Public Review Draft 9/22/2011

 EPA 453/R-XX-XXX 

 Month 2011 

Guidance for 1-Hour SO2 NAAQS SIP 

Submissions 
Public Review Draft 9/22/2011

i 

EPA 453/R-XX-XXX 

Month 2011 

 

Guidance for 1-Hour SO2 NAAQS SIP 

Submissions 

Office of Air Quality Planning and Standards 

U.S. Environmental Protection Agency 

Research Triangle Park, North Carolina 
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TABLE OF CONTENTS 

Preface................................................................. Page iii 

I. Purpose.................................................................................... Page 1 

II. Background ......................................................................... Page 1 

 A. Roles of EPA and state governments 

B. How this guidance applies to tribes 

III. Schedule for implementation of the 1-hour SO2 NAAQS........ Page 6 

IV. Section 110(a)(1) and (2) NAAQS maintenance/infrastructure 

elements................................................................. Page 8 

A. Section 110(a)(1)-(2) SIP submission 

B. Section 110(a)(1) SIPs 

C. Redesignation to attainment for areas designated as unclassifiable 

V. SO2 nonattainment area planning elements ....................... Page 27 

A. SO2 SIP planning 

B. New source review 

C. Conformity 

VI. Transition from the existing SO2 NAAQS to a revised SO2 

NAAQS........................................................................... Page 31 

VII. Redesignation to attainment for areas designated as 

nonattainment............................................................. Page 33 

A. Attainment of the NAAQS 

B. Consequences of a nonattainment area failing to attain 

by the statutory attainment date 

Appendix A 

Modeling Guidance for Nonattainment Areas and Section 110(a)(1) 

Plans.............................................................................. Page A-1 

Appendix B 

Guidance on Section 110(a)(2) Infrastructure SIP Elements ....... Page B-1 

Appendix C 

Guidance on Non-modeling Technical Demonstration of 

Attainment.................................................................... Page C-1 
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Preface 

In the final primary National Ambient Air Quality Standard for Sulfur Dioxide (SO2

NAAQS) rulemaking, published at 75 FR 35520 (June 22, 2010), EPA provided initial guidance 

in the preamble concerning the Clean Air Act (CAA) provisions that states and emissions 

sources would need to address when implementing the new 1-hour SO2 NAAQS. We stated that 

we intended to develop and seek public comment on additional guidance on modeling and the 

development of state implementation plans (SIPs) to be submitted under sections 110 and 191 of 

the CAA. To this end, EPA is issuing the draft guidance contained in this document so that 

states and other interested parties may comment on our preliminary recommendations for how to 

meet the section 110(a)(1) requirement that all areas submit a SIP no later than June 3, 2013, and 

the section 191 requirement that all areas that are designated as nonattainment under the 1-hour 

SO2 NAAQS submit a SIP providing for attainment of that standard. This draft guidance is not 

final agency action, and imposes no binding or enforceable requirements. We intend to re-issue 

this guidance, revised as appropriate, after receiving and considering the public comments that 

we receive on this preliminary draft. 

 In addition to this guidance document, EPA is also planning a rulemaking to address 

some of the 1-hour SO2 NAAQS implementation program elements. These elements include: (1) 

establishing that compliance with the 1-hour SO2 NAAQS is appropriately based on the results 

of both air quality modeling and monitoring; (2) establishing the modeling requirements 

necessary to determine compliance with the 1-hour SO2 NAAQS; (3) establishing the minimum 

scope of analysis required to demonstrate attainment and maintenance of the 1-hour SO2

NAAQS to comply with the SIP requirements in CAA section 110(a)(1); (4) establishing a 

reasonable time period for sources to comply with any new emissions limitations states need to 
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establish in the 110(a)(1) SIPs to demonstrate attainment and maintenance of the 1-hour SO2

NAAQS; (5) to set an attainment date for areas designated as unclassifiable; and (6) establishing 

the criteria for redesignating areas from “unclassifiable” to “attainment.” These elements are 

also addressed in this guidance document. EPA will consider any comments that we receive on 

these elements in this draft guidance as we develop the proposed rulemaking. EPA will re-issue 

the guidance as expeditiously as practicably following the public comment period so that states 

can use the guidance to begin developing their plans as soon as possible. EPA will also propose 

a rulemaking that would codify the hybrid modeling and monitoring implementation approach in 

order to ensure compliance with the 1-hour SO2 NAAQS in a timely manner. 

EPA developed the proposed hybrid modeling and monitoring approach to show 

compliance with the 1-hour SO2 NAAQS due to the unique challenges presented by this 

pollutant. EPA does not believe that this implementation approach is necessarily transferable to 

other NAAQS pollutants. For NAAQS pollutants other than SO2, air quality monitoring is more 

appropriate for determining whether all areas are attaining the NAAQS, and there is 

comparatively less dependence upon conducting refined air quality modeling. EPA recommends 

that states coordinate with their respective EPA Regional Offices during the SIP development 

process, as early and often as possible, in order to ensure that critical issues related to the 

implementation of this approach are resolved as expeditiously as practicable in order for the SIP 

to be submitted and approved in a timely manner. 
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I. Purpose 

This guidance document discusses the CAA statutory requirements that states need to 

address when implementing the new 1-hour SO2 NAAQS based on the structure outlined in the 

CAA and existing rules. It supplements existing guidance documents and regulations, and 

contains additional guidance on attainment modeling (Appendix A), and developing SIPs under 

sections 110(a)(1)-(2) and 191-192 of the CAA.1 In this document, we also describe in more 

detail how and when we expect states to demonstrate attainment, implementation, maintenance, 

and enforcement of the new 1-hour SO2 NAAQS. 

II. Background 

On June 2, 2010, the Administrator signed a notice establishing a new 1-hour primary 

SO2 NAAQS of 75 parts per billion (ppb), which is attained when the 3 year average of the 99th

percentile of 1-hour daily maximum concentrations does not exceed 75 ppb.2 EPA revised the 

primary SO2 NAAQS to provide the requisite protection of public health with an adequate 

margin of safety, as required by CAA section 109(b). 

We explained in the preamble to the final NAAQS rule that we anticipate that most areas 

will initially be designated under CAA section 107(d) as “unclassifiable” for the revised primary 

SO2 NAAQS, meaning that they would not initially be subject to the nonattainment area SIP 

requirements of CAA sections 191-192. This is because we do not expect most states to have 

either monitoring data or appropriate modeling information sufficient to determine whether those 

areas are meeting or not meeting the 1-hour SO2 NAAQS for initial designations 

recommendations due to EPA in June 2011. However, we anticipate that unclassifiable areas 

 

1

 See also SO2 Guideline Document, Office of Air Quality Planning and Standards, Research Triangle Park, NC 

27711, EPA-452/R-94-008, February 1994. 2

 EPA publicly disseminated the signed notice on June 3, 2010. The new 1-hour SO2 NAAQS became effective on 

August 23, 2010. 
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will contain sources of SO2 emissions that could potentially cause or contribute to a violation of 

the primary SO2 NAAQS. In such cases, EPA believes that a post-designation implementation 

approach that relies on robust state implementation plans submitted in accordance with CAA 

sections 110(a)(1)3 and (2)4 is appropriate and necessary to ensure public health protection under 

the 1-hour NAAQS. These SIPs have sometimes been referred to respectively as “maintenance” 

and “infrastructure” SIPs. To be approved, EPA believes such plans should provide for 

attainment and maintenance of the new 1-hour SO2 NAAQS as expeditiously as practicable. In 

the final NAAQS preamble we explained that we believe it is reasonable to expect any necessary 

emissions controls and limits to be in place and effective no later than five years after initial area 

designations (or by approximately August 2017), which is consistent with the timeframe that 

would otherwise apply to any area designated as nonattainment in 2012. 

For the primary 1-hour SO2 NAAQS, we anticipate that initial designations will be 

promulgated by June 2012, two years following promulgation of the NAAQS. In guidance 

issued on the 1-hour SO2 NAAQS designations process we indicated that an area that has 

monitoring data or refined modeling results showing a violation of the NAAQS would be 

designated as “nonattainment”; an area that has an appropriate demonstration showing no 

violations would be designated as “attainment”; and all other areas with sources that have the 

potential to cause or contribute to a violation of the NAAQS, which may include areas with SO2

monitors showing no violations but without refined modeling that shows no violations, would be 

designated as “unclassifiable.” Areas with no SO2 monitors would be designated as 

“unclassifiable” as well, absent any other appropriate data to support a designation of 

“attainment” or “nonattainment.” We anticipate that most areas of the country will be designated 

 

3

 CAA section 110(a)(1) directs states to submit, no later than three years following promulgation or revision of a 

primary NAAQS, SIPs that provides for “implementation, maintenance, and enforcement of” the NAAQS. 4

 CAA section 110(a)(2) lists the programmatic elements that each SIP must contain. 
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as “unclassifiable” due to a lack of both monitoring and refined modeling information, in 

advance of states conducting further refined modeling at the time of designation.5

 

Subpart 5, of Part D, of Title I of the CAA (sections 191 and 192) describes the 

requirements that apply to areas designated nonattainment. A substantial set of longstanding 

guidance has been issued and continues to reflect EPA recommendations for interpreting these 

requirements, most notably in the General Preamble published in the Federal Register on April 

16, 1992 (see, e.g., 57 FR 13498, at 13545). The guidance below addresses additional statutory 

state planning and control requirements that apply regardless of designation, such as 

infrastructure SIP elements. 

A. Roles of EPA and state governments 

The CAA assigns important roles to EPA and state governments to achieve the NAAQS. 

States have the primary responsibility for developing and implementing SIPs that contain 

measures necessary to achieve the air quality standards in each area once EPA has established 

the NAAQS. EPA provides technical tools, assistance, and guidance to states and tribes, 

including information on the potential control measures that may help areas attain the standards. 

Under section 110 of the CAA, and related provisions, states are directed to submit, for EPA 

approval, SIPs that provide for the attainment, implementation, maintenance, and enforcement of 

such standards through control programs directed at sources of SO2 emissions. (CAA sections 

110(a), and 191-192). If a state does not adopt and implement approved SIPs by the time periods 

provided in the CAA, EPA has the responsibility to adopt a federal implementation plan (FIP) to 

ensure that areas attain the NAAQS in an expeditious manner. 

 

 

5

 See guidance memorandum on designations entitled “Area Designations for the 2010 Revised Primary Sulfur 

Dioxide National Ambient Air Quality Standard, March 24, 2011.” 
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 States, in conjunction with EPA, also administer the prevention of significant 

deterioration (PSD) program for SO2. (CAA sections 160-169). The primary feature of this 

program is a requirement that certain stationary sources of air pollutants obtain a permit (“PSD 

permit”) before commencing construction. (CAA section 165(a)(1)). To obtain such a permit, 

one of the things that a permit applicant must show is that the proposed source will not cause or 

contribute to a violation of any NAAQS. (CAA section 165(a)(3); 40 CFR 52.21(k); 40 CFR 

51.166(k)). Most states implement the PSD permitting program under state regulations approved 

by EPA as part of a SIP. 

Federal rules supplement state emissions control measures and provide for nationwide or 

regional reductions in emissions of SO2 and other air pollutants. Under Title II of the CAA there 

are rules that limit the sulfur content of the fuel used by automobiles, trucks, buses, motorcycles, 

non-road engines and equipment, marine vessels and locomotives. Emissions of SO2 are also 

limited by new source performance standards (NSPS) for stationary sources under sections 111 

and 129 of the CAA; and the national emission standards for hazardous air pollutants (NESHAP) 

for stationary sources under section 112 of the CAA (such reductions resulting due to control of 

hazardous air pollutants (HAP) such as hydrogen chloride (HCl) under those rules). Title IV of 

the CAA, sections 402-416, specifically provides for major reductions in SO2 emissions from 

fossil-fuel fired power plants. EPA has also promulgated the Cross State Air Pollution Rule 

(CSAPR) to define additional SO2 emission reductions needed in the eastern United States by 

2012 and 2014 to eliminate significant contribution of upwind states to downwind states’ 

nonattainment, or inability to maintain, the PM2.5 NAAQS pursuant to CAA section 

110(a)(2)(D).6

 States may rely on these federal measures in their SO2 SIPs. 

 

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 The final Cross State Air Pollution Rule (CSAPR) was signed by the EPA Administrator on July 6, 2011, and was 

published at 76 FR 48208, August 8, 2011 
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B. How this guidance applies to tribes 

 CAA section 301(d) authorizes EPA to treat eligible Indian tribes in the same manner as 

states under the CAA and requires EPA to promulgate regulations specifying the provisions of 

the statute for which such treatment is appropriate. EPA has promulgated these regulations – 

known as the Tribal Authority Rule or TAR – at 40 CFR Part 49. (63 FR 7254, February 12, 

1998). The TAR establishes the process for Indian tribes to seek treatment-as-a-state eligibility 

and sets forth the CAA functions for which such treatment will be available. Under the TAR, 

eligible tribes may seek approval for all CAA and regulatory purposes other than a small number 

of functions enumerated at section 49.4. Implementation plans under section 110 are included 

within the scope of CAA functions for which eligible tribes may obtain approval. Section 110(o) 

also specifically describes tribal roles in submitting implementation plans. Eligible Indian tribes 

may thus submit tribal implementation plans (TIPs) covering their reservations and other areas 

under their jurisdiction. 

 The CAA and the TAR do not, however, direct tribes to apply for treatment as a state or 

implement any CAA program. In promulgating the TAR, EPA explicitly determined that it was 

not appropriate to treat tribes similarly to states for purposes of, among other things, specific 

plan submittal and implementation deadlines for NAAQS-related requirements. (40 CFR § 

49.4(a)). In addition, where tribes do seek approval of CAA programs, including section 110 

implementation plans, the TAR provides flexibility and allows them to submit partial program 

elements, so long as such elements are reasonably severable – i.e., “not integrally related to 

program elements that are not included in the plan submittal, and are consistent with applicable 

statutory and regulatory requirements.” (40 CFR § 49.7). 

 
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To date, very few tribes have sought treatment as a state for purposes of section 110 

implementation plans. In the event that a tribe is unable to develop a plan that addresses the 

implementation of the 1-hour SO2 NAAQS, the Administrator, pursuant to the discretionary 

authority explicitly granted to the Administrator under sections 301(a) and 301(d)(4) of the 

CAA, has the authority to promulgate a FIP to protect air quality. Consistent with the provisions 

of sections 304(a) and 301(d)(4), if a tribe does not submit a TIP meeting the completeness 

criteria of 40 CFR part 51, appendix V, or does not receive EPA approval of a submitted TIP, 

EPA has authority to develop a FIP to protect air quality in the affected area. (40 CFR 49.11(a)). 

In addition, upon request from a tribe that has undertaken the responsibility for developing a TIP 

to address the requirements to implement the 1-hour SO2 NAAQS, EPA will provide assistance 

as necessary to complete the development of the plan. 

III. Schedule for Implementation of the 1-hour SO2 NAAQS 

The timeline below shows the expected steps from promulgation of the new NAAQS 

through attainment, assuming timely action at each step and assuming that initial area 

designations are effective in August 2012: 
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Date Action 

June 2010 EPA issues 1-hour SO2 NAAQS. This action starts the 2-year clock for 

CAA section 107 initial area designations and the 3-year clock for CAA 

section 110(a)(1)-(2) SIP submissions. 

August 2010 Effective date of 1-hour SO2 NAAQS. Under the PSD program, EPA and 

states that issue PSD permits under delegated federal authority begin 

requiring a demonstration in the permitting record that the proposed 

source will not cause or contribute to a violation of the 1-hour SO2

NAAQS. States that issue PSD permits under a SIP-approved state 

program should also require such a demonstration for permits issued on 

or after August 23, 2010 to ensure that sources commencing construction 

in such states have permits that conform to the requirements of CAA 

section 165(a)(3). 

June 2011 States submit initial area designations recommendations, based on 

available monitoring data and any refined modeling they choose to 

perform for designations purposes. 

June 2012 EPA issues final initial area designations. Monitored or modeled 

violations would result in a designation of “nonattainment.” Areas would 

be designated “attainment” if they submit appropriate modeling data 

(where needed) showing evidence of no violations and if all available 

monitoring data indicate attainment for the affected area. Generally, all 

other cases would be initially designated “unclassifiable.” 

June 2013 All states submit CAA section 110(a)(1)-(2) SIPs. SIPs address PSD and 

infrastructure requirements, and provide for implementation and 

maintenance of the 1-hr NAAQS in all areas throughout the state, 

particularly in areas initially designated unclassifiable and attainment. 

SIPs for unclassifiable areas would rely on refined modeling and any 

monitoring that demonstrates attainment and maintenance of the new SO2

NAAQS as expeditiously as practicable, which EPA anticipates should 

be no later than the attainment deadline for areas initially designated 

nonattainment. SIPs should contain any additional federally enforceable 

control measures necessary to ensure maintenance of the NAAQS. 

Attainment demonstration SIPs for nonattainment areas have a separate 

deadline. 

February 2014 Assuming designations are effective in August 2012, states submit SIPs 

for nonattainment areas consistent with CAA section 191-192 that 

demonstrate attainment as expeditiously as practicable and no later than 5 

years from the effective date of designations (August 2017). 

June-December 

2014 

EPA approves or disapproves timely submitted (by June 2013) and 

complete CAA section 110(a)(1)-(2) SIPs. EPA’s action would be based 

on adequacy of the states’ modeling demonstrations (and any obtained 

monitoring) showing attainment as expeditiously as practicable and 

continued maintenance of the NAAQS. Demonstrations can take into 

account SO2 reductions from national and regional standards that require 

compliance by the attainment date. EPA would also have discretion to re-
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Date Action 

designate attainment and unclassifiable areas if SIPs are inadequate, as 

well as propose FIPs. 

February-August 

2015 

EPA approves or disapproves timely submitted (by February 2014) and 

complete CAA section 191-192 attainment SIPs for areas initially 

designated nonattainment, with similar remedies as discussed above if 

SIPs are deficient. 

June 2016 CAA section 110(c) deadline by which EPA must issue a final FIP for 

any area whose section 110(a)(1) SIP is disapproved in June 2014. 

February 2017 CAA section 110(c) deadline by which EPA must issue a FIP for a 

nonattainment area whose section 192 SIP is disapproved in February 

2015. 

August 2017 If designations are effective in August 2012, CAA section 192 would 

require nonattainment areas to attain the 1-hour SO2 NAAQS as 

expeditiously as practicable, but no later than August 2017. Depending 

on installation dates and impact of other regulations on SO2, as discussed 

below, many areas will likely attain earlier than August 2017. EPA 

expects unclassifiable areas to attain on a similar schedule as 

nonattainment areas. 

IV. Section 110(a)(1) and (2) NAAQS maintenance/infrastructure elements 

CAA section 110(a)(1) specifies that states are to develop plans to implement, maintain, 

and enforce the new SO2 NAAQS. For areas designated nonattainment, states will follow the 

more common and familiar attainment planning and emissions control requirements of CAA part 

D, section 172 and 191-192. For all other areas, states are expected to follow the more general 

requirements of CAA section 110(a). In the context of the 1-hour SO2 NAAQS, EPA believes 

this means states are expected to develop and submit SIPs showing that all areas of the state will 

attain and maintain the 1-hour SO2 NAAQS. This should be accomplished by evaluating 

whether significant sources of SO2 emissions, either individually or in combination with other 

nearby sources, cause or contribute to violations of the 1-hour SO2 NAAQS. Where sources of 

SO2 are determined to cause or contribute to NAAQS violations, states should include in the 

 
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110(a)(1) SIPs sufficient permanent and enforceable control measures to ensure the NAAQS is 

attained and maintained as expeditiously as practicable. 

For the short-term 1-hour SO2 standard, EPA believes it is more accurate and efficient to 

use modeling to assess medium to larger sources and to rely on monitoring for groups of smaller 

sources and sources that may not be as conducive to modeling. We expect that states would 

focus performance of attainment demonstration modeling on areas with larger sources (e.g., 

those sources emitting over 100 tons per year (tpy) of SO2), and any other sources that we 

anticipate to cause or contribute to a violation to determine compliance with the new SO2 

NAAQS7. 

Almost all SO2 in the United States is emitted by stationary sources. EPA has 

promulgated requirements for low sulfur motor fuels that have substantially reduced on-road 

vehicle contributions to ambient SO2 concentrations, such that on-road contributions may 

generally be addressed merely as part of the unmodeled background concentrations included in 

modeling analyses. Further restrictions will reduce other mobile source emissions (such as from 

ships and locomotives) in the next few years. Thus, only in exceptional cases would modeling of 

mobile source emissions be necessary. 

A. Section 110(a)(1)-(2) SIP submission 

The CAA establishes a 3-year deadline after revision of a NAAQS for states to submit 

SIPs under sections 110(a)(1) and (2) to EPA. Additionally, nonattainment area SIPs are due 

within 18 months from the effective date of the designation, under section 191 of the CAA. EPA 

anticipates that designations should be issued by June 2012 and take effect by August 2012. 

 

 

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 In cases where large sources of SO2 emissions are located on the borders of other states, EPA suggests that these 

states coordinate with each other during the development of the SIP submittals for the affected areas. 
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Therefore, the nonattainment area SIP submission deadline is anticipated to be by no later than 

February 2014. 

Section 192 requires nonattainment areas to achieve the 1-hour SO2 NAAQS as 

expeditiously as practicable, but no later than 5 years after designation, which EPA expects to be 

by no later than August 2017. Section 110(a)(1), unlike section 192, does not specify a deadline 

by which states are required to show when they have met the requirements to implement, 

maintain, and enforce a NAAQS. However, EPA believes it is generally reasonable for 

attainment to be achievable in unclassifiable areas within the same five year timeframe as is 

mandated for nonattainment areas. In many cases, federally promulgated regulations, including 

the CSAPR for the eastern United States, and nationally applicable regulations mandating 

maximum achievable control technology (MACT), may result in attainment sooner than the five 

year timeframe. These regulations are expected to reduce emissions from certain source 

categories emitting large amounts of SO2, such as Electric Generating Units (EGUs), industrial 

boilers, and solid waste incinerators. 

In order to demonstrate attainment and maintenance, SIPs for unclassifiable areas, just as 

for nonattainment areas, should include enforceable emissions limitations, timetables for 

compliance, appropriate testing/reporting to assure compliance, and include air quality modeling 

for SO2 sources showing that the SIP-adopted emissions limits are consistent with attainment of 

the 1-hour NAAQS. EPA believes it is reasonable to expect states to demonstrate, at a 

minimum, that major SO2 sources (≥ 100 tpy) are not causing or contributing to violations of the 

1-hour SO2 NAAQS. 

