MEMORANDUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

MAR 2 0 2015 

OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS 

SUBJECT: Updated Guidance for Area Designations for the 2010 Primary Sulfur Dioxide National 

Ambient Air Quality Standard 

FROM: Stephen D. Page 

Director 

TO: Regional Air Division Directors, Regions 1 - 10 

The purpose of this memorandum is to provide information on the schedule and process for designating 

areas for the 2010 primary sulfur dioxide (S02) national ambient air quality standard (NAAQS). In 

addition, it identifies factors for determining the boundaries for S02 areas designated nonattainment, 

attainment and unclassifiable. We recommend that states and tribes consider and address these factors 

when identifying boundaries for any updated area designation recommendations they choose to submit 

for the 2010 S02 NAAQS. Please share this information with the state and tribal agencies in your 

region. As explained below, this guidance updates and replaces the previous designation guidance for 

the 2010 S02 NAAQS, which was issued on March 24,201 1. In assessing the appropriate designation 

for a given area, the EPA will consider the state's previous designation recommendations and any 

updated recommendations, as well as any additional relevant information that we receive. 

On June 2, 2010, then-Administrator Jackson signed the revised primary S02 NAAQS (75 FR 35520, 

June 22, 201 0) after review of the existing primary S02 standards issued in 1971. The EPA established 

the revised primary S02 standard at 75 parts per billion (ppb) which is met at a monitoring site when 

the 3-year average of the 99th percentile of daily maximum 1-hour concentrations does not exceed 75 

ppb. The Administrator determined that this is the level necessary to provide protection of public 

health with an adequate margin of safety, especially for children, the elderly and those with asthma. 

These groups are particularly susceptible to the health effects associated with breathing S02. 

On August 5, 2013, the EPA published a notice announcing designations of29 areas in 16 states as 

nonattainrnent for the 2010 S02 standard, based on certified ambient air quality monitoring data for the 

years 2009- 2011 that showed these areas were violating the standard (78 FR 4 7191 ). However, at that 

time the EPA was not yet prepared to issue designations for the remaining areas. With stakeholder input, 

the EPA developed and proposed the S02 Data Requirements Rule (DRR) that would require states to 

gather and submit to the EPA additional information characterizing S02 air quality in areas with larger 

sources of S02 emissions. (79 FR 27445). The EPA intends to use this information to inform the 

designations of these areas. In the S02 DRR, as proposed, air agencies implementing the 1-hour S02 

standard would have the choice to use either monitoring or modeling to characterize S02 air quality in 

Internet Address (URL) • http://www.epa.gov 

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the vicinity of priority S02 sources, and submit the modeling and/or monitoring results to the EPA on 

the schedule specified in the rule. 

Following the initial August 2013 designations, three lawsuits were filed against the EPA in different 

U.S. District Courts, alleging that the agency had failed to perform a nondiscretionary duty under the 

Clean Air Act (CAA) by not designating all portions of the country by the June 2013 deadline. In an 

effort intended to resolve the litigation in one of those cases, plaintiffs Sierra Club and the Natural 

Resources Defense Council and the EPA filed with the U.S. District Court for the Northern District of 

California a proposed consent decree that specified a schedule for the EPA to complete the remaining 

S02 designations for the rest of country in three additional rounds. On March 2, 2015, the Court entered 

the consent decree and issued an enforceable order for the EPA to complete the area designations 

according to the consent decree schedule. 

General Approach and Schedule 

Pursuant to the court order, the EPA must complete the remaining designations on a schedule that 

contains three specific deadlines. By no later than July 2, 2016 (16 months from the court's order), the 

EPA must designate two groups of areas: (I) areas that have newly monitored violations of the 2010 

S02 standard and (2) areas that contain any stationary source that according to the EPA's Air Markets 

Database either emitted more than 16,000 tons ofS02 in 2012 or emitted more than 2,600 tons ofS02 

and had an annual average emission rate of at least 0.45 lbs S02/mmBtu 1 in 2012 and that has not been 

announced (as of March 2, 2015) for retirement. 2 The last two deadlines for completing remaining 

designations are December 31, 2017, and December 31, 2020. The designations completed by these later 

deadlines are expected to be informed by information provided by states pursuant to the anticipated S02 

DRR.3 However, even if the S02 DRR is notfinalized, the EPA must still complete designations 

pursuant to these deadlines. 

