1-hour averaging time would be appropriate (e.g., Colorado Petroleum
Association (CPA), Dow, NAM, Petroleum Association of Wyoming (PAW),
Utah Petroleum Association (UPA)).  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, and EPA’s
responses, are discussed in more detail above (section II.E) and in the
Response to Comments document.

c.	Conclusions on averaging time

In considering the most appropriate averaging time for the NO2 primary
NAAQS, the Administrator notes the available scientific evidence as
assessed in the ISA, the air quality analyses presented in the REA, the
conclusions of the policy assessment chapter of the REA, CASAC
recommendations, and public comments received.  These considerations are
described below.

When considering averaging time, the Administrator notes that the
evidence relating short-term (minutes to hours) NO2 exposures to
respiratory morbidity was judged in the ISA to be “sufficient to infer
a likely causal relationship” (ISA, section <IP_ADDRESS>) while the
evidence relating long-term (weeks to years) NO2 exposures to adverse
health effects was judged to be either “suggestive but not sufficient
to infer a causal relationship” (respiratory morbidity) or
“inadequate to infer the presence or absence of a causal
relationship” (mortality, cancer, cardiovascular effects,
reproductive/developmental effects) (ISA, sections <IP_ADDRESS>-<IP_ADDRESS>).
Thus, the Administrator concludes that these judgments most directly
support an averaging time that focuses protection on short-term
exposures to NO2.

As in past reviews of the NO2 NAAQS, the Administrator notes that it is
instructive to evaluate the potential for a standard based on annual
average NO2 concentrations, as is the current standard, to provide
protection against short-term NO2 exposures.  To this end, the
Administrator notes that Table 10-1 in the REA reported the ratios of
short-term to annual average NO2 concentrations.  Ratios of 1-hour daily
maximum concentrations (98th and 99th percentile) to annual average
concentrations across 14 locations ranged from 2.5 to 8.7 while ratios
of 24-hour average concentrations to annual average concentrations
ranged from 1.6 to 3.8 (see Thompson, 2008 for more details).  The
policy assessment chapter of the REA concluded that the variability in
these ratios across locations, particularly those for 1-hour
concentrations, suggested that a standard based on annual average NO2
concentrations would not likely be an effective or efficient approach to
focus protection on short-term NO2 exposures.  For example, in an area
with a relatively high ratio (e.g., 8), the current annual standard (53
ppb) would be expected to allow 1-hour daily maximum NO2 concentrations
of about 400 ppb.  In contrast, in an area with a relatively low ratio
(e.g., 3), the current standard would be expected to allow 1-hour daily
maximum NO2 concentrations of about 150 ppb.  Thus, for purposes of
protecting against the range of 1-hour NO2 exposures, the REA noted that
a standard based on annual average concentrations would likely require
more control than necessary in some areas and less control than
necessary in others, depending on the standard level selected.

In considering the level of support available for specific short-term
averaging times, the Administrator notes that the policy assessment
chapter of the REA considered evidence from both experimental and
epidemiologic studies.  Controlled human exposure studies and animal
toxicological studies provide evidence that NO2 exposures from less than
1-hour up to 3-hours can result in respiratory effects such as increased
airway responsiveness and inflammation (ISA, section <IP_ADDRESS>).
Specifically, the ISA concluded that NO2 exposures of 100 ppb for 1-hour
(or 200 ppb to 300 ppb for 30-min) can result in small but significant
increases in nonspecific airway responsiveness (ISA, section <IP_ADDRESS>).
In contrast, the epidemiologic literature provides support for
short-term averaging times ranging from approximately 1-hour up to
24-hours (ISA, section <IP_ADDRESS>).  A number of epidemiologic studies have
detected positive associations between respiratory morbidity and 1-hour
(daily maximum) and/or 24-hour NO2 concentrations.  A few epidemiologic
studies have considered both 1-hour and 24-hour averaging times,
allowing comparisons to be made.  The ISA reported that such comparisons
in studies that evaluate asthma emergency department visits failed to
reveal differences between effect estimates based on a 1-hour averaging
time and those based on a 24-hour averaging time (ISA, section <IP_ADDRESS>).
 Therefore, the ISA concluded that it is not possible, from the
available epidemiologic evidence, to discern whether effects observed
are attributable to average daily (or multi-day) concentrations (24-hour
average) or high, peak exposures (1-hour maximum) (ISA, section
<IP_ADDRESS>).

As noted in the policy assessment chapter of the REA, given the above
conclusions, the experimental evidence provides support for an averaging
time of shorter duration than 24 hours (e.g., 1-h) while the
epidemiologic evidence provides support for both 1-hour and 24-hour
averaging times.  The Administrator concludes that, at a minimum, this
suggests that a primary concern with regard to averaging time is the
level of protection provided against 1-hour NO2 concentrations.
However, she also notes that it is important to consider the ability of
a 1-hour averaging time to protect against 24-hour average NO2
concentrations.  To this end, the Administrator notes that Table 10-2 in
the REA presented correlations between 1-hour daily maximum NO2
concentrations and 24-hour average NO2 concentrations (98th and 99th
percentile) across 14 locations (see Thompson, 2008 for more detail).
Typical ratios ranged from 1.5 to 2.0, though one ratio (Las Vegas) was
3.1.  These ratios were far less variable than those discussed above for
annual average concentrations, suggesting that a standard based on
1-hour daily maximum NO2 concentrations could also be effective at
protecting against 24-hour NO2 concentrations.  The REA concluded that
the scientific evidence, combined with the air quality correlations
described above, support the appropriateness of a standard based on
1-hour daily maximum NO2 concentrations to protect against health
effects associated with short-term exposures.

Based on these considerations, the Administrator concludes that a
standard with a 1-hour averaging time can effectively limit short-term
(i.e., 1- to 24-hours) exposures that have been linked to adverse
respiratory effects.  This conclusion is based on the observations
summarized above and in more detail in the proposal, particularly that:
1) the 1-hour averaging time has been directly associated with
respiratory effects in both epidemiologic and experimental studies and
that 2) results from air quality analyses suggest that a 1-hour standard
could also effectively control 24-hour NO2 concentrations.  In addition,
the Administrator notes the support provided for a 1-hour averaging time
in comments from CASAC, States, environmental groups, and medical/public
health groups.  The Administrator notes that arguments offered by some
industry groups against setting a 1-hour NO2 standard generally focus on
commenters’ conclusions regarding uncertainties in the scientific
evidence.  As discussed in more detail above (section II.E.2), the
Administrator disagrees with the conclusions of these commenters
regarding the appropriate interpretation of the scientific evidence and
associated uncertainties.  Given these considerations, the Administrator
judges that it is appropriate to set a new NO2 standard with a 1-hour
averaging time.

3.	Form

This section discusses considerations related to the form of the 1-hour
NO2 primary NAAQS.  Specifically, this section summarizes the rationale
for the Administrator’s proposed decision regarding form (II.F.4.a;
see section II.F.3 of the proposal for more detail), discusses comments
related to form (II.F.4.b), and presents the Administrator’s final
conclusions regarding form (II.F.4.c).

a.	Rationale for proposed decision

When considering alternative forms in the proposal, the Administrator
noted the conclusions in the policy assessment chapter of the REA.
Specifically, she noted the conclusion that the adequacy of the public
health protection provided by the combination of standard level and form
should be the foremost consideration.  With regard to this, she noted
that concentration-based forms can better reflect pollutant-associated
health risks than forms based on expected exceedances.  This is the case
because concentration-based forms give proportionally greater weight to
years when pollutant concentrations are well above the level of the
standard than to years when the concentrations are just above the
standard, while an expected exceedance form would give the same weight
to years with concentrations that just exceed the standard as to years
when concentrations greatly exceed the standard.  The Administrator also
recognized the conclusion in the policy assessment chapter of the REA
that it is desirable from a public health perspective to have a form
that is reasonably stable and insulated from the impacts of extreme
meteorological events.  With regard to this, she noted that a form that
calls for averaging concentrations over three years would provide
greater regulatory stability than a form based on a single year of
concentrations.  Therefore, consistent with recent reviews of the O3 and
PM NAAQS, the proposal focused on concentration-based forms averaged
over 3 years, as evaluated in the REA.

In considering specific concentration-based forms, the REA focused on
98th and 99th percentile concentrations averaged over 3 years.  This
focus on the upper percentiles of the distribution is appropriate given
the reliance, in part, on NO2 health evidence from experimental studies,
which provide information on specific exposure concentrations that are
linked to specific health effects.  The REA noted that a 99th percentile
form for a 1-hour daily maximum standard would correspond approximately
to the 4th highest daily maximum concentration in a year (which is the
form of the current O3 NAAQS) while a 98th percentile form (which is the
form of the current short-term PM2.5 NAAQS) would correspond
approximately to the 7th or 8th highest daily maximum concentration in a
year (REA, Table 10-4; see Thompson, 2008 for methods).

Consideration in the REA of an appropriate form for a 1-hour standard
was based on analyses of standard levels that reflected the allowable
area-wide NO2 concentration, not the maximum allowable concentration.
Therefore, in their review of the final REA, CASAC did not have the
opportunity to comment on the appropriateness of specific forms in
conjunction with a standard level that reflects the maximum allowable
NO2 concentration anywhere in an area.  Given this, when considering
alternative forms for the 1-hour standard in the proposal, the
Administrator judged that it was appropriate to consider both forms
evaluated in the REA (i.e., 98th and 99th percentiles).  Therefore, she
proposed to adopt either a 99th percentile or a 4th highest form,
averaged over 3 years, and she solicited comment on both 98th percentile
and 7th or 8th highest forms.

b.	CASAC and public comments on form

In their letter to the Administrator, CASAC discussed the issue of form
within the context of the proposed approach of setting a 1-hour standard
level that reflects the maximum allowable NO2 concentration anywhere in
an area.  CASAC recommended that, for such a standard, EPA adopt a form
based on the 3-year average of the 98th percentile of the distribution
of 1-hour daily maximum NO2 concentrations.  Specifically, they stated
the following in their comments on the proposal (Samet, 2009):

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.

As indicated in their letter, CASAC concluded that the potential
instability in higher percentile NO2 concentrations near major roads
argues for a 98th, rather than a 99th, percentile form.  Several State
organizations and agencies (e.g., NESCAUM and agencies in IN, NC, SD,
VA) and industry groups (e.g., AAM, ACC, API, AirQuality Research and
Logistics (AQRL), CPA, Dow, ExxonMobil, IPAMS, PAW, UPA) also
recommended a 98th percentile form in order to provide regulatory
stability.  In contrast, a small number of State and local agencies
(e.g., in MO and TX), several environmental organizations (e.g., EDF,
EJ, GASP, NRDC), and medical/public health organizations (e.g., ALA,
ATS) recommended either a 99th percentile form or a more stringent form
(e.g., no exceedance) to further limit the occurrence of NO2
concentrations that exceed the standard level in locations that attain
the standard.

c.	Conclusions on form

The Administrator recognizes that there is not a clear health basis for
selecting one specific form over another.  She also recognizes that the
analyses of different forms in the REA are most directly relevant to a
standard that reflects NO2 concentrations permitted to occur broadly
across a community, rather than the maximum concentration that can occur
anywhere in the area.  In contrast, as discussed below (section
II.F.4.c), the Administrator has judged it appropriate to set a new
1-hour standard that reflects the maximum allowable NO2 concentration
anywhere in an area.  In light of this, the Administrator places
particular emphasis on the comments received on form from CASAC relating
to a 1-hour standard level that reflects the maximum allowable NO2
concentration anywhere in an area.  In particular, the Administrator
notes that CASAC recommended a 98th percentile form averaged over 3
years for such a standard, given the potential for instability in the
higher percentile concentrations around major roadways.