Several recently promulgated and forthcoming national and regional rules will likely 

result in significant reductions in SO2 emissions over the next several years. A limited 
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qualitative assessment based on the results of preliminary modeling of sample facilities indicates 

that effective SO2 source control, such as the installation of SO2 scrubbers,should generally 

suffice for areas to meet the 1-hour SO2 NAAQS (see Brode 2010b). Exceptions could include 

unique sources with specific characteristics that contribute to higher ambient impacts (short stack 

heights, complex terrain, etc.). These national and regional rules are anticipated to lead to SO2

reductions that will help achieve compliance with the new SO2 NAAQS prior to August 2017. 

If, upon EPA review of SIP submittals that rely upon those reductions or other local control 

measures, it appears that states will nevertheless fail to attain or maintain the NAAQS, the CAA 

provides authorities for EPA to address these situations, including, as appropriate, disapproving 

SIP submittals, re-designating areas to nonattainment, issuing SIP calls, and promulgating FIPs. 

EPA will make decisions concerning what actions should be taken on SIP submittals on a caseby-case basis. EPA will also continue to work with states during the SIP development process in 

order to address SIP deficiencies and to ensure that SIPs are approved in a timely manner. 

EPA expects that most areas of the country will be designated as unclassifiable for the 1-

hour SO2 NAAQS, due to a lack of both monitoring and refined modeling data concerning the 

attainment status of areas, in advance of states conducting further refined modeling at the time of 

designations. Areas that have both monitoring data and appropriate modeling results showing no 

violations would be designated as “attainment.” For areas designated as attainment, we expect 

that section 110(a)(1) SIPs only need to ensure any applicable SO2 emissions limits are 

permanent and enforceable, and address the “infrastructure” requirements set forth in section 

110(a)(2). In such areas EPA expects that the PSD program required by section 110(a)(2)(C) 

will regulate future growth and should provide for continued maintenance of the standard. 

 
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B. Section 110(a)(1) SIPs 

Section 110(a)(1) of the CAA directs each state, after reasonable notice and opportunity 

for public comment, to adopt and submit to EPA, within 3 years after promulgation, a SIP which 

provides for the implementation, maintenance, and enforcement of any new or revised NAAQS 

throughout the state. These plans should ensure timely attainment of the SO2 NAAQS on a 

schedule that is as expeditious as would be required if an area had been designated 

nonattainment (i.e., no later than five years). The section 110(a)(1) SIP should also contain the 

following elements: (1) an attainment emissions inventory; (2) a control strategy, as appropriate; 

(3) a maintenance demonstration using an EPA-approved air quality dispersion model, as 

appropriate; (4) a contingency plan; and (5) a plan for verification of continued attainment of the 

standard. These elements are discussed in detail below. 

1. Attainment emissions inventory 

The state should develop an accurate attainment inventory to identify the level of 

emissions in the area sufficient to attain the 1-hour SO2 NAAQS. This inventory should be 

consistent with EPA’s most recent guidance on emissions inventories currently available, and 

should include the emissions for the time period associated with the modeling and monitoring 

data showing attainment. Major source size thresholds for SO2 are currently listed as 100 tpy. 

However, in cases where sources, individually or collectively, that are below this level may 

potentially cause or contribute to a violation of the standard, these sources should also be 

included in the emissions inventory for the affected area. Future SO2 emissions projections in 

the attainment emissions inventory can account for modeled controlled maximum allowable 

emissions levels associated with a sources’ compliance with enforceable national, regional, or 

local rules that will occur by the SO2 NAAQS compliance date. 
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2. Maintenance demonstration 

In general, EPA does not believe that air quality monitoring alone is sufficient to 

demonstrate attainment and maintenance with the 1-hour SO2 NAAQS. EPA does not generally 

expect that monitoring data alone will fully characterize short-term ambient concentrations 

around major stationary sources of SO2, and as a result may not capture the maximum SO2

impacts. For SO2, the state could generally demonstrate attainment and maintenance of the 

NAAQS by using refined dispersion modeling (see Appendix W to 40 CFR Part 51) to show that 

the current or future mix of sources and enforceable emission rates in an identified area will not 

lead to a violation of the 1-hour SO2 NAAQS. 

In counties that do not have large sources, or any sources, of SO2, refined modeling 

specific to those counties may not be needed to show that the county or portions of the county 

are in attainment of the NAAQS. Appendix C contains guidance for states on making a nonmodeling technical attainment demonstration for areas meeting these criteria. 

a. Refined dispersion modeling 

With representative and appropriate meteorological and other input data, refined 

dispersion models are able to characterize air quality impacts from the modeled SO2 sources 

across the domain of interest on an hourly basis with a high degree of spatial resolution, 

overcoming the limitations of an approach based solely on monitoring. By simulating plume 

dispersion on an hourly basis across a grid of receptor locations, dispersion models are able to 

estimate the detailed spatial gradients of ambient concentrations resulting from SO2 emission 

sources across a full range of meteorological and source operating conditions. To capture such 

results on a monitor would normally require a prohibitively expensive air quality monitoring 

network. Further, as we have observed in prior actions (see., e.g., 43 FR 45993, 45997, 46000-
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03 (October 5, 1978)), monitoring data would not be adequate to demonstrate attainment if 

sources are using stacks with heights that are greater than good engineering practice (GEP), or 

other prohibited dispersion techniques, since in those cases monitoring would implicitly reflect 

credit for such practices in contravention of section 123 of the CAA and our regulations at 40 

CFR sections 51.100 and 51.118. 

Refined dispersion modeling for the section 110(a)(1) SIP should follow EPA’s 

Guideline on Air Quality Models, Appendix W to 40 CFR Part 51, which provides 

recommendations on modeling techniques and guidance for estimating pollutant concentrations 

in order to assess control strategies and determine emission limits. These recommendations were 

originally published in April 1978 and were incorporated by reference in the PSD regulations at 

40 CFR sections 51.166 and 52.21 in June 1978 (43 FR 26382-26388). The purpose of 

Appendix W is to promote consistency in the use of modeling within the air quality management 

process. Appendix W is periodically revised to ensure that new model developments or 

expanded regulatory requirements are incorporated. The most recent revision to Appendix W 

was published on November 9, 2005 (70 FR 68218), wherein EPA adopted the American 

Meteorological Society (AMS) and Environmental Protection Agency (EPA) Regulatory Model

(AERMOD) as the preferred dispersion model for a wide range of regulatory applications in all 

types of terrain. To support the promulgation of AERMOD as the preferred model, EPA 

evaluated the performance of the model across a total of 17 field study data bases (Perry, et al., 

2005; EPA, 2003), including several field studies based on model-to-monitor comparisons of 

SO2 concentrations from operating power plants. AERMOD is a steady-state plume dispersion 

model that employs hourly sequential preprocessed meteorological data to simulate transport and 

dispersion from multiple point, area, or volume sources for averaging times from one hour to 
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15 

multiple years, based on an advanced characterization of the atmospheric boundary layer. 

AERMOD also accounts for building wake effects (i.e., downwash) on plume dispersion. 

Appendix A contains modeling guidance supplemental to that provided in the preamble 

to the final rulemaking promulgating the 1-hour SO2 NAAQS. It clarifies our recommendations 

on how to conduct refined dispersion modeling under Appendix W to support the 

implementation of the new 1-hour SO2 NAAQS. Although AERMOD is identified as the 

preferred model under Appendix W for a wide range of applications and would be appropriate 

for most modeling applications to support the 1-hour SO2 NAAQS, Appendix W allows 

flexibility to consider the use of alternative models on a case-by-case basis when an adequate 

demonstration can be made that the alternative model performs better than, or is more 

appropriate than, the preferred model for a particular application. 

b. Source emissions threshold levels for the maintenance demonstrations 

 In most cases, we expect that the clearest way to determine whether an SO2 source has 

the potential to emit (PTE) at levels that could cause or contribute to a violation of the 1-hour 

SO2 NAAQS would be to conduct refined air quality dispersion modeling for the source in 

question in combination with any nearby sources. Based on data in EPA's 2008 National 

Emissions Inventory (NEI), there are over 33,000 SO2 point sources in the United States. It is 

unlikely that every one of these sources has the potential to emit at levels that would violate or 

contribute to violations of the 1-hour SO2 NAAQS. Several factors other than a source’s 

emissions help determine its impact on ground level SO2 concentrations. These factors include 

the influence of stack parameters (stack height, stack exit temperature, stack exit velocity, stack 

diameter), complex terrain (i.e., where receptor elevation is above the source elevation), building 

downwash, proximity of stacks to ambient air, as well as the urban or rural classification of the 
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16 

area around the source. The discussion that follows suggests some ways in which states and 

sources might consider managing the overall burden of assessing whether SO2 source emissions 

will adversely impact public health. 

EPA recognizes the resource burden potentially involved in conducting refined air quality 

modeling for every source of SO2 emissions. As one example, states might focus their limited 

modeling resources on the largest sources of SO2, such as those sources that are emitting at least 

100 tpy, in recognition that these sources emit nearly all the emissions of SO2 (just over 99% of 

all SO2 emissions in 2005 from point sources in the country). States might also focus on those 

sources located in areas where there is the highest potential for citizen exposure to elevated 

ambient SO2 levels. Therefore, EPA is seeking comments and recommendations on reasonable 

approaches to determining which sources should be modeled. 

EPA examined the 2005 NEI for purposes of judging how much of the inventory would 

be categorized as higher versus lower priority in such an approach. This inventory is based on 

actual emissions, which may not accurately reflect the distribution of sources on an allowable 

emissions basis, but EPA used this inventory because a comparable data base of allowable 

emissions is not available. The 2005 NEI identifies over 33,000 sources in the United States that 

emit SO2. Of these sources, about 1928 sources are estimated to have actual emissions above 

100 tpy. Furthermore, as noted above, the larger sources emit a preponderance of the SO2

emissions. Accordingly, we expect these sources also represent a substantial share of the areas 

with potential to violate the SO2 NAAQS. Thus, EPA believes that focusing limited resources 

on sources with emissions in excess of 100 tpy can address the areas of greatest concern with 

only a fraction of the workload that would be needed to address all sources that emit SO2. 
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Given the influence of stack parameters and other factors on the concentrations that result 

from a given quantity of emissions, EPA recognizes that an approach that only considers sources 

emitting more than a specified emissions threshold may in some cases overlook sources that have 

lower emissions but nevertheless have the potential to cause or contribute substantially to 

violations of the standard. EPA also recognizes that EPA and the states have substantial 

experience with modeling SO2 concentrations, and so EPA and the states have substantial 

experience in judging the kinds of circumstances that might be expected to have relatively high 

SO2 concentrations. This suggests the possibility of hybrid approaches in which the state 

addresses all sources emitting more than a given threshold, and in addition addresses sources that 

by reasonable judgment might be expected to contribute to ambient SO2 concentrations above 

the NAAQS. An example of this approach would be for a state to model all sources with 

emissions of at least 100 tpy as well as all smaller sources that are judged to be in circumstances 

that might create high ambient SO2 concentrations, such as low stack heights, building 

downwash, complex terrain, or clustering of multiple sources. States may wish to also place 

priority on conducting additional analyses in areas where significant population exposure to 

elevated ambient SO2 may be occurring. 

An approvable CAA section 110(a)(1) SIP would need to appropriately and accurately 

assess the degree that sources throughout the state cause or contribute to violations of the SO2

NAAQS, but it might not be necessary for states to address all sources of SO2 in an area in order 

to demonstrate attainment and maintenance of the NAAQS. However, any source that is 

modeled at less than its PTE for purposes of demonstrating attainment of the standard should 

receive a SIP enforceable emission or operating permit limit that is consistent with the modeled 

levels. These decisions, of course, should be made by the state on a case-by-case basis depending 
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on the attainment needs of the affected area(s). We are seeking input on approaches such as 

those discussed above for states to identify appropriate threshold levels, as well as any other 

emissions-related parameters, or guiding principles, that may help states and EPA better focus on 

the sources of SO2 most likely to cause or contribute to a violation of the new 1-hour SO2

NAAQS and impact the most populated areas. For example, we welcome any supportable 

information on the configurations and operating characteristics for typical SO2 sources that might 

be most likely to cause a violation (e.g., uncontrolled coal-fired power plants greater than 

250MW with stack height less than 50 feet). Conversely, we would also appreciate supportable 

information on the configurations and operating characteristics for SO2 sources that are not likely 

to cause a violation. 

3. Control strategy 

The SIP should reasonably attribute the expected improvement in the air quality for the 

affected area to SO2 emission reductions resulting from controls that are permanent and 

enforceable.8 If no emissions reductions are needed to ensure attainment and maintenance of the 

1-hour SO2 NAAQS, the SIP should indicate the mechanisms that the state will use to ensure the 

area remains in attainment. Attainment resulting from temporary reductions in emission rates 

(i.e., reduced production or shutdown due to temporary adverse economic conditions) or 

unusually favorable meteorological conditions would not qualify as an air quality improvement 

due to permanent and enforceable emissions reductions. In making the showing that the 

improvement in air quality (or already acceptable air quality) is due to permanent and 

 

 

8

 This is consistent with EPA’s existing policy on redesignations as stated in a memorandum titled “Section 107 

designations policy Summary.” (April 21, 1983). This memorandum indicates that in order for an area to be 

redesignated to attainment, the state should show that “actual enforceable emission reductions are responsible for the 

recent air quality improvement.” We expect to apply the principles of this guidance to the section 110(a)(1) SIP 

submittals. 
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enforceable measures, the state should estimate the percent emission reductions achieved from 

national, regional, and local control measures. 

The EPA expects that in order to meet the implementation, maintenance and enforcement 

plan goals of section 110(a)(1) for the 1-hour SO2 NAAQS, states should consider all control 

measures that are reasonable to implement in light of the attainment and maintenance needs for 

the affected area(s). Where additional controls are necessary to attain it would be appropriate for 

the level of controls in these areas to be similar to that required in areas that are designated as 

nonattainment, to provide for the attainment and maintenance of the 1-hour SO2 NAAQS as 

expeditiously as practicable. EPA expects that expeditious attainment in these areas will in 

many cases occur when national and regional control measures are implemented, and will be 

within 5 years of the effective date of designation of an area. This approach would allow states 

to take into consideration emission reductions that we expect to be achieved from the 

implementation of future controls from national and regional control measures well before 2017, 

as well as any needed local control measures. It would also help to reduce the risk of such areas 

failing to meet the NAAQS as expeditiously as nonattainment areas are expected to meet the 

standard. 

a. National measures under development that are expected to help reduce SO2 emissions 

 Several recent and upcoming EPA regulations have the potential to significantly reduce 

SO2 emissions in the United States. The first of these regulations is the CSAPR which contains 

federally-imposed restrictions on SO2 and NOx emissions from electricity generation units 

(EGUs) in 28 states in the eastern United States (76 FR 48208, August 8, 2011). This rule does 

not specify plant-specific emission limits. Instead, this rule establishes caps on total emissions 

from the set of affected facilities in each state, and provides for issuance of a number of 
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20 

emissions allowances in each state corresponding to the applicable caps. It allows trading of 

allowances, so that each source may have more or less emissions so long as the source emits no 

more than the number of allowances it holds either through issuance or through purchase. Thus, 

the rule does not require that a specific degree of emissions control will be achieved at any 

particular facility. Nevertheless, the emission caps are such that the rule will result in substantial 

additional control of SO2 and NOx emissions. This control is mandated by 2012 and 2014 

(representing start dates for two stages of caps under CSAPR), which is well within expected 

attainment timeframes for the 1-hour SO2 NAAQS. EPA expects these controls generally to 

suffice for achieving attainment. Therefore, for such facilities, negotiation and establishment of 

suitable emission limits will be far simpler than would be the case if the state and the company 

had to negotiate whether control equipment was necessary. Furthermore, the state has the option 

to negotiate with its sources to pursue a distribution of controls under CSAPR that optimizes the 

achievement of attainment of the SO2 standard. For example, the state has the option under 

CSAPR to revise the allocation of allowances to mirror the distribution of emissions that best 

serves the purpose of attaining the SO2 standard9. 

A second regulation that EPA is preparing would mandate maximum achievable control 

technology (MACT) at EGUs and is known as the Mercury and Air Toxics Standards (MATS). 

This regulation will be designed to address hazardous air pollutants (HAP) pursuant to section 

112 of the CAA. HAPs listed under section 112(b) that are emitted by EGUs include mercury, 

hydrochloric acid (HCl), hydrogen fluoride (HF), dioxin, and various metals. SO2 is a criteria 

 

9

 In general, cap and trade programs require emission reductions from among a set of sources but do not require 

controls at particular sources. SO2 concentrations are generally sensitive to emissions from individual nearby plants 

and less sensitive to region-wide restrictions common to trading programs. Therefore, to demonstrate attainment, it 

may be necessary to establish plant-specific SO2 limits irrespective of any emission reductions that the facility may 

be implementing to address trading program requirements. For such demonstrations the allowable emissions should 

reflect the specific limits given in an enforceable document (e.g. a rule or permit). 
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pollutant and is not a HAP listed under CAA section 112(b). Nevertheless, for facilities that 

comply with this regulation by installing exhaust gas scrubbing equipment in order to control 

HCl and acid gas emissions, EPA expects such add-on controls to also reduce emissions of SO2 

substantially. EPA proposed this rule on May 3, 2011 (76 FR 24976). EPA is on a court ordered 

time line to finalize this rule by November 2011. EPA anticipates that the implementation of this 

rule will be completed no later than February of 2015. 

 A third regulation, published on March 21, 2011 (76 FR 15608), is known as the 

Industrial Boilers MACT. This rule establishes limits for mercury, dioxin, HCl, and assorted 

metals for industrial boilers. This rule does not establish limits on emissions of the non-HAP 

SO2, but EPA expects this rule to result in widespread installation of exhaust gas scrubbing 

equipment that would reduce SO2 emissions substantially. EPA has published a notice deferring 

compliance dates under this rule, and the rule is undergoing further review by EPA, but the 

following discussion of the possible impact of the Industrial Boilers MACT reflects impacts that 

could result from the rule as published on March 21, 2011. 

b. SO2 limits for sources using control measures to meet the Cross-state Air Pollution 

Rule and MACT rules 

 EPA envisions that control measures that sources use to comply with requirements of 

these federal rules will facilitate attainment and maintenance of the 1-hour SO2 NAAQS, but 

states would need to ensure that the covered sources have, as appropriate, federally-enforceable 

1-hour SO2 emissions limits. As noted above, the CSAPR may result in many sources installing 

add-on controls or using other control measures, but the specific prospective impact of these 

rules at specific locations is uncertain. For SO2 controls to be creditable for SO2 NAAQS 

planning purposes, they need to apply to the source as enforceable 1-hour SO2 emission limits on 

the specific source. The controls and associated 1-hour SO2 emissions limits would need to 
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22 

become permanent and enforceable under the SIP, even if they might not be required to be so 

under the CSAPR. 

Unlike the CSAPR, the MACT rules would impose specific requirements, including HAP 

emissions limitations, for facilities in the subject source categories. While MACT standards 

generally do not specify the type of control measure or technology a source must use to meet an 

emission standard, they are based upon the HAP emissions performance that is achieved by an 

average of the best performing sources in the subject source category. Each facility that is 

subject to these rules would be subject to HAP emission limits that in many cases may 

necessitate installation of control equipment or the use of other control measures to substantially 

reduce SO2 emissions. However, as promulgated, the Industrial Boilers MACT does not set 

specific limits on emissions of SO2. Therefore, to rely upon SO2 reductions resulting from this 

rule to assure attainment and maintenance of the 1-hour SO2 NAAQS, states would need to also 

establish 1-hour SO2 emission limits to assure that any control measures used to comply with 

MACT requirements are designed and/or operated to provide the necessary control of SO2

emissions as well. If the MATS Rule is promulgated similarly, i.e., it sets emission limits for 

HAPs but does not set specific 1-hour SO2 emission limits, then states would need to set 1-hour 

SO2 emission limits for these facilities as well, to the extent such limits are needed to assure 

attainment and maintenance of the 1-hour SO2 NAAQS. 

c. What SO2 control levels does EPA expect sources will achieve under the CSAPR 

and the MACT rules? 

Source-specific factors can significantly influence the SO2 emissions of facilities subject 

to the CSAPR and pertinent MACT rules. When a facility opts to comply with the CSAPR by 

installing SO2 control equipment, the company may choose among various levels of SO2 control 

efficiency, taking into account the number of SO2 allowances it holds or plans to hold. Flue gas 
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23 

desulfurization systems that have been installed under other emissions trading programs, such as 

the Acid Rain Program and the Clean Air Interstate Rule (CAIR), have commonly achieved 

between 90 and 98 percent control efficiency. Similarly, controls for HAPs may achieve varying 

degrees of efficiency. For example, facilities that install flue gas scrubbing equipment to comply 

with HCl emission limits in a MACT regulation may have varying fuel chlorine content, leading 

to varying degrees of control needed to meet HCl limits, and may use varying degrees of reagent 

for SO2 removal. Controlled SO2 emissions are also a function of the fuel sulfur content and 

various other factors. Dry sorbent injection is another control option, achieving SO2 control 

efficiencies from 30 to 60 percent or higher. However, the actual post-control emission level 

that can be achieved at a particular facility is a function of several site-specific factors. The SIP 

to address the 1-hour SO2 emission limits for specific facilities would need to reflect sourcespecific factors influencing control efficiency as well as the attainment and maintenance needs of 

the area. 

4. Contingency Plan 

Establishing contingency measures for the 1-hour SO2 NAAQS may involve different 

considerations than for some other criteria pollutants. The analytical tools for quantifying the 

relationship between SO2 reductions and resulting air quality improvements produce relatively 

certain results compared to the tools for other pollutants. Also, controls for SO2 are generally 

well understood and are far less prone to compliance rate and control efficiency uncertainty 

relative to control plans for regional pollutants like ozone and fine particulate matter, since the 

latter typically involve many control assumptions for many small sources. Since SO2 control 

measures are based on what is directly and quantifiably necessary to attain the SO2 NAAQS, we 

expect that it would be unlikely for an area to implement the necessary and appropriate emission 
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24 

controls yet fail to attain and maintain the NAAQS. Thus if a state provides a modeling analysis 

showing that compliance with specific SO2 emission limits provides for attainment, if the 

pertinent source(s) continues to meet the limits, and there are no significant future changes to the 

area’s inventory of allowable emissions or background concentrations, there would generally be 

no cause for implementing contingency measures. Therefore, EPA believes it would be 

appropriate, specifically for SO2 programs, for states to adopt for contingency measure purposes 

a comprehensive program to identify sources of violations related to the SO2 NAAQS and to 

undertake an aggressive follow-up for compliance and enforcement, including expedited 

procedures for establishing enforceable consent agreements with affected sources pending the 

adoption of any needed revised SIP for the area in question. 

