From: Richard Wayland/RTP/USEPA/US

To: dominic_j._mancini@omb.eop.gov

Cc: Lydia Wegman/RTP/USEPA/US@EPA

Date: 12/12/2012 03:44 PM

Subject: Fw: Revised Section VIII of premable - Amendments to Ambient Monitoring and Reporting 

Requirements

Dom... may have missed the period after your initial. Hope you get this.

Chet

__________________

Dom,

As we discussed yesterday, this revised section does a few things:

1. We have provided more emphasis on ensuring near-road environments represent population 

exposures.

2. We have added a reference citing our modeling results regarding the expected lower nearroad gradients in the future.

3. We have substantially changed our presumption of micro- and middle scale sites as applicable 

to the annual NAAQS. Previously we drafted that near-road type locations are area wide. In this 

revised version we say that there are cases where unique locations exist in the near road 

environment and that the annual NAAQS should not apply to those cases. States should provide 

a recommendation on this in their annual plans.

4. We have provided concrete examples of the types of locations that should not be applicable to 

the annual PM2.5 NAAQS, even if they are at a NO2 near-road station.

The attached file is a redline strikeout compared to the section VIII file that you have from earlier. 

I think this addresses your concerns from yesterday.
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VIII. Amendments to Ambient Monitoring and Reporting Requirements

The EPA is finalizing a number of changes to the ambient air monitoring, 

reporting, and network design requirements associated with the PM NAAQS. Ambient 

PM monitoring data are used to meet a variety of monitoring objectives including 

determining whether an area is in violation of the PM NAAQS. Ambient PM monitoring 

data are collected by state, local, and tribal monitoring agencies (“monitoring agencies”) 

in accordance with the monitoring requirements contained in 40 CFR parts 50, 53, and 

58. This section discusses the monitoring changes that the EPA is finalizing to support 

the revised PM NAAQS summarized in sections III.F, IV.F, and VI.F above.

The monitoring changes being finalized primarily relate to the revised primary 

PM2.5 NAAQS. Several monitoring changes were proposed specifically in support of a 

potential secondary PM2.5 visibility index; however, as explained in Section VI, EPA is 

not finalizing a secondary NAAQS using a visibility index and therefore is not finalizing 

the monitoring changes that would have been necessary to support it. The EPA did not 

propose any monitoring changes associated with the PM10 NAAQS and is not adopting 

any in this final rule.

A. Issues Related to 40 CFR Part 53 (Reference and Equivalent Methods)

To be used in a determination of compliance with the PM NAAQS, PM data are 

typically collected using samplers or monitors employing an FRM or FEM. The EPA also 

allows use of alternative methods where explicitly stated in the monitoring methodology 

requirements (appendix C of 40 CFR part 58), such as PM2.5 ARMs which can be used to 

determine compliance with the NAAQS. The EPA prescribes testing and approval criteria 

for FRM and FEM methods in 40 CFR part 53.

1. PM2.5 and PM10-2.5 Federal Equivalent Methods 
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As described in the proposal, the EPA continues to believe that an effective PM2.5

monitoring strategy includes the use of both filter-based FRM samplers and wellperforming continuous PM2.5 monitors. Well-performing continuous PM2.5 monitors 

would include both non-designated continuous PM2.5 monitors and designated Class III1

continuous FEMs that meet the performance criteria described in table C-4 of 40 CFR 

part 53 when comparing to a collocated FRM operated by the monitoring agency. Only 

designated methods (i.e., FRMs, FEMs, and ARMs) are approved to be used in 

comparison to the NAAQS; however, non-designated methods may be useful to meet 

other monitoring objectives (e.g., reporting the AQI). The use of Class III continuous 

FEMs at SLAMS is described in more detail in section VIII.B.3.b.ii below. Monitoring 

agencies are encouraged to evaluate the quality of data being generated by FEMs and, 

where appropriate, to reduce the use of manual, filter-based samplers to improve 

operational efficiency and to lower overall operating costs. To encourage such a strategy, 

the EPA is working with numerous stakeholders including the monitoring committee of 

NACAA, instrument manufacturers, and monitoring agencies to support national data 

analyses of continuous PM2.5 FEM performance, and where such performance does not 

meet data quality objectives, to develop and institute a program of best practices to 

improve the quality and consistency of resulting data.

The EPA believes that progress is being made to implement well performing 

PM2.5 continuous FEMs across the nation. As noted in the proposal, the first few steps 

involved the EPA developing and approving the testing and performance criteria which 

 

1 Class III refers to those methods for PM2.5 or PM10-2.5 that are employed to provide 

PM2.5 or PM10-2.5 ambient air measurements representative of one-hour or less integrated 

PM2.5 or PM10-2.5 concentrations, as well as 24-hour measurements determined as, or 

equivalent to, the mean of 24 one-hour consecutive measurements.
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were finalized in 2006, followed by instrument companies performing field testing and 

submitting applications to the EPA, and the EPA review and approval, as appropriate, of 

Class III FEMs. In the current step, monitoring agencies are testing and assessing the data 

comparability from continuous PM2.5 FEMs.

While EPA did not propose any changes to the performance or testing criteria in 

40 CFR part 53 used to approve PM2.5 continuous FEMs, the EPA did propose an 

administrative change to part 53.9 – “Conditions of designations.” See 77 FR 39006. 

This section describes a number of conditions that must be met by a manufacturer as a 

condition of maintaining designation of an FRM or FEM. Subsection (c) of this section 

reads, “Any analyzer, PM10 sampler, PM2.5 sampler, or PM10-2.5 sampler offered for sale 

as part of a FRM or FEM shall function within the limits of the performance 

specifications referred to in 40 CFR 53.20(a), 53.30(a), 53.50, or 53.60, as applicable, for 

at least 1 year after delivery and acceptance when maintained and operated in accordance 

with the manual referred to in 40 CFR 53.4(b)(3).” The EPA’s intent in this requirement 

is to ensure that monitoring methods work within performance criteria, which includes 

methods for PM2.5 and PM10-2.5; however, there was no specific reference to performance 

criteria for Class II2and III PM2.5 and PM10-2.5 methods. The EPA proposed to link the 

performance criteria referred to in 40 CFR part 53.35 associated with Class II and III 

PM2.5 and PM10-2.5 methods with this requirement for maintaining designation of 

approved FEMs. The specific performance criteria identified in 40 CFR 53.35 for PM2.5

and PM10-2.5 methods are available in table C-4 to subpart C of 40 CFR part 53.

 

2 Class II refers to those methods for PM2.5 or PM10-2.5 in which integrated samples are 

taken by filtration and subjected to a subsequent filter conditioning process followed by a 

gravimetric mass determination, but which is not a Class I equivalent methods because of 

substantial deviations from the design specification of the sampler specified for reference 

methods in appendix L or O (as applicable) of part 50 of the CFR. 
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All comments received on this proposed change were supportive and EPA is 

finalizing this change. The implication of this change is that instrument manufacturers 

and air agencies operating the equipment will have a shared responsibility for approved 

FEMs to meet required performance criteria for at least the first 12 months of operation, 

which is the typical warranty period for an instrument. By having a shared responsibility 

for an FEM to meet the performance criteria, instrument companies and air agencies will 

both be motivated to ensure the best practices for installing, operating, and servicing an 

instrument are carried out according to the instrument company’s operating manual and 

other readily available materials3in support of each method.

2. Use of Chemical Speciation Network (CSN) Methods to Support the Proposed 

New Secondary PM2.5 Visibility Index NAAQS

The EPA had proposed to use CSN methods to support the proposed new 

secondary PM2.5 visibility index NAAQS; however, as explained in Section VI of this 

final rule, EPA is not finalizing the new secondary PM2.5 visibility index NAAQS and 

therefore has no need to finalize the CSN methods to support such a standard.

Despite our decision not to finalize formal requirements for CSN methods, this 

network remains a critical component in our PM monitoring program. The EPA, 

monitoring agencies, and external scientists and policy makers use PM2.5 data from the 

CSN to support several important monitoring objectives such as: development of 

modeling tools and the application of source apportionment modeling for control strategy 

development to implement the NAAQS; health effects and exposure research studies; 

assessment of the effectiveness of emission reductions strategies through the 

 

3 At the recent National Air Quality Conference in May of 2012, a training session on 

“Best Practices for Operating PM2.5 Continuous FEMs” was conducted. Presentations 

from this session are publically available on EPA’s web site at: 

http://www.epa.gov/ttn/amtic/2012present.html.
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characterization of air quality; and development of SIPs. The use of the CSN to support 

all of these objectives will continue.

B. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)

1. Terminology Changes

The EPA proposed to revise several terms associated with PM2.5 monitor 

placement to ensure consistency with other NAAQS and to conform with long-standing 

practices in siting of equipment by monitoring agencies (77 FR 39007).

The EPA proposed to revoke the term “community-oriented” and replace it with 

the term “area-wide.” The term “community-oriented,” while used within the description 

of the design criteria for PM2.5, is not defined and has not been used in the design criteria 

for other NAAQS pollutants. Appendix D to 40 CFR part 58 presents a functional usage 

of the term where sites at the neighborhood and urban scale area are considered to be 

“community-oriented.” In addition, population-oriented, micro-or middle-scale PM2.5

monitoring may also be considered “community-oriented” when determined by the 

Regional Administrator to represent many such locations throughout a metropolitan area. 

The EPA proposed to replace this usage of “community-oriented” with the term “areawide” in the text of the PM2.5 network design criteria and to define it in 40 CFR 58.1 to 

provide a more consistent usage of this concept throughout appendix D of 40 CFR part 

58. Specifically, the EPA proposed that the terminology would read – “Area-wide means 

all monitors sited at neighborhood, urban, and regional scales, as well as those monitors 

sited at either micro-or middle-scale that are representative of many such locations in the 

same CBSA.”

The EPA proposed to revoke the term “Community Monitoring Zone” (CMZ) and 

to remove references to it in 40 CFR part 58. Community monitoring zone is currently 
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defined as “an optional averaging area with established, well defined boundaries, such as 

county or census block, within an MPA4that has relatively uniform concentrations of 

annual PM2.5 as defined by appendix N of 40 CFR part 50 of this chapter. Two or more 

community oriented state and local air monitoring stations (SLAMS) monitors within a 

CMZ that meet certain requirements as set forth in appendix N of 40 CFR part 50 may be 

averaged for making comparisons to the annual PM2.5 NAAQS.” The EPA proposed to 

revoke this term and references to it since, as discussed in section VII.A.2 above, the 

EPA proposed to eliminate all references to the now-revoked spatial averaging option 

throughout appendix N.

The one comment directly addressing the proposed rule changes (from a state air 

agency) supported the proposal. A few industry commenters noted the change in the 

context of how monitoring data are used to compare to the NAAQS, but did not address 

the proposed specific terminology changes. However, as explained in section III.E.3.a, 

several industry commenters did provide comments critical of EPA’s proposal to revoke 

spatial averaging which is related to revoking the term “Community Monitoring Zone”. 

For the reasons explained above, the EPA is finalizing its proposed change to 

revoke the term “community-oriented” and to replace it with the term “area-wide.” The

EPA is also finalizing its proposal to revoke the term “Community Monitoring Zone” 

(CMZ) and references to it in 40 CFR part 58. 

2. Special Considerations for Comparability of PM2.5 Ambient Air Monitoring Data 

to the NAAQS

In general, ambient monitors must meet a basic set of requirements before the 

 

4 Monitoring Planning Area (MPA) means a contiguous geographic area with well 

established, well defined boundaries, such as a CBSA, county or State, having a common 

areas that is used for planning monitoring locations for PM2.5.
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resulting data can be used for comparison to the NAAQS. These requirements include 

the presence and implementation of an approved quality assurance project plan; the use 

of methods that are reference, equivalent, or other approved method as described in 

appendix C to 40 CFR part 58; and compliance with the probe and siting path criteria as 

described in appendix E to 40 CFR part 58. While these 40 CFR part 58 requirements 

apply to any monitor that provides data for comparison to the NAAQS, there are certain 

additional restrictions that apply only to PM2.5 monitoring5. These additional restrictions 

provide that sites must be “population-oriented” for comparison to either the 24-hour or 

annual NAAQS, and specifically for comparison to the annual NAAQS, sites must be 

sited to represent area-wide locations. There is a related provision that provides for 

comparing sites at micro- or middle-scales to the annual PM2.5 NAAQS when the site is 

determined by the Regional Administrator to represent a larger region of localized high 

ambient PM2.5 concentration. 

