United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA-454/R-98-001
June 1998
Air
r/EPA
GUIDELINE FOR SELECTING AND
MODIFYING THE OZONE
MONITORING SEASON BASED ON
AN 8-HOUR OZONE STANDARD
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EPA-454/R-98-001
Guideline for Selecting and Modifying
The Ozone Monitoring Season
Based On An 8-Hour Ozone Standard
United States Environmental Protection Agency
Office of Air Quality Planning and Standards
Emissions, Monitoring, and Analysis Division
Research Triangle Park, NC 27711
June 1998
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TABLE OF CONTENTS
Section Page
List of Figures v
List of Tables vii
Executive Summary ix
1.0 Introduction and Background 1
2.0 Methodology 2
3.0 Results 4
4.0 Criteria for Revision of the Ozone Monitoring Season 14
References 18
Appendix A: Histograms 19
Appendix B: Calculations 29
Appendix C: National Ambient Air Quality Standards for Ozone 33
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List of Figures
Figure No. Page
Figure 1. Current Ozone Monitoring Seasons (Based on the 1-Hour Ozone
NAAQS) 7
Figure 2. Calculated Ozone Monitoring Seasons (Based on the 8-Hour Ozone NAAQS) . 9
Figure 3. Proposed Ozone Monitoring Seasons (Based on the 8-Hour Ozone NAAQS and
Adjusted for Regional Uniformity) 11
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List of Tables
Table No. Page
Table 1. Summary Table of the Number of Daily 8-Hour Concentrations^ 0.080 ppm as a
Function of Month for the State of Illinois 3
Table 2. Ozone Monitoring Seasons Based on the 1-Hour and 8-Hour NAAQS 5
Table 3. States and Territories with No 8-Hour Values > 0.080 ppm 13
IV
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Executive Summary
This document provides guidance for the EPA Regional Offices to select and modify the
ozone monitoring seasons designated for each State in the Code of Federal Regulations (40 CFR
58, Appendix D). The guidance may be summarized as follows:
1. From AIRS, examine all ozone monitoring stations that collected data during the past 6-
year period (in this case 1990-1995 was used). For each State and Territory, determine
the number of days in each month, summed over the 6 years, in which at least one 8-hour
average ozone concentration exceeded 0.080 ppm; by using a value of 0.080 ppm rather
than 0.08 as articulated in the revised National Ambient Air Quality Standard (NAAQS)
for ozone, a degree of conservatism is introduced which ensures that monitoring occurs
during months that have the potential to violate the NAAQS.
2. Prepare a histogram of these values (See Appendix A of this document) for each State or
Territory. Choose a proposed ozone monitoring season which would be defined as the
continuous period that includes all months showing at least one 8-hour average
concentration > 0.080 ppm.
3. Extrapolate the ozone monitoring season to States and Territories for which data are
insufficient based on similarities in climatology. For each State and Territory not covered
by a monitoring histogram, this procedure produces an estimated histogram from which a
proposed ozone monitoring season is derived.
4. Table 2. depicts the proposed ozone monitoring season for each State and Territory based
on this analysis and compares it to the existing ozone monitoring season.
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5. Adjust the ozone monitoring seasons as follows (see also Table 2.):
a. Choose an appropriate composite monitoring season which includes both the 1-
hour and 8-hour monitoring seasons if both NAAQS still apply.
b. If the 1-hour NAAQS has been revoked, choose the 8-hour monitoring season as a
replacement for the 1-hour or "composite" monitoring season.
c. Adjust the monitoring seasons in neighboring States to reflect similar seasons in
areas of transport or within EPA Regional boundaries.1 Adjustments should only
be made to lengthen the ozone monitoring season, rather than to shorten it.
6. Make appropriate changes to the Code of Federal Regulations (CFR):
All official changes to the ozone monitoring season which will affect State and Local Air
Monitoring Stations (SLAMS) and National Air Monitoring Stations (NAMS) in a
county, group of counties, or entire State must be promulgated as changes to 40 CFR 58
to modify the table in Appendix D to 40 CFR 58, entitled "Ozone Monitoring Season by
State". The Regional Administrator is responsible for coordinating changes to the ozone
season with each affected State and he/she will prepare a rulemaking package and publish
it the Federal Register notice. Note that either the State, the EPA Regional Office, or
EPA Headquarters may request that a State's ozone season be revised and the attendant
data analysis and review be initiated. Any changes which affect NAMS, must be
concurred with by the Office of Air Quality Planning and Standards (OAQPS). This
concurrence will promote national consistency in ozone monitoring and also ensure that
appropriate changes will be made to the Aerometric Information Retrieval System (AIRS)
such that statistics are appropriately calculated. It is also suggested that changes to
scheduled ozone monitoring seasons should be incorporated into the comment section of
the monitoring site file on AIRS, for all affected monitors.