In considering this recommendation, the Administrator recognizes that
the public health protection provided by the 1-hour NO2 standard is
based on the approach used to set the standard and the level of the
standard (see below), in conjunction with the form of the standard.
Given that the Administrator is setting a standard that reflects the
maximum allowable NO2 concentration anywhere in an area, rather than a
standard that reflects the allowable area-wide NO2 concentration, she
agrees with CASAC that an appropriate consideration with regard to form
is the extent to which specific statistics could be unstable at
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 98th percentile of the annual distribution of
1-hour daily maximum NO2 concentrations.

4.	Level

As discussed below and in more detail in the proposal (section II.F.4),
the Administrator has considered two different approaches to setting the
1-hour NO2 primary NAAQS.  In the proposal, each of these approaches was
linked with a different range of standard levels.  Specifically, the
Administrator proposed to set a 1-hour standard reflecting the maximum
allowable NO2 concentration anywhere in an area and to set the level of
such a standard from 80 to 100 ppb.  The Administrator also solicited
comment on the alternative approach of setting a standard that reflects
the allowable area-wide NO2 concentration and setting the standard level
from 50 to 75 ppb.  This section summarizes the rationale for the
Administrator’s proposed approach and range of standard levels
(II.F.3.a), describes the alternative approach and range of standard
levels (II.F.3.b), discusses comments related to each approach and range
of standard levels (II.F.3.c), and presents the Administrator’s final
conclusions regarding the approach and level (II.F.3.d).

a.	Rationale for proposed decisions on approach and level

In assessing the most appropriate approach to setting the 1-hour
standard and the most appropriate range of standard levels to propose,
the Administrator considered the broad body of scientific evidence
assessed in the ISA, including epidemiologic and controlled human
exposure studies, as well as the results of exposure/risk analyses
presented in the REA.  In light of the body of available evidence and
analyses, as described above, the Administrator concluded in the
proposal that it is necessary to provide increased public health
protection for at-risk individuals against an array of adverse
respiratory health effects linked with short-term (i.e., 30 minutes to
24 hours) exposures to NO2.  Such health effects have been associated
with exposure to the distribution of short-term ambient NO2
concentrations across an area, including higher short-term (i.e., peak)
exposure concentrations, such as those that can occur on or near major
roadways and near other sources of NO2, as well as the lower short-term
exposure concentrations that can occur in areas not near major roadways
or other sources of NO2.  The Administrator’s proposed decisions on
approach and level, as discussed in detail in the proposal (section
II.F.4), are outlined below.

In considering a standard-setting approach, the Administrator was
mindful in the proposal that the available evidence and analyses from
the ISA and REA support the public health importance of
roadway-associated NO2 exposures.  The exposure assessment described in
the REA estimated that roadway-associated exposures account for the
majority of exposures to peak NO2 concentrations (REA, Figures 8-17,
8-18).  The ISA concluded (section 4.3.6) that NO2 concentrations in
heavy traffic or on freeways “can be twice the residential outdoor or
residential/arterial road level.”  In considering the potential
variability in the NO2 concentration gradient, the proposal noted that
available monitoring studies suggest that NO2 concentrations could be 30
to 100% higher than those in the same area but away from the road.

The Administrator also considered that millions of people in the United
States live, work, and/or attend school near important sources of NO2
such as major roadways (ISA, section 4.4), and that ambient NO2
concentrations in these locations vary depending on the distance from
major roads (i.e., the closer to a major road, the higher the NO2
concentration) (ISA, section 2.5.4).  Therefore, these populations,
which likely include a disproportionate number of individuals in groups
with higher prevalence of asthma and higher hospitalization rates for
asthma (e.g. ethnic or racial minorities and individuals of low
socioeconomic status ) (ISA, section 4.4), are likely exposed to NO2
concentrations that are higher than those occurring away from major
roadways.

Given the above considerations, the Administrator proposed an approach
to setting the 1-hour NO2 primary NAAQS whereby the standard would
reflect the maximum allowable NO2 concentration anywhere in an area.  In
many locations, this concentration is likely to occur on or near a major
roadway.  EPA proposed to set the level of the standard such that, when
available information regarding the concentration gradient around roads
is considered, appropriate public health protection would be provided by
limiting the higher short-term peak exposure concentrations expected to
occur on and near major roadways, as well as the lower short-term
exposure concentrations expected to occur away from those roadways.  The
Administrator concluded that this approach to setting the 1-hour NO2
NAAQS would be expected to protect public health against exposure to the
distribution of short-term NO2 concentrations across an area and would
provide a relatively high degree of confidence regarding the protection
provided against peak exposures to higher NO2 concentrations, such as
those that can occur around major roadways.  The remainder of this
section discusses the proposed range of standard levels.

In considering the appropriate range of levels to propose for a standard
that reflects the maximum allowable NO2 concentration anywhere in an
area, the Administrator considered the broad body of scientific evidence
and exposure/risk information as well as available information on the
relationship between NO2 concentrations near roads and those away from
roads.  Specifically, she considered the extent to which a variety of
levels would be expected to protect at-risk individuals against
increased airway responsiveness, respiratory symptoms, and
respiratory-related emergency department visits and hospital admissions.

After considering the scientific evidence and the exposure/risk
information (see sections II.B, II.C, and II.F.4.a.1 through II.F.4.a.3
in the proposal), as well as the available information on the NO2
concentration gradient around roadways (section II.A.2 above and in the
proposal), the Administrator concluded that the strongest support is for
a standard level at or somewhat below 100 ppb.  The Administrator’s
rationale in reaching this proposed conclusion is provided below.

The Administrator noted that a standard level at or somewhat below 100
ppb in conjunction with the proposed approach would be expected to limit
short-term NO2 exposures to concentrations that have been reported to
increase airway responsiveness in asthmatics (i.e., at or above 100
ppb).  While she acknowledged that exposure to NO2 concentrations below
100 ppb could potentially increase airway responsiveness in some
asthmatics, the Administrator also noted uncertainties regarding the
magnitude and the clinical significance of the NO2-induced increase in
airway responsiveness, as discussed in the policy assessment chapter of
the REA (section <IP_ADDRESS>, discussed in section II.F.4.e in the
proposal).  Given these uncertainties, the Administrator concluded in
the proposal that controlled human exposure studies provide support for
limiting exposures at or somewhat below 100 ppb NO2.

The Administrator also noted that a standard level at or somewhat below
100 ppb in conjunction with the proposed approach would be expected to
maintain peak area-wide NO2 concentrations considerably below those
measured in locations where key U.S. epidemiologic studies have reported
associations with more serious respiratory effects, as indicated by
increased emergency department visits and hospital admissions.
Specifically, the Administrator noted that 5 key U.S. studies provide
evidence for such associations in locations where the 99th percentile of
the distribution of 1-hour daily maximum NO2 concentrations measured at
area-wide monitors ranged from 93 to 112 ppb (Ito et al., 2007; Jaffe et
al., 2003; Peel et al., 2005; Tolbert et al., 2007; and a study by the
New York State Department of Health, 2006).  The Administrator concluded
that these studies provide support for a 1-hour standard that limits the
99th percentile of the distribution of 1-hour daily maximum area-wide
NO2 concentrations to below 90 ppb (corresponds to a 98th percentile
concentration of 85 ppb), and that limiting area-wide concentrations to
considerably below 90 ppb would be appropriate in order to provide an
adequate margin of safety.  The Administrator noted that, based on
available information about the NO2 concentration gradient around roads,
a standard level at or somewhat below 100 ppb set in conjunction with
the proposed approach would be expected to accomplish this.
Specifically, she noted that given available information regarding NO2
concentration gradients around roads (see section II.A.2), a standard
level at or below 100 ppb (with either a 99th or 98th percentile form)
would be expected to limit peak area-wide NO2 concentrations to
approximately 75 ppb or below.  Therefore, the Administrator concluded
that a standard level at or somewhat below 100 ppb under the proposed
approach would be expected to maintain peak area-wide NO2 concentrations
well below 90 ppb across locations despite the expected variation in the
NO2 concentration gradient that can exist around roadways in different
locations and over time.

The Administrator also noted that a study by Delfino provides mixed
evidence for effects in a location with area-wide 98th and 99th
percentile 1-hour daily maximum NO2 concentrations of 50 and 53 ppb,
respectively.  In that study, NO2 effect estimates were positive, but
some reported 95% confidence limits for the odds ratio (OR) that
included values less than 1.00.  Given the mixed results of the Delfino
study, the Administrator concluded that it may not be necessary to
maintain area-wide NO2 concentrations at or below 50 ppb to provide
protection against the effects reported in epidemiologic studies.

In addition to these evidence-based considerations, the Administrator
noted that a standard level at or somewhat below 100 ppb under the
proposed approach would be consistent with the results of the exposure
and risk analyses presented in the REA.  As discussed in section II.C of
the proposal, the results of these analyses provide support for setting
a standard that limits 1-hour area-wide NO2 concentrations to between 50
and 100 ppb.  As described above, a standard level of 100 ppb that
reflects the maximum allowable NO2 concentration would be expected to
maintain area-wide NO2 concentrations at or below approximately 75 ppb.
Given all of these considerations, the Administrator concluded in the
proposal that a standard level at or somewhat below 100 ppb (with a 99th
percentile form), in conjunction with the proposed approach, would be
requisite to protect public health with an adequate margin of safety
against the array of NO2-associated health effects.

In addition to the considerations discussed above, which support setting
a standard level at or somewhat below 100 ppb, the Administrator also
considered the extent to which available evidence could support standard
levels below 100 ppb.  The Administrator concluded that the evidence
could support setting the standard level below 100 ppb to the extent the
following were emphasized:

The possibility that an NO2-induced increase in airway responsiveness
could occur in asthmatics following exposures to concentrations below
100 ppb and/or the possibility that such an increase could be clinically
significant

The mixed results reported in the study by Delfino et al. (2002) of an
association between respiratory symptoms and the relatively low ambient
NO2 concentrations measured in the study area

Specifically, she noted that a standard level of 80 ppb (99th percentile
form), in conjunction with the proposed approach, could limit area-wide
NO2 concentrations to 50 ppb and would be expected to limit exposure
concentrations to below those that have been reported to increase airway
responsiveness in asthmatics.  For the reasons stated above, the
Administrator proposed to set the level of a new 1-hour standard between
80 ppb and 100 ppb.

b.	Rationale for the alternative approach and range of levels

As described above, the Administrator proposed to set a 1-hour NO2 NAAQS
reflecting the maximum allowable NO2 concentration anywhere in an area
and to set the level of such a standard from 80 to 100 ppb.  However,
prior to the proposal, the approach of setting a 1-hour NO2 NAAQS that
reflects the maximum allowable NO2 concentration anywhere in an area had
not been discussed by EPA in the REA or considered by CASAC.  Rather,
the potential alternative standards discussed in the REA, and reviewed
by CASAC, reflected allowable area-wide NO2 concentrations (i.e.,
concentrations that occur broadly across communities).

