change breakpoints at the higher end of the AQI scale (from 200 to
500), which would apply to State contingency plans or the Significant
Harm Level (40 CFR 51.16), because the information from this review
does not inform decisions about breakpoints at those higher levels.
    With regard to an AQI value of 50, the breakpoint between the good
and moderate categories, historically this value is set at the level of
the annual NAAQS, if there is one, or one-half the level of the short-
term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 12,
1998). Taking into consideration this practice, EPA proposed to set the
AQI value of 50 to be between 25 and 50 ppb SO2, 1-hour
average; stating that concentrations toward the lower end of this range
would be appropriate if the standard was set at the lower end of the
range of proposed standard levels, while concentrations toward the
higher end of this range would be more appropriate if the standard was
set at the higher end of the range of proposed standard levels. EPA
solicited comments on this range for an AQI value of 50 and the
appropriate basis for selecting an AQI value of 50.
    With regard to an AQI value of 150, the breakpoint between the
unhealthy for sensitive groups and unhealthy categories, historically
values between the short-term standard and an AQI value of 500 are set
at levels that are approximately equidistant between the AQI values of
100 and 500 unless there is health evidence that suggests a specific
level would be appropriate (63 FR 67829, Dec. 12, 1998). For an AQI
value of 150, EPA proposed to set the breakpoint within the range from
175 to 200 ppb SO2, 1-hour average, since it represents the
midpoint between the proposed range for the short-term

standard and the level of an AQI value of 200 (300 ppb SO2,
1-hour average).
    EPA received few comments on the proposed breakpoints. Consistent
with the level of the short-term primary SO2 standard
promulgated in this rule, EPA is setting the AQI value of 100, the
breakpoint between the moderate and unhealthy for sensitive groups
category, at 75 ppb, 1-hour average. EPA is setting the AQI value of
50, the breakpoint between the good and moderate categories, at 35 ppb
SO2, 1-hour average, which is approximately one-half the
level of the new short-term standard, since the annual SO2
standard is being revoked. EPA is setting the AQI value of 150, the
breakpoint between the unhealthy for sensitive groups and unhealthy
categories, at 185 ppb SO2, 1-hour average, which represents
the approximate midpoint between the level of the new short-term
standard (75 ppb SO2, 1-hour average) and the level of an
AQI value of 200 (300 ppb SO2, 1-hour average).
    EPA received comments from several State environmental
organizations and organizations of State and local air agencies about
forecasting and reporting the AQI for SO2. These commenters
expressed the view that forecasting hourly SO2
concentrations would be difficult. One commenter requested that EPA
delay the forecasting requirement for one year and other agencies
requested that EPA provide assistance in developing a forecast model.
Another commenter expressed the view that it is impractical to
incorporate SO2 into its forecasting and public health
notification program because SO2 does not behave like a
regional pollutant, and that exceedances may occur with little or no
warning and for two hours or less. This commenter requested EPA
consider the resources necessary for public communications at the State
and local levels, particularly in areas where other air quality
exceedances are relatively rare.
    EPA recommends and encourages air quality forecasting but it is not
required (64 FR 42548; August 4, 1999). We agree that there will be new
challenges associated with creating and communicating an SO2
forecast, and will work with State and local agencies that want to
develop an SO2 forecasting program on issues including, but
not limited to, forecasting air quality for short time periods. We plan
to work with State and local air agencies to figure out the best way to
present this information to the public using the AQI.
    With respect to the comment that it is impractical to incorporate
SO2 into a forecasting and public health notification
program because SO2 does not behave like a regional
pollutant, this final rule departs from the proposed rule in that it
allows for a combined monitoring and modeling approach. Because of
this, the monitoring network is not required to be wholly source-
oriented in nature. States have flexibility to allow required
monitoring sites to serve multiple monitoring objectives including
characterizing source impacts, highest concentrations, population
exposure, background, and regional transport. Further, EPA expects that
much of the existing network will be retained by States to satisfy the
minimum monitoring requirements. This means that it is unlikely that
AQI reporting and forecasting will be heavily driven by source-oriented
monitors. Rather, many of the existing monitors (a majority of which
are community-wide monitors) will remain in place, which prevents the
need for new geographic regions to be delineated. With respect to
concerns expressed about the resources required to report the AQI in
areas were exceedances of the standard are very rare, Appendix G to
Part 58 specifies that if the index value for a particular pollutant
remains below 50 for a season or year, then a State or local agency may
exclude the pollutant from the calculation of the AQI.

IX. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under section 3(f)(1) of Executive Order 12866 (58 FR 51735,
October 4, 1993), this action is an ``economically significant
regulatory action'' because it is likely to have an annual effect on
the economy of $100 million or more. Accordingly, EPA submitted this
action to the Office of Management and Budget (OMB) for review under EO
12866 and any changes made in response to OMB recommendations have been
documented in the docket for this action. In addition, EPA prepared a
Regulatory Impact Analysis (RIA) of the potential costs and benefits
associated with this action. However, the CAA and judicial decisions
make clear that the economic and technical feasibility of attaining the
national ambient standards cannot be considered in setting or revising
NAAQS, although such factors may be considered in the development of
State implementation plans to implement the standards. Accordingly,
although an RIA has been prepared, the results of the RIA have not been
considered by EPA in developing this final rule.
    When estimating the SO2- and PM2.5-related
human health benefits and compliance costs in Table 2 below, EPA
applied methods and assumptions consistent with the state-of-the-
science for human health impact assessment, economics and air quality
analysis. EPA applied its best professional judgment in performing this
analysis and believes that these estimates provide a reasonable
indication of the expected benefits and costs to the nation of the
selected SO2 standard and alternatives considered by the
Agency. The Regulatory Impacts Analysis (RIA) available in the docket
describes in detail the empirical basis for EPA's assumptions and
characterizes the various sources of uncertainties affecting the
estimates below.
    EPA's 2009 Integrated Science Assessment for Particulate Matter
concluded, based on the scientific literature, that a no-threshold log-
linear model most adequately portrays the PM-mortality concentration-
response relationship. Nonetheless, consistent with historical practice
and our commitment to characterizing the uncertainty in our benefits
estimates, EPA has included a sensitivity analysis with an assumed
threshold in the PM-mortality health impact function in the RIA. EPA
has included a sensitivity analysis in the RIA to help inform our
understanding of the health benefits which can be achieved at lower air
quality concentration levels. While the primary estimate and the
sensitivity analysis are not directly comparable, due to differences in
population data and use of different analysis years, as well as the
difference in the assumption of a threshold in the sensitivity
analysis, comparison of the two results provide a rough sense of the
proportion of the health benefits that occur at lower PM2.5
air quality levels. Using a threshold of 10 [mu]g/m\3\ is an arbitrary
choice (EPA could have assumed 6, 8, or 12 [mu]g/m\3\ for the
sensitivity analysis). Assuming a threshold of 10 [mu]g/m\3\, the
sensitivity analysis shows that roughly one-third of the benefits occur
at air quality levels below that threshold. Because the primary
estimates reflect EPA's current methods and data, EPA notes that
caution should be exercised when comparing the results of the primary
and sensitivity analyses. EPA appreciates the value of sensitivity
analyses in highlighting the uncertainty in the benefits estimates and
will continue to work to refine these analyses, particularly in those
instances in which air quality modeling data are available.
    Table 2 shows the results of the cost and benefits analysis for
each standard alternative. As indicated above, implementation of the
SO2 control

measures identified from AirControlNET and other sources does not
result in attainment with the all target NAAQS levels in several areas.
In these areas, additional unspecified emission reductions might be
necessary to reach some alternative standard levels. The first part of
the table, labeled Partial attainment (identified controls), shows only
those benefits and costs from control measures we were able to
identify. The second part of the table, labeled Unidentified Controls,
shows only additional benefits and costs resulting from unidentified
controls. The third part of the table, labeled Full attainment, shows
total benefits and costs resulting from both identified and
unidentified controls. It is important to emphasize that we were able
to identify control measures for a significant portion of attainment
for many of those counties that would not fully attain the target NAAQS
level with identified controls. Note also that in addition to
separating full and partial attainment, the table also separates the
portion of benefits associated with reduced SO2 exposure
(i.e., SO2 benefits) from the additional benefits associated
with reducing SO2 emissions, which are precursors to
PM2.5 formation--(i.e., the PM2.5 co-benefits).
For instance, for the selected standard of 75 ppb, $2.2 million in
benefits are associated with reduced SO2 exposure while $15
billion to $37 billion are associated with reduced PM2.5
exposure.

                                    Table 2--Monetized Benefits and Costs To Attain Alternate Standard Levels in 2020
                                                                 [Millions of 2006$] \a\
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                      Number of
                                   counties fully   Discount rate   Monetized SO2   Monetized PM2.5 co-       Costs                Net benefits
                                     controlled       (percent)       benefits         benefits,c,d
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                        Partial Attainment (identified controls)
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb...........................              40               3             \b\  $30,000 to $74,000..          $2,600  $27,000 to $71,000.
                                   ..............               7  ..............  $28,000 to $67,000..  ..............  $25,000 to $64,000.
75 ppb...........................              20               3             \b\  $14,000 to $35,000..            $960  $13,000 to $34,000.
                                   ..............               7  ..............  $13,000 to $31,000..  ..............  $12,000 to $30,000.
100 ppb..........................               6               3             \b\  $6,900 to $17,000...            $470  $6,400 to $17,000.
                                   ..............               7  ..............  $6,200 to $15,000...  ..............  $5,700 to $15,000.
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                  Unidentified Controls
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb...........................              16               3             \b\  $4,000 to $9,000....          $1,800  $2,200 to $7,200.
                                   ..............               7  ..............  $3,000 to $8,000....  ..............  $1,200 to $6,200.
75 ppb...........................               4               3             \b\  $1,000 to $3,000....            $500  $500 to $1,500.
                                   ..............               7  ..............  $1,000 to $3,000....  ..............  $500 to $2,500.
100 ppb..........................               3               3             \b\  $500 to $1,000......            $260  $240 to $740.
                                   ..............               7  ..............  $500 to $1,000......  ..............  $240 to $740.
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                     Full Attainment
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb...........................              56               3           $8.50  $34,000 to $83,000..          $4,400  $30,000 to $79,000.
                                   ..............               7  ..............  $31,000 to $75,000..  ..............  $27,000 to $71,000.
75 ppb...........................              24               3           $2.20  $15,000 to $37,000..          $1,500  $14,000 to $36,000
                                   ..............               7  ..............  $14,000 to $34,000..  ..............  $13,000 to $33,000.
100 ppb..........................               9               3           $0.60  $7,400 to $18,000...            $730  $6,700 to $17,000.
                                   ..............               7  ..............  $6,700 to $16,000...  ..............  $6,000 to $15,000.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Estimates have been rounded to two significant figures and therefore summation may not match table estimates.
\b\ The approach used to simulate air quality changes for SO2 did not provide the data needed to distinguish partial attainment benefits from full
  attainment benefits from reduced SO2 exposure. Therefore, a portion of the SO2 benefits is attributable to the known controls and a portion of the SO2
  benefits are attributable to the unidentified controls. Because all SO2-related benefits are short-term effects, the results are identical for all
  discount rates.
\c\ Benefits are shown as a range from Pope et al. (2002) to Laden et al. (2006). Monetized benefits do not include unquantified benefits, such as other
  health effects, reduced sulfur deposition, or improvements in visibility.
\d\ These models assume that all fine particles, regardless of their chemical composition, are equally potent in causing premature mortality because
  there is no clear scientific evidence that would support the development of differential effects estimates by particle type. Reductions in SO2
  emissions from multiple sectors to meet the SO2 NAAQS would primarily reduce the sulfate fraction of PM2.5. Because this rule targets a specific
  particle precursor (i.e., SO2), this introduces some uncertainty into the results of the analysis.

