GUIDELINE SERIES
OAQPS NO. 1.2-008
GUIDELINES FOR THE INTERPRETATION OF
AIR QUALITY STANDARDS, MDAD. 8/74.
(FINAL)
US. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina
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GUIDELINES FOR THE INTERPRETATION
OF AIR QUALITY STANDARDS
August 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
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
criteria for relating ambient air quality data to the NAAQS.
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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 derisity.
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 the 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 or
equal to .50 should be rounded up and those less than 0.50 should be
rounded down. The following examples should clarify these points.
Computed Value Rounded Value
79.50 80
80.12 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 signficant 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 than 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?
Recommendati on
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, noncompliance 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, and the
dilution capacity of the atmosphere favored the occurrence of high
SOp 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 7A: The NAAQS for CO and S02 include eight-hour and three-hour
averages, respectively. For such standards how is 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 hour.s-:are'ignored and then the counting
is resumed. In this way there is no problem with overlap.
Discussion
This issue has generated considerable interest concerning the
relative merits 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.
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.
While the proposed counting scheme determines the number of times
the standard is exceeded the second highest value is commonly used for
planning purposes. The determination of the second highest value in
the case of running averages has certain technical subtleties that are
discussed in detail in issue 7B.
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ISSUE 7B: When using running averages for 8-hour and 3-hour averages
how should the second highest value be determined?
Recommendati on
The second highest value should be determined so that there is
one other non-overlapping value that is at least as high as the second
highest value. Although this seems relatively straightforward the
following discussion indicates some of the subtleties involved.
Discussion
The use of running averages to determine compliance with specific
air quality standards necessitates that the number of values above
the standard be evaluated on the basis of non-overlapping time periods.
That is, any two values above the standard must be distinct and not have
any common hours. This can be achieved by a relatively straightforward
counting procedure. For example, in the case of 8-hour CO an 8-hour
average can be associated with each clock hour of the calendar year.
Then values above the 8-hour standard are counted sequentially beginning
with the first 8-hour average of the year. Each time a violation is
counted the next seven 8-hour values are ignored, and the counting
procedure resumes with the eighth 8-hour average. This counting pro-
cedure results in the maximum number of non-overlapping violations
of the 8-hour standard.
This count is all that is needed to evaluate compliance with the
8-hour standard because the standard is not to be exceeded more than
once per year; and, therefore, any count value greater than one is
sufficient to indicate non-compliance. However, it is also desirable
to employ the second highest 8-hour average to indicate the magnitude
of the problem. There are several ways to define the second highest
value, and three possible definitions will be indicated here in order
to briefly discuss their consistency with the counting procedure described
above. The three definitions considered for the second highest value
are: 1) the second highest 8-hour value of those counted as being
above the standard, 2) the second highest 8-hour value that does not
overlap the maximum 8-hour value, and 3) the maximum second highest
non-overlapping 8-hour average.
Annotated graphs of 8-hour CO are used to facilitate the discussion
of the consequences of each definition. For example, Figure 1 illustrates
that the first definition underestimates the magnitude of the problem
because the counting procedure may count the first time the standard
is exceeded and bypass the peak values. Therefore this definition is
inadequate.
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Although the second definition is intuitively appealing, Figure 2
illustrates that in some cases there could be two violations of the
standard, and yet the second highest value that does not overlap the
maximum is less than the standard. This can only occur in marginal
cases in which the standard is only exceeded during one fifteen hour
period in the year and that the maximum value occurs in the middle, of
this interval. Figure 3a and 3b show another case in which this
definition produces the peculiarity that a higher CO value may lower
the second highest value.
In order to avoid these inconsistencies it becomes necessary
to define the second highest value as the maximum second highest
non-overlapping value. What this means is that there is one 8-hour
value that is greater than or equal to the maximum second highest
value and that these two values are not overlapping. It is important
to recognize that the maximum second highest value may overlap the
maximum 8-hour value. However, as shown in Figure 2, there is still
one other 8-hour non-overlapping value that exceeds the maximum second
high.
With these subtleties in mind, it seems appropriate to use the
maximum second highest non-overlapping value as the second high. In
this way, the magnitude of the problem is properly assessed; and the
second high value is always consistent with the number of violations.
This definition of the second highest value is also consistent with the
approach used in determining control strategies on the basis of the roll'
back equation. It is this maximum second highest value that must be
reduced below the standard in order to satisfy the requirement that
the standard not be exceeded more than once per year.
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Figure 1
8-Hour
Average
Standard
„
12 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
HOUR
Using the counting procedure the violations are counted at
hours 3 and 12 as indicated by the x's. Note that the peak values
do not occur at these points.
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Figure 2
A / \
— -> L / ^-
A
/ V
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
There are two non-overlapping violations at hours 3 and 12, and
these are detected by the counting procedure. However, the maximum
occurs at hour 8 and the second is below the standard. However in
this case the maximum second maximum would be V2i which is above
the standard. Although V? overlaps the maximum, M, there is one eight-
hour average, namely Vi, that is at least as high as V? and the two
time periods are disjoint.
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Figure 3a
12
10
8
J~\ 7 \—
—i \_— .—^ / >—-.. •
1 2 3 4 5 6 7 8 9 10 IT 12 13 14 15 16 17 18 19 20 21 22 23 24
Figure 3b
14
12
10
8
12 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
In Figure 3a the maximum value is 12 as well as the second
highest value. However, in Figure 3b the maximum is now 14, and
the second highest value that does not overlap the 14 is below the
standard. Therefore the second highest value that does not overlap the
maximum can actually be lowered by having more high values. It should
be noted that in both of the above cases the maximum second hiahest
non-overlapping value is 12.
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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 underlying assumption in determining these probabilities is that
excursions above the standard occur randomly over the days of the year.
This is, of course, an oversimplification but is sufficient for the
purposes of this discussion.
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Probability of selecting two or more days when site is above standard
Sampling Frequency - days per year
Actual Number of excursions 61/365 122/365 183/365
2 0.03 0.11 0.25
4 0.13 a.41 0.69
6 0.26 0.65 0.89
8 0.40 0.81 0.96
10 0.52 0.90 0.99
12 0.62 0.95 0.99
14 0.71 0.97 0.99
16 0.78 0.98 0.99
18 0.83 0.99 0.99
20 0.87 0.99 0.99
22 0.91 0.99 0.99
24 0.93 0.99 0.99
26 0.95 0.99 0.99
From this table it is 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 No. 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 particulate 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.?
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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