Kantz, Marcus, Interoffice Memorandum, March 5, 1997.
VI
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1.0 Introduction and Background
Seasonal ozone monitoring requirements at State and Local Air Monitoring Stations
(SLAMS) [which by definition include the National Air Monitoring Stations (NAMS)] were first
introduced into regulation in 1986. Since that time, modifications to the original ozone
monitoring seasons have been made periodically as new data and information became available;
each of these changes has been incorporated into existing ambient air monitoring regulations2.
These seasons are important because they play a significant role in the estimation of annual ozone
NAAQS exceedances and provide the basis for calculations in the Aerometric Information
Retrieval System (AIRS) summary files. The monitoring seasons may also be tied to certain State
emission regulations.
For the initial formulation of the ozone monitoring seasons for each State, the District of
Columbia, and Territories held by the United States, the monthly ranges were principally based on
empirically-derived relationships between monthly mean daily maximum temperature and
observed peak ozone concentrations. The basic premise was that areas with monthly mean
maximum temperatures predominantly below 55 degrees Fahrenheit (°F) are expected to have
hourly ozone concentrations less than 0.08 ppm (i.e., significantly less than the hourly ozone
standard of 0.12 ppm). This relationship was determined empirically from observed ozone
concentrations and indicated by smog chamber studies.3
With the 1997 revisions to the National Ambient Air Quality Standards (NAAQS) for
ozone (i.e., the addition of a maximum 8-hour average ozone concentration of 0.08 ppm; see
Appendix C to this document), it has become necessary to redefine the ozone monitoring season
for purposes of demonstrating attainment. The principal motivation is to develop an
understanding of the characteristics of the ozone monitoring season based on the new standard.
Another motivation for this guideline document is that reductions in required monitoring can
240 CFR 58, Appendix D, Section 2.5
The 1-hour 0.12 ppm ozone standard was in place in 1979, before the establishment of ozone seasons.
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result in considerable cost savings. If the ozone monitoring season in any particular State can be
shortened even 1 month, significant potential savings could be realized.
Unlike previous techniques to define the ozone monitoring season, the technique adopted
by this study relies principally on six years of observed data from the SLAMS (including NAMS)
networks, in this case, for the period 1990-1995; these data are used to derive the statistics for
each State. Any extrapolation of the results was performed based on similarities in climatology.
Section 2. describes the methodology, Section 3. presents the resulting table of ozone monitoring
seasons by States, and Section 4. discusses the criteria for revising the ozone monitoring season.
2.0 Methodology
The specific steps in the method used to determine the ozone monitoring season are based on
existing data and the 8-hour average ozone standard of 0.08 ppm; these steps are outlined below:
1) Twenty-four 8-hour average ozone concentrations were calculated for each day and station
in the United States and its Territories using hourly data from the Aerometric Information
Retrieval System (AIRS) database for the 6-year period 1990-1995. From this database, a
second data set containing the peak 8-hour average concentrations for each day was
assembled.
2) The peak 8-hour average ozone concentrations for each station were examined and when
concentrations > 0.080 ppm were found, the day, month, and concentration values were
recorded. Note that the 8-hour standard includes a rounding procedure that rounds 0.084
ppm down and 0.085 ppm up.4 The chosen approach to establish the ozone monitoring
season, however, incorporates a more conservative approach and defines exceedances as >
440 CFR 50, Appendix I, Section 2.3
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0.080 ppm rather than 0.085 ppm. This convention ensures that time periods which have the
potential to exceed .085 ppm are included in the monitoring season.
3) These data were used to construct State-by-State histograms5 for the months in which the
8-hour concentrations were > 0.080 ppm in the 6-year period 1990-1995 using all available
data (i.e., in AIRS) for each State.
4) Since an uninterrupted monitoring season should be adopted which will capture all the
potential daily maximum 8-hour average ozone concentrations > 0.085 ppm, the ozone season
for the 8-hour NAAQS would be defined for each State from these histograms as beginning
with the first month with 8-hour values > 0.080 ppm and ending with the last month with 8-
hour values > 0.080 ppm. For example, Table 1. is a summary table of the frequency of 8-
hour values > 0.080 ppm for the State of Illinois and shows that values > 0.080 ppm occurred
from April through September. The ozone monitoring season for Illinois would then be
defined as the period April through September, April being the first month with values >
0.080 ppm and September being the last month with values > 0.080 ppm.
