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Implementation of the 2010 Primary 1-Hour SO2 NAAQS

Draft White Paper for Discussion

I. Overview

The purpose of this paper is to facilitate input from states, tribes, and other interested 

stakeholders on EPA’s implementation of the primary 1-hour sulfur dioxide (SO2) National 

Ambient Air Quality Standard (NAAQS). In particular, this paper focuses on the determination of 

whether the air quality in a given area currently meets the NAAQS. This determination occurs 

primarily in the context of designating areas as attainment, nonattainment, or unclassifiable; 

and in the context of redesignating areas from nonattainment, attainment, or unclassifiable to 

another status. EPA anticipates that some combination of enhanced ambient monitoring and 

technically credible dispersion modeling (the combination of which we refer to as a “hybrid 

approach”) is likely needed to make this determination in these contexts. To identify a 

workable approach, EPA is soliciting stakeholder input on how best to determine whether an 

area attains the SO2 NAAQS (e.g., by defining and establishing a robust, representative 

monitoring network for SO2 across the country, and/or by applying an appropriate modeling 

approach). EPA is also soliciting input on how best to implement such 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 to focus upcoming 

stakeholder discussions.

In EPA’s final SO2 NAAQS preamble and subsequent draft guidance in March and 

September 2011, EPA expressed its expectation that many areas would be initially designated 

as unclassifiable due to limitations in the scope of the ambient monitoring network and the 

short time available before which states could conduct modeling to support their designations 

recommendations due in June 2011. In order to address concerns about potential violations in 

these unclassifiable areas, EPA recommended that states submit substantive attainment 

demonstration SIPs based on air quality modeling by June 2013 (under Clean Air Act section 

110(a)(1)) that show how their 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 appropriate to further discuss potential implementation options. Thus, EPA recently wrote to 

state environmental commissioners and tribal leaders explaining that, for the time being, we 

intend to move forward with the process of designating areas under the NAAQS, but that we 

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 plans to move ahead 

expeditiously with nonattainment designations focused on areas with sufficient ambient air 

quality data, there will still be large portions of the country initially designated as unclassifiable, 

and there may be NAAQS violations in those areas. As we discuss the monitoring and modelingbased approaches in this paper for determining SO2 concentrations, EPA is also interested in 

discussing the future use of this information in resolving the unclassifiable status of areas of 
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concern by affirmatively redesignating such areas as either attainment or nonattainment. EPA 

is interested in whether this kind of approach would satisfactorily address concerns about 

unclassifiable areas, or whether other approaches to NAAQS implementation in such areas

should be explored (e.g., affirmative planning requirements to identify and correct NAAQS 

violations in unclassifiable areas). 

Finally, we note that the initial nonattainment designations will trigger planning 

requirements for those areas. EPA expects to provide assistance to states in implementing 

these nonattainment area requirements, but this document is not focused on identifying or 

resolving issues that may arise in developing SIPs showing future attainment of the NAAQS. We 

plan to provide separate guidance or rulemaking on attainment planning as soon as possible. 

Nevertheless we expect that these discussions could serve as an opportunity to identify such 

issues and inform our future efforts.

II. Introduction

In June 2010, EPA promulgated a revised primary SO2 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 rely on both monitoring and modeling data for the 

purposes of designating areas as being in attainment or nonattainment of the standard, to the 

extent states had time and were able to develop such data in time to inform their designations 

recommendations. The approach further addressed unclassifiable areas, which were expected 

to cover an extensive portion of the country, by recommending that states would submit SIPs 

showing future attainment of the NAAQS, not just 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 SO2 NAAQS, asking for comment on the latter. Since that 

time, EPA has heard from a number of states and other stakeholders who have expressed 

concern with several aspects of EPA’s proposed implementation approach.

On April 12, 2012, EPA announced in letters to state environmental commissioners and 

tribal leaders that it intends to move forward with the designations process as quickly as 

possible, focusing on areas with sufficient ambient air quality data. Because we still expect this 

process will result in significant portions of the country being initially designated as 

unclassifiable, we also announced we would initiate a stakeholder engagement process to 

consider and 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 

document focuses on addressing future-designated unclassifiable areas by developing 

approaches to modeling and/or monitoring that would enable EPA to affirmatively determine 

whether or not areas of concern are attaining the NAAQS. EPA also communicated in the April 

2012 letters that we expect all states by June 2013 to submit SIPs meeting the “infrastructure” 

SIP requirements under section 110(a)(2) of the Clean Air Act. However, given that we are 

undertaking a stakeholder outreach process to continue to develop possible approaches for 
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determining attainment with the SO2 NAAQS, we made it clear in the April announcement that 

we do not expect states by June 2013 to submit substantive CAA section 110(a)(1) attainment 

demonstration SIPs for unclassifiable areas. 

