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Implementation ofthe 2010 Primary 1-Hour502 NAAQS
DraftWhite Paperfor Discussion
I. Overview
The purpose ofthis paper is to facilitate input from states, tribes, and other interested stakeholders on EPA's implementation ofthe primary 1-hoursulfur dioxide (S02) National Ambient Air QualityStandard (NAAQS). In particular, this paperfocuses on the determination of whetherthe air quality in a given area currently meets the NAAQS. This determination occurs primarily in the context ofdesignating areas as attainment, nonattainment, or unclassifiable; and in the context of redesignating areas from nonattainment, attainment, or unclassifiable to anotherstatus. EPA anticipates that some combination ofenhanced ambient monitoring and technically credible dispersion modeling (the combination ofwhich we referto as a "hybrid approach") is likely needed to make this determination in these contexts. To identifya workable approach, EPA is soliciting stakeholder input on how best to determinewhetheran area attains the S02 NAAQS (e.g., by defining and establishing a robust, representative monitoring networkfor S02 across the country, and/or by applying an appropriate modeling approach). EPA is also soliciting inputon how best to implementsuch an approach (e.g., implementation options and SIP timelines for areas in which violations may be identified). This paper addresses these topics and poses several "charge" questions intended tofocus upcoming stakeholder discussions.
In EPA's final S02 NAAQS preamble and subsequent draftguidance in March and September 2011, EPA expressed its expectation that many areas would be initially designated as unclassifiable due to limitations in the scope ofthe ambient monitoring networkand the shorttime available before which states could conduct modeling to supporttheir designations recommendations due in June 2011. In orderto address concerns about potential violations in these unclassifiable areas, EPA recommended that states submit substantive attainment demonstration SIPs based on air quality modeling byJune 2013 {under Clean Air Act section 110(a)(1)) that show howtheir unclassifiable areas would attain and maintain the NAAQS in the future. As discussed below, commenters raised concerns with this approach, and we believe it is appropriatetofurther discuss potential implementation options. Thus, EPA recently wroteto state environmental commissioners and tribal leaders explaining that, for the time being, we intend to move forward with the process ofdesignating areas underthe NAAQS, butthatwe are no longer recommending that demonstrations showing future attainment in unclassifiable areas be included in 2013 SIP submittals. Nevertheless, EPA recognizes that, despite its current plansto move ahead expeditiously with nonattainment designations focused on areas with sufficient ambient air quality data, there will still be large portions ofthe country initially designated as unclassifiable, and there may be NAAQS violations in those areas. As we discussthe monitoring and modelingbased approaches in this paper for determining S02 concentrations, EPA is also interested in discussing thefuture use ofthis information in resolving the unclassifiable status ofareas of 

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concern by affirmatively redesignating such areas as either attainment or nonattainment. EPA is interested in whetherthis kind of approach would satisfactorily address concerns about unclassifiable areas, orwhether other approaches to NAAQS implementation in such areas should be explored (e.g., affirmative planning requirements to identifyand correct NAAQS violations in unclassifiable areas). Finally, we note thatthe initial nonattainmentdesignations will trigger planning requirements forthose areas. EPA expects to provide assistance to states in implementing these nonattainmentarea requirements, butthis document is notfocused on identifying or resolving issues that may arise in developing SIPs showing future attainment ofthe NAAQS. We plan to provide separate guidance or rulemaking on attainment planning as soon as possible. Neverthelesswe expect thatthese discussions could serve as an opportunityto identify such issues and inform ourfuture efforts.
II. Introduction
In June 2010, EPA promulgated a revised primary S02 NAAQS, designed as a new 1-hour standard. In the preamble to the final rule, EPA included a discussion of a possible implementation approach for the standard that differed in some ways from NAAQS implementation approaches taken for other pollutants. Specifically, EPA discussed a possible hybrid implementation approach that would relyon both monitoringand modeling data forthe purposes ofdesignating areas as being in attainment or nonattainment ofthe standard, to the extent states had time and were able to develop such data in timeto inform their designations recommendations. The approach further addressed unclassifiable areas, which were expected to coveran extensive portion ofthe country, by recommending that states would submit SIPs showing future attainment ofthe NAAQS, notjust for designated nonattainment areas but also for unclassifiable areas. Following this, in March and September 2011, EPA issued guidance on designations and SIP planning for the S02 NAAQS, asking for comment on the latter. Since that time, EPA has heard from a numberofstates and other stakeholders who have expressed concern with several aspects of EPA's proposed implementation approach. On April12, 2012, EPA announced in lettersto state environmental commissioners and tribal leaders that it intendsto move forward with the designations process as quickly as possible, focusing on areas with sufficient ambient air qualitydata. Because we still expect this process will result in significant portions ofthe country being initially designated as unclassifiable, we also announced we would initiate a stakeholderengagement process to considerand discuss possible alternative implementation options to those described by EPA in the preamble to the final NAAQS and subsequent guidance documents. As discussed above, this documentfocuses on addressing future-designated unclassifiable areas by developing approaches to modeling and/or monitoringthatwould enable EPA to affirmatively determine whether or notareas ofconcern are attaining the NAAQS. EPA also communicated in the April 2012 lettersthatwe expect all states byJune 2013 to submit SIPs meeting the "infrastructure" SIP requirements undersection 110(a)(2) ofthe Clean Air Act. However, given that we are undertaking astakeholder outreach process to continue to develop possible approaches for 

