5462
905R77109
GUIDELINE SERIES
OAQPS NO. 1.2-008
(Final)
February, 1977
GUIDELINES FOR THE INTERPRETATION
OF AIR QUALITY STANDARDS.
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
Prepared by Thomas Curran
February 1977
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Monitoring and Data Analysis Division
Research Triangle Park, North Carolina 27711
Agency
t
Chic^o, Iii^cis 606 W
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INTRODUCTION
This revised guideline document discusses a series of issues
concerning the interpretation of air quality data with respect to the
National Ambient Air Quality Standards (NAAQS). This revision super-
cedes the original August 1974 version of this document. The issues
presented deal with points of interpretation that have frequently
resulted in requests for further clarification. Each issue is pre-
sented with a recommendation and a discussion indicating our current
position. It is hoped that this document will continue to be useful
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
with the standards even though all other sites meet the
standard?
Recommendation
Each monitoring site within the AQCR must meet the standard or
the AQCR is in nonconformance with that standard. This same reason-
ing may also be applied to other geographical subdivisions, such as
counties or planning regions. It should be recognized that each moni-
toring site generally represents a much smaller spatial area than an
entire AQCR. Therefore, a violation at a single site need not dictate
an AQCR-wide SIP revision but may indicate that only a particular
localized area within this region requires further control.
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 harm-
ful 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 maxi-
mum 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 assuming, of course, that
data quality is maintained by use of an adequate quality assurance
program. 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
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relocation of monitoring stations not meeting the guideline criteria.
Prior to any relocation, careful attention should be given to what
extent the data from the monitor actually indicates a potential
problem with respect to the standards.
Although the status of an AQCR with respect to the standards has
several uses, it is often an inadequate indicator of air quality
improvement. For example, there has been no change in the status of the
N.Y.-N.J.-Conn. AQCR with respect to the annual TSP standard, but fewer
sites now exceed the standard and, therefore, it is estimated that
7 million fewer people in the area are now exposed to levels above
this standard compared to 1971.
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ISSUE 2: Should monitoring data be disqualified on the basis of non-
conformance to siting criteria?
Recommendation
Any disqualification of monitoring data because of non-conformance
to siting criteria should be treated on a case-by-case basis.
Discussion
The primary reason for requiring a case-by-case treatment of this
issue is because improper siting may overestimate or underestimate the
actual problem or, perhaps, make little difference. The central question
is whether the data are adequate to ensure the protection of human
health and welfare. The answer to this question can vary from one
situation to another. For example, improper siting that overestimates
the problem but still meets the standard would be adequate to show com-
pliance. On the other hand, improper siting that underestimates the
problem and yet violates the standard would suffice to establish the
need for control. In the event that the nature of the non-conformity
would have no serious impact, the data could be taken at face value.
These varying possibilities and the potential difficulties in determining
the degree of control required make it highly advisable that all moni-
toring stations satisfy the siting criteria. However, it is recognized
that in some cases practical constraints will make this difficult and
exceptions may be required.
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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 in an AQCR-wide monitoring network is allowed one
excursion per year above a short-term standard. If any site exceeds
the standard more than once per year, a violation has occurred. In
the special case of Supplementary Control System (SCS) networks around
a well-defined single source, it is recommended that the formal agree-
ment between the source and the appropriate control agency allow only
one excursion per year from the entire network around the source.
Discussion
A site-by-site interpretation leads to a clear indication of where
the violation has occurred and minimizes potential ambiguities in
developing control strategies. In contrast, a policy allowing com-
bination of data from different monitoring sites would be potentially
cumbersome (particularly for non-overlapping violations) and difficult
to interpret. For example, combination of high values from separate
sites could result in an AQCR being in violation even though each
individual county (or state) is in compliance. In such instances
where no one site is in violation, emission control strategies to
achieve ambient standards would be difficult to define. This potential
problem is more evident when noting that AQCR's in the west range up
to 450 miles in length and 92,000 square miles in area—larger than
39 of the 50 states.
