As an initial matter, the Administrator recognizes the robustness of the longstanding evidence, described in the ISA, of O3 effects on vegetation and associated terrestrial ecosystems. The newly available studies and analyses have strengthened the evidence for the current review that provides the foundation for the Administrator's consideration of O3 effects, associated public welfare protection objectives, and the revisions to the current standard needed to achieve those objectives. In light of the extensive evidence base in this regard, the Administrator focuses on protection against adverse public welfare effects of O3 related effects on vegetation. In so doing, she takes note of effects that compromise plant function and productivity, with associated effects on ecosystems. She is particularly concerned about such effects in natural ecosystems, such as those in areas with protection designated by Congress for current and future generations, as well as areas similarly set aside by states, tribes and public interest groups with the intention of providing similar benefits to the public welfare. She additionally recognizes that providing protection for this purpose will also provide a level of protection for other vegetation that is used by the public and potentially affected by O3 including timber, produce grown for consumption and horticultural plants used for landscaping.
A central issue in this review of the secondary standard, as in the last review (completed in 2008), has been consideration of the role for a cumulative seasonal exposure index. In the last review, the Administrator proposed such an index as one of two options for the form of a revised standard. The Administrator's decision in that review was to retain the existing form and averaging time, while revising the standard level to provide the desired level of protection. As described in section IV.A above, this decision was remanded to the EPA in 2013 by the D.C. Circuit. In the current review, the ISA evaluates the evidence and concludes that, among the approaches investigated, quantifying exposure with a cumulative seasonal index best captures the aspects of exposure that relate to effects on vegetation, particularly those related to growth and yield. The PA considered this finding both in the context of assessing potential impacts, and, conversely, the protection from such impacts that might be realized, as well as in the context of using a cumulative seasonal exposure index as a form for the secondary standard. In the proposal, the Administrator focused on the former context, as an exposure index, while additionally soliciting comment on use of the index as the form for the revised standard. Advice from CASAC, all of which was received prior to the proposal, has largely emphasized the latter context, and that was also the focus of some comments.
In considering revisions to the secondary standard that will specify a level of air quality to provide the necessary public welfare protection, the Administrator focuses on use of a cumulative seasonal exposure index, including specifically the W126 index as defined in the proposal, for assessing exposure, both for making judgments with regard to the potential harm to public welfare posed by conditions allowed by various levels of air quality and for making the associated judgments regarding the appropriate degree of protection against such potential harm. In so doing, the Administrator takes note of the conclusions in the ISA and PA, with which the CASAC concurred, that, based on the currently available evidence, a cumulative seasonal concentration-weighted index best captures the aspects of ecosystem exposure to O3 in ambient air that impact vegetation. In considering the public comments in this area, she notes the broad support for use of such a metric as an exposure index, with many additionally supporting its use as the form for a revised standard, in light of CASAC advice on that point. Thus, based on the substantial support in the evidence and CASAC advice, and in consideration of public comments, the Administrator concludes that it is appropriate to use such a cumulative seasonal concentration-weighted index for purposes of assessing the potential public welfare risks, and similarly, for assessing the potential protection achieved against such risks on a national scale.
The Administrator has considered conclusions of the ISA and PA, as well as advice from CASAC and public comments, regarding different cumulative, concentration-weighted metrics, and different temporal definitions of aspects of these metrics. The Administrator takes note of the PA conclusions in support of the W126 exposure index, recognized by the ISA for its strength in weighting potentially damaging O3 concentrations that contributes to the advantages it offers over other weighted cumulative indices. With regard to the relevant definitions for the temporal aspects of this index, conclusions in the ISA and PA, and such considerations in the last review, have led to a focus on a maximum 3-month, 12-hour index, defined by the 3-consecutive-month period within the O3 season with the maximum sum of W126-weighted hourly O3 concentrations during the period from 8:00 a.m. to 8:00 p.m. each day (as explained in section IV.A.1.c above). The Administrator takes note of the support in the ISA and PA, as well as CASAC recommendations for consideration of the W126 index defined in this way. While recognizing that no one definition of an exposure metric used for the assessment of protection for multiple effects at a national scale will be exactly tailored to every species or each vegetation type, ecosystem and region of the country, as discussed in section IV.C.2 above, the Administrator judges that on balance, a W126 index derived in this way, and averaged over three years, as discussed below, will be appropriate for such purposes.
In considering the appropriate exposure index to facilitate assessment of the level of protection afforded to the public welfare by alternative secondary standards in the proposal, the Administrator concluded that a 3-year average W126 index was appropriate for these purposes. A number of considerations raised in the PA influenced the Administrator's conclusion at the time of proposal, in combination with public welfare judgments regarding the weight to place on the evidence of specific vegetation-related effects estimated to result across a range of cumulative seasonal concentration-weighted O3 exposures and judgments on the extent to which such effects in such areas may be considered adverse to public welfare (December 17, 2014, 79 FR 76347, 75312). Some comments were received from the public on this aspect of the proposed decision, as discussed in section IV.C.2 above, and have been considered in the conclusions reached here.
The Administrator continues to place weight on key aspects raised in the PA and summarized in the proposal on the appropriateness of considering a 3-year average index. The Administrator notes the PA consideration of the potential for multiple consecutive years of critical O3 exposures to result in larger impacts on forested areas than intermittent occurrences of such exposures due to the potential for compounding effects on tree growth. The Administrator additionally notes the evidence, as considered in the PA and summarized in the proposal, for some perennial species of some effects associated with a single year's exposure of a critical magnitude that may have the potential for some "carry over" of effects on plant growth or reproduction in the subsequent season. Further, the Administrator notes the occurrence of visible foliar injury and growth or yield loss in annual plants or crops associated with exposures of a critical magnitude. While the Administrator appreciates that the scientific evidence documents the effects on vegetation resulting from individual growing season exposures of specific magnitude, including those that can affect the vegetation in subsequent years, she is also mindful, both of the strengths and limitations of the evidence, and of the information on which to base her judgments with regard to adversity of effects on the public welfare. The Administrator also recognizes uncertainties associated with interpretation of the public welfare significance of effects resulting from a single-year exposure, and that the public welfare significance of effects associated with multiple years of critical exposures are potentially greater than those associated with a single year of such exposure.
As she did for the proposal, the Administrator has considered advice from CASAC in this area, including the CASAC comments that it favors a W126-based secondary standard with a single year form, that its recommended range of levels relates to such a form, and that a lower range (e.g., with 13 ppm-hrs at the upper end) would pertain to a 3-year form. The Administrator also notes CASAC's recognition that her decision on use of a 3-year average over a single-year W126 index may be a matter of policy. While recognizing the potential for effects on vegetation associated with a single-year exposure, the Administrator concludes that use of a 3-year average metric can address the potential for adverse effects to public welfare that may relate to shorter exposure periods, including a single year.
While the Administrator recognizes the scientific information and interpretations, as well as CASAC advice, with regard to a single-year exposure index, she also takes note of uncertainties associated with judging the degree of vegetation impacts for annual effects that would be adverse to public welfare. Even in the case of annual crops, the assessment of public welfare significance is unclear for the reasons discussed below related to agricultural practices. The Administrator is also mindful of the variability in ambient air O3 concentrations from year to year, as well as year-to-year variability in environmental factors, including rainfall and other meteorological factors, that influence the occurrence and magnitude of O3-related effects in any year, and contribute uncertainties to interpretation of the potential for harm to public welfare over the longer term. As noted above, the Administrator also recognizes that the public welfare significance of effects associated with multiple years of critical exposures are potentially greater than those associated with a single year of such exposure. Based on all of these considerations, the Administrator recognizes greater confidence in judgments related to public welfare impacts based on a 3-year average metric. Accordingly, the considerations identified here lead the Administrator to conclude it is appropriate to use an index averaged across three years for judging public welfare protection afforded by a revised secondary standard.
In reaching a conclusion on the amount of public welfare protection from the presence of O3 in ambient air that is appropriate to be afforded by a revised secondary standard, the Administrator has given particular consideration to the following:  (1) the nature and degree of effects of O3 on vegetation, including her judgments as to what constitutes an adverse effect to the public welfare; (2) the strengths and limitations of the available and relevant information; (3) comments from the public on the Administrator's proposed decision, including comments related to identification of a target level of protection; and (4) CASAC's views regarding the strength of the evidence and its adequacy to inform judgments on public welfare protection. The Administrator recognizes that such judgments include judgments about the interpretation of the evidence and other information, such as the quantitative analyses of air quality monitoring, exposure and risk. She also recognizes that such judgments should neither overstate nor understate the strengths and limitations of the evidence and information nor the appropriate inferences to be drawn as to risks to public welfare. The CAA does not require that a secondary standard be protective of all effects associated with a pollutant in the ambient air but rather those known or anticipated effects judged adverse to the public welfare (as described in section IV.B.2 above). The Administrator additionally recognizes that the choice of the appropriate level of protection is a public welfare policy judgment entrusted to the Administrator under the CAA taking into account both the available evidence and the uncertainties.
The Administrator finds the coherence and strength of the weight of evidence concerning effects on vegetation from the large body of available literature compelling. The currently available evidence addresses a broad array of O3-induced effects on a variety of tree species across a range of growth stages (i.e., seedlings, saplings and mature trees) using diverse field-based (e.g., free air, gradient and ambient) and OTC exposure methods. The Administrator gives particular attention to the effects related to native tree growth and productivity, recognizing their relationship to a range of ecosystem services, including forest and forest community composition. She is also mindful of the significance of community composition changes, particularly in protected areas, such as Class I areas. At the same time, she recognizes, while the evidence strongly supports conclusions regarding O3 impacts on growth and the evidence showing effects on tree seedlings, as well as on older trees, there are limitations in our ability to predict impacts in the environment or to estimate air quality or exposures that will avoid such impacts. Such limitations relate to the variability of environmental factors or characteristics that can influence the extent of O3 effects.
In recognition of the CASAC advice and the potential for adverse public welfare effects, the Administrator has considered the nature and degree of effects of O3 on the public welfare. In so doing, the Administrator recognizes that the significance to the public welfare of O3-induced effects on sensitive vegetation growing within the U.S. can vary, depending on the nature of the effect, the intended use of the sensitive plants or ecosystems, and the types of environments in which the sensitive vegetation and ecosystems are located. Any given O3-related effect on vegetation and ecosystems (e.g., biomass loss, visible foliar injury), therefore, may be judged to have a different degree of impact on the public depending, for example, on whether that effect occurs in a Class I area, a residential or commercial setting, or elsewhere. The Administrator notes that such a distinction is supported by CASAC advice in this review. In her judgment, like those of the Administrator in the last review, it is appropriate that this variation in the significance of O3-related vegetation effects should be taken into consideration in making judgments with regard to the level of ambient O3 concentrations that is requisite to protect the public welfare from any known or anticipated adverse effects. As a result, the Administrator concludes that of those known and anticipated O3-related vegetation and ecosystem effects identified and discussed in this notice, particular significance should be ascribed to those that may occur on sensitive species that are known to or are likely to occur in federally protected areas such as Class I areas or on lands set aside by states, tribes and public interest groups to provide similar benefits to the public welfare, for residents on those lands, as well as visitors to those areas.
Likewise, the Administrator also notes that less protection related to growth effects may be called for in the case of other types of vegetation or vegetation associated with other uses or services. For example, the maintenance of adequate agricultural crop yields is extremely important to the public welfare and currently involves the application of intensive management practices. With respect to commercial production of commodities, the Administrator notes that judgments about the extent to which O3-related effects on commercially managed vegetation are adverse from a public welfare perspective are particularly difficult to reach, given that the extensive management of such vegetation (which, as CASAC noted, may reduce yield variability) may also to some degree mitigate potential O3-related effects. The management practices used on these lands are highly variable and are designed to achieve optimal yields, taking into consideration various environmental conditions. In addition, changes in yield of commercial crops and commercial commodities, such as timber, may affect producers and consumers differently, further complicating the question of assessing overall public welfare impacts. Thus, the Administrator concludes, while research on agricultural crop species remains useful in illuminating mechanisms of action and physiological processes, information from this sector on O3-induced effects is considered less useful in informing judgments on what specific standard would provide the appropriate public welfare protection. In so doing, the Administrator notes that a standard revised to increase protection for forested ecosystems would also be expected to provide some increased protection for agricultural crops and other commercial commodities, such as timber.