Distributing this paper will facilitate the process for obtaining stakeholder input. The 

paper describes concepts and discusses possible approaches for determining whether areas 

across the country are attaining the 1-hour SO2 NAAQS. Appendix A provides basic information 

about the current population of SO2 sources across the country, SO2 emissions from these 

facilities, and the current SO2 monitoring network. We anticipate that the ideas contained in 

this paper will provide a basis for productive discussions with stakeholders as we explore 

approaches to ensure attainment of the 1-hour primary SO2 NAAQS, increase public health 

protection, and achieve continued improvement in air quality across the country through SO2

emissions reductions. 

III. Background

In the proposal for the SO2 NAAQS (74 FR 64810, December 8, 2009), EPA outlined a 

possible implementation approach for the NAAQS that, similar to implementation approaches 

for other criteria pollutant NAAQS, would have relied primarily on monitoring for determining 

attainment of the standard. The proposal discussion did not address how EPA has historically 

made significant use of modeling in implementing the prior daily and annual SO2 NAAQS. 

During the public comment period, EPA heard from a number of commenters that the means of 

determining whether areas are either in attainment or nonattainment for the new standard 

described in the proposal were not appropriate or feasible. Specifically, numerous state and 

local government commenters expressed concerns regarding the perceived burdens of 

implementing the proposed monitoring network and the sufficiency of its scope for purposes of 

identifying violations. Some of these commenters suggested using modeling to determine the 

scope of monitoring requirements, or favored modeling over monitoring to determine 

attainment status. Partly in response to these comments, and after reconsidering the 

proposal’s monitoring-focused approach, specifically regarding how EPA has historically 

determined SO2 attainment status1

, the preamble to the final rule described how we 

anticipated using a hybrid analytic approach towards designations that combines the use of 

monitoring and available modeling to determine attainment with the new 1-hour SO2 NAAQS 

(75 FR 35570). We also discussed the use of section 110(a)(1) of the CAA as the mechanism for 

states to submit substantive attainment and 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 that this approach could better address 

several potentially problematic issues than would the narrower monitoring-focused approach 

discussed in the proposal for the SO2 NAAQS, including the unique source-specific impacts of 

 1 The final rule preamble (75 FR 35551) describes EPA’s historic use of modeling in determining attainment for the 

SO2 standard. This discussion – and the documents referenced therein – may be helpful background for discussing 

possible improved approaches.
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SO2 emissions and the special challenges SO2 emissions have historically presented in terms of 

monitoring short-term SO2 levels for comparison with the NAAQS in many situations (75 FR 

35550). We noted that we 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 under the CAA. We also explained that to 

ensure that all areas of the country attain the 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 of the NAAQS in unclassifiable areas (75 FR 

35573). These plans were to take into account anticipated future SO2 reductions from major 

rules such as the 2011 Cross-State Air Pollution Rule (CSAPR), the 2011 Mercury and Air Toxics 

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 

of the attainment and maintenance plans that would be submitted under section 110(a) of the 

CAA.

On March 24, 2011, EPA issued guidance to states for making initial area designation 

recommendations. As part of the guidance document, we described our intended approach at 

that time for making initial area designations based generally on a determination 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 of either monitoring 

and/or modeling data. The guidance included a detailed attachment addressing how modeling 

could be conducted to support initial designations, if a state chose to do so. We also noted that 

we were preparing separate guidance on developing SIP revisions for the SO2 NAAQS, and that 

we intended to seek public comment on the draft guidance document.

On September 22, 2011, we released the draft guidance on SIP submissions, which 

included further guidance 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/modeling approach for implementing the 

new 1-hour primary SO2 NAAQS. For areas initially designated as unclassifiable (i.e., areas 

without sufficient monitoring/modeling data to show nonattainment or demonstrate 

attainment), the draft guidance recommended that states submit substantive CAA section

110(a) SIPs that show how they would attain and maintain the NAAQS in the future. Because 

we are undertaking discussions that would affect this approach, we announced in letters to 

state environmental commissioners and tribal leaders in April 2012 that we no longer 

recommend such submittals. 

