FRMs, as set forth in several appendices to 40 CFR Part 50, serve
either or both of two primary purposes. The first is to provide a
specified, definitive methodology for routinely measuring
concentrations of various ambient air pollutants for comparison to the
NAAQS in Part 50 and for other air monitoring objectives. The second is
to provide a standard of comparison for determining equivalence to the
specified reference method of alternative and perhaps more practical
pollutant measurement methods (FEMs) that can be used in lieu of the
FRM for routine monitoring.
    Some of the FRMs contained in appendices to Part 50 (such as the
current SO2 FRM) are manual methods that are completely
specified within their respective appendices. Others (such as the ozone
FRM) are in the form of a measurement principle and associated
calibration procedure that must be implemented in a commercial FRM
analyzer model. Such FRM analyzers must be tested and shown to meet
explicit performance and other requirements that are set forth in 40
CFR Part 53 (Ambient Air Monitoring Reference and Equivalent Methods).
Each of these analyzer models is considered to be an FRM only upon
specific designation as such by EPA under the provisions of Part 53.
    From time to time, as pollutant measurement technology advances,
the reference methods in these Part 50 appendices need to be assessed
to determine if improved or more suitable measurement technology has
become available to better meet current FRM needs as well as potential
future FRM requirements. Such new technology can either be presented to
EPA for evaluation by an FEM applicant under Sec.  53.16, or (as in
this case) EPA can originate the process itself as provided in Sec.
53.7. If, after reviewing a new methodology, the Administrator
determines that the new methodology is substantially superior, Sec.
53.16 of Part 53 provides for supersession of FRMs under these
circumstances.
    The FRM for measuring SO2 in the ambient air was
promulgated on April 30, 1971 (36 FR 8186), in conjunction with EPA's
establishment (originally as 42 CFR Part 410) of the first national
ambient air quality standards (NAAQS) for six pollutants (including
sulfur dioxide) as now set forth in 40 CFR Part 50. This SO2
FRM is specified in Appendix A of Part 50 and identified as the
pararosaniline method. It is a manual, wet-chemical method requiring
sample air to be bubbled through an absorbing reagent
(tetrachloromecurate), which is then returned to a laboratory for
chemical analysis. At the time of its promulgation, the method was
considered the best available method and was in considerable use for
monitoring SO2 in the air. However, newly developed
automated continuous analyzers approved as FEMs rapidly supplanted use
of this manual method for air monitoring in the U.S. By the 1990's, the
FRM was no longer used at all in domestic air monitoring (EPA, 2009b),
and since then the method has been used mainly as a comparison
reference method for the testing and designation of candidate FEMs for
SO2 in accordance with 40 CFR Part 53.
    The pararosaniline manual FRM has served its role for many years,
but now a better method is needed that more fully meets the needs of
contemporary SO2 monitoring. The existing FRM is primarily a
24-hour integrated method, whereas a 1-hour SO2 FRM
measurement capability would be needed to implement the proposed 1-hour
SO2 NAAQS. Existing FEM analyzers can and do provide 1-hour
measurement capability, but EPA wishes to facilitate the approval of
new monitoring technologies as well. While the existing manual
reference method can produce 1-hour averages, it is clearly impractical
for routine use in making 1-hour SO2 measurements. Also, the
1-hour mode of the manual method is not a good standard for approving
new FEMs with 1-hour measurement capability, because scores of 1-hour
measurements would be needed during equivalency testing. Further, the
existing FRM is cumbersome to use and requires a mercury-containing
reagent that is potentially hazardous to operators or to the
environment if it is mishandled.
    These operational shortcomings suggest that the existing FRM should
be replaced with a more suitable

methodology. Fortunately, the existing SO2 instrumental
measurement technique based on the UVF measurement principle offers
superior performance and substantial operational advantages, as
reported in an FRM evaluation for EPA produced by Research Triangle
Institute (Rickman, 1987). Analyzers using this technique can well
provide the needed detection limits, precision, and accuracy and
fulfill other purposes of an FRM, including use as an appropriate
standard of reference for testing and designation of new FEM analyzers.
After reviewing these factors, EPA has determined that a new, automated
FRM for SO2 based on the UVF measurement principle should be
adopted. EPA is proposing to add the new FRM in a new Appendix A-1 to
Part 50.
    In association with the proposed new FRM, EPA is also proposing to
update the performance-based requirements for FEM SO2
analyzers currently in 40 CFR Part 53. These requirements were
established in the 1970's, based primarily on the wet-chemical
measurement technology available at that time. Those initial
requirements have become significantly outdated and should be modified
to match current technology, particularly because they would apply to
new FRM analyzers under the proposed new FRM. The better instrumental
performance available with the proposed new UVF reference method
technique allows the performance requirements for SO2 in 40
CFR Part 53 to be made more stringent for both FRM and FEM analyzers
(EPA, 2009c).
2. Proposed new FRM measurement technique
    Since the 1970's, a variety of measurement principles have been
successfully used to produce continuous analyzers for SO2,
some of which have qualified for EPA designation as equivalent methods
(found at http://www.epa.gov/ttn/amtic/files/ambient/criteria/
reference-equivalent-methods-list.pdf). These include methods based on
ultraviolet fluorescence, flame photometry, differential optical
absorption spectroscopy, coulometric and conductometric techniques, and
second derivative ultraviolet absorption spectrometry. Although some of
these techniques saw considerable utilization in the 1970's, their use
dwindled after the introduction of UVF analyzers because of various
shortcomings such as non-specificity for SO2, susceptibility
to interferences, marginal performance, or operational disadvantages
(e.g. requiring hydrogen gas or wet-chemical reagents). Consequently,
the UVF technique has emerged as the clearly dominant measurement
technique for SO2, providing a majority of the domestic air
monitoring data obtained over the last three decades, and virtually
100% of the current monitoring data (EPA, 2009b). As the proposed new
reference method, the UVF technique would be specified in performance-
based form, with a generic reference measurement principle and
associated calibration procedure in a new Appendix A-1 to 40 CFR Part
50. Associated performance requirements applicable to candidate UVF FRM
analyzers would be specified in 40 CFR Part 53. This form of the FRM is
consistent with that specified for FRMs for CO, O3, and
NO2 in Appendices C, D, and F (respectively) to 40 CFR Part
50.
    Reasonable commercial availability of high quality analyzers
utilizing the reference measurement principle that can be offered by
multiple manufacturers, ideally over many years, is an important aspect
of any new reference measurement principle. EPA has designated more
than a dozen UVF analyzers as equivalent to the current reference
method over the last 30 years. Although most of the early model UVF
analyzers are no longer in production, many have been replaced by
redesigned and improved models, and entirely new models continue to
become designated as FEMs. Currently, more than a half-dozen designated
FEM models offered by multiple manufacturers are commercially
available. The widespread use of the method has three important
technical advantages for an FRM: (1) A variety of analyzer models are
available and will likely continue to be available from multiple
manufacturers for many years to come, (2) analyzer manufacturers have
had (and continue to have) a strong marketing incentive to improve,
refine, perfect, and continue to market such analyzers, and (3) the
number of accumulated UVF field monitoring datasets (including related
QC data) provide an extensive, available performance track record that
can be evaluated to assess the performance of the analyzers in actual
monitoring use.
    The only other equivalent method measurement technique that has
even a small representation among currently available FEM analyzers is
the differential optical absorption spectrometric method. The open-path
nature of this method (measurement of pollutants in the open air
without a closed measurement cell) is not suitable for many of the
purposes of a reference method. Further, this method is only available
as two product models from two manufacturers, and very few State and
local monitoring agencies are using such analyzers.
    The UVF technique is not without some imperfections as a reference
method. Analyzers utilizing the technique are, to a limited degree,
susceptible to interference from aromatic hydrocarbon species and
potentially other compounds at existing levels or levels that may occur
at many monitoring sites. However, analyzer manufacturers have
developed very effective ways to reduce these potential limitations,
including careful selection of wavelengths, optimum optical design, and
sample air scrubbers, such that typical interferences are minimal.
    All UVF analyzers that have been designated as SO2 FEMs
have been tested and shown to meet the existing performance
requirements of 40 CFR Part 53. These include required testing for both
positive and negative potential interferents, minimum level of
measurement, zero and span drift, and precision. The results of these
tests have been submitted to EPA and are in the archived FEM
applications for these analyzers. Many newer models substantially
exceed those requirements, with sensitivities down to less than 1 ppb,
and typically commensurate levels of signal noise, precision, and zero
drift (EPA, 2009c). In addition, UVF analyzers can accommodate a wide
range of concentration measurement ranges. They are quite well suited
to measure high, short-term SO2 concentrations near sources,
and they can also be used to measure trace-level concentrations in
clean areas.
    For these reasons, EPA has decided to propose a new automated
SO2 FRM based on the UVF measurement technology. EPA is
confident that commercially available UVF instrument models would
provide capability to serve not only current monitoring and FRM
applications but anticipated monitoring and FRM needs well into future
years. EPA solicits comment on the proposal to promulgate an FRM for
SO2 that would be an automated method based on ultraviolet
fluorescence, which would be specified in the form of a reference
measurement principle and calibration procedure, as stated here, and
contained in a new Appendix A-1 to 40 CFR Part 50.
3. Technical description of the proposed UVF FRM
    The proposed new reference method is based on automated measurement
of the intensity of the characteristic fluorescence released by
SO2 in an ambient air sample when irradiated by ultraviolet
light. The SO2 fluorescence produced is also in the
ultraviolet range,