The Administrator also recognizes that O3-related effects on sensitive vegetation can occur in other areas that have not been afforded special federal or other protections, including effects on vegetation growing in managed city parks and residential or commercial settings, such as ornamentals used in urban/suburban landscaping or vegetation grown in land use categories involving commercial production of commodities, such as timber. For vegetation used for residential or commercial ornamental purposes, the Administrator believes that there is not adequate information at this time to establish a secondary standard based specifically on impairment of these categories of vegetation, but notes that a secondary standard revised to provide protection for sensitive natural vegetation and ecosystems would likely also provide some degree of protection for such vegetation.
Based on the above considerations, in identifying the appropriate level of protection for the secondary standard, the Administrator finds it appropriate to focus on sensitive trees and other native species known or anticipated to occur in protected areas such as Class I areas or on other lands set aside by the Congress, states, tribes and public interest groups to provide similar benefits to the public welfare, for residents on those lands, as well as visitors to those areas. In light of their public welfare significance, the Administrator gives particular weight to protecting such vegetation and ecosystems. Given the reasons for the special protection afforded such areas (identified in section I.A.3 above), she recognizes the importance of protecting these natural forests from O3-induced impacts, including those related to O3 effects on growth, and including those extending in scale from individual plants to the ecosystem. The Administrator also recognizes that the impacts identified for O3 range from those for which the public welfare significance may be more easily judged, but for which quantitative relationships with O3 in ambient air are less well established, such as impacts on forest community composition in protected wilderness areas, carbon storage and other important ecosystem services, to specific plant-level effects, such as growth impacts (in terms of RBL) in tree seedlings, for which our quantitative estimates are more robust.
For considering the appropriate public welfare protection objective for a revised standard, the Administrator finds appropriate and useful the estimates of tree seedling growth impacts (in terms of RBL) associated with a range of W126-based index values developed from the robust E-R functions for 11 tree species, that were described in the PA and proposal and are summarized in Table 5 above. In making judgments based on those observations, however, the Administrator has considered the broader evidence base and public welfare implications, including associated strengths, limitations and uncertainties. Thus, in drawing on estimates from this table, she is not making judgments simply about a specific magnitude of growth effect in seedlings that would be acceptable or unacceptable in the natural environment. Rather, the Administrator is using the estimates in the table, as suggested by CASAC and emphasized by some commenters, as a surrogate or proxy for consideration of the broader array of vegetation-related effects of potential public welfare significance, that include effects on growth of individual sensitive species and extend to ecosystem-level effects, such as community composition in natural forests, particularly in protected public lands, as well as forest productivity. In so doing, she notes that CASAC similarly viewed biomass loss as "a scientifically valid surrogate of a variety of adverse effects to public welfare" (Frey, 2014c, p. 10). Thus, in considering the appropriate level of public welfare protection for the revised standard, the Administrator gives primary attention to the relationship between W126 exposures and estimates of RBL in tree seedlings in Table 5, finding this to be a useful quantitative tool to inform her judgments in this matter.
In considering the RBL estimates in Table 5 above (drawn from the final PA), the Administrator takes note of comments from CASAC that also give weight to these relationships in formulating its advice and notes the CASAC comments on specific RBL values (Frey, 2014c). In so doing, she considers and contrasts comments and their context on RBL estimates of 2% and 6% for the median studied species.
With regard to the CASAC advice regarding 2% RBL for the median studied tree species, the Administrator notes, as an initial matter, the unclear basis for such a focus, as described in section IV.C.2 above and in the proposal. Further, she notes that the CASAC advice related to this RBL value was that it would be appropriate for the range of levels identified in the PA for the Administrator's consideration to "include[] levels that aim for not greater than 2% RBL for the median tree species" (Frey, 2014c, p. 14). As described in the proposal, the range identified in the PA, which the Administrator considered, extended down to W126 index levels for which the estimated RBL in the median tree species is less than or equal to 2%, consistent with the CASAC advice. In addition, the Administrator notes that only the lowest portion of this range (7-8 ppm-hrs) corresponds to an estimated RBL for the median tree species of less than or equal to 2%, with the remainder of CASAC's range (up to 15 ppm-hrs) associated with higher median RBL estimates. Thus, the Administrator understands CASAC to have identified 2% RBL for the median tree species as a benchmark falling within, and at one end of, the range of levels of protection that the CASAC considers appropriate for the revised standard to provide. However, the fact that the CASAC range included levels for which the RBL estimates were appreciably greater than 2% indicates that CASAC did not judge it necessary that the revised standard be based on the 2% RBL benchmark. Accordingly, the Administrator proposed revisions to the secondary standard based on options related to higher RBL estimates and associated exposures. After also considering public comments, the Administrator continues to consider the uncertainty regarding the extent to which associated effects on vegetation at lower O3 exposures would be adverse to public welfare to be too great to provide a foundation for public welfare protection objectives for a revised secondary standard.
With regard to the CASAC comments on a 6% RBL estimate, the Administrator takes particular note of their characterization of this level of effect in the median studied species as "unacceptably high" (Frey, 2014c, pp. iii, 13, 14). These comments were provided in the context of CASAC's considering the significance of effects associated with a range of alternatives for the secondary standard. Moreover, the range recommended by CASAC excluded W126 index values for which the median species was estimated to have a 6% RBL, based on the information before CASAC at the time (Frey, 2014c, p. 12-13). Accordingly, the EPA interprets these comments regarding 6% RBL to be of a different nature than the CASAC advice regarding a 2% median RBL, both because these two comments are framed to address different questions and because CASAC treated them differently in its recommended range.
In the Administrator's consideration of the RBL estimates to inform judgments on O3 exposures of concern to public welfare and the appropriate protection that the secondary standard should provide from such exposures, she has given particular consideration to the current evidence for the relationship of reduced growth of sensitive tree species with ecosystem effects (as described in the ISA), CASAC's view of 6% RBL for the median studied species as unacceptably high, and the role of the Administrator's judgments regarding public welfare impacts of effects in specially protected natural systems, such as Class I areas. With regard to a point of focus among the median RBL estimates extending below 6% for purposes of judging the appropriate public welfare protection objectives for a revised secondary standard, the Administrator is mindful of the CASAC advice to consider lower levels if using a 3-year average, rather than annual, W126 index value.
In considering the CASAC advice, the Administrator notes that her judgments on a 3-year average index focus on the level of confidence in conclusions that might be drawn with regard to single as compared to multiple year impacts, as described above. For example, the Administrator, while recognizing the strength of the evidence with regard to quantitative characterization of O3 effects on growth of tree seedlings and crops, and in addition to noting the additional difficulties for assessing the welfare impacts of O3 on crops, takes note of the uncertainty associated with drawing conclusions with regard to the extent to which small percent reductions in annual growth contribute to adverse effects on public welfare and the role of annual variability in environmental factors that affect plant responses to O3. Moreover, as explained above, the Administrator concludes that concerns related to the possibility of a single unusually damaging year, inclusive of those described by CASAC, can be addressed through use of a 3-year average metric. Thus, similar to the CASAC's view that a lower level would be appropriate with a 3-year form, the Administrator considers it appropriate to focus on a standard that would generally limit cumulative exposures to those for which the median RBL estimate would be somewhat lower than 6%.
In focusing on cumulative exposures associated with a median RBL estimate somewhat below 6%, the Administrator considers the relationships in Table 5, noting that the median RBL estimate is 6% for a cumulative seasonal W126 exposure index of 19 ppm-hrs. Considering somewhat lower values, the median RBL estimate is 5.7% (which rounds to 6%) for a cumulative seasonal W126 exposure index of 18 ppm-hrs and the median RBL estimate is 5.3% (which rounds to 5%) for 17 ppm-hrs. In light of her decision that it is appropriate to use a 3-year cumulative exposure index for assessing vegetation effects (described above), the potential for single-season effects of concern, and CASAC comments on the appropriateness of a lower value for a 3-year average W126 index, the Administrator concludes it is appropriate to identify a standard that would restrict cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-year W126 index, in nearly all instances. In reaching this conclusion, based on the current information to inform consideration of vegetation effects and their potential adversity to public welfare, she additionally judges that the RBL estimates associated with marginally higher exposures in isolated, rare instances are not indicative of effects that would be adverse to the public welfare, particularly in light of variability in the array of environmental factors that can influence O3 effects in different systems and uncertainties associated with estimates of effects associated with this magnitude of cumulative exposure in the natural environment.
While giving primary consideration to growth effects using the surrogate of RBL estimates based on tree seedling effects, the Administrator also recognizes the longstanding and robust evidence of O3 effects on crop yield. She takes note of CASAC concurrence with the PA description of such effects as of public welfare significance and agrees. As recognized in the proposal, the maintenance of adequate agricultural crop yields is extremely important to the public welfare. Accordingly, research on agricultural crop species remains important for further illumination of mechanisms of action and physiological processes. Given that the extensive management of such vegetation, which as CASAC noted may reduce yield variability, may also to some degree mitigate potential O3-related effects, however, judgments about the extent to which O3-related effects on crop yields are adverse from a public welfare perspective are particularly difficult to reach. Further, management practices for agricultural crops are highly variable and generally designed to achieve optimal yields, taking into consideration various environmental conditions. As a result of this extensive role of management in optimizing crop yield, the Administrator notes the potential for greater uncertainty with regard to estimating the impacts of O3 exposure on agricultural crop production than that associated with O3 impacts on vegetation in natural forests. For all of these reasons, the Administrator is not giving the same weight to CASAC's statement regarding crop yield loss as a surrogate for adverse effects on public welfare, or the magnitude that would represent an adverse impact to public welfare, as to the CASAC's comments on RBL as a surrogate for an array of growth-related effects. Similarly, given the considerations summarized above and in the proposal, the Administrator concludes that agricultural crops do not have the same need for additional protection from the NAAQS as forested ecosystems and finds protection of public welfare from crop yield impacts to be a less important consideration in this review for the reasons identified, including the extensive management of crop yields and the dynamics of agricultural markets. Thus, the Administrator is not giving a primary focus to crop yield loss in selecting a revised secondary standard. She notes, however, that a standard revised to increase protection for forested ecosystems would also be expected to provide some increased protection for agricultural crops.
The Administrator has additionally considered the evidence and analyses of visible foliar injury. In so doing, the Administrator notes the ISA conclusion that "experimental evidence has clearly established a consistent association of the presence of visible foliar injury symptoms with O3 exposure, with greater exposure often resulting in greater and more prevalent injury" (U.S. EPA, 2013, section 9.4.2, p. 9 - 41). The Administrator also recognizes the potential for this effect to affect the public welfare in the context of affecting values pertaining to natural forests, particularly those afforded special government protection, as discussed in section IV.A.3 above. However, she recognizes the significant challenges in judging the extent to which such effects should be considered adverse to public welfare, in light of the variability in the occurrence of visible foliar injury and the lack of clear quantitative relationship with other effects on vegetation, as well as the lack of established criteria or objectives that might inform consideration of potential public welfare impacts related to this vegetation effect.
