In the PA (U.S. EPA, 2022b, section <IP_ADDRESS>), the EPA analyzed the quality of data from FRM samplers and continuous PM2.5 FEM monitors operating in routine networks to determine whether they meet the MQOs for PM2.5 FRMs and FEMs (40 CFR Part 58, Appendix A, section <IP_ADDRESS>): "Measurement Uncertainty for Automated and Manual PM2.5 Methods. The goal for acceptable measurement uncertainty is defined for precision as an upper 90 percent confidence limit for the coefficient of variation (CV) of 10 percent and +-10 percent for total bias." When aggregating data across the country, all PM2.5 FRMs meet the MQOs for these methods. But of PM2.5 continuous FEMs aggregated across the country, some meet the MQOs, and others do not.
One of the major challenges to ensuring uniform data from PM methods is that there are no accepted standards against which to calibrate PM methods. This was discussed in the 2004 Air Quality Criteria for Particulate Matter (U.S. EPA, 2004b). PM reference methods typically include the design and performance requirements set forth in the 40 CFR Part 50. This is a contrast to FRMs and FEMs for gaseous NAAQS pollutants for which there are accepted calibration standards; in the case of ozone, there is even a standard reference photometer that can be used to calibrate approved methods in the field or laboratory. For PM monitoring methods, in the absence of accepted calibration standards, acceptable data quality is determined by comparing to other PM FRMs. One challenge to comparing to other PM FRMs during the initial field testing for purposes of FEM approval is that the dataset will in almost all cases be substantially more limited than what's available in routine networks once deployed. Thus, we seek to encourage instrument manufacturers of approved FEMs to evaluate data in routine networks and consider improvements to their FEM calibration, as needed.
The EPA is proposing to use routine and collocated FRM data operated by State, local and Tribal agencies as the basis to update factory calibrations. Routine State, local and tribal agency FRM data form the largest portion of the monitored air quality data used in epidemiologic studies that are being used to inform proposed decisions regarding the adequacy of the public health protection afforded by the primary PM2.5 NAAQS, as discussed in section II above. While the EPA is proposing to use routine FRM data, there are other reference datasets that could be considered. For example, the agency has an FRM audit program operated by independent operators and laboratories. This program is highly valuable to the success of the PM2.5 monitoring program by providing independent data to assess the quality of routinely operated FRMs and FEMs.  If we used the audit program data as the basis for calibrating continuous monitors, we would lose the ability to collect independent data from audit monitors to assess the operation of routine monitors. Therefore, by using routinely operated FRMs to calibrate continuous FEMs, the Agency will continue to maintain the independence of the FRM audit program to assess the quality of routinely operated FRM and continuous FEM data. The EPA also has chemical speciation data available at sites where the Chemical Speciation Network (CSN) or IMPROVE samplers are operated; however, these samplers use technologies that operate at different flow rates and with different-size selective devices than approved FRMs, and neither of these programs use FRMs as the basis to collect samples. Therefore, while CSN and IMPROVE data can be useful to help determine the aerosol chemistry of PM2.5 and may provide additional validation of collocated FRM or FEM data, by themselves these data are not appropriate to update factory calibration of continuous FEMs.
The EPA proposes to direct instrument companies and other interested stakeholders to the EPA's Air Quality System (AQS) database to access the valid routine network data that the Agency proposes to allow for use in updating factory calibration of continuous FEMs. There are several ways to obtain data from the AQS database, and many do not require registration. For example, daily processed datasets by year are publicly available at the web site of "Pre-Generated Data Fields." The data utilized would need to be valid PM FRM and FEM data that are collocated and aligned to the same date. For example, for PM2.5 mass concentrations, there are files by year for "PM2.5 FRM/FEM Mass" identified with a parameter code of 88101. This information, already aggregated to daily data, represent the time-period of midnight-to-midnight local standard time. While any years of data may be considered, instrument companies should normally use at least two years of recent data where we are past the certification period for the previous-year data, which is May 1[st] of each year. Including at least two years of data is intended to address cases where one of the years may have high or low air quality concentrations. Data in the current year and previous year when we are not past the May 1[st] certification date can be considered to test data with a correction established from a previous year or more than one year. If multiple factors are included, any new statistical correction or corrections should be based on one or more calendar years, with independent testing of that data on another year or more that was not used to develop the equation(s).
The EPA also encourages instrument companies to consider and implement all the ways to optimize PM2.5 FEMs. This may include, but is not limited to, whether a method's data can be improved by operating the FEM inside an HVAC-controlled shelter or outside with minimal or no HVAC control; optimizing heating of the airstream to avoid condensation while retaining semi-volatile PM captured on the FRM; and any specialized guidance or training that may help monitoring agencies optimize their data quality and comparability to collocated FRMs. Other options might include updates to unique coefficients used in the factory calibration such as the density of the aerosol, where applicable. Such changes would normally need to be approved by the EPA according to existing rules found at 40 CFR § 53.14.
Another challenge to consider is how to deal with potential outliers that may exist in the validated State, local, and Tribal agency network data available from AQS that would be used to establish new factory calibrations. One of the reasons to use data from the AQS database is that there are tens of thousands of collocated data pairs available that include many of the approved continuous PM2.5 FEMs. Having a large data set will diminish the effect of any one or more outliers. However, acknowledging that the goal of this proposed change is to update factory calibrations to increase the number of routinely operating FEMs meeting MQOs across the networks in which they are operated, we propose that instrument companies may, but are not required to, check for and exclude any potential outliers. Additionally, we propose that the range of data may be limited to those concentrations that are within the normal operating ranges of most sites, but this is not required. This approach, for example, could include 24-hour average PM2.5 concentrations up to the level of the primary 24-hour PM2.5 NAAQS or some percentile above that level (e.g., 125% of the 24-hour NAAQS). The rationale for this is that there are very few sites with routine concentrations above the level of the primary 24-hour PM2.5 NAAQS, and the establishment of any equation with this data would need to be constructed carefully to avoid having data below the primary annual PM2.5 NAAQS drive the coefficients used above the level of the primary 24-hour PM2.5 NAAQS.
Ideally, the geographic coverage of the data used in establishing a new factory calibration would be national in scope; however, instrument companies can only use the data that is available. For widely used PM2.5 FEMs, this will not be an issue, but for less-operated PM2.5 FEMs, there may be limitations in the geographic scope of data produced. Another challenge may be a large grouping of sites in one part of the country that drives development of an equation used across all networks. Instrument companies may limit the use of sites with large groupings in one or more geographical area so that the data are more geographically representative across the network so long as there is a reasonable rationale as to why data from certain sites are not being included. With a new factory calibration available, instrument companies will need to test the performance of the updated calibration across a variety of sites. Testing of an updated factory calibration can be accomplished by utilizing a different year or years other than the time-period used to establish the revised factory calibration or a subset of data across all years. Testing should also include the range of sites in which the method is used.
Building off the geographic location of the sites in which an updated factory calibration is tested with previously collected data, the EPA considered what performance level should be acceptable. Ideally, an updated factory calibration would work such that a significantly larger number of, or all, individual sites operating with the updated factory calibration would meet the MQOs. However, due to several complicating factors such as seasonal changes in temperature and humidity, elevation, differences in aerosol composition, and differences in concentration between more polluted urban sites and relatively cleaner rural sites (some of which read well below the proposed revisions to the level of the primary annual PM2.5 NAAQS discussed in section II above), the EPA should not expect that every site will necessarily meet the MQOs. Therefore, the goal of this proposal is to increase the number of routinely operating FEMs meeting MQOs across the networks in which they are operated, especially for sites near the level of the NAAQS proposed elsewhere in this proposal. Since there are multiple MQOs to consider, the EPA proposes to place the most attention on improvements to the bias MQO goal because this statistic will likely have the most influence on improving the resultant data collected. In attempting to address this goal, instrument companies may be interested in testing their original data used in field studies of their candidate FEMs with an updated factory calibration. While this could be a useful exercise to understand the sensitivity of the original and any updated factory calibration, the EPA proposes not to require meeting the performance criteria of the original field testing as a condition of approving an updated factory calibration.
Regarding how frequently factory calibrations should be updated, the EPA believes it would be most appropriate to not define a specific time-period for updates. Rather, updates should be based on the available of quality data being produced across the network. Monitoring agencies routinely check their data comparability to collocated FRMs, including as part of annual data certification where an AMP-256 report describing data quality is included as part of the certification package (§ 58.15(c)). In addition, monitoring agencies typically provide a more thorough review of their networks and accompanying data quality as part of the five-year assessments due to the EPA pursuant to 40 C.F.R § 58.10(d).
Another important aspect to implementing updated factory calibrations is the treatment of data already collected under the original factory calibration. There are two time periods to consider. First, there is the time-period before the EPA approves an updated to a factory calibration. We propose that data collected prior to an approved update to a factory calibration be allowed to remain as measured based on the factory calibration that was approved at the time the data was collected. Second, there is the time-period between when an updated factory calibration is approved by the EPA and when that updated calibration is implemented in the field. While ideally, this time-period would be short, there may be reasons why some agencies and the sites they run cannot easily update the firmware with the updated factory calibration. We solicit comment on how to handle these situations and whether there should be an allowance to correct such data.
The EPA sought early input from State, local, and Tribal monitoring agencies (NACAA Monitoring Committee 01/20/22; AAPCA Ambient Monitoring Committee 01/26/2022; Tribal air quality professionals call on 02/17/22) regarding how best to address the issue of some PM2.5 FEMs having bias issues. Many monitoring agencies identified that they strongly favor a national solution that can be accomplished and implemented through a firmware upgrade or similar resolution that is consistent with the approach described above. One State suggested that the EPA should consider and allow site-by-site corrections between FRM and collocated FEMs with ongoing collocation at a 1:6 sample frequency for FRMs. The rationale for site-by-site corrections was that there are differences in the types of aerosol composition and concentration between urban and rural locations and having site-by-site corrections would ensure that each type of location is individually calibrated to a collocated FRM rather than to a consistent factory calibration that may average out any differences. In contrast, other monitoring agencies expressed concern about the challenges of implementing a site-by-site approach, especially for those agencies who stated that they would not be able to redeploy the FRMs that would be necessary to perform the site-by-site corrections in their networks for reasons including no longer having FRMs, not having staff available to support and operate the FRMs, and no longer have gravimetric laboratory capacity to support a larger inventory of FRMs operating in their networks.
The CASAC also provided input on the FEM bias issue. As part of their review of the draft PA, the CASAC stated that "the FEM bias needs to be addressed to make the FRMs and FEMs more comparable" (Sheppard, 2022a, p. 2 of consensus responses). The CASAC offered two options for the EPA to consider. "One option would be to allow states to develop correction factors for co-located FRMs and FEMs. These correction factors could be used to adjust FEM concentrations downward (or upward) to be comparable to FRMs. Another option would be for the EPA to revise the `equivalency box' (EB) criteria used to judge whether the bias of a new continuous PM2.5 monitor relative to an FRM is acceptable during field testing" (Sheppard, 2022a, p. 2 of consensus responses). The CASAC's first option is consistent with the input received during early input described above. The EPA believes that the second option should be considered in future reviews of the PM NAAQS to help establish updated goals for data quality from PM2.5 FEMs. The existing network of commercially available PM2.5 FRMs and some of the continuous FEMs are already meeting the MQOs at the existing concentrations, which are at or below the proposed revisions to the level of the primary annual PM2.5 NAAQS discussed in section II above. However, the EPA recognizes that not all PM2.5 FEMs are meeting MQOs and, therefore, the EPA intends to address improvements to existing FEMs that are not meeting MQOs as described above.
In attempting to address the comparability of PM2.5 FEMs to collocated FRMs through our proposal to allow updates to factory calibrations, the EPA recognizes that other potential solutions do not need to be mutually exclusive. That is, there can be multiple approaches to improve the comparability of PM2.5 FRMs to continuous FEMs. Therefore, the EPA solicits comment on additional ways to improve PM2.5 data comparability between PM2.5 FRMs and collocated continuous FEMs.
The EPA encourages early dialogue with instrument companies considering an update to any part (e.g., hardware, software, and/or firmware revision) of an approved FEM designation.  Dialogue with the EPA as well as applications by instrument manufactures can be initiated by contacting the EPA ORD's Reference and Equivalent (R&E) Methods Designation program. The contact information for this can be found at 40 CFR § 53.4, "Applications for reference or equivalent method determinations."