but is measured at a longer wavelength. An analyzer implementing this
measurement principle would include a measurement cell, an ultraviolet
light source of appropriate wavelength, an ultraviolet detector system
with appropriate wavelength sensitivity, and a pump and flow control
system for sampling the ambient air. Generally, the analyzer also
requires a means to reduce concentrations of aromatic hydrocarbons and
possibly other compounds (depending on target wavelengths and other
parameters used) in the air sample to control for potential measurement
interferences. The analyzer is calibrated by referencing the
instrumental fluorescence measurements to SO2 standard
concentrations traceable to a NIST (National Institute of Standards and
Technology) primary standard for SO2. This generic
description of the FRM would be contained in Appendix A-1 to 40 CFR
Part 50 and would be coupled with explicit analyzer performance
requirements specified in Subpart B of 40 CFR Part 53. To qualify as an
FRM, an analyzer model based on this principle would have to be tested
in accordance with test procedures in Subpart B Part 53 and shown to
meet the performance requirements specified in that Subpart. EPA could
then designate the analyzer model as an FRM analyzer, as provided in
Part 53.
4. Implications to air monitoring networks
    Under Sec.  53.16, EPA must consider the benefits of a proposed
supersession of an existing reference method, the potential economic
consequences of such action for State and local monitoring agencies,
and any disruption of State and local air quality monitoring programs
that might result from such action. Supersession of an existing
reference method, as described in Sec.  53.16, presumes that the
existing FRM would be deleted from Part 50 and replaced with a new FRM,
and that all equivalent methods based on the old FRM would be
cancelled. In the case of SO2, essentially all current
domestic air monitoring activity is carried out using FEM UVF
analyzers. Cancellation of the FEM designations of all these analyzers
now would be potentially very disruptive to State, local, and other
monitoring networks, even though Sec.  53.16 alludes to a possible
transition period to allow monitoring agencies some period of time to
replace cancelled FEM analyzers.
    EPA recognizes that these existing SO2 FEMs are
providing monitoring data that are adequate for the current and the
proposed SO2 NAAQS and for many other purposes, and there
appears to be no need or purpose served by their withdrawal. Therefore,
in this case, EPA proposes instead to retain the existing manual FRM
for SO2 and to promulgate an entirely new automated FRM for
SO2. The new FRM description would be contained in a new
Appendix A-1 to 40 CFR Part 50, and the existing FRM would be re-
codified as Appendix A-2 to 40 CFR Part 50, with both reference methods
coexisting. Following adoption of the new Appendix A-1, new language
proposed for Sec.  53.2(a) and (b) would provide that new FRM and FEM
analyzers for SO2 be designated only with reference to the
proposed new Appendix A-1. At the same time, retention of the existing
SO2 reference method will preclude the need to cancel the
designations of all existing FEMs for SO2.
    Under this proposal, no monitoring agencies would be required to
change their SO2 monitoring procedures as a result of the
proposed changes, so it would have no economic costs for implementation
and no disruptive effects on state, local, or tribal air quality
monitoring programs. Further, since UVF FEM analyzers have been in
dominant use for many years, no bias or discontinuity in any aspect of
the monitoring data obtained subsequently would result from the
proposed change in the SO2 reference methodology.
    In conjunction with the proposed new FRM, EPA is also proposing to
adopt updated performance requirements in 40 CFR Part 53, applicable to
both FRM and FEM analyzers, consistent with the automated methods and
in anticipation of future NAAQS needs. This would ensure that, going
forward, all new SO2 monitors would have improved
performance. EPA believes that the proposal to retain the existing FRM
while adding the new FRM would provide for a smooth, evolutionary
transition from the older, manual FRM to the new, modern, automated FRM
and FEM technology and the associated better performance requirements,
with no immediate impact to current monitoring activities. For purposes
of comparing SO2 monitoring data to the SO2
NAAQS, the EPA believes that the UVF FEMs are appropriate for continued
use under the current standards and under the option being considered
for a new 1-hour averaged primary SO2 NAAQS. After several
years, at a time when either a new SO2 NAAQS would require
higher monitoring data quality or there would be no further potential
for disruption to monitoring agencies, EPA would plan to withdraw the
older reference method and it's associated FEMs.
5. Proposed revisions to 40 CFR Part 53
    Several amendments associated with the proposed new SO2
reference measurement principle are proposed to 40 CFR Part 53. The
most significant of these would update the performance requirements for
both new FRM and new FEM analyzers for SO2, as set forth in
proposed revised Table B-1. Based on typical performance capabilities
available for UVF analyzers, EPA is proposing to reduce the allowable
noise from 5 ppb to 1 ppb, the lower detectable limit from 10 ppb to 2
ppb, and the allowable interference equivalent limits from 20 ppb to 5 ppb for each interferent and from 60 ppb
to 20 ppb for the total of all interferents. Also, EPA proposes to
change the allowable zero drift limits from 20 ppb to
4 ppb, and to delete the specified limits for span drift at
20% of the upper range limit (URL) for SO2 analyzers. Review
of FEM analyzer performance test results has shown that the 20% URL
span limit requirements are unnecessary because drift performance
requirements are adequately covered by the zero drift and 80% URL span
drift limits. EPA proposes to change the lag time allowed from 20 to 2
minutes and change the rise and fall time limits from 15 to 2 minutes.
For precision, EPA proposes to change the form of the precision limit
specifications from ppm to percent (of the URL) for SO2
analyzers and to set the limit at 2 percent for both 20% and 80% of the
URL. Two percent is equivalent to 10 ppb for the standard (500 ppb)
range, which is equivalent to the existing limit value for precision at
20% of the URL, but would be a reduction from 15 ppb to 10 ppb for the
limit value at 80% of the URL. This change in units from ppm (or ppb as
given here) to percent makes the requirement responsive to higher and
lower measurement ranges. Also, a new footnote is proposed to be added
to Table B-1 to clarify how noise tests are to be carried out for
candidate analyzers having an adjustable or automatic time constant
capability.
    EPA recognizes that SO2 monitoring needs can vary
widely, from monitoring background levels in pristine areas to
measuring short-term (1-hour) or even very short-term (less than 1-
hour) high-level averages in the vicinity of substantial sources of
SO2. To address the need for more sensitive, lower
measurement ranges for SO2 analyzers, EPA is proposing a
separate set of performance requirements that would apply specifically
to narrower measurement ranges, i.e. ranges extending from zero to
concentrations

less than 0.5 ppm. These additional requirements are listed in the
proposed revised Table B-1. A candidate analyzer that meets the Table
B-1 requirements for the standard measurement range (0 to 0.5 ppm)
could optionally have one or more narrower ranges included in its FRM
or FEM designation by further testing to show that it meets these
supplemental, narrower-range requirements.
    At the other (high) end of the concentration measurement spectrum,
another related change proposed for Sec.  53.20 would allow optional
designation of measurement ranges for SO2 up to 2 ppm rather
than 1 ppm as is now permitted, and designation of these higher ranges
would be applicable to both FRM and FEM analyzers. Such higher ranges
are often needed for measurement of short-interval SO2
averages. Finally, EPA is proposing to clarify in Sec.  53.20 that
optional testing for auxiliary higher or lower measurement ranges (for
all gaseous pollutants) may include tests for only some of the
performance parameters, since the test results for the other
performance parameters carried out for the standard measurement range
would be technically applicable and adequate for the higher and/or
lower ranges as well.
    EPA believes that these changes in performance requirements are
appropriate, based on analyzer performance data available from analyzer
manuals and recent FEM applications. EPA solicits comments especially
from UVF instrument users and manufacturers on these proposed changes,
particularly in regard to whether they are reasonable, appropriate, of
significant benefit, and achievable without undue cost. Comments are
also requested on such issues as the trade off between a high
measurement range and the need for adequate resolution at
concentrations near the annual NAAQS, a similar trade off between noise
level and response time (some analyzers allow these parameters to be
adjusted by the operator or may adjust them automatically based on the
rate of change of the concentration level), and whether such
performance parameters should be addressed in more detail in 40 CFR
Part 53. In particular, should SO2 analyzer requirements
address the potential need for faster measurement response time to
permit more accurate monitoring of short-term intervals such as 5-
minute or 10-minute averages, and are the special performance
requirements EPA is proposing for measuring very low levels (trace
levels) of SO2 appropriate and effective?
    Another significant change proposed to 40 CFR Part 53 would add
some low and medium level 1-hour comparability tests to the Subpart C
comparability test requirements, as specified in Table C-1. These would
help to ensure that the 1-hour measurement performance of candidate
FEMs are adequate, relative to the FRM. Also, EPA proposes to amend
Table A-1 in Subpart A to reflect the new FRM description in proposed
new Appendix A-1 of 40 CFR Part 50. This table would also be amended to
correct some printing errors in the current table as well as to add new
entries related to the new FRM for lead in PM10 that was
recently promulgated. Other minor changes would be necessary in the
wording of a few sections of Subparts A and B due to the proposed
change in the nature of the SO2 FRM from a manual to an
automated method or to update the language. These changes are reflected
in the proposed regulatory text section of this notice.
    EPA proposes additional minor revisions to Tables B-2 and B-3 of
Subpart B. The changes proposed to Table B-2 would update some of the
analytical methods for generation or verification of SO2 and
interferent test concentrations and their associated references.
Similarly, Table B-3 would be updated to add a specific listing for
ultraviolet fluorescent methods and to add a few additional interferent
test species for some other measurement techniques that have been found
from experience to be needed.

B. Network design

1. Background
    The basic objectives of an ambient monitoring network, as noted in
40 CFR Part 58 Appendix D, include (1) providing air pollution data to
the general public in a timely manner, (2) supporting compliance with
ambient air quality standards and emissions strategy development, and
(3) providing support for air pollution research. The SO2
network was originally deployed to support implementation of the
SO2 NAAQS established in 1971. Although the SO2
standard was established in 1971, EPA did not establish uniform minimum
monitoring requirements for SO2 monitoring until May 1979.
From the time of the implementation of the 1979 monitoring rule,
through 2008, the SO2 network has steadily decreased in size
from approximately 1496 sites in 1980 to the approximately 488 sites
operating in 2008 (Watkins and Thompson, 2009). The reduction in
network size is due in part to the change in the source sector
contributions to the overall SO2 inventory and the general
decline of ambient SO2 levels over time. In the early
decades of the SO2 network, particularly the 1970s, there
was a wider variety of more ubiquitous SO2 sources in urban
areas, including residential coal and oil furnaces, when compared to
the stationary source, electric generating unit (EGU)-dominated
inventories of today (see below). The situation in the 1970s led to a
network design keyed on population, an appropriate approach at the time
considering the close proximity of sources and people, particularly in
urban, residential settings (Watkins and Thompson, 2009).
    An analysis of the approximately 488 monitoring sites comprising
the current (2008) SO2 monitoring network indicates that
just under half (46%) of the sites in the current SO2
network are reported to be for the assessment of concentrations for
general population exposure. As for the present day inventory, the 2005
NEI (http://www.epa.gov/ttn/chief/net/2005inventory.html) indicates
that SO2 emissions from EGUs contribute approximately 70% of
the anthropogenic SO2 emissions in the U.S. However, only
approximately one third (35%) of the network is reported to be
addressing locations of maximum (highest) concentrations, likely linked
to a specific source or group of sources such as EGUs.
    The current network supports the reporting of 1-hour data to EPA's
Air Quality System (AQS) database, as required in Sec.  58.12 of 40 CFR
Part 58, since the network utilizes the continuous UVF FEM, which can
provide time-resolved data averaged over periods as short as several
minutes. The routine submittal of hourly data by state, local, and
tribal air monitoring agencies to AQS is suitable for use in comparison
to both of the current primary 24-hour and annual NAAQS. There are a
few monitoring agencies who also report 5-minute data voluntarily to
AQS.
    The current network is sited at a variety of spatial scales;
however a majority of the network, just over sixty percent, is sited at
the neighborhood spatial scale\32\ (Watkins and Thompson,

2009). Although there are 488 SO2 monitors operating in the
network, there are currently no minimum monitoring requirements for
SO2 in 40 CFR part 58 Appendix D, other than the following
three: (1) SO2 must be monitored at National Core (NCore)
monitoring sites (discussed below), (2) the EPA Regional Administrator
must approve the removal of any existing monitors, and (3) any ongoing
SO2 monitoring must have at least one monitor sited to
measure the maximum concentration of SO2 in that area.
---------------------------------------------------------------------------

    \32\ Spatial scales are defined in 40 CFR Part 58 Appendix D,
Section 1.2, where the scales of representativeness include:
    1. Microscale--Defines the concentration in air volumes
associated with area dimensions ranging from several meters up to
about 100 meters.
    2. Middle scale--Defines the concentration typical of areas up
to several city blocks in size, with dimensions ranging from about
100 meters to 0.5 kilometers.
    3. Neighborhood scale--Defines concentrations within some
extended area of the city that has relatively uniform land use with
dimensions in the 0.5 to 4.0 kilometers range.
    4. Urban scale--Defines concentrations within an area of city-
like dimensions, on the order of 4 to 50 kilometers. Within a city,
the geographic placement of sources may result in there being no
single site that can be said to represent air quality on an urban
scale. The neighborhood and urban scales have the potential to
overlap in applications that concern secondarily formed or
homogeneously distributed air pollutants.
    5. Regional scale--Defines usually a rural area of reasonably
homogeneous geography without large sources, and extends from tens
to hundreds of kilometers.
---------------------------------------------------------------------------

    The SO2 monitors that are required as part of the
National Core monitoring network (NCore) were not required solely for
providing direct support of the SO2 NAAQS. The monitoring
rule promulgated in 2006 (71 FR 61236) and codified at 40 CFR Part 58
and its Appendices established the NCore multi-pollutant network
requirement to support integrated air quality management data needs.
Further, NCore is intended to establish long-term sites providing data
for air quality trends analysis, model evaluation, and, for urban
sites, tracking metropolitan air quality statistics. To do this, NCore
sites are required to measure various pollutants, including
SO2, but are not sited to monitor maximum concentrations of
SO2. NCore sites provide data representing concentrations at
the broader neighborhood and urban spatial scales. The data from the
NCore sites will be compared to the NAAQS although, as noted earlier,
NAAQS comparisons are not the primary objective of NCore. The NCore
network, which will be fully implemented by January 1, 2011, will
result in approximately 83 sites, each with an SO2 monitor,
with approximately 60 sites being located in urban areas.
    As set out in detail in section II.B of this notice, there is a
causal relationship between short-term SO2 exposure and
respiratory morbidity, with ''short-term'' meaning exposures from 5-10
minutes up to and including 24 hours. This finding is based primarily
on results from controlled human exposure studies of 5-10 minutes as
well as epidemiologic studies using mostly 1-hour daily maximum and 24-
hour average SO2 concentrations. Importantly, the ISA
described the controlled human exposure studies of 5-10 minutes as
being the ``definitive evidence'' for this conclusion (ISA, section
5.2). In addition, when describing epidemiologic studies observing
positive associations between ambient SO2 and respiratory
symptoms, the ISA stated ``that it is possible that these associations
are determined in large part by peak exposures within a 24-hour
period'' (ISA, section 5.2 at p. 5-5). The ISA also stated that the
respiratory effects following 5- to 10-minute SO2 exposures
in controlled human exposure studies provide a basis for a progression
of respiratory morbidity that could result in increased ED visits and
hospital admissions (ISA, section 5.2). Thus, the monitoring network to
support the proposed NAAQS should be focused on identifying the
expected maximum short-term concentrations in any particular area.
    The ISA (Section 2.1) indicates that point (i.e., stationary)
sources account for approximately 95% of the total anthropogenic
SO2 emissions in the U.S. According to the 2005 National
Emissions Inventory (http://www.epa.gov/ttn/chief/net/
2005inventory.html), electrical generating units (EGUs) emit
approximately 70% of the anthropogenic SO2 emissions in the
U.S. The 2005 NEI indicates that the total anthropogenic emission
inventory of SO2 is approximately 14,742 thousand tons per
year. Of those 14,742 thousand tons per year of emitted SO2,
approximately 85% were emitted by stationary sources that emit 100 or
more tons per year (comprising approximately 1,928 of the 32,988
facilities listed in the 2005 NEI). This information indicates that a
relatively small number (6%) of all SO2 emitting stationary
sources are responsible for a large majority of the total anthropogenic
emissions inventory (85%) in the U.S. Therefore, monitors sited to
reflect locations of expected maximum concentrations should be
primarily oriented towards locations influenced by one or a cluster of
high SO2 emitting sources.
    As noted in the key observations of the exposure analysis of the
REA (REA, Section 8.12), there are a variety of factors that influence
overall population exposure to ground-level concentrations in a given
area, including population density and proximity to sources, emissions
density in an area, and source specific emission parameters such as
stack height, among other factors. In general, however, it is expected
that any short-term peaks that may occur in an area are more likely to
occur nearer to a source or sources, or in an area where multiple
sources are significantly contributing to increased ground level
concentrations (an area with high emissions density).\33\ Given that
maximum ground-level concentrations of SO2 are usually
directly traceable to specific sources, or a cluster of sources, the
network design should support implementation of the proposed 1-hour
SO2 NAAQS by targeting maximum ground-level concentrations
in areas of both higher population and higher emissions.
---------------------------------------------------------------------------