Further, the Administrator takes note of the range of evidence on visible foliar injury and the various related analyses, including additional observations drawn from the WREA biosite dataset in response to comments, as summarized in section IV.C.2 above. In so doing, she does not agree with CASAC's comment that a level of W126 exposure below 10 ppm-hrs is required to reduce foliar injury, noting some lack of clarity in the WREA and PA presentation of the WREA cumulative proportion analysis findings and their meaning (described in section IV.C.2.4 above). She notes that the additional observations summarized in section IV.C.2 above indicate declines in proportions of sites with any visible foliar injury and biosite index scores with reductions in cumulative W126 exposure across a range of values extending at the high end well above 20 ppm-hrs, down past and including 17 ppm-hrs. In considering this information, however, the Administrator takes note of the current lack of robust exposure-response functions that would allow prediction of visible foliar injury severity and incidence under varying air quality and environmental conditions, as recognized in section IV.A.1.b above. Thus, while the Administrator notes that the evidence is not conducive to use for identification of a specific quantitative public welfare protection objective, due to uncertainties and complexities described in sections IV.A.1.b and IV.A.3 above, she concludes that her judgments above, reached with a focus on RBL estimates, would also be expected to provide an additional desirable degree of protection against visible foliar injury in sensitive vegetation. Accordingly, she considers that a conclusion on the appropriateness of selecting a standard that will generally limit cumulative exposures above 17 ppm-hrs to be additionally supported by evidence for visible foliar injury, while not based on specific consideration of this effect.
With the public welfare protection objectives identified above in mind, the Administrator turns to her consideration of form and level for the revised secondary standard. In considering whether the current form should be retained or revised in order to provide the appropriate degree of public welfare protection, the Administrator has considered the analyses of air quality data from the last 13 years that describe the cumulative exposures, in terms of a 3-year W126 index, occurring at monitoring sites across the U.S. when the air quality metric at that location, in terms of the current standard's form and averaging time, is at or below different alternative levels. The Administrator notes both the conclusions drawn from analyses of the strong, positive relationship between these metrics and the findings that indicate the amount of control provided by the 4[th] high metric.
The Administrator has also considered advice from CASAC and public commenters that support revision of the form to the W126 exposure index. The Administrator concurs with the underlying premise that O3 effects on vegetation are most directly assessed using a cumulative seasonal exposure index, specifically the W126 exposure index. The Administrator additionally recognizes, based on analyses of the last 13 years of monitoring data, and consideration of modeling analyses with associated limitations and uncertainties, that cumulative seasonal exposures appear to have a strong relationship with design values based on the current form and averaging time. She additionally notes the correlation of reductions in W126 index values with reductions in precursor emissions over the past decade that were targeted at meeting the current O3 standards (with 4[th] high form), which indicate the control of cumulative seasonal exposures that can be achieved with a standard of the current form and averaging time.
With regard to recommendations from the CASAC that the form for the revised secondary standard should be the biologically relevant exposure metric, and related comments from the public indicating that the secondary standard must have such a form, the Administrator disagrees. In so doing, she notes that CAA section 109 does not impose such a requirement on the form or averaging time for the NAAQS, as explained in IV.C.2 above. She further notes that the averaging time and form of primary standards are often not the same as the exposure metrics used in reviews of primary standards, in which specific information on quantitative relationships between different exposure metrics and health risk is more often available than it is in reviews of secondary NAAQS. As discussed in section IV.C.2 above, with examples, a primary standard with a particular averaging time and form may provide the requisite public health protection from health effects that are most appropriately assessed using an exposure metric of a different averaging time and form and indicator, and the same principle can apply when establishing or revising secondary standards. The Administrator recognizes that the exposure metric and the standard metric can be quite similar, as in the case of consideration of short-term health effects with the primary O3 standard. She also notes, however, as illustrated by the examples described in section IV.C.2 above, that it is not uncommon for the EPA to retain or adopt elements of an existing standard that the Administrator judges in combination across all elements, including in some cases a revised level, to provide the requisite protection under the Act, even if those elements do not neatly correspond to the exposure metric. Accordingly, she concludes that the Act does not require that the secondary O3 standard be revised to match the exposure metric identified as biologically relevant in this review, as long as the revised standard provides the degree of protection required under CAA section 109(b)(2).
Based on the considerations described here, including the use of an exposure metric that CASAC has agreed to be biologically relevant and appropriate, related considerations summarized in the proposal with regard to air quality analyses and common uses of exposure metrics in other NAAQS reviews, the Administrator finds that, in combination with a revised level, the current form and averaging time for a revised secondary standard can be expected to provide the desired level of public welfare protection. Accordingly, she next turns to the important consideration of a level that, in combination with the form and averaging time, will yield a standard that specifies the requisite air quality for protection of public welfare. In so doing, she has recognized the recommendation by CASAC for revision of the form and averaging time and provided the basis for her alternative view, as described above. Further, in the context of the Administrator's decision on objectives for public welfare protection of a revised secondary standard, and with consideration of the advice from CASAC on levels for a W126-based standard, the Administrator has also reached the conclusion, as described above, that in order to provide the appropriate degree of public welfare protection, the revised secondary standard should restrict cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-year average W126 index, in nearly all instances. Thus, the Administrator finds it appropriate to revise the standard level to one that, in combination with the form and averaging time, will exert this desired degree of control for cumulative seasonal exposures.
In considering a revised standard level, the Administrator has, in light of public comments, revisited the information she considered in reaching her proposed decision on a level within the range of 65 to 70 ppb, and additional information or insights conveyed with public comments. The primary focus of the Administrator's considerations in reaching her proposed decision was the multi-faceted analysis of air quality data from 2001 through 2013 documented in the technical memo in the docket (Wells, 2014a), as well as the earlier analyses and related information described in the PA (as summarized in section IV.E.4 of the proposal). This analysis describes the occurrences of 3-year W126 index values of a magnitude from 17 ppm-hrs through 7 ppm-hrs at monitor locations where O3 concentrations met different alternative standards with the current form and averaging time, and has been expanded in consideration of public comments to present in summary form the more extensive historical dataset accompanying this analysis (Wells, 2015b). Focusing first on the air quality analyses for the most recent period for which data are available (2011-2013) and with the protection objectives identified above in mind, the Administrator observes that across the sites meeting the current standard of 75 ppb, the analysis finds multiple sites distributed across different NOAA climatic regions with 3-year average W126 index values above 17 ppm-hrs, with the values at some sites extending well above 19 ppm-hrs. In comparison, she observes that across sites meeting an alternative standard of 70 ppb, the analysis for the period from 2011-2013 finds no occurrences of W126 values above 17 ppm-hrs and less than a handful of occurrences that equal 17 ppm-hrs. The nearly 600 monitors that would meet an alternative standard of 70 ppb during the 2011-2013 period are distributed across all nine NOAA climatic regions and 46 of the 50 states (Wells, 2015b).
The Administrator notes that some public commenters, who disagreed with her proposed decision on form and averaging time, emphasized past occurrences of cumulative W126 exposure values above the range identified in the proposal (of 13 to 17 ppm-hrs). For example, these commenters emphasize data from farther back across the full time period of the dataset analyzed in the technical memorandum (2001-2013), identifying a value of 19.1 ppm-hrs at a monitor for which the 4[th] high is 70 ppb for the 3-year period of 2006-2008. The Administrator notes, as discussed in section IV.C.2 above, that this was one of fewer than a handful of isolated occurrences of sites for which the 4[th] high was at or below 70 ppb and the W126 index value was above 17 ppm-hrs, all but one of which were below 19 ppm-hrs. The Administrator additionally recognizes her underlying objective of a revised secondary standard that would limit cumulative exposures in nearly all instances to those for which the median RBL estimate would be somewhat lower than 6%. She observes that the single occurrence of 19 ppm-hrs identified by the commenter among the nearly 4000 3-year W126 index values from across the most recently available 11 3-year periods years of data at monitors for which the 4[th] high metric is at or below 70 ppb is reasonably regarded as an extremely rare and isolated occurrence (Wells, 2015b). As such, it is unclear whether it would recur, particularly as areas take further steps to reduce O3 to meet revised primary and secondary standards. Further, based on the currently available information, the Administrator does not judge RBL estimates associated with marginally higher exposures in isolated, rare instances to be indicative of adverse effects to the public welfare. Thus, the Administrator concludes that a standard with a level of 70 ppb and the current form and averaging time may be expected to limit cumulative exposures, in terms of a 3-year average W126 exposure index, to values at or below 17 ppm-hrs, in nearly all instances, and accordingly, to eliminate or virtually eliminate cumulative exposures associated with a median RBL of 6% or greater.
The Administrator recognizes that any standard intended to exert a very high degree of control on cumulative seasonal exposures, with the objective of limiting exposures above 17 ppm-hrs across the U.S., in nearly all instances, will, due to regional variation in meteorology and sources of O3 precursors, result in cumulative seasonal exposures well below 17 ppm-hrs in many areas. Even implementation of a standard set in terms of the cumulative seasonal exposure metric, while limiting the highest exposures, would, due to regional variation in meteorology and sources of O3 precursors, result in many areas with much lower exposures. Such variation in exposures occurring under a specific standard is not unexpected and the overall distribution of exposures estimated to occur with air quality conditions associated with different alternative standards is a routine part of the consideration of public health protection in reviews of primary standards, and can also play a role in the review of secondary standards. For these reasons, and in light of the discussion in section IV.C.2.d above on consideration of "necessary" protection, the Administrator notes that an expectation of differing exposures is not, in itself, a basis for concluding that the air quality would be more (or less) than necessary (and thus not requisite) for the desired level of public welfare protection.
The Administrator has also considered the protection afforded by a revised standard against other effects studied in this review, such as visible foliar injury and reduced yield for agricultural crops, and also including those associated with climate change. While noting the evidence supporting a relationship of O3 in ambient air with climate forcing effects, as concluded in the ISA, the Administrator judges the quantitative uncertainties to be too great to support identification of a standard specific to such effects such that she concludes it is more important to focus, as she has done above, on setting a standard based on providing protection against vegetation-related effects which would be expected to also have positive implications for climate change protection through the protection of ecosystem carbon storage.
The Administrator additionally considers the extent of control for cumulative seasonal exposures exerted by a revised standard level of 65 ppb, the lower end of the proposed range. In focusing on the air quality analyses for the most recent period for which data are available, the Administrator observes that across the sites meeting a 4[th] high metric of 65 ppb, the analysis finds no occurrences of W126 values above 11 ppm-hrs and 15 occurrences of a value between 7 ppm-hrs and 11 ppm-hrs, scattered across four of the NOAA climatic regions. The Administrator finds these magnitudes of cumulative seasonal exposures to extend appreciably below the objectives she identified above for affording public welfare protection. In considering this alternative level, she additionally notes that data for only 217 monitors (less than 20 percent of the total with valid 4[th] high metric values) were at or below a 4[th] high value of 65 ppb during the period from 2011-2013. In so noting, she recognizes the appreciably smaller and less geographically extensive dataset available and the associated uncertainty for conclusions based on such an analysis.
Thus, based on the support provided by currently available information on air quality, the evidence base of O3 effects on vegetation and her public welfare policy judgments, and after carefully taking the above comments and considerations into account, fully considering the scientific views of the CASAC, and also taking note of CASAC's policy views, the Administrator has decided to retain the current indicator, form and averaging time and to revise the secondary standard level to 70 ppb. In the Administrator's judgment, based on the currently available evidence and quantitative exposure and air quality information, a standard set at this level, in combination with the currently specified form, averaging time and indicator would be requisite to protect the public welfare from known or anticipated adverse effects. A standard set at this level provides an appreciable increase in protection compared to the current standard. The Administrator judges that such a standard would protect natural forests in Class I and other similarly protected areas against an array of adverse vegetation effects, most notably including those related to effects on growth and productivity in sensitive tree species. The Administrator believes that a standard set at 70 ppb would be sufficient to protect public welfare from known or anticipated adverse effects and believes that a lower standard would be more than what is necessary to provide such protection. This judgment by the Administrator appropriately recognizes that the CAA 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. Accordingly, the Administrator concludes that it is appropriate to revise the level for the secondary standard to 70 ppb (0.070 ppm), in combination with retaining the current form, indicator, and averaging time, in order to specify the level of air quality that provides the requisite protection to the public welfare from any known or anticipated adverse effects associated with the presence of O3 in the ambient air.
