, 

### UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION 10 1200 Sixth**~venue.** Suite900 Seattle, Washington 98101-3140 I

OFFICE OFTHE REGIONAL ADMINISTRATOR

# A~R 04 

## 2011

The Honorable Sean Parnell Governor ofthe State ofAlaska P. O. Box 110001 Juneau, Alaska 99811-0001 Dear Governor Parnell: OnJune 3,2010, the U.S. Environmental Protection Agency (EPA) revisedthe heathbasedNational Ambient Air

### QualityStandard (NAAQS) for sulfur dioxide**(S~)** 

### by establishing anew I-hour standard at alevel of75 parts perbillion (ppb). This new 1-hourstandardwill protectpublic health by reducing people's exPosure to high short-term (5-minutes to 24-hours) concentrations ofS02•

### An areameets the

# new standard when the 3-year average ofthe 99th 

### percentile ofI-hourdaily maximum concentrations does notexceed 75 ppb. Section 107(d)(1) ofthe Clean AirAct(CAA) describes theprocess EPA follows for designating areas as meeting ornot meeting new orrevisedNAAQS. Within one year of establishing a new orrevised standard, each**~overnor**

### mustrecommend a designation of attainment, nonattainment orunclassifiable,**~ith** 

### appropriate boundaries, for areas withintheir jurisdiction. Therefore, we ask that 

### your stateprovide designation recommendations, including appropriate 

### areaboundaries, for all areas byJune 3, 2011, withrespect to

### the I-hour802 NAAQS. , The enclosed memorandum provides

# ~dance to state and local air

### pollution control agencies and tribes onthe timeline andprocess fordesignating areas underthe revised S02 standard. We plan to announce ourintended'designations no laterthan 

### 120 days priorto 

### promulgating final designations. Following the announcementofintended designations, states and tribes will have the opportunity to

### comment on any modifications EPA proposes to 

### their recommendations. EPA intendsto

### complete~designationsbyno later than 

### June 3,2012. I 

### Establishing areadesignations is akey step inthe process ofproviding the health protection intended by the CAA. Shouldyou have questionsregarding this matter, please do not hesitate to contactme or Steve Body ofthe 

## State & 

### Tribal AirPrograms Unit at

### 206-553-0782 or body.steve@epa.gov. Sincerdly,

DennisiJ. 

### Mclerran

## ~ Regio$!Administrator Enclosure 

## I

# ·~~t~~~UNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NC 27711



## MAR 242011 OFFICE OF AIR ()UAUTY PLANNING

AND STANDARDS



### MEMORANDUM SUBJECT: A.rea. Des

..ignation.S for th

..e 20I 0

### Rev12'~ 

# .) imary Sulfur Dioxide National Ambient Air Quality Standard

.**~** "_",Ne ,/ 

### FROM: Stephen D. Page, Direct'**~** Ofllce ofAir Quality Pla ni g and**St~ards** 

### TO: Regional Air Division Directors, Regions I-X

This memorandum provides infonnation on the schedule and process for designating areas for the purpose ofimplementingthe 2010 revised primary sulfur dioxide (S02) national ambient airquality standard (NAAQS). In addition, 

### it identiHes factors EPA intendsto evaluate indetermining boundaries torarcas designated nOllattainment. We recommend that 

### slates and tribes consider and address these factors when identifying boundaries tl.x their area designation recommendations. Please sharethis ini'{)nnation with the state and tribal agencies in your Region. On June 2, 20I0, AdministratorJackson signed therevised primary SO:: NAAQS (75 FR 35520. published on Junc 22,2010) afterreview ofthe existing two primary 

### S02 standards. promulgated on April 30, 1971 (36 FR 8187). EPAestablished the revised primary 

### S02 standard at 75 parts perbillion (ppb) which is attained when the 3-year average ofthe 991h percentile of I-hour dally maximum concentrations does not exceed 75 ppb. The Administrator has detenllined that this is the leveillcccssaryto provide protection ofpublic health with an adequate margin of

### safelY, espcciafly 

### tor children, the elderly and 

### those with asthma. These groups are particularly susceptible to the health effects associated with breathing SO;;. Gc@ral approach and schedule. Clean Air Act (CAA) section 107(d) directs states to submit their S02 designation recommendations to EPA by June 3. 2011. IfEPA intends to modifY any state'sboundary recommendation, 

### l:PA will notify the state no later than 120 days priorto its action to promulgate designations (Le., February 2012 for designations to be promulgated in June 2012), and the state 

### will have an opportunhyto comment on EPA's intended modifications and provide additional intormation for EPA to consider. Section 107(d)requires EPA to promulgate initial area designations by June 3, 2012, which is 2 years after promulgation ofthe revised primary standard. While the language in section 107 specifically addresses states, we intend to follow the same process for tribes, pursuant to section 301 (d) of the CAA and the Tribal Authority Rule (40 CFRPart49). Therefore, we intend to designate tribal areas, inconsultation with the tribes, on the same schedule as state designations. Ifa state ortribe does not submit designation recommendations, EPA will promulgate the designations that 

### it deems appropriate. Sections 111 through VI ofthe preamble to the final rule promulgating the revised primary S02 NAAQS describe the approach EPA anticipates using for designations for the I-hour S02 standard. EPA anticipates taking an analytic approach that uses both air quality monitoring and modeling information for designations. Suchan approach, ifadopted, would be consistent with EPA'shistoric practices for S02 NAAQS implementation. In that preamble we acknowledged that in some cases, monitoring data may 

### be themore technically appropriate information for determining compliance withthe I-hourNAAQS. (Seee.g., 75 FR at 35552, n. 22). We also recognized that a single monitormay generally not be adequate to fully characterize ambient concentrations ofS02, including the maximum ground level concentrations that exist around stationary S02 sources, particularly whenmeasuring for a 1-hour standard. (See 75 FRat 35551). Refined dispersionmodels are able to characterize S02 air quality impacts from the modeled sources across the domainofinterest on an hourly basis with a high degree ofspatial resolution, thus overcoming the limitations ofan approach based solely onmonitoring. Attachment 2 summarizes three possible designations and the criteria for initial designations ofthe I-hour S02 primary standard that EPA expects to apply. As stated in the preamble, we do not believe it would be realistic orappropriate to expect states to complete modeling for all significant sources ofS02 and assess the results .in time for the designation recommendations the Actrequires be submittedto EPA by June 3, 2011. (See 75 FR at 35570­ 71). Therefore, we do not generally expect states to provide refined dispersion modeling information along with their initial designation recommendations. However, EPA does intend to consider, as appropriate, available air quality monitoring and modeling information submitted by states or tribes in support oftheir recommendations. States and tribes should identify areas as attairunent, nonattainment orunclassifiable on the basis ofavailable information. Given the currently limited network ofS02monitors, and our expectation that states 

### will not yet have completed appropriate modeling ofall significant S02 sources, we anticipate that most areas ofthe country will be designated "unclassifiable." I

### f a state or tribe, following receipt ofanEPA 120-day letter, has additional infonnationthat it wants EPA to consider with respect to a designation recommendation that EPA plans to modify. we request that such information be submitted within 60 days after receiving EPA's letter. This will help ensure that EPAcan fully considerany such information priorto issuing tinal designations.
2Also, 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 EPA's letters responding to the recommendations made by states and tribes. Attachment 1isthis anticipated schedule. Thepreamble to the final NAAQS rulemaking includes a general discussion ofstates' statutory planning and emissions control responsibilities under each ofthe three possible designations. The CAA directs states with areas designated as "nonattainment" for S02to develop and submit a plan within 18 months after designation providing for attainment as expeditiously as practicable, but no later than 5 years after the initial designation date. (See CAA sections 191-193). The CAA also directs states to submit by June 3, 2013, a SIP demonstrating an adequate program to implement, maintain and enforce the S02NAAQS. Generally, these infrastructureplans for attainmentareas are not expected to include an attainment demonstration. However, in light ofthe incomplete monitoring and modeling data available at the time ofdesignations, for areas designated unclassifiable, we would expect states to include inthese plans demonstrations ofexpeditious attainment and maintenance ofthe S02 NAAQS. EPA is developing separate guidance ondeveloping SIPrevisions for the S02standard and we intend to seek public review and comment on that guidance document. Identifying an areathat is in violation orthe S02 NAAQS. Section 107(d){1) ofthe CAA defines an area as "nonattainn:tent" if

### itis violating the NAAQS orifit is contributing to a violation in a nearby area. Thus, the first step in making designations is to identify through monitoring orappropriate modeling areas violating the NAAQS. In assessingwhether monitoring data indicate a violation, EPA intends to use the most recent three consecutive years ofquality-assured, certified air quality data in the EPA Air Quality System (AQS), I using data from Federal Reference Method (FRM) and Federal Equivalent Method (FEM) monitors that are sited and operated in accordance with 40 CFR Parts 50 and 58. Procedures for using monitored air quality datato detennine whether a violation has occurred are given in 40 CFR Part 50 Appendix T, as revised in conjunction with the final rule tor the 2010 S02 NAAQS. We expect that in providingtheir recommendations to EPA, states and tribes would review available S02 monitoring data from 2008 through 2010. Priorto EPA issuing letters to states and tribes concem.ing any intended modifications to theirrecommendations, data from 2011 may become available. Ifthis is the case, EPA intends to also consider 2011 S02 airquality monitoring data in formulating any intended modifications to state and tribal recommendations. Air quality monitoringdata affected by exceptional events may be excludedfrom use in identifying a violation ifthey meet the criteria for exclusion, as speci11ed in the final rule

1 This information is available on EPA's websiteatwww.epa.gov/ttn/alrs/alrsaqs/. 3'TreatmentofData Influencedby Exceptional Events" (72 FR 13560; March 22, 2007) codified in 40 CFR Parts 50 and 51. In section VILB ofthe S02 NAAQS final rule preamble, we discussed schedules for states and tribesto flag data influenced by exceptional events and submit related documentation specificalIy for SOl data collected from 2008 through 2010 used in the initial designations process. These schedules are contained in Table 1of40 CFR 50.14 and require initial data flagging by October 1,2010and detailed documentation submittal. by June 

