, and/or attend school near major roads, or 

commute in vehicles on major roads, can be exposed to these elevated concentrations. 

For example, based on NO2 monitoring studies the ISA stated that NO2 concentrations in 

heavy traffic or on freeways “can be twice the residential outdoor or residential/arterial 

road level,” that “exposure in traffic can dominate personal exposure to NO2,” and that 

“NO2 levels are strongly associated with distance from major roads (i.e., the closer to a 

major road, the higher the NO2 concentration)” (ISA, sections 2.5.4, 4.3.6). In addition, a

considerable fraction of the population resides, works, or attends school near major 

roadways and these populations are likely to have increased exposure to NO2 (ISA, 

section 4.4). Based on data from the 2003 American Housing Survey, approximately 36 

million individuals live within 300 feet (~90 meters) of a four-lane highway, railroad, or 

airport (ISA, section 4.4).

8

 Furthermore, in California, 2.3% of schools with a total 

enrollment of more than 150,000 students were located within approximately 500 feet of 

high-traffic roads (ISA, section 4.4). 

(8) Comment: Harris County Public Health and Environmental Services (HCPHES) noted

that EPA’s REA “concludes that it is appropriate to focus on studies that evaluated NO2

health effect associations using both single- and multi-pollutant models. However, 

HCPHES believes that insufficient study and attention has been given to such synergistic 

effects. Because of this insufficiency, HCPHES believes that EPA should include an 

adequate margin of safety and set the one-hour standard sufficiently below the upper 

limit of 100 ppb leaning more towards the bottom end of the proposed range.”

Response: The Administrator’s decision on standard level, including her consideration of 

the requirement to provide an adequate margin of safety, is discussed in detail in section 

II.F.4.d of the final rule. 

(9) Comment: AASHTO recommended that EPA should not set its NO2 standard below 

background concentrations of NO2 in order to avoid making the standard unattainable. 

Response: As described in detail in section II.F.4.d of the final rule, the Administrator 

has set an NO2 NAAQS that, in her judgment, is requisite to protect public health, 

including the health of sensitive subpopulations, with an adequate margin of safety. In 

setting such a standard, the Administrator is prohibited from considering such factors as 

 

8

The most current American Housing Survey

(http://www.census.gov/hhes/www/housing/ahs/ahs.html) is from 2007 and lists a higher fraction 

of housing units within the 300 foot boundary. According to Table 1A-6 from that report

(http://www.census.gov/hhes/www/housing/ahs/ahs07/tab1a-6.pdf), out of 128.2 million total 

housing units in the United States, about 20 million were reported by the surveyed occupant or 

landlord as being within 300 feet of a 4-or-more lane highway, railroad, or airport. That 

constitutes 15.6% of the total housing units in the U.S. Assuming equal distributions, with a 

current population of 306.3 million, that means that there would be 47.8 million people meeting 

the 300 foot criteria.
projections of areas that will or will not attain the standard. However, EPA notes that we 

are unaware of any locations with nonanthropogenic background concentrations that 

would exceed the standard. 

3. Comments on the annual standard 

(1) Comment: With regard to an annual standard, CASAC and a number of public 

commenters (e.g., NACAA, NESCAUM; agencies from States including CA, IN, MO, 

NC, NY, SC, TX, VA; tribal organizations including Fon du Lac and NTAO; 

environmental/medical/public health groups including ACCP, ALA, AMA, ATS, CAC, 

EDF, EJ, GASP, NACPR, NAMDRC, NRDC; and industry groups including AAM, API, 

Dow, INGAA, UARG) agreed with the proposed decision to maintain an annual 

standard, though their recommendations with regard to the level of that annual standard 

differed (see below). 

In their comments on the final REA, CASAC recommended “retaining the current 

standard based on the annual average” based on the “limited evidence related to potential 

long-term effects of NO2 exposure and the lack of strong evidence of no effect” and that 

“the findings of the REA do not provide assurance that a short-term standard based on the 

one-hour maximum will necessarily protect the population from long-term exposures at 

levels potentially leading to adverse health effects” (Samet, 2008). A number of State 

agencies and organizations and industry groups also recommended maintaining the 

current level of the annual standard (i.e., 53 ppb). This recommendation was based on 

the conclusion that, while some evidence supports a link between long-term NO2

exposures and adverse respiratory effects, that evidence is not sufficient to support a 

standard level either higher or lower than the current level. 

In contrast, some environmental organizations and medical/public health organizations as 

well as a small number of States (e.g., ALA, EDF, EJ, NRDC, and organizations in CA) 

recommended setting a lower level for the annual standard. These commenters generally

supported their recommendation by pointing to the State of California’s annual standard 

of 30 ppb and to studies where long-term ambient NO2 concentrations have been 

associated with adverse respiratory effects such as impairments in lung function growth.9 

Response: EPA’s response to these comments, and the Administrator’s conclusions on 

the annual standard, are discussed in section II.G of the final rule. 

(2) Comment: Several petroleum industry commenters (API, UPA, IPAMS, PAW, UARG) 

claimed that EPA did not provide a rationale for changing the units of the annual 

standard. These commenters assert that the proposal to express the level as 53 ppb is 

different from the current standard which also expresses the level in terms of 

 

9

These groups also cited a recent study by Karr et al. (Influence of Ambient Air Pollutant 

Sources on Clinical Encounters for Infant Bronchiolitis. Am. J. Respir. Crit. Care. Med. (2009),

Aug. 27, epub ahead of print). 
micrograms/m3

(100 micrograms/m3). These commenters claim that by shifting to ppb, 

the stringency of the standard is increased for higher elevations (i.e., at sea level 53 ppb = 

100 microgram/m3, but at 5,000 ft, 53 ppb = 84 micrograms/m3). PAW specifically 

stated that “the change in the annual standard from the current mass and volume 

concentration to a strictly volume concentration represents a significant and unjustified 

tightening of the standard for high altitude areas, which includes many of the areas in 

which the oil & gas industry operates in the western states.” 

Response: The issue raised in the comments is attributed to the mathematical conversion 

between ppb (a mixing ratio, by volume) and micrograms per cubic meter (μg/m3), which 

is a mass-based metric. According to the ideal gas law (PV=nRT) the number of 

molecules (n) are proportional to their volumes (V) which in turn is proportional to their 

pressures (P). Since the total air pressure, and hence the concentration of air, decreases 

with altitude, a constant mixing ratio will not translate into a constant concentration. 

Typically, when converting from ppb to μg/m3, scientists standardize the process by 

using a standard pressure and temperature, e.g., 1 atm and 25 C, respectively, in the 

conversion calculations. This standardization results in the apparent difference in mass 

concentration provided as an example by the commenter. 

EPA notes that “parts per” concentration, such as parts per billion (ppb) or parts per 

million (ppm), etc., are mixing ratios, and are very common units for measuring gas 

phase species. Further, the use of mixing ratios is widespread for expressing the relative

amounts of species at various altitudes. The chemiluminescence FRM for the NO2

NAAQS provides in-situ concentration information, in a mixing ratio by volume (e.g., 

ppb), that is compensated for both temperature and pressure. As a result, 

chemiluminescence FRM-produced data, which measures concentrations from a known 

volume of air, is not subject to variation due to ambient temperature or pressure (e.g., 

altitude). 

The parenthetical in the existing standard was intended to be informative, indicating the 

mass-based measurement corresponding to the level of the standard at standard 

temperature and pressure. The parenthetical did not create a second standard, allowing 

compliance with either measurement depending on ambient temperature or pressure, and 

therefore deletion of the parenthetical is not a change to the standard. Since the FRM is 

not subject to variation due to ambient temperature or pressure, the ppb level is the only 

measurement relevant to attainment. However, EPA recognized that the presence of the 

parenthetical in the standard could be confusing, and determined it was appropriate to 

delete it to clarify the standard.
V. Technical Issues with Monitoring Requirements

A. Comments on near-road monitor siting requirements 

(1) Comment: Some state agencies (e.g., AASHTO, NYSDOT, and WIDNR) recommended 

that the maximum horizontal distance be increased from the proposed 50 meters to as 

much as 200 meters, noting that a 50 meter limit would be infeasible for many locations 

and could create a serious safety hazard for monitoring staff and the general public. In 

contrast, several environmental, and industry groups (e.g., GASP and AQRL) supported 

the proposed range, and in GASP’s case, supported reducing the maximum distance to 30 

meters or less in order to reduce the potential variation in roadway distance between 

monitoring sites and subsequent measurements of near-road NO2 concentrations. 

Response: EPA’s decision on the siting criteria for NO2 monitors requires, among others, 

that the near-road NO2 monitors be sited within 50 meters from the roadway. EPA 

discusses the basis for this decision in detail in section III.B.7 of the preamble to the final 

rule. 

(2) Comment: A limited number of comments were received recommending changes to the 

requirements for the vertical height of the NO2 monitor inlet manifold. Two state

organizations (NESCAUM and NYSDEC) noted that the proposed 2-7 meter vertical 

range could generate significant variation in the concentrations reported from monitors 

located at different heights. NESCAUM recommended that the height range be reduced 

to 2.5-3.5 meters. NYSDEC commented that the proposed vertical height range may not 

be practical for many sites, especially those located in urban areas. In addition, the ALA, 

EJ, EDF, and NRDC noted that: 

[T]he lower end of the proposed height of 2 to 7 meters appears to 

capture the highest NO2 concentrations, and more accurately 

represents human exposure at the breathing zone. Additional 

monitors at other relationships to these sources may be needed for 

research purposes, but they should be in addition to those designed 

to establish the peak exposures for NAAQS compliance purposes. 

Response: EPA’s decision on the siting criteria for NO2 monitors requires, among others, 

that the near-road NO2 monitors inlet manifolds be placed between 2 to 7 meters above 

the ground. EPA discusses the basis for this decision in detail in section III.B.7 of the 

preamble to the final rule. With regard to the ALA, EJ, EDF, and NRDC comment on 

needing additional monitors for research, EPA does not believe that this particular 

research issue is appropriate to address through this rulemaking, but rather through 

ongoing and future research that further characterizes the near-road environment. For the 

purposes of supporting the revised 1-hour NAAQS, EPA believes that the siting criteria 

are sufficient and appropriate. 
(3) Comment: Several groups that support the proposed two-tiered monitoring network 

requested modifications to the proposed criteria for determining where a near-road 

monitor should be located. Some environmental/public health organizations (ALA, EJ, 

NRDC, EDF) commented that “Near-road monitor placement should be determined not 

only by the highest AADT [annual average daily traffic] volumes in a given [core-based 

statistical area] CBSA, but also by the highest heavy-duty truck volumes.” The NYDOH, 

IADNR, and some industry groups (e.g., EEI and SRNS) recommend making population 

a prominent consideration when deciding where to locate monitors. HCPHES 

recommended that near-road monitoring stations should take into consideration the 

location of other major mobile and point sources for NO2 emissions, such as airports, 

seaports, and power plants, or vulnerable population groups like children, when 

determining the appropriate locations for near-road monitors. 