Given this, the Administrator noted in the proposal that comments
received on the approach to setting the 1-hour standard (i.e., from
CASAC and from members of the public) could provide important new
information for consideration.  Therefore, the Administrator also
solicited comment on the alternative approach of setting a 1-hour NO2
primary NAAQS that would reflect the allowable area-wide NO2
concentration, analogous to the standards evaluated in the REA, and with
a level set within the range of 50 to 75 ppb.  In discussing this
alternative approach with a standard level from 50 to 75 ppb, the
Administrator noted the following in the proposal:

Such a standard would be expected to maintain area-wide NO2
concentrations below peak 1-hour area-wide concentrations measured in
locations where key U.S. epidemiologic studies have reported
associations with respiratory-related emergency department visits and
hospital admissions.

Standard levels from the lower end of the range would be expected to
limit roadway-associated exposures to NO2 concentrations that have been
reported in controlled human exposure studies to increase airway
responsiveness in asthmatics.  Specifically, a standard level of 50 ppb
under this approach could limit near-road concentrations to between
approximately 65 and 100 ppb, depending on the relationship between
near-road NO2 concentrations and area-wide concentrations.

This alternative approach would provide relatively more confidence
regarding the degree to which a specific standard level would limit
area-wide NO2 concentrations and less confidence regarding the degree to
which a specific standard level would limit the peak NO2 concentrations
likely to occur near major roadways.

c.	Comments on approach and level

 In the proposal, each approach to setting the 1-hour standard, and each
range of standard levels, was linked to different requirements for the
design of the NO2 monitoring network.  Specifically, in conjunction with
the proposed approach (i.e., standard reflects the maximum allowable NO2
concentration anywhere in an area and the level is set within the range
of 80 to 100 ppb), the Administrator proposed to establish a 2-tiered
monitoring network that would include monitors sited to measure the
maximum NO2 concentrations anywhere in an area, including near major
roadways, and monitors sited to measure maximum area-wide NO2
concentrations.  In conjunction with the alternative approach (i.e.,
standard reflects the allowable area-wide NO2 concentration and the
level is set within the range of 50 to 75 ppb), the Administrator
solicited comment on a monitoring network that would only include
area-wide NO2 monitors.  Because of these linkages in the proposal, most
commenters combined their comments on the approach to setting a 1-hour
standard and on the standard level with their comments on the monitoring
requirements.  In this section, we discuss comments from CASAC and
public commenters on the approach to setting a 1-hour standard and on
the standard level.  Comments on the monitoring network are also
discussed in this section to the extent they indicate a preference for
either the proposed or alternative approach to setting the 1-hour
standard.  More specific comments on monitor placement and network
design are discussed below in section III.B.2 and in the Response to
Comments document.  EPA responses to technical comments on the
scientific evidence and the exposure/response information are discussed
above in section II.E.2 and in the Response to Comments document.  The
Administrator’s response to commenters’ views on the approach to
setting the 1-hour standard and on the standard level is embodied in the
discussed in section II.F.4.d.

i.	CASAC comments on the approach to setting the standard

A majority of CASAC and 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, though CASAC did not reach consensus
on this approach.  Specifically, in their letter to the Administrator
(Samet, 2009), 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 near-roadway exposures that may reach
levels in the range at which some individuals with asthma may be
adversely affected. Other members acknowledged the need for research and
development of near-road monitoring data for criteria pollutants in
general but favored retention of EPA's current area-wide monitoring for
NO2 regulatory purposes, due to the lack of epidemiological data based
on near-roadway exposure measurements and issues related to implementing
a near-road monitoring system for NO2.

Thus, the recommendation of the majority of 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 addition,
these CASAC Panel members noted important uncertainties with the
alternative approach.  Specifically, they stated the following (Samet,
2009):

Panel members also supported the proposed two-tiered approach because
basing regulations on area-wide monitoring alone was problematic. Such
an approach would require EPA to embed uncertainties and assumptions
about the relationship between area-wide and road-side monitoring into
the area-wide standard.

A 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.  In their view, the
use of area-wide concentrations in these studies introduces uncertainty
into the selection of a standard level for a standard that reflects the
maximum allowable NO2 concentration anywhere in an area and that is
linked with a requirement to place monitors near major roads.  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 [the standard] now relies” (Samet,
2009).  These CASAC Panel members also strongly supported obtaining
monitoring data near major roads, while recognizing uncertainties
associated with identifying appropriate monitoring sites near roads (see
section III.B.2 and the Response to Comments document for more
discussion of CASAC’s monitoring comments).

ii.	Public comments on the approach to setting the standard

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 a number of environmental organizations (e.g., CAC, EDF, EJ,
GASP, NRDC), by the ALA, and 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 supported the rationale in the proposal for
setting a 1-hour standard that reflects the maximum allowable NO2
concentration anywhere in an area.

	A number of State commenters expressed the view that area-wide monitors
should be used for attainment/non-attainment determinations (e.g.,
NACAA, NESCAUM and agencies in IL, IN, MI, MS, NC, NM, SC).  One State
commenter (NESCAUM) agreed with EPA concerns about near-road exposures
but concluded that it is premature to establish a large near-road
monitoring network at this time due to uncertainty regarding the
relationship between near-road and area-wide NO2 concentrations and the
variability in that relationship.  NESCAUM recommended that EPA work
with States to establish a targeted monitoring program in select urban
areas to gather data that would inform future modifications to the
monitoring network, but that “The existing area-wide monitoring
network should be used to identify initial nonattainment areas.”
Other State commenters also concluded that the most appropriate approach
would be to base non-attainment determinations only on area-wide
monitors.  Based on their monitoring comments, many of these commenters
appeared to support setting a 1-hour standard that reflects the
allowable area-wide NO2 concentration.  State concerns with the proposed
approach often included uncertainties associated with identifying and
accessing appropriate monitor sites near major roads, as well as
concerns related to implementation and cost to States (as discussed
further in the Response to Comments document, the Administrator may not
consider cost of implementation in decisions on a NAAQS).

One commenter (AAM) concluded that the focus of the proposed approach on
NO2 concentrations around major roadways is not justified 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.  This conclusion is based on an analysis of 42 existing NO2
monitors in 6 locations.  Comparing NO2 concentrations measured by these
monitors, some of which are close to roads and other of which are
farther from roads, led AAM to conclude that “roadside monitors are
not measuring high NO2 concentrations.”

We agree that there is uncertainty associated with estimates of
roadway-associated NO2 concentrations (see REA, sections 7.4.6 and
<IP_ADDRESS> 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:

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 can be approximately 30 to
100% higher than 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, while the analysis submitted by AAM 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 are likely to
reflect contributions from a combination of mobile and stationary
sources, with one or the other dominating depending on the proximity of
these sources to the monitors.  Therefore, we conclude that the analysis
submitted by AAM, which does not consider other relevant lines of
evidence and information, does not appropriately characterize the
relationship between NO2 concentrations near roads and those away from
roads. (See the Response to Comments document for a more detailed
discussion of AAM comments).

In addition, 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.
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.

iii.	CASAC comments on standard level

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).  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), 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, 2009) 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.

CASAC also recommended that this level be employed with a 98th
percentile form, in order to promote the stability of the standard (see
above for discussion of form).

iv.	Public comments on standard level

A number of State and local agencies and organizations expressed support
for setting the level of the 1-hour NO2 standard within the proposed
range of 80 to 100 ppb.  While some 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, a number of State commenters (e.g., NACAA, NESCAUM and
agencies in IL, NC, NM, TX, VA) recommended a standard level from 80 to
100 ppb in conjunction with a recommendation that only area-wide
monitors be deployed for purposes of determining attainment with the
standard.  Based on these monitoring comments, these State commenters
appear to favor an approach where a standard level from 80 to 100 ppb
would reflect the allowable area-wide NO2 concentration.  As discussed
above (and in more detail in section III.B.2 and the Response to
Comments document), State commenters often based these recommendations
on uncertainties in designing an appropriate national near-road
monitoring network.

A number of environmental organizations (e.g., CAC, 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 described below.

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, EJ,
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 lowest-observed 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 United States. 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.

The Administrator’s consideration of the Delfino study as it relates
to a decision on standard level is discussed below (section II.F.4.d).
Regarding the recommendation to set the level below 50 ppb based on mean
area-wide NO2 concentrations in epidemiologic study locations, we note
that the Administrator proposed to set a standard that reflects the
maximum allowable NO2 concentration anywhere in an area and to set the
form of that standard at the upper end of the distribution of 1-hour
daily maximum NO2 concentrations.  As described in the proposal, such a
standard, with a level from the proposed range of 80 to 100 ppb, would
be expected to maintain peak area-wide NO2 concentrations below the peak
area-wide concentrations measured in locations where key U.S.
epidemiologic studies have reported associations with
respiratory-related emergency department visits and hospital admissions.
 Because reducing NOX emissions to meet a 98th percentile NO2 standard
should lower the distribution of NO2 concentrations, including the mean,
a standard that limits the 98th percentile of the distribution of 1-hour
daily maximum concentrations would also be expected to limit mean
concentrations.  Therefore, although we acknowledge that the
relationship between peak and mean NO2 concentrations will likely vary
across locations and over time, if peak area-wide NO2 concentrations are
maintained below those in key epidemiologic study locations, mean
area-wide NO2 concentrations would also be expected to be maintained
below the mean area-wide concentrations in those locations (see ISA,
figure 2.4-13 for information on the relationship between peak and mean
NO2 concentrations).

As discussed above (section, II.E.2), a number of industry groups did
not support setting a new 1-hour NO2 standard.  However, several of
these groups (e.g., AAM, Dow, NAM, NPRA) also 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 emphasized uncertainties in the scientific evidence.
Specifically, as discussed in more detail above (section II.E.2), 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.  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.  As described above, 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.  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 above (section II.E.2) and in the Response to
Comments document and are summarized briefly below.

As noted in section II.E.2, we agree that the presence of co-pollutants
in the ambient air complicates the interpretation of epidemiologic
studies; however, our conclusions regarding causality are based on
consideration of the broad body of epidemiologic studies (including
those employing multi-pollutant models) as well as animal toxicological
and controlled human exposure studies.  The ISA concluded that this body
of evidence “supports a direct effect of short-term NO2 exposure on
respiratory morbidity at ambient concentrations below the current NAAQS
level” (ISA, p. 5-16).  In addition, the ISA (p. 5-15) concluded the
following:

[T]he strongest evidence for an association between NO2 exposure and
adverse human health effects comes from epidemiologic studies of
respiratory symptoms and ED visits and hospital admissions. These new
findings were based on numerous studies, including panel and field
studies, multipollutant studies that control for the effects of other
pollutants, and studies conducted in areas where the whole distribution
of ambient 24-h avg NO2 concentrations was below the current NAAQS level
of 0.053 ppm (53 ppb) (annual average).

Given that epidemiologic studies provide the strongest support for an
association between NO2 and respiratory morbidity, and that a number of
these studies controlled for the presence of other pollutants with
multi-pollutant models (in which NO2 effect estimates remained robust),
we disagree that NO2 epidemiologic studies should not be used to inform
a decision on the level of the 1-hour NO2 standard.