These provisions have been in the monitoring regulations since the inception of 

the PM2.5 NAAQS. Nonetheless, these provisions and the fact that such monitoring 

requirements are not found in the requirements for all other criteria pollutants have 

created areas of uncertainty for EPA and state, local, and tribal agencies that base 

implementation decisions on monitoring requirements through programs such as 

dispersion modeling, SIP planning, and the calculation of transportation conformity 

budgets. For example, in developing modeling guidance to support near-road 

transportation conformity modeling, EPA struggled to determine how the identification 

of acceptable PM2.5 receptor locations can be reconciled with the PM2.5 monitoring 

 

5

These are found in 40 CFR 58.30 (Special considerations for data comparisons to the 

NAAQS). 
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regulations that reference potentially acceptable (or unacceptable) monitoring locations 

that may, or may not, be considered unique for purposes of comparing to the annual 

PM2.5 NAAQS. Accordingly, the EPA proposed to revise these particular PM2.5

requirements for consistency with long-standing practices in all other NAAQS pollutant 

monitoring networks, and to ensure that interpretation of the monitoring rules does not 

cause ambiguity in implementation examples that also include the treatment of 

unmonitored areas (see 77 FR 39007-009). Each of these topics is described below.

a. Eliminating the term “population oriented” from section 58.30

The EPA proposed to remove the term “population oriented” from section 58.30

so that there would no longer be an explicit requirement that PM2.5 monitoring sites be 

“population-oriented” for comparison to the PM2.5 NAAQS. EPA noted that this 

requirement is not entirely consistent with the definition of “ambient” used in the 

NAAQS. The EPA’s definition of ambient air is specified in 40 CFR 50.1 – “Ambient air

means that portion of the atmosphere, external to buildings, to which the general public 

has access.” The EPA’s definition of “population-oriented” is provided in 40 CFR 58.1 -

“Population-oriented monitoring (or sites) means residential areas, commercial areas, 

recreational areas, industrial areas where workers from more than one company are 

located, and other areas where a substantial number of people may spend a significant 

fraction of their day.” The NAAQS establish standards for concentrations “in the

ambient air”6—i.e., air to which members of the public could be exposed— and all 

monitors used for NAAQS regulatory purposes must be representative of ambient air 

 

6

See 40 CFR part 50.
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concentrations.7 Consistent with this requirement and the long-standing practice of 

monitoring agencies locating ambient monitors, EPA’s experience is that PM2.5 monitors 

are placed in areas that are representative of population exposures. There are no PM2.5 

monitors currently operating as “non-population oriented” and EPA does not believe that 

the requirement for near-road monitoring (discussed in detail further below) will result in 

monitors that are not representative of population exposures. At the same time, the 

specification that certain PM2.5 monitors must be “population-oriented” in the rules has 

created substantial confusion in how to treat potential locations of exposure for NAAQSrelated regulatory requirements other than monitoring network design, such as in 

applying modeling as part of a PSD or SIP exercise.

8

 The EPA’s intention in proposing to remove the term “population oriented” from 

section 58.30 was to remove a potential source of inconsistency in the monitoring rules as 

they apply for all the NAAQS. As noted earlier, the NAAQS provide protection for the 

public health and welfare in areas where the public can be exposed. For all other criteria 

pollutants, the monitoring requirements have no such restriction on the comparability of a 

monitor. In the case of PM2.5 however, the additional restriction of monitors being 

required to be “population-oriented” for comparability to the NAAQS has existed. The 

term “population oriented” has lacked a quantitative definition (e.g., the interpretation of 

“substantial number” in the definition of “population-oriented”), therefore monitoring 

agencies and those stakeholders who based implementation strategies and decisions on 

monitoring regulations have been uncertain about which locations would meet 

 

7

See, e.g, 40 CFR 58.1 (defining “federal reference method” as “a method for sampling and analyzing the 

ambient air for an air pollutant...”)

8 Examples include dispersion modeling to support NAAQS attainment planning, associated SIP 

development, and the calculation of transportation conformity budgets.
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requirements described in §58.30, which do not exist for any other NAAQS. Monitoring 

agencies are also not in a position to precisely forecast where future residential, 

commercial, or recreational development may occur, therefore requiring that PM2.5

monitors that are to be compared to the NAAQS can only be located where “substantial 

numbers of people” live, work, or play (i.e., in the present tense) represents an unwise 

limitation on the flexibility of monitoring agencies to revise their PM2.5 networks to 

account for anticipated changes in demographics or development as well as a 

contradiction with the inherent applicability of the NAAQS in ambient air locations 

where the public has access (e.g., in any location outside the perimeter of a industrial 

facility). From an operational standpoint, we note that revoking this term would not

change the requirements in the PM2.5 network design criteria. To the extent that the 

phrase “population-oriented” served to emphasize the need for micro- or middle-scale 

monitors to be representative of locations with population exposure to be comparable to 

the annual NAAQS, the definition of ambient air, together with the requirement in 

revised section 58.30 that such sites must be “area-wide” to be comparable to the annual 

NAAQS, adequately serves the same purpose. By revising the PM2.5 monitoring rules to 

ensure consistency with the long-standing definition of ambient air applied to the other 

NAAQS pollutants, the EPA will be able to more clearly define how to treat potential 

exposure receptors for other NAAQS regulatory requirements, regardless of whether 

monitoring exists or not.

Public comments on this issue were supported by air agencies and public health 

and environmental groups. Two commenters from state agencies supported the proposed 

change, with one noting further that regardless of a change it is still the air agency’s 
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responsibility to plan a network with sites that are appropriate for comparison to the 

NAAQS. Several public health and environmental groups supported revoking 

“population oriented” as a condition for comparability of PM2.5 monitoring sites to the 

NAAQS stating that retaining such a policy is inconsistent with the text, purpose and 

intent of the Clean Air Act. Most industry commenters did not support revoking 

“population-oriented” as a condition for comparison to the NAAQS. Most of these 

comments raised concerns with using data from an area where potentially no one is 

exposed.

In considering these comments, the EPA agrees that it is appropriate for 

individual air agencies to provide a recommendation in the annual monitoring network 

plan regarding whether any site may or may not be appropriate for comparison to the 

PM2.5 (or any) NAAQS. The roles of the air agency and the EPA in this process of 

identifying whether a site is, or is not, consistent with the network plan requirements for a 

NAAQS are specified in the already-established monitoring requirements of §58.10. In 

this approval process, the air agency initiates the recommendations and the EPA has the 

responsibility to approve, as appropriate, any plans that provide for changes to the 

network. 

EPA disagrees with the industry comments. As noted above, monitors (including 

those for PM2.5) must already meet the test of being representative of ambient air to be 

compared to the NAAQS, and thus such monitors meeting this test will be sited in 

locations where people are already located, or where they could be exposed, whether or 

not the term “population oriented” appears in section 58.30. Moreover, as discussed 

below, comparisons to the annual PM2.5 NAAQS can be only be from monitors “that are 
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representative of area-wide air quality”. “Area-wide” monitors are those at the 

neighborhood scale or larger, or at smaller scales if they are representative of many such 

locations in the same CBSA. EPA anticipates that a monitor that is sited as 

representative of ambient air at the neighborhood scale or larger (or of ambient air at 

many smaller areas) will be representative of population exposure. This conclusion is 

further supported by the fact that all current monitors used for comparison with the PM2.5

NAAQS are designated as “population-oriented.”9 

After consideration of the public comments, the EPA is finalizing its decision to 

revoke use of “population-oriented” as a condition for comparability of PM2.5 monitoring 

sites to the NAAQS. The EPA concludes that the “population-oriented” language is 

unnecessary and inconsistent with other monitoring rules, and should therefore be 

removed.

b. Applicability of Micro- and Middle-scale Monitoring Sites to the Annual PM2.5

NAAQS

The EPA proposed language in 40 CFR section 58.30 to clarify when data from 

PM2.5 monitoring sites at micro- and middle-scale locations can be compared to the 

annual PM2.5 NAAQS. The EPA’s intent was to provide consistency and predictability in 

the interpretation of the monitoring regulations. The EPA’s current rules state that 

“PM2.5 data that are representative, not of area-wide but rather, of relatively unique 

population-oriented micro-scale, or localized hot spot, or unique population-oriented 

middle-scale impact sites are only eligible for comparison to the 24-hour PM2.5 NAAQS. 

For example, if the PM2.5 monitoring site is adjacent to a unique dominating local PM2.5

 

9 The last known non population-oriented site at Sun Metro in El Paso Texas (AQS ID: 

48-141-0053), was shut down in October 2010 and is in the process of being moved to a 

nearby neighborhood.
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source or can be shown to have average 24-hour concentrations representative of a 

smaller than neighborhood spatial scale, then data from a monitor at the site would only 

be eligible for comparison to the 24-hour PM2.5 NAAQS.” We proposed clarifying 

language to explicitly state that measuring PM2.5 in micro- and middle-scale 

environments near emissions of mobile sources, such as a highway, does not constitute 

being impacted by a “unique” source and so could be compared to the annual PM2.5

NAAQS. We explained that mobile sources are rather ubiquitous and there are many 

locations throughout an urban area where elevated exposures attributable to such sources 

could occur. Therefore, we proposed that in most cases the potential location for a PM2.5

monitoring site, including micro- and middle-scale sites near roadways, would be eligible 

for comparison to the annual NAAQS. We further noted that the existing definition of 

“middle scale” in appendix D to part 58 already indicates that traffic corridors can be 

middle scale, and hence not unique, and therefore comparable to the annual PM2.5 

NAAQS (as well as to the 24-hour PM2.5 NAAQS) (77 FR 39008). 

Air agencies that commented on this part of the proposed rule offered a variety of 

positions. One air agency stated that sites at these smaller scales should not be compared 

to the annual NAAQS. Another air agency stated that these sites should be considered for 

comparison with the annual PM2.5 NAAQS only when the air agency initiates a decision 

that such sites at these smaller scales are area-wide. A different air agency offered that all 

micro- and middle-scale sites should be compared to the annual NAAQS since the 

wording of the provision is problematic and will be difficult for agencies to implement.

Industry commenters were largely against finalizing such a provision. The major 

concern raised was that such a provision combined with other related provisions 
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represented an unwarranted tightening of the NAAQS. Some industry commenters 

pointed out that there are examples of unique locations in near road environments and as 

such EPA should not presume that PM2.5 monitors in these locations should be applicable

to the annual PM2.5 NAAQS.

In considering comments on this part of the rule, the EPA notes that there are 

already examples of where the States and EPA have determined certain micro- and 

middle-scale locations as applicable to the annual NAAQS and others where they were 

determined as not applicable to the annual PM2.5 NAAQS. These cases exist where the 

and as described in the proposal, EPA repeats that the rules already provide that microand middle-scale monitors are comparable to the NAAQS (i.e., both the 24-hour and 

annual) where the States proposed and the Regional Administrator determineds thate

either the micro-scale or middle-scale site did or did not represents many similar areas in 

a CBSA (40 CFR 58.30 and section 4.7 to Appendix D, part 58). The EPA also notes that 

the existing descriptions of the types of micro- and middle-scale sites which are unique 

and cited in §58.30 are not being amended and that data from these types of sites would 

remain as not comparable to the annual PM2.5 NAAQS. Accordingly, PM2.5 data that are 

representative, not of area-wide but rather, of relatively unique population-oriented 

microscale, or localized hot spots, or unique middle scale impact sites will only be 

eligible for comparison to the 24-hour NAAQS. Our proposal was to clarify language to 

explicitly state that measuring PM2.5 in micro- and middle-scale environments near 

emissions of mobile sources, such as a highway, does not constitute being impacted by a 

“unique” source and so the site could be compared to the annual PM2.5 NAAQS. 