Response: EPA has clarified that the selection criteria should include the consideration 

of localized factors when identifying locations of expected maximum concentrations. 

Near-road sites shall be selected by ranking all road segments within a CBSA by AADT 

and then identifying a location or locations adjacent to those highest ranked road 

segments, considering fleet mix, roadway design, congestion patterns, terrain, and 

meteorology, where maximum hourly NO2 concentrations are expected to be highest and 

the siting criteria can be met in accordance with 40 CFR Part 58 Appendix E. EPA 

discusses the basis for this decision in detail in section III.B.6 of the preamble to the final 

rule. 

(4) Comment: Many state agencies commented that EPA needed to provide greater guidance 

to state and municipal agencies about where to site near-road monitors. The CAC 

recommended that state discretion over the location of near-road monitors be limited 

“such that gaming is not tolerated.”

Response: EPA agrees that guidance will be useful in aiding the implementation of a 

near-road network. As noted in the section III.B.5 of the preamble to the final rule, EPA 

plans to assist state agencies in the network implementation process, particularly through 

guidance documentation that the Agency intends to prepare and share with state and local 

agencies early enough in the overall implementation process to be useful to states in 

developing their NO2 monitoring network. Further, EPA encourages state and local 

monitoring agencies to include or cooperate with transportation officials in the 

development of the near-road network. EPA believes that state and local monitoring 

agencies may benefit greatly from working with federal, state, and/or local transportation 

officials during the development of and long-term maintenance of a near-road network. 

EPA believes that the commenter’s concern that the state discretion allowed in the 

placement of near-road monitors may lead to installations in locations unlikely to violate 

the NAAQS, will be avoided due to the process outlined in 40 CFR §58.10 under which a 

state’s annual monitoring network plan must be made available for public inspection or 

comment and approved by the EPA Regional Administrator. This process will permit 

scrutiny of the monitor siting locations and assist in identifying and siting monitors in 

appropriate locations within CBSAs.
(5) Comment: CASAC advised that siting monitors based on traffic counts alone might miss 

locations where maximum NO2 concentrations would occur. CASAC recommends that 

EPA should use model results from Congestion Mitigation and Air Quality Land Use 

Regression modeling as well as Gaussian plume models and emissions inventories to 

determine where roadside monitors should be located. 

Response: EPA does not intend for AADT counts to be the sole basis for choosing a 

near-road site. EPA has clarified rule language to reflect that required near-road NO2

monitoring stations shall be selected by ranking all road segments within a CBSA by

AADT and then identifying a location or locations adjacent to those highest ranked road 

segments, considering fleet mix, roadway design, congestion patterns, terrain, and 

meteorology, where maximum hourly NO2 concentrations are expected to occur and the 

siting criteria can be met in accordance with 40 CFR Part 58 Appendix E. This issue is 

discussed in section III.B.6 of the preamble to the final rule. EPA also notes that air 

quality models, which were noted by the CASAC panel member to be considered for use 

in near-road site selection, are tools that EPA believes will be useful, and likely used by

some states to inform where near-road sites need to be placed

(6) Comment: EPA received a number of comments recommending using different 

population thresholds, or a different approach in requiring near-road monitors. Several 

commenters (e.g., NESCAUM, NYSDOT, NYSDEC) recommended using a higher 

population threshold than that proposed, for example, NYSDEC suggested the population 

threshold be increased to as much as 2.5 million. Conversely, CAC suggested that the 

proposed threshold may not be low enough. Other commenters (e.g., AASHTO and 

HCPHES) recommended that EPA use an AADT threshold (ranging from 100,000 to 

250,000 AADT), in lieu of a population threshold, to require near-road monitoring. 

Another commenter, San Joaquin Air Pollution Control District suggested the using a 

combination of population and AADT thresholds to require near-road monitors. 

Response: EPA’s decision is to require one near-road NO2 monitor in CBSAs with a 

population greater than or equal to 500,000 persons and a second near-road monitor is 

required in CBSAs with a population greater than or equal to 2,500,000 persons, or in any

CBSAs with one or more road segments with an Annual Average Daily Traffic (AADT) 

count greater than or equal to 250,000. EPA discusses the basis for this decision in detail 

in section III.B.2 of the preamble to the final rule. 

(7) Comment: EPA received comments suggesting that EPA require monitoring at sites nonnear-road locations. One commenter (Center on Race, Poverty, and the Environment) 

recommended that EPA account for populations in rural areas, such as those impacted by 

agricultural emissions. The Fond du Lac Band of Lake Superior Chippewa and the 

NTAA suggested that EPA expand monitoring coverage into rural areas and tribal lands. 

Other commenters (ATS, AMA, NAMDRC, NACPR, ACCP, and the Swinomish Tribe) 

also recommended that EPA consider sources other than mobile sources, such as large 

stationary sources. 
Response: As discussed in section II.F.4.d of the final rule, the Administrator has set a 

new 1-hour NO2 standard that reflects the maximum allowable NO2 concentration 

anywhere in an area. In her judgment, this new standard will protect public health, 

including the health of sensitive populations such as asthmatics, with an adequate margin 

of safety. It follows from this standard that monitors should be placed in locations where 

maximum NO2 concentrations are expected. The Administrator has judged that, in many

areas, these maximum concentrations are likely to occur around major roads (final rule, 

sections II.A.2 and II.F.4.d). Therefore, she is requiring NO2 monitors within 50 meters 

of major roads. In situations where maximum NO2 concentrations are likely to occur in 

locations other than near a major road (e.g., near stationary point or area sources, whether 

agricultural or industrial (such as those noted by the commenters), the Regional 

Administrator has the authority to require additional monitors in these locations. Further, 

the Administrator has also recognized that susceptible and vulnerable populations, which 

include asthmatics and disproportionately exposed groups, (as discussed in sections 

II.B.4 and II.F.4.d of the preamble to the final rule) are at particular risk of NO2-related 

health effects. The Administrator is therefore requiring the Regional Administrators, 

working in collaboration with states, to site forty monitors in appropriate locations, 

focusing primarily on protecting such susceptible and vulnerable communities. This 

decision is discussed in detail in section III.B.4.

(8) Comment: Two tribal commenters ( Fond Du Lac Band of Lake Superior Chippewa and 

the National Tribal Association) recommended that more monitoring stations be required 

for rural or tribal roadways since EPA recognized Native Americans as an at-risk 

population group. 

Response: As discussed in section II.F.4.d of the final rule, the Administrator has set a 

new 1-hour NO2 standard that reflects the maximum allowable NO2 concentration 

anywhere in an area. In her judgment, this new standard will protect public health, 

including the health of sensitive populations such as asthmatics, with an adequate margin 

of safety. It follows from this standard that monitors should be placed in locations where 

maximum NO2 concentrations are expected. The Administrator has judged that, in many

areas, these maximum concentrations are likely to occur around major roads (final rule, 

sections II.A.2 and II.F.4.d). Therefore, she is requiring NO2 monitors within 50 meters 

of major roads. In situations where maximum NO2 concentrations are likely to occur in 

locations other than near a major road (e.g., near multiple smaller roads and/or stationary

sources), the Regional Administrator will have the authority to require additional 

monitors in these locations. EPA notes that it is not in a position to require monitoring by 

tribes or on tribal land. However, EPA has and will continue to work with tribes in 

conducting ambient monitoring on tribal lands.

(9) Comment: One state agency, NYSDEC requested that CBSAs with populations larger 

than 2.5 million be allowed to have more than two near-road monitors.

Response: EPA’s decision is to require, at a minimum, one near-road NO2 monitor in 

CBSAs with a population greater than or equal to 500,000 persons and a second near-
road monitor in CBSAs with a population greater than or equal to 2,500,000 persons, or 

in any CBSAs with one or more road segments with an Annual Average Daily Traffic 

(AADT) count greater than or equal to 250,000. EPA discusses the basis for this decision 

in detail in section III.B.2 of the preamble to the final rule. EPA also notes that states 

always have the ability to conduct additional monitoring above the minimum monitoring 

requirements at their discretion.

(10) Comment: Some public health and environmental groups (e.g., ALA, CAC, and NRDC) 

suggested that near-road monitors should be required to be located on the downwind side 

of the target road. Conversely, several commenters (AQRL and NYSDEC) suggested 

that such a requirement may be over restrictive and not necessary. AQRL commented 

that siting monitors in downwind locations would not be feasible for all locations and 

EPA should allow upwind monitoring locations so long as upwind monitors are used in 

conjunction with air dispersion modeling.

Response: EPA is not going to finalize a requirement that near-road sites must be 

climatologically downwind of the target road segment because of the additional 

limitations this introduces to finding potential site candidates in exchange for what may

be a small increase in the opportunity to monitor peak NO2 concentrations. EPA 

discusses the basis for this decision in detail in section III.B.7 of the preamble to the final 

rule.

(11) Comment: Several commenters (e.g., CASAC, ACCP, ALA, AMS ATS, EDF, NACPR, 

NAMDRC, NRDC) made comments that the proposed network should form the basis for 

a broader near-road monitoring network that would encompass other pollutants as well, 

i.e., be a multi-pollutant near-road monitoring network. These organizations’ 

recommendations were based on the argument that it makes little sense to only monitor 

NO2 concentrations near roads when it is well-understood that many other pollutants, 

hazardous to the public’s health, are emitted by on-road vehicles along with NO2.

Response: The scope of this rulemaking pertains to the NO2 NAAQS and the monitoring 

associated with it. However, EPA agrees that multi-pollutant monitoring is desirable, and 

believes that the sites required in the near-road component of the NO2 network design 

could also be suitable for monitoring other pollutants including carbon monoxide, 

particulate matter (especially ultra-fine particulate matter), air toxics, and black carbon.

(12) Comment: Comments were submitted, particularly from state agencies (e.g., Michigan, 

Mississippi, and Tennessee), noting that one of the challenges to developing a near-road 

monitoring network was the potential safety hazard of having a monitoring station 

located within 50 meters of busy roadways. 

