would work in concert with such a standard to result in the intended
degree of protection. The data and analyses generated as a result of
this program will serve to inform the next review of the NAAQS for
oxides of nitrogen and sulfur. The information generated during the
field program can also be used to help state agencies and EPA better
understand how an AAI-based standard would work in terms of the
implementation of such a standard.
    Based on the above considerations, the Administrator is proposing
not to set a new multi-pollutant AAI-based secondary standard for
oxides of nitrogen and sulfur in this review. In reaching this
decision, the Administrator recognizes that the new NO2 and
SO2 primary 1-hour standards set in 2010, while not
ecologically relevant for a secondary standard, will nonetheless result
in reductions in oxides of nitrogen and sulfur that will directionally
benefit the environment by reducing NOy and SOx deposition to sensitive
ecosystems. EPA is proposing to revise the secondary standards by
adding secondary standards identical to the NO2 and
SO2 primary 1-hour standards set in 2010. More specifically,
EPA is proposing a 1-hour secondary NO2 standard set at a
level of 100 ppb and a 1-hour secondary SO2 standard set at
a level of 75 ppb. While this will not add secondary standards of an
ecologically relevant form to address deposition-related effects, it
will directionally provide some degree of additional protection. This
is consistent with the view that the current secondary standards are
neither sufficiently protective nor appropriate in form, but that it is
not appropriate to propose to set a new, ecologically relevant multi-
pollutant secondary standard at this time, for all of the reasons
discussed above.
    While not a basis for this decision, the Administrator also
recognizes that a new, innovative AAI-based standard would raise
significant implementation issues that would need to be addressed
consistent with the CAA requirements for implementation-related actions
following the setting of a new NAAQS. It will take time to address
these issues, during which the Agency will be conducting a field pilot
program to gather relevant data and the environment will benefit from
reductions in oxides of nitrogen and sulfur resulting from the new
NO2 and SO2 primary standards, as noted above, as
well as reductions expected to be achieved from EPA's Cross-State Air
Pollution Rule and Mercury and Air Toxics standards. These
implementation-related issues are discussed in more detail below in
section IV.A.5.
    The Administrator solicits comment on all aspects of this proposed
decision, including the framework and elements of a multi-pollutant
standard for oxides of nitrogen and sulfur to address deposition-
related effects on sensitive ecosystems, with a focus on aquatic
acidification, and the uncertainties and complexities associated with
the development of such a standard at this time. The Administrator also
solicits comment on the field pilot program and related monitoring
methods as discussed below in section IV.

IV. Field Pilot Program and Ambient Monitoring

    This section describes EPA's plans for a field pilot program and
the evaluation of monitoring methods for ambient air indicators of
NOy and SOx to implement the Administrator's
decision to undertake such a field monitoring program in conjunction
with her decision to propose not to set a new multi-pollutant secondary
standard in this review, as discussed above in section III.H. As noted
above and discussed below in section IV.A, the field pilot program is
intended to collect and analyze data so as to enhance our understanding
of the degree of protectiveness that would likely be afforded by a
standard based on the AAI as developed in the PA. Data generated by
this field program would also support development of an appropriate
monitoring network that would work in concert with such a standard to
result in the intended degree of protection. As discussed below in
section IV.B, the evaluation of monitoring methods focuses on the
development of Federal Reference Methods/Federal Equivalent Methods
(FRM/FEM) for NOy and SOx. The EPA notes that the
monitoring program described here is intended to be coordinated with
EPA's CASTNET as a supplement to existing monitoring programs and is
beyond the scope of the current CASTNET program.

A. Field Pilot Program

    This section presents the objectives of a field pilot program
(section IV.A.1) that would gather relevant field data over a 5-year
period in a sample of three to five sensitive ecoregions across the
country. An overview of the scope and structure of the field program,
with a focus on measurements of ambient air indicators of oxides of
nitrogen and

sulfur, is presented in section IV.A.2. Section IV.A.3 explains the
role of additional complementary measurements beyond the ambient air
indicators that would be included in the program, and section IV.A.4
discusses a parallel longer-term research agenda, both of which are
guided by the uncertainties discussed above in section III. Section
IV.A.5 identifies implementation challenges presented by an AAI-based
standard that could be addressed in parallel with a field pilot
program. Section IV.A.6 discusses engagement with stakeholder groups as
part of the planned pilot program.
1. Objectives
    Consideration of a new multi-pollutant standard to address
deposition-related effects on sensitive aquatic ecoregions raises
unique challenges relative to those typically raised in reviews of
existing NAAQS for which an established network of FRM/FEM monitors,
designed to measure the indicator pollutant, is generally available.
The primary goal of this field pilot program, and the related
monitoring program discussed in section IV.B, is to enhance our
understanding of the degree of protectiveness that would likely be
afforded by a standard based on the AAI, as described above in section
III, so as to aid the Agency in considering an appropriate multi-
pollutant standard that would be requisite to protect public welfare
consistent with section 109 of the CAA, through the following
objectives:
    (1) Evaluate measurement methods for the ambient air indicators of
NOy and SOx and consider designation of such
methods as FRMs;
    (2) Examine the variability and improve characterization of
concentration and deposition patterns of NOy and
SOx, as well as reduced forms of nitrogen, within and across
a number of sensitive ecoregions across the country;
    (3) Develop updated ecoregion-specific factors (i.e., F1 through
F4) for the AAI equation based in part on new observed air quality data
within the sample ecoregions as well as on updated nationwide air
quality model results and expanded critical load data bases, and
explore alternative approaches for developing such representative
factors;
    (4) Calculate ecoregion-specific AAI values using observed
NOy and SOx data and updated ecoregion-specific
factors to examine the extent to which the sample ecoregions would meet
a set of alternative AAI-based standards;
    (5) Develop air monitoring network design criteria for an AAI-based
standard;
    (6) assess the use of total nitrate measurements as a potential
alternative indicator for NOy;
    (7) Support related longer-term research efforts, including
enhancements to and evaluation of modeled dry deposition algorithms;
and
    (8) Facilitate stakeholder engagement in addressing implementation
issues associated with possible future adoption of an AAI-based
standard.
2. Overview of Field Pilot Program
    The CASTNET program (Figure IV-1) affords an available
infrastructure relevant to an AAI-based standard, given the location of
sites in some acid-sensitive ecoregions and various measurements of
sulfur and nitrogen species. The EPA plans to use CASTNET sites in
selected acid-sensitive ecoregions to serve as the platform for this
pilot program, potentially starting in late 2012 and extending through
2018. The CASTNET sites in three to five ecoregions in acid-sensitive
areas would collect NOy and SOx (i.e.,
SO2 and p-SO4) measurements over a 5-year period.
The initial step in developing a data base of observed ambient air
indicators for oxides of nitrogen and sulfur requires the addition of
NOy samplers at the pilot study sites so that a full
complement of indicator measurements are available to calculate AAI
values. These CASTNET sites would also be used to make supplemental
observations useful for evaluation of CMAQ's characterization of
factors F2 -F4 in the AAI equation.
    The selected ecoregions would account for geographic variability by
including regions from across the U.S., including the east, upper
midwest and west. Each selected region would have at least two existing
CASTNET sites. Each of the pilot CASTNET sites would be used to
evaluate the performance of the established methods, data retrieval and
reporting procedures used in the AAI equation.

