Comments. We received numerous comments supporting the proposed threshold. A number of commenters stated that the 0.5 deciview threshold is appropriate given the low triggering threshold for applicability established by Congress, and that the literature supports it as the minimum level of perceptibility.

Some commenters cited published documentation supporting their assertions that a minimum change in deciviews necessary for perceptibility is 0.5 deciviews. 32 33 Henry, R. C. " Just Noticeable Differences in Atmospheric Haze", Journal of the Air & Waste Management Association, 52: 1238 1243, October 2002.

100 Other commenters criticized the threshold as too low. They stated that a change of 0.5 deciviews is inconsistent with language in the regional haze rule pointing to 1.0 deciview as the appropriate perceptibility threshold, and they cited more recent literature justifying perceptibility as greater than a change of 1 deciview33.

One commenter said that we should allow States and regional planning organizations ( RPOs) the flexibility to determine appropriate visibility impact thresholds in light of current knowledge about a range of perceptibility thresholds. Another commenter said that we should explain our basis for establishing a threshold of a one time impact of greater than 0.5 deciviews, in light of the overall goal of the regional haze program. Yet another commenter said that the proposal would " change the regulatory role of the deciview metric by converting it into a regulatory 0.5 deciview standard ( versus a ` goal') for defining how States must exercise their authority and discretion in determining whether an individual source ` causes or contributes'

to visibility impairment in a Class I area."

Several commenters said that the 0.5 deciview threshold is too high. A recurring comment was that the statutory BART applicability test from CAA Section 169A( b)( 2)( A) contains two separate elements: " causation" of any visibility impairment and 101 " contribution" to any such impairment. Commenters pointed out that by setting a threshold of 0.5 deciviews, we had combined " cause or contribute" into a single test of causality, thus effectively eliminating the " contribution" element of the BART applicability test. The commenters asserted that a single BARTeligible source can " contribute" to visibility impairment with impacts much lower than 0.5 deciviews. They argued that we must set the minimum threshold for individual source contribution to visibility impairment at the lowest level detectable by modeling or other appropriate analysis, and that this minimum individual contribution level must in any event be set at no greater than a 0.1 deciview change relative to natural conditions, which is a clearly measurable level. One commenter suggested that a cause or contribute threshold be set at some percentage of the " just noticeable" change of 0.5 deciviews.

Another commenter said that in a case where multiple sources each have a visibility impact of less than a 0.5 deciview change,

but together result in a change of more than 0.5 deciview, each of these sources contributes to the resulting visibility impairment.

This commenter asserted that BART guidelines that result in exemptions for these " contributing" sources would subvert the goals of the regional haze program.

Similarly, several commenters suggested that if any combination of BART eligible sources causes visibility impairment in a Class I area of more than 0.5 deciviews ( by CALPUFF modeling for any 24 hour period, for example), that State should determine 102 that each individual source is subject to BART. Thus, the commenter added, the court's concern about the lack of " empirical evidence of a source's contribution to visibility impairment"

would be addressed.

Two commenters said that our requirement to use the maximum 24 hour value over the 5 year period of meteorological data in the modeling, as proposed, is too stringent, unreasonable,

inappropriate, and departs from the previous methodologies for the regional haze program. Additionally they said that the threshold is restrictive because the single highest 24 hour modeled impact over a three or five year period may be influenced by short term weather conditions, like high humidity, and the BART applicability determination should not be made based on a one time occurrence.

One commenter said that whatever the final threshold for a single source impact for BART sources, EPA should clarify that the purpose of this modeling assessment is to evaluate a source's anticipated contribution to uniform regional haze over the Class I area. EPA should state that the assumption of a uniform haze contribution based on CALPUFF modeling eliminates the need to assess issues related to the size of the Class I area, views within a Class I area, and weather impact interactions. Finally,

one commenter said that thresholds should be established separately for the eastern and western regions of the United States, as natural visibility conditions are established separately for eastern and western regions in the guidance. 34 Moreover, the fact that the ultimate purpose of the visibility provisions is expressed as a " goal" does not mean that all aspects of the program are merely aspirational. CAA section 169A( a)( 4) requires EPA to establish regulations to ensure that reasonable progress is made toward the national visibility goal, and 169A( b)( 2) provides that EPA must require SIPs to contain emission limits, schedules of compliance, and other measures as may be necessary to make reasonable progress towards meeting the goal.

35 If " causing" visibility impairment means causing a humanly perceptible change in visibility in virtually all situations ( i. e. a 1.0 deciview change), then " contributing" to visibility impairment must mean having some lesser impact on the conditions affecting visibility that need not rise to the level of human perception ( 0.5 deciviews or lower).

105 Final Rule. Today's guidelines advise States to use a deciview metric in defining " cause or contribute", as explained further below. The fact that the deciview is also used to track progress toward the goal of natural visibility does not in any way indicate that we are " converting" a " goal" into a requirement. 34 Use of the same metric in the " cause or contribute" context as used for establishing reasonable progress goals, tracking changes in visibility conditions, and defining baseline, current, and natural conditions simply provides for a consistent approach to quantifying visibility impairment.

In response to commenters who said we conflated the " cause or contribute" test, we are clarifying that for purposes of determining which sources are subject to BART, States should consider a 1.0 deciview change or more from an individual source to " cause" visibility impairment, and a change of 0.5 deciviews to " contribute" to impairment. 35 106 In a regulatory context, we believe that a State's decision as to an appropriate threshold for contribution could depend upon the number of sources affecting a class I area. To illustrate, if there were only one emissions source affecting visibility in a class I area, that source could have a deciview impact only slightly below the perceptibility threshold without contributing to noticeable impairment. However, if there were 100 sources each changing visibility by 0.1 deciviews, the total impact would be a 10 deciview change in visibility. In this hypothetical example,

all 100 sources would be contributing, in equal amounts, to substantial visibility impairment.

Because circumstances will vary in different locations, we believe that States should have discretion to set an appropriate threshold depending on the facts of the situation. We believe,

however, that it would be difficult for a State to justify a threshold higher than 0.5 deciviews for two reasons. First,

studies suggest that 0.5 deciviews represents a level of visibility impairment which can be perceived in some but not all situations. A source whose emissions are predicted to result in perceptible or nearly perceptible results should clearly be considered to contribute to impairment. Second, 0.5 deciviews represents one half of the 1.0 deciview level that we are equating with a single source " causing" visibility degradation. Typically,

there are multiple sources that affect visibility in class I areas, so a source causing a 0.5 deciview change can be expected to be contributing to noticeable visibility impairment. 36 All states are working together in regional planning organizations, and we expect that states will have modeling information that identifies sources affecting visibility in individual class I areas, and the magnitude of their impacts.

107 In determining whether the maximum threshold of 0.5 deciviews or a lower threshold is appropriate for purposes of BART, we believe that States should consider the number of emissions sources affecting the class I area and the magnitude of the individual sources' impacts. 36 In general, a larger number of sources causing impacts in a class I area may warrant a lower contribution threshold. In selecting a threshold, States may want to take into account the fact that individual sources have varying amounts of impact on visibility in class I areas. Depending on facts regarding the number of sources affecting a class I area and their modeled impacts, the State could set a threshold that captures those sources responsible for most of the total visibility impacts, while still excluding other sources with very small impacts.

We would note that under this guidance, States would have discretion in setting the threshold for " contributes to" based on modeled impacts of sources. Consistent with the American Corn Growers decision, we are not requiring States to find sources subject to BART regardless of their of their impact on Class I areas. We are suggesting that, in establishing a threshold for assessing contribution for BART, it may be logical to draw a line between " contribution" and " non contribution" based on the number 108 and magnitude of the various sources affecting the Class I areas at issue. Such an approach gives States the ability to assess the empirical evidence showing contribution and to design an appropriate regulatory regime in light of the nature of the problem. We note that for 750 MW power plants, such a line drawing exercise is likely to be unnecessary, as such sources will in most or all cases have impacts far exceeding 1.0 deciviews.

Finally, we disagree that separate threshold levels should be established based on geography because a unit change in visibility expressed in deciviews, perceived or measured, is the same regardless of geography. As explained in the 1999 Regional Haze Rule, the deciview can be used to express changes in visibility impairment in a way that corresponds to human perception in a linear manner. As a result, using the deciview as the metric for measuring visibility means, for example, that a one deciview change in a highly impaired environment would be perceived as roughly the same degree of change of one deciview in a relatively clear environment, and geography is not a factor.

Interpretation of CALPUFF Results The standard CALPUFF modeling run provides day by day estimates of a source's visibility effects over a five year period. In the proposed BART Guideline, we indicated that if the maximum daily visibility value at any receptor over the five years modeled is greater than the " cause or contribute" threshold, then the State should conclude that the source is subject to BART. A 109 number of commenters took issue with our proposal to use the 24

hour maximum modeled visibility impact over five years of meteorological data. Several of them pointed out, for example,

that the maximum modeled 24 hour impact may be an outlier unduly influenced by weather. We agree that the maximum modeled effect in a five year period could be the result of unusual meteorology.

We also recognize that, although CALPUFF is the best currently available tool for analyzing the visibility effects of individual sources, it is a model that includes certain assumptions and uncertainties. Thus, we agree with commenters that a state should not necessarily rely on the maximum modeled impact in determining whether a source may reasonably be anticipated to contribute to visibility impairment in a Class I area.

The final Guideline states that it would be reasonable for States to compare the 98th percentile of CALPUFF modeling results against the " contribution" threshold established by the State for purposes of determining BART applicability. Some stakeholders have argued for the 90th percentile value, or even lower,

contending that EPA should not use extreme cases to make BART applicability decisions. EPA agrees that, in most cases,

important public policy decisions should not be based on the extreme tails of a distribution. For two reasons, however, we have concluded that the 98th percentile is appropriate in this case.

First, Congress established a very low threshold for determining whether a source should be subject to BART review   110 whether it " may reasonably be anticipated to contribute to any impairment of visibility." The use of 90th percentile value would effectively disregard visibility effects that are predicted to occur on 36 or 37 days a year. We do not believe that such an approach would be consistent with the language of the statute.

Second, we note that the 98th percentile value would only be used to determine whether a particular BART eligible source would be subject to further review by the State. In determining what, if any, emission controls should be required, the state will have the opportunity to consider the frequency, duration, and intensity of a source's predicted effect on visibility.

We recognize that the use of the 98th percentile of modeled visibility values would appear to disregard as many as 7 days per year from consideration. On the other hand, there are other features of our recommended modeling approach that are likely to overstate the actual visibility effects of an individual source.

Most important, the simplified chemistry in the model tends to magnify the actual visibility effects of that source. Because of these features and the uncertainties associated with the model, we believe it is appropriate to use the 98th percentile   a more robust approach that does not give undue weight to the extreme tail of the distribution. In our judgement, this approach will effectively capture the sources that contribute to visibility impairment in a Class I area, while minimizing the likelihood that 37 H. R. Rep. No. 95 294 at 204 ( 1077).

111 the highest modeled visibility impacts might be caused by unusual meteorology or uncertain assumptions in the model.

Use of CALPUFF for Visibility Modeling Background. In providing the States with the option of making a determination as to which sources are subject to BART based on a consideration of each source's individual contribution to visibility impairment, we proposed that States use an air quality model such as CALPUFF. We also proposed that States use a CALPUFF or other EPA approved model in the BART analysis itself. The CALPUFF system, as explained in the 2004 reproposed guideline,

consists of a diagnostic meteorological model, a gaussian puff dispersion model with algorithms for chemical transformation and complex terrain, and a post processor for calculating concentration fields and visibility impacts.