Response: EPA notes that in all instances of field work, safety is a top priority. EPA 

believes that the safety issue raised in public comments is something that is an important 

component of near-road siting logistics, and that the near-road network can be safely 

implemented while still serving its intended purpose of providing data on the expected 

maximum NO2 concentrations that occur within 50 meters of heavily trafficked roads. 
EPA discusses this issue in further detail in section III.B.7 of the preamble to the final 

rule.

(13) Comment: The Clean Air Council (CAC) recommended the “EPA not permit alternative 

monitoring systems on a state-by-state basis if they would hinder uniformity to the extent 

that public understanding and accountability of government would be degraded.” In 

contrast, the Southeast Michigan Council of Governments (SEMCOG) suggested that, 

rather than requiring a near-road monitoring network, EPA should enable states to select 

monitoring locations based on each state’s specific needs and issues as part of the state 

implementation plan (SIP) process.

Response: EPA believes that it must use, and has chosen, a balanced approach in setting 

monitoring requirements that recognizes the need to have as nationally consistent 

monitoring network as possible, while allowing an appropriate amount of flexibility so 

that states are able to comply with monitoring requirements. EPA believes that the nearroad site selection process must include certain considerations such as AADT, fleet mix, 

roadway design, congestion patterns, terrain, and meteorology to identify where 

maximum hourly NO2 concentrations are expected occur. While states have flexibility in 

evaluating what specific thresholds or criteria are appropriate for their particular 

situations with respect to the metrics listed above, they are to make all effort to meet the 

primary objective of locating the site in a location of expected concentration. This 

flexibility is necessary when considering the reality of, for example, safety or site access, 

and other logistical issues that are inherent to installing a monitoring network that can 

differ on a case-by-case basis. EPA discusses issues of site selection and siting criteria in 

further detail in sections III.B.6 and III.B.7 of the preamble to the final rule.

(14) Comment: The Spokane Regional Clean Air Authority (SRCAA) noted that it might not 

be necessary for EPA to require an extensive near-road monitoring network if a smaller 

network can develop a strong statistical correlation between particular variables (i.e. 

traffic density, vehicle mix) and above-standard NO2 concentrations.

Response: EPA believes that the currently available data, and the number of variables 

that influence the concentration and behavior of NO2 in the near-road environment, 

including fleet mix, roadway design, congestion patterns, terrain, and meteorology, do 

not allow the development of a “strong,” and nationally applicable, statistical correlation 

that would allow the avoidance of what the commenter states is an “extensive” near-road 

network. (EPA estimates that approximately 126 near road monitors will be needed.) The 

reasoning behind the size and extent of the near-road monitors required in the two-tier 

network design, with respect to SRCAA’s comments, are discussed in detail in sections 

III.B.1 and III.B.2 of the preamble to the final rule..

(15) Comment: Several environmental and public health groups (e.g., ALA, EDF, EJ, NRDC)

pointed out that “only 58 of 489 total NO2 monitors are sited in areas of expected peak 

concentrations.”
Response: The relative current state of the NO2 network is detailed in the NOx Network 

Review and Background document (Watkins and Thompson, 2009), which is posted in 

the docket. This document provides information and summaries on what EPA believes 

the current monitoring network, according to state provided meta-data, is addressing.

B. Comments on area-wide monitor requirements 

A number of comments were received, particularly from state agencies, in support of 

maintaining the existing area-wide monitoring network. Comments specific to the siting or areawide monitors tended to focus on either (1) the need for more area-wide monitors than the 

proposed 52 or (2) the need to make sure that all area-wide monitors were measuring ambient 

NO2 concentrations rather than NO2 concentrations near major NO2 emissions sources.

(1) Comment: Several industry groups and state agencies (e.g., AAM, EMA, INDEM, and 

NYSDOT) provided specific comments that they preferred the alternative monitoring 

approach of using area-wide monitors to measure 1-hour NO2 concentrations due to the 

high cost of implementing, operating, and maintaining a near-road monitoring network 

and the uncertain benefits such a network would provide. In addition, one commenter 

recommended the area-wide approach because it provided greater flexibility for monitors 

to be located in places where more people are likely to be exposed to high concentrations 

of NO2.

Response: The Administrator has set a new 1-hour NO2 standard that reflects the 

maximum allowable NO2 concentration anywhere in an area. In her judgment, this new 

standard will protect public health, including the health of sensitive populations such as 

asthmatics, with an adequate margin of safety. It follows from this standard that monitors 

should be placed in locations where maximum NO2 concentrations are expected. The 

Administrator has judged that, in many areas, these maximum concentrations are likely to 

occur around major roads (final rule, sections II.A.2 and II.F.4.d). Therefore, the 

Administrator has concluded that a two-tier network design composed of (1) near-road 

monitors which would be placed in locations of expected maximum 1-hour NO2

concentrations near heavily trafficked roads in urban areas and (2) monitors located to 

characterize areas with the highest expected NO2 concentrations at the neighborhood and 

larger spatial scales (also referred to as “area-wide” monitors) are needed to implement 

the 1-hour NO2 NAAQS. The rationale for this decision is discussed in section III.B.1 of 

the preamble to the final rule.

(2) Comment: Several environmental, public health, and tribal organizations (ALA, EDF, 

EJ, NRDC, NTAA) commented that EPA should require more monitors than currently

proposed. Specifically, the ALA, EJ, NRDC, and EDF stated that they “oppose the 

proposed requirement to retain only 52 air monitors to measure area wide concentrations 

NO2” In addition, they commented: 

“[The] EPA should require states and local offices to review inventory data to identify

any potential NO2 hotspots outside of those large metropolitan areas. For instance, if a 

large power plant or any other source is creating elevated NO2 levels in proximity to 
homes, schools or other sensitive sites, in an area of less than one million people, EPA 

should consider requiring a monitor. In particular, certain large agricultural facilities may

emanate high concentrations of NO2 under certain conditions, such as wet weather. 

Many of these facilities are directly upwind of rural communities, meriting an NO2

monitor.” 

Response: EPA recognizes a variety of exposure scenarios can occur in an area such as 

the ones described by the commenters above. Therefore, the final rule authorizes 

Regional Administrators to require additional monitors above the minimum required 

number of monitors in circumstances such as those described by the commenter above 

where there is a likelihood of high concentrations of NO2 that approach or exceed the 

NAAQS. The size and extent of the required area-wide monitors in the network design 

are discussed in section III.B.3 of the preamble to the final rule. A more detailed 

discussion of the Regional Administrator authority and some examples where such 

authority may be exercised is in section III.B.4 in the preamble to the final rule. 

(3) Comment: One state agency provided comments opposing using only an area-wide 

network for monitoring NO2 on the grounds that the network represents an inadequate 

alternative to the creation of a near-road monitoring network. The commenter was 

critical of how useful dispersion modeling would be at identifying NO2 concentrations 

beyond a monitor’s immediate area. The commenter claimed there were too many

confounding factors to accurately estimate the NO2 concentrations near roadways and 

other NO2 emissions sources from an area-wide monitor.

Response: The Administrator has set a new 1-hour NO2 standard that reflects the 

maximum allowable NO2 concentration anywhere in an area. In her judgment, using 

only an area-wide network of NO2 monitors would not adequately support this new 

standard because maximum concentrations of NO2 are likely to occur around major roads 

(final rule, sections II.A.2 and II.F.4.d)]. The Administrator has concluded that a two-tier 

network design composed of (1) near-road monitors which would be placed in locations 

of expected maximum 1-hour NO2 concentrations near heavily trafficked roads in urban 

areas and (2) monitors located to characterize areas with the highest expected NO2

concentrations at the neighborhood and larger spatial scales (also referred to as “areawide” monitors) are needed to implement the 1-hour NO2 NAAQS. The rationale for this 

decision is discussed in section III.B.1 of the preamble to the final rule.

(4) Comment: One commenter, Dow, recommended that area-wide monitors should be 

located at least 1,000 meters from any major roads or intersections to ensure that the 

concentration of NO2 measured is representative of an area-wide concentration instead of 

“peak” near-road concentrations.

Response: EPA believes that Table E-1 of 40 CFR Part 58 Appendix E, which provides 

roadway set-back distances for neighborhood and larger scale sites is appropriate to use 

to ensure that any NO2 site that is intended as an area-wide site will be located at a 

sufficient distance from any major road. This issue is discussed section III.B.8.of the 

preamble to the final rule.
(5) Comment: NYSDEC encourages EPA to develop a table for defining the minimum 

distance to the nearest major roadway that can be used to calculate the area-wide NO2

concentrations, since specific distances are unlikely to be appropriate for all core based 

statistical areas (CBSAs). 

Response: EPA believes that existing data in 40 CFR Part 58 Appendix E, Table E-1, are 

appropriate for use in determining the minimum distance a monitoring site must be from 

a road with a given annual average daily traffic count to be considered a neighborhood or 

larger spatially representative site. If a state feels that an increased minimum distance 

compared to Table E-1 is appropriate for an area-wide site in one or more of their 

particular CBSAs, or on a case-by-case basis, the state can elect to site their area-wide 

monitors accordingly, so long as they continue to meet siting and network design 

requirements specified in 40 CFR Part 58.

(6) Comment: Several environmental and public health groups (e.g., ALA, EDF, EJ, NRDC) 

opposed the proposed requirement to retain only 52 air monitors to measure area wide 

concentrations NO2. 

Response: EPA is not requiring that the current NO2 network be reduced; rather, EPA 

has introduced minimum monitoring requirements where there was previously no 

minimum NO2 monitoring requirement. This issue is discussed in section III.B.8 of the 

preamble to the final rule. Further, the relative current state of the NO2 network is 

detailed in the NOx Network Review and Background document (Watkins and 

Thompson, 2009), which is posted in the docket. This document provides information 

and summaries on what EPA believes the current monitoring network, according to state 

provided meta-data, is addressing. 

C. Comments on the need for a research monitoring network

A number of commenters, primarily from industry groups and state agencies recommended that 

some form of near-road research monitoring network be utilized before a full-scale near-road 

monitoring network is implemented. 

(1) Comment: A subset of CASAC panel members recommended establishing a limited 

roadside monitoring network (20 to 50 monitors in different cities) for the purpose of 

informing the development of a national network. Specifically, these CASAC member 

recommended the following: 

 A small roadside network could provide more information than a national network of 

NOx monitors if the smaller network also monitored other roadway-associated 

pollutants, including CO and continuous particulate matter (PM) with speciation, and 

if the network also included detailed meteorological measurements and automatic 

traffic counters. They note that measuring only NO2 could lead to the mistaken 

impression that air quality is deteriorating as diesel particulate filters, which can 

increase the fraction of NOx that is emitted as NO2, become more common. 
 If EPA undertakes near-road monitoring, the Agency should seek the advice of 

technical experts, such as those in the CASAC Ambient Air Monitoring and Methods

(AAMM) subcommittee. 

