
[Federal Register Volume 88, Number 196 (Thursday, October 12, 2023)]
[Rules and Regulations]
[Pages 70595-70602]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2023-22531]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2022-0007; FRL-9344-02-OAR]
RIN 2060-AV63


Reference Measurement Principle and Calibration Procedure for the 
Measurement of Ozone in the Atmosphere (Chemiluminescence Method)

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The Environmental Protection Agency (EPA) is finalizing an 
update to the current ozone absorption cross-section to the recommended 
consensus-based cross-section value of 1.1329x10-17 cm\2\ 
molecule-1 or 304.39 atm-1 cm-1, with 
an uncertainty of 0.94 atm-1 cm-1. The new value 
is 1.2% lower than the current value of 308 atm-1 
cm-1 and reduces the uncertainty in the value to 0.31%. The 
adoption of this updated ozone absorption cross-section could result in 
increases in measured ozone concentrations but given the existing 
sources of potential variability in monitoring data, it is unlikely 
that there will be any consistent measurable and predictable effect on 
reported data. The EPA is also updating the dates of publication for 
two references associated with the updated cross-section value, adding 
a new reference, and making a technical correction to move three 
figures inadvertently placed in section 6.0 References to a new section 
7.0 Figures.

DATES: This final rule is effective on November 13, 2023.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2022-0007. All documents in the docket are 
listed on the https://www.regulations.gov website. Although listed in 
the index, some information is not publicly available, e.g., CBI or 
other information whose disclosure is restricted by statute. Certain 
other material, such as copyrighted material, is not placed on the 
internet and will be publicly available only in hard copy form. 
Publicly available docket materials are available electronically 
through https://www.regulations.gov.

FOR FURTHER INFORMATION CONTACT: Ms. Joann Rice, Office of Air Quality 
Planning and Standards, Air Quality Assessment Division, Ambient Air 
Monitoring Group (C304-06), Environmental Protection Agency, Research 
Triangle Park, North Carolina 27711; telephone number: (919) 541-3372; 
email address: [email protected].

SUPPLEMENTARY INFORMATION: 
    Organization of this document. The information in this preamble is 
organized as follows:

Table of Contents

I. Background
    Comments on the Proposed Rule
II. Statutory and Executive Orders Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 14094: Modernizing Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act (NTTAA)
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations
    K. Congressional Review Act (CRA)

I. Background

    In 1961, the ozone absorption cross-section was measured to be 
1.1476 x10-17 cm\2\ molecule-1 or 308.3 
atmosphere (atm)-1 centimeter (cm)-1 with a 
reported relative standard uncertainty of 1.4% (Hearn, 1961).\1\ In the 
1980s, the National Institute of Standards and Technology (NIST), in 
collaboration with the EPA, developed the Standard Reference Photometer 
(SRP), which is the international standard for the measurement of 
ozone. The SRP is based on ultraviolet (UV) photometry and uses this 
cross-section value as the reference value for UV ozone measurements. 
To establish and maintain traceability, the readings of an ozone 
analyzer are compared to a NIST-made ozone SRP through a hierarchy of 
standards. Efforts to improve the accuracy of the ozone absorption 
cross-section have continued over several years during which rigorous 
assessment of the bias and uncertainty in the value became a high 
priority.
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    \1\ Hearn A.G. (1961). Absorption of ozone in ultra-violet and 
visible regions of spectrum, Proc. Phys. Soc. 78 932, DOI: 10.1088/
0370-1328/78/5/340.
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    The Gas Analysis Working Group of the Consultive Committee for 
Metrology in Chemistry and Biology (CCQM-GAWG) of the Bureau of Weights 
and Measures in France (BIPM) convened a task group in 2016 to review 
all published measurements of the ozone cross-section since 1950. This 
task group was also charged with recommending a consensus-based cross-
section value and associated uncertainty for adoption in measurements 
of ozone concentrations by standard UV photometric instruments, 
including the SRP. (Hodges et al., 2019).\2\
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    \2\ Hodges, J.T., Viallon, J., Brewer, P.J., Drouin, B.J., 
Gorshelev, V., Janssen, C., Lee, S., Possolo, A., Smith, M.A.H., 
Walden, and Wielgosz, R.I. (2019). Recommendation of a consensus 
value of the ozone absorption cross-section at 253.65 nm based on a 
literature review, Metrologia, 56, 034001. https://doi.org/10.1088/1681-7575/ab0bdd.
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    After publication in Hodges et al., 2019, the CCQM-GAWG \3\ 
convened an international group of stakeholders in October 2020 to 
discuss adopting and implementing a globally coordinated change in the 
cross-section value for surface ozone monitoring. This group, 
representing several international and national metrology institutes, 
NIST, and environmental agencies including EPA, agreed to adopt and 
implement the new cross-section value as it represents a more accurate 
value with less