### 2DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION PRE-DECISIONAL determining attainmentwith the S02NAAQS, we made it clear in the April announcement that we do notexpect states byJune 2013 to submit substantive CAA section 110(a)(1) attainment demonstration SIPs for unclassifiable areas. Distributing this paperwill facilitate the process for obtaining stakeholder input. The paper describes concepts and discusses possible approaches for determiningwhether areas across the countryare attaining the 1-hourS02NAAQS. Appendix A provides basic information aboutthe current population ofS02sources across the country, S02emissions from these facilities, and the current S02monitoring network. We anticipate thatthe ideas contained in this paper will provide a basis for productive discussions with stakeholders as we explore approaches to ensure attainment ofthe 1-hour primary S02NAAQS, increase public health protection, and achieve continued improvement in air quality across the country through S02 emissions reductions.
Ill. Background
In the proposal for the S02 NAAQS (74 FR 64810, December 8, 2009), EPA outlined a possible implementation approach forthe NAAQS that, similarto implementation approaches for other criteria pollutant NAAQS, would have relied primarily on monitoringfordetermining attainment ofthe standard. The proposal discussion did not address how EPA has historically made significant use of modeling in implementing the prior daily and annual S02NAAQS. During the public comment period, EPA heard from a number ofcommenters thatthe means of determiningwhether areas are either in attainment or nonattainmentfor the new standard described in the proposal were notappropriate orfeasible. Specifically, numerous state and local government commenters expressed concerns regarding the perceived burdens of implementingthe proposed monitoring networkand the sufficiency of its scope for purposes of identifyingviolations. Some ofthese commenters suggested using modelingto determine the scope of monitoring requirements, orfavored modeling over monitoringto determine attainmentstatus. Partly in response to these comments, and after reconsidering the proposal's monitoring-focused approach, specifically regarding how EPA has historically determined S02 attainment status\the preamble to the final rule described howwe anticipated using a hybrid analytic approach towards designations that combinesthe use of monitoring and available modelingto determineattainmentwith the new 1-hourS02 NAAQS (75 FR 35570). We also discussed the use ofsection 110(a)(1) ofthe CAA as the mechanism for states to submit substantive attainmentand maintenance SIPs for areas initially designated as unclassifiable.
In the preamble to the final NAAQS, we provided preliminary rationale for the possible hybrid monitoring/modeling approach. We explained thatthis approach could betteraddress several potentially problematic issues than would the narrower monitoring-focused approach discussed in the proposal forthe S02 NAAQS, including the unique source-specific impacts of

1The final rule preamble (75 FR 35551) describes EPA's historic use of modeling in determining attainmentforthe S02 standard. This discussion -and the documents referenced therein
- may be helpful background for discussing possible improved approaches. 