In the time since states submitted their original designations recommendations in 20 II, they may have 

obtained additional monitoring and/or modeling information that may be relevant for future 

designations. The CAA section 107(d) does not require states to submit updated recommendations. 

However, if states would like to submit updated designation recommendations for the EPA to consider 

in the next rounds of designations, we will consider such information. 

For the round of designations to be completed by July 2, 2016, we request that states provide updated 

recommendations and supporting information to the EPA by September 18, 2015, to assure that the 

information can be fully considered by the EPA. If the EPA intends to modify any state's designation 

recommendation (original or updated), we will notify the state no later than 120 days prior to 

promulgating the final designations (e.g., no later than March 2, 2016, for designations due to be 

promulgated on or before July 2, 2016). States will then have an opportunity to comment on the EPA's 

intended modifications and provide additional information for the EPA to consider. While the language 

1 The abbreviation "lbs/mmBtu" means pounds per one million British thermal units. 2 A stationary source with a coal-fired unit that as of January I, 20 I 0, had a capacity of over 5 megawatts that otherwise 

meets the emissions criteria, is excluded from the July 2, 20 !6, deadline if it had announced by March 2, 2015, that it will 

cease burning coal at that unit through a company public announcement, public utilities commission filing, consent decree, 

public legal settlement, final state or federal permit filing or other similar means of communications. 3 As a legal matter, the deadlines in the court order apply even if the EPA does not finalize the SO, DRR. However, the 

general approach described in this memorandum reflects our expectation that the EPA will in fact finalize the S02 DRR. 

2 
in section I 07 specifically addresses states, we intend to follow the same process for tribes, pursuant to 

section 30l(d) of the CAA and the Tribal Authority Rule (40 CFR Part 49). Therefore, we intend to 

designate tribal areas, in consultation with the tribes, on the same schedule as state designations. If a 

state or tribe does not submit an updated designation recommendation, the EPA will promulgate the 

designations that it deems appropriate after considering the originally submitted recommendation and 

any relevant additional information available to the EPA. 

The states and tribes that choose to submit updated recommendations should identify areas as 

attainment, nonattainment or unclassifiable on the basis of currently available information. The EPA 

intends to send "120-day letters" to selected tribes by January 22,2016 (and not later than March 2, 

2016) notifying them of the EPA's intended designation decisions. !fa state or tribe has additional 

information that it wants the EPA to consider in response to a 120-day letter, we request that such 

information be submitted by April 8, 2016. This will help ensure that the EPA can fully consider any 

such information prior to issuing final designations. Also, although not required by statute, in order to 

consider public input in the designation process, we plan to provide a 30-day public comment period 

immediately following issuance of the EPA's 120-day letters responding to the recommendations made 

by states and tribes. Attachment I is the anticipated schedule for the round of designations that must be 

completed by July 2, 2016. 

We recognize that the timeline for designations by July 2, 2016, does not provide for establishment and 

use of data from new ambient monitors. Therefore, we anticipate that in many areas the most reliable 

information for informing these designations will be based on source modeling. The EPA has issued 

guidance on the use of source modeling for this purpose in the S02 NAAQS Designations Modeling 

Technical Assistance Document (Modeling TAD).4 

Following this next round, the EPA is also required by the court order to complete designations for 

remaining areas in up to two additional rounds. According to the court order, the EPA must sign for 

publication in the Federal Register no later than December 31, 2017, designations for remaining 

undesignated areas in which, by January I, 2017, states have not installed and begun operating an 

appropriate S02 monitoring network meeting the EPA specifications referenced in the EPA's anticipated 

S02 D RR. We expect this round to cover most remaining undesignated areas in the nation - those for 

which states choose to follow the modeling approach under the anticipated S02 ORR to address impacts 

from larger S02 sources, and those which do not contain S02 sources that would require further 

characterization under the anticipated S02 ORR. Finally, by no later than December 31, 2020, the EPA 

must issue designations for all remaining undesignated areas. This final round would cover areas for 

which states choose to follow and timely implement the monitoring approach under the anticipated S02 

ORR. We will provide more information regarding the approach and schedule for the remaining rounds 

of designations in the future. 