    \33\ There is inherent variability in where peak ground level
concentrations may occur in space and time from an individual source
or group of sources, due to multiple factors including tons emitted,
stack height, meteorology, among others. These factors are discussed
further in the Monitor Placement and Siting section of this chapter.
---------------------------------------------------------------------------

2. Proposed changes
    In conjunction with the proposed 1-hour primary NAAQS and (if EPA
should adopt a standard at the upper end of the range of levels for
which the Agency is soliciting comment) the potential retention of the
current 24-hour NAAQS, we are proposing a number of changes to the
SO2 monitoring network. As just noted, there are currently
minimum monitoring requirements for SO2 only at NCore sites.
The proposal for a new 1-hour NAAQS necessitates the re-introduction of
minimum monitoring requirements. An analysis of the approximately 488
monitoring sites comprising the current (2008) SO2
monitoring network indicates that just under half (~46%) of the sites
in the current SO2 network are reported to be for the
assessment of concentrations for general population exposure. The
current network was not originally deployed to address current short-
term, peak concentrations, such as those locations nearer to stationary
sources or in areas of higher emissions densities, where maximum hourly
and 5- to 10-minute concentrations are likely to occur. The Agency has
data indicating that only about one third of the existing
SO2 network may be source-oriented monitors and/or sited in
locations of expected maximum concentrations (Watkins and Thompson,
2009).
    To fully support the proposed SO2 NAAQS, the monitoring
network needs to identify where short-term, peak ground-level
concentrations--i.e. concentrations from 5 minutes to one hour (or
potentially up to 24 hours)--

may occur. Due to the multiple variables that affect ground level
SO2 concentrations caused by one or more stationary sources,
it is difficult to specify a source specific threshold, algorithm, or
metric by which to require monitoring in a rule such as this. To
achieve this goal, therefore, EPA is proposing a two-pronged network
design to ensure that States perform a sufficient amount of monitoring
of ambient concentrations of SO2 to determine attainment of
the proposed SO2 NAAQS that intends to prevent exposure to
peak concentrations. EPA anticipates this two-pronged network would
require approximately 345 monitors nationwide, providing data for
comparison with both the proposed 1-hour and the 24-hour standard if
retained. The network would be wholly comprised of monitors sited at
locations of expected maximum hourly concentrations. EPA is proposing
that the two prongs of this SO2 network design would be
distributed based on: (1) A Population Weighted Emissions Index (PWEI)
and (2) the state-level contribution to the national, SO2
emissions inventory. EPA notes that although we propose that the
network include a minimum number of required monitors, State, local,
and tribal agencies may conduct additional monitoring above the minimum
requirements. If those additional monitors satisfy all applicable
requirements in 40 CFR Part 58, the data from those monitors would be
comparable to the NAAQS. EPA estimates that one-half to two-thirds of
the monitors in the existing network (excluding any currently operating
NCore sites) may have to be moved in order to be counted towards the
requirement for monitors sited at locations of expected maximum short-
term concentrations of SO2.
    We solicit comment on whether the estimated 348 monitors required
by this proposal, distributed based on the two network design
components presented below, are too few, too many, or suitable to
establish a minimum network sufficient to meet the monitoring
objectives noted above, including supporting compliance with the
proposed 1-hour SO2 NAAQS.
    We propose that state and, where appropriate, local air monitoring
agencies submit a plan for deploying SO2 monitors in
accordance with the proposed requirements presented below by July 1,
2011. We also propose that the SO2 network being proposed be
physically established no later than January 1, 2013. Considering the
proposed timeline and criteria presented in the network design, we
solicit comment on whether alternative dates would be more appropriate
as deadlines for state and local monitoring agencies to submit a
monitoring plan. We also solicit comments on whether alternative dates
would be more appropriate as deadlines for state and local monitoring
agencies to physically deploy monitors.
a. Population weighted emissions index (PWEI) triggered monitoring
    The EPA proposes that the first prong of the ambient SO2
monitoring network account for SO2 exposure by requiring
monitors in locations where population and emissions may lead to higher
potential for population exposure to peak hourly SO2
concentrations. In order to do this, EPA has developed a Population
Weighted Emissions Index (PWEI) that uses population and emissions
inventory data at the CBSA \34\ level to assign required monitoring for
a given CBSA (population and emissions being obvious relevant factors
in prioritizing numbers of required monitors). The PWEI for a
particular CBSA is calculated by multiplying the population (using the
latest Census Bureau estimates) of a CBSA by the total amount of
SO2 emissions in that CBSA. The CBSA emission value is in
tons per year, and is calculated by aggregating the county level
emissions for each county in a CBSA. We then normalize by dividing the
resulting product of CBSA population and CBSA SO2 emissions
by 1,000,000 to provide a PWEI value, the units of which are millions
of people-tons per year. This calculation has been performed for each
CBSA and has been posted in the docket as ``CBSA PWEI Calculation,
2009''. EPA believes that using this PWEI metric to inform where
monitoring is required is more appropriate for the SO2
network design than utilizing a population-only type of approach, so
that we may focus monitoring resources in areas of the country where
people and emission sources are in greater proximity. In addition,
EPA's initial view is that this PWEI concept is appropriate for
SO2 but is not necessarily transferrable to the other
criteria pollutants. From a very broad vantage point, SO2 is
exclusively a primarily emitted pollutant (i.e. unlike PM2.5
and ozone there is no secondary formation of SO2), is almost
exclusively emitted by stationary sources (unlike NO2, CO,
PM2.5, thoracic coarse PM, and ozone), and is a gaseous
pollutant which is somewhat more subject to transport (unlike Pb in the
Total Suspended Particulate (TSP) and PM10 size fractions).
---------------------------------------------------------------------------

    \34\ CBSAs are defined by the U.S. Census Bureau, and are
comprised of both Metropolitan Statistical Areas and Micropolitan
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------

    We propose that the first prong of the SO2 network
design require monitors in CBSAs, according to the following criteria.
For any CBSA with a calculated PWEI value equal to or greater than
1,000,000, a minimum of three SO2 monitors are required
within that CBSA. For any CBSA with a calculated PWEI value equal to or
greater than 10,000, but less than 1,000,000, a minimum of two
SO2 monitors are required within that CBSA. For any CBSA
with a calculated PWEI value equal to or greater than 5,000, but less
than 10,000, a minimum of one SO2 monitor is required within
that CBSA. EPA believes that the monitors required within these
breakpoints provide a reasonable minimum number of monitors in a CBSA
that considers the combination of population and emissions that exist
in a given CBSA. This proposed requirement is based on factors that
will ensure highly populated areas will receive monitoring even if the
emissions in that area are moderate, which is appropriate given the
fact that the greater population creates increased potential for
exposure to those moderate sources. Additionally, this proposed
requirement also ensures that those areas with higher emissions or
emission densities, with moderate or modest populations will receive
monitoring since those increased emissions are likely to have a
significant impact on whatever population may exist nearby.
    EPA estimates that these criteria will result in 231 required sites
in 132 CBSAs. We propose that monitors triggered in this first prong of
the network design must be sited in locations of expected maximum 1-
hour concentrations, at the appropriate spatial scale\35\, within the
boundaries of a given CBSA. EPA also proposes that when state or local
agencies make selections for monitoring sites from a pool of similar
candidate site locations, they shall prioritize monitoring where the
maximum expected hourly concentrations occur in relative greater
proximity to populations. EPA believes that states will likely need to
use some form of quantitative analysis, such as

modeling, data analysis, or saturation studies to aid in determining
where ground-level SO2 maxima may occur in a given CBSA. The
selection of these sites shall be documented in the Annual Monitoring
Network Plan per Sec.  58.10, which includes a requirement for public
inspection or comment, and approval by the EPA Regional Administrator.
---------------------------------------------------------------------------

    \35\ Due to the variability in where maximum ground-level
concentrations may occur (discussed in the Monitor Siting and
Placement section of this chapter), the appropriate spatial scales
within which an SO2 monitor might be placed include the
microscale, middle, and neighborhood scales, which are defined in 40
CFR Part 58 Appendix D. [could also refer to the fn above where
these are described]
---------------------------------------------------------------------------

    EPA solicits comment on (1) the use of the Population Weighted
Emissions Index (PWEI), (2) the PWEI calculation method, (3) the PWEI
breakpoints that correlate to a number of required monitors, (4) the
requirement that the monitors shall be sited in locations of expected
maximum 1-hour concentration, and (5) that state or local agencies
making selections for monitoring sites from a pool of similar candidate
site locations shall prioritize monitoring where the maximum expected
hourly concentrations occur in relative greater proximity to
populations.
    EPA recognizes that CBSA populations and emissions inventories
change over time, suggesting a need for periodic review of the
monitoring network. At the same time, EPA recognizes the advantages of
a stable monitoring network. Therefore, while EPA currently provides
for updates of the NEI every 3 years, EPA believes that the current
network review requirements per Sec.  58.10 which requires an annual
network plan and recurring 5-year assessments provide a suitable
schedule for planning and assessing the monitoring network. Through the
5-year assessments, states will be in a position to review emissions
distributions from updated NEIs to calculate PWEI values for each CBSA
and subsequently assess whether the operational monitoring network
remains appropriate. EPA proposes that the number of sites required to
operate as a result of the PWEI values calculated for each CBSA be
reviewed and revised for each CBSA through the 5-year network
assessment cycle required in Sec.  58.10. EPA solicits comment on
whether such adjustments to the network should be required on a 5-year
cycle that matches the general frequency of network assessments or some
other frequency.
b. State-level emissions triggered monitoring
    As the second prong of the SO2 network, we are proposing
to require a monitor or monitors in each state, allocated by state-
level SO2 emissions. In this prong, EPA proposes to
distribute approximately 117 sites, based on the corresponding percent
contribution of each individual state to the national anthropogenic
SO2 emission inventory. This prong of the network design is
intended to allow a portion of the overall required monitors to be
placed where needed, independent of the PWEI, inside or outside of
CBSAs. EPA proposes to require monitors, using state boundaries as the
geographic unit for allocation purposes, in proportion to a state's SO2
emissions, i.e., a state with higher emissions will be required to have
a proportionally higher number of monitors. The proposed percent
contribution of individual states is based on the most recent NEI, with
SO2 emissions being aggregated by state. Each one percent
(after rounding) would correspond to one required monitor. For example,
according to the 2005 NEI, the State of Ohio contributes 8.66% of the
total anthropogenic SO2 inventory, which would correspond to
requiring nine monitors to be distributed within Ohio. Further, EPA
proposes that each state have at least one monitor required as part of
this second prong, even if a particular state contributes less than
0.5% of the total anthropogenic national emissions inventory. As a
result, approximately 117 monitoring sites would be required and
distributed based on state-level SO2 emissions in the most
recent NEI, which in this case, is the 2005 NEI. EPA solicits comment
on the use of state-level emission inventories based on the most recent
NEI to proportionally distribute approximately one third (117 sites) of
the required monitoring network.
    According to the most recent NEI, for this proposed second prong,
we estimate the state/percent contribution to the national inventory/
required monitor distribution to be:

Table 5--State-level Emission Triggered Monitors--This Table Shows State
 and Territory Level Contributions to the National SO2 Inventory and the
 Corresponding Number of Monitors Required for Each State as Proposed in
                 Prong 2 of the Proposed Network Design
------------------------------------------------------------------------
                                         Percent
                                     contribution to    Proposed number
        State or Territory           the national SO2     of required
                                        inventory           monitors
                                        (percent)
------------------------------------------------------------------------
Alabama...........................               4.02                  4
Alaska............................               0.46                  1
American Samoa....................                N/A                  1
Arizona...........................               0.60                  1
Arkansas..........................               0.77                  1
California........................               1.48                  1
Colorado..........................               0.54                  1
Connecticut.......................               0.23                  1
Delaware..........................               0.58                  1
District of Columbia..............               0.03                  1
Florida...........................               4.40                  4
Georgia...........................               5.07                  5
Guam..............................                N/A                  1
Hawaii............................               0.08                  1
Idaho.............................               0.16                  1
Illinois..........................               3.51                  4
Indiana...........................               7.10                  7
Iowa..............................               1.50                  2
Kansas............................               1.33                  1
Kentucky..........................               3.88                  4
Louisiana.........................               2.40                  2
Maine.............................               0.25                  1
Maryland..........................               2.58                  3
Massachusetts.....................               1.07                  1

Michigan..........................               3.32                  3
Minnesota.........................               1.05                  1
Mississippi.......................               0.81                  1
Missouri..........................                2.8                  3
Montana...........................               0.26                  1
Nebraska..........................               0.82                  1
Nevada............................               0.49                  1
New Hampshire.....................               0.43                  1
New Jersey........................               0.69                  1
New Mexico........................               0.32                  1
New York..........................               2.65                  3
North Carolina....................               4.40                  4
North Dakota......................               1.08                  1
Northern Mariana Islands..........                N/A                  1
Ohio..............................               8.66                  9
Oklahoma..........................               1.12                  1
Oregon............................               0.32                  1
Pennsylvania......................               7.96                  8
Puerto Rico.......................                N/A                  1
Rhode Island......................               0.06                  1
South Carolina....................               2.06                  2
South Dakota......................               0.19                  1
Tennessee.........................               2.63                  3
Texas.............................               6.34                  6
Utah..............................               0.35                  1
Vermont...........................               0.05                  1
Virgin Islands....................                N/A                  1
Virginia..........................               2.34                  2
Washington........................               0.45                  1
West Virginia.....................               3.63                  4
Wisconsin.........................               1.79                  2
Wyoming...........................               0.83                  1
------------------------------------------------------------------------

    EPA proposes siting requirements for this second prong of required
monitors to be the same as those in the first prong: siting in
locations of expected maximum 1-hour concentrations, at the appropriate
spatial scale, within the boundaries of a given state, and prioritizing
the selection of candidate sites where the maximum expected hourly
concentrations occur in greater proximity to populations. This again
would need to be determined case-by-case using quantitative analysis,
such as modeling, data analysis, or saturation studies to aid in
determining where ground-level SO2 maxima may occur in a
given state. We propose that these monitors can be located inside or
outside of CBSA boundaries. However, if a monitor required by the
second prong is placed inside a CBSA that already has a requirement for
monitoring due to the first prong of this network design, that monitor
would not be allowed to count towards satisfying the first prong
requirements. As noted for the first prong of required monitors, the
selection of these sites shall be documented in the Annual Monitoring
Network Plan per Sec.  58.10, which includes a requirement for public
inspection or comment, and approval by the EPA Regional Administrator.
    The EPA solicits comment on (1) the use of state-level emission
inventories to proportionally distribute required monitors, (2)
requiring each state to have at least one monitor under this prong of
the network design, and (3) requiring all monitors to be sited in
locations of expected maximum 1-hour concentration inside or outside of
CBSAs.
    EPA recognizes that emissions inventories change over time,
suggesting a need for periodic review of the monitoring network. At the
same time, EPA recognizes the advantages of a stable monitoring
network. Therefore, while EPA currently provides for updates of the NEI
every 3 years, EPA believes that the current network review
requirements per Sec.  58.10 which requires an annual network plan and
recurring 5-year assessments provide a suitable schedule for planning
and assessing the monitoring network. Through the 5-year assessments,
states will be in a position to review emissions distributions from
updated NEIs to assess whether the monitoring requirements remain
appropriate. EPA proposes that the number of sites required to operate
as a result of state-level emissions be reviewed and revised for each
state through the 5-year network assessment cycle required Sec.  58.10.
EPA solicits comment on whether such adjustments to the network should
be required on a 5-year cycle that matches the general frequency of
network assessments or some other frequency.
c. Monitor placement and siting
    Sites that are to be placed in locations of expected maximum 1-hour
concentrations, will also likely discern 5-minute peaks as well. EPA
expects that in general, these locations will be in proximity to larger
emitting sources (in tons per year) and/or areas of relatively high
emissions densities where multiple sources may be contributing to peak
ground-level concentrations. The variability in where such locations
exist relative to the

responsible emission source(s) depends on multiple factors including
the tonnage emitted by a source (or group of sources), stack height,
stack diameter, emission exit velocity, emission temperature, terrain,
and meteorology. Depending on these variables, plumes may heavily
fumigate areas immediately downwind of a source, or may never truly
touch down at all, dispersing into ambient air where SO2
concentrations continually decrease with increasing distance away from
the source. This is illustrated in an example where a relatively large
source with a tall stack height may not produce exceedingly high ground
level concentrations anywhere along its plume trajectory while a
smaller source with a relatively short stack may cause relatively
higher ground level concentrations under the same meteorological
conditions at the same location. The primary reason for this
variability is because the peak impacts of sources with higher stacks
will generally be farther downwind and may be more variably located
than is the case for sources with shorter stacks. Further, depending on
meteorology, an emission plume from an individual source may cause
increased ground-level concentrations at any heading, relative to the
parent source, corresponding to the prevailing winds.
    When analyzing a particular source, a state may find multiple
locations where peak ground-level concentrations may occur around an
individual source. EPA does not intend for multiple monitors to be
sited around or in proximity to one source. Not siting multiple
monitors around, or in proximity, to one source ensures that more
individual sources or groups of sources will receive attention by the
monitoring network. States always have the discretion to perform
additional monitoring above the minimum requirements to increase
monitoring around a particular source or group of sources.
    Due to the variability of how, when, where, and to what degree a
source or group of sources can contribute to peak, ground-level
SO2 concentrations, EPA expects that State and local
monitoring agencies will need to analyze all relevant information,
including available ambient and emissions data, and potentially use air
quality modeling or saturation studies to select appropriate monitoring
site locations. Further, due to the variability in where maximum
ground-level concentrations may occur, the appropriate spatial scales
within which a monitor might be placed include the microscale, middle,
and neighborhood scales, which are defined in 40 CFR Part 58 Appendix
D. EPA believes that states, in evaluating a source (or group of
sources) that contribute to a peak ground-level SO2
concentration that varies with space and time, should identify where
the highest concentrations are expected to occur in developing
candidate site locations. EPA proposes that when state and local
agencies make selections for monitoring sites from candidate site
locations, they shall prioritize monitoring where the maximum expected
hourly concentrations occur in greater proximity to populations. EPA
solicits comment on the role of population exposure in the site
selection process.
d. Monitoring required by the regional administrator
    In addition to the two prongs of the proposed SO2
network design, we propose that the Regional Administrator will have
discretion to require monitoring above these minimum requirements under
prongs 1 and 2, as necessary to address situations where the minimum
monitoring requirements are not sufficient to meet monitoring
objectives noted above. EPA recognizes that the minimum required
monitors in the proposed network design under the two prongs described
above are based on indicators that may not provide for all the
monitoring that may be necessary in an area. An example where EPA
envisions requiring an additional monitor might be a case where a
source having modest emissions still has high potential to cause a
violation of the NAAQS in a community or neighborhood. This situation
might occur where a modest SO2 source has, for example, a
low emission stack and/or is in an area where meteorological conditions
cause situations, such as inversions or stagnation, that might lead to
high ground-level concentrations of SO2. In this example,
such a monitor might be needed even though a state is fulfilling its
monitoring requirements under the first and second prongs of the
proposed network design. The purpose of this provision is to monitor in
and provide data for otherwise non-monitored locations that have the
potential to exceed the level of the NAAQS or that are perceived to
have higher exposure risks due to proximity to a source or sources. In
such an example, the Regional Administrators may make use of any
available data including existing model data, existing data analyses,
or screening tools such as AERSCREEN or SCREEN3, to inform a decision
of whether or not a monitor should be required for a given area or
location. Any monitor required through the Regional Administrator and
selected by the state or local agency would be included in the Annual
Monitoring Network Plan per Sec.  58.10, which includes a requirement
for public inspection or comment, and approval by the EPA Regional
Administrator. In any case, EPA encourages state, local, and tribal
monitoring agencies to provide input and information to the appropriate
Regional Administrators in determining whether additional monitors are
needed and the locations of such monitors. We solicit comment on the
proposal to allow Regional Administrators the discretion to require
monitoring above the requirements under prongs 1 and 2 for any area or
location where those monitoring requirements are not sufficient to meet
monitoring objectives.
    EPA notes that existing requirements detailed in Sec.  58.14(c)
address certain conditions where existing monitors can be shut down,
with EPA Regional Administrator approval. EPA is not reopening or
otherwise reconsidering this provision. However, this requirement is
noted here so that state or local agency requests to potentially
relocate SO2 monitors to meet the proposed requirements of
prongs 1 or 2 will be considered with the specific provisions of Sec.
58.14(c) in mind.
e. Alternative network design
    EPA solicits comments on alternative network designs, including
alternative methods to determine the minimum number of monitors per
state. We are particularly interested in whether a screening approach
for assessing the likelihood of a NAAQS exceedance could be developed
and serve as a basis for determining the number and location of
required monitors.
    More specifically, EPA requests comment on whether it should
utilize existing screening tools such as AERSCREEN or SCREEN3, which
use parameters such as effective stack height and emissions levels to
identify facilities with the potential to cause an exceedance of the
proposed standard. For that set of sources, EPA could then require
states to conduct more refined modeling (likely using the American
Meteorological Society (AMS)/EPA Regulatory Model (AERMOD)) to
determine locations where monitoring should be conducted. Any screening
or modeling would likely be carried out by states by using EPA
recommended models and techniques referenced by 40 CFR Part 51,
Appendix W, which provides guidance on air quality modeling. Such
screening or modeling uses facility emission tonnage, stack heights,
stack diameters, emission temperatures, emission velocities, and
accounts for local terrain and meteorology in determining where

expected maximum hourly concentrations may occur. In using this
approach, EPA would then require states to locate monitors at the point
of maximum concentration around sources identified as likely causing
NAAQS exceedances.
    This approach could lead to monitors being required at a
significantly larger number of locations than under the proposed
approach. For example, the NEI shows that 2,407 sources emit 50 tons
per year or more of SO2, while 1,928 sources emit 100 tons
per year or more of SO2. If, for example, the state
screening approach found that a substantial fraction of those 50 or 100
ton per year sources had a significant probability of violating the
NAAQS, states could be required to model, evaluate, and potentially
monitor a corresponding number of sources. EPA also notes that this
alternative approach would not distinctly use population as a factor
for where monitors should be placed. EPA solicits comment on the
resource implications for state and local agencies associated with this
approach.
    If EPA selects a standard level near the lower end of the proposed
range, it is likely that a greater number of areas would exceed the
NAAQS, leading to the need for additional monitors. A facility
screening approach, as described above would explicitly account for the
specific parameters of a facility, air quality information, and the
stringency of the standard for determining the number of monitors, in
contrast to the proposed approach. EPA solicits comment on how, in the
absence of a facility screening approach, the number of monitors
required nationwide could be adjusted if EPA finalizes a standard near
the lower end of the proposed range.