D. Decision on the Secondary Standard
For the reasons discussed above, and taking into account information and assessments presented in the ISA and PA, the advice and recommendations of CASAC, and the public comments, as well as public welfare judgments, the Administrator is revising the level of the current secondary standard. Specifically, the Administrator has decided to revise the level of the secondary standard to a level of 0.070 ppm, in conjunction with retaining the current indicator, averaging time and form. Accordingly the revised secondary standard is 0.070 ppm O3, as the annual 4[th] highest daily maximum 8-hour average concentration, averaged over three years.
V. Appendix U: Interpretation of the Primary and Secondary NAAQS for O3
A. Background
The EPA is finalizing the proposed Appendix U to 40 CFR Part 50: Interpretation of the Primary and Secondary National Ambient Air Quality Standards for Ozone. The proposed Appendix U addressed the selection of ambient O3 monitoring data to be used in making comparisons with the NAAQS, data reporting and data handling conventions for comparing ambient O3 monitoring data with the level of the NAAQS, and data completeness requirements. The EPA solicited public comment on four elements where the proposed Appendix U differed from Appendix P to 40 CFR Part 50, which addressed data handling conventions for the previous O3 NAAQS. These included the following: 1) the addition of a procedure to combine data collected from two or more O3 monitors operating simultaneously at the same physical location, 2) the addition of a provision allowing the Regional Administrator to approve "site combinations", or the combination of data from two nearby monitoring sites for the purpose of calculating a valid design value, 3) a change from the use of one-half of the method detection limit ((1/2) MDL) to zero (0.000 ppm) as the substitution value in 8-hour average data substitution tests, and 4) a new procedure for calculating daily maximum 8-hour average O3 concentrations for the revised NAAQS.
The EPA is also finalizing, as proposed, exceptional events scheduling provisions in 40 CFR 50.14 that will apply to the submission of information supporting claimed exceptional events affecting pollutant data that is intended to be used in the initial area designations for any new or revised NAAQS. The new scheduling provisions will apply to initial area designations for the 2015 O3 NAAQS.
B. Data Selection Requirements
The EPA proposed this section in Appendix U to clarify which data are to be used in comparisons with the revised O3 NAAQS. The EPA is finalizing this section in Appendix as proposed.
First, the EPA proposed to combine data at monitoring sites with two or more O3 monitoring instruments operating simultaneously into a single site-level data record for determining compliance with the NAAQS, and proposed an analytical approach to perform this combination (79 FR 75351-75352, December 17, 2014). Several commenters supported the EPA's proposed approach, including the State of Iowa, where 15 of the 20 monitoring sites currently operating two O3 monitors simultaneously are located. Commenters supporting the proposal noted that a similar approach is already being used for lead and particulate monitoring, and that the proposed approach will help states meet data completeness requirements. A few commenters supported the EPA's proposed approach with the additional restrictions that the monitoring instruments must use identical methods and be operated by the same monitoring agency.
One state objected to combining data across monitors because the secondary monitors at their sites were used only for quality assurance purposes and data from these monitors should not be combined with data reported from the primary monitors. The EPA notes that data collected to meet quality assurance requirements are stored in a separate location within the Air Quality System (AQS) database and are not used for determining compliance with the NAAQS. Thus, data from the secondary monitors would not be combined with data from the primary monitors at these sites.
Another commenter objected to the proposal because it would reduce the total number of comparisons made with the NAAQS. While this is true, the number of physical locations being compared with the NAAQS will not decrease under the proposed approach, and in fact may increase due to additional sites meeting the data completeness requirements.
Finally, one commenter cited the EPA's evaluation of collocated O3 monitoring data and precision data in the ISA (U.S. EPA, 2013, section 3.5.2), and stated that although the median differences in concentrations reported by the pairs of monitoring instruments were near zero, the extreme values were close to +/- 3.5%. The commenter argued that since the O3 NAAQS are based on the 4[th] highest annual value, data should not be combined across monitors because of the imprecision in the extreme values. The EPA disagrees, noting that the data presented in the ISA are based on hourly concentrations, while design values for the O3 NAAQS are based on a 3-year average of 8-hour average concentrations. Thus, the random variability in the hourly O3 concentration data due to monitoring imprecision will be reduced when concentrations are averaged for comparison with the NAAQS. Additionally, the precision data are typically collected at concentrations at or above the level of the NAAQS, thus the EPA expects that the level of precision documented in the ISA analysis is consistent with the level of precision in the 4[th] highest daily maximum concentrations used for determining compliance with the NAAQS.
The EPA is finalizing this addition in Appendix U as proposed. In addition, the AQS database will be updated to require state agencies to designate a primary monitor at O3 monitoring sites that report data under more than one Pollutant Occurrence Code (POC), a numeric indicator in AQS used to identify individual monitoring instruments. O3 design value calculations in AQS will be updated so that the data will automatically be combined across POCs at a site, and a single design value will be reported for each site. The EPA notes that the substitution approach described above will only be applied to design value calculations for the revised O3 standards, and that design values for previous O3 standards will continue to be calculated at the monitor level, in accordance with the applicable appendices of 40 CFR Part 50.
Second, the EPA proposed to add a provision in Appendix U that would allow the Regional Administrator to approve "site combinations", or to combine data across two nearby monitors for the purpose of calculating a valid design value. Although data handling appendices for previous O3 standards do not explicitly mention site combinations, the EPA has approved over 100 site combinations since the promulgation of the first 8-hour O3 NAAQS in 1997. Thus, the EPA's intention in proposing this addition was merely to codify an existing convention, and to improve transparency by implementing site combinations in AQS design value calculations.
Public commenters unanimously supported this proposed addition. Two commenters suggested that the EPA should require monitoring agencies to provide technical documentation supporting the similarities between sites approved for combining data, including a requirement for simultaneous monitoring whenever possible. One state requested that the EPA provide more detailed acceptability criteria for approving site combinations, while another state urged the EPA not to create a regulatory burden by prescribing detailed requirements codified in regulations.
The EPA is finalizing this addition as proposed in Appendix U. The EPA believes that approval of site combinations should be handled on a case-by-case basis, and that any requests for supporting documentation should be left to the discretion of the Regional Administrator. The EPA may issue future guidance providing general criteria for determining an acceptable level of similarity in air quality concentrations between monitored locations, but is not prescribing detailed criteria for approval of site combinations in this rulemaking.
Additionally, the AQS database will be updated with new fields for monitoring agencies to request site combinations, and an additional field indicating Regional Administrator approval. All pre-existing site combinations will be initially entered into the database as having already been approved by the Regional Administrator. Since this provision has already been used in practice under previous O3 standards, site combinations will be applied to AQS design value calculations for both the revised O3 standards and previous O3 standards.
C. Data Reporting and Data Handling Requirements
First, the EPA proposed a change in Appendix U to the pre-existing 8-hour average data substitution test (40 CFR Part 50, Appendix P, section 2.1) which is used to determine if a site would have had a valid 8-hour average greater than the NAAQS when fewer than 6 hourly O3 concentration values are available for a given 8-hour period. The EPA proposed to change the value substituted for the missing hourly concentrations from one-half of the method detection limit of the O3 monitoring instrument ((1/2) MDL) to zero (0.000 ppm).
Several commenters supported the proposed change, stating that the use of a constant substitution value instead of (1/2) MDL, which can vary across O3 monitoring methods, would simplify design value calculations. One commenter noted that with a substitution value of zero, the data substitution test for an 8-hour average value greater than the NAAQS is equivalent to a sum of hourly O3 concentrations greater than 0.567 ppm (i.e., if the sum is 0.568 ppm or higher, the resulting 8-hour average must be at least 0.071 ppm, which is greater than the revised O3 NAAQS of 0.070 ppm). Finally, one commenter opposed the proposed change in favor of some type of mathematical or statistical interpolation approach, but did not provide a specific recommendation.
The EPA is finalizing the proposed change in Appendix U, with the addition of a short clause making note of the equivalent summation approach described above. The purpose of the data substitution test is to identify 8-hour periods that do not meet the requirements for a valid 8-hour average, yet the reported hourly concentration values are so high that the NAAQS would have been exceeded regardless of the magnitude of the missing concentration values. The EPA believes that zero, being the lowest measured O3 concentration physically possible, is the most appropriate value to substitute in this situation.
Second, the EPA proposed a new procedure in Appendix U for determining daily maximum 8-hour O3 concentrations for the revised NAAQS. The EPA proposed to determine the daily maximum 8-hour O3 concentration based on 17 consecutive moving 8-hour periods in each day, beginning with the 8-hour period from 7:00 AM to 3:00 PM, and ending with the 8-hour period from 11:00 PM to 7:00 AM. In addition, the EPA proposed that a daily maximum value would be considered valid if 8-hour averages were available for at least 13 of the 17 consecutive moving 8-hour periods, or if the daily maximum value was greater than the level of the NAAQS. This procedure is designed to eliminate "double counting" exceedances of the NAAQS based on overlapping 8-hour periods from two consecutive days with up to 7 hours in common, which was allowed under previous 8-hour O3 NAAQS. A dozen public commenters expressed support for the proposed procedure, including several states.
One regional air quality management organization and three of its member states submitted similar comments stating that they agreed with the principle of eliminating "double counting" exceedances of the NAAQS based on overlapping 8-hour periods, but suggested an alternative calculation procedure that would accomplish the same objective. The alternative procedure iteratively finds the highest 8-hour period in a given year, then removes this 8-hour period and all other 8-hour periods associated with that day, including any overlapping 8-hour periods on adjacent days, from the data until a daily maximum value is determined for each day of the year with sufficient monitoring data. The commenters' procedure is more elegant than the iterative procedure the EPA examined in a previous data analysis supporting the proposal (Wells, 2014b, Method 1). However, the EPA compared the two procedures using the data from the original analysis and found the resulting annual 4[th] highest daily maximum values to be identical (Wells, 2015a). Additionally, the commenters' procedure suffers from the same limitations the EPA identified previously in the original analysis: added complexity in design value calculations, longer computational time, and challenges to real-time O3 data reporting systems, which would have to re-calculate 8-hour daily maximum values for the entire year each time the system was updated with new data.
Three states submitted comments stating that they agreed with the proposed calculation procedure, but disagreed with the proposed requirements for determining a valid daily maximum 8-hour O3 concentration. These states were primarily concerned that the proposed requirements would only allow a monitoring site to have four missing 8-hour averages during a day before the entire day would be invalidated, compared with six missing 8-hour averages allowed previously. One state also stated concerns that the proposed requirements would be more difficult to meet while maintaining compliance with existing monitoring requirements such as biweekly quality assurance checks. The EPA compared annual data completeness rates calculated using the Appendix U requirements to annual data completeness rates calculated using the requirements under the previous O3 standards across all U.S. monitoring sites based on data from 2011-2013 (Wells, 2015a). The mean annual data completeness rate was 0.1% higher under the proposed Appendix U requirements than under the previous O3 standards, and the median annual data completeness rates were identical. In addition, the EPA notes that the Appendix U requirements allow for biweekly quality assurance checks and other routine maintenance to be performed between 5:00 AM and 9:00 AM local time without affecting data completeness. Thus, the EPA does not believe that the proposed daily data completeness requirements in Appendix U will be more difficult for monitoring agencies to meet.
Finally, two public commenters opposed the proposed procedures for determining daily maximum 8-hour concentrations. These commenters expressed similar concerns, primarily that not considering 8-hour periods starting midnight to 6:00 AM is less protective of public health than the procedure used to determine daily maximum 8-hour concentrations for the previous O3 standards. The EPA believes that this approach provides the appropriate degree of protection for public health, noting that the hourly concentrations from midnight to 7:00 AM are covered under the 8-hour period from 11:00 PM to 7:00 AM, which is included in the design value calculations proposed in Appendix U. At the same time, the proposed approach ensures that individual hourly concentrations may not contribute to multiple exceedances of the NAAQS, which EPA believes is inappropriate given that people are only exposed once.
