ELECTRONIC CODE OF FEDERAL REGULATIONS

e-CFR Data is current as of March 27, 2014

Title 40: Protection of Environment 

PART 50—NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS 

APPENDIX N TO PART 50—INTERPRETATION OF THE NATIONAL AMBIENT AIR QUALITY STANDARDS FOR

PM2.5

1.0 GENERAL

(a) This appendix explains the data handling conventions and computations necessary for 

determining when the national ambient air quality standards (NAAQS) for PM2.5 are met, specifically 

the primary and secondary annual and 24-hour PM2.5 NAAQS specified in §50.7, 50.13, and 50.18. 

PM2.5 is defined, in general terms, as particles with an aerodynamic diameter less than or equal to a 

nominal 2.5 micrometers. PM2.5 mass concentrations are measured in the ambient air by a Federal 

Reference Method (FRM) based on appendix L of this part, as applicable, and designated in 

accordance with part 53 of this chapter; or by a Federal Equivalent Method (FEM) designated in 

accordance with part 53 of this chapter; or by an Approved Regional Method (ARM) designated in 

accordance with part 58 of this chapter. Only those FRM, FEM, and ARM measurements that are 

derived in accordance with part 58 of this chapter (i.e., that are deemed “suitable”) shall be used in 

comparisons with the PM2.5 NAAQS. The data handling and computation procedures to be used to 

construct annual and 24-hour NAAQS metrics from reported PM2.5 mass concentrations, and the 

associated instructions for comparing these calculated metrics to the levels of the PM2.5 NAAQS, are 

specified in sections 2.0, 3.0, and 4.0 of this appendix.

(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:

Annual mean refers to a weighted arithmetic mean, based on quarterly means, as defined in 

section 4.4 of this appendix.

The Air Quality System (AQS) is EPA's official repository of ambient air data.

Collocated monitors refers to two or more air measurement instruments for the same parameter 

(e.g., PM2.5 mass) operated at the same site location, and whose placement is consistent with §53.1 of 

this chapter. For purposes of considering a combined site record in this appendix, when two or more 

monitors are operated at the same site, one monitor is designated as the “primary” monitor with any 

additional monitors designated as “collocated.” It is implicit in these appendix procedures that the 

primary monitor and collocated monitor(s) are all deemed suitable for the applicable NAAQS 

comparison; however, it is not a requirement that the primary and monitors utilize the same specific 

sampling and analysis method.

Combined site data record is the data set used for performing calculations in appendix N. It 

represents data for the primary monitors augmented with data from collocated monitors according to 

the procedure specified in section 3.0(d) of this appendix.

Creditable samples are daily values in the combined site record that are given credit for data 

completeness. The number of creditable samples (cn) for a given year also governs which value in the 

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sorted series of daily values represents the 98th percentile for that year. Creditable samples include 

daily values collected on scheduled sampling days and valid make-up samples taken for missed or 

invalidated samples on scheduled sampling days.

Daily values refer to the 24-hour average concentrations of PM2.5 mass measured (or averaged 

from hourly measurements in AQS) from midnight to midnight (local standard time) from suitable 

monitors.

Data substitution tests are diagnostic evaluations performed on an annual PM2.5 NAAQS design 

value (DV) or a 24-hour PM2.5 NAAQS DV to determine if those metrics, which are judged to be based 

on incomplete data in accordance with 4.1(b) or 4.2(b) of this appendix shall nevertheless be deemed 

valid for NAAQS comparisons, or alternatively, shall still be considered incomplete and not valid for 

NAAQS comparisons. There are two data substitution tests, the “minimum quarterly value” test and the 

“maximum quarterly value” test. Design values (DVs) are the 3-year average NAAQS metrics that are 

compared to the NAAQS levels to determine when a monitoring site meets or does not meet the 

NAAQS, calculated as shown in section 4. There are two separate DVs specified in this appendix:

(1) The 3-year average of PM2.5 annual mean mass concentrations for each eligible monitoring 

site is referred to as the “annual PM2.5 NAAQS DV”.

