905R77109 

GUIDELINE SERIES 

OAQPS NO. 1.2-008 

(Final ) 

February, 1977 

GUIDELINES FOR THE INTERPRETATION 

OF AIR QUALITY STANDARDS. 

5462 

I • 1 

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 

Chi 6OC~ (UsF 
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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 supercedes 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 presented 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. 
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 sanie reasoning may also be applied to other geographical subdivisions, such as 

counties or planning regions. It should be recognized that each monitoring 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 NAAQS' 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 circumstances 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 maxinium 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 a.n 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 
relocation of monitoring stations not nieeting the guideline criteria. 

Prior to any relocation, careful attention should be given to what 

extent the data froin 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. 
ISSUE 2: Should monitoring data be disqualified on the basis of nonconformance 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 compliance. 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 monitoring stations satisfy the siting criteria. However, it is recognized 

that in some cases practical constraints will make this difficult and 

exceptions may be required. 
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 agreement 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 combination 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 
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 SO2 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 successfully 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. 
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. A11 

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 Ualue 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 

consistent with the standard. It should be noted that the parenthtical 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. 
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 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 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 unusuaily favorable for the dispersion of pollutants. Uence, a longer and more representative period of record is 

required to evaluate the status of this AQCR with respect to the NAAQS. 
ISSUE 6: The NAAQS are defined in trrms 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 conventional 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. 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. 
ISSUE 7A: The NAAQS for CO and S02 include an eight-hour CO standard 

and three- and twenty-four-hour SO2 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 ilust 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 Iias 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 t.ie 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. 

Recommendinn a running 24-average interpretation for S0 2represents 

a change and, thPrefore, certain points should be mentioned. There 

has been a considerable increase in the use of continuous S02 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, particularly around large isolated point sources, the fixed midnight-tomidnight 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 midnightto-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 
that in many areas of the country Sn alevels are sufficiently low and 

well behaved that the midnight-to-mi<~ night 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 overlapped 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. 
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 determine 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 regarding overlap also applies in determining the second highest value. 

Basically, there are two key points involved in selecting the appropriate second highest value: (1) consistency with the counting procedure, 

i.e., if a site has two or more non-overlapping values above the standard, 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 following 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 overlapall 

Order (mg/m3 ) Date & time of occurrence of the higher values? 

1 16 Dec. 8 10:01 am-6:00 pm 

2 15 Dec. 8 9:01 am-5:00 pm 

3 15 Dec. 8 11:01 am=7:00 pm 

4 14 Dec. 8 3:01 am-11:00 am 

5 13 Nov. 20 10:01 am-6:00 pm 

6 13 Piov. 11 11:01 am-7:00 pm 

7 13 Feb. 9 9:01-am-5:00 pm 

8 12 Nov. 11 10:01 am-6:00 pm 

9 12 Oct. 29 10:01 am-6:00 pm 

yes 

yes 

No (this is the second max) 
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 

allof 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/m3is 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/m3from 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. 
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. 

flicrimcinn 

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 1 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 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. 
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 Oat 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 monitoring. 
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 violatjon. 

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 uncontrollable 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. 
ISSUE 10: Should all available air quality data or only those derived 

from air quality surveillance systems, as specjfied in a 

state implementation plan (SIP), be used to determine compliance with NAAQS? 

Recommendation 

A11 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 Nationai Air Surveiliance 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. 
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). A11 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 in 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 ineteoro1ogical 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 becames 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.