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S02 emissions and the special challenges S02 emissions have historically presented in terms of monitoring short-term S02 levels for comparison with the NAAQS in many situations (75 FR 35550). We noted thatwe anticipated that many areas would be designated "unclassifiable," due to the short time period available to generate monitoring and/or modeling data before state designations recommendations were due underthe CAA. We also explained thatto ensure that all areas ofthe country attainthe NAAQS on a timely basis, EPA intended to emphasize the CAA section 110(a)(1) requirement that all states submit SIPs that show implementation, maintenance, and enforcement ofthe NAAQS in unclassifiable areas (75 FR 35573). These plans were to take intoaccount anticipated future S02 reductions from major rules such as the 2011 Cross-State Air Pollution Rule (CSAPR), the 2011 Mercury and AirToxics Standards (MATS), and the reconsidered boiler MACT rules; and they were to be informed by updated modeling guidance. We also committed to soliciting public comment on guidance regarding modeling and on additional implementation planning guidance, including the content ofthe attainment and maintenance plans that would be submitted under section llO(a) ofthe CAA. On March 24, 2011, EPA issued guidance to states for making initial area designation recommendations. As part ofthe guidance document, we described our intended approach at that time for making initial area designations based generally on adetermination that relied on a combination of both monitoring and, as available, modeling data. We reiterated that many areas would need to be designated initially "unclassifiable" due to a lack ofeither monitoring and/or modeling data. The guidance included a detailed attachment addressing how modeling could be conducted to support initial designations, ifastate chose to do so. We also noted that we were preparing separate guidance on developing SIP revisions forthe S02 NAAQS, and that we intended to seek public comment on the draftguidance document. On September 22, 2011, we released the draftguidance on SIP submissions, which included furtherguidance on air quality modeling for nonattainment areas and on developing attainment demonstrations under section 110(a)for unclassifiable areas. We also provided further rationale for considering a hybrid monitoring/modelingapproach for implementingthe new 1-hour primary S02 NAAQS. For areas initially designated as unclassifiable (i.e., areas without sufficient monitoring/modelingdata to show nonattainmentordemonstrate attainment), the draftguidance recommended that states submit substantive CAA section 110(a) SIPs that show howtheywould attain and maintain the NAAQS in the future. Because we are undertaking discussions thatwould affectthis approach, we announced in letters to state environmental commissioners and tribal leaders in April 2012 thatwe no longer recommend such submittals.
IV. Issues
A. Key comments from stakeholders
EPA has received comments on S02 NAAQS implementation from state, local, and tribal governments; environmental groups; and industry stakeholders in several settings since we 4DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION PRE-DECISIONAL

issued the final S02 NAAQS inJune 2010. The public comment period forthe September 2011 draftguidance for SIP submissions offered an opportunityfor stakeholders to provide detailed written comf11ents on the proposed 1-hourS02 NAAQS implementation approach. In light of these comments, we present concepts in this paperfor further exploration in upcoming stakeholder discussions. Stakeholder concerns generally related to one or more ofthefollowing aspects ofthe proposed approach: 

### 1. Difficulty in meeting the suggested approach for attainment demonstrations and in demonstrating attainment everywhere based on air quality modeling, especially in multi-source areas; 2. Time and resource burden associated with conducting attainment demonstration modeling and establishing newcontrol requirements for significant S02 sources located outside ofdesignated nonattainmentareas byJune 2013; and, 3. Legal and policy objections to EPA's authorityto expect modeling-based attainment demonstrationsto satisfy the CAA section 110(a)(1) "implementation, enforcement, and maintenance" requirement for areas designated unclassifiable, especially in areas where nearby monitors indicate noexceedances.
Morespecifically, comments from states and other stakeholders asked that we reassess what may be reasonable for implementingthe 1-hourprimary S02 standard. First, some commenters have suggested thatwe may have underestimated the resources needed for widespread modeling, analysis, and rule development that may be necessary tofollow our recommendations regarding CAA section llO(a) SIPs. This concern is compounded by the level of effort that would be necessary byJune 2013 to model emissions from numerous sources of S02 located across the country in orderto support such submittals. Second, many commenters have asserted that modeling attainment is difficult due to the purported conservative nature of modeling, even in areas that are not monitoring violations. For example, modeling ambient S02 concentrations at a large number of receptor locations across an area may provide a more "conservative" assessment ofambient levels than a single or limited number ofambient monitors, especiallywhen maximum allowable emissions are modeled. Third, commenters have stated that the time discussed underthe proposed approach for states to submit SIPs and demonstrate attainment is generally insufficient. Commenters claim this is especially true for unclassifiable areas, where, despite the large geographic scope and numberofsources, the SIP submission deadline under CAA section llO(a) is actuallysooner than would be the deadline for nonattainment areas under CAA section 191(a). While a few commenters expressed concern about nonattainmentarea planning requirements, the majority expressed concerns aboutthe burden ofdeveloping SIPs for unclassifiable areas. B. EPA's evaluation ofcomments and understanding ofthe issues Manycommenters have asserted thatthe proposed implementation approach involving CAA section 110(a)(1) SIPs could place more burden on states to address areas designated 