### 1. 2011. This should assure that any exceptional events claim asserted by a state ortribe can be fully considered by EPA before final designations. States and tribes may also chooseto use available airquality modeling results to indicate a violation ofthe NAAQS. Attachment 3 provides further guidance on the appropriate refined dispersion modeling analysis that could be used to support designation recommendations. Such modeling could include usingtheAERMOD dispersion model, \\ith allowable source emissions and emissions limitation credit for stacks no higher than good engineering practice. As noted above (and in the preamble to the final 802primary NAAQS rulemaking), we recognizethat 

### it is notrealistic to expect states ortribesto complete this type ofmodetingfor all significant sources ofS02 in the time available for providingdesignation recommendations. Wherethe time and resources to conduct refined dispersion modeling are limited, we believe 

### it is reasonable to focus first onthe most significant sources ofS02emissions, and onthose sources that are most likely to contribute to a violation. Werecognize that this approach means that all areas where S02 NAAQS violations maybe occurring might not be identitied in the initial round ofarea designations. States are expectedto address any such areas in the course ofdeveloping the SIPs due by June 

### 3, 2013. Identifying attainment areas. EPA may initially designate an area as_attainment if

### it is clear that it meets the S02NAAQS. EPA doesnot believe itwouldbe appropriate to do so without appropriate refined dispersion modeling and, where avaiJable, airquality monitoring data indicatingno violations ofthe NAAQS. In the absence ofinformation clearly demonstrating a designation of"attainment" or "nonattainment,"EPA intends to designate the area as "unclassifiable." Determining nonattainment area boundaries. As a pollutant that arises from direct emissions, S02concentrations are highest relatively closeto the source(s) and much lower at greaterdistances due to dispersion. Thus, S02 concentration patterns resemble those ofother directly emitted pollutants like lead and differ from those ofphotochemically-formed (secondary) pollutants such as ozone. Accordingly, consistent with our approach underother NAAQS, we expect to consider the countyline as the starting point for determining S02 nonattainment areas. As discussed further in Attachment 2, EPAintends to consider several factors when determining the final nonattainment boundaries. We believe 

### it is appropriate to evaluate each potential nonattainmentarea on a case-by-case basis, and to recognize that area4specific analyses conducted by states, tribes and/or EPA may support a boundary with either a larger or smaller areathan the countyboundary, A nonattainment area should contain the areaviolating the NAAQS (e.g., the area around a violating monitor), as well as any adjacent areas (e.g., counties or portionsthereof) that contain emissions sources contributing to the violation. (See CAA section 107(d)(1)(A)(i». Consequently, we recommend that states and tribesbase their boundary recommendations on an evaluation offive factors: 1) air quality data; 2) emissions-related data; 3) meteorology; 4) geography/topography and 5)jurisdictional boundaries, as well as other available data. Dispersion modeling, as discussed in Attachment 3, can be a helpful tool in this evaluation because 

### itallows the model userto simultaneously assess multiple factors. States and tribes may identify and evaluateotherrelevant factors orcircumstances specific to a particular area. While EPA generally believes that in the absence ofother relevant information it is appropriate to use county boundaries to define nonattainment area';, we recognize thatthe fivefactor analysis and other infomlation may support designating only a portion ofa county as "nonattainment." For example, a topographical teature may divide a county into two separate air basins, orcontributing sources may be clustered in only a portion ofa county. For defining partial county boundaries, EPA recommends the use ofwell·deJinedjurisdictional lines such as township borders, immovable landmarks such as major roadways orotherpermanentand readily identifiable boundaries. Determining attainment area boundaries. In areas without a violating monitor, refined dispersion modeling could be used to help determine that an area with S02 sources is in attainmentfor the 1-hour S02NAAQS. An attainment area boundary cannot contain any area that exceeds the NAAQS orany area containing sources that are causing or contributing to a violating area. (See CAA section 107(d)(1)(A)(i). County boundaries may be appropriate for defining attainment areas in the absence ofother information that would help define a more specific boundary around the modeled source(s). While we believe this memorandum provides helpful guidance onhow boundaries would be determined for S02designations, the guidance contained herein is not binding on states, tribes the public or EPA. The final basis for detemlining nonattainment area boundaries will be addressed in EPA's action to initially designate areas under the 2010 S02 standard. When EPA promulgates designations, those determinations will be final and binding on states, tribes, the public and EPA. Attachment 1is a timeline ofkey dates in the designations process 

### tor therevised 2010 S02 NAAQS. Attachment 2 identifies the primary five factors that EPA plans to consider in evaluating and making decisions onnonattainment area boundaries. Attachment 3 is the modeling guidancethat states and tribes should use to support designation recommendations, 5including appropriate area boundaries. Staffmembers at EPA's Office ofAir Quality }llanning and Standards are available for ass.istance and consultation throughout the designations process. General questions on this guidance may be directed to Valerie Broadwell (919) 541-3310 orDoug Solomon (919) 541­ 4132. Modeling-related questions may be directed to James Thurman (919) 541-2703. Attachments: 3

cc: Scott Mathias, OAQPS Lydia Wegman, OAQPS Richard Wayland, OAQPS Greg Green, OAQPS Margo Oge, OTAQ Kevin McLean, OGe Sara Schneeberg, OGe

6

### ATTACHMENT 1

TIMELINE FOR2010 

### Primary S02NAAQS DESI(;NATION-PROCESS Milestone Date* EPApromulgates S02NAAQS June 3, 2010 States and tribes flag exceptional event-influenced S02monitoring data from 2008-2009 October 1,2010

States and tribes flag exceptional event-influenced S02 monitoring data from 2010; provide detailed documentation to support a1l2008-2010 claims No later than June 1,2011

States and tribes submit recommendations for area designations to EPA No later than June 3, 2011

EPA notifies states and tribes concerning any intended modifications to their recommendations (120.day letters) ola February 3, 2012 (no later than 120 days prior to final designations)

EPA publishes public notice ofstate and tribal recommendations and EPA's intended modifications and initiates 30-daypublic comment period ola February 20, 2012

Endof30-day public comment period ola March 20,2012 States and tribes submit additional information to demonstrate why an EPA modification is inappropriate olaApril 3,2012

EPA promulgates final S02areadesignations No later than June 3, 2012 

### '" ola = on orabout Note: This schedule assumes EPA has sufficient information to promulgate designations within 2 years. In the event EPA determines that insufficient information is available to do so, the Clean Air Act allows EPA to extendthe designationprocess, but no later thanJune 3, 2013.

### ATTACHMENT2

.Determining Designations and 

### AppropriateArea Boundaries for the l·hour, 75 ppb SOl

### NAAQS



### Nonattainment**-.~-~** 

### Attainment Unclassifiable (all other areas) An area' where monitoring data Q.t an appropriate modeling analysis indicate a violation. An area that has no monitored violations and which has an appropriate modeling analysis, ifneeded, and any other relevant infonnation demonstrating no violations. An area that has no monitored violations and lacks an appropriate modeling analysis, ifneeded, or other appropriate infonnation sufficientto support an alternate designation.

Attainment area boundaries. Areas designated as "attainment" should be supported by inlunnation clearly demonstrating that there are no violations oCthe 

### S02NAAQS inside the area boundary. This couldconsist ofappropriate airquality dispersion modeling and, where available. air quality monitoring data. As provided in Attachment 3, appropriate modeling would include using the AERMOD dispersion model, with allowable source emissions and emissions limitation credit for stacks no higher than good engineering practice. Countyboundaries may be appropriate for defining attainment areas inthe absence ofother infonnationthat would help define a more specific boundary around the modeled source(s). In the absence ofinformation clearly demonstrating a designation of"attainment" or "nonattainment" EPA intends to designate tbe area as ··unclassifiable." Nonattainment area**boundarie~.** EPA intends to lJse the county as tbe analytical starting point Jur assessing the appropriate geographic boundaries ofa 

### S02 nonattainment area. As a framework for area-specific analyses to supportfinal boundary determinations, we intend to evaluate the five factors listed below, as well as otherrelevant available information. The purpose ofevaluatingthese factors is to determine the appropriate boundaries encompassingthe area meeting the CAA's definition of"nonattainment area" i.e., an area violating the S02 standard and any nearby areas contributing to tbe violating area. The modeling guidance in Attachment 3 discusses howmodeling could be used to address several ofthese factors simultaneously. When considered as a whole, results may support nonattainment boundaries that are either larger or smallerthan the analytical starting point. L 

### Air quality data. We intend to review 

### 802 air quality monitoring data, including the design value calculated for each monitorin the area, for the most recent 3-yearperiod. Areas where monitoring data indicate a violation ofthe I-hour, 75 ppb primary 

### S02 standard 

### will be designated as "nonattainment." Source-oriented modeling,may also be used to assess air quality in a particular location. Attachment 3 provides further guidance on using refined dispersion modeling for this type ofassessment 

### 2. Emissions-related data (location ofsources and potential contribution to ambient S02 concentrations). We intcndto examine allowable emissions 0[S02 from sources located inand around the violating area. Significant emissions levels in a nearby area indicate the potentia] for the area to contribute to observed or modeled violations ofthe NAAQS. We intend to review data from the latest National Emissions Inventory orother relevant sources ofthe data, such as state inventories orinventories from otherfederal sources. We would also consider any additional information we receive onfederally-enforceable emissions controls that are not reflected in recent inventories but which will require compliance before final designations are issued. 3. Meteorology (weather/transportpatterns)

.. We intend to evaluate meteorological data to help determine howweather conditions, including wind speed and direction, affect the plume ofsources contributing to ambient**S~** concentrations. Where feasible, we would consider results from source-oriented dispersion modeling. 4. Geograpby/topography (mountain ranges or other airbasinboundaries). We intend to examine the physical features ofthe land that might affect the distribution ofS02overan area. Mountains or otherphysical features may affect the distribution ofemissions, and may help define boundaries. 5. Jurisdictional boundaries (e.g., counties, air districts, pre-existing nonattainment areas, reservations, metropolitanplanning organizations). Oncethe geographic area associated with the area violatingthe S02 standard and the nearby area contributing to violations are determined, we intend to consider existingjurisdictional boundaries for the purposes of providing a clearlydefined legal boundary for carrying out the air quality planning and enforcement functions for the nonattainment area. Ifan existingjurisdictional boundary is used to help define the nonattainment area, it should encompass all ofthe area that has been identitied as meetingthe nonattainment definition. Where existingjurisdictional boundaries are not adequate to describe the nonattainment area, other clearly defined andpermanent landmarks orgeographic coordinates may be used. EPA plans to consider these tactors. along with any otherrelevant information, in determining whether to make modifications to the 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 EPA to consider. Any information provided to support a boundary recommendation for a nonattainment area should showthat: 1) 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 the monitored or modeled violation.