[GRAPHIC] [TIFF OMITTED] TP01AU11.030

    Over the course of this 5-year pilot program, the most current
national air quality modeling, based on the most current national
emissions inventory, would be used to develop an updated set of F2--F4
factors. A parallel multi-agency national critical load data base
development effort would be used as the basis for calculating updated
F1 factors. As discussed above in section III.B, these factors would be
based on average parameter values across an ecoregion. Using this new
set of F factors, observations of NOy and SOx
derived from the pilot program, averaged across each ecoregion, would
be used to calculate AAI values in the sample ecoregions. The data from
the pilot program would also be used to examine alternative approaches
to generating representative air quality values, such as examining the
appropriateness of spatial averaging in areas of high spatial
variability.
3. Complementary Measurements
    Complementary measurements may be performed at some sites in the
pilot network to reduce uncertainties in the recommended methods and
better characterize model performance and application to the AAI. The
CASAC Air Monitoring and Methods Subcommittee (AMMS) advised EPA that
such supplemental measurements were of critical importance in a field
measurement program related to an AAI-based standard (Russell and
Samet, 2011b).
    Candidate complementary measurements to address sulfur, in addition
to those provided by the CASTNET filter pack (CFP), include trace gas
continuous SO2 and speciated PM2.5 measurements.
The co-located deployment of a continuous SO2 analyzer with
the CFP for SO2 will provide test data for determining
suitability of continuous SO2 measurements as a Federal
Equivalent Method (FEM), as well as producing valuable time series data
for model evaluation purposes. The weekly averaging time provided by
the CFP adequately addresses the annual-average basis of an AAI-based
secondary standard, but would not be applicable to short-term (i.e., 1-
hour) averages associated with the primary SO2 standard.
Conversely, because of the low concentrations associated with many
acid-sensitive ecoregions, existing SO2 Federal Reference
Methods (FRMs) designated for use in determining compliance with the
primary standard would not necessarily be appropriate for use in
conjunction with an AAI-based secondary standard.
    Co-locating the PM2.5 sampler used in the EPA Chemical
Speciation Network and the Interagency Monitoring of Protected Visual
Environments (IMPROVE) network at pilot network sites would allow for
characterizing the relationship between the CFP-derived p-
SO4 and the speciation samplers used throughout the state
and local air quality networks. Note that CASTNET already has several
co-located IMPROVE chemical speciation samplers. Because the AAI
equation is based on concentration of p-SO4, the original
motivation for capturing all particle size fractions is not as
important relative to simply capturing the concentration of total p-
SO4.
    Candidate measurements to complement oxidized nitrogen
measurements, in addition to the CFP, include a mix of continuous and
periodic sampling for the dominant NOy species, namely NO,
true NO2, PAN,

HNO3, and p-NO3. While there are several
approaches to acquiring these measurements, perhaps the most efficient
strategy would take advantage of the available CFP for total nitrate,
and add a three-channel chemiluminescence instrument that will cycle
between NOy, true NO2 and NO by adding photolytic
detection for true NO2. Other options for measuring true
NO2 would include adding either a stand-alone photolytic or
cavity ring-down spectroscopy instrument. Measurements of PAN may be
acquired either on a periodic basis through canister sampling and
subsequent laboratory analysis or through emerging in-situ sampling and
analysis methods. Although the CFP yields a reliable measurement of
total nitrate, the t-NO3 (i.e., the sum of HNO3
and p-NH4) value, strong consideration may be given to
direct measurement of HNO3, which has the highest deposition
velocity of all the dominant NOy species. Similar to the use
of continuous SO2 data, these speciated NOy data
serve two purposes: evaluating total NOy instrument behavior
and evaluating air quality models. The measurement of individual
NOy species can be used to generate site-specific
NOy values for comparison to modeled NOy, and
will likely provide insight into and improvement of modeled dry
deposition.
    The CASAC AMMS (Russell and Samet, 2011b) recommended that EPA
consider the use of t-NO3 obtained from CASTNET sampling as
an indicator for NOy, reasoning that t-NO3 is
typically a significant fraction of deposited oxidized nitrogen in
rural environments and CASTNET measurements are widely available.
Collection of this data would support further consideration of using
the CFP for t-NO3 as the indicator of oxides of nitrogen for
use in an AAI-based secondary standard.
    The CASAC AMMS also recommended that total NHx
(NH3 and p-NH4) be considered as a proxy for
reduced nitrogen species, reasoning that the subsequent partitioning to
NH3 and p-NH4 may be estimated using equilibrium
chemistry calculations. Reduced nitrogen measurements are used to
evaluate air quality modeling which is used in generating factor F2.
Additional studies are needed to determine the applicability of
NHx measurements and calculated values of NH3 and
NH4 to the AAI.
    The additional supplemental measurements of speciated
NOy, continuous SO2 and NHx will be
used in future air quality modeling evaluation efforts. Because there
often is significant lag in the availability of contemporary emissions
data to drive air quality modeling, the complete use of these data sets
will extend beyond the 5-year collection period of the pilot program.
Consequently, the immediate application of those data will address
instrument performance comparisons that explore the feasibility of
using continuous SO2 instruments in rural environments, and
using the speciated NOy data to assess NOy
instrument performance. Although contemporary air quality modeling will
lag behind measurement data availability, the observations can be used
in deposition models to compare observed transference ratios with the
previously calculated transference ratios to test temporal stability of
the ratios.
    An extended water quality sampling effort should parallel the air
quality measurement program to address some of the uncertainties
related to factor F1 and the representativeness of the nth percentile
critical load as discussed in section III.B.5.b.i. The objective of the
water quality sampling would be to develop a larger data base of
critical loads in each of the pilot ecoregions such that the nth
percentile can adequately be characterized in terms of representing all
water bodies. Opportunities to leverage and perhaps enhance existing
ecosystem modeling efforts enabling more advanced critical load
modeling and improved methods to estimate base cation production would
be pursued. For example, areas with ongoing research studies producing
data for dynamic critical load modeling would be considered when
selecting the pilot ecoregions.
4. Complementary areas of research
    The EPA recognizes that a source of uncertainty in an AAI-based
secondary standard that would not be directly addressed in the pilot
program stems from the uncertainty in the model used to link
atmospheric concentrations to dry deposition fluxes. Currently, there
are no ongoing direct dry deposition measurement studies at CASTNET
sites that can be used to evaluate modeled results. It was strongly
recommended by CASAC AMMS that a comprehensive sampling-intensive study
be conducted in at least one, preferably two sites in different
ecoregions to assess characterization of dry deposition of sulfur and
nitrogen. These sites would be the same as those for the complementary
measurements described above, but they would afford an opportunity to
also complement dry deposition process research that benefits from the
ambient air measurements collected in the pilot program. The concerns
regarding uncertainties underlying an AAI-based secondary standard
suggest that research that includes dry deposition measurements and
evaluation of dry deposition models should be a high priority.
    Similar leveraging should be pursued with respect to ecosystem
research activities. For example, studies that capture a suite of soil,
vegetation, hydrological, and water quality properties that can help
evaluate more advanced critical load models would complement the
atmospheric-based pilot program. In concept, such studies could provide
the infrastructure for true multi-pollutant, multi-media ``super''
sites assuming the planning, coordination, and resource facets can be
aligned. While this discussion emphasizes the opportunity of leveraging
ongoing research efforts, consideration could be given to explicitly
including related research components directly in the pilot program.
5. Implementation challenges
    The CAA requires that once a NAAQS is established, designation and
implementation must move forward. With a standard as innovative as the
AAI-based standard considered in this review, the Administrator
believes that its success will be greatly improved if, while additional
data are being collected to reduce the uncertainties discussed above,
the implementing agencies and other stakeholders have an opportunity to
discuss and thoroughly understand how such a standard would work. And
since, as noted above, emissions reductions that are directionally
correct to reduce aquatic acidification will be occurring as a result
of other CAA programs, the Administrator believes that this period of
further discussion will not delay progress but will ensure that once
implementation is triggered, agencies will be prepared to implement it
successfully.
    Consideration of an AAI-based secondary standard for oxides of
nitrogen and sulfur would present significant implementation challenges
because it involves multiple, regionally-dispersed pollutants and
relatively complex compliance determinations based on regionally
variable levels of NOy and SOx concentrations
that would be necessary to achieve a national ANC target. The
anticipated implementation challenges fall into three main categories:
monitoring and compliance determinations for area designations, pre-
construction permit application analyses of individual source impacts,
and State Implementation Plan (SIP) development. Several overarching
implementation questions that we

