In the 2012 and 2020 reviews, quantitative analyses for PM-related visibility effects focused on daily visibility impairment, given the short-term nature of PM-related visibility effects. The evidence and information available in this reconsideration continues to provide support for the short-term (i.e., hourly or daily) nature of PM-related visibility impairment. As such, the quantitative analyses presented in the PA continue to focus on daily visibility impairment and utilize a two-phase assessment approach for visibility impairment, consistent with the approaches taken in past reviews. First, the PA considers the appropriateness of the elements (indicator, averaging time, form, and level) of the visibility index for providing protection against PM-related visibility effects. Second, recent air quality was used to evaluate the relationship between the current secondary 24-hour PM2.5 standard and the visibility index. The information available since the 2012 review includes an updated equation for estimating light extinction, summarized in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>) and described in the 2019 ISA (U.S. EPA, 2019a, section <IP_ADDRESS>), as well as more recent air monitoring data, that together allow for development of an updated assessment of PM-related visibility impairment in study locations in the U.S.
a. Target Level of Protection in Terms of a PM2.5 Visibility Index
In evaluating the adequacy of the current secondary PM standards, the PA first evaluates the appropriateness of the elements (indicator, averaging time, form, and level) identified for a distinct secondary standard to protect against visibility effects. In previous reviews, the visibility index was set at a level of 30 dv, with estimated light extinction as the indicator, a 24-hour averaging time, and a 90[th] percentile form, averaged over three years.
With regard to an indicator for the visibility index, the PA recognizes the lack of availability of methods and an established network for directly measuring light extinction (U.S. EPA, 2022b, section <IP_ADDRESS>). Therefore, consistent with previous reviews, the PA concludes that a visibility index based on estimates of light extinction by PM2.5 components derived from an adjusted version of the original IMPROVE algorithm to be the most appropriate indicator for the visibility index in this reconsideration. As described in section <IP_ADDRESS> of the PA, the IMPROVE algorithm estimates light extinction using routinely monitored components of PM2.5 and PM10-2.5, along with estimates of relative humidity (U.S. EPA, 2022b, section <IP_ADDRESS>).
With regard to averaging time, the PA notes that the evidence continues to provide support for the short-term nature of PM-related visibility effects. Given that there is no new information available regarding the time periods during which visibility impairment occurs or public preferences related to specific time periods for visibility impairment, the PA concludes that it is appropriate to continue to focus on daily visibility impairment. In so doing, the PA relies on analyses that were conducted in the 2012 review that showed relatively strong correlations between 24-hour and sub-daily (i.e., 4-hour average) PM2.5 light extinction that indicated that a 24-hour averaging time is an appropriate surrogate for the sub-daily time periods relevant for visual perception (U.S. EPA, 2011, Figures G-4 and G-5; Frank, 2012). These analyses continue to provide support for a 24-hour averaging time for the visibility index in this reconsideration. Consistent with previous reviews, the PA also notes that the 24-hour averaging time may be less influenced by atypical conditions and/or atypical instrument performance than a sub-daily averaging time (85 FR 82740, December 18, 2020; 78 FR 3226, January 15, 2013).
With regard to the form for the visibility index, the available information continues to provide support for a 3-year average of annual 90[th] percentile values. Given that there is no new information to inform selection of an alternate form, as in previous reviews, the PA notes that the 3-year average form provides stability from the occasional effect of inter-annual meteorological variability that can result in unusually high pollution levels for a particular year (85 FR 82741, December 18, 2020; 78 FR 3198, January 15, 2013; U.S. EPA, 2011, p. 4-58). In so doing, the PA considers the evaluation in the 2010 Urban-Focused Visibility Assessment (UFVA) of three different statistical forms: 90[th], 95[th], and 98[th] percentiles (U.S. EPA, 2010a, Chapter 4). In considering this evaluation of statistical forms from the 2010 UFVA, consistent with the 2011 PA, the PA notes that the Regional Haze Program targets the 20 percent most impaired days for visibility improvements in visual air quality in Federal Class I areas and that the median of the distribution of these 20 percent most impaired days would be the 90[th] percentile. The 2011 PA also noted that strategies that are implemented so that 90 percent of days would have visual air quality that is at or below the level of the visibility index would reasonably be expected to lead to improvements in visual air quality for the 20 percent most impaired days. Additionally, as in the 2011 PA, the PA recognizes that the available public preference studies do not address frequency of occurrence of different levels of visibility (U.S. EPA, 2022b, section <IP_ADDRESS>). Therefore, the analyses and consideration for the form of a visibility index from the 2011 PA continue to provide support for a 90[th] percentile form, averaged across three years, in defining the characteristics of a visibility index in this reconsideration.
With regard to the level for the visibility index, the PA recognizes that there is an additional public preference study (Malm et al., 2019) available in this reconsideration. As noted above, however, this study differs from the previously available public preference studies in several ways which makes it difficult to integrate this newly available study with the previously available studies. Most significantly, this study was evaluated public preferences for visibility in the Grand Canyon, perhaps the most notable Class I area in the country for visibility purposes. Therefore, the PA concludes that the Grand Canyon study is not directly comparable to the other available preferences studies and public preferences of visibility impairment in the Malm et al. (2019)are not appropriate to consider in identifying a range of levels for the target level of protection against visibility impairment for this reconsideration of the secondary PM NAAQS.
Therefore, the PA continues to rely on the same studies and the range of 20 to 30 dv identified from those studies in previous reviews. With regard to selecting the appropriate target level of protection for visibility impairment within this range, the PA notes that in previous reviews, a level at the upper end of the range (i.e., 30 dv) was selected given the uncertainties and limitations associated with the public preference studies (U.S. EPA, 2022b, section <IP_ADDRESS>). However, the PA also recognizes that (1) the degree of protection provided by a secondary PM NAAQS is not determined solely by any one element of the standard but by all elements (i.e., indicator, averaging time, form, and level) being considered together, and (2) decisions regarding the adequacy of the current secondary standards is a public welfare policy judgment to be made by the Administrator. As such, the Administrator may judge that a target level of protection below the upper end of the range (i.e., less than 30 dv) is appropriate, depending on his public welfare policy judgments, which draw upon the available scientific evidence for PM-related visibility effects and on analyses of visibility impairment, as well as judgments about the appropriate weight to place on the range of uncertainties inherent in the evidence and analyses.
In considering the available public preference studies, consistent with past reviews, the PA concludes that it is reasonable to consider a range of 20 to 30 dv for selecting a target level of protection, including a high value of 30 dv, a midpoint value of 25 dv, and a low value of 20 dv. A target level of protection at or in the upper end of the range would focus on the Washington, DC, preference study results (Abt Associates, 2001; Smith and Howell, 2009) which identified 30 dv as the level of impairment that was determined to be "acceptable" by at least 50 percent of study participants. The public preferences of visibility impairment in the Washington, DC, study are likely to be generally representative of urban areas that do not have valued scenic elements (e.g., mountains) in the distant background. This would be more representative of areas in the middle of the country and many areas in the eastern U.S., as well as possibly some areas in the western U.S.
A target level of protection in the middle of the range would be most closely associated with the level of impairment that was determined to be "acceptable" by at least 50 percent of study participants in the Phoenix, AZ, study (BBC Research & Consulting, 2003), which was 24.3 dv. This study, while methodologically similar to the other public preference studies, included participants that were selected as a representative sample of the Phoenix area population and used computer-generated images to depict specific uniform visibility impairment conditions. This study yielded the best results of the four public preference studies in terms of the least noisy preference results and the most representative selection of participants. Therefore, based on this study, the use of 25 dv to represent a midpoint within the range of target levels protection is well supported.
A target level of protection at or just above the lower end of the range would focus on the Denver, CO, study, but may not be as strongly supported as higher levels within the range (Ely et al., 1991). Older studies, such as those conducted in Denver, CO (Ely et al., 1991), and British Columbia, Canada (Pryor, 1996), used photographs that were taken at different times of the day and on different days to capture a range of light extinction levels needed for the preference studies. Compared to studies that used computer-generated images (i.e., those in Phoenix, AZ, and Washington, DC) there was more variability in scene appearance in these older studies that could affect preference rating and includes uncertainties associated with using ambient measurements to represent sight path-averaged light extinction values rather than superimposing a computer-generated amount of haze onto the images. When using photographs, the intrinsic appearance of the scene can change due to meteorological conditions (i.e., shadow patterns and cloud conditions) and spatial variations in ambient air quality that can result in ambient light extinction measurement not being representative of the sight-path-averaged light extinction. Computer-generated images, such as those generated with WinHaze, do not introduce such uncertainties, as the same base photograph is used (i.e., there is no intrinsic change in scene appearance) and the modeled haze that is superimposed on the photograph is determined based on uniform light extinction throughout the scene.
In addition to differences in preferences that may arise from photographs versus computer-generated images, urban visibility preference may differ by location, and such differences may arise from differences in the cityscape scene that is depicted in the images. These differences are related to the perceived value of objects and scenes that are included in the image, as objects at a greater distance have a greater sensitivity to perceived visibility changes as light extinction is changed compared to similar scenes with objects at shorter distances. For example, a person (regardless of their location) evaluating visibility in an image with more scenic elements such as mountains or natural views may value better visibility conditions in these images compared to the same level of visibility impairment in an image that only depicts urban features such as buildings and roads. That is, if a person was shown the same level of visibility impairment in two images depicting different scenes  -  one with mountains in the background and urban features in the foreground and one with no mountains in the background and nearby buildings in the image without mountains in the distance  -  may find the amount of haze to be unacceptable in the image with the mountains in the distance because of a greater perceived value of viewing the mountains, while finding the amount of haze to be acceptable in the image with the buildings because of a lesser value of viewing the cityscape or an expectation that such urban areas may generally have higher levels of haze in general. This is consistent when comparing the differences between the Denver, CO, study results (which found the 50% acceptance criteria occurred at the best visual air quality levels among the four cities) and the Washington, DC, results (which found the 50% acceptability criteria occurred at the worst visual air quality levels among the four cities). These results may occur because the most prominent and picturesque feature of the cityscape of Denver is the visible snow-covered mountains in the distance, while the prominent and picturesque features of the Washington, DC, cityscape are buildings relatively nearby without prominent and/or values scenic features that are more distant. Given these variabilities in preferences it is unclear to what extent, the available evidence provides strong support for a target level of protection at the lower end of the range. Future studies that reduce sources of noisiness and uncertainty in the results could provide more information that would support selection of a target level of protection at or just above the lower end of the range.
Taken together, the PA concludes that available information continues to support a visibility index with estimated light extinction as the indicator, a 24-hour averaging time, and a 90[th] percentile form, averaged over three years, with a level within the range of 20 to 30 dv.
b. Relationship between the PM2.5 Visibility Index and the Current Secondary 24-Hour PM2.5 Standard
The PA presents quantitative analyses based on recent air quality that evaluate the relationship between recent air quality and calculated light extinction. As in previous reviews, these analyses explored this relationship as an estimate of visibility impairment in terms of the 24-hour PM2.5 standard and the visibility index. Generally, the results of the updated analyses are similar to those based on the data available at the time of the 2012 and 2020 reviews (U.S. EPA, 2022b, section <IP_ADDRESS>). As discussed in section V.C.1.a above, the PA concludes that the available evidence continues to support a visibility index with estimated light extinction as the indicator, a 24-hour averaging time, and a 90[th] percentile form, averaged over three years, with a level within the range of 20 to 30 dv. These analyses evaluate visibility impairment in the U.S. under recent air quality conditions, particularly those conditions that meet the current standards, and the relative influence of various factors on light extinction. Given the relationship of visibility with short-term PM, we focus particularly on the short-term PM standards. Compared to the 2012 review, updated analyses incorporate several refinements, including (1) the evaluation of three versions of the IMPROVE equation to calculate light extinction (U.S. EPA, 2022b, Appendix D, Equations D-1 through D-3) in order to better understand the influence of variability in equation inputs; (2) the use of 24-hour relative humidity data, rather than monthly average relative humidity as was used in the 2012 review (U.S. EPA, 2022b, section <IP_ADDRESS>, Appendix D); and (3) the inclusion of the coarse fraction in the estimation of light extinction (U.S. EPA, 2022b, section <IP_ADDRESS>, Appendix D). The analyses in the reconsideration are updated from the 2012 and 2020 reviews and include 60 monitoring sites that measure PM2.5 and PM10 and are geographically distributed across the U.S. in both urban and rural areas (U.S. EPA, 2022b, Appendix D, Figure D-1).
