Technology Transfer Network / NAAQS
Ozone Implementation
Guideline on Data Handling Conventions
for the 8-hour Ozone NAAQS
Note: EPA no longer updates this information, but it may be useful as a reference or resource.
EPA-454/R-98-017 December 1998
U.S. Environmental Protection Agency Office of Air Quality Planning and Standards Research Triangle Park, North Carolina 27711
December 1998
Disclaimer
This draft has been reviewed and approved for publication by the U.S. Environmental Protection Agency's Office of Air Quality Planning and Standards. Mention of trade names or commercial products are not intended to constitute endorsement or recommendation for use.
Contents
Introduction p1 *(page in printed document) Data Handling Conventions p1 What monitors does this guidance cover? p1 How do I compute the daily maximum 8-hour average ozone concentration? p2 What is a valid ozone monitoring day? p2 How many monitoring days are required? p2 What is a complete data year? p2 Can I exclude ozone monitoring data affected by natural events such as stratospheric ozone intrusion from computations used to determine compliance with the NAAQS? p3 Is any credit given toward meeting the 75% minimum data completeness requirement for days with missing monitoring data that would have had low ozone concentrations? p3 How do I find the 4th highest daily maximum 8-hour average ozone concentration at an ambient air quality monitoring site? p4 How many significant figures must I carry in the computations? p4 What is the attainment test? p4 What is the minimum number of years of data needed to demonstrate attainment with the 8-hour ozone NAAQS? p4 How do I compute the average annual 4th highest 8-hour daily maximum ozone concentration at an ambient air quality monitoring site? p4 What is the ozone air quality design value? p5 What if only 1 or 2 years of data are available at the site? p5 How do I judge the air quality status of ozone monitoring sites that have stopped monitoring? p5 What if 3 years of ozone monitoring data are available but some years do not meet the data completeness requirements? p6 How do I judge the air quality status of an area that has more than one ozone monitoring site? p7 How do I compute the area's design value? p7 References p7 Appendix I to 40CFR, Part 50 p8 * page number in printed document
Guideline on Data Handling Conventions for the 8-Hour Ozone
National Ambient Air Quality Standard (NAAQS)
Introduction
This guideline covers the data handling and ambient air quality monitoring data completeness requirements for the 8-hour ozone NAAQS as given in Appendix I. As background for the discussion that follows, the 8-hour primary and secondary ambient ozone air quality standards (40CFR50.10) are:
(a) The level of the national 8-hour primary and secondary ambient air quality standards for ozone, measured by a reference method based on Appendix D to this part and designated in accordance with part 53 of this chapter, is 0.08 parts per million (ppm), daily maximum 8-hour average.
(b) The 8-hour primary and secondary ozone ambient air quality standards are met at an ambient air quality monitoring site when the average of the annual fourth-highest daily maximum 8-hour average ozone concentration is less than or equal to 0.08 ppm, as determined in accordance with Appendix I to this part.
For completeness, the full text of Appendix I (40CFR50.10, Appendix I) is provided as an attachment to this guideline. This guideline uses the ‘plain English' question and answer (Q & A) format to present the data handling and data completeness issues of concern.
Data Handling Conventions
Q: What monitors does this guidance cover?
This guidance on data handling and data completeness requirements of the 8-hour ozone NAAQS is intended for all ozone monitors that are designated as either National Air Monitoring Stations (NAMS) or State and Local Air Monitoring Stations (SLAMS). All NAMS and SLAMS sites must meet uniform monitor siting and quality assurance requirements established in the monitoring regulations (40CFR58). This guidance also applies to all other ozone monitoring sites, including special purpose monitors (SPM), that have recorded at least three years of ambient ozone data and meet all the monitoring and quality assurance requirements for NAMS and SLAMS.
Q: How do I compute the daily maximum 8-hour average ozone concentration?