This approach to contingency measures for the SO2 NAAQS would not, however, 

preclude a state from requiring additional contingency measures that are enforceable and 

appropriate for a particular source or source category. As stated previously, a contingency 

measure for an SO2 SIP might be a consent agreement requiring a source to reduce emissions 

further in the event that certain contingency provisions are triggered. Alternatively, the source 

might adopt a contingency measure such as switching to low sulfur coal or reducing load until 

more permanent measures can be put into place to address emissions interfering with 

maintenance of the NAAQS. In either case, the contingency measures identified in the SIP 

should be implemented as expeditiously as practicable following the determination that the 

affected area is failing to attain or maintain the standard. 

5. Verification of continued attainment 

States should provide an indication of how they will track the progress of the section 

110(a)(1) plan. In most cases, tracking compliance with applicable emission limits, along with 
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25 

tracking whether background concentrations are changing, should suffice for verifying continued 

attainment. We expect that a need for more thorough tracking of emissions and air quality would 

arise mainly in areas with multiple area sources, areas with sources for which emissions are 

difficult to track, and areas where the success of the attainment plan may be particularly 

uncertain. 

C. Redesignation to attainment for areas designated as unclassifiable 

The CAA does not clearly specify the conditions for redesignating an area from 

“unclassifiable” to “attainment.” CAA section 107(d)(3)(E) only addresses conditions for 

redesignating areas from “nonattainment” to “attainment.” Nonetheless, EPA believes that for 

the 1-hour SO2 NAAQS the criteria closely derived from section 107(d)(3)(E) would be 

appropriate. Accordingly, for an area initially designated as “unclassifiable” to be redesignated 

as attainment, EPA expects to apply the following criteria to meet the principles of CAA section 

107(d)(3): (1) EPA has determined that the area is attaining the NAAQS; (2) the area has a fully 

approved section 110(a)(1) SIP; (3) EPA has determined that the attainment-level air quality in 

the affected area is attributable to any necessary permanent and enforceable emissions measures; 

(4) the area has met all of the requirements of section 110 of the CAA; and (5) the state has 

verified implementation of the approved 110(a)(1) SIP control measures. The following 

subsections provide additional details on the anticipated criteria for redesignation to attainment. 

a. Attainment of the NAAQS 

EPA would consider redesignating areas from unclassifiable to attainment if all relevant 

information indicates that the area is attaining the 1-hour SO2 NAAQS. There are two 

components involved in making this demonstration that should be considered interdependently. 

The first component is monitoring data, in situations where there is one or more SO2 air quality 
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monitors located within the affected area. Monitoring data indicate attainment of the NAAQS if 

the 3-year average of the annual 99th percentile of 1-hour daily maximum concentrations does 

not exceed 75 ppb. The second component is modeling data. Since modeling is generally an 

essential means of addressing whether existing emissions limits suffice to provide for attainment 

at all ambient locations around sources, states may in most cases address this component by 

attesting that the relevant sources of SO2 in the area are in compliance with limits that have been 

shown by suitable modeling to provide for attainment. The justification for redesignation from 

unclassifiable to attainment would need to include modeling information except where modeling 

is shown to be inappropriate (if, for example, sources are poorly characterized) or unnecessary (if 

an unclassifiable area is found to have only de minimis emissions). In most cases, EPA expects 

the modeling submitted as a part of the attainment demonstration for the plan would suffice to 

address this requirement. 

b. Approved 110(a)(1) SIP meeting the requirements of section 110(k) of the CAA 

The 110(a)(1) SIP for the area should be fully approved under section 110(k) of the CAA 

and satisfy all of the requirements that apply to the affected area. An area could not be 

redesignated to attainment if a required element of the plan is the subject of a disapproval action, 

a finding of failure to submit or to implement the SIP, a partial approval, a conditional approval, 

or a limited approval. 

c. Attainment-level air quality for the affected area must be attributable to 

permanent and enforceable measures 

The state should be able to reasonably demonstrate that attainment-level air quality is 

attributable to SIP-enforced emissions limits, where necessary. In making this showing, the state 

should provide reliable estimates of the percent reduction in SO2 emissions from national, 

regional, and local measures that have been implemented in the affected area. 
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d. The area meets all of the requirements of section 110 of the CAA 

For the purposes of redesignation, a state should meet all of the requirements of section 

110 of the CAA prior to the submittal of the redesignation request. This would include meeting 

all of the requirements under section 110(a)(2) addressing the general infrastructure requirements 

necessary to provide for the implementation of the 1-hour SO2 NAAQS. 

e. Verification of attainment

The state would need to verify implementation of all control measures identified in the 

approved 110(a)(1) SIP, that all affected sources are in compliance with the control measures, 

and that any emissions reductions projected in the approved plan for the affected area have been 

realized. 

V. SO2 nonattainment area planning elements 

A. SO2 SIP planning 

Any state containing an area designated as nonattainment for the SO2 NAAQS is directed 

by the CAA to develop and submit to EPA a SIP meeting the requirements of subpart 5, of part 

D, of Title I of the CAA, providing for attainment by the applicable statutory attainment date. 

CAA sections 191(a) and 192(a). As indicated in section 191(a), all components of the SO2 part 

D SIP are to be submitted within 18 months of the effective date of an area’s designation as 

nonattainment. 

Section 172 of the CAA addresses the general requirements for areas designated as 

nonattainment. Section 172(c) directs states with nonattainment areas to submit a SIP that 

contains an attainment demonstration showing that the affected area will attain the standard by 

the applicable statutory attainment date. To be approved, the SIP would need to show that the 

area will attain the standard as expeditiously as practicable, and “provide for the implementation 
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of all reasonably available control measures [RACM] as expeditiously as practicable (including 

such reductions in emissions from existing sources in the area as may be obtained through the 

adoption, at a minimum, of reasonably available control technology [RACT]).” (CAA section 

172(c)(1)). 

 SIPs submitted under part D of the CAA also need to provide for reasonable further 

progress (RFP). (CAA section 172(c)(2)). Given the fact that SO2 concentrations are often 

dominated by emissions from a limited number of sources, and emissions controls often yield 

swift and dramatic air quality improvement, EPA expects to continue its historic policy of 

treating as RFP for SO2 “adherence to an ambitious compliance schedule.” (74 FR 13547, in the 

General Preamble published on April 16, 1992). 

As a general matter, nonattainment area SIPs would need to include contingency 

measures that would be implemented in the event that an area fails to meet RFP or fails to attain 

the standards by its attainment date. (CAA section 172(c)(9)). These contingency measures 

should be fully adopted rules or control measures that take effect without further action by the 

state or the Administrator. However, given the relative certainty of attainment of the SO2

NAAQS when sources are complying with emission limits found to provide for attainment, EPA 

expects to continue to address the requirement for SO2 contingency measures as meaning “that 

the state agency [or other applicable authority] has a comprehensive program to identify sources 

of violations of the SO2 NAAQS and to undertake an aggressive follow-up for compliance and 

enforcement, including expedited procedures for establishing enforceable consent agreements 

pending the adoption of revised SIPs.” (57 FR 13547, April 16, 1992). 

 Emission inventories are also critical for the efforts of state, local, and federal agencies to 

attain and maintain the NAAQS. Section 191(a) in conjunction with section 172(c) directs areas 
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designated as nonattainment for SO2 to submit an emission inventory to EPA no later than 18 

months after designation as nonattainment. Sections 191(a) and 172(c) also direct states to 

submit such periodic emission inventories for SO2 nonattainment areas as EPA may determine 

necessary to assure compliance with part D, of title I of the CAA. Where so determined, EPA 

may direct that the periodic inventory include emissions of SO2 for point, nonpoint, mobile, and 

area sources. 

B. New source review 

Part D of title I of the CAA prescribes the procedures and conditions under which a new 

major stationary source or major modification may obtain permits to construct or modify in an 

area designated nonattainment for any criteria pollutant. States that already have a 

nonattainment area new source review (NA NSR) permitting program for SO2 nonattainment 

areas may be able to use the existing program to authorize the construction and modification of 

major stationary sources of SO2. Because there are very few existing SO2 nonattainment areas, 

many states may not have NA NSR rules that apply to SO2. Such rules would need to be 

developed specifically for any nonattainment for the new 1-hour SO2 NAAQS. The NA NSR 

permitting requirements in section 172(c)(5) and 173 of the CAA are among “the requirements 

of this part” to be submitted to EPA as part of a revised SIP for a nonattainment area within 18 

months of the effective date of a designation or redesignation to nonattainment. Until such time 

that EPA approves a state’s revised SIP containing a NA NSR program for SO2, on and after the 

effective date of a nonattainment designation for the new 1-hour SO2 NAAQS, states may rely 

on the nonattainment area permitting requirements under 40 CFR part 51 appendix S to govern 

review and approval of permits for major stationary sources of SO2. 
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In general, the NA NSR program is to ensure that the construction and modification of 

major stationary sources of SO2 will not interfere with RFP toward the attainment of the new 

1-hour SO2 NAAQS. More specifically, the statutory NA NSR requirements include but are not 

limited to: 

 The installation of Lowest Achievable Emissions Rate (LAER) control technology; 

 The acquisition of emissions reductions to offset new emissions of nonattainment 

pollutant(s); 

 Certification that all major sources owned and operated in the state by the same owner are 

in compliance with all applicable requirements under the CAA; 

 A demonstration via an alternatives and siting analysis that the benefits of a proposed 

source significantly outweigh the environmental and social costs imposed as a result of its 

location, construction, or modification; and 

 An opportunity for public comment on the proposed permit. 

The NA NSR requirements apply on a pollutant-specific basis with respect to each 

nonattainment pollutant emitted by a source in amounts greater than the applicable major source 

threshold for the pollutant. (40 CFR 51.165(a)(1)(iv)). In areas that are designated as 

nonattainment for the 1-hour SO2 NAAQS, the major source threshold is 100 tpy of SO2. 

Similarly, NA NSR requirements apply to major modifications of such a source that result in a 

significant net emissions increase of the nonattainment pollutant. EPA’s NA NSR requirements 

at 40 CFR 51.165(a) currently define as “significant” a net emissions increase of at least 40 tpy 

SO2 (the significant emissions rate or SER for SO2). 

 
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C. Conformity 

Section 176(c) of the CAA requires that all federal actions conform to an applicable 

implementation plan developed pursuant to section 110 and part D of the CAA. The EPA rules 

at 40 CFR part 93 developed under the authority of section 176(c) prescribe the criteria and 

procedures for demonstrating and assuring conformity of federal actions to a SIP. Each federal 

agency must determine that any actions covered by the general conformity rule conform to the 

applicable SIP before the action is taken. The criteria and procedures for conformity apply only 

in nonattainment areas and those nonattainment areas redesignated to attainment since 1990 

(“maintenance areas”) with respect to the criteria pollutants under the CAA10. The general 

conformity rules apply one year following the effective date of designations for any new or 

revised NAAQS. The general conformity determination examines the impacts of direct and 

indirect emissions related to federal actions. The general conformity rule provides several 

options to satisfy air quality criteria, such as modeling or offsets, and requires the federal action 

to also meet any applicable SIP requirements and emissions milestones. The general conformity 

rule also requires that notices of draft and final general conformity determinations be provided 

directly to air quality regulatory agencies and to the public by publication in a local newspaper. 

Transportation conformity is required under CAA section 176(c) to ensure that federally 

supported highway and transit project activities are consistent with (“conform to”) the purpose of 

the SIP. Transportation conformity applies to areas that are designated nonattainment, and those 

areas redesignated to attainment after 1990 (“maintenance areas” with plans developed under 

CAA section 175A) for transportation-related criteria pollutants. Due to the relatively small 

amounts of sulfur in gasoline and on-road diesel fuel, transportation conformity does not apply to 

the SO2 NAAQS. (40 CFR 93.102(b)(1)). 

 

10 Criteria pollutants are those pollutants for which EPA has established a NAAQS under section 109 of the CAA. 
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VI. Transition from the existing SO2 NAAQS to a revised SO2 NAAQS 

In implementing the new 1-hour SO2 NAAQS, states will need to insure that the health 

protection provided under the prior SO2 NAAQS continues to be achieved as well as maintained. 

This means that states should continue implementing attainment and maintenance SIPs 

associated with the prior 24-hour and annual primary SO2 NAAQS until such time as they are 

subsumed by any new EPA-approved SIPs reflecting planning and control requirements 

associated with the new NAAQS. 

CAA section 110(l) provides that EPA may not approve a SIP revision if it interferes 

with any applicable requirement concerning attainment and RFP, or any other applicable 

requirement under the CAA. In addition, section 193 of the CAA prohibits the modification of a 

control, or a control requirement, in effect or required to be adopted as of November 15, 1990 

(i.e., prior to the promulgation of the Clean Air Act Amendments of 1990), unless such a 

modification would ensure equivalent or greater emissions reductions. 

In the final SO2 NAAQS, EPA provided that the prior 24-hour and annual primary SO2

NAAQS will remain in effect for one year following the effective date of the initial area 

designations under section 107(d)(1) for the new SO2 NAAQS before being revoked in most 

attainment areas. (40 CFR 50.4(e)). Any existing SIP provisions under CAA sections 110, 191 

and 192 associated with the annual and 24-hour SO2 NAAQS would need to remain in effect, 

even after the 24-hour and annual primary SO2 NAAQS are no longer in effect, including all 

currently implemented planning and emissions control obligations. This includes both those in 

the state’s SIP and those that have been promulgated by EPA in FIPs. This will ensure that both 

the new nonattainment NSR requirements and the general conformity requirements for a revised 

standard are in place so that there will be no gap in the public health protections provided by 
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these two programs. It will also ensure that all nonattainment areas under the annual and/or 24-

hour NAAQS and all areas for which SIP calls have been issued will continue to be protected by 

currently required control measures. 

EPA also provided that the annual and 24-hour NAAQS remain in place for any current 

nonattainment area, or any area for which a state has not fulfilled the requirements of a SIP call, 

until the affected area submits, and EPA approves, a SIP with an attainment, implementation, 

maintenance and enforcement SIP which fully addresses the attainment and maintenance 

requirements of the new SO2 NAAQS. (40 CFR 50.4(e)). The regulatory text provides that such 

a SIP must be submitted and approved pursuant to CAA section 191. This means that the SIP 

would need to meet part D nonattainment planning requirements under the new SO2 NAAQS, as 

described above11. 

It is important to understand that for PSD purposes, even after the time that the annual 

and 24-hour SO2 NAAQS are no longer in effect, the annual and 24-hour SO2 increments 

contained in the CAA and PSD regulations will remain in effect. Thus, the owner or operator of 

a new or modified source will still be required to demonstrate compliance with the annual and 

24-hour SO2 increments, even when the corresponding SO2 NAAQS no longer apply. The 

annual and 24-hour increments are established in section 163 of the CAA. Thus, they remain 

applicable even after EPA has revoked the SO2 NAAQS for the annual and 24-hour averaging 

times. EPA does not believe that it can eliminate the annual and 24-hour SO2 increments 

without appropriate legislative changes to the statutory SO2 increments. For example, in 1990, 

 

11 The areas that are currently designated as nonattainment for the pre-existing SO2 primary NAAQS are Hayden, 

AZ; Armstrong, PA; Laurel, MT; Piti, GU; and Tanguisson, GU. The areas that are designated nonattainment for 

both the primary and the secondary standards are East Helena, MT, Salt Lake Co, MT, Toole Co, UT, and Warren 

Co, NJ. ( See http://www.epa.gov/oar/oaqps/greenbk/lnc.html). The Billings/Laurel, MT area is the only area 

currently subject to a SIP call. 
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Congress amended the CAA to make clear that EPA could substitute PM10 increments for the 

statutory PM increments that were based on the TSP indicator. 

VII. Redesignation to attainment for areas designated as nonattainment 

The latest date by which an area designated as nonattainment is required to attain the SO2

NAAQS is determined from the effective date of the nonattainment designation for the affected 

area. As stated previously, assuming that designations for the 1-hour SO2 NAAQS are effective 

in August 2012, states would submit SIPs for nonattainment areas by no later than February 

2014. For areas designated nonattainment for the revised SO2 NAAQS, approvable SIPs would 

need to provide for attainment of the NAAQS as expeditiously as practicable, but no later than 5 

years from the effective date of the nonattainment designation for the area, or in this case by no 

later than August 2017. (CAA section 192(a)). The EPA expects to determine, under CAA 

section 179(c), whether an area has demonstrated attainment of the new SO2 NAAQS by 

evaluating air quality monitoring and modeling data consistent with 40 CFR part 50, Appendix T 

and 40 CFR part 51, Appendix W. 

A. Attainment of the NAAQS 

There are five basic conditions provided under section 107(d)(3)(E) of the CAA that a 

nonattainment area would need to meet in order for EPA to redesignate it as attainment: 

 EPA determines that the area has met the SO2 NAAQS; 

 EPA has fully approved the SIP for the area; 

 EPA determines the improvement in air quality in the affected area is due to permanent 

and enforceable reductions in emissions; 

 EPA has fully approved a maintenance plan meeting the requirements of section 175A of 

the CAA for the area; and 
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 the state containing the area has met all applicable requirements under section 110 and 

part D of the CAA. 

These conditions are discussed in more detail in the 1994 SO2 Guideline Document. (See

http://www.epa.gov/ttn/oarpg/t1pgm.html). 

B. Consequences of a nonattainment area failing to attain by the statutory attainment date 

 Any SO2 nonattainment area that fails to attain by its statutory attainment date would 

become subject to the requirements of sections 179(c) and (d) of the CAA. EPA is required to 

make a finding of attainment or failure to attain no later than 6 months after the specified 

attainment date and publish a notice in the Federal Register. If EPA finds the area has failed to 

attain, the state would then need to submit an implementation plan revision no later than one year 

following publication of the Federal Register notice making the determination of the area’s 

failure to attain. This submission would need to demonstrate that the standard will be attained as 

expeditiously as practicable, but no later than 5 years from the date of EPA’s publication of the 

notice that the area failed to attain. In addition, section 179(d)(2) provides that the SIP revision 

is to include any specific additional measures as may be reasonably prescribed by EPA, 

including “all measures that can be feasibly implemented in the area in light of technological 

achievability, costs, and any nonair quality and other air quality-related health and environmental 

impacts.” 
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Appendix A

Modeling Guidance for Nonattainment Areas and Section 110(a)(1) plans 

1. Purpose 

On June 2, 2010, Administrator Jackson signed a final rulemaking notice that revised the 

primary SO2 NAAQS (75 FR 35520, published on June 22, 2010) after review of the existing 

two primary SO2 standards, promulgated on April 30, 1971 (36 FR 8187).12 EPA established 

the revised primary SO2 standard at 75 parts per billion (ppb) which is attained when the 3-year 

average of the 99th percentile of 1-hour daily maximum concentrations does not exceed 75 ppb. 

In the final rule preamble, EPA outlined a possible analytic approach to determining compliance 

with the new NAAQS that would include the use of both modeling and monitoring. EPA 

explained that this analytic approach to determining compliance with the new 1-hour NAAQS 

could be the technically appropriate and accurate means of assessing peak 1-hour SO2

concentrations, and would be consistent with historic (past and more recent) implementation 

practice of using models to determine compliance with the SO2 NAAQS. 

This guidance explains the expected application of dispersion models to support the SIP 

process regarding: 

1. The use of modeling in the development of CAA Section 191-192 SIPs for 

nonattainment areas, and 

2. The use of modeling in the development of Section 110(a)(1) and (2) SIPs 

(commonly referred to as “maintenance” and “infrastructure” SIPs). 

While this guidance explains the expected general application of dispersion models, 

there will be applications of dispersion models unique to specific areas, (i.e., there may be areas 

of the country where it is necessary to model unique specific sources or types of sources). In 

such cases, there should be consultation with the state or appropriate reviewing authority with 

the appropriate Regional Office modeling contact to discuss how best to model a particular 

source. 

2. Guidance on Air Quality Models 

This guidance is based on and is consistent with EPA’s Guideline on Air Quality Models, 

also published as Appendix W of 40 CFR Part 51. Appendix W is the primary source of 

information on the regulatory application of air quality models for State Implementation Plan 

(SIP) revisions for existing sources and for New Source Review (NSR) and Prevention of 

Significant Deterioration (PSD) programs. Air quality modeling in this SIP process would need 

to employ air quality dispersion models13 that properly address the source-oriented nature of SO2

and, thus, should rely upon the principles and techniques in Appendix W. 

 

12 EPA publicly disseminated a copy of the signed notice on June 3, 2010, and therefore treats June 3, 2010, as the 

date of the rule’s promulgation, for purposes of the deadlines in CAA section 107(d) and 110(a)(1). 

13 Dispersion modeling uses mathematical formulations to characterize the atmospheric processes that disperse a 

pollutant emitted by a source. Based on emissions and meteorological inputs, a dispersion model can be used to 

predict concentrations at selected downwind receptor locations. 
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Appendix W was originally published in April 1978 and was incorporated by reference in 

the regulations for the Prevention of Significant Deterioration of Air Quality, Title 40, Code of 

Federal Regulations (CFR) sections 51.166 and 52.21 in June 1978 [43 FR 26382-26388]. The 

purpose of Appendix W guidelines is to promote consistency in the use of modeling within the 

air quality management process. These guidelines are periodically revised to ensure that new 

model developments or expanded regulatory requirements are incorporated. 

Clarifications and interpretations of modeling procedures become official EPA guidance 

through several courses of action: 1) the procedures are published as regulations or guidelines; 2) 

the procedures are formally transmitted as guidance to Regional Office managers; 3) the 

procedures are formally transmitted as guidance to Regional Modeling Contacts as a result of a 

Regional consensus on technical issues; or 4) the procedures are a result of decisions by the 

EPA’s Model Clearinghouse that effectively establish national precedent. Formally located in 

the Air Quality Modeling Group (AQMG) of EPA’s Office of Air Quality Planning and 

Standards (OAQPS), the Model Clearinghouse is the single EPA focal point for the review of 

criteria pollutant modeling techniques for specific regulatory applications. Model Clearinghouse 

and related Clarification memoranda involving decisions with respect to interpretation of 

modeling guidance are available at the Support Center for Regulatory Atmospheric Modeling 

(SCRAM) website.14

Recently issued EPA guidance of relevance for consideration in modeling for attainment 

and maintenance demonstrations includes: 

 “Applicability of Appendix W Modeling Guidance for the 1-hour SO2 NAAQS” 

August 23, 2010—confirming that Appendix W guidance is applicable for NSR/PSD 

permit modeling for the new SO2 NAAQS (U.S EPA, 2010a). 

 “Additional Clarification Regarding Application of Appendix W Modeling Guidance for 

the 1-hour NO2 National Ambient Air Quality Standard” March 1, 2011– provides 

additional guidance regarding NO2 permit modeling and also relevant to SO2 (U. S. EPA, 

2011a). 

 “Area designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air 

Quality Standards” March 24, 2011 – provides guidance for designating areas for the 

purpose of implementing the 2010 revised primary SO2 NAAQS, including modeling 

guidance (Attachment 3) (U. S. EPA, 2011b). 