In addition, we agree that uncertainty exists regarding the extent to
which the NO2-induced increase in airway responsiveness is adverse (REA,
section <IP_ADDRESS>); however, as discussed in detail above (section
II.E.2), we disagree with the conclusion by many industry commenters
that this effect is not adverse in asthmatics following exposures from
100 to 600 ppb NO2.  Specifically, we do not agree that the approach
taken in the study by Goodman et al. (2009), which was used by many
industry commenters to support their conclusions, was appropriate.  The
authors of the Goodman study used data from existing NO2 studies to
characterize the dose-response relationship of NO2 and airway
responsiveness and to calculate the magnitude of the NO2 effect.  Given
the protocol differences in existing studies of NO2 and airway
responsiveness, we do not agree that it is appropriate to base such an
analysis on these studies.

The Administrator’s consideration of these uncertainties, within the
context of setting a standard level, is discussed in the next section.

d.	Conclusions on approach and standard level

Having carefully considered the public comments on the appropriate
approach and level for a 1-hour NO2 standard, as discussed above, the
Administrator believes the fundamental conclusions reached in the ISA
and REA remain valid.  In considering the approach, the Administrator
continues to place primary emphasis on the conclusions of the ISA and
the analyses of the REA, both of which focus attention on the importance
of roadways in contributing to peak NO2 exposures, given that
roadway-associated exposures can dominate personal exposures to NO2.  In
considering the level at which the 1-hour primary NO2 standard should be
set, the Administrator continues to place primary emphasis on the body
of scientific evidence assessed in the ISA, as summarized above in
section II.B, while viewing the results of exposure and risk analyses,
discussed above in section II.C, as providing information in support of
her decision.

With regard to her decision on the approach to setting the 1-hour
standard, the Administrator continues to judge it appropriate to provide
increased public health protection for at-risk individuals against an
array of adverse respiratory health effects linked with short-term
exposures to NO2, where such health effects have been associated with
exposure to the distribution of short-term ambient NO2 concentrations
across an area.  In protecting public health against exposure to the
distribution of short-term NO2 concentrations across an area, the
Administrator is placing emphasis on providing a relatively high degree
of confidence regarding the protection provided against exposures to
peak concentrations of NO2, such as those that can occur around major
roadways.  Available evidence and information suggest that roadways
account for the majority of exposures to peak NO2 concentrations and,
therefore, are important contributors to NO2-associated public health
risks.  In reaching this conclusion, the Administrator notes the
following:

Mobile sources account for the majority of NOX emissions (ISA, Table
2.2-1).

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).

The exposure assessment presented in the REA estimated that
roadway-associated exposures account for the majority of exposures to
peak NO2 concentrations (REA, Figures 8-17, 8-18).

Monitoring studies suggest that NO2 concentrations near roads can be
considerably higher than those in the same area but away from the road
(e.g., by 30-100%, see section II.A.2).

In their comments on the approach to setting the 1-hour NO2 standard,
the majority of CASAC Panel members emphasized the importance of setting
a standard that limits roadway-associated exposures to NO2
concentrations that could adversely affect asthmatics.  These CASAC
Panel members favored the proposed approach, including its focus on
roads.

In addition, the Administrator notes that a considerable fraction of the
population resides, works, or attends school near major roadways and
that 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).
 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).  Of this population, which likely
includes a disproportionate number of individuals in groups with higher
prevalence of asthma and higher hospitalization rates for asthma (e.g.
ethnic or racial minorities and individuals of low socioeconomic status)
(ISA, section 4.4), asthmatics and members of other susceptible groups
(e.g., children, elderly) will have the greatest risks of experiencing
health effects related to NO2 exposure.  In the United States,
approximately 10% of adults and 13% of children have been diagnosed with
asthma, and 6% of adults have been diagnosed with COPD (ISA, section
4.4).

In considering the approach to setting the 1-hour standard, the
Administrator also notes that concerns with the proposed approach
expressed by the minority of CASAC Panel members included concern with
the uncertainty in the relationship between near-road and area-wide NO2
concentrations, given that U.S. epidemiologic studies have been based on
concentrations measured at area-wide monitors.  However, as discussed by
the majority of CASAC Panel members, a similar uncertainty would be
involved in setting a standard with the alternative approach (Samet,
2009).  The Administrator agrees with the majority of CASAC members and
concludes that uncertainty in the relationship between near-road and
area-wide NO2 concentrations should be considered regardless of the
approach selected to set the standard.  She recognizes that this
uncertainty can and should be taken into consideration when considering
the level of the standard.

In drawing conclusions on the approach, the Administrator has considered
the extent to which each approach, in conjunction with the ranges of
standard levels discussed in the proposal, would be expected to limit
the distribution of NO2 concentrations across an area and, therefore,
would be expected to protect against risks associated with NO2
exposures.  Specifically, she has considered the extent to which a
standard set with each approach would be expected to limit maximum NO2
concentrations and area-wide NO2 concentrations.

With regard to expected maximum concentrations, the Administrator notes
the following:

A standard reflecting the maximum allowable NO2 concentration anywhere
in an area would provide a relatively high degree of confidence
regarding the level of protection provided against peak exposures, such
as those that can occur on or near major roadways.  A standard level
from anywhere within the proposed range (i.e., 80 to 100 ppb) would be
expected to limit exposures to NO2 concentrations reported to increase
airway responsiveness in asthmatics.

A standard reflecting the allowable area-wide NO2 concentration would
not provide a high degree of confidence regarding the extent to which
maximum NO2 concentrations would be limited.  Maximum NO2 concentrations
would be expected to be controlled to varying degrees across locations
depending on the NO2 concentration gradient around roads.  Most standard
levels within the range considered in the proposal with this option
(i.e., 50 to 75 ppb) would not be expected to limit the occurrence,
across locations, of NO2 concentrations that have been reported to
increase airway responsiveness in asthmatics.

With regard to expected area-wide concentrations, the Administrator
notes the following:

The extent to which a standard reflecting the maximum allowable NO2
concentration anywhere in an area would be expected to limit area-wide
NO2 concentrations would vary across locations, e.g., depending on the
NO2 concentration gradient around roads.  However, in conjunction with a
standard level from anywhere within the proposed range (i.e., 80-100
ppb), such an approach would be expected to maintain area-wide NO2
concentrations below those measured in locations where key U.S.
epidemiologic studies have reported associations between ambient NO2 and
respiratory-related hospital admissions and emergency department visits
(based on available information regarding the NO2 concentration gradient
around roads as discussed below).

A standard reflecting the maximum allowable area-wide NO2 concentration
would provide a relatively high degree of certainty regarding the extent
to which area-wide NO2 concentrations are limited.  In conjunction with
a standard level from anywhere within the range of levels discussed in
the proposal (i.e., 50-75 ppb) with this alternative approach, such a
standard would be expected to maintain area-wide NO2 concentrations
below those measured in locations where key U.S. epidemiologic studies
have reported associations between ambient NO2 and respiratory-related
hospital admissions and emergency department visits.

Given the above considerations, the Administrator notes that both
approaches, in conjunction with appropriate standard levels, would be
expected to maintain area-wide NO2 concentrations below those measured
in locations where key U.S. epidemiologic studies have reported
associations between ambient NO2 and respiratory-related hospital
admissions and emergency department visits.  In contrast, only a
standard reflecting the maximum allowable NO2 concentration anywhere in
an area would be expected to effectively and efficiently (i.e., the
standard would be neither more nor less stringent than necessary) limit
peak exposures to NO2 at concentrations that have been reported to
increase airway responsiveness in asthmatics.  Only the very lowest part
of the range considered for the area-wide approach would be expected to
effectively limit such peak exposures.  After considering the evidence
and uncertainties, and the advice of the CASAC Panel, the Administrator
judges that the most appropriate approach to setting a 1-hour standard
to protect against the distribution of short-term NO2 concentrations
across an area, including the higher concentrations that can occur
around roads and result in elevated exposure concentrations, is to set a
standard that reflects the maximum allowable NO2 concentration anywhere
in an area.

In considering the level of a 1-hour NO2 standard that reflects the
maximum allowable NO2 concentration anywhere in an area, the
Administrator notes that there is no bright line clearly directing the
choice of level.  Rather, the choice of what is appropriate is a public
health policy judgment entrusted to the Administrator.  This judgment
must include consideration of the strengths and limitations of the
evidence and the appropriate inferences to be drawn from the evidence
and the exposure and risk assessments.  Specifically, the Administrator
notes the following:

Controlled human exposure studies have reported that various NO2
exposure concentrations increased airway responsiveness in mostly mild
asthmatics (section II above and II.B.1.d in proposal).  These studies
can inform an evaluation of the risks associated with exposure to
specific NO2 concentrations, regardless of where those exposures occur
in an area.  Because concentrations evaluated in controlled human
exposure studies are at the high end of the distribution of ambient NO2
concentrations (ISA, section <IP_ADDRESS>), these studies most directly
inform consideration of the risks associated with exposure to peak
short-term NO2 concentrations.

Epidemiologic studies (section II.B.1.a and b) 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 full range of 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 both
higher than (e.g., around major roadways) and lower than the area-wide
concentrations measured in study locations.  Epidemiologic studies then
evaluate whether area-wide NO2 concentrations are associated with the
risk of respiratory morbidity.  Therefore, in considering these
epidemiologic studies within the context of the level of a standard that
reflects the maximum allowable NO2 concentration anywhere in an area,
available information on the distribution of NO2 concentrations across
areas (e.g., concentration gradients around roads) should be considered.
 Considering available information on the relationship between maximum
and area-wide NO2 concentrations allows consideration of the extent to
which a particular standard level would be expected to protect the
public health against risks associated with the distribution of NO2
concentrations across an area, as indicated by area-wide concentrations
measured in epidemiologic study locations.

The risk and exposure analyses presented in the REA provide information
on the potential public health implications of setting standards that
limit area-wide NO2 concentrations to specific levels.  While the
Administrator acknowledges the uncertainties associated with these
analyses which, as discussed in the REA, could result in either over- or
underestimates of NO2-associated health risks, she judges that these
analyses are informative for considering the relative levels of public
health protection that could be provided by different standards.

The Administrator’s consideration of the controlled human exposure
evidence, epidemiologic evidence, and exposure/risk information are
discussed below specifically with regard to a decision on the level of a
standard that reflects the maximum allowable NO2 concentration anywhere
in an area.

In considering the potential for controlled human exposure studies of
NO2 and airway responsiveness to inform a decision on standard level,
the Administrator notes the following:

NO2-induced increases in airway responsiveness, as reported in
controlled human exposure studies, are logically linked to the adverse
respiratory effects that have been reported in NO2 epidemiologic
studies.

The meta-analysis of controlled human exposure data in the ISA reported
increased airway responsiveness in a large percentage of asthmatics at
rest following exposures at and above 100 ppb NO2, the lowest NO2
concentration for which airway responsiveness data are available in
humans.

This meta-analysis does not provide any evidence of a threshold below
which effects do not occur.  The studies included in the meta-analysis
evaluated primarily mild asthmatics while more severely affected
individuals could respond to lower concentrations.  Therefore, it is
possible that exposure to NO2 concentrations below 100 ppb could
increase airway responsiveness in some asthmatics.

In considering the evidence, the Administrator recognizes that the
NO2-induced increases in airway responsiveness reported for exposures to
NO2 concentrations at or above 100 ppb could be adverse for some
asthmatics.  However, she also notes that important uncertainties exist
with regard to the extent to which NO2-induced increases in airway
responsiveness are adverse.  Specifically, she notes the following with
regard to these uncertainties:

The magnitude of the NO2-induced increase in airway responsiveness, and
the extent to which it is adverse, cannot be quantified from the ISA
meta-analysis (REA, section <IP_ADDRESS>).