Table 1. Summary Table of the Number of Daily 8-Hour Concentrations > 0.080 ppm as
a Function of Month for the State of Illinois.
Month
Count
Jan
0
Feb
0
Mar
0
Apr
1
May
61
Jun
482
Jul
197
Aug
204
Sep
58
Oct
0
Nov Dec
0 0
5) Extrapolation of ozone monitoring seasons adopted from the histogram data to States
where data do not exist was principally based on similarities in climatology. The
climatological data used for this part of the project were monthly wind rose data, monthly
maximum, minimum, and average surface temperature data, and monthly average
See Appendix A to this guideline.
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precipitation data provided in the Climatic Atlas for the United States (1997) and Local
Climatological Data (LCD) for the period 1900-1995 (National Climate Date Center,
1997).
For a given station, the 24 8-hour averaged ozone concentrations for a given day were
calculated as shown in Appendix B to this guideline
3.0 Results
Table 2. provides the 8-hour ozone monitoring seasons for each State based on the histogram
data located in Appendix A to this guideline and contains comparisons to the current ozone
season based on the 1-hour NAAQS. This table also contains the "adjusted" 8-hour monitoring
seasons discussed in Section 4. of this document. Figure 1. shows the current ozone monitoring
seasons based on the 1-hour standard; Figure 2. depicts the seasons for the 8-hour NAAQS which
result from following the guidance in this document; and Figure 3. displays the 8-Hour ozone
monitoring seasons following an adjustment for regional uniformity. Section 4.0, "Criteria for
Revisions of the Ozone Monitoring Season" explains the methodology for calculating a new
ozone monitoring season.
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Table 2. Ozone Monitoring Seasons Based on the 1-Hour and 8-Hour NAAQS
State
Alabama
Alaska2
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
District Of Columbia
Florida
Georgia
Hawaii3
Idaho2
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana"
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota2
Ohio
Oklahoma
Oregon
Pennsylvania
Puerto Rico3
Rhode Island
South Carolina
South Dakota2
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wyoming2
American Somoa"
Guam"
Virgin Islands"
A - Season Based
on the 1-hour
NAAQS1
April - October
April - October
January - December
March - November
January - December
March - September
April - September
April - October
April - October
January - December
April - October
January - December
April - October
April - October
April - September
April - October
April - October
April - October
January - December
April - September
April - October
April - September
April - September
April - October
April - October
April - October
June - September
April - October
January - December
April - September
April - October
January - December
April - October
April - October
May - September
April - October
March - November
April - October
April - October
January - December
April - September
April - October
June - September
April - October
Jan-Dec & Mar-Oct.
May - September
April - September
April - October
April - October
April - October
April 15-October 15
April - October
January - December
January - December
January - December
B - Season Based on
the 8-Hour NAAQS
March - October
June - September
April - October
March - October
January - December
May - September
April - October
April - October
April - September
March - October
March - September
January - December
June - September
April - September
April - October
May - September
April - September
April - October
January - November
April - October
April - September
April - October
April - October
May - August
March - October
April - October
June - September
June -September
January - August
May - September
April - October
June - October
April - October
April - October
June - September
April - October
March - October
May - September
April - September
January - December
April - September
April - November
June - September
April - November
January - November
May - September
April - October
April - September
May - August
April - October
April - October
June - September
January - December
January - December
January - December
Difference
(B-A)
(Months)
+1
-3
-5
-1
0
-2
+1
0
-1
-4
0
0
-3
-1
+1
-2
-1
0
-1
+1
-1
+1
+1
-3
+1
0
0
-3
-4
-1
0
-7
0
0
-1
0
-1
-2
-1
0
0
+1
0
+1
-1/+3
0
+1
-1
-3
0
+1
-3
0
0
0
C - Adjusted 8-
Hour Monitoring
Season5
March - October
June - September
April - October
March - October
January - December
May - September
April - October
April - October
April - October
March - October
March - October
January - December
June - September
April - October
April - October
May - September
April - October
April - October
January - November
April - October
April - October
April - October
April - October
May - September
March - October
April - October
June - September
June -September
January - August
April - October
April - October
April - October
April - October
April - October
June - September
April - October
March - October
May - September
April - October
January - December
April - October
April - November
June - September
April - November
January - November
May - September
April - October
April - October
May - September
April - October
April - October
June - September
January - December
January - December
January - December
Difference
(C-A)
(Months)
+1
-3
-5
-1
0
-2
+1
0
0
-4
+1
0
-3
0
+1
-2
0
0
-1
+1
0
+1
+1
-2
+1
0
0
-3
-4
+1
0
-5
0
0
-1
0
-1
-2
0
0
+1
+1
0
+1
-1
0
+1
0
-2
0
+1
-3
0
0
0
Total Months | 430 385 -45 | 399 | -31
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Mo CFR 58, Appendix D.