In summary, the EPA is proposing that valid State, local, and Tribal air monitoring data generated in routine networks and submitted to the EPA may be used to update factory calibrations included as part of approved FEMs. This approach, initiated by instrument manufacturers, subject to EPA approval, would be implemented as a national solution in factory calibrations of approved FEMs through a firmware update. This would apply to any PM FEM methods (i.e., PM10, PM2.5, and PM10-2.5). As part of this process, the EPA proposes that a range of data based on the most representative concentrations up to all available concentrations may be used in developing and testing a new factory calibration, that a representative set of geographic locations can be used, that outliers may be included or not included, that a new factory calibration should be developed using data from at least two years and tested on a separate year(s) of data, that updates to factory calibrations can occur as often as needed, and should be evaluated by monitoring agencies as part of routine data assessments such as during certification of data and five year assessments, that the EPA recognizes only data from existing operating sites is available, and that an updated factory calibration does not have to work with the original field study data submitted that led to the designation as an FEM. The EPA solicits input on this approach and any alternatives that would lead to more sites meeting the bias MQO with automated FEMs, especially for those sites that are near the level of the primary annual PM2.5 NAAQS, as proposed to be revised in section II above.
4. Proposed Amendment to the PM2.5 Monitoring Network Design Criteria to address At-Risk Communities
To enhance protection of air quality in communities subject to disproportionate air pollution risk, particularly in light of the proposed range for a revised PM2.5 annual standard, the EPA proposes to modify our PM2.5 monitoring network design criteria to include an environmental justice factor that accounts for proximity of populations at increased risk of adverse health effects from PM2.5 exposures to sources of concern. Specifically, the EPA proposes to modify our existing requirement (40 CFR Part 58, Appendix D, section 4.7.1(b)(3)): "For areas with additional required SLAMS, a monitoring station is to be sited in an area of poor air quality," to additionally address at-risk communities with a focus on anticipated exposures from local sources of emissions. The scientific evidence evaluated in the 2019 ISA and ISA Supplement indicates that sub-populations at potentially greater risk from PM2.5 exposures include: children, lower socioeconomic status (SES) populations, minority populations (particularly Black populations), and people with certain preexisting diseases (particularly cardiovascular disease and asthma). The EPA is proposing that communities with relatively higher proportions of sub-populations at greater risk from PM2.5 exposure within the jurisdiction of a state or local monitoring agency should be considered "at-risk communities" for these purposes.
The PM2.5 network design criteria has led to a robust national network of PM2.5 monitoring stations. These monitoring stations are largely in Core-Based Statistical Areas (CBSAs) across the country that include many PM2.5 monitor sites in at-risk communities.  Many of the epidemiologic studies evaluated in the 2019 ISA and ISA Supplement, including those that provide evidence of disparities in PM2.5 exposure and health risk in minority populations and low SES populations, often use data from these existing PM2.5 monitoring sites. However, we anticipate that if the level of the annual NAAQS is lowered, characterizing localized air quality issues may become even more important around local emission sources. The EPA believes that adding a network design requirement to specifically locate monitors in at-risk communities will improve our characterization of exposures for at-risk communities where localized air quality issues may exist. Requiring the siting of PM2.5 monitoring stations in at-risk communities allows other methods to be operated alongside PM2.5 measurements to support multiple monitoring objectives (40 CFR part 58, Appendix D, section 1.1). The EPA believes that it is appropriate to formalize the monitoring network's characterization of PM2.5 concentrations in communities at increased risk, to provide these areas with the level of protection intended with the PM2.5 NAAQS. The addition of this requirement will also lead to enhanced local data that will allow regulatory air quality agencies to assist communities to reduce exposures and to help inform future implementation and reviews of the NAAQS.
As described in section II.B.2 above and in more detail in the PA (U.S. EPA, 2022b, section 3.3.2), the public health implications of health effects associated with PM2.5 in ambient air are dependent on the type and severity of effects, as well as the size of the population affected and whether there are populations and/or lifestages at increased risk of a PM2.5-related health effect. The 2019 ISA cites extensive evidence indicating that "both the general population as well as specific populations and lifestages are at risk for PM2.5-related health effects" (U.S. EPA, 2019, p. 12-1). Factors that may contribute to increased risk of PM2.5-related health effects include lifestage, pre-existing diseases (cardiovascular disease and respiratory disease), race/ethnicity, and socioeconomic status. The increased risk faced by these sub-populations raises environmental justice concerns. Section II of this preamble, section 12.5 of the 2019 ISA (U.S. EPA, 2019a) and section 3.3.3 of the ISA Supplement (U.S. EPA, 2022a) provide extensive discussion on the evidence for disparities in PM2.5 exposures and PM2.5-related health risks of these sub-populations.
Consistent with the requirement of the Clean Air Act to protect sensitive sub-populations, the EPA is particularly concerned with protecting sub-populations identified as being at higher risk of adverse health effects from PM2.5 exposure in the 2019 ISA, ISA Supplement and PA (U.S. EPA, 2019a; U.S. EPA, 2022a; U.S. EPA, 2022b). The EPA finds it appropriate to better characterize the localized air quality in communities with relatively higher proportions of these sub-populations to ensure these sub-populations receive the intended level of protection of a revised NAAQS proposed earlier in section II. Thus, the EPA is proposing to modify the PM2.5 ambient monitoring network design criteria to add a provision pertaining to sub-populations identified as at increased risk for PM2.5 exposures and health risks associated with PM2.5 ("at-risk communities").
An enhanced network should include representation of at-risk communities who live near emission sources of concern such as, but not limited to, major ports, rail yards, airports, industrial areas, or major transportation corridors. The EPA finds it appropriate, in light of the evidence of increased risk to these communities, to better characterize exposures given proximity to local sources of concern. For example, the EPA believes it is worthwhile to characterize localized ambient concentrations occurring when there are emission sources located in a part of a metropolitan area that are different than the design value site of the same metropolitan area. Thus, while there may be sites with higher overall maximum concentrations in another part of the same metropolitan area, those sites are covered by our long-standing existing requirement that monitors be placed "...in the area of expected maximum concentration" [§58, Appendix D, Section 4.7.1(b)(1)].
PM2.5 concentrations have generally trended down when averaged across all monitoring sites over the last two decades since PM2.5 measurements commenced nationally in 1999. This downward trend has resulted in lower background concentrations being measured upwind of urban areas; however, the impact of local emissions on PM2.5 may not be known if there is not a requirement to monitor ambient air in these areas. For example, the presence of new local sources of fine particle air pollution proximate to at-risk communities, such as significant increases in heavy duty truck traffic since monitors were originally sited, should be taken into consideration. As explained in the PA (U.S. EPA, 2022b), measured PM2.5 at near-road monitoring stations include an increment relative to other sites in the same CBSA. The near-road sites will complement any new or moved sites located to specifically address at-risk communities near sources of concern. We anticipate the significance of local emissions may increase if, as proposed, the level of the annual PM2.5 NAAQS is lowered. Thus, the EPA seeks to support communities with at-risk populations in proximity to local sources of concern so that they have access to PM2.5 NAAQS-comparable data to ensure compliance with the PM2.5 NAAQS and for other data uses.
To successfully select and deploy an ambient air monitoring station, monitoring agencies must comply with the requirements of the EPA network design criteria (40 CFR Part 58, Appendix D, section 4.7), consider input from the community and other interested stakeholders, and then overlay the requirements and input with logistically available options in the neighborhoods they intend to monitor. Often, monitoring agencies partner with schools and other government agencies that have access to property in a neighborhood so that the desired monitoring stations can be sited, deployed, and maintained. Locating monitoring stations in neighborhoods should be done in a way that provides a good representation of the particulate matter exposures of the communities in which they are located. Alternatively, monitoring stations can be located directly next to emission sources of concern. However, these locations, known as "source-oriented" sites, may not necessarily represent the exposures in community or the effect of a multitude of emissions that can impact a neighborhood.
To ensure monitoring sites are appropriately representing exposure in at-risk communities, we propose that sites represent "area-wide" air quality near local sources of concern. Sites representing "area-wide" air quality are those monitors sited at neighborhood, urban, and regional scales, as well as those monitors sited at either micro- or middle-scale that are identified as being representative of many such locations in the same MSA. Most existing as well as new or moved sites are expected to be neighborhood-scale, which means that the monitoring stations would typically represent conditions throughout some reasonably homogeneous urban sub-region with dimensions of a few kilometers [§58, Appendix D, section 4.7.1(c)(3)]. Additionally, as described in §58.30, sites representing "area-wide" air quality have a long-standing applicability to both the annual and 24-hour PM2.5 NAAQS. Siting in a community representing "area-wide" air quality as proposed is consistent with other network design objectives pursuant to which we locate monitors where people live, work, and play.
The types of sites that are minimally required as part of the PM2.5 network design are associated with two geopolitical levels: MSAs and states. The minimum number and type of sites that are required within an MSA are a function of the population of the MSA, based on the latest available information from the Census Bureau, and the design value of the existing network of PM2.5 sites reported for that MSA. MSAs with design values at or above 85% of any PM2.5 NAAQS are required to operate one more site than those MSAs with values that are less than 85% of any PM2.5 NAAQS (40 CFR Part 58, Appendix D, Table D-5). Each MSA required to operate at least one monitoring station is to site the monitor at neighborhood or larger scale in an area of expected maximum concentration. MSAs with a population of 1 million or more are required to operate a PM2.5 monitor at a NO2 near-road station in the same MSA. Thus, according to Table D-5 of Appendix D to Part 58, only those MSAs with a population of greater than 1 million with the most recent 3-year design value greater than or equal to 85% of any PM2.5 NAAQS are required to operate at least three PM2.5 monitoring stations. Since one of these sites would be the site in the area of expected maximum concentration, which most often will be the design value site, and the other the near-road site, only the third location would not address either of those two requirements.
The requirement for a third monitoring station in a MSA, where it exists, would take on the revised network design requirement to address at-risk communities near sources of concern. Many existing sites in the area of expected maximum concentration or near-road sites that are located in at-risk communities. Thus, having multiple sites located in at-risk communities may be appropriate so long as each siting criteria is achieved. Also, while we are proposing this modification to our network design criteria, we recognize that the number of monitors to support key monitoring objectives, including addressing at-risk communities, could go well beyond what is currently minimally required. Many monitoring agencies already operate more monitoring sites than are minimally required and we expect this to continue in considering siting monitors in at-risk communities. Thus, the existing and robust network of almost 1,000 PM2.5 sites nationally will continue to protect all populations at the level of the NAAQS discussed in section II of this proposal, by always having at least one site in the area of expected maximum concentration for each CBSA where monitoring is required. Many existing and a few new sites will form an important sub-component of the PM2.5 network by characterizing air quality in at-risk communities, particularly with respect to sources of concern.
Monitoring requirements applicable at the state level include measuring regional background and regional transport (40 CFR Part 58, Appendix D, section 4.7.3). These required sites at the state level are largely located in rural areas and may include use of IMPROVE samplers or continuous PM2.5 monitors. The sites required at the state level complement sites required at the MSA level. Together the sites already required at the state level combined with existing siting requirements at the MSA level as well as the proposed revisions described herein to address at-risk communities will achieve several monitoring objectives, including comparison to the NAAQS and AQI. The availability of data from regional background and regional transport sites compared to data from design value sites already allow for calculating incremental exposure in communities with the highest design value location. With the proposed addition of a siting requirement for at-risk communities and the use of data from these sites compared to select regional background and regional transport sites as well as other sites in the same MSA, we can assess the incremental burden of exposure from local emissions to at-risk communities.