anticipate will be addressed in parallel with the field pilot program's
five-year data collection period include:
    (1) What are the appropriate monitoring network density and siting
requirements to support a compliance system based on ecoregions?
    (2) Given the unique spatial nature of the secondary standard
(e.g., ecoregions), what are the appropriate parameters for
establishing nonattainment areas?
    (3) How can new or modified major sources of oxides of nitrogen and
oxides of sulfur emissions assess their ambient impacts on the standard
and demonstrate that they are not causing or contributing to a
violation of the NAAQS for preconstruction permitting? To what extent
does the fact that a single source may be impacting multiple areas,
with different acid sensitivities and variable levels of NOy
and SOx concentrations that would be necessary to achieve a
national ANC target, complicate this assessment and how can these
additional complexities best be addressed?
    (4) What additional tools, information, and planning structures are
needed to assist states with SIP development, including the assessment
of interstate pollutant transport and deposition?
    (5) Would transportation conformity apply in nonattainment and
maintenance areas for this secondary standard, and, if it does, would
satisfying requirements that apply for related primary standards (e.g.,
ozone, PM2.5, and NO2) be demonstrated to satisfy
requirements for this secondary standard?
6. Final Monitoring Plan Development and Stakeholder Participation
    The existing CASTNET sampling site infrastructure provides an
effective means of quickly and efficiently deploying a monitoring
program to support potential implementation of an AAI-based secondary
standard, and also provides an additional opportunity for federally
managed networks to collaborate and support the states, local agencies
and tribes (SLT) in determining compliance with a secondary standard. A
collaborative effort would help to optimize limited federal and SLT
monitoring funds and would be beneficial to all involved. The CASTNET
is already a stakeholder-based program with over 20 participants and
contributors, including federal, state and tribal partners.
    The CASAC AMMS generally endorsed the technical approaches used in
CASTNET, but concerns were raised by individual representatives of
state agencies concerning the perception of EPA-controlled management
aspects of CASTNET and data ownership. Potential approaches to resolve
these issues will be developed and evaluated in existing National
Association of Clean Air Agencies (NACAA)/EPA ambient air monitoring
workgroups. The EPA Office of Air and Radiation (which includes the
Office of Air Quality Planning Standards, OAQPS; and the Office of
Atmospheric Program's Clean Air Markets Division, OAP-CAMD), and their
partners on the NACAA monitor steering committee will develop a
prioritized specific plan that identifies the three to five ecoregions
and the instrumentation to be deployed. The EPA anticipates that a cost
estimate of the plan with priorities and options will be developed by
January, 2012. Although this pilot program is focused on data
collection, the plan will include details of the data analysis
approaches as well as a vehicle that incorporates engagement from those
within EPA and SLTs to foster progress on the implementation questions
noted above in section IV.A.5.
    If an AAI-based secondary standard were to be set in the future,
deployment of a full national network would follow the pilot monitoring
program. The number of sites deployed in the network will lead to
increased confidence in capturing spatial patterns of air quality.
Recognizing that this section presents the general elements of the
field pilot programs, EPA intends to develop a more detailed field
pilot program plan through a process that will engage the air quality
management and research (atmospheric and ecosystem) communities, as
well as other federal agencies, state and local agencies, and non-
government based centers of expertise. The EPA is seeking comment and
input on all aspects of this field pilot program.

B. Evaluation of Monitoring Methods

    The EPA generally relies on monitoring methods that have been
designated as FRMs or FEMs for the purpose of determining the
attainment status of areas with regard to existing NAAQS. Such FRMs or
FEMs are generally required to measure the air quality indicators that
are compared to the level of a standard to assess compliance with a
NAAQS. Prior to their designation by EPA as FRM/FEMs through a
rulemaking process, these methods must be determined to be applicable
for routine field use and need to have been experimentally validated by
meeting or exceeding specific accuracy, reproducibility, and
reliability criteria established by EPA for this purpose. As discussed
above in section III.A, the ambient air indicators being considered for
use in an AAI-based standard include SO2, particulate
sulfate (p-SO4), and total reactive oxides of nitrogen
(NOy).
    The CASTNET provides a well established infrastructure that would
meet the basic location and measurement requirements of an AAI-based
secondary standard given the rural placement of sites in acid sensitive
areas. In addition, CFPs currently provide very economical weekly,
integrated average concentration measurements of SO2, p-
SO4, ammonium ion (NH4) and t-NO3, the
sum of HNO3 and p-NO3.
    While routinely operated instruments that measure SO2,
p-SO4, NOy and/or t-NO3 exist,
instruments that measure p-SO4, NOy, t-
NO3, or the CFP for SO2 have not been designated
by EPA as FRMs or FEMs. The EPA's Office of Research and Development
has initiated work that will support future FRM designations by EPA for
SO2 and p-SO4 measurements based on the CFP. Such
a designation by EPA could be done for the purpose of facilitating
consistent research related to an AAI-based standard and/or in
conjunction with setting and supporting an AAI-based secondary
standard.
    Based on extensive review of literature and available data, the EPA
has identified potential methods that appear suitable for measuring
each of the three components of the indicators. These three methods are
being considered as new FRMs to be used for measuring the ambient
concentrations of the three components that would be needed to
determine compliance with an AAI-based secondary standard.
    For the SO2 and p-SO4 measurements, EPA is
considering the CFP method, which provides weekly average concentration
measurements for SO2 and p-SO4. This method has
been used in the EPA's CASTNET monitoring network for 15 years, and
strongly indicates that it will meet the requirements for use as an FRM
for the SO2 and p-SO4 concentrations for an AAI-
based secondary standard.
    Although the CFP method would provide measurements of both the
SO2 and p-SO4 components in a unified sampling
and analysis procedure, individual FRMs will be considered for each.
The EPA recognizes that an existing FRM to measure SO2
concentrations using ultra-violet fluorescence (UVF) exists (40 CFR
Part 50, Appendix A-1) for the purpose of monitoring compliance for the
primary SO2 NAAQS. However, several factors suggest that the
CFP method would be