Default values for the continental United States, including Alaska, have been used for this State.
Default values for the tropical latitudes have been used for this State or Territory.
No data were available for this State or Territory and the ozone monitoring season was based on climatology.
Ozone monitoring seasons for certain States were adjusted to match ozone seasons in neighboring States within transport regions and to
better reflect months with ozone conducive meteorology. Specifically, the following adjustments were made: the ozone monitoring seasons
were extended in the States of New Hampshire, Rhode Island, Georgia, Illinois, Minnesota, Washington, Pennsylvania, Maryland,
District of Columbia, Virginia, and New Mexico.
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Table 3. lists the States having no 8-hour values > 0.080 ppm over the 6-year period
1990-1995. For those States having no exceedances found in the continental United States,
including Alaska, a default ozone monitoring season of June through September, the heart of the
summer or growing season, was applied. For the States or Territories (i.e., Hawaii, Puerto Rico,
Guam, American Somoa, and the Virgin Islands) in the tropical latitudes, the default ozone
monitoring season was January through December because meteorological conditions (i.e.,
temperature, winds, etc.) are conducive to creating an episode on a year around basis at those
latitudes.
Table 3. States and Territories with No 8-Hour Values > 0.080 ppm.
Alaska Hawaii
Idaho North Dakota
Puerto Rico South Dakota
Wyoming
Montana, Guam, American Somoa, and the Virgin Islands were States/Territories for
which data were not available to create histograms of 8-hour values > 0.080 ppm. Based on the
Climatic Atlas for the United States (1997) and the LCDs (National Climatic Data Center, 1997)
for Montana and the surrounding region, Montana has a climate similar in aspects to North
Dakota, South Dakota, Wyoming, and Idaho, all of which border Montana. North Dakota, South
Dakota, Wyoming, and Idaho are listed in Table 3. as having no 8-hour ozone values > 0.080
ppm so that the default ozone monitoring season of June through September was applied. Based
on climatology, the ozone monitoring season of June through September was also applied to
Montana.
10
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4.0 Criteria for Revisions of the Ozone Monitoring Season
According to the provisions of 40 CFR 58.13(a)(3), the Regional Administrator (RA) has
the authority to exempt particular periods or seasons from the requirements to collect ambient air
quality data at SLAMS. Appendix H of 40 CFR 50 also mentions such waivers for continuous
ozone monitoring requirements areas where it can be demonstrated that exceedances of the ozone
NAAQS are extremely unlikely. Such exemptions or waivers take the form of a formal change to
40 CFR 58, Appendix D, published as a final rule in the Federal Register by the RA. For example
in April 1989, Region VT officially modified Texas' ozone monitoring season which was originally
designated as the entire year, by publishing a notice of change in the Federal Register. Since this
change involved NAMS, it was coordinated with EPA headquarters. The RA revised the ozone
monitoring season to a 245 day time period of March - October for the seven more northern
regions of Texas.
The primary objective for ozone monitoring is protection of public health and the
determination of all ambient concentrations greater than the level of the NAAQS. Accordingly, a
simple rule may be followed to determine the seasonal requirement for ozone monitoring: any
location for which an ozone monitor is determined to be necessary, an uninterrupted particular
time of the year must be monitored if it has the potential for ozone exceedances.
There are two basic constraints regarding time and geographic coverage. As in the case
for the 1986 promulgated ozone monitoring seasons, the period of required monitoring must be
uninterrupted (e.g., April through November). The required monitoring period cannot have any
gaps (e.g., it cannot be April through September plus November). Single counties should not
have different ozone seasons; instead, seasons should be crafted for multi-county areas. In
particular, ozone nonattainment areas comprised of multiple counties, should have the same ozone
monitoring seasons to avoid questions during ozone attainment reviews and enforcement/sanction
actions. It is also suggested that EPA Regional Offices with States located in the ozone transport
regions consider having the same or similar ozone monitoring seasons to make data analysis more
11
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meaningful. At most, two regions within each State would be acceptable; this alternative is
principally intended for large States (e.g., Texas and California).