The EPA is finalizing as proposed in Appendix U the procedure for determining daily maximum 8-hour concentrations. The EPA does not believe that daily maximum 8-hour concentrations for two consecutive days should be based on overlapping 8-hour periods, since the exposures experienced by individuals only occur once. The EPA believes that the new procedure will avoid this outcome while continuing to make use of all hourly concentrations in determining attainment of the standards, without introducing unnecessary complexity into design value calculations, and without creating additional difficulties for monitoring agencies to meet the data completeness requirements.
D. Exceptional Events Information Submission Schedule
The "Treatment of Data Influenced by Exceptional Events; Final Rule" (72 FR 13560, March 22, 2007), known as the Exceptional Events Rule and codified at 40 CFR 50.14, contains generic deadlines for an air agency to submit to the EPA specified information about exceptional events and associated air pollutant concentration data. As discussed in this section and in more detail in the O3 NAAQS proposal, without revisions to 40 CFR 50.14, an air agency may not be able to flag and submit documentation for some relevant data either because the generic deadlines may have already passed by the time a new or revised NAAQS is promulgated or because the generic deadlines require submission of documentation at least 12 months prior to the date by which the EPA must make a regulatory decision, which may be before air agencies have collected some of the potentially affected data. Specific to the revised O3 NAAQS, revisions to 40 CFR 50.14 are needed because it is not possible for air agencies to flag and submit documentation for any exceptional events that occur in October through December of 2016 by 1 year before the designations are made in October 2017, as is required by the existing generic schedule.
The EPA is finalizing exceptional events scheduling provisions in 40 CFR 50.14, as proposed and as supported by multiple commenters, that will apply to the submission of information supporting claimed exceptional events affecting pollutant data that is intended to be used in the initial area designations for any new or revised NAAQS. The new scheduling provisions will apply to initial area designations for the revised O3 NAAQS. The provisions that we are promulgating use a "delta schedule" that calculates the timelines associated with flagging data potentially influenced by exceptional events, submitting initial event descriptions and submitting exceptional events demonstrations based on the promulgation date of a new or revised NAAQS. The general data flagging deadlines in the Exceptional Events Rule at 40 CFR 50.14(c)(2)(iii) and the general schedule for submission of demonstrations at 40 CFR 50.14(c)(3)(i) continue to apply to data used in regulatory decisions other than those related to the initial area designations process under a new or revised NAAQS.
The EPA acknowledges the concern raised by several commenters that a strengthened O3 NAAQS may result in numerous demonstrations for exceptional events occurring between 2014 and 2016, the data years that the EPA will presumably use for initial area designation decisions made in October 2017. Commenters noted that the proposed schedule is particularly burdensome for agencies needing to submit exceptional events packages for the third year to be used in a 3-year design value (i.e., 2016 data). Several commenters recommended that the EPA either establish no defined schedule for data flagging and exceptional events demonstration submittal or allow a minimum of 2 years from the setting of any new or revised NAAQS for air agencies to provide a complete exceptional events demonstration. Given the CAA requirement that the EPA follow a 2-year designations schedule, the EPA cannot remove submittal schedules entirely for data influenced by exceptional events or provide a minimum 2-year period from the setting of a new or revised NAAQS for documentation submittal. Neither of these options would ensure that the EPA has time to consider event-influenced data in initial area designation decisions. Rather, the EPA is promulgating in this action an exceptional events schedule that provides air agencies with the maximum amount of time available to prepare exceptional events demonstrations and will still allow the EPA sufficient time to consider such exceptional events demonstrations in the designations process in advance of the date by which the EPA must send 120-day notification letters to states. The EPA recognizes that the schedule promulgated in this action is compressed, particularly for the third year of data to be used in a 3-year design value, and we will work cooperatively with air agencies to accommodate this scenario.
Under the schedule promulgated in this action and assuming initial area designation decisions in October 2017 for the revised O3 NAAQS, affected air agencies would need to flag data, submit initial event descriptions and submit demonstrations for exceptional events occurring in 2016 by May 31, 2017. This schedule provides approximately 5 months between the EPA's receipt of the demonstration package and the expected date of designation decisions and approximately 1 month between the EPA's receipt of a package and the date by which the EPA must notify states and tribes of intended modifications to the Governors' recommendations for designations (i.e., 120-day letters).
While, for the third year of data anticipated to be used in a 3-year design value for the revised O3 NAAQS, the promulgated schedule provides for demonstration submission 5 months after the end of the calendar year, the EPA expects that most submitting agencies will have additional time to prepare documentation as we expect the majority of potential O3-related exceptional events to occur during the warmer months (e.g., March through October). Additionally, the EPA will soon propose rule revisions to the 2007 Exceptional Events Rule and will release through a Federal Register Notice of Availability a draft guidance document to address Exceptional Events Rule criteria for wildfires that could affect O3 concentrations. We expect to promulgate Exceptional Events Rule revisions and finalize the new guidance document before the October 2016 date by which states, and any tribes that wish to do so, are required to submit their initial designation recommendations for the revised O3 NAAQS. Considered together, the EPA believes the exceptional events scheduling dates promulgated in this action, the upcoming Exceptional Events Rule revisions, the forthcoming guidance, and the existing guidance and examples of submitted demonstrations currently on the EPA's exceptional events website at http://www.epa.gov/ttn/analysis/exevents.htm, will help air agencies submit information in a timely manner.
Applying the "delta schedule" promulgated in this action for air quality data collected in 2013 through 2014 that could be influenced by exceptional events and be considered during the initial area designations process for the revised O3 NAAQS, results in extending to July 1, 2016, the otherwise applicable generic deadlines of July 1, 2014, and July 1, 2015, respectively, for flagging data and providing an initial description of an event (40 CFR 50.14(c)(2)(iii)). The schedule promulgated in this action also results in a July 1, 2016, date for flagging data and providing an initial description of an event for air quality data collected in 2015. The July 1, 2016, date for data collected in 2015 is the same as that which would apply under the existing generic deadline in the 2007 Exceptional Events Rule. Under the schedule promulgated in this action, October 1, 2016 is the deadline for submitting exceptional events demonstrations for data years 2013 through 2015. As noted previously, under the schedule promulgated in this action, affected air agencies would need to flag, submit initial event descriptions and submit demonstrations for exceptional events occurring in 2016 by May 31, 2017. The EPA believes these revisions will provide adequate time for air agencies to review potential O3 exceptional events influencing compliance with the revised O3 NAAQS, to notify the EPA by flagging the relevant data and providing an initial event description in AQS, and to submit documentation to support exceptional events demonstrations. The schedule revisions promulgated in this action will also allow the EPA to consider and act on the submitted information during the initial area designation process.
While the EPA will make every effort to designate areas for any new or revised NAAQS on a 2-year schedule, the EPA recognizes that under some circumstances we may need up to an additional year for the designations process to ensure that air agencies and the EPA base designations decisions on complete and sufficient information. The promulgated schedule accounts for the possibility that the EPA might announce after promulgating a new or revised NAAQS that we are extending the designations schedule beyond 2 years using authority provided in CAA section 107(d)(B)(i). If the EPA determines that we will follow a 3-year designation schedule, the deadline is 2 years and 7 months after promulgation of a new or revised NAAQS for states to flag data influenced by exceptional events, submit initial event descriptions and submit exceptional events demonstrations for the last year of data that will be used in the designations (e.g., if the EPA were to designate areas in October 2018, the exceptional events submittal deadline for 2017 data would be May 31, 2018). If the EPA notifies states and tribes of a designations schedule between 2 and 3 years, the deadline for states to flag data affected by exceptional events, submit initial event descriptions, and submit exceptional events demonstrations associated with data from the last year to be considered would be 5 months prior to the date specified for designation decisions.
Therefore, using the authority provided in CAA section 319(b)(2) and in the 2007 Exceptional Events Rule at 40 CFR 50.14(c)(2)(vi), the EPA is modifying the schedule for flagging data and submitting exceptional events demonstrations considered for initial area designations by replacing the deadlines and information in Table 1 in 40 CFR 50.14 with the deadlines and information presented in Table 6. As we did in the O3 NAAQS proposal, we are also providing Table 7 to illustrate how the promulgated schedule might apply to the designations process for the revised O3 NAAQS and to designations processes for other future new or revised NAAQS.
Additionally, in conjunction with promulgating exceptional events schedules for initial area designations for new or revised NAAQS, the EPA, as proposed, is removing obsolete regulatory language in 40 CFR 50.14(c)(2)(iv) and (v) and 40 CFR 50.14(c)(3)(ii) and (iii) associated with exceptional events schedules for all historical standards.
Table 6. Schedule for Flagging and Documentation Submission for Data Influenced by Exceptional Events for Use in Initial Area Designations
                    Exceptional Events / Regulatory Action
                    Exceptional Events Deadline Schedule[d]

Flagging and initial event description deadline for data years 1, 2 and 3.[a]
If state and tribal initial designation recommendations for a new/revised NAAQS are due August through January, then the flagging and initial event description deadline will be the July 1 prior to the recommendation deadline. If state and tribal recommendations for a new/revised NAAQS are due February through July, then the flagging and initial event description deadline will be the January 1 prior to the recommendation deadline.
Exceptional events demonstration submittal deadline for data years 1, 2 and 3.[a]
No later than the date that state and tribal recommendations are due to the EPA.
Flagging, initial event description and exceptional events demonstration submittal deadline for data year 4[b] and, where applicable, data year 5.[c]
By the last day of the month that is 1 year and 7 months after promulgation of a new/revised NAAQS, unless either option a or b applies.
a.	If the EPA follows a 3-year designation schedule, the deadline is 2 years and 7 months after promulgation of a new/revised NAAQS.
b.	If the EPA notifies the state/tribe that it intends to complete the initial area designations process according to a schedule between 2 and 3 years, the deadline is 5 months prior to the date specified for final designations decisions in such EPA notification.
[a] Where data years 1, 2, and 3 are those years expected to be considered in state and tribal recommendations.
[b] Where data year 4 is the additional year of data that the EPA may consider when it makes final area designations for a new/revised NAAQS under the standard designations schedule.
[c] Where data year 5 is the additional year of data that the EPA may consider when it makes final area designations for a new/revised NAAQS under an extended designations schedule.
[d] The date by which air agencies must certify their ambient air quality monitoring data in AQS is annually on May 1 of the year following the year of data collection as specified in 40 CFR 58.15(a)(2). In some cases, however, air agencies may choose to certify a prior year's data in advance of May 1 of the following year, particularly if the EPA has indicated its intent to promulgate final designations in the first 8 months of the calendar year. Data flagging, initial event description and exceptional events demonstration deadlines for "early certified" data will follow the deadlines for "year 4" and "year 5" data.

Table 7. Examples by Month of Applying the Promulgated Revised Schedule for Flagging and Documentation Submission for Data Influenced by Exceptional Events for Use in Initial Area Designations

Month of NAAQS Promulgation, State and Tribal Recommendation, and Final Designations
                    Exceptional Events / Regulatory Action
                    Exceptional Events Deadline Schedule[c]
                                      Oct
                                      Nov
                                      Dec
                                      Jan
                                      Feb
                                      Mar
                                      Apr
                                    May[d]
                                    Jun[d]
                                    Jul[d]
                                    Aug[d]
                                      Sep
                                      Oct

                                   Oct 2015
                                   Nov 2015
                                   Dec 2015
                                   Jan 2016
                                   Feb 2016
                                   Mar 2016
                                   Apr 2016
                                   May 2016
                                   Jun 2016
                                   Jul 2016
                                   Aug 2016
                                   Sep 2016
                                   Oct 2016
Flagging and initial event description deadline for data years 1, 2, and 3.[a]
If state and tribal initial designation recommendations for a new/revised NAAQS are due August through January, then the flagging and initial event description deadline will be the July 1 prior to the recommendation deadline. If state and tribal recommendations for a new/revised NAAQS are due February through July, then the flagging and initial event description deadline will be the January 1 prior to the recommendation deadline.