B. Paperwork Reduction Act

    The information collection requirements in this final rule have
been submitted for approval to the Office of Management and Budget
(OMB) under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The
Information Collection Request (ICR) document prepared by EPA for these
revisions to part 58 has been assigned EPA ICR number 2370.02. The
information collected under 40 CFR part 53 (e.g., test results,
monitoring records, instruction manual, and other associated
information) is needed to determine whether a candidate method intended
for use in determining attainment of the NAAQS in 40 CFR part 50 will
meet the design, performance, and/or comparability requirements for
designation as a Federal reference method (FRM) or Federal equivalent
method (FEM). We do not expect the number of FRM or FEM determinations
to increase over the number that is currently used to estimate burden
associated with SO2 FRM/FEM determinations provided in the
current ICR for 40 CFR part 53 (EPA ICR numbers 2370.01). As such, no
change in the burden estimate for 40 CFR part 53 has been made as part
of this rulemaking.
    The information collected and reported under 40 CFR part 58 is
needed to determine compliance with the NAAQS, to characterize air
quality and

associated health impacts, to develop emissions control strategies, and
to measure progress for the air pollution program. The amendments would
revise the technical requirements for SO2 monitoring sites,
require the siting and operation of additional SO2 ambient
air monitors, and the reporting of the collected ambient SO2
monitoring data to EPA's Air Quality System (AQS). The ICR is estimated
to involve 102 respondents for a total approximate cost of $15,203,762
(total capital, and labor and non-labor operation and maintenance) and
a total burden of 207,662 hours. The labor costs associated with these
hours is $11,130,409. Included in the $15,203,762 total are other costs
of other non-labor operations and maintenance of $1,104,377 and
equipment and contract costs of $2,968,975. In addition to the costs at
the State and local air quality management agencies, there is a burden
to EPA for a total of 14,749 hours and $1,060,621. Burden is defined at
5 CFR 1320.3(b). State, local, and Tribal entities are eligible for
State assistance grants provided by the Federal government under the
CAA which can be used for monitors and related activities. An agency
may not conduct or sponsor, and a person is not required to respond to,
a collection of information unless it displays a currently valid OMB
control number. The OMB control numbers for EPA's regulations in 40 CFR
are listed in 40 CFR part 9.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of this rule on small
entities, small entity is defined as: (1) A small business that is a
small industrial entity as defined by the Small Business
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not
dominant in its field.
    After considering the economic impacts of this final rule on small
entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This final
rule will not impose any requirements on small entities. Rather, this
rule establishes national standards for allowable concentrations of
SO2 in ambient air as required by section 109 of the CAA.
American Trucking Ass'ns v. EPA, 175 F.3d 1027, 1044-45 (DC Cir. 1999)
(NAAQS do not have significant impacts upon small entities because
NAAQS themselves impose no regulations upon small entities). Similarly,
the amendments to 40 CFR Part 58 address the requirements for States to
collect information and report compliance with the NAAQS and will not
impose any requirements on small entities.

D. Unfunded Mandates Reform Act

    This action is not subject to the requirements of sections 202 and
205 of the UMRA. EPA has determined that this final rule does not
contain a Federal mandate that may result in expenditures of $100
million or more for State, local, and Tribal governments, in the
aggregate, or the private sector in any one year. The revisions to the
SO2 NAAQS impose no enforceable duty on any State, local or
Tribal governments or the private sector. The expected costs associated
with the monitoring requirements are described in EPA's ICR document,
but those costs are not expected to exceed $100 million in the
aggregate for any year. Furthermore, as indicated previously, in
setting a NAAQS, EPA cannot consider the economic or technological
feasibility of attaining ambient air quality standards. Because the CAA
prohibits EPA from considering the types of estimates and assessments
described in section 202 when setting the NAAQS, the UMRA does not
require EPA to prepare a written statement under section 202 for the
revisions to the SO2 NAAQS.
    With regard to implementation guidance, the CAA imposes the
obligation for States to submit SIPs to implement the SO2
NAAQS. In this final rule, EPA is merely providing an interpretation of
those requirements. However, even if this rule did establish an
independent obligation for States to submit SIPs, it is questionable
whether an obligation to submit a SIP revision would constitute a
Federal mandate in any case. The obligation for a State to submit a SIP
that arises out of section 110 and section 191 of the CAA is not
legally enforceable by a court of law, and at most is a condition for
continued receipt of highway funds. Therefore, it is possible to view
an action requiring such a submittal as not creating any enforceable
duty within the meaning of U.S.C. 658 for purposes of the UMRA. Even if
it did, the duty could be viewed as falling within the exception for a
condition of Federal assistance under U.S.C. 658.
    EPA has determined that this final rule contains no regulatory
requirements that might significantly or uniquely affect small
governments because it imposes no enforceable duty on any small
governments. Therefore, this rule is not subject to the requirements of
section 203 of the UMRA.

E. Executive Order 13132: Federalism

    This final rule does not have federalism implications. It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. The rule does not alter the
relationship between the Federal government and the States regarding
the establishment and implementation of air quality improvement
programs as codified in the CAA. Under section 109 of the CAA, EPA is
mandated to establish NAAQS; however, CAA section 116 preserves the
rights of States to establish more stringent requirements if deemed
necessary by a State. Furthermore, this rule does not impact CAA
section 107 which establishes that the States have primary
responsibility for implementation of the NAAQS. Finally, as noted in
section E (above) on UMRA, this rule does not impose significant costs
on State, local, or Tribal governments or the private sector. Thus,
Executive Order 13132 does not apply to this rule.

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

    Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by Tribal officials in the development of regulatory
policies that have Tribal implications.'' This final rule does not have
Tribal implications, as specified in Executive Order 13175. It does not
have a substantial direct effect on one or more Indian Tribes, on the
relationship between the Federal government and Indian Tribes, or on
the distribution of power and responsibilities between the

Federal government and Tribes. The rule does not alter the relationship
between the Federal government and Tribes as established in the CAA and
the TAR. Under section 109 of the CAA, EPA is mandated to establish
NAAQS; however, this rule does not infringe existing Tribal authorities
to regulate air quality under their own programs or under programs
submitted to EPA for approval. Furthermore, this rule does not affect
the flexibility afforded to Tribes in seeking to implement CAA programs
consistent with the TAR, nor does it impose any new obligation on
Tribes to adopt or implement any NAAQS. Finally, as noted in section E
(above) on UMRA, this rule does not impose significant costs on Tribal
governments. Thus, Executive Order 13175 does not apply to this rule.

G. Executive Order 13045: Protection of Children From Environmental
Health and Safety Risks

    This action is subject to Executive Order (62 FR 19885, April 23,
1997) because it is an economically significant regulatory action as
defined by Executive Order 12866, and we believe that the environmental
health risk addressed by this action has a disproportionate effect on
children. The final rule will establish uniform national ambient air
quality standards for SO2; these standards are designed to
protect public health with an adequate margin of safety, as required by
CAA section 109. The protection offered by these standards may be
especially important for asthmatics, including asthmatic children,
because respiratory effects in asthmatics are among the most sensitive
health endpoints for SO2 exposure. Because asthmatic
children are considered a sensitive population, we have evaluated the
potential health effects of exposure to SO2 pollution among
asthmatic children. These effects and the size of the population
affected are discussed in chapters 3 and 4 of the ISA; chapters 3, 4,
7, 8, 9 of the REA, and sections II.A through II.E of this preamble.

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

    This rule is not a ``significant energy action'' as defined in
Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR
28355; May 22, 2001) because it is not likely to have a significant
adverse effect on the supply, distribution, or use of energy. The
purpose of this rule is to establish revised NAAQS for SO2.
The rule does not prescribe specific control strategies by which these
ambient standards will be met. Such strategies will be developed by
States on a case-by-case basis, and EPA cannot predict whether the
control options selected by States will include regulations on energy
suppliers, distributors, or users. Thus, EPA concludes that this rule
is not likely to have any adverse energy effects.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (NTTAA), Public Law 104-113, section 12(d) (15 U.S.C. 27)
directs EPA to use voluntary consensus standards in its regulatory
activities unless to do so would be inconsistent with applicable law or
otherwise impractical. Voluntary consensus standards are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) that are developed or adopted by
voluntary consensus standards bodies. The NTTAA directs EPA to provide
Congress, through OMB, explanations when the Agency decides not to use
available and applicable voluntary consensus standards.
    This final rulemaking involves technical standards with regard to
ambient monitoring of SO2. The use of this voluntary
consensus standard would be impractical because the analysis method
does not provide for the method detection limits necessary to
adequately characterize ambient SO2 concentrations for the
purpose of determining compliance with the final revisions to the
SO2 NAAQS.

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

    Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes
Federal executive policy on environmental justice. Its main provision
directs Federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
    EPA has determined that this final rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health effects on
any population, including any minority or low-income population. The
final rule will establish uniform national standards for SO2
in ambient air.