[[Page 70596]]

uncertainty and is an advancement and improvement in the UV photometer 
measurement method.
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    \3\ https://www.bipm.org/en/committees/cc/ccqm/wg/ccqm-gawg-ozone-tg.
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    40 CFR part 50, appendix D, ``Reference Measurement Principle and 
Calibration Procedure for the Measurement of Ozone in the Atmosphere,'' 
currently provides EPA's ozone calibration procedure with a stated 
value of 308  4 atm-1 cm-1. This 
final action updates the ozone absorption cross-section to align with 
the BIPM CCQM-GAWG's updated international cross-section value of 
304.39 atm-1 cm-1 with an uncertainty of 0.94 
atm-1 cm-1 at standard temperature and pressure 
of 0 [deg]C and 1 atmosphere. The EPA agrees that the new cross-section 
value results in an improvement in the accuracy of surface ozone 
monitoring measurements by reducing uncertainty and is finalizing the 
change in appendix D of part 50 to this more accurate consensus value.
    The updated value reduces the uncertainty to 0.31% from the current 
1.4%. The value is also 1.2% lower than the current value of 308 
atmosphere atm-1 cm-1, a change that could result 
in increases in measured ozone concentrations. However, there are 
several factors that EPA believes make it unlikely that this change 
will have a measurable, predictable influence on any particular set of 
ozone monitoring data.
    Design values, the metric used to compare ambient ozone 
concentrations measured at a monitor to the National Ambient Air 
Quality Standard (NAAQS) to determine compliance, are determined using 
the data reporting, data handling, and computation procedures provided 
in 40 CFR part 50, appendix U, ``Interpretation of the Primary and 
Secondary National Ambient Air Quality Standards for Ozone.''
    Multiple factors can contribute to variability in monitoring data 
and ultimately design values, including, but not limited to, the 
precision of the monitoring method, the acceptance criteria for 
Standard Reference Photometer (SRP) calibration and verification, the 
acceptance criterion for bench and field standards used to calibrate 
ozone monitors in the field, how agencies perform calibration and 
adjust analyzer response, the precision and bias acceptance criteria in 
EPA's Quality Assurance (QA) Handbook,\4\ data handling and computation 
procedures in Appendix U, and meteorology.
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    \4\ Quality Assurance Handbook for Air Pollution Measurement 
Systems, Volume II, EPA-454/B-17-001, Jan. 2017, available at: 
https://www.epa.gov/sites/default/files/2020-10/documents/final_handbook_document_1_17.pdf.
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    The inherent precision (variability) of the measurements from 
analyzers used to measure ozone is about 1 ppb, or 0.001 ppm. The variability in the measurement in either the 
positive or negative direction should be considered relative to the 
change in monitoring data due to the new cross-section value.
    When the new cross-section value is implemented, all SRPs 
maintained by BIPM, NIST, and the EPA will be updated to incorporate 
the new value. The update will be achieved through software/firmware 
modification and will not require any hardware changes. The EPA is 
planning to update all Agency's SRPs simultaneously, instead of through 
a phased approach, to minimize disruption of the SRP network. To 
establish and maintain traceability, the readings of an ozone analyzer 
are compared through a hierarchy of standards to a NIST ozone SRP. The 
process of using NIST-traceable standards to verify the ozone 
concentrations is implemented for all regulatory network ozone 
analyzers used for comparison to the NAAQS. There are 12 SRPs within 
the EPA's network: three at EPA's Office of Research and Development 
(ORD) and nine at various EPA Regional offices and the California Air 