However, in light of public comments pointing out that there are cases where near-road 
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environments can be considered a unique location; EPA is not finalizing this part of the 

rule language. However, we conclude that a near-road environment is not one of these 

unique or relatively unique areas, since roads are ubiquitous throughout a metropolitan 

area. Examples of such locations that are considered unique and should therefore not be 

considered applicable to the annual PM2.5 NAAQS are explained later in section 

VIII.B.3.b.i. We are consequently adding to the regulatory text in section 58.30 a 

clarifying example on the treatment of near road monitoring sites, indicating that a site 

“located in a near-road environment where there are many other similar locations in the 

same CBSA “... would be considered representative of an area-wide location and, 

therefore, eligible for comparison to the annual PM2.5 NAAQS.” As noted in the 

preamble to the proposed rule (77 FR 39008-09), air agencies and EPA will use the 

annual monitoring network plan described in 40 CFR 58.10 for identification and 

approval of sites that are suitable and sites that are not suitable for comparison 

withagainst the annual PM2.5 NAAQS.

The EPA disagrees with those comments indicating that this change would 

represent a tightening of the NAAQS. As explained in section III.E.3.a on the form of 

the annual NAAQS, EPA carefully considered that areas such as traffic corridors were 

potential high exposure areas, including at-risk populations, and therefore protections 

afforded by the standard, including the revised form, are appropriate for these locations. 

Monitoring in such areas as traffic corridors does not make the annual standard more 

stringent, but rather affords the populations of such middle- and micro-scale areas (where 

determined to represent area-wide air quality) the requisite level of protection from longterm exposure to PM2.5. In addition, as noted above, the change primarily provides an 
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example of how to treat certain monitors in micro- and middle-scale environments that 

have always been eligible for comparison to the annual NAAQS (in cases where they 

where they were approved by the Regional Administrator as representing many areas 

within the CBSA), not the basic question of whether such monitors in micro- and middlescale environments were ever eligible for comparison to either the annual or 24-hour 

NAAQS.

3. Changes to Monitoring for the National Ambient Air Monitoring System

a. Background

As described in appendix D to 40 CFR part 58, the ambient air monitoring 

networks must be designed to meet three basic monitoring objectives: 

(a) Provide air pollution data to the general public in a timely manner. Data can be 

presented to the public in a number of attractive ways including through air quality maps, 

newspapers, Internet sites, and as part of weather forecasts and public advisories. 

(b) Support compliance with ambient air quality standards and emissions strategy 

development. Data from FRM, FEM, and ARM monitors for NAAQS pollutants will be 

used for comparing an area’s air pollution levels against the NAAQS. Data from monitors 

of various types can be used in the development of attainment and maintenance plans. 

SLAMS, and especially National Core Monitoring Network (NCore)10 station data, will 

be used to evaluate the regional air quality models used in developing emission strategies 

and to track trends in air pollution abatement control measures’ impact on improving air 

quality. In monitoring locations near major air pollution sources, source-oriented 

 

10 NCore is a multi-pollutant network that integrates several advanced measurements for 

particles, gases and meteorology (U.S. EPA, 2011a, Appendix B, section B.4). 

Measurements required at NCore include PM2.5 mass and speciation, PM10-2.5 mass, 

ozone, CO, SO2, NO, NOy, and basic meteorology. 
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monitoring data can provide insight into how well industrial sources are controlling their 

pollutant emissions. 

(c) Support for air pollution research studies. Air pollution data from the NCore 

network can be used to supplement data collected by researchers working on health 

effects assessments and atmospheric processes or for monitoring methods development 

work. 

To support the air quality management work indicated in the three basic air 

monitoring objectives, a network must be designed with a variety of types of monitoring 

sites. Monitoring sites must be capable of informing managers about many things 

including the peak air pollution levels, typical levels in populated areas, air pollution 

transported into and outside of a city or region, and air pollution levels near specific 

sources. Following is a listing of six general site types: (a) sites located to determine the 

highest concentrations expected to occur in the area covered by the network (highest 

concentration); (b) sites located to measure typical concentrations in areas of high 

population density (population oriented); (c) sites located to determine the impact of 

significant sources or source categories on air quality (source impact or source oriented); 

(d) sites located to determine general background concentration levels (general 

background); and (e) sites located to determine the extent of regional pollutant transport 

among populated areas (regional transport); and in support of secondary standards

(welfare related impacts).

b. Primary PM2.5 NAAQS

The EPA proposed to add a near-road component to the PM2.5 network design 

criteria and to clarify the use of approved PM2.5 continuous FEMs at SLAMS.
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i. Addition of a Near-road Component to the PM2.5 Monitoring Network

The EPA proposed to add a near-road component to the PM2.5 monitoring network 

(77 FR 39009). The EPA explained that there are gradients in near-roadway PM2.5 that 

are most likely to be associated with heavily travelled roads (particularly those with 

significant heavy-duty diesel activity), and that the largest numbers of impacted 

populations are located in the largest CBSAs in the country (Ntziachristos et al., 2007; 

Ross et al., 2007; Yanosky et al., 2008; Zwack et al., 2011). The EPA further noted that 

by adding a modest number of PM2.5 monitoring sites that are leveraged with 

measurements of other pollutants in the near-road environment, a number of key 

monitoring objectives will be supported, including collection of NAAQS comparable 

data in the near-road environment, support for long-term health studies investigating 

adverse effects on people, providing a better understanding of pollutant gradients 

impacting neighborhoods that parallel major roads, availability of data to validate 

performance of models simulating near-road dispersion, characterization of areas with 

potentially elevated concentrations and/or poor air quality, implementation of a multipollutant paradigm as stated in the NO2 NAAQS proposed rule (74 FR 34442, July 15, 

2009), and monitoring goals consistent with existing objectives noted in the specific 

design criteria for PM2.5 described in appendix D, 4.7.1(b) to 40 CFR part 58. 

The monitoring methods that are appropriate for this purpose are an FRM, FEM, 

or ARM. The EPA recognized that there are limitations in the ability of some of these 

methods to accurately measure PM2.5 mass due to the incomplete retention of semivolatile material on the sampling medium (U.S. EPA, 2009a, section 3.4.1.1). This 

limitation is relevant to the near-road environment as well as to other environments 
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where PM is expected to have semi-volatile components. The EPA also recognized that 

continuous PM2.5 FEMs, which provide mass concentration data on an hourly basis, are 

better suited to accomplish the goals of near-road monitoring as they will complement the 

time resolution of the other air quality measurements and traffic data collected at the 

same sites. In this regard, particular PM2.5 FEMs are generally better suited for near-road 

monitoring than FRMs. However, filter-based FRMs do offer some advantages which 

may be highly desirable for near-road monitoring, such as readily available filters for 

later chemical analysis such as for elemental composition by x-ray fluorescence and 

black carbon (BC) by transmissometry. As a result of these tradeoffs, monitoring 

agencies are encouraged to select one or more PM2.5 methods for deployment at near–

road monitoring stations that best meet their agencies monitoring objectives while 

ensuring that at least one of those methods is appropriate for comparison to the NAAQS 

(i.e., a FRM, FEM, or ARM). The EPA believes that by allowing monitoring agencies to 

choose the FRM, FEM, or ARM method(s) that best fits their needs, whether filter-based 

or continuous, the data will still be able to meet the objectives cited above while ensuring 

maximum flexibility for the monitoring agencies in the operation of their network.

Additionally, the EPA noted that the near-road sites would provide a valuable 

platform for evaluating emerging monitoring technologies and for measuring other 

pollutants besides PM2.5 mass to enhance knowledge of exposure in the near road 

environment and to support the characterization and comparison of specific method 

readings in an emission-rich environment. Further, in its response to the EPA on a 

“Review of the “Near-road Guidance Document – Outline” and “Near-road Monitoring 

Pilot Study Objectives and Approach” (U.S. EPA, 2010i), the CASAC AAMMS cited 
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several other measurements that may be useful or potentially linked to health and welfare 

effects such as BC, ultrafine particles, and particle size distribution (Russell and Samet, 

2010b, pp. xi and xii). The EPA agrees with these recommendations and encourages 

monitoring agencies to include these measurements, and others cited in the Subcommittee 

letter, where possible, in addition to the PM2.5 mass measurement. The EPA also 

encourages monitoring agencies to explore partnerships with instrument manufacturers 

and researchers to use the sites to evaluate the performance of emerging PM2.5

monitoring methods in the near-road environment, especially potential or current FEMs 

able to provide temporally resolved data and capture the semi-volatile components of 

PM2.5. Such emerging PM2.5 methods could be operated as SPMs11 to provide 

comparisons to the EPA approved methods supporting compliance to advance the 

understanding of instrument performance in the near-road environment. Monitoring 

agencies are also encouraged to partner with instrument manufacturers and researchers to 

operate monitors able to measure other PM properties relevant for the near-road 

environment (e.g., ultrafine particles, BC) to provide additional information about 

exposure to PM in this environment. The EPA is interested in supporting monitoring 

agencies willing to operate and report the data from these supplemental monitors. EPA 

notes that the implementation of additional measurements, while encouraged, is 

completely voluntary to ensure maximum flexibility for state monitoring programs. 

The EPA believes that requiring a modest network of near-road compliance PM2.5

monitors is necessary to provide characterization of concentrations in near-road 

 

11 SPMs means a monitor included in an air agency’s monitoring network that the agency 

has designated a a special purpose monitor in its monitoring network plan and in the Air 

Quality System database, and which the air agency does not count toward when showing 

compliance with the minimum monitoring requirements in appendix D to Part 58.
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environments including for comparison to the NAAQS. These long-term monitors will 

supplement shorter-term networks operated by researchers to support the tracking of 

long-term trends12 of near-road PM2.5 mass concentrations and other pollutants in nearroad environments where people are exposed. Therefore, the EPA proposed to require 

near-roadway monitoring of PM2.5 at one location within each CBSA with a population 

of one million persons or greater. The EPA believes that this network will be adequate to 

support the NAAQS since the largest CBSAs are likely to have greater numbers of 

exposed populations, a higher likelihood of elevated near-road PM2.5 concentrations, and

a wide range of diverse situations with regard to traffic volumes, traffic patterns, roadway 

designs, terrain/topography, meteorology, climate, surrounding land use and population 

characteristics. Given the latest population data available, the proposed requirement 

would result in approximately 52 required near-road PM2.5 monitors across the country. 

An indirect benefit of this network design is that monitoring agencies in these largest 

CBSAs are more likely to already have redundant monitors that could be relocated to the 

near-road environment, reducing costs for equipment and ongoing operation.13 While 

only a single near-road PM2.5 monitor is required within each of the CBSAs, agencies 

may elect to add additional PM2.5 monitoring sites in near-road environments.

While the EPA recognized that the location of maximum concentration of PM2.5

exposure from roadway sources might differ from the maximum location of NO2 or other 

pollutants, the EPA proposed to require that near-road PM2.5 monitors be collocated with 

 

12 For example, the emissions used for the PM NAAQS RIA modeling show that nationwide on-road 

primary PM2.5 emissions are expected to be reduced by 63% between 2007 and 2020. Additionally, the 

elemental carbon portion of the on-road emissions is expected to drop by 81% between 2007 and 2020. 

Therefore, we expect that measured near-road PM2.5 gradients will be much lower in the future as elemental 

carbon is a large fraction of the existing gradient. 

13 EPA Regional Administrator approval would be required prior to the discontinuation of 

SLAMS monitors, based on the criteria described in 40 CFR 58.14(c).
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the planned NO2 monitors. The NO2 network design considers multiple factors that are 

also relevant for PM2.5 concentrations (i.e., average annual daily traffic, fleet mix, 

roadway design, congestion patterns, terrain, and meteorology) and significant thought 

and review has already gone into its design, including pilot studies at five locations, and 

the development of a technical assistance document in conjunction with the affected 

monitoring agencies and the CASAC AAMMS (Russell and Samet, 2010b) to support 

deployment. Further, this collocation will allow multiple pollutants to be tracked in the 

near-road environment. To the extent that air agencies are still determining the optimum 

location for their multi-pollutant14near-road monitoring stations, EPA encourages 

consideration of sites that best reflect measurement of maximum concentrations 

associated with exposure of people living in areas that parallel major roads, to maximize 

the value of the data for use later in health studies. Therefore, while compromises may be 

necessary when siting a multi-pollutant near road monitoring station, on balance, the EPA 

believes this is the most efficient and beneficial approach for deployment of this 

component of the network.