References

American Lung Association. (2001). Urban air pollution and health
inequities: a workshop report. Environ Health Perpect. 109(S3):357-
374.
American Thoracic Society. (1985). Guidelines as to what constitutes
an adverse respiratory health effect, with special reference to
epidemiologic studies of air pollution. Am Rev Respir Dis. 131:666-
668.
American Thoracic Society. (2000). What constitutes an adverse
health effect of air pollution? Am J Respir Crit Care Med. 161:665-
673.
Brode (2010). Representativeness of Ambient Monitoring Data for
SO2 Epidemiological Studies. Sulfur Dioxide Review
Docket. Docket ID No. EPA-HQ-OAR-2007-0352. Available at http://
www.regulations.gov.
EPA. (1982). Air Quality Criteria for Particulate Matter and Sulfur
Oxides. U.S. EPA, Research Triangle Park, NC: Office of Health and
Environmental Assessment. Available at: http://cfpub.epa.gov/ncea/
cfm/recordisplay.cfm?deid=46205.
EPA. (1986). Second Addendum to Air Quality Criteria for Particulate
Matter and Sulfur Oxides (1982): Assessment of Newly Available
Health Effects Information. US EPA, Research Triangle Park, NC:
Office of Health and Environmental Assessment. Available at: http://
cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=39469.
EPA. (1994a). Supplement to the Second Addendum (1986) to Air
Quality Criteria for Particulate Matter and Sulfur Oxides (1982).
Research Triangle Park, NC: Office of Health and Environmental
Assessment, Environmental Criteria and Assessment Office. EPA-600/
FP-93/002. Available at: http://cfpub.epa.gov/ncea/cfm/
recordisplay.cfm?deid=96580.
EPA. (1994b). Review of the National Ambient Air Quality Standards
for Sulfur Oxides: Assessment of Scientific and Technical
Information, Supplement to the 1986 OAQPS Staff Paper Addendum.
Research Triangle Park, NC: Office of Air Quality Planning and
Standards. EPA-452/R-94/013. Sulfur Dioxide Review Docket. Docket ID
No. EPA-HQ-OAR-2007-0352. Available at http://www.regulations.gov.
EPA. (2005). Review of the National Ambient Air Quality Standards
for Particulate Matter: Policy Assessment of Scientific and
Technical Information--OAQPS Staff Paper. Research Triangle Park,
NC: Office of Air Quality Planning and Standards. EPA-452/R-05-005a.
Available at: http://www.epa.gov/ttn/naaqs/standards/pm/data/
pmstaffpaper_20051221.pdf.

EPA (2006). Air Quality Criteria for Ozone and Related Photochemical
Oxidants (Final); Available at: http://www.epa.gov/ttn/naaqs/
standards/ozone/s_o3_cr_cd.html.
EPA. (2007a). SO2 NAAQS Review Plan--Draft. US EPA,
Research Triangle Park, NC: National Center for Environmental
Assessment. Available at http://www.epa.gov/ttn/naaqs/standards/so2/
s_so2_cr_pd.html.
EPA. (2007b). Sulfur Dioxide Health Assessment Plan: Scope and
Methods for Exposure and Risk Assessment. US EPA Research Triangle
Park, NC: Office of Air Quality Planning and Standards, Research
Triangle Park. Available at: http://www.epa.gov/ttn/naaqs/standards/
so2/s_so2_cr_pd.html.
EPA (2007c). Review of the National Ambient Air Quality Standards
for Pb: Policy Assessment of Scientific and Technical Information.
OAQPS Staff paper. Office of Air Quality Planning and Standards,
Research Triangle Park, NC. EPA-452/R-07-013. Available at: http://
www.epa.gov/ttn/naaqs/standards/pb/data/20071101_pb_staff.pd.
EPA. (2007d). Review of the National Ambient Air Quality Standards
for Ozone: Assessment of Scientific and Technical Information, OAQPS
Staff paper. Office of Air Quality Planning and Standards, Research
Triangle Park, NC. EPA-452/R-07-007a. Available at: http://epa.gov/
ttn/naaqs/standards/ozone/s_o3_cr_sp.html.
EPA. (2007e). Review of the National Ambient Air Quality Standards
for Ozone: Ozone Health Risk Assessment for Selected Urban Areas.
Office of Air Quality Planning and Standards, Research Triangle
Park, NC. EPA 452/R-07-009. Available at: http://www.epa.gov/ttn/
naaqs/standards/ozone/s_o3_cr_td.html.
EPA. (2008a). Integrated Science Assessment (ISA) for Sulfur
Oxides--Health Criteria (Final Report). EPA/600/R-08/047F. Available
at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=198843.
EPA. (2008b). Integrated Science Assessment (ISA) for Oxides of
Nitrogen--Health Criteria (Final Report). EPA/600/R-08/071 .
Available at: http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_
cr_isi.html.
EPA. (2008c). Risk and Exposure Assessment to Support the Review of
the NO2 Primary National Ambient Air Quality Standard.
EPA-452/R-08-008a; Available at: http://www.epa.gov/ttn/naaqs/
standards/nox/s_nox_cr_rea.html.
EPA. (2009a). Risk and Exposure Assessment to Support the Review of
the SO2 Primary National Ambient Air Quality Standards-
Final Report. EPA-452/R-09-007; Available at: http:// www.epa.gov/
ttn/naaqs/standards/so2/s_so2_cr_rea.html.
EPA. (2009b). Modern SO2 Instrument Performance Data.
Spreadsheet of performance data for existing UVF analyzers. Office
of Research and Development. Sulfur Dioxide Review Docket. Docket ID
No. EPA-HQ-OAR-2007-0352. Available at http://www.regulations.gov.
EPA. (2010). Responses to Significant Comments on the 2009 Proposed
Rule on the Primary National Ambient Air Quality Standards for
Sulfur Dioxide (December 8, 2009; 74 FR 64810) Sulfur Dioxide Review
Docket. Docket ID No. EPA-HQ-OAR-2007-0352. Available at http://
www.regulations.gov. Also available at: http://www.epa.gov/ttn/
naaqs/standards/so2/s_so2_cr_rc.html.
Henderson. (2006). Letter to EPA Administrator Stephen Johnson:
Clean Air Scientific Advisory Committee's (CASAC) Peer Review of the
Agency's 2nd Draft Ozone Staff Paper. EPA-CASAC-07-001. October 24,
2006. Sulfur Dioxide Review Docket. Docket ID No. EPA-HQ-OAR-2007-
0352-0044. Available at http://www.regulations.gov.
Henderson. (2008a). Letter to EPA Administrator Stephen Johnson:
Clean Air Scientific Advisory Committee's (CASAC) Peer Review of
EPA's Integrated Science Assessment (ISA) for Sulfur Oxides--Health
Criteria (Second External Review Draft, May 2008). EPA-CASAC-08-017
August 8, 2008. Sulfur Dioxide Review Docket. Available at http://
www.regulations.gov.
Henderson. (2008b). Letter to EPA Administrator Stephen Johnson:
Clean Air Scientific Advisory Committee's (CASAC) Peer Review of
EPA's Risk and Exposure Assessment to Support the Review of the
SO2 Primary National Ambient Air Quality Standards (First
Draft, July 2008). EPA-CASAC-08-019. August 22, 2008. Sulfur Dioxide
Review Docket. Docket ID No. EPA-HQ-OAR-2007-0352-0034. Available at
http://www.regulations.gov.
Ito K. (2007). Characterization of PM2.5, gaseous
pollutants, and meteorological interactions in the context of time-
series health effects models. J Expos Sci Environ Epidemiol. 17:S45-
S60.
Jaffe DH, Singer ME, Rimm AA. (2003). Air pollution and emergency
department visits for asthma among Ohio medicaid recipients, 1991-
1996. Environ Res. 91:21-28.
Johns. (2009). Presentation and analysis of controlled human
exposure data described in Table 3-1 of the 2008 Integrated Science
Assessment (ISA) for Sulfur Oxides; April 29, 2009. Available at:
http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_rea.html.
Johns and Simmons (2009). Memorandum to the Sulfur Oxides NAAQS
Review Docket. Quality Assurance Review of Individual Subject Data
Presented in Table 3-1 of the 2008 Integrated Science Assessment
(ISA) for Sulfur Oxides. Air Quality Criteria for Sulfur Oxides
Docket. Docket ID No. EPA-HQ-ORD-2006-0260-0036. Available at http:/
/www.regulations.gov.
Linn WS, Avol EL, Peng RC, Shamoo DA, Hackney JD. (1987). Replicated
dose-response study of sulfur dioxide effects in normal, atopic, and
asthmatic volunteers. Am Rev Respir Dis. 136:1127-1134.
NY DOH. (2006). A Study of Ambient Air Contaminants and Asthma in
New York City. ATSDR Final Report NTIS PB2006-113523.
Albany, NY; New York State Energy Research and Development
Authority; New York State Department of Health, for Atlanta, GA;
Agency for Toxic Substances and Disease Registry; U.S. Department of
Health and Human Services.
Samet JM. (2009). Letter to EPA Administrator Lisa P. Jackson: Clean
Air Scientific Advisory Committee's (CASAC) Review of EPA's Risk and
Exposure Assessment to Support the Review of the SO2
Primary National Ambient Air Quality Standards: Second Draft. EPA-
CASAC-09-007, May 18, 2009. Sulfur Dioxide Review Docket. Docket ID
No. EPA-HQ-OAR-2007-0352-0035. Available at http://
www.regulations.gov.
Schwartz J. (1995). Short term fluctuations in air pollution and
hospital admissions of the elderly for respiratory disease. Thorax.
50:531-538.
Thompson R. (2009). Sulfur Dioxide Descriptive Statistics Tables.
Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, Research Triangle Park, NC. Sulfur Dioxide Review
Docket. Docket ID No. EPA-HQ-OAR-2007-0352-0036. Available at http:/
/www.regulations.gov.
Thompson R and Stewart MJ. (2009). Air Quality Statistics for Cities
Referenced in Key U.S. and Canadian Hospital Admission and Emergency
Department Visits for All Respiratory Causes and Asthma. Office of
Air Quality Planning and Standards, U.S. Environmental Protection
Agency, Research Triangle Park, NC. Sulfur Dioxide Review Docket.
Docket ID No. EPA-HQ-OAR-2007-0352-0018. Available at http://
www.regulations.gov.
Watkins and Thompson. (2009). SO2 Network Review and
Background; OAQPS; Office of Air Quality Planning and Standards,
U.S. Environmental Protection Agency, Research Triangle Park, NC.
Sulfur Dioxide NAAQS Review Docket. (OAR-2005-0352). Sulfur Dioxide
Review Docket. Docket ID No. EPA-HQ-OAR-2007-0352-0037. Available at
http://www.regulations.gov.
Wilson AM, Wake CP, Kelly T, Salloway JC. (2005). Air pollution,
weather, and respiratory emergency room visits in two northern New
England cities: An ecological time-series study. Environ Res.
97:312-321.