Resources Board (CARB). One of ORD's SRPs is sent to NIST to be re-
verified against the NIST SRP annually. That SRP serves as the 
reference for the two other ORD SRPs. Each SRP in the U.S. is re-
verified against one of ORD's three SRPs annually. Under normal 
verification operations, implementing the ozone standards traceability 
process for the entire SRP network could take 2 or more years starting 
from when the SRP software/firmware is updated. During this time, the 
implementation progress and monitoring data collected with the new 
cross-section will need to be tracked.
    The acceptance criteria used in comparing the SRPs (Level 1 
standards) to each other is a slope of 1.00  0.01 (or 1%) 
and an intercept 0.00  1 ppb. Field and bench standards 
(Level 2 standard) used to calibrate ozone analyzers in the field have 
acceptance criteria for the slope of 1.00  0.03 (or 3%) and 
an intercept of 0  3 ppb. The 1.2% change in cross-section 
value is well within the 3% acceptance for Level 2 standards.
    The goal for annual measurement uncertainty for ozone in 40 CFR 
part 58, ``Ambient Air Quality Surveillance,'' is an upper 90 percent 
confidence limit for the coefficient of variation of 7% for precision 
and for bias an upper 95 percent confidence limit of 7%. Bias and 
precision estimates are determined using data obtained from the 
comparison of the ozone analyzer response to one-point Quality Control 
(QC) checks using a Level 2 calibration standard. The 1.2% change in 
cross-section value is well within the bias and precision goal of 7%. 
Data reported to the EPA's Air Quality System by state, local, and 
tribal monitoring agencies is used to assess bias and precision. The 
2021 national average precision for all ozone analyzers in the U.S. is 
2.3% and the national average bias is 1.6%.\5\ The 1.2% change is, 
therefore, within the national precision and less than the national 
bias.
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    \5\ Data obtained on 9/1/2022 from EPA's Ozone Data Quality 
Dashboard: https://sti-r-shiny.shinyapps.io/ozone_dashboard/.
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    The QA Handbook, Volume II, Appendix D Validation Template \6\ also 
specifies critical criteria for monitoring organizations to maintain 
the integrity and evaluate the quality of the data collected by the 
analyzer. The critical criteria are a one-point QC check (every 14 days 
at a minimum) < 7.1% difference or < 1.5 ppb 
difference, whichever is greater; zero drift <  3.1 ppb 
(over a 24-hour period) or < 5.1 ppb (>24 hours and up to 
14 days); and span check drift over a 14-day period of < 7.1%. Any change to monitoring data due to the new cross-section 
is also well within the 7.1% acceptance criteria. Monitoring 
organizations may manually adjust the analyzer response while others 
may institute automated adjustment through use of a data acquisition or 
data handling system. Automated adjustments to the ozone analyzer data 
are not recommended because the monitoring agency may not know if the 
standard being used for monitor comparison, or the analyzer, has 
degraded or drifted.
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    \6\ Appendix D, Measurement Quality Objectives and Validation 
Templates: https://www.epa.gov/sites/default/files/2020-10/documents/app_d_validation_template_version_03_2017_for_amtic_rev_1.pdf.
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    Ozone analyzers are calibrated or verified every 182 days if one-
point zero and span checks are performed every 14 days, and every 365 
days if one-point zero and span checks are done daily. The acceptance 
criteria for multi-point calibration are all points < 2.1% 
or <= 1.5 ppb difference of the best fit straight line, 
whichever is greater, and a slope of 1  0.05 or 5%. The 
1.2% change is also well within this acceptance criteria for ozone 
monitor calibration.
    Ozone design values are computed as the 3-year average of the 
annual 4th highest daily maximum 8-hour value