In addition to using data from the robust network of almost 1,000 PM2.5 sites for NAAQS and AQI purposes, having a stable network of long-term sites is especially valuable for trends and as an input to long term health and epidemiology studies that support reviews of the PM NAAQS. Therefore, while we are proposing to add a PM2.5 network design criteria to address at-risk communities, many sites are likely already in valuable locations meeting one of the existing network design criteria (i.e., being in an area-wide area of expected maximum concentration or collocated with near-road sites) and supporting multiple monitoring objectives. Also, in many communities there may already be sites meeting the network design criteria we are proposing for at-risk communities. Thus, acknowledging the value of having long-term data from a consistent set of network sites, on balance the EPA believes that the movement of sites should be minimized, especially in MSA's with a small number of sites. However, a small number of new sites are expected to be required due to the existing minimum monitoring requirements (Table D-5 of Appendix D to Part 58) and the revised primary annual PM2.5 NAAQS proposed in section II of this proposal. Also, sites do on occasion need to move due to loss of leases, no longer meeting siting criteria, or other reasons. For any of these cases, we believe it is appropriate to include prioritizing establishing sites in at-risk communities near sources of concern, should new sites be established, or existing locations be lost, and replacement sites need to be identified. Therefore, the EPA proposes that annual monitoring network plans [40 CFR 58.10(a)(1)] that include the few newly required sites and five-year assessments [40 CFR 58.10(d)] include a provision to examine the ability of existing and proposed sites to support air quality characterization for areas with at-risk populations in the community and the objective discussed herein.
Assessing and prioritizing at-risk communities for monitoring can be accomplished through several approaches. The most critical aspect of prioritizing which communities to monitor is their representation of the at-risk populations described earlier in this section. The other major consideration is whether the community is near source(s) of concern. While many CBSA's have one or more sources of concern described above, some CBSA's will not have the level of emissions from sources of concern that result in an elevated level of measured PM2.5 concentrations in surrounding communities. Since one of our other siting criteria to "..be in the area of expected concentration" [§58, Appendix D, Section 4.7.1(b)(1)] ensures there is a monitoring site in the community with the highest exposure in each CBSA with a monitoring requirement, on balance the EPA believes we should include being in an at-risk community for CBSAs with a third site requirement when there are no sources of concern identified in a CBSA or such sources do exist but are not expected to lead to elevated levels of measured PM2.5 concentrations.
To identify at-risk communities to consider for the proposed monitoring requirement, tools such as the EPA's EJSCREEN are available. The EPA solicits comment on other tools and/or datasets that can could be utilized to identify the at-risk communities described above. With information on at-risk communities, monitoring agencies need data that can best inform where there may be elevated levels of exposures from sources of concern. While we use FRMs and FEMs to determine compliance with the NAAQS, there are several additional datasets available that may be useful in evaluating the potential for elevated levels of exposure to communities near sources of concern. Potential datasets include non-regulatory data (CSN, IMPROVE, and AQI non-regulatory PM2.5 continuous monitors), modelling data  -  which utilizes emission inventory and meteorological data, emerging sensor networks such as used in the EPA's AirNow fire and smoke map, and satellites  -  which measure radiance and with computational algorithms are then used to estimate PM2.5 from aerosol optical depth (AOD). The 2019 ISA and PA (U.S. EPA, 2019a; U.S. EPA, 2022b) include details on each these, except for the AirNow fire and smoke map, which first became operational in 2020. Each of these datasets have advantages and disadvantages, especially when attempting to determine exposure concentrations for the averaging times of the PM2.5 NAAQS described in section II (i.e., annual NAAQS and 24-hour NAAQS). The EPA solicits comment on datasets most useful to identify communities with high exposures for PM2.5 NAAQS (i.e., annual or 24-hour), including any discussion on limitations or advantages of the dataset of interest. The EPA is soliciting comment on the use of these datasets for the purpose of identifying communities where the proposed monitoring requirement would apply and not for the purpose of satisfying the proposed monitoring requirement.
The monitoring methods appropriate for use at these proposed sites are FRMs and automated continuous FEMs. These are the methods that are eligible to compare to the PM2.5 NAAQS, which will be the primary objective for collecting this data. There are several other monitoring objectives that would benefit from use of automated continuous FEMs. For example, having hourly data available from automated continuous FEMs would allow sites to provide data in near-real time to support forecasting and near real-time reporting of the AQI. Automated continuous methods are also useful to support evaluation of other methods such as low-cost sensors. When used in combination with on-site wind speed and wind direction measurements, automated FEMs can provide useful pollution roses indicating the origin of emissions that affect a community. Additionally, when collocated with continuous carbon methods such as an aethalometer, automated FEMs can help identify potential local carbon sources contributing to increased exposure in the community. The EPA and the CASAC worked collaboratively in 2010 (Russell and Samet, 2010) to define a list of measurements that would be useful to implement in the near-road environment, and a subset of these measurements may additionally be of value to characterize the exposure in at-risk communities. While either FRMs or automated FEMs may be used at a site for comparison to the PM2.5 NAAQS, the EPA encourages use of automated continuous FEMs at sites in at-risk communities.
Although there are only a few new sites required, plus any potentially moved sites in cases where a site lease is lost, EPA believes we should build upon our existing regulatory process for selecting and approving these sites (40 CFR 58.10). For example, the timeline to implement the proposed PM2.5 sites in at-risk communities should allow monitoring agencies enough time for communities and other interested parties to provide their input regarding moving or adding new sites, while also minimally disrupting ongoing operations of monitoring agency programs. Another important factor is to ensure all existing PM2.5 sites have data available for comparison to a revised PM2.5 NAAQS, which is discussed in section II of this proposal. With a final rule from this proposal expected in 2023, we believe it would be appropriate to provide at least 12 months from the effective date of a final rule for monitoring agencies to initiate planning to implement these measures by seeking input from communities and other interested parties, and to consider revisions to their PM2.5 networks or explain how the existing network meets the objectives of this proposed modification. Thus, the EPA proposes that monitoring agencies identify their initial approach to the question of whether any new or moved sites are needed and to identify the potential communities in which the agencies are considering adding monitoring, if applicable, as well as identifying how they intend to meet the proposed revised criteria for PM2.5 network design to address at-risk communities. These aspects that will potentially affect the siting of new and moved sites should be addressed in the agencies' annual monitoring network plans due to each applicable EPA Regional office no later than July 1, 2024 (40 CFR 58.10). Specifics on the resulting proposed new or moved sites for PM2.5 network design to address at-risk communities would need to be detailed in the annual monitoring network plans due to each applicable EPA Regional office no later than July 1, 2025 (40 CFR 58.10). We are proposing that any new or moved sites would be required to be implemented and fully operational no later than 24 months from the date of approval of a plan or January 1, 2027, whichever comes first, but the EPA solicits comment on whether less time is needed (e.g., 12 months from plan approval and/or January 1, 2026).
In summary, the EPA is proposing to modify our PM2.5 network design criteria to include an environmental justice factor to address at-risk communities with a focus on exposures from sources of concern. While this proposal would require that sites be located in at-risk communities, particularly those whose air quality is potentially affected by local sources of concern, such sites should still meet the requirement for being considered "area-wide" air quality. Specific areas of interest we seek comment on include how to identify at-risk communities, the sources of concern important to consider, the datasets to identify communities with high exposures, and the most useful measurements to collocate with PM2.5 in at-risk communities. The EPA seeks comment on these areas of interest as well as the proposed modification of our PM2.5 network design objectives and implementation as described herein.
5. Proposed Revisions to Probe and Monitoring Path Siting Criteria
The EPA is proposing changes to monitoring requirements in the subpart Appendix E --  Probe and Monitoring Path Siting Criteria for Ambient Air Quality Monitoring. Since 2006, multiple rule revisions were made to establish siting requirements for PM10−2.5 and O3 monitoring sites (71 FR 2748, January 17, 2006), Near-Road NO2 monitoring sites (75 FR 6535, February 9, 2010), Near-Road CO monitoring sites (76 FR 54342, August 31, 2011), and Near-Road PM2.5 monitoring sites (78 FR 3285, January 15, 2013). Through these multiple revisions to the regulatory text, some requirements were inadvertently omitted, and, over time, the clarity of this Appendix was reduced through these omissions that, in a few instances, led to unintended and conflicting regulatory requirements. The EPA proposes to reinstate portions of previous Probe and Monitoring Path Siting Criteria Requirements from previous rulemaking where appropriate to restore the original intent. The proposed changes that affect the overall Appendix follow, while those specific to the various sections of the Appendix will be addressed under a specific section heading. The EPA notes that the entire regulatory text section for Appendix E is being reprinted with this proposal because this section is being reorganized for clarity in addition to being selectively revised as described in detail below. The EPA is soliciting comment on the specific provisions of Appendix E proposed for revision. However, there are a number of provisions that are being reprinted in the regulatory text solely for clarity to assist the public in understanding the changes being proposed and reconciling requirements between different portions of the regulation text; the EPA is not soliciting comment on those provisions and considers changes to those provisions to be beyond the scope of this proposed rulemaking.
a. Providing Separate Section for Open Path Monitoring Requirements
The current Appendix E regulation combines open path monitor siting requirements with requirements for siting samplers and monitors that utilize probe inlets. While this approach allowed the EPA to promulgate an abbreviated regulation for probe-siting requirements, the EPA now has determined that the clarity of the requirements for each monitoring method type has been diminished by this combination. As such, the EPA is proposing to relocate all open path monitor siting criteria requirements to a separate section in this Appendix. Providing separate sections for these distinct monitoring method types will allow the EPA to more clearly articulate minimum technical siting requirements for each. Further rationale for creating these separate sections is that the regulatory monitoring community has not submitted to AQS measurement results from open path monitors since 2009. Because these open path monitoring methods are rarely used for monitoring to compare to the NAAQS, the EPA believes that moving the open path siting criteria to their own section will make clearer the probe siting criteria for the ambient air monitoring methods that are now most commonly utilized by monitoring organizations.
b. Amending Distance Precision for Spacing Offsets
The EPA proposes to require that when rounding is performed to assess compliance with these siting requirements, the distance measurements will be rounded such as to retain at least two significant figures. The EPA proposes to communicate this rounding requirement in the regulatory text using footnotes in Table E-1, Table E-2, and Table E-3 of the current regulation.
c. Clarifying Summary Table of Probe Siting Criteria
To provide additional specificity and flexibility to the summary table for probe siting criteria (see current Table E-4 in Appendix E), the EPA proposes to change the ">" (greater than) symbols to ">=" (greater than or equal to) symbols. This minor revision will more clearly express the EPA's intent that the distance offsets provided in the current Table E-4 in Appendix E are acceptable for NAAQS compliance monitoring.
d. Adding Flexibility for the Spacing from Minor Sources
Current requirements for the spacing of probe inlets and monitoring paths from minor sources of SO2 and NO2 stipulate that the probe inlets and monitoring paths must be away from these minor sources (see current section 3(b) in Appendix E). The EPA proposes to clarify and provide flexibility by changing this requirement to a goal. The EPA proposes to replace the "must" in this regulation with a "should". As stated in Section 1(c) of the current rule, a "must" defines a requirement while a "should" specifies a goal. Since the current rule does not specify how far the probe must be spaced from such minor sources, the EPA proposes that a "should" in this regulation is more appropriate. Minor sources can have adverse impacts on the representativeness of the ambient pollutant concentrations sampled by the probe inlet. As such, the EPA recommends that sites with these minor sources be avoided whenever practicable and probe inlets spaced as far from these minor sources as possible when alternative monitoring stations are not suitable.
e. Amendments and Clarification for the Spacing from Obstructions and Trees
The EPA proposes to clarify and redefine that the minimum arc required to be free of obstructions for a probe inlet or monitoring path is 270 degrees. Currently this portion of the regulation (see current section 4(b) of Appendix E) specifies 180 degrees as this minimum arc. However, this requirement is inconsistent with the requirement found in footnote 5 of Table E-4 in Appendix E that specifies the probe inlet or monitoring path must have unrestricted airflow of 270 degrees around the probe and 180 degrees for the arc is only allowed if the probe is on the side of a building or a wall. These inconsistent regulatory requirements were introduced in the 2006 rulemaking when the 270-degree requirement was omitted from the regulation text of section 4(b) (see 71 FR 61236, October 17, 2006).