When considering boundaries for updated area designation recommendations, the EPA recommends that 

states and tribes analyze and consider the following five factors: ambient air quality data or dispersion 

modeling, emissions and emissions-related data, meteorology, geography/topography and jurisdictional 

boundaries. (See Attachment 2). In addition, the EPA has provided two TAOs to assist states and tribes 

with assessing modeling and monitoring data when determining if an area is meeting or not meeting the 

4 The Modeling TAD is available on the EPA's website at 

http://www. epa.gov/airqualityl.vu!furdioxide/pdf,/S02ModelingTAD.pdf 

3 
!-hour S02 standard. These documents can be found on the EPA's website at 

http :1 I epa. gov/ a irqua l i tylsulfurdioxide/impl ement. ht mi. 

Identifying an Area that is in Violation of the S02 NAAOS 

Section 107(d)(l) of the CAA defines an area as "nonattainment" if it is violating the NAAQS or if it is 

contributing to a violation in a nearby area. Thus, the first step in making designation decisions is to 

identify areas for which monitoring data or appropriate modeling information indicate a violation of the 

NAAQS. 

Monitor-based Violations. In assessing whether monitoring data indicate a violation, the EPA intends to 

use the most recent three consecutive years of quality-assured, certified ambient air quality data in the 

EPA Air Quality System (AQS)5 using data from Federal Reference Method and Federal Equivalent 

Method monitors that are sited and operated in accordance with 40 CFR Parts 50 and 58. Procedures for 

using monitored air quality data to determine whether a violation has occurred are given in 40 CFR Part 

50 Appendix T, as revised in conjunction with the 20 I 0 S02 NAAQS. We expect that in providing any 

updated recommendations to the EPA for the areas to be designated by July 2, 2016, states and tribes 

would review available S02 monitoring data fi·om 2012-2014. Prior to the EPA issuing letters to states 

and tribes concerning any intended modifications to state recommendations, certified S02 monitoring 

data from 2015 may become available. If this is the case, the EPA intends to also consider certified 2015 

S02 ambient air quality monitoring data in formulating any intended modifications to state and tribal 

recommendations. 

Ambient air quality monitoring data affected by exceptional events may be excluded from use in 

identifying a violation if they meet the criteria for exclusion, as specified in the final rule "Treatment of 

Data Influenced by Exceptional Events" (72 FR 13560; March 22, 2007) codified in 40 CFR parts 50 

and 51. In section VII.B of the S02 NAAQS final rule preamble, we discussed schedules for states and 

tribes to flag data influenced by exceptional events and submit related documentation specifically for 

S02 data used in the initial designations process. As previously noted, the EPA did not designate all 

areas of the country in August 2013 and, consistent with the court order, intends to complete 

designations in multiple rounds beginning on or about July 2, 2016, through on or about December 31, 

2020. This extended designations schedule introduces additional years of monitoring data in which S02 

concentrations may be influenced by exceptional events. Because the previously promulgated 

exceptional events data flagging and documentation submittal dates associated with S02 data that could 

be considered in the initial area designations process have already passed, the EPA is recommending 

new submittal dates in this guidance for these future data. State and tribal air agencies who wish the 

EPA to be able to fully consider excluding exceptional event-affected S02 data for purposes of 

designations are asked to flag exceptional event data and submit an initial description in the EPA's 

AQS, and also submit the associated exceptional event demonstrations no later than 5 months prior to 

the date of final designation decisions. Additionally, air agencies are asked to notify their reviewing 

EPA Regional Office as soon as possible about their intent to prepare and submit an exceptional event 

documentation. 