 Siting criteria should include consideration of such factors as vehicle mix, location of 

susceptible or vulnerable populations (e.g., schools, hospitals), local physical features 

such as urban street canyons.

 Such a network should be established quickly to capture ongoing changes in mobile 

source emissions and should be funded primarily by EPA. 

Response: The Administrator has judged that a regulatory, two-tier network design, 

which includes near-road monitors in urban areas (not a research network), is necessary

to support the intent of the revised NAAQS to protect against risks associated with 

exposures to peak concentrations of NO2 anywhere in an area. This rationale is discussed 

in section III.B.1 of the preamble to the final rule. EPA believes that it will be useful to 

provide guidance to ensure consistent implementation of the monitoring network and has 

committed to engage the CASAC AAMM subcommittee on this effort, and discusses this 

issue in section III.B.5 of the preamble to the final rule. In regard to measuring other 

roadway-associated pollutants, the scope of this rulemaking pertains to the NO2 NAAQS 

and the monitoring associated with it. However, EPA agrees that multi-pollutant 

monitoring is desirable, and believes that the sites required in the near-road network 

design could be suitable for other pollutants including carbon monoxide, particulate 

matter (especially ultra-fine particulate matter), air toxics, and black carbon. 

(2) Comment: Some industry commenters (e.g., CE, EEI, NAM, NPRA, NMC, OIPA, POH, 

SRNS, SCCC, and SCMA) provided support for a research monitoring network due to 

perceived weaknesses in the scientific basis for implementing an extensive near-road 

monitoring network. Generally indicating that a research network would allow EPA to 

gain a better understanding of near-road peak NO2 concentrations and their impact on 

public health before requiring the development of a large and expensive monitoring 

network. For example, NAM commented:

If EPA concludes that NO2 near roadway monitoring is required, 

the NAM recommends at a minimum, EPA should consider 

conducting a near roadway "special purpose" monitoring network 

program. Conducting such a near road monitoring program would 

allow EPA to collect necessary data that can be used to better 

understand the health impacts associated with short term NO2

exposures.

Response: The administrator has judged that a regulatory, two-tier network design, 

which includes near-road monitors in urban areas (not a research network), is necessary

to support the intent of the revised NAAQS to protect against risks associated with 

exposures to peak concentrations of NO2 anywhere in an area. EPA believes that the 

existing near-road research provides a sufficient base of information to implement an 

appropriately designed near-road monitoring network, and the collective experience that 
exists in the ambient monitoring community will allow for successful implementation of 

that network. EPA discusses the basis and rationale for the two-tier network design, 

which include near-road monitors for regulatory purposes in section III.B.2 of the 

preamble to the final rule. 

(3) Comment: Some state groups (e.g., AASHTO, ILEPA, MIDEQ, MSDEQ, NACAA, 

NCDENR, NESCAUM, NMED, SCDHEC, SJAQMD, and VDOT) focused their support 

for a research monitoring program by identifying a need to develop more experience 

siting near-road monitors. Creating greater certainty about where and how near-road 

monitors ought to be used could reduce variations in measurement results driven solely

by differences in monitor siting. These comments are exemplified by NACAA’s 

comment:

...a major new network – particularly one that is inherently 

complicated and untried – should not be rolled out without the 

benefit of an effective near-road monitoring research program that 

can address many of the relevant data questions, and inform the 

specific siting requirements of the rule.

Response: The Administrator has judged that a regulatory, two-tier network design, 

which includes near-road monitors in urban areas (not a research network), is necessary

to support the intent of the revised NAAQS to protect against risks associated with 

exposures to peak concentrations of NO2 anywhere in an area. EPA believes that the 

existing near-road research provides a sufficient base of information to implement a 

appropriately designed near-road monitoring network, and the collective experience that 

exists in the ambient monitoring community will allow for successful implementation of 

that network. EPA discusses the basis and rationale for the two-tier network design, 

which include near-road monitors for regulatory purposes in section III.B.2 of the 

preamble to the final rule.

(4) Comment: The CASAC panel members who support the alternative approach, which is 

coupled to a monitoring network that includes only area-wide monitors, stated that 

roadside monitors will likely detect extreme concentrations of NO2 at certain times (e.g., 

heavy traffic, idling truck near monitor) and more effort is needed to characterize these 

measurements and their relationship to traditional area-wide measurements.

Response: EPA believes that some urban areas that will have both area-wide monitors 

(whether required or not) and near-road monitors may be able to provide data to increase 

the understanding or possibly quantify the relationship (at least for that particular area) 

between near-road and area-wide concentrations. Intense investigation of the roadway

gradient has been, and will continue to be best served by research studies that utilize 

transect monitoring and/or saturation monitoring around target road segments. 

(5) Comment: The CASAC panel members who support the alternative approach, which is 

coupled to a monitoring network that includes only area-wide monitors, stated that the 
siting of the near-road monitors will require consideration of many factors as the 

monitors will be sensitive to the microscale environment of the location. Because the 

monitors will capture concentrations at the extreme end of the range, the data from any 

two given monitors may not be directly comparable if the characteristics of their sites 

vary greatly.

Response: EPA understands that near-road sites will be microscale in nature, and 

concentrations at the site will be subject to factors including AADT, fleet mix, roadway

design, congestion patterns, terrain, and meteorology. As a result, EPA would expect site 

to site differences corresponding to variability in the above factors. However, the intent 

of the revised primary NO2 NAAQS is to protect against the maximum allowable NO2

concentration anywhere in an area, which includes ambient air on and around roads. The 

required near-road monitoring sites are intended to measure the maximum expected 

concentrations, considering the above factors. 

(6) Comment: The CASAC panel members who support the alternative approach, which is 

coupled to a monitoring network that includes only area-wide monitors, stated that siting 

monitors based on traffic count alone may miss locations where maximum concentrations 

of NO2 occur. EPA should investigate model results from Congestion Mitigation and Air 

Quality (CMAQ) Land Use Regression modeling as well as Gaussian plume models and 

emissions inventories to determine where roadside monitors should be located.

Response: As noted in section III.B.6 of the preamble to the final rule, EPA does not 

intend for AADT counts to be the sole basis for choosing a near-road site. EPA 

understands that there are other factors that can influence which road segment in a CBSA 

may be the actual location where the maximum NO2 concentrations could occur. These 

factors include vehicle fleet mix, roadway design, congestion patterns, terrain, and 

meteorology. When states identify their top-ranked road segments by AADT, EPA 

intends for states to evaluate all of the factors listed above, which influence where the 

location of expected maximum NO2 concentration may occur, when evaluating a pool of 

candidate near-road monitoring sites. Further, EPA expects that modeling will be a tool 

used by states to assist in informing where near-road monitoring sites should go.

(7) Comment: The CASAC panel members who support the alternative approach, which is 

coupled to a monitoring network that includes only area-wide monitors, commented that 

diesel particle filters increase the fraction of NO2 in NOx and may lead to higher 

measured concentrations of NO2 in places where vehicles are actually cleaner.

Response: EPA notes that the required near-road NO2 monitors are to be sited in 

locations that are expected to measure maximum NO2 concentrations. The issue of 

whether the fleet, or a component of the fleet, travelling on the target road is relatively 

dirty or clean is part of consideration of fleet mix that states are to consider when 

identifying near-road site locations. 
(8) Comment: Many state and industry commenters (e.g., NACAA, NESCAUM, many

states, Consumers Energy, EA, EEI, NAM) recommended that a near-road research 

monitoring program be implemented before a full-scale near-road monitoring network is 

required. Specifically, EA recommended that a near-road monitoring network not be 

required until enough near-road monitoring data was available to ensure that EPA’s 

proposed NO2 level is not set below background NO2 levels. 

Response: The EPA believes that is has sufficient understanding of the near-road 

environment from existing research to propose a near-road network that is appropriate for 

regulatory use. This issue is discussed in issue in section III.B.2 of the preamble to the 

final rule. In addition, as discussed in section II.F.4.d of the preamble to the final rule, 

the Administrator has set the level of the 1-hour standard at 100 ppb to protect public 

health with an adequate margin of safety. Her decision is based on consideration of 

available health evidence, exposure/risk information, the advice of CASAC, and input 

from the public.

D. Justification of meteorological measurements

Most commenters did not mention the issue of meteorological measurements but the comments 

received on the subject were unanimously against requiring monitoring stations to include threedimensional anemometry equipment.

Comment: CASAC Panel members and a number of state commenters (e.g., AKDEC, 

NCDENR, SCDHEC, and WIDNR) questioned the need for meteorological 

measurements at the NO2 monitoring sites. These commenters called into question the 

ability of three-dimensional anemometry to provide the microscale meteorological 

information EPA desires if the monitors are to be located close to the ground and other 

structures, and if they are averaging measurements over hour long periods of time. 

SCDHEC commented that the recording of air turbulence data at monitor stations ought 

to be encouraged but not required unless EPA were willing to fully fund the purchase, 

installation, and operation of the necessary equipment. These commenters concluded that 

the requirement to include equipment for measuring meteorological characteristics would 

be an unnecessary expense.

Response: EPA has chosen not to finalize a requirement for meteorological 

measurements, including three-dimensional anemometry, at near-road NO2 monitoring 

stations, but does encourage states to perform meteorological monitoring to better 

characterize the behavior of NO2 cocentrations in the near-road environment. EPA 

discusses its rationale for this decision in section III.B.9 of the preamble to the final rule.

E. Monitoring technology 
Commenters (CASAC Panel members, some state agencies, and some industry groups) who 

expressed an opinion regarding monitoring techniques generally agreed that the proposed 

chemiluminescence approach is appropriate. However, some groups (e.g., CASAC and industry) 

recommended that EPA promote the development of additional methods for measuring true NO2

that would not be subject to positive interference. 

(1) Comment: Several commenters (e.g., MODNR, SRNS, Teledyne API, and UARG) 

recommended that EPA encourage the development of alternative monitoring 

technologies, which includes the photolytic-chemiluminescence method or cavity ringdown spectroscopy, and to permit the use of these technologies for monitoring if the new 

technologies provide comparable or better monitoring results than existing FRM/FEM 

[Federal Reference Method/Federal Equivalent Method]. NCDENR urged EPA to 

maximize the flexibility in the choice of FRM and FEM technology available to 

demonstrate compliance so long as the technology provides comparable results. 