superior to that UVF FRM for monitoring compliance with an AAI-based
secondary standard and will be discussed in more detail below.
    For monitoring the NOy component, a continuous analyzer
for measuring NOy is commercially available and is
considered to be suitable for use as an FRM. This method is similar in
design to the existing NO2 FRM (described in 40 CFR Part 50,
Appendix F), which is based on the ozone chemiluminescence measurement
technique. The method is adapted to and further optimized to measure
all NOy. However, this NOy method requires
further evaluation before it can be fully confirmed as a suitable FRM.
The EPA is currently completing a full scientific assessment of the
NOy method to determine whether it would be appropriate to
consider for designation by EPA as an FRM. Specific details on these
three methods are given below.
    On February 16, 2011, EPA presented this set of potential FRMs to
the CASAC AMMS for their consideration and comment. In response, the
CASAC AMMS stated that, overall, it believes that EPA's planned
evaluation of methods for measuring NOy, SO2 and
p-SO4 as ambient air indicators is a suitable approach in
concept. On supporting the CFP method as a potential FRM for
SO2, CASAC stated that they felt that the CFP is adequate
for measuring long-term average SO2 gas concentrations in
rural areas with low levels (less than 5 parts per billion by volume
(ppbv)) and is therefore suitable for consideration as an FRM. For p-
SO4, CASAC generally supports the use of the CFP as a
potential FRM for measuring p-SO4 for an AAI-based secondary
standard. The method has been relatively well-characterized and
evaluated, and it has a documented, long-term track record of
successful use in a field network designed to assess spatial patterns
and long-term trends.
    On supporting the photometric NOy method as a potential
FRM, CASAC concluded that the existing NOy method is
generally an appropriate approach for the indicator. However, CASAC
agrees that additional characterization and research is needed to fully
understand the method in order to designate it as a FRM. The EPA is now
soliciting public comment on these methods as to their adequacy,
suitability, and relative merits as FRMs for purposes of monitoring to
determine compliance with an AAI-based secondary standard.
1. Potential FRMs for SO2 and p-SO4
    The CFP is a combined, integrated sampling and analysis method
based on the well-established measurement technology that has been used
extensively in EPA's CASTNET monitoring network (see http://www.epa.gov/castnet). This method is in current use at over 80
monitoring sites and has been in use at not less than 40 sites for over
15 years. This method employs a relatively simple and inexpensive
sampler and uses four 47-mm filters placed in an open-faced filter pack
to simultaneously collect integrated filter samples for the
SO2 and p-SO4 components. In addition, the CFP is
also capable of the collection of t-NO3, the sum of
HNO3 and p-NO3.
    The first stage of the filter pack assembly contains a Teflon[reg]
filter that collects p-SO42- and p-
NO3, the second stage contains a nylon filter that collects
SO2 (as SO42-) and HNO3,
and the third stage contains two cellulose fiber filters impregnated
with potassium carbonate (K2CO3) that collect any
remaining SO2 (as SO42-). The sampler
collects 1-week integrated samples at a very low, controlled flow rate
(1.5 or 3 L/min) in an attempt simulate actual deposition. Weekly
averaged SO2 and p-SO4 concentrations could then
be averaged over a 1-year period to calculate annual average values.
    Upon sample completion, the species-specific filters are extracted,
with subsequent analysis by the well-established and documented ion
chromatographic (IC) analytical technique. During the IC analysis, an
aliquot of a filter extract is injected into a stream of eluent (ion
chromatography mobile phase, generally a millimolar-strength solution
of carbonate-bicarbonate) and passed through a series of ion
exchangers. The anions of interest are separated on the basis of their
relative affinities for a low capacity and the strongly basic anion
exchanger (guard and separator column). The separated anions are
directed onto a cation exchanger (suppressor column) where they are
converted to their highly conductive acid form, and the eluent is
converted to a weakly conductive form. The now-separated anions, each
in their acid form, are measured by conductivity. They are identified
on the basis of retention time compared to that of standards and
quantified by measurement of peak area compared to the peak areas of
calibration standards.
    Calibration and quality assurance for the method are applied to the
sample filters, the analytical processes, and the flow rate measurement
and control aspects of the sampler. Overall method performance is
typically assessed with collocated samplers. These quality assurance
techniques are routinely used and have proved adequate for other types
of FRMs and equivalent methods in air monitoring network service.
    The measurement and analytical procedures and past performance data
associated with the CFP method are well documented and available
through Quality Assurance Performance Plans (QAPPs), Standard Operating
Procedures (SOPs) and annual reports (US EPA, 2010a and 2010b). The
accumulated database on the CFP method is substantial and indicates
that the method is sound, stable and has good reliability in routine,
field operation. Data quality assessment results show the method to
have good reproducibility, with collocated and analytical precision
values in the range of 2 percent to 10 percent (excluding very low
concentration measurements near the method detection limits; US EPA
2010b).
    Data quality objectives (DQOs) for a new FRM would be based upon
current DQOs being used for this method by EPA's OAP/CAMD and the NPS,
the federal managers of CASTNET (US EPA, 2010a). In its current state,
the CFP method is expected to meet or exceed (as past CASTNET data have
indicated; US EPA, 2010b) the expected FRM DQOs, even when deployed in
new monitoring networks outside of CASTNET. In addition, CASTNET
samples have agreed favorably with other measures of SO2 and
p-SO4 in comparison studies. For example, in direct
comparison with an annular denuder sampler (ADS) method, CASTNET/ADS
ratios for SO2 and p-SO4 were generally on the
order of 0.9-1.1 (Lavery et al, 2009; Sickles et al, 1999; Sickles et
al, 2008), thus illustrating the accuracy of the CFP method in the
determination of long-term average SO2 and p-SO4
concentrations. The EPA believes that the CFP method would be fully
adequate as an FRM in determining yearly average SO2 and p-
SO4 concentrations for compliance determination purposes.
    The EPA recognizes that an existing FRM for SO2 has
proven adequate for the purposes of monitoring compliance for the
primary SO2 NAAQS, specifically the newly-promulgated 1-hour
standard. However, this FRM is better suited to the shorter-term,
higher concentration primary and secondary SO2 NAAQS, and
there is substantial uncertainty as to the adequacy of this
SO2 FRM for monitoring the lower concentrations relevant to
determining compliance with an AAI-based secondary standard. The
performance specifications for SO2 FRM analyzers (40 CFR
Part 53, Table B-1) require a lower detectable limit (LDL) of 0.002

ppm for the standard measurement range and 0.001 ppm for the lower
measurement range. These requirements correspond to mass per unit
volume concentrations of 5.24 and 2.62 [micro]g/m\3\, respectively.
Analysis of 2009 CASTNET data shows that of the 84 CASTNET sampling
sites, 63 measured annual average SO2 concentrations below
even the lower of these LDL requirements of 2.62 [micro]g/m\3\ for the
lower range SO2 FRM (US EPA, 2010a). In addition, 11 of the
84 sites measured annual (2009) average SO2 concentrations
very near or below the manufacturers' reported detection limits for
trace level UVF SO2 monitors. Further, it is likely that the
number of sites with annual average SO2 concentration below
both the SO2 FRM LDL and the manufacturers reported
detection limits will increase due to expected declines in mean
SO2 concentrations (US EPA, 2010b). For these reasons, EPA
is considering the CFP method for use as the FRM for monitoring the
SO2 component of an ambient air indicator for oxides of
sulfur, with a recommendation for additional study and data collection
to evaluate further the possible applicability of the continuous UVF
SO2 FRM for this purpose.
2. Potential FRM for NOy
    Atmospheric concentrations of NOy are measured
continuously by an analyzer that photometrically measures the light
intensity, at wavelengths greater than 600 nanometers (nm), resulting
from the chemiluminescent reaction of ozone (O3) with NO in
sampled air. This method is very similar to the chemiluminescence NO/
NO2 analyzers widely used to collect NO2
monitoring data for determining compliance with the NO2
NAAQS. The various oxides of nitrogen species, excluding NO, are first
quantitatively reduced to NO by means of a catalytic converter. These
species include NO2, HNO2, PANs, HNO3
and p-NO3. The NO, which commonly exists in ambient air,
passes through the converter unchanged, and, when combined with the NO
resulting from the catalytic conversion of the other oxides of
nitrogen, a measurement of the total NOy concentration
results. To maximize the conversion of the more chemically active
oxides of nitrogen species, the converter is located externally, at or
near the air sample inlet probe. This location minimizes losses of
these active species that could otherwise occur from chemical reactions
and wall losses in the sample inlet line.
    The NOy analyzer is a suitable, commercially produced
continuous chemiluminescence analyzer that includes an ozone generator,
a reaction cell, a photometric detector, wavelength filters as
necessary to reduce sensitivity to wavelengths below 600 nanometer
(nm), a pump and flow control system to draw atmospheric air through
the converter and into the reaction cell, a suitable converter, a
system to control the operation of the analyzer, and appropriate
electronics to process and quantitatively scale the photometric
signals. The converter contains a catalyst such as molybdenum and is
heated to an optimum temperature designed to optimize the conversion of
the various oxides of nitrogen to NO. It is connected to the analyzer
via suitable lengths of Teflon[reg] tubing. Hourly NOy
measurements obtained by the analyzer would be averaged over the same
7-day period used by the CFP method to measure the SO2 and
p-SO4 components, with further averaging over a 1-year
period.
    Commercial NOy analyzers are currently available, and
the analyzers have been used for a variety of monitoring applications.
During the 2006 TexAQS Radical and Aerosol Measurement Project (TRAMP),
Luke et al., 2010, compared measured NOy concentrations
obtained with an NOy instrument based upon the above
mentioned methodology with the sum of measured individual
NOy species (i.e., NOyi =
NO+NO2+HNO3+PANs+HNO2+p-
NO3). This comparison yielded excellent overall agreement
during both day ([NOy](ppb) = [NOyi](ppb) x 1.03-0.42; r\2\ = 0.9933)
and night time ([NOy](ppb) = [NOyi](ppb) x 1.01-0.18; r\2\ = 0.9975)
periods (Luke et al, 2010). The results of this study show that this
NOy method is capable of the accurate determination of all
the atmospherically relevant NOy components, resulting in an
accurate determination of total NOy concentrations. The
NOy instruments have been routinely operated in networks
such as SouthEastern Aerosol Research and Characterization (SEARCH),
dating back several years. In addition, state monitoring agencies
across the U.S. have begun, starting in 2009, the routine operation of
commercially available NOy instrumentation in anticipation
of EPA's NCore network transitioning to full operation in 2011.
    These initial assessments described above are promising and
indicate that the photometric NOy method appears to be
accurate, reliable, and capable of routine network operation. As a
result, the method is most likely capable for use as an FRM for
determining atmospheric NOy concentrations as a component in
determining compliance with an AAI-based secondary standard. However,
as described below, this continuous method for NOy requires
additional time for further evaluation before it can be fully confirmed
for adoption as a FRM. The EPA has identified measurement uncertainties
and some remaining science questions associated with this method. Among
these are: (a) The ability of the method to capture all components of
NOy relevant to nitrogen deposition, (b) the efficiency of
the molybdenum converter in converting all oxides of nitrogen to NO for
detection (excluding NO2, as this conversion is already well
documented), (c) appropriate inlet height specifications to minimize
any bias associated with vertical concentration gradients of key
NOy components, (d) identification and quantification of
potential measurement interferences in the NOy
determination, and (e) development and demonstration of effective
calibration/challenge procedures to best represent the various mixtures
of NOy components that are expected to be present in the
different air sheds across the U.S.
    To address these NOy method uncertainties and to fully
assess this method for use as the NOy FRM, EPA has developed
a detailed research plan (Russell and Samet, 2011b) which was presented
to the CASAC AMMS on February 16, 2011. In response, CASAC recognized
the need for, and supported the general outline of EPA's research plan
to evaluate the NOy method for potential designation as an
FRM (US EPA, 2011). In addition, the CASAC AMMS suggested additional
areas of research associated with the photometric NOy method
that warrant further assessment prior to final designation of the
method as the NOy FRM. These include operation of the method
during extremely low temperature conditions to investigate possible
condensation in sample lines, method detection limits relative to low
levels expected in remote areas, and ambient-based method evaluations
in various air sheds across the U.S. In response to these CASAC AMMS
suggestions, EPA is carrying out studies, in addition to the tasks
outlined in the research plan, for the NOy method. The
results of these studies will likely take a year or more to become
available. As noted previously, EPA anticipates that these results will
be favorable and will confirm the adequacy of the NOy method
as a suitable FRM for determining compliance with an AAI-based
secondary standard.

