! i 

### UNITED STATES ENVIRON ENTAL PROTECTION AGENCY RE ION 10 1200 Sixth

.venue, Suite 900 

# f
Seattle, Washington 98101-3140

OFFICE OF THE 

## APR 04 2011 REGIONAL ADMINISTRATOR


### The Honorable C.L. "Butch" Otter Governor ofthe State ofIdaho P. O. Box 83720 Boise, Idaho 83720 DearGovernor Otter: On

### June 3, 2010, the U.S.**Environm~tal** 

### Protection Agency (EPA) revised the heathbasedNational Ambient Air Quality Standard (NAAQS) for sulfurdioxide (S(h)byestablishing anew I-hourstandard at alevel of75 partsperbillion (Ppb). This new I-hourstandard will protect public healthbyreducingpeople's

### e~bosure to high short-tenn (5-minutes to 24-hours) concentrations ofS02. 

### An areameets the neW standard when the 3-yearaverage ofthe 99th 

### percentileofI-hourdaily maximum concenttations does not exceed 75 ppb. Section 107(d)(I} ofthe Clean AirAJt (CAA) describes theprocess EPA follows for designating areas as meeting ornot meeting anew orrevised NAAQS. Within one year of establishing anew orrevised standard, each governor mustrecommend adesignation of attainment, nonattainment orunc1assifiable, with appropriatebOWldaries, for areas within their jurisdiction. Therefore, we askthat your stat

# rprovide designation recommendations, including appropriate areaboundaries, for all areas by**~une** 

### 3,2011,withrespect to the I-hour S02 NAAQS. The enclosed memorandumprovides bidanceto state andlocal airpollution control agencies and tribes on the timeline andprocess for designating areas underthe revised S02 standard. Weplanto announce ourintended.designations no laterthan 120 days priorto promulgating final designations. Following the announcement ofintended designations, states andtribes will have the opportunity to commenton anymodifications EPAproposes to their recommendations. EPA intends to completeIdesignationsbyno laterthan June 3, 2012. Establishing areadesignations is akey step in

### theprocess ofproviding thehealth protection intendedbythe

## eAA. !

### Should**yo~** 

### have questions regarding this

### matter, please do not hesitate to contactme or Steve Bodyofthe**~te** 

### & 

### Tribal AirPrograms Unit at206-553-0782 or body.steve@epa.gov.· I I 

### Sincerely,

,


### Dennis'J, Mclerran RegionW Administrator I 

### EnclosureUNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NC 27711



## MAR 242011 OFf'"ICE Of' AIR QUALITY PLANNING

AND STANOAROS



### MEMORANDUM SUBJECT: Area 

### DCSl,'gnation.s, ihrthe 2010 "f,

### {CVII2S .) imary Sulfur Dioxide National

... 

### , Ambient Air Quality Standard 

### I'" /
...**~** , 

### FROM: Stephen D. Page,**Direct~ '~**  Office of

### Air Quality PIa nj g and**S~ards** 

### TO: Regional Air Division Directors, 

### Regions I-X

This memorandum provides information on the schedule und process for designating areas for the purpose ofimplementing the 2010 revised primary sulfur dioxide (S02)national ambient air quality standard (NAAQS). 

### In addition, 

### it identifies factors EPA intends to cyaluate 

### in detcnnining boundaries for areas designated nonauainment 

### We recommend 

### that states and tribes consider and address these factors when identifying boundaries for their area designation recommendations. 

### Please share

### this infonnationwith the 

### state and tribal 

### agencies in your Region. On June 2, 20 10. AdministratorJitckson signed the rC\'ised primary SO;! NAAQS (75 FR 

### 35520, published on June 22, 2010) afterreview ofthe existingtwo 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 99!11 percentile of 1**~hour** daily 

### maximum concentrations does not exceed ppb. The Administrator has detemlined that this is thelevel necessary to provide protection ofpublic health with an adequate margin of

### safety, especially for children, the elderly and those with asthma. These groups are particularly susceptible to the health effects associated with breathing S(h**Gen~r~1** upprqach and schedule. Clean Air Act (CAA) section 107(d) directs states to 