When light extinction was calculated using the revised IMPROVE equation, in areas that meet the current 24-hour PM2.5 standard for the 2017-2019 time period, all sites have light extinction estimates at or below 26 dv (U.S. EPA, 2022b, Figure 5-3). For the four locations that exceed the current 24-hour PM2.5 standard, light extinction estimates range from 22 dv to 27 dv (U.S. EPA, 2022b, Figure 5-3). These findings are consistent with the findings of the analyses using the same IMPROVE equation in the 2012 review with data from 102 sites with data from 2008-2010 and in the 2020 review with data from 67 sites with data from 2015-2017. The analyses presented in the PA indicate similar findings to those from the analyses in the 2012 and 2020 reviews, i.e., the updated quantitative analysis shows that the 3-year visibility metric was no higher than 30 dv at sites meeting the current secondary PM standards, and at most such sites the 3-year visibility index values are much lower (e.g., an average of 20 dv across the 60 sites).
When light extinction was calculated using the revised IMPROVE equation, the resulting 3-year visibility metrics are nearly identical to light extinction estimates calculated using the original IMPROVE equation (U.S. EPA, 2022b, Figure 5-4), but some sites are just slightly higher. Using the revised IMPROVE equation, for those sites that meet the current 24-hour PM2.5 standard, the 3-year visibility metric is at or below 26 dv. For the four locations that exceed the current 24-hour PM2.5 standard, light extinction estimates range from 22 dv to 29 dv (U.S. EPA, 2022b, Figure 5-4). These results are similar to those for light extinction calculated using the original IMPROVE equation, and those from previous reviews.
When light extinction was calculated using the refined equation from Lowenthal and Kumar (2016), the resulting 3-year visibility metrics are slightly higher at all sites compared to light extinction estimates calculated using the original IMPROVE equation (U.S. EPA, 2022b, Figure 5-5). These higher estimates are to be expected, given the higher OC multiplier included in the IMPROVE equation from Lowenthal and Kumar (2016), which reflects the use of data from remote areas with higher concentrations of organic PM when validating the equation. As such, it is important to note that the Lowenthal and Kumar (2016) version of the equation may overestimate light extinction in non-remote areas, including the urban areas in the updated analyses in this reconsideration.
Nevertheless, when light extinction is calculated using the Lowenthal and Kumar (2016) equation for those sites that meet the current 24-hour PM2.5 standard, the 3-year visibility metric is generally at or below 28 dv. For those sites that exceed the current 24-hour PM2.5 standard, three of these sites have a 3-year visibility metric ranging between 26 dv and 30 dv, while one site in Fresno, California that exceeds the current 24-hour PM2.5 standard and has a 3-year visibility index value of 32 dv (compared to 29 dv when light extinction is calculated with the original IMPROVE equation) (see U.S. EPA, 2022b, Appendix D, Table D-3). At this site, it is likely that the 3-year visibility metric using the Lowenthal and Kumar (2016) equation would be below 30 dv if PM2.5 concentrations were reduced such that the 24-hour PM2.5 level of 35 ug/m[3] was attained.
In considering visibility impairment under recent air quality conditions, the PA recognizes that the differences in the inputs to equations estimating light extinction can influence the resulting values. For example, given the varying chemical composition of emissions from different sources, the 2.1 multiplier in the Lowenthal and Kumar (2016) equation may not be appropriate for all source types. At the time of the 2012 review, the EPA judged that a 1.6 multiplier for converting OC to OM was more appropriate, for the purposes of estimating visibility index at sites across the U.S., than the 1.4 or 1.8 multipliers used in the original and revised IMPROVE equations, respectively. A multiplier of 1.8 or 2.1 would account for the more aged and oxygenated organic PM that tends to be found in more remote regions than in urban regions, whereas a multiplier of 1.4 may underestimate the contribution of organic PM found in remote regions when estimating light extinction (78 FR 3206, January 15, 2013; U.S. EPA, 2012, p. IV-5). The available scientific information and results of the air quality analyses indicate that it may be appropriate to select inputs to the IMPROVE equation (e.g., the multiplier for OC to OM) on a regional basis rather than a national basis when calculating light extinction. This is especially true when comparing sites with localized PM sources (such as sites in urban or industrial areas) to sites with PM derived largely from biogenic precursor emissions (that contribute to widespread secondary organic aerosol formation), such as those in the southeastern U.S. The PA notes, however, that conditions involving PM from such different sources have not been well studied in the context of applying a multiplier to estimate light extinction, contributing uncertainty to estimates of light extinction for such conditions.
At the time of the 2012 review, the EPA noted that PM2.5 is the size fraction of PM responsible for most of the visibility impairment in urban areas (77 FR 38980, June 29, 2012). Data available at the time of the 2012 review suggested that, generally, PM10-2.5 was a minor contributor to visibility impairment most of the time (U.S. EPA, 2010a) although the coarse fraction may be a major contributor in some areas in the desert southwestern region of the U.S. Moreover, at the time of the 2012 review, there were few data available from PM10-2.5 monitors to quantify the contribution of coarse PM to calculated light extinction. Since that time, an expansion in PM10-2.5 monitoring efforts has increased the availability of data for use in estimating light extinction with both PM2.5 and PM10-2.5 concentrations included as inputs in the equations. The analysis in the 2020 review addressed light extinction at 20 of the 67 PM2.5 sites where collocated PM10-2.5 monitoring data were available. Since the 2020 review, PM10-2.5 monitoring data are available at more locations and the analyses presented in the PA include those for light extinction estimated with coarse and fine PM at all 60 sites. Generally, the contribution of the coarse fraction to light extinction at these sites is minimal, contributing less than 1 dv to the 3-year visibility metric (U.S. EPA, 2020a, section <IP_ADDRESS>). However, the PA notes that in the updated quantitative analyses, only a few sites were in locations that would be expected to have high concentrations of coarse PM, such as the Southwest. These results are consistent with those in the analyses in the 2019 ISA, which found that mass scattering from PM10−2.5 was relatively small (less than 10%) in the eastern and northwestern U.S., whereas mass scattering was much larger in the Southwest (more than 20%) particularly in southern Arizona and New Mexico (U.S. EPA, 2019a, section <IP_ADDRESS>, p. 13-36).
Overall, the findings of these updated quantitative analyses are generally consistent with those in the 2012 and 2020 reviews. The 3-year visibility metric was generally below 26 dv in most areas that meet the current 24-hour PM2.5 standard. Small differences in the 3-year visibility metric were observed between the variations of the IMPROVE equation, which may suggest that it may be more appropriate to use one version over another in different regions of the U.S. based on PM characteristics such as particle size and composition to more accurately estimate light extinction.
2. Non-Visibility Effects
Consistent with the evidence available at the time of the 2012 and 2020 reviews, and as described in detail in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>), the data remain insufficient to conduct quantitative analyses for PM effects on climate and materials. For PM-related climate effects, as explained in more detail in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>.1), our understanding of PM-related climate effects is still limited by significant key uncertainties. The recently available evidence does not appreciably improve our understanding of the spatial and temporal heterogeneity of PM components that contribute to climate forcing (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). Significant uncertainties also persist related to quantifying the contributions of PM and PM components to the direct and indirect effects on climate forcing, such as changes to the pattern of rainfall, changes to wind patterns, and effects on vertical mixing in the atmosphere (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). Additionally, while improvements have been made to climate models since the completion of the 2009 ISA, the models continue to exhibit variability in estimates of the PM-related climate effects on regional scales (e.g., ~100 km) compared to simulations at the global scale (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). While our understanding of climate forcing on a global scale is somewhat expanded since the 2012 review, significant limitations remain to quantifying potential adverse PM-related climate effects in the U.S. and how they would vary in response to incremental changes in PM concentrations across the U.S. As such, while recent research is available on climate forcing on a global scale, the remaining limitations and uncertainties are significant, and the recent global scale research does not translate directly for use at regional spatial scales. Therefore, the evidence does not provide a clear understanding at the necessary spatial scales for quantifying the relationship between PM mass in ambient air and the associated climate-related effects in the U.S. that would be necessary for informing consideration of a national PM standard on climate in this reconsideration (U.S. EPA, 2022b, section <IP_ADDRESS>.1; U.S. EPA, 2019a, section 13.3).
For PM-related materials effects, as explained in more detail in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>.2), the available evidence has been somewhat expanded to include additional information about the soiling process and the types of materials impacted by PM. This evidence provides some limited information to inform dose-response relationships and damage functions associated with PM, although most of these studies were conducted outside of the U.S. where PM concentrations in ambient air are typically above those observed in the U.S. (U.S. EPA, 2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section 13.4). The evidence on materials effects characterized in the 2019 ISA also includes studies examining effects of PM on the energy efficiency of solar panels and passive cooling building materials, although the evidence remains insufficient to establish quantitative relationships between PM in ambient air and these or other materials effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2). While the available evidence assessed in the 2019 ISA is somewhat expanded since the time of the 2012 review, quantitative relationships have not been established for PM-related soiling and corrosion and frequency of cleaning or repair that further the understanding of the public welfare implications of materials effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section 13.4). Therefore, there is insufficient information to inform quantitative analyses assessing materials effects to inform consideration of a national PM standard on materials in this reconsideration (U.S. EPA, 2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section 13.4).
D. Proposed Conclusions on the Secondary PM Standards
In reaching proposed conclusions on the current secondary PM standards (presented in section IV.D.3), the Administrator has taken into account policy-relevant evidence- and quantitative information-based considerations discussed in the PA (summarized in section IV.D.2), as well as advice from the CASAC and public comment on the standards received thus far in the reconsideration (section IV.D.1). In general, the role of the PA is to help "bridge the gap" between the Agency's assessment of the current evidence and quantitative analyses, and the judgments required of the Administrator in determining whether it is appropriate to retain or revise the NAAQS. Evidence-based considerations draw upon the EPA's integrated assessment of the scientific evidence of PM-related welfare effects presented in the 2019 ISA and ISA Supplement (summarized in section V.B above) to address key policy-relevant questions in the reconsideration. Similarly, the quantitative information-based considerations (summarized in section V.C above) focused on the potential for PM-related welfare effects under recent air quality conditions for the purposes of addressing the policy-relevant questions.
This approach to reviewing the secondary standards is consistent with the requirements of the provisions of the CAA related to the review of the NAAQS and with how the EPA and the courts have historically interpreted the CAA. As discussed in section I.A above, these provisions require the Administrator to establish secondary standards that, in the Administrator's judgment, are requisite (i.e., neither more nor less stringent than necessary) to protect the public welfare from known or anticipated adverse effects associated with the presence of the pollutant in ambient air. Consistent with the Agency's approach across all NAAQS reviews, the EPA's approach to informing these judgments is based on a recognition that the available welfare effects evidence generally reflects a continuum that includes ambient air exposures for which scientists generally agree that effects are likely to occur through lower levels at which the likelihood and magnitude of response become increasingly uncertain. The CAA does not require the Administrator to establish secondary standards at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect the public welfare from known or anticipated adverse effects.
The proposed decision on the adequacy of the current secondary standards described below is a public welfare policy judgment by the Administrator that draws upon the scientific evidence for the relevant welfare effects, quantitative analyses of air quality, as available, and judgments about how to consider the uncertainties and limitations that are inherent in the scientific evidence and quantitative analyses. The four basic elements of the NAAQS (i.e., indicator, averaging time, form, and level) have been considered collectively in evaluating the public welfare protection afforded by the current standard against PM-related visibility, climate and materials effects. The Administrator's final decision will additionally consider public comments received on this proposed decision.
1. CASAC Advice in this Reconsideration
The CASAC provided its advice regarding the current secondary standards in the context of its review of the draft PA (Sheppard, 2022a). In its comments on the draft PA, the CASAC first recognized the scientific evidence is sufficient to support a causal relationship between PM and visibility effects, climate effects and materials effects.