IV. Issues

A. Key comments from stakeholders

EPA has received comments on SO2 NAAQS implementation from state, local, and tribal 

governments; environmental groups; and industry stakeholders in several settings since we 
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issued the final SO2 NAAQS in June 2010. The public comment period for the September 2011 

draft guidance for SIP submissions offered an opportunity for stakeholders to provide detailed 

written comments on the proposed 1-hour SO2 NAAQS implementation approach. In light of 

these comments, we present concepts in this paper for further exploration in upcoming 

stakeholder discussions. Stakeholder concerns generally related to one or more of the following 

aspects of the 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 new control requirements for significant SO2 sources located 

outside of designated nonattainment areas by June 2013; and,

3. Legal and policy objections to EPA’s authority to expect modeling-based attainment 

demonstrations to satisfy the CAA section 110(a)(1) “implementation, enforcement, and 

maintenance” requirement for areas designated unclassifiable, especially in areas where 

nearby monitors indicate no exceedances.

More specifically, comments from states and other stakeholders asked that we reassess 

what may be reasonable for implementing the 1-hour primary SO2 standard. First, some

commenters have suggested that we may have underestimated the resources needed for 

widespread modeling, analysis, and rule development that may be necessary to follow our 

recommendations regarding CAA section 110(a) SIPs. This concern is compounded by the level 

of effort that would be necessary by June 2013 to model emissions from numerous sources of 

SO2 located across the country in order to 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 SO2 concentrations at a large number of receptor 

locations across an area may provide a more “conservative” assessment of ambient levels than 

a single or limited number of ambient monitors, especially when maximum allowable emissions 

are modeled. 

Third, commenters have stated that the time discussed under the 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 number of sources, the SIP submission deadline under CAA section 110(a) is actually sooner 

than would be the deadline for nonattainment areas under CAA section 191(a). While a few 

commenters expressed concern about nonattainment area planning requirements, the majority 

expressed concerns about the burden of developing SIPs for unclassifiable areas.

B. EPA’s evaluation of comments and understanding of the issues

Many commenters have asserted that the 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 longer term to ensure these areas also attain 

the SO2 NAAQS and achieve appropriate public health protection. The potential approaches 

focus on how to determine current attainment, rather than future attainment, in order to 

consider whether a more workable approach can be developed that significantly reduces the 

extent of unclassifiable areas, identifies and addresses sources of concern that are causing 

NAAQS violations, and avoids the issues that gave rise to many of the concerns expressed about 

the suggested CAA section 110(a) approach. 

The options presented below are presented for the purpose of eliciting further 

comment and discussion with stakeholders, but should not be considered as limiting the range 

of options that could be discussed, nor as 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 draft guidance, we 

have identified for further 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 of the 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 of the 

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 SO2 NAAQS by pursuing an adequately updated and expanded monitoring network to 

collect ambient air quality data. Such an approach would be conceptually similar to our 

implementation approach for other NAAQS in that it would 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 SO2 NAAQS we recognized that the proposed

network might not be sufficiently expansive and discussed the potential resource burden of 

establishing a sufficient monitoring network in light of the unique source-specific impacts of 

SO2. Subsequently, some stakeholders asserted that the hybrid modeling/monitoring approach 

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. Monitoring Options

Under a monitoring-focused approach, the question turns to the nature of a potential

monitoring network for implementing this NAAQS. A map of the current national SO2
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monitoring network is provided in Figure 1. Currently, there are about 440 monitors in 

operation around the country, with not more than 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 of the number of monitors in the network would be necessary to 

determine whether national ambient SO2 levels are meeting the 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 SO2 source in the country. 

Figure 1

There are two alternatives introduced in this paper for expanding the SO2 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 of the current monitoring network is situated to 

characterize sources with high SO2 emissions. It may be appropriate to focus the monitoring 

network on the major sources of SO2 emissions across the country. EPA could establish through 

rulemaking an SO2 emissions threshold above which an SO2 monitor would be installed. For 

example if a threshold of 2,000 tons of SO2 per year (tpy) were deemed appropriate, that would 

address close to 600 of the SO2 sources in the country, or 93% of the emissions. The monitors 

in the current network could be reallocated to the identified sources, and new monitors could 

be installed and operated at the remaining sources. This alternative could provide a 
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straightforward approach to implementing the SO2 NAAQS and at the same time provide a 

more robust monitoring network across the country. 