The following discussion will focus on procedures to justify changes in the current ozone
seasons. States are encouraged to review the duration of their current ozone monitoring season
as part of their annual network review. With the significant amount of monitoring being
performed in most States and Territories, the potential for ozone exceedances can be determined
using the ozone monitoring data available for the State or Territory. In those few cases where
ozone monitoring data are not available, application of results from areas with similar
climatological conditions and underlying precursor emissions is recommended.
Ambient concentrations produced in a monitoring area can provide the basis for revisions
to existing ozone monitoring seasons. A review of historical ozone data for this purpose must be
based on 6 years of the most recent data, to ensure that both favorable and unfavorable
meteorological conditions are represented. In addition, these data should be representative of
both current and expected near-term future conditions. This will help to anticipate the effects of
possible growth in ozone precursor emissions.
Historical ozone data would be examined using the histogram approach outlined in
Section 2. Based on the outcome of such a review, ozone monitoring must be maintained during
those calendar periods in which the 8-hour averaged ozone concentration > 0.080 ppm have been
observed and therefore has the potential to exceed the 8-hour NAAQS of 0.08 ppm. To capture
those periods when the 8-hour values may be > 0.08 ppm, the ozone season should be defined
from these histograms as beginning with the first month with 8-hour values > 0.080 ppm and
ending with the last month with values > 0.080 ppm. If because of urban growth or
meteorological conditions or both, 8-hour values > 0.080 ppm begin to appear at the boundaries
of the designated ozone monitoring season, the ozone monitoring season should be extended one
month beyond the designated boundary of the season. For example, the present analysis indicates
that the ozone monitoring season for Illinois after adjustment is April through October (See Table
12
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2.). If in the future, 8-hour values > 0.080 ppm are found in early April and data are not available
prior to April, then the RA should consider extending the ozone monitoring season to include
March. It is recommended that the historical data should be reviewed every three years after any
change in season is made to see if a further extensions are required. Similarly, if no 8-hour values
> 0.080 ppm are found in April6, then the ozone monitoring season should be revised to May
through October.
If a State does not collect ozone data and changes in the ozone monitoring season have
been observed for States that have similar climatology, then those changes in the ozone
monitoring season should also be implemented for the State that does not collect ozone data. For
example, if identical changes in the ozone monitoring season were noted for North Dakota, South
Dakota, Wyoming, and Idaho, which monitor ozone, then a similar change should be implemented
in the ozone monitoring season for Montana which does not presently collect ozone data, but
which has a similar climatology as North Dakota, South Dakota, Wyoming, and Idaho according
to the Climatic Atlas for the United States (1997).
Additionally, ozone monitoring seasons may be adjusted to reflect similar seasons in areas
of transport or within EPA Regional boundaries. This will simplify not only the management of
the ozone monitoring program, but will also ensure that data is being procured in neighboring
States for later comparisons. Table 2. and Figure 3. illustrate calculated ozone seasons based on
the 8-hour NAAQS and seasons adjusted for regional uniformity, respectively; note that for these
purposes, seasons were only adjusted upward, i.e., the longer period was chosen. When any area
is subject to both the 1-hour and 8-hour NAAQS, the monitoring season should also reflect the
longer of the two periods, or a different period which would overlap the sum of the two. For
example:
Note that only one value > 0.080 ppm was found for Illinois in April in the 6-year period 1990-1995.
13
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If a State's 1-hour ozone monitoring season was April to October and its 8-hour
season was April to November, the appropriate composite season would be April to
November. If the 1-hour season was April to October and the 8-hour season was
May to November, the appropriate choice would be April to November.
14
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REFERENCES
Federal Register. Vol. 50, No. 46, March 8, 1985.
Federal Register. Vol. 51, No. 53, March, 19, 1986.
Federal Register. Part 58, Appendices A through G, July 1, 1995.
Local Climatological Date, US Department of Commerce, National Climatic Data Center, Federal
Plaza, 151 Palton Ave., Asheville, NC 28801-5001
US Department of Commerce, Climatic Atlas for the United States. NOAA, 80 pg., 1997
US EPA, Guideline on Modification to Monitoring the Seasons for Ozone. Technical Support
Division, OAQPS, 47 pg., March 1990.
Kantz, Marcus, Interoffice Memorandum, March 5, 1997.