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Review and
Executive Order 13563: Improving Regulation and Regulatory Review

    Under Executive Order 12866 (58 FR 51735, October 4, 1993), this
action is a ``significant regulatory action.'' Accordingly, EPA
submitted this action to the Office of Management and Budget (OMB) for
review under Executive Orders 12866 and 13563 (76 FR 3821, January 21,
2011), and any changes made in response to OMB recommendations have
been documented in the docket for this action.

B. Paperwork Reduction Act

    This action does not impose an information collection burden under
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
Burden is defined at 5 CFR 1320.3(b). There are no information
collection requirements directly associated with the establishment of a
NAAQS under section 109 of the CAA.

C. Regulatory Flexibility Act

    For purposes of assessing the impacts of today's rule on small
entities, small entity is defined as: (1) A small business that is a
small industrial entity as defined by the Small Business
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not
dominant in its field.
    After considering the economic impacts of today's proposed rule on
small entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This
proposed rule will not impose any requirements on small entities.
Rather, this rule establishes national standards for allowable
concentrations of oxides of nitrogen and sulfur in ambient air as
required by section 109 of the CAA. See also American Trucking
Associations v. EPA. 175 F. 3d at 1044-45 (NAAQS do not have
significant impacts upon small entities because NAAQS themselves impose
no regulations upon small entities). We continue to be interested in
the potential impacts of the proposed rule on small entities and
welcome comments on issues related to such impacts.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and Tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to state, local, and tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
1 year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and to
adopt the least costly, most cost-effective or least burdensome
alternative that achieves the objectives of the rule. The provisions of
section 205 do not apply when they are inconsistent with applicable
law. Moreover, section 205 allows EPA to adopt an alternative other
than the least costly, most cost-effective or least burdensome
alternative if the Administrator publishes with the final rule an
explanation why that alternative was not adopted. Before EPA
establishes any regulatory requirements that may significantly or
uniquely affect small governments, including tribal governments, it
must have developed under section 203 of the UMRA a small government
agency plan. The plan must provide for notifying potentially affected
small governments, enabling officials of affected small governments to
have meaningful and timely input in the development of EPA regulatory
proposals with significant Federal intergovernmental mandates, and
informing, educating, and advising small governments on compliance with
the regulatory requirements.
    This action contains no Federal mandates under the provisions of
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), 2 U.S.C.
1531-1538 for state, local, or tribal governments or the private
sector. Therefore, this action is not subject to the requirements of
sections 202 or 205. Furthermore, as indicated previously, in setting a
NAAQS EPA cannot consider the economic or technological feasibility of
attaining ambient air quality standards; although such factors may be
considered to a degree in the development of state plans to implement
the standards. See also American Trucking Associations v. EPA, 175 F.
3d at 1043 (noting that because EPA is precluded from considering costs
of implementation in establishing NAAQS, preparation of a Regulatory
Impact Analysis pursuant to the Unfunded Mandates Reform Act would not
furnish any information which the court could consider in reviewing the
NAAQS). Accordingly, EPA has determined that the provisions of sections
202, 203, and 205 of the UMRA do not apply to this proposed decision.
The EPA acknowledges, however, that any corresponding revisions to
associated state implementation plan (SIP) requirements and air quality
surveillance requirements, 40 CFR part 51 and 40 CFR part 58,
respectively, might result in such effects. Accordingly, EPA will
address, as appropriate, unfunded mandates if and when it proposes any
revisions to 40 CFR parts 51 or 58.

E. Executive Order 13132: Federalism

    This proposed rule does not have federalism implications. It will
not have substantial direct effects on the states, on the relationship
between the national government and the states, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132 because it does not contain
legally binding requirements. Thus, the requirements of Executive Order
13132 do not apply to this rule.
    EPA believes, however, that this proposed rule may be of
significant interest to state governments. As also noted in section E
(above) on UMRA, EPA recognizes that states will have a substantial
interest in this rule and any corresponding revisions to associated SIP
requirements and air quality surveillance requirements, 40 CFR part 51
and 40 CFR part 58, respectively. Therefore, in the spirit of Executive
Order 13132 and consistent with EPA policy to promote communications
between EPA and state and local governments, EPA specifically solicits
comment on this proposed rule from state and local officials.

F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

    Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.'' This rule concerns the
establishment of national standards to address the public welfare
effects of oxides of nitrogen and sulfur.

    This action does not have Tribal implications, as specified in
Executive Order 13175 (65 FR 67249, November 9, 2000). It does not have
a substantial direct effect on one or more Indian tribes, since tribes
are not obligated to adopt or implement any NAAQS. Thus, Executive
Order 13175 does not apply to this rule.

G. Executive Order 13045: Protection of Children from Environmental
Health & Safety Risks

    This action is not subject to EO 13045 because it is not an
economically significant rule as defined in EO 12866.

H. Executive Order 13211: Actions that Significantly Affect Energy
Supply, Distribution or Use

    This action is not a ``significant energy action'' as defined in
Executive Order 13211 (66 FR 28355, May 22, 2001), because it is not
likely to have a significant adverse effect on the supply,
distribution, or use of energy. This action concerns the establishment
of national standards to address the public welfare effects of oxides
of nitrogen and sulfur. This action does not prescribe specific
pollution control strategies by which these ambient standards will be
met. Such strategies will be developed by states on a case-by-case
basis, and EPA cannot predict whether the control options selected by
states will include regulations on energy suppliers, distributors, or
users.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (NTTAA), Public Law 104-113, 12(d) (15 U.S.C. 272 note)
directs EPA to use voluntary consensus standards in its regulatory
activities unless to do so would be inconsistent with applicable law or
otherwise impractical. Voluntary consensus standards are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) that are developed or adopted by
voluntary consensus standards bodies. The NTTAA directs EPA to provide
Congress, through OMB, explanations when the Agency decides not to use
available and applicable voluntary consensus standards.
    The EPA is not aware of any voluntary consensus standards that are
relevant to the provisions of this proposed rule. The EPA welcomes any
feedback on such standards that may be applicable.