Model-based Violations. States and tribes may also use air quality modeling results to indicate a 

violation of the S02 NAAQS. The EPA believes that dispersion modeling is an appropriate tool to 

5 S02 air quality data is available from the EPA's website at www.epa.gov/ttnlairs/airsaqsl. 

4 
indicate a violation of the S02 NAAQS, and it has used dispersion modeling in the past for S02 

designations. The previously mentioned December 2013 Modeling TAD provides further 

recommendations on developing an appropriate refined dispersion modeling analysis to support 

designation recommendations. Such modeling could include using the AERMOD dispersion model or 

other appropriate dispersion model. The Modeling TAD updates the March 24, 2011, Modeling 

Guidance for S02 NAAQS Designations in four respects, all to ensure that a modeling approach better 

simulates a monitoring approach. Specifically, the Modeling TAD recommends: 

• using actual emissions as an input for assessing violations to provide results that reflect current 

actual air quality (i.e., modeling that simulates a monitor); 

• using 3 years of modeling results to calculate a simulated design value consistent with the 3-year 

monitoring period required to develop a design value for comparison to the NAAQS; 

• placing receptors only in locations where a monitor could be placed; and 

• using actual stack heights rather than following the Good Engineering Practice stack height 

policy when using actual emissions. 

Identifying Attainment Areas 

The EPA may designate an area as attainment if it is clear that it meets the S02 NAAQS and does not 

contribute to a violation in a nearby area. An area may be demonstrated in attainment if the most recent 

3 years of ambient air quality monitoring data indicate no violations and if the monitoring network in the 

area is sufficient to be compared to the NAAQS per the S02 NAAQS Designations Monitoring TAD6 

An area may also be demonstrated in attainment if appropriate modeling analysis indicates no violations 

ofthe 2010 S02 NAAQS. (Using allowable emissions in modeling would be an appropriate approach to 

support an attainment determination.) In either case, it will also be necessary to show that sources in the 

area are not contributing to a violation in a nearby area. In the absence of information clearly 

demonstrating a designation of "attainment" or "nonattainment," the EPA intends to designate the area 

as "unclassifiable"7 when it takes action pursuant to the court order. 

Determining N onattainment Area Boundaries 

Ambient S02 is a pollutant that arises from direct emissions, and S02 concentrations are generally 

expected to be highest relatively close to the source(s) and lower at farther distances due to dispersion. 

Thus, S02 concentration patterns resemble those of other directly emitted pollutants like lead and differ 

from those of photochemically-formed (secondary) pollutants such as ozone. Accordingly, as explained 

in the 2011 guidance, we expect to continue to consider county boundaries as the analytical starting 

point for determining S02 nonattainment areas. 

We believe it is appropriate to evaluate each potential nonattainment area on a case-by-case basis to 

determine an appropriate boundary that satisfies the statutory conditions. A nonattainment area should 

contain the area violating the NAAQS (e.g., the area around a violating monitor or encompassing 

6 The Monitoring TAD is available on the EPA's website at 

http://www. epa.govlairqualitylsu!furdioxidelpdfilS02MonitoringTAD.pdf 7 While states have and may continue to submit designations recommendations identifying areas as "attainment," the EPA 

expects to continue its traditional approach, where appropriate, of using a designation category of"unclassifiable/attainmentn 

for areas that the EPA determines meet the NAAQS. The EPA expects to reserve the category "unclassifiable" for areas 

where the EPA cannot determine based on available information whether the area is meeting or not meeting the NAAQS or 

where the EPA cannot determine whether the area contributes to a violation in a nearby area. 

