& EPA
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA-454/R-98-017
December 1998
Air
GUIDELINE ON DATA HANDLING
CONVENTIONS FOR THE 8-HOUR
OZONE NAAQS
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EPA-454/R-98-017
December 1998
GUIDELINE ON DATA HANDLING
CONVENTIONS FOR THE 8-HOUR
OZONE NAAQS
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
December 1998
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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.
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Contents
Introduction 1
Data Handling Conventions 1
What monitors does this guidance cover? 1
How do I compute the daily maximum 8-hour average ozone concentration? 2
What is a valid ozone monitoring day? 2
How many monitoring days are required? 2
What is a complete data year? 2
Can I exclude ozone monitoring data affected by natural events such as stratospheric ozone
intrusion from computations used to determine compliance with the
NAAQS? 3
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? 3
How do I find the 4th highest daily maximum 8-hour average ozone concentration at an
ambient air quality monitoring site? 4
How many significant figures must I carry in the computations? 4
What is the attainment test? 4
What is the minimum number of years of data needed to demonstrate attainment with the
8-hour ozone NAAQS? 4
How do I compute the average annual 4th highest 8-hour daily maximum ozone
concentration at an ambient air quality monitoring site? 4
What is the ozone air quality design value? 5
What if only 1 or 2 years of data are available at the site? 5
How do I judge the air quality status of ozone monitoring sites that have stopped
monitoring? 5
What if 3 years of ozone monitoring data are available but some years do not meet the data
completeness requirements? 6
How do I judge the air quality status of an area that has more than one ozone monitoring
site? 7
How do I compute the area's design value? 7
References 7
Appendix I to 40CFR, Part 50 8
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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.
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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
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(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
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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,
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Year
1993
1994
1995
Percent
Valid Days
100%
96%
98%
8-hour Daily Maximum Concentrations
(ppm)
1st
0.092
0.090
0.087
2nd
0.091
0.089
0.085
3rd
0.090
0.086
0.083
4th
0.089
0.084
0.081
the average annual 4th maximum ozone concentration is,
Avg 4th max = (0.089+ 0.084+ 0.081)73 =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
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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
1993
1994
1995
Percent
Valid Days
96%
74%
89%
8 -hour Daily Maximum Ozone Concentrations
(ppm)
1st
0.105
0.090
0.103
2nd
0.103
0.085
0.101
3rd
0.103
0.082
1.101
4th
0.102
0.080
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.
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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.
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Appendix I to 40CFR, Part 50
Interpretation of the 8-Hour Primary and Secondary National
Ambient Air Quality Standards for Ozone
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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
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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)(l) 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.
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Example 1. Ambient monitoring site attaining the primary and secondary O3 standards.
Year
1993
1994
1995
Average
Percent
Valid Days
100%
96%
98%
98%
1st
Highest
Daily Max
8 -hour
Cone.
(ppm)
0.092
0.090
0.087
2nd Highest
Daily Max
8 -hour
Cone.
(ppm)
0.091
0.089
0.085
3rd Highest
Daily Max
8 -hour
Cone.
(ppm)
0.090
0.086
0.083
4th Highest
Daily Max
8 -hour
Cone.
(ppm)
0.088
0.084
0.080
0.084
5th Highest
Daily Max
8 -hour
Cone.
(ppm)
0.085
0.080
0.075
(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 O3 standards.
Year
1993
1994
1995
Average
Percent
Valid Days
96%
74%
98%
89%
1st
Highest
Daily Max
8 -hour
Cone.
(ppm)
0.105
0.090
0.103
2nd Highest
Daily Max
8 -hour
Cone.
(ppm)
0.103
0.085
0.101
3rd Highest
Daily Max
8 -hour
Cone.
(ppm)
0.103
0.082
0.101
4th Highest
Daily Max
8 -hour
Cone.
(ppm)
0.102
0.080
0.097
0.093
5th Highest
Daily Max
8 -hour
Cone.
(ppm)
0.102
0.078
0.095
11
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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.
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TECHNICAL REPORT DATA
(Please read Instructions on reverse before completing)
1. REPORT NO.
EPA-454/R-98-017
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
Document is available electronically at "http://www.epa.gov/ttn/oarpg/"
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
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Ozone Compliance
Ozone Standard
Ambient Air Quality Standards (NAAQS)
Air Pollution Control
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
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