C. Data reporting

    SO2 UV fluorescence FEMs are continuous gas analyzers,
producing updated data values on the order of every 20 seconds. Data
values are typically aggregated into minute averages and then compiled
into hourly averages for reporting purposes. EPA proposes to retain the
existing requirement that State and local monitoring agencies report
hourly SO2 data to AQS within 90 days of the end of each
calendar quarter. EPA encourages monitoring agencies to voluntarily
report their pre-validated data on an hourly basis to EPA's real time
AIRNow data system.
    The definitive evidence for the ISA's conclusion of causal
association between short-term SO2 exposure and respiratory
morbidity is from controlled human exposure studies of 5-10 minutes in
exercising asthmatics (ISA, section 5.2). The REA therefore assessed
exposure and risks associated with 5-minute SO2
concentrations above 5-minute health effect benchmark levels derived
from these controlled human exposure studies. In performing these
analyses, the REA noted that: (1) The majority of the current
SO2 monitoring network reported 1-hour SO2
concentrations (REA section 7.2.3); (2) very few state and local
agencies in the U.S. voluntary reported ambient 5-minute SO2
concentrations, as such reporting is not required (REA, section
<IP_ADDRESS>); and (3) the lack of 5-minute monitoring data necessitated
the use of statistically estimated 5-minute SO2
concentrations derived from reported 1-hour SO2 levels (see
REA section 7.2.3) in order to expand the geographic scope of the
exposure and risk analyses. Thus given the demonstrated importance of
5-minute SO2 concentrations, EPA proposes that State and
local agencies shall report to AQS the maximum 5-minute block average
of the twelve 5-minute block averages of SO2 for each hour,
in addition to the existing requirement to report the 1-hour average.
    EPA solicits comment on the proposed requirement for state and
local monitoring agencies to report both hourly average and the maximum
5-minute block average out of the twelve 5-minute block averages of
SO2 for each hour. EPA also solicits comment on the
advantages and disadvantages of alternatively requiring state and local
agencies to report all twelve 5-minute SO2 values for each
hour. Having all twelve 5-minute SO2 values for each hour
would provide more detailed information for health research purposes
and provide additional information to help inform the next review of
the SO2 standard. We also solicit comment on alternatively
requiring state and local agencies to report the maximum 5-minute
concentration in an hour based on a moving 5-minute averaging period
rather than time block averaging.
    EPA notes the potential resource burden with the proposed
requirement to report 5-minute average values in addition to 1-hour
average values, as is currently required. Accordingly, we solicit
comment on the magnitude and importance of this resource burden,
recognizing that monitoring agencies utilize a variety of automated
data acquisition and management programs, and that the resulting burden
of validating and reporting 5-minute data may vary from a relatively
trivial matter to an issue of greater importance, depending on the
procedures utilized within each agency's data reporting process.
    As a part of the larger data quality performance requirements of
the ambient monitoring program, we are proposing data quality
objectives (DQOs) for the proposed SO2 network. The DQOs are
meant to identify measurement uncertainty for a given pollutant method.
We propose a goal for acceptable measurement uncertainty for
SO2 methods to be defined for precision as an upper 90
percent confidence limit for the coefficient of variation (CV) of 15
percent and for bias as an upper 95 percent confidence limit for the
absolute bias of 15 percent. We solicit comment on the proposed DQOs
and on what the acceptable measurement uncertainty should be.

IV. Proposed Appendix T--Interpretation of the Primary NAAQS for Oxides
of Sulfur and Proposed Revisions to the Exceptional Events Rule

    The EPA is proposing to add Appendix T, Interpretation of the
Primary National Ambient Air Quality Standards for Oxides of Sulfur, to
40 CFR Part 50 in order to provide data handling procedures for the
proposed SO2 1-hour primary standard. The proposed Sec.
50.11 which sets the averaging period, level, indicator and form of the
NAAQS refers to this Appendix T. The proposed Appendix T would detail
the computations necessary for determining when the proposed 1-hour
primary SO2 NAAQS is met. The proposed Appendix T also would
address data reporting, data completeness considerations, and rounding
conventions.
    Two versions of the proposed Appendix T are printed at the end of
this notice. The first applies to a 1-hour primary standard based on
the annual 4th high value form, while the second applies to a 1-hour
primary standard based on the 99th percentile daily value form. (As
explained in section II.F. 3 above, EPA is proposing alternative forms
here based on technical analysis that they are equally effective.) The
discussion here addresses the first of these versions, followed by a
brief description of the differences found in the second version.
    For the proposed 1-hour primary standard, EPA is proposing data
handling procedures, a proposed addition of a cross-reference to the
Exceptional Events Rule, a proposed addition to allow the Administrator
discretion to consider otherwise incomplete data to be complete, and a
proposed provision addressing the

possibility of there being multiple SO2 monitors at one
site.
    The EPA is also proposing SO2-specific changes to the
deadlines in 40 CFR 50.14, by which states must flag ambient air data
that they believe have been affected by exceptional events and submit
initial descriptions of those events, and to the deadlines by which
states must submit detailed justifications to support the exclusion of
that data from EPA determinations of attainment or nonattainment with
the NAAQS. The deadlines now contained in 40 CFR 50.14 are generic, and
are not always appropriate for SO2 given the anticipated
schedule for the designations of areas under the proposed
SO2 NAAQS.

A. Background

    The general purpose of a data interpretation appendix is to provide
the practical details on how to make a comparison between multi-day and
possibly multi-monitor ambient air concentration data and the level of
the NAAQS, so that determinations of attainment and nonattainment are
as objective as possible. Data interpretation guidelines also provide
criteria for determining whether there are sufficient data to make a
NAAQS level comparison at all.
     The regulatory language for the current SO2 NAAQS,
originally adopted in 1977, contains data interpretation instructions
only for the issue of data completeness. This situation contrasts with
the situations for ozone, PM2.5, PM10, and most
recently Pb for which there are detailed data interpretation appendices
in 40 CFR Part 50 addressing issues that can arise in comparing
monitoring data to the NAAQS. EPA has used its experience developing
and applying these other data interpretation appendices to develop the
proposed text for Appendix T.
    An exceptional event is defined in 40 CFR 50.1 as an event that
affects air quality, is not reasonably controllable or preventable, is
an event caused by human activity that is unlikely to recur at a
particular location or is a natural event, and is determined by the
Administrator in accordance with 40 CFR 50.14 to be an exceptional
event. Air quality data that is determined, under the procedural steps
and substantive criteria specified in section 50.14, to have been
affected by an exceptional event may be excluded from consideration
when EPA makes a determination that an area is meeting or not meeting
the associated NAAQS. The key procedural deadlines in section 50.14 are
that a State must notify EPA that data have been affected by an event,
i.e., ``flag'' the data in the Air Quality Systems (AQS) database, and
provide an initial description of the event by July 1 of the year after
the data are collected, and that the State must submit the full
justification for exclusion within 3 years after the quarter in which
the data were collected. However, if a regulatory decision based on the
data, for example a designation action, is anticipated, the schedule is
shortened and all information must be submitted to EPA no later than a
year before the decision is to be made. This generic schedule presents
problems when a NAAQS has been recently revised, as discussed below.

B. Interpretation of the primary NAAQS for oxides of sulfur

    The purpose of a data interpretation rule for the SO2
NAAQS is to give effect to the form, level, averaging time, and
indicator specified in the proposed regulatory text at 40 CFR 50.11,
anticipating and resolving in advance various future situations that
could occur. The proposed Appendix T provides definitions and
requirements that apply to the proposed 1-hour primary standard for
SO2. The requirements concern how ambient data are to be
reported, what ambient data are to be considered (including the issue
of which of multiple monitors' data sets will be used when more than
one monitor has operated at a site), and the applicability of the
Exceptional Events Rule to the primary SO2 NAAQS.
1. 1-hour primary standard based on the annual 4th high value form
    With regard to data completeness for the proposed 1-hour primary
standard, the proposed Appendix follows past EPA practice for other
NAAQS pollutants by requiring that in general at least 75% of the
monitoring data that should have resulted from following the planned
monitoring schedule in a period must be available for the key air
quality statistic from that period to be considered valid. For the
proposed 1-hour primary SO2 NAAQS, the key air quality
statistics are the daily maximum 1-hour concentrations in three
successive years. It is important that sampling within a day encompass
the period when concentrations are likely to be highest and that all
seasons of the year are well represented. Hence, the 75% requirement is
proposed to be applied at the daily and quarterly levels. EPA invites
comment on the proposed completeness requirements.
    Recognizing that there may be years with incomplete data, the
proposed text provides that a design value derived from incomplete data
will nevertheless be considered valid in either of two situations.
    First, if the design value calculated from at least four days of
monitoring observations in each of these years exceeds the level of the
1-hour primary standard, it would be valid. This situation could arise
if monitoring was intermittent but high SO2 levels were
measured on enough hours and days for the mean of the three annual 4th
highest values to exceed the standard. In this situation, more complete
monitoring could not possibly have indicated that the standard was
actually met.
    Second, we are proposing a diagnostic data substitution test which
is intended to identify those cases with incomplete data in which it
nevertheless is very likely, if not virtually certain, that the daily
1-hour design value would have been observed to be below the level of
the NAAQS if monitoring data had been minimally complete.
    The diagnostic test would be applied only if there is at least 50%
data capture in each quarter of each year and if the 3-year mean of the
observed annual 4th highest maximum hourly values in the incomplete
data is below the NAAQS level. The test would substitute a high
hypothetical concentration for as much of the missing data as needed to
meet the 100% requirement in each quarter. The value that is
substituted for the missing values is the highest daily maximum 1-hour
observed in the same quarter, looking across all three years under
evaluation. If the resulting 3-year design value is below the NAAQS, it
is highly likely that the design value calculated from complete data
would also have been below the NAAQS, so the original design value
indicating compliance would be considered valid.
    It should be noted that one possible outcome of applying the
proposed substitution test is that a year with incomplete data may
nevertheless be determined to not have a valid design value and thus to
be unusable in making 1-hour primary NAAQS compliance determinations
for that 3-year period. EPA invites comment on incorporating the
proposed substitution test into the final rule.
    EPA is proposing that the Administrator have general discretion to
use incomplete data to calculate design values that would be treated as
valid for comparison to the NAAQS despite the incompleteness, either at
the request of a state or at her own initiative. Similar provisions
exist already for the PM2.5 and lead NAAQS, and EPA has
recently proposed such provisions to accompany the proposed 1-hour
NO2 and SO2 NAAQS. The Administrator would

consider monitoring site closures/moves, monitoring diligence, and
nearby concentrations in determining whether to use such data.
2. 1-hour primary standard based on the annual 99th percentile daily
value form
    The second version of the proposed Appendix T appearing at the end
of this notice contains proposed interpretation procedures for a 1-hour
primary standard based on the 99th percentile daily value form. The 4th
high daily value form and the 99th percentile daily value form would
yield the same design value in a situation in which every hour and day
of the year has reported monitoring data, since the 99th percentile of
365 daily values is the 4th highest value. However, the two forms
diverge if data completeness is 82% or less, because in that case the
99th percentile value is the 3rd highest (or higher) value, to
compensate for the lack of monitoring data on days when concentrations
could also have been high.
    Logically, provisions to address possible data incompleteness under
the 99th percentile daily value form should be somewhat different from
those for the 4th highest form. With a 4th highest form, incompleteness
should not invalidate a design value that exceeds the standard, for
reasons explained above. With the 99th percentile form, however, a
design value exceeding the standard stemming from incomplete data
should not automatically be considered valid, because concentrations on
the unmonitored days could have been relatively low, such that the
actual 99th percentile value for the year could have been lower, and
the design value could have been below the standard. The second
proposed version of Appendix T accordingly has somewhat different
provisions for dealing with data incompleteness. One difference is the
addition of another diagnostic test based on data substitution, which
in some cases can validate a design value based on incomplete data that
exceeds the standard.
    The second version of the proposed Appendix T provides a table for
determining which day's maximum 1-hour concentration will be used as
the 99th percentile concentration for the year. The proposed table is
similar to one used now for the 24-hour PM2.5 NAAQS, which
is based on a 98th percentile form, but adjusted to reflect a 99th
percentile form for the 1-hour primary SO2 standard. The
proposed Appendix T also provides instructions for rounding (not
truncating) the average of three annual 99th percentile hourly
concentrations before comparison to the level of the primary NAAQS.