The guidance listed above, in addition to other relevant support documents can be found 

on the SCRAM website at http://www.epa.gov/ttn/scram/so2_modeling_guidance.htm. This 

website will be made publicly available at the time of release of this SO2 implementation 

guidance document. 

The following sections will refer to the relevant sections of Appendix W and other 

existing guidance with summaries as necessary. Please refer to those original guidance 

 

 

14 The Support Center for Regulatory Atmospheric Modeling (SCRAM) website is available at: 

http://www.epa.gov/ttn/scram/. 
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documents for full discussion and consult with the appropriate EPA Regional Modeling Contact 

if questions arise about interpretation on modeling techniques and procedures15. 

3. Model selection

Preferred air quality models for use in regulatory applications are addressed in Appendix 

A of EPA's Guideline on Air Quality Models. If a model is to be used for a particular 

application, the user should follow the guidance on the preferred model for that application. 

These models may be used without an area specific formal demonstration of applicability as long 

as they are used as indicated in each model summary of Appendix A. Further recommendations 

for the application of these models to specific source problems are found in subsequent sections 

of Appendix W. In 2005, EPA promulgated the American Meteorological 

Society/Environmental Protection Agency Regulatory Model (AERMOD) as the Agency’s 

preferred near-field dispersion modeling for a wide range of regulatory applications in all types 

of terrain based on extensive developmental and performance evaluation. 

For SIP development under the 1-hour SO2 primary NAAQS, AERMOD should be used 

unless use of an alternative model can be justified (Section 3.2, Appendix W), such as the 

Buoyant Line and Point Source Dispersion Model (BLP). As outlined in the August 23, 2010 

clarification memo “Applicability of Appendix W Modeling Guidance for the 1-hour SO2

National Ambient Air Quality Standard”, AERMOD is the preferred model for single source 

modeling to address the 1-hour SO2 NAAQS as part of the NSR/PSD permit programs (U.S. 

EPA, 2010a). AERMOD is appropriate for the SIP development process because SO2

concentrations result from direct emissions from combustion sources so that concentrations are 

highest relatively close to sources and are much lower at greater distances due to dispersion. 

Given the source-oriented nature of this pollutant (see, e.g., 75 FR at 35570), dispersion models 

are the most appropriate air quality modeling tools to predict the near-field concentrations of this 

pollutant. 

The AERMOD modeling system includes several components. The regulatory 

components are: 

 AERMOD: the dispersion model (U.S. EPA, 2004a; U.S. EPA, 2011c) 

 AERMAP: the terrain processor for AERMOD (U.S. EPA, 2004b, U.S. EPA, 2011d) 

 AERMET: the meteorological data processor for AERMOD (U.S. EPA, 2004c; U.S. 

EPA, 2011e) 

 BPIPPRIME: the building input processor (U.S. EPA, 2004d) 

and non-regulatory components are: 

 AERSURFACE: the surface characteristics processor for AERMET (U.S. EPA, 2008) 

 AERSCREEN: a recently released screening version of AERMOD (U.S. EPA, 2011f) 

 

15 List of Regional Modeling Contacts by EPA Regional Office is available from SCRAM website at: 

http://www.epa.gov/ttn/scram/guidance_cont_regions.htm
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Before running AERMOD, the user should become familiar with the user’s guides associated 

with the modeling components listed above and the AERMOD Implementation Guide (AIG) 

(U.S. EPA, 2009). The AIG lists several recommendations for applications of AERMOD which 

would be applicable for SIP modeling. 

 

4. Modeling Framework 

 Figure 1 presents a flow chart of the SIP modeling framework from identifying sources 

and emissions inputs to design value calculations. The general steps include the following: 

1. Gather statewide information about SO2 sources including source emissions and 

locations, as well as other pertinent source characteristics (e.g., building information 

for modeling building downwash). While maximum allowable emissions would be 

used in the refined SIP modeling demonstration, initial information gathering could 

be focused on actual emissions for larger sources of 100 tpy or more as an initial 

screening of sources to possibly include in refined modeling 16; 

2. Map selected sources to identify any possible geographic clusters of sources within 

the state. It may also be useful to map monitor locations to identify source clusters 

around monitors, especially monitored areas that are expected to be designated 

nonattainment. The mapping of the sources and monitors will aid in identification of 

the appropriate modeling domain for each geographic area to be addressed in the SIP 

demonstration. The sources to be explicitly modeled within each area should include 

the larger sources and others that potentially contribute to violations for the state to 

have the greatest flexibility in determining controls across sources, as necessary, to 

attain the NAAQS; 

3. Beginning with the maximum allowable emissions or federally enforceable emission 

limits, apply control strategies that may be employed from nationally enforceable 

rules17; 

4. Input the initially controlled emissions along with receptors, meteorology, and 

background concentrations into the dispersion model and calculate design values 

based on cumulative concentrations (all modeled sources and background); 

5. If there are no predicted exceedances of the NAAQS from the initial dispersion 

modeling results, the area has demonstrated attainment; 

6. If there are predicted exceedances of the NAAQS, additional control strategies would 

need to be implemented on sources and assessed, which may necessitate re-running 

the dispersion model; 

7. If additional controls result in no predicted violations of the NAAQS, the area has 

demonstrated attainment. 

8. If there are still predicted violations of the NAAQS, continue to assess additional 

controls until no predicted violations occur. 

 

16 Please note that sources may have actual emissions much lower than their allowable limits, so it may be prudent to 

gather information about sources that are lower than 100 tpy of actual emissions. 

17 See Section B.3 of the SO2 SIP guidance document for more information about national rules. 
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Figure 1. Flowchart of SO2 Modeling Framework for SIP Demonstration. 

The following sections provide details of the SO2 modeling framework and each element 

in the modeling analysis for the SIP development effort. Section 5 describes the modeling 

domain and receptor grid. Section 6 describes the input emissions and controls, while Section 7 

describes meteorological inputs. Section 8 describes the inclusion of background, and Section 9 

describes the calculations of the design values. 

5. Modeling domain 

Selection of the modeling domain is important in terms of how many sources to explicitly 

model and what kind of receptor network to create. Two questions may arise in model domain 

selection: 

1. Where to center the modeling domain?, and 

2. How large should the modeling domain be? (i.e., in terms of the number of sources to 

model and size of the receptor network in order to account for the areas of impact). 
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If the nonattainment or unclassifiable area boundaries were determined using the five 

designations factors or refined dispersion modeling outlined in the March 24, 2011 designations 

guidance memo (U.S. EPA, 2011b), the modeling domain should encompass the nonattainment 

or unclassifiable area and be centered on the center of the designated area. The designated area 

should already include the sources thought to cause or contribute to a NAAQS exceedance, if a 

nonattainment area, or include sources, if any, that are thought to influence an unclassifiable area 

and be eligible for consideration in application of controls. 

However, if the nonattainment or unclassifiable area is the presumptive county boundary 

and no efforts were made in the designations process to alter the designations area size or 

boundaries, states will need to consider the two questions above and consider the emission 

source selection process discussed in Section 5.1 below. If the modeling is being performed 

around a violating monitor, the domain should be centered on the violating monitor. If the 

modeling is being done for an area without a violating monitor, the domain should be centered 

on the dominant source or cluster of sources in an area, that is, the source or sources expected to 

contribute the most to SO2 air quality levels. In both cases, the domain should then extend to 

include nearby sources that are thought to cause or contribute to a potential NAAQS violation or 

unclassifiable area, as explained further below in Section 5.1. 

As stated in Section 4 and shown in Figure 1, the first step of the SIP modeling exercise 

is to determine the sources to include in the modeling. The determination of sources to include 

in modeling is a multi-step process. If modeling is being performed for a violating monitor, the 

first basic step would be to consider those sources within the nonattainment area defined in 

designations or those thought to cause or contribute to violations within 50 km of the monitor, 

the nominal distance within in which EPA considers most steady state Gaussian plume models to 

be applicable. States will need to consider sources throughout their state and determine the areas 

to focus on that appropriately account to these SO2 sources 

5.1. Determining sources to model 

As stated above, the determination of sources to explicitly model for each area is a multistep process: 

1. As discussed in Section 4 and Figure 1, the first step in the implementation modeling is 

to gather statewide information about SO2 sources and map them along with any 

nonattainment monitors to identify clusters of sources and, thereby, the geographic 

area(s) of concern. If monitors are mapped with the sources, clusters of sources can be 

identified around those monitored areas. It is reasonable to initially focus on the most 

significant sources of SO2 emissions, e.g., sources emitting more than 100 tons 

(maximum allowable) 18 per year. Please note, however, that smaller sources can also 

cause or contribute to a NAAQS violation (75 FR 35552), especially sources with short 

stacks and/or located in complex terrain (i.e., where receptor elevation is above stack 

height), or sources with high maximum hourly emissions rates. As a first-tier screen it 

may be reasonable to use actual emissions in the screening process. If actual emissions 

 

18 See Section 6.1 for details regarding the use of maximum allowable vs. actual emissions. 
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are used, it may be necessary to map sources with actual emissions less than 100 

tons/year, bearing in mind that actual emissions may be much less than maximum 

allowable or federally enforceable permit limit emissions, which will be the emissions 

inputs for the implementation modeling. States should use best professional judgment in 

determining an emissions threshold for mapping, or consult the appropriate Regional 

Office modeling contact. 

2. Once the geographic areas of concern has been identified, the spatial distribution of 

sources within either the nonattainment area or 50 km of the violating monitor or 

dominant source should be spatially analyzed to determine what source(s) will need to 

be explicitly modeled. Again, it is reasonable to initially focus on the most significant 

sources of SO2 emissions, e.g., sources emitting greater than 100 tons (maximum 

allowable) per year. While the analysis begins with a nonattainment area or initially 

assumes an area within 50 km of a violating monitor or dominant source, it may not be 

necessary to explicitly model all sources within these initial areas of consideration based 

on steps 3 and 4 below. 

3. Sources should be examined and every effort made to determine if any sources can be 

accounted for without explicitly modeling them, i.e., use of monitored background 

concentrations (if available). Accounting for such sources through the use of a 

background monitor will depend upon whether that monitor reflects impacts from those 

sources. 

4. Sources found not to be represented by monitored background may also be examined 

through the use of screening models to see if they should or should not be included in 

the refined modeling. We recommend the use of EPA’s new screening model 

AERSCREEN (U.S. EPA, 2011f, U. S. EPA, 2011g) and following recommendations 

based on pre-existing screening guidance (U.S. EPA, 1992). For small isolated sources, 

screening may be useful on a source by source basis. However, for a cluster of small 

sources, their cumulative impact should also be assessed. Individual sources may not be 

significant by themselves, but in clusters together they may potentially cause or 

contribute to a NAAQS violation. Although AERSCREEN does not output a design 

value concentration based on the 99th percentile form of the 1-hour SO2 standard, it does 

output the overall maximum 1-hour concentration which could be used as a conservative 

estimate for comparison with the NAAQS and EPA’s suggested interim significant 

impact level (SIL) for the 1-hour SO2 NAAQS of 3 ppb19. If the maximum 1-hour 

concentration output from AERSCREEN violates the NAAQS, it does not mean that the 

source is in nonattainment, but that the source should be evaluated using refined 

dispersion modeling (See Step 3 below for more details). 

Figures 2 and 3 provide a hypothetical example of the screening process by mapping 

sources to identify possible nonattainment or unclassifiable areas and those sources to explicitly 

model. Figure 2 shows a partial map of a state with sources of 10 tons/year or greater of actual 

emissions indicated by circles of varying size based on emissions intensity. Also shown in 

 

 

19 The 3 ppb interim SIL for new 1-hour SO2 NAAQS was suggested by EPA for states to consider using for the 

PSD program in the August 23, 2010 memorandum “Guidance Concerning the Implementation of the 1-hour SO2

NAAQS for the Prevention of Significant Deterioration Program” 
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Figure 2, are violating SO2 monitors as represented by stars. Based on this figure, an example 

application of the four step process is described below. 

Step 1: In Figure 2, clusters of sources, especially around monitors can be seen. Any violating 

monitor is a nonattainment area with a geographic area defined in the designations process that 

constitutes the potential modeling domain. For example, for monitor 1, the modeling domain 

should encompass the 100+ ton per year sources within the designated nonattainment area. The 

modeling domain for monitor 2 could be the cluster of 100+ ton emitters in its vicinity, and the 

same for monitor 3. The cluster of sources northeast of monitor 3 could be combined into a 

separate area centered on the large 10,000+ ton emitter. To the southeast of monitor 3, the large 

10,000+ ton source may be the center of a potential modeling domain that includes the monitor. 

Alternatively, the modeling domain could be centered at monitor 3 and include all large sources 

in its vicinity. These are just illustrative examples only and the states may have their own 

criteria for initially creating modeling domains. 

Figure 2. Example of screening process: Mapping of spatial distribution of sources to 

determine geographic areas of focus for SIP implementation. 
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Step 2: Figure 3 shows source emissions ranging from less than 100 to over 10,000 tons per 

year within the potential area to be modeled centered around a violating monitor (or dominant 

source(s). There are three 10,000+ ton emitters near the center and two 100 to 1,000 ton emitters 

west-southwest of the domain center. At this point, it could be initially assumed that all facilities 

greater than 100 tpy should be included in refined modeling before completing steps 3 and 4 

below. 

Figure 3. Illustrative example of possible modeling domain centered on violating monitor 

(star) with emissions (circles) within 50 km (large circle) and 10 km (inner circle). 

Note: Other monitors are shown by plus sign and asterisks, while shaded contours represent terrain. 

Step 3: Determine whether any source or sources can be accounted for by a representative 

background monitor. In Figure 3, there are two other monitors in the area, one north and one 

south-west of the domain center. The south-western monitor may be representative of the two 

sources in that part of the domain so that these sources would not be explicitly modeled and 

instead accounted for through inclusion of ambient data from this monitor. Background 

concentrations should be calculated following the guidance in Section 7 of this document. 

Step 4: Screening modeling may be used to determine additional sources or combinations of 

sources to be excluded from refined modeling, especially smaller sources whose impacts may be 
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largely dependent on their stack parameters (height, exit velocity, etc.). AERSCREEN could be 

used to exclude such sources through screening modeling. AERSCREEN does not output an 

SO2 design value but does output the overall maximum 1-hour concentration for an individual 

stack. If a facility contains more than one emission point or stack, each stack should be 

processed in AERSCREEN and the maximum 1-hour concentrations can be added together to 

represent impacts from the whole facility after running AERSCREEN. While AERSCREEN can 

be used with the surface characteristics of the source being screened, given the documented 

sensitivity of AERMOD to surface characteristics (Brode et al., 2008), it may be useful to also 

model the source in AERSCREEN using the surface characteristics of the meteorological site 

being used in the refined modeling, to ensure that the source is below de minimis impact levels 

with either set of surface characteristics. When analyzing AERSCREEN output, the following 

general criteria could be followed: 

1. If the facility’s maximum 1-hour concentration exceeds 75 ppb, then the source 

should be included in refined dispersion modeling. 

2. If the facility’s maximum 1-hour concentration is below 75 ppb but above the 

suggested interim 1-hour significant impact level of 3 ppb or the state’s 1-hour SIL, it 

should be included in the refined modeling. 

3. If the facility’s maximum 1-hour concentration is below the suggested interim 1-hour 

significant impact level or the state’s 1-hour SIL, that source may not have to be 

included in refined modeling. However, the facility should not be excluded on the 

sole basis of being below the SIL without first looking at surrounding sources and 

their maximum 1-hour concentrations. The case may arise when there are several 

small sources that singularly are below the SIL but their cumulative impact may lead 

to concentrations that contribute to violations of the NAAQS. 

In summary, for the example in Figure 3, the smaller sources of emissions to the north of 

the domain center may be excluded from refined modeling if screening indicates they are not 

likely to cause or contribute to potential NAAQS violations. Other larger sources to the west 

southwest of the monitor may be represented by background monitors. The largest emitters 

within 10 km of the violated monitor would be included in refined modeling as they are likely 

contributing to potential NAAQS violations. 

This is just one example of how to determine the modeling domain and sources to 

explicitly model. In some cases, an analysis out to 50 km may not be needed. Please consult 

with the appropriate EPA Regional Office modeler if there is uncertainty in deciding which 

sources to explicitly model, which sources to represent based on background monitoring, and/or 

which to exclude from refined modeling based on screening modeling. 

5.2. Receptor grid 

The model receptor grid is unique to the particular situation and depends on the size of 

the modeling domain, the number of modeled sources, and complexity of the terrain. Receptors 

should be placed in areas that are considered ambient air (i.e., where the public generally has 

access) and placed out to a distance such that areas of violation can be detected from the model 

output to help determine the size of nonattainment areas. Receptor placement should be of 
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sufficient density to provide resolution needed to detect significant gradients in the 

concentrations with receptors placed closer together near the source to detect local gradients and 

placed farther apart away from the source. In addition, the user should place receptors at key 

locations such as around facility fence lines (which define the ambient air boundary for a 

particular source) or monitor locations (for comparison to monitored concentrations for model 

evaluation purposes). The receptor network should cover the modeling domain. States may 

already have existing receptor placement strategies in place for regulatory dispersion modeling 

under NSR/PSD permit programs. If this strategy is considered adequate for the implementation 

modeling, states should continue with their respective receptor placement strategies. 

If modeling indicates elevated levels of SO2 (near the standard) near the edge of the 

receptor grid, consideration should be given to expanding the grid or conducting an additional 

modeling run centered on the area of concern. As noted above, terrain complexity should also be 

considered when setting up the receptor grid. If complex terrain is included in the model 

calculations, AERMOD requires that receptor elevations be included in the model inputs. In 

those cases, the AERMAP terrain processor (U.S. EPA, 2004b; U.S EPA, 2011d) should be used 

to generate the receptor elevations and hill heights. The latest version of AERMAP (version 

09040 or later) can process either Digitized Elevation Model (DEM) or National Elevation Data 

(NED) data files. The AIG recommends the use of NED data since it is more up to date than 

DEM data, which is no longer updated (Section 4.3 of the AIG). 

6. Source inputs 

This section provides guidance on source characterization to develop appropriate inputs 

for dispersion modeling with the AERMOD modeling system. Section 6.1 provides guidance on 

use of allowable vs. actual emission levels, Section 6.2 discusses control strategies for emissions, 

Section 6.3 covers guidance on Good Engineering Practice (GEP) stack heights, Section 6.4 

discusses dispersion techniques, Section 6.5 provides details on source configuration and source 

types, Section 6.6 provides details on urban/rural determination of the sources, and Section 6.7 

provides general guidance on source grouping, which may be important for design value 

calculations. 

6.1. Allowable vs. Actual emissions 

Consistent with past SO2 modeling guidance (Section 4.5.2 of U.S. EPA (1994)) and 

regulatory modeling for other programs (Appendix W, Section 8.1), dispersion modeling for the 

purposes of SIP development should be based on the use of maximum allowable emissions or 

federally enforceable permit limits. Also consistent with past and current guidance, in the 

absence of allowable emissions or federally enforceable permit limits, potential to emit 

emissions (i.e., design capacity) should be used. Because of the short-term nature of the new 

SO2 NAAQS, the maximum short term or hourly emission rate should be input into AERMOD 

for each modeled hour. As stated in the August 23, 2010 memo (U. S. EPA, 2010a), 

“Since short-term SO2 standards (≤ 24 hours) have been in existence for decades, existing 

SO2 emission inventories used to support modeling for compliance with the 3-hour and 

24-hour SO2 standards should serve as a useful starting point, and may be adequate in 
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many cases for use in assessing compliance with the new 1-hour SO2 standard since 

issues identified in Table 8-2 of Appendix W related to short-term vs. long-term emission 

estimates may have already been addressed.”20 

The necessary emissions information for attainment demonstration modeling should be 

available from existing SO2 inventories used for permitting or SIP demonstrations. If short-term 

emissions are not readily available, they may be calculated using the methodology shown in 

Table 8-1 of Appendix W. For the short term NAAQS standards this is a product of the 

maximum allowable emission limit or federally enforceable emission limit, the operating level 

and operating factor. The operating level is defined in Section 8.1 of Appendix W as the actual 

or design capacity (whichever is greater) or federally enforceable permit condition. Appendix W 

also recommends modeling at 50% and 75% of capacity to determine the load that may cause the 

highest concentration because changes in stack parameters in loads less than 100% of capacity 

may cause higher ground level concentrations. The operating factor is assumed to be continuous 

operation21. For an example, calculation of short term emissions, see the June 28, 2010 

memorandum “Applicability of Appendix W Modeling Guidance for the 1-hour NO2 National 

Ambient Air Quality Standard” (U. S. EPA, 2010b). Although the example is for NO2, the 

calculation methodology would be the same for SO2. 

Regarding the use of allowable emissions and the modeling of intermittent emissions 

sources from such sources as emergency generators and startup/shutdown emissions, the 

inclusion of such emissions for the purpose of modeling for SO2 attainment demonstrations 

should follow the recommendations in the March 1, 2011 memo “Additional Clarification 

Regarding Application of Appendix W Modeling Guidance for the 1-hour NO2 National 

Ambient Air Quality Standard” (U. S. EPA, 2011a). As stated in this memo, EPA believes the 

most appropriate data to use for compliance demonstrations for the 1-hour NO2 NAAQS are 

those based on emissions scenarios that are continuous enough or frequent enough to contribute 

significantly to the annual distribution of maximum daily 1-hour concentrations. Although the 

referenced guidance in this memo is for NO2 permit modeling, the common 1 hour averaging 

time and form of both the NO2 and SO2 standards makes this modeling guidance applicable to 

the 1-hour SO2 NAAQS and, thus, applicable to SO2 modeling in support of attainment and 

maintenance demonstrations. For more details, refer to the NO2 memo. If any questions arise 

regarding preparation of emissions inputs for dispersions modeling including intermittent 

emissions from sources, then users should consult the appropriate EPA Regional Modeling 

Contact. 

6.2. Controls 

Once sources have been identified within the nonattainment or unclassifiable area that 

may cause or contribute to NAAQS violations, the process of identifying and modeling the effect 

of control strategies begins. In some cases, control of one source may allow an area to be in 

 

20 The August 23, 2010 memo refers to modeling for PSD and Table 8-2 refers to PSD applications. 

21 As stated in Table 8-1 of Appendix W, “If an operation does not occur for all hours of the time period of 

consideration (e.g. 3 or 24 hours) and the source operation is constrained by a federally enforceable permit 

condition, an appropriate adjustment to the modeled emission rate may be made. (e.g., if operation is only 8 a.m. to 

4 p.m. each day, only these hours will be modeled with emissions from the source. Modeled emissions should not 

be averaged across non-operating time periods.” 
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attainment, while in other cases, controls could be implemented on several sources to share the 

control responsibility to demonstrate the area to be in attainment. As stated in Section B.1 of the 

SIP guidance document, states should develop an accurate attainment inventory to identify the 

level of emissions in the area sufficient to attain the 1-hour SO2 NAAQS and be consistent with 

EPA’s most recent guidance on emissions inventories. These emissions are maximum allowable 

emissions levels that reflect enforceable national, regional, or local rules that will be in place 

within the timeframe for demonstrating attainment of the standard. When modeling with 

emissions from the emissions inventory, the input emissions should be reflective of implemented 

control strategies that will allow the area to be in attainment of the NAAQS. The controlled 

emissions should be tested using Table 8-1 of Appendix W. See Section B.3 of the SIP guidance 

document for more information about control strategies. 