The regional haze rule addresses visibility impairment caused by emissions of fine particles and their precursors. As fine particle precursors, such as SO2 or NOx, are dispersed, they react in the atmosphere with other pollutants to form visibilityimpairing pollutants. In fact, Congress implicitly recognized in 1977 the role of chemical transformation in creating visibility impairment, when it stated that the " visibility problem is caused primarily by emissions of SO2, [ NOx], and particulate matter." 37 In most cases, to predict the impacts of a source's specific contribution to visibility impairment, a State will need a tool 38 To determine whether a BART eligible source " may reasonably be anticipated to cause or contribute to any visibility impairment in any Class I area," it may not 112 that takes into account not only the transport and diffusion of directly emitted PM2.5 but also one that can address chemical transformation.

Because the air quality model CALPUFF is currently the best application available to predict the impacts of a single source on visibility in a Class I area, we proposed that a CALPUFF assessment be used as the preferred approach first, for determining whether an individual source is subject to BART, and second, in the BART determination process. The CALPUFF assessment is specific to each source, taking into account the individual source's emission characteristics, location, and the particular meteorological, topographical, and climatological conditions of the area in which the source is located, any of which may have an impact on the transport of PM2.5 and its precursors. CALPUFF can be used to estimate not only the effects of directly emitted PM2.5 emissions from a source, but also to predict the visibility impacts from the transport and chemical transformation of fine particle precursors.

The CALPUFF model is generally intended for use on scales from 50 km to several hundred kilometers from a source. As a general matter, States will typically need to assess the impacts of potential BART sources on Class I areas located more than 50 km from the source. 38 However, in situations where the State is always be sufficient for the State to predict the impacts of a BART eligible source only on the nearest Class I area ( or on the nearest receptor in the nearest Class I area). The particular meteorological and topographical conditions, for example, could mean that a source's greatest impacts occurred at a Class I area other than the nearest one.

39 PLUVUEII is a model used for estimating visual range reduction and atmospheric discoloration caused by plumes resulting from the emissions of particles, nitrogen oxides, and sulfur oxides from a single source. The model predicts the transport, dispersion, chemical reactions, optical effects and surface deposition of point or area source emissions. It is available at http:// www. epa. gov/ scram001/ tt22. htm# pluvue.

113 assessing visibility impacts for source receptor distances less than 50 km, we proposed that States use their discretion in determining visibility impacts, giving consideration to both CALPUFF and other EPA approved methods. As an example, we suggested that States could use an appropriate local scale plume impact model, such as PLUVUEII39, to determine whether a source's emissions are below a level that would be reasonably anticipated to cause or contribute to visibility impairment in any Class I area.

Comments. A number of States, environmental groups, and some industry commenters strongly supported the use of CALPUFF as proposed. Many commenters supported the use of CALPUFF but indicated that States must have the flexibility to use additional tools for their individual source analyses. Some suggested options for the " cause or contribute" determination were the use of photochemical grid models, or more simplified, non modeling approaches. Commenters claimed that States must have the option 114 to incorporate advances in science and technologies into models or other applications that may produce more accurate simulations of meteorology, chemistry, and visibility impairment. Other industry groups and States argued that CALPUFF has significant limitations,

especially simulating complex atmospheric chemistry, and that EPA's recommendation of CALPUFF as the preferred approach is therefore inappropriate.

Another issue raised by commenters was the use of CALPUFF for estimating secondary particulate matter formation. Commenters recognized that CALPUFF was incorporated into the " Guideline on Air Quality Models" at 40 CFR Part 51, Appendix W in April 2003 as the preferred model for Prevention of Significant Deterioration ( PSD) increment and National Ambient Air Quality Standards ( NAAQS)

compliance assessments of long range transport of primary emissions of SO2 and PM2.5. However, commenters stated that CALPUFF has not been incorporated into the Guideline on Air Quality Models for predicting the secondary formation of PM. The commenters remarked that EPA guidance indicates that photochemical grid models be used to simulate secondary PM formation and concluded on this basis that the application of CALPUFF as we proposed is in conflict with our guidance.

Final rule. We believe that CALPUFF is an appropriate application for States to use for the particular purposes of this rule, to determine if an individual source is reasonably anticipated to cause or contribute to impairment of visibility in 40 The model code and its documentation are available at no cost for download from http:// www. epa. gov/ scram001/ tt22. htm# calpuff.

115 Class I areas, and to predict the degree of visibility improvement which could reasonably be anticipated to result from the use of retrofit technology at an individual source. We encourage States to use it for these purposes40.

CALPUFF is the best modeling application available for predicting a single source's contribution to visibility impairment. It is the only EPA approved model for use in estimating single source pollutant concentrations resulting from the long range transport of primary pollutants. In addition, it can also be used for some purposes, such as the visibility assessments addressed in today's rule, to account for the chemical transformation of SO2 and NOx. As explained above, simulating the effect of precursor pollutant emissions on PM2.5 concentrations requires air quality modeling that not only addresses transport and diffusion, but also chemical transformations. CALPUFF incorporates algorithms for predicting both. At a minimum,

CALPUFF can be used to estimate the relative impacts of BARTeligible sources. We are confident that CALPUFF distinguishes,

comparatively, the relative contributions from sources such that the differences in source configurations, sizes, emission rates,

and visibility impacts are well reflected in the model results.

States can make judgements concerning the conservativeness or overestimation, if any, of the results. In fact, although we 116 focused on the use of CALPUFF for primary pollutants in revising the Guideline of Air Quality Modeling, section 7.2.1. e. of the Guideline states:

e. CALPUFF ( Section A. 3) may be applied when assessment is needed of reasonably attributable haze impairment or atmospheric deposition due to one or a small group of sources. This situation may involve more sources and larger modeling domains than that to which VISCREEN ideally may be applied. The procedures and analyses should be determined in consultation with the appropriate reviewing authority ( paragraph 3.0( b) and the affected FLM( s).

We believe that our proposed use of CALPUFF is thus fully in keeping with the Guideline on Air Quality Models, especially in light of the low triggering threshold for determining whether a source is reasonably anticipated to cause or contribute to visibility impairment in a Class I area, and the fact that the modeling results are used as only one of five statutory criteria evaluated to determine BART emission limits.

Even so, as commenters point out, CALPUFF has not yet been fully evaluated for secondary pollutant formation. For the specific purposes of the regional haze rule's BART provisions,

however, we have concluded that CALPUFF is sufficiently reliable to inform the decision making process.

EPA revised the Guideline on Air Quality Models in 2003 ( 40 CFR Part 51, Appendix W, April 2003), in part, to add CALPUFF to 41 Under CAA section 165( a), a major emitting facility may not be constructed unless the owner or operator of the facility demonstrates that the emissions from the facility will not cause or contribute air pollution in excess of an increment or NAAQS.

117 the list of approved models for particular uses. At that time,

we considered comments that CALPUFF should be approved for use in predicting the impact of secondary emissions on particulate matter concentrations. As we stated in the revision, CALPUFF represents a substantial improvement in methods for assessing long range transport of air pollutants. However, as explained in the response to comments for that rulemaking, the modeling results in the context of a PSD review may be used as the sole determining factor in denying a source a permit to construct. 41 Although its use in simulating long range transport is beneficial, given the significance of the modeling results in assessing increment consumption due to a single source's impacts, we made a determination that it would not be appropriate in the rulemaking revising Appendix W to approve CALPUFF for use in modeling secondary emissions.

In contrast to the significance of the modeling results in the PSD context, the use of CALPUFF in the context of the regional haze rule is not determinative of a source's ability to construct or operate. A State may use CALPUFF to determine whether a source can reasonably be anticipated to cause or contribute to visibility impairment and so should be subject to additional review to determine if the source should be subject to control. 42 CALPUFF Analysis in Support of the Regional Haze Rule, U. S. Environmental Protection Agency, April 15, 2005, Docket No. OAR 2002 0076.

43 Ibid.

118 Based on our analysis of the power plants covered by the guidelines, we believe that all but a handful of these plants have impacts of greater than 1.0 deciview on one or more Class I areas. 42 In fact, we anticipate that most of these plants are predicted to have much higher maximum impacts. 43 Because of the scale of the predicted impacts from these sources, CALPUFF is an appropriate or a reasonable application to determine whether such a facility can reasonably be anticipated to cause or contribute to any impairment of visibility. In other words, to find that a source with a predicted maximum impact greater than 2 or 3 deciviews meets the low triggering threshold in the Act does not require the degree of certainty in the results of the model that might be required for other regulatory purposes.

In the unlikely case that a State were to find that a 750 MW power plant's predicted contribution to visibility impairment is within a very narrow range between exemption from or being subject to BART, the State can work with EPA and the FLM to evaluate the CALPUFF results in combination with information derived from other appropriate techniques for estimating visibility impacts to inform the BART applicability determination. Similarly for other types of BART eligible sources, States can work with the EPA and FLM to 119 determine appropriate methods for assessing a single source's impacts on visibility.

As discussed in in section E. below we also recommend that the States use CALPUFF as a screening application in estimating the degree of visibility improvement that may reasonably be expected from controlling a single source in order to inform the BART determination. As we noted in 2004, this estimate of visibility improvement does not by itself dictate the level of control a State would impose on a source; " the degree of improvement in visibility which may reasonably be anticipated to result from the use of [ BART]," is only one of five criteria that the State must consider together in making a BART determination.

The State makes a BART determination based on the best estimates available for each criterion, and as the CAA does not specify how the State should take these factors into account, the States are free to determine the weight and significance to be assigned to each factor. American Corn Growers v. EPA [ cite to dissent, fn.

10]. CALPUFF accordingly is an appropriate application for use in combination with an analysis of the other statutory factors, to inform decisions related to BART.

We understand the concerns of commenters that the chemistry modules of the CALPUFF model are less advanced than some of the more recent atmospheric chemistry simulations. To date, no other modeling applications with updated chemistry have been approved by EPA to estimate single source pollutant concentrations from long 120 range transport. In its next review of the Guideline on Air Quality Models, EPA will evaluate these and other newer approaches and determine whether they are sufficiently documented,

technically valid, and reliable to approve for general use. In the meantime, as the Guideline makes clear, States are free to make individualized showings that these or other alternative approaches are valid and appropriate for their intended applications.

Theoretically, the CALPUFF chemistry simulations, in total,

may lead to model predictions that are generally overestimated at distances downwind of 200 km. Again, States can make judgements concerning the conservativeness or overestimation, if any, of the results.

The use of other models and techniques to estimate if a source causes or contributes to visibility impairment may be considered by the State, and the BART guidelines preserve a State's ability to use other models. Regional scale photochemical grid models may have merit, but such models have been designed to assess cumulative impacts, not impacts from individual sources.

Such models are very resource intensive and time consuming relative to CALPUFF, but States may consider their use for SIP development in the future as they are adapted and demonstrated to be appropriate for single source applications and such use has been approved by the appropriate EPA regional offices. However,

to date, regional models have not been evaluated for single source applications. Their use may be more appropriate in the cumulative 44For regional haze applications, regional scale modeling typically involves use of a photochemical grid model that is capable of simulating aerosol chemistry, transport, and deposition of airborne pollutants, including particulate matter and ozone. Regional scale air quality models are generally applied for geographic scales ranging from a multi state to the continental scale. Because of the design and intended applications of grid models, they may not be appropriate for BART assessments, so States should consult with the appropriate EPA Regional Office prior to carrying out any such modeling.

121 modeling options Options 1 and 2 discussed above44. In evaluating visibility improvement as one of the five factors to consider in setting BART controls, other models, used in combination with CALPUFF, may be helpful in providing a relative sense of the source's visibility impact and can aid in informing the BART decision. A discussion of the use of alternative models is given in the Guideline in Appendix W, section 3.2.

The Use of Natural Conditions in Determining Visibility Impacts for Reasonable Progress and Comparison to Threshold values Background. As set out in section 169A( a) of the CAA and stated in the 1999 Regional Haze Rule, a return to natural visibility conditions, or the visibility conditions that would be experienced in the absence of human caused impairment, is the ultimate goal of the regional haze program. To implement the CAA goal, the regional haze rule requires that a comparison with natural conditions for the 20 percent best and worst days to calculate " reasonable progress" determinations. Default values 45 Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule ( U. S. EPA. September 2003). http:// www. epa. gov/ ttncaaa1/ t1/ memoranda/ rh_ envcurhr_ gd. pdf Natural background conditions, expressed in deciviews, are defined for each Class I area. EPA has issued guidance for estimating natural background conditions which has estimates of default conditions as well as measures to develop refined estimates of natural conditions.