The new Aerometric Information Retrieval System (AIRS) will do this computation for you. First, running 8-hour averages are computed from the hourly ozone concentration data for each hour of the year and the result is stored in the first, or start, hour of the 8-hour period. In the event that only 6 (or 7) hourly averages are available, the 8-hour average is computed using 6 (or 7) as the divisor. Generally, if fewer than 6 hours of ozone concentration data are available, then the 8-hour daily maximum concentration for the day is assigned the value of ‘ missing.' However, just because we are missing more than 2 hours of monitoring data, we do not want to ignore 8-hour concentrations that clearly exceed the level of the standard. When more than 2 hours of monitoring data are missing, we compute the 8-hour average concentration by substituting one-half the minimum detectable limit for the missing hourly concentrations and using 8 as the divisor. In this case, if the computed 8-hour average concentration is greater than 0.08 ppm, it is retained, otherwise the 8-hour concentration for this 8-hour period is assigned the value of ‘missing.' In all cases, the computed 8-hour average ozone concentration is reported in parts per million (ppm) to three decimal places (the insignificant digits to the right of the third decimal place are truncated).
Q: What is a valid ozone monitoring day?
An ozone monitoring day is counted as a valid ozone monitoring day if 8-hour averages are available for at least 18 of the 24 possible 8-hour average periods that start within the day, or the daily maximum 8-hour average concentration for the day is greater than 0.08 ppm. The new AIRS automatically computes the number of valid ozone monitoring days.
Q: How many monitoring days are required?
Ozone monitoring seasons vary by state. The designated ozone monitoring seasons for the National Air Monitoring Stations (NAMS) and State and Local Air Monitoring Stations (SLAMS) are listed in the ambient monitoring rule (40CFR58, Appendix D) and are also included in AIRS. These same ozone seasons are used to compute the data completeness requirements for special purpose monitors (SPM) used to demonstrate compliance with the standard.
Q: What is a complete data year?
For the purpose of judging compliance with the standard, a valid year must have valid 8-hour daily maximum ozone concentrations for at least 75 percent (74.5 % rounds up) of the required monitoring days in the ozone season designated for NAMS or SLAMS monitoring sites. All three years must average at least 90 percent data completeness (89.5% rounds up) to demonstrate attainment.
Q: Can I exclude ozone monitoring data affected by natural events such as stratospheric ozone intrusion from computations used to determine compliance with the NAAQS?
Yes, monitoring data affected by natural events, such as stratospheric ozone intrusion, can be excluded when determining compliance with the 8-hour ozone NAAQS under certain conditions. One of these conditions is that the requesting agency must document that this rare natural event had a significant impact on ozone levels and must submit the request to the appropriate EPA Regional Office. Another is that the decision to exclude or make adjustments to the monitoring data affected by stratospheric ozone intrusion, or other exceptional natural events, is subject to the approval of the appropriate Regional Administrator. Upon EPA Regional Office concurrence, the data must be appropriately flagged in AIRS as an exceptional event if the annual 4th highest daily maximum concentration is to be computed correctly. In the case of stratospheric ozone intrusion, although this event is exceptional, the circumstances or the criteria under which it occurs are difficult to measure and document given generally available meteorological data. Stratospheric ozone intrusions are infrequent and very localized events of short duration, and this fact makes it difficult to use currently available airport meteorological data to determine whether a stratospheric ozone intrusion has occurred. Therefore, determining whether a stratospheric ozone intrusion, or any other natural event, has significantly affected measured ozone concentrations should be a case-by-case decision based on reasonable judgment regarding the season of the year, time of day, and accompanying meteorological conditions associated with the ozone measurement in question. (see EPA, 1986)
Q: Is any credit given toward meeting the 75% minimum data completeness requirement for days with missing monitoring data that would have had low ozone concentrations?