[[Page 70597]]

measured at each monitoring site. Appendix U provides for three levels 
of truncation for the hourly, daily 8-hour maximum, and design value 
calculations. Hourly averaged ozone monitoring data are to be reported 
in ppm to the third decimal place, with additional digits to the right 
truncated (e.g., 0.070 ppm). In assessing how and if the updated cross-
section value may affect ozone design values, it is important to note 
that other factors, including meteorology, can also influence design 
values. The effects of meteorology on hourly ozone concentrations can 
contribute to an increase or decrease in design values for a site 
because formation of ozone is heavily dependent on meteorological 
conditions. Interannual meteorological variations are known to affect 
daily and seasonal average ozone concentrations. Therefore, while we do 
not have reason to believe this proposal will significantly increase 
design values, meteorology would be a confounding factor in determining 
the effect on 3-year design values.
    Taking these factors into consideration, the EPA believes it is 
unlikely that the cross-section change will have a measurable, 
predictable influence on any given ozone design value or monitoring 
data set.
    Because the EPA believes that adoption of the new cross-section 
will improve the accuracy of measured ozone values and is unlikely to 
have a measurable, predictable influence on any given monitor or design 
value, the EPA is finalizing its proposal to revise the current ozone 
absorption cross-section to the recommended international consensus-
based cross-section value of 304.39 atm-1 cm-1, 
with an uncertainty of 0.94 atm-1 cm-1.
    Ozone analyzers are traceable to a NIST standard reference UV-based 
photometer with a specified ozone UV absorption cross-section value. 
The absorption cross-section value stated this appendix (304.39 
atm-1 cm-1  0.94 atm-1 
cm-1) will be implemented January 1, 2025, with an 
additional year for state, local, and tribal monitoring agencies to 
complete implementation, to January 1, 2026. Until January 1, 2025, the 
previous ozone absorption cross-section value (308  4 
atm-1 cm-1) will be used. After January 1, 2025, 
both cross-section values, 304.39  0.94 atm-1 
cm-1 and 308  4 atm-1 
cm-1, may be used. After January 1, 2026, only the cross-
section value of 304.39  0.94 atm-1 
cm-1 may be used. EPA recognizes the challenges, complexity, 
and time it will take to develop guidance and complete implementation 
of the updated cross-section value and is, therefore, delaying the 
proposed implementation start date of January 1, 2024, until January 1, 
2025, with an additional year (to January 1, 2026) to complete 
implementation.
    The EPA is including an additional published reference for the 
research done to support the cross-section change in 40 CFR part 50, 
appendix D, section 6.0 References: Hodges, J.T., Viallon, J., Brewer, 
P.J., Drouin, B.J., Gorshelev, V., Janssen, C., Lee, S., Possolo, A., 
Smith, M.A.H., Walden, and Wielgosz, R.I., ``Recommendation of a 
consensus value of the ozone absorption cross-section at 253.65 nm 
based on a literature review,'' Metrologia, 56 (2019) 034001, https://doi.org/10.1088/1681-7575/ab0bdd. The EPA is also changing the 
publication dates of two existing references associated with the 
updated cross-section value in 40 CFR part 50, appendix D, section 6.0 
References.

