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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