Appendix N to Part 50—Interpretation of the National Ambient Air Quality Standards for PM2.5

1.0 General

1. (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, 50.18, and 50.20. 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 to 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. Only those FRM and FEM 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.
2. (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.

3. (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 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. (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. (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).

   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

4. (a) Section 58.30 of this chapter provides special considerations for data comparisons to the annual PM2.5 NAAQS.
5. (b) Monitors meeting the network technical requirements detailed in § 58.11 of this chapter are suitable for comparison with the NAAQS for PM2.5.

6. (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

7. (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.
8. (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/m ³) 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 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.
9. (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/m ³). 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.

10. (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. (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. (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.”
    3. (3) In certain circumstances, including but not limited to site closures or relocations, data from two nearby sites may be combined into a single site data record for the purpose of calculating a valid design value. The appropriate Regional Administrator may approve such site combinations if the Regional Administrator determines that the measured concentrations do not differ substantially between the two sites, taking into consideration factors such as distance between sites, spatial and temporal patterns in air quality, local emissions and meteorology, jurisdictional boundaries, and terrain features.

11. (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

12. (a) Levels of the primary and secondary annual PM2.5 NAAQS are specified in §§ 50.7, 50.13, 50.18, and 50.20 as applicable.
13. (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 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.

14. (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.

    1. (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.

    2. (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.

15. (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.

16. (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

17. (a) Levels of the primary and secondary 24-hour PM2.5 NAAQS are specified in §§ 50.7, 50.13, 50.18, and 50.20 as applicable.
18. (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 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.

19. (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.

    1. (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.

20. (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.

21. (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.

22. (a) Annual PM2.5 NAAQS DVs shall be rounded to the nearest tenth of a µg/m ³ (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).

23. (b) Twenty-four-hour PM2.5 NAAQS DVs shall be rounded to the nearest 1 µg/m ³ (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.

24. (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:

    

    Where:

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

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

    xi q,y = the i ^th value in quarter q for year y.

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

    

    Where:

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

    nQ,y = the number of quarters Q in year y with at least one daily value; and

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

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

    

    Where:

    X = the annual PM2.5 NAAQS DV; and

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

27. (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

28. (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 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 n ^th 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

29. (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

    

    Where:

    P 0.98 = the 24-hour PM2.5 NAAQS DV; and

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

30. (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, as amended at 82 FR 14327, Mar. 20, 2017; 89 FR 16381, Mar. 6, 2024]