2

### ATTACHMENT 3 Modeling Guidancefor SOlNAAQS Designations 1. Purpose 

### On June 2, 2010, 

### AdministratorJackson signed a final ruJemaking notice that revised the primary S02 NAAQS (75 

### FR 35520, 

### published on June 22. 2010) 

### afterreview ofthe existing two primary SOi 

### standards, promulgated on April 30, 1971 

### (36 FR 8187).1 EPA established the revised primary S02standard at 75 

### parts per billion (ppb) which is attained when the 3-year average ofthe 99th 

### percentile ofI-hourdaily maximum concentrations does not exceed 75 

### ppb, 

### In the final rule preamble, EPA outlined an expected analytic approach to determining compJiance with the new NAAQS that would include the use ofboth modeling and monitoring. EPAbelieves this analytic approach to determining compliance with the new I-hour NAAQS would be the generally more technically appropriate and accurate means ofassessing peak 1­ hour SOl 

### concentrations, and would be

### consistent with historic (past and more recent) implementation practice ofusing models to determine compliance with the SOl NAAQS. While this guidance explains the use ofmodeling for NAAQS designations, it does not preclude the fact that monitoring data may be more technically appropriate than modeling in some cases. In cases wherethere is complete air quality data from FRM orFEM S02monitors, that data would be considered by EPA in designating areas as attainment or nonattainment. (See 75 

### FR at 35570). 

### The guidance presented here is for cases where modeling is used in support of the designations process. Dispersion modeling could be 

### used in these initial designations to a limited degree (as could monitoring) but wouLd likely be used to a larger extent subsequently as the basis for redesignation ofnonattainment and unclassifiable areas to attainment. As the preamble to the rule promulgating the new I-hour S02 NAAQS noted, EPA does not think it realistic or appropriate to expect states to complete modeling for all significant sources ofS02 and assess the results in time for the designation recommendations the Act requires be submitted by June 2011. 

### (Sec 75 

### FR at 35570-71). 

### Therefore, we do 

### not generally expect states to provide modeling information along with their initial designation recommendations. lIowever, EPA does intend to consider, as appropriate. available monitoring data and modeling information submitted by states ortribes in support oftheir recommendations. This guidance explains the expected application ofdispersion models to support the designations process regarding:

I EPA publiclydisseminated a copy ofthesigned notice onJune 3,2010,and therefore treats June 2, 2010, as the date oftherule's promulgation, for purposesofthe deadlines in 

### eMsection 1 07(d). 11. the use ofmodeling to infonnthe nonattainment boundaries for areas with violating ambient airquality monitors 

### ifthe presumptive county boundaries are not used (either to expand the boundaries outside the county orshrink the boundary within the county); and 

### 2. The use ofmodeling in areas without a violating monitor as evidence ofattainment ofthe NAAQS (showing no violations or contributions to violations ofthe standard). This guidance isconsistent with EPA's Guideline onAirQuality Mode!s, orAppendix W to 

### 40 CFR Part 51, and other relevant modeling guidance issued to support regulatory programs. When the need for interpretation ofthis guidance arises, the user should consult with the appropriate Regional Modeling Contact2

, Also as indicated inthe preamble ofthe I-hour 802NAAQ8 final nile, we intend to issue additional guidance describingthe development ofan approvable 11O(a)(1) implementation pJans for areas designated"unc1assifiable"thatwill include technical direction on how to conduct refined dispersion modeling to demonstrate future NAAQS attainment.



### 2. Guidance on Air Quality Models Much ofthis guidance is based onEPA's Guideline onAir Quality Models, also published asAppendix W of

### 40 CPR Part 51. Appendix W is the primary source ofinfonnation onthe regulatory application ofairquality models for State Implementation Plan (SIP) revisions for existing sources and for New Source Review (NSR) and Prevention ofSignificant Deterioration (PSD) programs. Airquality modelin

# f in support ofthis designations process would need to employ airquality dispersion models that properly address the source-oriented nature ofSO::! and, thus, should rely upon the principles and techniques in Appendix W. Appendix W was originallypublished in Apri1 1978 and was incorporated by reference in the regulations for the Prevention ofSignificant Deterioration ofAir Quality, Title

### 40, Code of Federal Regulations (CFR) sections 51.166 and 52.2] in June 1978 (43 FR 26382-26388). The purpose ofAppendix W guidelines is to promote consistency in the use ofmodeling within the air quality management process. These guidelines are periodicallyrevised to ensure that new model developments orexpanded regulatory requirements are incorporated.
Clarifications and interpretations ofmodeling procedures become official EPA guidance through several courses ofaction: 1) the procedures are published as regulations orguidelines; 2)

2 List ofRegional Modeling Contacts by EPA Regional Office is availablefrom SCRAM websiteat:

3Dispersion modeling uses mathematical formulations to characterize the atmospheric processes that disperse a pollutantemitted by a source. Based on emissions and meteorological inputs,a dispersion model canbeusedto predictconcentrations atselected downwindreceptorlocations. 2the procedures are formally transmitted as guidance 10 Regional Office managers; 3) the procedures are formally transmitted as guidance to Regional Modeling Contacts as a result ofa Regional consensus ontechnical issues; or4) the procedures are a result ofdecisions by the EPA'sModel Clearinghouse that effectively establish national precedent. Formally located in the Air Quality Modeling Group (AQMG) ofEPA's Office ofAir Quality Planning and Standards (OAQPS), the Model Clearinghouse is the single EPA focal point for the review of criteriapollutant modelingtechniques 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.4 Recently issued EPA guidance ofrelevance for consideration inmodeling for designations includes:

• "Applicability ofAppendix W Modeling Guidance for the 1-hour 

### S02 NMQS" August 23, 

### 201O-confirmingthat Appendix W guidance is applicable for NSR/PSD permit modeling for the new 

### S02NAAQS. • "Additional Clarification Regarding Application ofAppendix W Modeling Guidance for the I-hour

### N02 National Ambient Air Quality Standard" March 1, 

### 2011- provides additional guidance regarding 

### N02 permit modeling and also relevantto 

### S02.
The foJlowing sections will providereference to the relevant sections ofAppendix Wand other existing guidance with summaries asnecessary. Please refer to those original guidance documents for full discussion and consult with the appropriate EPARegional Modeling Contact ifquestions arise about interpretation on modeling techniques and procedures.



### 3. Model selection Preferred air quality models for use in regulatory applications are addressed in Appendix A ofEPA's GUIDELINE ON AIR QUALITY MODELS. Ifamodel is to 

### be used for a particular application, the user should folJow the guidance onthe preferred model for that application. These models may beused without an area specific formal demonstration of applicability as long as they arc used as indicated in each model summary ofAppendix A. Further recommendations for the application ofthese models to specific source problems are found in subsequent sections ofAppendix W. In 

### 2005, the American Meteorological Society/Environmental ProtectionAgency Regulatory Model (AERMOD) was promulgated as EPA'spreferred near-field dispersion modeling for a wide range ofregulatory applications in all types ofterrainbased on extensive developmental and perfonnance evaluation.

4TheSupportCenter for RegulatoryAtmospheric Modeling (SCRAM) websiteis availableat: httn;Lb1'\:Y'N,!.:fL'W4QY/Ulli.gId.ml· 

### 3For area designations under the l·hour S02primary NAAQS, AERMOD should be used unless use ofan alternative model can bejustified (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 ofAppendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard", AERMOD is the preferred model for single source modeling to address the I-hour S02NAAQS as partoftheNSR/PSD permit programs. AERMOD is appropriate to inform this designations process because S02 concentrationsresult 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-orientednature ofthispollutant (See, e.g., 75 FR at 35570), dispersion models are the most appropriate airquality modelingtools to predict the near-field concentrations ofthis pollutant. The AERMOD modeling system includes several components. The regulatory components are:

• AERMOD: the dispersionmodel (U.S. EPA, 2004a) • AERMAP: the terrain processor for AERMOD (U.S. EPA, 2004b) • AERMET: the meteorological data processor for AERMOD (U.S. EPA, 2004c) • 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 ofAERMOD (U.S. EPA, 2011 b) Before running AERMOD, the user should become familiar with the user's guides associated with the modeling components listed above and theAERMOD Implementation Guide (AlG) (U.S. EPA, 2009). The AIG lists several recommendations for applications ofAERMOD which would be applicable for designations modeling.



### 4. 

### Modeling domain Selection ofthe modeling domain is important in terms ofhow many sources to explicitly model and what kind ofreceptor network to create. Two questions may arise in model domain selection: 1. Where to centerthe modeling domain?, and 2. How large should the modeling domain be? (i.e., in terms ofthe number ofsources to model and size ofthe receptor network in orderto account for the areas ofimpact).

