February 6, 2013 Page 1

Next Steps for Area Designations and Implementation of the Sulfur Dioxide National 

Ambient Air Quality Standard

   

This paper describes the EPA’s updated strategy for completing initial area designations under

the June 2010 1‐hour primary sulfur dioxide (SO2) NAAQS.  Note that this strategy anticipates further

rulemaking and development of guidance by EPA, and may be further refined in response to additional

feedback from stakeholders as those products are developed.  This paper does not reflect final agency

action of any kind, and does not impose any legally binding or enforceable requirements.  The strategy

reflects a consideration of several factors, including:

 input received from air agencies1 and other stakeholders;

 consideration of the approach followed for other NAAQS, which is based on identifying NAAQS

violations in a manner consistent with the scope of the ambient monitoring network for the

relevant pollutant;

 recognition of the use of modeling for SO2 designations in the past (and the importance of using

modeling in the PSD program);  

 a focus on priority sources based on the magnitude of source emissions and populations in

proximity of emissions, with recognition of resource limitations;

 sufficient time for air agencies to develop the appropriate data to characterize air quality, and

 respect for the responsibilities of air agencies and the federal government established in the

designation process under the CAA.

This strategy addresses areas that are not currently associated with a violating monitor and is a

complement to our separate effort to designate those areas with violating monitors in June 2013.

Updated Strategy

EPA has developed an updated SO2 designations strategy regarding which sources and areas

would be addressed, how air quality would be characterized for those sources and areas, and when key

steps would be taken in the overall process. This strategy contains the following key advantages:

 Complements initial designations that will be made in June 2013 for areas with violating

monitors by identifying approaches that would be adopted in new regulatory provisions for

characterizing air quality in additional areas;

 Provides an expeditious but workable timeframe for designating additional areas once new

regulatory provisions are in place, recognizing the additional time needed for air agencies to

monitor air quality near key sources (or alternatively to characterize air quality through air

quality modeling);  

 Provides flexibility for air agencies to determine the most appropriate and effective approach

for characterizing air quality in their jurisdictions – through monitoring, modeling, or a mix of

both;

 Provides an orderly process for completing designations that maintains the respective roles of

air agencies and EPA; and

                                                           

1 The use of the term “air agencies” in this document is intended to include state air agencies, as well as local and

tribal air agencies that implement the SO2 NAAQS.
February 6, 2013 Page 2

 Provides incentives and time for air agencies and sources to reduce emissions early and

potentially avoid nonattainment designation in certain areas, improving protection of public

health sooner than would be otherwise required.  This would occur if air agencies and sources

take action to limit emissions (e.g., to comply with the Mercury and Air Toxics Standards (MATS)

or other requirements) and demonstrate attainment with the 1‐hour SO2 NAAQS before future

designation milestone dates pass.   

Important features of this strategy are described in more detail below.

Focus on Monitoring, with Flexibility for Modeling

EPA believes the starting point for future SO2 designations should be, as with other NAAQS, a

monitoring network to adequately characterize air quality in areas of concern. This strategy for SO2

seeks to allow regulators to characterize air quality for the SO2 NAAQS in a manner consistent with the

general monitoring network coverage of other NAAQS. As with all implementing programs to attain any

NAAQS, there are practical resource constraints to consider when deploying an adequate ambient air

quality monitoring network.  Several air agency and industry representatives have suggested that area

designations should be based on ambient monitoring only.  However, the current monitoring network

provides relatively limited geographic coverage, and many monitors in the existing network are not sited

with the objective of characterizing source‐oriented maximum concentrations.  Air agencies may be able

to move monitors to other locations within their own boundaries, but feedback from air agency

representatives indicates that these cases would be very limited.  Regarding new monitor locations,

preliminary estimates indicate that a new SO2 monitoring site can cost an air agency anywhere from

$50,000 to $100,000 in capital costs.  For example, an additional 200 new, source oriented SO2

monitoring sites would cost $10‐20 million.  Some air agencies have indicated that certain sources have

funded monitoring costs in the past, and they may look to sources to help fund SO2monitoring in the

future.  However, it appears that a strategy devoted exclusively to characterizing air quality through

monitoring may not be viable in a number of jurisdictions that lack such funding.

Therefore, while focusing on monitoring as a starting point, EPA recognizes that there are

factors and circumstances unique to each air agency that will influence the number of new monitors the

air agency may be able to deploy.  For this reason, as initially indicated in the preamble to the final rule

for the 2010 SO2 NAAQS2

, and consistent with past practice for SO2, this updated strategy would

maintain air agencies’ flexibility to use modeling to characterize “actual” air quality around a source or

source region as a surrogate for ambient monitoring.  In the 1970’s and 1980’s, under certain

circumstances air quality modeling data were used to characterize SO2 concentrations for the area

designations process. Although modeling also poses resource challenges for air agencies, it can allow for

characterization of air quality around sources where monitoring is impractical.   