2. Population-focused Reallocation and Expansion

A second alternative could resemble the current minimum monitoring requirements 

discussed in the final SO2 NAAQS rule using the population weighted emissions index (PWEI) 

based on population and emissions inventory data at the core-based statistical area (CBSA) 

level. This approach would assign a required number of monitors for a given CBSA based on 

relevant factors such as population and SO2 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 at which to site monitors in these PWEI areas, an SO2

emissions threshold could be used to focus the placement of monitors around the higher SO2 

emissions in the PWEI areas. For example, a threshold of 750 tpy would provide for monitoring

of approximately 400 sources in those PWEI areas responsible for 56% of the SO2 emissions. In 

addition, to monitor air quality near the largest sources of SO2 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 of the PWEI areas. For example, 

deploying monitors near sources with emissions over 5,000 tpy would approximately address 

an additional 170 sources outside of the PWEI areas and 34% of SO2 emissions. This approach 

would allow states to focus their resources in areas with the highest risk to public health and 

with the largest sources of emissions. Under this example, the resulting minimum network 

would consist of over 550 sites nationwide and would provide monitoring of sources 

responsible for 90% of the total national emissions.

3. Other Alternatives and Considerations

In addition to the alternatives presented above, there are other threshold alternatives 

EPA could consider. EPA could select thresholds based on annual emissions at various levels 

(see Appendix). EPA could also consider selection of thresholds 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 a source’s SO2 emissions, but also 

the source’s characteristics (e.g., stack height, source configuration, etc.). Another alternative 

may be a tiered approach, where states would monitor sources with emissions above a high 

threshold and then have a range of options for assessing air quality at sources with emissions in 

a given range below that threshold, or vice-versa. 

 2 The PWEI for a particular CBSA was proposed to be calculated by multiplying the population (using the latest 

Census Bureau estimates) of a CBSA by the total amount of SO2 emissions in that CBSA. The CBSA SO2 emission 

value would be in tons per year, and calculated by aggregating the county level emissions for each county in a 

CBSA. We would then divide the resulting product of CBSA population and CBSA SO2 emissions by 1,000,000 to 

provide a PWEI value, the units of which would be millions of people-tons per year.
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For any source-focused monitoring alternative, the question arises as to how many 

monitors are needed to characterize the air quality around an SO2 source. EPA is considering 

establishing approaches that would allow sources to be characterized by just a few monitors, 

or, if appropriate, a single properly-sited monitor. Under such approaches, modeling (or other 

analyses – e.g., examination of wind roses, identification of sensitive receptor locations, 

assessment of nearby terrain features, and estimation of distance of maximum impacts) would 

likely need to be completed to site these monitors at the areas of highest concentration. This 

modeling might be less resource intensive than attainment demonstration modeling, but would 

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 authority to 

require additional monitoring in certain circumstances, such as in areas impacted by major 

industrial point sources or a combination of sources that are not required to monitor under the 

other monitoring provisions. Also, attention should be given to the need to collect site-specific 

meteorological data, as well as ambient SO2 data, in order to support data analyses (e.g., 

generation of pollution roses) and, if necessary, dispersion modeling.

Pursuing any of these approaches could require EPA to make revisions to the SO2

monitoring regulations to address revised monitoring requirements, including any new 

requirements based on appropriate emissions thresholds.

4. Key Questions

In order for EPA to determine the feasibility of using monitors principally to determine 

attainment or nonattainment of the standard, stakeholder input is needed on the following key 

questions:

a. Are the conceptual monitoring networks described above sufficient to determine 

whether ambient SO2 levels meet the NAAQS and are protective of public health 

without the need for additional modeling? If not, then what enhancements 

should be made to them? In what situations should meteorological data 

collection also be required?

b. What is an appropriate number of 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 identify the 

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 SO2 monitoring network? Would 

stakeholders be willing to conduct monitoring at new locations, or provide 

funding to assist states in conducting such monitoring? If so, what type of 

agreement would be needed between states and stakeholders to ensure the 

monitoring would be done?

f. For potential stakeholder operated monitors, what kind of oversight would the 

states need to perform? Would EPA perform additional oversight? Would 

someone audit these facility monitoring programs and associated monitors? 