With regard to visibility effects, the CASAC recognized that the identification of a target level of protection for the visibility index is based on a limited number of studies and suggested that "additional region- and view-specific visibility preference studies and data analyses are needed to support a more refined visibility target" (Sheppard, 2022a, p. 21 of consensus responses). While the CASAC did not recommend revising either the target level of protection for the visibility index or the level of the current 24-hour PM2.5 standard, they did state that a visibility index of 30 deciviews "needs to be justified" and "[i]f a value of 20-25 deciviews is deemed to be an appropriate visibility target level of protection, then a secondary 24-hour PM2.5 standard in the range of 25-35 ug/m[3] should be considered" (Sheppard, 2022a, p. 21 of consensus responses).
The CASAC also recognized the limited availability of monitoring methods and networks for directly measuring light extinction. As such, they suggest that "[a] more extensive technical evaluation of the alternatives for visibility indicators and practical measurement methods (including the necessity for a visibility FRM) is need for future reviews" (Sheppard, 2022a, p. 22 of consensus letter). The majority of the CASAC "recommend[ed] that an FRM for a directly measured PM2.5 light extinction indicator be developed" to inform the consideration of the protection afforded by the secondary PM standards against visibility impairment, the minority of the CASAC "believe that a light extinction FRM is not necessary to set a secondary standard protective of visibility" (Sheppard, 2022a, p. 22 of consensus responses).
With regard to climate and materials effects, the CASAC noted that substantial uncertainties remain in the scientific evidence for these effects. The CASAC suggested a number of areas for future research to further inform our understanding of these effects, including more climate-related research and research that would allow for quantitative assessment of the relationship between materials effects and PM in ambient air.
2. Evidence- and Quantitative Information-Based Considerations in the Policy Assessment
The secondary PM standards include the 24-hour PM2.5 standard, with its level of 35 ug/m[3] as the 98[th] percentile, averaged over three years; the annual PM2.5 standard, with its level of 15.0 ug/m[3] as the annual mean, averaged over three years; and the 24-hour PM10 standard, with its level of 150 ug/m[3], not to be exceeded more than once per year on average over three years. Together, these standards provide protection against both long-term average and short-term peak PM concentrations. For example, the 24-hour PM2.5 standard is most effective at limiting peak 24-hour PM2.5 concentrations, but in doing so, also has an effect on annual average PM2.5 concentrations. Additionally, the annual standard is most effective in controlling "typical" or average PM2.5 concentrations, but also provides some measure of protection against peak exposures.
The PA considers the degree to which the available scientific evidence and quantitative information supports or calls into question the adequacy of the protection afforded by the current secondary PM standards. In doing so, the PA considers the evidence assessed in the 2019 ISA and ISA Supplement, including the extent to which the evidence for PM-related visibility impairment, climate effects, or materials effects alters key conclusions from the 2020 review. The PA also considers quantitative analyses of visibility impairment and the extent to which they may indicate different conclusions from those in the 2020 review regarding the degree of protection from adverse effects provided by the current secondary standards.
Consistent with the approaches used in previous reviews, the quantitative analyses in the PA utilized a two-phase assessment for visibility impairment. First, the PA considered the appropriateness of the elements (indicator, averaging time, form, and level) of the visibility index for providing protection against PM-related visibility effects. Second, the PA evaluated the relationship between the current secondary 24-hour PM2.5 standard and the visibility index.
With regard to the appropriateness of the visibility index and its target level of protection against PM-related visibility effects, the PA notes that there is limited information available in this reconsideration beyond that available in previous reviews to inform conclusions on the elements (indicator, averaging time, form, and level) of the visibility index (described in more detail in section V.C.1.a above). In considering the available information, the PA concludes that the available information continues to support a visibility index with estimated light extinction as the indicator, a 24-hour averaging time, and a 90[th] percentile form, averaged over three years, with a level within the range of 20 to 30 dv.
With regard to the relationship between the current secondary 24-hour PM2.5 standard and the visibility index, the PA presents updated analyses based on recent air quality information, with a focus on locations meeting the current secondary 24-hour PM2.5 and PM10 standards. In the absence of advances in the monitoring methods for directly measuring light extinction, and given the lack of a robust monitoring network for the routine measurement of light extinction across the U.S. (section V.B.1.a), as in previous reviews, the PA analyses use calculated light extinction to estimate PM-related visibility impairment (U.S. EPA, 2022b, section <IP_ADDRESS>). Compared to the 2012 review, updated analyses incorporate several refinements. These include (1) the evaluation of three versions of the IMPROVE equation to calculate light extinction (U.S. EPA, 2022b, Appendix D, Equations D-1 through D-3) in order to better understand the influence of variability in equation inputs; (2) the use of 24-hour relative humidity data, rather than monthly average relative humidity as was used in the 2012 review (U.S. EPA, 2022b, section <IP_ADDRESS>, Appendix D); and (3) the inclusion of the coarse fraction in the estimation of light extinction (U.S. EPA, 2022b, section <IP_ADDRESS>, Appendix D). The PA's updated analyses include 60 monitoring sites that measure PM2.5 and PM10 that are geographically distributed across the U.S. in both urban and rural areas (U.S. EPA, 2022b, Appendix D, Figure D-1).
In areas that meet the current 24-hour PM2.5 standard for the 2017-2019 time period, all sites have light extinction estimates at or below 26 dv using the original and revised IMPROVE equations (U.S. EPA, 2022b, section <IP_ADDRESS>). In addition, the four locations that exceeds the current 24-hour PM2.5 standard have light extinction estimates that range from 22 to 27 dv when using the original IMPROVE equation (U.S. EPA, 2022b, Figure 5-3) and from 22 to 29 dv when using the revised IMPROVE equation (U.S. EPA, 2022b, Figure 5-4). The analyses presented in the PA indicate similar findings to those from the analyses in the 2012 and 2020 reviews, i.e., the updated quantitative analysis shows that the 3-year visibility metric was no higher than 30 dv (the upper end of the range of target levels of protection) at sites meeting the current secondary PM standards, and at most such sites the 3-year visibility index values are much lower (e.g., an average of 20 dv across the 60 sites).
When light extinction is calculated using the updated IMPROVE equation from Lowenthal and Kumar (2016), the resulting 3-year visibility metrics are slightly higher at all sites compared to light extinction calculated using the original and revised IMPROVE equations (U.S. EPA, 2022b, Figure 5-5). The slightly higher estimates of light extinction are consistent with the higher OC multiplier included in the IMPROVE equation from Lowenthal and Kumar (2016), reflecting the use of data from remote areas with higher concentrations of organic PM when validating that equation. As such, it is important to note that the Lowenthal and Kumar (2016) version of the IMPROVE equation may overestimate light extinction in non-remote areas, including in the urban areas included in the analyses presented in the PA.
Nevertheless, when light extinction is calculated using the Lowenthal and Kumar (2016) equation for those sites that meet the current 24-hour PM2.5 standard, the 3-year visibility metric is generally at or below 28 dv. For the sites that exceed the current 24-hour PM2.5 standard, three of the sites have a 3-year visibility metric ranging between 26 dv and 30 dv, while one site in Fresno, California that exceeds the current 24-hour PM2.5 standard has a 3-year visibility index value of 32 dv (compared to 29 dv when light extinction is calculated with the original IMPROVE equation) (see U.S. EPA, 2022b, Appendix D, Table D-3). At this site, it is likely that the 3-year visibility metric using the Lowenthal and Kumar (2016) equation would be below 30 dv if PM2.5 concentrations were reduced such that the 24-hour PM2.5 level of 35 ug/m[3] was attained.
In the 2012 review, the EPA noted that PM2.5 is the size fraction of PM responsible for most of the visibility impairment in urban areas (77 FR 38980, June 29, 2012). Data available at the time of the 2012 review suggested that PM10-2.5 is often a minor contributor to visibility impairment (U.S. EPA, 2010a), though it may make a larger contribution in some areas in the desert southwestern region of the U.S. However, at the time of the 2012 review, there were few data available from PM10-2.5 monitors to quantify the contribution of coarse PM to calculated light extinction. Since that time, an expansion in PM10-2.5 monitoring efforts has increased the availability of data for use in estimating light extinction with both PM2.5 and PM10-2.5 concentrations included as inputs in the equations. The analysis in the 2020 review addressed light extinction at 20 of the 67 PM2.5 sites where collocated PM10-2.5 monitoring data were available. Since the 2020 review, PM10-2.5 monitoring data are available at more locations and the analyses presented in the PA include those for light extinction estimated with coarse and fine PM at all 60 sites. Generally, the contribution of the coarse fraction to light extinction at these sites is minimal, contributing less than 1 dv to the 3-year visibility metric, as assessed and presented in the 2020 PA (U.S. EPA, 2020a, section <IP_ADDRESS>). However, the PA notes that in the updated quantitative analyses, only a few sites were in locations that would be expected to have high concentrations of coarse PM, such as the Southwest. These results are consistent with those in the analyses in the 2019 ISA, which found that mass scattering from PM10−2.5 was relatively small (less than 10%) in the eastern and northwestern U.S., whereas mass scattering was much larger in the Southwest (more than 20%) particularly in southern Arizona and New Mexico (U.S. EPA, 2019a, section <IP_ADDRESS>, p. 13-36).
In summary, the findings of these updated quantitative analyses are generally consistent with those in the 2012 and 2020 reviews. The 3-year visibility metric was generally below 26 dv in most areas that meet the current 24-hour PM2.5 standard when light extinction is calculated using the original and revised IMPROVE equations, and generally at or below 28 dv when using the Lowenthal and Kumar (2016) equation to estimate light extinction. Small differences in the 3-year visibility metric were observed between the variations of the IMPROVE equation. When light extinction is calculated using the revised IMPROVE equation, there is a generally +-1-2 dv at the study locations compared to light extinction calculated using the original IMPROVE equation (U.S. EPA, 2022b, Appendix D, Table D-3). When light extinction is calculated using the Lowenthal and Kumar (2016) equation, the difference compared to using either the original or revised IMPROVE equation generally ranges from no difference to up to 4 dv greater in areas that meet the current secondary 24-hour PM2.5 standard (U.S. EPA, 2022b, Appendix D, Table D-3). As noted in previous reviews, a change of 1 to 2 dv in light extinction under many viewing conditions will be perceived as a small, but noticeable, change in the appearance of a scene, regardless of the initial amount of visibility impairment (U.S. EPA, 2004a; U.S. EPA, 2010a). Given that there is more variability when estimating light extinction using the Lowenthal and Kumar (2016) IMPROVE equation compared to the original or revised IMPROVE equations, it is important to recognize that the PA notes that the Lowenthal and Kumar (2016) equation may not be appropriate for all locations and source types. For example, the larger multiplier used in the Lowenthal and Kumar (2016) may be more appropriate for estimating light extinction in more remote areas where there is more aged and oxygenated organic PM compared to in urban areas. As such, the PA recognizes that one version of the IMPROVE equation is not necessarily more accurate or precise in estimating light extinction, and that differences in locations may support the selection of inputs to the IMPROVE equation or of the appropriate IMPROVE equation to estimate light extinction on a regional basis rather than on a national basis. Overall, regardless of the IMPROVE equation that is used to estimate light extinction, in areas that meet the current 24-hour PM2.5 standards, the 3-year visibility metric is at or below 28 dv, which is in the upper range of levels for the target level of protection identified from the public preference studies (i.e., 20 to 30 dv). In fact, even in areas that exceed the secondary 24-hour PM2.5 standard, and regardless of the IMPROVE equation that is used to calculate light extinction, all study locations have 3-year visibility index values at or below 30 dv, which is the upper end of the range of target levels of protection.