Based on the considerations above, EPA believes that a dual‐pathway approach would be most

appropriate.  This approach allows flexibility to use either monitoring or modeling for the

characterization of current SO2 concentrations. To facilitate the dual‐pathway approach, EPA would

issue updated rules and guidance to recommend both an acceptable SO2 monitoring network for a

source or source region, and acceptable SO2 modeling for designation purposes.  The modeling

guidelines would make clear that for designation purposes, actual emissions from recent years could be

                                                           

2 75 FR 35520, June 22, 2010.
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used in the modeling analyses to characterize current air quality, as this more closely matches the actual

air quality that would be characterized by a monitoring network.   

“Thresholds” for Sources Subject to the Program

In the May‐June 2012 stakeholder meetings, EPA presented information indicating that there

are 20,000+ SO2 sources nationally, but that based on 2008 emissions data, a much smaller number ‐ 

about 480 sources with actual emissions exceeding 2800 tons per year ‐ account for 90% of national SO2

emissions.   A number of stakeholders commented that, given constraints on resources for

characterizing air quality through either monitoring or modeling, focusing on the largest sources of

emissions (e.g., those included by the “90%” threshold) is a reasonable principle for prioritizing which

sources should be evaluated for purposes of assessing attainment of the 1‐hour SO2 NAAQS.  Using more

recent 2011 emissions data for electric generating units and 2008 data for non‐EGUs, we now estimate

that about 540 sources, each emitting over 1900 tons of SO2 per year, account for 90% of national SO2

emissions.     

   

One important monitoring objective for a NAAQS with localized impacts (such as SO2 or lead) is

to characterize air quality near the largest emitters of the pollutant.  Another important NAAQS

monitoring objective – and one that some stakeholders suggested that EPA should give more weight ‐‐ is

to characterize air quality in populated areas, often accomplished through the use of population

thresholds in urbanized areas [e.g., Core Based Statistical Areas (CBSAs)] to help assess broad population

exposures.  Monitoring requirements for several other NAAQS (e.g., 8‐hour ozone, PM2.5, CO, NO2)

include provisions to monitor air quality in urban areas above specific population levels.  Consistent with

these two basic objectives that are considered in ambient monitoring network design, EPA suggests

proposing the use of a “two‐pronged” emissions threshold for this SO2 designation approach, to apply

whether a state elects to use all monitoring or all modeling or a combination of both approaches.

For example, under such an approach, a lower threshold (e.g., 2000‐3000 tons per year) would

apply to sources located in more densely populated areas (e.g., CBSAs having 1,000,000 or more

persons); and a higher threshold (e.g., 5000‐10,000 tons per year) would apply to sources located in less

densely populated areas outside of such CBSAs.  To illustrate potential coverage of possible options, a

two‐pronged threshold including 3000+ ton sources located in CBSA’s with a population of 1,000,000,

and 10,000+ ton sources outside of these CBSA’s, would cover 202 sources and 66% of national

emissions.  A two‐pronged threshold including 2000+ ton sources located in CBSA’s with a population of

1,000,000, and 5000+ ton sources outside of these CBSA’s, would cover 341 sources and 81% of national

emissions.  Potential threshold options would be presented in a future rulemaking.  (Note that source

and emissions estimates will change to some degree with final 2011 national emission inventory data,

which will be available later this year.)   

In a future rulemaking, factors to consider in selecting appropriate thresholds could include the

comprehensiveness of the total emissions represented; the comparability of source coverage under this

approach with typical source coverage of an ambient monitoring network; emission levels for sources in

areas with monitored violations; and emission levels associated with “well‐controlled” sources.  Upon

analysis of such factors, EPA would expect to propose a range of threshold options for a minimum level

of coverage (preliminary estimates suggest that this range could cover sources accounting for 66% to

90% of national SO2 emissions).  In addition, the basis for the emissions that would be compared to the
February 6, 2013 Page 4

threshold (e.g., highest of the most recent three years of data) would need to be defined in the

rulemaking.  

Implementation Timeline  

The EPA recognizes the need to begin implementation of the updated strategy now to avoid

unwanted delay in addressing SO2 sources in unmonitored areas.  (Note that EPA will designate a

number of nonattainment areas in the next few months based on air quality monitoring data showing

violations of the NAAQS, and air agencies will be developing attainment plans for these areas in parallel

with addressing the remaining areas discussed in this strategy).  While we intend to act quickly on the

remaining areas, EPA believes, and many stakeholders strongly agree, that some aspects of such a

strategy should be adopted through notice‐and‐comment rulemaking.   

