GUIDELINE SERIES
OAQPS NO. 1.2-008 (Revised)
INTERIM GUIDELINES FOR THE
INTERPRETATION OF AIR QUALITY STANDARDS
VS. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
Research Triangle Park, North Carolina
SUBJECT: "Interim Guidelines for the Interpretation ' i\pX^: May 13, 1974
of Air Quality "Standards" ,
FROM: Robert E. Neligan, Director
Monitoring and Data Analysis Division
TO: See Below
I am enclosing a revised copy of our guideline document entitled
"Guidelines for the Interpretation of Air Quality Standards." As
you know, this document was circulated in draft form for review, and
we received extensive comments from numerous sources including our
regional offices and various state and local agencies. The present
version incorporates many of these comments; and since in some cases
these changes have resulted in different recommendations, I would
like to release this updated version as an interim guideline. This
should serve to indicate that the previous draft version is superseded
and therefore clarify our position on these issues. Prior to doing
this., however, I would appreciate receiving a final review from you
indicating either your concurrence or what .changes you feel are essen-
tial before we issue the document as an interim^guideline.
Since you and members of your staff have provided,,valuable input
in the preparation of this document, I will briefly indicate the major
points of the reviews we have received: ' V-
i
(1) Although few agreed totally with the document, a;lmost all
reviewers indicated that they were glad to see such a document attempted.
i
(2) Many commentators indicated a preference for "parts per million"
rather than "yg/m3." At the present time we are remaining with the "yg/m3"
since this has been indicated as Agency policy.
(3) Overwhelmingly negative reviews were received on the use of
3-fixed 8-hour periods rather than running averages. Many held that
the use of running averages had not caused any major difficulties for
them. In addition to other factors, this input and the recommendation
of Dr. John Knelson of HSL/NERC have resulted in our change on this
position. Therefore, the present document suggests running 8-hour averages,
(4) Reviews were negative on permitting compliance with "once per
year" standards to be established by every sixth day sampling. Some
suggested that this was tantamount to redefining the standard. Our
present position is that intermittent sampling data is sufficient to
show compliance unless predictive equations show that the standard
was exceeded. In such a case, more frequent monitoring might be
required, but no violation would be declared solely on the basis of
predicted values. :
EPA Form 1320-6 (R«v. 6-72)
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-2-
These points are, of course, discussed in more detail in the
document. Once again, I appreciate the assistance you have given
us in this matter.
Robert E. Neligan
Enclosure
Addressees
Donald Goodwin, Director, ESED
Joseph Padgett, Director, SASD
•Jean Schuencr.an, Director, CPDD
B. J. Steigerwald, Director, OAQPS
Donald Walters, OAQPS
Edv/ard Tuerk, OAWM
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INTERIM
GUIDELINES FOR THE INTERPRETATION
OF AIR QUALITY STANDARDS
March 1974
U. S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Monitoring and Data Analysis Division
Research Triangle Park, North Carolina 27711
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INTRODUCTION
This guideline document discusses a series of issues concerning
the interpretation of air quality data as it relates to the National
Ambient Air Quality Standards (NAAQS). The issues presented! deal
i
with points of interpretation that have frequently resulted in
requests for further clarification. This document states each issue
with a recommendation and a discussion indicating our current
position. It is hoped that this document will serve as a useful step
in the evolutionary development of a uniform and consistent set of
i
criteria for relating ambient air quality data to the NAAQS.1
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ISSUE 1: Given that there are a number of monitoring sites within an
Air Quality Control Region (AQCR), does each of these sites
have to meet the National Ambient Air Quality Standards
(NAAQS)? In particular, if only one of these sites exceeds
a standard, does that mean that the AQCR is in nonconformance
of the standards even though all other sites meet the
standard?
Recommendation
Each monitoring site within the AQCR must meet the standard or
the region is in nonconformance with that standard.
Discussion
The NAAQS1 were defined to protect human health and welfare. The
presence of one monitoring site within an AQCR violating any given
standard indicates that receptors are being exposed to possibly harmful
pollutant concentrations.
Concentrations in excess of standard values at a single monitoring
station may result from the effect of a small, nearby source which is
insignificant in terms of the total emission inventory, or the station
in violation may be so located that the probability that individuals
would be exposed for prolonged periods is negligible. Such circum-
stances do not mitigate the recommended interpretation of the question
raised by this issue since NAAQS are generally interpreted as being set
to protect health and welfare regardless of the population density.