### submit their SO:: designation

### recommendations to EPA by June 3, 

### 2011. IfEPA intends to modify' any stnte's boundary recommendation, EPA 

### will notify t.he state no later than 120 days prior to its action to promulgatedesignations (i.e., February 2012 for designations to be promulgated in June 201 and the state \"lill have an opportunity to comment on EPA's intended modifications and provide additional Information for EPA to consider. Section 1 07(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 30I(d) of the CAA and the Tribal Authority Rule (40 CFRPart49). Therefore, we intend to designate tribal areas. in consultation 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 III 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 anticipatestaking an analytic approach that uses both air quality monitoring and modeling information for designations. Suchan approach, ifadopted, wouldbeconsistent with EPA'shistoric practices for S02 NAAQS implementation. In that preamble we acknowledged that in some cases, monitoringdata may be the more technically appropriate information for determining compliance with the I-hourNAAQS. (See e.g., 75 FR at 35552, n. 22), We also recognized that a single monitor may generally not be adequate to fully characterize ambient concentrations ofS02. including the maximum ground level concentrations that existaround stationary 802 sources, particularly when measuring for a I-hour standard. (See 75 FR at 35551). Refined dispersionmodelsare able to characterize S02 air quality impacts from the modeled sources across the domain ofinterest on an hourly basis with a high degree ofspatial resolution, thus overcomingthe limitations ofan approach based solely on monitoring. Attachment 2 summarizes three possible designations and the criteriafor 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 rea1istic orappropriate 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 to 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 airquality monitoring and modeling information submitted by states ortribes in support oftheir recommendations. States and tribes should identify areas as attainment, 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 anticipatethat most areas ofthecountry will be designated "unclassifiable." Ifa state ortribe, following receipt ofanEPA 120-dayletter, has additional information that it wants EPA toconsider with respect to a designation recommendation that EPA plans to modify, we request that such informationbe submitted within 60 days after receiving EPA's letter. This will help ensure that EPA can fully consider any suchinformation prior to issuing final designations. 2

### Also, although not required by statute. in order to considerpublic input in the designation process, we plan to provide a 30-day public commentperiod immediately following issuance of EPA's letters responding to the recommendations made by states and tribes. Attachment 1is this anticipated schedule. The preamble 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 aplan within 18 months after designationproviding for attainment as expeditiously as practicable, but no laterthan 5years afterthe 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 SOlNAAQS. Generally, these infrastructure plans for attainment areas are not expectedto 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 in these plans demonstrations ofexpeditious attainment and maintenance ofthe S02 NAAQS. EPA is developing separate guidance on developing S1P revisions for the S02standard and we intend to seek pubJic review and comment on that guidance document. Identifying an area that is inviolation ofthe S02.NAAQS. Section I07(d)(1) ofthe CAA defines an area as ''"nonattainment'' ifit is violating the NAAQS or ifit is contributingto a violation in anearby area. Thus, the first step in making designations is to identify through monitoring or appropriate modeling areas violating the NAAQS. In assessing whether 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 airquality datato determine whether a violation has occurred are given in 40 CFR Part 50 Appendix T, as revised in conjunction with the final rule for the 2010 S02 NAAQS. We expect that in providing theirrecommendations to EPA, states and tribes would review available S02 monitoring data from 2008 through 2010. Priorto EPA issuing letters to states and tribes concerning any intended modifications to theirrecommendations, data from 2011 may become available. If

### this is the case, EPA intends to also consider2011**S~** 

### air quality monitoring data in formulating any intended modifications to state and tribal recommendations. Airquality monitoring data affected by exceptional events may be excluded from use in identifying a violation ifthey meet the criteria for exclusion, as specified in the final rule

I This information is available on EPA's website atwww.epa.gov/ttn/alrs/airsaqsj

. 3

### 'Treatment ofData Influenced by Exceptional Events" (72 FR )3560; March 22,2007) codified in 40 CFR Parts 50 and 5'1. In section VII.S ofthe S02 NAAQS final rule preamble, we discussed schedules for states and tribes to flag data influenced by exceptional events and submit related documentation specifically for S02 data collected from 2008 through 2010 used in the initial designations process. These schedules are contained in Table I of40 

### CFR 50.14 and require initial data flagging by October I, 20I0 

### and detailed documentation submittal by June 1, 2011. This should assure that any exceptional events claim asserted by a stale or tribe can be fully considered by EPA before final designations. States and tribes may also choose to use available air quality modeling results to indicate aviolation ofthe NAAQS. Attachment 3provides further guidance on the appropriate refined dispersion modeling analysis that could be used to support designation recommendations. Such modeling could include using the AERMOD dispersion model. with allowable source emissions and emissions limitation credit for stacks no higherthan good engineering practice. As noted above (and in the preamble to the final S02primary NAAQS ruJemaking), we recognize that it