Thus, EPA would begin implementing this strategy by developing a “data requirements”

rulemaking directing air agencies to characterize air quality by a date certain for areas with sources

covered by the relevant thresholds, in support of new designations recommendations.  The rulemaking

would also set forth other relevant milestones for implementing the SO2 designations strategy in these

areas, including deadlines for air agencies to recommend nonattainment area boundaries based on the

characterization of air quality.  To characterize the source areas covered by the rule, the air agency

would have the flexibility to choose those sources for which it would install new monitors, and those for

which it would conduct modeling to characterize air quality.  Monitoring would take longer than

modeling because the air agency would need time to site and deploy the new monitors and collect three

years of data.  Modeling analyses can be done much sooner.   

Accordingly, the strategy envisions a future round of designations based first on modeling

information, and a later round based on new monitoring information. The rulemaking would set forth a

process for air agencies to identify which covered sources or source regions will be monitored and which

will be modeled. Air agencies could choose to characterize air quality for additional sources as well.  The

final designation date in turn will establish the due date for submittal of attainment plans for

nonattainment areas designated based on the new data.

Alternatively, air agencies also would have the incentive to work with sources in these areas to

avoid a nonattainment designation by establishing and submitting to EPA enforceable emission

limitations ensuring that attainment with the SO2 NAAQS (in the form of permit limits, source‐specific

SIP revisions, or other permanent and enforceable legal documents) occurs prior to the date that final

designations based on modeling information are issued. Note that in areas with multiple nearby sources

contributing to the potential nonattainment problem, the air agencies would need to coordinate with all

of the sources in the area to ensure the timely adoption and implementation of such enforceable

emission limitations and controls in order for the area to avoid a potential nonattainment designation.

Notably, as the timeline below indicates, we expect the modeling‐based designations to be completed in

2017, which is after the current MATS compliance deadline of April 2015, or April 2016 (if a source

requests and is granted a 1‐year extension to install controls). Therefore, based on the anticipated

timeline presented below, this approach would allow air agencies to take into consideration emission

reduction measures that will be implemented to comply with that rule.  
February 6, 2013 Page 5

   After considering stakeholder input, the EPA believes that a workable implementation timeline3

for carrying out this updated strategy would involve the following milestones:

 4/2013:  EPA issues draft technical assistance documents for air quality modeling and

monitoring for public review

 7/2013:  EPA issues final technical assistance documents for modeling and monitoring  

 Late 2013: EPA proposes data requirements rule for public comment

 Late 2014: EPA finalizes data requirements rule with appropriate revisions based on public

comments

 2015:  Air agency, in consultation with sources and EPA, identifies those sources and areas

that will deploy new monitoring, and others that instead will be subject to modeling to

characterize air quality

 1/2016: Air agency identifies which sources will deploy new monitoring; air agency also

submits modeling protocol for sources to be modeled

 6/2016: Air agency submits updated monitoring plan  

 1/2017: Air agency has new monitors deployed and operational.  Air agency submits

modeling analyses for selected sources and nonattainment area boundary

recommendations as appropriate.  Alternatively, air agency submits enforceable emission

limitations and modeling showing attainment for areas to avoid nonattainment designation.

 8/2017:  EPA issues 120‐day letters as part of designation process for newly modeled areas

 12/2017: EPA issues final designations for modeled areas  

 8/2019:  SIP attainment demonstrations are due for “modeled” areas that were designated

nonattainment in 12/2017

 5/2020: Air agencies certify 2019 monitoring data and submit designation recommendations

based on monitoring data

 8/2020:  EPA issues 120‐day letters as part of designation process for areas with new

monitoring data

 12/2020:  EPA issues final designations for rest of country, including for new monitored

areas

 8/2022: SIP attainment demonstrations are due for areas designated as nonattainment in

12/2020 based on monitoring data

Background

Designed to protect sensitive individuals from respiratory effects associated with short‐term

exposures to SO2, the SO2 standard was established with a 1‐hour averaging time.4

  The reaction of SO2

with other pollutants in the atmosphere and the contribution of SO2 to regional air pollution problems

such as fine particle formation and acidic deposition are well‐understood.  However, the highest

ambient concentrations of gaseous SO2 emissions generally occur relatively close to one or a few key

                                                           