                                     July 1, 2016
(data years 2013, 2014, 2015)
                                     July 1, 2016
(data years 2013, 2014, 2015)
                                     July 1, 2016
(data years 2013, 2014, 2015)
                                     July 1, 2016
(data years 2013, 2014, 2015)
                                      Jan 1, 2017
(data years 2013, 2014, 2015)
                                      Jan 1, 2017
(data years 2013, 2014, 2015)
                                      Jan 1, 2017
(data years 2013, 2014, 2015)
                                      Jan 1, 2017
(data years 2013, 2014, 2015)
                                      Jan 1, 2017
(data years 2014, 2015, 2016)
                                      Jan 1, 2017
(data years 2014, 2015, 2016)
                                     July 1, 2017
(data years 2014, 2015, 2016)
                                     July 1, 2017
(data years 2014, 2015, 2016)
                                     July 1, 2017
(data years 2014, 2015, 2016)
Exceptional events demonstration submittal deadline  for data years 1, 2, and 3.[a]
No later than the date that state and tribal recommendations are due to EPA.
                                      by Oct 2016
(data years 2013, 2014, 2015)
                                      by Nov 2016
(data years 2013, 2014, 2015)
                                     by Dec  2016
(data years 2013, 2014, 2015)
                                      by Jan 2017
(data years 2013, 2014, 2015)
                                      by Feb 2017
(data years 2013, 2014, 2015)
                                      by Mar 2017
(data years 2013, 2014, 2015)
                                      by Apr 2017
(data years 2013, 2014, 2015)
                                      by May 2017
(data years 2013, 2014, 2015)
                                     by June 2017
(data years 2014, 2015, 2016)
                                     by July 2017
(data years 2014, 2015, 2016)
                                      by Aug 2017
(data years 2014, 2015, 2016)
                                      by Sep 2017
(data years 2014, 2015, 2016)
                                      by Oct 2017
(data years 2014, 2015, 2016)
AQS quality assurance and data certification
Annually on May 1 of the year following the year of data collection
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
                                                                          May 1
Flagging, initial event description and exceptional events demonstration submittal deadline for data year 4[b] and, where applicable, data year 5.[c]
By the last day of the month that is 1 year and 7 months after promulgation of a new/revised NAAQS, unless either option a or b applies.
a.	If the EPA follows a 3 year designation schedule, the deadline is 2 years and 7 months after promulgation of a new/revised NAAQS.
b.	If the EPA notifies the state/tribe that it intends to complete the initial area designations process according to a schedule between 2 and 3 years, the deadline is 5 months prior to the date specified for final designations decisions in such EPA notification.
                                               by May 31, 2017
(data year 2016)
                                              by June 30, 2017
(data year 2016)
                                              by July 31, 2017
(data year 2016)
                          by Aug 31, 2017
(data year 2016 and potentially 2017)
                          by Sep 30, 2017
(data year 2016 and potentially 2017)
                          by Oct 31, 2017
(data year 2016 and potentially 2017)
                          by Nov 30, 2017
(data year 2016 and potentially 2017)
                          by Dec 31, 2017
(data year 2016 and potentially 2017)
                                               by Jan 31, 2018
(data year 2017)
                                            by Feb 28/29, 2018
(data year 2017)
                                               by Mar 31, 2018
(data year 2017)
                                               by Apr 30, 2018
(data year 2017)
                                               by May 31, 2018
(data year 2017)
State & Tribal Recommendations to EPA
                                   Oct 2016
                                   Nov 2016
                                   Dec 2016
                                   Jan 2017
                                   Feb 2017
                                   Mar 2017
                                   Apr 2017
                                   May 2017
                                   June 2017
                                   July 2017
                                   Aug 2017
                                   Sep 2017
                                   Oct 2017
EPA notifies States/Tribes of intended modifications to recommendations (EPA sends 120-day letters)
                                   June 2017
                                   July 2017
                                   Aug 2017
                                   Sep 2017
                                   Oct 2017
                                   Nov 2017
                                   Dec 2017
                                   Jan 2018
                                   Feb 2018
                                   Mar 2018
                                   Apr 2018
                                   May 2018
                                   June 2018
Administrator Promulgates Final Designations
                                   Oct 2017
                                   Nov 2017
                                   Dec 2017
                                   Jan 2018
                                   Feb 2018
                                   Mar 2018
                                   Apr 2018
                                   May 2018
                                   June 2018
                                   July 2018
                                   Aug 2018
                                   Sep 2018
                                   Oct 2018
[a] Where data years 1, 2, and 3 are those years expected to be considered in state and tribal recommendations.
[b] Where data year 4 is the additional year of data that the EPA may consider when it makes final area designations for a new/revised NAAQS under the standard designations schedule.
[c] Where data year 5 is the additional year of data that the EPA may consider when it makes final area designations for a new/revised NAAQS under an extended designations schedule.
[d] The date by which air agencies must certify their ambient air quality monitoring data in AQS is annually on May 1 of the year following the year of data collection as specified in 40 CFR 58.15(a)(2). In some cases, however, air agencies may choose to certify a prior year's data in advance of May 1 of the following year, particularly if the EPA has indicated its intent to promulgate final designations in the first 8 months of the calendar year. Data flagging, initial event description and exceptional events demonstration deadlines for "early certified" data will follow the deadlines for "year 4" and "year 5" data.

VI. Ambient Monitoring Related to O3 Standards
A. Background
The EPA proposed to revise the state-by-state O3 monitoring seasons; the PAMS monitoring requirements; the Federal Reference Method (FRM) for measuring O3; and the Federal Equivalent Method (FEM) performance requirement specifications for automated O3 analyzers. The EPA also proposed to make additional minor changes to the FEM analyzer performance testing requirements for NO2 and particulate matter in part 53.
The EPA is finalizing changes to the length of the required O3 monitoring season for 32 states and the District of Columbia. Section VI.B of this preamble provides an overview of the proposed changes to the length of the required O3 monitoring seasons, a summary of significant public comments and our responses, and a summary of the final decisions made to the O3 monitoring seasons for each state.
The EPA is finalizing changes to the PAMS monitoring requirements in 40 CFR part 58, Appendix D Section 5. Section VI.C of this preamble provides background on the PAMS program and current monitoring requirements, a summary of the proposed changes to the PAMS requirements, a summary of significant public comments and our responses, and a summary of the changes to the PAMS requirements in this final rule.
The EPA is finalizing changes to the FRM for O3 in Section VI.D of this preamble and to the associated FEM performance requirement specifications for automated O3 analyzers in Section VI.E. A summary of significant public comments and our responses are provided and a summary of the final changes to the FRM and FEM requirements in this final rule. The EPA is also finalizing minor additional changes to Part 53 including conforming changes to the FEM performance testing requirements in Table B-1 and Figure B-5 for NO2; extending the period of time for the Administrator to take action on a request for modification of a FRM or FEM from 30 days to 90 days; and removing an obsolete provision for manufacturers to submit Product Manufacturing Checklists for fine and coarse particulate matter monitors.
B. Revisions to the Length of the Required O3 Monitoring Seasons
Unlike the ambient monitoring requirements in 40 CFR part 58 for other criteria pollutants that mandate year-round monitoring at State and Local Air Monitoring Stations (SLAMS), O3 monitoring is only required during the seasons of the year that are conducive to O3 formation. These seasons vary in length from place - to-place as the conditions conducive to the formation of O3 (i.e., seasonally-dependent factors such as ambient temperature, strength of solar insolation, and length of day) differ by location. In some locations, conditions conducive to O3 formation are limited to the summer months of the year. In other states with warmer climates (e.g., California, Nevada, and Arizona), the currently required O3 season is year-round. Elevated levels of winter-time O3 have also been measured in some western states where precursor emissions can interact with sunlight off the snow cover under very shallow, stable boundary layer conditions (U.S. EPA 2013a).
The EPA has determined that the proposed lengthening of the O3 monitoring seasons in 32 states and the District of Columbia is appropriate. Ambient O3 concentrations in these areas could approach or exceed the level of the NAAQS, more frequently and during more months of the year compared with the current season lengths. It is important to monitor for O3 during the periods when ambient concentrations could approach the level of the NAAQS to ensure that the public is informed when exposure to O3 could reach or has reached a level of concern.
The EPA completed an analysis to address whether extensions of currently required monitoring seasons are appropriate (Rice, 2014). In this analysis, we used all available data in AQS, including data from monitors that collected O3 data year-round during 2010-2013. More than half of O3 monitors are voluntarily operated on a year-round basis by monitoring agencies. We determined the number of days where one or more monitors had a daily maximum 8-hour O3 average equal to or above 0.060 ppm in the months outside each state's current O3 monitoring season and the pattern of those days in the out-of-season months. We believe that a threshold of 0.060 ppm, taking into consideration reasonable uncertainty, serves as an appropriate indicator of ambient conditions that may be conducive to the formation of O3 concentrations that approach or exceed the NAAQS. We also considered regional consistency, particularly for those states with little available data. We note that seasonal O3 patterns vary year-to-year due primarily to highly variable meteorological conditions conducive to the formation of elevated O3 concentrations early or late in the season in some years and not others. The EPA believes it is important that O3 monitors operate during all periods when there is a reasonable possibility of ambient levels approaching the level of the NAAQS.
Basing O3 monitoring season requirements on the goal of ensuring monitoring when ambient O3 levels approach or exceed the level of the NAAQS supports established monitoring network objectives described in Appendix D of Part 58, including the requirement to provide air pollution data to the general public in a timely manner and to support comparisons of an area's air pollution levels to the NAAQS. The operation of O3 monitors during periods of time when ambient levels approach or exceed the level of the NAAQS ensures that unusually sensitive people and sensitive groups are alerted to O3 levels of potential health concern allowing them to take precautionary measures. The majority of O3 monitors in the U.S. report to AIRNOW, as well as to state-operated websites and automated phone reporting systems. These programs support many objectives including real-time air quality reporting to the public, O3 forecasting, and the verification of real-time air quality forecast models.
1. Proposed Changes to the Length of the Required O3 Monitoring Seasons
The EPA proposed to extend the length of the required O3 monitoring season in 32 states and the District of Columbia. The proposed changes were an increase of one month for 22 states (Connecticut, Delaware, Idaho, Illinois, Iowa, Kansas, Maryland, Massachusetts, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Texas (northern portion only), Virginia, and West Virginia) and the District of Columbia, an increase of one and one half months for Wisconsin, an increase of two months for four states (Indiana, Michigan, Montana, and North Dakota), an increase of four months for Florida and South Dakota, an increase of five months for Colorado, and an increase of seven months for Utah. For Wyoming, we proposed to add three months at the beginning of the season and remove one month at the end of the season, resulting in a net increase of two months. Ozone season requirements are currently split by Air Quality Control Region (AQCR) in Louisiana and Texas. We proposed lengthening the required season in the northern part of Texas (AQCR 022, 210, 211, 212, 215, 217, and 218) by one month and leaving the year-round O3 season in the southern part of Texas (AQCRs 106, 153, 213, 214, and 216) unchanged. No changes were proposed for the AQCRs in Louisiana. As noted earlier, in a few states with limited available data and few exceedance days outside the currently-required season (Iowa, Missouri, and West Virginia), the proposed changes were made by considering supporting information from the surrounding states. These changes involved the proposed addition of one month (March) to the currently-required O3 seasons for these states.
The EPA also proposed that O3 monitors at all National Core Multipollutant Monitoring Stations (NCore) be operated year-round, January through December, regardless of the length of the required O3 season for the remainder of the SLAMS within each state.