122 for natural visibility conditions are provided in EPA guidance. 45 In the 2004 reproposal of the BART guidelines, we proposed that changes in visibility, expressed in deciviews, should be determined by comparing the impact from a single source to natural visibility conditions. That impact should then be compared to a threshold impact, also expressed in deciviews, to assess if a BART eligible source should be subject to a BART review.

Comments. Opposing commenters said that a return to natural conditions is unattainable as it would require the elimination of every manmade source, and that changes should be compared against currently existing conditions. They added that true " natural conditions" cannot be verified, do not account for manmade emissions from other countries, and are not a realistic target for improvement. Further, they argued that natural conditions are a " goal" representing a benchmark that is relevant to the States'

determination, under the regional haze program, of the level of " reasonable progress" to achieve; however they stated that there is no legal requirement ( and there could not be a legal requirement) that the natural conditions goal ultimately must be achieved. Several commenters added that current visibility 123 conditions make more sense as a baseline because sources that are subject to BART today will likely not be in operation in the 2064 time frame. A commenter added that using current visibility conditions for the analysis will give a more realistic,

realworld prediction of whether controlling the source pursuant to BART will actually improve visibility. The commenter said that Congress did not intend for sources to have to consider retrofitting controls under the BART provision if those sources currently are not impacting real world visibility. Other utility groups stated that in addition to international emissions, the estimated natural visibility conditions failed to account for natural phenomena such as sea salt, wildfires, and natural organics. One commenter noted that natural visibility estimates will be revised and refined over time and it would be unwise to compare impacts and improvements to a moving baseline.

On the other hand, numerous commenters supported the use of natural visibility conditions as a baseline for measuring visibility improvements. Several environmental groups said that any increase in the baseline beyond natural visibility conditions will unlawfully distort and weaken the BART requirement by effectively raising the applicability threshold in less protected,

highly polluted areas, which would be illogical. Further, they pointed out that these BART eligible sources clearly are contributing to the very manmade visibility impairment that the Act is explicitly designed to remedy by a return to natural conditions. They added that measuring natural conditions as 46 See also our explanation of the CAA goal provided in the regional haze rule at 64 FR 126.

124 opposed to some other baseline condition is a more appropriate approach, given that the planning goal is to achieve natural visibility by the end of the program. They also added that a baseline other than natural conditions would never assure " reasonable progress".

Finally, two commenters asked for clarification on the values for natural conditions to be used for estimating changes in visibility. The commenters appeared to assume that we intended for the comparison to be done for natural visibility conditions on the 20% best days.

Final Rule. We disagree with commenters saying that the use of natural conditions as the baseline for making visibility impact determinations is inappropriate. The visibility goal of the CAA is both the remedying of existing impairment, and prevention of future impairment. The court, in American Corn Growers, upheld our interpretation of that goal as the return to natural visibility conditions. 46 Long term regional haze strategies are developed to make " reasonable progress" towards the CAA goal, and States must demonstrate reasonable progress in their regional haze State implementation plans ( SIPs). Since the BART program is one component of that demonstration, visibility changes due to BART are appropriately measured against the target of natural conditions. 125 In establishing the goal of natural conditions, Congress made BART applicable to sources which " may be reasonably anticipated to cause or contribute to any impairment of visibility at any Class I area". Congress used sweeping terms ( cause or contribute to any impairment at any Class I area) to describe the low triggering threshold for BART review. Using existing conditions as the baseline for single source visibility impact determinations would be fundamentally at odds with the low triggering threshold for BART review, because it would create the following untenable paradox: the dirtier the existing air, the less likely it would be that any control is required from any one source. This is true because of the nonlinear nature of visibility impairment. In other words, as a Class I area becomes more polluted, any individual source's contribution to changes in impairment becomes geometrically less. Therefore the more polluted the Class I area would become, the less control would seem to be needed from an individual source. We agree that this kind of calculation would essentially raise the " cause or contribute" applicability threshold to a level that would never allow enough emission control to significantly improve visibility. Such a reading would render the visibility provisions meaningless, as EPA and the States would be prevented from assuring " reasonable progress" and fulfilling the statutorily defined goals of the visibility program. Conversely, measuring improvement against clean 126 conditions would ensure reasonable progress toward those clean conditions.

We also believe that using existing visibility conditions rather than natural background conditions as the reference point for determining whether a BART eligible source is subject to BART would be inconsistent with the analytical framework suggested by the American Corn Growers decision. In that decision, the D. C.

Circuit held that EPA may not require states, in the course of determining whether a BART eligible source is subject to BART ( or in the course of evaluating visibility improvements as one of the five factors for determining what BART is) to also consider visibility improvement from the application of controls to all other BART eligible sources. In other words, the court held that visibility benefits of controlling emissions from the source in question should be considered in isolation, without regard to the potential benefits from reducing emission at other BART eligible units. If the benefits of controlling emissions from other sources are not to be considered in determining BART, neither should the detrimental affects of emissions from other sources be allowed to mask the potential benefits from controlling emissions at the source in question. Otherwise, a skewed framework is created, in which the State could never require BART at any source, because by artificially assuming that no other reductions would occur elsewhere, it would become impossible to show that controlling any one source would produce benefits. The most equitable way to escape this conundrum is to evaluate each 47 Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule ( U. S. EPA. September 2003). http:// www. epa. gov/ ttncaaa1/ t1/ memoranda/ rh_ envcurhr_ gd. pdf Natural background conditions, expressed in deciviews, are defined for each Class I area. EPA has issued guidance for estimating natural background conditions which has estimates of default conditions as well as measures to develop refined estimates of natural conditions.

127 source's impact without regard to other sources   in other words,

as it would be against natural background conditions.

With regard to BART eligible sources not being in operation for the duration of the program, a State, in making BART determinations, is explicitly directed by the CAA to account for the remaining useful life of a source. Thus, States may factor into their reasonable progress estimates those shut downs that are required and effected in permit or SIP provisions. In addition,

as provided for under our guidance47, proper accounting for international emissions and natural phenomena is in the 5 year SIP progress report, not in the setting of natural visibility estimates. Finally, these final BART guidelines use the natural visibility baseline for the 20% best visibility days for comparison to the " cause or contribute" applicability thresholds.

We believe this estimated baseline is likely to be reasonably conservative and consistent with the goal of natural conditions.

Modeling Protocol Background. The 2004 guidelines proposed that a written modeling protocol be submitted for assessing visibility impacts from sources at distances greater than 200 km from a Class I area. 128 The proposal indicated that the protocol should include a description of the methods and procedures to follow, for approval by the appropriate reviewing authority; critical items to include in the protocol are meteorological and terrain data,

sourcespecific information ( stack height, temperature, exit velocity,

elevation, and allowable emission rate of applicable pollutants),

and receptor data from appropriate Class I areas.

Comments. All of the comments supported the development of a written modeling protocol. Industry, Federal, and State commenters said a modeling protocol should be required of all States and stakeholders who are performing the BART modeling analysis. Commenters said the protocol should allow all interested parties an opportunity to understand the modeling approach and how the results will be used, and that the State should provide opportunity for comments on the procedures prior to the publication of the final results.

Many utility groups commented that the protocol should provide States with flexibility, and that the choice of models should be at the States' ( or RPOs') discretion. Some commenters stressed that it is important that states and sources retain the flexibility to decide how to set up and run the selected model,

while others asked for specific guidance on the setup of CALPUFF or other approved models, including on specific parameters ( e. g.

how to adjust for cases where sources are greater than 200 km from a Class I area). 129 Regarding the approval of a modeling protocol, some commenters said that the protocol should be approved by EPA.

Others stated, however, that we should have only an advisory role in development of the protocol. They said that States are in a better position to determine which modeling input values best reflect conditions in their States.

Several commenters representing environmental groups said we should develop a CALPUFF protocol that must be followed and should include, among other items, meteorological data ( i. e., where available 5 years of data should be used), emissions reported for the same meteorological years, documented source parameters, model physical parameters, and assumed background concentrations for ozone and ammonia ( based on nearby reliable observations and/ or regional modeling results). They added that a protocol developed by EPA would help to produce consistent BART determinations across various sources and geographic areas for both shorter and longer distances. FLMs stated that this is also an appropriate time to create regional modeling platforms for CALPUFF, which would allow States and sources to run the model more expeditiously and more consistently. They recommended that we consider a multi agency process to reach agreement on an appropriate modeling protocol prior to allowing BART applicability and control determinations to be based on model results. FLMs added that it would be helpful to establish a national procedure for this process, including a methodology for establishing natural background conditions,

background ammonia concentrations, and determining sulfuric acid 48Interagency Workgroup on Air Quality Modelig ( IWAQM) Phase 2 Summary Report and Recommendations for Modeling Long Range Transport Impacts, U. S. Environmental Protection Agency, EPA 454/ R 98 019, December 1998.

130 emission rates. Such a process, they said, could reasonably be engaged in prior to deadlines for state implementation plans, and would not delay implementation of the BART guidelines. The FLMs noted that consistent, nationally applicable guidance is essential, and that once it is developed, virtually no deviations should be allowed. Finally, they added that the CALPUFF modeling exercises should follow the Interagency Workgroup on Air Quality Modeling ( IWAQM) Phase 2 Summary Report and Recommendations for Modeling Long Range Transport Impacts48, but that we, in consultation with the FLMs and States, should also publish additional guidance to address more recent issues such as particle speciation, emission rate averaging times, and " natural obscuration." Another State commenter said that The Guideline on Air Quality Models ( CFR Part 51, Appendix W) should be included along with the IWAQM Report as a reference for CALPUFF setup.

One RPO commented that we should provide data, perhaps using example facilities, to demonstrate the effect of the process so that States can get a better feeling for which sources are likely to fall below the 0.5 deciview threshold. This would help States understand the net effect of all of the parameters chosen in the exemption process. 131 Commenters also said that we should continuously revise modeling protocols by providing a modeling clearinghouse to States, and further, that we should consider new models for use,

such as the Community Multiscale Air Quality ( CMAQ) model.

There were specific comments requesting guidance for calculating visibility impacts and other general modeling concerns. One technical comment was that the guidelines should specify that the IMPROVE monitor is the receptor by which modeled visibility impacts should be evaluated with the CALPUFF model.

Another commenter suggested using recent scientific evidence to update the light extinction coefficients used by CALPUFF to calculate visibility changes. These commenters also stated that CALPUFF might be improved by capping the relative humidity to lower values than are currently used.

Additional commenters representing utility organizations discussed how to identify Class I areas that should be modeled.

They said that the guidelines should require sources to model only the nearest Class I area ( or possibly the two closest), and one commenter said that we should provide a reasonable methodology to minimize the effort needed to address impacts from BART eligible sources on multiple Class I areas.

Final Rule. We agree that States should adopt a modeling protocols should be required for all modeling demonstrations,

regardless of the distance from the BART eligible source and the Class I area impacted. We are therefore dropping the 200 km and greater distance requirement from the guidelines. As noted in the 132 2004 re proposal, we believe that potential uncertainties in model performance may be greater at distances greater than 200 km for a source. However, a mechanism ( i. e. a modeling protocol) is needed in all modeling situations to ensure that the model and the BART exemption test are applied consistently and equitably across BART eligible sources nationwide. A modeling protocol for all cases will ensure agreement on the databases used, and the modeling techniques and the overall technical approach used prior to the actual analyses. Thus, misunderstandings concerning model results will be mitigated, and this may reduce the need for additional analyses. We favor coordination among States, EPA regions, RPOs, and other federal agencies to agree on a modeling protocol( s) which would provide consistent application of the model. Such coordination may relieve the administrative burden of examining and approving a protocol for each separate BART eligible source. A protocol developed by and acceptable to all stakeholders would expedite the modeling process and further ensure nationwide consistency in application of the model.