J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations

    Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes
federal executive policy on environmental justice. Its main provision
directs federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
    EPA has determined that this proposed rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it retains the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or low-
income population.

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List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: July 12, 2011.
Lisa P. Jackson,
Administrator.

    For the reasons set forth in the preamble, part 50 of chapter 1 of
title 40 of the code of Federal regulations is proposed to be amended
as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY
STANDARDS

    1. The authority citation for part 50 continues to read as follows:

    Authority: 42 U.S.C. 7401, et seq.
    2. Section 50.5 is amended by revising paragraphs (b) and (c) and
by adding paragraphs (d) and (e) to read as follows:

Sec.  50.5  National secondary ambient air quality standards for sulfur
oxides (sulfur dioxide).

* * * * *
    (b) The level of the national secondary 1-hour ambient air quality
standard for oxides of sulfur is 75 parts per billion (ppb, which is 1
part in 1,000,000,000), measured in the ambient air as sulfur dioxide
(SO2).
    (c) The levels of the standards shall be measured by a reference
method based on Appendix A-1 or A-2 of this part, or by a Federal
Equivalent Method (FEM) designated in accordance with part 53 of this
chapter.
    (d) To demonstrate attainment with the 3-hour secondary standard,
the second-highest 3-hour average must be based upon hourly data that
are at least 75 percent complete in each calendar quarter. A 3-hour
block average shall be considered valid only if all three hourly
averages for the 3-hour period are available. If only one or two hourly
averages are available, but the 3-hour average would exceed the level
of the standard when zeros are substituted for the missing values,
subject to the rounding rule of paragraph (a) of this section, then
this shall be considered a valid 3-hour average. In all cases, the 3-
hour block average shall be computed as the sum of the hourly averages
divided by 3.
    (e) The 1-hour secondary standard is met at an ambient air quality
monitoring site when the three-year average of the annual 99th
percentile of the daily maximum 1-hour average concentrations is less
than or equal to 75 ppb, as determined in accordance with Appendix T of
this part.
    3. Section 50.11 is revised to read as follows:

Sec.  50.11  National primary and secondary ambient air quality
standards for oxides of nitrogen (with nitrogen dioxide as the
indicator).

    (a) The level of the national primary and secondary annual ambient
air quality standards for oxides of nitrogen is 53 parts per billion
(ppb, which is 1 part in 1,000,000,000), annual average concentration,
measured in the ambient air as nitrogen dioxide.
    (b) The level of the national primary and secondary 1-hour ambient
air quality standards for oxides of nitrogen is 100 ppb, 1-hour average
concentration, measured in the ambient air as nitrogen dioxide.
    (c) The levels of the standards shall be measured by:
    (1) A reference method based on appendix F to this part; or
    (2) A Federal equivalent method (FEM) designated in accordance with
part 53 of this chapter.
    (d) The annual primary and secondary standards are met when the
annual average concentration in a calendar year is less than or equal
to 53 ppb, as determined in accordance with Appendix S of this part for
the annual standard.
    (e) The 1-hour primary and secondary standards are met when the
three-year average of the annual 98th percentile of the daily maximum
1-hour average concentration is less than or equal to 100 ppb, as
determined in accordance with Appendix S of this part for the 1-hour
standard.
    4. Appendix S is amended as follows:
    a. by revising paragraph 1.(a),
    b. by revising the definition of ``Design values'' under paragraph
1.(c),
    c. by revising paragraph 2.(b),
    d. by revising paragraphs 3.1(a) through (d),
    e. by revising paragraphs 3.2(a) through (e),
    f. by revising paragraph 4.1(b),
    g. by revising paragraph 4.2(c),
    h. by revising paragraph 5.1(b), and
    i. by revising paragraph 5.2(b) to read as follows:

Appendix S to Part 50--Interpretation of the Primary and Secondary
National Ambient Air Quality Standards for Oxides of Nitrogen (Nitrogen
Dioxide)

    1. General.
    (a) This appendix explains the data handling conventions and
computations necessary for determining when the primary and
secondary national ambient air quality standards for oxides of
nitrogen as measured by nitrogen dioxide (``NO2 NAAQS'')
specified in Sec.  50.11 are met. Nitrogen dioxide (NO2)
is measured in the ambient air by a Federal reference method (FRM)
based on appendix F to this part or by a Federal equivalent method
(FEM) designated in accordance with part 53 of this chapter. Data
handling and computation procedures to be used in making comparisons
between reported NO2 concentrations and the levels of the
NO2 NAAQS are specified in the following sections.
* * * * *
    (c) * * *
    Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
specified in section 5 of this appendix. The design values for the
primary and secondary NAAQS are:
    (1) The annual mean value for a monitoring site for one year
(referred to as the ``annual primary or secondary standard design
value'').
    (2) The 3-year average of annual 98th percentile daily maximum
1-hour values for a monitoring site (referred to as the ``1-hour
primary or secondary standard design value'').
* * * * *
    2. Requirements for Data Used for Comparisons With the
NO2 NAAQS and Data
Reporting Considerations.
* * * * *
    (b) When two or more NO2 monitors are operated at a
site, the state may in advance designate one of them as the primary
monitor. If the state has not made this designation, the
Administrator will make the designation, either in advance or
retrospectively. Design values will be developed using only the data
from the primary monitor, if this results in a valid design value.
If data from the primary monitor do not allow the development of a
valid design value, data solely from the other monitor(s) will be
used in turn to develop a valid design value, if this results in a
valid design value. If there are three or more monitors, the order
for such comparison of the other monitors will be determined by the
Administrator. The Administrator may combine data from different
monitors in different years for the purpose of developing a valid 1-
hour primary or secondary standard design value, if a valid design
value cannot be developed solely with the data from a single
monitor. However, data from two or more monitors in the same year at
the same site will not be combined in an attempt to meet data
completeness requirements, except if one monitor has physically
replaced another instrument permanently, in which case the two
instruments will be considered to be the same monitor, or if the
state has switched the designation of the primary monitor from one
instrument to another during the year.
* * * * *
    3. Comparisons with the NO2 NAAQS.
    3.1 The Annual Primary and Secondary NO2 NAAQS.
    (a) The annual primary and secondary NO2 NAAQS are
met at a site when the valid annual primary standard design value is
less than or equal to 53 parts per billion (ppb).
    (b) An annual primary or secondary standard design value is
valid when at least 75 percent of the hours in the year are
reported.
    (c) An annual primary or secondary standard design value based
on data that do not meet the completeness criteria stated in section
3.1(b) may also be considered valid with the approval of, or at the
initiative of, the Administrator, who may consider factors such as
monitoring site closures/moves, monitoring diligence, the
consistency and levels of the valid concentration measurements that
are available, and nearby concentrations in determining whether to
use such data.
    (d) The procedures for calculating the annual primary and
secondary standard design values are given in section 5.1 of this
appendix.
    3.2 The 1-Hour Primary and Secondary NO2 NAAQS.
    (a) The 1-hour primary or secondary NO2 NAAQS is met
at a site when the valid 1-hour primary or secondary standard design
value is less than or equal to 100 parts per billion (ppb).
    (b) An NO2 1-hour primary or secondary standard
design value is valid if it encompasses three consecutive calendar
years of complete data. A year meets data completeness requirements
when all 4 quarters are complete. A quarter is complete when at
least 75 percent of the sampling days for each quarter have complete
data. A sampling day has complete data if 75 percent of the hourly
concentration values, including state-flagged data affected by
exceptional events which have been approved for exclusion by the
Administrator, are reported.
    (c) In the case of one, two, or three years that do not meet the
completeness requirements of section 3.2(b) of this appendix and
thus would normally not be useable for the calculation of a valid 3-
year 1-hour primary or secondary standard design value, the 3-year
1-hour primary or secondary standard design value shall nevertheless
be considered valid if one of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of
three consecutive years have at least one reported hourly value, and
the design value calculated according to the procedures specified in
section 5.2 is above the level of the primary or secondary 1-hour
standard.
    (ii) (A) A 1-hour primary or secondary standard design value
that is below the level of the NAAQS can be validated if the
substitution test in section 3.2(c)(ii)(B) results in a ``test
design value'' that is below the level of the NAAQS. The test
substitutes actual ``high'' reported daily maximum 1-hour values
from the same site at about the same time of the year (specifically,
in the same calendar quarter) for unknown values that were not
successfully measured. Note that the test is merely diagnostic in
nature, intended to confirm that there is a very high likelihood
that the original design value (the one with less than 75 percent
data capture of hours by day and of days by quarter) reflects the
true under-NAAQS-level status for that 3-year period; the result of
this data substitution test (the ``test design value'', as defined
in section 3.2(c)(ii)(B)) is not considered the actual design value.
For this test, substitution is permitted only if there are at least
200 days across the three matching quarters of the three years under
consideration (which is about 75 percent of all possible daily
values in those three quarters) for which 75 percent of the hours in
the day, including state-flagged data affected by exceptional events
which have been approved for exclusion by the Administrator, have
reported concentrations. However, maximum 1-hour values from days
with less than 75 percent of the hours reported shall also be
considered in identifying the high value to be used for
substitution.
    (B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture but at least 50 percent data capture, including state-
flagged data affected by exceptional events which have been approved
for exclusion by the Administrator; if any quarter has less than 50
percent data capture then this substitution test cannot be used.
Identify for each quarter (e.g., January-March) the highest reported
daily maximum 1-hour value for that quarter, excluding state-flagged
data affected by exceptional events which have been approved for
exclusion by the Administrator, looking across those three months of
all three years under consideration. All daily maximum 1-hour values
from all days in the quarter period shall be considered when
identifying this highest value, including days with less than 75
percent data capture. If after substituting the highest non-excluded
reported daily maximum 1-hour value for a quarter for as much of the
missing daily data in the matching deficient quarter(s) as is needed
to make them 100 percent complete, the procedure in section 5.2
yields a recalculated 3-year 1-hour standard ``test design value''
below the level of the standard, then the 1-hour primary or
secondary standard design value is deemed to have passed the
diagnostic test and is valid, and the level of the standard is
deemed to have been met in that 3-year period. As noted in section
3.2(c)(i), in such a case, the 3-year design value based on the data
actually reported, not the ``test design value'', shall be used as
the valid design value. (iii) (A) A 1-hour primary or secondary
standard design value that is above the level of the NAAQS can be
validated if the substitution test in section 3.2(c)(iii)(B) results
in a ``test design value'' that is above the level of the NAAQS. The
test substitutes actual ``low'' reported daily maximum 1-hour values
from the same site at about the same time of the year (specifically,
in the same three months of the