List of Subjects

40 CFR Part 50

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

40 CFR Part 53

    Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental

relations, Reporting and recordkeeping requirements.

40 CFR Part 58

    Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.

    Dated: June 2, 2010.
Lisa P. Jackson,
Administrator.

0
For the reasons stated in the preamble, title 40, chapter I of the Code
of Federal Regulations is amended as follows:

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

0
1. The authority citation for part 50 continues to read as follows:

    Authority: 42 U.S.C. 7401, et seq.

0
2. Section 50.4 is amended by adding paragraph (e) to read as follows:

Sec.  50.4  National primary ambient air quality standards for sulfur
oxides (sulfur dioxide).

* * * * *
    (e) The standards set forth in this section will remain applicable
to all areas notwithstanding the promulgation of SO2
national ambient air quality standards (NAAQS) in Sec.  50.17. The
SO2 NAAQS set forth in this section will no longer apply to
an area one year after the effective date of the designation of that
area, pursuant to section 107 of the Clean Air Act, for the
SO2 NAAQS set forth in Sec.  50. 17; except that for areas
designated nonattainment for the SO2 NAAQS set forth in this
section as of the effective date of Sec.  50. 17, and areas not meeting
the requirements of a SIP call with respect to requirements for the
SO2 NAAQS set forth in this section, the SO2
NAAQS set forth in this section will apply until that area submits,
pursuant to section 191 of the Clean Air Act, and EPA approves, an
implementation plan providing for attainment of the SO2
NAAQS set forth in Sec.  50.17.

0
3. Section 50.14 is amended by revising paragraph (c)(2)(vi) to read as
follows:

Sec.  50.14  Treatment of air quality monitoring data influenced by
exceptional events.

* * * * *
    (c) * * *
    (2) * * *
    (vi) 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 exceptional event data, providing initial data descriptions
and providing detailed data documentation in AQS for the initial
designations of areas for those NAAQS. Table 1 provides the schedule
for submission of flags with initial descriptions in AQS and detailed
documentation. These schedules shall apply for those data which will or
may influence the initial designation of areas for those NAAQS. EPA
anticipates revising Table 1 as necessary to accommodate revised data
submission schedules for new or revised NAAQS.

Table 1--Schedule of Exceptional Event Flagging and Documentation Submission for Data To Be Used in Designations
                                       Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
                                            Air quality
    NAAQS Pollutant/standard/(level)/     data collected   Event flagging & initial     Detailed documentation
            promulgation date              for calendar      description deadline         submission deadline
                                               year
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m3)             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 \1\
                                                    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, 2010. \a\
----------------------------------------------------------------------------------------------------------------
SO 2/1-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, 2012,          or March 31, 2012,
                                                           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.

* * * * *

0
4. A new 50.17 is added to read as follows:

Sec.  50.17  National primary ambient air quality standards for sulfur
oxides (sulfur dioxide).

    (a) The level of the national primary 1-hour annual ambient air
quality standard for oxides of sulfur is 75 parts per billion (ppb,
which is 1 part in 1,000,000,000), measured in the ambient air as
sulfur dioxide (SO2).
    (b) The 1-hour primary standard is met at an ambient air quality
monitoring site when the three-year average of the annual (99th
percentile) of the daily maximum 1-hour average concentrations is less
than or equal to 75 ppb, as determined in accordance with Appendix T of
this part.
    (c) The level of the standard shall be measured by a reference
method based on Appendix A or A-1 of this part, or by a Federal
Equivalent Method (FEM)

designated in accordance with part 53 of this chapter.

0
5. Add Appendix A-1 to Part 50 to read as follows:

Appendix A-1 to Part 50--Reference Measurement Principle and
Calibration Procedure for the Measurement of Sulfur Dioxide in the
Atmosphere (Ultraviolet Fluorescence Method)

1.0 Applicability

    1.1 This ultraviolet fluorescence (UVF) method provides a
measurement of the concentration of sulfur dioxide (SO2)
in ambient air for determining compliance with the national primary
and secondary ambient air quality standards for sulfur oxides
(sulfur dioxide) as specified in Sec.  50.4, Sec.  50.5, and Sec.
50.17 of this chapter. The method is applicable to the measurement
of ambient SO2 concentrations using continuous (real-
time) sampling. Additional quality assurance procedures and guidance
are provided in part 58, Appendix A, of this chapter and in
Reference 3.

2.0 Principle

    2.1 This reference method is based on automated measurement of
the intensity of the characteristic fluorescence released by
SO2 in an ambient air sample contained in a measurement
cell of an analyzer when the air sample is irradiated by ultraviolet
(UV) light passed through the cell. The fluorescent light released
by the SO2 is also in the ultraviolet region, but at
longer wavelengths than the excitation light. Typically, optimum
instrumental measurement of SO2 concentrations is
obtained with an excitation wavelength in a band between
approximately 190 to 230 nm, and measurement of the SO2
fluorescence in a broad band around 320 nm, but these wavelengths
are not necessarily constraints of this reference method. Generally,
the measurement system (analyzer) also requires means to reduce the
effects of aromatic hydrocarbon species, and possibly other
compounds, in the air sample to control measurement interferences
from these compounds, which may be present in the ambient air.
References 1 and 2 describe UVF method.
    2.2 The measurement system is calibrated by referencing the
instrumental fluorescence measurements to SO2 standard
concentrations traceable to a National Institute of Standards and
Technology (NIST) primary standard for SO2 (see
Calibration Procedure below).
    2.3 An analyzer implementing this measurement principle is shown
schematically in Figure 1. Designs should include a measurement
cell, a UV light source of appropriate wavelength, a UV detector
system with appropriate wave length sensitivity, a pump and flow
control system for sampling the ambient air and moving it into the
measurement cell, sample air conditioning components as necessary to
minimize measurement interferences, suitable control and measurement
processing capability, and other apparatus as may be necessary. The
analyzer must be designed to provide accurate, repeatable, and
continuous measurements of SO2 concentrations in ambient
air, with measurement performance as specified in Subpart B of Part
53 of this chapter.
    2.4 Sampling considerations: The use of a particle filter on the
sample inlet line of a UVF SO2 analyzer is required to
prevent interference, malfunction, or damage due to particles in the
sampled air.

3.0 Interferences

    3.1 The effects of the principal potential interferences may
need to be mitigated to meet the interference equivalent
requirements of part 53 of this chapter. Aromatic hydrocarbons such
as xylene and naphthalene can fluoresce and act as strong positive
interferences. These gases can be removed by using a permeation type
scrubber (hydrocarbon ``kicker''). Nitrogen oxide (NO) in high
concentrations can also fluoresce and cause positive interference.
Optical filtering can be employed to improve the rejection of
interference from high NO. Ozone can absorb UV light given off by
the SO2 molecule and cause a measurement offset. This
effect can be reduced by minimizing the measurement path length
between the area where SO2 fluorescence occurs and the
photomultiplier tube detector (e.g. <5 cm). A hydrocarbon scrubber,
optical filter and appropriate distancing of the measurement path
length may be required method components to reduce interference.

4.0 Calibration Procedure

    Atmospheres containing accurately known concentrations of sulfur
dioxide are prepared using a compressed gas transfer standard
diluted with accurately metered clean air flow rates.
    4.1 Apparatus: Figure 2 shows a typical generic system suitable
for diluting a SO2 gas cylinder concentration standard
with clean air through a mixing chamber to produce the desired
calibration concentration standards. A valve may be used to
conveniently divert the SO2 from the sampling manifold to
provide clean zero air at the output manifold for zero adjustment.
The system may be made up using common laboratory components, or it
may be a commercially manufactured system. In either case, the
principle components are as follows:
    4.1.1 SO2 standard gas flow control and measurement
devices (or a combined device) capable of regulating and maintaining
the standard gas flow rate constant to within 2 percent
and measuring the gas flow rate accurate to within 2,
properly calibrated to a NIST-traceable standard.
    4.1.2 Dilution air flow control and measurement devices (or a
combined device) capable of regulating and maintaining the air flow
rate constant to within 2 percent and measuring the air
flow rate accurate to within 2, properly calibrated to a
NIST-traceable standard.
    4.1.3 Mixing chamber, of an inert material such as glass and of
proper design to provide thorough mixing of pollutant gas and
diluent air streams.
    4.1.4 Sampling manifold, constructed of glass,
polytetrafluoroethylene (PTFE TeflonTM), or other
suitably inert material and of sufficient diameter to insure a
minimum pressure drop at the analyzer connection, with a vent
designed to insure a minimum over-pressure (relative to ambient air
pressure) at the analyzer connection and to prevent ambient air from
entering the manifold.
    4.1.5 Standard gas pressure regulator, of clean stainless steel
with a stainless steel diaphragm, suitable for use with a high
pressure SO2 gas cylinder.

4.1.6 Reagents

    <IP_ADDRESS> SO2 gas concentration transfer standard
having a certified SO2 concentration of not less than 10
ppm, in N2, traceable to a NIST Standard Reference
Material (SRM).
    <IP_ADDRESS> Clean zero air, free of contaminants that could cause a
detectable response or a change in sensitivity of the analyzer.
Since ultraviolet fluorescence analyzers may be sensitive to
aromatic hydrocarbons and O2-to-N2 ratios, it
is important that the clean zero air contains less than 0.1 ppm
aromatic hydrocarbons and O2 and N2
percentages approximately the same as in ambient air. A procedure
for generating zero air is given in reference 1.