In the case of SCS networks around a well-defined single source,
the SCS agreement becomes the focal point. In these instances
accountability is clear and the recommended interpretation is that
only one excursion be allowed for the entire network. The reason
for this distinction in the case of SCS networks is that the intended
purpose of such networks is to prevent excursions by requiring
emission reductions at the source on an intermittent basis. In
keeping with this philosophy, this interpretation is intended to avoid
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indirectly allowing a source to selectively exceed the standard once
in all directions. With SCS, the source operates within a framework
giving more capability with respect to factors such as wind direction
and to some extent can choose to control or not control based upon
such information. This interpretation is consistent with EPA's
approach where SCS networks are concerned, e.g., in each of the
regulations we have proposed or promulgated which involve SCS at
smelters (see Nevada S02 Control Strategy, February 6, 1975, F.R. at
p. 5508; Arizona SO? Control Strategy, October 22, 1975, F.R. at p. 49362;
and New Mexico S02 Control Strategy, May 2, 1975, F.R. at p. 19211).
We also note that state agencies in Texas and Washington have success-
fully enforced against sources for causing air quality concentrations
in excess of the NAAQS, allowing the standards to be exceeded one
time per year for each source.
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ISSUE 4; 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 signifi-
cant figures to be used in comparisons, many computational details
are minimized while still maintaining a level of protection that is
consistent with the standard. It should be noted that the parenth-
tical 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, many require additional significant
figures to be equivalent.
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ISSUE 5: What period of record of air quality data is necesary 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.
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 circumstances
affecting emissions. Case-by-case treatment would allow greater flexi-
bility 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) SO? concentrations during one heating season in a
northern AQCR are lower than the short-term standards. If it can be
shown that the number of heating degree days, the industrial activity,
and 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 pollu-
tants. 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 tprms 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 routine 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 con-
ventional 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 require-
ments 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. For example, when
new stations begin monitoring, a running 4-quarter or 12-month period
may provide the most timely initial evaluation of compliance. This
same flexibility may also be employed when developing control strategies,
or considering possible variances, to ensure that the standards will be
met.
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ISSUE 7A: The NAAQS for CO and S02 include an eight-hour CO standard
and three- and twenty-four-hour SO? standards. 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 nust 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 is no problem with overlap. It should
be noted that a clock-hour is the smallest time interval suggested
for reporting data.
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
outweighed by the following properties of running averages: (1) run-
ning averages afford more protection than fixed averages and this
additional margin appears warranted, (2) running averages more accur-
ately reflect the dosage to receptors, and (3) running averages
provide more equitable control from one region to another due to
differences in diurnal patterns.
Recommending a running 24-average interpretation for S02 represents
a change and, therefore, certain points should be mentioned. There
has been a considerable increase in the use of continuous SO? monitors
and the promulgation of the equivalency regulations has provided the
mechanism to establish that continuous methods are equivalent to the
Federal Reference Method (24-hour bubbler). In some cases, particu-
larly around large isolated point sources, the fixed midnight-to-
midnight interpretation can result in second high values 30 to 40
percent lower than the second highest non-overlapping 24-hour average.
.As a consequence, a site that appears to be in compliance using midnight-
to-midnight values may actually have other 24-hour averages well above the
standard. Therefore, to ensure adequate protection, the running 24-
hour average interpretation is recommended. However, it is recognized
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that in many areas of the country SOo levels are sufficiently low and
well behaved that the midnight-to-mioriight computations are adequate to
show compliance. Therefore, it is only necessary to compute running
24-hour averages for borderline situations. Because 24-hour bubblers
are seldom the only monitors in areas where this problem arises (due
to the 3-hour standard), such data continue to be generally adequate
for demonstrating compliance.
The counting procedures for 3-hour or 24-hour standards are similar
to that described for 8-hour CO with the obvious change of ignoring
the next two 3-hour averages or the next twenty-three 24-hour averages,
respectively, when counting values above the corresponding standard.
It is worth noting that in applying this counting procedure, the
maximum and second highest values could be ignored because they are over-
lapped by a counted value. Therefore, the sole function of this procedure
is to count the number of non-overlapping violations. The identification
of the maximum and second highest values is an independent procedure
as described in the following discussion of Issue 7B.
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ISSUE 7B: When using running averages for 24-, 8-, and 3-hour averages,
how should the second highest value be determined?
Recommendation
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
In using running average values to determine compliance with these
multiple-hour air quality standards, the adopted convention is to deter-
mine violations on the basis of non-overlapping time periods. That is,
any two values above the standard must be distinct and not share any
common hours, as indicated in Issue 7A. This same consideration regard-
ing overlap also applies in determining the second highest value.
Basically, there are two key points involved in selecting the appro-
priate second highest value: (1) consistency with the counting procedure,
i.e., if a site has two or more non-overlapping values above the stand-
ard, then the second highest value should be above the standard and (2) the
second highest value should accurately reflect the relative magnitude of
the problem. Both of these characteristics can be achieved by using the
maximum second highest non-overlapping value. This is relatively simple
to determine and the method can perhaps best be illustrated in the follow-
ing example using 8-hour CO averages.