4

### Ifthe modeling is being pcrformed to inform the nonattainmcnt boundary around a violating monitor, the domain should be centered on the violating monitor. Ifthe modeling is being done to show compliance with the NAAQS in the absence ofa violating monitor, the domain should be centered on the dominant source in an area, that is, the source or sources expected to contribute the most to S02 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, as explained further below in Section4.1. The determination ofsources to include in modeling is amulti-step process. Ifmodeling is being performed for a violating monitor, the first basic step would be to gather information and analyze the emission sources within 50 km ofthe monitor, which is the nominal distance at which EPA considers most steady-state Gaussian plume models are applicable. In some cases where large S02 sources are scattered outside ofthe 50 km radius, it may be necessary to extend the modeling domain beyond 50 km or conduct mUltiple AERMOD modeling exercises with the overall region broken down to several AERMOD runs covering different areas ofthe potential nonattainment area. For these situations, consultation with the appropriate EPA Regional Modeling Contact is recommended.

4.1 Determining 

### sources to model 

### As stated above, the determination ofsources to explicitly model is a multi-step process: 1. The spatial distribution ofall sources within 50 km ofthe violating monitor or dominant source should be analyzed and initially assumed to be included in refined dispersion modeling. Forthe purposes ofdesignations it 

### is reasonable to initially focus on the most significant sources ofS02 emissions, e.g., sources emitting greater than 100 tons per year. Please note, however, that sources less 

### than 100 tons can be potential contributors to a NAAQS violation, especially sources with short stacks andior located in complex terrain (Le., where receptorelevation is above stack height). 2. Sources should be examined and attempts made to determine ifany sources can be accounted for without explicitly modeling them, i.e., use ofmonitored background concentrations. Accounting for such sources through the use ofa background monitor will depend upon how well that monitor reflects impacts fromthose sources. 3. Sources found not to be representative by monitored background should also be examined through the use ofscreening models to see ifthey should or should not be included in the refined modeling. We recommend the use ofEPA'snew screeningmodel AERSCREEN (US. EPA, 2011b) and following recommendations based onpre-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 ofsmall sources, their cumulative impact should also be assessed. Individual sources may not be significant by themselves, but together they could cause a NAAQS violation or significantly contribute to aNAAQS violation. Although AERSCREEN does not output a design value
5

### conce,ntration based on the 99th 

### percentile form ofthe I-hour S02 standard, it**do~s** 

### output the overall maximum I-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 I-hour S02 NAAQS of3 ppb5• 

### Ifthe maximum I-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 3below for more details).

Figure 1 shows a hypothetical monitor with circles of50

### Ian and 10 

### km radii centered over it. Based on this figure, an example application ofthese three steps is described below. Step 1: Figure I shows facility emissions ranging from less than one ton to over 100 tons per year within 50 km ofthe violating monitor. Most ofthe smaller facilities (less than tcn tons) are located north ofthe violating monitor. There are two 100+ ton emitters near the monitor and two 100+ ton emitters west-southwest ofthe monitor. At this point, it

### could be initially assumed that all faciHties should be included in refined modeling. Step 2: Detennine whether any source or sources can be accounted for by a representative background monitor. In Figure 1) 

### there are two othermonitors in the area, one north and one south ofthe violating monitor. The northernmost monitor may be representative ofthe facilities north (white and yellow dots) ofthe violating monitor and the southern monitor may be representative ofthe sources southeast (white and blue dots) ofthe violating monitor. Background concentrations should be

### calculated following the guidance in Section 7below. Step 3: 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 largely dependent on their stackparameters (height, exit velocity. etc.). AERSCREEN could be used to eliminate such sources through screening modeling. AERSCREEN does not output an S02 design value but does outputthe overall maximum I-hourconcentration for an individual stack. Ifa facility contains more than one emission point or stack, each stack should be processed in AERSCREEN and the maximum I-hour concentrations can be added togetherto represent impacts from the whole facility after running AERSCREEN. While AERSCREEN can be used with the surface characteristics ofthe source being screened, given the documented sensitivity ofAERMOD to surface characteristics (Brode et aI., 2008), it 

### may be useful to also model the source in AERSCREEN using the surface characteristics ofthe meteorological site being used in the refined modeling as well. to ensure that the source is below de minimis impact levels with either set ofsurface characteristics.

5The 3 ppb interim SIL for new 1-hourS02 NAAQS was providedby EPA for states to consider using for PSD programin theAugust 23, 2010memorandumuGuidance Concerningthe Implementation ofthe 1-hourS02 NAAQS forthe Prevention ofSignificant Deterioration ProgramU 6FIGURE 1. HYPOTHETICAL EXAMPLE OF 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.

When analyzing AERSCREEN output, the following general criteria could be followed:

• Ifthe facility's maximum I-hour concentration exceeds 75 ppb, then the source should be included in refined dispersion modeling. • Ifthe tacility's maximum I-hourconcentration is below 75 ppb but above the suggested interim I-hour significant impact level of3 ppb orthe state's 1-hour 

### SIL, 

### it should be included in the refined modeling. • Ifthe facility's maximum I-hour concentration is below the suggested interim I-hour significant impact level orthe state's I-hour 

### SIL. that source may not have to be
7included in refined modeling. However, the facility should not be excluded on the sole basis ofbeing belowthe SIL without first looking at surrounding sources and their maximum I-hour concentrations. The case may arise when there are several small sources that singularly are belowthe SIL buttheircumulative impact may lead to concentrations that contribute to violations oftheNAAQS.
In summary for the example in Figure 1, the smallersources below I ton ofemissions to the north ofthe monitor maybe best represented with the use ofbackground monitor concentrations. Other sources between 1 and 10 tons thatare not represented by background monitors could be excludedbased on screening results, depending on their stack parameters and terrain. The smaller sources (less than I ton) within 10 km ofthe monitor location may also screen out. The 100+ton sources nearthe edge ofthe 50 km domain should be included in refined modeling. The largest emitters very close to the sources should be included in refined modeling as they are likely contributing to potential NAAQS violations and are not reflected in background monitors. This isjust one example ofhow to detennine the modeling domain and sources to model. In some cases, an analysis outto 50

### km may not be needed. Please consult with the appropriate EPARegional Officemodeler ifthereis uncertainty in deciding which sources toexplicitly model, which sources to represent based on background monitoring, andlorwhich to exclude from refinedmodeljng using screening modeling.



### 4.2 Receptor grid The model receptor grid is unique to the particular situation and depends on the size of the modeling domain, the number ofmodeled sources, and complexity ofthe 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 suchthat areas ofvioladon can be detected from the model output to help determine the size ofnonattainment areas. Receptor placement should be of sufficient density to provide resolution needed to detect significant gradients in the concentrations with receptors placed closer together nearthe 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) ormonitor locations (tor comparison to monitored concentrations for model evaluation purposes). Thereceptor network should cover the modeling domain. An example receptor grid for a single source is shown in Figure 2awith an example grid with multiple sources shown in Figure 2b. In Figure 2a, receptors are located every 50 m within one kilometer ofthe source and then every 100m from one to two kilometers. From two to 10 km, the receptor spacing is 250 m and every 500 m outside of1 0 km ofthe source. The modeling domain is centered onan isolatedfacility and extends out to 10 km in the east-west and north-south direction. Figure 2b shows an example grid for a multi-source area. Two sources are modeled with a fine grid ofreceptors J km (50 m spacing) around each source embedded within a lOxI 0 8

### km grid (250 m spacing). The lOx 1 0 km grid is then embedded within a 20x20km grid with coarser spacing (500 m). Ifmodeling indicates elevated levels ofS02 (ncar the standard) nearthe edge ofthe receptor grid, consideration ofexpandingthe grid or conducting an additional modelingrun centered on the area ofconcern should be investigated. As noted above, terrain complexity should also be considered when setting up the receptor grid. 

### Ifcomplex terrain is included in the model calculations, AERMOD requires thatreceptorelevations be included in the model inputs. In those cases, the AERMAPterrain processor (U.S. EPA, 2004b) shouldbe used to generate the receptorelevations and

### hill heights. The latest version ofAERMAP (09040) can process either Digitized Elevation Model (OEM) orNational Elevation Data(NED) data files. The AIG recommends the use ofNED data since it is more up to date than OEM data, which is no longer updated (Section4.3 ofthe AlG).



### 5. Source inputs This section provides guidance on source characterization to develop appropriate inputs for dispersion modeling with the AERMOD modeling system. Section 

### 5.] provides guidance on use ofallowable vs. actual emission levels, Section 5.2 covers guidance onGood Engineering Practice (GEP) stack heights, Section 5.3 provides details on source configuration and 

### source types, Section 5,4 provides details on urban/rural determination ofthe sources, and Section 5.5 provides general guidance on source grouping, which may be important for design value calculations.