4.2 Procedure

    4.2.1 Obtain a suitable calibration apparatus, such as the one
shown schematically in Figure 1, and verify that all materials in
contact with the pollutant are of glass, TeflonTM, or
other suitably inert material and completely clean.
    4.2.2 Purge the SO2 standard gas lines and pressure
regulator to remove any residual air.
    4.2.3 Ensure that there are no leaks in the system and that the
flow measuring devices are properly and accurately calibrated under
the conditions of use against a reliable volume or flow rate
standard such as a soap-bubble meter or a wet-test meter traceable
to a NIST standard. All volumetric flow rates should be corrected to
the same reference temperature and pressure by using the formula
below:
[GRAPHIC] [TIFF OMITTED] TR22JN10.000

Where:

Fc = corrected flow rate (L/min at 25 [deg]C and 760 mm Hg),
Fm = measured flow rate, (at temperature, Tm and
pressure, Pm),
Pm = measured pressure in mm Hg, (absolute), and
Tm = measured temperature in degrees Celsius.

    4.2.4 Allow the SO2 analyzer under calibration to
sample zero air until a stable response is obtained, then make the
proper zero adjustment.
    4.2.5 Adjust the airflow to provide an SO2
concentration of approximately 80 percent of the upper measurement
range limit of the SO2 instrument and verify that the
total air flow of the calibration system exceeds the demand of all
analyzers sampling from the output manifold (with the excess
vented).
    4.2.6 Calculate the actual SO2 calibration
concentration standard as:

[GRAPHIC] [TIFF OMITTED] TR22JN10.001

Where:

C = the concentration of the SO2 gas standard
Fp = the flow rate of SO2 gas standard
Ft = the total air flow rate of pollutant and diluent gases

    4.2.7 When the analyzer response has stabilized, adjust the
SO2 span control to obtain the desired response
equivalent to the calculated standard concentration. If substantial
adjustment of the span control is needed, it may be necessary to re-
check the zero and span adjustments by repeating steps 4.2.4 through
4.2.7 until no further adjustments are needed.
    4.2.8 Adjust the flow rate(s) to provide several other
SO2 calibration concentrations over the analyzer's
measurement range. At least five different concentrations evenly
spaced throughout the analyzer's range are suggested.
    4.2.9 Plot the analyzer response (vertical or Y-axis) versus
SO2 concentration (horizontal or X-axis). Compute the
linear regression slope and intercept and plot the regression line
to verify that no point deviates from this line by more than 2
percent of the maximum concentration tested.

    Note: Additional information on calibration and pollutant
standards is provided in Section 12 of Reference 3.

5.0 Frequency of Calibration

    The frequency of calibration, as well as the number of points
necessary to establish the calibration curve and the frequency of
other performance checking will vary by analyzer; however, the
minimum frequency, acceptance criteria, and subsequent actions are
specified in Reference 3, Appendix D: Measurement Quality Objectives
and Validation Template for SO2 (page 9 of 30). The
user's quality control program should provide guidelines for initial
establishment of these variables and for subsequent alteration as
operational experience is accumulated. Manufacturers of analyzers
should include in their instruction/operation manuals information
and guidance as to these variables and on other matters of
operation, calibration, routine maintenance, and quality control.

6.0 References for SO2 Method

1. H. Okabe, P. L. Splitstone, and J. J. Ball, ``Ambient and Source
SO2 Detector Based on a Fluorescence Method'', Journal of
the Air Control Pollution Association, vol. 23, p. 514-516 (1973).
2. F. P. Schwarz, H. Okabe, and J. K. Whittaker, ``Fluorescence
Detection of Sulfur Dioxide in Air at the Parts per Billion Level,''
Analytical Chemistry, vol. 46, pp. 1024-1028 (1974).
3. QA Handbook for Air Pollution Measurement Systems--Volume II.
Ambient Air Quality Monitoring Programs. U.S. EPA. EPA-454/B-08-003
(2008).
BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TR22JN10.002

[GRAPHIC] [TIFF OMITTED] TR22JN10.003

BILLING CODE 6560-50-C

0
6. Appendix A to Part 50 is redesignated as Appendix A-2 to Part 50.

0
7. Appendix T to Part 50 is added to read as follows:

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

1. General

    (a) This appendix explains the data handling conventions and
computations necessary for determining when the primary national
ambient air quality standards for Oxides of Sulfur as measured by
Sulfur Dioxide (``SO2 NAAQS'') specified in Sec.  50.17
are met at an ambient air quality monitoring site. Sulfur Dioxide
(SO2) is measured in the ambient air by a Federal
reference method (FRM) based on appendix A or A-1 to this part or by
a Federal equivalent method (FEM) designated in accordance with part
53 of this chapter. Data handling and computation procedures to be
used in making comparisons between reported SO2
concentrations and the levels of the SO2 NAAQS are
specified in the following sections.
    (b) Decisions to exclude, retain, or make adjustments to the
data affected by exceptional events, including natural events, are
made according to the requirements and process deadlines specified
in Sec. Sec.  50.1, 50.14 and 51.930 of this chapter.
    (c) The terms used in this appendix are defined as follows:
    Daily maximum 1-hour values for SO2 refers to the
maximum 1-hour SO2 concentration values measured from
midnight to midnight (local standard time) that are used in NAAQS
computations.
    Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
specified in section 5 of this appendix. The design value for the
primary 1-hour NAAQS is the 3-year average of annual 99th percentile
daily maximum 1-hour values for a monitoring site (referred to as
the ``1-hour primary standard design value'').
    99th percentile daily maximum 1-hour value is the value below
which nominally 99 percent of all daily maximum 1-hour concentration
values fall, using the ranking and selection method specified in
section 5 of this appendix.
    Pollutant Occurrence Code (POC) refers to a numerical code (1,
2, 3, etc.) used to distinguish the data from two or more monitors
for the same parameter at a single monitoring site.
    Quarter refers to a calendar quarter.
    Year refers to a calendar year.

2. Requirements for Data Used for Comparisons With the SO2
NAAQS and Data Reporting Considerations

    (a) All valid FRM/FEM SO2 hourly data required to be
submitted to EPA's Air Quality System (AQS), or otherwise available
to EPA, meeting the requirements of part 58 of this chapter
including appendices A, C, and E shall be used in design value
calculations. Multi-hour average concentration values collected by
wet chemistry methods shall not be used.
    (b) Data from two or more monitors from the same year at the
same site reported to EPA under distinct Pollutant Occurrence Codes
shall not be combined in an attempt to meet data completeness
requirements. The Administrator will combine annual 99th percentile
daily maximum concentration values from different monitors in
different years, selected as described here, for the purpose of
developing a valid 1-hour primary standard design value. If more
than one of the monitors meets the completeness requirement for all
four quarters of a year, the steps specified in section 5(a) of this
appendix shall be applied to the data from the monitor with the
highest average of the four quarterly completeness values to derive
a valid annual 99th percentile daily maximum concentration. If no
monitor is complete for all four quarters in a year, the steps
specified in section 3(c) and 5(a) of this appendix shall be applied
to the data from the monitor with the highest average of the four
quarterly completeness values in an attempt to derive a valid annual
99th percentile daily maximum concentration. This paragraph does not
prohibit a monitoring agency from making a local designation of one
physical monitor as the primary monitor for a Pollutant Occurrence
Code and substituting the 1-hour data from a second physical monitor
whenever a valid concentration value is not obtained from the
primary monitor; if a monitoring agency substitutes data in this
manner, each substituted value must be accompanied by an

AQS qualifier code indicating that substitution with a value from a
second physical monitor has taken place.
    (c) Hourly SO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.

3. Comparisons With the 1-Hour Primary SO2 NAAQS

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

4. Rounding Conventions for the 1-Hour Primary SO2 NAAQS

    (a) Hourly SO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
    (b) Daily maximum 1-hour values and therefore the annual 99th
percentile of those daily values are not rounded.
    (c) The 1-hour primary standard design value is calculated
pursuant to section 5 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).

5. Calculation Procedures for the 1-Hour Primary SO2 NAAQS

    (a) Procedure for identifying annual 99th percentile values.
When the data for a particular ambient air quality monitoring site
and year meet the data completeness requirements in section 3(b), or
if one of the conditions of section 3(c) is met, or if the
Administrator exercises the discretionary authority in section 3(d),
identification of annual 99th percentile value is accomplished as
follows.
    (i) The annual 99th percentile value for a year is the higher of
the two values resulting from the following two procedures.

    (1) Procedure 1. For the year, determine the number of days with
at least 75 percent of the hourly values reported.
    (A) For the year, determine the number of days with at least 75
percent of the hourly values reported including State-flagged data
affected by exceptional events which have been approved for
exclusion by the Administrator.
    (B) For the year, from only the days with at least 75 percent of
the hourly values reported, select from each day the maximum hourly
value excluding State-flagged data affected by exceptional events
which have been approved for exclusion by the Administrator.
    (C) Sort all these daily maximum hourly values from a particular
site and year by descending value. (For example: (x[1], x[2], x[3],
* * *, x[n]). In this case, x[1] is the largest number and x[n] is
the smallest value.) The 99th percentile is determined from this
sorted series of daily values which is ordered from the highest to
the lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days with
valid data for year y (cny). The corresponding ``n''
value in the right column identifies the rank of the annual 99th
percentile value in the descending sorted list of daily site values
for year y. Thus, P0.99, y = the nth largest value.
    (2) Procedure 2. For the year, determine the number of days with
at least one hourly value reported.
    (A) For the year, determine the number of days with at least one
hourly value reported including State-flagged data affected by
exceptional events which have been approved for exclusion by the
Administrator.
    (B) For the year, from all the days with at least one hourly
value reported, select from each day the maximum hourly value
excluding State-flagged data affected by exceptional events which
have been approved for exclusion by the Administrator.
    (C) Sort all these daily maximum values from a particular site
and year by descending value. (For example: (x[1], x[2], x[3], * *
*, x[n]). In this case, x[1] is the largest number and x[n] is the
smallest value.) The 99th percentile is determined from this sorted
series of daily values which is ordered from the highest to the
lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days with
valid data for year y (cny). The corresponding ``n''
value in the right column identifies the rank of the annual 99th
percentile value in the descending sorted list of daily site values
for year y. Thus, P0.99,y = the nth largest value.
    (b) The 1-hour primary standard design value for an ambient air
quality monitoring site is mean of the three annual 99th percentile
values, rounded according to the conventions in section 4.

                                 Table 1
------------------------------------------------------------------------
                                                     P0.99,y is the nth
  Annual number of days with valid data for year    maximum value of the
                    ``y'' (cny)                     year, where n is the
                                                        listed number
------------------------------------------------------------------------
1-100.............................................                     1
101-200...........................................                     2
201-300...........................................                     3
301-366...........................................                     4
------------------------------------------------------------------------

PART 53-AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS

0
8. The authority citation for part 53 continues to read as follows:

    Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec.
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat.
1713, unless otherwise noted.