C. Exceptional events information submission schedule

    The Exceptional Events Rule at 40 CFR 50.14 contains generic
deadlines for a state to submit to EPA specified information about
exceptional events and associated air pollutant concentration data. A
state must initially notify EPA that data have been affected by an
event by July 1 of the calendar year following the year in which the
event occurred; this is done by flagging the data in AQS and providing
an initial event description. The state must also, after notice and
opportunity for public comment, submit a demonstration to justify any
claim within 3 years after the quarter in which the data were
collected. However, if a regulatory decision based on the data (for
example, a designation action) is anticipated, the schedule to flag
data in AQS and submit complete documentation to EPA for review is
shortened, and all information must be submitted to EPA no later than
one year before the decision is to be made.
    These generic deadlines are suitable for the period after initial
designations have been made under a NAAQS, when the decision that may
depend on data exclusion is a redesignation from attainment to
nonattainment or from nonattainment to attainment. However, these
deadlines present problems with respect to initial designations under a
newly revised NAAQS. One problem is that some of the deadlines,
especially the deadlines for flagging some relevant data, may have
already passed by the time the revised NAAQS is promulgated. Until the
level and form of the NAAQS have been promulgated a state does not know
whether the criteria for excluding data (which are tied to the level
and form of the NAAQS) were met on a given day. Another problem is that
it may not be feasible for information on some exceptional events that
may affect final designations to be collected and submitted to EPA at
least one year in advance of the final designation decision. This could
have the unintended consequence of EPA designating an area
nonattainment because of uncontrollable natural or other qualified
exceptional events.
    The Exceptional Events Rule at Sec.  50.14(c)(2)(v) indicates
``when EPA sets a NAAQS for a new pollutant, or revises the NAAQS for
an existing pollutant, it may revise or set a new schedule for flagging
data for initial designation of areas for those NAAQS.''
    For the specific case of SO2, EPA anticipates that the
signature date for the revised SO2 NAAQS will be June 2,
2010 (a date specified by Consent Decree), that state/tribal
designations recommendations will be due by June 2, 2011, and that
initial designations under the revised NAAQS will be made by June 1,
2012 (since June 2, 2012 would be on a Saturday) and will be based on
air quality data from the years 2008-2010 or 2009-2011 if there is
sufficient data for these data years. (See Section VI below for more
detailed discussion of the designation schedule and what data EPA
intends to use.) Under the current rule, because final designations
would be made by June 1, 2012, all events to be considered during the
designations process would have to be flagged and fully documented by
states one year prior to designations, by June 1, 2011. A state would
not be able to flag and submit documentation regarding events that
occurred between June to December 2011 by one year before designations
are made in June 2012.
    EPA is proposing revisions to 40 CFR 50.14 only to change
submission dates for information supporting claimed exceptional events
affecting SO2 data. The proposed rule text at the end of
this notice shows the changes that would apply if a revised
SO2 NAAQS is promulgated by June 2, 2010, and designations
are made two years after such promulgation. For air quality data
collected in 2008, we propose to extend the generic July 1, 2009
deadline for flagging data (and providing a brief initial description
of the event) to October 1, 2010. EPA believes this extension would
provide adequate time for states to review the impact of exceptional
events from 2008 on the revised standard and notify EPA by flagging the
relevant data in AQS. EPA is not proposing to change the foreshortened
deadline of June 1, 2011 for submitting documentation to justify an
SO2-related exceptional event from 2008. We believe the
generic deadline provides adequate time for states to develop and
submit proper documentation.
    For data collected in 2009, EPA proposes to extend generic deadline
of July 1, 2010 for flagging data and providing initial event
descriptions to October 1, 2010. EPA is retaining the deadline of June
1, 2011 for states to submit documentation to justify an
SO2-related exceptional event from 2009. EPA plans to assist
the states by providing at the time of signature our assessment of
which monitoring sites and days have exceeded the NAAQS in 2008 and
2009. For data collected in 2010, EPA is proposing a deadline of June
1, 2011 for flagging data and providing initial event descriptions and

for submitting documentation to justify exclusion of the flagged data.
EPA believes that this deadline provides states with adequate time to
review and identify potential exceptional events that occur in calendar
year 2010, even for those events that might occur late in the year. EPA
believes these deadlines will be feasible because experience suggest
that exceptional events affecting SO2 data are few in number
and easily assessed, so no state is likely to have a large workload.
    If a state intends 2011 data to be considered in SO2
designations, 2011 data must be flagged and detailed event
documentation submitted 60 days after the end of the calendar quarter
in which the event occurred or by March 31, 2011, whichever date occurs
first. Again, EPA believes these deadlines will be feasible because
experience suggest that exceptional events affecting SO2
data are few in number and easily assessed, so no state is likely to
have a large workload.
    Table 6 summarizes the proposed designation deadlines discussed in
this section and provides designation schedule information from recent,
pending or prior NAAQS revisions for other pollutants. If the
promulgation date for a revised SO2 NAAQS occurs on a
different date than June 1, 2010 (i.e. if the consent decree should be
amended--which EPA does not presently anticipate), EPA will revise the
final SO2 exceptional event flagging and documentation
submission deadlines accordingly, consistent with this proposal, to
provide states with reasonably adequate opportunity to review,
identify, and document exceptional events that may affect an area
designation under a revised NAAQS. EPA invites comment on these
proposed changes in the exceptional event flagging and documentation
submission deadlines for the revised SO2 NAAQS shown in
Table 6.

      Table 6--Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
                                 Designations Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
                                          Air quality data
   NAAQS pollutant/ standard/(level)/      collected for     Event flagging & initial    Detailed documentation
           promulgation date               calendar year       description deadline       submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m\3\)             2004-2006  October 1, 2007 \a\......  April 15, 2008 \a\.
 Promulgated October 17, 2006.
Ozone/8-Hr Standard (0.075 ppm)                  2005-2007  June 18, 2009 \a\........  June 18, 2009 \a\.
 Promulgated March 12, 2008.
                                                      2008  June 18, 2009 \a\........  June 18, 2009 \a\.
                                                      2009  60 Days after the end of   60 Days after the end of
                                                             the calendar quarter in    the calendar quarter in
                                                             which the event occurred   which the event occurred
                                                             or February 5, 2010,       or February 5, 2010,
                                                             whichever date occurs      whichever date occurs
                                                             first \b\.                 first \b\.
NO2/1-Hour Standard (80-100 Ppb, Final                2008  July 1, 2010 \a\.........  January 22, 2011 \a\.
 Level Tbd).
                                                      2009  July 1, 2010 \a\.........  January 22, 2011 \a\.
                                                      2010  April 1, 2011 \a\........  July 1, 2011 \a\.
SO2/1-Hour Standard (50-100 PPB, Final                2008  October 1, 2010 \b\......  June 1, 2011 \b\.
 Level Tbd).
                                                      2009  October 1, 2010 \b\......  June 1, 2011 \b\.
                                                      2010  June 1, 2011 \b\.........  June 1, 2011 \b\.
                                                      2011  60 Days after the end of   60 Days after the end of
                                                             the calendar quarter in    the calendar quarter in
                                                             which the event occurred   which the event occurred
                                                             or March 31, 2011,         or March 31, 2011,
                                                             whichever date occurs      whichever date occurs
                                                             first \b\.                 first \b\.
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published In the original rulemaking, or are being proposed elsewhere
  and are shown in this table for informational purposes--the agency is not opening these dates for comment
  under this rulemaking.
\b\ Indicates change from general schedule In 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
  initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
  notably, for data used by EPA for redesignations to attainment.

V. Designations for the SO2 NAAQS

    After EPA establishes or revises a NAAQS, the CAA directs EPA and
the states to begin taking steps to ensure that the new or revised
NAAQS is met. The first step is to identify areas of the country that
do not meet the new or revised NAAQS. This step is known as the initial
area designations.
    Section 107(d)(1)(A) of the CAA provides that, ``By such date as
the Administrator may reasonably require, but not later than 1 year
after promulgation of a new or revised NAAQS for any pollutant under
section 109, the Governor of each state shall * * * submit to the
Administrator a list of all areas (or portions thereof) in the state''
that designates those areas as nonattainment, attainment, or
unclassifiable. The CAA section 107(d)(1)(A)(i) defines an area as
nonattainment if it is violating the NAAQS or if it is contributing to
a violation in a nearby area.
    Section 107(d)(1)(B)(i) further provides, ``Upon promulgation or
revision of a NAAQS, the Administrator shall promulgate the
designations of all areas (or portions thereof) * * * as expeditiously
as practicable, but in no case later than 2 years from the date of
promulgation. Such period may be extended for up to one year in the
event the Administrator has insufficient information to promulgate the
designations within 2 years. By no later than 120 days prior to
promulgating designations, EPA is required to notify states of any
intended modifications to their boundaries as EPA may deem necessary.
States then have an opportunity to comment on EPA's intended decisions.
(See section 107(d)(1)(B)(ii).) Whether or not a state provides a
recommendation, EPA must promulgate the designation that the Agency
deems appropriate.
    Therefore, following promulgation of any revised SO2
NAAQS in June 2010, EPA must promulgate initial designations by June
2012, or, by June 2013 in the event that the Administrator has
insufficient information to promulgate initial designations within 2
years. Along with the proposal to set a

new 1-hour primary SO2 NAAQS, elsewhere in this action, EPA
is proposing new SO2 ambient air monitoring network
requirements. As proposed, any new monitors would be deployed no later
than January 1, 2013. Compliance with the proposed 1-hour
SO2 NAAQS would be determined based on 3 years of complete,
quality assured, certified monitoring data. We do not expect newly
sited monitors for the proposed new network to generate sufficient
monitoring data for EPA to use in determining whether areas are in
compliance with the revised SO2 NAAQS by the statutory
deadline for EPA to complete initial designations, even if EPA were to
take an additional third year. Therefore, EPA intends to complete the
designations on a 2-year schedule, by June 2012, based on 3 years of
complete, quality assured, certified air quality monitoring data from
the current monitoring network.
    EPA expects to base designations on air quality data from the years
2008-2010 or 2009-2011. Because the new monitoring network requirements
would not apply until January 1, 2013, EPA expects that many
SO2 monitors now operating will continue in operation at
their current locations at least through the end of 2011.\36\ The
SO2 monitors in the current network were generally sited to
measure the highest 24-hour and annual average SO2
concentrations. However, all of the monitors report hourly data. EPA
estimates that around 488 monitors operated in 2008. EPA believes at
least one third of the monitors meet the proposed network design
requirements and therefore would not need to be moved. Additional
monitors may be retained in their current locations if they are
measuring high levels of SO2. If a monitor in the existing
network indicates a violation of the 1-hour SO2 NAAQS, EPA
intends to designate the area nonattainment, regardless of whether or
not the monitor is located such that it could be counted towards
meeting the proposed new network requirements. However, if the monitor
indicates that the monitoring site meets the 1-hour SO2
NAAQS, EPA's decision on the designation of the area would be made on a
case-by-case basis. One possible outcome is that the area may be
designated as unclassifiable because EPA would be unable to determine
whether the area is violating the 1-hour SO2 NAAQS, or
contributing to a violation in a nearby area, because of a lack of a
complete monitoring network meeting the new network requirements.
---------------------------------------------------------------------------

    \36\ EPA Regional Administrator approval will be required for
any state to discontinue an existing monitoring site, and EPA does
not expect that it will before 2011 approve discontinuation of
monitoring at any site which appears to have a substantial
likelihood of violating the 1-hour NAAQS.
---------------------------------------------------------------------------

    Accordingly, state Governors would need to submit their initial
designation recommendations to EPA no later than June 2011. If the
Administrator intends to modify any state recommendation, EPA would
notify the state's Governor no later than February 2012, 120 days prior
to promulgating the final designations. States would then have an
opportunity to comment on EPA's tentative decisions before EPA
promulgates the final designations in June 2012.
    While CAA section 107 specifically addresses states, EPA intends to
follow the same process for tribes to the extent practicable, pursuant
to section 301(d) of the CAA regarding tribal authority, and the Tribal
Authority Rule (63 FR 7254; February 12, 1998). Pursuant to the Tribal
Authority Rule, Tribes are not subject to the schedule requirements
that apply to states. However, EPA intends to promulgate designations
for Tribal land as well as state land according to the schedule
mandated for state land, so EPA encourages Tribes that wish to provide
input on EPA's designations to provide this input on the schedule
mandated for states.