With regard to PM-related climate effects, the PA recognizes that while the evidence base has expanded since the completion of the 2009 ISA, the recent evidence has not appreciably improved the understanding of the spatial and temporal heterogeneity of PM components that contribute to climate forcing (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). Despite continuing research, there are still significant limitations in quantifying the contributions of PM and PM components to the direct and indirect effects on climate forcing (e.g., changes to the pattern of rainfall, changes to wind patterns, effects on vertical mixing in the atmosphere) (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). In addition, while a number of improvements and refinements have been made to climate models since the 2012 review, these models continue to exhibit variability in estimates of the PM-related climate effects on regional scales (e.g., ~100 km) compared to simulations at the global scale (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). While recent research has added to the understanding of climate forcing on a global scale, there remain significant limitations to quantifying potential adverse effects from PM on climate in the U.S. and how they would vary in response to incremental changes in PM concentrations in the U.S. Overall, the PA recognizes that while new research is available on climate forcing on a global scale, the remaining uncertainties and limitations are significant, and the new global scale research does not translate directly to use at regional spatial scales. Thus, the evidence does not provide a clear understanding at the spatial scales needed for the NAAQS of a quantitative relationship between concentrations of PM mass in ambient air and the associated climate-related effects (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). The PA concludes that the evidence does not call into question the adequacy of the current secondary PM standards for climate effects.
With regard to materials effects, the PA notes the availability of recent evidence in this reconsideration related to the soiling process and the types of materials that are affected. Such evidence provides some limited information to inform dose-response relationships and damage functions associated with PM, though most recent studies have been conducted outside the U.S. in areas where PM concentrations in ambient air are higher than those observed in the U.S. (U.S. EPA, 2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section 13.4). The recent evidence includes studies examining PM-related effects on the energy efficiency of solar panels and passive cooling building materials, though there remains insufficient evidence to establish quantitative relationships between PM in ambient air and these or other materials effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2). While recent research has expanded the body of evidence for PM-related materials effects, the PA recognizes the lack of information to inform quantitative analyses assessing materials effects or the potential public welfare implications of such effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2). Thus, the PA concludes that the evidence does not call into question the adequacy of the current secondary PM standards for materials effects.
Overall, the PA recognizes that the newly available welfare effects evidence, critically assessed in the 2019 ISA as part of the full body of evidence, and visibility effects evidence, assessed in the ISA Supplement, reaffirms the conclusions on the visibility, climate, and materials effects of PM as recognized in the 2012 and 2020 reviews (U.S. EPA, 2022b, sections <IP_ADDRESS>., <IP_ADDRESS>, and 5.5). Further, there is a general consistency of the currently available evidence with the evidence that was available in previous reviews, including with regard to key aspects of the decision to retain the standards in the 2012 and 2020 reviews (U.S. EPA, 2022b, sections <IP_ADDRESS>, <IP_ADDRESS>, and 5.5). The quantitative analyses for visibility impairment for recent air quality conditions indicate that estimated light extinction in areas meeting the current secondary 24-hour PM2.5 standards have a 3-year visibility index at or below 30 dv (i.e., the upper end of the range of target levels of protection identified in the 2012 and 2020 reviews) and most areas have 3-year visibility index values at or below the midpoint of the range of target levels of protection (i.e., 25 dv) (U.S. EPA, 2022b, sections <IP_ADDRESS> and 5.5). Collectively, the PA finds that the evidence and quantitative information-based considerations support consideration of retaining the current secondary PM standards, without revision (U.S. EPA, 2022b, section 5.5).
3. Administrator's Proposed Decision on the Current Secondary PM Standards
This section summarizes the Administrator's considerations and conclusions related to the current secondary PM2.5 and PM10 standards and presents his proposed decision that no change is required for those standards at this time. The CAA provisions require the Administrator to establish secondary standards that, in the judgment of the Administrator, are requisite to protect public welfare from known or anticipated adverse effects associated with the presence of the pollutant in the ambient air. In so doing, the Administrator seeks to establish standards that are neither more nor less stringent than necessary for this purpose. The Act does not require that standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect the public welfare from known or anticipated adverse effects. The final decision on the adequacy of the current secondary standards is a public welfare policy judgment to be made by the Administrator. The decision should draw on the scientific information and analyses about welfare effects, and associated public welfare significance, as well as judgments about how to consider the range and magnitude of uncertainties that are inherent in the scientific evidence and analyses. This approach is based on the recognition that the available evidence generally reflects a continuum that includes ambient air exposures at which scientists agree that effects are likely to occur through lower levels at which the likelihood and magnitude of responses become increasingly uncertain. This approach is consistent with the requirements of the provisions of the Clean Air Act related to the review of NAAQS and with how the EPA and the courts have historically interpreted the Act.
      Given these requirements, the Administrator's final decision in this reconsideration will be a public welfare policy judgment that draws upon the scientific and technical information examining PM-related visibility impairment, climate effects and materials effects, including how to consider the range and magnitude of uncertainties inherent in that information. The Administrator recognizes that his final decision will be based on an interpretation of the scientific evidence and technical analyses that neither overstates nor understates their strengths and limitations, nor the appropriate inferences to be drawn.
As an initial matter in considering the secondary standards, the Administrator notes the longstanding body of evidence for PM-related visibility impairment. As in previous reviews, this evidence continues to demonstrate a causal relationship between ambient PM and effects on visibility (U.S. EPA, 2019a, section 13.2). The Administrator recognizes that visibility impairment can have implications for people's enjoyment of daily activities and for their overall sense of well-being. Therefore, as in previous reviews, he considers the degree to which the current secondary standards protect against PM-related visibility impairment. In so doing, and consistent with previous reviews, the Administrator considers the protection provided by the current secondary standards against PM-related visibility impairment in conjunction with the Regional Haze Program as a means of achieving appropriate levels of protection against PM-related visibility impairment in urban, suburban, rural, and Federal Class I areas across the country. Programs implemented to meet the secondary PM NAAQS, along with the requirements of the Regional Haze Program established for protecting against visibility impairment in Class I areas, would be expected to improve visual air quality across all areas.
In addition, the Administrator notes that the Regional Haze Program was established by Congress specifically to achieve "the prevention of any future, and the remedying of existing, impairment of visibility in mandatory Class I areas, which impairment results from man-made air pollution," and that Congress established a long-term program to achieve that goal (CAA section 169A). The Administrator finds that in adopting section 169A, Congress set a goal of eliminating anthropogenic visibility impairment at Class I areas, as well as a framework for achieving that goal which extends well beyond the planning process and timeframe for attaining secondary NAAQS. Thus, recognizing that the Regional Haze Program will continue to contribute to reductions in visibility impairment in Class I areas, the Administrator proposes to conclude that addressing visibility impairment in Class I areas is beyond the scope of the secondary PM NAAQS and that setting the secondary PM NAAQS at a level that would remedy visibility impairment in Class I areas would result in standards that are more stringent than is requisite.
In further considering what standards are requisite to protect against adverse public welfare effects from visibility impairment, the Administrator adopts an approach consistent with the approach used in previous reviews (section V.A.1.b). That is, he first identifies an appropriate target level of protection in terms of a PM visibility index that accounts for the factors that influence the relationship between particles in the ambient air and visibility (i.e., size fraction, species composition, and relative humidity). He then considers air quality analyses examining the relationship between this PM visibility index and the current secondary 24-hour PM2.5 standard in locations meeting the current 24-hour PM2.5 and PM10 standards (U.S. EPA, 2022b, section <IP_ADDRESS>).
To identify a target level of protection, the Administrator first considers the characteristics of the visibility index and defines its elements (indicator, averaging time, form, and level). With regard to the indicator for the visibility index, the Administrator recognizes that there is a lack of availability of methods and an established network for directly measuring light extinction, consistent with the conclusions reached in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>) and with the CASAC's recommendation for additional research on direct measurement methods for light extinction (Sheppard, 2022a, p. 21 of consensus responses). He notes that in the 2012 and 2020 reviews, given the lack of such monitoring data, the EPA used an index based on estimates of light extinction by PM2.5 components calculated using an adjusted version of the original IMPROVE algorithm. As described above (sections V.B.1.a and V.D.2), this algorithm allows the estimation of light extinction using routinely monitored components of PM2.5 and PM10-2.5, along with estimates of relative humidity. While revisions have been made to the IMPROVE algorithm since the 2012 review (U.S. EPA, 2022b, section <IP_ADDRESS>), the Administrator recognizes that our fundamental understanding of the relationship between ambient PM and light extinction has changed little since the 2012 review. He further recognizes that the results of the quantitative analyses in the PA that examined three versions of the IMPROVE equation indicate that there are very small differences in estimates of light extinction between the equations, and that it is not always clear that one version of the IMPROVE equation is more appropriate for estimating light extinction across the U.S. than other versions of the IMPROVE equation. He does, however, recognize that the PA suggests that it may be appropriate to select inputs to the IMPROVE equation (e.g., the multiplier for OC to OM) on a regional basis rather than a national basis when calculating light extinction (U.S. EPA, 2022b, section <IP_ADDRESS>), and he further notes the CASAC's recognition that PM-visibility relationships are region specific (Sheppard, 2022a, p. 21 of consensus responses). In the absence of a robust monitoring network to directly measure light extinction (sections V.B.1.a and V.D.2), he preliminarily judges that estimated light extinction, as calculated using one or more versions of the IMPROVE algorithms, continues to be the most appropriate indicator for the visibility index in this reconsideration.
In further defining the characteristics of a visibility index based on estimates of light extinction, the Administrator considers the appropriate averaging time, form, and level of the index. With regard to the averaging time and form, the Administrator notes that in previous reviews, a 24-hour averaging time was selected and the form was defined as the 3-year average of annual 90[th] percentile values. The Administrator recognizes that the evidence available in this reconsideration and described in the PA continue to provide support for the short-term nature of PM-related visibility effects. In so doing, he relies on analyses of 24-hour and sub-daily PM2.5 light extinction to inform his conclusions on averaging time. The Administrator notes that there are strong correlations between 24-hour and sub-daily (i.e., 4-hour average) PM2.5 light extinction), indicating that a 24-hour averaging time is an appropriate surrogate for the sub-daily time periods relevant for visual perception (U.S. EPA, 2011, Appendix G, section G.4). He further recognizes that the longer averaging time may be less influenced by atypical conditions and/or atypical instrument performance. Considering this information, and noting that the CASAC did not provide advice or recommendations with regard to the averaging time of the visibility index, the Administrator preliminarily judges that it the 24-hour averaging time continues to be appropriate for the visibility index.
With regard to the form of the visibility index, the Administrator notes that consistent with the approach taken in other NAAQS, including the current secondary 24-hour PM2.5 NAAQS, a multi-year percentile form offers greater stability to the air quality management process by reducing the possibility that statistically unusual indicator values will lead to transient violations of the standard. Using a 3-year average provides stability from the occasional effects of inter-annual meteorological variability that can result in unusually high pollution levels for a particular year (U.S. EPA, 2011, p. 4-58). In considering the percentile that would be appropriate with the 3-year average, the Administrator first notes that the Regional Haze Program targets the 20% most impaired days for improvements in visual air quality in Class I areas. Based on analyses examining 90[th], 95[th], and 98[th] percentile forms, the Administrator preliminarily judges that a focus similar to the Regional Haze Program focused on improving the 20% most impaired days suggest that the 90[th] percentile, which represents the median of the 20% most impaired days, such that 90% of days have visual air quality that is at or below the target level of protection of the visibility index, would be reasonably expected to lead to improvements in visual air quality for the 20% most impaired days (U.S. EPA, 2011, p. 4-59). In the analyses of percentiles, the results suggest that a higher percentile value could have the effect of limiting the occurrence of days with peak PM-related light extinction in areas outside of Federal Class I areas to a greater degree. However, the Administrator preliminarily concludes that it is appropriate to balance concerns about focusing on the group of most impaired days with concerns about focusing on the days with peak visibility impairment. Additionally, the Administrator notes that the CASAC did not provide advice or recommendations related to the form of the visibility index. Therefore, the Administrator preliminarily judges that it remains appropriate to define a visibility index in terms of a 24-hour averaging time and a form based on the 3-year average of annual 90[th] percentile values.