Response: The FRM/FEM program ensures that only evaluated and approved methods 

may be used for comparison to the NAAQS (see 40 CFR Part 53). EPA only approves a 

particular method for use in measuring oxides of nitrogen once it has been shown to meet 

requirement set forth in 40 CFR Part 50 Appendix F, with testing and data provided 

usually provided by instrument vendors. The use of FRM/FEMs and furthering method 

technologies is discussed in section III.A.1 of the preamble to the final rule.

(2) Comment: One industry commenter, Aerodyne, expressed concern that current 

NO2 monitoring equipment and siting protocols will not be suitable for use in near-road 

applications without proper reconfiguration. The commenter notes that the method by

which the chemiluminescence FRM determines NO2 concentrations, using alternating 

measurements of NO and NOX and subsequently using the difference to indicate NO2, 

may not be appropriate because this technique does not possess the time response 

required to accurately track the plumes of combustion gas emitted by passing vehicles. 

The commenter notes that these plumes “tend to be short-lived, with durations on the 

order of 3 to 30 seconds.” As a result, the commenter asserts that the values reported by 

chemiluminescence FRMs “...can be quite inaccurate because of the inability of the 

alternating channel monitors to properly average these plumes.” The commenter goes on 

to note that this issue “...begs the question of whether any of this affects reported 1 hour 

average NO2 concentrations, the subject of the proposed regulations.”

Response: The “alternating measurement” technique utilized by the chemiluminescence 

FRM is asserted by the commenter to cause variability in measuring short duration 

plumes or ‘bursts’ of NO2. EPA believes this variability would be most apparent when 

there is only one plume, or a few individual plumes, that may pass the monitor for a short 

period of time, but is unlikely to cause a discernable effect in the 1-hour average. EPA 

believes that this particular characteristic of the chemiluminescence FRM does not 

warrant a new measurement approach for near-road monitors because the averaging time 

of the NAAQS is 1 hour. The NAAQS is a 1-hour standard, which EPA believes is a 

long enough averaging period where short-term variations, on the order of 3 to 30 
seconds will be averaged out, an are not expected to have a discernible effect. The 

commenter states that “It has always been (rightly) assumed that when reporting 24 hour 

and /or yearly averages, the short term upsets observed in the NO2 data have no 

discernible effect.” EPA notes that states calculate 24-hour and annual averages from the 

1 hour averages they acquire each day. 

The commenter suggests several alternatives to relying on a single 

chemiluminescence analyzer at near-road sites. EPA has expressed its desire to further 

the development of alternative methods in determining NO2 concentrations, which is 

discussed in section III.A.1 of the preamble to the final rule, and also is open to 

alternative applications of existing methods, all of which would need to follow the 

process for submission and approval described in 40 CFR part 53.

(3) Comment: Some industry commenters expressed concern that open path monitors might 

not be appropriate for use in near-road applications. 

Response: EPA notes that path integrated optical remote sensing techniques, also known 

as open-path methods, are not typically used by states at State and Local Ambient 

Monitoring Station (SLAMS) sites. However, there are approved open-path methods 

capable of providing hourly NO2 concentrations that can be compared to both the annual 

and 1-hour standard. EPA believes that an appropriate path length is between 50 meters 

to 300 meters, where 50 corresponds to the maximum distance away from the edge of the 

nearest traffic lane of a nearby road segment and 300 meters corresponds to the 

traditional maximum path length at micro- or middle scales site. EPA recognizes that if 

open path methods are going to be used in the near-road environment, issues regarding 

path orientation, safety, and optimal path length within the allowable range will likely be 

considered in the monitoring guidance that will need to be developed to assist states in 

the implementation of the network design.

(4) Comment: One industry commenter, AQRL, requested that EPA set guidelines for the 

maximum allowable inlet length and sample residence time.

Response: EPA has only chosen to set a maximum residence time of NO2 in the sample 

line between the inlet probe and the analyzer of 20 seconds. EPA discusses this issue in 

section III.B.7 of the preamble to the final rule. 

(5) Comment: API commented that, because state and local air monitoring agencies have 

little experience in near-road NO2 monitoring, EPA needs to be prepared to support 

significant field validation and statistical analysis to assess monitor performance and 

dispersion and photochemical modeling analyses to assist in siting monitors. 

Response: EPA believes it has systems in place that can provide “field validation and 

statistical analysis” of any ambient monitoring network. Field validation and statistical 

analysis starts with the initial testing of instruments to ensure that they are operating 

properly and providing precise and unbiased measurements before monitoring begins. All 
monitoring organizations are required to have quality management plans and quality

assurance project plans that provide the necessary elements to help in their achievement 

of the NO2 data quality goals. In addition, the quality assurance (QA) requirements in 40 

CFR part 58 Appendix A provide the quality control samples, the auditing activities and 

the appropriate statistical analysis that monitoring organizations and EPA can use to 

judge whether the monitoring systems are under control from a data quality standpoint.

(6) Comment: Several state commenters (e.g., MODNR and NCDENR) suggest that the 

current bias measurement uncertainty criteria used for other NAAQS will be appropriate 

and acceptable for use with a 1-hr NO2 standard.

Response: EPA is finalizing the approach to develop data quality objectives, and is 

changing the proposed goal for measurement uncertainty, to match those criteria of the 

NCore network and the historical approach for NO2, where the goals for acceptable 

measurement uncertainty for NO2 methods for precision is an upper 90 percent 

confidence limit for the coefficient of variation (CV) of 10 percent and for bias is an 

upper 95 percent confidence limit for the absolute bias of 15 percent.

This issue is discussed section III.C of the preamble to the final rule.

(7) Comment: A state commenter, IADNR, recommend allowing provisions for mobile 

monitoring to determine NO2 exposure to motorists. 

Response: The use of mobile monitoring may be a very beneficial tool to assist states in 

locating candidate near-road NO2 sites; however, EPA does not believe that mobile 

monitoring is suitable for regulatory monitoring. The network design is predicated on 

placing monitors in the location of expected maximum concentration, which is inherently

in a generally fixed location. Further, much consideration would have to be given to 

issues regarding the deviation from the standard practice of using fixed point (or fixed 

path-integrated) data for comparison to the NAAQS. Another issue that would arise 

would be regarding data completeness and representation, e.g. at what time and where the 

data are valid, and whether the data are representative of a given location. EPA is 

encouraged by the interest in mobile monitoring, but we believe that such monitoring is 

currently best suited for non-regulatory informational purposes.

F. Timing of monitor deployment

In general, the majority of comments about the timing of monitor deployment, State

Implementation Plan (SIP) development, and determination of attainment were received from 

state agencies and industry groups recommending that EPA keep the proposed current 

implementation deadlines as proposed or extend the deadlines further. A few environment and 

public health groups provided comments urging EPA to move the implementation deadlines 

forward. [who were the commenters among the states, industry and enviros?]

Comment: A lage majority of state and industry groups who made comments on network 

implementation recommended that the deadline for installing near-road monitors remain 

January 1, 2013, or that EPA provide the maximum amount of time possible. They
claimed that the current January 1, 2013 deadline provides insufficient time to plan and

implement such a complicated network of monitors. SJAQMD went further, 

recommending that EPA allow for schedule relief if a state can demonstrate that monitor 

installation delays are the result of delays in obtaining permits or rights-of-way for the 

near-road monitors. Conversely, the environmental and public health groups who made 

comments on network implementation recommended that urged “EPA to seek 

Congressional funding for an expanded network and to set a deadline for deployment of 

no later than January 1, 2012.” 

Response: EPA recognizes the need to aid state agencies in the network implementation 

process, particularly through guidance documentation that will be developed in 

partnership between EPA and various stakeholders including NACAA and the states. 

Further, EPA has modified the timelines for implementation by changing the date by

which state and, when appropriate, local air monitoring agencies shall provide a plan for 

deploying monitors in accordance with required network design from July 1, 2011 to July

1, 2012. However, EPA is finalizing the date by which state and, when appropriate, local 

air monitoring agencies shall establish the required NO2 monitoring network as January

1, 2013, as was proposed. The basis and rationale for this decision is discussed in 

section III.B.5 of the preamble to the final rule.

G. Other monitoring issues

Many comments about the monitoring network associated with a one-hour standard were also 

submitted, including comments on what should be considered an exceptional event and the 

appropriate requirements for using data from monitors for evaluating attainment.

(1) Comment: Some commenters urged EPA to create criteria for defining exceptional events 

for near-road monitors that included events like road construction.

Response: The Exceptional Events Rule (EER) and the accompanying preamble (72 FR 

13560, March 22, 2007) set forth and explain the criteria and procedures that must be 

used to determine whether an event qualifies as an exceptional event and the 

documentation that must be submitted to support an exceptional events claim. For an 

exceptional event claim to be approved and data affected by such an event to be excluded 

from consideration in any NAAQS attainment designations, the data would need to meet 

the criteria and procedures established in the rule. EPA believes that these criteria and 

procedures are sufficient to address any exceptional events claims that may arise for NO2 

NAAQS. Additions or modifications to the EER are not needed for any exceptional 

events claims that may arise for NO2. In addition, the current exceptional event data 

flags available in the AQS--unique traffic disruption and other-- provide sufficient 

flexibility to address near-road exceptional events. 

(2) Comment: One state commenter, NYSDOT, recommended (if EPA chose to finalize the 

two-tier network design which included near-road monitoring) that “EPA establish 

national guidance so there is reasonable uniformity among EPA regions in the 

implementation of these provisions.”
Response: EPA notes that network consistency, from site to site, is achieved in ambient 

monitoring networks by the adherence to monitoring regulations and through the use of 

guidance. EPA has detailed siting criteria as specific as we believe is necessary and 

appropriate to develop a consistent monitoring network , while allowing some necessary

flexibility for logistical considerations that will occur on a case-by-case basis. Further, 

EPA recognizes the need to aid state agencies in the network implementation process, 

particularly through guidance documentation that will be developed in partnership 

between EPA and various stakeholders including NACAA and the states. This issue is 

discussed in section III.B.5 of the preamble to the final rule.

(3) Comment: Some commenters recommended that the annual primary standard design 

value be calculated by averaging all 1-hour values for the year, while a third commenter 

recommended the design value be calculated by averaging within calendar quarters 

before averaging across calendar quarters.

Response: This issue is discussed in the final notice (Section IV.A).