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unclassifiable than to address areas with other evidence of a NAAQS violation. In recognition of these concerns, we have identified potential approaches that place priority on addressing nonattainment area problems first (consistent with traditional NAAQS implementation efforts), while also addressing unclassifiable areas over the longertermto ensure these areas also attain the S02 NAAQS and achieve appropriate public health protection.The potential approaches focus on howto determine current attainment, rather than future attainment, in orderto consider whether a more workable approach can be developed that significantly reduces the extent of unclassifiable areas, identifies and addresses sources ofconcern that are causing NAAQS violations, and avoids the issues that gave rise to many ofthe concerns expressed about the suggested CAA section llO(a) approach.
The options presented below are presented for the purpose of eliciting further comment and discussion with stakeholders, butshould not be considered as limitingthe range ofoptionsthat could be discussed, noras endorsing or rejecting any particular idea at this time.
V. Implementation Concepts and Issues for Consideration
After considering the comments received on the September 

### 2011 draftguidance, we have identifiedforfurther discussion two conceptual approaches for addressing the many areas still expected to be initially designated "unclassifiable" for the 1-hour NAAQS. While this document provides a general outline ofthe approaches, there are several issues and key questions which need to be addressed before revised implementation guidance (or rules, if necessary) can be developed. These issues and key questions will be the focus ofthe stakeholder discussions.
For most air pollutants, EPA relies on ambient monitoring data for determining whether an area is currently attaining the NAAQS. The same approach could be taken to address the 1- hour S02 NAAQS by pursuing an adequately updated and expanded monitoring networkto collect ambient air quality data. Such an approach would be conceptually similar to our implementation approach for other NAAQS in that itwould rely on ambient monitoring data to evaluate air quality, serve as the basis for initial area designations (if developed on time), and inform ongoing compliance with the NAAQS. Although EPA originally proposed an expanded monitoring network, in the preamble to the final 

### S02 NAAQS we recognized thatthe proposed network might not be sufficiently expansive and discussed the potential resource burden of establishing a sufficient monitoring network in light ofthe unique source-specific impacts of 

### S02• Subsequently, some stakeholders asserted thatthe hybrid modeling/monitoringapproach we proposed in response to this concern could also be overly burdensome. Thus we are exploring alternative monitoring and modeling approaches to adequately measure air quality and protect public health.
A. MonitoringOptions
Under a monitoring-focused approach, the question turns tothe nature of a potential monitoring networkfor implementing this NAAQS. A map ofthe current national 

### S02 

### 6DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION PRE-DECISIONAL monitoring network is provided in Figure 1. Currently, there are about

### 440 monitors in operation around the country, with not morethan 

### 1/3 being source-oriented or at high concentration sites. As such, it is likely that a significant reallocation of monitors in the existing network plus expansion ofthe number of monitors in the networkwould be necessary to determine whether national ambient 502 levels are meetingthe NAAQS and protective of public health under a monitoring-focused approach. However, it might not be necessary or feasible to expect monitoring for every single S02 source in the country. Figure 1 Active 502 Sites

There are two alternatives introduced in this paper for expandingthe 502 monitoring network: (1) a national network reallocation and expansion and (2) a population-focused reallocation and expansion.
1. National Network Reallocation and Expansion As previously stated, not more than 

### 1/3 ofthe current monitoring network is situated to characterize sources with high 502 emissions. It may be appropriate to focus the monitoring network on the majorsources of S02 emissions across the country. EPA could establish through rulemaking an 502 emissions threshold above which an 502 monitorwould be installed. For example ifa threshold of 

### 2,000 tons of502 peryear (tpy) were deemed appropriate, thatwould address close to 

### 600 ofthe S02 sources in the country, or93% ofthe emissions. The monitors in the current networkcould be reallocated tothe identified sources, and new monitors could be installed and operated atthe remaining sources. This alternative could provide a 