There are also inconsistent requirements in the current regulation regarding the spacing of probe inlets from the driplines of trees. Section 5(a) of Appendix E requires the probe inlet must be no closer than 10 meters to the driplines of any trees, while footnote 3 of Table E-4 of the Appendix E qualifies that this minimum 10-meter offset is only required when the tree also acts as an obstruction.
f. Reinstating Minimum 270-Degree Arc and Clarifying 180-Degree Arc in Regulatory Text
The EPA proposes to correct identified inconsistencies in this regulation by reinstating the 270-degree requirement in section 4(b) of Appendix E. Additionally, the EPA proposes to further clarify this regulation by stating that the continuous 180-degree minimum arc of unrestricted airflow provision is reserved for monitors sited on the side of a building or a wall to comply with network design criteria requirements specified in Appendix D of part 58. Examples include CO monitoring in urbanized areas that relies on monitoring in street canyons and near-road monitoring where a continuous arc of 270 degrees of unrestricted airflow is not routinely possible given limited monitor siting options.
g. Clarification on Obstacles that Act as an Obstruction
The EPA proposes to clarify the definitions of "obstructions" and "obstacles" in the regulatory text (see section 4 of the current Appendix E). While obstacles should be avoided as much as is practicable, logistical constraints may dictate that some obstacles are present within the vicinity of the monitoring probe inlet. Obstructions to the air flow of the probe inlet are those obstacles that are horizontally closer than twice the vertical distance the obstacle protrudes above the probe inlet and can be reasonably thought to scavenge reactive gases or to restrict the airflow for any pollutant. The EPA does not generally consider objects or obstacles such as flag poles or site towers for NOy convertors or towers for meteorological sensors, etc. to be obstructions.
h. Amending and Clarifying the 10-meter Tree Dripline Requirement
The EPA proposes to reconcile the conflicting requirements in section 5(a) and Table E-4, footnote 3 of the current regulation by deleting the qualification in footnote 3 of Table E-4 to require that the probe inlet must always be no closer than 10 meters to the tree dripline. The EPA also proposes to reinstate the goal that was omitted from section 5(a) during previous rule revisions, that monitor probe inlets should be at least 20 meters from the driplines of trees. Additionally, the EPA proposes to clarify section 5(a) of the current regulation by adding that when the tree or group of trees is considered an obstruction, then the regulatory requirements of section 4(a) apply.
i. Amending Spacing Requirement for Microscale Monitoring
To obtain representative ambient air monitoring measurements for source-oriented and microscale air monitoring stations, it is important to have unobstructed airflow between the monitor's probe inlet and the source under investigation. This reasoning was used by the EPA when near-road NO2 monitoring stations were required to have an unobstructed airflow between the monitor probe and the outside nearest edge of the traffic lane (see current section 4(d) of this regulation). To assist in further clarifying the monitoring siting criteria for the spacing from obstructions and spacing from trees, the EPA proposes to change from a goal to a requirement that microscale sites for any pollutant shall have no trees or shrubs blocking the line-of-sight fetch between the monitor's probe inlet and the source under investigation. The EPA proposes to communicate this requirement by changing the "should" to a "shall" in the regulatory text of section 5(c). The EPA does not consider small obstacles such as shrubs that are below this fetch to adversely impact the representativeness of the air quality measurements results. This proposed revision of section 4(d) will bring more consistency to Appendix E.
j. Amending Waiver Provisions
The EPA believes the effects of any requirements in this proposal that may be considered to be new are minor. While we are attempting to clarify probe and siting criteria as part of our monitoring regulations, the Agency fully intends to maintain waiver provisions that exist in the regulation for these siting criteria (see current section 10). For cases where long-term trend sites or monitors that determine the design value for their area cannot reasonably meet these regulatory siting requirements, the EPA encourages monitoring organizations to work with their respective EPA Regional Offices to determine if a waiver from these siting criteria is appropriate.
Even though the current regulation adequately and clearly identifies which monitoring situations are eligible for the EPA to consider waiving the requirements for probe-siting criteria (see current section 10), these waiver provisions are silent regarding how long an approved waiver remains in force and effect. Environmental conditions (e.g., airflow due to changes in growth of trees, shrubs, construction of buildings or other obstructions) around monitoring stations are prone to change over time. As such, the EPA has identified that previously approved waivers should be periodically reevaluated to ensure that the conditions upon which the original waiver was approved still exist and that the siting conditions have not degraded to an unacceptable level. The EPA proposes to modify section 10.3 of the current regulation to state that waivers from the probe-siting criteria must be renewed minimally every 5 years. Ideally, sites needing a waiver renewal should be inspected by the EPA such as during a Technical Systems Audit (TSA) typically conducted at a subset of sites within each Primary Quality Assurance Organization (PQAO) every three years. However, virtual inspections may also be acceptable using documentation such as photos and traffic counts. Dates for the most recent approval of a waiver must then be included in the applicable network assessment and annual monitoring network plan. The EPA proposes to revise 58.10(b)(10) of the regulation to maintain consistency in the regulation text for probe siting criteria requirements and annual monitoring network plans. This proposal leverages the existing annual assessment requirements found in parts 58.10(a)(1) and 58.10(d).
k. Broadening of Acceptable Probe Materials
The current regulatory specifications for acceptable probe materials for sampling reactive gases are limited to borosilicate glass, FEP, or their equivalent (see section 9 of the regulation). The EPA's selection of "or its equivalent" in the current regulatory text was intended to allow flexibility to monitoring organizations when selecting suitable sampling train materials. In practice, however, this text has resulted in potentially suitable materials not being used for sampling trains due to concerns that the material may not meet these regulatory requirements. The current requirements for acceptable probe materials were promulgated in 1979. Since 1979, several potential alternatives to borosilicate glass and FEP were developed and are commercially available.
Because some of these alternative materials have advantages over the currently approved materials (e.g., cost and durability), the EPA has received numerous inquiries from monitoring organizations regarding the regulatory suitability of these materials. Monitoring organizations have expressed particular interest in the potential use of PVDF (polyvinylidene fluoride) which is marketed under the registered tradename of Kynar[(R)] by Arkema Inc. (Colombes, France). In response to these inquiries, the EPA's Office of Research and Development (ORD) recently designed and conducted a laboratory study to determine the transport efficiency of O3, SO2, NO2, and CO through several candidate tubing materials (Johnson, 2022). Based on these tests results, the EPA is proposing to revise Section 9 of the current regulation to add PVDF, PTFE, and PFA to the list of approved materials for efficiently transporting gaseous criteria pollutants. The EPA also proposes to clarify that the residence-time criteria for sampling reactive gas through these approved materials applies to all O3, SO2, and NO2 monitors. In conjunction with the previously approved borosilicate glass and FEP materials, including these three new materials would provide monitoring organizations with a wider variety of efficient sampling and transport materials needed for conducting NAAQS compliance monitoring.
The EPA has also studied and approved the use of Nafion[TM] upstream of ozone analyzers to minimize measurement bias associated with high ambient RH levels (U.S. EPA, 2020b). Minimal loss of ozone occurred in these systems as long as the Nafion[TM] system was conditioned beforehand. Nafion[TM] is composed primarily of PTFE and can be considered equivalent to PTFE. It has been shown in ORD's recent tests described above to exhibit virtually no loss of ozone at 20 second residence times.
D. Taking Comment on Incorporating data from Next Generation Technologies
1. Background on use of FRM and FEM monitors
The EPA approves FRM and FEM monitors for criteria pollutant measurements in the Federal Register after careful review of applications describing extensive testing of the methods operation and performance. The siting of these monitors across State, local, and Tribal networks is subject to detailed requirements for network design detailed in Appendix D to 40 CFR part 58 with probe and siting criteria described in Appendix E for 40 CFR part 58. The operation of these monitors is subject to extensive quality assurance requirements detailed in Appendix A to 40 CFR part 58, which ensures data quality statistics are produced to inform the quality of the data needed to ensure regulatory grade decisions are made with data of known quality. The EPA believes these requirements are important for ensuring the degree of accurate and precise data which is appropriate for regulatory decision-making, particularly decisions about attainment or nonattainment of the NAAQS. However, the EPA also recognizes that the capital and operating costs of these monitors is substantial, which requires the EPA and states to prioritize where monitors should be deployed. The EPA recognizes that making use of broader air quality data sets which are less expensive can provide important benefits, even if the EPA does not consider those datasets suitable for all regulatory purposes. In some circumstances in the past, for example, the EPA has used non-FRM monitoring to inform decisions about the boundaries of a nonattainment area, although the data was not sufficient to support a finding that an area was in nonattainment. Likewise, the EPA has incorporated sensor data into its fire and smoke map for the purpose of informing the public of potential imminent health risks, even though that data would not be comparable to the NAAQS for purposes of determining attainment. There are multiple uses of air quality data and the EPA believes there may be additional opportunities to develop broader air quality datasets which provide benefits to the EPA and the public even where the data is not from FRM/FEM monitors and is not suitable for comparison to the NAAQS.
2. Next Generation Technologies: Data Considerations
The EPA and our State, local, and Tribal partners in cooperation with other Federal Agencies have made great strides in integrating data from routine air monitoring methods with data from next generation technologies to address emerging air quality issues. For example, the EPA and USFS, in consultation with other partners, launched the publicly available AirNow Fire and Smoke Map, which has received over 26 million page views since its release in July 2020. This fire and smoke map has been an invaluable tool for the public, providing refined spatial information on current Air Quality Index (AQI) conditions, fire and smoke plumes locations, actions for communities to take based on local air quality, and links to Smoke Forecast Outlooks developed by specially trained air resource advisors. Data are brought together from multiple systems including permanent and temporary PM2.5 continuous monitoring sites, sensors, and satellite derived fire and smoke data. With the success of the fire and smoke map and a robust and growing network of PM2.5 continuous FEMs and sensor network data, as well as existing and future satellites products, the EPA is interested in considering further enhancements to the evolution of data products to meet new and emerging non-regulatory air quality data needs. Below we describe each of the major data sets, their advantages, and any challenges to their use. We then solicit input on additional approaches and/or products to incorporating data from next generation technologies that can help address important non-regulatory air quality data needs.
3. PM2.5 continuous FEMs
As described in the PA, State, local, and Tribal monitoring agencies are using an increasing number of PM2.5 continuous FEMs. These methods are primarily deployed to meet two monitoring objectives: first, to compare to the NAAQS, and second, to report and support forecasting of the AQI. PM2.5 continuous FEMs have some key advantages over FRMs, most notably that they provide automated hourly measurement of PM2.5 available in near real time. The continuous PM2.5 data are reported as soon as practicable after the end of each hour, usually within 5-10 minutes, and are used in multiple applications of real-time data such as such as by State, local, and Tribal web sites[,], the EPA's AirNow web site, and national media outlets. Recent improvements in the availability and exchange of near real-time data through a dedicated AirNow Application Programming Interface (API) allow for efficient exchange of data between the EPA, other federal agencies, and commercial data providers such as low-cost sensor networks. The efficient exchange of data through the AirNow API was a key advancement in the successful implementation of the EPA AirNow's fire and smoke map. The PM2.5 continuous FEM data are critical to "ground truthing" other datasets such as sensors and satellites for two important reasons. First, PM2.5 continuous FEMs are subject to extensive regulatory-grade quality assurance and quality control as required by Appendix A to 40 CFR Part 58. Second, PM2.5 continuous FEMs are located in accordance with strict siting criteria according to Appendix E to 40 CFR Part 58. The siting criteria assure that measured data represent ambient air at ground level where people are breathing and are thus exposed to particle pollution. The EPA and State, local, and Tribal agencies are working to upgrade many existing FRM-only sites with PM2.5 continuous FEMs through use of American Rescue Plan funds. Despite these investments, there are major challenges to monitoring agencies' ability to have enough trained and available staff to support their regulatory monitoring networks, especially in remote locations, and to have the capital resources to implement new monitoring stations. So, while there may be some improvements to the existing network of almost 1,000 PM2.5 regulatory-grade monitoring stations, regulatory instruments will not produce data everywhere that it is desired. Thus, the integration of PM2.5 continuous FEMs with other datasets is an important opportunity to address existing and emerging air quality data needs for non-regulatory purposes.
4. PM2.5 satellite products
Satellite-based instruments provide measurements of radiance that can be used to calculate the aerosol optical depth (AOD) of the atmosphere. For over a decade, satellite AOD values have been used in models that incorporate multiple datasets to predict surface level PM2.5 concentrations over the U.S. (hereafter, satellite-PM2.5). Despite some heterogeneity in performance under varying conditions, the satellite-PM2.5 datasets have significantly advanced in terms of accuracy in recent years (Di et al., 2019; van Donkelaar et al., 2019; Zhang and Kondragunta, 2021). The EPA is using satellite-PM2.5 datasets in a variety of contexts. Satellite-PM2.5 data was included in a comparative analysis of hybrid modeling methods in the PA (U.S. EPA, 2022b). The EPA is also working with NASA and NOAA to use satellite-PM2.5 in the AirNow system. The EPA also uses satellite AOD and many other satellite data products in the development of our photochemical modeling platforms that are used in regulatory and policy assessments both by the EPA and by our State and local partners.