(4) Comment: Several comments were made in regard to EPA’s data completeness test. For 

example, three commenters agreed that the completeness test ought to stay at 75% for 

each quarter while a fourth commenter recommended the percentage be increased to 

82%.

Response: This issue is discussed in the final notice (Section IV.B).

(5) Comment: A comment from a State agency noted that the proposed 75% capture 

requirement was inappropriate and should not be used for a 1-hr NO2 standard. 

Response: This issue is discussed in the final notice (Section IV.B). 

(6) Comment: A commenter requested that EPA truncate NO2 concentration measurements 

rather than rounding them.

Response: EPA agrees with this comment, as reflected in the final notice. 

(7) Comment: An industry group requested that EPA further clarify the guidelines for 

situations where incomplete monitoring data could be considered valid. 

Response: The rule text identifies factors for the administrator to consider in determining 

when to use such data. No further clarification is necessary because the variety and 

uniqueness of such situations require that they be treated on a case by case basis.
(8) Comment: One commenter expressed concern that the use of near-road monitoring 

would lead to near-source monitoring for other point sources of NO2 and other pollutant 

emissions.

Response: EPA is establishing a two-tier monitoring network, which includes both nearroad and area-wide monitoring, under this rule. EPA does not believe that near-road 

monitoring will directly result in non-near-road monitoring. However, EPA recognizes 

that in certain circumstances, there can be an area or areas of expected maximum 

concentrations of NO2 due to non-road, point, or area sources, that may not be monitored 

even though a state is fulfilling its minimum monitoring requirements. EPA has included 

a mechanism to deal with such circumstances by providing the Regional Administrator 

with the authority to require additional monitors above those minimally required. This 

issue is discussed in section III.B.4 of the preamble to the final rule. 

(9) Comment: A State agency recommended that EPA allow averaging across monitors for 

design value calculations and substitution of values from additional monitors for the 

primary monitor, for example, if the primary monitor is taken offline for quality control 

(QC) during a high pollutant period.

Response: This issue is discussed in the final notice (Section IV.B). 
VI. Air Quality Index 

(1) Comment: Many State agency commenters discouraged the use of near-road 

concentrations in calculating the AQI since near-road NO2 concentrations do not 

represent exposure of the general public to ambient NO2. Several other commenters, 

including NACAA, expressed concern that using near-road monitors for calculating the 

AQI could diminish the value of the index as a metric for reporting air quality to the 

general public by making it sensitive to NO2 concentrations that are present only in small 

areas. CASAC also raised questions about how the AQI would be affected by roadside 

monitors. An additional commenter noted that it would be difficult to forecast the AQI 

for regions where near-road monitoring stations existed, since near-road NO2

concentrations are likely to be unpredictable.

Response: The response to this comment on the AQI is discussed in section VII of the 

final rule. 

(2) Comment: The ALA, EJ, EDF, NRDC support setting the 100 level of the AQI at 50 ppb 

and the level of the “moderate” category well below 50 ppb (i.e., 25 ppb). These 

commenters disagree with EPA’s proposal to maintain the breakpoints at the higher end 

of the AQI scale (i.e., 200 to 500). They note that the 500 level has not been changed in 

the last 22 years, and EPA cannot claim that it represents the most up-to-date scientific 

evidence without more extensive review. They also note that the proposed 200 level for 

NO2 is too high, relative to the 100 level, given the relationship between these levels for 

other pollutants. As a result, these commenters recommend that EPA “establish a 

meaningful and practical scale of levels of concern and graduated cautionary statements 

for both sensitive groups as well as the general population” based on the current scientific 

evidence.

In addition, NACAA expressed support for the “EPA’s proposed range of 0.040 to 0.053 

ppm for the AQI value of 50 and 0.360 and 0.370 ppm for the AQI value of 150.” But 

NACAA did express some concern that it might make for a “confusing outreach message 

if the AQI level of 50 is set below the annual standard of 0.053 ppm” since this would 

mean that state and local air agencies would “be forecasting moderate air quality even 

though an area is meeting the annual standard.”

Response: With respect to an AQI value of 100, EPA has concluded that it is appropriate 

in this case to set this value at 100 ppb NO2, 1-hour average, the level of the short-term 

standard. With respect to an AQI value of 50, EPA agrees with the NACAA comment 

which noted it might make for a confusing outreach message if this value is set below the 

level of the annual standard. This method of structuring the index is appropriate in the 

case where a short-term standard is set to protect against the health effects associated 

with short-term exposures and/or an annual standard is set to protect against health 

effects associated with long-term exposures. In such cases, the short-term standard in 
effect defines a level of health protection provided against short-term risks and thus can 

be a useful benchmark against which to compare daily air quality concentrations.

With respect to the breakpoints at the higher end of the AQI scale (i.e., 200 to 500), EPA 

acknowledges that these breakpoints have been in place for a long period of time, and 

that an AQI value of 200 is higher for NO2, relative to the 100 level, than it is for the 

other pollutants in the AQI. EPA does not agree that there is new health evidence that 

would provide support for changing these levels. The levels at the upper end of the AQI 

are linked to a related program designed to prevent air pollution emergencies. Some state 

and local air agencies are required to have plans to take actions at these levels, thus it is 

important to have a firm basis for revising these levels. 

Significant harm levels (SHL) are those ambient concentrations of air pollutants that 

present an imminent and substantial endangerment to public health or welfare, or to the 

environment, as established in 40 CFR part 51.151. The SHL is typically set at the same 

ambient concentration of a pollutant as the AQI value of 500. Appendix L of 40 CFR 

part 51 includes example emergency episode plans as part of the Prevention of Air 

Pollution Emergency Episodes program. This program requires specified areas to have 

contingency plans in place and to implement these plans during episodes when high 

levels of air pollution, approaching the SHL, are in danger of being reached or have been 

reached. The Appendix L example links the AQI with emergency episodes. AQI values 

of 200, 300 and 400 are the basis for the example Alert, Warning and Emergency episode 

levels included in 40 CFR part 51, Appendix L. In this guidance, the Alert level 

corresponds to the breakpoint between the Unhealthy and Very Unhealthy categories, and 

the Warning level corresponds to the breakpoint between the Very Unhealthy and 

Hazardous categories. The Hazardous category ranges up to the SHL.

 
VII. Comments on the Process for Reviewing the NO2 Primary 

NAAQS

A number of comments were received recommending changes to the current rulemaking process, 

either for NAAQS in general or the NO2 NAAQS in particular. 

(1) Comment: A number of industry groups and state agencies recommended that the 

proposed near-road monitoring network be considered in a separate rulemaking from the 

proposed new 1-hr NO2 standard, because it represents a dramatic shift in the method of 

monitoring ambient air quality from previous NAAQS.

Response: EPA notes that separating the monitoring issues from the NAAQS revision is 

not consistent with recently finalized or planned NAAQS reviews. EPA has promulgated 

monitoring rules as part of, or simultaneously with the PM and Pb rules, and has 

proceeded to include the monitoring rule as part of this primary NO2, as well as the 

primary SO2, the secondary NOx/SOx, and CO revisions. Monitoring issues were 

handled separately from the 2008 ozone NAAQS revision, however EPA intends that 

future ozone NAAQS reviews will integrate monitoring into the action.

(2) Comment: Some commenters (e.g., NACAA, NESCAUM, Roger McClellan) noted that 

EPA had originally intended to publish an advance notice of proposed rulemaking 

(ANPR) and that because no ANPR was published, the public and CASAC had limited 

opportunity to comment on the range of available options. Other comments expressed 

disapproval with the limited amount of time for stakeholders to comment on EPA’s ISA 

and REA; the late notifications that EPA provided when comment periods were extended; 

and the inclusion of new sections and studies in the final versions of the ISA and REA, 

which denied stakeholders the opportunity to comment on these additions. 

Response: The Clean Air Act requires EPA to review and to revise, as appropriate, the 

NAAQS every five years. As a result of a lawsuit challenging EPA’s failure to complete 

its review of the air quality criteria for NOX and the NAAQS for NO2, inter alia, EPA 

agreed in a consent decree to a schedule for this review of the NO2 primary NAAQS. 

Pursuant to CAA § 113, EPA provided an opportunity for comment on the consent 

decree. This schedule was approved by the United States District Court for the District of 

Columbia.

We disagree that EPA’s decision to not publish an ANPR limited the opportunity for 

meaningful comment on the rulemaking. EPA on occasion uses an ANPR to seeking 

input from stakeholders before issuing a proposed rule, but EPA is not required to publish 

an ANPR. Moreover, EPA provided for stakeholder comment throughout the review 

process, including 60 days of comment and held two public hearings on the proposed 

rule. To the extent that “new sections and studies” were added to the final version of the 

ISA and REA, stakeholders had the opportunity to comment on this material following 

the notice of proposed rulemaking.
(3) Comment: API commented that EPA did not not adequately address comments provided 

by other government agencies on the draft proposed rule. 

Response: EPA engaged in extensive discussions with other agencies as part of the 

interagency review process prior to the publication of the proposed rule. Under section 

307(d) of the CAA, EPA is under no obligation to provide written responses to comments 

provided by other agencies during the interagency review process. The scientific 

rationale for the standards is clearly provided in the record for the final rule. 

(4) Comment: AASHTO requested that if EPA creates a 1-hr NO2 standard more stringent 

than 80 ppb, an additional public comment period be created to allow stakeholders to 

comment directly on the specific standard level selected. 

Response: As described in section II.F.4.d of the final rule, the Administrator has set the 

level of the 1-hour standard at 100 ppb. 

(5) Comment: AGCA commented that EPA’s Regulatory Impact Analysis (RIA) did not 

provide enough information to predict which areas of the country will not be in 

attainment with a new standard if the proposed near-road monitoring network is also 

included. This lack of information has the potential to negatively affect AGCA members 

because it prevents the organization from assessing the potential impact of a new nearroad monitoring network on the road construction industry.

Response: We developed the best estimate we were able to develop of future design 

values at currently existing monitors, based on emission inventory projections for 2020, 

using the best available emission data and control strategy information. What we are not 

able to do is to credibly estimate specific design values at locations where monitors do 

not currently exist, but will in the future. States will determine where to place those 

monitors as they implement the NAAQS. In addition, we did not model near-roadway

concentrations as a function of traffic density; this RIA is a national scale analysis.
VIII. Interpretation of the Clean Air Act 

(1) Comment: Numerous comments were received from industry groups and state agencies 

expressing concern with EPA using near-road measurements of NO2 concentrations to 

evaluate area-wide attainment with an NO2 NAAQS. Many of the commenters note that 

EPA’s proposed near-road monitoring network focuses on monitoring roadways likely to 

have high NO2 concentrations without requiring any demonstration that large and 

adjacent populations are exposed to similarly high concentrations. Consequently, the 

commenters argue that EPA has proposed a monitoring network using rationale that does 

not reflect the purpose of the CAA and NAAQS, which is to protect the public from 

exposure to unhealthy concentrations of pollutants found within “ambient air” rather than 

just “air”.