calendar) for unknown values that were not successfully measured.
Note that the test is merely diagnostic in nature, intended to
confirm that there is a very high likelihood that the original
design value (the one with less than 75 percent data capture of
hours by day and of days by quarter) reflects the true above-NAAQS-
level status for that 3-year period; the result of this data
substitution test (the ``test design value'', as defined in section
3.2(c)(iii)(B)) is not considered the actual design value. For this
test, substitution is permitted only if there are a minimum number
of available daily data points from which to identify the low
quarter-specific daily maximum 1-hour values, specifically if there
are at least 200 days across the three matching quarters of the
three years under consideration (which is about 75 percent of all
possible daily values in those three quarters) for which 75 percent
of the hours in the day have reported concentrations. Only days with
at least 75 percent of the hours reported shall be considered in
identifying the low value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture. Identify for each quarter (e.g., January-March) the
lowest reported daily maximum 1-hour value for that quarter, looking
across those three months of all three years under consideration.
All daily maximum 1-hour values from all days with at least 75
percent capture in the quarter period shall be considered when
identifying this lowest value. If after substituting the lowest
reported daily maximum 1-hour value for a quarter for as much of the
missing daily data in the matching deficient quarter(s) as is needed
to make them 75 percent complete, the procedure in section 5.2
yields a recalculated 3-year 1-hour standard ``test design value''
above the level of the standard, then the 1-hour primary or
secondary standard design value is deemed to have passed the
diagnostic test and is valid, and the level of the standard is
deemed to have been exceeded in that 3-year period. As noted in
section 3.2(c)(i), in such a case, the 3-year design value based on
the data actually reported, not the ``test design value'', shall be
used as the valid design value.
    (d) A 1-hour primary or secondary standard design value based on
data that do not meet the completeness criteria stated in 3.2(b) and
also do not satisfy section 3.2(c), may also be considered valid
with the approval of, or at the initiative of, the Administrator,
who may consider factors such as monitoring site closures/moves,
monitoring diligence, the consistency and levels of the valid
concentration measurements that are available, and nearby
concentrations in determining whether to use such data.
    (e) The procedures for calculating the 1-hour primary and
secondary standard design values are given in section 5.2 of this
appendix.
    4. Rounding Conventions.
    4.1 Rounding Conventions for the Annual Primary and Secondary
NO2 NAAQS.
* * * * *
    (b) The annual primary or secondary standard design value is
calculated pursuant to section 5.1 and then rounded to the nearest
whole number or 1 ppb (decimals 0.5 and greater are rounded up to
the nearest whole number, and any decimal lower than 0.5 is rounded
down to the nearest whole number).
    4.2 Rounding Conventions for the 1-hour Primary and Secondary
NO2 NAAQS.
* * * * *
    (c) The 1-hour primary or secondary standard design value is
calculated pursuant to section 5.2 and then rounded to the nearest
whole number or 1 ppb (decimals 0.5 and greater are rounded up to
the nearest whole number, and any decimal lower than 0.5 is rounded
down to the nearest whole number).
    5. Calculation Procedures for the Primary and Secondary
NO2 NAAQS.
    5.1 Procedures for the Annual Primary and Secondary
NO2 NAAQS.
* * * * *
    (b) The annual primary or secondary standard design value for a
site is the valid annual mean rounded according to the conventions
in section 4.1.
    5.2 Calculation Procedures for the 1-hour Primary and Secondary
NO2 NAAQS.
* * * * *
    (b) The 1-hour primary or secondary standard design value for a
site is the mean of the three annual 98th percentile values, rounded
according to the conventions in section 4.
* * * * *

    5. Appendix T is amended as follows:
    a. by revising paragraph 1.(a),
    b. by revising the definition of ``Design values'' under paragraph
1.(c),
    c. by revising paragraph 2.(b),
    d. by revising paragraphs 3.(a) through (e),
    e. by revising paragraph 4.(c), and
    f. by revising paragraph 5.(b) to read as follows:

Appendix T to Part 50--Interpretation of the Primary and Secondary
National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur
Dioxide)