With regard to the level of the visibility index, the Administrator first notes that the information that is available regarding the range of levels of visibility impairment judged to be acceptable by at least 50% of study participants in the visibility preference studies is largely the same as was in previous reviews. As such, the Administrator notes that the PA identifies a range of 20 to 30 dv as appropriate for considering the level for the visibility index. Furthermore, the Administrator notes that a level at the upper end of the range (i.e., 30 dv) was selected for the 2012 and 2020 reviews, given the uncertainties and limitations associated with the public preference studies (U.S. EPA, 2022b, section <IP_ADDRESS>). In considering the available public preference studies and the range of target levels of protection derived from the studies, the Administrator notes that, while methodologically similar, the studies have inherent differences that impact the responses from the study participants. He notes that the images used to evaluate public preferences differed significantly depending on geographical location, and that public preferences for visual air quality can vary depending on the scenic elements depicted in the images. He also recognizes that the older studies (i.e., those in Denver, CO, and British Columbia, Canada) used photographs, paired with ambient measurements of light extinction, as opposed to the computer-generated images in more recent studies (i.e., those in Phoenix, AZ, and Washington, DC), which introduces more variability in scene appearance that can influence preferences. Furthermore, the distances of objects depicted in the images can influence the perceived visibility changes, as objects at a greater distance have more sensitivity to changes in visibility impairment compared to those at shorter distances. The Administrator recognizes that these differences, and the uncertainties and limitations that result from them, are important to consider when identifying a target level of protection for the visibility index, particularly in identifying the appropriate level of protection that would be neither more nor less stringent than necessary for a national standard.
In addition to the methodological differences across the public preferences studies, the Administrator takes note of the uncertainties and limitations associated with the studies and discussed in the PA. In particular, the Administrator notes that available studies may not capture the full range of visibility preferences in the U.S. population, particularly given the potential for preferences to vary based on the visibility conditions commonly encountered and the types of scenes being viewed and factors that are not captured by the methods used in available preference studies may influence people's judgments on acceptable visibility, including the duration of visibility impairment, the time of day during which light extinction is greatest, and the frequency of episodes of visibility impairment (U.S. EPA, 2022b, section <IP_ADDRESS>).
In considering the appropriate target level of protection for the visibility index, the Administrator also takes note of the CASAC's advice. Specifically, he notes that the CASAC recognizes that such a judgment is based on a limited number of visibility preference studies, with studies conducted in the western U.S. reporting public preferences for visibility impairment associated with the lower end of the range of levels, while studies conducted in the eastern U.S. reporting public preferences associated with the upper end of the range. While the CASAC did not specifically recommend a level for the visibility index, they did state that a visibility index of 30 deciviews "needs to be justified" (Sheppard, 2022a, p. 21 of consensus responses). In considering the available information and the CASAC's advice, the Administrator notes that the public preference studies were conducted in several geographical areas across the U.S., and while they provide insight to regional preferences for visibility impairment, none of these studies identify a specific level of visibility impairment that would be perceived as "acceptable" or "unacceptable" across the whole U.S. population. The Administrator notes that there have long been significant questions about how to set a national standard for visibility that is not overprotective for some areas of the U.S. In establishing the Regional Haze Program to improve visibility in Class I areas, Congress noted that "as a matter of equity, the national ambient air quality standards cannot be revised to adequately protect visibility in all areas of the country." H.R. Rep. 95-294 at 205. Similarly, in the 1997 review, the Administrator at that time noted significant differences in visibility in the eastern U.S. compared to the western U.S. due to background conditions, found that a standard set to protect against visibility impairment nationwide would be significantly overprotective and not justified for some parts of the country, and concluded it was appropriate to rely on the Regional Haze Program in conjunction with the secondary PM NAAQS to achieve the requisite degree of protection from visibility impairment (62 FR 38652, July 18, 1997). For the reasons noted above, the Administrator is not seeking to set a standard that would eliminate visibility impairment in Class I areas, but significant uncertainties remain regarding how to judge visibility impairment across the entire range of daily outdoor activities for Americans across the country. Thus, the Administrator recognizes that there are substantial uncertainties and limitations in the public preference studies that should be considered when selecting a target level of protection for the visibility index. The Administrator proposes to conclude that the uncertainties and variability inherent in the public preference studies warrant setting a higher target level of protection than if the underlying methods and results from the public preference studies were more consistent. In so doing, the Administrator first preliminarily judges that, consistent with similar judgments in past reviews, it is appropriate to recognize that the secondary 24-hour PM2.5 standard is intended to address visibility impairment across a wide range of regions and circumstances, and that the current standard works in conjunction with the Regional Haze Program to improve visibility, and therefore, it is appropriate to establish a target level of protection based on the upper end of the range of levels. In considering the information available in this reconsideration and the CASAC's advice, the Administrator proposes to conclude that the protection provided by a visibility index based on estimated light extinction, a 24-hour averaging time, and a 90[th] percentile form, averaged over 3 years, set at a level of 30 dv (the upper end of the range of levels) would be requisite to protect public welfare with regard to visibility impairment.
Having provisionally concluded that it remains appropriate in this reconsideration to define the target level of protection in terms of a visibility index based on estimated light extinction as described above (i.e., with a 24-hour averaging time; a 3-year, 90[th] percentile form; and a level of 30 dv), the Administrator next considers the degree of protection from visibility impairment afforded by the existing secondary standards. He considers the updated analyses of PM-related visibility impairment presented in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>), which reflect several improvements over the 2012 review. Specifically, the updated analyses examine multiple versions of the IMPROVE algorithm, including the version incorporating revisions since the 2012 review (section V.B.1.a). This approach provides an improved understanding of how variation in equation inputs impacts calculated light extinction (U.S. EPA, 2022b, Appendix D). In addition, all of the sites included in the analyses had PM10-2.5 data available, which allows for better characterization of the influence of the coarse fraction on light extinction (U.S. EPA, 2022b, section <IP_ADDRESS>).
The Administrator notes that the results of these updated analyses are consistent with the results from the 2012 and 2020 reviews. Regardless of the IMPROVE equation used, these analyses demonstrate that the 3-year visibility metric is at or below 28 dv in all areas meeting the current 24-hour PM2.5 standard (section V.C.1.b). Given the results of these analyses, the Administrator concludes that the updated scientific evidence and technical information support the adequacy of the current secondary PM2.5 and PM10 standards to protect against PM-related visibility impairment. While the inclusion of the coarse fraction had a relatively modest impact on calculated light extinction in the analyses presented in the PA, he nevertheless recognizes the continued importance of the PM10 standard given the potential for larger impacts in locations with higher coarse particle concentrations, such as in the southwestern U.S., for which only a few sites met the criteria for inclusion in the analyses in the PA (U.S. EPA, 2019a, section <IP_ADDRESS>; U.S. EPA, 2022b, section <IP_ADDRESS>).
With regard to the adequacy of the secondary 24-hour PM2.5 standard, the Administrator notes that the CASAC stated that "[i]f a value of 20-25 deciviews is deemed to be an appropriate visibility target level of protection, then a secondary 24-hour PM2.5 standard in the range of 25-35 ug/m[3] should be considered" (Sheppard, 2022a, p. 21 of consensus responses). The Administrator recognizes that the CASAC recommended the Administrator provide additional justification for a visibility index target of 30 dv but did not specifically recommend that he choose an alternative level for the visibility index. The Administrator has considered the CASAC's advice, together with the available scientific evidence and quantitative information in reaching his proposed conclusions. The Administrator recognizes conclusions regarding the appropriate weight to place on the scientific and technical information examining PM-related visibility impairment including how to consider the range and magnitude of uncertainties inherent in that information is a public welfare policy judgment left to the Administrator. As such, the Administrator notes his conclusion on the appropriate visibility index (i.e., with a 24-hour averaging time; a 3-year, 90[th] percentile form; and a level of 30 dv) and his conclusions regarding the quantitative analyses of the relationship between the visibility index and the current secondary 24-hour PM2.5 standard. In so doing, he proposes to conclude that  the current secondary standards provide requisite protection against PM-related visibility effects. With respect to non-visibility welfare effects, the Administrator considers the evidence for PM-related impacts on climate and on materials and concludes that it is generally appropriate to retain the existing secondary standards and that it is not appropriate to establish any distinct secondary PM standards to address non-visibility PM-related welfare effects. With regard to climate, he recognizes that a number of improvements and refinements have been made to climate models since the time of the 2012 review. However, despite continuing research and the strong evidence supporting a causal relationship with climate effects (U.S. EPA, 2019a, section 13.3.9), the Administrator notes that there are still significant limitations in quantifying the contributions of the direct and indirect effects of PM and PM components on climate forcing (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). He also recognizes that models continue to exhibit considerable variability in estimates of PM-related climate impacts at regional scales (e.g., ~100 km), compared to simulations at the global scale (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). The resulting uncertainty leads the Administrator to preliminarily conclude that the scientific information available in this reconsideration remains insufficient to quantify, with confidence, the impacts of ambient PM on climate in the U.S. (U.S. EPA, 2022b, section <IP_ADDRESS>.1) and that there is insufficient information at this time to base a national ambient standard on climate impacts.
With respect to materials effects, the Administrator notes that the available evidence continues to support the conclusion that there is a causal relationship with PM deposition (U.S. EPA, 2019a, section 13.4). He recognizes that deposition of particles in the fine or coarse fractions can result in physical damage and/or impaired aesthetic qualities. Particles can contribute to materials damage by adding to the effects of natural weathering processes and by promoting the corrosion of metals, the degradation of painted surfaces, the deterioration of building materials, and the weakening of material components. While some recent evidence on materials effects of PM is available in the 2019 ISA, the Administrator notes that this evidence is primarily from studies conducted outside of the U.S. in areas where PM concentrations in ambient air are higher than those observed in the U.S. (U.S. EPA, 2019a, section 13.4). Given the limited amount of information on the quantitative relationships between PM and materials effects in the U.S., and uncertainties in the degree to which those effects could be adverse to the public welfare, the Administrator preliminarily judges that the scientific information available in this reconsideration remains insufficient to quantify, with confidence, the public welfare impacts of ambient PM on materials and that there is insufficient information at this time to support a distinct national ambient standard based on materials impacts.
Taken together, the Administrator proposes to conclude that the scientific and technical information for PM-related visibility impairment, climate impacts, and materials effects, with its attendant uncertainties and limitations, supports the current level of protection provided by the secondary PM standards as being requisite to protect against known and anticipated adverse effects on public welfare. For visibility impairment, this proposed conclusion reflects his consideration of the evidence for PM-related light extinction, together with his consideration of updated analyses of the protection provided by the current secondary PM2.5 and PM10 standards. For climate and materials effects, this conclusion reflects his preliminary judgment that, although it remains important to maintain secondary PM2.5 and PM10 standards to provide some degree of control over long- and short-term concentrations of both fine and coarse particles, it is generally appropriate not to change the existing secondary standards and that it is not appropriate to establish any distinct secondary PM standards to address PM-related climate and materials effects at this time. As such, the Administrator recognizes that current suite of secondary standards (i.e., the 24-hour PM2.5, 24-hour PM10, and annual PM2.5 standards) together provide such control for both fine and coarse particles and long- and short-term visibility and non-visibility (e.g., climate and materials) effects related to PM in ambient air. His proposed conclusions on the secondary standards are consistent with advice from the CASAC, which noted substantial uncertainties remain in the scientific evidence for climate and materials effects. Thus, based on his consideration of the evidence and analyses for PM-related welfare effects, as described above, and his consideration of CASAC advice on the secondary standards, the Administrator proposes not to change those standards (i.e., the current 24-hour and annual PM2.5 standards, 24-hour PM10 standard) at this time. The Administrator solicits comments on this proposed conclusion.
The Administrator additionally recognizes that the available evidence on visibility impairment generally reflects a continuum and that the public preference studies did not identify a specific level of visibility impairment that would be perceived as "acceptable" or "unacceptable" across the whole U.S. population. However, he notes a judgment of a target level of protection, below 30 dv and down to 25 dv, could be supported if more weight was put on the public preference study performed in the Phoenix, AZ, study (BBC Research & Consulting, 2003), which yielded the best results of the four public preference studies in terms of the least noisy preference results and the most representative selection of participants. While the Administrator notes that CASAC did not recommend revising the level of the current 24-hour PM2.5 standard, the Administrator recognizes that, should an alternative level be considered for the visibility index, that the CASAC recommends also considering revisions to the secondary 24-hour PM2.5 standard (Sheppard, 2022a, p. 21 of consensus responses). Thus, the Administrator solicits comment on the appropriateness of a target level of protection for visibility below 30 dv and down as low as 25 dv, and of revising the level of the current secondary 24-hour PM2.5 standard to a level as low as 25 ug/m[3]. Any comments on such revisions should include an explanation of the basis for the commenters' views.