Although air quality improvement should be stressed in areas of maximum
concentrations and areas of highest population exposure, the goal of
ultimately achieving standards should apply to all locales. Data from
monitoring sites are the only available measure of air quality and must
be accepted at face value. Attention is thus focused on t|he selection
of monitoring sites in terms of the representativeness of the air they
sample. This is discussed in more detail in the guideline series
document entitled "Guidance for Air Quality Monitoring Network Design
and Instrument Siting," (OAQPS No. 1.2-012). Consideration should be
given to the relocation of monitoring stations not meeting the guideline
criteria.
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ISSUE 2: How many significant figures should be employed when making
comparisons with the NAAQS and what system of units should
be used?
Recommendation
Comparisons with the standards should be made after converting
the raw data to micrograms (or milligrams) per cubic meter. All
comparisons are made after rounding the air quality value to the
nearest integer value in micrograms per cubic meter (or milligrams per
cubic meter for carbon monoxide). The rounding convention to be
employed is that values whose fractional part is greater than 0.50
should be rounded up and those less than 0.50 should be rounded down.
Any value whose fractional part equals 0.50 should be rounded to the
nearest even integer. The following examples should clarify these
points.
Computed Value Rounded Value
79.50 80
80.12 80
80.50 80
80.51 81
81.50 82
Discussion
By letting the standard itself dictate the number of significant
figures to be used in comparisons, many computational details are
minimized while still maintaining a level of protection that is con-
sistent with the standard. It should be noted that the parenthetical
expressions given in the NAAQS indicating parts per million (ppm) may
be used as a guide but in some cases, such as the annual standard for
sulfur dioxide, may require additional significant figures to be
equivalent.
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ISSUE 3: Short-term standards are specified as concentrations which
are not to be exceeded more than once per year. How is
this to be interpreted when analyzing data obtained from
multiple monitoring sites?
Recommendation
Each site is allowed one excursion above the standard per year.
If any site exceeds the standard more than once per year, a violation
has occurred.
Discussion
By examining each site separately, data processing problems are
lessened and, more importantly, regions employing more thari the
required.minimum number of monitoring sites would not be unduly
penalized.
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ISSUE 4: How should compliance with the NAAQS by July 1975 and 1977
be determined?
Recommendation
Base the preliminary determination of compliance on adherence
to the implementation plan emission reduction schedules. Confirm
compliance with NAAQS by air quality surveillance during the
calendar year 1976. However, noncompllance with short-term standards
can be determined during the last six months of 1975 if two concentra-
tions in excess of the standards occur. Similarly, for AQCRs or
states which do not have to achieve NAAQS until 1977, compliance
would be based on data obtained in 1978.
Discussion
Implementation plans based on bringing many individual or cate-
gories of sources into compliance with emission regulations by
July 1975 have been granted at least conditional approval. However,
a twelve-month period of air quality surveillance is required to
determine annual average air quality values. Further, the calendar
year has been recommended as the time unit for the calculation of
annual average concentrations (see Issue 5). Obviously the calendar
year of data required to demonstrate that annual NAAQS have been
achieved by the control activities fully implemented by July 1975
cannot begin before 1 January 1976. Noncompliance with short period
standards can be determined in less than a calendar year by the
occurrence of two concentrations in excess of the NAAQS. Before an
AQCR can be said to be in compliance with short-term NAAQS, a full
twelve-month period of air quality surveillance records, encompassing
all four seasons, must be available for examination.
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ISSUE 5: What period of record of air quality data is necessary
to establish the status of an AQCR with respect to the NAAQS?
Recommendation
Each AQCR should be treated as a separate case in establishing
its status with respect to the NAAQS (this issue should be considered
in conjunction with Issue 4).
Discussion
Although each AQCR would be examined individually, the gradual
establishment of precedents would eventually provide consistency.