VI. Clean Air Act Implementation Requirements

    This section of the preamble discusses the Clean Air Act (CAA)
requirements that states and emissions sources would need to address
when implementing new or revised SO2 NAAQS based on the
structure outlined in the CAA and existing rules.\37\ The EPA believes
that there are sufficient guidance documents and regulations currently
in place to fully implement the proposed revision to the SO2
NAAQS.\38\ However, EPA may provide additional guidance in the future,
as necessary, to assist states and emissions sources to comply with the
CAA provisions for implementing a new or revised SO2 NAAQS.
---------------------------------------------------------------------------

    \37\ Since EPA is proposing to take comments on retaining the
current 24-hr standards without revision if the 1-hr standard is set
at 100-150 ppb, the discussion in this section relates to
implementation of the proposed 1-hour standard and the possible
retention or revocation of the current 24-hr standard.
    \38\ See SO2 Guideline Document, Office of Air
Quality Planning and Standards, Research Triangle Park, NC 27711,
EPA-452/R-94-008, February 1994.
---------------------------------------------------------------------------

    The CAA assigns important roles to EPA, states and tribal
governments to achieve the NAAQS. States have the primary
responsibility for developing and implementing state implementation
plans (SIPs) that contain state measures necessary to achieve the air
quality standards in each area once EPA has established the NAAQS. EPA
provides assistance to states and tribes by providing technical tools,
assistance, and guidance, including information on the potential
control measures that may assist in helping areas attain the standards.
    Under section 110 of the CAA, 42 U.S.C. 7410, and related
provisions, states are directed to submit, for EPA approval, SIPs that
provide for the attainment and maintenance of such standards through
control programs directed at sources of SO2 emissions. If a
state fails to adopt and implement the required SIPs by the time
periods provided in the CAA, EPA has the responsibility under the CAA
to adopt a federal implementation plan (FIP) to assure that areas
attain the NAAQS in an expeditious manner. The states, in conjunction
with EPA, also administer the prevention of significant deterioration
(PSD) program for SO2. See sections 160-169 of the CAA, 42
U.S.C. 7470-7479. In addition, federal programs provide for nationwide
reductions in emissions of SO2 and other air pollutants
under Title II of the Act, 42 U.S.C. 7521-7574. These programs involve
limits on the sulfur content of the fuel used by automobiles, trucks,
buses, motorcycles, non-road engines and equipment, marine vessels and
locomotives. EPA is also in the process of establishing limits on the
sulfur content of the fuel used by ocean going vessels. Emissions
reductions for SO2 are also obtained from implementation of
the new source performance standards (NSPS) for stationary sources
under sections 111 and 129 of the CAA, 42 U.S.C. 7411 and 7429; and the
national emission standards for hazardous air pollutants (NESHAP) for
stationary sources under section 112 of the CAA, 42 U.S.C. 7412.

A. How this rule applies to tribes

    CAA section 301(d) authorizes EPA to treat eligible Indian tribes
in the same manner as states (TAS) under the CAA and requires EPA to
promulgate regulations specifying the provisions of the statute for
which such treatment is appropriate. EPA has promulgated these
regulations--known as the Tribal Authority Rule or TAR--at 40 CFR Part
49. See 63 FR 7254 (February 12, 1998). The TAR establishes the process
for Indian tribes to seek TAS eligibility and sets forth the CAA
functions for which TAS will be available. Under the TAR, eligible
tribes may seek approval for all CAA and regulatory purposes other than
a small number of functions enumerated at section 49.4. Implementation
plans

under section 110 are included within the scope of CAA functions for
which eligible tribes may obtain approval. Section 110(o) also
specifically describes tribal roles in submitting implementation plans.
Eligible Indian tribes may thus submit implementation plans covering
their reservations and other areas under their jurisdiction.
    The CAA and TAR do not, however, direct tribes to apply for TAS or
implement any CAA program. In promulgating the TAR EPA explicitly
determined that it was not appropriate to treat tribes similarly to
states for purposes of, among other things, specific plan submittal and
implementation deadlines for NAAQS-related requirements. 40 CFR
49.4(a). In addition, where tribes do seek approval of CAA programs,
including section 110 implementation plans, the TAR provides
flexibility and allows them to submit partial program elements, so long
as such elements are reasonably severable--i.e., ``not integrally
related to program elements that are not included in the plan
submittal, and are consistent with applicable statutory and regulatory
requirements''. 40 CFR 49.7.
    To date, very few tribes have sought TAS for purposes of section
110 implementation plans. However, some tribes may be interested in
pursuing such plans to implement today's proposed standard, once it is
promulgated. In several sections of this preamble, EPA describes the
various roles and requirements states will address in implementing
today's proposed standard. Such references to states generally include
eligible Indian tribes to the extent consistent with the flexibility
provided to tribes under the TAR. Where tribes do not seek TAS for
section 110 implementation plans, EPA under its discretionary authority
will promulgate FIPs as ``necessary or appropriate to protect air
quality.'' 40 CFR 49.11(a). EPA also notes that some tribes operate air
quality monitoring networks in their areas. For such monitors to be
used to measure attainment with the proposed revised primary NAAQS for
SO2, the criteria and procedures identified in this proposed
rule would apply.

B. Attainment dates

    The latest date by which an area is required to attain the
SO2 NAAQS is determined from the effective date of the
nonattainment designation for the affected area. For areas designated
nonattainment for the revised SO2 NAAQS, SIPs must provide
for attainment of the NAAQS as expeditiously as practicable, but no
later than 5 years from the effective date of the nonattainment
designation for the area. See section 192(a) of the CAA. The EPA will
determine whether an area has demonstrated attainment of the
SO2 NAAQS by evaluating air quality monitoring data
consistent with the form of the NAAQS for SO2, if revised,
which will be codified at 40 CFR part 50, Appendix T.
1. Attaining the NAAQS
    In order for an area to be redesignated as attainment, it must meet
five conditions provided under section 107(d)(3)(E) of the CAA. This
section requires that:
     EPA must have determined that the area has met the
SO2 NAAQS;
     EPA has fully approved the state's implementation plan;
     The improvement in air quality in the affected area is due
to permanent and enforceable reductions in emissions;
     EPA has fully approved a maintenance plan for the area;
and
     The state(s) containing the area have met all applicable
requirements under section 110 and part D.
2. Consequences of failing to attain by the statutory attainment date
    Any SO2 nonattainment area that fails to attain by its
statutory attainment date would be subject to the requirements of
sections 179(c) and (d) of the CAA. EPA is required to make a finding
of failure to attain no later than 6 months after the specified
attainment date and publish a notice in the Federal Register. The state
would then need to submit an implementation plan revision no later than
one year following the effective date of the Federal Register notice
making the determination of the area's failure to attain. This
submission must demonstrate that the standard will be attained as
expeditiously as practicable, but no later than 5 years from the
effective date of EPA's finding that the area failed to attain. In
addition, section 179(d)(2) provides that the SIP revision must include
any specific additional measures as may be reasonably prescribed by
EPA, including ``all measures that can be feasibly implemented in the
area in light of technological achievability, costs, and any nonair
quality and other air quality-related health and environmental
impacts.''

C. Section 110(a)(1) and (2) NAAQS infrastructure requirements

    Section 110(a)(2) of the CAA directs all states to develop and
maintain a solid air quality management infrastructure, including
enforceable emission limitations, an ambient monitoring program, an
enforcement program, air quality modeling capabilities, and adequate
personnel, resources, and legal authority. Section 110(a)(2)(D) also
requires state plans to prohibit emissions from within the state which
contribute significantly to nonattainment or maintenance areas in any
other state, or which interfere with programs under part C of the CAA
to prevent significant deterioration of air quality or to achieve
reasonable progress toward the national visibility goal for Federal
class I areas (national parks and wilderness areas).
    Under sections 110(a)(1) and (2) of the CAA, all states are
directed to submit SIPs to EPA which demonstrate that basic program
elements have been addressed within 3 years of the promulgation of any
new or revised NAAQS. Subsections (A) through (M) of section 110(a)(2)
set forth the elements that a state's program must contain in the
SIP.\39\ The list of section 110(a)(2) NAAQS implementation
requirements are the following:
---------------------------------------------------------------------------

    \39\ Two elements identified in section 110(a)(2) are not listed
below because, as EPA interprets the CAA, SIPs incorporating any
necessary local nonattainment area controls would not be due within
3 years, but rather are due at the time the nonattainment area
planning requirements are due. These elements are: (1) Emission
limits and other control measures, section 110(a)(2)(A), and (2)
Provisions for meeting part D, section 110(a)(2)(I), which requires
areas designated as nonattainment to meet the applicable
nonattainment planning requirements of part D, title I of the CAA.
---------------------------------------------------------------------------

     Ambient air quality monitoring/data system: Section
110(a)(2)(B) requires SIPs to provide for setting up and operating
ambient air quality monitors, collecting and analyzing data and making
these data available to EPA upon request.
     Program for enforcement of control measures: Section
110(a)(2)(C) requires SIPs to include a program providing for
enforcement of SIP measures and the regulation and permitting of new/
modified sources.
     Interstate transport: Section 110(a)(2)(D) requires SIPs
to include provisions prohibiting any source or other type of emissions
activity in the state from contributing significantly to nonattainment
or interfering with maintenance of the NAAQS in another state, or from
interfering with measures required to prevent significant deterioration
of air quality or to protect visibility.
     Adequate resources: Section 110(a)(2)(E) directs states to
provide assurances of adequate funding, personnel and legal authority
to implement their SIPs.
     Stationary source monitoring system: Section 110(a)(2)(F)
directs

states to establish a system to monitor emissions from stationary
sources and to submit periodic emissions reports to EPA.
     Emergency power: Section 110(a)(2)(G) directs states to
include contingency plans, and adequate authority to implement them,
for emergency episodes in their SIPs.
     Provisions for SIP revision due to NAAQS changes or
findings of inadequacies: Section 110(a)(2)(H) directs states to
provide for revisions of their SIPs in response to changes in the
NAAQS, availability of improved methods for attaining the NAAQS, or in
response to an EPA finding that the SIP is inadequate.
     Consultation with local and Federal government officials:
Section 110(a)(2)(J) directs states to meet applicable local and
Federal government consultation requirements when developing SIPs and
reviewing preconstruction permits.
     Public notification of NAAQS exceedances: Section
110(a)(2)(J) directs states to adopt measures to notify the public of
instances or areas in which a NAAQS is exceeded.
     PSD and visibility protection: Section 110(a)(2)(J) also
directs states to adopt emissions limitations, and such other measures,
as may be necessary to prevent significant deterioration of air quality
in attainment areas and protect visibility in Federal Class I areas in
accordance with the requirements of CAA Title I, part C.
     Air quality modeling/data: Section 110(a)(2)(K) requires
that SIPs provide for performing air quality modeling for predicting
effects on air quality of emissions of any NAAQS pollutant and
submission of data to EPA upon request.
     Permitting fees: Section 110(a)(2)(L) requires the SIP to
include requirements for each major stationary source to pay permitting
fees to cover the cost of reviewing, approving, implementing and
enforcing a permit.
     Consultation/participation by affected local government:
Section 110(a)(2)(M) directs states to provide for consultation and
participation by local political subdivisions affected by the SIP.