### 5.1 Allowable VB. 

### Actual emissions Consistent withpast S02modeling guidance (Section4.5.2 orD.S. EPA (1994) and regulatory modeling for otherprograms (Appendix W, Section 8.1), dispersion modeling for the purposes ofdesignations should be based onthe use ofmaximum allowable emissions or federally enforceable permit limits. Also consistent with past and current guidance, in the absence ofallowable emissions orfederally enforceable permit limits, potential to emit emissions (Le., design capacity) should be used. Because ofthe short-term nature ofthe new S02NAAQS, the maximum shortterm orhourly emission rate should be input into AERMOD for each modeled hour. As stated in the August 23, 20I 0 memo, "Since**short~terrn** S02 standards ($24 hours) have beenin existence for decades, existing S02 emission inventories used to support modeling for compliance with the**3~hour** and 24-hour S02 standards should serve as a useful starting point, and may be adequate in many cases for use in assessing compliance with the new I-hour S02 standard since issues identified in Table**8~2** ofAppendix W related to short-term vs. long-term emission estimates may have already been addressed." 9

# a**X~n(l!rrl)** 

# b

·20 •••• , •

..**~** ... ""

.......**~~~,** I+ ••**~i** •• .,. .. ·r.·.·,., ·20 ·15 .10 

### ..s 0 5 10 15 20 X-dnldlonfilm)
FIGURE 2, EXAMPLE RECEPTOR GRIDS WITH (A) A GRID CENTERED ON AN ISOLATED SOURCE WITH FENCELINE RECEPTORS SHOWN IN BLUE AND THE EMISSION POINTS SHOWN IN BLACK. AND (B) A GRID WITH MULTIPLE SOURCES, 

### 10Theexisting S02 inventories used for permitting or SIP demonstrati.ons should contain the necessary emissions information for designations-related modeling. Ifshort-term emissions are notreadily available, they may be calculated using the methodology shown in Table 8-2 of Appendix W. For an example calculation ofshort term emissions, see theJune 28, 20I 0 memorandum "Applicability ofAppendix W Modeling Guidance for the 1-hourN02 National Ambient Air Quality Standard." Although the example is for N02, the calculation methodology would be the same for S02. Regardingthe use ofallowable emissions and modeling ofintermittent emissions sources, from such sources as emergency generators and startup/shutdown**emissions~** the inclusion ofsuch emissions for the purpose ofmodeling for S02 designations should follow the recommendations in the March 1, 2011 memo"Additional Clarification RegardingApplication ofAppendix W Modeling Guidance for the 1-hourNO:! National Ambient Air Quality Standard." As statedin this memo, EPA believes the most appropriate data to use fbr compliance demonstrations for the I-hour N02NAAQS are those based on emissions scenarios that are continuous enough or frequent enough to contribute significantly tothe annual distribution ofmaximumdaily I-hourconcentrations. Although the referenced guidance in this memo is for N02 permit modeling, the common I hour averaging time and form ofboth the N02 and S02 standards makes this modeling guidance applicable to the I-hour S02 NAAQS and, thus, applicable to S02 modelingin support ofdesignations. Formore details, refer to the N02 memo. Ifany questions arise regarding preparation ofemissions inputs for dispersions modeling including intermittent emissions from sources, then users should consult the appropriate EPA Regional Modeling Contact.

5.2 Good Engineering Practice**«~EP)** stackheight Consistent with previous S02 modeling guidance (U.S. EPA, 1994) and Section 6.2.2 of Appendix W, [or stacks withheightsthat are within the limits ofGood Engineering Practice (GEP), actual heights should be used in modeling. Under EPA's regulations at 40 CFR 51.100, GEP height, Hg, is determinedto be the greater of:

• 65 m, measured from the ground-level elevation at the base ofthe stack; • For stacks in existence onJanuary 12, 1979, and for which the owner or operator had obtained all applicable permits orapprovals required under40 CFRParts 51 and 52 

### H

g

### =2.5H provided the owner oroperator produces evidence thatthis equation was actually relied on in designing the stack orestablishing an emission limitation to ensure protection against downwash;

11For all other stacks, 

### Hg=H 

### + l.SL, where H is the height ofthe nearby structure(s) measured from the ground-level elevation at the base ofthe stack and 

### L is the lesser dimension ofheight orprojected width of nearby structure(s), or

• the height demonstrated by a fluid model or a field study approved by EPA orthe State/local agency which ensures that the emissions from a stack do notresult in excessive concentrations ofany airpollutant as a result ofatmospheric downwash, wakes, eddy etTccts created by the source itself, nearby structures ornearby terrain features. For more details about GEP, see the Guideline for Determination ofGood Engineering Practice Stack Height Technical Support Document (U.S. EPA, 1985). Ifstack heights exceed GEP, then OEPheights should be used with the individual stack's otherparameters (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). Ifbuilding downwash is being considered, the BPIPPRIMEprogram (U.S. EPA, 2004d) should be used to input building parameters for AERMOD. More information about buildings and stacks is in Section 5.3.



### 5.3 Source configurations and source types An accurate characterization ofthe 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 orpotential emissions levels at each stack, the stack'sparameters suchas exit temperature, diameter, and exit velocity should reflect those emissions levels. Accurate locations (Le. latitude and longitude or Universal Transverse Mercator ([JTM) coordinates and datum)6 ofthe modeled emission sources are also important, as this can affect the impact ofan emission source onreceptors, determination ofstackbase elevation. and relative location to any nearby building structures. Not onlyare accurate stack locations needed, but accurate information for any nearby buildings is important. This information would include location and orientationrelative to stacks and building size parameters (height, and comer coordinates oftiers) as these parameters are input into BPIPPRIMEto calculatebuilding

6 Latitudes andlongitudes to four decimal places position a stackwithin 30feet ofits actual location and five decimal placesplacea stackwithinthreefeet oritsactual location. Users should usethe greatestprecision available. 

### 12parameters for AERMOO. Ifstack locations and orbuilding information are not accurate, downwash will not be accurately accounted for in AERMOO. Emission source type characterization within themodeling environment is also important. As stated in the AERMOD User'sGuide (U.S. EPA, 2004a). emissions 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 arc not part ofthe regulatory default option in AERMOD because the user must invoke the BETA option in the model options keyword MODELOPT while not including the "OF AULT'° modeling option for these options to work properly. While most sources canbe characterized as POINT sources, some sources, such as fugitive releases or nonpoint sources (emissions from ports, airports, or smaller point sources with no accurate locations) may be best characterized as VOLUME orAREA type sources. Ifquestions arise about proper source characterization ortyping, users should consult the appropriate EPA Regional Modeling Contact.



### 5.4 Urban/rural determination For any dispersion modeling exercise, the urban orrural detenninationofa source is important in determining the boundary layer characteristics that affect the model's prediction of downwind concentrations. Figure 3 gives example maximum I-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 3a, the urban concentration is much higher than the rural concentration for distances less than 750 m from the stack but then drops belowthe rural concentration beyond 750 m. For thetaller stack in Figure 3b, the urban concentration ismuch higherthanthe rural concentration even as distances increase from the source. These profiles show that the urban or rural designation ofa source can be quite important. In addition, for S02 modeling, the urban/rural detennination is important because AERMOD invokes a 4-hourhalflife7 for urban S02 sources. Thiswould only

### be done for urban sources when the POLLUTlD keyword 

### in AERMOD is set to "802"and the MODEI"OPT keyword includes the OFAULT option. Rural sources within the same AERMOD runwould not be affected. Ifthe DFAULToption is not included with the MODELOPTkeyword, the 4-hour halflife would not beused and the user would specify the 4-hourhalflife using the HALFLlFE or DCAYCOEFF keywords in orderto account for the chemical transformation. See Section 3.2.6 ofthe AERMOD User'sGuide (U.S. EPA, 2004a) for more details about these keywords. lethe user invokes the HALFLIFE orDCAYCOEEF option, then any rural sources included in

., Overa 4-hourperiod, SOz concentrations decrease by halffrom theinitial value. 

### 13the modeling would need to 

### be run in separate AERMOD runs so thatthey are not subject to the 4-hourhalflife. Note that ifthe DFAUl:Toption is used. the rural sources would not need to 

### be in a separaterun from the urban sources. Determining whether a source is urban orrural canbe done using the methodology outlined in Section 7.2.3 ofAppendix Wand recommendations outlined in Sections 5.1 through 5.3 in the AlG (U.S. EPA, 2009). In summary, there are two methods ofurban/rural classification described in Section 7.2.3 ofAppendix W. The first method ofurban determination is a land use method (Appendix W. Section 7.2.3c). In the landuse method, the user analyzes the land use within a 3 km radius ofthe source using the meteorological land use scheme described byAuer (1978). Using this methodology, a source is considered urban ifthe land use types, II (heavy industrial), 12 (light-moderate industrial), Cl (commercial), R2 (common residential), and R3 (compact residential) are 50% or more ofthe area within the 3 km radius circle. Otherwise, the source isconsidered 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 of3 

### km radius is used. Ifthe popUlation density within the circle is greater than 750peoplclkm2

, then the source is considered urban. Otherwise, the source is modeled as a rural source. Ofthe two methods, the land use method is considered more definitive (Section 7.2.3e, Appendix W). Caution should beexercised 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 ofwater orother non-urban land use categoryto 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 ofAppendix W states, "Population density shouldbe 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, butthe area is sufficiently built-up so that the urban land use criteria would be satisfied

..." With eilher method, Section 7.2.3(1) ofAppendix W recommends modeling all sources within an urban complex as urban, even 

### ifsome sources withinthe complex would 

### be considered rural using either the land use orpopulation density method. Anotherconsiderationthat may need attention by the user and is discussed in Section 5.1 ofthe AIG relates to tall stacks located within oradjacent to small to moderate size urban areas. In such cases, the stack height oreffective plume height for very buoyant sources may extend above the urban boundary layerheight. The application oftheurban option in AERMOD tor these types ofsources may artificially limit the plume height. The use ofthe urban option may not be appropriate for these sources, since the actual plume is likely to be transported overthe urban boundary layer. Section 5.1 ofthe AIG gives details on detennining iCa tall stack should be modeled as urban orrural, based on comparing the stack oreffective 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 189m (based ona
14

### 10m 1000,...-,,----1"---

. .

_-­ 

# a**900~** I

700

j j , I J l**~** 500 1000 15(1() :woo 2500 3000 3500**~** 4500 5000 OistllllCt' (III) 

### 100m 

# b

6

2

1 L**~** ! t

.. ..**~** 500 1000 1500 2000 2500 lOOO 3500 4000 4500 5000 Distance(m)

FIGURE 3. URBAN (REO) AND RURAL (BLUE) CONCENTRATION PROFILES FOR (A) 10 M BUOYANT STACK RELEASE, AND (B) 100 M BUOYANT STACK RELEASE. 