List the 9 highest 8-hour averages for the year in order starting
with the highest as shown in the table below:
Listing of the 9 Highest 8-Hour CO Averages in Order
8-Hour average Does it overlap all
Order (mg/m3) Date & time of occurrence of the higher values?
1
2
3
4
5
6
7
8
9
16
15
15
14
13
13
13
12
12
Dec.
Dec.
Dec.
Dec.
Nov.
Nov.
Feb.
Nov.
Oct.
8 10:01 am-6:00 pm
8 9:01 am-5:00 pm
8 11:01 am-7:00 pm
8 3:01 am-ll:00 am
20 10:01 am-6:00 pm
11 11:01 am-7:00 pm
9 9:01-am-5:00 pm
11 10:01 am-6:00 pm
29 10:01 am-6:00 pm
— —
yes
yes
No (this is
the second max)
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For each of these 8-hour values include the time of occurrence.
As indicated, the first value is the maximum 8-hour average for the year.
For each of the remaining values, answer the question, "Does it overlap
all of the higher values?" The first value for which the answer is "no"
is the proper second highest value. A point of interest in this example
is that 14 mg/m3 is the proper second highest value, even though it
overlaps the maximum because there is one other non-overlapping value at
least as high (namely, 15 mg/m^ from 11:01 a.m. to 7:00 p.m. on December 8),
This particular example was constructed to illustrate this possibility,
although it remains to be seen how often this would occur in actual
practice. Nevertheless, it demonstrates that this particular algorithm
for determining the second highest value will yield the proper value for
use in control strategies.
It should be noted that with 9 values on the list, even the extreme
case of 9 consecutive values must result in two disjoint 8-hour intervals.
A similar procedure is used for 3-hour or 24-hour averages using a list
of the 4 highest or 25 highest values, respectively.
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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 generally 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 con-
ducted. 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 diffi-
cult. 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 frequencies. 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 0.41 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.
For example, if an every-sixth-day schedule is used and an area
actually has eight days per year above the standard, there is only a
40 percent chance that the data will indicate a violation. Although
this may appear unacceptably low, it applies to one particular year and
the probability that the site would appear to meet the standard two
years in a row is less than 30 percent.
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. For example,
if the results of a predictive equation indicate a violation, more
frequent monitoring might be advisable. At the present time it is
difficult to suggest a predictive equation that has equal validity at
.all sites. In cases where existing monitoring data are not adequate for
judging air quality status with respect to the standards, two options
are possible: (1) additional (or more frequent) monitoring and/or
(2) the use of predictive equations. The decision as to which approach
will be used in a particular application depends on the degree of urgency
'associated with the decision, the validity of any predictive models
proposed, and the availability of resources to conduct additional moni-
toring.
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ISSUE 9: How should participate matter, CO and other pollutant con-
centrations 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 are considered
to be violations regardless of the source. These standards are intended
to protect human health and welfare and from this viewpoint the origin
of the pollutant is irrelevant. However, as indicated in CFR 51.12(d),
the source becomes relevant when considering control strategies.
Detailed information establishing that violations are due to uncon-
trollable natural sources may be used in determining the feasibility
of modifying control stragegies. In general, reasonably available
control technology would be expected for all existing sources and
best available technology for new sources impacting the non-attainment
monitor.
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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 com-
pliance with NAAQS?
Recommendation
All available valid air quality data can be used to determine
compliance with the 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 accord-
ance 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 or year?
Recommendation
Partial daily monitoring of pollutants subject to short-term
NAAQS is not recommended (except non-methane 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
this should be in a non-peak period. Partial annual monitoring during
only certain seasons is not recommended tn general, but it is recognized
that in certain limited cases such a scheme may be adequate for highly
seasonal pollutants.
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
concentrations 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.
With respect to partial annual monitoring, a better argument can
be made that such schemes can result in significant resource savings.
If this can be accomplished with little or no loss of information,
then such monitoring is adequate. Obviously, this depends upon the
degree of seasonality of the pollutant and the intended purpose of the
data. If the primary purpose of the monitor is to assess compliance,
then it is only necessary to show that the data will be sufficient to
document status with respect to standards. However, if the monitor
is intended to assess trends, it becomes much more difficult to
justify partial annual monitoring and this would seldom be acceptable
unless all values in the omitted season are near the minimum detectable
limit.
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