Yes, days with missing monitoring data may be counted toward meeting the minimum 75 percent data completeness requirement, but only if there is no clear pattern to the timing of the missing measurements and the missing measurements can be assumed to be less than the level of the NAAQS. Meteorological data, or ambient measurements from nearby monitoring sites, may be sufficient to demonstrate that missing ozone data at a particular monitoring location would not likely have exceeded the level of the standard. For example, if you can demonstrate that meteorological conditions on days with missing ozone measurements at a site were not conducive to concentrations above the level of the ozone standard, then you may be able to count the day toward meeting the minimum data completeness requirements. The demonstration must follow EPA guidance on the use of statistical analyses of air quality and meteorological data (EPA, 1998) and the results of the analyses must be submitted to the appropriate Regional Administrator for approval. Once approved, days with missing monitoring data that are assumed to be less than the level of the NAAQS must be appropriately flagged in AIRS to ensure that the percent data completeness calculations are computed correctly.
Q: How do I find the 4th highest daily maximum 8-hour average ozone concentration at an ambient air quality monitoring site?
The simplest way is to look at the annual 4th highest 8-hour daily maximum ozone concentration in a standard AIRS summary report. Otherwise, you simply rank the 8-hour daily maximum ozone concentrations from highest to lowest and identify the 4th highest value.
Q: How many significant figures must I carry in the computations?
Three. The remaining right-most digits are truncated. The units for all computations are parts per million (ppm). AIRS sets this automatically when it computes the daily maximum 8-hour concentrations and for all subsequent computations.
Q: What is the attainment test?
The 8-hour primary and secondary ozone ambient air quality standards are met at an ambient air quality monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentration is less than or equal to 0.08 ppm (i.e., the site is said to be in attainment). Because the third decimal digit, in ppm, is rounded, 0.084 ppm is the largest concentration that is less than, or equal to, 0.08 ppm.
Q: What is the minimum number of years of data needed to demonstrate attainment with the 8-hour ozone NAAQS?
Three complete years of ozone monitoring data are required to demonstrate attainment at a monitoring site.
Q: How do I compute the average annual 4th highest 8-hour daily maximum ozone concentration at an ambient air quality monitoring site?
The average annual 4th highest 8-hour daily maximum concentration is simply the average of the annual 4th highest 8-hour daily maximum concentrations from each of the last 3 years. For example,
Year Percent
Valid Days 8-hour Daily Maximum Concentrations(ppm)
1st 2nd 3rd 4th
1993 100% 0.092 0.091 0.090 0.089
1994 96% 0.090 0.089 0.086 0.084
1995 98% 0.087 0.085 0.083 0.081
the average annual 4th maximum ozone concentration is,
Avg 4th max = (0.089 + 0.084 + 0.081)/3 = 0.08466... = 0.084 ppm
which rounds to 0.08 ppm and the site meets the standard.
Q: What is the ozone air quality design value?
For a concentration-based standard, the air quality design value for a site is defined as the standard-related test statistic. Thus, the 3-year average annual fourth-highest daily maximum 8-hour average ozone concentration (e.g., the 0.084 ppm value calculated in the previous question) is the air quality design value for a site.
Q: What if only 1 or 2 years of data are available at the site?
If only 2 years of data are available at a NAMS or SLAMS monitoring site that is still operating, then you must wait until the 3rd year of data is available before you can determine the air quality status of this monitoring site. Because the form of the standard is an average concentration, the air quality status of a monitoring site cannot be determined using only one year of ambient data. Monitoring sites with only 1 or 2 years of data that indicate a potential to violate the ozone NAAQS should continue to operate so that their air quality status can be determined based on three complete years of data.
Q: How do I judge the air quality status of ozone monitoring sites that have stopped monitoring?
The appropriate EPA Regional Office must be notified prior to shutting down a NAMS site, or a SLAMS site that has exceeded the level of the ozone NAAQS. The air quality status at a NAMS or SLAMS site that has stopped monitoring after two years must be handled on a case by case basis. Factors to consider are the reasons that the site stopped monitoring, the magnitude of the ozone concentrations measured, and the likelihood that additional monitoring data may be available in future years.
Q: What if 3 years of ozone monitoring data are available but some years do not meet the data completeness requirements?