This option would consider differences in monitoring coverage,
meteorology, the type and mix of sources, and unusual economic
circumstances. Case by case treatment would allow greater flexibility
in examining borderline cases, such as annual averages which fluctuate
around the standard, or short-term excursions above the air, quality
standards. Use of this option is illustrated by the following examples;
(1) S02 concentrations during the heating season in a northern AQCR
are lower than the short-term standards. If it can be shown that the
number of hearing degree days, the industrial activity, and1 the
dilution capacity of the atmosphere favored the occurrence 'of high
S02 concentrations, then the status of the AQCR with respect to the
NAAQS would be evaluated accordingly, (2) eight-hour average CO
concentrations in an AQCR fluctuate about the standard. The period
of record was unusually favorable for the dispersion of pollutants.
Hence, a longer and more representative period of record is required
to evaluate the status of this AQCR with respect to the NAAQS.
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ISSUE 6: The NAAQS are defined in terms of a year, i.e., annual mean
concentrations and short-term concentrations not to be
exceeded more than once per year. What is meant by the term
"year" and how frequently should air quality summaries be
prepared to conform to that definition?
Recommendation
The term "year" means a calendar year and air quality summaries
should be prepared for that period.
Discussion
While pollutant exposures may overlap calendar years, the use of
a calendar year for air quality summaries remains a simple and conven-
tional practice. Indeed, inquiries concerning air quality are most
frequently expressed in terms of a calendar year. The data do not
warrant quarterly evaluation of compliance or noncompliance with NAAQS,
nor would it be reasonable to revise emission control requirements on
a quarterly basis. This of course does not remove the need for
continual appraisal of air quality on a quarterly or monthly basis to
assess both status and progress with respect to the standards. Such
efforts are obviously useful and sometimes necessary to ensure that
standards are met on a calendar year basis.
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ISSUE 7: The NAAQS for CO and SOg include eight-hour and three-hour
averages, respectively. For such standards how 1s the
time Interval defined?
Recommendation
Compliance with these standards should be judged on the basis of
running averages starting at each clock-hour. However, in determining
violations of the standard the problem of overlap must be considered.
This point can best be illustrated by consideration of the 8-hour CO
average. In order to exceed the 8-hour CO standard twice there must
be two 8-hour averages above the standard and the time periods for
these averages must not contain any common hourly data points. A
simple counting procedure for this interpretation for 8-hour CO is to
proceed sequentially through the data and each time a violation is
recorded the next seven clock hours are ignored and then the counting is
resumed. In this way there 1s no problem with overlap.
Discussion
This issue has generated considerable interest concerning the
relative mints of fixed versus running averages. At the present time
the computational advantages of the fixed interval approach are out-
weighed by the following properties of running averages: (1) running
averages afford more protection than fixed averages and this additional
margin appears warranted, (2) running averages more accurately reflect
the dosage to receptors and (3) running averages provide more equitable
control from one region to another due to differences in diurnal
patterns.
While the proposed counting scheme determines the number of times
the standard is exceeded the second highest value is commonly used for
planning purposes. In the case of 8-hour CO this should be computed
by determining the,maximum 8-hour average and then selecting the
Second highest 8-hour average that does not overlap the time interval
associated with the maximum.
In discussing this issue there are certain related points that
are worth mentioning. It should be noted that a clock-hour is the
smallest time interval suggested for reporting data and that 24-hour
averages are Interpreted as daily averages. Factors influencing these
suggestions include computational complexity,, differences in reporting
intervals for various measurement methods, and the need to maintain
both uniform -and consistent control from one region to another.
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Issue 8: The chances of detecting violations of 24-hour maximum
standards depend considerably upon the frequency with which
the air is monitored. In view of this, how should data
obtained from intermittent monitoring be interpreted?
Recommendation
•Sampling at monitoring sites which yields only partial annual
coverage is not necessarily sufficient to show compliance with "once
per year" standards. Although noncompliance will not be declared on
the basis of predicted values, it is possible that predicted values in
excess of the standard may necessitate more frequent sampling at a
particular site.
Discussion
Ideally, continuous monitoring of all pollutants would be conducted.
However, except for those pollutants specified in Federal regulations,
EPA does not currently require continuous monitoring. Thus, one is
left with either (1) predictive equations employing data from partial
annual coverage, or (2) the data collected through partial annual
coverage. Since the accuracy of predictive equations is not well
established, the remaining alternative is to judge compliance on the
basis of partial annual coverage; however, states at their option,
could sample more frequently than the required minimum. Partial annual
coverage schedules make detection of short-term violations difficult.