Coordinated efforts could help develop standardized data sets to provide the added benefit of reducing the cost and time needed for a modeling analysis and facilitating comparability between individual analyses. A centralized regional database, accessible on a web site, may provide quality assured upper air, surface,

precipitation, monitoring, land use, and terrain data already in model ready format. Some of the RPOs, for instance, have developed data sets for model inputs that can be used by States, 133 further reducing time and cost constraints while standardizing the model input requirements. The use of predefined receptor networks developed by the FLMs can also relieve resource constraints.

In developing a modeling protocol, we also encourage States to use the framework provided for model setup in EPA's IWAQM.

CALPUFF model users may find default settings in that document which may be appropriate for their modeling situations and add an element of consistency to model applications. The Guideline on Air Quality Models ( CFR Part 51, Appendix W) also provides useful guidance.

We do, however, understand and agree that a modeling protocol should provide flexibility to States for setting up and running the model, as the diversity of the nation's topography and climate, and variations in source configurations and operating characteristics, dictate against a strict modeling " cookbook". A State may need to address site specific circumstances at individual sources potentially affecting a specific Class I area.

For example, in a particular area a State may have available ammoniaemissions data, that is more representative of the modeling domain, which may supplement the model defaults. States should work with the appropriate EPA regional office and Federal Land Managers in adjusting the model input parameters. The modeling input recommendations in the IWAQM report are designed for visibility impact applications, and those defaults allow for tailoring for a given application ( e. g. puff splitting). The 49 http:// www. src. com/ calpuff/ calpuff1. htm.

50Analysis in Support of Changes to the Regional Haze Rule, U. S. Environmental Protection Agency, April 15, 2005, Docket No. OAR 2002 0076.

134 model developers website49 also has a series of frequently asked questions with answers to assist users in tailoring model applications.

We agree that we should have only an advisory role in development of the protocol as the States better understand the BART eligible source configurations and the geophysical and meteorological data affecting their particular Class I area( s).

We believe, however, that EPA should have final approval of State protocols so that the model, and the BART control decision which depends, in part, on the model results, is applied equitably across all regions. EPA approval will help ensure that vastly different modeling results are not produced for modeled BARTeligible sources that have similar emission configurations and operate under comparable geophysical and meteorological conditions.

In the protocol approval process, we support the idea of designing example runs, as we have done in our example analysis for EGUs50, so that States may get a better understanding of what visibility impacts might be expected from a particular type of source or sources. Once a protocol has been approved, a State may be able to use example runs as a proxy in making BART determinations which could potentially eliminate the need for 135 case by case review for every BART eligible source. A common sense approach should be taken, particularly where an analysis may add a significant resource burden to a State. For example, if there are multiple Class I areas in relatively close proximity to a BART eligible source, a State may model a full field of receptors at the closest Class I area. Then a few strategic receptors may be added at the other Class I areas ( perhaps at the closest point to the source, a receptor at the highest and lowest elevation in the Class I area, a receptor at the IMPROVE monitor,

and a few receptors that are expected to be at the approximate plume release height). If the highest modeled impacts are observed at the nearest Class I area, a State may choose not to analyze the other Class I areas any further and additional analyses might be unwarranted.

As models are revised and advances in science are incorporated into the models, we can make certain that revisions to protocols are made accordingly. We will work closely with States and FLMs, as should States; we expect that States will also work closely with FLMs throughout the protocol development and approval process. We expect a similar protocol development and approval process for other models that may be used, once those models are developed to predict and track single source impacts,

demonstrate acceptable model performance, and are approved by us for use. States should contact the appropriate FLM and EPA regional office for the latest guidance and modeling updates. 136 Alternatives for determining visibility impacts from individual sources Background. In the 2004 reproposal, we requested comment on the following alternatives to CALPUFF modeling for determining whether individual sources cause or contribute to visibility impairment: look up tables developed from screening level air quality modeling; running CALPUFF in a simpler screening mode than the preferred approach; a source ranking methodology; and an emissions divided by distance ( Q/ D) method. Except for the simplified CALPUFF approach, all alternatives were based on developing a relationship between source emissions and the source's distance to a Class I area. Each of these approaches was intended to reduce the resource burden on States.

Comments. Some commenters supported the use of alternative approaches, while others suggested that the alternatives could be used either in conjunction, or in hierarchical fashion, with modeling approaches. Many commenters were opposed to their use.

The opposing comments were consistent in stating that the alternatives were inappropriate because they did not account for important factors such as terrain, local meteorological data,

prevailing wind directions ( which influence pollutant transport),

and differences in stack release parameters. Commenters added that there is no direct connection between emissions, distance,

and visibility impairment, and that the methods treat SO2 and NOx equally for impairment estimates. 51 CALPUFF Analysis in Support of the Regional Haze Rule, U. S. Environmental Protection Agency, April 15, 2005, Docket No. OAR 2002 0076.

137 Final Rule. We disagree that the alternatives are necessarily inappropriate, but we share most of the concerns articulated by the opposing commenters. We believe that alternatives should not be used to exempt a source from BART review without more rigorous evaluations and sensitivity tests showing that the results are at least as conservative as the CALPUFF model. We know of at least one study showing that, for one location and for one year, there is no guarantee that the simplified CALPUFF technique is as conservative as the preferred approach51. While we are not adopting in the guideline any specific alternative to modeling for large power plants greater than 750MW, a State may develop its own alternative approach for the other source categories to determine if a source would be subject to BART, provided that the alternative demonstrates a sufficient basis to determine clearly that the source causes or contributes to visibility impairment, or that more refined analysis is warranted. Use of an alternative approach could be a conservative non modeling method for easing a State's resource burden. We believe conservatism is needed because of the purpose of the test: i. e. solely to determine if a closer look at the source is warranted.

E. The BART Determination Prcess 138 Background. CAA section 169A( g)( 7) directs States to consider five factors in making BART determinations. The regional haze rule codified these factors in 40 CFR 51.308( e)( 1)( ii)( B),

which directs States to identify the " best system of continuous emissions control technology" taking into account " the technology available, the costs of compliance, the energy and nonair quality environmental impacts of compliance, any pollution control equipment in use at the source, and the remaining useful life of the source." Section IV. of the BART Guidelines provides a stepby step guide to conducting a BART determination which takes these factors into account.

This section of the preamble addresses a number of issues relative to the process for conducting a BART determination contained in Section IV of the BART Guidelines.

1. What is meant by " technical feasibility of the control options" in step 2 of the BART determination?

Comments. We received several comments on this discussion,

both on the 2001 proposal and on the 2004 reproposal. One commenter recommended that the concept of available technology for regional haze should be expanded to include those in the pilot scale testing phase, because these guidelines will precede the installation of controls by about 10 years. Other commenters believed that the discussion of technical feasibility introduced terms and concepts that were not clear, for example, what is meant by " commercial demonstration." One commenter raised issues with 139 deeming technologies used in foreign countries " available" unless their performance has been demonstrated in the United States. A few commenters expressed concern with the provision in the guidelines that new technologies should be considered up to the time of a State's public comment period on the BART determination.

The commenter believed that this could create an endless review loop for States if new technologies continually became available.

Final rule. In the final guidelines, we have retained the language that was in the proposed guidelines. Because the guidelines call for consideration of technologies that become available by the time of the State's public comment process on the BART determination, technologies should be considered that become available well after we finalize the BART guidelines. We continue to believe that States need not consider technologies to be " commericially available" if they are only in the pilot test phase as of the date the State conducts its public comment period. We also note, for clarity, that the Guidelines state that technologies need to be both licensed and commercially available ( i. e. commercially demonstrated and sold).

2. What is meant by " ranking of all technically feasible control options" in step 2 of the BART determination?

Comments. The comments on this section of the guidelines, as proposed in 2001 and reproposed in 2004, generally were directed at issues related to sources emitting more than one pollutant.

While our proposed guidelines call for a separate ranking for each 140 pollutant, a number of comments suggested approaches for considering combinations of pollutants in developing a ranking of control measures. Some comments recommended that the guidelines consider control measures favorably if the total visibility impairing pollutant reduction is greater than for a technology reducing emissions of only one pollutant. Other comments suggested developing a weighting approach. In this approach, a method would be developed under which emissions of pollutants with greater contributions to light extinction would be given a greater weight than emissions of other pollutants.

Final rule. We agree with comments that there could be situations where the control technology selected as BART could involve judgments on controls for a combination of pollutants.

Nonetheless, we continue to believe that the most practical approach to taking this into account as a general matter is to begin with a separate ranking for each pollutant. We do not believe that it would be feasible to develop a weighted ranking system given the regional differences in contributions to extinction from various constituents. As noted above, however,

States may find that it is appropriate to place greater overall emphasis on controlling emissions of direct PM, SO2, and NOx than on controlling emissions of VOCs and ammonia.

32. How should the costs of control be estimated in step 4 of the BART determination? 141 Comments. This section of the guidelines remained unchanged between the 2001 proposal and the 2004 reproposal. Comments varied, ranging from questioning the reliance on EPA's Air Pollution Control Cost Manual ( hereafter called the " Control Cost Manual") to requesting that we not include the concept of incremental cost effectiveness in the guidelines. A commenter expressed concerns that incremental cost effectiveness calculations, the cost of implementing each succeeding control option, is too dependent on the number of interim options included in the analysis. Moreover, the commenter believed that incremental cost calculations increase the complexity of the analysis, and they also increase the possibility for inconsistent cost results.

Final rule. We have finalized this section of the guidelines with minor changes to how it was proposed. We believe that the Control Cost Manual provides a good reference tool for cost calculations and it serves to provide for consistency. If there are elements or sources that are not addressed by the Control Cost Manual or there are additional cost methods that could be used, we believe that these could serve as useful supplemental information to that available from using the methods in the Control Cost Manual.

In addition, the guidelines continue to include both average and incremental costs. We continue to believe that both average and incremental costs provide information useful for making 142 control determinations. However, we believe that these techniques should not be misused. For example, a source may be faced with a choice between two available control devices, control A and control B, where control B achieves slightly greater emission reductions. The average cost ( total annual cost/ total annual emission reductions) for each may be deemed to be reasonable.

However, the incremental cost ( total annual costA B / total annual emission reductionsA B) of the additional emission reductions to be achieved by control B may be very great. In such an instance, it may be inappropriate to choose control B, based on its high incremental costs, even though its average cost may be considered reasonable.

Finally, it is important to note that, while BART determinations are focused at individual sources, it is likely that in response to SIP requirements, States will be making BART determinations for many units in a subject source category all at the same time. In doing so, States are likely to compare costs across each source category as well as looking at costs for individual units in order to respond to SIP requirements in an efficient manner ( from the State's perspective). This also mirrors how we used cost information in setting the presumptive limits for EGUs described earlier.

43. How should " remaining useful life" be considered in step 4 of the BART determination?

Comments. We received a number of comments on the issue of remaining useful life, both on the 2001 proposal and on the 2004 143 reproposal. One commenter asserted that remaining useful life should not be considered in the cost analysis and that if a source is in operation at the time of a State's SIP submittal, it must have plans to install controls. Other commenters believed that,

to the extent that assertions regarding a plant's remaining useful life influences the BART decision, there must be an enforceable requirement for the plant to shut down by that date. Other comments questioned whether Congress intended enforceable restrictions in order to take into account the remaining useful life and whether EPA had the authority under the CAA to require plant shutdowns.

A number of comments were received regarding our request for comments on how to provide flexibility for situations where market conditions change. Some comments interpreted this provision as a loophole that would allow sources to continue operation for a number of years without BART. Another comment supported the concept of allowing a source to later change its mind, so long as BART is installed.