### 7DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION PRE-DECISIONAL straightforward approach to implementingthe S02 NAAQS and at the same time provide a more robust monitoring networkacross the country.
2. Population-focused Reallocation and Expansion A second alternative could resemble the current minimum monitoring requirements discussed in the final S02 NAAQS rule using the population weighted emissions index (PWEI) based on population and emissions inventory data atthe core-based statistical area (CBSA) level. This approach would assign a required numberof monitorsfor a given CBSA based on relevant factors such as population and S02 emissions in the area. 2 The PWEI index provides an indication of population weighted exposure, and can help EPA and states prioritize limited monitoring resources with an emphasis on public health protection.
To identify priority locations atwhich to site monitors in these PWEI areas, an S02 emissions threshold could be used tofocus the placement of monitors around the higher S02 emissions in the PWEI areas. For example, a threshold of 750 tpywould provide for monitoring of approximately400 sources in those PWEI areas responsible for 56% ofthe S02 emissions. In addition, to monitorair quality near the largest sources of S02 that are located in areas not covered by the population-oriented PWEI approach, an emission threshold could be set to prioritize the location of an additional set of monitors outside ofthe PWEI areas. For example, deploying monitors near sources with emissions over 5,000 tpy would approximately address an additional170 sources outside ofthe PWEI areas and 34% of S02 emissions. This approach would allow states tofocus their resources in areas with the highest risk to public health and with the largest sources ofemissions. Underthis example, the resulting minimum network would consist of over 550 sites nationwide and would provide monitoring ofsources responsible for 90% ofthe total national emissions.
3. OtherAlternatives and Considerations In additionto the alternatives presented above, there are otherthreshold alternatives EPA could consider. EPA could select thresholds based on annual emissions atvarious levels (see Appendix). EPA could also consider selection ofthresholds based on hourly emissions, but for most sources, hourly data are much more limited. EPA could also consider screening out sources based on thresholds that take into account not only asource's 502 emissions, but also the source's characteristics (e.g., stack height, source configuration, etc.). Another alternative may be atiered approach, where states would monitorsources with emissions above a high threshold and then have a range ofoptionsfor assessing air quality at sources with emissions in a given range belowthatthreshold, orvice-versa.

2 The PWEI for a particular CB5A was proposed to be calculated by multiplying the population (using the latest Census Bureau estimates) ofa CBSA bythe total amount of502 emissions in that CBSA. The CB5A 502 emission value would be in tons peryear, and calculated by aggregating the county level emissions foreach county in a CBSA. Wewould then divide the resulting product of CBSA population and CBSA S02 emissions by 1,000,000 to provide a PWEI value, the units ofwhich would be millions ofpeople-tons peryear. 

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For anysource-focused monitoringalternative, the question arises as to how many monitors are needed to characterize the air quality around an S02 source. EPA is considering establishing approaches that would allow sources to be characterized byjustafew monitors, 

### or, ifappropriate, a single properly-sited monitor. Under such approaches, modeling (or other analyses- e.g., examination ofwind roses, identification ofsensitive receptor locations, assessment'ofnearby terrain features, and estimation ofdistance of maximum impacts) would likely need to be completed to site these monitors atthe areas of highest concentration. This modeling might be less resource intensive than attainment demonstration modeling, butwould require resources to complete the analysis.
In addition to any potentially revised monitoring network, as with all other NAAQS monitoring requirements, EPA Regional Administrators and states would have the authorityto require additional monitoring in certain circumstances, such as in areas impacted by major industrial pointsources ora combination ofsources that are not required to monitor underthe other monitoring provisions. Also, attention should be given to the need to collect site-specific meteorological data, as well as ambient S02data, in orderto support data analyses (e.g., generation ofpollution roses) and, ifnecessary, dispersion modeling. Pursuing any ofthese approaches could require EPA to make revisions tothe S02 monitoring regulations to address revised monitoring requirements, including any new requirements based on appropriate emissions thresholds.
4. Key Questions In orderfor EPAto determinethe feasibility of using monitors principally to determine attainment or nonattainmentofthe standard, stakeholder input is needed on the following key questions:

a. Are the conceptual monitoring networks described above sufficient to determine whether ambient S02 levels meetthe NAAQS and are protective of public health without the need for additional modeling? Ifnot, then what enhancements should be made to them? In what situations should meteorological data collection also be required?
b. What is an appropriate numberof monitors to site around a source to assess air quality?

c. Is it reasonable for states to consider relocating monitors within their states? What are potential barriers to relocation (e.g., 