As stated in Section 4 and shown in Figure 1, the initial modeling for the SIP can include 

the national rules that will be in place by the attainment date. Therefore, if these initial controls 

allow for the area to be in attainment by the attainment date, additional controls may not be 

necessary. However, if additional controls are necessary to achieve attainment, identifying 

additional sources to control may be necessary (see Figure 1). Often these sources can be 

determined by analyzing spatial relationships between the sources and receptors whose 

concentrations exceed the NAAQS. 

When identifying sources to control, control of a particular source that leaves other 

sources unchanged will obviously change hourly cumulative concentrations. This in turn, may 

change the daily maximum 1-hour concentration for a given receptor and day, and subsequently 

may alter the annual distribution of daily maximum 1-hour concentrations and the magnitude of 

the 4th highest concentration from the annual distribution. This would change the cumulative 

design value which may still be above the NAAQS but now driven by a different source that 

earlier may not have been identified as a source to control. Analysis of sources to control may 

need to be extended to percentiles below the 99th percentile of the annual distribution of daily 

maximum 1-hour concentrations. While the cumulative (all sources) design value is based on 

the multiyear average 99th percentile (i.e. 4th highest) of the annual distribution of the daily 

maximum 1-hour concentrations, lower percentiles, such as 98th (8th highest),

 97th (11th highest), 

etc. can exceed the NAAQS level. A modeling analysis should include multiyear averages of the 

daily maximum 1-hour concentrations beginning with the 4th highest and proceeding downward 

to 5th, 6th, 7th, etc. highest until the cumulative impact at all receptors is below the NAAQS. 

Since the form of the standard is based on the maximum 1-hour concentration for each day, the 

analysis should be limited to the daily maximum 1-hour concentrations, not for example, 2nd or 

3rd highest of the 1-hour concentrations for a day. Also, the rank, 4th, 5th, etc. highest, should be 

consistent across all averaged years, i.e. the Nth highest from the distribution of daily maximum 

1-hour concentrations should be chosen for each year, not 4th highest for one year, and 5th highest 

for another year. AERMOD (version 11059 or later) has an output option, MAXDCONT, to aid 

in this type of analysis and is discussed in more detail in Section 8.2. Such an analysis will lead 

to a more comprehensive identification of sources to possibly control to reach attainment of the 

NAAQS. 

 
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6.3. Good Engineering Practice (GEP) stack height 

Consistent with previous SO2 modeling guidance (U.S. EPA, 1994) and Section 6.2.2 of 

Appendix W, for stacks with heights that are within the limits of Good Engineering Practice 

(GEP), actual heights should be used in modeling. Under EPA’s regulations at 40 CFR 51.100, 

GEP height, Hg, is determined to be the greater of: 

 65 m, measured from the ground-level elevation at the base of the stack; 

 For stacks in existence on January 12, 1979, and for which the owner or operator had 

obtained all applicable permits or approvals required under 40 CFR Parts 51 and 52 

Hg=2.5H 

provided the owner or operator produces evidence that this equation was actually relied 

on in designing the stack or establishing an emission limitation to ensure protection 

against downwash; 

For all other stacks, 

Hg=H + 1.5L, 

where H is the height of the nearby structure(s) measured from the ground-level elevation 

at the base of the stack and L is the lesser dimension of height or projected width of 

nearby structure(s), or 

 the height demonstrated by a fluid model or a field study approved by EPA or the 

State/local agency which ensures that the emissions from a stack do not result in 

excessive concentrations of any air pollutant as a result of atmospheric downwash, 

wakes, eddy effects created by the source itself, nearby structures or nearby terrain 

features. 

For more details about GEP, see the Guideline for Determination of Good Engineering Practice 

Stack Height Technical Support Document (U.S. EPA, 1985). 

If stack heights exceed GEP, then GEP heights should be used with the individual stack’s 

other parameters (temperature, diameter, exit velocity). For stacks modeled with actual heights 

below GEP, building downwash should be considered as this can impact concentrations near the 

source (Section 6.2.2b, Appendix W). If building downwash is being considered, the 

BPIPPRIME program (U.S. EPA, 2004d) should be used to input building parameters for 

AERMOD. More information about buildings and stacks is in Section 6.5. 

6.4. Dispersion techniques 

As stated in past SO2 modeling guidance (U .S. EPA, 1994), the CAA and EPA regulations 

generally prohibit stationary sources from taking credit for dispersion techniques in determining 

 
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allowable emission limitations. As stated in Section 5.3 of the 1994 SO2 modeling guidance 

prohibited dispersion techniques are: 

 Using that portion of a stack in excess of good engineering practice stack height 

 Varying the pollutant emission rate according to atmospheric conditions or ambient 

concentrations of that pollutant (referred to as intermittent or supplemental control systems – 

ICS or SCS) or, 

 Increasing final exhaust gas plume rise by manipulating source process parameters, exhaust 

gas parameters, stack parameters or combining exhaust gases from several existing stacks 

into one stack, or other selective handling of exhaust gas streams so as to increase the 

exhaust gas plume rise. 

Exceptions to the prohibitions are: 

 Merging of gas streams in original design and construction, or as part of a change that 

includes installation of controls and a net reduction in allowable emissions affected by 

the change 

 Utilizing techniques which increase final, exhaust gas plume rise, provided facility-wide 

allowable emissions of SO2 are less than 5,000 tons per years 

 Smoke management techniques involved in agricultural or silvicultural programs 

 Episodic restrictions on residential wood burning and open burning and, 

 Reheating after a pollution control system 

6.5. Source configurations and source types 

An accurate characterization of the modeled facilities is critical for refined dispersion 

modeling, including accurate stack parameters and physical plant layout. Accurate stack 

parameters should be determined for the emissions being modeled. Since modeling would be 

done with maximum allowable or potential emissions levels at each stack, the stack’s parameters 

such as exit temperature, diameter, and exit velocity should reflect those emissions levels. 

Accurate locations (i.e. latitude and longitude or Universal Transverse Mercator (UTM) 

coordinates and datum)22 of the modeled emission sources are also important, as this can affect 

the impact of an emission source on receptors, determination of stack base elevation, and relative 

location to any nearby building structures. Not only are accurate stack locations needed, but 

accurate information for any nearby buildings is important. This information would include 

location and orientation relative to stacks and building size parameters (height, and corner 

coordinates of tiers) as these parameters are input into BPIPPRIME to calculate building 

parameters for AERMOD. If stack locations and or building information are not accurate, 

downwash will not be accurately accounted for in AERMOD. 

Emission source type characterization within the modeling environment is also important. 

As stated in the AERMOD User’s Guide (U.S. EPA, 2004a; U.S. EPA, 2011c), emissions 

 

22 Latitudes and longitudes to four decimal places position a stack within 30 feet of its actual location and five 

decimal places place a stack within three feet of its actual location. Users should use the greatest precision 

available. 
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sources can be characterized as several different source types: POINT sources, capped stacks 

(POINTCAP), horizontal stacks (POINTHOR), VOLUME sources, OPENPIT sources, 

rectangular AREA sources, circular area sources (AREACIRC), and irregularly shaped area 

sources (AREAPOLY). Note that POINTCAP and POINTHOR are not part of the regulatory 

default option in AERMOD because the user must invoke the BETA option in the model options 

keyword MODELOPT while not including the “DFAULT” modeling option for these options to 

work properly. While most sources can be characterized as POINT sources, some sources, such 

as fugitive releases or nonpoint sources (emissions from ports/ships, airports, or smaller point 

sources with no accurate locations) may be best characterized as VOLUME or AREA type 

sources. Sources such as flares can be modeled in AERMOD using the parameter input 

methodology described in Section 2.1.2 of the AERSCREEN User’s Guide (U. S. EPA, 2011f). 

If questions arise about proper source characterization or typing, users should consult the 

appropriate EPA Regional Modeling Contact. 

6.6. Urban/rural determination 

For any dispersion modeling exercise, the urban or rural determination of a source is 

important in determining the boundary layer characteristics that affect the model’s prediction of 

downwind concentrations. Figure 4 gives example maximum 1-hour concentration profiles for a 

10 meter stack (Figure 3a) and a 100 m stack (Figure 3b) based on urban vs. rural designation. 

The urban population used for the examples is 100,000. In Figure 4a, the urban concentration is 

much higher than the rural concentration for distances less than 750 m from the stack but then 

drops below the rural concentration beyond 750 m. For the taller stack in Figure 4b, the urban 

concentration is much higher than the rural concentration even as distances increase from the 

source. These profiles show that the urban or rural designation of a source can be quite 

important. 

In addition, for SO2 modeling, the urban/rural determination is important because 

AERMOD invokes a 4-hour half life23 for urban SO2 sources. This would only be done for 

urban sources when the POLLUTID keyword in AERMOD is set to “SO2” and the MODELOPT 

keyword includes the DFAULT option. Rural sources within the same AERMOD run would not 

be affected. If the DFAULT option is not included with the MODELOPT keyword, the 4-hour 

half life would not be used and the user would specify the 4-hour half life using the HALFLIFE 

or DCAYCOEFF keywords in order to account for the chemical transformation. See Section 

3.2.6 of the AERMOD User’s Guide (U.S. EPA, 2004a) for more details about these keywords. 

If the user invokes the HALFLIFE or DCAYCOEEF option, then any rural sources included in 

the modeling would need to be run in separate AERMOD runs so that they are not subject to the 

4-hour half life. Note that if the DFAULT option is used, the rural sources would not need to be 

in a separate run from the urban sources. Determining whether a source is urban or rural can be 

done using the methodology outlined in Section 7.2.3 of Appendix W and recommendations 

outlined in Sections 5.1 through 5.3 in the AIG (U.S. EPA, 2009). In summary, there are two 

methods of urban/rural classification described in Section 7.2.3 of Appendix W. 

The first method of urban determination is a land use method (Appendix W, Section 

7.2.3c). In the land use method, the user analyzes the land use within a 3 km radius of the source 

 

23 Over a 4-hour period, SO2 concentrations decrease by half from the initial value. 
Public Review Draft 9/22/2011

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using the meteorological land use scheme described by Auer (1978). Using this methodology, a 

source is considered urban if the land use types, I1 (heavy industrial), I2 (light-moderate 

industrial), C1 (commercial), R2 (common residential), and R3 (compact residential) are 50% or 

more of the area within the 3 km radius circle. Otherwise, the source is considered a rural 

source. The second method uses population density and is described in Section 7.2.3d of 

Appendix W. As with the land use method, a circle of 3 km radius is used. If the population 

density within the circle is greater than 750 people/km2

, then the source is considered urban. 

Otherwise, the source is modeled as a rural source. Of the two methods, the land use method is 

considered more definitive (Section 7.2.3e, Appendix W). 

Caution should be exercised with either classification method. As stated in Section 5.1 of 

the AIG (U.S. EPA, 2009), when using the land use method, a source may be in an urban area 

but located close enough to a body of water or other non-urban land use category to result in an 

erroneous rural classification for the source. The AIG in Section 5.1 cautions users against using 

the land use scheme on a source by source basis, but advises considering the potential for urban 

heat island influences across the full modeling domain. When using the population density 

method, Section 7.2.3e of Appendix W states, “Population density should be used with caution 

and should not be applied to highly industrialized areas where the population density may be low 

and thus a rural classification would be indicated, but the area is sufficiently built-up so that the 

urban land use criteria would be satisfied...” With either method, Section 7.2.3(f) of Appendix 

W recommends modeling all sources within an urban complex as urban, even if some sources 

within the complex would be considered rural using either the land use or population density 

method. 
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Figure 4. Urban (red) and rural (blue) concentration profiles for (a) 10 m buoyant stack 

release, and (b) 100 m buoyant stack release. 
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Another consideration that may need attention by the user and is discussed in Section 5.1 

of the AIG relates to tall stacks located within or adjacent to small to moderate size urban areas. 

In such cases, the stack height or effective plume height for very buoyant sources may extend 

above the urban boundary layer height. The application of the urban option in AERMOD for 

these types of sources may artificially limit the plume height. The use of the urban option may 

not be appropriate for these sources, since the actual plume is likely to be transported over the 

urban boundary layer. Section 5.1 of the AIG gives details on determining if a tall stack should 

be modeled as urban or rural, based on comparing the stack or effective plume height to the 

urban boundary layer height. The 100 m stack illustrated in Figure 3b, may be such an example 

as the urban boundary layer height for this stack would be 189 m (based on a population of 

100,000) and equation 104 of the AERMOD formulation document (Cimorelli, et al., 2004). 

This equation is: 

4

1















 

o

iuc iuo P

P

z z

 where ziuo is a reference height of 400 m corresponding to a reference population Po of 2,000,000 

people. 

 Given that the stack is a buoyant release, the plume may extend above the urban 

boundary layer and may be best characterized as a rural source, even if it were near an urban 

complex. Exclusion of these elevated sources from application of the urban option would need 

to be justified on a case-by-case basis in consultation with the appropriate reviewing authority. 

AERMOD requires the input of urban population when utilizing the urban option. 

Population can be entered to one or two significant digits (i.e., an urban population of 1,674,365 

can be entered as 1,700,000). Users can enter multiple urban areas and populations using the 

URBANOPT keyword in the runstream file (U.S. EPA, 2004a; U.S. EPA, 2011c). If multiple 

urban areas are entered, AERMOD requires that each urban source be associated with a 

particular urban area or AERMOD model calculations will abort. Urban populations can be 

determined by using a method described in Section 5.2 of the AIG (U.S. EPA, 2009). 

6.7. Source groups 

In AERMOD, individual emission sources’ concentration results can be combined into 

groups using the SRCGROUP keyword (Section 3.3.11 of the AERMOD User’s Guide (U.S, 

EPA, 2004a). The user can automatically calculate a total concentration (from all sources) using 

the SRCGROUP ALL keyword. For the purposes of attainment demonstrations and design value 

calculations, source group ALL should be used, especially if all sources in the modeling domain 

are modeled in one AERMOD run. Design values should be calculated from the total 

concentrations (all sources and background). For the purposes of SIP modeling, individual 

source contributions outputs to the total concentration may be necessary to determine the 

effectiveness of control strategies. To avoid any confusion, source groups that are used to 

calculate the design value concentrations or determine source contributions to design values 

should be mutually exclusive (i.e. an emission source should not be in two source groups). This 

would be especially important if the design value concentrations are calculated outside of 

AERMOD by adding the individual groups together to calculate a total concentration (See 
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Section 8.1 of this document for examples). If individual source groups that are used in design 

value concentrations are not mutually exclusive, there would be double counting of 

concentrations when calculating design values either in AERMOD or outside of AERMOD.

7. Meteorological data 

Section 7 gives guidance on the selection of meteorological data for input into 

AERMOD. Much of the guidance from Section 8.3 of Appendix W is applicable to SIP 

modeling and is summarized here. In Section 7.2.1, the use of a new tool, AERMINUTE (U.S. 

EPA, 2011h), is introduced. AERMINUTE is an AERMET pre-processor that calculates hourly 

averaged winds from ASOS (Automated Surface Observing System) 1-minute winds. 

7.1. Surface characteristics and representativeness 

The selection of meteorological data that are input into a dispersion model should be 

considered carefully. The selection of data should be based on spatial and climatological 

(temporal) representativeness (Appendix W, Section 8.3). The representativeness of the data is 

based on: 1) the proximity of the meteorological monitoring site to the area under consideration, 

2) the complexity of terrain, 3) the exposure of the meteorological site, and 4) the period of time 

during which data are collected. Sources of meteorological data are: National Weather Service 

(NWS) stations, site-specific or onsite data, and other sources such as universities, Federal 

Aviation Administration (FAA), military stations, and others. Appendix W addresses spatial 

representativeness issues in Sections 8.3.a and 8.3.c. 

Spatial representativeness of the meteorological data can be adversely affected by large 

distances between the source and receptors of interest and the complex topographic 

characteristics of the area (Appendix W, Section 8.3.a and 8.3.c). If the modeling domain is 

large enough such that conditions vary drastically across the domain then the selection of a 

single station to represent the domain should be carefully considered. Also, care should be taken 

when selecting a station if the area has complex terrain. While a source and meteorological 

station may be in close proximity, there may be complex terrain between them such that 

conditions at the meteorological station may not be representative of the source. An example 

would be a source located on the windward side of a mountain chain with a meteorological 

station a few kilometers away on the leeward side of the mountain. Spatial representativeness 

for off-site data should also be assessed by comparing the surface characteristics (albedo, Bowen 

ratio, and surface roughness) of the meteorological monitoring site and the analysis area. When 

processing meteorological data in AERMET (U.S. EPA, 2004c; U.S. EPA, 2011e), the surface 

characteristics of the meteorological site should be used [Section 8.3.c of Appendix Wand the 

AERSURFACE User’s Guide (U.S. EPA 2008)]. Spatial representativeness should also be 

addressed for each meteorological variable separately. For example, temperature data from a 

meteorological station several kilometers from the analysis area may be considered adequately 

representative, while it may be necessary to collect wind data near the plume height (Section 

8.3.c of Appendix W). 

Surface characteristics can be calculated in several ways. For details see Section 3.1.2 of 

the AIG (U.S. EPA, 2009). EPA has developed a tool, AERSURFACE (U.S. EPA, 2008) to aid 
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in the determination of surface characteristics. The current version of AERSURFACE uses 1992 

National Land Cover Data. Note that the use of AERSURFACE is not a regulatory requirement 

but the methodology outlined in Section 3.1.2 of the AIG should be followed unless an 

alternative method can be justified. 

7.2. Meteorological inputs 

Appendix W states in Section 8.3.1.1 that the user should acquire enough meteorological 

data to ensure that worst-case conditions are adequately represented in the model results. 

Appendix W states that 5 years of NWS meteorological data or at least one year of site-specific 

data should be used(Section 8.3.1.2, Appendix W) and should be adequately representative of the 

study area. If one or more years (including partial years) of site-specific data are available, those 

data are preferred. While the form of the SO2 NAAQS contemplates obtaining three years of 

monitoring data, this does not preempt the use of 5 years of NWS data or at least one year of 

site-specific data in the modeling. The 5-year average based on the use of NWS data, or an 

average across one or more years of available site specific data, serves as an unbiased estimate of 

the 3-year average for purposes of modeling demonstrations of compliance with the NAAQ (See 

the August 23, 2010 Clarification Memorandum on “Applicability of Appendix W Modeling 

Guidance for the 1-hour SO2 National Ambient Air Quality Standard” (U. S. EPA, 2010a). See 

the memorandum for more details on the use of 5 years of NWS data or at least one year of sitespecific data and applicability to the NAAQS.

7.2.1. NWS data 

NWS data are available from the National Climatic Data Center (NCDC) in many 

formats, with the most common one in recent years being the Integrated Surface Hourly data 

(ISH). Most available formats can be processed by AERMET. As stated in Section 7.1, when 

using data from an NWS station alone or in conjunction with site-specific data, the data should 

be spatially and temporally representative of conditions at the modeled sources. 

 

A recently discovered issue with ASOS is that 5-second wind data that are used to 

calculate the 2-minute average winds are truncated rather than rounded to whole knots. For 

example, a wind of 2.9 knots is reported as 2 knots, not 3 knots. To account for this truncation of 

NWS winds (either standard observation or AERMINUTE output), an adjustment of 1⁄2 knot or 

0.26 m/s is added to the winds in stage 3 AERMET processing. For more details refer to the 

AERMET User’s Guide Addendum (U.S. EPA, 2011e) and/or the appropriate EPA Regional 

Modeling Contact. 

7.2.1.1. AERMINUTE 

In AERMOD, concentrations are not calculated for variable wind (i.e., missing wind 

direction) and calm conditions, resulting in zero concentrations for those hours. Since the SO2

NAAQS is a one hour standard, these light wind conditions may be the controlling 

meteorological circumstances in some cases because of the limited dilution that occurs under low 

wind speeds which can lead to higher concentrations. The exclusion of a greater number of 

instances of near-calm conditions from the modeled concentration distribution may therefore 
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lead to underestimation of daily maximum 1-hour concentrations for calculation of the design 

value. 

To address the issues of calm and variable winds associated with the use of NWS 

meteorological data, EPA has developed a preprocessor to AERMET, called AERMINUTE 

(U.S. EPA, 2011h) that can read 2-minute ASOS winds and calculate an hourly average. 

Beginning with year 2000 data, NCDC has made the 1-minute wind data, reported every minute 

from the ASOS network freely available. The AERMINUTE program reads these 2-minute 

winds and calculates an hourly average wind. In AERMET, these hourly averaged winds replace 

the standard observation time winds read from the archive of meteorological data. This results in 

a lower number of calms and missing winds and an increase in the number of hours used in 

averaging concentrations. For more details regarding the use of NWS data in regulatory 

applications see Section 8.3.2 of Appendix W and for more information about the processing of 

NWS data in AERMET and AERMINUTE, see the AERMET (U.S. EPA, 2004c; U. S. EPA, 

2011e) and AERMINUTE User’s guides (U.S. EPA, 2011h). 

Since the release of AERMINUTE in 2011, some permitting agencies have expressed 

concern that the inclusion of AERMINUTE output in AERMOD will lead to an increase in the 

conservatism of AERMOD output. This perceived increase in conservatism is due to an increase 

in hours with lower wind speeds input into AERMOD. The purpose of AERMINUTE is not to 

lead more conservative concentration estimates, but to increase the data quality and 

representativeness of the meteorological inputs into AERMOD. Concentrations are not 

calculated for hours with reported calm winds or variable winds. These calm or variable winds 

are due to the METAR reporting code used to report ASOS observations. In the METAR coding 

used to report surface observations beginning July 1996, a calm wind is defined as a wind speed 

less than 3 knots and is assigned a value of 0 knots. The METAR code also introduced the 

variable wind observation that may include wind speeds up to 6 knots, but the wind direction is 

reported as missing, if the wind direction varies more than 60 degrees during the 2-minute 

averaging period for the observation. These are often hours of interest because these are light 

wind conditions and important for an hourly SO2 standard. With the use of AERMINUTE, 

hourly averages can be calculated for those hours with reported calm or missing winds, because 

the 2-minute average winds in the one-minute data files have not been subjected to the METAR 

coding. In effect, AERMINUTE is obtaining data that was unavailable because of METAR 

coding, making the meteorological data more representative of the area. 