### is not realistic to expect states ortribes to complete this type ofmodeling for all significant sources 0[S02 in the time available for providing designation recommendations. Where the time and resources to conduct refined dispersion modeling are limited, we believe it is reasonable to focus first onthe most signiticant sources ofS02 emissions, and on those sources that are most likely to contribute to aviolation. We recognize that this approach means that all areas where S02 NAAQS violations may be occurring might not be identified in the initial round ofarea designations. States are expected to 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 ifit

### is clearthat it meets the S02 NAAQS. EPA does not believe it would be appropriate to do so without appropriate refined dispersion modeling and, where available, air quality monitoring data indicating no violations ofthe NAAQS. In the absence ofinformation clearly demonstrating adesignation of"attainmentH 

### or "nonattainment,"EPA intends to designate the area as "unclassifiable." Determining nonattainment area boundaries. As apollutant that arises from direct emissions, S02concentrations are highest relatively close to 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, consistentwith our approach under other NAAQS, we expect to considerthe county line as the starting point for determining S02 nonattainment areas. As discussed further in Attachment 2, 

### EPA intends to consider several factors when determining the final nonattainmcnt boundaries. We believe it is appropriate to evaluate each potential nonattainment area on acase-by-case basis, andto recognize that area4specific analyses conducted by states, tribes and/or EPA may support a boundary with either a larger or smaller area than the county boundary. A nonattainment area should contain the area violating the NAAQS (e.g., the area around a violating monitor), as well as any adjacent areas (e.g., counties orportionsthereot) that contain emissions sources contributing to the violation. (See CAA section 107(d)(J)(A)(i». Consequently, we recommend that states and tribes base their boundary recommendations on an evaluation offive factors; 1) air quality data; 2)**emissions~relatcd** 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 it allows the model userto simultaneously assess multiple factors. States and tribes may identify and evaluate otherrelevant factors orcircumstances specific to a particulararea. While EPA generally believes that in the absence ofother relevant information it is appropriate to use county boundaries to de.fine nonattainment areas, we recognize that the fivefactor analysis and other information may support designating only a portion ofa county as "nonattainment." Forexample, a topographical feature may divide a county into two separate air basins, orcontributing sources may be clustered in only a portion ofacounty. For defining partial county boundaries, EPA recommends the use ofwell-definedjurisdictional lines such as township borders, immovable landmarks such as major roadways orother permanentand 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 SO] sources is in attainment for the I-hour SOz NAAQS. An attainment area boundary cannotcontain any area that exceeds the NAAQS oranyarea containing sourcesthat are causing or contributing to a violating area. (See CAA section 107(d)(l)(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 S02 designations, the guidance contained herein is not binding on states, tribes the public orEPA. The final basis for determining 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 f()r the revised 2010 S02NAAQS. Attachment 2 identifies the primary five factors that EPA plans to consider in evaluating and making decisions on nonattainment area boundaries. Attachment 3 is the modeling guidance that states and tribes should use to support designation recommendations, 

### 5

### including appropriate area boundaries. Staffmembers at EPA's Oflice ofAir Quality Planning and Standards are available for assistance and consultation throughout the designations process. General questions on this 

### guidance may be directed 

### to Valerie Broadwell (919) 541-3310 

### or Doug 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 Grecn, OAQPS Margo Oge, OTAQ Kevin McLean, OGC Sara Sclmeeberg, OGC

6

### ATTACHMENT 1

TIMELINEFOR

### 2010 Primary S02NAAQS DESIGNATION PROCESS Milestone Date* EPApromulgates S02NAAQS June 3, 2010 States and tribes flag exceptional event-influenced S02 monitoringdata from 2008·2009 October 1,2010
States and tribes flag exceptional event-influenced S02 monitoring data from 20 10; provide detailed documentation to support all 2008-2010 claims No later than Junc 1,2011

Statesandtribes submitrecommendations for area designations to EPA No latcrthan June 3, 2011
EPA notifies states and tribes concerning any intended modifications to their recommendations (120·day letters) o/a February 3, 20]2 (no later than 120 days priorto linal designations)
EPA publishes public notice ofstate and tribal recommendations and EPA's intended modifications and initiates 30-daypublic comment period a/aFebruary 20, 2012

Endof30-daypublic commentperiod ola March 20, 2012 States andtribes submit additional infonnationto demonstrate whyan EPA modification is inappropriate o/a April 3, 2012

EPA promulgates final S02areadesignations No later than June 3, 20]2

,.. ola on orabout Note: This schedule assumes EPA has sufficient infonnation to promulgate designations within 2 years. In the event EPA detenninesthat insufficient infonnation is available to do so, the Clean AirAct alJows EPA to extend the designationprocess, but no latcr than June 3, 2013.