The NO2-induced increase in airway responsiveness in resting asthmatics
was typically not accompanied by increased respiratory symptoms, even
following exposures to NO2 concentrations well above 100 ppb (ISA,
section <IP_ADDRESS>).

The increase in airway responsiveness that was reported for resting
asthmatics was not present in exercising asthmatics (ISA, Table 3.1-3).

Taking into consideration all of the above, the Administrator concludes
that existing evidence supports the conclusion that the NO2-induced
increase in airway responsiveness at or above 100 ppb presents a risk of
adverse effects for some asthmatics, especially those with more serious
(i.e., more than mild) asthma.  The Administrator notes that the risks
associated with increased airway responsiveness cannot be fully
characterized by these studies, and thus she is not able to determine
whether the increased airway responsiveness experienced by asthmatics in
these studies is an adverse health effect.  However, based on these
studies, the Administrator is concluding that asthmatics, particularly
those suffering from more severe asthma, warrant protection from the
risk of adverse effects associated with the NO2-induced increase in
airway responsiveness.  Therefore, the Administrator concludes that the
controlled human exposure evidence supports setting a standard level no
higher than 100 ppb to reflect a cautious approach to the uncertainty
regarding the adversity of the effect.  However, those uncertainties
lead her to also conclude that this evidence does not support setting a
standard level lower than 100 ppb.

In considering the more serious health effects reported in NO2
epidemiologic studies, as they relate to the level of a standard that
reflects the maximum allowable NO2 concentration anywhere in an area,
the Administrator notes the following:

A cluster of 5 key U.S. epidemiologic studies (Ito et al., 2007; Jaffe
et al., 2003; Peel et al., 2005; Tolbert et al., 2007; and a study by
the New York State Department of Health, 2006) provide evidence for
associations between NO2 and respiratory-related emergency department
visits and hospital admissions in locations where 98th percentile 1-hour
daily maximum NO2 concentrations measured at area-wide monitors ranged
from 85 to 94 ppb.  The Administrator judges it appropriate to place
substantial weight on this cluster of key U.S. epidemiologic studies in
selecting a standard level, as they are a group of studies that reported
positive, and often statistically significant, associations between NO2
and respiratory morbidity in multiple cities across the United States.

A single study (Delfino et al., 2002) provides mixed evidence for NO2
effects (i.e., respiratory symptoms) in a location with a 98th
percentile 1-hour daily maximum NO2 concentration, as measured by an
area-wide monitor, of 50 ppb.  In that study, most of the reported NO2
effect estimates were positive, but not statistically significant.
Given the variability in the NO2 effect estimates in this study, as well
as the lack of studies in other locations with similarly low NO2
concentrations, the Administrator judges it appropriate to place limited
weight on this study, compared to the cluster of 5 studies as noted
above.

Given these considerations, the Administrator concludes that the
epidemiologic evidence provides strong support for setting a standard
that limits the 98th percentile of the distribution of 1-hour daily
maximum area-wide NO2 concentrations to below 85 ppb.  This judgment
takes into account the determinations in the ISA, based on a much
broader body of evidence, that there is a likely causal association
between exposure to NO2 and the types of respiratory morbidity effects
reported in these studies.  Given the considerations discussed above,
the Administrator judges that it is not necessary, based on existing
evidence, to set a standard that maintains peak area-wide NO2
concentrations to below 50 ppb.

In considering specific standard levels supported by the epidemiologic
evidence, the Administrator notes that a level of 100 ppb, for a
standard reflecting the maximum allowable NO2 concentration anywhere in
the area, would be expected to maintain area-wide NO2 concentrations
well below 85 ppb, which is the lowest 98th percentile concentration in
the cluster of 5 studies.  With regard to this, she specifically notes
the following:

If NO2 concentrations near roads are 100% higher than concentrations
away from roads, a standard level of 100 ppb would limit area-wide
concentrations to approximately 50 ppb.

If NO2 concentrations near roads are 30% higher than concentrations away
from roads, a standard level of 100 ppb would limit area-wide
concentrations to approximately 75 ppb.

The Administrator has also considered the NO2 exposure and risk
information within the context of the above conclusions on standard
level.  Specifically, she notes that the results of exposure and risk
analyses were interpreted as providing support for limiting area-wide
NO2 concentrations to no higher than 100 ppb.  Specifically, these
analyses estimated that a standard that limits area-wide NO2
concentrations to approximately 100 ppb or below would be expected to
result in important reductions in respiratory risks, relative to the
level of risk permitted by the current annual standard alone.  As
discussed above, a standard level of 100 ppb, for a standard that
reflects the maximum allowable NO2 concentration, would be expected to
maintain area-wide NO2 concentrations to within a range of approximately
50 to 75 ppb.  Given this, the Administrator concludes 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.

Finally, the Administrator notes that a standard level of 100 ppb for a
standard that reflects the maximum allowable NO2 concentration anywhere
in an area is consistent with the consensus recommendation of CASAC.

Given the above considerations and the comments received on the
proposal, the Administrator determines that the appropriate judgment,
based on the entire body of evidence and information available in this
review, is a standard that reflects the maximum allowable NO2
concentration anywhere in an area and with a level of 100 ppb.
Specifically, she concludes that such a standard, with the averaging
time and form discussed above, will provide a significant increase in
public health protection compared to that provided by the current annual
standard alone and would be expected to protect against the respiratory
effects that have been linked with NO2 exposures in both controlled
human exposure and epidemiologic studies.  This includes focusing on and
limiting exposures at and above 100 ppb for the vast majority of people,
including those in at-risk groups, and maintaining area-wide NO2
concentrations well below those in locations where key U.S.
epidemiologic studies have reported that ambient NO2 is associated with
clearly adverse respiratory health effects, as indicated by increased
hospital admissions and emergency department visits.

In setting the standard level at 100 ppb rather than a lower level, the
Administrator also acknowledges the uncertainties associated with the
scientific evidence.  She notes that a 1-hour standard with a level
lower than 100 ppb would only result in significant further public
health protection if, in fact, there is a continuum of serious, adverse
health risks caused by exposure to NO2 concentrations below 100 ppb
and/or associated with area-wide NO2 concentrations well-below those in
locations where key U.S. epidemiologic studies have reported
associations with respiratory-related emergency department visits and
hospital admissions.  Based on the available evidence, the Administrator
does not believe that such assumptions are warranted.  Taking into
account the uncertainties that remain in interpreting the evidence from
available controlled human exposure and epidemiologic studies, the
Administrator notes that the likelihood of obtaining benefits to public
health with a standard set below 100 ppb decreases, while the likelihood
of requiring reductions in ambient concentrations that go beyond those
that are needed to protect public health increases.

Therefore, the Administrator judges that a standard reflecting the
maximum allowable NO2 concentration anywhere in an area set at 100 ppb
would be sufficient to protect public health with an adequate margin of
safety, including the health of at-risk populations, from adverse
respiratory effects that have been linked with short-term exposures to
NO2 and for which the evidence has been judged sufficient to support a
likely causal relationship with NO2 exposures.  The Administrator does
not believe that a lower standard level is needed to provide this degree
of protection.  These conclusions by the Administrator appropriately
consider the requirement for a standard that is neither more nor less
stringent than necessary for this purpose and recognizes that the CAA
does not require that primary standards be set at a zero-risk level or
to protect the most sensitive individual, but rather at a level that
reduces risk sufficiently so as to protect the public health with an
adequate margin of safety.

G.	Annual Standard

In the proposal, the Administrator noted that some evidence supports a
link between long-term exposures to NO2 and adverse respiratory effects
and that CASAC recommended in their comments prior to the proposal that,
in addition to setting a new 1-hour standard to increase public health
protection, the current annual standard be retained.  CASAC’s
recommendation was based on the scientific evidence and on their
conclusion that a 1-hour standard might not provide adequate protection
against exposure to long-term NO2 concentrations (Samet, 2008b).

With regard to an annual standard, CASAC and a large 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
the National Tribal Air Organization; environmental/medical/public
health groups including ACCP, ALA, AMA, ATS, CAC, EDF, EJ, GASP, NACPR,
NAMDRC, NRDC) 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).

As noted above, 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,
2008b).  A number of State agencies and organizations 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
addition, a number of industry groups (e.g., AAM, API, Dow, INGAA, UARG)
recommended retaining the level of the current annual standard but, as
described above, did so within the context of a recommendation that EPA
should not set a new 1-hour standard.

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.

As discussed above (II.B.3), the evidence relating long-term NO2
exposures to adverse health effects was judged in the ISA to be either
“suggestive but not sufficient to infer a causal relationship”
(respiratory morbidity) or “inadequate to infer the presence or
absence of a causal relationship” (mortality, cancer, cardiovascular
effects, reproductive/developmental effects) (ISA, sections
<IP_ADDRESS>-<IP_ADDRESS>).  In the case of respiratory morbidity, the ISA
(section <IP_ADDRESS>) concluded that “The high correlation among
traffic-related pollutants made it difficult to accurately estimate the
independent effects in these long-term exposure studies.”  Given these
uncertainties associated with the role of long-term NO2 exposures in
causing the reported effects, the Administrator concluded in the
proposal that, consistent with the CASAC recommendation, existing
evidence is not sufficient to justify setting an annual standard with
either a higher or lower level than the current standard.  Commenters
have not submitted any new analyses or information that would change
this conclusion.  Therefore, the Administrator does not agree with the
commenters who recommended a lower level for the annual standard.

The Administrator judges that her conclusions in the proposal regarding
the annual standard remain appropriate.  Specifically, she continues to
agree with the conclusion that, though some evidence does support the
need to limit long-term exposures to NO2, the existing evidence for
adverse health effects following long-term NO2 exposures does not
support either increasing or decreasing the level of the annual
standard.  In light of this and considering the recommendation from
CASAC to retain the current level of the annual standard, the
Administrator judges it appropriate to maintain the level of the annual
standard at 53 ppb.

H.	Summary of final decisions on the primary NO2 standard

For the reasons discussed above, and taking into account information and
assessments presented in the ISA and REA, the advice and recommendations
of the CASAC, and public comments, the Administrator has decided to
revise the existing primary NO2 standard.  Specifically, the
Administrator has determined that the current annual standard by itself
is not requisite to protect public health with an adequate margin of
safety.   In order to provide protection for asthmatics and other
at-risk populations against an array of adverse respiratory health
effects related to short-term NO2 exposure, the Administrator is
establishing a short-term NO2 standard defined by the 3-year average of
the 98th percentile of the yearly distribution of 1-hour daily maximum
NO2 concentrations.  She is setting the level of this standard at 100
ppb, which is to reflect the maximum allowable NO2 concentration
anywhere in an area.  In addition to setting a new 1-hour standard, the
Administrator retains the current annual standard with a level of 53
ppb.  The new 1-hour standard, in combination with the annual standard,
will provide protection for susceptible groups against adverse
respiratory health effects associated with short-term exposures to NO2
and effects potentially associated with long-term exposures to NO2.