 

7.2.2. Site-specific data 

The use of site-specific meteorological data is the best way to achieve spatial 

representativeness. AERMET can process a variety of formats and variables for site-specific 

data. The use of site-specific data for regulatory applications is discussed in detail in Section 

8.3.3 of Appendix W. Due to the range of data that can be collected onsite and the range of 

formats of data input to AERMET, the user should consult Appendix W, the AERMET User’s 

Guide (U.S. EPA, 2004c; U. S. EPA, 2011e), and Meteorological Monitoring Guidance for 

Regulatory Modeling Applications (U.S. EPA, 2000). Also, when processing site-specific data 

for an urban application, Section 3.3 of the AERMOD Implementation Guide offers 

recommendations for data processing. In summary, the guide recommends that site-specific 
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turbulence measurements should not be used when applying AERMOD’s urban option, in order 

to avoid double counting the effects of enhanced turbulence due to the urban heat island. 

7.2.3. Areas without representative meteorological data 

In areas with SO2 sources where the state has determined that there is no representative 

meteorological data, it may be difficult to perform accurate refined dispersion modeling for the 

implementation modeling. In nonattainment or unclassifiable areas composed of isolated 

sources, it may be possible to use AERSCREEN (U.S. EPA, 2011g) to conservatively determine 

the attainment status of an area. As noted in Section 5.1, AERSCREEN does not output a design 

value metric to compare to the SO2 NAAQS but does output the maximum 1-hour concentration 

which can be used as a conservative estimate to compare to the NAAQS. Any use of 

AERSCREEN or screening meteorology in the absence of hourly representative meteorological 

data should be considered carefully and in consultation with the appropriate Regional Office 

modeling contact. 

Currently, the screening meteorology created by the MAKEMET processor for use with 

AERSCREEN cannot be used to calculate an SO2 design value. If screening meteorology is used 

in AERMOD, the SO2 design value cannot be calculated. AERMOD will abort processing if 

screening meteorology is used and an SO2 design value is requested in the input file. 

7.2.4. Upper air data 

AERMET requires full upper air soundings to calculate the convective mixing height. 

For AERMOD applications in the U.S., the early morning sounding, usually the 1200 UTC 

(Universal Time Coordinate) sounding, is typically used for this purpose. Upper air soundings 

can be obtained from the Radiosonde Data of North America CD for the period 1946-1997. 

Upper air soundings for 1994 through the present are also available for free download from the 

Radiosonde Database Access website. Users should choose all levels or mandatory and 

significant pressure levels24 when selecting upper air data. Selecting mandatory levels only 

would not be adequate for input into AERMET as the use of just mandatory levels would not 

provide an adequate characterization of the potential temperature profile. 

8. Background concentrations 

The inclusion of ambient background concentrations is important in determining 

cumulative impacts. The modeled contribution to the cumulative analysis should follow the 

form of the standard and be calculated as described in Section 2.6.1.2 of the August 23, 2010 

clarification memo on “Applicability of Appendix W Modeling Guidance for the 1-hour SO2

National Ambient Air Quality Standard” (U. S. EPA, 2010a) This memo suggested a “first tier” 

approach to including a uniform monitored background contribution based on adding the overall 

highest hourly background SO2 concentration from a representative monitor to the modeled 

design value. We recognize that this approach could be overly conservative in many cases and 

 

24 By international convention, mandatory levels are in millibars: 1,000, 850, 700, 500, 400, 300, 200, 150, 100, 50, 

30, 20, 10,7 5, 3, 2, and 1. Significant levels may vary depending on the meteorological conditions at the upper-air 

station 
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may also be prone to reflecting source-oriented impacts, increasing the potential for doublecounting of modeled and monitored contributions. As discussed in EPA’s March 1, 2011 memo 

“Additional Clarification Regarding Application of Appendix W Modeling Guidance for the 1-

hour NO2 Ambient Air Quality Standard” (U. S. EPA, 2011a), and the March 24, 2011 SO2

designations guidance memo (U.S. EPA, 2011b) we recommend a less conservative “first tier” 

approach for a uniform monitored background concentration based on the monitored design 

values for the latest 3-year period, regardless of the years of meteorological data used in the 

modeling. Adjustments to this approach may be considered in consultation with the appropriate 

EPA Regional Modeling Contact with adequate justification and documentation of how the 

background concentration was calculated. 

Section 8.2.2 of Appendix W gives guidance on background concentrations for isolated 

single sources and is also applicable for multi-source areas. One option is, as described in 

Section 8.2.2.b: 

“Use air quality data in the vicinity of the source to determine the background 

concentration for the averaging times of concern. Determine the mean background 

concentration at each monitor by excluding concentrations when the source in question is 

impacting the monitor... For shorter time periods, the meteorological conditions 

accompanying concentrations of concern should be identified. Concentrations for 

meteorological conditions of concern, at monitors, not impacted by the source in 

question, should be averaged for separate averaging time to determine the average 

background value. Monitoring sites inside a 90° degree sector downwind of the source 

may be used to determine the area of impact.” 

When no monitors are located in the vicinity of the sources being modeled a “regional 

site” (i.e., one that is located away from the area of interest but is impacted by similar natural and 

distant man-made sources) may be used to determine background (Section 8.2.2.c, Appendix W). 

In multi-source areas, background includes two components, nearby sources and other sources 

(Section 8.2.3 of Appendix W). Nearby sources are those sources that are expected to cause a 

significant concentration gradient in the vicinity of the source or sources under consideration, 

and should be explicitly modeled. Identification of nearby sources calls for professional 

judgment and consultation with the appropriate EPA Regional Modeling Contact. For other 

sources, such as natural sources, minor sources and distant major sources, the methodology of 

Section 8.2.2 should be used. 

EPA’s March 24, 2011 memo for SO2 designations guidance (U.S. EPA, 2011b) 

describes an appropriate methodology of calculating temporally varying background monitored 

concentrations by hour of day and season (excluding periods when the source in question is 

expected to impact the monitored concentration). The methodology for SO2 is to use the 99th

percentile concentration for each hour of the day by season and average across three years, 

excluding periods when the dominant source(s) are influencing the monitored concentration (i.e., 

99th percentile, or 4th highest, concentrations for hour 1 for January or winter, 99th percentile 

concentrations for hour 2 for January or winter, etc.). Recent updates included in AERMOD 

allow for the inclusion of temporally varying background concentrations in the design value 

 
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calculation in combination with modeling results. See the AERMOD User’s Guide Addendum 

for more details (U. S. EPA, 2011c). 

As an illustrative example Figure 5 shows the 1-hour SO2 NAAQS level, the design 

value (the 3-year average of the 99th percentile of the annual distribution of daily maximum 1-

hour concentrations), and 3-year averages of the 99th percentile concentrations by season and 

hour of day. To calculate the 99th percentile concentration for a season and hour of day 

combination (no consideration for day of week), the second highest concentration for that 

combination should be selected. Also shown are 3-year averages of the 99th percentile 

concentration by hour of day (across all seasons), and the average concentration by hour of day 

across the three years25. In this example, the winter background concentrations show a distinct 

diurnal variability, with less for each of the other seasons. 

Figure 5. SO2 monitored concentrations for various averaging times. 

In summary background concentrations can be included as: 

 Highest monitored hourly SO2 background concentration 

 “First tier” approach based on monitored design values added to modeled design values; 

or 

 

25Modelers should use the 1st-highest value for more detailed pairings, such as month by hour-of-day or season by 

hour-of-day and day-of-week (consider day of week in calculating values). 
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 Temporally varying based on the 99th percentile monitored concentrations by hour of day 

and season added to modeled design values. 

9. Determining design value metrics 

Refined dispersion modeling for SIPs will provide predictions of SO2 design values at 

each receptor that includes contributions from all modeled sources and background. Based on 

the form of the 1-hour SO2 NAAQS, the design value should be calculated as the average of the 

99th percentile of the annual distribution of daily maximum 1-hour concentrations averaged 

across the modeled years. 

9.1. Design value calculation methodology 

Whether design values are calculated within AERMOD or outside of AERMOD, to 

calculate a design value to compare against the standard, the following steps should be followed: 

1. At each receptor, for each hour of the modeled period, calculate a total concentration 

across all sources including background concentrations if applicable. This can be done in 

AERMOD using SRCGROUP ALL or by adding individual source groups outside of 

AERMOD, using hourly POSTFILEs. If the user is totaling the concentrations outside of 

AERMOD, the source groups need to be mutually exclusive, i.e. no one source should be 

in multiple source groups. 

2. From the total concentrations calculated in step 1, obtain the 1-hour maximum 

concentration at each receptor for each modeled day. 

3. From the output of step 2, for each year modeled, calculate the 99th percentile (4th

highest) daily maximum 1-hour concentration at each receptor. If modeling 5 years of 

meteorological data, this results in five 99th percentile concentrations at each receptor. 

4. Average the 99th percentile (or 4th highest) concentrations across the modeled years to 

obtain a design value at each receptor. 

5. Modeled source contributions to a NAAQS violation can be determined by analyzing the 

hourly concentrations from the individual source groups corresponding to the same hour 

as the 4th daily maximum 1-hour concentration from each year. See 75 FR at 35540. For 

example, a receptor has a 5-year average design value of 200.8 mg/m3

 (or approximately 

77 ppb) and AERMOD was modeled for the period January 1, 2005 through December 

31, 2009 for four source groups. From the AERMOD output, the user can determine the 

date of the 4th highest daily maximum 1-hour concentrations that are used to calculate the 

5-year average design value. Table 1 shows the 4th highest daily maximum 1-hour 

concentrations for each year and associated dates that are used in the design value 

calculation. 
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Table 1. 4th highest daily maximum 1-hour concentrations (μg/m3) for 2005-2009. 

Date 

(YYMMDDHH) 

Concentration 

05080101 200.1 

06073105 201.5 

07080403 207.1 

08072705 197.1 

09080104 198.1 

5-YEAR AVG. 200.8 

If output by source group is available, the user can extract each source group’s 

concentration at each of the hours listed in Table 1. Table 2 shows example source contributions 

for each hour shown in Table 1 and indicates that Source 1 is the main contributor to the design 

value for all hours. 

Table 2. Source contributions to 4th highest daily maximum 1-hour concentrations (μg/m3

) 

and 5-year average design values. 

Date 

(YYMMDDHH) 

TOTAL SOURCE 1 SOURCE 2 SOURCE 3 SOURCE 4 

05080101 200.1 155.1 25.1 1.5 18.4 

06073105 201.5 157.4 26.2 0.5 17.4 

07080403 207.1 161.5 20.5 2.1 23.0 

08072705 197.1 159.2 23.1 1.7 13.1 

09080104 198.1 155.3 22.6 2.0 18.2 

5-YEAR AVG. 200.8 157.7 23.5 1.6 18.0 

When calculating design values and there are exceedances of the NAAQS, one may need 

to consider other percentiles below the 99th percentile as well. Examining percentiles below the 

99th percentile would be useful in the context of determining sources that may be significant 

contributors to a NAAQS violation, i.e. a source’s contribution may be above the SIL. There 

may be cases in which a source is not a significant contributor to the design value as defined in 

the NAAQS, but may be a significant contributor at a lower percentile that is still above the 

NAAQS level. Sources that fit this category should not be immediately discounted when 

determining sources to control for attaining the NAAQS. To calculate design values based on 

other percentiles, one can just step down through the 5th, 6th, 7th, etc. highest of the annual 

distributions of daily maximum 1-hour concentrations in steps 3 through 5 in the five steps listed 

above until no concentrations exceed the NAAQS level. The individual sources’ contributions 

can then be determined to be significant or not. 

9.2. Running AERMOD and implications for design value calculations 

Recent enhancements to AERMOD include options to aid in the calculation of design 

values for comparison with the SO2 NAAQS. These enhancements include: 
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 The output of daily maximum 1-hour concentrations by receptor for each day in the modeled 

period for a specified source group. This is the MAXDAILY output option in AERMOD. 

 The output, for each rank specified on the RECTABLE output keyword, of daily maximum 

1-hour concentrations by receptor for each year for a specified source group. This is the 

MXDYBYYR output option. 

 The MAXDCONT option, which shows the contribution of each source group to the high 

ranked values for a specified target source group, paired in time and space. The user can 

specify a range of ranks to analyze, or specify an upper bound rank, i.e. 4th highest, and a 

lower threshold value, such as the NAAQS for the target source group. The model will 

process each rank within the range specified, but will stop after the first rank (in descending 

order of concentration) that is below the threshold, specified by the user. A warning message 

will be generated if the threshold is not reached within the range of ranks analyzed (based on 

the range of ranks specified on the RECTABLE keyword). This option may be needed to aid 

in determining which sources should be considered for controls. 

For more details about the enhancements see the AERMOD User’s guide Addendum (U. S. 

EPA, 2011c). 

Ideally, all explicitly modeled sources, receptors, and background should be modeled in 

one AERMOD run for all modeled years. In this case, the use of the one of the above output 

options can be used in AERMOD to calculate design values for comparison to the NAAQS and 

determine the area’s attainment status and/or inform attainment/nonattainment boundaries. The 

use of these options in AERMOD allows AERMOD to internally calculate concentration metrics 

that can be used to calculate design values and therefore lessen the need for large output files, i.e. 

hourly POSTFILES. 

However, there may be situations where a single AERMOD run with all explicitly 

modeled sources is not possible. These situations often arise due to runtime or storage space 

considerations during the AERMOD modeling. Sometimes separate AERMOD runs are done 

for each facility or group of facilities, or by year, or the receptor network is divided into separate 

sub-networks. In some types of these situations, the MAXDAILY, MXDYBYYR, or 

MAXDCONT output option may not be an option for design value calculations, especially if all 

sources are not included in a single run. If the user wishes to utilize one of the three output 

options, then care should be taken in developing the model inputs to ensure accurate design value 

calculations. 

Situations that would effectively preclude the use of the MAXDAILY, MXDYBYYR, 

and MAXDCONT option to calculate meaningful AERMOD design value calculations include 

the following examples: 

 Separate AERMOD runs for each source or groups of sources. 

o SIP modeling includes 10 facilities for five years of NWS data and each facility is 

modeled for five years in a separate AERMOD run, resulting in 10 separate 

AERMOD runs. 

 
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 Separate AERMOD runs for each source and each modeled year. 

o 10 facilities are modeled for 5 years of NWS data. Each facility is modeled 

separately for each year, resulting in fifty individual AERMOD runs.

In the two situations listed above, the MAXDAILY, MXDYBYYR, or, MAXDCONT 

option would not be useful as the different AERMOD runs do not include a total concentration 

with contributions from all facilities. In these situations the use of hourly POSTFILES, which 

can be quite large, and external post-processing would be needed to calculate design values. 

Situations in which the MAXDAILY, MXDYBYYR, or, MAXDCONT options may be 

used but may necessitate some external post-processing afterwards to calculate a design value 

include: 

 The receptor network is divided into sections and an AERMOD run, with all sources and 

years, is made for each sub-network. 

o A receptor network of 20,000 receptors is divided into four 5,000 receptor subnetworks. Ten facilities are modeled with five years of NWS data in one 

AERMOD run for each receptor network, resulting in four AERMOD runs. After 

the AERMOD runs are complete, the MAXDAILY, MXDYBYYR, or, 

MAXDCONT results for each network can be re-combined into the larger 

network. 

 All sources and receptors are modeled in an AERMOD run for each year. 

 Ten facilities are modeled with five years of NWS data. All facilities are modeled with 

all receptors for each year individually, resulting in five AERMOD runs. MAXDAILY, 

MXDYBYYR, or, MAXDCONT output can be used and post-processed to generate the 

necessary design value concentrations. The receptor network is divided and each year is 

modeled separately for each sub-network with all sources. 

Ten facilities are modeled with five years of NWS data for 20,000 receptors. The 

receptor network is divided into four 5,000 receptor networks. For each subnetwork, all ten facilities are modeled for each year separately, resulting in twenty 

AERMOD runs. MAXDAILY, MXDYBYYR, or, MAXDCONT output can be 

used and post-processed to generate the necessary design value concentrations. 

10. Documentation 

It is expected that the state would submit a modeling and analysis protocol that details the 

methodology and model inputs before commencement of the modeling exercise. This 

information should support the states’ implementation plans and provide a basis for EPA’s 

review and evaluation. The protocol should include the following: 

 Characterization of the nonattainment problem or characterization of the modeled area in 

absence of a violating monitor, 
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 An emissions analysis around the violating monitor or area under consideration for the 

attainment and maintenance demonstration in absence of a violating monitor, and 

 Methodology for preparing air quality and meteorology inputs including choice of 

meteorological data and representativeness of the data. 

Additionally, post-modeling documentation should include: 

 Summary and analysis of modeling results, and 

 Provision of modeling data inputs and outputs in electronic form. 

A meeting with the appropriate EPA Regional Modeling Contact and other technical and 

planning staff to discuss the modeling and analysis protocol is recommended before submitting 

the protocol and beginning any refined modeling. For example modeling protocols, please see 

the SCRAM website on SO2 Implementation at: 

http://www.epa.gov/ttn/scram/so2_modeling_guidance.htm.

11. Summary 

 

In summary, we emphasize the following key points of this modeling guidance: 

 AERMOD is EPA’s preferred near-field dispersion model for regulatory applications and 

is applicable for SO2 SIPs modeling consistent with EPA’s Guideline on Air Quality 

Models, also published as Appendix W of 40 CFR Part 51. 

 Sources should be modeled with maximum allowable 1-hour or short-term emission rates 

in the SIP modeling based on continuous operations at the source. 

 It is reasonable to initially focus on larger emitters, i.e. –100 or more tons per year but 

smaller sources, especially those with short stacks and/or located in complex terrain can 

possibly cause or contribute to NAAQS violations. 

 Modeling should be done with five years of representative NWS meteorological data or 

at least one year of site specific meteorology. 

 Background concentrations can be included as: 

o “First tier” approach based on monitored design values added to modeled design 

values; or 

o Temporally varying based on the 99th percentile monitored concentrations by hour 

of day and season added to modeled design values. 

 States should submit a modeling and analysis protocol that details the methodology and 

model inputs before commencement of the modeling exercise. This information should 

support the states’ recommended SIPs, and provide a basis for EPA’s evaluation of them. 

 At any time during the SIP process when there are questions regarding modeling or 

interpretation of this guidance, the appropriate EPA Regional Modeling Contact should 

be consulted. 
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12. References 

Auer, Jr., A.H., 1978: Correlation of Land Use and Cover with Meteorological Anomalies. 

Journal of Applied Meteorology, 17(5), 636-643. 

Brode, R., K. Wesson, J. Thurman, and C. Tillerson, 2008: AERMOD Sensitivity to the Choice 

of Surface Characteristics, Paper 811, Air And Waste Management Association Annual 

Conference. 

Cimorelli, A. J., S. G. Perry, A. Venkatram, J. C. Weil, R. J. Paine, R. B. Wilson, R. F. Lee, W. 

D. Peters, R. W. Brode, and J. O. Paumier, 2004. AERMOD: Description of Model 

Formulation, EPA-454/R-03-004. U.S. Environmental Protection Agency, Research 

Triangle Park, NC. http://www.epa.gov/ttn/scram/7thconf/aermod/aermod_mfd.pdfU.S. 

EPA, 1985: Guideline for Determination of Good Engineering Practice Stack Height 

(Technical Support Document for the Stack Height Regulations), Revised. EPA-450/4-

80-023R. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711.

http://www.epa.gov/ttn/scram/guidance/guide/gep.pdf 

U.S. EPA, 1992: Screening Procedures for Estimating the Air Quality Impact of Stationary 

Sources. EPA-454/R-92-019. U.S. Environmental Protection Agency, Research Triangle 

Park, NC 27711. http://www.epa.gov/ttn/scram/guidance/guide/scrng.wpd 

U.S. EPA, 1994: SO2 Guideline Document. EPA-452/R-95-008. U.S. Environmental Protection 

Agency, Research Triangle Park, NC 27711. 

U.S. EPA, 2000: Meteorological Monitoring Guidance for Regulatory Modeling Applications. 

EPA-454/R-99-005. U.S. Environmental Protection Agency, Research Triangle Park, NC 

27711. http://www.epa.gov/ttn/scram/guidance/met/mmgrma.pdf 

U.S. EPA, 2004a: User's Guide for the AMS/EPA Regulatory Model – AERMOD. EPA-454/B03-001. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711.

http://www.epa.gov/ttn/scram/models/aermod/aermod_userguide.zip 

U.S. EPA, 2004b: User's Guide for the AERMOD Terrain Preprocessor(AERMAP). EPA454/B-03-003. U.S. Environmental Protection Agency, Research Triangle Park, North 

Carolina 27711. 

http://www.epa.gov/ttn/scram/models/aermod/aermap/aermap_userguide.zip 

U.S. EPA, 2004c: User’s Guide for the AERMOD Meteorological Preprocessor (AERMET). 

EPA-454/B-03-002. U.S. Environmental Protection Agency, Research Triangle Park, 

NC 27711. http://www.epa.gov/ttn/scram/7thconf/aermod/aermet_userguide.zip 

U.S. EPA, 2004d: User's Guide to the Building Profile Input Program. EPA-454/R-93-038. U.S. 

Environmental Protection Agency, Research Triangle Park, North Carolina 27711. 

U.S. EPA, 2005. Guideline on Air Quality Models. 40 CFR Part 51 Appendix W. 

http://www.epa.gov/ttn/scram/guidance/guide/appw_05.pdf 

U.S. EPA, 2008: AERSURFACE User’s Guide. EPA-454/B-08-001. U.S. Environmental 

Protection Agency, Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/7thconf/aermod/aersurface_userguide.pdf 

U.S. EPA, 2009: AERMOD Implementation Guide. U.S. Environmental Protection Agency, 

Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/7thconf/aermod/aermod_implmtn_guide_19March

2009.pdf 

U.S. EPA, 2010a: Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National 

Ambient Air Quality Standard. Tyler Fox Memorandum dated August 23, 2010, 
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Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/ClarificationMemo_AppendixW_Hourly-SO2-

NAAQS_FINAL_08-23-2010.pdf 

U.S. EPA, 2010b: Applicability of Appendix W Modeling Guidance for the 1-hour NO2

National Ambient Air Quality Standard. Tyler Fox Memorandum dated June 28, 2010, 

Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/ClarificationMemo_AppendixW_Hourly-NO2-

NAAQS_FINAL_06-28-2010.pdf 

U.S. EPA, 2011a: Additional Clarification Regarding the Application of Appendix W Modeling 

Guidance for the 1-hour NO2 National Ambient Air Quality Standard. Tyler Fox 

Memorandum dated March 1, 2011, Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/Additional_Clarifications_AppendixW_Hourly-NO2-

NAAQS_FINAL_03-01-2011.pdf 

U.S. EPA, 2011b: Area Designations for the 2010 Revised Primary Sulfur Dioxide National 

Ambient Air Quality Standards. Stephen D. Page Memorandum dated March 24, 2011, 

Research Triangle Park, North Carolina 27711.

http://www.epa.gov/air/sulfurdioxide/pdfs/20110411so2designationsguidance.pdf 

U.S. EPA, 2011c: Addendum – User’s Guide for the AMS/EPA Regulatory Model – AERMOD. 