The EPA notes that the planned 52 near-road monitors represent a small number 

of the total approximate 900 operating PM2.5 monitoring stations across the country. The 

EPA could have proposed more near-road sites, however, the addition of sites in lower 

population CBSAs is not expected to lead to much if any difference in characterization of 

air quality since the bump in PM2.5 concentration associated with near-road environments 

in lower population CBSAs, which typically have correspondingly less travelled roads, is 

expected to be very small. The EPA could also have proposed multiple sites in larger 

 

14 NO2, CO, and now PM2.5 measurements are all expected to be collocated at near-road 

monitoring stations.
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CBSAs; however, State monitoring programs are already working towards representative 

near-road monitoring stations and there is a synergistic value in ensuring these 

measurements are collocated with multiple other measurements to serve the monitoring 

objectives noted above. Since EPA has already finalized requirement of CO monitoring 

at near-road stations in CBSA’s with a population of 1 million or more at sites that are 

collocated with NO2, there would be less value in requiring any more than 52 PM2.5 

monitors as any more stations will not have CO for use in multi-pollutant monitoring 

objectives (e.g., health studies and model evaluation).

Ideally, near-road sites would be located at the elevation and distance from the 

road where maximum concentration of PM2.5 levels exposure occurs in this environment, 

representing locations where populations are exposed; for example, in apartments and 

other housing; schools located along major roadways; industrial parks where workers 

exposed; and in recreational areas such as greenways, bikeways, and other park facilities 

that are often developed along roads.and within reasonable proximity to another areawide PM2.5 compliance monitoring site at which a similar PM monitor is used (i.e., for 

comparison purposes). However, Sspecific to probe and siting criteria for near-road PM2.5 

monitors, which is explained later in this section, EPA did not set additional criteria on

what the elevation and distance requirements should be, beyond what is already defined 

for PM2.5 or near-road NO2 monitors for reasons explained above. Also, the EPA did not 

propose that the near-road PM2.5 monitors be located within a specific distance of other 

area-wide sites; however, monitoring agencies are encouraged to consider that a nearroad site selected in accordance with monitoring requirements and also located in 

proximity to a robust area-wide site, such as an NCore station, would provide useful 
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information in characterizing the near-road contribution to multiple pollutants, including 

PM2.5 and tracking the decreasing trend that is expected in the PM2.5 near-road gradient 

over time.

The timeline to implement the near-road PM2.5 monitors should be as minimally 

disruptive to on-going operations of monitoring agency programs as possible recognizing 

monitoring agencyies resource constraints, while still meeting the need to collect for 

near-road PM2.5 data in a timely fashion. Since the near-road PM2.5 monitors were

proposed to be collocated with the emerging near-road NO2 network that was scheduled 

to be operational by January 1, 201315, the EPA believes it is appropriate to wait until 

after the near-road NO2 network is established before implementing the near-road PM2.5

monitors. Therefore, the EPA proposed that each PM2.5 monitor planned for collocation 

with a near-road NO2 monitoring site be implemented no later than January 1, 2015.

The EPA received comments from a number of air agencies, industrial groups, 

and environmental and public health organizations on its proposal to require PM2.5

monitoring in near-road environments.

Among comments from air agencies, several commenters did not support the 

addition of near road monitoring citing the challenges of siting these stations and the 

additional cost it would require to operate the monitors. Several air agencies recognized 

the value of adding monitors to provide better characterization of exposures in near-road 

environments, but recommended a slower deployment of the PM2.5 monitors so that it can 

be phased in over a multi-year period. Several air agencies recommended that the PM2.5

monitoring in the near-road environment be deployed on a phased-in schedule with the 

 

15 The EPA has proposed a revised timeline for deployment of the near-road NO2

monitors, where all CBSAs with one million or more people are to have their first nearroad NO2 station operational by January 1, 2014 (77 FR 64244, October 19, 2012).
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first such monitors being required no sooner than one year after deployment of the NO2

sites. These air agencies stated that phasing in of the PM2.5 monitors in the near road 

environment would allow more time to learn and share information on what worked best 

in deploying the NO2 monitors at near-road monitoring stations, since NO2 is the first 

pollutant required to be monitored at near-road stations. A few air agencies identified a 

need to more clearly support or require the maintenance of as much of the existing 

network of neighborhood scale PM2.5 monitoring sites as possible in regulatory text. 

These neighborhood scale PM2.5 sites were identified by commenters as the most broadly 

representative sites for characterizing CBSA wide exposures that are supportive of a 

number of monitoring objectives. A few air agencies also identified a need for flexibility 

in the proposed network design requirement that PM2.5 near-road monitors must be 

collocated with the NO2 monitors in the near-road environment. The commenters 

suggested allowing flexibility for air agencies to meet the requirement for PM2.5 in a 

near-road environment by siting at a different near-road location where PM2.5

concentrations are expected to be high.

Most industry commenters did not support the addition of near-road monitoring 

for PM2.5, again arguing that using data from such monitors, for comparison to the 

NAAQS, combined with other changes (i.e., elimination of “population-oriented” as a 

criteria for comparison to the NAAQS and the elimination of spatial averaging) would 

represent, in their judgement, a tightening of the PM2.5 NAAQS. A few of these 

commenters asserted that monitoring in the near-road environment is not representative 

of ambient air exposures. A few industry comments noted that if the EPA required PM2.5

monitoring in the near-road environment, any data collected should not be used for 
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comparison to the NAAQS. One commenter stated it had no problem with monitoring in 

the near-road environment, so long as any such monitoring used to compare to the PM2.5 

annual NAAQS is population–oriented. One commenter stated that the decision to colocate with NO2 monitors was based on convenience and the intent of the NO2 near-road 

monitoring is to find the highest micro-scale concentrations within a few meters of the 

most heavily travelled expressways, representing a unique situation. 

Environmental and public health groups strongly support the addition of PM2.5 

monitoring to the near-road environment. Commenters cited the large number of people 

that live in proximity to major roadways16 in their support for adding these monitors, that 

such protection of people in these environments in long overdue, and that such data 

therefore be used for comparison to the NAAQS.

Regarding comments from air agencies that the near-road monitors are 

challenging to site and that there is additional cost in operating these monitors, the EPA 

maintains that the major challenges in siting would already be accomplished by 

implementing the required NO2 monitoring stations in near-road environments since the 

EPA fully expects that the PM2.5 monitors will be placed at the NO2 near roadway 

stations and has revised the PM2.5 monitoring requirements consistent with that 

expectation. The EPA also points out that the requirements for the minimum number of 

PM2.5 monitors is unchanged and that in most cases the addition of near-road PM2.5

monitors can be accomplished by relocating an existing monitor, with no net increase in 

 

16 One study identified that 45 million Americans live within 300 feet of a major roadway

or other source of mobile emissions. The commenters information is based on the 

American Housing Survey, which is available on the web at: 

http://www.census.gov/housing/ahs/data/ahs2009.html. The survey provides an estimate 

of the county’s housing units in the U.S. that are located with 300 feet of a highway with 

four or more lanes, or a railroad, or an airport.

 
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monitors. Thus, while we are requiring a new component of the PM2.5 monitoring 

network, the overall size of the network is expected to remain about the same, and we 

expect that air agencies can meet this requirement by relocating existing lower-priority 

monitors. In considering comments from air agencies on a schedule for implementing 

PM2.5 monitors at near road monitoring stations, the EPA is persuaded by commenters 

from air agencies who stated that a phased deployment of the PM2.5 monitors would be a 

better approach as it would allow agencies to learn from the deployment of the NO2

monitors and a first phase of PM2.5 monitors. Phasing in the deployment of monitors is 

also consistent with previous CASAC advice (Russell and Samet, 2010b) on a schedule 

for deployment of near-road NO2 monitors.

Regarding comments from air agencies on maintaining the neighborhood scale 

monitoring stations as the largest part of the network as these sites are the most broadly 

representative of exposures across CBSAs, the EPA supports such a goal. Neighborhood 

scale monitoring sites remain the backbone of the PM2.5 monitoring network and they 

will continue to represent over two thirds of the operating network following the 

deployment of the near-road monitors. The EPA expects that each CBSA required to 

monitor for PM2.5 will maintain its existing highest concentration area-wide monitoring 

site (referred to as the design value site) and not attempt to move such sites to near-road 

environments. Maintaining the area-wide and largely neighborhood scale design value 

sites is critical to the long-standing goal of using data to support a variety of monitoring 

objectives. The EPA also recognizes that while every PM2.5 monitor has value in some 

capacity at its current location, air agencies are expected to recommend relocation of 

monitors that are relatively low in priority to meet the near-road requirement.
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Regarding comments from air agencies on the need for flexibility in the network 

design requirement that PM2.5 near-road monitors must be collocated with the NO2

monitors in the near-road environment, the EPA points out that it prefers to maintain this 

requirement so that the multi-pollutant data are available to support the monitoring 

objectives cited above. However, the EPA also recognizes there may be cases where an 

air agency recommends siting their near-road PM2.5 monitor in another high 

concentration near-road environment. The EPA believes such cases will be very limited, 

but that these situations can be supported in one of two ways. First, EPA and the air 

agency can use their discretion to site two near-road PM2.5 monitors in the area. Second, 

the EPA can use its discretion in approving a deviation from the PM2.5 monitoring 

requirements as already exists in the network design criteria. Such deviations are to be 

approved by the Regional Administrator as described in section 4.7.1 of Appendix D to 

part 58.

Regarding the comment that PM2.5 monitors in near-road environments were sited 

for convenience, which due to siting with NO2 monitors a few meters from the road 

presents a unique situation, the EPA disagrees that these monitors were sited solely for 

convenience or that they would represent a unique situation within an urban area. On the 

initial point, the EPA believes that the characterization of representative maximum PM2.5 

concentrations due to on-road mobile sources and the appropriate location of such PM2.5

monitors will be the same approximate locations that are the focus of the near-road NO2

network. This is due to the fact that PM2.5, like NOX, is disproportionately influenced by 

heavy duty (HD) vehicles which are predominantly diesel fueled, when compared to light 

duty (LD) vehicles which are primarily gasoline fueled. Specifically, for both PM2.5 and 
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NOx, HD vehicles emit more of these two pollutants and their precursors on a per vehicle 

basis than LD vehicles. The EPA recognized this fact in the near-road NO2 network by 

requiring states to consider the fleet mix of candidate road segments where near-road 

monitoring might occur. In the design of the NO2 near-road network where the PM2.5

monitors will be installed, states were instructed to place a higher priority on those highly 

trafficked roads which have more diesel fueled vehicles using a metric called the fleet 

equivalent average annual daily traffic.

17

 As such, the Agency believes it is appropriate 

that required near-road PM2.5 monitors would be located with near-road NO2 monitors as 

they are similarly influenced not only by fleet mix but also by total traffic count, 

congestion patterns, roadway design, terrain, and meteorology. On the second point with 

regard to such sites representing a unique situation within an urban area, EPA again

points out that the determination of a near-road micro- or middle-scale site being 

considered to represent “area-wide” air quality or “unique” will be made on a case by 

case basis with the monitoring agency providing such recommendations in their annual 

monitoring network plans described in §58.10. Examples of such “unique” micro- and 

middle-scale locations are provided later in this section.these major roadways are 

ubiquitous throughout metropolitan areas and as such represent many such locations that 

occur within an urban area and the resulting exposure of the population in residential, 

commercial and recreational areas.

We do not accept the comment that siting some monitors in near roadway 

environments makes the standard impermissibly stricter. A significant fraction of the 

population lives in proximity to major roads. These exposures occur in locations that 

 

17 See the Near-road NO2 Monitoring Technical Assistance Document at: 

http://www.epa.gov/ttn/amtic/files/nearroad/NearRoadTAD.pdf
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represent ambient air for which the agency has a responsibility to ensure the public is 

protected with an adequate margin of safety. Ignoring monitoring results from such areas 

(or not monitoring at all) would abdicate this responsibility. Put another way, monitoring 

in such areas does not make the standard stricter, but rather affords requisite protection to 

the populations, among them at-risk populations, exposed to fine particulate in these 

areas. Thus, the EPA has made a determination to protect all area-wide locations, 

including those locations with populations living near major roads that are representative 

of many such locations throughout an area. As discussed above, EPA concludes that the 

requirement to locate monitors to represent ambient air, along with other siting 

requirements, will ensure that monitors represent PM2.5 concentrations in areas of 

potential public exposure.