III.	Ambient Monitoring and Reporting Requirements

The EPA is finalizing several changes to the ambient air monitoring,
reporting, and network design requirements for the NO2 NAAQS. This
section discusses the changes we are finalizing which are intended to
support the proposed 1-hour NAAQS and retention of the current annual
NAAQS as discussed in Section II. Ambient NO2 monitoring data are used
to determine whether an area is in violation of the NO2 NAAQS. Ambient
NO2 monitoring data are collected by state, local, and Tribal monitoring
agencies (“monitoring agencies”) in accordance with the monitoring
requirements contained in 40 CFR parts 50, 53, and 58

A.	Monitoring Methods

We are finalizing the proposed changes regarding the NO2 Federal
Reference Method (FRM) or Federal Equivalent Method (FEM) analyzers.
Specifically, we are continuing to use the NO2 chemiluminescence FRM and
are finalizing the requirement that any NO2 FRM or FEM used for making
primary NAAQS decisions must be capable of providing hourly averaged
concentration data.  The following paragraphs provide background and
rationale for the continued use of the chemiluminescence FRM and the
decision to finalize the proposed changes.

1.	Chemiluminescence FRM and Alternative Methods

The current monitoring method in use by most State and local monitoring
agencies is the gas-phase chemiluminescence FRM (40 CFR Part 50,
Appendix F), which was implemented into the NO2 monitoring network in
the early 1980s.  EPA did not propose to discontinue using the
chemiluminescence FRM, although we received comment raising concerns
about using a method that is subject to known interferences from certain
species of oxides of nitrogen known as NOZ.  Important components of
ambient NOZ include nitrous acid (HNO2), nitric acid (HNO3), and the
peroxyacetyl nitrates (PANs).

The issue of concern in public comments is that the reduction of NO2 to
NO on the MoOX converter substrate used in chemiluminescent FRMs is not
specific to NO2; hence, chemiluminescence method analyzers are subject
to varying interferences produced by the presence in the air sample of
the NOZ species listed above and others occurring in trace amounts in
ambient air.  This interference is often termed a “positive
artifact” in the reported NO2 concentration since the presence of NOZ
results in an over-estimate in the reported measurement of the actual
ambient NO2 concentration.  This interference by NOZ compounds has long
been known and evaluated (Fehsenfeld et al., 1987; Nunnermacker et al.,
1998; Parrish and Fehsenfeld, 2000; McClenny et al., 2002; U.S.
Environmental Protection Agency, 1993, 2006a).  Further, as noted in the
ISA, it appears that interference by NOZ on chemiluminescence FRMs is
not more than 10 percent of the reported NO2 concentration during most
or all of the day during winter (cold temperatures), but larger
interference ranging up to 70 percent can be found during summer (warm
temperatures) in the afternoon at sites away and downwind from strong
emission sources.

The EPA acknowledges that the NOZ interference in the reported NO2
concentrations collected downwind of NOX source areas and in relatively
remote areas away from concentrated point, area, or mobile sources is
significantly larger than the NOZ interference in NO2 measurements taken
in urban cores or other areas with fresh NOX emissions.  To meet the
primary objective of monitoring maximum NO2 concentrations in an area
the EPA is requiring NO2 monitors to be placed in locations of the
expected highest concentrations, which will generally place monitors
near fresh NOX sources or in areas of dense NOX emissions.  Therefore,
EPA believes that the positive artifact issue, although present, is
small, relative to the actual NO2 being measured..  As a result EPA
believes the chemiluminescence FRM is suitable for continued use in the
ambient NO2 monitoring network, as the potential positive bias from NOZ
species is not significant enough to discontinue using the
chemiluminescence FRM.

EPA also received support through public comments in furthering the
development of alternative methods in determining NO2 concentrations.
Such alternative methods suggested included the
photolytic-chemiluminescent method and cavity ring-down spectroscopy.
As a result, EPA, both the Office of Research and Development and the
Office of Air, will further efforts, working with commercial and
industrial vendors, to identify and evaluate such new technologies.
These efforts may include potentially field testing instruments and
possibly characterizing methods in a laboratory setting of potential
future reference and equivalent methods that may more accurately and/or
directly measure NO2 .

2.	Allowable FRM and FEMs for comparison to the NAAQS

The current CFR language does not prohibit the use of any particular FRM
or FEM to be used in comparison to the standard.  There are designated
wet chemical methods that are only able to report ambient concentration
values averaged across multiple hours.  With the establishment of a
1-hour NAAQS, any FRM or FEM which is a wet chemical based method would
not be appropriate for use in determining compliance of the 1-hour
NAAQS.  EPA addressed this issue by proposing that only those methods
capable of providing 1-hour measurements be comparable to the NAAQS.

a.	Proposed changes to FRM and FEMs that may be compared to the NAAQS

EPA proposed that only those FRMs or FEMs that are capable of providing
hourly averaged concentration data may be used for comparison to the
NAAQS.

b.	Comments

EPA did not receive any substantive public comments that objected to
only allowing those FRMs or FEMs that are capable of providing hourly
averaged concentration data may be used for comparison to the NAAQS, and
received some comments that supported this concept.

c.	Decisions on allowable FRM and FEMs for comparison to the NAAQS

As a result of the supporting public comments, and a lack of public
objections, EPA is finalizing the proposed changes to 40 CFR Part 58
Appendix C to only allow data from FRM or FEMs that are capable of
providing hourly data to be used for comparison to the NAAQS.

B.	Network Design

With the establishment of a 1-hour NO2 NAAQS intended to limit exposure
to maximum concentrations that may occur anywhere in an area, EPA
recognizes that the current NO2 network is inadequate to assess
compliance with the revised NAAQS.  As a result, EPA is promulgating new
NO2 network design requirements.  The following sections provide
background, rationale, and details for the final changes to the NO2
network design requirements.

1.	Two-tiered Network Design

The ISA and REA indicate that one of the largest factors affecting
ambient exposures to NO2 above health benchmarks concentrations are
mobile source emissions, particularly at locations near major roads.
Information in the ISA and the REA shows that concentrations of mobile
source pollutants, including NO2, typically display peak concentrations
on or immediately adjacent to roads, producing a gradient in pollutant
concentrations where concentrations decrease with increasing distance
from roads (ISA sections 2.5.4 and 4.3.6 and table 2.2-1; REA section
7.3.2 and Figures 8-17 and 8-18).  Since the intent of the revised NAAQS
is to limit exposure to NO2 concentrations that occur anywhere in an
area, monitors intended to measure the maximum allowable NO2
concentration in an area should include measurements of the peak
concentrations that occur on and near roads due to on-road mobile
sources.  This need to measure maximum concentrations is the basis for
the first tier of the proposed network design, which focuses monitoring
near highly trafficked roads in urban areas where peak NO2
concentrations are expected to occur. The basis for the second tier of
the network design is to utilize monitors sited to measure area-wide
concentrations to characterize the wider area impact of NO2 sources on
urban populations.  Area-wide monitoring of NO2 also serves to maintain
continuity in collecting data to inform long-term pollutant
concentration trends analysis and health and scientific research.

This section discusses the two-tier network design approach versus the
alternative network design which was also presented for comment in
conjunction with a solicitation for comment on an alternative NAAQS. The
details of the two-tier network design, including how many monitors are
required, where they are to be located, and the related siting criteria
are discussed in subsequent sections.

a.	Proposed Two-tier Network Design

EPA proposed 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).  As an alternative, and in
conjunction with a solicitation for comment on an alternative NAAQS, EPA
solicited comment on a network comprised of only area-wide monitors.

b.	Comments

EPA received many comments on the overall two-tier network design, with
commenters who made statements with a relatively clear position on the
issue generally falling into four categories: (1) those who support the
adoption of the proposed two-tier design approach, (2) ) those who
support the adoption of the two-tier concept, but with modifications,
(3) those who only supported the adoption of the alternative network
design, and (4) those who encourage EPA to commit to further research
the near-road environment by monitoring near-roads, but not to use
near-road data for regulatory purposes, and therefore supported the
alternative network design.

Those commenters who generally support the proposed two-tier network as
proposed, which requires monitoring near-roads and area-wide monitoring,
included the Clean Air Scientific Advisory Committee (although there was
not a consensus, there was a majority in support of the proposed network
design), health groups (e.g. American Thoracic Society, American Medical
Association, and the National Association for the Medical Direction of
Respiratory Care, the American Association of Cardiovascular and
Pulmonary Rehabilitation, and the American College of Chest Physicians),
several state groups (e.g. the Central Pennsylvania Clean Air Board ,
the New York City Law Department and the Metropolitan Washington Air
Quality Committee), and some industry commenters (e.g. American
Chemistry Council, The Clean Energy Group, and Dow Chemical).

Those commenters who support the adoption of the two-tier network design
concept, but suggest modifications to the actual design include some
health and environmental groups (e.g. the American Lung Association,
Earthjustice, the Environmental Defense Fund, and the National Resource
Defense Council), some states (e.g. California, the Central Pennsylvania
Clean Air Board, Harris County (Texas), Iowa, New York, San Joaquin Air
Pollution Control District, Spokane Regional Clean Air Agency, the Texas
Commission on Environmental Quality, and Wisconsin), and some industry
commenters, including the American Petroleum Institute and the Utility
Air Regulatory Group, who are cited by other industry commenters, which
make statements in context that indicate the two-tier network is an
acceptable approach..  The comments and suggestions made by this group
of commenters are discussed in greater detail in the following sections.

Those commenters who only supported the adoption of the alternative
network design included state and industry groups (e.g. the state of
Indiana, the New York Department of Transportation, Alliance of
Automobile Manufacturers, and the Engine Manufacturers Association).
These commenters typically made comments on the two-tier network design,
but did not do so in a context that clearly supported near-road
research.

EPA received comments from some states or state groups (e.g. National
Association of Clean Air Agencies (NACAA), the Northeast States for
Coordinated Air Use Management (NESCAUM), and 10 other individual states
or state groups) and industry commenters (e.g. American Association of
State Highway Transportation Officials, Edison Electric, and the
National Association of Manufacturers) that encouraged EPA to further
research the near-road environment, but not to require near-road monitor
produced data for regulatory purposes, and therefore supported the
adoption of the alternative network design for regulatory purposes.  For
example, with regard to implementing the two-tier network design that
includes near-road regulatory monitoring, the National Association of
Clean Air Agencies (NACAA) stated that “…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.”
The National Association of Manufacturers stated that “Conducting such
a near road [research] 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.”  EPA notes that the
existing scientific research referenced in the proposal document and
this document show that there are on- and near-road peaks of NO2
concentrations, relative to upwind or background levels, which exist due
to on-road mobile source emissions.  This research, as a body of
evidence, also identifies the multiple local factors that affect how,
where, and when peak NO2 concentrations occur on or near a particular
road segment.  These factors include traffic volume, fleet mix, roadway
design, congestion patterns, terrain, and meteorology.  Although the
near-road environment may be complex with regard to mobile source
pollutant behavior, there is sufficient information from existing
research on what factors to consider when identifying locations of
expected maximum concentrations due to on-road mobile sources, and EPA
and states have access to these data.  Further, EPA notes that near-road
monitoring is not a new concept for the ambient air monitoring community
as near-road carbon monoxide monitoring has been a part of routine
networks for nearly three decades and therefore there is some experience
within the whole of EPA and state and local agencies on conducting
ambient monitoring near-roads.  EPA believes that the existing research
provides sufficient information with which we may require a near-road
monitoring network and the collective experience that exists in the
ambient monitoring community will allow for successful implementation of
that network.  EPA also believes that the two-tier network design will
provide a network that has a reasonable degree of similarity across the
country where the required near-road monitors are targeting the maximum
NO2 concentrations in an area particularly attributable to on-road
mobile sources.  The process using the factors noted above in the
selection of a near-road monitoring site and the associated monitor
siting criteria are explained in subsequent sections.