Comments on the Proposed Rule

    On February 24, 2023, the EPA proposed to update the current ozone 
absorption cross-section (88 FR 11835) and solicited comment on the 
proposed update. The EPA received two comments by the close of the 
public comment period on March 27, 2023. One commenter expressed 
concern that the proposed target date of January 1, 2024, provides 
insufficient time to implement the new cross-section value and noted 
that monitoring equipment that is no longer supported by manufacturers 
would require monitoring agencies to purchase new ozone monitoring 
equipment.
    In further consideration of global implementation of the updated 
cross-section value, the international task group leading 
implementation and the EPA recognize the challenges, complexity, and 
time it will take to implement the updated value and are accordingly 
delaying the implementation start date from January 2024 until January 
2025 with an additional year (to January 2026) to complete 
implementation. Regarding the assertion that some monitoring agencies 
will be required to purchase new equipment, existing equipment will be 
adjusted by firmware updates if available. Where firmware updates are 
not available for certain monitors, those monitors may instead be 
calibrated against ozone transfer standards, which are calibrated 
directly back to a Standard Reference Photometer (SRP) using the 
updated cross-section value. Therefore, the purchase of new equipment 
should not be required.
    A second comment on the proposed cross-section value assumed that 
the percentage increase in monitoring data would be 0.00086 ppm at the 
current level of the standard (0.070 ppm). The commenter noted that, if 
that increase had been applied to the health studies upon which the 
current NAAQS is based, ``a NAAQS closer to 71 ppb very well could have 
been chosen based on the monitoring data.'' The commenter also noted 
that under the current ozone reconsideration, the Clean Air Science 
Advisory Committee (CASAC) and EPA ``must'' consider the ozone cross-
section change on monitoring data and health effect studies and, if not 
considered, the NAAQS may be ``artificially lowered'' or more 
stringent.
    The EPA disagrees that this change will make the NAAQS ozone 
standard more stringent. As described in the proposed action, at the 
current level of the standard (0.070 ppm), 0.00086 ppm is within the 
current precision of the measurement method which is +/- 0.001 ppm. 
Moreover, when viewed in conjunction with the current monitor 
calibration acceptance criteria \7\, the use of truncation conventions 
for the ozone hourly, daily 8-hour maximum, and design value 
calculations, and other unpredictable factors, EPA disagrees with the 
commenter's suggestion that the change will result in any consistent 
measurable and predictable effect on reported data. This inherent 
measurement variability is already included in the measurements that 
have been and are being used in health effects research studies related 
to the ozone NAAQS. The CASAC is aware of this action, which is 
required to bring the U.S. into alignment with international monitoring 
standards.
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    \7\ See QA Handbook, Vol. II, App. D, Measurement Quality 
Objectives and Validation Templates, available at https://www.epa.gov/sites/default/files/2020-10/documents/app_d_validation_template_version_03_2017_for_amtic_rev_1.pdf.
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    No other comments were received. The EPA is finalizing this action 
as proposed.

II. Statutory and Executive Orders Reviews

    Additional information about these statutes and Executive Orders 
can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 14094: Modernizing Regulatory Review

    This action is not a significant regulatory action as defined by 
Executive Order 12866, as amended by

[[Page 70598]]

Executive Order 14094 and was, therefore, not subject to a requirement 
for Executive Order 12866 review.

B. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under 
the PRA. This action revises the ozone absorption cross-section and 
revise and amend relevant references. It does not contain any 
information collection activities.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. In 
making this determination, the EPA concludes that the impact of concern 
for this rule is any significant adverse economic impact on small 
entities and that the agency is certifying that this rule will not have 
a significant economic impact on a substantial number of small entities 
if the rule has no net burden on the small entities subject to the 
rule. This action updates the ozone absorption cross-section value for 
surface ozone monitoring under 40 CFR part 50, and we anticipate that 
there will be minimal costs associated with this change. We have, 
therefore, concluded that this action will have no net regulatory 
burden for all directly regulated small entities.

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
UMRA, 2 U.S.C. 1531-1538 and does not significantly or uniquely affect 
small governments. This action imposes no enforceable duty on any 
state, local, or tribal governments, or the private sector.

E. Executive Order 13132: Federalism

    This action 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.