E. Proposed Decisions on the Secondary PM Standards
Taking the above considerations into account, upon reconsidering the public welfare protection provided by the current secondary PM standards for the known and anticipated adverse effects within the scope of this reconsideration, in light of the currently available scientific evidence and quantitative information, the Administrator proposes not to change the current secondary PM standards at this time. In the Administrator's preliminary judgment, such a suite of secondary PM standards and the rationale supporting not revising the current standards are reasonably judged to reflect the appropriate consideration of the strength of the available evidence and other information and their associated uncertainties and the advice of CASAC.
The Administrator recognizes that the final suite of standards will reflect his ultimate judgment in the final rulemaking, and in the on-going review of the secondary NAAQS for oxides of nitrogen, oxides of sulfur and PM, as to the suite of secondary PM standards that are requisite to protect the public welfare from known or anticipated adverse effects associated with the pollutant's presence in the ambient air. The final judgment to be made by the Administrator will appropriately consider the requirement for standards that are neither more nor less stringent than necessary and will recognize that the CAA does not require that secondary standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect the public welfare from known or anticipated adverse effects.
The Administrator also solicits comment on whether it would be appropriate to revise the current secondary 24-hour PM2.5 standard, in conjunction with considering a lower target level of protection for the visibility index below 30 dv, and as low as 25 dv. The Administrator takes note that, while the CASAC did not recommend changes to the current level of 35 ug/m3 for the secondary 24-hour PM2.5 standard, they indicated that alternative levels should be considered if a lower target level of protection (i.e., lower than 30 dv) for the visibility index was judged to be appropriate. Thus, the Administrator additionally solicits comment on the appropriateness of revising the level of the current secondary 24-hour PM2.5 standard to a level as low as 25 ug/m[3]. Any comments on such revisions should include an explanation of the basis for the commenters' views.
Having reached the proposed decision described here based on interpretation of the  welfare effects evidence for this reconsideration, as assessed in the 2019 ISA and ISA Supplement, and the quantitative analyses of visibility impairment in the PA; the evaluation of policy-relevant aspects of the evidence and quantitative analyses in the PA; the advice and recommendations from the CASAC; public comments received to date in this reconsideration; and the public welfare policy judgments described above, the Administrator recognizes that other interpretations, assessments and judgments might be possible. Therefore, the Administrator solicits comment on the array of issues associated with reconsideration of the secondary PM standards, including public welfare and science policy judgments inherent in his proposed decision, as described above, and the rationales upon which such views are based.
VI. Interpretation of the NAAQS for PM
A. Proposed Amendments to Appendix K: Interpretation of the NAAQS for Particulate Matter
The EPA proposes to revise Appendix K to make the PM10 data handling procedures for the 24-hour PM10 standards specified in 40 CFR 50.6 more consistent with those for other NAAQS pollutants and to codify existing practices. The proposed revisions, which describe site-level computations, site-to-site combinations, and daily validity requirements are discussed in more detail below.
 Updating design value calculations to be on a site-level basis
First, the EPA proposes to require PM10 design values be calculated on a site-level basis. Past practice has been to calculate a monitor-level design value for each individual PM10 monitor when more than one monitor is located at a single site; however, this practice is inconsistent with the data handling for PM2.5 and several other NAAQS pollutants. This inconsistency with PM2.5 has led to public confusion about the applicable PM10 design value and data completeness criteria at a site because operators are more accustomed to site-level monitoring requirements. To resolve this confusion, the EPA believes it would be appropriate to identify a single design value for each site; the EPA is proposing an analytic approach to combine data collected from multiple PM10 monitors collocated at a site to obtain a single set of daily PM10 concentration data for that site. This proposal to move from monitor-level to site-level PM10 design values is supported by the high level of consistency in the measurement data obtained across the various federal reference and equivalent PM10 monitoring instruments currently in operation (U.S. EPA, 2009a, section <IP_ADDRESS>).
The proposed approach would provide for monitoring agencies to designate in their annual network plan one monitor as the primary monitor for each site. Once a primary monitor has been determined for a site, missing daily PM10 concentrations for the primary monitor would be substituted from any other monitors located at the site. In the event of two or more monitors operating at the same site, missing daily PM10 concentrations for the primary monitor would be substituted with daily values averaged across the other collocated monitors. The EPA notes that at the time of this proposal, there were more than 100 sites nationwide with two or more monitors operating simultaneously.
This proposed approach for combining data across collocated monitors at a site is consistent with the existing approach described in Appendix N to Part 50 for the current PM2.5 NAAQS. The EPA invites public comment on the scientific validity of combining data across PM10 monitors and the merits of the proposed approach for combining data across multiple PM10 monitors collocated at a site.
2. Codifying site combinations to maintain a continuous data record
Second, and complementary to the first proposed revision described above, the EPA proposes to maintain the existing practice of combining data from nearby monitoring sites to determine a valid design value, known as a "site combination." Site combinations typically involve situations where one site closes and another begins monitoring a short distance away within a few days, and the monitoring agency wishes to combine the data from the two sites to maintain a continuous data record. The EPA Regional offices have approved over ten site combinations for PM10 since the promulgation of the 1987 PM10 NAAQS; these will be considered approved site combinations if these revisions are promulgated.
Relatedly, the EPA proposes to maintain the existing practice of allowing monitoring agencies to submit site combination requests to the appropriate Regional Administrator through the EPA's Air Quality System (AQS) database. Site combinations may be approved by the Regional Administrator after they determine that the measured air quality concentrations do not differ substantially between the two sites. To make this determination for a requested site combination, the Regional Administrator may request additional information from the Agency including detailed information on the locations and distance between the two sites, levels of ambient concentrations measured at the two sites, and local emissions or meteorology data. To improve transparency, the EPA will make records of all approved site combinations available in the AQS database and will update design value calculations in AQS when approved site combinations are implemented. The EPA invites public comment on the merits of the proposed process for approving site combinations to obtain valid design values for the PM10 NAAQS.
3. Clarifying daily validity requirements for continuous monitors
Third, the EPA proposes to maintain the existing practice of considering daily averages to be valid if at least 75 percent of the hourly averages (i.e., 18 hourly values) for the 24-hour period are available unless a substitution test can show validity on days with seven or more missing hours.
B. Proposed Amendments to Appendix N: Interpretation of the NAAQS for PM2.5
The EPA proposes to revise Appendix N by updating references to the proposed revision(s) of the standards and changing data handling provisions related to combining data from nearby monitoring sites to codify existing practices that are currently being implemented as EPA standard operating procedures.
1. Updating references to the proposed revision(s) of the standards
The EPA proposes to maintain the existing practice of combining data from nearby monitoring sites to determine a valid design value, known as a "site combination." Site combinations typically involve situations where one site closes and another begins monitoring a short distance away within a few days, and the monitoring agency wishes to combine the data from the two sites to maintain a continuous data record. The EPA Regional offices have approved over 40 site combinations for PM2.5 since the promulgation of the 1997 PM2.5 NAAQS; these will be considered approved site combinations if these revisions are promulgated.
2. Codifying site combinations to maintain a continuous data record
Relatedly, the EPA proposes to maintain the existing practice of allowing monitoring agencies to submit site combination requests to the appropriate Regional Administrator through the EPA's Air Quality System (AQS) database. Site combinations may be approved by the Regional Administrator after they determine that the measured air quality concentrations do not differ substantially between the two sites. To make this determination for a requested site combination, the Regional Administrator may request additional information from the Agency including detailed information on the locations and distance between the two sites, levels of ambient concentrations measured at the two sites, and local emissions or meteorology data. To improve transparency, the EPA will make records of all approved site combinations available in the AQS database and will update design value calculations in AQS when approved site combinations are implemented. The EPA invites public comment on the merits of the proposed process for approving site combinations to obtain valid design values for the PM2.5 NAAQS.
VII. Proposed Amendments to Ambient Monitoring and Quality Assurance Requirements
The EPA is proposing revisions to ambient air monitoring requirements for PM to improve the usefulness of and appropriateness of data used in regulatory decision making. These proposed changes focus on ambient monitoring requirements found in 40 CFR Part 50 (Appendix L), Part 53, and Part 58 with associated appendices (A, B, C, D, and E). These proposed changes include addressing updates in the approval of reference and equivalent methods, updates in quality assurance statistical calculations to account for lower concentration measurements, updates to support improvements in PM methods, a revision to the PM2.5 network design to account for at-risk populations, and updates to the Probe and Monitoring Path Siting Criteria for NAAQS pollutants.
The EPA last completed revisions to PM ambient air monitoring regulations as a part of the PM NAAQS review completed in 2012 (78 FR 3085, January 15, 2013). This final rulemaking included revisions to ensure the suite of standards for PM provide requisite protection of public health and welfare as well as corresponding revisions to the data handling conventions for PM and to the ambient air monitoring, reporting, and network design requirements. Other pollutant-specific monitoring updates have occurred in conjunction with revisions to the NAAQS. In such cases, the monitoring revisions were typically finalized as part of the final rulemaking for the NAAQS. Specific proposed changes are described below.
A. Proposed Amendment in 40 CFR Part 50 (Appendix L): Reference Method for the Determination of Fine Particulate Matter as PM2.5 in the Atmosphere  -  addition of the Tisch cyclone as an approved second stage separator.
The EPA is proposing a technical change to Appendix L to include the addition of an alternative PM2.5 particle size separator to that of the WINS and the VSCC size separators. The new separator is the TE-PM2.5C cyclone manufactured by Tisch Environmental Inc., Cleves, Ohio, and has been shown to have performance equivalent to that of the originally specified WINS impactor with regards to aerodynamic cutpoint and PM2.5 concentration measurement. In addition, the new TE-PM2.5C has a service interval comparable to the VSCC separator and is significantly longer than the service interval for the WINS. Generally, the TE-PM2.5C is also physically interchangeable with the WINS and VSCC where both are manufactured for the same sampler. The proposal would allow the WINS, VSCC, or TE-PM2.5C to be used in a PM2.5 FRM sampler. As is the case for the WINS and VSCC, the TE-PM2.5C is now also an approved size separator for candidate PM2.5 FEMs. Currently, the EPA has designated one PM2.5 sampler configured with TE-PM2.5C separator as a Class II PM2.5 equivalent method and one as a PM10-2.5 equivalent method. Upon promulgation of this proposed change to Appendix L, these instruments would be redesignated as PM2.5 and PM10-2.5 FRMs, respectively. Owners of such samplers would contact the sampler manufacturer to receive a new reference method label for the samplers.
B. Proposed Amendments to Ambient Air Monitoring Reference and Equivalent Methods in 40 CFR Part 53
The EPA is proposing clarifications to the regulations associated with submittal of candidate FRM and FEM applications for review by the EPA. Revisions are also proposed in instances where current regulatory specifications are no longer pertinent and require updating. In addition, the EPA has compiled a list of noted minor errors to correct in regulations associated with the testing requirements and acceptance criteria for federal reference methods (FRMs) and federal equivalent methods (FEMs) in Part 53. These errors are typically not associated with the content of Federal Register notices but often relate to transcription errors and typographical errors in the electronic CFR (eCFR) and printed versions of the CFR.
1. Update to program title and delivery address for FRM and FEM Application and Modification Requests
The EPA is proposing to update the name of the program and delivery address for the EPA review of FRM and FEM Applications and Modification Requests (§53.4). These revisions are due solely to organizational changes and do not affect the structure or role of the Reference and Equivalent Methods Designation Program in reviewing new FRM and FEM application requests and requests to modify existing designated instruments.
2. Requests for delivery of a candidate FRM or FEM instrument
As part of the current applicant review process, §53.4(d) allows the EPA to request only candidate PM2.5 FRMs and Class II or Class III equivalent methods for test purposes. The EPA proposes to revise this section to allow the EPA to request any candidate FRM, FEM, or a designated FRM or FEM associated with a Modification Request, regardless of NAAQS pollutant type or metric.