We do recognize, however, the possibility that some near-road monitoring 

stations may be representative of relatively unique locations versus the more 

representative area-wide situation mentioned above. This could occur because an air 

agency made a siting decision based on NO2 criteria that resulted in the characterization 

of a microscale environment that is not considered area-wide for PM2.5; for example, due 

to proximity to a unique source like a tunnel entrance, nearby major point source, or other 

relatively unique microscale hot spot. In these types of scenarios, air agencies would 

identify the site as a unique monitor comparable only to the 24-hour PM2.5 NAAQS per 

the language in section 58.30, and not comparable to the annual NAAQS, through the 

Annual Monitoring Network Plan process described earlier. Although EPA expects most 

near-road PM2.5 monitors to be sited to represent area-wide conditions, since a vast 
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majority of the near-road stations have yet to be installed, we believe that providing such 

clarity and flexibility in siting and NAAQS comparability is warranted.

After careful consideration of the public comments, the EPA is finalizing its 

decision to add PM2.5 monitors to the near-road monitoring stations. The EPA is 

finalizing this decision as the near-road environment is an area where significant public 

exposure can occur, recognizing that this is a gap in the current PM2.5 monitoring 

networks, and because these PM2.5 monitors will be collocated with NO2 monitors in the 

near-road environment, there will not be a significant additional burden on the air 

agencies18. However, in recognition of the comments from air agencies above, EPA is 

finalizing a revised and phased schedule for deployment of the PM2.5 monitors at nearroad stations. A minimum of one PM2.5 monitor in each CBSA with a population greater 

than or equal to≥ 2.5 million is to be collocated at a near-road NO2 monitoring station 

and must to be operational by January 1, 2015. The remaining CBSAs (i.e., those CBSAs 

with populations greater than or equal to ≥1M, but less than 2.5M) must operational by 

January 1, 2017. This schedule will ensure that air agencies have sufficient time to learn 

from deployment of the NO2 monitors in near-road environments, that the highest 

population CBSAs begin operating their PM2.5 monitors in near-road environments first, 

and that the remaining PM2.5 monitors are deployed on the same schedule as the CO 

monitors (also, required by January 1, 201719). In consideration of the comments 

regarding maintaining neighborhood scale monitoring sites as the largest portion of the 

network, the EPA is revising the wording of a requirement that requires at least one site 

 

18 The incremental one-time cost of moving the 52 monitors required to be located in the 

near-road environment is described in section X.B – Paperwork reduction Act. 

19 76 FR 54294, August 31, 2011
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to be in an area-wide location of expected maximum concentration, to wording that states 

that such sites must be in an area-wide location of expected maximum concentration

while also being at the neighborhood or larger scale of representation. 

ii. Use of PM2.5 Continuous FEMs at SLAMS

The EPA proposed that each agency specify its intention and rationale to use or 

not use data from continuous PM2.5 FEMs that are eligible for comparison to the NAAQS 

as part of its annual monitoring network plan due to the applicable EPA Region Office by 

July 1 each year. The proposal also provided that the EPA Regional Administrator would 

be responsible for approving annual monitoring network plans where agencies have 

provided a recommendation that certain PM2.5 FEMs be considered ineligible for 

comparison to the NAAQS. 

In 2006, the EPA finalized new performance criteria for approval of continuous 

PM2.5 monitors as either Class III FEMs or ARMs. At the time of proposal, the EPA had

already approved six PM2.5 continuous FEMs20 and there are nearly 200 of these monitors 

already operating in State, local, and Tribal networks. Monitoring agencies have been 

deploying and field-testing these units over the last couple of years and the EPA recently 

compiled an assessment of the FEM data in relationship to collocated FRMs (Hanley and 

Reff, 2011; U.S. EPA, 2011a, pp. 4-50 to 4-51). As described in the proposal (FR 

38983),the EPA found that some sites with continuous PM2.5 FEMs have an acceptable 

degree of comparability with collocated FRMs, while others had poor data comparability 

that would not meet the performance criteria used to approve the FEMs (71 FR 61285-

61286, Table C-4, October 17, 2006). The EPA is encouraging use of the FEM data from 

 

20 The EPA maintains a list of approved Reference and Equivalent Methods on its web 

site at: http://www.epa.gov/ttn/amtic/criteria.html. 
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those sites with acceptable data comparability including for purposes of comparison to 

the NAAQS. For sites with unacceptable data comparability, the EPA is working closely 

with the monitoring committee of the NACAA, instrument manufacturers, and 

monitoring agencies to document best practices on these methods to improve the 

comparability and consistency of resulting data wherever possible. The EPA believes that 

the performance of many of these continuous PM2.5 FEMs at locations with poor data 

comparability can be improved to a point where the acceptance criteria noted above can 

be met.

Given the varying data comparability of continuous PM2.5 FEMs noted above, we 

believe that a need exists for flexibility in the approaches for how such data are used, 

particularly for the objective of determining NAAQS compliance. Accordingly, we 

proposed that monitoring agencies address the use of data from PM2.5 continuous FEMs 

in their annual monitoring network plans due to the applicable EPA Regional Office by 

July 1 of each year for any cases where the agency believes that the data generated by 

PM2.5 continuous FEMs in their network should not to be compared to the NAAQS. The 

annual network plans would include assessments such as comparisons of continuous 

FEMs to collocated FRMs, and analyses of whether the resulting statistical performance 

would meet the established approval criteria. Based on these quantitative analyses, 

monitoring agencies would have the option of requesting that data from continuous FEMs 

be excluded from NAAQS comparison subject to EPA approval; however, these data 

could still be utilized for other objectives such as AQI reporting. 

The issue exists of whether such data use provisions should be prospective only 

(i.e., future NAAQS comparability excluded based on an analysis of recent past 
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performance) or a combination of retrospective and prospective (i.e., the implications of 

unacceptable FEM performance impacting usage of previously collected data as well as 

future data). In the proposal, the EPA stated that in most cases, monitoring agencies 

should be restricted to addressing prospective data issues to provide stability and 

predictability in the long-term PM2.5 data sets used for supporting attainment decisions. 

However, in the first year after this proposed option would become effective, we 

indicated it would be appropriate to provide monitoring agencies with a one-time 

opportunity to review already reported continuous PM2.5 FEM data and request that data 

with unacceptable performance be restricted (retrospectively) from NAAQS 

comparability. Accordingly, in the first year after this rule becomes effective, we 

proposed that monitoring agencies have the option of requesting in their annual 

monitoring network plans that a portion or all of the existing continuous PM2.5 FEM data, 

as applicable, as well as future data, be restricted from NAAQS comparability for the 

period of time that the plan covers.21 In the proposal we stated that annual monitoring 

network plans in subsequent years would only need to cover new data for the period of 

time that the plan covers. 

As noted above, in cases where an agency is operating a PM2.5 continuous FEM 

that is not meeting the expected performance criteria used to approve the FEMs (71 FR 

61285 to 61286, Table C-4, October 17, 2006) when compared to their collocated FRMs, 

an agency can recommend that the data not be used for comparison to the NAAQS. 

However, all required SLAMS would still be required to have an operating FRM (or 

other well performing FEM) to ensure a data record is available for comparison to the 

 

21 Data from any PM2.5 monitor being used to meet minimum monitoring requirements 

could not be restricted from NAAQS comparability.
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NAAQS. In cases where a PM2.5 continuous FEM was not meeting the expected 

performance criteria, and the Regional Administrator has approved a recommendation 

that the FEM data not be considered eligible for comparison to the NAAQS, the data 

would still be required to be loaded to AQS; however, these data would be identified 

distinctly from data used for comparison to the NAAQS. 

The goal of proposing to allow monitoring agencies the opportunity to 

recommend not having data from PM2.5 continuous FEMs as comparable to the NAAQS 

is to ensure that only high quality data (i.e., data from FRMs which are already well 

established and new continuous FEMs that meet the performance criteria used to approve 

FEMs when compared to collocated FRMs operated in each agencies network) are used 

when comparing data to the PM2.5 NAAQS. Under the current monitoring regulations, a 

monitoring agency can identify a PM2.5 continuous FEM as an SPM, which allows the 

monitor to be operated for up to 24 months without its data being used in comparison to 

the NAAQS. While 24 months should be sufficient time to operate the monitor across all 

seasons, assess the data quality, and in some cases resolve operational issues with the 

instrument, it may still leave some agencies with monitors whose data are not sufficiently 

comparable to data from their FRMs. In these cases there may be a disincentive to 

continue operating the PM2.5 continuous FEM, especially in networks where the 

monitoring data are near the level of the NAAQS. With the proposed provision, where a 

monitoring agency can recommend not having data from PM2.5 continuous FEMs be

comparable to the NAAQS, a monitoring agency can continue to operate their PM2.5

continuous FEM to support other monitoring objectives (e.g., diurnal characterization of 

PM2.5, AQI forecasting and reporting), while working through options for improved data 
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comparability while still providing data for comparison to the NAAQS from an FRM.

The EPA believes that an assessment of FEM performance should include several 

elements based on the original performance criteria. The Agency also believes that 

certain modifications to the performance criteria are appropriate in recognition of the 

differences between how monitoring agencies operate routine monitors and how 

instrument manufacturers conduct required FRM and FEM testing protocols. The details 

below summarize these issues. 

The EPA proposed to use the performance criteria used to approve the FEMs (71 

FR 61285 to 61286, Table C-4, October 17, 2006) for those agencies that recommend not 

having data from PM2.5 continuous FEMs be comparable to the NAAQS. To 

accommodate how routine monitoring networks operate, the EPA proposed that agencies 

seeking to demonstrate insufficient data comparability base their assessment mainly on 

collocated data from FRMs and continuous FEMs at monitoring stations in their network. 

The EPA does not believe it is practical to utilize the requirement in table C-4 of 40 CFR 

part 53 for having multiple FRMs and FEMs at each site since such arrangements are not 

typically found in monitoring agency networks. Accordingly, the requirement for 

assessing intra-method replicate precision would be inapplicable. Another consideration 

is the range of 24-hour data concentrations, for instance, the performance criteria in table 

C-4 of 40 CFR part 53, provides for an acceptable concentration range of 3 to 200 μg/m3. 

However, the EPA notes that during an evaluation of data quality from two FEMs (U.S.

EPA, 2011a, p. 4-50), the Agency found that including low concentration data was

helpful for understanding whether an intercept or slope was driving a potential bias in an 

instrument. Therefore, the EPA proposed that agencies may include low concentration 
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data (i.e., below 3 μg/m3) for purposes of evaluating the data comparability of continuous 

FEMs. With regard to the minimum number of samples needed for the assessment, the 

EPA notes that a minimum of 23 sample pairs are specified for each season in table C-4 

of 40 CFR part 53. Having 23 sample pairs per season should be easily obtainable within 

one year for sites with a FRM operating on at least a 1 in 3 day sample frequency and we 

proposed that this requirement be applicable to the assessments being discussed here. For 

sites on a one in 6 day sampling frequency, two years of data may be necessary to meet 

this requirement. The EPA recognizes that it would be best to assess the data based on the 

most recently available information; however, having data across all seasons in multiple 

years will provide a more robust data set for use in the data comparability assessment; 

therefore, the EPA proposed that data quality assessments be permitted to utilize up to the 

last three years of data for purposes of recommending not having data from PM2.5

continuous FEMs be comparable to the NAAQS.