Each satellite data product has its own strengths and limitations. One strength is the spatial coverage, which can be once-a-day globally for polar orbiting satellites or over a fixed field of view continuously for geostationary satellites. Satellite-PM2.5 data has the limitation that it is not a direct measurement of PM2.5 concentrations, but rather is derived through a model that connects the total column AOD to surface PM2.5. In addition, the satellite products are only capable of making daytime measurements because they rely on sunlight. In fact, most satellite-PM2.5 data products use the surface monitor network as an input. As such, the satellite-PM2.5 data does not substitute for a ground-based monitor; rather it complements the monitor network. The EPA continues to explore ways to use the wealth of data from satellites to address important air quality questions consistent with their strengths and limitations.
5. Use of air sensors
The term "air sensor" is a simplified way of referring to a class of technology that has expanded on the market in recent years and has common traits of directly reading a pollutant in the air, being smaller in size, and often sold at lower prices that support a wider number of monitoring locations than possible in the past. As explained on the EPA's Air Sensor Toolbox website, air sensor monitors that are lower in cost, portable, and generally easier to operate than regulatory-grade monitors are widely used in the United States to understand air quality conditions. Many refer to this class of technology as "low-cost air sensors," "air sensor devices," or "air quality sensors." Potential uses for these non-regulatory air sensor technologies include, but are not limited to, science education, supplementing regulatory air quality measurements, conducting research, measuring local air quality to better understand sources of pollution, locating leaks at industrial facilities, and emergency response.
The growth in use of sensors included in the EPA's fire and smoke map provides a platform to build upon. There are thousands of PM sensors whose data are coordinated and overlaid with routine and temporary PM2.5 continuous monitors as well as satellite-derived data on fires and smoke. Sensors offer an opportunity to supplement higher-cost regulatory monitoring to provide data for the non-regulatory uses as described above. However, there are several challenges to using sensors. Each commercially available PM sensor appears to have its own data quality challenges depending on season, aerosol encountered, and meteorological conditions (typically temperature and relative humidity). The EPA has gone to considerable length to ensure the PM2.5 sensor data on the fire and smoke map have a correction available with collocated FRMs and FEMs. This was possible due to the large number of air sensors that are the same make and model located across the country. Thus, an important challenge for the use of sensors is the spatial richness in sensor networks needed to make integrating the dataset with other monitoring data viable. Even with corrected sensor data in hand, publicly shared sensor data lacks reliability and accountability for ensuring that basic siting criteria are met. Sensors are often installed by members of the public who share data to the sensor network, which is generally understood as implicitly representing that the sensor is located in ambient air although, in fact, the sensor may be located inside a home or next to a highly localized source of emissions such as the flue of a home heating system. In areas with many reporting sensors, these concerns about siting may be lessened through site-to site comparison of data; however, the absence of any confirmed information about siting presents challenges for use of sensor data.
6. Summary
The near real-time integration of data from PM2.5 continuous monitors, sensors, and satellites has been proven through use of the EPA's fire and smoke map. This mapping product is possible though the use of API's where data sets are automatically shared on pre-specified computer servers. Given the success of the fire and smoke map, the EPA is interested in pursuing additional approaches and/or products that can help address important non-regulatory air quality data needs. Therefore, the EPA solicits comment on the most important data uses and data sets to consider in future products. Such approaches and/or products could utilize historical or near real-time data. For example, what are the advantages and disadvantages of using existing data and tools to identify PM hot spots across an area of interest? Could satellite data or a combined surface layer (PM2.5 FRM and FEM data, sensor data, and satellite data) be useful in siting regulatory monitors? Could combined surfaces layers be useful in determining the boundaries of nonattainment areas? Could combined surface layers be useful in exploring potential emission sources to consider in SIP planning? To what extent would requirements for data formats, units, or timescales of interest need to evolve to best address these needs? What other datasets should the EPA consider merging with the data sets listed above to help better inform air quality management, including prioritizing network investments for potential new sites such as in at risk communities described elsewhere in this proposal? The EPA seeks input and prioritization on each of these questions to help improve the utility of data to better support air quality management to improve public health and the environment.
VIII. Clean Air Act Implementation Requirements for the PM NAAQS
The proposed revision to the primary annual PM2.5 NAAQS discussed in section II above, if finalized, would trigger a process under which states will make recommendations to the Administrator regarding area designations. States also will be required to review their existing section 110 infrastructure state implementation plans and modify them if necessary to implement a revised NAAQS. A revised primary annual PM2.5 NAAQS will need to be incorporated into the implementation of applicable air permitting requirements and the transportation conformity and general conformity processes, and states will need to review existing regulations for these programs that already cover PM2.5 to determine the extent to which any changes are needed. This section provides background information for understanding the possible implications of the proposed NAAQS changes and describes the EPA's plans for providing states guidance needed to assist their implementation efforts. This section also describes existing EPA interpretations of CAA requirements and other EPA guidance relevant to implementation of a revised PM2.5 NAAQS. Given the strong scientific evidence for disparities in PM2.5 exposures and PM2.5-related health risk among certain populations (as discussed in section II of this notice), the EPA included in its 2016 PM2.5 State Implementation Plan (SIP) Requirements Rule (which was written to be applicable for any future NAAQS revisions) included a number of key recommendations for states to advance environmental justice through their attainment planning process. In addition, as discussed throughout this section, environmental justice considerations are evaluated with regard to the several specific program elements of the overall implementation process.
With respect to the topics covered in this section, the EPA welcomes the public to provide input to the Agency through comments. However, because these issues are not relevant to the establishment of a revised primary annual PM2.5 NAAQS, and because no specific revisions are proposed for the regulations implementing the PM2.5 NAAQS (i.e., 40 CFR 51, subpart Z), the EPA does not expect to respond to these comments in the final action on this proposal (nor is it required to do so).
 Designation of Areas
After the EPA establishes or revises a NAAQS, the CAA requires the EPA and the states to take steps to ensure that the new or revised NAAQS is met. The first step, known as the initial area designations, involves identifying areas of the country that either meet or do not meet the new or revised NAAQS, along with the nearby areas contributing to the violations.
Section 107(d)(1) of the CAA states that, "By such date as the Administrator may reasonably require, but not later than 1 year after promulgation of a new or revised national ambient air quality standard for any pollutant under section 109, the Governor of each state shall ... submit to the Administrator a list of all areas (or portions thereof) in the State" and that making recommendations for whether the EPA should designate those areas as nonattainment, attainment, or unclassifiable. The CAA provides the EPA discretion to require states to submit their designations recommendations within a reasonable amount of time not exceeding 1 year. The CAA also stipulates that "the Administrator may not require the Governor to submit the required list sooner than 120 days after promulgating a new or revised national ambient air quality standard." Section 107(d)(1)(B)(i) further provides, "Upon promulgation or revision of a NAAQS, the Administrator shall promulgate the designations of all areas (or portions thereof) ... as expeditiously as practicable, but in no case later than 2 years from the date of promulgation. Such period may be extended for up to one year in the event the Administrator has insufficient information to promulgate the designations." With respect to the NAAQS setting process, courts have interpreted the term "promulgation" to be signature and widespread dissemination of a final rule. One way the EPA intends to account for environmental justice in the implementation process is to promptly issue designations in accordance with the statutory requirements to ensure expeditious public health protections for all populations, including those currently experiencing disparities in PM2.5 exposures and PM2.5-related health risk.
If the EPA agrees with the designation recommendation of the state, then it may proceed to promulgate the designations for such areas. If, however, the EPA disagrees with the state's recommendation, then the EPA may elect to make modifications to the recommended designations. By no later than 120 days prior to promulgating the final designations, the EPA is required to notify states of any intended modifications to the designations of any areas or portions thereof, including the boundaries of areas, as the EPA may deem necessary. States then have an opportunity to comment on the EPA's tentative designation decision. If a state elects not to provide designation recommendations, then the EPA must timely promulgate the designation that it deems appropriate. While section 107(d) of the CAA specifically addresses the designations process for states, the EPA intends to follow the same process for tribes to the extent practicable, pursuant to section 301(d) of the CAA regarding tribal authority, and the Tribal Authority Rule (63 FR 7254, February 12, 1998). To provide clarity and consistency in doing so, the EPA issued a guidance memorandum to our Regional Offices on working with tribes during the designations process (Page, 2011a).
Monitoring data are currently available from numerous existing PM2.5 Federal Equivalent Methods (FEM) and Federal Reference Methods (FRM) sites to determine compliance with the proposed revised PM2.5 primary annual NAAQS. As discussed in section II above, the EPA is proposing to: (1) revise the level of the primary annual PM2.5 standard and retain the current primary 24-hour PM2.5 standard (section II.D.3); and (2) not change the current secondary annual and 24-hour PM2.5 standards at this time (section V.D.3). Consistent with the process used in previous area designations efforts, the EPA will evaluate each area on a case-by-case basis considering the specific facts and circumstances unique to the area to support area boundaries decisions for the revised standard. Section 107(d) explicitly requires that the EPA designate as nonattainment not only the area that is violating the pertinent standard, but also those nearby areas that contribute to the violation in the violating area. For the reason noted earlier, the EPA believes it is important to consider environment justice within the framework of this area-specific analysis. Consistent with past practice, the EPA plans to address issues relevant to area designations more fully in a separate designations-specific memorandum around the time of promulgation of any revised PM2.5 NAAQS. For informational purposes, the public can comment on the process and schedule for the initial area designations and nonattainment boundary setting effort associated with a new or revised PM2.5 NAAQS. As noted above, the EPA does not expect to respond to these comments in the final regulatory action establishing the NAAQS.
As in past iterations of the PM2.5 NAAQS, the EPA intends to make the designations for any revised NAAQS based on the most recent 3 years of complete and valid air quality data. Accordingly, the EPA recommends that states base their initial designation recommendations on the most current available 3 years of complete and valid air quality data. The EPA intends to use available air quality data from the current PM2.5 mass and speciation monitoring networks and other technical information. The EPA will then base the final designations on 3 consecutive years of certified air quality monitoring data, likely 2021-2023.
In some areas, state or tribal air agencies may have flagged air quality data for certain days in the Air Quality System due to potential impacts from exceptional events (i.e., such as wildfires or high wind dust storms). Air quality concentrations on such days may affect the calculation of design values for regulatory air monitoring sites in determining whether such sites may violate the revised PM2.5 NAAQS, and therefore could influence the initial area designations for this revised NAAQS. Under the 2016 Exceptional Events Rule (see "Treatment of Data Influenced by Exceptional Events; Final Rule," 81 FR 68216, October 3, 2016), an air agency may submit to the EPA a demonstration with supporting information and analyses for each day the air agency claims should be excluded from design value calculations for regulatory purposes. The EPA has provided a number of tools to assist air agencies in preparing their demonstrations and will continue to work with air agencies as they identify, prepare and submit exceptional events demonstrations. The EPA recognizes that some areas and stakeholders may be concerned about wildland and prescribed fire related impacts to designations and or other forthcoming actions of regulatory significance for which a state may want to submit an exceptional events demonstration. The EPA has already issued guidance addressing development of exceptional events demonstrations for both wildland and prescribed fires. These documents, among other tools, are available on the EPA's website identified above. The air agency is required to follow the exceptional events demonstration submission deadlines that are identified in Table 2 to 40 CFR section 50.14(c)(2)(vi)  -  "Schedule for Initial Notification and Demonstration Submission for Data Influenced by Exceptional Events for Use in Initial Area Designations." Further, the EPA has notified states of areas subject to mitigation plan provisions. Within 2 years of the notification, if the air agency has not submitted a required mitigation plan, the EPA will not concur with the air agency's request to exclude data until the required plan is submitted and verified.
As noted earlier, the EPA intends to provide designation guidance to the states and tribes around the time of the promulgation of a revised NAAQS, to assist in formulating these recommendations. With regard to the area designations process, if, after evaluating the state recommendations in light of the technical factors, the Administrator intends to modify any state area recommendation, the EPA will notify the appropriate state Governor no later than 120 days prior to making final designations decisions. A state that believes the Administrator's intended modification is inappropriate will have the opportunity to demonstrate to the EPA why it believes its original recommendation (or a revised recommendation) is more appropriate before final designations are promulgated. The Administrator will take any additional input from the state into account in making final designation decisions. If the Administrator departs from the stated intentions in the initial 120-day notification letter in a way that does not match the most recently received recommendation from the Governor (or tribe) as of the date of the final designation, the Administrator will provide an additional 120-day notification letter notifying the Governor of such modifications. The EPA invites preliminary comment on all aspects of the designation process at this time, which the Agency will consider in developing any updated guidance.