The entries in the following table are the probabilities of choosing
two or more days on which excursions have occurred for different numbers
of actual excursions above the standard and different sampling frequen-
cies. The assumption underlying these probabilities is that a monitoring
site excursions above the standard occur randomly over the days of the
year.
Probability of selecting two or
Actual Number of excursions
2
4
6
8
10
• 12
14
16
18
20
22
24
26
more days
Sampling
61/3fi5
0.03
0.13
0.26
0.40
0.52
0.62
0.71
0.78
0.83
0.87
0.91
0.93
0.95
when site is
Frequency -
1Z2/3-65-
0.11
0.41
0.65
0.81
0.11
0.95
0.97
0.98
0.99
0.99
0.99
0.99
0.99
above standard
days per year
183/365
0.25
0.69
0.89
0.96
0.25
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
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From this table it 1s clear that the frequency of sampling must
be considered in Judging compliance with "once per year" standards.
The present recommendation was selected so that more frequent monitoring
does not Inherently penalize a given area. At the same time a certain
degree of flexibility in the use of predictive equations such as the one
discussed by Larsen ("A Mathematical Model for Relating Air Quality
Measurements to Air Quality Standards," EPA Publication NOi AP-89) is
left to those who evaluate compliance. At the present time it is
difficult to suggest a predictive equation that has equal validity at
all sites. It is felt that this determination should be made on a case
by case basis after a detailed evaluation of the site in question.
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ISSUE 9: How should participate matter, CO and other pollutant
concentrations resulting from severe recurring dust storms,
forest fires, volcanic activity and other natural sources
be taken into account in determining compliance with NAAQS?
Recommendation
Regardless of the source, ambient pollutant concentrations
exceeding a NAAQS constitute a violation.
Discussion
Ambient pollutant concentrations exceeding the NAAQS and resulting
from emissions from natural sources constitute a violation. However,
such violations should not be used as a basis for developing or
revising an existing, across-the-board control strategy.
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ISSUE 10: Should all available air quality data or only those
derived from air quality surveillance systems, as
specified in a state implementation plan (SIP), be
used to determine compliance with NAAQS?
Recommendation
All available valid air quality data representative of the
exposure of receptors can be used to determine compliance with NAAQS.
This includes data obtained from the air quality surveillance system
specified in the applicable SIP, data obtained from the National Air
Surveillance Network (NASN), data obtained by industry monitoring
stations, data obtained from monitoring stations installed and
operated by concerned citizens, etc.
Discussion
NAAQS have been established to protect the health and welfare
of the population. If the NAAQS have validity, the violation of
a standard at any point in the AQCR is significant. Even though a
station is not part of the established surveillance network, if
acceptable methods, procedures, calibrations and recordings have been
used and can be verified, and the station is located in accordance with
applicable criteria for representativeness, the data from that station
should be used for the determination of conformity with NAAQS.
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ISSUE 11: May monitoring for certain pollutants be restricted to
only a portion of the day? For example, in the case
of oxidant, which has a clear diurnal pattern, would it
suffice to monitor only during the hours from 8 a.m. to
6 p.m. E .S.T.?
Recommendation
Partial daily monitoring of pollutants subject to short-term
NAAQS is not allowed (except nonmethane hydrocarbons where 6-9 a.m.
is specified in the NAAQS). All hours of the day must be monitored,
except perhaps for one hour missed during instrument calibration, and
reported, and will be used in evaluating compliance.
Discussion
While specific pollutants show rather consistent diurnal patterns
of concentration, particularly when mean hourly values are considered,
the concentration patterns are subject to modification with both seasonal
and short period changes of meteorological conditions. This is most
noticeable when a region is subjected to episode conditions. In
addition, the actual local time of occurrence of periods of high concen-
trations will vary from AQCR to AQCR and perhaps from monitoring station
to monitoring station within an AQCR. Extensive study of patterns and
trends exhibited by pollutant concentrations within each AQCR would be
required to select the portion of the day to be monitored if partial
monitoring were allowed. Further, monitoring data for the full twenty-
four hour period will help determine the extent and duration of
episodes and contribute to the determination of the need for emergency
control measures.
It should be noted that automatic monitoring devices used to
obtain sequential hourly data are seldom amenable to shut-down and
subsequent start-up without a warm-up and stabilization period.
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