3 Reminder: this timeline does not include SIP and attainment dates for the areas with current monitored

violations that will be designated in June 2013.  Also, this timeline does not address what action EPA will take if,

based on three full years of data, an existing monitor shows a new violation of the 1‐hour NAAQS.  Consistent with

this updated strategy, in those cases, EPA will work with the affected air agencies to designate the area as

nonattainment or the air agency can avoid a nonattainment designation by establishing and submitting to EPA

enforceable emission limitations ensuring attainment, as discussed above.   4 The design value for a monitor is violating the SO2 primary standard if the average of the 99th percentile of daily

maximum 1‐hour values for three consecutive years exceeds 75 parts per billion.   
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SO2 sources in an area, often within 10‐20 kilometers of that parent source or sources.  Thus, from an air

quality management perspective, the SO2 NAAQS is considered to be a “source‐oriented” NAAQS rather

than a “regional” one (i.e., more similar to the lead NAAQS than the ozone NAAQS).  Strategies to attain

the SO2 NAAQS are expected to be focused on key point sources.  The largest sources of SO2 include

coal‐fired electric utilities and industrial boilers, refineries, pulp and paper related industries, and

chemical manufacturing.

The traditional NAAQS implementation process begins with the area designations process

described in section 107 of the Clean Air Act (CAA), which generally relies on air quality concentrations

to be characterized by ambient monitoring data collected by the air agency to identify areas that are

exceeding the relevant standard.  Typically, the air agency provides EPA with area recommendations and

supporting technical information; EPA considers this information and issues a letter to the air agency at

least 120 days prior to finalizing the designation which describes its intended designation and

boundaries; and the air agency in some cases provides EPA with additional comments or suggested

modifications during the 120‐day period.  (As noted above, air quality modeling data has been used to

characterize SO2 concentrations for the area designations process in some cases, and was later used as

the basis for SIP Calls.)

The preamble to the final SO2 NAAQS noted that although the current SO2 ambient monitoring

network included 400+ monitors nationwide, the scope of the network had certain limitations, and

approximately two‐thirds of the monitors are not located to characterize maximum concentration

source‐oriented impacts. It was observed that some areas without monitoring likely have concentrations

violating the NAAQS.  To address these potential public health impacts, the SO2 NAAQS preamble and

subsequent draft guidance issued in September 2011 recommended that air agencies submit

substantive attainment demonstration SIPs based on air quality modeling by June 2013 [under Clean Air

Act section 110(a)(1)] that would show how areas expected to be designated unclassifiable and have

sources emitting over 100 tons of SO2 per year would attain and maintain the NAAQS in the future.   

A number of commenters expressed concern with this suggested implementation approach,

particularly with the number of sources to be modeled (more than 1680 sources had emissions

exceeding 100 tons in 2008), and the recommended SIP submission date for areas without monitoring

being before the SIP due date for violating areas with monitoring data.  In response, EPA Assistant

Administrator Gina McCarthy sent letters to state Environmental Commissioners on April 12, 2012

indicating that EPA wanted to further consult with stakeholders regarding how to best implement this

standard and protect public health in an effective manner.  The letters also stated that the Agency

would not expect air agencies to submit attainment demonstrations by June 2013 for areas not

designated as “nonattainment” based on ambient monitoring data.  EPA developed a white paper on

possible implementation approaches and proceeded to convene three stakeholder meetings in May‐

June 2012 with environmental group representatives; state, local, and tribal air agency representatives;

and industry representatives.  On July 27, 2012, EPA also announced that it was extending the deadline

for SO2 NAAQS area designations by an additional year, to June 3, 2013, based on the unavailability of

data.5

                                                           

5 77 FR 46295, August 3, 2012.
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Input from Stakeholders

A number of important comments were provided to EPA in the stakeholder meetings and in

writing, and have informed this updated SO2NAAQS implementation strategy.6

  Key themes included:

 Several air agency and industry representatives supported basing designations on monitoring data

only.   

 Other air agencies, however, cited concerns about the cost of establishing new monitoring sites and

supported flexibility to use monitoring or modeling to characterize air quality for the designations

process.  However, if modeling is used to characterize air quality for designations, then there was

strong sentiment that it should be based on modeling of actual emissions (not allowable emissions).  

 Environmental groups strongly supported the use of modeling to characterize air quality for future

designations.

 All stakeholder groups supported a “threshold” concept to focus implementation on the largest

emissions sources and/or sources located in areas with higher population.

 Air agencies asked for sufficient time to conduct the necessary monitoring or modeling, citing the

large number of sources to be addressed (even with a threshold), limited resources, and the

stringency of the 1‐hour standard.

 Many stakeholders stated that any new modeling or monitoring requirements should be established

through a notice‐and‐comment rulemaking process.

                                                           

6 See EPA’s SO2 implementation website for more information on EPA’s May 2012 white paper on SO2

implementation, summaries of the three stakeholder meetings, and the docket including written comments on the

white paper at:   http://www.epa.gov/oaqps001/sulfurdioxide/implement.html.