Subpart A--[Amended]

0
9. Section 53.2 is amended by revising paragraphs (a)(1) and (b) to
read as follows:

Sec.  53.2  General requirements for a reference method determination.

* * * * *
    (a) Manual methods--(1) Sulfur dioxide (SO2) and Lead.
For measuring SO2 and lead, appendixes A-2 and G of part 50
of this chapter specify unique manual FRM for measuring those
pollutants. Except as provided in Sec.  53.16, other manual methods for
lead will not be considered for a reference method determination under
this part.
* * * * *
    (b) Automated methods. An automated FRM for measuring
SO2, CO, O3, or NO2 must utilize the
measurement principle and calibration procedure specified in the
appropriate appendix to part 50 of this chapter (appendix A-1 only for
SO2 methods) and must have been shown in accordance with
this part to meet the requirements specified in this subpart A and
subpart B of this part.

0
10. Section 53.8 is amended by revising paragraph (c) to read as
follows:

Sec.  53.8  Designation of reference and equivalent methods.

* * * * *
    (c) The Administrator will maintain a current list of methods
designated as FRM or FEM in accordance with this part and will send a
copy of the list to any person or group upon request. A copy of the
list will be available via the Internet and may be available from other
sources.

0
11. Table A-1 to Subpart A is revised to read as follows:

  Table A-1 to Subpart A of Part 53--Summary of Applicable Requirements for Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Applicable subparts of part 53
     Pollutant          Reference or       Manual or automated   Applicable part 50 appendix -----------------------------------------------------------
                         equivalent                                                               A         B         C         D         E         F
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...............  Reference...........  Manual..............  A-2
                                          Automated...........  A-1                           [check]   [check]
                    Equivalent..........  Manual..............  A-1                           [check]             [check]
                                          Automated...........  A-1                           [check]   [check]   [check]
CO................  Reference...........  Automated...........  C                             [check]   [check]
                    Equivalent..........  Manual..............  C                             [check]             [check]
                                          Automated...........  C                             [check]   [check]   [check]
O3................  Reference...........  Automated...........  D                             [check]   [check]
                    Equivalent..........  Manual..............  D                             [check]             [check]
                                          Automated...........  D                             [check]   [check]   [check]
NO2...............  Reference...........  Automated...........  F                             [check]   [check]
                    Equivalent..........  Manual..............  F                             [check]             [check]
                                          Automated...........  F                             [check]   [check]   [check]
Pb................  Reference...........  Manual..............  G
                    Equivalent..........  Manual..............  G                             [check]             [check]
                                          Automated...........  G                             [check]             [check]
PM10-Pb...........  Reference...........  Manual..............  Q
                    Equivalent..........  Manual..............  Q                             [check]             [check]

                                          Automated...........  Q                             [check]             [check]
PM10..............  Reference...........  Manual..............  J                             [check]                       [check]
                    Equivalent..........  Manual..............  J                             [check]             [check]   [check]
                                          Automated...........  J                             [check]             [check]   [check]
PM2.5.............  Reference...........  Manual..............  L                             [check]                                 [check]
                    Equivalent Class I..  Manual..............  L                             [check]             [check]             [check]
                    Equivalent Class II.  Manual..............  L\1\                          [check]             [check]\            [check]   [check]\
                                                                                2\                          1 2\
                    Equivalent Class III  Automated...........  L\1\                          [check]             [check]             [check]   [check]\
                                                                                1\
PM10	2.5..........  Reference...........  Manual..............  L, O                          [check]                                 [check]
                    Equivalent Class I..  Manual..............  L, O                          [check]             [check]             [check]
                    Equivalent Class II.  Manual..............  L, O                          [check]             [check]\            [check]   [check]\
                                                                                2\                          1 2\
                    Equivalent Class III  Automated...........  L\1\, O\1\                    [check]             [check]             [check]   [check]\
                                                                                1\
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator.
2. Alternative Class III requirements may be substituted.

Subpart B--[Amended]

0
12. Section 53.20 is amended by revising paragraph (b) and Table B-1 in
paragraph (c) to read as follows:

Sec.  53.20  General provisions.

* * * * *
    (b) For a candidate method having more than one selectable
measurement range, one range must be that specified in table B-1
(standard range for SO2), and a test analyzer representative
of the method must pass the tests required by this subpart while
operated in that range. The tests may be repeated for one or more
broader ranges (i.e., ones extending to higher concentrations) than the
range specified in table B-1, provided that the range does not extend
to concentrations more than four times the upper range limit specified
in table B-1. For broader ranges, only the tests for range
(calibration), noise at 80% of the upper range limit, and lag, rise and
fall time are required to be repeated. The tests may be repeated for
one or more narrower ranges (ones extending to lower concentrations)
than that specified in table B-1. For SO2 methods, table B-1
specifies special performance requirements for narrower (lower) ranges.
For methods other than SO2, only the tests for range
(calibration), noise at 0% of the measurement range, and lower
detectable limit are required to be repeated. If the tests are
conducted or passed only for the specified range (standard range for
SO2), any FRM or FEM method determination with respect to
the method will be limited to that range. If the tests are passed for
both the specified range and one or more broader ranges, any such
determination will include the additional range(s) as well as the
specified range, provided that the tests required by subpart C of this
part (if applicable) are met for the broader range(s). If the tests are
passed for both the specified range and one or more narrower ranges,
any FRM or FEM method determination for the method will include the
narrower range(s) as well as the specified range. Appropriate test data
shall be submitted for each range sought to be included in a FRM or FEM
method determination under this paragraph (b).
    (c) * * *

                                               Table B-1--Performance Specifications for Automated Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                        SO 2
                                                            ----------------------------                                         Definitions and test
       Performance parameter                Units \1\                           Lower        O 3          CO         NO 2             procedures
                                                             Std. range \3\   range 2 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range...........................  ppm...................         0-0.5       <0.5        0-0.5           0-50     0-0.5    Sec. 53.23(a).
2. Noise...........................  ppm...................           0.001      0.0005       0.005          0.5       0.005  Sec. 53.23(b).
3. Lower detectable limit..........  ppm...................           0.002      0.001        0.010          1.0       0.010  Sec. 53.23(c).
4. Interference equivalent
    Each interferent...............  ppm...................      0.005  minus>0.00  minus>0.02    minus>1.0  minus>0.02
                                                                                5
    Total, all interferents........  ppm...................          --         --            0.06           1.5       0.04   Sec. 53.23(d).
5. Zero drift, 12 and 24 hour......  ppm...................      0.004  minus>0.00  minus>0.02    minus>1.0  minus>0.02
                                                                                2
6. Span drift, 24 hour
    20% of upper range limit.......  Percent...............          --         --       20.0   minus>10.0  minus>20.0
    80% of upper range limit.......  Percent...............      3.0    minus>3.0   minus>5.0     minus>2.5  minus>5.0
7. Lag time........................  Minutes...............           2          2           20               10      20      Sec. 53.23(e).
8. Rise time.......................  Minutes...............           2          2           15                5      15      Sec. 53.23(e).
9. Fall time.......................  Minutes...............           2          2           15                5      15      Sec. 53.23(e).
10. Precision
    20% of upper range limit.......  ppm...................          --         --            0.010          0.5       0.020  Sec. 53.23(e).
                                     Percent...............           2          2       ..........  ...........  ..........  Sec. 53.23(e).
    80% of upper range limit.......  ppm...................          --         --            0.010          0.5       0.030  Sec. 53.23(e).
                                     Percent...............           2          2           --               --      --      Sec. 53.23(e).
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas.
  Percent means percent of the upper range limit.
2. Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range shows that
  the lower range specification is met for each of these test parameters.

3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct
  suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are
  within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct
  calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM
  designation.
4. For nitric oxide interference for the SO2 UVF method, interference equivalent is 0.003 ppm for the lower range.

* * * * *

0
13. Section 53.21 is amended by revising paragraph (a) to read as
follows:

Sec.  53.21  Test conditions.

    (a) Set-up and start-up of the test analyzer shall be in strict
accordance with the operating instructions specified in the manual
referred to in Sec.  53.4(b)(3). Allow adequate warm-up or
stabilization time as indicated in the operating instructions before
beginning the tests. The test procedures assume that the test analyzer
has an analog measurement signal output that is connected to a suitable
strip chart recorder of the servo, null-balance type. This recorder
shall have a chart width of a least 25 centimeters, chart speeds up to
10 cm per hour, a response time of 1 second or less, a deadband of not
more than 0.25 percent of full scale, and capability either of reading
measurements at least 5 percent below zero or of offsetting the zero by
at least 5 percent. If the test analyzer does not have an analog signal
output, or if other types of measurement data output are used, an
alternative measurement data recording device (or devices) may be used
for the tests, provided it is reasonably suited to the nature and
purposes of the tests and an analog representation of the analyzer
measurements for each test can be plotted or otherwise generated that
is reasonably similar to the analog measurement recordings that would
be produced by a conventional chart recorder.
* * * * *

0
14. Section 53.22(d) is amended by revising Table B-2 to read as
follows:

Sec.  53.22  Generation of test atmospheres.