    1. General.
    (a) This appendix explains the data handling conventions and
computations necessary for determining when the primary and
secondary national ambient air quality standards for Oxides of
Sulfur as measured by Sulfur Dioxide (``SO2 NAAQS'')
specified in Sec.  50.17 and Sec.  50.5 (b), respectively, are met
at an ambient air quality monitoring site. Sulfur dioxide
(SO2) is measured in the ambient air by a Federal
reference method (FRM) based on appendix A-1 or A-2 to this part or
by a Federal equivalent method (FEM) designated in accordance with
part 53 of this chapter. Data handling and computation procedures to
be used in making comparisons between reported SO2
concentrations and the levels of the SO2 NAAQS are
specified in the following sections.
* * * * *
    (c) * * *
    Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
specified in section 5 of this appendix. The design value for the
primary and secondary 1-hour NAAQS is the 3-year average of annual
99th percentile daily maximum 1-hour values for a monitoring site
(referred to as the ``1-hour primary standard design value'').
* * * * *
    2. Requirements for Data Used for Comparisons With the
SO2 NAAQS and Data Reporting Considerations.
* * * * *
    (b) Data from two or more monitors from the same year at the
same site reported to EPA under distinct Pollutant Occurrence Codes
shall not be combined in an attempt to meet data completeness
requirements. The Administrator will combine annual 99th percentile
daily maximum concentration values from different monitors in
different years, selected as described here, for the purpose of
developing a valid 1-hour primary or secondary standard design
value. If more than one of the monitors meets the completeness
requirement for all four quarters of a year, the steps specified in
section 5(a) of this appendix shall be applied to the data from the
monitor with the highest average of the four quarterly completeness
values to derive a valid annual 99th percentile daily maximum
concentration. If no monitor is complete for all four quarters in a
year, the steps specified in section 3(c) and 5(a) of this appendix
shall be applied to the data from the monitor with the highest
average of the four quarterly completeness values in an attempt to
derive a valid annual 99th percentile daily maximum concentration.
This paragraph does not prohibit a monitoring agency from making a
local designation of one physical monitor as the primary monitor for
a Pollutant Occurrence Code and substituting the 1-hour data from a
second physical monitor whenever a valid concentration value is not
obtained from the primary monitor; if a monitoring agency
substitutes data in this manner, each substituted value must be
accompanied by an AQS qualifier code indicating that substitution
with a value from a second physical monitor has taken place.
* * * * *
    3. Comparisons with the 1-hour Primary and Secondary
SO2 NAAQS.
    (a) The 1-hour primary or secondary SO2 NAAQS is met
at an ambient air quality monitoring site when the valid 1-hour
primary or secondary standard design value is less than or equal to
75 parts per billion (ppb).
    (b) An SO2 1-hour primary or secondary standard
design value is valid if it encompasses three consecutive calendar
years of complete data. A year meets data completeness requirements
when all 4 quarters are complete. A quarter is complete when at
least 75 percent of the sampling days for each quarter have complete
data. A sampling day has complete data if 75 percent of the hourly
concentration values, including State-flagged data affected by
exceptional events which have been approved for exclusion by the
Administrator, are reported.

    (c) In the case of one, two, or three years that do not meet the
completeness requirements of section 3(b) of this appendix and thus
would normally not be useable for the calculation of a valid 3-year
1-hour primary or secondary standard design value, the 3-year 1-hour
primary or secondary standard design value shall nevertheless be
considered valid if one of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of
three consecutive years have at least one reported hourly value, and
the design value calculated according to the procedures specified in
section 5 is above the level of the primary or secondary 1-hour
standard.
    (ii) (A) A 1-hour primary or secondary standard design value
that is equal to or below the level of the NAAQS can be validated if
the substitution test in section 3(c)(ii)(B) results in a ``test
design value'' that is below the level of the NAAQS. The test
substitutes actual ``high'' reported daily maximum 1-hour values
from the same site at about the same time of the year (specifically,
in the same calendar quarter) for unknown values that were not
successfully measured. Note that the test is merely diagnostic in
nature, intended to confirm that there is a very high likelihood
that the original design value (the one with less than 75 percent
data capture of hours by day and of days by quarter) reflects the
true under-NAAQS-level status for that 3-year period; the result of
this data substitution test (the ``test design value'', as defined
in section 3(c)(ii)(B)) is not considered the actual design value.
For this test, substitution is permitted only if there are at least
200 days across the three matching quarters of the three years under
consideration (which is about 75 percent of all possible daily
values in those three quarters) for which 75 percent of the hours in
the day, including State-flagged data affected by exceptional events
which have been approved for exclusion by the Administrator, have
reported concentrations. However, maximum 1-hour values from days
with less than 75 percent of the hours reported shall also be
considered in identifying the high value to be used for
substitution.
    (B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved
for exclusion by the Administrator; if any quarter has less than 50
percent data capture then this substitution test cannot be used.
Identify for each quarter (e.g., January-March) the highest reported
daily maximum 1-hour value for that quarter, excluding State-flagged
data affected by exceptional events which have been approved for
exclusion by the Administrator, looking across those three months of
all three years under consideration. All daily maximum 1-hour values
from all days in the quarter period shall be considered when
identifying this highest value, including days with less than 75
percent data capture. If after substituting the highest reported
daily maximum 1-hour value for a quarter for as much of the missing
daily data in the matching deficient quarter(s) as is needed to make
them 100 percent complete, the procedure in section 5 yields a
recalculated 3-year 1-hour standard ``test design value'' less than
or equal to the level of the standard, then the 1-hour primary or
secondary standard design value is deemed to have passed the
diagnostic test and is valid, and the level of the standard is
deemed to have been met in that 3-year period. As noted in section
3(c)(i), in such a case, the 3-year design value based on the data
actually reported, not the ``test design value'', shall be used as
the valid design value.
    (iii) (A) A 1-hour primary or secondary standard design value
that is above the level of the NAAQS can be validated if the
substitution test in section 3(c)(iii)(B) results in a ``test design
value'' that is above the level of the NAAQS. The test substitutes
actual ``low'' reported daily maximum 1-hour values from the same
site at about the same time of the year (specifically, in the same
three months of the calendar) for unknown hourly values that were
not successfully measured. Note that the test is merely diagnostic
in nature, intended to confirm that there is a very high likelihood
that the original design value (the one with less than 75 percent
data capture of hours by day and of days by quarter) reflects the
true above-NAAQS-level status for that 3-year period; the result of
this data substitution test (the ``test design value'', as defined
in section 3(c)(iii)(B)) is not considered the actual design value.
For this test, substitution is permitted only if there are a minimum
number of available daily data points from which to identify the low
quarter-specific daily maximum 1-hour values, specifically if there
are at least 200 days across the three matching quarters of the
three years under consideration (which is about 75 percent of all
possible daily values in those three quarters) for which 75 percent
of the hours in the day have reported concentrations. Only days with
at least 75 percent of the hours reported shall be considered in
identifying the low value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture. Identify for each quarter (e.g., January-March) the
lowest reported daily maximum 1-hour value for that quarter, looking
across those three months of all three years under consideration.
All daily maximum 1-hour values from all days with at least 75
percent capture in the quarter period shall be considered when
identifying this lowest value. If after substituting the lowest
reported daily maximum 1-hour value for a quarter for as much of the
missing daily data in the matching deficient quarter(s) as is needed
to make them 75 percent complete, the procedure in section 5 yields
a recalculated 3-year 1-hour standard ``test design value'' above
the level of the standard, then the 1-hour primary or secondary
standard design value is deemed to have passed the diagnostic test
and is valid, and the level of the standard is deemed to have been
exceeded in that 3-year period. As noted in section 3(c)(i), in such
a case, the 3-year design value based on the data actually reported,
not the ``test design value'', shall be used as the valid design
value.
    (d) A 1-hour primary or secondary standard design value based on
data that do not meet the completeness criteria stated in 3(b) and
also do not satisfy section 3(c), may also be considered valid with
the approval of, or at the initiative of, the Administrator, who may
consider factors such as monitoring site closures/moves, monitoring
diligence, the consistency and levels of the valid concentration
measurements that are available, and nearby concentrations in
determining whether to use such data.
    (e) The procedures for calculating the 1-hour primary or
secondary standard design values are given in section 5 of this
appendix.
    4. Rounding Conventions for the 1-hour Primary and Secondary
SO2 NAAQS.
* * * * *
    (c) The 1-hour primary or secondary standard design value is
calculated pursuant to section 5 and then rounded to the nearest
whole number or 1 ppb (decimals 0.5 and greater are rounded up to
the nearest whole number, and any decimal lower than 0.5 is rounded
down to the nearest whole number).
    5. Calculation Procedures for the 1-hour Primary and Secondary
SO2 NAAQS.
* * * * *
    (b) The 1-hour primary or secondary standard design value for an
ambient air quality monitoring site is the mean of the three annual
99th percentile values, rounded according to the conventions in
section 4.

[FR Doc. 2011-18582 Filed 7-29-11; 8:45 am]
BILLING CODE 6560-50-P