We noted that the EPA Regional Administrators have previously approved deviations from the required O3 monitoring seasons as allowed by paragraph 4.1(i) of 40 CFR part 58, Appendix D. We proposed to retain the rule language permitting such deviations from the required O3 monitoring seasons, but note that finalized changes to O3 monitoring season requirements would revoke all existing Regional Administrator-granted waiver approvals. As appropriate, monitoring agencies could seek new approvals for seasonal deviations. Any seasonal deviations based on the Regional Administrator's waiver of requirements must be described in the state's annual monitoring network plan and updated in the AQS.
Given the timing of the final rulemaking and any associated burden on state/local monitoring agencies to implement the extended O3 seasons, we proposed that implementation of the revised O3 seasons would become effective at SLAMS (including NCore sites) on January 1, 2017. We solicited comment on whether the revised seasons could be implemented beginning January 1, 2016, for all monitors or for a subset of monitors, such as those currently operating year-round or on a schedule that corresponds to the proposed O3 season.
2. Comments on the Length of the Required O3 Monitoring Seasons
We received several comments on the proposed revisions to O3 monitoring seasons. Several commenters supported the proposed O3 season length changes and agreed that O3 monitoring seasons should reflect the times of year when O3 may approach or exceed the level of the NAAQS. A few commenters noted the complexities that would arise in the implementation of multi-state planning agreements if states that shared an MSA had different required O3 monitoring seasons. Two state agencies that supported season length changes also recommended changes to neighboring states' O3 seasons. New York recommended that Connecticut's proposed O3 season be further extended (adding the month of October) to match the proposed season in New York (March  -  October) because they share a major MSA and nonattainment area, and the highest design value monitor in the nonattainment area is often in Connecticut. The results from the EPA's analysis did not support the addition of October for Connecticut. However, the EPA recognizes the value in having a consistent O3 season across planning areas and we recommend that monitoring agency representatives from New York and Connecticut contact their respective EPA Regional Office to jointly develop a monitoring plan to provide coverage of the MSA for a longer period of time. Consistent with the results from the EPA's analysis and consistent with our proposal, the EPA is finalizing the March  -  October season in New York and the March  -  September season in Connecticut.
Although no changes were proposed for Arkansas, the Arkansas Department of Environmental Quality recommended that the O3 season in the nonattainment area that includes Crittenden County, Arkansas (March  -  November) be consistent with the O3 seasons in Tennessee (March  -  October) and Mississippi (March  -  October) by either shortening the O3 season in Arkansas or lengthening the O3 season by one month in Tennessee and Mississippi. Based on the results from the EPA's analysis and consistent with our proposal, the EPA is not finalizing any changes to the current O3 seasons in Arkansas, Tennessee, or Mississippi. There is currently one monitor operating in Crittenden County. We recommend that Arkansas work with their EPA Regional Administrator to consider a waiver for the monitor(s) in Crittenden County to allow a deviation (shortened season) from the required O3 season if the agency demonstrates that such a deviation is appropriate for consistency in the nonattainment area.
Two commenters noted the need to extend seasons to capture wintertime O3 events. One commenter urged the EPA to extend monitoring to year-round in the intermountain west (specifically Wyoming) to adequately capture summer and winter O3 problem days and noted especially two monitors in the Pinedale area of Wyoming that should be operated year-round. The EPA's analysis showed that there were no days that were >= 0.060 ppm in Wyoming for the months of October  -  December and that the Wyoming Department of Environmental Quality is currently operating about 70% of their O3 monitors year-round including all O3 monitors in Sublette County, which includes the Pinedale area. Another commenter supported lengthening the seasons for states in the western U.S. where wintertime O3 could be an issue in light of the unique and growing O3 pollution problems caused by oil and gas development activities. They also recommended that the EPA expand the O3 monitoring season to year-round for North Dakota, South Dakota, and Montana beyond what was proposed. The number of observed days that were >= 0.060 ppm in the months outside the season proposed for these states (one day for North Dakota and no days observed for South Dakota and Montana) do not support a further extension to the length of the O3 monitoring season beyond what was proposed. These states are already operating a large percentage of their monitors year-round (89% in North Dakota, 100% in South Dakota, and 78% in Montana). The EPA is finalizing the seasons as proposed in Wyoming (January  -  September), North Dakota (March  -  September), South Dakota (March  -  October), and Montana (April  -  September). The EPA encourages these states to continue year-round operation of their monitors to determine what areas are affected by elevated levels of winter-time O3.
The commenters who opposed lengthening the O3 monitoring seasons noted concerns with the threshold (0.060 ppm) used as the basis for the changes and the length of time (2010-2013) for which ambient data were retrieved and analyzed. Many of those with concerns recommended that levels in the proposed range (e.g., 0.065 ppm or 0.070 ppm) or the current NAAQS level of 0.075 ppm be used as the appropriate threshold for determining the O3 season. With regard to the 0.060 ppm threshold used, this value represents 86 percent of the NAAQS level of 0.070 ppm. This percentage is consistent with the 85 percent threshold used to require additional O3 monitoring based on Appendix D requirements, which include the MSA population and design value. As noted previously, year-to-year variability occurs in seasonal O3 patterns based on highly variable and unpredictable meteorological conditions, which can support the formation of early or late season elevated O3 concentrations in some years and not in other years. This threshold serves as an appropriate indicator of ambient conditions that may be conducive to the formation of O3 concentrations that approach or exceed the level of the NAAQS.
	Certain logistical complexities were noted if longer seasons were required, including site access during winter and the challenge of getting the monitoring equipment ready in time. Four states noted concerns with operator safety and anticipated their inability to access sites due to early spring snowfall. The EPA agrees that site access could be an issue depending on weather conditions and notes that specific site monitoring season deviations may be appropriate. We suggest that this be addressed through the monitoring season waiver process with the EPA Regional Administrator. Any deviations based on the Regional Administrator's waiver of requirements must be described in the state's annual monitoring network plan and updated in AQS.
Several commenters had concerns about the additional cost and resources needed to expand the O3 monitoring seasons. There was some disagreement with the EPA's total annual average cost estimate of $230,000 which took into account the number of O3 monitors already operating year-round across the country. Commenters noted specifically that the proposed extension of required monitoring seasons would increase operational costs and potentially impact the resources available for other monitoring efforts. The added cost of operating O3 monitors over a longer period was noted by some commenters, referencing both the cost of staff to operate the monitors, as well as the additional wear and tear those O3 monitors would experience over a longer operational period. They noted that extending their required monitoring season by adding the month of March would increase staffing requirements for monitor operation and quality assurance. They also noted that the life expectancy of equipment would be reduced due to increased wear and tear. The EPA acknowledges that operational costs for O3 monitoring networks will incrementally increase in states where required seasons have been lengthened. We encourage monitoring agencies to review available technology and operational procedures to institute practices that could potentially reduce such costs, such as the automation of quality control and calibration checks and remote access to evaluate monitor operations. As noted earlier, all states operated at least a portion of their O3 monitoring network outside of the required O3 season during the 2010-2013 data period and reported the data to AQS. In addition, many states are operating more than the minimum number of monitors required to support the basic monitoring objectives described in 40 CFR part 58, Appendix D. Some states have a large percentage of their total O3 monitors operating outside the currently-required O3 season and some states have a small percentage. In situations where states are already operating a large number of their O3 monitors outside their current O3 season, the actual cost increase will be less. In cases where states have a small number of monitors operating outside their current O3 season, in addition to automation and remote access, those states could investigate with their Regional Administrator the process in 40 CFR part 58.14 for reducing the total number of operating monitors that are above the number required by 40 CFR, part 58, appendix D to offset the cost of extending the O3 monitoring season in their state.
Two commenters had concerns about the 4-year period of time evaluated in the EPA's analysis and noted that the 4-year period of time evaluated does not take into account meteorological anomalies and other weather induced situations and is not consistent with the 3 years used to calculate design values. One state agency's comments referenced their own analysis showing concentrations going back 20 years. They noted that 2010 was an unusual year and inclusion of such an unusual year in the 4-year period (2010-2013) of the EPA's analysis provides too much weight on those data. As noted earlier, year-to-year variability occurs in seasonal O3 patterns based on variable meteorological conditions and given the impracticality of forecasting such conditions that affect O3 photochemistry, the EPA believes it is important that O3 monitors operate when there is a reasonable possibility of ambient levels approaching the level of the NAAQS. Another state agency commented that 4 years appeared to be an unusual number of years given that design values are based on 3 years. To support the proposed rule in 2014, the EPA's analysis of O3 seasons began in 2013. At that time the EPA's analysis considered the most recent 3 years of certified data (2010  -  2012) and updated the analysis to add a 4[th] year (2013) when the data were quality-assured, certified, and available in AQS. We used 4 years of data, including the most recent year (2013) to include an additional year of potentially-variable meteorological conditions to propose changes to the seasons. The EPA treated all years equally and did not put any more weight on the 2010 data than any of the other years used in the analysis. The EPA believes that using recently-available data across multiple years to capture varying meteorological conditions was appropriate to support the decisions on extending the O3 seasons. One commenter disagreed with the EPA's definition of year-round (at least 20 daily observations in all 12 months of at least 1 year of the 4-year period). The definition of year-round was used to estimate the number of monitors being operated outside a state's required O3 season and also used for the EPA's Information Collection Request (ICR). All available data in AQS were used for the O3 season analysis, including data from year-round monitors.
Two commenters noted that "regional consistency" is not a scientific reason and is not needed for making changes to the O3 seasons. One commenter noted that significant geographical, meteorological and demographic differences exist between neighboring states that may not warrant identical monitoring seasons. The EPA notes that regional consistency was considered, but only important for a few states where little data were available and the neighboring states had more available data and a sufficient number of days that were >= 0.060 ppm to support the proposed O3 season changes. Regional consistency was not important for other states.
Some commenters expressed support for the proposed requirement that NCore O3 sites operate year-round. They questioned whether data from NCore stations outside the O3 season will be used for designations and requested that the EPA exclude those data from the designations process. Consistent with the designations process for all criteria pollutants, the states, tribes, and the EPA use all data available in AQS that meet the quality assurance requirements in 40 CFR part 58, Appendix A for the designations process. Given that O3 data from NCore stations will meet these requirements, there is no rational basis for excluding these data from comparison to the NAAQS. Accordingly, such data from NCore stations cannot be excluded and will be treated in a manner equivalent to all other O3 data in AQS. The EPA expects that the highest O3 values will occur during the required O3 season; therefore, we don't anticipate that NCore data from the out-of-season months will contribute to the design value used in the designations process. The EPA is finalizing the requirement for year-round O3 monitoring at NCore stations.
The EPA Regional Administrators have previously approved deviations from the required O3 monitoring seasons through rulemakings (64 FR 3028, January 20, 1999; 67 FR 57332, September 10, 2002; and 69 FR 52836, August 30, 2004). The current ambient monitoring rule, in paragraph 4.1(i) of 40 CFR part 58, Appendix D (71 FR 61319, October 17, 2006), allows the EPA Regional Administrators to approve changes to the O3 monitoring season without rulemaking. The EPA is retaining the rule language allowing such deviations from the required O3 monitoring seasons without rulemaking. In the finalized revision to paragraph 4.1(i) of 40 CFR part 58, Appendix D, the EPA is clarifying the minimum considerations that should be taken into account when reviewing requests, and clarifying that changes to the O3 seasons finalized in this rule revoke all previously approved seasonal deviations. The EPA recognizes that the court ordered deadline for this final rule will result in the revocation of all O3 season when this final rule becomes effective. We encourage monitoring agencies with existing waivers to engage their EPA Regions as soon as possible to evaluate whether new or continued waivers are appropriate given the level of the revised O3 NAAQS.
