GUIDELINE SERIES
OAQPS NO. 1-2008
DRAFT
GUIDELINES FOR 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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DRAFT
GUIDELINES FOR INTERPRETATION
OF
AIR QUALITY STANDARDS
Monitoring and Data Analysis Division
Office of Air Quality Planning and Standards
Environmental Protection Agency
October 30, 1973 <£
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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 ENTIRE AQCR HAS VIOLATED THE STANDARD
EVEN THOUGH ALL OTHER SITES MEET THE STANDARD?
RECOMMENDATION: Each monitoring site within the AQCR must meet the
standard or the region is in violation of that
standard.
DISCUSSION:
The NAAQS's 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 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. The forthcoming guideline
document concerning the location of monitoring
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instruments should be consulted in evaluating
sites now in use. Consideration should be given
to the relocation of monitoring stations if the
guideline criteria are not met.
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ISSUE 2: HOW SHOULD MEASURED AMBIENT AIR QUALITY LEVELS BE
REPORTED FOR COMPARISON WITH THE NAAQS? IS THE LEVEL
.SPECIFIED BY THE STANDARD TO BE CONSIDERED EXACT,
E.G., IS 75.0 pg/m3 THE PRIMARY ANNUAL NAAQS FOR
TOTAL SUSPENDED PARTICULATE (TSP)?
RECOMMENDATION:
DISCUSSION:
Each measurement should be converted to ug/m ; the
same number of significant figures shall .be
carried after the conversion as are available from
the original instrument reading or analysis technique.
Computed averages (arithmetic or geometric) will
carry one significant figure more than the number
set from which the average is derived.
An excursion will be deemed to occur if and only if,
the converted, rounded measurement is one unit
above the standard.
This procedure is the most direct, easily understood
technique for summarizing and evaluating air quality
data and will be easily understood in presentations
in which comparisons with air quality standards are
made. In addition, Federal regulations are specific
with respect to the measure of air quality used
(e.g.: arithmetic mean, geometric mean) and the
standard to which it is compared. The use of any
other rule is, in effect, an amendment to Federal
regulations. Conventions concerning the use of
significant figures will be treated in greater detail
in the forthcoming Guidelines for the Evaluation of
Air Quality Data. The feasibility of returning to
ppm by volume as the unit of expression for gaseous
pollutants is being examined.
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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.
For any level of air quality, the expected number
of excursions above the standard in an AQCR is
a direct function of the number of monitoring
sites in operation. Hence, examining each site
separately tends to adjust for varying 'numbers
of sites among regions.
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ISSUE 4: WHAT PFJIIOD OF RECORD OF AIR QUALITY DATA IS
NECESSARY TO nSTA!Jl.;:'U TIM: STATUS OF AN AQCU WITH
• RESPECT TO 'HIE 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 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) SO-
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 heating degree-days,
the industrial activity, and the dilution capacity
of the atmosphere favored the occurrence of high
SO- 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 pollutant =>. Hence, a longer and more representative
period of record is required to evaluate the status
of this AQCR with respect to the NAAQS.
1 This issue should be considered in conjunction with Issue 10.
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ISSUE 5: THE NAAQS ARE DEFINED IN TERMS OF A YEAR, I.E.,
ANNUAL MF.AN CONCENTRATIONS AND SHORT-TERM
CONCENTRATIONS NOT TO BE EXCEEDED MOKE THAN 01ICE
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 tern "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 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
non-compliance with NAAQS, nor would it be reasonable
to revise emission control requirements on a quarterly
basis. Since all of EPA's summary files are structured
around the calendar year, redefinition of the term
"year" would necessitate revision of current data
handling procedures.
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DRAFT
ISSUE 6: THE NAAQS's FOR CO AND S02 INCLUDE EIGHT-HOUR AND
THREE-HOUR AVERAGES, RESPECTIVELY. FOR SUCH
STANDARDS HOW IS THE TIME INTERVAL DEFINED?.
RECOMMENDATION: Tine is defined as discrete intervals beginning
at midnight. Thus, there are three such intervals
for CO and eight for S0_.
DISCUSSION: This option keeps all of the data within one calendar
day (and therefore year). It is computationally easy
to handle and minimizes the redundancy of overlapping
time periods.
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ISSUE 7: 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: Partial annual coverage is sufficient to show
compliance; predictive equations to calculate
expected maximum concentrations, etc. should not
be employed.
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 (a) predictive
equations employing data from partial annual
. coverage, or (b) 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, regions 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
vhich excursions have occurred for different numbers
of actual excursions above the standard and different
sampling frequencies. The assumption underlying
these probabilities is that at a monitoring site
excursions above the standard occur randomly over
the days of the year.
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i/KAf'T
PROBABILITY OF SCLECTIIIC TV.'O OR MORE DAYS WHEN SITE
IS ABOVE ST/uNDARD
Sampling Frequency - Days per year
Actual no,
of excursions 61/365 122/365 183/365
2
4
6
8
10
12
14
16 '
18
20
22
24
26
.03
.13
.26
.40
.52
.62
.71
.78
.83
.87
.91
.93
.95
.11
•41
.65
.81
.90
.95
.97
.98
.99
.99
.99
.99
.99
.25
.69
.89
.96
.99
.99
.99
.99
.99
.99
.99
.99
.99
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ISSUE 8: HOW SHOULD PARTICULATE MATTER, CO AND OTTIER 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 concen-
trations 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 9: 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 will 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
compliance (or violations).
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ISSUE 10: HOW SHOULD COMPLIANCE WITH THE NAAQS BY JULY
1975 BE DETERMINED?
RECOMMENDATION: Base the >reliminary 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, non-compliance
with short term standards can be determined during
the last six months of 1975 if two concentrations
in excess of the standards occur.
DISCUSSION:
Implementation plans based on bringing many individual
or categories 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.
Non-compliance 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 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. L.S.T.?
A
RECOMMENDATION:
DISCUSSION:
Partial daily monitoring of pollutants subject to
short term NAAQS is not allowed. 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.
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 noticable
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 startup without
a warm-up and stabilization period.
1 Except nonmethane hydrocarbons where 6-9 A.M. is specified in
the NAAQS.
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