 Section 110(a)(1) and (2) Infrastructure SIP Requirements
The CAA directs states to address basic SIP requirements to implement, maintain, and enforce the NAAQS. Under CAA sections 110(a)(1) and (2), states are required to have state implementation plans that provide the necessary air quality management infrastructure including, among other things, enforceable emissions limitations, an ambient monitoring program, an enforcement program, air quality modeling capabilities, and adequate personnel, resources, and legal authority. After the EPA promulgates a new or revised NAAQS, states are required to make a new SIP submission to establish that they meet the necessary structural requirements for such new or revised NAAQS or make changes to do so. The EPA refers to this type of SIP submission as an "infrastructure SIP submission." Under CAA sections 110(a)(1), all states are required to make these infrastructure SIP submissions within 3 years after promulgation of a new or revised primary standard. While the CAA authorizes the EPA to set a shorter time for states to make these SIP submissions, the EPA does not currently intend to do so.
Under CAA section 110(a)(1) and (2), states are required to make SIP submissions that address a number of requirements pertaining to implementation, maintenance, and enforcement of a new or revised NAAQS. The specific subsections in CAA section 110(a)(2) require states to address a number of requirements, as applicable:  (A) Emissions limits and other control measures, (B) Ambient air quality monitoring/data system, (C) Programs for enforcement of control measures and for construction or modification of stationary sources, (D)(i) Interstate pollution transport; and (D)(ii) Interstate and international pollution abatement, (E) Adequate resources and authority, conflict of interest, and oversight of local governments and regional agencies, (F) Stationary source monitoring and reporting, (G) Emergency episodes, (H) SIP revisions, (I) Plan revisions for nonattainment areas, (J) Consultation with government officials, public notification, PSD and visibility protection, (K) Air quality modeling and submission of modeling data, (L) Permitting fees, and (M) Consultation and participation by affected local entities.  These requirements apply to all SIP submissions in general, but the EPA has provided specific guidance to states concerning its interpretation of these requirements in the specific context of infrastructure SIP submissions for a new or revised NAAQS (Page, 2013).
The EPA interprets the CAA such that two elements identified in section 110(a)(2) are not subject to the 3-year submission deadline of section 110(a)(1) and thus states are not required to address them in the context of an infrastructure SIP submission. The elements pertain to part D, in title I of the CAA, which addresses plan requirements for nonattainment areas. Therefore, for the reasons explained below, the following section 110(a)(2) elements are considered by the EPA to be outside the scope of infrastructure SIP actions: (1) the portion of section 110(a)(2)(C), programs for enforcement of control measures and for construction or modification of stationary sources that applies to permit programs applicable in designated nonattainment areas, (known as "nonattainment new source review") under part D; and (2) section 110(a)(2)(I), which requires a SIP submission pursuant to part D, in its entirety. The EPA does not expect states to address the requirement for a new or revised NAAQS in the infrastructure SIP submissions to include regulations or emissions limits developed specifically for attaining the relevant standard in areas designated nonattainment for the proposed revised PM2.5 NAAQS. States will be required to submit infrastructure SIP submissions for a revised PM2.5 NAAQS before they are required to submit nonattainment plan SIP submissions to demonstrate attainment with the same NAAQS. States are required to submit nonattainment plans to provide for attainment and maintenance of a revised PM2.5 NAAQS within 18 months from the effective date of nonattainment area designations as required under CAA section 189(a)(2)(B). The EPA reviews and acts upon these later SIP submissions through a separate process. For this reason, the EPA does not expect states to address new nonattainment area emissions controls per section 110(a)(2)(I) in their infrastructure SIP submissions.
One of the required infrastructure SIP elements is that each state's SIP must contain adequate provisions to prohibit, consistent with the provisions of title I of the CAA, emissions from within the state that will significantly contribute to nonattainment in, or interfere with maintenance by, any other state of the primary or secondary NAAQS. This element is often referred to as the "good neighbor" or "interstate transport" provision. The provision has two prongs: significant contribution to nonattainment (prong 1) and interference with maintenance (prong 2). The EPA and states must give independent significance to prong 1 and prong 2 when evaluating downwind air quality problems under CAA section 110(a)(2)(D)(i)(I). Further, case law has established that the EPA and states must implement requirements to meet interstate transport obligations in alignment with the applicable statutory attainment schedule of the downwind areas impacted by upwind-state emissions. Thus, the EPA anticipates that states will need to address interstate transport obligations associated with any revised PM NAAQS, if finalized, in alignment with the provisions of subpart 4 of part D of the CAA, as discussed in more detail in section VIII.C below. Specifically, states must implement any measures required to address interstate transport obligations as expeditiously as practicable and no later than the next statutory attainment date, i.e., for this NAAQS revision, if finalized, as expeditiously as practicable but no later than the end of the sixth calendar year following nonattainment area designations. See CAA section 188(c).
The EPA anticipates developing further information and coordinating with states with respect to the requirements of CAA section 110(a)(2)(D)(i)(I) for implementation of any revised PM NAAQS. We note that states may elect to make SIP submissions that address certain infrastructure SIP elements separately from the others. In recent years, due in part to the complexity of addressing interstate transport obligations, some states have found it efficient to make SIP submissions to address the interstate transport provisions separately from other infrastructure SIP elements.
It is the responsibility of each state to review its air quality management program's existing SIP provisions in light of each new or revised NAAQS to determine if any revisions are necessary to implement a new or revised NAAQS. Most states have revised and updated their SIPs in recent years to address requirements associated with other revised NAAQS. For some states, it may be the case that for a number of infrastructure elements, the state may believe it already has adequate state regulations already adopted and approved into the SIP to address a particular requirement with respect to any revised PM2.5 NAAQS. For such portions of the state's infrastructure SIP submission, the state may provide an explanation of how its existing SIP provisions are adequate.
If a state determines that existing SIP-approved provisions are adequate in light of the revised PM2.5 NAAQS with respect to a given infrastructure SIP element (or sub-element), then the state may make a SIP submission "certifying"  that the existing SIP contains provisions that address those requirements of the specific section 110(a)(2) infrastructure elements. In the case of such a certification submission, the state does not have to include a copy of the relevant provision (e.g., rule or statute) itself. Rather, the state in its infrastructure SIP submission may provide citations to the SIP-approved state statutes, regulations, or non-regulatory measures, as appropriate, which meet the relevant CAA requirement. Like any other SIP submission, that state can make such a certification only after it has provided reasonable notice and opportunity for public hearing. This "reasonable notice and opportunity for public hearing" requirement for infrastructure SIP submissions is to meet the requirements of CAA sections 110(a), and section 110(l). Under the EPA's regulations at 40 CFR part 51, if a public hearing is held, an infrastructure SIP submittal must include a certification by the state that the public hearing was held in accordance with the EPA's procedural requirements for public hearings. See 40 CFR part 51, appendix V, paragraph 2.1(g), and see 40 CFR 51.102.
In consultation with its EPA Regional office, a state should follow all applicable EPA regulations governing infrastructure SIP submissions in 40 CFR part 51 - e.g., subpart I (Review of New Sources and Modifications), subpart J (Ambient Air Quality Surveillance), subpart K (Source Surveillance), subpart L (Legal Authority), subpart M (Intergovernmental Consultation), subpart O (Miscellaneous Plan Content Requirements), subpart P (Protection of Visibility), and subpart Q (Reports). For the EPA's general criteria for infrastructure SIP submissions, refer to 40 CFR part 51, Appendix V, Criteria for Determining the Completeness of Plan Submissions. The EPA recommends that states electronically submit their infrastructure SIPs to the EPA through the State Plan Electronic Collaboration System (SPeCS), an online system available through the EPA's Central Data Exchange.
 Implementing any Revised PM2.5 NAAQS in Nonattainment Areas
Part D of the CAA describes the various program requirements that apply to nonattainment areas for different NAAQS. Section 172 (found in subpart 1 of part D) includes general SIP requirements, and sections 188-190 (found in subpart 4 of part D) include SIP requirements that specifically govern implementation for the PM10 and PM2.5 NAAQS. All PM2.5 nonattainment areas are initially classified as Moderate per CAA section 188(a). Under section 189(a)(2), states are required to submit attainment plan SIP submissions to the EPA within 18 months of the effective date of area designations. These plans need to show how the nonattainment area will attain the primary PM2.5 standards "as expeditiously as practicable," but presumptively by no later than the end of the 6[th] calendar year after the effective date of designations. For example, if the EPA finalizes nonattainment designations for a revised PM2.5 NAAQS in 2024, then the outermost statutory Moderate area attainment date would be December 31, 2030. If the state fails to attain the standard by the end of the 6[th] calendar year after the effective date of designations, the EPA is required to reclassify the area to Serious, and the state then must attain the standard by the end of the 10[th] calendar year after the effective date of designations (e.g., December 31, 2034).
On August 24, 2016, the EPA issued a detailed SIP Requirements Rule for implementing the PM2.5 NAAQS (81 FR 58010, August 24, 2016) (PM2.5 SIP Requirements Rule). It provides guidance and establishes additional regulatory requirements for states regarding development of attainment plans for nonattainment areas for the 1997, 2006, and 2012 revisions of the PM2.5 NAAQS. The EPA also intended this implementation rule to apply to nonattainment areas designated pursuant to any future revisions of the PM2.5 NAAQS. The rule covers a number of SIP requirements for nonattainment areas, including a nonattainment area emissions inventory, policies regarding PM2.5 precursor pollutants (i.e., SO2, NOx, VOC, and ammonia), control strategies (such as reasonably available control measures and reasonably available control technology), air quality modeling, attainment demonstrations, reasonable further progress requirements, quantitative milestones, and contingency measures. Guidance provided in the PM2.5 SIP Requirements Rule is supplemented by other EPA guidance documents, including guidance on emissions inventory development (80 FR 8787, February 19, 2015; U.S. EPA, 2017), optional PM2.5 precursor demonstrations (U.S. EPA, 2019b), and guidance on air quality modeling for meeting air quality goals for the ozone and PM2.5 NAAQS and regional haze program (U.S. EPA, 2018b).
Under the basic approach outlined in the PM2.5 SIP Requirements Rule, a state would first develop an updated emissions inventory of sources and emissions activities in the nonattainment area. It would then use air quality modeling or other tools to estimate the air quality improvement that can be expected in the nonattainment area by the attainment year due to enforceable and existing "on the books" federal, state, and local emissions reduction measures. The state also would work with the regulated community and other stakeholders to evaluate potential control measures for emissions sources and activities in the nonattainment area, and identify the additional reasonably available control measures (RACM) and reasonably available control technology (RACT) that can be implemented by these sources in order to attain the standard as expeditiously as practicable, but no later than by the end of the 6[th] calendar year after the effective date of designations.
The evaluation of air quality improvement associated with potential future emissions reductions is commonly performed with sophisticated air quality modeling tools. Given that fine particle concentrations are affected both by regionally-transported pollutants (e.g., SO2 and NOx emissions from power plants) and emissions of direct PM2.5 and other pollutants from local sources in the nonattainment area (e.g., steel mills, rail yards, highway mobile sources), the EPA recommends the use of regional photochemical models (such as CMAQ and CAMx), in combination with source-oriented dispersion models (such as AERMOD), as needed, to develop PM2.5 attainment strategies for any revised PM2.5 NAAQS. The EPA SIP modeling guidance provides details on the development of attainment demonstrations, and the EPA will continue to assist air agencies in modeling and technical analyses (80 FR 8787, February 19, 2015; U.S. EPA, 2017).
The PM2.5 SIP Requirements Rule provides recommendations to states regarding when and how to consider environmental justice in the context of PM2.5 attainment planning. Some of the considerations for states include: (1) identifying areas with overburdened communities where more ambient monitoring may be warranted; (2) targeting emissions reductions that may be needed to attain the PM2.5 NAAQS; and (3) increasing opportunities for meaningful involvement for overburdened populations (80 FR 58010, 58136, August 25, 2016). The EPA expects states to consider these and other factors as part of their SIP development process.