Some industry commenters (e.g. Engine Manufacturers Association, S.C.
Chamber of Commerce, S.C. Manufacturers Alliance) who supported the
adoption of the alternative network design suggested that monitoring in
the near-road environment would not be indicative of exposure for
general populations, and that EPA should not focus on the near-road
micro-environment when requiring monitoring.  For example, the South
Carolina Chamber of Commerce and the South Carolina Manufacturers
Alliance both state that “It appears the proposed monitoring network
will result in a collection of microscale data, which is not at all
representative of air quality relevant to population exposure.”  EPA
notes that the intent of near-road monitoring is to support the NAAQS in
preventing exposure to NO2 concentrations that may occur anywhere in an
area.  EPA recognizes that there is variability in the properties (such
as traffic counts, fleet mix, and localized features) among the road
segments that may exist in an area, but on the whole, roads are
ubiquitous, particularly in urban environments, thus much of the public
may be exposed to near-road environments.  The US Department of
Transportation (US DOT) Federal Highway Administration (FHWA) “Status
of the Nation’s Highways, Bridges, and Transit: 2006 Conditions and
Performance” document (  HYPERLINK
"http://www.fhwa.dot.gov/policy/2006cpr/es02h.htm"
http://www.fhwa.dot.gov/policy/2006cpr/es02h.htm ) states that “while
urban mileage constitutes only 24.9 percent of total (US) mileage, these
roads carried 64.1 percent of the 3 trillion vehicles miles (VMT)
travelled in the United States in 2004.”  The document also states
that “urban interstate highways made up only 0.4 percent of total (US)
mileage but carried 15.5 percent of total VMT.”  These statements
indicate how much more traffic volume exists on roads in urban areas
versus the more rural areas that have significant amounts mileage of the
total public road inventory.  The 2007 American Housing Survey (
HYPERLINK "http://www.census.gov/hhes/www/housing/ahs/ahs07/ahs07.html"
http://www.census.gov/hhes/www/housing/ahs/ahs07/ahs07.html ) estimates
that over 20 million housing units are within 300 feet (~91 meters) of a
4-lane highway, airport, or railroad.  Although that survey includes
airports and railroads, roads are the most pervasive of the three,
indicating that a significant number of the general population live near
roads.  Resultantly, EPA believes that near-road monitors are indicative
of exposure for a significant component of the general population.

EPA received many comments from states and industry on the two-tier
network design specifically concerning the funding to implement and
operate the monitoring network.  These comments on funding permeated
almost all of the comments form state and local monitoring agencies,
regardless of their position on the network design.  EPA notes that it
has historically funded part of the cost of the installation and
operation of monitors used to satisfy Federal monitoring requirements.
EPA understands these concerns although the CAA requirements from which
this final rule derives (Secion 103 (c)(2) of the CAA) are note
contingent on EPA providing funding to states to assist in meeting
monitoring requirements.  However, EPA plans to work with state and
local air agencies in identifying available State and Tribal Air Grant
(STAG) funds and planning for increasing resource needs in future
budgets to better accommodate the implementation costs that new minimum
monitoring requirements such as those in this rule incur.

c.	Conclusions Regarding the Two-tier Network Design

As noted in Section II.F.3 above, the Administrator is setting a new
1-hour NO2 standard that reflects the maximum allowable NO2
concentration anywhere in an area.  After considering NO2 monitoring
studies assessed in the ISA and discussed in the REA, as well as
modeling simulations presented in the REA, the Administrator concluded
that maximum NO2 concentrations are expected to occur around major roads
in many areas (see section II.F.3 for more detail).  Because of this,
and because a considerable fraction of the population lives, works, or
attends school near major roadways, the Administrator concludes that it
is appropriate to require NO2 monitors near major roads in urban areas.
Monitoring NO2 concentrations by major roadways will provide additional
protection for asthmatics and members of other susceptible groups that
may experience greater risks of health effects related to NO2 exposure.
The Administrator notes that this decision makes progress in identifying
and addressing the health burdens faced by communities
disproportionately impacted by pollution from major roadways.

Although near-road monitoring is a new approach for NO2, it is a form of
source-oriented monitoring, which is not a new concept with regard to
monitoring to characterize stationary source impacts.  The novel aspect
of this NO2 network design is that the near-road monitoring component is
targeting mobile sources instead of stationary sources. The required
near-road NO2 monitors are to be located in locations of expected
maximum 1-hour concentrations (discussed in a subsequent section) and
will likely represent the highest NO2 concentrations in an area directly
attributable to mobile sources or a group of sources that includes
mobile sources.  Unlike a network with monitors focused primarily on
stationary sources, if a near-road monitor shows data for a given area
indicating concentrations below the NAAQS, it is likely that all
near-road concentrations throughout the area are likely below the NAAQS.
 In addition, if a near-road monitor shows data that are above the
NAAQS, it is probable that measures that may be taken to bring
concentrations back down below the NAAQS will likely affect some or all
of the mobile sources in the area, reducing mobile source emissions
across an area.

EPA also recognizes the need to have monitors away from roads that
represent area-wide concentrations that serve multiple monitoring
objectives including comparison to the NAAQS, photochemical pollutant
assessment, aiding in ozone forecasting, and characterization of point
and area source impacts.

After considering the scientific data and the public comments regarding
the proposed network design, EPA is selecting to implement the 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).

2.	First Tier (Near-road Monitoring Component) of the NO2 Network Design

The following paragraphs provide background, rationale, and details for
the final changes to the first tier of the two-tier NO2 network design.
In particular, this section will focus on the thresholds that trigger
monitoring requirements.  Near-road site selection and siting criteria
details will be discussed in subsequent sections.

a.	Proposed First Tier (Near-road Monitoring Component) of the Network
Design

	EPA proposed that the first tier of the two-tier NO2 monitoring network
design focus monitors in locations of expected maximum 1-hour
concentrations near major roads in urban areas.  As noted in previous
section, the ISA and REA indicate that one of the largest factors
affecting ambient exposures to NO2 above health benchmarks
concentrations are mobile source emissions, particularly at locations
near major roads.  Since the combination of increased mobile source
emissions and increased urban population densities can lead to increased
exposures and associated risks, urban areas are the appropriate areas to
concentrate required near-road monitoring efforts. Therefore, we
proposed that one near-road NO2 monitor be required in CBSAs with a
population greater than or equal to 350,000 persons.  Based on 2008
Census Bureau statistics, this will result in approximately 143 sites in
as many CBSAs.

We also proposed that a second near-road monitor be 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.  Based on 2008
Census Bureau statistics and data from the 2007 Highway Performance
Monitoring System (HPMS) maintained by the US Department of
Transportation (DOT) Federal Highway Administration (FHWA), this
particular element of the minimum monitoring requirements will add
approximately 24 sites to the approximate 143 near-road sites in CBSAs
that already will have one near-road monitor required due to the 350,000
population threshold. Of the 24 additional sites, two sites are
triggered due to the 250,000 AADT threshold and are attributed to the
Las Vegas, Nevada and Sacramento, California CBSAs. Overall, this first
tier of the proposed network design is estimated to require 167
near-road sites in 143 CBSAs.

b.	Comments

EPA received comments from some industry and public health groups
supporting our proposed population thresholds which trigger minimum
near-road monitoring requirements.  For example, Dow Chemical Company
stated that “Dow comments that the proposed population thresholds are
reasonable for implementation of the new network design and that we
don't see a need to establish a threshold lower than 350,000 people for
the lower bound.”

EPA also received comments from some states and state groups suggesting
that a combination of population and AADT counts or just AADT should be
used to trigger minimum near-road monitoring requirements.  For example,
the San Joaquin Air Pollution Control District in California suggested
that we modify minimum monitoring requirements so that one near-road NO2
monitor be required for any CBSA with a population of 350,000 people and
that had one or more road segments that had AADT counts of 125,000 or
more.  In another example, Harris County Public Health and Environmental
Services suggested that “…rather than specifying population limits
for the monitoring, HCPHES supports a metric like the Annual Average
Daily Traffic (AADT) as a threshold for requiring a near-road monitor.
An initial focus on an AADT in excess of 250,000 is acceptable as a
starting point but EPA should revisit that level and consider lowering
it to 100,000 in five years.”  EPA notes that, in general, roads with
higher AADT counts have relatively higher amounts of mobile source
emissions, which provide an increased potential to lead to relatively
higher on-road and roadside NO2 concentrations.  This concept is
supported, for example, by Gilbert et al. 2007, who state that the NO2
concentrations analyzed in their study are significantly associated with
distance from the nearest highway and the traffic count on the nearest
highway.  EPA believes that part of the intent of suggestions to include
AADT as part of, or distinctly as the requirement threshold is to
further increase the focus of the near-road network to locations where
NO2 concentrations are expected to be highest.  However, EPA chose the
population threshold approach to trigger near-road monitoring in lieu of
a population-AADT count combination or a distinct AADT count threshold
because of the uncertainty in defining which AADT counts would be
appropriate to use in such an approach.  The depth of knowledge and
certainty in the understanding of what particular AADT count might be
expected to contribute to some specific NO2 concentration is not
sufficient to translate into a specific, nationally applicable AADT
count threshold that could be used as part of a population-AADT
threshold approach to require near-road monitors.  Therefore, EPA chose
not to utilize a population-AADT or an AADT-only threshold to trigger
near-road monitoring because of the lack of a quantitative, nationally
applicable relationship between a certain AADT and an expected NO2
concentration.

A few commenters suggested focusing more near-road monitors only in the
larger CBSAs.  For example, the New York Department of Environmental
Conservation (NY DEC) suggested to require, at minimum, two near-road
monitors in any CBSA of 2,500,000 people or more, but none in CBSAs
below that population threshold.  In their comments they point out the
variety of near-road environments that exist in the larger CBSAs such as
New York City.  EPA concurs with NY DEC, and notes that the larger
CBSAs, such as those with a population of 2,500,000 or more persons, are
more likely to have a greater number of major roads across a potentially
larger geographic area, and a corresponding increase in potential for
exposure in different settings.  This is the reasoning behind the
requirement for two monitors in CBSAs with more than 2,500,000 people.
EPA also believes that having multiple monitors in the largest CBSAs
will also allow better understanding of the differences that may exist
between different roads in the same CBSAs due to fleet mix, congestion
patterns, terrain, or geographic locations within those individual
CBSAs. However, EPA believes that a network with less overall monitors
in a fewer number of CBSAs than that which was proposed would lead to an
insufficiently sized network and would be an unbalanced approach for a
regulatory network intended to support the revised NAAQS.  Considering
the potential variability that may exist from one CBSA to the next with
regard to the number of roads, the populations that live near roads, the
fleet mix, the terrain, and the local meteorology, EPA believes a
network that has more monitors in a greater number of CBSAs is more
appropriate to fulfill the intent of the revised NAAQS, where the public
are to be protected from NO2 concentrations exceeding the NAAQS anywhere
in an area.