While three complete years of ozone monitoring data are required to demonstrate attainment at a monitoring site, fewer than 3 complete years of data may be sufficient to demonstrate nonattainment. Incomplete data may not be used to excuse a clear nonattainment situation. In a year with missing monitoring data, the true annual 4th highest 8-hour daily maximum concentration will always be at least as high, or higher, than the observed annual 4th highest daily maximum 8-hour average ozone concentration. Thus, calendar years with less than 75% data completeness must be included in the attainment analysis computation if the 3-year average of the annual fourth highest daily maximum 8-hour average ozone concentration is greater than 0.08 ppm. For example,
Year Percent
Valid Days 8-hour Daily Maximum Concentrations(ppm)
1st 2nd 3rd 4th
1993 96% 0.105 0.103 0.103 0.102
1994 74% 0.090 0.085 0.082 0.080
1995 89% 0.103 0.101 1.101 0.097
the 3-year average annual 4th highest daily maximum 8-hour ozone concentration is,
Avg 4th max = (0.102 + 0.080 + 0.097)/3 = 0.093 ppm
which rounds to 0.09 ppm and the site has failed the attainment test, even though the 1994 ozone monitoring data do not meet the 75% data completeness requirement.
Q: How do I judge the air quality status of an area that has more than one ozone monitoring site?
The data handling procedures described in this guidance are applied on an individual basis at each monitor in the area. An area is in compliance with the 8-hour ozone NAAQS if, and only if, every monitoring site in the area meets the NAAQS.
Q: How do I compute the area's design value?
The air quality design value for a site is the 3-year average annual fourth-highest daily maximum 8-hour average ozone concentration. The air quality design value for the area is the highest design value among all sites in the area.
References
1. "Guideline on the Identification and Use of Air Quality Data Affected by Exceptional Events", EPA-450/4-86-007, U.S. Environmental Protection Agency, Research Triangle Park, N.C., July 1986.
2. "Guideline on Accounting for Missing Data to Meet the Data Completeness Requirements of the 8-Hour Ozone NAAQS", U.S. Environmental Protection Agency, Research Triangle Park, N.C., in preparation.
Appendix I to Part 50--Interpretation of the 8-Hour Primary and
Secondary National Ambient Air Quality Standards for Ozone
1. General.
This appendix explains the data handling conventions and
computations necessary for determining whether the national
8-hour primary and secondary ambient air quality standards for
ozone specified in Sec. 50.10 are met at an ambient ozone air
quality monitoring site. Ozone is measured in the ambient air by
a reference method based on Appendix D of this part. Data
reporting, data handling, and computation procedures to be used
in making comparisons between reported ozone concentrations and
the level of the ozone standard are specified in the following
sections. Whether to exclude, retain, or make adjustments to the
data affected by stratospheric ozone intrusion or other natural
events is subject to the approval of the appropriate Regional
Administrator.
2. Primary and Secondary Ambient Air Quality Standards for
Ozone.
2.1 Data Reporting and Handling Conventions.
2.1.1 Computing 8-hour averages. Hourly average
concentrations shall be reported in parts per million (ppm) to
the third decimal place, with additional digits to the right
being truncated. Running 8-hour averages shall be computed from
the hourly ozone
concentration data for each hour of the year and the result shall
be stored in the first, or start, hour of the 8-hour period. An
8-hour average shall be considered valid if at least 75% of the
hourly averages for the 8-hour period are available. In the event
that only 6 (or 7) hourly averages are available, the 8-hour
average shall be computed on the basis of the hours available
using 6 (or 7) as the divisor. (8-hour periods with three or more
missing hours shall not be ignored if, after substituting
one-half the minimum detectable limit for the missing hourly
concentrations, the 8-hour average concentration is greater than
the level of the standard.) The computed 8-hour average ozone
concentrations shall be reported to three decimal places (the
insignificant digits to the right of the third decimal place are
truncated, consistent with the data handling procedures for the
reported data.)