3. Amendments to requirements for submission of materials in §53.4(b)(7) for language and format
The EPA proposes to amend §53.4(b)(7) of 40 CFR, which specifies the format(s) in which all submissions must be received, to specify that all written application materials must be submitted to the EPA in English in MS Word format and that submitted data must be submitted in MS Excel format.
4. Amendment to designation of reference and equivalent methods
The EPA proposes to clarify the terms of new FRM and FEM methods (§53.8(a)) to ensure that candidate samplers and analyzers are not publicly announced, marketed, or sold as FRMs until the EPA's approval has been formally announced in the Federal Register.
5. Amendment to one test field campaign requirement for Class III PM2.5 FEMs
Field comparability tests for candidate Class III PM2.5 FEMs include the requirement that a total of five field campaigns must be conducted at four separate sites: A, B, C, and D. The Site D specifications of §53.35(b)(1)(ii)(D) require that the site "...shall be in a large city east of the Mississippi River, having characteristically high sulfate concentrations and high humidity levels." However, dramatic decreases in ambient sulfate concentration make it difficult for applicants to routinely meet the high sulfate concentration requirement. Therefore, the EPA proposes to revise the Site D specifications to read "...shall be in a large city east of the Mississippi River, having characteristically high humidity levels."
6. Amendment to use of monodisperse aerosol generator
Wind tunnel evaluation of candidate PM10 inlets and evaluation of candidate PM2.5 fractionators under static conditions requires the generation and use of monodisperse calibration aerosols of specified aerodynamic sizes. In the current regulations (§53.61(g)), the TSI Incorporated Vibrating Orifice Aerosol Generator (VOAG) is the only approved monodisperse generator for this purpose. However, TSI Incorporated no longer manufacturers nor supports the VOAG. Therefore, the EPA proposes to add a commercially available monodisperse aerosol generator -- the Model 1520 Fluidized Monodisperse Aerosol Generator, MSP Corporation, Shoreview, MN - to the list of approved generators for this purpose.
7. Corrections to 40 CFR Part 53 (Reference and Equivalent Methods)
Certain provisions of §53.14, Modification of a reference or equivalent method, incorrectly state an EPA response deadline of 30 days for receipt of modification materials in response to an EPA notice. Per a 2015 amendment, (80 FR 65460, 65416; October. 26, 2015), all EPA response deadlines for modifications of reference or equivalent methods are 90 days from day of receipt.
The EPA proposes corrections to the following tables: Table A-1 to Subpart A of Part 53 - Summary of Applicable Requirements for Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants identifies the applicable Part 50 appendices and Part 53 subparts for each criteria pollutant. The four rows in the section for PM10-2.5 erroneously do not include the footnote instruction that the aforementioned pollutant alternative Class III requirements may be substituted in regard to Part 50 Appendix O - Reference Method for the Determination of Coarse Particulate Matter as PM10-2.5 in the Atmosphere.
Table B-1 SO2 states the interference equivalent for each interferent is +-0.005 ppm for both the standard- and lower-range limits, with the exception of nitric oxide (NO) for the lower-range limit per note 4. When testing the lower range of SO2, the limit for NO is +-0.003 ppm, therefore an incorrect lower limit (+-0.0003) is currently stated in note 4 for this exception to the SO2 lower-range limit.
The EPA proposes corrections to the following figures: After the EPA received an inquiry regarding the interaction of NO and O3, the EPA investigated the interferent testing requirements stated by 40 CFR Part 53, Subpart B. The EPA has determined that during the 2011 SO2 amendment and subsequent 2015 O3 amendment, several typographical errors were introduced into Table B-3, the most significant of which is the omission of note 3, which instructs the applicant to not mix the pollutant with the interferent. Additionally, Appendix A to Subpart B of Part 53 provides figures depicting optional forms for reporting test results. Figure B-3 lists an incorrect formula: the lower detectible limit section is missing the proper operator in the LDL calculation formula and Figure B-5 lists an incorrect calculation metric: there is a typesetting error in the calculation of the standard deviation. The EPA proposes to correct the typesetting errors.
The EPA proposes correcting typesetting errors in several formulas provided throughout §53.43.
C. Proposed Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)
1. Quality Assurance Requirements for Monitors Used in Evaluations for National Ambient Air Quality Standards
The EPA has evaluated the quality system as part of the PM NAAQS reconsideration and identified several areas that could be improved in light of lower average ambient PM2.5 concentrations across the country and the proposed more revised primary annual PM2.5 NAAQS described in section II above. Thus, we assessed PM2.5 concentration data across a range of values to determine if any changes were warranted to their use in the statistics used to evaluate the data qualify in the PM2.5 network. This section describes that work and any proposed changes as a result. Other changes proposed in this section include clarifications and other improvements that will better assist with the consistency and operations of quality assurance programs.
a. Quality System Requirements
The EPA has reconsidered the Appendix A, <IP_ADDRESS> goal for acceptable measurement uncertainty for automated and manual PM2.5 methods currently stated as an upper 90 percent confidence limit for the coefficient of variation (CV) of 10 percent and +-10 percent for total bias. The average PM2.5 concentrations across the nation have steadily declined since the promulgation of the first PM2.5 standard (U.S. EPA, 2022, section 2.3). As ambient concentrations decrease, the bias is inflated using the current bias statistic in 4.2.5. The EPA has developed a new bias statistic to minimize the effect of low PM2.5 concentrations on bias and is proposing to revise section 4.2.5 to implement this new bias statistic. The EPA has concluded that with this change to the bias statistic, the coefficient of variation (CV) of 10 percent and +-10 percent for total bias is still an acceptable goal for estimating total bias in the networks. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
The EPA is proposing to update and clarify ambient air monitoring requirements found in Appendix A, section 2.6.1 pertaining to EPA Protocol Gas standards used for ambient air monitoring and the Ambient Air Protocol Gas Verification Program. Appendix A would be revised to clarify that in order to participate in the Ambient Air Protocol Gas Verification Program, producers of Protocol Gases must adhere to the requirements of 40 CFR section 75.21(g), and only regulatory ambient air monitoring programs may submit cylinders for assay verification to the EPA Ambient Air Protocol Gas Verification Program. The EPA is proposing to include an allowable uncertainty of +-2.0 percent for EPA Protocol Gas standards used in ambient air monitoring. This allowable uncertainty limit would match the existing limit set by the EPA's continuous emission monitoring program found in Part 75, Appendix A, Section 5.1.4(b) and would make the EPA's regulations of quality assurance of ambient air monitors more uniform and consistent.
b. Measurement Quality Check Requirements
The EPA is proposing to remove section <IP_ADDRESS> from Appendix A. This provision in the quality assurance requirements for ambient air monitoring allows for NO2 compressed gas standards to be used to generate audit standards. However, NO2 compressed gas standards are not currently designated by the EPA's Office of Research and Development (ORD) as an EPA Protocol Gas Standard. As such, this provision conflicts with paragraph 2.6.1 of Appendix A that requires that any standard used for generating test atmospheres be an EPA Protocol Gas Standard. The EPA is aware that there is a need for NO2 compressed gas standards for direct read NO2 monitoring methods. If these NO2 compressed gas standards can, in the future, be proven to be stable and approvable as EPA Protocol Gas Standards, the EPA will consider restoring this provision to Appendix A.
The EPA is proposing to revise the requirement in section <IP_ADDRESS> pertaining to the validation of the gaseous cylinders used for the National Performance Audit Program (NPAP). The EPA proposes to change the requirement for annual verification to the ORD-recommended certification periods for standards identified in Table 2-3 of the EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards (Appendix A, 6.0(4). These ORD-recommended periods are based on the periods for which similar gas mixtures over specific concentration ranges have been shown to be stable, as documented in the peer-reviewed literature or in concentration stability data submitted by NIST and specialty gas producers and reviewed by the EPA. In effect, this would decrease the cost and burden on the Protocol Gas Verification Program (PGVP), which performs these verifications annually. The EPA anticipates this will also decrease the delay in returning tanks back to the auditors. This would provide auditors with longer periods with valid certifications to perform audits without annual interruptions for the verification process.
The EPA is proposing to adjust the minimum value required by Appendix A, section 3.2.4 to be considered valid sample pairs for the PM2.5 Performance Evaluation Program (PEP) from 3 ug/m[3] to 2 ug/m[3]. As discussed above, ambient PM2.5 concentrations have decreased, and many samples being collected now are below the 3 ug/m[3] threshold and deemed invalid for purposes of a valid audit sample. Therefore, decreasing this threshold from 3 ug/m[3] to 2 ug/m[3] would increase the number of valid PEP sample pairs collected, which would reduce the number of re-audits that need to be performed to compensate for invalid sample pairs. Inclusion of values down to 2 ug/m3 would represent the concentrations occurring in routine monitoring operations and are included in annual mean concentrations of the networks. Reducing the number of re-audits would reduce audit costs to monitoring organizations while better representing the data in the networks. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order:  Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
c. Calculations for Data Quality Assessments
The EPA is proposing to update the Appendix A, section 4.2.1, Equation 6 and Equation 7 for calculating the Collocated Quality Control Sampler Precision Estimate for PM10, PM2.5 and Pb. The proposed changes are
Equation 6

to

and
Equation 7

to

These new statistics are designed to address the inflated precision values that result from using these calculations to compare low concentrations that are now observed in the networks. The current precision estimate uses a relative percent difference (RPD) when comparing two collocated samplers. As the two numbers used in the comparison get smaller, the statistic generally produces a result that is inflated. A precision statistic calculated for low-concentration data may show poor agreement even if the nominal values are relatively close to each other. By using the square root in the denominator in these statistics, the variability is more constant across all concentrations thereby reducing the inflated effect. The EPA believes this proposed change would provide the correct context for considering inflated RPDs when calculating the bias estimate. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
The EPA is proposing to update the Appendix A, 4.2.5, Equation 8 calculation for the Performance Evaluation Programs Bias Estimate for PM2.5 from

to

Again, because the average ambient PM concentrations across the nation have steadily declined since the promulgation of the PM2.5 standard, the current method of calculation may not be appropriate for determining bias for these lower ambient concentrations and newer sampling methodologies. The current bias estimate uses a percent difference (PD), referenced in Appendix A, section 4.1.1, when comparing an audit sampler against a routine sampler. As the two numbers used in the comparison get smaller, the statistic generally produces a result that is inflated. A bias statistic calculated for low-concentration data may show poor agreement even if the nominal values are relatively close to each other. This may be misleading when trying to assess bias and summarizing data to be used in decision making. The EPA believes this proposed change would provide the correct context for considering inflated RPDs when calculating the bias estimate. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
d. References
The EPA proposes to update the references and hyperlinks in Appendix A, section 6. Several of the reference documents have been updated and the web locations have changed. This proposal provides accuracy in identifying and locating essential supporting documentation so that historical documents that do not represent current practices are not used. The EPA believes that it is important that interested parties -- especially ambient air monitoring organizations and stakeholders -- have the most current materials that provide clarifications and guidance on the interpretation of the regulations.
The EPA is also proposing to add a footnote to Table A-1 of Appendix A to Part 58 - Minimum Data Assessment Requirements for NAAQS Related Criteria Pollutant Monitors. The proposed footnote would clarify the allowable time (i.e., every two weeks, once a month, once a quarter, once every 6 months, or distributed over all 4 quarters depending on the check) between checks and encourage monitoring organizations to perform data assessments at regular intervals. The EPA believes this proposal is appropriate because the current stipulation is unclear regarding the specified interval for required verifications. For example, under the current flow rate verification for PM10 (low vol.), PM2.5, and Pb-PM10, a flow check could be performed on April 1 and not checked again until May 31, leaving approximately two months between checks. Following this practice would leave large intervals of time between verifications, and if a check fails using the described practice, an unacceptably large data loss could result. Also, a check could be performed on the last day of a QC check interval and then on the first day of the following interval, with only a day or two between checks. This is not the intended practice for QC measures that are meant to ensure equipment is continually operating properly over an operational period. For this reason, the EPA is proposing to clarify the allowable time between checks.