Response: We note that ambient air includes the outdoor air present around roads as well 

as the air that intrudes from outdoors into homes, buildings, or vehicles on/near those 

roads. As discussed in the final rule, people who spend time on or around major roads 

(e.g., because they live there, go to school there, or commute in vehicles on major roads) 

can be exposed to elevated concentrations of ambient NO2. 

(2) Comment: A number of industry groups provided comments maintaining that EPA may

have overstepped its regulatory authority with its proposed 1-hr NO2 standard. 

Comments included claims that the health effects from short-term exposure to high 

concentrations of NO2 fail to constitute the “adverse health effects” called for in the 

CAA. Other comments noted that EPA is authorized by the CAA to set its NAAQS at 

levels intended to protect sensitive subpopulations, not the most sensitive individual. In

addition, some commenters noted that NAAQS are not intended to eliminate all risks. 

Response: The issues raised by these commenters are addressed in sections II.E.2, II.E.3, 

II.F.4 in the final rule. 

(3) Comment: Roger McClellan provided some general comments on the role of CASAC 

and concluded that--

In offering recommendations for a specific upper bound for the 

Standard, the Committee moved beyond advising on the science 

informing the policy judgments inherent in setting the Standard, to 

taking on a role reserved for the Administrator – the setting of the 

Standard. The Clean Air Act wisely delegates the setting of the 

Standard to the Administrator and calls for a Clean Air Scientific 

Advisory Committee, not a Clean Air Standard Setting Committee.

Response: Section 109(d)(2)(B) of the Clean Air Act, and the charter of CASAC, 

provide that CASAC shall review the air quality criteria and the NAAQS and 

“recommend to the Administrator any new national ambient air quality standards and 
revisions of existing criteria and standards as may be appropriate.” EPA believes that 

CASAC fulfilled this duty and provided helpful advice as a committee on the science 

issues raised by this review. Of course, the Administrator cannot, and did not, delegate 

the setting of the standard to CASAC.

(4) Comment: NPRA commented that EPA lacks the legal authority to withhold a 

designation of “attainment” from districts unless the districts cannot be classified on the 

basis of available information as meeting or not meeting the national air quality standard. 

The commenter argued that, at present, EPA cannot know whether it will lack the 

necessary information in 2012. According to NPRA, “it is clear from the statute that for 

NO2 EPA must designate areas as in attainment that have attaining monitors absent an 

affirmative showing that such an area is contributing to downwind nonattainment in another 

area..” In addition, NPRA commented that section 107(d) of the CAA requires that EPA 

“consider the recommendations of States with regard to the designation of areas as 

attainment, nonattainment and unclassifiable.”

One state agency agreed with EPA’s proposal to designate all areas currently in 

attainment with the NO2 standard as “unclassifiable” until a near-road network has been 

established and measurements have been evaluated.

Response: (Implementation group) 

(5) Comment: Several environmental groups (e.g., ALA, EDF, EJ, NRDC) noted that the 

CAA requires that NAAQS be set to protect public health with an adequate margin of 

safety without consideration of cost and that, in setting the NAAQS, EPA must err on the 

side of protecting public health. These groups stated that this includes protecting 

sensitive subpopulations and guarding against “potential” health effects. 

Response: EPA agrees that the NAAQS must be requisite (i.e., no less and no more

stringent than necessary) to protect the public health, including the health of sensitive 

populations, with an adequate margin of safety. The Administrator’s consideration of 

this issue in the current review is described in section II.F.4.d of the final rule. 
IX. Comments on Implementation 

(1) Comment: A minority of CASAC Panel members posed the question of what would be 

an effective control strategy to reduce NO2 concentrations around major roads. A 

number of public commenters also expressed the concern that states have little ability to 

reduce on-road emissions from motor vehicles. Several went on to opine that it is the 

role of the federal government to reduce emissions from motor vehicles. One pointed out 

that their state has already implemented measures to reduce on-road emissions of NOX as 

part of their plan to attain the ozone standard. One commenter opined that states have 

few options to reduce on-road emissions beyond federal requirements. Another 

commenter suggested that one option for states would be to adopt California’s mobile 

source regulations including those that apply to non-road equipment. The commenter is 

concerned that adoption of California’s non-road regulations would place a significant 

burden on the construction industry. The commenter is also concerned that states may

attempt to reduce emissions from non-road equipment by imposing operating restrictions, 

requiring older equipment to be replaced or mandating retrofits for older equipment.

Response: EPA agrees that it is a federal responsibility to implement regulations that 

reduce emissions from new light- and heavy-duty vehicles. However, states have 

authority that can be used to reduce emissions from in-use light- and heavy-duty vehicles. 

For example, states can implement programs to retrofit heavy-duty diesel vehicles to 

reduce their NOX emissions. States can implement regulations to reduce or eliminate 

long-duration idling of heavy-duty diesel vehicles. They can implement an inspection 

and maintenance program for light-duty vehicles. Additionally, states can implement a 

wide range of programs to improve travel efficiency. States can implement strategies 

such as congestion pricing, programs to reduce trips for commuting purposes, and 

measures to improve the operational efficiency of the transportation network. States can 

also work with freight shippers to improve the efficiency of goods movement in an area.

EPA agrees that a number of states have already implemented controls on on-road 

sources. The emissions reductions attributable to those measures will help such areas to 

attain the NO2 NAAQS and can be included in a SIP for NO2. If such an area was 

designated nonattainment for NO2, EPA would expect the state to evaluate mobile source 

controls that it had not yet implemented and to evaluate the potential for controlling 

emissions from other sources that are contributing to the nonattainment problem.

With regard to the comments related to the adoption of California’s on-road and nonroad regulations by other states and the possibility that some states may choose to reduce 

NOX emissions by requiring older non-road equipment to be replaced or equipped with 

retrofits or impose operating restrictions on non-road equipment, EPA expects each state 

with a designated NO2 nonattainment area to develop a SIP that brings the area into 

attainment by the applicable deadline and that each state would evaluate the potential for 

controlling emissions from all sources that are contributing to the nonattainment problem. 

CAA section 177 allows states to adopt California’s standards that apply to new motor 

vehicles or new motor vehicle engines and CAA section 209(e)(2)(B) allows states to 
adopt California’s standards that apply to new or used non-road engines and equipment. 

California, and therefore other states, are prohibited by CAA section 209(e)(1) from 

promulgating standards for new construction and farm equipment with engines smaller 

than 175 hp and from new locomotives and locomotive engines. It is possible that a state 

could choose to adopt California’s standards that apply to new on-road and/or other nonroad equipment in order to attain the NO2 NAAQS. States may also choose to reduce 

emissions from non-road equipment through operating restrictions, or requiring older 

equipment to be replaced to the extent that such measures are not federally pre-empted. 

EPA would expect that before any state included any of these types of measures in a SIP 

for NO2 the state would have evaluated a wide range of potential control measures and 

concluded that such measures are necessary to bring the area into attainment by the 

applicable deadline.

(2) Comment: Some commenters stated that modeling for permitting of the short term 1-hr 

NO2 standard could likely show that stationary sources violate the standard as opposed to 

near-road mobile sources. Others requested that if EPA requires near-roadway

monitoring for NO2, it should provide clarifying guidance stating that near-road monitors 

would not be applicable for NSR or other point source permit applications. 

Response: The permitting regulations for Prevention of Significant Deterioration of Air 

Quality (PSD) require applicants to provide 1 year of air quality data representative of the 

air quality in the area that the proposed new or modified source will impact. Typically, 

such monitoring data is used to represent background air quality levels that are 

considered along with modeled concentrations of a pollutant to show that the source’s 

predicted impact will not cause or contribute to a violation of any NAAQS. We agree 

that the ambient data collected from near-road monitoring sites for NO2 may not be 

representative of background air quality levels throughout the area of concern. However, 

if used correctly, such data could be useful in assessing the source’s contribution in 

specific areas of potentially high ambient NO2 concentrations largely attributable to nearroad mobile source emissions. EPA will give further consideration to this comment as 

we evaluate any NSR guidance and regulations that may be needed to implement a 1-

hour NO2 NAAQS.

(3) Comment: API recommends that EPA update the Guidelines on Air Quality Models to 

address screening and refined models for modeling peak short-term concentrations in 

order to accurately predict short-term NO2 concentrations in diverse environmental 

settings. 

Response: The EPA agrees with this comment and intends to review the need to revise 

the Guideline on Air Quality Models to address issues associated with the promulgation 

of a short-term NO2 NAAQS.

(4) Comment: API also recommends that EPA develop a database of NO2 to NOX emissions 

ratios for the variety of sources that would be affected by any new NO2 standards. The 

database could also include methods to make use of data from existing ambient 
monitoring networks to provide realistic background ozone and NO2 concentrations 

required for the short-term NO2 modeling framework. 

Response: The EPA will review the need for the development of a database of NO2 to 

NOX emissions ratios sources following the promulgation of the NO2 NAAQS.

(5) Comment: CAC disagrees with EPA’s decision not to impose non-attainment 

classifications on areas with measured near-road NO2 concentrations in excess of the new 

NO2 standard, and urges EPA to provide a graduated non-attainment classification system 

for the new standard. According to the Council, “a classification system defining higher 

levels of non-attainment with increasingly stringent requirements at those levels is one 

that allows for finer calibration of air quality regulatory response defined at the federal 

level.” 

Response: As stated in the proposed rule, Section 192(a), of part D, of the CAA 

specifically provides an attainment date for areas designated as nonattainment for the 

NO2 NAAQS. Therefore, EPA has legal authority to classify NO2 nonattainment areas, 

but the 5 year attainment date addressed under section 192(a) cannot be extended 

pursuant to section 172(a)(2)(D). Based on this limitation, EPA proposed not to establish 

classifications within the 5 year interval for attaining any new or revised NO2 NAAQS. 

See the preamble to the final rule for consequences for areas failing to meet the 

attainment date. Once EPA makes a determination that an area has failed to meet its 

attainment date, the State will be requested to submit a SIP revision which must show 

that it can attain the standard as expeditiously as practicable, but no later than 5 years 

from the determination of failure to attain. 