### cost, agreement with local community)? Is it reasonable for states to consider transferring their monitors to other states?
d. What kind of modeling (or other analyses) would be necessary to identifythe location of maximum impact? What information and resources are necessary to
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complete such modeling? What is a reasonable schedule for completing this modeling?

e. What options exist for paying for the expanded S02 monitoring network? Would stakeholders be willing to conduct monitoring at new locations, or provide funding to assist states in conducting such monitoring? Ifso, whattype of agreement would be needed between states and stakeholders to ensure the monitoringwould be done?
f. For potential stakeholder operated monitors, what kind ofoversight would the states need to perform? Would EPA perform additional oversight? Would someone auditthese facility monitoring programs and associated monitors? Whattype ofagreement would be needed between the states and stakeholders to insure the monitoringwas carried out? How can we best ensure thatthese data are made public (e.g., require submittal toAQS)?
B. Modeling Options
The monitoring alternatives discussed above would identifya set of priority sources for monitoring. In recognition ofthe limitations of monitoringto identify high concentrations in thevicinity ofS02 sources and the resource burden associated with establishing a robust monitoring network, EPA believes that modeling may need to be used to supplement or in lieu of monitoringwhere appropriate. States may preferto model instead of monitoratsources where modeling is technically straightforward (e.g., consistentwith the design ofthe model) and ifthey do not have enough resources to install the number of needed monitors. At the same time, EPA recognizes that modeling these sources may also impose additional resource burdens on states, and modeling may not be appropriate in some situations.
Underthe modeling alternative, states could modelthe sources that otherwise would have required one or more monitors under potentially revised minimum monitoring requirements. This modeling would use the AERMOD dispersion model, and EPA is exploring several options for the emissions levels thatwould be modeled to characterize current air quality, including: (1) actual emissions, where states would need to provide some ongoing assurance that the area's current air quality is being evaluated against the NAAQS similar to monitoring (in addition to assurance that results are not inappropriately affected by dispersion), and (2) allowable source emissions (e.g., federally enforceable permit limits or potential to emit). Recognizing the stakeholder interest in modeling actuals, EPA could develop approaches for states to providethis assurance by (1) setting enforceable emission limits for the sources equal totheiractual emissions such that future levels would be noworse than current actuals, (2) periodically modeling levels ofactual emissions to continue to ensure they are not violating the NAAQS, and/or (3) periodically reporting on whetherthe sources' actual emissions have increased relative to the prior modeling assessment, and in cases where the emissions increase has created the potential forviolation ofthe NAAQS, completing additional modelingto determine whetherthe higher emissions level resulted in a modeled violation in 

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the area or in an affected area. Othertechnical modeling issues which need to be addressed include receptor placement, the spatial extent ofthe modeling, which other nearby sources need to be modeled, estimating background concentrations, and use ofsite
-specificversus National Weather Service meteorological data

. In addition, it may be necessary to ensure that actual and allowable emissions for these sources are not inappropriately affected bydispersion techniques, such as stack heights greater than those considered within the limits ofgood engineering practice (GEP). Moreover, in situations where it is found, perhaps through New Source Review (NSR) or Prevention of Significant Deterioration (PSD) modeling, that existing, new or modified sources in designated attainment or unclassifiable areas would cause NAAQS violations, EPA and states could act to revise the SIP through developing furtheremissions limits and a newdemonstration offuture attainment. Any thresholds setforth in a potentially revised monitoring rule could also apply to modeling requirements.
For states that have unmonitored areas with no S02 sources, Appendix Cin the September

### 2011 draftguidance discusses an example ofa possible non-modeling option that could allowthem to demonstrate attainmentwithout needing to complete modeling. This, or variants on it, could also be considered underthis option.
EPA recognizes that some ofthe issues discussed here may also be relevant to attainmentdemonstration modeling for nonattainment SIPs and may come up in stakeholder discussions and be relevant forconsideration in future attainment planning guidance.


### 1. Key Questions In orderfor EPA to evaluate the feasibility of using modeling in conjunction with monitoringto determine either attainment or nonattainmentofthe standard, stakeholder input is needed on the following key questions:

a. Should some criteria (e.g., the PWEI concept) be used to identify prioritysources to be modeled in an area where there is no nearby monitor?
b. How should the modeling be performed- i.e., what changes to the March 24, 

### 2011 guidance should be made, such as the use ofsize cut-offs and use of actual emissions?

c. Are there situations where modeling is preferable to monitoring? Ifso, then what are these situations? Should EPA require modeling in certain situations, or is monitoring alone always a sufficient option for areas ofconcern?
d. Are there situations where monitoring is preferable to modeling? Ifso, then what are these situations? Should EPA require monitoring in certain situations, or is modeling alone always a sufficient option for areas of concern?