We received three comments for and three comments against early implementation of the revised O3 seasons by the start of the applicable O3 season in each state by January 1, 2016. Those commenters in favor of early implementation of the revised O3 seasons are already operating a large percentage of O3 monitors year-round or outside the current O3 monitoring season in their state. Those commenters against early implementation cited concerns with the need for additional time to implement the revised O3 seasons, especially in areas where access in order to service and support the monitoring equipment may be problematic during winter weather conditions, and the undue burden on already constrained state resources. One commenter noted that given the date for the final rule (October 1, 2015) that there is insufficient time for public review of their annual monitoring network plan due July 1, 2015, for early implementation in 2016. The EPA encourages those agencies who are able to implement the O3 season changes early to do so by the start of the applicable O3 season in their state in 2016. However, taking into consideration the timing and potential burden on monitoring agencies, the EPA is finalizing the requirement for implementing the revised O3 seasons no later than the start of the applicable O3 monitoring season in 2017, as proposed.
3.  Final Decisions on the Length of the Required O3 Monitoring Seasons
Final changes to the required O3 monitoring seasons are summarized in this section as well as in revised Table D-3 in 40 CFR part 58, Appendix D.
Detailed state-by-state technical information has been placed in the docket to document the basis for the EPA's decision on each state. This information includes state-by-state maps and number of days that were >= 0.060 ppm; distribution charts of the number of days that were >= 0.060 ppm by month and state; and detailed information regarding AQS site IDs, dates and concentrations of all occurrences of the 8-hour daily maximum of at least 0.060 ppm between 2010 and 2013. Summaries have also been prepared for each state including the former and proposed O3 monitoring seasons.
	No changes to the required O3 monitoring season were proposed or finalized for these states: Alabama, Alaska, Arizona, Arkansas, California, Georgia, Hawaii, Kentucky, Northern Louisiana (AQCR 019, 022), Southern Louisiana (AQCR 106), Maine, Mississippi, Nevada, New Mexico, Oklahoma, Oregon, Tennessee, Southern Texas (AQCR 106, 153, 213, 214, 216), Vermont, Washington, Puerto Rico, Virgin Islands, Guam, and American Samoa; however, all existing O3 season deviations or waivers are revoked.
Changes to the required O3 monitoring seasons are finalized as follows for these states and the District of Columbia and all existing O3 season deviations or waivers are revoked.
Colorado: Proposed addition of January, February, October, November, and December is finalized. The required season is revised to January  -  December.
Connecticut: Proposed addition of March is finalized, revising season to March  -  September.
Delaware: Proposed addition of March is finalized, revising season to March  -  October.
District of Columbia: Proposed addition of March is finalized, revising season to March  -  October.
Florida: Proposed addition of January, February, November, and December is finalized. The required season is revised to January  -  December.
Idaho: Proposed addition of April is finalized, revising season to April  -  September.
Illinois: Proposed addition of March is finalized, revising season to March  -  October.
Indiana: Proposed addition of March and October, revising season to March  -  October.
Iowa: Proposed addition of March is finalized, revising season to March  -  October.
Kansas: Proposed addition of March is finalized, revising season to March  -  October.
Maryland: Proposed addition of March is finalized, revising season to March  -  October.
Massachusetts: Proposed addition of March is finalized, revising season to March  -  September.
Michigan: Proposed addition of March and October is finalized, revising season to March  -  October.
Minnesota: Proposed addition of March is finalized, revising season to March  -  October.
Missouri: Proposed addition of March is finalized, revising season to March  -  October.
Montana: Proposed addition of April and May is finalized, revising season to April  -  September.
Nebraska: Proposed addition of March is finalized, revising season to March  -  October.
New Hampshire: Proposed addition of March is finalized, revising season to March  -  September.
New Jersey: Proposed addition of March is finalized, revising season to March  -  October.
New York: Proposed addition of March is finalized, revising season to March  -  October.
North Carolina: Proposed addition of March is finalized, revising season to March  -  October.
North Dakota: Proposed addition of March and April is finalized, revising season to March  -  September.
Ohio: Proposed addition of March is finalized, revising season to March  -  October.
Pennsylvania: Proposed addition of March is finalized, revising season to March  -  October.
Rhode Island: Proposed addition of March is finalized, revising season to March  -  September.
South Carolina: Proposed addition of March is finalized, revising season to March  -  October.
South Dakota: Proposed addition of March, April, May, and October is finalized, revising season to March  -  October.
Texas (Northern AQCR 022, 210, 211, 212, 215, 217, 218): Proposed addition of November is finalized, revising season to March  -  November.
Utah: Proposed addition of January, February, March, April, October, November, and December is finalized. The required season is revised to January  -  December.
Virginia: Proposed addition of March is finalized, revising season to March  -  October.
West Virginia: Proposed addition of March is finalized, revising season to March  -  October.
Wisconsin: Proposed addition of March and April 1  -  15 is finalized, revising season to March  -  October 15.
Wyoming: Proposed addition of January, February, March, and removal of October is finalized, revising season to January  -  September.
Finally, we are finalizing the required O3 monitoring season for all NCore stations to be year-round (January  -  December) regardless of the required monitoring season for the individual state in which the NCore station is located.
C. Revisions to the PAMS Network Requirements
Section 182 (c)(1) of the CAA required the EPA to promulgate rules for enhanced monitoring of O3, NOx, and volatile organic compounds (VOC) for nonattainment areas classified as serious (or above) to obtain more comprehensive and representative data on O3 air pollution. In addition, Section 185B of the CAA required the EPA to work with the National Academy of Sciences (NAS) to conduct a study on the role of O3 precursors in tropospheric O3 formation and control. As a result of this study, the NAS issued the report entitled, "Rethinking the Ozone Problem in Urban and Regional Air Pollution", (NAS, 1991).
In response to the CAA requirements and the recommendations of the NAS report, on February 12, 1993 (58 FR 8452), the EPA revised the ambient air quality surveillance regulations to require Photochemical Assessment Monitoring Stations (PAMS) in each O3 nonattainment area classified as serious, severe, or extreme ("PAMS areas"). As noted in the EPA's Technical Assistance Document (TAD) for Sampling and Analysis of Ozone Precursors (U.S. EPA, 1998), the current objectives of the PAMS program are to: 1) provide a speciated ambient air database that is both representative and useful in evaluating control strategies and understanding the mechanisms of pollutant transport by ascertaining ambient profiles and distinguishing among various individual volatile organic compounds (VOCs); 2) provide local, current meteorological and ambient data to serve as initial and boundary condition information for photochemical grid models; 3) provide a representative, speciated ambient air database that is characteristic of source emission impacts to be used in analyzing emissions inventory issues and corroborating progress toward attainment; 4) provide ambient data measurements that would allow later preparation of unadjusted and adjusted pollutant trends reports; 5) provide additional measurements of selected criteria pollutants for attainment/nonattainment decisions and to construct NAAQS maintenance plans; and 6) provide additional measurements of selected criteria and non-criteria pollutants to be used for evaluating population exposure to air toxics as well as criteria pollutants.
 The original requirements called for two to five fixed sites per PAMS area depending on the area's population. Four types of PAMS sites were identified including upwind (Type 1), maximum precursor emission rate (Type 2), maximum O3 concentration (Type 3), and extreme downwind (Type 4) sites. Each PAMS site was required to measure O3, nitrogen oxide (NO), NO2, speciated VOCs, selected carbonyl compounds, and selected meteorological parameters. In addition, upper air meteorological monitoring was required at one site in each PAMS area.
In the October 17, 2006 monitoring rule (71 FR 61236), the EPA revised the PAMS requirements to only require two sites per PAMS area. The intent of the revision was to "allow PAMS monitoring to be more customized to local data needs rather than meeting so many specific requirements common to all subject O3 nonattainment areas; the changes also gave states the flexibility to reduce the overall size of their PAMS programs -- within limits -- and to use the associated resources for other types of monitoring they consider more useful." In addition to reducing the number of required sites per PAMS area, the 2006 revisions also limited the requirement for carbonyl measurements (specifically formaldehyde, acetaldehyde, and acetone) to areas classified as serious or above for the 8-hour O3 standards. This change was made in recognition of carbonyl sampling issues which were believed to cause significant uncertainty in the measured concentrations.
Twenty-two areas were classified as serious or above O3 nonattainment at the time the PAMS requirements were promulgated in 1993. On July 18, 1997 (62 FR 38856), the EPA revised the averaging time of the O3 NAAQS from a 1-hour averaging period to an 8-hour averaging period. On June 15, 2005 (70 FR 44470), the EPA revoked the 1-hour; however, PAMS requirements were identified as requirements that had to be retained in the anti-backsliding provisions included in that action. Therefore, PAMS requirements continue to be applicable to areas that were classified as serious or above nonattainment for the 1-hour O3 standards as of June 15, 2004. Currently, 25 areas are subject to the PAMS requirements with a total of 75 sites. As will be discussed in detail later, the current PAMS sites are concentrated in the Northeast U.S. and California with relatively limited coverage in the rest of the country (Cavender, 2014).
The first PAMS sites began operation in 1994, and have been in operation for over 20 years. Since the start of the program, there have been many changes to the nature and scope of the O3 problem in the U.S. as well as to our understanding of it. The O3 standards has been revised multiple times since the PAMS program was first implemented. On July 18, 1997, the EPA revised the O3 NAAQS to a level of 0.08 parts per million (ppm), with a form based on the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentration. On March 28, 2008 (73 FR 16436), the EPA revised the O3 standards to a level of 0.075 ppm, with a form based on the 3-year average of the annual fourth-highest daily maximum 8-hour average O3 concentration. These changes in the level and form of the O3 NAAQS, along with notable decreases in O3 levels in most parts of the U.S., have changed the landscape of O3 NAAQS violations in the U.S. At the time of the first round of designations for the 8-hour standards (June 15, 2005), only 5 areas were classified as serious or above for the 8-hour standards as compared to 22 areas that were classified as serious or above for the 1-hour standards. While the number of serious and above areas decreased, the number of nonattainment areas remained nearly the same. In addition to the change in the landscape of O3 nonattainment issues, much of the equipment used at PAMS sites is outdated and in need of replacement. New technologies have been developed since the inception of the PAMS program that should be considered for use in the network to simplify procedures and improve data quality. For these reasons, the EPA determined that it would be appropriate to re-evaluate the PAMS program as explained below.
In 2011, the EPA initiated an effort to re-evaluate the PAMS requirements in light of changes in the needs of PAMS data users and the improvements in monitoring technology. The EPA consulted with the Clean Air Science Advisory Committee (CASAC), Air Monitoring and Methods Subcommittee (AMMS) to seek advice on potential revisions to the technical and regulatory aspects of the PAMS program; including changes to required measurements and associated network design requirements. The EPA also requested advice on appropriate technology, sampling frequency, and overall program objectives in the context of the most recently revised O3 NAAQS and changes to atmospheric chemistry that have occurred over the past 10-15 years in the significantly impacted areas. The CASAC AMMS met on May 16 and May 17, 2011, and provided a report with their advice on the PAMS program on September 28, 2011 (U.S. EPA, 2011f). In addition, the EPA met multiple times with the National Association of Clean Air Agencies (NACAA) Monitoring Steering Committee (MSC) to seek advice on the PAMS program. The MSC includes monitoring experts from various State and local agencies actively engaged in ambient air monitoring and many members of the MSC have direct experience with running PAMS sites. Specific advice obtained from the CASAC AMMS and the MSC that was considered in making the proposed changes to the PAMS requirements is discussed in the appropriate sections below.
Based on the findings of the PAMS evaluation and the consultations with the CASAC AMMS and NACAA MSC, the EPA proposed to revise several aspects of the PAMS monitoring requirements including changes in 1) network design, 2) VOC sampling, 3) carbonyl sampling, 4) nitrogen oxides sampling, and 5) meteorology measurements. The following paragraphs summarize the proposed changes, the comments received, and the final changes and supporting rationale.
1. Network Design