EPA proposed to require a second monitor in CBSAs with a population
greater than or equal to 2,500,000 persons, to allow for further
characterization of larger urban areas that are more likely to have a
greater number of major roads across a potentially larger geographic
area, and a corresponding increase in potential for exposure.  The
additional provision for any CBSAs with one or more road segments with
an Annual Average Daily Traffic (AADT) count greater than or equal to
250,000 to have a second monitor if they do not already have two
near-road monitors required due their population is to cover situations
where a relatively less populated area has a very highly trafficked
road.  In this case, EPA believes that because those road segments with
250,000 AADT have been identified by US DOT FHWA (  HYPERLINK
"http://www.fhwa.dot.gov/policyinformation/tables/02.cfm"
http://www.fhwa.dot.gov/policyinformation/tables/02.cfm ) as being the
top 0.03 percent of the most traveled public road segments, that they
are the most heavily trafficked roads in the country.  These roads
segments may correspond to the greatest potential for high exposures
directly connected to motor vehicle emissions in the entire country.
Typically, these very highly trafficked roads are in the largest
populated CBSAs, such as those with 2,500,000 people or more, and are
somewhat atypical for CBSAs with less than 2,500,000 people.  As a
result, EPA believes it is appropriate to require a second monitor in a
CBSA that has one or more road segments with 250,000 AADT counts or more
if they do not already have two near-road monitors required due their
population.

 EPA received a few comments requesting explanation of the reasoning
behind the selection of the population thresholds that trigger minimum
monitoring requirements.  For example, the New York Department of
Transportation suggested that this final rule explain the basis for the
350,000 and 2,500,000 population thresholds that will establish
near-road monitors.  In using the population threshold approach to
trigger monitoring requirements, EPA proposed that those CBSAs with
populations equal to or greater than 350,000 and 2,500,000 shall have
one or two required near-road monitors, respectively.  The selection of
the CBSA population level of 350,000 to trigger the requirement of one
near-road monitor was chosen to require monitoring in CBSAs that will
provide near-road monitoring data with a more diverse array of CBSAs in
terms of population, potential fleet mixes, geographic extent, and
geographic setting, from across the country relative to a network
triggered by a higher population threshold.  EPA reviewed the spatial
extent and the population included in CBSAs of certain sizes when making
selecting the population thresholds.  There are 143 CBSAs with 350,000
people across the country (including territories) which contain
approximately 72% of the total population according to 2008 U.S. Census
Bureau estimates (www.census.gov).  This collection of CBSAs
collectively represents territory in 44 states, the District of
Columbia, and Puerto Rico. For comparison, there are 52 CBSAs with
1,000,000 or more people, which contain approximately 55% of the total
population.  This particular collection of CBSAs collectively represents
territory in only [SPACE RESERVED FOR DATA – NEED TO VERIFY IN
ArcGIS].  EPA believes that selecting 350,000 as a threshold to
minimally require one near-road monitor provides a sufficiently sized
network with good spatial representation across the country in a diverse
set of CBSAs that represent a significant portion of the population.
The selection of the 2,500,000 population threshold to trigger a second
near-road monitor, as noted earlier in this section, is based on the
fact that the larger urban areas in the country are more likely to have
a greater number of major roads across a potentially larger geographic
area, and have a corresponding increase in potential for exposure in
different settings.

EPA received many state comments suggesting adjustments to the overall
near-road network components’ size, without giving distinct
suggestions on how to do so.  For example, the Regional Air Pollution
Control Agency, who represents a portion of Ohio, stated “Given the
fairly standard fleet of vehicles on the nation's major highways, we
urge EPA to consider the need for 142 near-roadway monitors. Perhaps a
limited number of monitors across the country would suffice to
sufficiently characterize near-roadway NO2 levels.”  This group of
commenters had various reasons for why they suggested the network be
reduced, including funding concerns (discussed above), safety issues
(discussed below), the perceived need to do additional research
monitoring (discussed above), problems with state implementation plans
(discussed in ??), and designation issues (discussed in ??).  EPA
understands the issues and concerns presented, and believe that many of
these issues are addressed throughout this document.  However, in regard
to reducing the size of the first tier of the network design, EPA
believes that a significantly smaller network than what was proposed
would be insufficient in supporting the revised NAAQS by assuring that a
significant amount of populations across the country are informed and
ultimately protected by having near-road NO2 data that can be compared
to the NAAQS.  The intent of the first tier of the network design is to
require near-road monitoring in CBSAs that will provide near-road
monitoring data with a more diverse array of CBSAs in terms of
population, potential fleet mixes, geographic extent, and geographic
setting, from across the country.

c.	Conclusions Regarding the First Tier (Near-road Monitoring Component)
of the Network Design

	EPA notes that there is a significant lack of routine near-road NO2
monitoring data from different locations around the country.  If the
proposed monitoring network is to support the intent of the revised
NAAQS, preventing exposures to NO2 concentrations above the NAAQS
anywhere in an area, and recognizing that mobile sources have been
identified as a major contributor to exposure risks, a sufficiently
sized network including near-road monitors, such as that proposed, is
appropriate.. The number and distribution of the monitors in the
proposed network will allow states and EPA to gather data for comparison
to the NAAQS and ultimately inform the public on what NO2 concentrations
populations in a diverse array of urban areas, that contain over 72% of
the total U.S. population (based on 2008 U.S. Census Bureau estimates),
may routinely be exposed to on and near roads.  Therefore, we are
finalizing the thresholds that will trigger minimally required near-road
NO2 monitors as they were proposed, where one near-road NO2 monitor is
required in CBSAs with a population greater than or equal to 350,000
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.

3.	Second Tier (Area-wide Monitoring Component) of the Network Design

The following paragraphs provide background, rationale, and details for
the final changes to the second tier of the two-tier NO2 network design.
In particular, this section will focus on the threshold that triggers
monitoring requirements.  Area-wide site selection and siting criteria
details will be discussed in a subsequent section.

a.	Proposed Second Tier (Area-wide Monitoring Component) of the Network
Design

	As the second tier of the proposed two-tier network design, EPA
proposed to require monitors to characterize the highest expected NO2
concentrations at the neighborhood and larger (area-wide) spatial scales
in a area.  This component of the two-tier network design provides
information on area-wide type exposures that may occur in an area due to
an individual or a group of point, area, on-road, and/or non-road
sources.  Further, area-wide sites serve multiple monitoring objectives
aside from NAAQS comparison including photochemical pollutant
assessment, aiding in ozone forecasting, aiding in particulate matter
precursor analysis and particulate matter forecasting.  We proposed to
require one area-wide monitoring site in each CBSA with a population
greater than or equal to 1,000,000. These area-wide sites are proposed
to be sited to represent an area of highest concentration at the
neighborhood or larger spatial scales. Based on 2008 Census Bureau
statistics, this minimum monitoring requirement is expected to trigger
52 area-wide monitors in as many CBSAs.  EPA also proposed to allow any
current photochemical assessment monitoring station (PAMS) sites that
are situated to address the highest NO2 concentrations in an urban area
and sited at neighborhood or urban scales to satisfy this proposed
area-wide monitoring requirement.

b.	Comments

	Most commenters who made comments on area-wide monitoring did so in
regard to their support of the adoption of the alternative area-wide
network design and not specifically about the area-wide monitoring
component of the proposed two-tier network design.  However, EPA did
receive comments from health groups on area-wide monitoring in context
of the proposed network design.  The health group commenters, including
the American Lung Association, Earthjustice, the Environmental Defense
Fund, and the National Resource Defense Council, stated they “oppose
the proposed requirement to retain only 52 air monitors to measure
area-wide concentrations of NO2.”  EPA understands the perceived
concern to be that we are actively reducing the number of required
area-wide monitors.

Prior to this rulemaking, the 2006 monitoring rule (REFERENCE) removed
minimum monitoring requirements for NO2, however the rule has had a
limited impact to date, evidenced by the fact that the size of the NO2
network has remained relatively steady, with around 400 monitors, a
majority of which are area-wide monitors, were operating in 2008
(Watkins and Thompson, 2008).  This is due to multiple reasons, but in
large part due to the fact that area-wide monitors serve multiple
monitoring objectives, as noted earlier, which includes photochemical
pollutant assessment, aiding in pollutant forecasting, and in some cases
in support to ongoing health research.  Noting that states always have
the right to conduct monitoring above the minimum Federal requirements,
and recognizing the objectives of programs such as the Photochemical
Assessment Monitoring Stations (PAMS), EPA believes that the actual
number of area-wide monitors that will operate in the NO2 network will
be greater than the minimally required 52 sites.   As a result, EPA does
not believe that a modification to the two-tier network design that
increases the number of required area-wide NO2 sites as was proposed is
necessary.

c.	Conclusions on the Second Tier (Area-wide Monitoring Component) of
the Network Design

	EPA recognizes that significant portion of the existing NO2 monitoring
network can be characterized as area-wide monitors and that these
monitoring sites serve multiple monitoring objectives, as noted above,
even thought they were not previously required to operate.  In order to
ensure that a minimum number of area-wide monitors continue operating
into the future, we are finalizing the proposed minimum monitoring
requirements for area-wide monitors, where one area-wide monitor is
required in any CBSA with 1,000,000 people or more.  Since there were no
objections to allowing PAMS stations that meet siting criteria
(discussed in a subsequent section) to satisfy minimum monitoring
requirements for area-wide monitors, we are finalizing that allowance as
proposed.  In addition, EPA encourages states to use the network
assessment process to review existing area-wide NO2 sites to help
determine what monitors might meet minimum monitoring requirements and
whether or not other existing monitors warrant continued operation.  EPA
would expect that this process would occur gradually over several years,
but would greatly assist in streamlining the NO2 network, as states move
to meet new minimum monitoring requirements and modify the network to
reflect the intent of the revised NAAQS and the needs of other ongoing
monitoring objectives.

4.	Regional Administrator Authority

The following paragraphs provide background, rationale, and details for
the final changes to Regional Administrator’s authority to use their
discretion in requiring additional NO2 monitors in addition to those
required in the minimum monitoring requirements of the two-tier network
design.

a.	Proposed Regional Administrator Authority

	EPA proposed that the Regional Administrator will have discretion to
require monitoring above the minimum requirements as necessary to
address situations where the required near-road monitors do not
represent a location or locations where the expected maximum hourly NO2
concentrations exist in a CBSA.  We also proposed to allow Regional
Administrators the discretion to require additional near-road monitoring
sites to address situations where minimum monitoring requirements are
not sufficient to meet monitoring objectives, such as a situation where
there is a variety of exposure potential in an area due to variety in
the amount or types of fleet mix, congestion patterns, terrain, or
geographic areas within a CBSA. Finally, we proposed that EPA Regional
Administrators have the discretion to require additional area-wide NO2
monitoring sites above the minimum monitoring requirements where the
minimum monitoring requirements for area-wide monitors are not
sufficient to meet monitoring objectives.

b.	Comments

	EPA received comments from some state groups (e.g. the New York
Department of Environmental Conservation, New York Department of
Transportation, and the New York City Law Department) and an industry
group (the Council of Industrial Boiler Operators) who requested greater
clarification on how Regional Administrators may use their authority to
require additional monitors above the minimum requirements.  For
example, the Council of Industrial Boiler Operators stated that “This
[Regional Administrator Authority] unreasonably vests an unbounded
amount of discretion in EPA to determine when "minimum monitoring
requirements are not sufficient" and which neighborhoods are "uniquely
affected," and impose additional monitoring requirements where all
applicable monitoring requirements are already met by the State and
local agency.”

 	EPA believes that although minimum monitoring requirement do provide a
sufficiently sized and targeted monitoring network, a nationally
applicable network design can not always be guaranteed to account for
all the potential pollution exposure situations that can exist in every