What type of agreement would be needed between the states and stakeholders 

to insure the monitoring was carried out? How can we best ensure that these 

data are made public (e.g., require submittal to AQS)?

B. Modeling Options

The monitoring alternatives discussed above would identify a set of priority sources for 

monitoring. In recognition of the limitations of monitoring to identify high concentrations in 

the vicinity of SO2 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 monitoring where appropriate. States may prefer to model instead of monitor at sources 

where modeling is technically straightforward (e.g., consistent with the design of the model) 

and if they 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. 

Under the modeling alternative, states could model the 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 that would 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 provide this assurance by (1) setting enforceable emission limits for 

the sources equal to their actual emissions such that future levels would be no worse than 

current actuals, (2) periodically modeling levels of actual emissions to continue to ensure they 

are not violating the NAAQS, and/or (3) periodically reporting on whether the sources’ actual 

emissions have increased relative to the prior modeling assessment, and in cases where the 

emissions increase has created the potential for violation of the NAAQS, completing additional 

modeling to determine whether the higher emissions level resulted in a modeled violation in 
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the area or in an affected area. Other technical modeling issues which need to be addressed 

include receptor placement, the spatial extent of the modeling, which other nearby sources 

need to be modeled, estimating background concentrations, and use of site-specific versus 

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 by dispersion techniques, such as stack heights greater 

than those considered within the limits of good 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 further emissions limits and a new demonstration of future 

attainment. Any thresholds set forth in a potentially revised monitoring rule could also apply to 

modeling requirements. 

For states that have unmonitored areas with no SO2 sources, Appendix C in the 

September 2011 draft guidance discusses an example of a possible non-modeling option that 

could allow them to demonstrate attainment without needing to complete modeling. This, or 

variants on it, could also be considered under this option.

EPA recognizes that some of the issues discussed here may also be relevant to 

attainment demonstration modeling for nonattainment SIPs and may come up in stakeholder 

discussions and be relevant for consideration in future attainment planning guidance. 

1. Key Questions

In order for EPA to evaluate the feasibility of using modeling in conjunction with 

monitoring to determine either attainment or nonattainment of the standard, stakeholder 

input is needed on the following key questions:

a. Should some criteria (e.g., the PWEI concept) be used to identify priority sources 

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 of size cut-offs and use of actual 

emissions?

c. Are there situations where modeling is preferable to monitoring? If so, then 

what are these situations? Should EPA require modeling in certain situations, or 

is monitoring alone always a sufficient option for areas of concern? 

d. Are there situations where monitoring is preferable to modeling? If so, 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 willing to conduct, or provide funding to assist states in 

conducting, any new modeling? If so, what type of agreement would be needed 

between states and stakeholders to insure modeling would be done?

C. Implementation Options

Regardless of the attainment determination approach that is ultimately used in a given 

context, EPA intends to move forward with proposing nonattainment area designations focused 

on areas with sufficient ambient air quality data. As noted above, this will trigger planning 

requirements, the implementation of which is important, but is not the primary focus of this 

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 

initial step 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 or approval process, followed by the deployment or redeployment of 

monitors and/or the conducting of modeling to determine which areas are attaining and which 

are not. The timing of these actions is an important discussion topic, and involves consideration 

of the practical realities of enhancing the ambient monitoring network (such as cost, timing, 

etc.) and conducting necessary modeling, as well as the importance of being able to use the 

new approach to determine as expeditiously as practicable whether there are NAAQS violations 

occurring. Appropriate timing for these actions will need to be discussed in the context of the 

approach that EPA recommends (e.g., how many monitors, schedule for deployment, data 

collections, etc.).

Under a monitoring-focused alternative, an area designated unclassifiable might either 

have no monitor, or it may have a monitor or monitors that do not show violations of the 

NAAQS at the time of designation. Following designation, states with unclassifiable areas might 

be required by revised rules to site new monitors or relocate existing monitors. EPA could also 

potentially direct states (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 of the monitors. Any data 

ultimately collected by the comprehensive monitoring network could be used, as appropriate, 

for area redesignations or for any necessary future SIP calls.