The EPA recognizes that only a portion of continuous PM2.5 FEMs will be 

collocated with FRMs, and it would be impractical to restrict the applicability of data 

comparability assessments to only those sites that had collocated FRM and FEM 

monitors. In these cases, the monitoring agency will be permitted to group the sites that 

are not collocated with an FRM with another similar site that is collocated with an FRM 

for purposes of recommending that the data are not eligible for use in comparison to the 

NAAQS. Monitoring agencies may recommend having PM2.5 continuous FEM data 

eligible for comparison to the NAAQS from locations where the method has been 

demonstrated to provide acceptable data comparability, while also recommending not 

having it eligible in other types of areas where the method has not been demonstrated to 
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meet data comparability criteria. For example, a rural site may be more closely associated 

with aged particles where volatilization issues are minimized resulting in acceptable data 

comparability between filter-based and continuous methods, while a highly populated 

urban site with fresh emissions with higher volatility may result in higher readings on the 

PM2.5 continuous FEM that would not meet the expected performance criteria as 

compared to a collocated FRM. In all cases where a monitoring agency chose to group 

sites for purposes of identifying a subset of PM2.5 continuous FEMs that would not be 

comparable to the NAAQS, the assessment submitted with the annual monitoring 

network plan would have to provide sufficient detail to support the identification of 

which combinations of method and sites would, and would not, be comparable to the 

NAAQS, as well as the rationale and quantitative basis for the grouping and 

recommendation.

Most comments received on this issue were from air agencies. All air agencies 

either supported the proposal or supported it with a recommendation to continue to allow 

for retrospective assessments to be used such that data would not be compared to the 

NAAQS, if such an assessment showed that the data were not of sufficient comparability 

to a collocated FRM such that the continuous FEM should not be compared to the 

NAAQS. One air agency supported the proposal, except though it had reservations about 

how to best group sites together when a particular PM2.5 continuous FEM is not 

collocated with a FRM. 

The EPA notes the support by air agencies to finalize this provision. EPA also 

notes that all commenters who offered input on the retrospective use of assessments were 

supportive of allowing continued retrospective assessments in annual monitoring 
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networks plans so that data may be recommended as excluded from comparison to the 

NAAQS under certain provisions. However, the EPA has some reservations about how 

and under what circumstances such an allowance should be made. The EPA notes the 

concern expressed from one agency about how to best group sites together when 

considering an assessment.

On the issue of whether to allow data collected to be retrospectively excluded 

from comparison to the NAAQS, the EPA notes there are a number of considerations, 

including that several air agencies support such a policy. The EPA has evaluated how this 

issue can be achieved and believes that some consideration should be allowed, but also 

wants to ensure there is a consistent and easily recognizable interpretation of such cases 

where air agencies recommend excluding already collected and reported data. To help 

illustrate the possible outcomes of how this could work consider the following examples. 

Example 1: An agency finds that the bias between a collocated PM2.5 continuous FEM 

and FRM are acceptable, but near the limit of that acceptability and then finds a year later 

that the assessment indicates that the bias is just outside the limit of that acceptability. 

Such relatively small changes where an assessment indicates flipping in or out of the 

acceptable bias are in themselves acceptable since the overall Data Quality Objectives 

(DQOs) can still be met. The overall DQOs can still be met since there are a number of 

other factors that feed into the DQOs such as precision, data completeness, and especially 

sample frequency, which when operating a continuous FEM is a daily sample. Daily 

sampling provides less uncertainty than sampling at the one-in-three day or one-in-six 

day sampling frequencies, which are routinely employed by filter-based FRM samplers. 

Therefore, in this example the existing data should still be compared to the NAAQS, but 
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the air agency should thoughtfully consider whether to recommend22 and the EPA will 

consider whether to approve that any new data from PM2.5 continuous FEMs are used in 

comparison to the NAAQS. If an air agency recommends to not use a PM2.5 continuous 

FEM for comparison to the NAAQS, it would need to ensure another approved method 

(i.e, a filter-based FRM/FEM or other continuous FEM which is performing within 

acceptable performance criteria) is operating at the site or sites of interest. This would be 

expected for all SLAMS, but at a minimum the design value monitoring station for the 

area of interest would be required to have another approved PM2.5 method (i.e., an FRM, 

other filter-based FEM, or other continuous FEM or ARM with acceptable data 

comparability) operating on the required sample frequency or more often for that 

location. Example 2: A PM2.5 continuous FEM operated by an air agency is found to have 

a significant bias compared to a collocated FRM. If the air agency finds cause to 

invalidate the data (e.g., a flow sensor is found to be outside of acceptable limits), then it 

should invalidate the relevant data (i.e., data from the period going back to the last 

successful flow check or audit or other information that points to a cause that the flow 

sensor is not meeting its performance criteria) for all data uses and there is no follow-up 

issue of retrospective analysis. A case of finding cause to invalidate data would be based 

on validation criteria found in an air agencies approved quality assurance project plan 

(QAPP). Example 3: A PM2.5 continuous FEM operated by an air agency and previously 

identified as appropriate to compare to the NAAQS, is found to have a significant and 

unacceptable bias compared to a collocated FRM and there is no other reason to 

invalidate the data. That is, all other information points to the data being valid; however, 

there has been a significant shift in the comparability of the PM2.5 continuous FEM 

 

22 Through the annual monitoring network plan explained in §58.10.
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compared to a collocated FRM (which itself is found to be operating correctly and data 

are valid). A significant shift in the comparability would be noticeable by comparing 

assessments for a site from one year to the next and seeing a significant and unacceptable 

change in one of the key statistical metrics used in the evaluation (i.e., additive or 

multiplicative bias). Such a case of retrospectively recommending not using PM2.5

continuous FEM data should also take into account all other available information that 

can help inform approving such a recommendation as part of an annual monitoring 

network plan. For example, do data from the PM2.5 performance evaluation program data 

also suggest an unacceptable bias for a specific period of interest with this method as 

used in the air agencies network? Note: This type of assessment is often limited by the 

small number of samples taken in the PEP program relative to the large number of 

collocated samples expected when an FRM and PM2.5 continuous FEM are collocated. In 

this type of example, the air agency might want to recommend not using the continuous 

FEM data for comparison to the NAAQS; however, the continuous FEM data could be 

appropriate for use in reporting the Air Quality Index (AQI) or other data uses either as is 

or if statistically correlated23 and corrected back to the collocated FRM. So in this last 

example, the PM2.5 continuous FEM data would be stored separately in the EPA’s data 

system so that they are eligible for use in AQI calculations, but not used in comparison to 

the NAAQS, if approved by the EPA. Again, the air agency should thoughtfully consider 

and state its position and rationale in the annual monitoring network plan on whether any 

future data should be compared to the NAAQS. 

 

23 The EPA has had a long-standing policy of allowing PM2.5 continuous data to be

statistically correlated and corrected to use in AQI reporting. A report is available on this:

See “Data Quality Objectives (DQOs) for Relating Federal Reference Method (FRM) and 

Continuous PM2.5 Measurements to Report an Air Quality Index (AQI), EPA-454/B-02-

002, November 2002”.
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Another issue to consider is the transparent and consistent use of PM2.5

continuous FEM data from a method where one air agency recommends using the data 

for comparison to the NAAQS and another specifically recommends to not use it for 

comparison to the NAAQS. The use of the annual monitoring plans ensures that the 

process is transparent; however, it may not ensure a consistent approach if one agency 

recommends exclusion of data and another agency does not. For example, consider two 

adjacent air agencies operating the same make and model of a PM2.5 continuous FEM, 

where one air agency recommends using data and the other air agency recommends not 

using it for comparison to the NAAQS. While on its face it may seem straightforward 

that a method with acceptable comparability to a collocated FRM should perform 

similarly in other air agency networks where they have similar aerosol composition and 

climate, in practice there are a number of other variables that affect data comparability. 

Such factors that lead to differences in comparability might include differences in 

installation, training, development of SOPs, control of shelter conditions, maintenance of 

the continuous FEM, and performance of the FRMs which are being used as the basis of 

comparison to the continuous FEM. Also, there may be cases where the concentration 

levels are so far away from the level of the NAAQS (either substantially higher or lower) 

that it would not matter if the data are excluded or not, the same NAAQS determination 

would result. The EPA has considered these issues and in general believes that it would 

still be acceptable for one agency to use data for comparison to the NAAQS, while 

another agency does not, even if it’s the same method used in adjacent air agency 

networks. 

On the issue of grouping sites for purposes of allowing monitors that are not 
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collocated to be included when recommending a method should not be compared to the 

NAAQS, the EPA believes that it is not necessary to provide specific details on what 

criteria are necessary to group sites as air agencies are in the best position to determine a 

recommendation of when such sites should or should not be grouped. However, to 

illustrate examples of possible ways to group sites, the air agency could take into account 

factors such as whether the sites are all in either a rural or urban location, since urban 

locations tend to be impacted more directly by fresh emissions which are known to be 

more volatile, or whether there is consistency in the climate for the sites of interest as 

might be the case for sites near a large water body or at a high altitude. The EPA will 

consider these issues when evaluating air agency requests for approval.

The EPA is finalizing its proposal to allow each air agency to specify its intention 

to use or not use data from continuous PM2.5 FEMs that are eligible for comparison to the 

NAAQS as part of their annual monitoring network plan due to the applicable EPA 

Region Office by July 1 each year where adequate FRM data are available. The EPA’s 

approval of an annual monitoring network plan24 as a whole, or in part, will constitute 

concurrence with an air agency’s recommendation to use or not use data from continuous 

PM2.5 FEMs as eligible for comparison to the NAAQS, unless otherwise noted in the 

approval of the plan. The absence of an air agency statement specifying a position on use 

of data from a continuous PM2.5 FEM for comparison to the NAAQS will be interpreted 

as meaning that all such data are applicable for comparison to the NAAQS following the 

provisions in Part 50, Appendix N on data handling and Part 58 on the monitoring 

requirements. In finalizing this decision the EPA will ensure, as proposed, that air 

 

24 Approval of an annual monitoring network plan is subject to approval of the EPA 

Regional Administrator as provided for in §58.10(a)(2).
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agencies can identify already collected data from PM2.5 continuous FEMs that should not 

be used for comparison to the NAAQS. After considering comments in support of 

allowing additional retrospective assessments, the EPA is also finalizing an approach of 

allowing for the continued use of retrospective assessments to inform when already 

collected data should not be compared to the NAAQS, if there has been a significant 

change in the assessment of that data from previous years. 

c. Revoking PM10-2.5 Speciation Requirements at NCore Sites

The EPA proposed to revoke the requirement for PM10-2.5 speciation monitoring 

as part of the current suite of NCore monitoring requirements. The requirement to 

monitor for PM10-2.5 mass (total) at all NCore multi-pollutant sites remains. PM10-2.5 mass 

monitoring commenced on January 1, 2011 as part of the nationwide startup of the NCore 

network (U.S. EPA, 2011a, p. 1-15).

As part of the process to further define appropriate techniques for PM10-2.5 

speciation monitoring, a public consultation with the CASAC AAMMS on monitoring 

issues related to PM10-2.5 speciation was held in February 2009 (74 FR 4196, January 23, 

2009). The subcommittee noted the lack of consensus on appropriate sampling and 

analytical methods for PM10-2.5 speciation and expressed concern that the Agency’s 

commitment to launch the PM10-2.5 monitoring network without sufficient time to analyze 

the data from a planned pilot project was premature (Russell, 2009). Based on the noted 

lack of consensus on PM10-2.5 speciation monitoring techniques, the Agency did 

implement a small pilot monitoring project to evaluate the available monitoring and 

analytical technologies.
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The EPA pilot monitoring project was completed in 2011, with plans to analyze 

the data and prepare a final report on findings and recommendations in 2013. At that 

time, the EPA will consider what PM10-2.5 speciation sampling techniques, analytical 

methodologies, and monitoring design strategies would be most appropriate as part of a 

potential nation-wide monitoring deployment. Such a deployment could be based on the 

NCore multi-pollutant framework or some other strategy that allows flexibility and 

targets measurements in areas with higher levels of coarse particles. 

All comments received from air agencies and multi-state organizations were 

supportive of the removal of the PM10-2.5 speciation requirement. 

A few industry commenters raised concerns about the availability of PM10-2.5

speciation data for research purposes. One environmental group opposed revoking the 

PM10-2.5 speciation requirement and expressed the need for PM10-2.5 data to support health 

effects research and future regulatory efforts. 