EPA-454/B-03-001, Research Triangle Park, North Carolina 27711. 

http://www.epa.gov/ttn/scram/models/aermod/aermod_userguide.zip 

U.S. EPA, 2011d: Addendum - User's Guide for the AERMOD Terrain Preprocessor 

(AERMAP). EPA-454/B-03-003. U.S. Environmental Protection Agency, Research 

Triangle Park, North Carolina 27711. 

http://www.epa.gov/ttn/scram/models/aermod/aermap/aermap_userguide.zip 

U.S. EPA, 2011e: Addendum - User’s Guide for the AERMOD Meteorological Preprocessor 

(AERMET). EPA-454/B-03-002. U.S. Environmental Protection Agency, Research 

Triangle Park, NC 27711. 

http://www.epa.gov/ttn/scram/7thconf/aermod/aermet_userguide.zip 

U.S EPA, 2011f: AERSCREEN User’s Guide. EPA-454-/B-11-001. U.S. Environmental 

Protection Agency, Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/models/screen/aerscreen_userguide.pdf 

U.S. EPA, 2011g: AERSCREEN Released as the EPA Recommended Screening Model. Tyler 

Fox Memorandum dated April 11, 2011, Research Triangle Park, North Carolina 27711.

http://www.epa.gov/ttn/scram/20110411_AERSCREEN_Release_Memo.pdf 

U.S EPA, 2011h: AERMINUTE User’s Guide. U.S. Environmental Protection Agency, 

Research Triangle Park, North Carolina 27711. 

http://www.epa.gov/ttn/scram/7thconf/aermod/aerminute_v11059.zip 
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B-1 

Appendix B

Guidance on Section 110(a)(2) Infrastructure SIP Requirements

The Clean Air Act (CAA) directs states to address basic SIP requirements to assure 

attainment and maintenance of the standards. States are to submit these SIPs, pursuant to CAA 

sections 110(a)(1) and 110(a)(2), within 3 years after promulgation of a new or revised primary 

standard. Many of the section 110(a)(2) SIP elements relate to the general information and 

authorities that constitute the "infrastructure" of a state's air quality management program, and 

these have been in place since the initial SIPs were submitted in response to the 1970 CAA. It is 

the responsibility of each state to review its air quality management program's infrastructure SIP 

provisions in light of each new or revised NAAQS. 

 States should review and revise, as appropriate, their existing SO2 SIPs to ensure that 

they are adequate to address the 2010 1-hour SO2 NAAQS. States should, in consultation with 

EPA Regional Offices, follow applicable EPA regulations governing SIP submittals in 40 CFR 

Part 51. See, e.g., Subpart H ("Prevention of Air Pollution Emergency Episodes"), Subpart I 

("Review of New Sources and Modifications"), Subpart J (Ambient Air Quality Surveillance), 

Subpart K (Source Surveillance), Subpart L (Legal Authority), Subpart M ("Intergovernmental 

Consultation"), Subpart O (Miscellaneous Plan Content Requirements), Subpart P ("Protection of 

Visibility"), and Subpart Q ("Reports"). 

 For many infrastructure SIP elements, a SIP submittal should refer to and include 

citations to relevant state regulations. See, e.g., our guidance below regarding elements (F), (H), 

(J), and (M). For EPA's general criteria for SIP submittals, refer to 40 CFR Part 51, Appendix V, 

"Criteria for Determining the Completeness of Plan Submissions." For example, in accordance 

with Appendix V, paragraph 2.1(d), a SIP submittal would include a copy of the actual 

regulation that the state is submitting for approval and incorporation by reference into its SIP, if 

a copy of that regulation has not already been provided by the state. 

 Pursuant to section 110(a), states must provide reasonable notice and opportunity for 

public hearing for all infrastructure SIP submittals. Pursuant to EPA's regulations at 40 CFR 

Part 51, a SIP submittal is to include a certification by the state that the public hearing was held 

in accordance with EPA's procedural requirements for public hearings. See paragraph 2.1(g) of 

Appendix V to Part 51 and 40 CFR 51.102. 
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B-2 

 If a state believes that its existing (already approved) infrastructure SIP is adequate with 

respect to one or more section 110(a)(2) infrastructure elements (or sub-elements), then the 

state's SIP submission may be a certification that the existing SIP contains provisions that fully 

address those section 110(a)(2) infrastructure elements (or sub-elements) as applicable for the 

2010 1-hour SO2 NAAQS.26 Such certification (e.g., in the form of a letter to EPA from the 

Governor or her/his designee) should cite the applicable provisions in the existing SIP and 

provide a specific description of how compliance with each element is achieved. EPA's policy 

position is that a state's certification made in connection with the 2010 1-hour SO2 NAAQS 

should be included in a SIP submittal only after the state has provided public notice and 

opportunity for public hearing on its certification. 

 Section 110(a)(2) of the CAA directs all states to develop and maintain an air quality 

management infrastructure that includes enforceable emission limitations, an ambient monitoring 

program, an enforcement program, air quality modeling capabilities, and adequate personnel, 

resources, and legal authority. Section 110(a)(2)(D) also directs SIPs to prohibit emissions, from 

within the state, that contribute significantly to nonattainment in any other state or that interfere 

with maintenance in any other state, or that interfere with programs under part C of the CAA to 

prevent significant deterioration of air quality or to protect visibility in any other state. 

 Two elements identified in section 110(a)(2) include requirements that are not governed 

by the 3-year submission deadline of section 110(a)(1). The requirements pertain to part D, of 

title I of the CAA, which addresses plan requirements for nonattainment areas. Therefore, the 

following section 110(a)(2) elements are considered by EPA to be outside the scope of 

infrastructure SIP actions: (1) section 110(a)(2)(C) to the extent it refers to permit programs 

(known as "nonattainment new source review") required under part D; and 

(2) section 110(a)(2)(I) in its entirety. EPA does not expect infrastructure SIP submittals to 

include regulations or emission limits developed specifically for attaining the relevant standard. 

Those submittals are due at the time the nonattainment area planning requirements are due 

(18 months following designation). 

 

 

26 This certification does not serve as a demonstration for purposes of CAA section 110(l) that the state has in fact 

attained and maintained the 1-hour SO2 NAAQS. That demonstration, which is part of a required attainment plan 

for unclassifiable areas, is specific to the 2010 1-hour SO2 NAAQS and is addressed elsewhere in our 

implementation guidance. 
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Except as described above, subsections (A) through (M) of section 110(a)(2) set forth the 

infrastructure elements that a SIP should address in order to be approved by EPA. The elements 

are presented below in context of the 2010 1-hour SO2 NAAQS. 

Section 110(a)(2)(A): Emission limits and other control measures

“Each such plan shall [. . .] include enforceable emission limitations and 

other control measures, means, or techniques (including economic incentives 

such as fees, marketable permits, and auctions of emissions rights), as well as 

schedules and timetables for compliance, as may be necessary or appropriate to 

meet the applicable requirements of this chapter.” 

 EPA would not expect infrastructure SIP submissions to identify nonattainment 

emissions controls. Emissions limitations and other control measures to attain the 2010 NAAQS 

in areas designated nonattainment for the 2010 1-hour SO2 NAAQS are on a different schedule 

from the section 110(a) infrastructure elements and will be reviewed and acted upon through a 

separate process. However, the infrastructure SIP submission should include a list or table 

referencing all SO2 emission reduction measures adopted and relied on by the state to meet other 

CAA requirements. Such SO2 emission reduction measures may be a required part of a 

"maintenance track" attainment plan for the SIP submittal due in June 2013. The measures 

identified by the state should show, through dispersion modeling, that affected areas would be 

projected to be in compliance with the NAAQS as expeditiously as practicable, which we would 

expect to be no later than the year 2017. The attainment plan elements are specific to the 2010 1-

hour SO2 NAAQS and are addressed elsewhere in our implementation guidance. 

There are two issues that generally fall under this particular element of section 

110(a)(2)(A) for which we are not prepared to issue guidance at this time. They are: (1) how 

states would need to address previously approved emissions limitations that may treat startup, 

shutdown and malfunction (SSM) events inconsistently with our longstanding guidance on 

excess emissions; and (2) how states would need to address previously approved variance 

provisions and “director’s discretion” provisions that do not comport with EPA policy. We are 

currently discussing options for resolving these issues, taking into consideration several actions 
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B-4 

on state provisions relating to SSM and director's discretion in which EPA is currently engaged27

(e.g., infrastructure SIPs for Utah and North Dakota). Also, EPA has proposed to enter a 

settlement agreement that would obligate EPA to respond by August 31, 2012, to a petition for 

rulemaking filed by the Sierra Club that concerns SSM provisions in 39 states' SIPs . (See notice 

published in the Federal Register on September 1, 2011, at 76 FR 54465.) Under terms of this 

"Ozone Settlement Agreement," by August 31, 2012 EPA would either grant or deny the SSM 

petition with respect to states' individual provisions and further would take final action (such as 

an SSM SIP Call) with respect to those provisions on which we grant the petition. 

Nevertheless, in the meantime EPA wishes to provide infrastructure SIP guidance to the 

extent possible. Therefore, as general guidance, EPA can advise that states not make 

infrastructure SIP submissions that rely on previously approved but potentially flawed 

provisions. Further, we wish to make clear that for infrastructure SIP submissions such as for the 

2008 Pb NAAQS, any "new" (i.e., not already SIP-approved) provisions should be consistent 

with EPA's longstanding policies on SSM and director's discretion, which are briefly 

summarized as follows.28 See page 25652 at http://69.175.53.6/register/2011/may/05/2011-

10995.pdf. Because excess emissions might aggravate air quality so as to prevent attainment and 

maintenance of the NAAQS and compliance with other CAA requirements, EPA would view all 

periods of excess emissions as violations of the applicable emission limitation. Therefore, new 

provisions as part of an approvable SIP submittal could not exempt from enforcement excess 

emissions that may occur at a facility during a period of startup or shutdown. Further, new 

provisions as part of an approvable SIP submittal could not automatically exempt from 

enforcement excess emissions claimed to result from an equipment malfunction. In addition, 

new provisions as part of an approvable SIP submittal could not allow a state air director the 

discretion to determine whether an instance of excess emissions is a violation of an emission 

limitation, because such a determination could bar EPA and citizens from enforcing applicable 

requirements. 

 

27 See e.g., infrastructure SIPs for Utah and North Dakota. EPA has also proposed to enter a settlement agreement 

that would obligate EPA to respond by August 31, 2012, to a petition for rulemaking filed by the Sierra Club that 

concerns SSM provisions in 39 states' SIPs . (See notice published in the Federal Register on September 1, 2011, at 

76 FR 54465.) 

28 For further description of EPA's policy on SSM and director's discretion, see, e.g., a memorandum dated 

September 20, 1999, entitled, "State Implementation Plans: Policy Regarding Excess Emissions During 

Malfunctions, Startup, and Shutdown," from Steven A. Herman, Assistant Administrator for Enforcement and 

Compliance Assurance, and Robert Perciasepe, Assistant Administrator for Air and Radiation. 
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Section 110(a)(2)(B): Ambient air quality monitoring/data system

“Each such plan shall [. . .] provide for establishment and operation of 

appropriate devices, methods, systems, and procedures necessary to (i) monitor, 

compile, and analyze data on ambient air quality, and (ii) upon request, make 

such data available to the Administrator.” 

 To meet section 110(a)(2)(B) requirements for this NAAQS, the state should provide in 

the SIP for a monitoring system to: 

 Monitor air quality for SO2 at appropriate locations throughout the state using EPA 

approved Federal Reference Method or equivalent monitors. States would need to 

install new SO2 monitors in accordance with recent revisions to the SO2 monitoring 

network requirements. 

 Submit data to EPA’s Air Quality System (AQS) in a timely manner, in accordance 

with EPA's air quality data reporting regulations. See 40 CFR 51.320, "Annual air 

quality data report." 

 Submit approvable annual monitoring plans to EPA that describe how the state has 

complied with monitoring requirements and explain any proposed changes to the 

network. 

 Provide the EPA Regional Office prior notification of any planned changes to 

monitoring sites or to the network plan. 

Section 110(a)(2)(C): Programs for enforcement, PSD, and NSR

“Each such plan shall [. . .] include a program to provide for the 

enforcement of the measures described in subparagraph (A), and regulation of the 

modification and construction of any stationary source within the areas covered 

by the plan as necessary to assure that national ambient air quality standards are 

achieved, including a permit program as required in parts C and D of this 

subchapter.” 

 The Prevention of Significant Deterioration (PSD) and nonattainment New Source 

Review (NNSR) programs contained in parts C and D of title I of the CAA, and collectively 

referred to as the major New Source Review (NSR) program, govern preconstruction review and 
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B-6 

permitting of any new or modified major stationary sources of air pollutants regulated under the 

CAA as well as any precursors to the formation of that pollutant when identified for regulation 

by the Administrator.29 The EPA rules addressing these programs can be found generally at 40 

CFR 51.166 and 52.21 (for PSD), and 51.165, 52.24, and Part 51, Appendix S (for NNSR). 

 To meet section 110(a)(2)(C) requirements for this NAAQS, the SIP submittal should: 

 Reference relevant state and federal regulations that provide for enforcement of SO2

emission limits and control measures. 

 Identify the various state regulations that govern permitting of new and modified 

stationary sources (minor and major) of SO2 in the state. 

 Revise its PSD program regulations to address any applicable EPA amendments to 

SO2 PSD rules within 3 years from the date of such amendments. 

 For areas subject to a state’s SIP-approved PSD program, the state should demonstrate 

that it is authorized to implement its existing PSD permit program to ensure that the construction 

and modification of major stationary sources does not cause or contribute to a violation of the 

new 1-hour primary SO2 NAAQS. The state's PSD program should ensure that new or modified 

sources will apply the Best Available Control Technology to reduce SO2 emissions in 

accordance with CAA sections 165(a)(3) and (4). 

 The state's PSD program should apply to sources that emit greenhouse gases (GHG) in 

accordance with EPA's Tailoring Rule.30 Among other things, the state's PSD program must 

either: (i) limit PSD applicability to GHG-emitting sources by adopting the applicability 

thresholds included in the Tailoring Rule; or (ii) adopt lower GHG thresholds and show that the 

state has adequate personnel and funding to administer and implement those lower thresholds. 

Otherwise, the state is directed to remove from EPA’s consideration for approval that portion of 

the SIP (or SIP submission) for which EPA rescinded our previous approval of the PSD program 

(in a rulemaking referred to as the "GHG PSD SIP Narrowing Rule").31 To request such 

removal, a state may choose to follow the example of the letter request submitted by South 

 

29 The terms “major” and “minor” categorize a stationary source, for NSR applicability purposes, in terms of an 

annual emissions rate (tons per year, tpy) for a pollutant. Generally, a minor source is any source that is not 

“major.” “Major” is defined in the applicable NSR regulations—PSD or nonattainment NSR. 

30 Prevention of Significant Deterioration and Title V Greenhouse Gas Tailoring Rule; Final Rule. 75 FR 31514 

(June 3, 2010). 

31 Limitation of Approval of Prevention of Significant Deterioration Provisions Concerning Greenhouse GasEmitting Sources in State Implementation Plans; Final Rule, 75 FR 82536 (December 30, 2010). 
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B-7 

Carolina.32

 If a state lacks a SIP-approved PSD program, it is subject to a federal implementation 

plan (FIP), and major stationary sources within its jurisdiction are subject to the federal PSD 

requirements in 40 CFR 52.21. Some states are subject to a FIP for PSD permitting of all 

regulated NSR pollutants, and fewer states are subject to a FIP for PSD permitting that is limited 

to GHG. Note that a state subject to a FIP for PSD permitting, whether applicable to all 

regulated NSR pollutants or limited to GHG, remains obligated to adopt and submit a PSD 

program for EPA approval that applies to all regulated NSR pollutants, including GHG. Until a 

state provides such a program, its SIP would not be approvable with respect to 

section 110(a)(2)(C).33

 EPA has historically and recently interpretated that the status of an infrastructure SIP's 

approval with respect to the state's PSD program would not adversely affect a state's request for 

redesignation of an area from nonattainment to attainment. In the past, EPA has approved area 

redesignations without having approved infrastructure SIPs, on the basis that infrastructure SIPs 

do not involve nonattainment requirements, and with the expectation that infrastructure SIP 

requirements regarding PSD will be addressed for those areas once redesignation has become 

effective. 

 Minor NSR programs are subject to the statutory requirements in section 110(a)(2)(C) of 

the CAA, which requires “...regulation of the modification and construction of any stationary 

source ...as necessary to assure that the [NAAQS] are achieved.” These programs are to be 

established in each state within 3 years of the promulgation of a new or revised NAAQS. 

 EPA has not proposed to amend the PSD regulations with regard to the new 1-hour SO2

NAAQS. As an interim measure, we have issued SO2 modeling guidance to supplement the 

guidance contained in EPA’s Guideline on Air Quality Models (40 CFR Part 51, Appendix S), 

which will assist states and prospective sources in carrying out the analyses necessary to satisfy 

 

32 South Carolina's letter request can be found at http://www.regulations.gov, at EPA-R04-OAR-2010-0721-0006. 33 For sources subject to a FIP for PSD permitting, either the EPA Regional Office or the state acting as EPA's 

delegate is the permitting authority. EPA recognizes that many states have been implementing a PSD FIP program 

for some time. When a state is already subject to a FIP for PSD permitting (whether or not the state has been 

delegated authority to implement the PSD FIP), and EPA disapproves this element of the state's infrastructure SIP 

submittal, we expect the permitting authority would simply continue implementation of the PSD FIP, and EPA 

would have no additional FIP obligations. In addition, the state would not be subject to any potential mandatory 

sanctions in connection with such disapproval, as such sanctions do not apply to infrastructure SIP deficiencies. 
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B-8 

the PSD requirements for SO2.

34 That supplemental guidance recommended use of an interim 

significant impact level (SIL) of 3 ppb for modeling the 1-hour SO2 impacts of proposed new 

major stationary sources and major modifications. States may wish to consider use of this 

recommended interim SIL to help determine the scope of the modeling analysis needed with 

regard to the 1-hour SO2 NAAQS. States may also wish to use this recommended interim SIL to 

determine whether the proposed source’s emissions, in conjunction with other emissions in the 

area, would cause or contribute to a violation of the 1-hour SO2 NAAQS. 

Section 110(a)(2)(D)(i): Interstate transport provisions

“Each such plan shall [. . . ] contain adequate provisions: 

(i) prohibiting, consistent with the provisions of this subchapter, any 

source or other type of emissions activity within the state from emitting any air 

pollutant in amounts which will— 

(I) contribute significantly to nonattainment in, or interfere with 

maintenance by, any other state with respect to any such national primary or 

secondary ambient air quality standard, or 

(II) interfere with measures required to be included in the applicable 

implementation plan for any other state under part C of this subchapter to prevent 

significant deterioration of air quality or to protect visibility.” 

 Section 110(a)(2)(D)(i) provides for SIPs to include provisions prohibiting any source or 

other type of emissions activity in one state from contributing significantly to nonattainment, or 

interfering with maintenance, of the NAAQS in another state. (The preceding sub-elements, 

from subsection (2)(D)(i)(I), respectively refer to what may be called prongs 1 and 2.) Further, 

this section directs SIPs to include provisions prohibiting any source or other type of emissions 

activity in one state from interfering with measures required to prevent significant deterioration 

of air quality, or from interfering with measures required to protect visibility (i.e., measures to 

 

34 See EPA memorandum titled “Guidance Concerning the Implementation of the 1-hour SO2 NAAQS for the 

Prevention of Significant Deterioration Program,” dated August 23, 2010. In this guidance, EPA recommends an 

interim screening tool to help determine when a source’s proposed emissions increase would be considered to have a 

significant hourly ambient impact as part of the required source impact analysis for the new 1-hour SO2 NAAQS. 
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B-9 

address regional haze) in any state. (The preceding sub-elements, from subsection (2)(D)(i)(II), 

respectively refer to what may be called prongs 3 and 4.) 

 To address prongs 1 and 2 of section 110(a)(2)(D), the state’s submission should include 

an assessment whether or not emissions from SO2 sources located close to state borders (e.g., 

within the 50-km modeling domain used to assess source-specific SO2 impacts35) have associated 

interstate transport impacts and if so, address the impacts. The assessment might include, but is 

not limited to, information concerning emissions in the state, meteorological conditions in the 

state and the potentially impacted states, monitored ambient concentrations in the state and the 

potentially impacted states, the distance to the nearest area that is not attaining the NAAQS in 

another state, and air quality modeling. For areas designated as unclassifiable, this assessment 

could also be informed by the analysis conducted to satisfy 110(a)(1) maintenance SIP 

requirements. 

 EPA has proposed several rules that will or may reduce emissions of SO2, such as the 

CSAPR, the Industrial Boilers MACT, and the MATS. The emissions reductions achieved under 

these rules should result in reduced impacts from transported SO2 emissions and states should 

consider these measures, among others measures, in fulfilling the section 110(a)(2)(D)(i)(I) 

requirements. 

 Under section 110(a)(2)(D)(i)(II), the PSD sub-element (prong 3) may be met by the 

state’s confirmation in a SIP submission that new major sources and major modifications in the 

state are subject to PSD and (if the state contains a nonattainment area for the relevant pollutant) 

NNSR programs that implement the 2010 1-hour SO2 NAAQS.36

 As described more fully under element (C) of this infrastructure SIP guidance, if a state 

lacks a SIP-approved PSD program, it is subject to a FIP, and major stationary sources within its 

jurisdiction are subject to the federal PSD requirements in 40 CFR 52.21. States relying on a FIP 

(whether applicable to all regulated NSR pollutants or limited to GHG) to satisfy the PSD subelement of section 110(a)(2)(D)(i)(II) have programs that are technically deficient and not 

approvable. Although these programs are deficient and these states have not "submitted" 

anything to EPA, EPA would not be required to take further action with respect to this sub-

 

35 See Section 5 Modeling domain in Appendix A, “Modeling Guidance for Nonattainment Areas and Section 

110(a)(1) Plans.” 

36 Memorandum issued by William T. Harnett, Director, OAQPS/AQPD, “Guidance on SIP Elements Required 

Under Sections 110(a)(1) and (2) for the 2006 24-Hour Fine Particle (PM2.5) National Ambient Air Quality 

Standards (NAAQS),” dated September 25, 2009. 
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B-10 

element because the federal rules represent a FIP that fully addresses PSD. In addition, 

mandatory sanctions would not apply because the deficiencies are neither with regard to a 

required submittal under part D nor in response to a SIP call under section 110(k)(5). As 

described in this infrastructure SIP guidance for element (C), such states remain obligated to 

adopt and submit a PSD program for EPA approval that applies to all regulated NSR pollutants. 