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e. What options exist for paying for the new modeling analyses? Would stakeholders be willingto conduct, or provide fundingto assist states in conducting, any new modeling? Ifso, whattype ofagreement would be needed between states and stakeholders to insure modeling would be done? 

### C. Implementation Options Regardless ofthe attainment determination approach that is ultimately used in a given context, EPA intends to move forward with proposing nonattainmentarea designations focused on areas with sufficient ambient air quality data. As noted above, this will trigger planning requirements, the implementation ofwhich is important, but is notthe primary focus ofthis document. Nevertheless, it is important to consider how best to implement expeditiously whatever improved approach EPA ultimately recommends for unclassifiable areas. It is possible that an initialstep could be issuance of EPA guidance or rulemaking · adopting minimum monitoring/modeling/hybrid requirements for determining attainment.There would then likely be some state adoption orapproval process, followed by the deployment or redeployment of monitorsand/orthe conducting of modelingto determinewhich areas are attaining and which are not.The timing ofthese actions is an important discussion topic, and involves consideration ofthe practical realities ofenhancing the ambient monitoring network (such as cost, timing, etc.) and conducting necessary modeling, as well as the importance of being able to use the newapproach to determine as expeditiously as practicable whetherthere are NAAQS violations occurring. Appropriate timingfor these actions will need to be discussed in the context ofthe approach that EPA recommends (e.g., how many monitors, schedule for deployment, data collections, etc.).
Under a monitoring-focused alternative, an area designated unclassifiable mighteither have no monitor, or it may have a monitoror monitors that do notshow violations ofthe NAAQS atthetime ofdesignation. Following designation, states with unclassifiable areas might be required by revised rules to site new monitors or relocate existing monitors. EPA could also potentially directstates (through rulemaking or other procedures, such as SIP Calls) to submit a plan for siting the monitors required by a revised monitoring rule, including requiring modeling (or other analyses) to demonstrate the appropriate placement ofthe monitors. Any data ultimatelycollected bythe comprehensive monitoring networkcould be used, as appropriate, for area redesignations orfor any necessary future SIP calls.
EPA recognizes that any approach that would depend on the siting ofadditional monitors could impose additional resource burdens on states in orderto install, maintain, and operate those devices. We acknowledged this issue in the preamble to the final S02 NAAQ5, in which we indicated that EPA and states may not have adequate resources to site sufficient monitors to properly characterize air quality around major502 sources. Therefore, an alternative thatwould rely upon expanding the 502 monitoring network deserves further discussion with stakeholders. In particular, EPA would like to consider potential means of alleviating some ofthe cost burden on states, for example by having sources cover capital costs 

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and/oroperating costs for monitors in their areas. Considering the importance of monitoring the ambient air around these sites and continuing state budget concerns, states might reasonably expect sources to workjointlywith the appropriate state or local agency to install and operate these monitors. The source and the state would need to worktogether in siting the monitors and analyzing the data, which could then be reported to states and to EPA

. Regarding the role of modeling, as noted above, EPA recognizes that modeling may also present resource burdens on the states. Under a modeling alternative, modeling could, for example, be completed for sources identified by minimum monitoring requirements butfor which monitorswere not deployed. Some consideration would need to be given towhen this modeling would need to be completed. Similar to a potential monitoring network, the modeling could be done in a phased approach, having states model first the sources in areas with the highest population or some other determinant ofgreatest public health risk. There may also be cost burdens onthe states associated with data collection, such as more representative meteorological data. Considering the potential importance ofa modeling option in some locations and continuing state budget concerns, states might reasonably expect sources to workjointlywith the appropriate state or local agency to both collect the necessary data, butalso perform some or all ofthe modeling analysis.
Whatever approach is developed (e.g., monitoring-focused, modeling-focused, or allowing states flexibilityto determine whetherto use monitoring and/or modeling), there are practical considerations about howquickly monitors can be deployed and modeling can be completed. Depending on the significance ofthe changes being made, a phased
-in approach maywell be warranted. This phased approach could require thefirst monitorsto be sited (or the first modeling to be conducted for sources) in areas with the highest population, highest emissions, orsome other determinant ofgreatest public health risk. Finally, EPA will need to decide on requirements regarding what SIP revisions will be needed to carry out the newapproach. As referred to above, one approach would be to amend the minimum requirements for monitoring networks (and any modeling requirements as part of a hybrid approach), specify the appropriate deadlines, and then act promptly on that data to identifyany new NAAQS violations in unclassifiable areas and redesignate those areas to nonattainment, or do aSIP call for those areas
. This approach is straightforward and most closely resembles our approach to implementing other NAAQS. Nevertheless, some stakeholders suggested thatwe should explore approaches that do not involve additional nonattainment designations