The EPA has considered the comments from air agencies that were all supportive 

of revoking the requirement, as well as the industry and environmental groups concerns 

that PM10-2.5 speciation data will not be available for research. In considering these 

comments, the EPA recognizes the importance of efforts to develop and evaluate 

speciation monitoring approaches for PM10-2.5 given that there is relatively little 

information available on the chemical and biological composition of PM10-2.5 and on the 

health effects associated with the various components (U.S. EPA, 2009a, section 2.3.4). 

Without more information on the chemical speciation of PM10-2.5, the apparent variability 

in associations with health effects across locations is difficult to characterize (U.S. EPA, 

2009a, section 6.5.2.3). However, the EPA believes that until a final report on the 
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findings from the pilot study is completed in 2013 and the results of the study can be 

considered, PM10-2.5 speciation is not ready for nationwide deployment. Therefore, the 

EPA is finalizing its decision to revoke the PM10-2.5 speciation requirement at NCore 

stations. Given the continued importance of characterizing PM10-2.5 species, and given 

that ongoing and future research will likely further inform the development of speciation 

methods, the appropriateness of requiring speciation monitoring for PM10-2.5 will be 

revisited in future reviews.

d. Measurements for the Proposed New PM2.5 Visibility Index NAAQS

The EPA proposed requirements for sampling of PM2.5 chemical speciation in 

states with large CBSAs. However, as explained in section VI, the EPA is not finalizing 

the proposed secondary PM2.5 visibility index NAAQS and therefore is not finalizing the 

proposed monitoring changes associated with that standard. 

4. Revisions to the Quality Assurance Requirements for SLAMS, SPMs, and PSD

a. Quality Assurance Weight of Evidence

The EPA proposed to use a weight-of-evidence approach for determining whether 

the quality of data is appropriate for regulatory decision-making purposes. While the 

EPA believes that it is essential to require a minimum set of checks and procedures in 

appendix A to support the successful implementation of a quality system, the success or 

failure of any one check or series of checks should not preclude the EPA from 

determining that data are of acceptable quality to be used for regulatory decision-making 

purposes. Accordingly, the EPA proposed to include additional wording in appendix A to 

clarify the role that appendix A generated data quality indicators have in the overall 

quality system that supports ambient air monitoring activities. 
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The EPA received eight comments on the weight of evidence approach with the 

majority of commenters endorsing the approach. One commenter felt that the “paragraph, 

as written, undermines the importance of the quality control/quality assurance system 

dictated in Part 58.” Some that supported the approach also provided a word of caution 

that “while a common sense approach to the assessment of quality data is important, 

minimum requirements are necessary to ensure scientifically-defensible data is being 

used in decision making”. The EPA agrees with the commenter’s points that data should 

be subject to a minimum set of requirements for data collection, reporting and quality. In 

developing the weight of evidence approach, the EPA is not attempting to diminish the 

requirements of appendix A but rather ensure that other elements of a quality system that 

air agencies implement and are documented in their QAPP can also be used when judging 

whether data are valid for a particular monitoring objective. While the EPA considers the 

appendix A requirements the minimum for reporting, it is not the only data that the EPA 

and the air agencies use to judge quality. Therefore, if an appendix A requirement for 

some reason is not complete, the EPA concludes that it should not necessarily be the sole 

reason to declare the data invalid or unusable. One commenter who felt that the approach 

may be appropriate also suggested that the language of the proposal was vague and may 

weaken the ability of air monitoring agencies to validate their own data and instead 

allows the EPA to make decisions regarding data validity. In the majority of cases when 

the quality of ambient air data is called into question, the EPA Regions and air agencies

work together and reach consensus on data usability. The EPA agrees that the air 

agencies know more about their data and it is the air agencies responsibility to certify the 

data as valid. In most cases, the EPA and the air agencies will be in agreement on the 
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validity and usability of this data. However, since the EPA is responsible for making final 

regulatory decisions concerning the NAAQS, in rare cases it may ultimately have to 

make a validity decision that the air agencies may not agree with. After consideration of

the general support received, the EPA will finalize the language as proposed. For the 

reasons explained above, the EPA concludes that this will not undermine the quality 

assurance system, but rather strengthen it.

A few commenters, although supporting the weight of evidence approach, also 

commented that appendix A minimum requirements should not only apply to all air 

quality data collected by state, local, and tribal agencies, but also to “secondary” data 

collected by other monitoring efforts. The EPA understands that this term is used by these 

commenters to either represent the Chemical Speciation and IMPROVE Network data 

being used to calculate light extinction for the secondary PM2.5 visibility index NAAQS, 

or for criteria pollutant data collected by entities other than the state, local or tribal 

monitoring organizations. The EPA agrees with the comments that the appendix A 

requirements must apply to the CSN and IMPROVE data, if the data were being used for 

comparison to the secondary NAAQS, and included the term “PM2.5 CSN” to refer to 

both networks. However, since as explained in Section VI, the secondary PM2.5 visibility 

index NAAQS is not being finalized, the EPA will be removing any text related to the 

CSN and IMPROVE requirements from appendix A. If the term is being used by 

commenters to refer to criteria pollutant data collected by entities other than the state, 

local or tribal monitoring organizations then the appendix A requirements, as has always 

been the case, apply to those monitors.

b. Quality Assurance Requirements for the Chemical Speciation Network
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The EPA proposed to include requirements for flow rate verifications and flow 

rate audits for the PM2.5 CSN. Air agencies currently perform these audits even though 

they are not currently required. Thus, although they would be considered a new 

requirement, they are not new implementation activities. In addition, the CSN already 

includes six collocated sites which the EPA proposes to include in the 40 CFR part 58 

appendix A requirements. The EPA proposed that PSD sites would not be required to 

collocate a second set of instruments for speciated PM2.5 mass monitoring.

There were no comments that specifically addressed the addition of collocation 

and flow rate requirements in appendix A for the chemical speciation network (CSN). 

Since these flow rates have historically been included in the Agencies’ CSN Network 

Quality Assurance Project Plan and implemented for many years, air agencies may not 

have considered them any additional burden on the program. However, as explained in 

Section VI, the secondary PM2.5 visibility index NAAQS is not being finalized; therefore,

the EPA will not include these QA requirements into appendix A since the networks will 

not produce data to be used for NAAQS decisions. 

c. Waivers for Maximum Allowable Separation of Collocated PM2.5 Samplers and 

Monitors

The EPA proposed to allow waivers, when approved by the EPA Regional 

Administrator, for collocation of PM2.5 samplers and monitors of up to 10 meters so long 

as the site is at a neighborhood scale or larger. The EPA proposed to allow waivers for 

the maximum allowable distance associated with collocated PM2.5 samplers and

monitors. Ensuring PM2.5 continuous FEMs and PM2.5 FRMs meet collocation 

requirements (i.e., 1 to 4 meters for PM2.5 samplers with flow rates of less than 200 

liters/minute) can be challenging, since in some cases multiple instruments, FEMs
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installed in the shelter and FRMs installed on a platform, are being sited at the same 

station. The EPA believes that instruments spaced farther apart could be maintained

within the operational precision of the instruments, especially at sites located at larger 

scales of representation (e.g., neighborhood scale and larger). 

All comments received responded in support of the requirement allowing up to 10 

meter horizontal spacing for sites at a neighborhood or larger scale of representation. The 

EPA received no negative comments on this part of the proposal. During stakeholder 

presentations of the proposal, the EPA received a verbal comment that air agencies were 

also having difficulty meeting the one meter vertical criteria since PM2.5 FEMs are 

typically housed in shelters with inlets extending through shelter roofs while the 

collocated FRM monitors are placed outside, usually on platforms somewhat lower to the 

ground. After considering this comment, and further discussion with EPA Office of 

Research and Development on spacing requirements, the agency will amend the appendix

A requirements to allow for a 1-3 meter vertical spacing which may be approved by the 

Regional Administrator for sites at a neighborhood or larger scale of representation. In 

addition, the language will be amended to allow for waiver approvals during annual 

network plan approval processes. Alternatively, the existing waiver provision outlined in 

paragraph 10 of Appendix E may be used. 

5. Revisions to Probe and Monitoring Path Siting Criteria 

a. Near-road Component to the PM2.5 Monitoring Network

The EPA proposed that the probe and siting criteria for the near-road component 

of the PM2.5 monitoring network design follow the same probe and siting criteria as the 

NO2 near-road monitoring sites. These requirements would provide that the monitoring 
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probe be sited “...as near as practicable to the outside nearest edge of the traffic lanes of 

the target road segments; but shall not be located at a distance greater than 50 meters, in 

the horizontal, from the outside nearest edge of the traffic lanes of the target road 

segment” (section 6.4 of appendix E to 40 CFR part 58). 

The EPA received comments from several stakeholders on the probe and siting 

criteria for PM2.5 monitors in the near-road environment. One public health group offered 

detailed comments on the probe and siting criteria for PM2.5 monitors in near-road 

environments. While the commenter offered support for collocating the PM2.5 monitors 

with NO2 monitors in the near-road environment, there was concern expressed regarding 

allowing monitors at sites of more than 15 meters from the traffic corridor which is the 

source of the air quality concern. The commenter points out that the EPA’s existing rules 

for siting localized hot spot sites in areas of highest concentration require sites at 

microscale locations which provide for a distance of no more than 15 meters from a 

major roadway. Several air agencies offered consistent comments that the inlet of the 

PM2.5 monitors should be the same as that of the near-roadway NO2 monitors; however, 

one of the commenters suggested that the requirements for distance to the nearest vertical 

wall or obstruction should match the requirements for current micro and middle scale 

installations of PM2.5 monitors. The concern expressed is that a wall or obstruction may 

disrupt the normal downwind flow across a roadway. 

In reviewing comments on probe and monitoring path criteria for PM2.5 monitors 

in near road environments, and whether to make any changes, the EPA has several issues 

to consider. One of the most important things to consider is what the intended network 

design of these monitors should be. As stated in the proposal our goal is to “better 
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understand the health impacts of these (near-road PM2.5) exposures,” that a number of 

monitoring objectives can be supported by having near-road PM2.5 monitors, and that 

while it might be that the location of maximum concentration of PM2.5 exposure from 

near-roadway sources might differ from the maximum location of NO2 or other 

pollutants, we proposed to require that the near-road PM2.5 monitors be collocated with 

the planned NO2 monitors. The EPA did not propose to change the distance from 

obstructions for PM2.5 monitors in its proposal. 

As we stated in the proposal, the planned NO2 monitors are using several relevant 

factors that are also relevant for siting of PM2.5 (e.g., average annual daily traffic and fleet 

mix [accounting for heavy duty vehicles] by road segment) and that significant thought 

and review are going into the design of the near-road stations. Therefore, the EPA is not 

persuaded that we should provide any additional constraints to the siting of the station 

(i.e., the distance from the roadway) than is already provided for in the NO2 near-road 

monitor probe and monitoring path siting criteria. The EPA is concerned that additional 

constraints (i.e., to require sites within 15 meters of the road), might have some 

advantages, but also might unnecessarily eliminate otherwise useful near-road locations 

that on balance might be a better candidate location. 

The EPA recognizes that there may be cases where the physical location of a 

near-road monitoring station is farther than 15 meters, but no greater than 50 meters from 

the roadway, but such cases are presumed to still be the most useful location for the siting 

of the NO2 monitors, which we then proposed to collocate with PM2.5. Regardless of the 

actual distance of the inlet for the PM2.5 monitor at the near-road monitoring station, so 

long as it is collocated with the approved near-road station for NO2 and meets existing 
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criteria, the EPA will consider this site to be appropriate as a near-road PM2.5 monitoring 

station. As explained in the proposal, there are a number of reasons to collect multipollutant data in the near-road environment. The EPA believes that these sites will be 

sufficient as representative maximum concentration sites for NO2 and PM2.5 in the nearroad environment. As noted above, where an air agency believes a different location is a 

more appropriate site for a near-road PM2.5 monitor, the EPA can use its discretion in 

approving a deviation from the PM2.5 monitoring requirements as already exists in the 

network design criteria. A deviation would be appropriate for consideration where, for 

example, a state provides quantitative evidence demonstrating that peak ambient PM2.5

concentrations would occur in a near-road location which meets siting criteria but is not a 

near-road NO2 monitoring site. Such deviations are to be approved by the Regional 

Administrator as described in section 4.7.1 of Appendix D to part 58.