Until a state provides such a program, its SIP would not be approvable with respect to prong 3, 

the PSD sub-element of section 110(a)(2)(D)(i)(II). 

 The sub-element providing for protection of visibility (prong 4) under 

section 110(a)(2)(D)(i)(II) may be met by the state’s confirmation that an approved regional haze 

SIP (or FIP) is in place. The development of the regional haze SIPs occurred in a collaborative 

environment among the states. Through this process, the states coordinated on emissions 

controls to protect visibility on an interstate basis. The regional haze rule also specifically 

requires that a state participating in a regional planning process include "all measures needed to 

achieve its apportionment of emission reduction obligations agreed upon through that process."37 

We anticipate that an approved regional haze SIP (or FIP) addressing this requirement will 

ensure that emissions from sources within the state are not interfering with measures to protect 

visibility in other states. 

Section 110(a)(2)(D)(ii): Interstate and international transport provisions

 “Each such plan shall [. . .] contain adequate provisions insuring 

compliance with the applicable requirements of sections 126 and 115 (relating to 

interstate and international pollution abatement).” 

 Section 126(a) of the CAA directs each SIP to include provisions requiring a new or 

modified source to notify neighboring states of potential impacts from the source. States with 

SIP-approved PSD programs should have a regulatory provision in place, consistent with 40 

CFR 51.166(q)(2)(iv), that requires such notification of other state and local agencies. States 

relying on federal program requirements of 40 CFR 52.21(q), which provide for notification of 

affected state and local air agencies, to satisfy this element have programs that are technically 

deficient and not approvable. Although these programs are deficient and these states have not 

 

37 40 CFR 51.308(d)(3)(ii). 
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"submitted" anything to EPA, EPA would not be required to take further action with respect to 

this element because the federal rules represent a FIP that fully addresses the notification issue. 

In addition, mandatory sanctions would not apply because the deficiencies are neither with 

regard to a required submittal under part D nor in response to a SIP call under section 110(k)(5). 

As described in this infrastructure SIP guidance for element (C), such states remain obligated to 

adopt and submit a PSD program for EPA approval that applies to all regulated NSR pollutants, 

including GHG. Until a state provides such a program, its SIP would not be approvable with 

respect to section 110(a)(2)(D)(ii). 

 Sections 126(b) and 126(c) of the CAA affect a state only if the Administrator has been 

petitioned to make a finding of violation that is related to either interstate transport or 

international transport of emissions from sources in the state. Thus, unless a state has been the 

subject of such a petition, the state has no continuing obligations under sections 126(b) or 126(c).

 Section 115 of the CAA authorizes the Administrator to require a state to revise its SIP 

under certain conditions to alleviate international transport. Because there are no pending 

actions pursuant to section 115 of the CAA, at this time EPA has no reason to approve or 

disapprove any existing state rules with regard to section 115 provisions. 

Section 110(a)(2)(E): Adequate personnel, funding, and authority

“Each such plan shall [. . .] provide: 

(i) necessary assurances that the state (or, except where the Administrator 

deems inappropriate, the general purpose local government or governments, or a 

regional agency designated by the state or general purpose local governments for 

such purpose) will have adequate personnel, funding, and authority under state 

(and, as appropriate, local) law to carry out such implementation plan (and is not 

prohibited by any provision of Federal or state law from carrying out such 

implementation plan or portion thereof), 

(ii) requirements that the state comply with the requirements respecting 

state boards under section 128, and 
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(iii) necessary assurances that, where the state has relied on a local or 

regional government, agency, or instrumentality for the implementation of any 

plan provision, the state has responsibility for ensuring adequate implementation 

of such plan provision.” 

 The SIP should assure that the state has adequate authority under its rules and regulations 

to carry out the state's SIP obligations with respect to the 2010 SO2 NAAQS and to revise the 

SIP as necessary. See EPA's regulations at 40 CFR Part 51, subpart L (“Legal Authority”) and 

subpart O (“Miscellaneous Plan Content Requirements”). For example: 

 In accordance with EPA's regulations at subpart L, the SIP should show that the state 

has the legal authority to carry out the plan; the provisions of the state's laws or 

regulations that provide that authority are to be specifically identified in the SIP, and 

copies of the laws or regulations should be included in the SIP submittal. See 40 

CFR sections 51.230 through 51.231. 

 A state may assign responsibility for carrying out a portion of a SIP to a state 

government agency other than the state air pollution control agency, if the SIP 

demonstrates that such other agency has the necessary legal authority. Similarly, the 

state may authorize a local agency to carry out a SIP or portion of a SIP within the 

local agency's jurisdiction, if the SIP demonstrates that the local agency has the 

necessary legal authority; however, the authorizing state is not relieved of 

responsibility for carrying out the SIP. See 40 CFR 51.232, "Assignment of legal 

authority to local agencies." 

In accordance with EPA's regulations at subpart O, the SIP submittal should include 

copies of rules and regulations that show that the state has adopted the emission 

limitations and other measures necessary for attainment and maintenance of the 2010 

SO2 NAAQS. See 40 CFR 51.281, "Copies of rules and regulations." 

 Further, the SIP should assure that the state has adequate funding and personnel to 

implement the SO2 NAAQS. See EPA's regulations at 40 CFR Part 51, subpart M 

("Intergovernmental Consultation") and subpart O ("Miscellaneous Plan Content 

Requirements"). For example: 
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 In accordance with EPA's regulations at subpart M, the SIP should identify the 

organizations that will participate in developing, implementing, and enforcing the 

SIP. The SIP should identify the responsibilities of such organizations and include 

related agreements among the organizations. See 40 CFR 51.240, "General plan 

requirements." 

 In accordance with EPA's regulations at subpart O, the SIP should describe resources 

for carrying out the SIP. Resources to be described include: (1) those available to the 

state (and local agencies, where appropriate) as of the date of SIP submittal; (2) those 

considered necessary during the 5 years following SIP submittal; and (3) projections 

regarding acquisition of the described resources. See 40 CFR 51.280, "Resources." 

Section 110(a)(2)(F): Stationary source monitoring and reporting

“Each such plan shall [. . .] require, as may be prescribed by the 

Administrator: 

(i) the installation, maintenance, and replacement of equipment, and the 

implementation of other necessary steps, by owners or operators of stationary 

sources to monitor emissions from such sources, 

(ii) periodic reports on the nature and amounts of emissions and 

emissions-related data from such sources, and 

(iii) correlation of such reports by the state agency with any emission 

limitations or standards established pursuant to this chapter, which reports shall 

be available at reasonable times for public inspection.” 

 The SIP should provide citations to the state's regulations for source monitoring, 

recordkeeping, and reporting requirements applicable to SO2. 

 In accordance with EPA regulations at 40 CFR Part 51, subpart K ("Source 

Surveillance"), the SIP should provide for monitoring the status of sources' compliance with the 

SO2 NAAQS. For example, the SIP should include provisions for owners or operators of 

stationary sources to maintain records of emissions and other information as may be necessary to 

enable the state to determine whether the sources are in compliance, and the SIP should further 
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B-14 

include provisions for the sources to periodically report that information to the state. See 40 

CFR 51.211, "Emission reports and recordkeeping." 

 The SIP should include provisions for stationary sources subject to the SO2 NAAQS to 

install, calibrate, maintain, and operate equipment for continuously monitoring and recording 

emissions, and the SIP should further include provisions for the sources to maintain the 

monitoring data and periodically submit it to the state. See 40 CFR 51.214, "Continuous 

emission monitoring." 

 In accordance with EPA regulations at 40 CFR Part 51, subpart A ("Air Emissions 

Reporting Requirements") and subpart Q ("Reports"), the responsible state agency should 

analyze the SO2 emissions data and correlate such data with applicable emission limitations or 

standards. The SIP should provide for periodic reporting of emissions inventory data by the state 

to the Administrator (through the appropriate Regional Office). See e.g., 40 CFR 51.321. All 

reports should be made available to the public. 

Section 110(a)(2)(G): Emergency episodes

“Each such plan shall provide for authority comparable to that in section 

303 of this title and adequate contingency plans to implement such authority.” 

 Section 303 of the CAA provides authority to the EPA Administrator to restrain any 

source from causing or contributing to emissions which present an “imminent and substantial 

endangerment to public health or welfare, or the environment.” Each state should have an 

approved SO2 emergency episode plan in place. The plan should be updated as appropriate for 

purposes of implementing the 2010 SO2 NAAQS. As part of the infrastructure SIP submittal, the 

plan should be consistent with emergency episode requirements in 40 CFR 51.150 through 

51.153. Appendix L to 40 CFR Part 51 includes example regulations for Prevention of Air 

Pollution Emergency Episodes and can be used for an example. The emergency episode plan 

should include provisions to trigger successive actions to prevent air quality concentrations from 

reaching the “significant harm level” (SHL), which represents an imminent and substantial 

endangerment to public health. 

 Unless EPA in the future proposes and promulgates regulations to revise the emergency 

episode requirements related to section 110(a)(2)(G), EPA recommends that states follow the 

guidance as stated in 40 CFR 51.150. 
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 To address the section 110(a)(2)(G) element, states with air quality control regions 

identified as either Priority I, Priority IA, or Priority II under the “Prevention of Air Pollution 

Emergency Episode” rules at 40 CFR 51.150 should develop emergency episode contingency 

plans. 

 A Priority I Region means any area with ambient concentration greater than 

100 μg/m3 (0.04 ppm) annual arithmetic mean; 455 μg/m3 (0.17 ppm) 24-hour 

maximum. 

 A Priority IA Region means any area that is classified as Priority I primarily because 

of emissions from a single point source. 

 A Priority II Region means any area that is not classified as a Priority I Region and 

has ambient concentrations between 60-100 μg/m3

 (0.02-0.04 ppm) annual arithmetic 

mean; 260-445 μg/m3

 (0.10-0.17 ppm) 24-hour maximum; any concentration above 

1,300 μg/m3 (0.50 ppm) 3-hour average. 

 Areas that do not meet the above criteria are classified as Priority III and are not 

required to submit contingency plans under the regulations. 

 Significant harm level for SO2: In accordance with 40 CFR 51.151, each plan for a 

Priority I region should include a contingency plan that provides for taking action necessary to 

prevent ambient pollutant concentrations at any location in the region from reaching the SHL for 

SO2 of 2.620 μg/m3 (1.0 ppm) 24-hour average. 

 Exemption of Contingency Plans for areas designated as “attainment” and 

“unclassifiable”: Per 40 CFR 51.152(d)(1), areas designated as attainment or unclassifiable 

under section 107 of the CAA may, at the Administrator’s discretion, be exempted from the 

requirements of 40 CFR 51.152 as the regulation relates to the requirement to develop 

contingency plans required under Priority I, IA, and II Regions. 

 Appendix L to 40 CFR Part 51 provides example regulations that states can use to inform 

decisions concerning air pollution emergency episodes. The example regulations provided in 

Appendix L reflect generally recognized ways of preventing air pollution from reaching levels 

that would cause imminent and substantial endangerment to the health of persons located within 

affected areas. Subpart H of 40 CFR Part 51 directs states to have emergency episode plans that 

contain alert levels, but it does not require adoption of the regulations as stated in Appendix L. 
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Section 110(a)(2)(H): Future SIP revisions

“Each such plan shall [. . .] provide for revision of such plan— 

(i) from time to time as may be necessary to take account of revisions of 

such national primary or secondary ambient air quality standard or the 

availability of improved or more expeditious methods of attaining such standard, 

and 

(ii) except as provided in paragraph (3)(C), whenever the Administrator 

finds on the basis of information available to the Administrator that the plan is 

substantially inadequate to attain the national ambient air quality standard which 

it implements or to otherwise comply with any additional requirements 

established under this chapter (CAA).” 

 The SIP should provide citations to the state regulatory provisions requiring the state to 

(1) revise its CAA section 110 plan from time to time as may be necessary to take into account 

revisions of such primary or secondary NAAQS or the availability of improved or more 

expeditious methods of attaining such standards; and (2) revise its CAA section 110 plan in the 

event the Administrator finds the plan to be substantially inadequate to attain the NAAQS. See 

40 CFR 51.104, “Revisions.” 

Section 110(a)(2)(I): Nonattainment area plan or plan revision under Part D 

“Each such plan shall [. . .] in the case of a plan or plan revision for an 

area designated as a nonattainment area, meet the applicable requirements of 

part D of this subchapter (relating to nonattainment areas).” 

 As noted in the introductory text of this appendix, EPA does not expect infrastructure SIP 

submissions to address subsection 110(a)(2)(I). Nonattainment area plans required under part D 

are required on a different schedule from the section 110 infrastructure elements and will be 

reviewed and acted upon through a separate process. 
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Section 110(a)(2)(J): Consultation with government officials, public notification, PSD and 

visibility protection

 “Each such plan shall [. . .] meet the applicable requirements of 

section 121 of this title (relating to consultation), section 127 of this title (relating 

to public notification), and part C of this subchapter (relating to prevention of 

significant deterioration of air quality and visibility protection).” 

 The SIP should reference the state rules that provide a process of consultation with 

general purpose local governments, designated organizations of elected officials of local 

governments, and any federal land manager having authority over federal land to which the plan 

applies, consistent with the requirements of CAA section 121. 

 The SIP should provide citations to regulations requiring the state to regularly notify the 

public of: instances or areas in which any primary NAAQS was exceeded; the associated health 

hazards; and ways in which the public can participate in regulatory and other efforts to improve 

air quality. See 40 CFR 51.285, “Public notification.” 

 Pursuant to the CAA, a SIP should allow a state to implement any new PSD requirements 

that are triggered upon the effective date of any new NAAQS. However, sources in a state may 

be subject to the federal PSD requirements pursuant to 40 CFR 52.21, if a state does not have a 

SIP-approved PSD program. As described in this infrastructure SIP guidance for element (C), 

such states remain obligated to adopt and submit a PSD program for EPA approval that applies 

to all regulated NSR pollutants, including GHG. Until a state provides such a program, its SIP 

would not be approvable with respect to section 110(a)(2)(J). 

 With regard to the requirement of the plan to meet the applicable requirements for 

visibility protection, EPA would not expect to treat this provision as applicable for purposes of 

the infrastructure SIP approval process. EPA recognizes that states are subject to visibility 

protection and regional haze program requirements under part C of the Act (which includes 

sections 169A and 169B). However, in the event of the establishment of a new primary 

NAAQS, the visibility protection and regional haze program requirements under part C do not 

change. Thus, EPA has concluded that there are no new applicable visibility protection 

obligations under section 110(a)(2)(J) as a result of the 2010 SO2 NAAQS. 
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Section 110(a)(2)(K): Air quality modeling/data

“Each such plan shall [. . .] provide for— 

(i) the performance of such air quality modeling as the Administrator may 

prescribe for the purpose of predicting the effect on ambient air quality of any 

emissions of any air pollutant for which the Administrator has established a 

national ambient air quality standard, and 

(ii) the submission, upon request, of data related to such air quality 

modeling to the Administrator.” 

 The SIP should demonstrate that the state has the authority and technical capability to 

conduct air quality modeling in order to assess the effect on ambient air quality of relevant 

pollutant emissions; and that the state can provide relevant data as part of the permitting and 

NAAQS implementation processes. The SIP should also provide that, upon request, the state 

will submit current and future data relating to such air quality modeling to the Administrator. 

EPA anticipates that the predominant type of air quality modeling to be conducted with respect 

to implementing the SO2 NAAQS will be source-oriented dispersion modeling with models such 

as AERMOD. 

Section 110(a)(2)(L): Permitting fees

“Each such plan shall require the owner or operator of each major 

stationary source to pay to the permitting authority, as a condition of any permit 

required under this chapter, a fee sufficient to cover— 

(i) the reasonable costs of reviewing and acting upon any application for 

such a permit, and 

(ii) if the owner or operator receives a permit for such source, the 

reasonable costs of implementing and enforcing the terms and conditions of any 

such permit (not including any court costs or other costs associated with any 

enforcement action), until such fee requirement is superseded with respect to such 
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sources by the Administrator’s approval of a fee program under subchapter 

(title) V of this chapter.” 

 The SIP should provide citations to the regulations providing for collection of permitting 

fees under the state’s EPA-approved title V permit program. See 40 CFR 70.9 (“Fee 

determination and certification”); 40 CFR Part 70, Appendix A (“Approval Status of State and 

Local Operating Permits Programs”). 

Section 110(a)(2)(M): Consultation/participation by affected local entities

 “Each such plan shall [. . .] provide for consultation and participation by 

local political subdivisions affected by the plan.” 

 To satisfy this element (M), and in accordance with EPA's regulations at 40 CFR Part 51, 

subpart M ("Intergovernmental Consultation"), the SIP should identify the organizations that will 

participate in developing, implementing, and enforcing the SIP. Further, the SIP should identify 

the responsibilities of such organizations and include related agreements among the 

organizations. See 40 CFR 51.240, "General plan requirements." 

 The SIP should identify policies or procedures requiring consultation and participation by 

local political subdivisions affected by the SIP. For example, the SIP should provide a citation to 

the state regulations that provide notice and opportunity for public hearing in accordance with 

EPA regulations at 40 CFR Part 51, subpart F ("Procedural Requirements"). Prior to submitting 

a SIP revision or a compliance schedule, the CAA and EPA regulations direct states to provide 

notice, provide the opportunity to submit written comments, and allow the public the opportunity 

to request a public hearing. See 40 CFR 51.102, "Public hearings." 
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Appendix C

Guidance on Non-modeling Technical Demonstration of Attainment

A state may have counties that do not have large sources, or any sources, of SO2. For 

these counties, refined modeling may not be needed to show that all or portions are in attainment 

of the SO2 NAAQS. This section offers guidance for conducting a non-modeling technical 

demonstration in cases where there are: (a) existing modeling results of nearby SO2 sources, or 

(b) no existing modeling results or plans to model nearby (i.e. within 50 km) SO2 sources. At any 

time during the SIP process when there are questions regarding non-modeling technical 

demonstrations or interpretation of this guidance, the appropriate EPA Regional Modeling 

Contact should be consulted. The first step in both cases is to map the SO2 sources surrounding 

the county, extending out to 50 km beyond the county boundaries, if necessary. 

A. Approach with existing modeling of nearby sources 

 If there are dispersion model results of those SO2 sources within the buffer around the 

county boundary from other modeling efforts (i.e., modeling of nonattainment areas or large 

sources), those results may be used to determine the spatial extent of the impact of a particular 

source or sources. If a nearby source, or sources, has modeled SO2 NAAQS violations, 

singularly or in a cumulative sense, the following questions should be considered: 

 

1. How far do those violations occur from the sources? 

2. How far are these sources from the county in question? 

If violations occur at a distance that does not impact the county in question or parts of the 

county not already part of an existing nonattainment area, then it may be a reasonable conclusion 

that the sources will not cause or contribute to a violation in the portion of the county in question 

and that the unaffected portions of the county could be considered in attainment with the 1-hour 

SO2 standard. 

B. Approach with no modeling of nearby sources 

In the absence of any prior modeling efforts, analysts may need to rely on their best 

professional judgment and consider the following questions for each SO2 source within the 50 

km county buffer: 

1. How far is the source from the county in question? 

2. What is the maximum allowable emission rate of the source? More emissions mean 

higher concentrations. 

3. If the source has stacks, what are the stack parameters (height, temperature, exit velocity, 

and diameter)? A short stack may have local impacts but may not have impacts that 

extend that far, while a tall stack will have impacts farther away. If there are any 

available modeled results for similar stacks, what are their impacts and can they inform 

any nonattainment problem associated with the source in question? 
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4. If the source has fugitive emissions, how far can the impacts from those sources 

potentially extend? 

5. Would downwash play a role in the dispersion of SO2 from the source such that it 

impacts the county in question? 

6. What are the terrain features around the source and county in question? Terrain can 

affect the travel of a plume from a source. 

7. What is the meteorology around the source? Meteorology, together with terrain can 

influence the impact of a source in a particular location. A wind rose may be useful for 

this question to see if the wind blows from any of the sources toward the county. 

Based on the answers to the questions above, analysts should then ask “Can this source 

potentially cause or contribute to a violation of the 1-hour SO2 NAAQS in this county?” If the 

answer to that question is “no,” for all of the sources thought to influence a county’s attainment 

status, then it may be a reasonable conclusion that the county in question is in attainment. 

However, if there is a question that a source could cause or contribute to a violation of the 1-hour 

SO2 NAAQS in the county, then it may be necessary to perform refined dispersion modeling of 

the sources thought to influence the county. 

C. Example of non-modeling approach 

Figure C.1 shows an example of the methodologies outlined above for Collingsworth 

County, TX. This example is for illustrative purposes only. In Figure C.1, the total 2005 NEI 

SO2 point source emissions in tons per year are shown for each surrounding county. While the 

displayed emissions are from the NEI, the exercise should be carried out using the maximum 

allowable or potential to emit emissions. Also shown in Figure C.1 is a 50 km buffer around 

Collingsworth County. The colored dots represent individual SO2 facilities with colors denoting 

2005 NEI annual SO2 emissions (tons). 

 In Figure C.1, it can be seen that there are a few 100+ ton emitters near the 50 km buffer 

of Collingsworth County (i.e., only in Gray and Beckham Counties). Other than those two 

counties, all of the other counties within the 50 km buffer of Collingsworth County contain 

sources that are below 100 tons. If modeling was done for the 100+ ton sources in Gray and 

Beckham Counties, the existing modeling could show if there are any predicted violations that 

extend into Collingsworth County (See Section A). If the results indicate such potential 

violations in Collingsworth County, then portions of Collingsworth County may already be 

designated as nonattainment as part of the overall area. The attainment status of the remainder of 

Collingsworth County could be analyzed using the methodology described in Section B. If 

existing modeling does not show modeled violations in Collingsworth County, the attainment 

status for the entire county could be determined using methodology described in Section B. 

Also shown in Figure C.1 are two wind roses showing 2006-2010 data for the National 

Weather Service ASOS stations in the area (i.e., Borger, TX in the northwest and Clinton, OK in 

the east). These wind roses could be used in the analyses of the other counties around 

Collingsworth County. Given that most of the surrounding counties contain emissions sources of 

less than one ton, they should not present a nonattainment problem for Collingsworth County. 

Also, given that the 30 ton emitter in Greer County, OK is outside the 50 km buffer of 
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Collingsworth County, it may not present a nonattainment problem in Collingsworth County 

given the distance from the county and prevailing wind patterns. If there is any question about 

the potential impact of a source to a low emitting county’s nonattainment status the seven 

questions in Section B should be considered. 

A similar approach could be applied in the other low emitting counties (e.g., Wheeler, 

Donley, Hall, etc). 

Figure C.1. Illustrative example of non-modeling technical analysis for Collingsworth County, 

TX.