Appendix T to Part 50 - Interpretation of the Primary National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide)
40:2.0.1.1.1.0.1.20.21 : Appendix T
Appendix T to Part 50 - Interpretation of the Primary National
Ambient Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide)
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining when the primary national
ambient air quality standards for Oxides of Sulfur as measured by
Sulfur Dioxide (“SO2 NAAQS”) specified in § 50.17 are met at an
ambient air quality monitoring site. Sulfur Dioxide (SO2) is
measured in the ambient air by a Federal reference method (FRM)
based on appendix A or A-1 to this part or by a Federal equivalent
method (FEM) designated in accordance with part 53 of this chapter.
Data handling and computation procedures to be used in making
comparisons between reported SO2 concentrations and the levels of
the SO2 NAAQS are specified in the following sections.
(b) Decisions to exclude, retain, or make adjustments to the
data affected by exceptional events, including natural events, are
made according to the requirements and process deadlines specified
in §§ 50.1, 50.14 and 51.930 of this chapter.
(c) The terms used in this appendix are defined as follows:
Daily maximum 1-hour values for SO2 refers to the maximum
1-hour SO2 concentration values measured from midnight to midnight
(local standard time) that are used in NAAQS computations.
Design values are the metrics (i.e., statistics)
that are compared to the NAAQS levels to determine compliance,
calculated as specified in section 5 of this appendix. The design
value for the primary 1-hour NAAQS is the 3-year average of annual
99th percentile daily maximum 1-hour values for a monitoring site
(referred to as the “1-hour primary standard design value”).
99th percentile daily maximum 1-hour value is the value
below which nominally 99 percent of all daily maximum 1-hour
concentration values fall, using the ranking and selection method
specified in section 5 of this appendix.
Pollutant Occurrence Code (POC) refers to a numerical
code (1, 2, 3, etc.) used to distinguish the data from two
or more monitors for the same parameter at a single monitoring
site.
Quarter refers to a calendar quarter.
Year refers to a calendar year.
2. Requirements for Data Used for Comparisons With the SO2 NAAQS
and Data Reporting Considerations
(a) All valid FRM/FEM SO2 hourly data required to be submitted
to EPA's Air Quality System (AQS), or otherwise available to EPA,
meeting the requirements of part 58 of this chapter including
appendices A, C, and E shall be used in design value calculations.
Multi-hour average concentration values collected by wet chemistry
methods shall not be used.
(b) Data from two or more monitors from the same year at the
same site reported to EPA under distinct Pollutant Occurrence Codes
shall not be combined in an attempt to meet data completeness
requirements. The Administrator will combine annual 99th percentile
daily maximum concentration values from different monitors in
different years, selected as described here, for the purpose of
developing a valid 1-hour primary standard design value. If more
than one of the monitors meets the completeness requirement for all
four quarters of a year, the steps specified in section 5(a) of
this appendix shall be applied to the data from the monitor with
the highest average of the four quarterly completeness values to
derive a valid annual 99th percentile daily maximum concentration.
If no monitor is complete for all four quarters in a year, the
steps specified in section 3(c) and 5(a) of this appendix shall be
applied to the data from the monitor with the highest average of
the four quarterly completeness values in an attempt to derive a
valid annual 99th percentile daily maximum concentration. This
paragraph does not prohibit a monitoring agency from making a local
designation of one physical monitor as the primary monitor for a
Pollutant Occurrence Code and substituting the 1-hour data from a
second physical monitor whenever a valid concentration value is not
obtained from the primary monitor; if a monitoring agency
substitutes data in this manner, each substituted value must be
accompanied by an AQS qualifier code indicating that substitution
with a value from a second physical monitor has taken place.
(c) Hourly SO2 measurement data shall be reported to AQS in
units of parts per billion (ppb), to at most one place after the
decimal, with additional digits to the right being truncated with
no further rounding.
3. Comparisons With the 1-Hour Primary SO2 NAAQS
(a) The 1-hour primary SO2 NAAQS is met at an ambient air
quality monitoring site when the valid 1-hour primary standard
design value is less than or equal to 75 parts per billion
(ppb).
(b) An SO2 1-hour primary standard design value is valid if it
encompasses three consecutive calendar years of complete data. A
year meets data completeness requirements when all 4 quarters are
complete. A quarter is complete when at least 75 percent of the
sampling days for each quarter have complete data. A sampling day
has complete data if 75 percent of the hourly concentration values,
including State-flagged data affected by exceptional events which
have been approved for exclusion by the Administrator, are
reported.