Final rule. We have retained the approach in the proposed guidelines, including the provision for flexibility for sources to continue operating, with BART in place, should conditions change.

We believe that the CAA mandates consideration of the remaining useful life as a separate factor, and that it is appropriate to consider in the analysis and that the effects of remaining useful life on costs may not be disregarded. We believe that, because the source would not be allowed to operate after the 5 year point 144 without such controls, the option for providing flexibility would not create a loophole for sources. Moreover, any source operating after this point without BART controls in place would be subject to enforcement actions for violating the BART limit. For any source that does not agree to shut down before the 5 year point,

the State should identify a specific BART emission limit that would apply after this point in time.

54. How should " visibility impacts" be considered in step 5 of the BART determination?

Background. The fifth statutory factor addresses the degree of improvement in visibility which may reasonably be anticipated to result from the use of the " best control technology" for sources subject to BART. The 2004 reproposal focuses on the use of single source emissions modeling to evaluate the BART control options. As part of the BART determination, we proposed that a State or individual source would run CALPUFF, or another EPAapproved model, to estimate, in deciviews, a BART source's visibility impact at a Class I area. The source would run the model once using its allowable emission rates, and then again at the various post control emissions rates being evaluated for the BART determination. The 24 hour model results would then be tabulated for the pre and post control scenarios, for the average of the 20 percent worst modeled days at each receptor, over the time period of meteorology modeled. The difference in the averages for each receptor is the expected degree of improvement in visibility. Alternatively, the proposal requested comment on 145 the option of using the hourly modeled impacts from CALPUFF at each receptor and determining the improvement in visibility based on the number of hours above the 0.5 deciview threshold for both the pre and post control model runs. We also requested comment on combinations of the proposed and alternative options and on the use of the simpler screening version of CALPUFF to do the analysis.

Comments. Several environmental groups said that issues relating to the determination of visibility improvement for evaluating BART controls are in many ways the same as for determining which BART eligible sources are subject to BART.

Thus, the commenter pointed out, the issues concerning the BART applicability test, discussed in section D., are all equally applicable here, including comments on: using the 0.5 deciview threshold on an aggregate basis for determining visibility impairment and potential exemption for BART eligible sources, use of a natural visibility baseline versus current visibility, using a substantially lower deciview threshold than 0.5 deciviews to determine the contribution to visibility impairment by an individual source, and demonstration of those thresholds by means of appropriate modeling rather than other less reliable and more subjective techniques.

An industry commenter claimed that the Corn Growers case emphasized the fact that the CAA clearly provides that BART determinations should balance the visibility benefits of controls 146 comprehensively against their burdens; the commenter noted that this is not mentioned in our proposal; the commenter said that although the proposal would allow States to run the CALPUFF model,

it fails to specify how they might consider the results.

One State commenter opposed the use of visibility modeling for the purpose of informing the choice of control option, stating that it is unnecessary, confusing and without adequate standards or guidance for implementation. The State added that the analysis of control options in the BART process should yield the greatest,

most cost effective control efficiency for NOx and SO2 at or above our presumptive levels of control. Moreover, it said that analysis of the degree of visibility improvement may result in very small increments of visibility improvements within Class I areas from an individual source, thus tilting the selection to the lower control efficiency option. The State added that we should remove this criterion from the analysis to ensure that the best cost effective controls will result. Another State agency said that modeling impacts should not be considered in BART determinations because they are not considered when determining BACT for the PSD program.

A variety of commenters pointed out several areas where the guidelines should be improved or clarified in regard to the degree of visibility improvement determination:

° We should clarify that the analysis is pollutant specific ( e. g., the modeling evaluation of a BART control option for 147 SO2 reduction should not be combined with the modeling evaluation of a BART control option for NOx.)

° We should clarify that only the closest Class I area must be modeled.

° We should describe CALPUFF as one possible model to use,

rather than as the only model that may be used.

° States and sources should have the flexibility to perform multiple modeling runs based on different levels of available control.

° Predicted visibility improvements that are imperceptible should be given no weight in determining the level of control that constitutes BART.

° States should be allowed to establish a factor for the required degree of visibility improvement.

Several industry and utility commenters expressed concern about using allowable emission rates to predict visibility impacts for BART control options; they argued that actual emission rates should be considered instead. Three commenters stated that we must make clear that States should use emission rates that will be permissible at the time BART controls take effect, not current emissions rates.

Additional comments from utilities, industry, and one State opposed the approach wherein the results from the 20 percent worst modeled days ( pre and post control) were used to evaluate the visibility improvements expected from the various control options.

Some believed this was too stringent, while others said it was not 148 stringent enough. Two utilities added that the criteria should use the 20 percent worst days based on monitored data, not modeled data. An environmental group stated that sources should not be limited to just the worst days, but the improvements should be based upon controls reducing visibility impairment on any day.

The commenter added that this rationale ignores the middle 60 percent of days in which visibility may worsen, because sources may increase emissions on these days as a trade off for cutting emissions on the worst days. The commenter further argued that there are no data to support our assertion that improvement on the worst days means improvement on other days. They noted that default " natural condition" deciview values for Class I areas in our natural conditions guidance exist only for the average of the 20 percent best and worst days. The commenter added that we used the average default natural conditions ( for the 20% best days) for the visibility impairment analysis, but there are no default " maximum 24 hour" values in the guidance.

Nine commenters supported implementation of visibility improvement thresholds, which were not proposed in 2004. A State commenter said it is unclear how the modeled net visibility improvement would be specifically utilized in the BART analysis,

and requested a target level of improvement or a de minimis level by which to measure improvement. Two industry commenters suggested alternatives to the 24 hour value. One said that setting a threshold for comparison, as in the BART applicability test, is more appropriate than the overall comparison of the 20 149 percent worst case days, and that the threshold for comparison should be on at least a daily average ( or longer), not an hourly average, due to the possibility of short term spikes based on certain meteorological conditions.

These commenters also said that a comparison of the number of days above or below a certain threshold is preferable since below a certain threshold, the impacts of visibility are not perceptible; unlike concentration levels of certain pollutants ( i. e., ozone) which do not have a threshold below which there are no effects, there are concentration levels of particulate below which there is no visibility impact. They also asserted that comparing the number of days would allow for a more complete picture of how controls would potentially improve visibility. As noted previously, a small number of unusual meteorological conditions can produce significant spikes on a single day or days.

Since the overall goal of the regional haze rule is long term visibility improvement, they said that a comparison of the total number of days exceeding a threshold over multiple years will provide a better overall indicator of visibility improvement. One commenter suggested that if we retain the maximum 24 hour value for the visibility impairment analysis, we should at least allow the use of only 1 year, rather than 5 years, of meteorological data. That would simplify the modeling and would lessen the chance that one day with atypical, extreme conditions would dictate the result. 52 Pitchford, M. and Malm, W., " Development and Applications of a Standard Visual Index," Atmospheric Environment, V. 28, no. 5, March 1994.

53 Henry, R. C. " Just Noticeable Differences in Atmospheric Haze", Journal of the Air & Waste Management Association, 52: 1238 1243, October 2002.

150 One FLM supported our proposed method to determine visibility improvement associated with installation of BART.

However, with regard to the use of hourly data instead of 24 hour data for the degree of visibility improvement assessment, another FLM said that while hourly model data are, by their nature, less reliable in predicting actual conditions, a measure that reports the total number of hours above a given threshold would still be a useful measure of the long term effect of BART control. They said we should require States to report a combination of measures of the visibility improvement expected from BART. Such measures would be the change in the 20 percent worst days as well as a metric that examines the amount of time during a year that the source's visibility impact would exceed a threshold with and without BART.

Another utility commenter added that, if a BART control option would result in no perceptible improvement in visibility at a Class I area, then it is not a cost effective option. This commenter said that based on Pitchford and Malm ( 1994) 52 and Henry ( 2002) 53 a 2 deciview threshold of perception would be appropriate,

with a 1 deciview threshold providing a margin of safety. Another commenter said that we should clarify that visibility improvement 151 differences among BART control options should be considered insignificant if the differences are less than the perceptibility threshold level, which should be set in excess of 1 deciview.

Other commenters said the minimum threshold should be 1 deciview.

Final Rule. We disagree with the comment that modeling should not be part of a BART review because it is not considered for BACT. CAA section 169A( g)( 2) clearly requires an evaluation of the expected degree of improvement in visibility from BART controls. All five statutory factors, including costeffectiveness and expected visibility improvement, should be reflected in the level of BART control that the State implements.

We believe that modeling, which provides model concentration estimates that are readily converted to deciviews, is the most efficient way to determine expected visibility improvement.

For the purposes of determining visibility improvement,

States may evaluate visibility changes on a pollutant specific basis. If expected improvement is shown from the various control choices, the State can weigh the results with the other four BART determination factors when establishing BART for a particular source. For example, a State can use the CALPUFF model to predict visibility impacts from an EGU in examining the option to control NOx and SO2 with SCR technology and a scrubber, respectively. A comparison of visibility impacts might then be made with a modeling scenario whereby NOx is controlled by combustion controls. If expected visibility improvements are significantly different under one control scenario than under another, then a 152 State may use that information, along with information on the other BART factors, to inform its BART determination.

Even though the visibility improvement from an individual source may not be perceptible ( i. e. the expected improvement may be less than or equal to 0.5 deciviews), it should still be considered in setting BART because the contribution to haze may be significant relative to other source contributions in the Class I area. Thus, we disagree that the degree of improvement should be contingent upon perceptibility. Failing to consider less

thanperceptible contributions to visibility impairment would ignore the CAA's intent to have BART requirements apply to sources that contribute to, as well as cause, such impairment.

We disagree that actual emission rates should be used to predict visibility impacts. A source may be operating at its full permitted level at any time, and visibility impairment is appropriately measured against that allowable emission rate ( hourly), as visitors may experience visibility in Class I areas at any time. Although we are not requiring States to use allowable emission rates to predict the anticipated future visibility impacts of BART controls, we disagree that daily average actual emission rates should be used to make this assessment. Emissions from a source can vary widely on a day to day basis; during peak operating days, the 24 actual emission rate could be more than double the daily average. On the other hand,

in the long term, estimating visibility impacts based on allowable emission rates for every hour of the year may unduly inflate the 153 maximum 24 hour modeled impairment estimate from a BART eligible source. Where States have information on a source's daily emissions, an emission rate based on the maximum actual emissions over a 24 hour period for the most recent five years may be a more appropriate gauge of a source's potential impact as it would ensure that peak emission conditions are reflected, but would likely not overestimate a source's potential impact on any given day. We have accordingly included this change to the final guidelines. We recommend that the State use the highest worst 24

hour average actual emission rate, for the most recent three or five year period of meteorological data, to characterize the maximum potential benefit.

We do not believe that using the maximum 24 hour average over the meteorological period modeled ( three or five years) is too restrictive in determining if a BART eligible source is subject to a BART review, or for evaluating the degree of visibility improvement expected from BART reductions. A scenic vista is observed instantaneously, which may suggest an even shorter averaging time for estimating impairment; models, however, must account for chemical transformations and long range transport which occur over time. A 24 hour averaging time may be the most practical way to simulate chemical and physical processes, but we do agree that averaging times of even 1 hour have merit. [ insert information regarding modeling meteorology] In response to the comment about reliance on monitoring rather than modeling receptors, a monitoring site may not coincide with the location of 154 the maximum modeled impact in a Class I area. Consequently, we believe that sole reliance on monitored data to assess maximum visibility impacts is not appropriate. In addition, monitoring data may not be useful for predicting future visibility impacts.

We agree with commenters that the difference between the preand post control scenarios on the average of the 20 percent worst modeled days may not be the best approach in determining visibility improvement. The importance of an average as a data point can depend on the variation in the data as a whole. In other words, there may be a few days of very high impairment in a Class I area, but many days of lower impairment. The average would tend towards the lower values, but that is not meaningful to an observer that may be experiencing impairment on one of the worst days. Also, average visibility is considered uniformly across the Class I area, and in this context, it is an average due to the pollutant contributions of all sources around the Class I area ( not the average due just to one source, and the average of the 20% worst modeled days would be of minimal value).