Science Applications International Corporation, Guideline for Selecting and Modifying the Ozone
Monitoring Season. A Report Prepared for the US EPA, Raleigh, North Carolina, January 1997.
15
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Appendix A
Histograms
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In the following appendix, the histograms of the daily maximum 8-hour averaged ozone
concentrations that are > 0.080 ppm are presented. A histogram is presented for each State and
Territory that collected ozone data from 1990 through 1995 and that had one or more 8-hour
values > 0.080 ppm1.
1-Histograms could not be created for those States that did not collect ozone data for the analysis period, specifically
Montana. Ozone data has been reported for Alaska, Idaho, Hawaii, North Dakota, Puerto Rico, South Dakota, and
Wyoming; however, none of these monitors showed 8-hour averaged ozone values > 0.080 ppm.
17
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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
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Jan Feb Mar Apr May Jun Jul
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:t of Columbia
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
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-------
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Appendix B
Calculations
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Calculating 24 8-Hour Concentrations
For a given station, the 24 8-hour average ozone concentrations for a given day were calculated
in the following manner. The first 8-hour averaged value, nl5 was calculated using hourly
concentrations starting from midnight, Cm,: i.e.,
|i, = (Cm + C, + C2 + C3 + C4 + C5 + C6 + C7 )/8,
where m refers to midnight and 1, 2, 3 ....... refers to 1:00 AM, 2:00 AM, 3:00 AM ..... This
procedure is repeated until there are 24 8-hour averaged concentrations; i.e.,
|i2 = (C, + C2 + C3 + C4 + C5 + C6 + C7 + C8 )/8,
|i3 = (C2 + C3 + C4 + C5 + C6+ C7 +C8 + C9 )/8,
N
N
= (C23 + Cm + Q + C2 + C3 + C4 + C
For 8-hour values calculated for 4:00 p.m. -midnight, the calculations used hourly ozone
concentrations that were obtained in the early morning hours of the following day to acquire 24 8-
hour averaged concentrations for each day. This method is consistent with the standard data
reporting criteria which notes that "8-hour averages are recorded in the start hour...".7
740 CFR 50, Appendix I
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Appendix C
National Ambient Air Quality Standards for Ozone
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40 CFR Part 50
National Ambient Air Quality Standards for Ozone
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
SUMMARY: This document describes EPA's decision to revise the national ambient air quality standards (NAAQS) for ozone
based on its review of the available scientific evidence linking exposures to ambient to adverse health and welfare effects at
levels allowed by the current standards. The current 1-hour primary standard is replaced by an 8-hour standard at a level of 0.08
parts per million (ppm) with a form based on the 3-year average of the annual fourth-highest daily maximum 8-hour average
concentrations measured at each monitor within an area. The new primary standard will provide increased protection to the
public, especially children and other at-risk populations, against a wide range of -induced health effects, including decreased
lung function, primarily in children active outdoors; increased respiratory symptoms, particularly in highly sensitive individuals;
hospital admissions and emergency room visits for respiratory causes, among children and adults with pre-existing respiratory
disease such as asthma; inflammation of the lung, and possible long-term damage to the lungs. The current 1 -hour secondary
standard is replaced by an 8-hour standard identical to the new primary standard. The new secondary standard will provide
increased protection to the public welfare against -induced effects on vegetation, such as agricultural crop loss, damage to forests
and ecosystems, and visible foliar injury to sensitive species.
EFFECTIVE DATE: This rule is effective September 16, 1997.
PART SO-NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS
(Revised in 62FR3855, July 18, 1997)
The revised Section 50.9 follows:
Sec. 50.9 - National 1-hour Primary and Secondary Ambient Air Quality Standards for Ozone.
(a) The level of the national 1 -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 0.12 parts per
million (235 g/m3). The standard is attained when the expected number of days per calendar year with maximum hourly average
concentrations above 0.12 parts per million (235 g/m3) is equal to or less than 1, as determined by Appendix H to this part.
(b) The 1 -hour standards set forth in this section will no longer apply to an area once EPA determines that the area has
air quality meeting the 1-hour standard. Area designations are codified in 40 CFR part 81.
Section SO.lOwas added to read as follows:
Sec. 50.10 - National 8-hour Primary and Secondary Ambient Air Quality Standards for Ozone.
(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.
Appendix I was added as follows:
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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 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.
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(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.
Example 1. Ambient Monitoring Site Attaining the Primary and Secondary Ozone 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 Ozone Standards
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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
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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