. For example, EPA could allow states to remedyviolations in unclassifiable areas by demonstrating attainment before a redesignation occurs
. Instead, EPA could bui ld in affirmative requirements for unclassifiable areas to discover and correct violations, similarto the original approach we previously recommended (but did not require) for demonstrating attainment under CAA section 110(a)(1). Alternatively, EPA could tryto integrate such unclassifiable requirements into permitting actions (e

.g., title V renewals) already happening in these areas.

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1. Key Questions
a. In what form should EPA set forth the revised approach? Would rules need to be revised? Which ones? How should states adopt the new approach, and how much time is needed for this?
b. What is a reasonable schedule for 1) designing a sufficient monitoring network; and 2) deploying a new monitoror moving a monitorfrom an existing location? (What can be done to initiate monitoringas quickly as possible to collect sufficient data to make attainment/nonattainment determinations?) Is a phased approach useful?

c. By what date should the modeling be completed and submitted to EPA? Is a phased approach useful?
d. Once the modeling/monitoring data are in, how should states and EPA use these data to address violations in unclassifiable areas? Is redesignating the most workable approach? What should be the timing forthese redesignations? Is the timing ofthe next S02 NAAQS revision a consideration?

e. Alternatively, should EPA consider approaches to identify and address violations in unclassifiable areas that do not involve redesignating these areas as "nonattainment"? Which alternative approaches are most promising?
f. Is it possible to develop an attainment determination approach that provides reasonable assurance that sources ofconcern that are causing violations will be identified and addressed?
g. How should EPA address unclassifiable areas with no emissions or shown to have no monitored or modeled violations? What requirements, ifany, are appropriate to support designating these areas as attainment? Is this necessary?
VI. Comments
Additional information concerning the stakeholder process and opportunities to provide comments can be found on the following website: http://www.epa.gov/air/sulfurdioxide/implement.html



### 14DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION

APPENDIX A S02 Emissions
NumberofSources Total Emissions All S02 Sources in 2008 NEI 7500 9,448,382 emitting 1ton ormore 502 sources> 100tons 1684 9,374,962 S02 sources> 250 tons 1268 9,307,657 S02 sources> 1000tons 779 9,042,377 S02 sources> 2000 tons 585 8,767,914 S02 sources> 2880 tons 479 8,508,328 S02 sources>5000tons 352 8,029,246 [Source: http:/ /www.epa.gov/ttn/chief/net/2008inventory.html]**SO~** Ambient Monitoring Network Current numberof monitors nationally: 441 Numberof monitors in 1980: 15003

. PRE-DECISIONAL

Percent

100%

99% 98% 96% 93% 90% 85%

Current minimum monitoring requirements: 129 monitors required in 104CBSAs4
For sake of comparison, the followingtable provides the current numberof ambient monitors nationallyfor each criteria pollutant:
Number ofMonitored Sites Pollutant (as ofend of2011) 5 Carbon Monoxide 330 lead 198 (forTSP in local conditions) Nitrogen Dioxide 397 Ozone 1291 PM2.5 868 PMlO 684 502 441**~** Ambient Monitoring Data
Based on ambient monitoring data from 2008-2010, there were about 70 monitors located in 

### GO areas with 1-hourS02 concentrations exceeding the level ofthe standard.

3 Source: http://www.epa.gov/ttn/ecas/regdata/RIAs/pSO,ch2 11-16-09.pdf 4 Based on 2010 census data and 2008 NEI data

. 5 Based on AirData files pulled from AQS in February 2012. Please notethese are sites, and not necessarily individual monitors. 15