Because each Class I area is unique, we believe States should have flexibility to assess visibility improvements due to BART controls by one or more methods, or by a combination of methods, and we agree with the commenters suggestions to do so. Although the analysis using the 20 percent average worst modeled days ( pre and post control) may not capture the data maxima, its use may be reasonable in evaluating the fifth factor as part of its BART determination. We believe the maximum 24 hour modeled impact can 155 be an appropriate measure in determining the degree of visibility improvement expected from BART reductions ( or for BART applicability). We have pointed out, however, that States should have flexibility when evaluating the fifth statutory factor is not necessarily based on a one time test for a maximum 24 hour impact.

A State is encouraged to account for the magnitude, frequency, and duration of the contributions to visibility impairment caused by the source based on the natural variability of meteorology. These are important elements to consider as they would provide useful information on both the short term peak impact and long term average assessments which are critical in making the visibility assessment. components of impairment when making the visibility improvement determination; both " average" and " peak" tests are crucial in making the assessment.

We agree with the suggestion that the use of a comparison threshold, as is done for determining if BART eligible sources should be subject to a BART determination, is an appropriate way to evaluate visibility improvement. However, we believe the States should have flexibility in setting absolute thresholds,

target levels of improvement, or de minimis levels since the deciview improvement must be weighed among the five factors, and States are free to determine the weight and significance to be assigned to each factor. For example, a 0.3 deciview improvement may merit a stronger weighting in one case versus another, so one " bright line" may not be appropriate. 54CALPUFF Analysis in Support of the Regional Haze Rule, U. S. Environmental Protection Agency, April 15, 2005, Docket No. OAR 2002 0076.

55 Ibid.

156 In addition, comparison thresholds can be used in a number of ways in evaluating visibility improvement ( e. g. the number of days or hours that the threshold was exceeded, a single threshold for determining whether a change in impacts is significant, a threshold representing an x percent change in improvement, etc.).

In our example modeling analysis of a hypothetical source54, we used three different 24 hour thresholds ( 1.0, 0.5, and 0.1 deciviews) and examined the number of days that those thresholds were exceeded for a source with a 90 percent change, for example,

in SO2 emissions ( i. e. 10,000 TPY and 1,000 TPY). The number of days that the thresholds were exceeded in the 10,000 TPY case was substantial, and the visibility improvement due to the reduction in emissions was dramatic ( i. e. the number of days exceeding the thresholds dropped considerably) 55.

Other ways that visibility improvement may be assessed to inform the control decisions would be to examine distributions of the hourly or daily impacts, compare the hourly values to thresholds, determine if the time of year is important ( e. g. high impacts are occurring during tourist season), consideration of the cost effectiveness of visibility improvements ( i. e. the cost per change in deciview), using the measures of deciview improvement identified by the State, or simply compare the worst case days for 157 the pre and post control runs. States may develop other methods as well.

65. In what sequence should alternatives be assessed in step 5 of the BART determination?

Background. Both the 2001 proposal and the 2004 reproposal requested comments on two options for evaluating the ranked options. Under the first option, which was the preferred approach, States would use a sequential process for conducting the impacts analysis, beginning with a complete evaluation of the most stringent control option. If a State determines that the most stringent alternative in the ranking does not impose unreasonable costs of compliance, taking into account both average and incremental costs, the analysis begins with a presumption that this level is selected. Under this option, States would then proceed to consider whether energy and non air quality environmental impacts would justify selection of an alternative control option. If there are no outstanding issues regarding energy and non air quality environmental impacts, the analysis is ended and the most stringent alternative is identified as the " best system of continuous emission reduction." If a State determines that the most stringent alternative is unacceptable due to such impacts, this approach would require them to document the rationale for this finding for the public record. Then, the next most effective alternative in the listing becomes the new control candidate and is similarly evaluated. This process would continue until the State identifies a technology which does not pose 158 unacceptable costs of compliance, energy and/ or non air quality environmental impacts.

We also requested comment on an alternative decision making approach that would not begin with an evaluation of the most stringent control option. For example, States could choose to begin the BART determination process by evaluating the least stringent technically feasible control option or by evaluating an intermediate control option drawn from the range of technically feasible control alternatives. Under this approach, States would then consider the additional emissions reductions, costs, and other effects ( if any) of successively more stringent control options. Under such an approach, States would still be required to ( 1) display and rank all of the options in order of control effectiveness and to identify the average and incremental costs of each option; ( 2) consider the energy and non air quality environmental impacts of each option; and ( 3) provide a justification for adopting the technology selected as the " best"

level of control, including an explanation as to the basis for rejecting the other more stringent control technologies identified in the BART determination were rejected.

In selecting a " best" alternative, the proposed guidelines included a discussion on whether the affordability of controls should be considered. We noted in the guidelines that, as a general matter, for plants that are essentially uncontrolled at present and emit at much greater levels per unit of production than other plants in the category, EPA is unlikely to accept as 159 BART any analysis that preserves a source's uncontrolled status.

The proposed guidelines noted, however, that we recognize there may be unusual circumstances that justify taking into consideration the conditions of the plant and the economic effects of requiring the use of a given control technology. These effects would include effects on product prices, the market share, and profitability of the source. We did not intend, for example, that the most stringent alternative must always be selected if that level would cause a plant to shut down, while a slightly lesser degree of control would not have this effect.

Comments. We received comments supporting both of the approaches for evaluating ranked control alternatives. Many commenters, including commenters from State agencies, were supportive of the first approach. Comments from State air quality agencies were strongly supportive of this approach. These commenters believed that this approach is consistent with past approaches by States for considering control options for case

bycase determinations, is well understood by all parties, and thus easier to implement. The first approach also was strongly supported in comments from environmental organizations and private citizens. Some comments noted that the plain terminology " best"

suggests that there must be a sound reason for not using the most stringent control level.

Many comments from industrial trade organizations were critical of the first approach and believed that any requirement to use this approach would reduce State discretion because this 160 approach, in the judgment of the commenters, would amount to use of the most stringent alternative as a default. Some of these comments asserted that the approach in option 1 would shift the BART analysis away from a cost benefit approach mandated by the CAA towards a BACT like technology analysis. Other commenters believed that EPA should recognize that BART, as a control requirement for retrofitting existing sources, should differ from BACT or other controls for new equipment. A number of comments,

in supporting the second approach, believed that this approach provides greater consideration of the incremental cost of each succeeding option.

Final rule. In the final guidelines, we have noted decided that States should retain the discretion to evaluate control options in whatever order they choose, so long as the State fully documents its analysis of the CAA factors. If it chooses a control alternative other than the " best" emissions control, it should document its justification, based on the factors, for not choosing each available more stringent option. We do not agree with commenters who asserted that the method for assessing BART controls for existing sources should consider all of the statutory factors. must differ from the method of assessing controls for new sources; so long as the factors are analyzed properly, there is no inherent need for a different process.

7. What should be the presumptive limits for SO2 and NOx for utility boilers? 161 Background. In the 2004 reproposal, we proposed that States,

as a general matter, should require EGUs greater than 250 MW in size at power plants larger than 750 MW to control 95 percent of their SO2 emissions, or control to within an SO2 emission range of 0.1 to 0.15 lb/ mmBtu. We also proposed to establish a rebuttable presumption that States should impose these BART SO2 limits on all EGUs greater than 250 MW, regardless of the size of the power plant at which they are located.

For NOx, we proposed that sources currently using controls such as SCRs to reduce NOx emissions during part of the year should be required to operate those controls year round. For power plants without post combustion controls, we proposed to establish a presumptive emissions limit of 0.20 lbs/ mmbtu for EGUs greater than 250 MW in size. We requested comment on the rate of NOx emissions that can be achieved with combustion modifications on specific types of boilers. Many commenters responded both in favor and in opposition to these proposed BART presumptive limits.

Comments. A number of utility groups said the presumptive SO2 emissions control approach inappropriately ignores the need for a visibility impact evaluation which is required in step 5 of the proposed case by case BART engineering analysis. They said that setting presumptive limits infringes on a state's authority to establish BART on a case by case basis considering not only visibility improvement, but the other statutory factors as well.

The commenters said that visibility is both Class I area and 162 source specific, which is the reason Congress gave the States the lead role and discretion in the BART program to determine which sources need to install or upgrade controls. Through the use of presumptions and default values, however, our prescriptive process, as proposed, would make the installation of maximum controls more likely without regard to visibility benefits.

Instead, they argued, we should give the states maximum flexibility to use the five statutory factors in their BART determinations. Commenters said sources must be allowed to assess the visibility improvements of a variety of control options.

Several utilities raised concern that sources with existing controls should not be required to meet the presumptive limits without the chance to evaluate the degree of visibility improvement expected from the additional emission reduction requirements. They said that if a source can demonstrate a reduction in visibility impairment below the specified threshold ( whether that threshold is our currently proposed 0.5 deciview or an alternative level) with less stringent controls, then neither we nor States should impose, by default, more stringent reduction requirements.

Commenters from industry, utilities, and States said that we had not indicated what previously controlled sources must do to comply with BART, while we had determined what controls are necessary for uncontrolled sources. They were concerned that the guidelines would lead States to require previously controlled sources to remove the controls and replace them with even newer 163 controls at great cost and very little, if any, improvement in emission levels and visibility in Class I areas. Commenters added that States should be able to use their discretion to determine whether additional controls are needed.

Some commenters were concerned that the proposed rule would require some plants to install SCR to meet the NOx control level proposed, as the potential retrofit of SCR technology for the BART determination may be supported by the degree of visibility improvement expected. They said that the guidelines indicate that if a State finds that a source's visibility contribution warrants the installation of SCR, then SCR may be imposed. The commenter added, however, that the guidelines also need to provide for instances where the visibility condition warrants a lesser control level than what would be achieved by advanced combustion control;

the commenter claimed there was reference to this concept in the preamble but not the guidelines.

Final rule. In these guidelines, we are finalizing specific presumptive limits for SO2 and NOx for certain EGUs based on fuel type, unit size, cost effectiveness, and the presence or absence of pre existing controls. The presumptive limits finalized in today's rulemaking reflect highly cost effective technologies as well as provide enough flexibility for States to take particular circumstances into account.

The presumptive limits apply to EGUs at power plants with a total generating capacity in excess of 750 MW. As explained in greater detail below, for these sources we are establishing a BART 164 presumptive emission limit for coal fired EGUs greater than 200 MW in size without existing SO2 control. These EGUs should achieve either 95 percent or greater SO2 removal, or an emission rate of 0.15 lb SO2/ mmBtu or lower, unless a State determines that an alternative control level is justified based on a careful consideratino of the statutory factors. For NOx, we are establishing a set of BART presumptive emission limits for coalfired EGUs greater than 200 MW in size based upon boiler size and coal type, and based upon whether selective catalytic reduction ( SCR) or selective noncatalytic reduction ( SNCR) are already employed at the source. See section d. below for a table listing those specific limits. Based on our analysis of emissions from power plants, we believe that applying these highly cost effective controls at the large power plants covered by the guidelines would result in significant improvements in visibility and help to ensure reasonable progress toward the national visibility goal.

States, as a general matter, must require owners and operators of greater than 750 MW power plants to meet these BART emission limits. We are establishing these requirements based on the consideration of certain factors discussed below. Although we believe that these requirements are extremely likely to be appropriate for all greater than 750 MW power plants subject to BART, a State may establish different requirements if the State can demonstrate that an alternative determination is justified based on a consideration of the five statutory factors. evidence before it. Our intent is that a State will face a high hurdle if 165 it chooses to justify a BART determination less than these requirements for a power plant greater than 750 MW in size.