The PM2.5 SIP Requirements Rule outlines some examples of how states can implement these recommendations. For instance, states can use modeling and screening tools to better understand where sources of PM2.5 or PM2.5 precursor emissions are located and identify areas that may be candidates for additional ambient monitoring. Furthermore, once these target areas are identified, states can prioritize direct PM2.5 or PM2.5 precursor control measures and enforcement strategies in these areas to reduce ambient PM2.5 and achieve the NAAQS. The EPA recognizes that states have flexibility under the CAA to concentrate state resources on controlling sources of PM2.5 emissions that directly and adversely affect certain populations currently experiencing disparities in PM2.5 exposures and PM2.5-related health risk, thereby maximizing health benefits for those populations. Moreover, states can establish opportunities to bolster meaningful involvement in a number of ways, such as communicating with communities with disparities in exposures and risks in appropriate languages and developing enhanced notice-and-comment opportunities for those communities.
As previously mentioned, the 2016 PM2.5 SIP Requirements Rule is structured in such a way that it provides guidance and regulatory requirements for remaining nonattainment areas for the 1997, 2006, and 2012 revisions of the PM2.5 NAAQS, as well as for nonattainment areas designated pursuant to any future revisions of the PM2.5 NAAQS. Thus, the EPA is not proposing changes to the current PM2.5 SIP Requirements Rule in this proposed rulemaking, and therefore is not requesting comment on that rule.
 Implementing the Primary and Secondary PM10 NAAQS
As summarized in sections III.C.3 and III.D.3 above, the EPA is proposing to retain the current primary and secondary 24-hour PM10 standards to protect against the health effects associated with short-term exposures to thoracic coarse particles and against the welfare effects considered in this reconsideration (i.e., visibility, climate, and materials effects). The EPA intends to retain the existing implementation strategy for meeting the CAA requirements for the PM10 NAAQS. States and emissions sources should continue to follow the existing guidance and regulations for implementing the current standards.
 Prevention of Significant Deterioration and Nonattainment New Source Review Programs for the Proposed Revised Primary Annual PM2.5 NAAQS
The CAA, at parts C and D of title I, contains preconstruction review and permitting programs applicable to new major stationary sources and major modifications of existing major sources. The preconstruction review of each new major stationary source and major modification applies on a pollutant-specific basis, and the requirements that apply for each pollutant depend on whether the area in which the source is situated is designated as attainment (or unclassifiable) or nonattainment for that pollutant. In areas designated attainment or unclassifiable for a pollutant, the Prevention of Significant Deterioration (PSD) requirements under part C apply to construction at major sources. In areas designated nonattainment for a pollutant, the Nonattainment New Source Review (NNSR) requirements under part D apply to major source construction. Collectively, those two sets of permit requirements are commonly referred to as the "major New Source Review" or "major NSR" programs.
Until the EPA designates an area with respect to the proposed revised PM2.5 NAAQS, the NSR provisions applicable under an area's designation for the 1997, 2006, and 2012 PM2.5 NAAQS would continue to apply. See 40 CFR 51.166(i)(2) and 52.21(i)(2). That is, for areas designated as attainment/unclassifiable for the 1997, 2006, and 2012 PM2.5 NAAQS, PSD will apply to new major stationary sources and major modifications that trigger major source permitting requirements for PM2.5. For areas designated nonattainment for the 1997, 2006, or 2012 PM2.5 NAAQS, NNSR requirements will apply for new major stationary sources and major modifications that trigger major source permitting requirements for PM2.5. When the new designations for the proposed revised PM2.5 NAAQS, if finalized, become effective, those designations will further inform whether PSD or NNSR applies to PM2.5 in a particular area.  New major sources and major modifications will be subject to the PSD program requirements for PM2.5 if they are located in an area that does not have a current nonattainment designation under CAA section 107 for PM2.5.
The EPA has assessed the proposed revision of the level of the primary annual PM2.5 NAAQS and is not proposing any changes to the NSR program regulations as part of this proposal to revise the PM2.5 NAAQS. Sources and reviewing authorities will be able to use existing NSR regulatory provisions. Under the PSD program, the applicant must demonstrate that the new or modified source emissions increase does not cause or contribute to a NAAQS violation. In 2017, the EPA revised the Guideline on Air Quality Models (published as Appendix W to 40 CFR Part 41) to address primary and secondary PM2.5 impacts in making this demonstration and has since provided associated technical guidance, models and tools, such as the recent "Final Guidance for Ozone and Fine Particulate Matter Permit Modeling" (July 29, 2022). The EPA will consider whether changes or updates to PSD program guidance or associated tools are warranted as a result of the proposed revision to the primary annual PM2.5 NAAQS, should it be finalized, and would communicate such changes through separate action(s) following promulgation of a revised standard.
The statutory requirements for a PSD permit program set forth under part C of title I of the CAA (sections 160 through 169) are addressed by the EPA's PSD regulations found at 40 CFR 51.166 (minimum requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD permitting program for permits issued under the EPA's federal permitting authority). These regulations already apply for PM2.5 in areas that have been designated attainment or unclassifiable for PM2.5 whenever a proposed new major source or major modification triggers PSD requirements for PM2.5.
      For PSD, a "major stationary source" is one with the potential to emit 250 tons per year (tpy) or more of any regulated NSR pollutant, unless the new or modified source is classified under a list of 28 source categories contained in the statutory definition of "major emitting facility" in section 169(1) of the CAA. For those 28 source categories, a "major stationary source" is one with the potential to emit 100 tpy or more of any regulated NSR pollutant. A "major modification" is a physical change or a change in the method of operation of an existing major stationary source that results, first, in a significant emissions increase of a regulated NSR pollutant and, second, in a significant net emissions increase of that pollutant. See 40 CFR 51.166(b)(2)(i), 40 CFR 52.21(b)(2)(i). The EPA PSD regulations define the term "regulated NSR pollutant" to include any pollutant for which a NAAQS has been promulgated and any pollutant identified in the EPA regulations as a constituent or precursor to such pollutant. See 40 CFR 51.166(b)(49), 40 CFR 52.21(b)(50). These regulations identify SO2 and NOX as precursors to PM2.5 in all attainment and unclassifiable areas. See 40 CFR 51.166(b)(49)(i), 40 CFR 52.21(b)(50)(i). Thus, for PM2.5, the PSD program currently requires the review and control of emissions of direct PM2.5 emissions and SO2 and NOX (as precursors to PM2.5), as applicable. Among other things, for each regulated NSR pollutant emitted or increased in a significant amount, the PSD program requires a new major stationary source or a major modification to apply the "best available control technology" (BACT) and to conduct an air quality impact analysis to demonstrate that the proposed major stationary source or major modification will not cause or contribute to a violation of any NAAQS or PSD increment. See CAA section 165(a)(3) - (4), 40 CFR 51.166(j) - (k), 40 CFR 52.21(j) - (k). The PSD requirements may also include, in appropriate cases, an analysis of potential adverse impacts on Class I areas. See CAA section 162(a) and 165, 40 CFR 51.166(p); 40 CFR 52.21(p)). The EPA has developed the Guideline on Air Quality Models and other documents to, among other things, provide methods and guidance for demonstrating compliance with the PM2.5 NAAQS and PSD increments for PM2.5.
The EPA has historically interpreted the requirement for an air quality impact analysis under CAA section 165(a)(3) and the implementing regulations to include a requirement to demonstrate that emissions from the proposed facility will not cause or contribute to a violation of any NAAQS that is in effect as of the date a PSD permit is issued, except to the extent that a pending permit application was subject to grandfathering provisions that the EPA had established through rulemaking. The EPA is not proposing such provisions for this action. In past NAAQS revision rules, including the 2012 PM2.5 NAAQS (78 FR 3086, January 15, 2013) and 2015 Ozone NAAQS (80 FR 65292, October 26, 2015), the EPA included limited grandfathering provisions that exempted certain pending PSD permit actions (those that had reached a particular stage in the permitting process at the time the revised NAAQS was promulgated or became effective) from the requirement to demonstrate that the proposed emissions increases would not cause or contribute to a violation of the revised NAAQS. In August 2019, the U.S. Court of Appeals for the D.C. Circuit vacated the grandfathering provision in the PSD rules applicable to the 2015 Ozone NAAQS, finding that the provision contradicted "Congress's `express policy choice' not to allow construction which will `cause or contribute to' nonattainment of `any' effective NAAQS, regardless of when they are adopted or when a permit was completed." Murray Energy Corp. v. EPA, 936 F.3d 597, 627 (D.C. Cir. 2019). Based on that court decision, the EPA is not proposing any grandfathering provision for this proposed PM2.5 NAAQS revision, if finalized. Accordingly, PSD permits issued on or after the effective date of any final revised PM2.5 NAAQS would require a demonstration that the proposed emissions increases would not cause or contribute to a violation of the revised PM2.5 NAAQS.
The EPA anticipates that, if this rule is finalized as proposed, the existing PM2.5 air quality in some areas will not be in attainment of the new revised primary annual PM2.5 NAAQS, and that these areas will be designated as "nonattainment" at a later date, consistent with the designation process described in the preceding sections. However, until such nonattainment designation occurs, proposed new major sources and major modifications located in any area currently designated attainment or unclassifiable for PM2.5 will continue to be subject to the PSD program requirements for PM2.5. This raises the question as to how a source can be issued a PSD permit in light of known existing ambient violations of the revised NAAQS. Section 165(a)(3)(B) of the CAA states that a proposed source may not construct unless it demonstrates that it will not cause or contribute to a violation of any NAAQS. This statutory requirement is implemented through a provision contained in the PSD regulations at 40 CFR 51.166(k) and 52.21(k). If a source cannot make this demonstration, or if its initial air quality impact analysis shows that the source's impact would cause or contribute to a violation, a PSD permit may not be issued unless the permit applicant compensates for the adverse impact that would otherwise cause or contribute to a violation of the NAAQS. While the PSD regulations do not explicitly specify remedial actions that a prospective source can take to address such a situation, the EPA has historically recognized in regulations, and through other actions, that sources applying for PSD permits may utilize offsets as part of the required PSD demonstration under CAA section 165(a)(3)(B).
Part D of title I of the CAA includes preconstruction review and permitting requirements applicable to new major stationary sources and major modifications located in areas designated nonattainment for a pollutant for which a NAAQS has been established (i.e., a criteria pollutant). The relevant part D requirements are typically referred to as the NNSR program. The EPA's regulations for the NNSR programs are contained in 40 CFR 51.165, 52.24 and part 51, Appendix S. Specifically, the EPA has developed minimum program requirements for an NNSR program that is approvable in a SIP, and those requirements, which include requirements for PM2.5, are contained in 40 CFR 51.165. In addition, 40 CFR part 51, Appendix S contains requirements constituting an interim NNSR program. This program enables NNSR permitting in nonattainment areas by states that lack a SIP-approved NNSR permitting program during the time between the date of the relevant designation and the date that the EPA approves into the SIP a NNSR program. See 40 CFR part 51, Appendix S, Part I; 40 CFR 52.24(k).
For NNSR, "major stationary source" is generally defined as a source with the potential to emit at least 100 tpy of the regulated NSR pollutant for which the area is designated nonattainment. In some cases, however, the CAA and the NNSR regulations define "major stationary source" for NNSR in terms of a lower rate dependent on the pollutant and degree of nonattainment in the area. For PM2.5, in addition to the general threshold level of 100 tpy, a lower major source threshold of 70 tpy applies in Serious PM2.5 nonattainment areas pursuant to subpart 4 of part D, title I of the CAA. See 40 CFR 51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 CFR part 51, Appendix S, II.A.4.(i)(a)(7) and (8).
Under the NNSR program, direct PM2.5 emissions and emissions of each PM2.5 precursor are reviewed separately in accordance with the applicable major source threshold. For example, the threshold for Serious PM2.5 nonattainment areas is 70 tpy of direct PM2.5, as well as for the PM2.5 precursors SO2, NOX, VOC, and ammonia. See 40 CFR 51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 CFR part 51, Appendix S, II.A.4.(i)(a)(7) and (8). For modifications, NNSR applies to proposed physical changes or changes in the method of operation of an existing stationary source where (1) the source is major for the nonattainment pollutant (or a precursor for that pollutant) and (2) the physical change or change in the method of operation of a major stationary source results, first, in a significant emissions increase of a regulated NSR pollutant and, second, in a significant net emissions increase of that same nonattainment pollutant (or same precursor for that pollutant). See 40 CFR 51.165(a)(1)(v)(A); 40 CFR part 51, Appendix S, II.A.5.(i).