5 
modeled violations), as well as any nearby areas (e.g., counties or portions thereof) that contain 

emissions sources contributing to the monitored or modeled violations. (See CAA section 

107(d)(l)(A)(i)). We recommend that states and tribes base their updated boundary recommendations on 

an evaluation of five factors: 1) air quality data or dispersion modeling results; 2) emissions-related data; 

3) meteorology; 4) geography and topography; and 5) jurisdictional boundaries, as well as other 

available data. These factors are discussed in more detail in Attachment 2. Dispersion modeling, as 

discussed in the Modeling TAD, can be a helpful tool in this evaluation because it allows the model user 

to simultaneously assess multiple factors. States and tribes may identify and evaluate other relevant 

factors or circumstances specific to a particular area. 

While the EPA generally believes that in the absence of other relevant information it is appropriate to 

use county boundaries to define nonattainment areas, we recognize that the five-factor analysis and other 

information may support a nonattainment area consisting of only a portion of a county. For example, a 

topographical feature may divide a county into two separate air basins, or contributing sources may be 

clustered in only a pmtion of a county. 8 For detlning partial county boundaries, the EPA recommends 

the use of well-defined jurisdictional lines such as township borders or other well-established 

geopolitical boundaries, and immovable landmarks such as major roadways or other permanent and 

readily identifiable physical features. 

Determining Attainment Area Boundaries 

An attainment ar·ea cannot contain any area that violates the NAAQS or contributes to a violation of the 

NAAQS in a nearby area. (See CAA section 107(d)(l)(A)(i)). County boundaries may be appropriate for 

defining attainment areas in the absence of any other relevant information that would help define a more 

specific boundary around the SOz source(s) in question. To detlne more specitlc boundaries, we 

recommend an evaluation of the flve factors mentioned previously, and, in particular, the use of 

dispersion modeling, as discussed in the Modeling TAD, to simultaneously assess multiple factors. 

While we believe this memorandum provides helpful guidance on how boundaries would be determined 

for S02 designations, the guidance contained herein is not binding on states, tribes the public or the 

EPA. The final basis for determining area boundaries will be addressed in the EPA's regulatory action to 

designate areas under the 2010 S02 NAAQS. When the EPA promulgates designations, those 

determinations will be final and binding on states, tribes, the public and the EPA. 

Attachment 1 is a timeline of key dates in the round of designations for the revised 2010 S02 NAAQS 

that must be completed by July 2, 2016. Attachment 2 identifles the primary five factors that the EPA 

plans to consider in evaluating and making decisions on appropriate nonattainment and attainment area 

boundaries. The EPA recommends that states and tribes also address these factors and follow the 

Monitoring and Modeling T ADs if they choose to submit updated designation recommendations. 

8 In the case of a contributing SO, source located very near a county boundary or state border, it may be appropriate for the 

corresponding nonattainment area to include portions of multiple counties or multiple states. 

6 
Staff members at the EPA's Office of Air Quality Planning and Standards are available for assistance 

and consultation throughout the designations process. General questions on this guidance may be 

directed to Andy Chang (919) 541-2416. Modeling-related questions may be directed to James Thurman 

(919) 541-2703. Monitoring-related questions may be directed to Nealson Watkins (919) 541-5522. 

Attachments (2) 

7 
ATTACHMENT 1 

TIMELINE FOR 2010 PRIMARY S02 NAAQS DESIGNATION PROCESS- ROUND 2-

AREAS AS SOCIA TED WITH JULY 2, 2016, COURT-ORDERED DEADLINE' 

Milestone Dateh 

Court Order March 2, 20 15 

States flag relevant exceptional event- No later than July I, 2015 

influenced so2 monitoring data from 2012 

and 20 13; provide detailed documentation to 

support all claims 

States may submit updated recommendations No later than September 18,2015 

and supporting information for area 

designations to the EPA 

States flag relevant exceptional event- No later than February 2, 2016 

influenced S02 monitoring data from 2014 

and 2015; provide detailed documentation to 

support all claims 

The EPA notifies states concerning any January 22, 20 16 

intended modifications to their (no later than 120 days prior to final 

recommendations (120-day letters) designations) 