EPA recognizes that any approach that would depend on the siting of additional 

monitors could impose additional resource burdens on states in order to install, maintain, and 

operate those devices. We acknowledged this issue in the preamble to the final SO2 NAAQS, in 

which we indicated that EPA and states may not have adequate resources to site sufficient 

monitors to properly characterize air quality around major SO2 sources. Therefore, an 

alternative that would rely upon expanding the SO2 monitoring network deserves further 

discussion with stakeholders. In particular, EPA would like to consider potential means of 

alleviating some of the cost burden on states, for example by having sources cover capital costs 
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and/or operating 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 work jointly with the appropriate state or local agency to install 

and operate these monitors. The source and the state would need to work together 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 but for 

which monitors were not deployed. Some consideration would need to be given to when 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 of greatest public health risk. There 

may also be cost burdens on the states associated with data collection, such as more 

representative meteorological data. Considering the potential importance of a modeling option 

in some locations and continuing state budget concerns, states might reasonably expect 

sources to work jointly with the appropriate state or local agency to both collect the necessary 

data, but also perform some or all of the modeling analysis. 

Whatever approach is developed (e.g., monitoring-focused, modeling-focused, or 

allowing states flexibility to determine whether to use monitoring and/or modeling), there are 

practical considerations about how quickly monitors can be deployed and modeling can be 

completed. Depending on the significance of the changes being made, a phased-in approach

may well be warranted. This phased approach could require the first monitors to be sited (or 

the first modeling to be conducted for sources) in areas with the highest population, highest 

emissions, or some other determinant of greatest public health risk.

Finally, EPA will need to decide on requirements regarding what SIP revisions will be 

needed to carry out the new approach. 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 

identify any new NAAQS violations in unclassifiable areas and redesignate those areas to 

nonattainment, or do a SIP call for those areas. This approach is straightforward and most 

closely resembles our approach to implementing other NAAQS. Nevertheless, some 

stakeholders suggested that we should explore approaches that do not involve additional 

nonattainment designations. For example, EPA could allow states to remedy violations in 

unclassifiable areas by demonstrating attainment before a redesignation occurs. Instead, EPA 

could build in affirmative requirements for unclassifiable areas to discover and correct 

violations, similar to the original approach we previously recommended (but did not require) 

for demonstrating attainment under CAA section 110(a)(1). Alternatively, EPA could try to 

integrate such unclassifiable requirements into permitting actions (e.g., title V renewals) 

already happening in these areas. 
DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION PRE-DECISIONAL

14

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 monitor or moving a monitor from an existing location? 

(What can be done to initiate monitoring as 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 for these redesignations? Is the 

timing of the next SO2 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 of concern 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, if any, 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
DRAFT FOR DISCUSSION NOT FINAL AGENCY ACTION PRE-DECISIONAL

15

APPENDIX A

SO2 Emissions

Number of Sources Total Emissions Percent

All SO2 Sources in 2008 NEI 

emitting 1 ton or more

7500 9,448,382 100%

SO2 sources > 100 tons 1684 9,374,962 99%

SO2 sources > 250 tons 1268 9,307,657 98%

SO2 sources > 1000 tons 779 9,042,377 96%

SO2 sources > 2000 tons 585 8,767,914 93%

SO2 sources > 2880 tons 479 8,508,328 90%

SO2 sources > 5000 tons 352 8,029,246 85%

 [Source: http://www.epa.gov/ttn/chief/net/2008inventory.html]

SO2 Ambient Monitoring Network

Current number of monitors nationally: 441

Number of monitors in 1980: 15003

Current minimum monitoring requirements: 129 monitors required in 104 CBSAs4

For sake of comparison, the following table provides the current number of ambient monitors 

nationally for each criteria pollutant:

Pollutant

Number of Monitored Sites

(as of end of 2011)5

Carbon Monoxide 330

Lead 198 (for TSP in local conditions)

Nitrogen Dioxide 397

Ozone 1291

PM2.5 868

PM10 684

SO2 441

SO2 Ambient Monitoring Data

Based on ambient monitoring data from 2008-2010, there were about 70 monitors located in 

60 areas with 1-hour SO2 concentrations exceeding the level of the standard.

 3 Source: http://www.epa.gov/ttn/ecas/regdata/RIAs/pSO2ch2_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 note these are sites, and not necessarily 

individual monitors.