* * * * *
    (d) * * *

                       Table B-2--Test Atmospheres
------------------------------------------------------------------------
           Test gas                  Generation          Verification
------------------------------------------------------------------------
Ammonia.......................  Permeation device.   Indophenol method,
                                 Similar to system    reference 3.
                                 described in
                                 references 1 and 2.
Carbon dioxide................  Cylinder of zero     Use NIST-certified
                                 air or nitrogen      standards whenever
                                 containing CO2 as    possible. If NIST
                                 required to obtain   standards are not
                                 the concentration    available, obtain
                                 specified in Table   2 standards from
                                 B-3.                 independent
                                                      sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Carbon monoxide...............  Cylinder of zero     Use a FRM CO
                                 air or nitrogen      analyzer as
                                 containing CO as     described in
                                 required to obtain   reference 8.
                                 the concentration
                                 specified in Table
                                 B-3.
Ethane........................  Cylinder of zero     Gas chromatography,
                                 air or nitrogen      ASTM D2820,
                                 containing ethane    reference 10. Use
                                 as required to       NIST-traceable
                                 obtain the           gaseous methane or
                                 concentration        propane standards
                                 specified in Table   for calibration.
                                 B-3.
Ethylene......................  Cylinder of pre-     Do.
                                 purified nitrogen
                                 containing
                                 ethylene as
                                 required to obtain
                                 the concentration
                                 specified in Table
                                 B-3.
Hydrogen chloride.............  Cylinder\1\ of pre-  Collect samples in
                                 purified nitrogen    bubbler containing
                                 containing           distilled water
                                 approximately 100    and analyze by the
                                 ppm of gaseous       mercuric
                                 HCL. Dilute with     thiocyante method,
                                 zero air to          ASTM (D612), p.
                                 concentration        29, reference 4.
                                 specified in Table
                                 B-3.
Hydrogen sulfide..............  Permeation device    Tentative method of
                                 system described     analysis for H2S
                                 in references 1      content of the
                                 and 2.               atmosphere, p.
                                                      426, reference 5.
Methane.......................  Cylinder of zero     Gas chromatography
                                 air containing       ASTM D2820,
                                 methane as           reference 10. Use
                                 required to obtain   NIST-traceable
                                 the concentration    methane standards
                                 specified in Table   for calibration.
                                 B-3.
Naphthalene...................  1. Permeation        Use NIST-certified
                                 device as            standards whenever
                                 described in         possible. If NIST
                                 references 1 and 2.  standards are not
                                2. Cylinder of pre-   available, obtain
                                 purified nitrogen    2 standards from
                                 containing 100 ppm   independent
                                 naphthalene.         sources which
                                 Dilute with zero     agree within 2
                                 air to               percent, or obtain
                                 concentration        one standard and
                                 specified in Table   submit it to an
                                 B-3..                independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Nitric oxide..................  Cylinder\1\ of pre-  Use NIST-certified
                                 purified nitrogen    standards whenever
                                 containing           possible. If NIST
                                 approximately 100    standards are not
                                 ppm NO. Dilute       available, obtain
                                 with zero air to     2 standards from
                                 required             independent
                                 concentration.       sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Nitrogen dioxide..............  1. Gas phase         1. Use an FRM NO2
                                 titration as         analyzer
                                 described in         calibrated with a
                                 reference 6.         gravimetrically
                                2. Permeation         calibrated
                                 device, similar to   permeation device.
                                 system described    2. Use an FRM NO2
                                 in reference 6..     analyzer
                                                      calibrated by gas-
                                                      phase titration as
                                                      described in
                                                      reference 6.
Ozone.........................  Calibrated ozone     Use an FEM ozone
                                 generator as         analyzer
                                 described in         calibrated as
                                 reference 9.         described in
                                                      reference 9.
Sulfur dioxide................  1. Permeation        Use an SO2 FRM or
                                 device as            FEM analyzer as
                                 described in         described in
                                 references 1 and 2.  reference 7.
                                2. Dynamic dilution
                                 of a cylinder
                                 containing
                                 approximately 100
                                 ppm SO2 as
                                 described in
                                 Reference 7..

Water.........................  Pass zero air        Measure relative
                                 through distilled    humidity by means
                                 water at a fixed     of a dew-point
                                 known temperature    indicator,
                                 between 20[deg]      calibrated
                                 and 30[deg] C such   electrolytic or
                                 that the air         piezo electric
                                 stream becomes       hygrometer, or wet/
                                 saturated. Dilute    dry bulb
                                 with zero air to     thermometer.
                                 concentration
                                 specified in Table
                                 B-3.
Xylene........................  Cylinder of pre-     Use NIST-certified
                                 purified nitrogen    standards whenever
                                 containing 100 ppm   possible. If NIST
                                 xylene. Dilute       standards are not
                                 with zero air to     available, obtain
                                 concentration        2 standards from
                                 specified in Table   independent
                                 B-3.                 sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Zero air......................  1. Ambient air
                                 purified by
                                 appropriate
                                 scrubbers or other
                                 devices such that
                                 it is free of
                                 contaminants
                                 likely to cause a
                                 detectable
                                 response on the
                                 analyzer.
                                2. Cylinder of       ...................
                                 compressed zero
                                 air certified by
                                 the supplier or an
                                 independent
                                 laboratory to be
                                 free of
                                 contaminants
                                 likely to cause a
                                 detectable
                                 response on the
                                 analyzer.
------------------------------------------------------------------------
\1\ Use stainless steel pressure regulator dedicated to the pollutant
  measured.
Reference 1. O'Keefe, A. E., and Ortaman, G. C. ``Primary Standards for
  Trace Gas Analysis,'' Anal. Chem. 38, 760 (1966).
Reference 2. Scaringelli, F. P., A. E. Rosenberg, E., and Bell, J. P.,
  ``Primary Standards for Trace Gas Analysis.'' Anal. Chem. 42, 871
  (1970).
Reference 3. ``Tentative Method of Analysis for Ammonia in the
  Atmosphere (Indophenol Method)'', Health Lab Sciences, vol. 10, No. 2,
  115-118, April 1973.
Reference 4. 1973 Annual Book of ASTM Standards, American Society for
  Testing and Materials, 1916 Race St., Philadelphia, PA.
Reference 5. Methods for Air Sampling and Analysis, Intersociety
  Committee, 1972, American Public Health Association, 1015.
Reference 6. 40 CFR 50 Appendix F, ``Measurement Principle and
  Calibration Principle for the Measurement of Nitrogen Dioxide in the
  Atmosphere (Gas Phase Chemiluminescence).''
Reference 7. 40 CFR 50 Appendix A-1, ``Measurement Principle and
  Calibration Procedure for the Measurement of Sulfur Dioxide in the
  Atmosphere (Ultraviolet FIuorscence).''
Reference 8. 40 CFR 50 Appendix C, ``Measurement Principle and
  Calibration Procedure for the Measurement of Carbon Monoxide in the
  Atmosphere'' (Non-Dispersive Infrared Photometry)''.
Reference 9. 40 CFR 50 Appendix D, ``Measurement Principle and
  Calibration Procedure for the Measurement of Ozone in the
  Atmosphere''.
Reference 10. ``Standard Test Method for C, through C5 Hydrocarbons in
  the Atmosphere by Gas Chromatography'', D 2820, 1987 Annual Book of
  Aston Standards, vol 11.03, American Society for Testing and
  Materials, 1916 Race St., Philadelphia, PA 19103.

0
15. Section 53.23(d) is amended by revising Table B-3 to read as
follows:

Sec.  53.23  Test procedures.

* * * * *
    (d) * * *

                                                                 Table B-3--Interferent Test Concentration,\1\ Parts per Million
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                   Hydro-            Hydro-                                                                                Carbon
      Pollu- tant             Analyzer type       chloric   Ammo-     gen     Sulfur  Nitrogen   Nitric   Carbon   Ethy-    Ozone      M-    Water vapor    mon-    Meth-    Ethane  Naphthalene
                                                    acid     nia    sulfide  dioxide   dioxide   oxide   dioxide    lene             xylene                oxide     ane
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...................  Ultraviolet fluorescence  .......  .......  \5\ 0.1      \4\       0.5      0.5  .......  .......      0.5      0.2       20,000  .......  .......  .......   \6\ 0.05
                                                                                0.14
SO2...................  Flame photometric.......  .......  .......     0.01      \4\  ........  .......      750  .......  .......  .......   \3\ 20,000       50  .......  .......  ...........
                                                                                0.14
SO2...................  Gas chromatography......  .......  .......      0.1      \4\  ........  .......      750  .......  .......  .......   \3\ 20,000       50  .......  .......  ...........
                                                                                0.14
SO2...................  Spectrophotometric-wet        0.2      0.1      0.1      \4\       0.5  .......      750  .......      0.5  .......  ...........  .......  .......  .......  ...........
                         chemical                                               0.14
                         (pararosanaline).
SO2...................  Electrochemical.........      0.2      0.1      0.1      \4\       0.5      0.5  .......      0.2      0.5  .......   \3\ 20,000  .......  .......  .......  ...........
                                                                                0.14
SO2...................  Conductivity............      0.2      0.1  .......      \4\       0.5  .......      750  .......  .......  .......  ...........  .......  .......  .......  ...........
                                                                                0.14
SO2...................  Spectrophotometric-gas    .......  .......  .......      \4\       0.5  .......  .......  .......      0.5      0.2  ...........  .......  .......  .......  ...........
                         phase, including DOAS.                                 0.14
O3....................  Chemiluminescent........  .......  .......  \3\ 0.1  .......  ........  .......      750  .......      \4\  .......   \3\ 20,000  .......  .......  .......  ...........
                                                                                0.08
O3....................  Electrochemical.........  .......  \3\ 0.1  .......      0.5       0.5  .......  .......  .......      \4\  .......  ...........  .......  .......  .......  ...........
                                                                                0.08
O3....................  Spectrophotometric-wet    .......  \3\ 0.1  .......      0.5       0.5  \3\ 0.5  .......  .......      \4\  .......  ...........  .......  .......  .......  ...........
                         chemical (potassium                                                                                0.08
                         iodide).
O3....................  Spectrophotometric-gas    .......  .......  .......      0.5       0.5      0.5  .......  .......      \4\     0.02       20,000  .......  .......  .......  ...........
                         phase, including                                                                                0.08
                         ultraviolet absorption
                         and DOAS.
CO....................  Infrared................  .......  .......  .......  .......  ........  .......      750  .......  .......  .......       20,000   \4\ 10  .......  .......  ...........
CO....................  Gas chromatography with   .......  .......  .......  .......  ........  .......  .......  .......  .......  .......       20,000   \4\ 10  .......      0.5  ...........
                         flame ionization
                         detector.
CO....................  Electrochemical.........  .......  .......  .......  .......  ........      0.5  .......      0.2  .......  .......       20,000   \4\ 10  .......  .......  ...........
CO....................  Catalytic combustion-     .......      0.1  .......  .......  ........  .......      750      0.2  .......  .......       20,000   \4\ 10      5.0      0.5  ...........
                         thermal detection.
CO....................  IR fluorescence.........  .......  .......  .......  .......  ........  .......      750  .......  .......  .......       20,000   \4\ 10  .......      0.5  ...........
CO....................  Mercury replacement-UV    .......  .......  .......  .......  ........  .......  .......      0.2  .......  .......  ...........   \4\ 10  .......      0.5  ...........
                         photometric.
NO2...................  Chemiluminescent........  .......  \3\ 0.1  .......      0.5   \4\ 0.1      0.5  .......  .......  .......  .......       20,000  .......  .......  .......  ...........
NO2...................  Spectrophotometric-wet    .......  .......  .......      0.5   \4\ 0.1      0.5      750  .......      0.5  .......  ...........  .......  .......  .......  ...........
                         chemical (azo-dye
                         reaction).
NO2...................  Electrochemical.........      0.2  \3\ 0.1  .......      0.5   \4\ 0.1      0.5      750  .......      0.5  .......       20,000       50  .......  .......  ...........
NO2...................  Spectrophotometric-gas    .......  \3\ 0.1  .......      0.5   \4\ 0.1      0.5  .......  .......      0.5  .......       20,000       50  .......  .......  ...........
                         phase.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Concentrations of interferent listed must be prepared and controlled to 10 percent of the stated value.
2. Analyzer types not listed will be considered by the Administrator as special cases.
3. Do not mix with the pollutant.