In addition, while States are not required to follow these guidelines for EGUs located at power plants with a generating capacity of less than 750 MW , based on our analysis detailed below, we believe that States will find these same presumptive controls to be highly cost effective, and to result in a significant degree of visibility improvement, for most EGUs greater than 200 MW, regardless of the size of the plant at which they are located. A State is free to reach a different conclusion if the State believes that an alternative determination is justified based on a consideration of the five statutory factors.

evidence before it. Nevertheless, our analysis indicates that these controls are likely to be among the most cost effective controls available for any source subject to BART, and that they are likely to result in a significant degree of visibility improvement.

The rest of this section discusses these presumptive limits for SO2 and NOx for EGUs and the additional visibility impact and cost effectiveness analyses we have performed since proposal of the guidelines in 2004.

g. Visibility Analysis for SO2 and NOx emissions from EGUs 56Summary of Technical Analyses for the Proposed Rule, Mark Evangelista, U. S. Environmental Protection Agency, April 12, 2004, Docket No. OAR 2002 0076.

57 CALPUFF Analysis in Support of the Regional Haze Rule, U. S. Environmental Protection Agency, April 15, 2005, Docket No. OAR 2002 0076.

58 Reference Mark's new TSD 166 In the 2004 reproposal, our preliminary CALPUFF modeling56 suggested that controlling a single 250 MW EGU at a 90 percent level would improve visibility substantially from that source.

Based on the expected degree of improvement in visibility and the use of highly effective control technologies that are available for sources of this capacity and greater, we concluded that the specific control levels in the proposal were appropriate. Even at that level of control however, our analysis indicated that emissions from the source might still cause a perceptible impact on visibility.

Following comments that we had ignored the need to consider the degree of improvement in visibility which could reasonably be anticipated from the use of the presumptive control technologies,

we undertook a more comprehensive modeling analysis of the anticipated visibility impacts of controlling large EGUs. Based on this modeling analysis, we anticipate that a majority of the currently uncontrolled EGUs at power plants covered by the guideline are predicted to have 24 hour maximum impacts of greater than a change of 2 or 3 deciviews57. Our modeling examples58 included scenarios that were was representative of a typical EGUs, 167 but, in our first hypothetical run # 1, we conservatively assumed SO2 emissions of 10,000 tons per year ( TPY) and NOx emissions of approximately 3,500 TPY. Such levels of emissions are well below those that may be expected of an uncontrolled 200 MW EGU. Not only were the maxima considerable, but tThe number of days during any year that such sources are predicted to have visibility impacts of greater than 0.5 deciviews or even 1.0 deciview were 29 days and 12 days on average, respectively, at 50km from a hypothetical Class I area in the East; if the 98th percentile were considered, there would be five days above a 1.0 deciview change.

substantial. Furthermore, when this same source was modeled with SO2 emissions of 1,000 TPY ( which is a reduction of 90 percent

see run # 5), the source was still a significant contributor to visibility impairment, even though maximum visibility was very much improved.

The modeled emission rates in the example were conservative;

for much larger EGUs with capacities of 750 MW or more, and emission rates much higher than those which were modeled,

visibility degradation is expected to be far worse. Clearly there is a substantial degree of visibility improvement which is likely from emission reductions at these sources.

Although we are confident that the EGUs for which we are establishing presumptive limits each have a significant impact on visibility at one or more Class I areas, a State retains the option and flexibility to conduct its own analysis or allow a source to demonstrate, after submitting an approved modeling 59 Technical Support Document for BART SO2 Limits for Electric Generating Units, Memorandum to Docket OAR 2002 0076, April 1, 2005.

168 protocol, that it should not be subject to BART ( based on its visibility effectsimpacts). Based on our modeling results, we believe that such a showing would be extremely difficult for these large EGUs.

b. BART Presumptive Limits for SO2 from Coal fired Units For currently uncontrolled coal fired EGUs greater than 200 MW in size located at power plants greater than 750MW, we are establishing a presumptive BART limits of 95 percent or greater SO2 removal, or an emission rate of 0.15 lb SO2/ mmBtu or lower. We are not establishing a presumptive limit for EGUs with existing post combustion SO2 controls or for EGUs that burn oil.

In 2004, we proposed presumptive limits for SO2 of 95 percent control or a comparable performance level of 0.1 to 0.15 lbs per million BTU as controls that would be achievable and costeffective We requested comment on the removal effectiveness of flue gas desulfurization (" FGD" or " scrubber" controls) for various coal types and sulfur content combinations. Having considered the comments received, we have determined that there is ample data to support the determination that the BART presumptive limits outlined in today's action are readily achievable by new wet or semi dry FGD systems across a wide range of coal types and sulfur contents based on proven scrubber technologies currently operational in the electric industry. 59 60 Ibid.

169 We agree with the commenters who stated that our dual recommendation provided equity across sources burning coals of varying sulfur content. We believe the presumptive limits provide enough flexibility that absent uniqueextreme circumstances, any BART eligible coal fired EGU will be able to achieve one of the limits with a new FGD system. We expect that BART eligible EGUs burning medium to high sulfur coal will be able to achieve a removal efficiency of 95% in a cost effective manner by utilizing various wet FGD technologies, and that those EGUs burning lower sulfur coals could meet the emission limit of 0.15lb/ mmBtu in a cost effective manner by utilizing dry FGD technologies. As described below, EPA's unit specific economic modeling showed that the majority of BART eligible units greater than 200 MW can meet the presumptive BART limit at a cost of $ 400 to $ 2000 per ton of SO2 removed.

Some commenters expressed concerns that the proposed limits were too stringent in particular for: ( 1) EGUs less than 750 MW in size, ( 2) EGUs burning low sulfur coals, and ( 3) EGUs burning lignite coals. However, numerous examples exist of smaller EGUs and EGUs burning low sulfur or lignite coals achieving these SO2 limits at reasonable cost. 60 We recognize that semi dry FGD systems are most commonly utilized on units burning lower sulfur coals and are not typically designed for removal efficiencies of 95 percent or greater. However, we believe that most of these 61 PQA Analysis 170 EGUs can readily achieve the presumptive emission rate limit of 0.15 lb SO2/ mmBtu. An analysis of EPA's RACT/ BACT/ LEAR Clearinghouse Dry FGD cost effectiveness data ranged from $ 393 to $ 2132 per ton SO2 removed, with an average cost effectiveness of $ 792 per ton61.

We received a few comments expressing the belief that the presumptive limits should be more stringent, given that BART emission limits will not be fully implemented until 2013 or 2014.

We recognize that while some scrubber units currently achieve reductions greater than 95%, not all units can do so. The individual units that currently achieve greater than 95% control efficiencies do not necessarily represent the wide range of unit types across the universe of BART eligible sources. An analysis of the Department of Energy's U. S. FGD Installation Database supports our belief that 95 percent removal efficiencies would be obtainable by all types of EGUs burning medium and high sulfur coal by 2014, including BART eligible EGUs. In addition, we note that the presumption does not limit the States' ability to consider whether more ( or less) stringenta different level of controls are is appropriate in a particular case. If, upon closer examination of an individual EGU, a State determines that a different emission limit is appropriate based upon its analysis of 62 Ibid.

63 Ibid.

171 the five factors, then the State may apply a more or less stringent limit.

Our analysis of presumptive BART limits accounted for variations in existing SO2 controls. We accordingly considered ( 1) coal fired EGUs without existing SO2 controls, and ( 2)

coalfired EGUs with existing SO2 controls. This analysis consisted of the following key elements: ( 1) identification of all potentially BART eligible EGUs, and ( 2) technical analyses and industry research to determine applicable and appropriate SO2 control options, ( 3) economic analysis to determine cost effectiveness for each potentially BART eligible EGU, and ( 4) evaluation of historical emissions and forecast emission reductions for each potentially BART eligible EGU. 62 We identified 491 potentially BART eligible coal fired units based on the following criteria: ( 1) the unit was put in place between August 7, 1962 and August 7, 1977, and ( 2) the unit had the potential to emit more than 250 tons annually of SO2. Our assessment of potential controls included various industry case studies, technical papers, public comments, BACT analyses, and historical Acid Rain emissions data. Our analysis is described in detail in the TSD. 63 172 We calculated cost effectiveness and projected SO2 emission reductions on a per unit basis based on removal efficiencies of 90 percent for dry FGD systems, in particular spray dry lime systems,

and 95 percent for wet FGD systems, in particular limestone forced oxidation systems. Based on our analysis, the average cost effectiveness for controlling all BART eligible EGUs greater than 200 MW without existing SO2 controls was estimated to $ 919 per ton of SO2 removed. Moreover, the range of costs effectiveness numbers demonstrates that the majority of these units can meet the presumptive limits at a cost of $ 400 to $ 2000 per ton of SO2 removed.

Unit Capacity ( MW) Tons ( K) of SO2 Emitted in 2001 % of BART Eligible Coal fired Unit's 2001 Emissions Calculated Average Cost Effectiveness for MW Grouping ($/ ton SO2 removed) % of Estimated Removable BART SO2 Emissions From coal fired Units*

< 50 MW 26 0.4% 1962 0.9%

50  100 MW 93 1.4 2399 1.6%

100  150 MW 171 2.5% 1796 2.2%

150  200 MW 235 3.5% 1324 3.4%

200  250 MW 253 3.8% 1282 3.1%

250  300 MW 281 3.2% 1128 4.0%

> 300 MW 5712 85.2% 84.8%

All Units 6707 100% 984 100%

BART Units (> 200MW) 6246 92.2% 919 91.9%

Figure 1. 173 In establishing presumptive BART limits, we were cognizant of the fact that upgrading an existing scrubber system is typically considered more cost effective than constructing a new scrubber system. However, due to the diverse and complex nature of upgrading existing FGD systems ( scrubber type, reagents, online year, absorber characteristics, current operating procedures,

etc.), there is no single solution or standard appropriate for all EGUs. As a result, we are not including specific numerical presumptive limits for EGUs with pre existing scrubbers. However,

for scrubbers currently achieving removal efficiencies of at least 50 percent, we strongly encourage recommend States to evaluate a range of scrubber upgrade options available for improving the SO2 removal performance of existing units. There are numerous scrubber enhancements available to upgrade the average removal efficiencies of all types of existing scrubber systems, and the guidelines contains a discussion of the options that States should evaluate in making BART determinations for EGUs with existing scrubbers.

The guidelines do not require EGUs with existing FGD systems to remove these controls and replace them with new controls, but the guidelines do state that coal fired EGUs with existing SO2 controls achieving removal efficiencies of less than 50 percent should consider constructing a new FGD system to meet the presumptive limits of 95 percent removal or 0.15 lb/ mmBtu in addition to evaluating the suit of upgrade options. For these 174 EGUs, the suite of available " upgrades" may not be sufficient to remove significant SO2 emissions in a cost effective manner, and States may determine that these EGUs should be retrofitted with new FGD systems.

c. BART Limits for SO2 from Oil Fired Units We are not establishing a presumptive BART limit for SO2 from oil fired EGUs. The guidelines state that the most appropriate control option for oil fired EGUs, regardless of capacity, is to set limits on the sulfur content of the fuel oil burned in the unit.

Commenters suggested EPA evaluate two primary control options for BART oil burning units: ( 1) sulfur content fuel oil limitations, and ( 2) flue gas desulfurization systems. We have been unable to find any FGD application in the U. S. electric industry on an oil fired unit. As a result, our analysis for oilfired units focused on benchmarking previously imposed fuel oil restrictions on the electric industry and ( 2) a regional economic analysis of switching from high sulfur to low sulfur fuel oil.

Our study of currently imposed fuel oil restrictions on the electric industry suggested that all BART eligible EGUs currently have some sort of imposed sulfur content or emission rate limitation. Of the 74 BART eligible oil burning EGUs, 32 currently have sulfur fuel oil restrictions of less than 1%, and 67 have some sort of sulfur content limitation. In addition, our economic analysis suggests that switching to low sulfur fuel oil 175 is a cost effective method in reducing SO2 emission from oil fired units.