### 15population of 100,000) and equation 104 ofthe AERMOD fonnulation document (Cimorelli, ct al., 2004). This equation is:

(1) where Ziuo is a reference height of400m corresponding to a reference population Po of2,000,000 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 nearan urban complex. Exclusion ofthese elevated sources from application ofthe urban option would need to bejustified on a case-by-case basis in consultation with the appropriate EPA Regional Modeling Contact. AERMOD requires the input ofurban population when utilizing the urban option. PopUlation can be entered to one ortwo significant digits (Le., 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 inthe runstream file (U.S. EPA, 2004a). Ifmultiple urban areas are entered, AERMODrequires that each urban source be associated with a particular urban area or AERMOD model calculations 

### will abort. Urban populations can be dctennined by using a method described in Section 5.2 ofthe AIG (U.S. EPA, 2009),



### 5.5 Source groups In AERMOD, individual emission sources' concentration results can be combined into groups using the SRCGROUPkeyword (Section 3.3.11 ofthe AERMOD User's Guide (U.S, EPA,2004a). The usercan automatical.ly calculate a total concentration (from all sources) using the SRCGROUP ALL keyword. Forthe purposes ofdesignations and design value calculations, source group ALL should be used, especially 

### ifall sources in the modeling domain are modeled inoneAERMOD run. Design values should be calculated from the total concentrations (all sources and background). For the purposes ofdesignations modeling, individual source contributions outputs to the total concentration may not be necessary. However, ifindividual facility contributions are needed for deciding which facilities to include in the nonattainmentor attainment area, source groups by facility should he used. To avoid any confusion, source groups that are used to calculate the design value concentrations ordetennine source contributions to design values should be mutually exclusive (Le. an emission source should not be in two source groups), This would be especially important 

### ifthe design value concentrations are calculated outside ofAERMOD by addingthe individual groups together to calculate a total concentration (See Section 8.1 ofthis docwnent for examples). Ifindividual source groups that are used in design value concentrations are not mutually exclusive, there would 

### be double


### 16counting ofconcentrations when calculatingdesign values either in AERMOD or outside of AERMOD.



### 6. 

### Meteorological data Section 6 gives guidance onthe selection ofmeteorological data for input into AERMOD. Much ofthe guidance from Section 8.3 ofAppendix W is applicable to designations modeling and is summarized here, In Section 6.2.1, the usc ofa new tool, AERMINUTE (U.S. EPA, 2011a), is introduced. AERMINUTE is an AERMET pre-processorthat calculates hourly averaged winds from ASOS (Automated Surface Observing System) l-minute winds.



### 6.1 Surface characteristics and representativeness The selection ofmeteorological datathat are input into a dispersion model should be considered carefully. The selection ofdata should bebased on spatial and climatological (temporal) representativeness (Appendix W, Section 8.3). The representativeness ofthe data is based on: l)the proximityofthemeteorological monitoring site tothe area under consideration, 2) the complexity ofterrain, 3) the exposure ofthe meteorological site, and 4)the period oftime during whichdata are collected. Sources ofmeteorological data are: National Weather Service (NWS) stations, site-specific oronsite 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 ofthe meteorological datacan be adversely affected by large distmlces between the source and receptors ofinterest and the complex topographic characteristicsofthe area (Appendix W, Section 8.3.a and 8.3.c). Ifthe modeling domain is large enoughsuchthat conditions vary drastically across the domain then the selection ofa single stationto representthe domain should be carefully considered. Also, care should be taken when selecting a station ifthe area has complex terrain. While a source and meteorological station may be inclose proximity, there may becomplex terrain between them such that conditions at the meteorological stationmay not berepresentative ofthe source. An example would 

### be a source located on the windward side ofa mountain chain with a meteorological station a few kilometers away onthe leeward side ofthe mountain. Spatial representativeness for off-site data should also be assessed by comparing the surface characteristics (albedo, Bowen ratio, and surface roughness) ofthe meteorological monitoring site and the analysis area. When processing meteorological data in AERMET (U.S. EPA, 2004c), the surface characteristics ofthe meteorological site should be used [Section 8.3.c ofAppendix Wand the AERSURFACE User's Guide (U.S. EPA 2008)]. Spatial representativeness should also be addressed 

### tor each meteorological variable separately. For example, temperature data from a meteorological station several kilometers from the analysis areamay 

### be considered adequately representative, while it may be necessary to collect wind datanearthe plume height (Section 8.3.c ofAppendix W). 17Surface characteristics canbe 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 in the detennination ofsurface characteristics. The current version ofAERSURF ACE uses 1992 National Land Cover Data. Note that the usc ofAERSURF ACE is not a regulatory requirement but the methodology outlined in Section 3.1.2 ofthe AIG should be followed unless an alternative method can bejustified.



### 6.2 Meteorological inputs Appendix W states in Section 8.3.1.1 thatthe user should acquire enough meteorological datatoensure that worst-case conditions are adequately represented in the model results. Appendix W states that 5 years ofNWS meteorological dataorat least one year ofsite-specific data shouldbe used(Section 8.3.1.2, Appendix W) and should be adequately representative ofthe study area. Ifone ormore years (including partial years) ofsite-specific dataare available, those data are preferred. While the fonn ofthe S02 NAAQS contemplates obtainingthree years of monitoring data, this does notpreempt the use of5 years ofNWS dataorat least one year of site-specific data in the modeling. The 5-year average based on the use ofNWS data, oran average across one ormore years ofavailable site specific data, serves as an unbiased estimate of the 3-yearaverage for purposes ofmodeling demonstrations ofcompliance withthe NAAQ (See the August 23, 2010 Clarification Memorandum on "Applicability ofAppendix W Modeling Guidance for the I-hour S02 National AmbientAir Quality Standard"). See the memorandum for more details on the use of5 years ofNWS data orat least one year ofsite-specific dataand applicabilityto the NAAQS.



### 6.2.1 NWS data NWS data are available from the National Climatic Data Center (NCDC) in many fonnats, with the most common one in recent years being the Integrated Surface Hourlydata (ISH). Most available formats can be processed by AERMET. As stated in Section 6.1, when using data from an NWS station alone or in conjunction with site-specific data, the data should be spatially and temporally representative ofconditions at the modeled sources. A recently discovered issue with ASOS is that 5-second wind data that arc used to calculate the 2-minute average winds are truncated rather than rounded to whole knots. For example, a wind of2.9 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

### Y2 knot or 0.26 

### mls is added to the winds in stage 3 AERMET processing. Formore details refer to the AERMET User'sGuide (U.S. EPA, 2004c) and/or the appropriate EPA Regional Modeling Contact.



### 186.2.1.1 AERMINUTE In AERMOD, concentrations are not calculated for variable wind (Le., missing wind direction) and calmconditions, resulting in zero concentrations for those hours. Since the S02 NAAQS is a one hour standard, these lightwind conditions may be the controlling meteorological circumstances in some cases because ofthe limited dilutionthat occurs under low wind speeds which can lead to higher concentrations. The excl.usion ofa greater numberof instances ofnear-calm conditions from the modeled concentration distributionmay therefore lead to underestimation ofdaily maximum I-hour concentrations for calculation ofthe design value. To address the issues ofcalm and variable winds associated with the use ofNWS meteorological data, EPA has developed a preprocessor to AERMET, called AERMINUTE (U.S. EPA, 2011a) that can read 2-minute ASOS winds and calculate an hourly average. Beginning with year2000 data, NCDC has made freely available, the I-minute winds, reported every minute from the ASOS network. The AERMINUTE program reads these 2-minute winds and calculates an hourly average wind. In AERMET (U.S. EPA, 2004c), these hourly averaged winds replace the standard observation time winds read from the archive ofmeteorological data. This results in a lower number ofcalms and missing winds and an increase in the number of hours used in averaging concentrations. Formore details regarding the use ofNWS data in regulatory applications see Section 8.3.2 ofAppendix Wand f'Or more infonnation about the processing ofNWS data in AERMET and AERMINUTE, seethe AERMET (U.S. EPA, 2004c) and AERMINUTE User's guides (U.S. EPA, 2011a). '

6.2.2 Site-specific data The use ofsite-specific meteorological data is the best way to achieve spatial representativeness. AERMET can process a variety offonnats and variables for**site~specific** data. The use ofsite-specific data for regulatory applications is discussed in detail in Section 8.3.3 ofAppendix W. Due to the range ofdata that canbe collected onsite and the range of fonnats ofdata inputto AERMET, the user should consult Appendix W, the AERMET User's Guide (U.S. EPA, 20040), 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 ofthe AERMOD Implementation Guide offersrecommendations f'Or data processing. In summary, the guide recommends that site-specificturbulence measurements should not be used when applying AERMOD's urban option, inorderto avoid doubJe counting the effects ofenhanced turbulence due to the urban heat island.



### 19

### 6.2.3 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 ofNorth 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 levels8 when selecting upper airdata. Selecting mandatory levels only would notbe adequate for input into AERMET as the use of just mandatory levels wouJd not provide an adequate characterization ofthe potential temperature profile.