2. Quality Assurance Requirements for Prevention of Significant Deterioration (PSD) Air Monitoring
This section on Quality Assurance Requirements for Prevention of Significant Deterioration (PSD) Air Monitoring was developed in parallel to the proposed changes associated with Appendix A. Thus, this section includes similar detail and proposed changes for evaluating quality system statistics for PM2.5, clarifications, and other improvements that will better assist with the consistency and operations of quality assurance programs for PSD.
a. Quality System Requirements
The EPA has reconsidered the Appendix A, section <IP_ADDRESS> goal for acceptable measurement uncertainty for automated and manual PM2.5 methods currently stated as an upper 90 percent confidence limit for the CV of 10 percent and +-10 percent for total bias. The average PM concentrations across the nation have steadily declined since the promulgation of the first PM2.5 standard (U.S. EPA, 2022, section 2.3). As ambient concentrations decrease, the bias is inflated using the current bias statistic in section 4.2.5. Using a new statistic to replace the existing statistic in section 4.2.5 developed to eliminate the effect of low concentrations on bias, the EPA has concluded that the coefficient of variation (CV) of 10 percent and +-10 percent for total bias is still an acceptable goal for estimating total bias in the networks. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
The EPA is proposing to update and clarify ambient air monitoring requirements found in Appendix A, section 2.6.1 pertaining to EPA Protocol Gas standards used for ambient air monitoring and the Ambient Air Protocol Gas Verification Program. Appendix A would be revised to clarify that in order to participate in the Ambient Air Protocol Gas Verification Program, producers of Protocol Gases must adhere to the requirements of 40 CFR section 75.21(g), and only regulatory ambient air monitoring programs may submit cylinders for assay verification to the EPA Ambient Air Protocol Gas Verification Program. The EPA is proposing to include an allowable uncertainty of +-2.0 percent for EPA Protocol Gas standards used in ambient air monitoring. This allowable uncertainty limit would match the existing limit set by the EPA's continuous emission monitoring program found in Part 75, Appendix A, Section 5.1.4(b) and would make the EPA's regulations more uniform and consistent.
b. Measurement Quality Check Requirements
The EPA is proposing to remove section <IP_ADDRESS> from Appendix A. This provision in the quality assurance requirements for ambient air monitoring allows for NO2 compressed gas standards to be used to generate audit standards. However, NO2 compressed gas standards are not currently designated by the EPA's ORD as an EPA Protocol Gas Standard. As such, this provision conflicts with paragraph 2.6.1 of Appendix A that requires that any standard used for generating test atmospheres be an EPA Protocol Gas Standard. The EPA is aware that there is a need for NO2 compressed gas standards for direct read NO2 monitoring methods. If these NO2 compressed gas standards can, in the future, be proven to be stable and approvable as EPA Protocol Gas Standards, the EPA will consider restoring this provision to Appendix A.
The EPA is proposing to revise the requirement in section <IP_ADDRESS> pertaining to the validation of the gaseous cylinders used for the NPAP. The EPA proposes to change the requirement for annual verification to the ORD-recommended certification periods for standards identified in Table 2-3 of the EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards (Appendix A, 6.0(4)). These ORD-recommended periods are based on the periods for which similar gas mixtures over specific concentration ranges have been shown to be stable, as documented in the peer-reviewed literature or in concentration stability data submitted by NIST and specialty gas producers and reviewed by the EPA. In effect, this would decrease the cost and burden on the PGVP, which performs these verifications annually. The EPA anticipates this will also decrease the delay in returning tanks back to the auditors. This would provide auditors with longer periods with valid certifications to perform audits without annual interruptions for the verification process.
The EPA is proposing to adjust the minimum value required by Appendix A, section 3.2.4 to be considered valid sample pairs for the PM2.5 Performance Evaluation Program (PEP) from 3 ug/m[3] to 2 ug/m[3]. As discussed above, ambient PM2.5 concentrations have decreased, and many samples being collected now are below the 3 ug/m[3] threshold and deemed invalid for purposes of a valid audit sample. Therefore, decreasing this threshold from 3 ug/m[3] to 2 ug/m[3] would increase the number of valid PEP sample pairs collected, which would reduce the number of re-audits that need to be performed to compensate for invalid sample pairs. Inclusion of values down to 2 ug/m3 would represent the concentrations occurring in routine monitoring operations and are included in annual mean concentrations of the networks. Reducing the number of re-audits would reduce audit costs to monitoring organizations while better representing the data in the networks. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
c. Calculations for Data Quality Assessments
The EPA is proposing to update the Appendix A, section 4.2.5, Equation 8 calculation for the Performance Evaluation Programs Bias Estimate for PM2.5 from

to

Again, because the average ambient PM concentrations across the nation have steadily declined since the promulgation of the PM2.5 standard, the current method of calculation may not be appropriate for determining bias for these lower ambient concentrations and newer sampling methodologies. The current bias estimate uses a PD, referenced in Appendix A, section 4.1.1, when comparing an audit sampler against a routine sampler. As the two numbers used in the comparison get smaller, the statistic generally produces a result that is inflated. A bias statistic calculated for low-concentration data may show poor agreement even if the nominal values are relatively close to each other. This may be misleading when trying to assess bias and summarizing data to be used in making decisions. The EPA believes this proposed change would provide the correct context for considering inflated RPDs when calculating the bias estimate. The technical justification and background for this change is documented in a technical memorandum to the docket for this rulemaking titled Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring Network.
d. References
The EPA proposes to update the references and hyperlinks in Appendix A, section 6. Several of the reference documents have been updated and the web locations have changed. This proposal provides accuracy in identifying and locating essential supporting documentation so that historical documents that do not represent current practices are not used. The EPA believes that it is important that interested parties -- especially ambient air monitoring organizations and stakeholders -- have the most current materials that provide clarifications and guidance on the interpretation of the regulations.
The EPA is also proposing to add a footnote to Table A-1 of Appendix A to Part 58 - Minimum Data Assessment Requirements for NAAQS Related Criteria Pollutant Monitors. The proposed footnote would clarify the allowable time (i.e., every two weeks, once a month, once a quarter, once every six months, or distributed over all four quarters depending on the check) between checks and encourage monitoring organizations to perform data assessments at regular intervals. The EPA believes this proposal is appropriate because the current stipulation is unclear regarding the specified interval for required verifications. For example, under the current flow rate verification for PM10 (low vol.), PM2.5, and Pb-PM10, a flow check could be performed on April 1 and not checked again until May 31, leaving approximately two months between checks. Following this practice would leave large intervals of time between verifications, and if a check fails using the described practice, an unacceptably large data loss could result. Also, a check could be performed on the last day of a QC check interval and then on the first day of the following interval, with only a day or two between checks. This is not the intended practice for quality control measures that are meant to ensure equipment is continually operating properly over an operational period. For this reason, the EPA is proposing to clarify the allowable time between checks.
3. Proposed Amendments to PM Ambient Air Quality Methodology
a. Proposal to revoke Approved Regional Methods (ARMs)
The EPA is proposing to remove provisions for approval and use of Approved Regional Methods (ARMs) throughout parts 50 and 58 of the CFR. ARMs are continuous PM2.5 methods that have been approved specifically within a State or local air agency monitoring network for purposes of comparison to the NAAQS and to meet other monitoring objectives. However, at this time, there are no approved ARMs, nor does the EPA anticipate any will be requested. There are, however, more than a dozen approved FEMs for PM2.5. These approved FEMs are eligible for comparison to the NAAQS and to meet other monitoring objectives.
The EPA first proposed a process to approve and use ARMs in January of 2006 (71 FR 2709, January 17, 2006). At that time, there were no approved continuous PM2.5 methods available to compare to the NAAQS. The hope was that approved ARMs would quickly start the use of PM2.5 continuous methods that worked well in monitoring agency networks, since the benefits of regulatory-grade automated methods were not available at that time to air agency programs. It was hoped that the benefits of automated PM2.5 methods  -  including real-time data reporting of PM2.5 to support forecasting and reporting of the AQI while also providing a regulatory dataset eligible for comparison to the PM2.5 NAAQS  -  would encourage the development of ARMs. The idea to encourage ARMs was conceived following review of data across the country demonstrating that some agencies were achieving acceptable data comparability with their PM2.5 methods compared to collocated FRMs; however, those methods did not necessarily provide consistent data across the country. At that time, there were no approved PM2.5 continuous FEMs and it was unclear how soon any might be approved. However, by March 2008, the EPA's Reference and Equivalent Methods program had approved the first PM2.5 continuous FEM (73 FR 13224, March 12, 2008).  Over the next eight years, an additional 12 PM2.5 continuous FEMs were approved. With many commercially available PM2.5 continuous FEMs available to air agencies, almost all agencies soon began implementing one or more PM2.5 FEMs in their network. By 2020, monitoring agencies were reporting PM2.5 continuous FEM data from 660 sites across the country (U.S. EPA, 2022, section <IP_ADDRESS>). Therefore, with a large and growing network of PM2.5 continuous FEMs and no approved applications for ARMs in the 16 years that this provision has been available, the EPA is proposing to remove this provision, including any related language, and to instead rely on the existing network of approved PM2.5 FEMs and future approved FEMs. The EPA notes that although references to ARMs occur across part 50 and part 58, the EPA is not reopening the substance of the provisions where these references occur and is only proposing regulatory text for these provisions for the purpose of removing the reference to ARMs.
b. Proposal for Calibration of PM Federal Equivalent Methods (FEMs)
The EPA is proposing to modify its specifications for PM FEMs described in Appendix C to Part 58. Specifically, 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 improve the PM concentration measurement performance of approved FEMs. This approach, initiated by instrument manufacturers, 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). The EPA is proposing this modification because there are some approved PM FEMs that are not currently meeting measurement quality objectives (MQOs) when evaluating data nationally (U.S. EPA, 2022, section <IP_ADDRESS>) meaning that an update to a factory calibration may be appropriate; however, there is not a clearly defined process to update the calibration of an FEM. While there are several types of data available to use as the reference for such updates (e.g., routinely operated FRMs, audit program FRMs, and chemical speciation sampler data), we are proposing to use routinely operated State, local, and Tribal FRMs as the basis of comparison upon which to calibrate FEMs. The goal of updating factory calibrations would be to increase the number of routinely operating FEMs meeting MQOs across the networks in which they are operated. The EPA has received input from CASAC (Sheppard, 2022, p. 2 of consensus responses) and State, local, and Tribal agencies (NACAA Monitoring Committee 01/20/22; AAPCA Ambient Monitoring Committee 01/26/2022; Tribal air quality professionals call on 02/17/22), all of which expressed strong interest in improving FEM data comparability to collocated FRMs. While there are other approaches that could improve data comparability between PM FEMs and collocated FRMs, The EPA believes that this approach represents the most reliable approach to update FEM factory calibrations, since the existing FRM network data that meets MQOs would be used to set updated factory calibrations. While the Agency is proposing to add this language to more expressly define a process to update factory calibrations of approved PM FEMs, the EPA believes that the existing rules for updating approved FRMs and FEMs found at 40 CFR § 53.14 may also continue to be utilized for this purpose as appropriate. This section allows instrument manufactures to submit to the EPA a "Modification of a reference or equivalent method." Submitting a modification request may be appropriate to ensure an approved FEM continues to meet the 40 CFR § 53.9, "Conditions of designation". Specifically, 40 CFR § 53.9(c) requires that, "Any analyzer, PM10 sampler, PM2.5 sampler, or PM10-2.5 sampler offered for sale as part of an FRM or FEM shall function within the limits of the performance specifications referred to in § 53.20(a), § 53.30(a), § 53.35, § 53.50, or § 53.60, as applicable, for at least 1 year after delivery and acceptance when maintained and operated in accordance with the manual referred to in § 53.4(b)(3)." Thus, instrument manufactures are encouraged to seek improvements to their approved FEM methods as needed to continue to meet data quality needs as operated across the network.  Instrument manufactures have an option to pursue that now and may have an additional option in the future should we finalize this proposal for calibration of PM FEMs.