The EPA publishes public notice of state o/a February 3, 2016 

recommendations and the EPA's intended 

modifications and initiates 30-day public 

comment period 

End of 30-day public comment period o/a March 4, 2016 

States and tribes submit additional o/a April 8, 2016 

information, if desired, to demonstrate why an 

EPA modification is inappropriate 

The EPA signs notice promulgating final S02 No later than July 2, 2016 

area designations (no later than 16 months 

from court order) 

' DesrgnatJOns f01 Round 3 must be p10mulgated by December 3!, 20 !7. Desrgnatrons f01 Round 4 must be promulgated by 

December 3!, 2020. 

b o/a = on or about 

1 
ATTACHMENT 2 

Determining Area Designations and Appropriate Area Boundaries for 

the 2010 S02 NAAQS 

Designation Categories 

Nonattainment Attainment1 Unclassifiable 

An area that the EPA has An area that the EPA has An area where the EPA cannot 

determined violates the 2010 determined meets the 2010 SO, determine based on available 

so, NAAQS, based on the most NAAQS and does not contribute information whether the area is 

recent three years of ambient air to a violation of the NAAQS in or is not meeting the 20 1 0 S02 

quality monitoring data or an a nearby area based on either: a) NAAQS and whether the area 

appropriate modeling analysis, the most recent 3 years of contributes to a violation in a 

or that the EPA has determined ambient air quality monitoring nearby area. 

contributes to a violation in a data from a monitoring network 

nearby area. in area that is sufficient to be 

compared to the NAAQS per 

EPA interpretations in the 

Monitoring TAD, orb) an 

appropriate modeling analysis. 

Nonattainment Area Boundaries. The EPA intends to use the county as the analytical starting 

point for assessing the appropriate geographic boundaries of a SOz nonattainment area. The five 

factors listed below comprise a framework for area-specific analyses to support final boundary 

determinations. The information addressing these factors could include appropriate air quality 

dispersion modeling as recommended by the S02 Designations Modeling TAD2 or, where 

available, ambient air quality monitoring data as recommended by the S02 Designations 

Monitoring TAD.3 For modeled violations, the portion of the modeling domain encompassing 

the modeled violations is an appropriate reference point for determining a nonattainment area 

boundary. 

Attainment Area Boundaries. Areas designated as attainment should be supported by information 

clearly demonstrating that there are no violations of the S02 NAAQS inside the area boundary, 

and that the included portions do not contain emission sources that may contribute to monitored 

or modeled violations outside the area boundary. County boundaries may be appropriate for 

defining attainment areas in the absence of any other relevant information that would help define 

1 While states have and may continue to submit designations recommendations identifying areas as "attainment,'' the 

EPA expects to continue its traditional approach, where appropriate, of using a designation category of 

"unclassifiable/attainment" for areas that the EPA determines meet the NAAQS. The EPA expects to reserve the 

category "unclassifiable 11 for areas where the EPA cannot determine based on available information whether the area 

is meeting or not meeting the NAAQS or where the EPA cannot determine whether the area contributes to a 

violation in a nearby area. 2 The Modeling TAD is available on the EPA's website at 

http://www. epa. go vi a irqua! itylsu/fiJrdioxi de/pdfs/SO 2Modeli ngTA D.pdf 3 The Monitoring TAD is available on the EPA's website at 

http://www.epa.gov/airquality/sulfiirdioxide/pdfs/S02MonitoringTAD.pdf 

1 
a more specific boundary around the S02 source(s) in question. As recommended in the 

Modeling TAD, appropriate modeling for establishing attainment area boundaries would include 

using the AERMOD dispersion model, with actual or allowable source emissions. Where 

appropriate modeling is conducted, the portion of the modeling domain demonstrated to be free 

of violations, and free of sources that may contribute to any nearby violations is an appropriate 

reference point for determining the boundary of an attainment area. 

Developing Supporting Information (Factor Analysis). As a framework for area-specific analyses 

to support final boundary determinations, the EPA intends to evaluate the five factors listed 

below, as well as other relevant available information. The purpose of evaluating these factors is 

to determine the appropriate boundaries encompassing the area meeting the CAA's definitions. 