2.1.2 Daily maximum 8-hour average concentrations.
(a) There are 24 possible running 8-hour average ozone
concentrations for each calendar day during the ozone monitoring
season. (Ozone monitoring seasons vary by geographic location as
designated in part 58, Appendix D to this chapter.) The daily
maximum 8-hour concentration for a given calendar day is the
highest of the 24 possible 8-hour average concentrations computed
for that day. This process is repeated, yielding a daily maximum
8-hour average ozone concentration for each calendar day with
ambient ozone monitoring data. Because the 8-hour averages are
recorded in the start hour, the daily maximum 8-hour
concentrations from two consecutive days may have some hourly
concentrations in common. Generally,overlapping daily maximum
8-hour averages are not likely, except in those non-urban
monitoring locations with less pronounced diurnal variation in
hourly concentrations.
(b) An ozone monitoring day shall be counted as a valid day
if valid 8-hour averages are available for at least 75% of
possible hours in the day (i.e., at least 18 of the 24 averages).
In the event that less than 75% of the 8-hour averages are
available, a day shall also be counted as a valid day if the
daily maximum 8-hour average concentration for that day is
greater than the level of the ambient standard.
2.2 Primary and Secondary Standard-related Summary
Statistic. The standard-related summary statistic is the annual
fourth-highest daily maximum 8-hour ozone concentration,
expressed in parts per million, averaged over three years. The
3-year average shall be computed using the three most recent,
consecutive calendar years of monitoring data meeting the data
completeness requirements described in this appendix. The
computed 3-year average of the annual fourth- highest daily
maximum 8-hour average ozone concentrations shall be expressed to
three decimal places (the remaining digits to the right are
truncated.)
2.3 Comparisons with the Primary and Secondary Ozone
Standards.
(a) The primary and secondary ozone ambient air quality
standards are met at an ambient air quality monitoring site when
the 3-year average of the annual fourth-highest daily maximum
8-hour average ozone concentration is less than or equal to 0.08
ppm. The number of significant figures in the level of the
standard dictates the rounding convention for comparing the
computed 3-year average annual fourth-highest daily maximum
8-hour average ozone concentration with the level of the
standard. The third decimal place of the computed value is
rounded, with values equal to or greater than 5 rounding up.
Thus, a computed 3-year average ozone concentration of 0.085 ppm
is the smallest value that is greater than 0.08 ppm.
(b) This comparison shall be based on three consecutive,
complete calendar years of air quality monitoring data. This
requirement is met for the three year period at a monitoring site
if daily maximum 8-hour average concentrations are available for
at least 90%, on average, of the days during the designated ozone
monitoring season, with a minimum data completeness in any one
year of at least 75% of the designated sampling days. When
computing whether the minimum data completeness requirements have
been met, meteorological or ambient data may be sufficient to
demonstrate that meteorological conditions on missing days were
not conducive to concentrations above the level of the standard.
Missing days assumed less than the level of the standard are
counted for the purpose of meeting the data completeness
requirement, subject to the approval of the appropriate Regional
Administrator.
(c) Years with concentrations greater than the level of the
standard shall not be ignored on the ground that they have less
than complete data. Thus, in computing the 3-year average fourth
maximum concentration, calendar years with less than 75% data
completeness shall be included in the computation if the average
annual fourth maximum 8-hour concentration is greater than the
level of the standard.
(d) Comparisons with the primary and secondary ozone
standards are demonstrated by examples 1 and 2 in paragraphs
(d)(1) and (d) (2) respectively as follows:
(1) As shown in example 1, the primary and secondary
standards are met at this monitoring site because the 3-year
average of the annual fourth-highest daily maximum 8-hour average
ozone concentrations (i.e., 0.084 ppm) is less than or equal to
0.08 ppm. The data completeness requirement is also met because
the average percent of days with valid ambient monitoring data is
greater than 90%, and no single year has less than 75% data
completeness.
Example 1. Ambient monitoring site attaining the primary
and secondary ozone standards
Percent Highest Daily Maximum 8-hour
Valid Concentration (ppm)
Year Days 1st 2nd 3rd 4th 5th
---------------------------------------------
1993 100% 0.092 0.091 0.090 0.088 0.085
1994 96% 0.090 0.089 0.086 0.084 0.080
1995 98% 0.087 0.085 0.083 0.080 0.075
---------------------------------------------
Average 98% 0.084
[Examples 1 and 2 have been reformatted to fit on a 8.5" by 11"
page in letter size (portrait).]