### 7. Background concentration The inclusion ofambient background concentrations tothe model results is important in determining cumulative impacts. The modeled contribution to the cumulative analysis should follow the form ofthe standard and be calculated as described in Section 2.6.1.2 ofthe August 23, 2010 clarification memo on "ApplicabilityofAppendix W Modeling Guidance for the 1­ hour 

### 802 National Ambient Air Quality Standard." This memo suggested a "first tier" approach to including a uniform monitored background contribution based on adding the overall highest hourly background 

### S02 concentration from a representative monitorto the modeled design value. We recognize thatthis approach could be overly conservative in many cases and may also be prone to reflecting source-oriented impacts, increasingthe potential for double-counting of modeled and monitored contributions. As discussed in EPA's March 1,2011 memo "Additional Clarification Regarding Application ofAppendix W Modeling Guidance for the I-hour N02 Ambient Air Quality Standard," werecommend a less conservative "firsttier" approach for a uniform monitored background concentration based onthe monitored design values for the latest 3-yearperiod, regardless ofthe years ofmeteorological data used in the modeling. Adjustments to this approach may be considered in consultation with the appropriate EPA Regional Modeling Contact with adequatejustification and documentation ofhowthe background concentration was calculated. Section 8.2.2 ofAppendix W gives guidance on background concentrations for isolated single sources and is also applicable for multi-source areas. pne option is, as described in Section 8.2.2.b:

8 By international convention, mandatory levels are in millibars: 1,000,850,700,500,400, 300, 200, 150, 100,50,30,20,10,75,3,2,and 1. Significantlevels mayvary depending on the meteorological conditions at theupper-air station 

### 20"Use airquality data in the vicinity ofthe source to determine the background concentration for the averaging times ofconcern. Determine the mean background concentration at each monitor by excluding concentrations when the source in question is impacting the monitor

... For shortertime periods, the meteorological conditions accompanying concentrations ofconcern should be identified. Concentrations tor meteorological conditions ofconcern, atmonitors, not impacted by the source in question, should be averaged for separate averagingtime to determine the average background value. Monitoring sites inside a 

### 900 degree sector**down~ind** ofthe source may be used to determine the area ofimpact." When no monitors are located in the vicinity ofthe sources beingmodeled a "regional site" (i.e., one that is located away from the area ofinterestbut 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 ofAppendix W). Nearby sources are those sources that arc expected to cause a significant concentration gradient inthe vicinity ofthe source or sources under consideration, and should be explicitly modeled. Identification ofnearby sources calls for professional judgment and consuJtation 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 1,2011 memo "Additional Clarification Regarding Application of Appendix W Modeling Guidance for the I-hourN02 Ambient Air Quality Standard," describes an appropriate methodology ofcalculating temporally varying background monitored concentrations by hourofday and season (excludingperiods when the source in question is expected to impact the monitored concentration). The methodology for 

### N02 is to use the 98

### th percentile concentration for each hour ofthe day by season and average across three years. This same methodology is applicable to S02 designations modeling based on use ofthe 

### 99th percentile by hour ofday and season for background concentration excluding periods when the dominant source(s) are influencing the monitored concentration (Le., 

### 99th percentile, or

### 4th highest, concentrations for hour 1 for January orwinter, 

### 99tb percentile concentrations torhour 2 for January or winter, etc.). Recent updates included in AERMOD allow for the inclusion of temporally varying background concentrations in the design value calculation in combination 

### with modeling results.
An illustrative example is shown in Figure 4. Shown arc the NAAQS standard concentration, the monitor's 3-yearaverage design value, and 3-year averages ofthe 

### 99th percentile concentrationsby season and hour ofday. To calculate the 

### 99th percentile concentration for a season and hour ofday combination, the second highest concentration for that combination should be selected. Also shown are 3-year averages ofthe 

### 99th percentile concentration 

### by hour ofday (across all seasons), and the average concentration by hour ofday



### 21across the three years9• In this example, the winter background concentrations show a distinct diurnal variability, with less for eachofthe other seasons.

20

1**"v,."",i.~~--L-.....l-....j** 5 6 7 6 9 1011121314151617181920212223 Hourof day

FIGURE 4. S02 MONITORED CONCENTRATIONS FOR VARIOUS AVERAGING TIMES.

It should be also noted here that the conventions regarding reporting time differ between ambient air quality monitoring, where the observation time is based on the hour-beginning convention, and meteorological monitoringwhere the observation is based on the hour-ending time. Thus, ambient monitoring data reported for hour 00 shouldbepaired with meteorological data for hour 01, etc. This is important when incorporating time-varying background

9 Modelers should use the lSI-highestvalue for moredetailed pairings, suchas monthby hour-of-dayor season byhour-of·dayand day-of-week 

### 22concentrations in the AERMOD calculations, which allow for temporally varying background concentrations.



### 8. Determiningdesign value mctrics Designations modeling will provide predictions ofS02 design values at eachreceptor that includes contributions from all modeled sources and backgrowld. Based on the form ofthe 1­ hour S02 NAAQS. the design value shouldbe calculated as the average ofthe 99th percentile of the annual distribution ofdaily maximum I-hour concentrations averaged across the modeled years.



### 8.1 Design value calculation methodology Whether design values are calculated within AERMOD oroutside ofAERMOD, to calculate a design value to compare againstthe standard, the following steps should be rollowed: 1. At each receptor, for eachhour ofthe modeled period, calculate a total concentration across all sources includingbackground concentrations ifapplicable. This can be done in AERMOD using SRCGROUP ALL orby adding individual source groups outside of AERMOD, using hourly POSTFILEs. Ifthe user is totaling the concentrations outside of AERMOD, the source groups need to be mutually exclusive, Le. no one source should be 

### in mUltiple source groups. 2. Fromthetotal concentrations calculated instep), obtain the I-hourmaximum concentration at eachreceptorfor each modeled day. 3. Fromthe output ofstep 2, for each yearmodeled, calculate the 99th percentile (4UI highest) daily maximum I-hourconcentrationat each receptor. Ifmodeling 5 years of meteorological data, this results in five 99th percentile concentmtions at each receptor. 4. Average the 99th percentile (or 4th highest) concentrations acrossthe modeled years to obtain a designvalue ateach receptor. 5. Modeled source contributions to a NAAQS violation canbedetermined by analyzingthe hourly concentrations from the individual source groups corresponding to the same hour as the 4th daily maximum 1-hourconcentration from each year. (See 

### 15 FR at 35540). Forexample, a receptor has a 5-yearaverage design value of200.8 mg/m 3 (or approximately 71 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 ofthe4th highest daily maximum I-hour concentrations that arc used tocalculate the 5-year average design value. Table 1 shows the 4th highestdaily maximum I-hour concentrations for each year and associated dates that are used in the design value calculation.

23TABLE 1. 4TH HIGHEST DAILY MAXIMUM**1~HOUR** CONCENTRATIONS (I.IG/M3) FOR 2005-2009. Date Concentration**~.** (YYMMDDHH)

05080101 200.1 06073105 201.5 07080403 207.1 08072705**197~** 1 .,,­ 09080104 198.1 5-YEARAVG. 200.8

Ifoutput by source group is available, the user can extract each source group's concentration at each ofthe hours listed in Table I. Table 

### 2 shows example source contributions for eachhour shown in Table 1and indicates that Source 1is the main contributor to the design value for all hours.

TABLE 2. SOURCE CONTRIBUTIONS TO 4TH HIGHEST DAILY MAXIMUM**1~HOUR.** CONCENTRATIONS (I.IG/M) AND 5·YEAR AVERAGE DESIGN VALUES. TOTAL SOURCE 1 SOURCE 3 SOURCE 4 SOURCE 2 

# i;; Date ' MMDDHH) 

# I 05080101 200.1 155.1 1.5 18.4 25.1 17.4 06073105 201.5 0.5 157.4 26.2 07080403 207.1 2.1 23.0 161.5 20.5 .­ 13.1 197.1 159.2 23.1 1.7**~072705 ~-----j**  09080104 198.1 ! 20

### 15**§~ -~**  1[d 5-YEARAVG. 1.6 18.0 200.8 157.7 23.5



### 8.2 RunningAERMOD and implications for design value calculations Recent enhancementsto AERMOD include options to aid in the calculation ofdesign values for comparison withthe S02NAAQS. These enhancements include:

• The output ofdaily maximum I-hourconcentrations 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 outputkeyword, ofdaily maximum I-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 ofeach source group to the high ranked values for a specified target source group, paired in time and space. The user can specify a range ofranks to analyze, or specify an upper boundrank. i.e. 4th highest, and a lowerthreshold value, such as the NAAQS for the target source group. The model will 24process each rank within the range specified, but will stop after the 1irst rank (in descending order ofconcentration) that is below the threshold, specified by the user. A warning message 

### will be generated ifthe threshold is not reached within the range ofranks analyzed (based on the range ofranks specified on the RECTABLE keyword). This option may be needed to aid in determining which sources to include in a nonattainment area. Ideally, all explicitly modeled sources, receptors, andbackground should be modeled in one AERMOD run for all modeled years. In this case, the use ofthe one ofthe above output options can be used in AERMOD to calculate design values for comparison to the NAAQS and detennine the area's attainment status and/or infonn attainmentlnonattainment boundaries. The use ofthese options in AERMOD allows AERMOD to internally calculate concentration metrics that canbe 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 preferred. 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 oftacilities, orby year, orthe receptor network is divided into separate sub-networks. In some types ofthese situations, the MAXDAILY, MXDYBYYR, or MAXDCONT output option may not 

### be an option for design value calculations, especially 

### ifall sources are not included in a single run. 

### Ifthe user wishes to utilize one ofthe three output options, then care should be taken in developingthemodel inputsto ensure accurate design value calculations. Situations that would effectively preclude the use ofthe MAXDAILY, MXDYBYYR, and MAXDCONT option to calculate meaningful AERMOD design valuecalculations include the following examples:

• Separate AERMOD nms toreach source orgroups ofsources.

o Designations modeling includes 10 facilities for five years ofNWS data and each facility is modeled for five years in a separate AERMOD run, resulting in 10 separate AERMOD runs. .
• Separate AERMOD runs for each source and each modeled year.

o 10 facilities are modeled for 5 years ofNWS 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 tacilities. In these situationsthe use ofhourly POSTFILES, which can be quite large, and external post-processing would be needed to calculate design values.



### 25Situationsthat may use the MAXDAlL Y, MXDYBYYR, or, MAXDCONT option 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 network. o 	 A receptor network of20,000 receptors is divided into four 5,000 receptor subnetworks. Ten facilities are modeled with five years ofNWS 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 bere-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 ofNWS data. All facilities are modeled with all receptors for each year individually, resulting in five AERMODruns. MAXDAILY, MXDYBYYR, or, MAXDCONToutput canbe used and post-processed to generate the necessarydesign 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 ofNWS data for 20,000 receptors. The receptornetwork is divided into four 5,000 receptor networks. Foreach subnetwork, all tcn facilities are modeled for each year separately, resulting in twenty AERMOD runs. MAXDAILY, MXDYBYYR, or, MAXDCONToutput can be used and post-processed to generate the necessary design value concentrations.



### 9. Use ofmodeling results to inform nonattainmentJattainment boundaries Dispersion modeling is a tool that could be used to examinethe spatial extent ofpotential violations ofthe l·hourS02NAAQS. Thus, in accordance with this guidance. refined dispersion modeling could 

### be used to in1brm boundary determinations in support ofthe S02 designations process, i.e. 1. For an area that contains a violating monitor, modelingcould be used to inform decisions on the appropriatenonattainment boundary inconjunction with other factors listed in Attachment 2. 

### 2. Foran areawithout a violating monitor, modeling could be used as evidenceofan area's attainment status and also to infonn decisions on the appropriate (attainment or nonattainment) boundary. The shape and size ofthe nonattainment or attainment areais recommended by the state and eitheradopted ormodified by EPA. For initial designations, 

### it is expected that states will focus on areas with violating monitors. Ifa county contains a violating monitor, that county would be 26considered in nonattainment. Ifthere are no violatingmonitors and no dispersion modeling results to show attainment ornonattaimnent, that county would general1y be considered unclassifiable.



### 9.1 Nonattainmentarea boundaries For nonattainment areas (those with a violating monitor). modeling could beused to refine the nonattainment area boundaries from the presumptive county boundaries in conjunction with other factors such as those listed in Attachment 2. This could include reducing the nonattainment area from the presumptive county to a smaner area orexpandingthe boundary beyond the county 

### ifsources outsidethe county contribute to a NAAQS violation in the county. A nonattainment area boundary should contain the areathat exceeds the NAAQS and include sources that may cause orcontribute to a NAAQS exceedance. Figure4 shows a hypothetical example ofmodeling ofan area that exceeds the NAAQS (either through monitoring or modeling). In eachpanel ofFigure 5, the black dot represents the emission source. In Figure 5a, the contours in orange and red are design values that exceed the NAAQS. Figures 5b-5d show different approaches to establishingthe nonattainment boundary so that the orange andred contours arc within the boundary. Tn Figure 5b, the hypothetical nonattainment boundary is a circle, centered on thearea shown as violating the NAAQS, while Figure 5c shows the hypothetical nonattainment boundary as a rectangle. Finally, Figure 5d shows a hypothetical nonattainment boundary as an irregular polygon in shape, perhapsbased onjurisdictional boundaries orotherlandmarks such as roads. Figure 6 illustrates a hypothetical example for a multi-source situation that is ill nonattainment. In the example, there are five sources (denoted by blue dots) in a modeling domain that covers four counties (A, B, C, and D). The modeling domain is centered on the violating monitor (star). The orange contour represents concentrations above the NAAQS. As in the single source example shown in Figure 5, the nonattainmcnt areacould 

### be circular, rectahgular. orirregularly shaped usingjurisdictional boundaries. In this example, the hypothetical nonattainment boundary would be defined by the northern portion ofCounty A and the southern portion ofCOWlty C. Since multiple sources are involved, the hypothetical nonattainment boundary should beextended to cover those sources that cause orcontribute to a NAAQS violation. In this hypothetical example. Sources 2 and 5 are the largest contributing sources to the potential NAAQS violation so the nonattainment boundaries wouldinclude those two sources.

27

# c 

# a b



# d

FIGURE 5. HYPOTHETICAL EXAMPLE OF A MODELED NAAQS VIOLATION (RED AND ORANGE CONTOURS) AND POSSIBLE NONATTAINMENT AREA BOUNDARIES DEFINED BY (B) CIRCLE, (C) RECTANGLE, AND (D) AN IRREGULAR POLYGON.

28

# a 

# b



# c d

FIGURE 6. HYPOTHETICAL EXAMPLE OF A MULTI-SOURCE AREA WITH MODELED NAAQS VIOLATIONS (ORANGE CONTOUR) AND POSSIBLE NONATTAINMENT AREA BOUNDARIES DEFINED BY (8) CIRCLE. (C) RECTANGLE. AND (D) AN IRREGULAR POLYGON.



### 9.2 Attainment area boundaries In areas without a vioJating monitor, modeling couJd be usedto help determine that an area with S02 emitting sources is in attainment for the J·hour S02NAAQS. An attainment area boundary could not contain any area that exceeds the NAAQS or any area containing sources that arecausing or contributing to a violating area. When considering attainment area boundaries, there will be no predicted area ofviolation from dispersion modeling so that other factors would need to be considered iftheboundary is not detemlined by using the county presumptive boundary. Figure 7 iUustrates a group ofsources where a monitored design value does not exceed the NAAQS and modeling also does not show any concentration levels in excess ofthe NAAQS. In this case, the state could recommendthat county A be considered attainment, since the monitor and modeling do not show violations ofthe NAAQS. Also, 

### ifthere are other 29sources in the remaining three counties (Le., B, 

### C. or D) and their modeled concentration levels do not showviolations ofthe NAAQS,then these counties could also be recommended as part of the attainmentarea.



## D


# B

A



# - -
FIGURE 7. HYPOTHETICAL EXAMPLE FOR AN AREA WITH A MONITOR (STAR) THAT DOES NOT VIOLATE THE NAAOS AND MODELING RESULTS FOR SOURCES (BLUE DOTS) THAT DO NOT SHOW A VIOLATION OF THE NAAOS.



### 10. Documentation Itisexpected that the state would submit a modeling and analysis protocol that details the methodology and model inputs before commencement ofthe modeling exercise. This information should support the states' recommended designations, and provide a basis for EPA's evaluation ofthe recommendations. The protocol should include the following:

30• Characterizationofthe nonattainmentproblemor characterization ofthe modeled area in absence ofa violating monitor, • An emissions analysis around the violating monitor or area under consideration for designations in absence ofa violatingmonitor. and • Methodology for preparing air quality andmeteorology inputs including choice of meteorological data and representativeness ofthe data. Additionally, the documentation should include:

• Summary and analysis ofmodeling results, and • Provision ofmodeling data inputs and outputs in electronic form. A meeting with the appropriate EPA Regional Modeling Contact and othertechnical and planning staffto discuss the modeling and analysis protocol is recommended before submitting the protocol and beginning any refined modeling.



### 11. Summary In summary, we emphasize the following key points ofthis modeling guidance:

• AERMOD is EPA'spreferred near-field dispersion model for regulatory applications and is applicable for S02 designations modeling consistent with EPA's 

### Guideline onA;r 

### Quality Models, also published as Appendix W of

### 40 CFRPart 51.
• Sources should be modeled with maximum allowable 1**~hour** or shorHerm emission rates in the designations modeling based on continuous operations at the source. • Modeling should be done 

### with five years ofrepresentative NWS meteorological data or at least one year ofsite specific meteorology.
• Background concentrations can be included as: o "First tier" approach based on monitored design values added to modeled design values; or o Temporallyvarying based on the 99th percentile monitored concentrations by hour ofday and season added to modeled design values.
• Dispersion modeling results couldbe used to inform the nonattainmcnt or attainment
areas in conjunctionwith other designations factors.
• States should submit a modeling and analysis protocol that details the methodology and model inputs before commencement ofthe modeling exercise. This information should support the states' recommended designations, and provide a basis for EPA's evaluation ofthe recommendations.·



### 31• At anytime duringthe designations process when there are questions regarding modeling
or interpretation ofthis guidance, the appropriate EPA Regional Modeling Contact should

### be consulted.



### 12. References Auer, Jr., A.H., 1978: Correlation ofLand Use and Cover with Meteorological Anomalies. Journal ofApplied Meteorology, 17(5),636·643. Brode, R., 

### K. Wesson, 

### 1. Thurman. and C. Tillerson, 2008: AERMOD Sensitivity to the Choice of Surface Characteristics, Paper 811, Air And Waste Management Association Annual Conference. Cimorelli, A. 1., S. G. Perry, A. Venkatram, J. C. Weil, R.J. Paine, 

### R. B. Wilson, R. F. Lee, W. D. Peters,

### R. W. Brode, and 

### 1. O. Paumier, 2004: AERMOD: Description ofModel Formulation, EPA-454/R-03-004. U.S. Environmental Protection Agency, Research Triangle Park, NC.
U.S. EPA, 1985: Guideline for Determination ofGood EngineeringPractice Stack Height (Technical Support Documentfor the Stack Height Regulations), Revised. EPA-450/4­ 80-023R. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. U.S. EPA, 1992: ScreeningProcedures for Estimatingthe Air Quality Impact ofStationary Sources. EPA-454/R-92-019. U.S. Environmental Protection Agency, Research Triangle Park, 

### NC 27711. U.S. EPA, 1994: S02 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 ModelingApplications. EPA-454/R-99-005. U.S. Environmental Protection Agency, Research Triangle Park, NC 2771 

### L 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. U.S. EPA, 2004b: User's Guide for the AERMOD Terrain Preprocessor(AERMAP). EPA

### 454/8-03-003. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711. 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.


### 32U.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 Carolina27711. U.S. EPA, 2005. Guideline (mAir Quality Models. 40 CPR Part 51 Appendix W. U.S. EPA, 2008: AERSURFACE User's Guide. EPA-4S4/B-08-00L U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711. U.s. EPA, 2009: AERMOD Implementation Guide. U.s. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 

## t. U.S EPA, 2011a: AERMfNUTE User's Guide. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711. U.S EPA, 20l1b: AERSCREEN User's Guide. EPA-454-/B-II-001. U.S. Environmental Protection Agency, ResearchTriangle Park, North Carolina 27711.



### 33