(c) In the case of one, two, or three years that do not meet the
completeness requirements of section 3(b) of this appendix and thus
would normally not be useable for the calculation of a valid 3-year
1-hour primary standard design value, the 3-year 1-hour primary
standard design value shall nevertheless be considered valid if one
of the following conditions is true.
(i) At least 75 percent of the days in each quarter of each of
three consecutive years have at least one reported hourly value,
and the design value calculated according to the procedures
specified in section 5 is above the level of the primary 1-hour
standard.
(ii)(A) A 1-hour primary standard design value that is equal to
or below the level of the NAAQS can be validated if the
substitution test in section 3(c)(ii)(B) results in a “test design
value” that is below the level of the NAAQS. The test substitutes
actual “high” reported daily maximum 1-hour values from the same
site at about the same time of the year (specifically, in the same
calendar quarter) for unknown values that were not successfully
measured. Note that the test is merely diagnostic in nature,
intended to confirm that there is a very high likelihood that the
original design value (the one with less than 75 percent data
capture of hours by day and of days by quarter) reflects the true
under-NAAQS-level status for that 3-year period; the result of this
data substitution test (the “test design value”, as defined in
section 3(c)(ii)(B)) is not considered the actual design value. For
this test, substitution is permitted only if there are at least 200
days across the three matching quarters of the three years under
consideration (which is about 75 percent of all possible daily
values in those three quarters) for which 75 percent of the hours
in the day, including State-flagged data affected by exceptional
events which have been approved for exclusion by the Administrator,
have reported concentrations. However, maximum 1-hour values from
days with less than 75 percent of the hours reported shall also be
considered in identifying the high value to be used for
substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture but at least 50 percent data capture, including
State-flagged data affected by exceptional events which have been
approved for exclusion by the Administrator; if any quarter has
less than 50 percent data capture then this substitution test
cannot be used. Identify for each quarter (e.g., January-March) the
highest reported daily maximum 1-hour value for that quarter,
excluding State-flagged data affected by exceptional events which
have been approved for exclusion by the Administrator, looking
across those three months of all three years under consideration.
All daily maximum 1-hour values from all days in the quarter period
shall be considered when identifying this highest value, including
days with less than 75 percent data capture. If after substituting
the highest reported daily maximum 1-hour value for a quarter for
as much of the missing daily data in the matching deficient
quarter(s) as is needed to make them 100 percent complete, the
procedure in section 5 yields a recalculated 3-year 1-hour standard
“test design value” less than or equal to the level of the
standard, then the 1-hour primary standard design value is deemed
to have passed the diagnostic test and is valid, and the level of
the standard is deemed to have been met in that 3-year period. As
noted in section 3(c)(i), in such a case, the 3-year design value
based on the data actually reported, not the “test design value”,
shall be used as the valid design value.
(iii)(A) A 1-hour primary standard design value that is above
the level of the NAAQS can be validated if the substitution test in
section 3(c)(iii)(B) results in a “test design value” that is above
the level of the NAAQS. The test substitutes actual “low” reported
daily maximum 1-hour values from the same site at about the same
time of the year (specifically, in the same three months of the
calendar) for unknown hourly values that were not successfully
measured. Note that the test is merely diagnostic in nature,
intended to confirm that there is a very high likelihood that the
original design value (the one with less than 75 percent data
capture of hours by day and of days by quarter) reflects the true
above-NAAQS-level status for that 3-year period; the result of this
data substitution test (the “test design value”, as defined in
section 3(c)(iii)(B)) is not considered the actual design value.
For this test, substitution is permitted only if there are a
minimum number of available daily data points from which to
identify the low quarter-specific daily maximum 1-hour values,
specifically if there are at least 200 days across the three
matching quarters of the three years under consideration (which is
about 75 percent of all possible daily values in those three
quarters) for which 75 percent of the hours in the day have
reported concentrations. Only days with at least 75 percent of the
hours reported shall be considered in identifying the low value to
be used for substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture. Identify for each quarter (e.g., January-March) the
lowest reported daily maximum 1-hour value for that quarter,
looking across those three months of all three years under
consideration. All daily maximum 1-hour values from all days with
at least 75 percent capture in the quarter period shall be
considered when identifying this lowest value. If after
substituting the lowest reported daily maximum 1-hour value for a
quarter for as much of the missing daily data in the matching
deficient quarter(s) as is needed to make them 75 percent complete,
the procedure in section 5 yields a recalculated 3-year 1-hour
standard “test design value” above the level of the standard, then
the 1-hour primary standard design value is deemed to have passed
the diagnostic test and is valid, and the level of the standard is
deemed to have been exceeded in that 3-year period. As noted in
section 3(c)(i), in such a case, the 3-year design value based on
the data actually reported, not the “test design value”, shall be
used as the valid design value.