(2) As shown in example 2, the primary and secondary
standards are not met at this monitoring site because the 3-year
average of the fourth-highest daily maximum 8-hour average ozone
concentrations (i.e., 0.093 ppm) is greater than 0.08 ppm. Note
that the ozone concentration data for 1994 is used in these
computations, even though the data capture is less than 75%,
because the average fourth-highest daily maximum 8-hour average
concentration is greater than 0.08 ppm.
Example 2. Ambient Monitoring Site Failing to Meet the Primary
and Secondary Ozone Standards
Percent Highest Daily Maximum 8-hour
Valid Concentration (ppm)
Year Days 1st 2nd 3rd 4th 5th
---------------------------------------------
1993 96% 0.105 0.103 0.103 0.102 0.102
1994 74% 0.090 0.085 0.082 0.080 0.078
1995 98% 0.103 0.101 0.101 0.097 0.095
---------------------------------------------
Average 89% 0.093
[Examples 1 and 2 have been reformatted to fit on a 8.5" by 11"
page in letter size (portrait).]
3. Design Values for Primary and Secondary Ambient Air
Quality Standards for Ozone.
The air quality design value at a monitoring site is defined
as that concentration that when reduced to the level of the
standard ensures that the site meets the standard. For a
concentration-based standard, the air quality design value is
simply the standard- related test statistic. Thus, for the
primary and secondary ozone standards, the 3-year average annual
fourth-highest daily maximum 8- hour average ozone concentration
is also the air quality design value for the site.
[52 FR 24664, July 1, 1987; 52 FR 29467, Aug. 7, 1987]
TECHNICAL REPORT DATA
1. REPORT NO. EPA-454/R-98-017 2. 3. RECIPIENT'S ACCESSION NO. 4. TITLE AND SUBTITLE Guideline on Data handling Conventions for the 8-Hour Ozone NAAQS 5. REPORT DATE December 1998 6. PERFORMING ORGANIZATION CODE 7. AUTHOR(S) Warren P. Freas 8. PERFORMING ORGANIZATION REPORT NO. 9. PERFORMING ORGANIZATION NAME AND ADDRESS Air Quality Trends Analysis Group (MD-14) U.S. Environmental Protection Agency Office of Air Quality Planning and Standards Research Triangle Park, NC 27711 10. PROGRAM ELEMENT NO. 11. CONTRACT/GRANT NO. 12. SPONSORING AGENCY NAME AND ADDRESS Director Office of Air Quality Planning and Standards Office of Air and Radiation U.S. Environmental Protection Agency Research Triangle Park, NC 27711 13. TYPE OF REPORT AND PERIOD COVERED 14. SPONSORING AGENCY CODE EPA/200/04 15. SUPPLEMENTARY NOTES 16. ABSTRACT
This report provides technical guidance on the data handling conventions for the 8-hour ozone National Ambient Air Quality Standards (NAAQS). The report is written in the "plain English" question and answer (Q&A) style. The following data handling issues are addressed:
1) Rounding conventions 2) Missing data 3) Treatment of exceptional natural events 4) Data completeness requirements for computing averages 5) Minimum years of data required to demonstrate compliance with the NAAQS 6) Use of data from Special Purpose Monitors (SPM) 7) The attainment test used to judge compliance with the NAAQS 17. KEY WORDS AND DOCUMENT ANALYSIS a. DESCRIPTORS Ozone Compliance Ozone Standard Ambient Air Quality Standards (NAAQS) b. IDENTIFIERS/OPEN ENDED TERMS Air Pollution Control c. COSATI Field/Group 18. DISTRIBUTION STATEMENT Release Unlimited 19. SECURITY CLASS (Report) Unclassified 21. NO. OF PAGES 16 20. SECURITY CLASS (Page) Unclassified 22. PRICE EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE