1

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

RESEARCH TRIANGLE PARK, NC 27711

MEMORANDUM

SUBJECT: Assessment of PM2.5 data to determine the number of sites that would be 

potentially required to increase their sample frequency to daily.

FROM: Tim Hanley (OAQPS/AQAD/AAMG)

TO: Ambient Monitoring Rule Docket (EPA-HQ-OAR-2013-0619)

This memorandum details how PM2.5 data were assessed to determine the number of PM2.5 SLAMS 

stations that would be required to transition to daily sampling if they are not already on daily sampling as 

a result of a proposed requirement affecting the length of time a site must stay on daily sampling (i.e., 

proposed to stay on daily sampling for a three year period). As part of a proposed rule - Revisions to 

Ambient Monitoring Quality Assurance and Other Requirements – 79 FR 54356 - OAQPS received

comments related to the proposed length of time PM2.5 SLAMS would be expected to operate on a daily 

sampling frequency. Of specific concern was whether clarifying the length of time to operate a PM2.5 

FRM or FEM to run on a daily sampling frequency would result in increased burden to a large number 

monitoring agencies. To understand the extent of how many sites could potentially be affected by this 

requirement, we have analyzed site level design values and present our findings below.

BACKGROUND

The existing §58.12 provides that required SLAMS must operate on a daily sampling schedule. The 

existing requirement is provided below:

“58.12 Operating Schedules – (d)(iii) Required SLAMS stations whose measurements determine the 24-hour design 

value for their area and whose data are within plus or minus 5 percent of the level of the 24-hour PM2.5 NAAQS must 

have an FRM or FEM operate on a daily schedule if that area's design value for the annual NAAQS is less than the 

level of the annual PM2.5 standard. A continuously operating FEM or ARM PM2.5 monitor satisfies this requirement 

unless it is identified in the monitoring agency's annual monitoring network plan as not appropriate for comparison 

to the NAAQS.”

In the proposed rule - Revisions to Ambient Monitoring Quality Assurance and Other Requirements – 79 

FR 54356, EPA sought to clarify the length of time a site is to maintain the daily sampling frequency, 

which was proposed to be 3 years.

1

 In response to this proposed clarification, some commenters believed 

that this could add a large burden to their workload. To understand the extent to which this clarification 

may impact monitoring agencies, an assessment of the PM2.5 site level design values was performed to 

determine the number of sites not already on a daily sampling frequency that would be expected to 

operate on a daily sampling frequency for a three year-period.

 

1 The length of time for the increased sampling frequency was not specified in the existing rule language, providing 

the potential for inconsistency in interpretation.
2

DATA ANALYSIS

PM2.5 design values were downloaded from EPA’s web site at http://www.epa.gov/airtrends/values.html. 

This PM2.5 file was last updated on August 8th, 2014 using data downloaded from the EPA’s AQS 

database. The file includes PM2.5 design values for the three year period of 2011 through 2013 by site 

with design values calculated according to EPA’s data interpretation rules2. In our data analysis, the 

PM2.5 site level design values were evaluated by looking for the total number of sites, number of valid 

design value sites, and number of invalid design value sites that meet each of the following criteria:

1. Total number of sites available to assess;

2. Number of sites with a 24-hour design value3 of 34, 35, or 36 μg/m3;

3. Number of sites additionally having an annual design value of <12.0 μg/m3;

4. Number of sites with all the criteria above and without another site in the same CBSA already on a 

daily sampling schedule;

5. Number of sites with all the criteria above and without another site in the same CBSA that is more than 

5% above the level of the daily NAAQS. 

We considered both sites with design values that are complete as well as those where there was not 

enough data to provide a valid design value. The results of this assessment are summarized in the 

following table:

Table of number of PM2.5 sites according to identified criteria:

2011 - 2013 PM2.5 Design Value's (DV)

sites according to the following criteria

Total Number 

of Sites

Number of 

Valid Design 

Values

Number of 

Invalid Design 

Values

Total number of sites in DV listing 980 697 283

Number of sites with a 24-hour DV of 34, 35, 36 10 9 1

Number of sites with a 24-hour DV of 34, 35, 36 and 

annual DV of 12.0 or lower 9 8 1

Number of CBSA's with a 24-hour DV of 34, 35, 36 

and annual DV of 12.0 or lower 7 6 1

Number of CBSA's with a 24-hour DV of 34, 35, 36 

and annual DV of 12.0 or lower, and not already on 

daily sampling

5 4 1

Number of CBSA's with a 24-hour DV of 34, 35, 

36; annual DV of 12.0 or lower; not already on 

daily sampling; and no other site in area with DV 

more than 5% above the 24-hour NAAQS.

4 3 1

 

 

2 Appendix N to 40 CFR Part 50 – Interpretation of the National Ambient Air Quality Standards for PM2.5

3 See http://www.epa.gov/ttnamti1/files/ambient/pm25/NAAQS%20Comparisons-FAQ-010907.pdf
3

In the following table we list the four individual sites out of 980 where the requirement to move to daily 

sampling would potentially4 be required to be maintained for a three year period. Again, we point out that 

the requirement to move to daily sampling was already in place.

Table of Sites with a 24-hour DV of 34, 35, 36; annual DV of 12.0 or lower; not already on 

daily sampling; and no other site in area with DV more than 5% above NAAQS.

State CBSA Site ID

2011-

2013 

Annual 

Design 

Value 

(μg/m3)

INVALID

2011-2013 

Annual 

Design 

Value 

(μg/m3)

2011-

2013 

24-hr 

Design 

Value 

(μg/m3)

INVALID

2011-2013 

24-hr 

Design 

Value 

(μg/m3)

Would 

site 

need to 

go to 

daily?

California 060631006 10.2 36 Yes

Oregon 410250003 9.5 35 Yes

Oregon

PortlandVancouverBeaverton, 

OR-WA

410670004 8.2 34 Yes

Pennsylvania

PhiladelphiaCamdenWilmington, 

PA-NJ-DEMD

421010048 10.9 35 Yes

SUMMARY

As detailed above using data from 2011 to 2013, there are only four PM2.5 sites in the country out of 980 

sites that are not currently operating on a daily sample frequency that would potentially be required to 

move to a daily sampling frequency for a three-year period. Given the very small number of sites that 

would potentially be required to go to a daily sampling frequency for a three-year period, we believe that 

likelihood of such occurances affecting a monitoring agency are relatively small.

 

4 Note: we are not affirmatively requiring that these sites move to a daily sampling frequency as a result of this 

memo. This memo was produced to examine the potential burden of required daily sampling frequency being 

maintained for at least a three year period. Sample frequency changes are to be identified and approved as 

appropriate in each agencies annual monitoring network plan according to §58.10.