(2) The 3-year average of annual 98th percentile 24-hour average PM2.5 mass concentration 

values recorded at each eligible monitoring site is referred to as the “24-hour (or daily) PM2.5 NAAQS 

DV”.

Eligible sites are monitoring stations that meet the criteria specified in §58.11 and §58.30 of this 

chapter, and thus are approved for comparison to the annual PM2.5 NAAQS. For the 24-hour PM2.5

NAAQS, all site locations that meet the criteria specified in §58.11 are approved (i.e., eligible) for 

NAAQS comparisons.

Extra samples are non-creditable samples. They are daily values that do not occur on scheduled 

sampling days and that cannot be used as make-up samples for missed or invalidated scheduled 

samples. Extra samples are used in mean calculations and are included in the series of all daily values 

subject to selection as a 98th percentile value, but are not used to determine which value in the sorted 

list represents the 98th percentile.

Make-up samples are samples collected to take the place of missed or invalidated required 

scheduled samples. Make-up samples can be made by either the primary or the collocated monitor. 

Make-up samples are either taken before the next required sampling day or exactly one week after the 

missed (or voided) sampling day.

The maximum quarterly value data substitution test substitutes actual “high” reported daily PM2.5

values from the same site (specifically, the highest reported non-excluded quarterly value(s) (year non

-specific) contained in the combined site record for the evaluated 3-year period) for missing daily 

values.

The minimum quarterly value data substitution test substitutes actual “low” reported daily PM2.5

values from the same site (specifically, the lowest reported quarterly value(s) (year non-specific) 

contained in the combined site record for the evaluated 3-year period) for missing daily values.

98th percentile is the smallest daily value out of a year of PM2.5 mass monitoring data below 

which no more than 98 percent of all daily values fall using the ranking and selection method specified 

in section 4.5(a) of this appendix.

Primary monitors are suitable monitors designated by a state or local agency in their annual 

network plan (and in AQS) as the default data source for creating a combined site record for purposes 

of NAAQS comparisons. If there is only one suitable monitor at a particular site location, then it is 

presumed to be a primary monitor.

Quarter refers to a calendar quarter (e.g., January through March).

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Quarterly data capture rate is the percentage of scheduled samples in a calendar quarter that 

have corresponding valid reported sample values. Quarterly data capture rates are specifically 

calculated as the number of creditable samples for the quarter divided by the number of scheduled 

samples for the quarter, the result then multiplied by 100 and rounded to the nearest integer.

Scheduled PM2.5 samples refers to those reported daily values which are consistent with the 

required sampling frequency (per §58.12 of this chapter) for the primary monitor, or those that meet 

the special exception noted in section 3.0(e) of this appendix.

Seasonal sampling is the practice of collecting data at a reduced frequency during a season of 

expected low concentrations.

Suitable monitors are instruments that use sampling and analysis methods approved for NAAQS 

comparisons. For the annual and 24-hour PM2.5 NAAQS, suitable monitors include all FRMs, and all 

FEMs/ARMs except those specific continuous FEMs/ARMs disqualified by a particular monitoring 

agency network in accordance with §58.10(b)(13) and approved by the EPA Regional Administrator 

per §58.11(e) of this chapter.

Test design values (TDV) are numerical values that used in the data substitution tests described 

in sections 4.1(c)(i), 4.1(c)(ii) and 4.2(c)(i) of this appendix to determine if the PM2.5 NAAQS DV with 

incomplete data are judged to be valid for NAAQS comparisons. There are two TDVs: TDVmin to 

determine if the NAAQS is not met and is used in the “minimum quarterly value” data substitution test 

and TDVmax to determine if the NAAQS is met and is used in the “maximum quarterly value” data 

substitution test. These TDV's are derived by substituting historically low or historically high daily 

concentration values for missing data in an incomplete year(s).

Year refers to a calendar year.

2.0 MONITORING CONSIDERATIONS

(a) Section 58.30 of this chapter provides special considerations for data comparisons to the 

annual PM2.5 NAAQS.

(b) Monitors meeting the network technical requirements detailed in §58.11 of this chapter are 

suitable for comparison with the NAAQS for PM2.5.

(c) Section 58.12 of this chapter specifies the required minimum frequency of sampling for PM2.5. 

Exceptions to the specified sampling frequencies, such as seasonal sampling, are subject to the 

approval of the EPA Regional Administrator and must be documented in the state or local agency 

Annual Monitoring Network Plan as required in §58.10 of this chapter and also in AQS.

3.0 REQUIREMENTS FOR DATA USE AND DATA REPORTING FOR COMPARISONS WITH THE NAAQS FOR

PM2.5

(a) Except as otherwise provided in this appendix, all valid FRM/FEM/ARM PM2.5 mass 

concentration data produced by suitable monitors that are required to be submitted to 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 the DV calculations. Generally, EPA will only use such data if they have 

been certified by the reporting organization (as prescribed by §58.15 of this chapter); however, data 

not certified by the reporting organization can nevertheless be used, if the deadline for certification has 

passed and EPA judges the data to be complete and accurate.

(b) PM2.5 mass concentration data (typically collected hourly for continuous instruments and daily 

for filter-based instruments) shall be reported to AQS in micrograms per cubic meter (μg/m3

) to at least 

one decimal place. If concentrations are reported to one decimal place, additional digits to the right of 

the tenths decimal place shall be truncated. If concentrations are reported to AQS with more than one 

decimal place, AQS will truncate the value to one decimal place for NAAQS usage (i.e., for 

implementing the procedures in this appendix). In situations where suitable PM2.5 data are available to 

EPA but not reported to AQS, the same truncation protocol shall be applied to that data. In situations 

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where PM2.5 mass data are submitted to AQS, or are otherwise available, with less precision than 

specified above, these data shall nevertheless still be deemed appropriate for NAAQS usage.

(c) Twenty-four-hour average concentrations will be computed in AQS from submitted hourly 

PM2.5 concentration data for each corresponding day of the year and the result will be stored in the 

first, or start, hour (i.e., midnight, hour ‘0’) of the 24-hour period. A 24-hour average concentration shall 

be considered valid if at least 75 percent of the hourly averages (i.e., 18 hourly values) for the 24-hour 

period are available. In the event that less than all 24 hourly average concentrations are available (i.e., 

less than 24, but at least 18), the 24-hour average concentration shall be computed on the basis of the 

hours available using the number of available hours within the 24-hour period as the divisor (e.g., 19, if 

19 hourly values are available). Twenty-four-hour periods with seven or more missing hours shall also 

be considered valid if, after substituting zero for all missing hourly concentrations, the resulting 24-

hour average daily value is greater than the level of the 24-hour PM2.5 NAAQS (i.e., greater than or 

equal to 35.5 μg/m3

). Twenty-four hour average PM2.5 mass concentrations that are averaged in AQS 

from hourly values will be truncated to one decimal place, consistent with the data handling procedure 

for the reported hourly (and also 24-hour filter-based) data.

(d) All calculations shown in this appendix shall be implemented on a site-level basis. Site level 

concentration data shall be processed as follows:

(1) The default dataset for PM2.5 mass concentrations for a site shall consist of the measured 

concentrations recorded from the designated primary monitor(s). All daily values produced by the 

primary monitor are considered part of the site record; this includes all creditable samples and all extra 

samples.

(2) Data for the primary monitors shall be augmented as much as possible with data from 

collocated monitors. If a valid daily value is not produced by the primary monitor for a particular day 

(scheduled or otherwise), but a value is available from a collocated monitor, then that collocated value 

shall be considered part of the combined site data record. If more than one collocated daily value is 

available, the average of those valid collocated values shall be used as the daily value. The data 

record resulting from this procedure is referred to as the “combined site data record.”

(e) All daily values in a combined site data record are used in the calculations specified in this 

appendix; however, not all daily values are given credit towards data completeness requirements. 

Only creditable samples are given credit for data completeness. Creditable samples include daily 

values in the combined site record that are collected on scheduled sampling days and valid make-up 

samples taken for missed or invalidated samples on scheduled sampling days. Days are considered 

scheduled according to the required sampling frequency of the designated primary monitor with one 

exception. The exception is, if a collocated continuous FEM/ARM monitor has a more intensive 

sampling frequency than the primary FRM monitor, then samples contributed to the combined site 

record from that continuous FEM/ARM monitor are always considered scheduled and, hence, also 

creditable. Daily values in the combined site data record that are reported for nonscheduled days, but 

that are not valid make-up samples are referred to as extra samples.

4.0 COMPARISONS WITH THE ANNUAL AND 24-HOUR PM2.5 NAAQS

4.1 Annual PM2.5 NAAQS

(a) The primary annual PM2.5 NAAQS is met when the annual PM2.5 NAAQS DV is less than or 

equal to 12.0 μg/m3

 at each eligible monitoring site. The secondary annual PM2.5 NAAQS is met when 

the annual PM2.5 NAAQS DV is less than or equal to 15.0 μg/m3

 at each eligible monitoring site.

(b) Three years of valid annual means are required to produce a valid annual PM2.5 NAAQS DV. A 

year meets data completeness requirements when quarterly data capture rates for all four quarters are 

at least 75 percent. However, years with at least 11 creditable samples in each quarter shall also be 

considered valid if the resulting annual mean or resulting annual PM2.5 NAAQS DV (rounded according 

to the conventions of section 4.3 of this appendix) is greater than the level of the applicable primary or 

secondary annual PM2.5 NAAQS. Furthermore, where the explicit 75 percent data capture and/or 11 

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sample minimum requirements are not met, the 3-year annual PM2.5 NAAQS DV shall still be 

considered valid if it passes at least one of the two data substitution tests stipulated below.

(c) In the case of one, two, or three years that do not meet the completeness requirements of 

section 4.1(b) of this appendix and thus would normally not be useable for the calculation of a valid 

annual PM2.5 NAAQS DV, the annual PM2.5 NAAQS DV shall nevertheless be considered valid if one 

of the test conditions specified in sections 4.1(c)(i) and 4.1(c)(ii) of this appendix is met.

(i) An annual PM2.5 NAAQS DV that is above the level of the NAAQS can be validated if it passes 

the minimum quarterly value data substitution test. This type of data substitution is permitted only if 

there are at least 30 days across the three quarters of the three years under consideration (e.g., 

collectively, quarter 1 of year 1, quarter 1 of year 2 and quarter 1 of year 3) from which to select the 

quarter-specific low value. Data substitution will be performed in all quarter periods that have less than 

11 creditable samples.

Procedure: Identify for each deficient quarter (i.e., those with less than 11 creditable samples) the 

lowest reported daily value for that quarter, looking across those three months of all three years under 

consideration. If after substituting the lowest reported daily value for a quarter for (11− cn) daily values 

in the matching deficient quarter(s) (i.e., to bring the creditable number for those quarters up to 11), 

the procedure yields a recalculated annual PM2.5 NAAQS test DV (TDVmin) that is greater than the 

level of the standard, then the annual PM2.5 NAAQS DV is deemed to have passed the diagnostic test 

and is valid, and the annual PM2.5 NAAQS is deemed to have been violated in that 3-year period.

(ii) An annual PM2.5 NAAQS DV that is equal to or below the level of the NAAQS can be validated 

if it passes the maximum quarterly value data substitution test. This type of data substitution is 

permitted only if there is at least 50 percent data capture in each quarter that is deficient of 75 percent 

data capture in each of the three years under consideration. Data substitution will be performed in all 

quarter periods that have less than 75 percent data capture but at least 50 percent data capture. If any 

quarter has less than 50 percent data capture then this substitution test cannot be used.

Procedure: Identify for each deficient quarter (i.e., those with less than 75 percent but at least 50 

percent data capture) the highest reported daily 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 quarters of all three years under consideration. If after substituting the highest 

reported daily PM2.5 value for a quarter for all missing daily data in the matching deficient quarter(s) 

(i.e., to make those quarters 100 percent complete), the procedure yields a recalculated annual PM2.5

NAAQS test DV (TDVmax) that is less than or equal to the level of the standard, then the annual PM2.5

NAAQS DV is deemed to have passed the diagnostic test and is valid, and the annual PM2.5 NAAQS 

is deemed to have been met in that 3-year period.

(d) An annual PM2.5 NAAQS DV based on data that do not meet the completeness criteria stated 

in 4(b) and also do not satisfy the test conditions specified in section 4(c), may also be considered 

valid with the approval of, or at the initiative of, the EPA Administrator, who may consider factors such 

as monitoring site closures/moves, monitoring diligence, the consistency and levels of the daily values 

that are available, and nearby concentrations in determining whether to use such data.

(e) The equations for calculating the annual PM2.5 NAAQS DVs are given in section 4.4 of this 

appendix.

4.2 Twenty-four-hour PM2.5 NAAQS

(a) The primary and secondary 24-hour PM2.5 NAAQS are met when the 24-hour PM2.5 NAAQS 

DV at each eligible monitoring site is less than or equal to 35 μg/m3

.

(b) Three years of valid annual PM2.5 98th percentile mass concentrations are required to produce 

a valid 24-hour PM2.5 NAAQS DV. A year meets data completeness requirements when quarterly data 

capture rates for all four quarters are at least 75 percent. However, years shall be considered valid, 

notwithstanding quarters with less than complete data (even quarters with less than 11 creditable 

samples, but at least one creditable sample must be present for the year), if the resulting annual 98th 

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percentile value or resulting 24-hour NAAQS DV (rounded according to the conventions of section 4.3 

of this appendix) is greater than the level of the standard. Furthermore, where the explicit 75 percent 

quarterly data capture requirement is not met, the 24-hour PM2.5 NAAQS DV shall still be considered 

valid if it passes the maximum quarterly value data substitution test.

(c) In the case of one, two, or three years that do not meet the completeness requirements of 

section 4.2(b) of this appendix and thus would normally not be useable for the calculation of a valid 24-

hour PM2.5 NAAQS DV, the 24-hour PM2.5 NAAQS DV shall nevertheless be considered valid if the 

test conditions specified in section 4.2(c)(i) of this appendix are met.

(i) A PM2.5 24-hour mass NAAQS DV that is equal to or below the level of the NAAQS can be 

validated if it passes the maximum quarterly value data substitution test. This type of data substitution 

is permitted only if there is at least 50 percent data capture in each quarter that is deficient of 75 

percent data capture in each of the three years under consideration. Data substitution will be 

performed in all quarters that have less than 75 percent data capture but at least 50 percent data 

capture. If any quarter has less than 50 percent data capture then this substitution test cannot be 

used.

Procedure: Identify for each deficient quarter (i.e., those with less than 75 percent but at least 50 

percent data capture) the highest reported daily PM2.5 value for that quarter, excluding state-flagged 

data affected by exceptional events which have been approved for exclusion by the Regional 

Administrator, looking across those three quarters of all three years under consideration. If, after 

substituting the highest reported daily maximum PM2.5 value for a quarter for all missing daily data in 

the matching deficient quarter(s) (i.e., to make those quarters 100 percent complete), the procedure 

yields a recalculated 3-year 24-hour NAAQS test DV (TDVmax) less than or equal to the level of the 

standard, then the 24-hour PM2.5 NAAQS DV is deemed to have passed the diagnostic test and is 

valid, and the 24-hour PM2.5 NAAQS is deemed to have been met in that 3-year period.

(d) A 24-hour PM2.5 NAAQS DV based on data that do not meet the completeness criteria stated 

in section 4(b) of this appendix and also do not satisfy the test conditions specified in section 4(c) of 

this appendix, may also be considered valid with the approval of, or at the initiative of, the EPA 

Administrator, who may consider factors such as monitoring site closures/moves, monitoring diligence, 

the consistency and levels of the daily values that are available, and nearby concentrations in 

determining whether to use such data.

(e) The procedures and equations for calculating the 24-hour PM2.5 NAAQS DVs are given in 

section 4.5 of this appendix.

4.3 Rounding Conventions. For the purposes of comparing calculated PM2.5 NAAQS DVs to the 

applicable level of the standard, it is necessary to round the final results of the calculations described 

in sections 4.4 and 4.5 of this appendix. Results for all intermediate calculations shall not be rounded.

(a) Annual PM2.5 NAAQS DVs shall be rounded to the nearest tenth of a μg/m3

 (decimals x.x5 and 

greater are rounded up to the next tenth, and any decimal lower than x.x5 is rounded down to the 

nearest tenth).

(b) Twenty-four-hour PM2.5 NAAQS DVs shall be rounded to the nearest 1 μg/m3

 (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).

4.4 Equations for the Annual PM2.5 NAAQS.

(a) An annual mean value for PM2.5 is determined by first averaging the daily values of a calendar 

quarter using equation 1 of this appendix:

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View or download PDF

Where:

X̅q,y = the mean for quarter q of the year y;

nq = the number of daily values in the quarter; and

xi q,y = the ith value in quarter q for year y.

(b) Equation 2 of this appendix is then used to calculate the site annual mean:

View or download PDF

Where:

X̅y = the annual mean concentration for year y (y = 1, 2, or 3); and

X̅q,y = the mean for quarter q of year y (result of equation 1).

(c) The annual PM2.5 NAAQS DV is calculated using equation 3 of this appendix:

View or download PDF

Where:

X̅ = the annual PM2.5 NAAQS DV; and

X̅y = the annual mean for year y (result of equation 2)

(d) The annual PM2.5 NAAQS DV is rounded according to the conventions in section 4.3 of this 

appendix before comparisons with the levels of the primary and secondary annual PM2.5 NAAQS are 

made.

4.5 Procedures and Equations for the 24-Hour PM2.5 NAAQS

(a) When the data for a particular site and year meet the data completeness requirements in 

section 4.2 of this appendix, calculation of the 98th percentile is accomplished by the steps provided in 

this subsection. Table 1 of this appendix shall be used to identify annual 98th percentile values.

Identification of annual 98th percentile values using the Table 1 procedure will be based on the 

creditable number of samples (as described below), rather than on the actual number of samples. 

Credit will not be granted for extra (non-creditable) samples. Extra samples, however, are candidates 

for selection as the annual 98th percentile. [The creditable number of samples will determine how 

deep to go into the data distribution, but all samples (creditable and extra) will be considered when 

making the percentile assignment.] The annual creditable number of samples is the sum of the four 

quarterly creditable number of samples.

Procedure: Sort all the daily 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 

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value.) The 98th percentile value 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 for the annual creditable number of samples for year y (cny) (e.g., for 120 creditable samples 

per year, the appropriate range would be 101 to 150). The corresponding “n” value in the right column 

identifies the rank of the annual 98th percentile value in the descending sorted list of site specific daily 

values for year y (e.g., for the range of 101 to 150, n would be 3). Thus, P0.98, y = the nth largest value 

(e.g., for the range of 101 to 150, the 98th percentile value would be the third highest value in the 

sorted series of daily values.

TABLE 1

Annual number of creditable 

samples for year y (cny)

The 98th percentile for year y (P0.98,y), is the nth maximum 24-

hour average value for the year where n is the listed number

1 to 50 1

51 to 100 2

101 to 150 3

151 to 200 4

201 to 250 5

251 to 300 6

301 to 350 7

351 to 366 8

(b) The 24-hour PM2.5 NAAQS DV is then calculated by averaging the annual 98th percentiles 

using equation 4 of this appendix: P0.98,y

View or download PDF

Where:

P̅0.98 = the 24-hour PM2.5 NAAQS DV; and

P0.98, y = the annual 98th percentile for year y

(c) The 24-hour PM2.5 NAAQS DV is rounded according to the conventions in section 4.3 of this 

appendix before a comparison with the level of the primary and secondary 24-hour NAAQS are made.

[78 FR 3277, Jan. 15, 2013]

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