D. Attainment planning requirements

1. SO2 nonattainment area SIP requirements
    Any state containing an area designated as nonattainment with
respect to the SO2 NAAQS would need to develop for
submission to EPA a SIP meeting the requirements of part D, Title I, of
the CAA, providing for attainment by the applicable statutory
attainment date. See sections 191(a) and 192(a) of the CAA. As
indicated in section 191(a), all components of the SO2 part
D SIP must be submitted within 18 months of the effective date of an
area's designation as nonattainment.
    Section 172 of the CAA addresses the general requirements for areas
designated as nonattainment. Section 172(c) directs states with
nonattainment areas to submit a SIP which contains an attainment
demonstration showing that the affected area will attain the standard
by the applicable statutory attainment date. The SIP must show that the
area will attain the standard as expeditiously as practicable, and must
``provide for the implementation of all Reasonably Available Control
Measures (RACM) as expeditiously as practicable (including such
reductions in emissions from existing sources in the area as may be
obtained through the adoption, at a minimum, of Reasonably Available
Control Technology (RACT)).''
    SIPs required under Part D of the CAA must also provide for
reasonable further progress (RFP). See section 172(c)(2) of the CAA.
The CAA defines RFP as ``such annual incremental reductions in
emissions of the relevant air pollution as are required by part D, or
may reasonably be required by the Administrator for the purpose of
ensuring attainment of the applicable NAAQS by the applicable
attainment date.'' See section 171 of the CAA. Historically, for some
pollutants, RFP has been met by showing annual incremental emission
reductions sufficient to maintain generally linear progress toward
attainment by the applicable attainment date.
    All SO2 nonattainment area SIPs must include contingency
measures which must be implemented in the event that an area fails to
meet RFP or fails to attain the standards by its attainment date. See
section 172(c)(9) of the CAA. These contingency measures must be fully
adopted rules or control measures that take effect without further
action by the state or the Administrator. The EPA interprets this
requirement to mean that the contingency measures must be implemented
with only minimal further action by the state or the affected sources
with no additional rulemaking actions such as public hearings or
legislative review.
    Emission inventories are also critical for the efforts of state,
local, and federal agencies to attain and maintain the NAAQS that EPA
has established for criteria pollutants including SO2.
Section 191(a) in conjunction with section 172(c) requires that areas
designated as nonattainment for SO2 submit an emission
inventory to EPA no later than 18 months after designation as
nonattainment. In the case of SO2, sections 191(a) and
172(c) also direct states to submit periodic emission inventories for
nonattainment areas. The periodic inventory must include emissions of
SO2 for point, nonpoint, mobile, and area sources.
2. New source review and prevention of significant deterioration
requirements
    The Prevention of Significant Deterioration (PSD) and nonattainment
New Source Review (NSR) programs contained in parts C and D of Title I
of the CAA govern preconstruction review of any new or modified major
stationary sources of air pollutants regulated under the CAA as well as
any precursors to the formation of that pollutant when identified for
regulation by the Administrator.\40\ The EPA rules addressing these
programs can be found at 40 CFR 51.165, 51.166, 52.21, 52.24, and Part
51, appendix S.
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    \40\ The terms ``major'' and ``minor'' define the size of a
stationary source, for applicability purposes, in terms of an annual
emissions rate (tons per year, tpy) for a pollutant. Generally, a
minor source is any source that is not ``major.'' ``Major'' is
defined by the applicable regulations--PSD or nonattainment NSR.
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    The PSD program applies when a major source located in an area that
is designated as attainment or unclassifiable for any criteria
pollutant is constructed or undergoes a major modification.\41\ The
nonattainment NSR program applies on a pollutant-specific basis when a
major source constructs or modifies in an area that is designated as
nonattainment for that pollutant. The minor NSR program addresses major
and minor sources that undergo construction or modification activities
that do not qualify as major, and it applies, as necessary to assure
attainment, regardless of the designation of the area in which a source
is located.
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    \41\ In addition, the PSD program applies to non-criteria
pollutants subject to regulation under the Act, except those
pollutants regulated under section 112 and pollutants subject to
regulation only under section 211(o).
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    PSD permit requirements are effective on the promulgation date of a
new or revised standard. SIPs that address the PSD requirements related
to attainment areas are due no later than 3 years after the
promulgation of a revised NAAQS for SO2. The PSD
requirements include but are not limited to the following:
     Installation of Best Available Control Technology (BACT);
     Air quality monitoring and modeling analyses to ensure
that a project's emissions will not cause or contribute to a violation
of any NAAQS

or maximum allowable pollutant increase (PSD increment);
     Notification of Federal Land Manager of nearby Class I
areas; and public comment on the permit.
    If EPA establishes a 1-hour NAAQS for SO2, the owner or
operator of any major stationary source or major modification locating
in an attainment or unclassifiable area for SO2 will be
required, as a prerequisite for a PSD permit, to demonstrate that the
emissions increases from the new or modified source will not cause or
contribute to a violation of the that new NAAQS. The EPA does not
anticipate that this will pose a technical problem, since the modeling
capability and SO2 emissions input data already exist.
Depending on the final form of the 1-hour NAAQS, it may be necessary to
make adjustments to the AERMOD modeling system to accommodate the form
of the standard; however, EPA anticipates that any such adjustments can
be readily accomplished in coordination with the promulgation of any
new NAAQS for SO2 in time to enable states to implement such
standard via the PSD program. The analyses for the 1-hour NAAQS will be
in addition to the existing demonstration of compliance for the annual
and 24-hour SO2 NAAQS, which will continue to be required
unless EPA revokes these standards in conjunction with its promulgation
of a new 1-hour NAAQS for SO2.
    The owner or operator of a new or modified source will still be
required to demonstrate compliance with the annual and 24-hour
SO2 increments, even if their counterpart NAAQS are revoked.
The annual and 24-hour increments are established in the CAA and will
need to remain in the PSD regulations because EPA does not interpret
the Clean Air Act to authorize EPA to remove them. It appears necessary
for Congress to amend the Act to make appropriate changes to the
statutory SO2 increments, perhaps similar to the way the Act
was amended to accommodate PM10 increments in lieu of the
statutory TSP increments. If we establish a new 1-hour SO2
NAAQS, EPA will consider the need to adopt new 1-hour SO2
increments.
    In association with the requirement to demonstrate compliance with
the NAAQS and increments, the owner or operator of a new or modified
source must submit for review and approval a source impact analysis and
an air quality analysis. The source impact analysis, primarily a
modeling analysis, must demonstrate that allowable emissions increases
from the proposed source or modification, in conjunction with emissions
from other existing sources will not cause or contribute to either a
NAAQS or increment violation. The air quality analysis must assess the
ambient air quality in the area that the proposed source or
modification would affect.
    For the air quality analysis, the owner or operator must submit in
its permit application air quality monitoring data that shall have been
gathered over a period of one year and is representative of air quality
in the area of the proposed project. If existing data representative of
the area of the proposed project is not available, new data may need to
be collected by the owner or operator of the source or modification.
Where data is already available, it might be necessary to evaluate the
location of the monitoring sites from which the SO2 data
were collected in comparison to any new siting requirements associated
with the 1-hour NAAQS. If existing sites are inappropriate for
providing the necessary representative data, then new monitoring data
will need to be collected by the owner or operator of the proposed
project.
    Historically, EPA has allowed the use of several screening tools to
help facilitate the implementation of the new source review program by
reducing the permit applicant's burden, and streamlining the permitting
process for de minimis circumstances. These screening tools include a
significant emissions rate (SER), significant impact levels (SILs), and
a significant monitoring concentration (SMC). The SER, as defined in
tons per year for each regulated pollutant, is used to determine
whether any proposed source or modification will emit sufficient
amounts of a particular pollutant to require the review of that
pollutant under the NSR permit program. EPA will consider whether to
evaluate the existing significant emissions rate (SER) for
SO2 to see if it would change substantially based on the
NAAQS levels for the 1-hour averaging period. Historically, we have
defined a de minimis pollutant impact as one that results in a modeled
ambient impact of less than approximately 4% of the short-term NAAQS.
The current SER for SO2 (40 tpy) is based on the impact on
the 24-hour SO2 NAAQS. See, 45 FR 52676, 52707 (August 7,
1980). We have typically used the most sensitive averaging period to
calculate the SER, and we may want to evaluate the new 1-hour period
for SO2 because it is likely to represent most sensitive
averaging period for SO2.
    The SIL, expressed as an ambient pollutant concentration ([mu]g/
m\3\), is used to determine whether the impact of a particular
pollutant is significant enough to warrant a complete air quality
impact analysis for any applicable NAAQS and increments. EPA has
promulgated regulations under 40 CFR 51.165(b) which include SILs for
SO2 to determine whether a source's impact would be
considered to cause or contribute to a NAAQS violation for either the
3-hour, 24-hour or annual averaging periods. These SILs were originally
developed in 1978 to limit the application of air quality dispersion
models to a downwind distance of no more than 50 kilometers or to
``insignificant levels.'' See, 43 FR 26398, June 19, 1978. Through
guidance, EPA has also allowed the use of SILs to determine whether or
not it is necessary for a source to carry out a comprehensive source
impact analysis and to determine the extent of the impact area in which
the analysis will be carried out. The existing SILs for SO2
were not developed on the basis of specific SO2 NAAQS
levels, so if the existing NAAQS are not being revised, there is
probably no need to revise the existing SILs. Even if we decide to
revoke any of the existing NAAQS, the corresponding SIL should still be
useful for increment assessment. A SIL for the 1-hour averaging period
does not exist, and would need to be developed for use with modeling
for 1-hour SO2 NAAQS and increments (if and when developed).
    Finally, the SMC, also measured as an ambient pollutant
concentration ([mu]g/m\3\), is used to determine whether it may be
appropriate to exempt a proposed project from the requirement to
collect ambient monitoring data for a particular pollutant as part of a
complete permit application. EPA first defined SMCs for regulated
pollutants under the PSD program in 1980. See, 45 FR 52676, 52709-10
(August 7, 1980). The existing SMC for SO2, based on a 24-
hour averaging period, may need to be re-evaluated to consider the
effect of basing the SMC on the 1-hour averaging period, especially in
light of the fact that we may revoke the NAAQS for the 24-hour
averaging period. Third, even if the 1-hour averaging period does not
indicate the need for a revised SMC for SO2, the fact that
the original SMC for SO2 is based on 1980 monitoring data
(Lowest Detectable Level, correction factor of ``5''), could be a basis
for revising the existing value. More up-to-date monitoring data and
statistical analyses of monitoring accuracy may yield a different--
possibly lower--correction factor today. A new 1-hour NAAQS would not
necessarily cause this result, but may provide a ``window

of opportunity'' to re-evaluate the SMC for SO2. See
sections II.E.2 and II.F.2 above.
    As a means of reducing the permit applicant's burden, and to
streamline permitting, permit authorities use screening tools referred
to as significant impact levels (SILs) and a significant monitoring
concentration (SMC). EPA issued unofficial SO2 SILs for the
3-hour (secondary standard), 24-hour and annual averaging periods.
These SILs were developed in 1978 to limit the application of air
quality dispersion models to a downwind distance of no more than 50
kilometers or to ``insignificant levels.'' See, 43 FR 263--, 26398,
(June 19, 1978). These values were not developed on the basis of
specific SO2 NAAQS levels, so if the existing NAAQS are not
being revised, there is probably no need to revise the existing SILs.
Even if we decide to revoke any of the existing NAAQS, the
corresponding SIL should still be useful for increment assessment. A
SIL for the 1-hour averaging period does not exist, and would need to
be developed for use with modeling for the 1-hour SO2 NAAQS
and increments (if and when developed).
    States which have areas designated as nonattainment for the
SO2 NAAQS are directed to submit, as a part of the SIP due
18 months after an area is designated as nonattainment, provisions
requiring permits for the construction and operation of new or modified
stationary sources anywhere in the nonattainment area. Prior to
adoption of the SIP revision addressing major source nonattainment NSR
for SO2 nonattainment areas, the requirements of 40 CFR part
51, appendix S will apply. Nonattainment NSR requirements include but
are not limited to:
     Installation of Lowest Achievable Emissions Rate (LAER)
control technology;
     Offsetting new emissions with creditable emissions
reductions;
     A certification that all major sources owned and operated
in the state by the same owner are in compliance with all applicable
requirements under the CAA;
     An alternative siting analysis demonstrating that the
benefits of a proposed source significantly outweigh the environmental
and social costs imposed as a result of its location, construction, or
modification; and
     Public comment on the permit.
    Minor NSR programs must meet the statutory requirements in section
110(a)(2)(C) of the CAA which requires ``* * * regulation of the
modification and construction of any stationary source * * * as
necessary to assure that the [NAAQS] are achieved.'' These programs
must be established in each state within 3 years of the promulgation of
a new or revised NAAQS.
3. General conformity
    Section 176(c) of the CAA requires that all federal actions conform
to an applicable implementation plan developed pursuant to section 110
and part D of the CAA. The EPA rules developed under section 176(c)
prescribe the criteria and procedures for demonstrating and assuring
conformity of federal actions to a SIP. Each federal agency must
determine that any actions covered by the general conformity rule
conform to the applicable SIP before the action is taken. The criteria
and procedures for conformity apply only in nonattainment areas and
those areas redesignated attainment since 1990 (``maintenance areas'')
with respect to the criteria pollutants under the CAA \42\: carbon
monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone
(O3), particulate matter (PM2.5 and PM10), and
sulfur dioxide (SO2). The general conformity rules apply one
year following the effective date of designations for any new or
revised NAAQS.\43\
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    \42\ Criteria pollutants are those pollutants for which EPA has
established a NAAQS under section 109 of the CAA.
    \43\ Transportation conformity is required under CAA section
176(c) (42 U.S.C. 7506(c) to ensure that federally supported highway