4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.
6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate
  response for interference

* * * * *

Subpart C [Amended]

0
16. Section 53.32 is amended by revising paragraph (e)(2) to read as
follows:

Sec.  53.32  Test procedures for methods for SO2, CO,
O3, and NO2.

* * * * *
    (e) * * *
    (2) For a candidate method having more than one selectable range,
one range must be that specified in table B-1 of subpart B of this
part, and a test analyzer representative of the method must pass the
tests required by this subpart while operated on that range. The tests
may be repeated for one or more broader ranges (i.e., ones extending to
higher concentrations) than the one specified in table B-1 of subpart B
of this part, provided that such a range does not extend to
concentrations more than four times the upper range limit specified in
table B-1 of subpart B of this part and that the test analyzer has
passed the tests required by subpart B of this part (if applicable) for
the broader range. If the tests required by this subpart are conducted
or passed only for the range specified in table B-1 of subpart B of
this part, any equivalent method determination with respect to the
method will be limited to that range. If the tests are passed for both
the specified range and a broader range (or ranges), any such
determination will include the broader range(s) as well as the
specified range. Appropriate test data shall be submitted for each
range sought to be included in such a determination.
* * * * *

0
17. Table C-1 to Subpart C is revised to read as follows:

          Table C-1 to Subpart C of Part 53--Test Concentration Ranges, Number of Measurements Required, and Maximum Discrepancy Specifications
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Simultaneous measurements required               Maximum
                                                                                ----------------------------------------------------    discrepancy
                   Pollutant                       Concentration range, parts per            1-hour                    24-hour           specification,
                                                           million (ppm)           ----------------------------------------------------     parts per
                                                                                First set    Second set   First set    Second set       million
--------------------------------------------------------------------------------------------------------------------------------------------------------
Ozone..........................................  Low 0.06 to 0.10.................            5            6  ...........  ...........              0.02
                                                 Med. 0.15 to 0.25................            5            6  ...........  ...........              0.03
                                                 High 0.35 to 0.46................            4            6  ...........  ...........              0.04
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
                                                    Total.........................           14           18  ...........  ...........  ................
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
Carbon monoxide................................  Low 7 to 11......................            5            6  ...........  ...........               1.5
                                                 Med. 20 to 30....................            5            6  ...........  ...........               2.0
                                                 High 25 to 45....................            4            6  ...........  ...........               3.0
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
                                                    Total.........................           14           18  ...........  ...........  ................
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
Sulfur dioxide.................................  Low 0.02 to 0.05.................            5            6            3            3              0.02
                                                 Med. 0.10 to 0.15................            5            6            2            3              0.03
                                                 High 0.30 to 0.50................            4            6            2            2              0.04
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
                                                    Total.........................           14           18            7            8  ................
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
Nitrogen dioxide...............................  Low 0.02 to 0.08.................  ...........  ...........            3            3              0.02
                                                 Med. 0.10 to 0.20................  ...........  ...........            2            2              0.02
                                                 High 0.25........................  ...........  ...........            2            2              0.03
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
                                                    Total.........................  ...........  ...........            7            8  ................
--------------------------------------------------------------------------------------------------------------------------------------------------------

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

0
The authority citation for part 58 continues to read as follows:

    Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.

Subpart B [AMENDED]

0
19. Section 58.10, is amended by adding paragraph (a)(6) to read as
follows:

Sec.  58.10  Annual monitoring network plan and periodic network
assessment.

* * * * *
    (a) * * *
    (6) A plan for establishing SO2 monitoring sites in
accordance with the requirements of appendix D to this part shall be
submitted to the EPA Regional Administrator by July 1, 2011 as part of
the annual network plan required in paragraph (a) (1). The plan shall
provide for all required SO2 monitoring sites to be
operational by January 1, 2013.
* * * * *

0
20. Section 58.12 is amended by adding paragraph (g) to read as
follows:

Sec.  58.12  Operating Schedules

* * * * *
    (g) For continuous SO2 analyzers, the maximum 5-minute
block average concentration of the twelve 5-minute blocks in each hour
must be collected except as noted in Sec.  58.12 (a).
* * * * *

0
21. Section 58.13 is amended by adding paragraph (d) to read as
follows:

Sec.  58.13  Monitoring network completion.

* * * * *
    (d) The network of SO2 monitors must be physically
established no later than January 1, 2013, and at that time, must be
operating under all of the requirements of this part, including the

requirements of appendices A, C, D, and E to this part.

0
22. Section 58.16 is amended by adding paragraph (g) to read as
follows:

Sec.  58.16  Data submittal and archiving requirements.

* * * * *
    (g) Any State or, where applicable, local agency operating a
continuous SO2 analyzer shall report the maximum 5-minute
SO2 block average of the twelve 5-minute block averages in
each hour, in addition to the hourly SO2 average.

0
23. Appendix A to Part 58 is amended as by adding paragraph <IP_ADDRESS> to
read as follows:

Appendix A to Part 58--Quality Assurance Requirements for SLAMS, SPMs
and PSD Air Monitoring

* * * * *
    <IP_ADDRESS> Measurement Uncertainty for SO2. The goal for
acceptable measurement uncertainty for precision is defined as an
upper 90 percent confidence limit for the coefficient of variation
(CV) of 10 percent and for bias as an upper 95 percent confidence
limit for the absolute bias of 10 percent.
* * * * *

0
24. Appendix D to Part 58 is amended as by revising paragraph 4.4 to
read as follows:

Appendix D to Part 58--Network Design Criteria for Ambient Air Quality
Monitoring

* * * * *

4.4 Sulfur Dioxide (SO2) Design Criteria.

    4.4.1 General Requirements. (a) State and, where appropriate,
local agencies must operate a minimum number of required
SO2 monitoring sites as described below.
    4.4.2 Requirement for Monitoring by the Population Weighted
Emissions Index. (a) The population weighted emissions index (PWEI)
shall be calculated by States for each core based statistical area
(CBSA) they contain or share with another State or States for use in
the implementation of or adjustment to the SO2 monitoring
network. The PWEI shall be calculated by multiplying the population
of each CBSA, using the most current census data or estimates, and
the total amount of SO2 in tons per year emitted within
the CBSA area, using an aggregate of the most recent county level
emissions data available in the National Emissions Inventory for
each county in each CBSA. The resulting product shall be divided by
one million, providing a PWEI value, the units of which are million
persons-tons per year. 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 100,000, 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 100,000, a minimum of
one SO2 monitor is required within that CBSA.
    (1) The SO2 monitoring site(s) required as a result
of the calculated PWEI in each CBSA shall satisfy minimum monitoring
requirements if the monitor is sited within the boundaries of the
parent CBSA and is one of the following site types (as defined in
section 1.1.1 of this appendix): population exposure, highest
concentration, source impacts, general background, or regional
transport. SO2 monitors at NCore stations may satisfy
minimum monitoring requirements if that monitor is located within a
CBSA with minimally required monitors under this part. Any monitor
that is sited outside of a CBSA with minimum monitoring requirements
to assess the highest concentration resulting from the impact of
significant sources or source categories existing within that CBSA
shall be allowed to count towards minimum monitoring requirements
for that CBSA.
    4.4.3 Regional Administrator Required Monitoring. (a) The
Regional Administrator may require additional SO2
monitoring stations above the minimum number of monitors required in
4.4.2 of this part, where the minimum monitoring requirements are
not sufficient to meet monitoring objectives. The Regional
Administrator may require, at his/her discretion, additional
monitors in situations where an area has the potential to have
concentrations that may violate or contribute to the violation of
the NAAQS, in areas impacted by sources which are not conducive to
modeling, or in locations with susceptible and vulnerable
populations, which are not monitored under the minimum monitoring
provisions described above. The Regional Administrator and the
responsible State or local air monitoring agency shall work together
to design and/or maintain the most appropriate SO2
network to provide sufficient data to meet monitoring objectives.
    4.4.4 SO2 Monitoring Spatial Scales. (a) The
appropriate spatial scales for SO2 SLAMS monitors are the
microscale, middle, neighborhood, and urban scales. Monitors sited
at the microscale, middle, and neighborhood scales are suitable for
determining maximum hourly concentrations for SO2.
Monitors sited at urban scales are useful for identifying
SO2 transport, trends, and, if sited upwind of local
sources, background concentrations.
    (1) Microscale--This scale would typify areas in close proximity
to SO2 point and area sources. Emissions from stationary
point and area sources, and non-road sources may, under certain
plume conditions, result in high ground level concentrations at the
microscale. The microscale typically represents an area impacted by
the plume with dimensions extending up to approximately 100 meters.
    (2) Middle scale--This scale generally represents air quality
levels in areas up to several city blocks in size with dimensions on
the order of approximately 100 meters to 500 meters. The middle
scale may include locations of expected maximum short-term
concentrations due to proximity to major SO2 point, area,
and/or non-road sources.
    (3) Neighborhood scale--The neighborhood scale would
characterize air quality conditions throughout some relatively
uniform land use areas with dimensions in the 0.5 to 4.0 kilometer
range. Emissions from stationary point and area sources may, under
certain plume conditions, result in high SO2
concentrations at the neighborhood scale. Where a neighborhood site
is located away from immediate SO2 sources, the site may
be useful in representing typical air quality values for a larger
residential area, and therefore suitable for population exposure and
trends analyses.
    (4) Urban scale--Measurements in this scale would be used to
estimate concentrations over large portions of an urban area with
dimensions from 4 to 50 kilometers. Such measurements would be
useful for assessing trends in area-wide air quality, and hence, the
effectiveness of large scale air pollution control strategies. Urban