(6) Comment: One commenter opined that states that fail to develop SIPs or meet EPA’s 

CAA deadlines would be subject to “numerous federal sanctions” including emissions 

caps, limiting economic development and the loss of federal highway transportation 

dollars. The commenter specifically states that a construction ban would impede projects 

that would improve municipal water supplies and wastewater treatment.

Response: CAA section 179 provides EPA with only two sanctions. These are the 2:1 

offset sanction that applies to new or modified major stationary sources and the highway

funding sanction. These sanctions are only applied in specific circumstances, such as 

when a state has failed to submit a required SIP or when a SIP revision is disapproved. 

The CAA no longer contains a sanction specifically aimed at limiting development. The 

CAA Amendments of 1990 do not include the construction ban that applied in certain 

situations under the 1977 CAA. Therefore, EPA disagrees that, if the available sanctions 

are imposed in an NO2 nonattainment area, they would result in a de facto construction 

ban and they would not delay municipal water or wastewater projects. It must be noted 

that there is no sanction for failing to reach attainment by the CAA deadline. For areas 

that fail to attain by the applicable deadline CAA section 179(d) requires that such areas 

submit a revised SIP within one year after EPA publishes a notice finding that the area 

failed to attain. In the case of NO2, this revised SIP would have to demonstrate 
attainment as expeditiously as practicable, but no later than 5 years after the date of the 

Federal Register notice in which EPA made the finding that the area had failed to attain.

(7) Comment: One commenter pointed out that a new 1-hr standard for NO2 will complicate 

permitting of small sources that may contribute to local NO2 emissions.

Response: Section 110(a)(2)(C) of the CAA requires states to regulate the construction 

and modification of any stationary source as necessary to assure that NAAQS are 

achieved. Consequently, the addition of a 1-hour NO2 NAAQS would require that the 

necessary consideration be given to preventing sources from violating a 1-hour NO2

NAAQS. We do not, however, view this as a complication of the permitting process. In 

some instances, a modeling analysis may be required to make the necessary compliance 

demonstration, while in other cases modeling may not be required. Such decisions as to 

the level of analysis needed to make the necessary compliance demonstration must be 

made by the applicable state or local agency permitting authority.

(8) Comment: Several industry commenters requested that EPA slow the timeline for 

implementing a near-road monitoring network and designating roadway areas, because 

they believe EPA lacks significant information about the implementation and 

performance of a national, near-road monitoring network. 

Response: EPA believes that there is sufficient information to implement a near-road 

NO2 network and that the timeline for implementation will allow EPA to consult CASAC 

and collaborate with states in the development of guidance that will assist in the 

implementation of a network that is as consistent as possible. This issue is discussed in 

section III.B.5 of the preamble to the final rule. 

(9) Comment: Numerous comments were received, primarily from state agencies, noting the 

significant cost of a near-road monitoring networks and requesting that the federal 

government pay for a large percentage of the network’s cost by funding the monitors 

through Section 103 of the CAA, rather than section 105. Many commenters claimed 

that without federal funds, and because of the tight budgets faced by state governments 

presently, the funding of a near-road monitoring network would result in less spending on 

other valuable environmental programs.

Response: EPA understands the resource concerns of the states and intends to work with 

states in identifying available funds and assess the increased resource needs that may be 

needed for network implementation. This issue is discussed in section III.B.1 of the 

preamble to the final rule. 

(10) Comment: Several commenters claim that EPA significantly underestimates the cost of 

near-road monitors for many locations where monitors are likely to be installed.

Response: EPA used direct quotes or its best estimate for the variety of capital and 

logistical costs that will be associated with implementing the network design. EPA must 

make its estimates as nationally applicable as possible, and cannot anticipate the 
individual variations in resource needs, or in the actual costs themselves, that may occur 

from one location of the country to the next. EPA intends to work with states in 

identifying available funds and consider the increased resource needs that may be needed 

for network implementation.

(11) Comment: State agencies requested that EPA provide additional guidelines for the 

criteria that would be used by Regional Administrators in determining when additional 

near-road monitors would be required or when requests for additional monitors will be 

granted. An additional comment requested that EPA provide further guidance and 

oversight of Regional Administrators to ensure the consistent implementation of any

near-road monitoring network across the country. State agencies also requested that the 

Regional Administrator be required to coordinate with the state government in deciding 

where near-road monitors would be placed.

Response: EPA has provided multiple examples of situations where Regional 

Administrator may use their discretion to require additional monitors above the minimum 

required in section III.B.4 of the preamble to the final rule. In particular, EPA notes that 

in situations where a Regional Administrator may consider the need for additional 

monitoring, EPA expects that the state and the Regional Administrator would work 

together to evaluate quantitative evidence that suggests an area may warrant additional 

monitoring. 

(12) Comment: AASHTO recommended that state and local air monitoring agencies be 

required to coordinate with their respective DOTs when developing a near-road 

monitoring plan. 

Response: Although can not require state and local air agencies to work with another 

state or local entity, EPA believes that state and local air monitoring agencies can greatly 

benefit from collaboration with their counterparts in state or local transportation 

authorities. Such collaboration may help in site identification, site access, and ensuring 

public and worker safety at near-road sites.

(13) Comment: Several State Agencies commented on the interaction between NOX and O3. 

For example, AASHTO requested that EPA clarify how areas can overcome potential 

negative impacts on ambient O3 of increased control of NOX emissions. In contrast, one 

State Agency (HCPHES) commented that a more stringent NO2 standard will help them 

attain the O3 NAAQS because NO2 is a precursor to O3. 

Response: It has long been recognized that reducing NOX emissions will result in lower 

concentrations of regional O3 (National Research Council, 1991). This has been

confirmed in numerous photochemical model simulations and assorted ambient data 

analyses over the past two decades (EPA 2005b; EPA, 2008d). As a result, efforts to 

reduce NOX emissions are the foundation of Federal and State actions to attain the ozone 

NAAQS. However, because of the complex chemistry of ozone formation, there are 

some specific instances in which certain NOX reductions can lead to localized ozone 

increases. In these areas, the local planning process will need to consider the 
multipollutant aspects of air quality management and derive the most appropriate set of 

local control measures needed to attain multiple NAAQS simultaneously.

(15) Comment: One state agency recommended that state and local DOTs should be required 

to perform air quality impact analyses for NO2 prior to road construction projects as well 

as conduct regular mobile monitor testing to demonstrate that the construction projects 

are not causing violations of the 1-hr NO2 standard.

Response: Any such requirement for state and local DOT’s to perform air quality impact 

analysis prior to road construction or to conduct any testing would have to be established 

and carried out under state or local authority. 

(16) Comment: One commenter encouraged EPA to develop a 1-hr NO2 significant impact 

level (SIL) without which, “any increase in NO2 in a nonattainment area (or a near 

non-attainment area) could result in denial of the proposed permit, even if the increase is 

not significant or offset with offsite emission reduction credits” (CAAPCOA).

Response: This issue is discussed in section VI.D.2 of the preamble to the final rule.

(17) Comment: The North Carolina Department of Environment and Natural Resources 

(NCDENR) supports the use of a weighted annual mean (quarterly average) for the 

annual primary standard.

Response: This issue is discussed in the final notice (Section IV.A).

(18) Comment: SJVAPCD recommended that EPA include guidelines for near-road 

monitoring that allows for monitoring to stop at locations where sufficient data has been 

collected to conclude that measured concentrations of NO2 are less than 85% of the 

standard level. 

Response: 40 CFR § 58.14(c), explains the process by which states may request approval 

for discontinuing monitoring at an individual site. The discontinuation is subject to 

Regional Administrator approval, and requires the satisfaction of multiple requirements 

which are spelled out in the regulation text.

(19) Comment: CASAC Panel members who supported the alternative approach, as well as 

some public commenters (e.g., States) also raised questions related to how nonattainment areas would be designated if roadside monitors measure violations. For 

example, some commenters questioned whether an entire county could be out of 

attainment based on a roadside monitor. 

Response: This comment is addressed in sections V.B and V.C of the final notice. 
(20) Comment: The ALA, EJ, EDF, and NRDC also recommended that EPA “formulate a 

rational basis for re-designation based on limited monitoring data and available 

information on mobile source emissions from the relevant roadways.” Under this 

approach, EPA is encouraged to “commit to designating near roadway areas no later than 

the end of 2013.” These same groups also commented, “We believe it is reasonable 

to...promulgate a nonattainment SIP submittal deadline of no later than the end of 2015.” 

Other environmental groups (e.g., CAC) also commented that the timing for designations 

should be accelerated. 

Response: Section 110(d)(1)(B) of the CAA requires the EPA to designate areas as 

attainment, nonattainment or unclassifiable no later than 2 years following promulgation 

of a new or revised NAAQS (the CAA provides the Agency an additional third year 

from promulgation should there be insufficient information 

on which to make compliance determinations). The EPA intends to finalize initial 

designations for the revised NO2 NAAQS in January 2012. 

A near-roadway monitoring network is not expected to be fully deployed until January

2013. For this reason, EPA will proceed with initial designations using air quality data 

from the existing, area-wide NO2 monitoring network in order to complete designations 

by January 2012. Once the near-roadway network is fully deployed and 3 years of air

quality data are available, the EPA intends to redesignate areas, as appropriate, based on 

the most recent air quality data from the new monitoring network. For both initial 

designations and redesignations, EPA will use monitoring data to identify violations of 

the standards. EPA would then consider a variety of other factors in determining which 

nearby areas contribute to a violation in setting the boundaries. 

The response to commenters’ requests that EPA both shorten and extend SIP and 

attainment deadlines is provided in the implementation section of the preamble. 

(21) Comment: One commenter (City of NY) recommended that EPA should develop a 

screening approach to estimate short-term NO2 concentrations, and provide source-type 

specific N02/NOX ratios with the NOX emission factors. 

Response: It is EPA's intention to utilize the current guidance and policies to implement 

the revised NAAQS for NO2. However, we will be reviewing the need to provide 

additional technical as well as policy guidance following the promulgation of the 

NAAQS. 

(22) Comment: API recommended that EPA “follow the precedent that was set for PM2.5 by

postponing the use of dispersion modeling to evaluate short-term ambient NO2

concentrations until appropriate modeling tools are developed, evaluated, peer-reviewed, 

and subjected to public comment and review. 

Response: It is EPA's intention to utilize the current guidance and policies to implement 

the revised NAAQS for NO2. However, we will be reviewing the need to provide 
additional technical as well as policy guidance following the promulgation of the 

NAAQS. 
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