The guidance in the Modeling TAD discusses how modeling could be used to address several of 

these factors simultaneously. When considered as a whole, results may support boundaries that 

are either larger or smaller than the analytical starting point. 

1. Ambient air quality data or dispersion modeling results. We intend to review S02 

ambient air quality monitoring data, including the design value calculated for each monitor in 

the area, for the most recent 3-year period. Areas where monitoring data indicate a violation 

of the !-hour, 75 ppb primary S02 standard will be designated as "nonattainment." Sourceoriented modeling may also be used to assess air quality in a particular location. The 

Modeling TAD provides further recommendations on using refined dispersion modeling for 

this type of air quality assessment. An area may be demonstrated to meet the S02 NAAQS if 

appropriate modeling analysis based on either actual or allowable emissions from relevant 

sources indicates no violations of the S02 NAAQS. As explained in the Monitoring TAD, 

data from a properly sited monitoring network may also be sufficient to establish that an area 

meets the S02 NAAQS. In either case, to be designated attainment it will be necessary to 

show that sources in the area are not contributing to a violation in a nearby area. 

2. Emissions-related data (location of sources and potential contribution to ambient S02 

concentrations). We intend to examine actual emissions ofS02 from sources located in and 

around the violating area. Significant emissions levels in a nearby area indicate potential for 

the area to contribute to observed or modeled violations of the NAAQS. We intend to review 

data from the latest National Emissions Inventory or other relevant sources of the data, such 

as state inventories or inventories from other federal sources. We would also consider any 

additional information we receive on federally-enforceable emissions controls that are not 

reflected in recent inventories but which will require compliance before final designations are 

issued. 

3. Meteorology (weather and transport patterns). We intend to evaluate meteorological data to 

help determine how weather conditions, including wind speed and direction, affect the plume 

of sources contributing to ambient S02 concentrations. This factor also can be assessed in the 

context of source-oriented dispersion modeling as recommended in the Modeling TAD. 

4. Geography and topography (mountain ranges or other air basin boundaries). We intend to 

examine the physical features of the land that might affect the distribution of S02 over an 

2 
area. Mountains or other physical features may affect the distribution of emissions, and may 

help define area boundaries. 

5. Jurisdictional boundaries (e.g., counties, air districts, pre-existing nonattainment areas, 

reservations, metropolitan planning organizations). For nonattainment areas, once the 

geographic area associated with the area violating the SOz NAAQS and the nearby area 

contributing to violations are determined, we intend to consider existing jurisdictional 

boundaries for the purposes of providing a clearly defined legal boundary for carrying out the 

air quality planning and enforcement functions for the area. If an existing jurisdictional 

boundary is used to help define the nonattainment area, it should encompass all of the area 

that has been identified as meeting the nonattainment definition. Where existing 

jurisdictional boundaries are not adequate to describe the nonattainment area, other clearly 

defined and permanent landmarks or physical features may be used. In certain cases 

geographic coordinates may be appropriate, but geopolitical boundaries are preferred. For 

attainment areas, these same jurisdictional considerations may play a role in determining 

appropriate boundaries. 

The EPA plans to consider these factors, along with any other relevant information, in 

determining whether to make modifications to the designation and area boundary 

recommendations made by states and tribes. The factors listed above, while generally 

comprehensive, are not intended to be exhaustive. States and tribes may submit additional 

information they believe is relevant for the EPA to consider. Any information provided to 

support a boundary recommendation for a nonattainment area should show that: I) violations are 

not occurring in nearby portions that are excluded from the recommended nonattainment area; 

and 2) the excluded portions do not contain emission sources that contribute to a monitored or 

modeled violation. Any information provided to support a boundary recommendation for an 

attainment area should show that: I) violations are not occurring in the area; and 2) the included 

portions do not contain emission sources that may contribute to monitored or modeled violations 

outside the area boundary. In the absence of information clearly demonstrating that an area and 

its associated boundary are properly designated "attainment" or "nonattainment," the EPA 

intends to designate the area as "unclassifiable." 

3