For example, to qualify as a major modification for SO2 (as a PM2.5 precursor) in a moderate PM2.5 nonattainment area, the existing source would have to have the potential to emit 100 tpy or more of SO2, and the project would have to result in an increase in SO2 emissions of 40 tpy or more. See 40 CFR 51.165(a)(1)(x)(A). New major stationary sources and major modifications for PM2.5 subject to NNSR must comply with the "lowest achievable emission rate" (LAER) as defined in the CAA and NNSR rules, as well as performing other analyses as required under section 173 of the CAA.
Following the promulgation of any revised NAAQS for PM2.5, some new nonattainment areas for PM2.5 may result. Where a state does not have an NNSR program or where the current NNSR program does not apply to PM2.5, that state will be required to submit the necessary SIP revisions to ensure that new major stationary sources and major modifications for PM2.5 undergo preconstruction review pursuant to the NNSR program. States are required to submit nonattainment plans to provide for attainment and maintenance of a revised PM2.5 NAAQS within 18 months from the effective date of nonattainment area designations as required under CAA section 189(a)(2)(B). Therefore, states whose existing NNSR program requirements, if any, cannot be interpreted to apply to the revised primary annual PM2.5 NAAQS at that time will be allowed to issue the necessary permits in accordance with the applicable nonattainment permitting requirements contained in 40 CFR part 51, Appendix S, which would apply to the revised PM2.5 NAAQS upon its effective date. See 73 FR 28321, 28340, May 16, 2008.
Finally, the EPA recommends that, where appropriate, PSD and NNSR permitting authorities assess impacts to communities with environmental justice concerns. For example, this may include conducting a demographic analysis to inform development of a plan for community outreach and engagement, conducting a cumulative emissions impact analysis, or considering the environmental and social costs imposed on the impacted community when conducting an alternative sites analysis.  Another option could be improving the understanding of the potential impact of minor sources by generating an emissions inventory for such minor sources, including sources that are not currently required to report emissions, to generate options on how emissions can be reduced in the target area. See 81 FR 58010, 58137. The EPA anticipates developing further information and consulting with permitting authorities on how to best address environmental justice in the permitting process.
 Transportation Conformity Program
Transportation conformity is required under CAA section 176(c) to ensure that transportation plans, transportation improvement programs (TIPs) and federally supported highway and transit projects will not cause or contribute to any new air quality violation, increase the frequency or severity of any existing violation, or delay timely attainment or any required interim emissions reductions or other milestones. Transportation conformity applies to areas that are designated as nonattainment or nonattainment areas that have been redesignated to attainment with an approved CAA section 175A maintenance plan (i.e., maintenance areas) for transportation-related criteria pollutants: carbon monoxide, ozone, NO2, PM2.5, and PM10. Transportation conformity for any new or revised NAAQS for PM2.5 does not apply until one year after the effective date of the nonattainment designation for that NAAQS. See CAA section 176(c)(6) and 40 CFR 93.102(d)). The EPA's Transportation Conformity Rule establishes the criteria and procedures for determining whether transportation activities conform to the SIP. The EPA is not proposing changes to the transportation conformity rule in this proposed rulemaking. The EPA notes that the transportation conformity rule already addresses the PM2.5 and PM10 NAAQS. However, in the future, the EPA will review the need to issue or revise guidance describing how the current conformity rule applies in nonattainment and maintenance areas for any new or revised primary or secondary PM NAAQS, as needed.
 General Conformity Program
The general conformity program implements CAA section 176(c) and requires that Federal agencies do not adopt, accept, approve, or fund activities that are not consistent with state air quality goals. General conformity applies to any federal action (e.g., funding, licensing, permitting, or approving) if 1) the action takes place in a nonattainment or maintenance area for any of the criteria pollutants and 2) it is not a Federal Highway Administration (FHWA) or Federal Transit Administration (FTA) project as defined in 40 CFR 93.101 (these projects are covered under the transportation conformity program described above).
The EPA's General Conformity Rule establishes the criteria and procedures for determining if a federal action conforms to the applicable attainment plan. General conformity for any revised PM2.5 NAAQS does not apply until one year after the effective date of the nonattainment designation for that NAAQS. The EPA is not proposing changes to the general conformity rule in this proposed rulemaking. The EPA notes that the general conformity rule already addresses the PM2.5 and PM10 NAAQS. However, in the future, the EPA will review the need to issue or revise guidance describing how the current conformity rule applies in nonattainment and maintenance areas for any new or revised primary or secondary PM NAAQS, as needed.
IX. Statutory and Executive Order Reviews
Additional information about these statutes and Executive Orders can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.
A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review
This action is an economically significant regulatory action that was submitted to the Office of Management and Budget (OMB) for review. Any changes made in response to OMB recommendations have been documented in the docket. The EPA prepared an illustrative analysis of the potential costs and benefits associated with this action. This analysis is contained in the document, "Regulatory Impact Analysis for the Proposed Reconsideration of the National Ambient Air Quality Standards for Particulate Matter," which is available in the Regulatory Impact Analysis (RIA) docket (EPA - HQ - OAR - 2019-0587) and briefly summarized below.  The RIA estimates the costs and monetized human health benefits in 2032, after implementing existing and expected regulations and assessing emissions reductions to meet the current annual and 24-hour particulate matter NAAQS (12/35 ug/ - m - 3), associated with applying national control strategies for the proposed annual and 24-hour alternative standard levels of 10/35 ug/ - m - [3] -  and 9/35 ug/m[3], as well as the following two more stringent alternative standard levels: (1) an alternative annual standard level of 8 ug/ - m - 3 -  in combination with the current 24-hour standard (i.e., 8/35 ug/ - m - [3] - ), and (2) an alternative 24-hour standard level of 30 ug/ - m - [3] in combination with the proposed annual standard level of 10 ug/ - m - [3] -  (i.e., 10/30 ug/ - m - 3 - ). . Table 2 provides a summary of the estimated monetized benefits, costs, and net benefits associated with applying national control strategies  -  - toward reaching  alternative standard levels. However, the CAA and judicial decisions make clear that the economic and technical feasibility of attaining ambient standards are not to be considered in setting or revising NAAQS, although such factors may be considered in the development of state plans to implement the standards. Accordingly, although an RIA has been prepared, the results of the RIA have not been considered in issuing this proposed rule.
Table 2  -  Estimated Monetized Benefits, Costs, and Net Benefits of the Illustrative Control Strategies Applied Toward the Primary Alternative Annual and Daily Standard Levels of 10/35 g/ - m - 3 - , 10/30 g/ - m - 3 - , 9/35 g/ - m - 3 - , and 8/35 g/ - m - 3 -  in 2032 for the U.S. (millions of 2017$)

                                     10/35
                                     10/30
                                     9/35
                                     8/35
                                  Benefits[a]
                              $8,500 and $17,000
                              $9,600 and $20,000
                              $21,000 and $43,000
                              $46,000 and $95,000
                                   Costs[b]
                                      $95
                                     $260
                                     $390
                                    $1,800
                                 Net Benefits
                              $8,400 and $17,000
                              $9,300 and $19,000
                              $20,000 and $43,000
                              $44,000 and $93,000
Notes: Rows may not appear to add correctly due to rounding. We focus results to provide a snapshot of costs and benefits in 2032, using the best available information to approximate social costs and social benefits recognizing uncertainties and limitations in those estimates. The estimated costs and monetized human health benefits associated with applying national control strategies do not fully account for all the emissions reductions needed to reach the proposed and more stringent alternative standard levels for some standard levels analyzed.
[a] We assume that there is a cessation lag between the change in PM exposures and the total realization of changes in mortality effects. Specifically, we assume that some of the incidences of premature mortality related to PM2.5 exposures occur in a distributed fashion over the 20 years following exposure, which affects the valuation of mortality benefits at different discount rates. Similarly, we assume there is a cessation lag between the change in PM exposures and both the development and diagnosis of lung cancer. The benefits are associated with two point estimates from two different epidemiologic studies, and we present the benefits calculated at a real discount rate of 3 percent. The benefits exclude additional health and welfare benefits that could not be quantified.
b The costs are annualized using a 7 percent interest rate.

B. Paperwork Reduction Act (PRA)
This action does not impose an information collection burden under the PRA. There are no information collection requirements directly associated with a proposed decision to revise or retain a NAAQS under section 109 of the CAA.
C. Regulatory Flexibility Act (RFA)
I certify that this action will not have a significant economic impact on a substantial number of small entities under the RFA. This action will not impose any requirements on small entities. Rather, this proposed rule establishes national standards for allowable concentrations of PM in ambient air as required by section 109 of the CAA. See also American Trucking Associations v. EPA, 175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant impacts upon small entities because NAAQS themselves impose no regulations upon small entities), rev'd in part on other grounds, Whitman v. American Trucking Associations, 531 U.S. 457 (2001).
D. Unfunded Mandates Reform Act (UMRA)
This action does not contain any unfunded mandate as described in the Unfunded Mandates Reform Act (UMRA), 2 U.S.C. 1531 -- 1538, and does not significantly or uniquely affect small governments. Furthermore, as indicated previously, in setting a NAAQS the EPA cannot consider the economic or technological feasibility of attaining ambient air quality standards, although such factors may be considered to a degree in the development of state plans to implement the standards. See also American Trucking Associations v. EPA, 175 F. 3d at 1043 (noting that because the EPA is precluded from considering costs of implementation in establishing NAAQS, preparation of the RIA pursuant to the Unfunded Mandates Reform Act would not furnish any information that the court could consider in reviewing the NAAQS).
The EPA acknowledges, however, that if corresponding revisions to associated SIP requirements and air quality surveillance requirements are proposed at a later time, those revisions might result in such effects. Any such effects would be addressed as appropriate if and when such revisions are proposed.
E. Executive Order 13132: Federalism
This action does not have federalism implications. It will not have substantial direct effects on the states, on the relationship between the national government and the states, or on the distribution of power and responsibilities among the various levels of government.  However, the EPA recognizes that states will have a substantial interest in this action and any future revisions to associated requirements.
F. Executive Order 13175: Consultation and Coordination with Indian Tribal Governments
This action does not have tribal implications, as specified in Executive Order 13175. It does not have a substantial direct effect on one or more Indian Tribes as tribes are not obligated to adopt or implement any NAAQS. In addition, tribes are not obligated to conduct ambient monitoring for PM or to adopt the ambient monitoring requirements of 40 CFR part 58. Thus, Executive Order 13175 does not apply to this action. However, consistent with the EPA Policy on Consultation and Coordination with Indian Tribes, the EPA will offer government-to-government consultation with tribes as requested.
G. Executive Order 13045: Protection of Children from Environmental Health Risks and Safety Risks
This action is subject to Executive Order 13045 because it is an economically significant regulatory action as defined by Executive Order 12866, and the EPA believes that the environmental health or safety risk addressed by this action may have a disproportionate effect on children. The Policy on Children's Health also applies to this action. Accordingly, we have evaluated the environmental health or safety effects of PM exposures on children. The protection offered by these standards may be especially important for children because childhood represents a lifestage associated with increased susceptibility to PM-related health effects. Because children have been identified as a susceptible population, we have carefully evaluated the environmental health effects of exposure to PM pollution among children. Children make up a substantial fraction of the U.S. population, and often have unique factors that contribute to their increased risk of experiencing a health effect due to exposures to ambient air pollutants because of their continuous growth and development. As described in the 2019 Integrated Science Assessment, children may be particularly at risk for health effects related to ambient air PM2.5 exposures compared with adults because they have (1) a developing respiratory system, (2) increased ventilation rates relative to body mass compared with adults, and (3) an increased proportion of oral breathing, particularly in boys, relative to adults. More detailed information on the evaluation of the scientific evidence and policy considerations pertaining to children, including an explanation for why the Administrator judges the proposed standards to be requisite to protect public health, including the health of children, with an adequate margin of safety, are contained in sections II.B and II.D of this preamble. Copies of all documents have been placed in the public docket for this action.
H. Executive Order 13211: Actions Concerning Regulations that Significantly Affect Energy Supply, Distribution or Use