As approximately 43 percent of the BART eligible oil units currently have a sulfur content limitation that is either equivalent to, or more stringent than, one percent sulfur by weight, the guidelines require States to consider a 1 percent or lower sulfur by weight fuel oil restriction on all BART eligible EGUs as part of their BART analysis, and recommends that States establish appropriate and sustainable sulfur content fuel oil restrictions, taking into account fuel oil availability. States should accordingly evaluate a one percent sulfur content limitation as a starting point of their BART determination for oil fired EGUs subject to BART.

d. BART Presumptive Limits for NOx from Coal fired Units In the 2004 reproposal, in discussing NOx controls on EGUs,

we explained that there are two somewhat distinct approaches to reducing emissions of NOx at existing sources. One is to use combustion controls ( including careful control of combustion air and low NOx burners). The other approach is removal technology applied to the flue gas stream ( such as SCRs and SNCRs).

For EGUs currently using controls such as SCRs or SNCRs to reduce NOx during part of the year, we are establishing a presumption that use of these same controls year round is BART.

( No commenters took issue with the idea of running existing SCRs or SNCRs year round. Some commenters supported year round 64 See Technical Support Document for BART NOx Limits for Electric Generating Units and Technical Support Document for BART NOx Limits for Electric Generating Units Excel Spreadsheet, Memorandum to Docket OAR 2002 0076, April 15, 2005.

176 operation of these controls.) Although only a few BART eligible sources currently have SNCRs installed, we note that States may wish to consider SCR as an alternative to annual operation of SNCR in light of the relatively high operating costs associated with SNCR.

For sources without post combustion controls ( i. e., SCRs and SNCRs), we are establishing a presumption as to the appropriate BART limits for coal fired units based on boiler design and coal type. These presumptions apply to EGUs greater than 200 MW at power plants with a generating capacity greater than 750 MW and are based on control strategies that are generally cost effective for all such units.

In 2004 we noted that, unlike the methods for controlling SO2 ( which fall within a fairly narrow range of cost effectiveness and control efficiencies), the removal efficiencies and costs associated with the control techniques for NOx vary considerably,

depending on the design of the boiler and the type of coal used.

In response to comments on the proposal, we have performed additional analyses of all individual BART eligible coal fired units64 and our analyses indicated that both cost effectiveness and post control rates for NOx do depend largely on boiler design and 177 type of coal burned. Based on these analyses, we believe that States should carefully consider the specific NOx rate limits for different categories of coal fired utility units, differentiated by boiler design and type of coal burned, set forth below as likely BART limits.

In today's action, EPA is setting presumptive NOx limits for EGUs larger than 750 MW. EPA's analysis indicates that the large majority of the units can meet these presumptive limits at relatively low costs. Because of differences in individual boilers, however, there may be situations where the use of such controls would not be technically feasible and/ or cost effective.

For example, certain boilers may lack adequate space between the burners and before the furnace exit to allow for the installation of over fire air controls. Our presumption accordingly may not be appropriate for all sources. As noted, the NOx limits set forth here today are presumptions only; in making a BART determination,

States have the ability to consider the specific characteristics of the source at issue and to find that the presumptive limits would not be appropriate for that source.

The table below indicates the types of boilers installed at the 491 BART eligible coal fired EGUs. Dry bottom wall fired boiler units and tangentially fired boiler units make up a large majority of the total BART eligible EGUs.

Table 1. Population of BART eligible coal fired EGUs. 65 The current combustion control technology EPA analyzed for cyclone units is coal reburning.

178 Boiler Type Number Number Number All units Units > 200 MW Units > 200 MW at 750 MW plants Cyclone 56 35 19 Cell Burner 35 35 29 Dry Bottom  Wall fired 188 121 77 Dry Bottom Turbo fired 14 10 4 Stoker 5 0 0 Tangentially fired 186 164 112 Wet Bottom 6 5 5 Other 1 0 0 Total BART eligible coalfired EGUs 491 370 246 For all types of boilers other than cyclone units, the limits in Table 2 are based on the use of current combustion control technology. Current combustion control technology is generally,

but not always, more cost effective than post combustion controls such as SCRs. For cyclone boilers, SCRs were found to be more cost effective than current combustion control technology; 65 thus the NOx limits for cyclone units are set based on using SCRs.

SNCRs are generally not cost effective except in very limited applications and therefore were not included in EPA's analysis.

The types of current combustion control technology options assumed include low NOx burners, over fire air, and coal reburning. 179 We are establishing presumptive NOx limits in the guidelines that we have determined are cost effective for most units for the different categories of units below, based on our analysis of the expected costs and performance of controls on BART eligible units greater than 200 MW, and expect that for many EGUs, States will establish BART limits at least as stringent as these. We assumed that coal fired EGUs would have space available to install separated over fire air. Based on the large number of units of various boiler designs that have installed separated over fire air, we believe this assumption to be reasonable. It is possible,

however, that some EGUs may not have adequate space available. In such cases, other NOx combustion control technologies could be considered such as Rotating Opposed Fire Air (" ROFA"). The limits provided were chosen at levels that approximately 75 percent of the units could achieve with current combustion control technology. The costs of such controls in most cases range from just over $ 100 to $ 1000 per ton. Based on our analysis, however,

we concluded that approximately 25 percent of the units could not meet these limits with current combustion control technology.

However, our analysis indicates that all but a very few of these units could meet the presumptive limits using advanced combustion controls such as rotating opposed fire air (" ROFA"), which has already been demonstrated on a variety of coal fired units. Based on the data before us, the costs of such controls in most cases are less than $ 1500 per ton. 66 No Cell burners, dry turbo fired units, nor wet bottom units burning lignite were identified as BART eligible, thus no presumptive limit was determined. Similarly, no wetbottom units burning sub bituminous were identified as BARTeligible

67 These limits reflect the design and technological assumptions discussed in the technical support document for NOx limits for these guidelines, e. g., EPA assumed space would be available for over fire air. See Technical Support Document for BART NOx Limits for Electric Generating Units and Technical Support Document for BART NOx Limits for Electric Generating Units Excel Spreadsheet, Memorandum to Docket OAR 2002 0076, April 15, 2005.

180 Table 2. Presumptive NOx emission limits for BART eligible coal fired units66 Unit type Coal type NOx presumptive limit ( lb/ mmbtu) 67 Dry bottom wallfired Bituminous 0.39 Sub bituminous 0.23 Lignite 0.29 Tangential fired Bituminous 0.28 Sub bituminous 0.15 Lignite 0.17 Cell Burners Bituminous 0.40 Sub bituminous 0.45 Dry turbo fired Bituminous 0.32 Sub bituminous 0.23 Wet bottom tangential fired Bituminous 0.62 Table 3: Average Cost effectiveness of NOx controls for BARTeligible coal fired units. 181 Unit type Coal type # units nation wide National Average ($/ ton)

Dry bottom wall fired Bituminous 114 1229 Sub bituminous 66 576 Lignite 3 1296 Tangentia l fired Bituminous 105 567 Sub bituminous 72 281 Lignite 9 614 Cell Burners Bituminous 32 1287 Sub bituminous 3 1021 Dry turbofired Bituminous 7 775 Sub bituminous 7 599 Wet bottom Bituminous 6 378 Cyclones ( with SCR) All 56 900 The advanced combustion control technology we used in our analysis, ROFA, is recently available and has been demonstrated on a variety of unit types. It can achieve significantly lower NOx emission rates than conventional over fire air and has been installed on a variety of coal fired units including T fired and wall fired units. We expect that not only will sources have gained experience with and improved the performance of the ROFA technology by the time units are required to comply with any BART requirements, but that more refinements in combustion control 68 See Technical Support Document for BART NOx Limits for Electric Generating Units and Technical Support Document for BART NOx Limits for Electric Generating Units Excel Spreadsheet, Memorandum to Docket OAR 2002 0076, April 15, 2005.

69 Id.

70 Reporting requirements for the Acid Rain Program and NOx SIP Call affected sources, see 40 CFR 75 subpart G ( parts 7562 64), and EPA Clean Air Markets Division Website, data and maps page ( www. epa. gov/ airmarkets).

182 technologies will likely have been developed by that time. As a result, we believe our analysis and conclusions regarding NOx limits are conservative. 68 For those units that cannot meet the presumptive limits using current combustion control technology,

States should carefully consider the use of advanced combustion controls such as ROFA in their BART determination.

A detailed discussion of our analysis is in the docket. 69 For data on emissions and existing control technology in use at the BART eligible EGUs, we used EPA's Clean Air Markets Division database. 70 C. Selective Catalytic Reduction (" SCR") and Cyclone Units.

We also analyzed the installation of SCRs at BART eligible EGUs, applying SCR to each unit and fuel type. The costeffectiveness was generally higher than for current combustion control technology except for one unit type, cyclone units.

Because of the relatively high NOx emission rates of cyclone units, SCR is more cost effective. Our analysis indicated that 71 See Technical Support Document for BART NOx Limits for Electric Generating Units and Technical Support Document for BART NOx Limits for Electric Generating Units Excel Spreadsheet, Memorandum to Docket OAR 2002 0076, April 15, 2005.

72 Id.

183 the cost effectiveness of applying SCR on coal fired cyclone units is typically less than $ 1500 a ton, and that the average costeffectiveness is $ 900 per ton. 71 As a result, we are establishing a presumptive NOx limit for cyclone units based on the use of SCR.

For other units, we are not establishing presumptive limits based on the installation of SCR. Although States may in specific cases find that the use of SCR is appropriate, we have not determined that SCR is generally cost effective for BART across unit types.

Oil and Gas fired Units For oil fired and gas fired units, we recommend that States consider requiring these units to installbelieve that installation of current combustion control technology is highly cost effective and should be considered in determining BART for these sources. but we are not establishing a presumption for these units. We performed an analysis of BART eligible oil and gas fired units similar to the analysis done for coal fired units. Our analysis indicated that a number of units can make significant reductions in NOx emissions which are cost effective through the application of current combustion control technology. 72 However, for a number of units, the use of combustion controls does not appear to be 184 cost effective. As a result, we determined that it would be inappropriate to establish a general presumption regarding likely BART limits. As a result, the guidelines only indicate that States should carefully consider the installation of current combustion control technology on oil and gas fired units.

IV. How does today's rule affect States options for using alternative strategies in lieu of source by source BART?

A. Background Over the past several years, there have been a number of rule makings and court decisions on the subject of BART and BARTalternative programs. In order to understand today's actions, it is useful to again review the regulatory and litigation history,

with a specific focus on BART alternative issues.

As noted in part I of this preamble, the 1999 regional haze rule included provisions for BART, codified at 40 CFR 51.308( e),

and in definitions that appear in 40 CFR 51.301. Among these provisions was section 308( e)( 2), allowing States to implement cap and trade programs, or other alternative programs, in lieu of BART. Section ( e)( 2) provided that trading program alternatives must be demonstrated to achieve greater reasonable progress than BART, and provided the general parameters for making this demonstration. Of particular relevance, section 185 ( e)( 2) directed States, in the course of estimating emissions reductions anticipated from source by source BART, to determine what comprises BART based on the four non visibility factors, and then estimate visibility improvements based on the application of BART to all sources subject to BART. In other words, section ( e)( 2) indicated that states should use what has since been termed a " group BART" approach to estimating the source by source BART benchmark, for comparison to the alternative program. Section ( e)( 2) did not prescribe the specific criteria to be used to compare the progress estimated from source by source BART to that anticipated from the trading program. The preamble discussion indicated that the comparison should be based on both emission reductions and visibility improvement, but did not provide further specificity. See 64 FR 35741   35743.

Specific criteria for making the comparison to programs was proposed in the BART Guidelines ( 40 CFR 51 App. Y) in 2001. These criteria   sometimes referred to as the " better than BART test"

consist of the following. First, if the geographic distribution of emissions reductions from the two programs is expected to be similar, the comparison can be made based on emissions alone.