(d) A 1-hour primary standard design value based on data that do
not meet the completeness criteria stated in 3(b) and also do not
satisfy section 3(c), may also be considered valid with the
approval of, or at the initiative of, the Administrator, who may
consider factors such as monitoring site closures/moves, monitoring
diligence, the consistency and levels of the valid concentration
measurements that are available, and nearby concentrations in
determining whether to use such data.
(e) The procedures for calculating the 1-hour primary standard
design values are given in section 5 of this appendix.
4. Rounding Conventions for the 1-Hour Primary SO2 NAAQS
(a) Hourly SO2 measurement data shall be reported to AQS in
units of parts per billion (ppb), to at most one place after the
decimal, with additional digits to the right being truncated with
no further rounding.
(b) Daily maximum 1-hour values and therefore the annual 99th
percentile of those daily values are not rounded.
(c) The 1-hour primary standard design value is calculated
pursuant to section 5 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).
5. Calculation Procedures for the 1-Hour Primary SO2 NAAQS
(a) Procedure for identifying annual 99th percentile
values. When the data for a particular ambient air quality
monitoring site and year meet the data completeness requirements in
section 3(b), or if one of the conditions of section 3(c) is met,
or if the Administrator exercises the discretionary authority in
section 3(d), identification of annual 99th percentile value is
accomplished as follows.
(i) The annual 99th percentile value for a year is the higher of
the two values resulting from the following two procedures.
(1) Procedure 1. For the year, determine the number of
days with at least 75 percent of the hourly values reported.
(A) For the year, determine the number of days with at least 75
percent of the hourly values reported including State-flagged data
affected by exceptional events which have been approved for
exclusion by the Administrator.
(B) For the year, from only the days with at least 75 percent of
the hourly values reported, select from each day the maximum hourly
value excluding State-flagged data affected by exceptional events
which have been approved for exclusion by the Administrator.
(C) Sort all these daily maximum hourly values from a particular
site and year by descending value. (For example: (x[1], x[2], x[3],
* * *, x[n]). In this case, x[1] is the largest number and x[n] is
the smallest value.) The 99th percentile is determined from this
sorted series of daily values which is ordered from the highest to
the lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days
with valid data for year y (cny). The corresponding “n” value in
the right column identifies the rank of the annual 99th percentile
value in the descending sorted list of daily site values for year
y. Thus, P0.99, y = the nth largest value.
(2) Procedure 2. For the year, determine the number of
days with at least one hourly value reported.
(A) For the year, determine the number of days with at least one
hourly value reported including State-flagged data affected by
exceptional events which have been approved for exclusion by the
Administrator.
(B) For the year, from all the days with at least one hourly
value reported, select from each day the maximum hourly value
excluding State-flagged data affected by exceptional events which
have been approved for exclusion by the Administrator.
(C) Sort all these daily maximum values from a particular site
and year by descending value. (For example: (x[1], x[2], x[3], * *
*, x[n]). In this case, x[1] is the largest number and x[n] is the
smallest value.) The 99th percentile is determined from this sorted
series of daily values which is ordered from the highest to the
lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days
with valid data for year y (cny). The corresponding “n” value in
the right column identifies the rank of the annual 99th percentile
value in the descending sorted list of daily site values for year
y. Thus, P0.99,y = the nth largest value.
(b) The 1-hour primary standard design value for an ambient air
quality monitoring site is mean of the three annual 99th percentile
values, rounded according to the conventions in section 4.
Annual number of days with
valid data for year “y” (cny) |
P0.99,y is the nth
maximum value of the year, where n is the listed number |
1-100 |
1 |
101-200 |
2 |
201-300 |
3 |
301-366 |
4 |
[75 FR 35595, June 23, 2010]