While it is still desirable for the near-road stations to be as close to the road as 

practical, there may be differences in the scale of representation of the near-road PM2.5

monitor from one location to another, while the NO2 near-road monitors are at the same 

scale of representation (i.e., micro-scale) in different locations. This is a result of the 

scale of representation being based on the pollutant at a location and not the location 

alone. Therefore, in cases where the station is 20 meters from a major road, the NO2

measurement may still be micro-scale, while the PM2.5 measurement would be middlescale if the average daily traffic count were sufficiently large enough.25 If a site with both 

measurements were 10 meters from a major road they would both be expected to be 

micro-scale sites.

 

25 See Table E-1 in Appendix E to Part 58 for defining the scale of representation of a PM 

sampler based on its distance to the nearest traffic lane and average daily traffic count.
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In considering the comment on distance from obstructions, the EPA notes that a 

monitoring station with multiple measurements is effectively considered collocated for 

those measurements, even though the actual location of the inlets is slightly different 

from each other within the station. For example, a gas monitor (e.g., for carbon 

monoxide) may be pulling ambient air from a manifold with an inlet located on one part 

of a station roof, while a PM monitor is pulling air directly from its inlet located a few 

meters away on the same roof. The EPA believes it is appropriate and consistent with the 

public comment above on distance from obstructions to maintain the existing 

requirements for distance from obstructions on a pollutant by pollutant basis, even if they 

are different for PM2.5 and NO2 monitors that will be at the same station. Air agencies 

will need to consider these distances from obstructions for each pollutant inlet probe (i.e., 

>1 meter for NO2 monitors and >2 meters for PM2.5 monitors) in locating monitors at the 

station.

The EPA is maintaining the existing probe and siting criteria for PM2.5 monitors; 

however, we are finalizing the provision that the required near-road component of the 

PM2.5 monitoring network design shall be collocated with a required NO2 monitor at 

near-road monitoring station. These near-road NO2 monitoring stations are to be sited 

“...as near as practicable to the outside nearest edge of the traffic lanes of the target road 

segments; but shall not be located at a distance greater than 50 meters, in the horizontal, 

from the outside nearest edge of the traffic lanes of the target road segment” (section 6.4 

of appendix E to 40 CFR part 58). The EPA is retaining the existing requirement that 

PM2.5 inlets, including those at near road stations, must be >2 meters from obstructions.

b. CSN Network
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As explained in Section VI, the EPA is not finalizing the proposed secondary 

standard based on a PM2.5 visibility index and therefore will not be finalizing probe and 

siting criteria associated with the use of CSN measurements. 

c. Reinsertion of Table E-1 to Appendix E

The EPA proposed to reinsert table E-1 to appendix E of 40 CFR part 58. This 

table presents the minimum separation distance between roadways and probes or 

monitoring paths for monitoring neighborhood and urban scale ozone (O3) and oxides of 

nitrogen (NO, NO2, NOx, NOy). This table was inadvertently removed during a previous 

CFR revision process. 

The only comments received on this topic were supportive of the reinsertion of 

table E-1; therefore, the EPA is finalizing the reinsertion of this table, unchanged from its 

prior iteration, back into the CFR. 

6. Additional Ambient Air Monitoring Topics

a. Annual Monitoring Network Plan and Periodic Assessment

In October of 2006, the EPA finalized new requirements for each state, or where 

applicable, local agency to perform and submit to their EPA Regional Offices an 

Assessment of the Air Quality Surveillance System (40 CFR 58.10). This assessment is 

required every five years. The first required five year assessments were due to EPA 

Regional Offices by July 1, 2010. The assessments are intended to provide a 

comprehensive look at each monitoring agency’s ambient air monitoring network to 

ensure that the network is meeting the minimum monitoring objectives defined in 

appendix D to 40 CFR part 58, whether new sites are needed, whether existing sites are 

no longer needed and can be terminated, and whether new technologies are appropriate 
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for incorporation into the ambient air monitoring network.26

Since each agency has completed its first required five-year assessment, and 

several monitoring rule requirements have either been added or changed since this 

requirement was added in 2006, the EPA thought it was appropriate to review this 

requirement and solicit comment on any possible changes the EPA should consider that 

may improve the usefulness of the assessments. Specifically, the EPA solicited comment 

on ways to either streamline or add additional criteria for future assessments. 

The EPA also proposed to remove references to “community monitoring zones” 

and “spatial averaging” in the annual monitoring network plans due to EPA Regional 

Offices by July 1 of each year. The Agency proposed to remove these references since, as 

discussed in section VII.A.2 above, the EPA proposed to remove all references to the 

spatial averaging option throughout 40 CFR part 50 appendix N. Consistent with these 

changes, the EPA also proposed to remove references to community monitoring zones 

under the annual monitoring network plans described in 40 CFR 58.10. 

The EPA received comments from several air agencies on the five year 

assessments. Most comments on the five year assessments focused on the type and 

usefulness of assessment tools made available to air agencies during the last review. Of 

specific note were concerns that assessment tools used to evaluate networks on a regional

or national basis do not provide the spatial resolution necessary to adequately assess state 

networks on a scale most useful to air agencies. This is especially true when attempting to 

evaluate smaller scale monitoring or pollutant gradients associated with near-road and 

source oriented monitoring. Suggestions for improvement identified the need for the EPA 

 

26 The EPA provides a link to these assessments on EPA’s website at: 

http://www.epa.gov/ttn/amtic/plans.html. A detailed description of the requirements for 

the assessments is described in 40 CFR 58.10.
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to work closely with air agencies early in the next cycle of assessments (due in 2015) so 

that any tools developed can be of benefit to the questions air agencies need to address 

for their programs. The EPA did not receive any comments on removing references to 

community monitoring zones specifically as it pertains to their listing in the annual 

monitoring network plans described in 40 CFR 58.10.27

The EPA took comment on potential improvements to the five year assessments. 

All the recommendations received focused on the types of assessments to perform and 

ensuring that the EPA works closely with air agencies so that assessments will be of 

benefit to the air agencies. No specific recommendations were made to add or remove 

any of the requirements of the five year assessments and consequently the EPA is not 

making any changes. The EPA intends to work with air agencies to ensure future tools 

are as helpful as practible. 

Consistent with the decision to end the practice of spatial averaging, the EPA is 

finalizing the removal of language that references “community monitoring zones” and 

“spatial averaging” in the annual monitoring network plans due to EPA Regional Offices 

by July 1 of each year. 

b. Operating Schedules

The EPA generally requires PM2.5 SLAMS to operate on at least a 1-day-in-3 

sampling schedule, unless a reduced sampling frequency is approved such as might be the 

case with a site that has a collocated continuous operating PM2.5 monitor.28 However, in 

the 2006 monitoring rule amendments, the EPA finalized a new requirement for the 

 

27 Comments on the substantive question of whether to revoke references to community 

monitoring zones were addressed in section VIII.B.1.

28 All NCore stations must operate on at least a one-in-three day sample frequency for 

filter-based PM sampling.
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operating schedule of PM2.5 SLAMS sites (40 CFR 58.12). The new requirement stated 

that sites with a design value within plus or minus five percent of the 24-hour PM2.5

NAAQS must have an FRM or FEM operating on a daily sampling schedule. This 

requirement was included to minimize any statistical error associated with the form of the 

24-hour PM2.5 NAAQS (i.e., the 98th percentile). In section III.F, the Administrator is 

finalizing revisions to the level of the primary annual PM2.5 NAAQS. Accordingly, 

possible changes to sampling frequency requirements were also considered. 

The EPA had previously considered how sample frequency affects the Data 

Quality Objectives in a consultation with the CASAC AAMMS in September of 2005 (70 

FR 51353 to 51354, August 30, 2005). As a result of that consultation, the EPA proposed 

(71 FR 2710 to 2808, January 17, 2006) and finalized (71 FR 61236 to 61328, October 

17, 2006) changes to the sample frequency requirements as part of the monitoring rule 

changes in 2006. In that work, the EPA demonstrated that having a higher sample count 

is generally more useful to minimize uncertainty for a percentile standard than an annual 

average. Given the decision to strengthen the primary annual PM2.5 NAAQS and the 

known burden of performing daily sampling using the filter-based samplers that are still a 

mainstay in monitoring agency networks, the issue of needing daily sampling for sites 

that have design values close to the level of the 24-hour PM2.5 standard was reconsidered 

if the site already has a design value above the level of the primary annual PM2.5

NAAQS. 

In a related issue, since the EPA finalized the requirement for daily sampling at 

sites within 5 percent of the 24-hour PM2.5 NAAQS in 2006, there has been confusion 

over the procedures for adjusting sample frequencies, where necessary, to account for 
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variations in year-to-year design values. Therefore, the EPA proposed to revise this 

requirement in the following ways: (1) The EPA proposed that monitors would only be 

required to operate on a daily schedule if their 24-hour design values were within five 

percent of the 24-hour PM2.5 NAAQS and the site had a design value that was not above 

the level of the annual PM2.5 NAAQS. (2) The EPA proposed that review of data for 

purposes of determining applicability of this requirement at a minimum be included in 

each agency’s annual monitoring network plan described in 40 CFR 58.10 based on the 

three most recent years of ambient data that were certified as of the May 1 annual 

deadline. However, monitoring agencies may request changes to sample frequency at any 

time of the year by submitting such a request to their applicable EPA Regional Office. 

Changes in sampling frequency are expected to take place by January 1 of the following 

year. Increased sampling is expected to be conducted for at least three years, unless a 

reduction in sampling frequency has been approved in a subsequent annual monitoring 

network plan or otherwise approved by the Regional Administrator. 

Comments received on the sample frequency requirements for PM2.5 were from 

air agencies, who were generally supportive of the EPA’s proposed approach.

The EPA is finalizing its proposal to modify the sample frequency requirements 

for triggering daily sampling so that only those areas with 24-hour design values within 

five percent of the 24-hour PM2.5 NAAQS and where the design value site is not above 

the level of the annual PM2.5 NAAQS would be required to operate on a daily sample 

frequency. The EPA is also finalizing all other aspects of this part of the proposal.

c. Data Reporting and Certification for CSN and IMPROVE Data
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The EPA is not finalizing its proposal on minor changes to reporting and 

certification of data associated with CSN and IMPROVE networks since as explained in 

Section VI, EPA is not finalizing a secondary standard to support visibility impairment 

that would have used CSN and IMPROVE data.

d. Requirements for Archiving Filters

The EPA proposed to extend the requirement for archival of PM2.5, PM10, and 

PM10-2.5 filters from manual low-volume samplers (samplers with a flow rate of less than 

200 liters/minute) at SLAMS from one year after data collection to five years after data 

collection. The archive of low-volume PM filters is an important resource for on-going 

research and development of emission control strategies and for use in health and 

epidemiology research. During a workshop on Ambient Air Quality Monitoring and 

Health Research in 2008, retaining filters for laboratory analysis was identified as a key 

recommendation to provide daily measurements of metals and elements (U.S. EPA, 

2008d, pp. 17 to 21). The EPA’s previous requirement of one-year is not sufficiently long 

for retrospective analysis of important episodes and for use in long-term epidemiology 

research. Since initially requiring filter archival of low-volume PM filters in 1997, the 

EPA has always recommended longer archiving of filters and most agencies are already 

doing so. However, a small number of agencies have reported discarding older filters, 

despite the minimal cost of storing these filters. Since cold storage of a large number of 

filters may be cost prohibitive and of little benefit in retaining key aerosol species in the 

x-ray fluorescence (XRF) analyses, the EPA proposed to minimize the costs of retaining 

filters by only requiring cold storage during the first year after sample collection. 

All comments received on this issue were from air agencies, which were largely 
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supportive of such a change to this requirement. One air agency did report that it would 

present a hardship to store filters for such a long period of time as they did not have the 

room to support such a requirement.

The EPA is finalizing the requirement for archival of PM2.5, PM10, and PM10-2.5

filters from manual low-volume samplers (samplers with a flow rate of less than 200 

liters/minute) at SLAMS for a minimum of five years after data collection, with cold 

storage only required for the first 12 months of archiving. EPA will work closely with air 

agencies through its EPA Regional Offices and laboratories to support any air agency 

unable to store filters for the new five year requirement.