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

    This action does not have tribal implications as specified in 
Executive Order 13175. This action updates a reference measurement 
principle and calibration procedure for the measurement of ambient 
ozone under 40 CFR part 50. Thus, Executive Order 13175 does not apply 
to this action.

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

    The EPA interprets Executive Order 13045 as applying only to those 
regulatory actions that concern environmental health or safety risks 
that the EPA has reason to believe may disproportionately affect 
children, per the definition of ``covered regulatory action'' in 
section 2-202 of the Executive Order. This action is not subject to 
Executive Order 13045 because it does not concern an environmental 
health risk or safety risk.

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

    This action is not subject to Executive Order 13211, because it is 
not a significant regulatory action under Executive Order 12866.

I. National Technology Transfer and Advancement Act (NTTAA)

    This rulemaking involves technical standards. The EPA used 
voluntary consensus standards in the preparation of this measurement 
principle and procedure; it is the benchmark against which all ambient 
ozone monitoring methods are compared. This action is simply updating 
the reference measurement principle in light of updated information.

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) 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 (people of color) and low-income 
populations.
    The EPA believes that this type of action does not concern human 
health or environmental conditions and, therefore, cannot be evaluated 
with respect to potentially disproportionate and adverse effects on 
people of color, low-income populations and/or indigenous peoples. This 
regulatory action is an update to a previously promulgated analytical 
method and does not have any impact on human health or the environment.

K. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule 
report to each house of the Congress and to the Comptroller General of 
the United States. This action is not a ``major rule'' as defined by 5 
U.S.C. 804(2).

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Ozone.

Michael S. Regan,
Administrator.

    For the reasons set forth in the preamble, the EPA amends title 40, 
chapter I of the Code of Federal Regulations 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. Amend appendix D to part 50 by:
0
a. Revising sections 2.2, 4.1 and 4.5.3.10;
0
b. Revising references 13. and 14. in section 6.0;
0
c. Removing figures 1., 2., and 3. in section 6.0;
0
d. Adding reference 15 in section 6.0; and
0
e. Adding section ``7.0 Figures.''.
    The revisions and addition read as follows:

Appendix D to Part 50--Reference Measurement Principle and Calibration 
Procedure for the Measurement of Ozone in the Atmosphere 
(Chemiluminescence Method)

* * * * *
    2.0 Measurement Principle.
* * * * *
    2.2 The measurement system is calibrated by referencing the 
instrumental chemiluminescence measurements to certified O3 
standard concentrations generated in a dynamic flow system and assayed 
by ultraviolet (UV) photometry to be traceable to a National Institute 
of Standards and Technology (NIST) standard reference photometer for 
O3 (see section 4, Calibration Procedure, below) with a 
specified ozone absorption cross-section value. The absorption cross-
section value stated in section 4.1 and section 4.5.3.10 of this 
appendix (304.39 atm-1 cm-1  0.94 
atm-1 cm-1) will be implemented January 1, 2025, 
with an additional year to complete implementation (January 1, 2026). 
Until January 1, 2025, the previous ozone absorption cross-section 
value, 308  4

[[Page 70599]]

atm-1 cm-1, will be used. After January 1, 2025, 
both cross-section values, 304.39  0.94 atm-1 
cm-1 and 308  4 atm-1 
cm-1, may be used. After January 1, 2026, only the cross-
section value of 304.39  0.94 atm-1 
cm-1 may be used.
* * * * *
    4.0 Calibration Procedure.
    4.1 Principle. The calibration procedure is based on the 
photometric assay of O3 concentrations in a dynamic flow 
system. The concentration of O3 in an absorption cell is 
determined from a measurement of the amount of 254 nm light absorbed by 
the sample. This determination requires knowledge of (1) the absorption 
coefficient ([alpha]) of O3 at 254 nm, (2) the optical path 
length (l) through the sample, (3) the transmittance of the sample at a 
nominal wavelength of 254 nm, and (4) the temperature (T) and pressure 
(P) of the sample. The transmittance is defined as the ratio I/
I0, where I is the intensity of light which passes through 
the cell and is sensed by the detector when the cell contains an 
O3 sample, and I0 is the intensity of light which 
passes through the cell and is sensed by the detector when the cell 
contains zero air. It is assumed that all conditions of the system, 
except for the contents of the absorption cell, are identical during 
measurement of I and I0. The quantities defined above are 
related by the Beer-Lambert absorption law,
BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TR12OC23.000

Where:

[alpha] = absorption coefficient of O3 at 254 nm = 304.39 
atm-1 cm-1, with an uncertainty of 0.94 
atm-1 cm-1 at 0 [deg]C and 1 atm. 
1, 2, 3, 4, 5, 6, 7, 15

c = O3 concentration in atmospheres, and

l = optical path length in cm.

    A stable O3 generator is used to produce O3 
concentrations over the required calibration concentration range. Each 
O3 concentration is determined from the measurement of the 
transmittance (I/I0) of the sample at 254 nm with a 
photometer of path length l and calculated from the equation,
[GRAPHIC] [TIFF OMITTED] TR12OC23.001

    The calculated O3 concentrations must be corrected for 
O3 losses, which may occur in the photometer, and for the 
temperature and pressure of the sample.
* * * * *
    4.5 Procedure.
* * * * *
    4.5.3.10. Calculate the O3 concentration from equation 
4. An average of several determinations will provide better precision.
[GRAPHIC] [TIFF OMITTED] TR12OC23.002

Where:

[O3]OUT = O3 concentration, ppm

[alpha] = absorption coefficient of O3 at 254 nm = 304.39 
atm-1 cm-1 at 0 [deg]C and 1 atm

l = optical path length, cm

T = sample temperature, K

P = sample pressure, torr

L = correction factor for O3 losses from 4.5.2.5 = (1-
fraction of O3 lost).

    Note: Some commercial photometers may automatically evaluate all 
or part of equation 4. It is the operator's responsibility to verify 
that all of the information required for equation 4 is obtained, 
either automatically by the photometer or manually. For 
``automatic'' photometers which evaluate the first term of equation 
4 based on a linear approximation, a manual correction may be 
required, particularly at higher O3 levels. See the photometer 
instruction manual and Reference 13 for guidance.

* * * * *
    6.0 References.
* * * * *
    13. Technical Assistance Document for the Calibration of Ambient 
Ozone Monitors, EPA publication number EPA-454/B-22-003, January 2023.
    14. QA Handbook for Air Pollution Measurement Systems--Volume II. 
Ambient Air Quality Monitoring Program. EPA-454/B-17-001, January 2017.

[[Page 70600]]

    15. Hodges, J.T., Viallon, J., Brewer, P.J., Drouin, B.J., 
Gorshelev, V., Janssen, C., Lee, S., Possolo, A., Smith, M.A.H., 
Walden, and Wielgosz, R.I., Recommendation of a consensus value of the 
ozone absorption cross-section at 253.65 nm based on a literature 
review, Metrologia, 56 (2019) 034001. [Available at https://doi.org/10.1088/1681-7575/ab0bdd.]
    7.0 Figures.
    [GRAPHIC] [TIFF OMITTED] TR12OC23.003
    
Figure 1. Gas-phase chemiluminescence analyzer schematic diagram, where 
PMT means photomultiplier tube.

[[Page 70601]]

[GRAPHIC] [TIFF OMITTED] TR12OC23.004

Figure 2. Schematic diagram of a typical UV photometric calibration 
system.

[[Page 70602]]

[GRAPHIC] [TIFF OMITTED] TR12OC23.005

Figure 3. Schematic diagram of a typical UV photometric calibration 
system (Option 1).

[FR Doc. 2023-22531 Filed 10-11-23; 8:45 am]
BILLING CODE 6560-50-C