### ATTACHMENT 2 DeterminingDesignations and AppropriateAreaBoundaries forthe 1-hour, 75 ppbS02NAAQS



### Nonanainment Altainmellt Unclassifiable 

### (all other areas) An area where monitoring An area that has no monitored violations data Q! an appropriate and which has an appropriate modeling modeling analysis indicate a analysis, ifneeded, and any other violation. 

# I**~1e".nt**i._.dondemon,""in.no VIolatIOns. An area that has no monitored violations and lacks an appropriate modeling analysis, ifneeded. or other appropriate information sufficientto support an alternate designation.

wm
__ 

### Attainment area boundaries. Areas designated as "attainment" should be supported by information clearly demonstrating thatthere are no violations ofthe S02NAAQS 

### inside the area boundary. This could consist 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. County boundaries may be appropriate for defining attainment areas inthe absence ofother information that would help define a more specific boundary around the modeled source(s). In the absence ofinformation clearly demonstrating a designation of"attainment" OT

### "nonattainment," EPA intends to designate the area as "unclassifiable." Nonattainment area**boulldarie~.** 

### EPA intends to use the county a..<; 

### the analytical starting point il)r assessing the appropriate geographic boundaries ofa S02 

### nonattainment area. As a framework for area-specific analyses to support final boundary detemlinations, we intend to evaluate the five factors listed below, as well as other relevant available information. 'Ihe purpose ofevaluatingthese factors is to determine the appropriate boundaries encompassing the area meeting the CANs definition of"nonattainment area" i.e., an area violating the S02 

### standard and any nearby areas contributing to the violating area. The modeling guidam;e in Attachment 3discusses how modeling 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. 1. Air quality data. We intend to review S02 air quality monitoring data, including the design value calculated for each monitor in the area. for the most recent 3-year period. Areas where monitoring data indicate a violation ofthe I-hour, 75 ppb primary S02 

### standard will be**designate~** 

### 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 ofsourcesand potential contribution to ambient 802 

### concentrations). We intendto examine allowable emissions of802 

### from sources located in

### and around the violating area. Significant emissions levels in a nearby area indicate the potential for the area to contribute to observed or modeled violations ofthe NAAQS. We intend to review data from the latest National Emissions Inventory or other relevant sources ofthe data, such as state inventories orinventories from other federal sources. We would also consider any additional information we receive on federally-enforceable emissions controls that are not reflected in recent inventories but which will require compliance before final designations are issued. 3. Meteorology 

### (weather/transportpatterns). We intend to evaluate meteorological data to help determine how weather conditions, including wind speed and direction, affect the plume ofsources contributing to ambient S02 concentrations. Where feasible, we would consider results from source-oriented dispersion modeling. 4. Geography/topography 

### (mountain ranges or other air basin boundaries), We intend to examine the physical features ofthe land that might affect the distribution ofS02 

### over an area. Mountains or otherphysical features may affect the distribution ofemissions, and may help define boundaries. 5. Jurisdidional boundaries 

### (e.g., counties, air districts, pre-existing nonattainment areas, reservations, metropolitan planning organizations). Once the geographic area associated with the area violating the S02 standard and the nearby area contributing to violations are determined, we intend to consider existingjurisdictional boundaries for the purposes of providing aclearly defined legal boundary for carrying out the airquality 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 meeting the nonattainment definition. Where existingjurisdictional boundaries are not adequate to describe the nonattainment area, otherclearly defined and permanent landmarks or geographic coordinates may be used. EPA plans to consider these tactors, along with any other relevant 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 infonnation they believe is relevant for EPA to consider. Any 

### information providedto support a boundary recommendation for a nonattainment area should show that: 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

### ATTACHMENT3 Modeling.Guidancefor SOlNAAQS Designations 1. Purpose OnJune 2, 2010, Administrator Jackson signed 

### afinal rulemaking notice that revisedthe primary S02 NAAQS (75 FR35520, published on June 22, 2010) after review ofthe existing two primary SOi standards, promulgated on April 30, 1971 (36 FR 8187).1 EPA established the revised primary S02standard at 75 partsper billion (Ppb) which is attained whenthe 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 detennining compliance with the newNAAQS that would include the use ofbothmodeling and monitoring. EPA believes this analytic approach to detennining compliance with the new I-hourNAAQS would be the generallymore technically appropriate andaccurate means ofassessing peak 1­ hour S02 concentrations, and wouldbe consistent with historic (past and more recent) implementation practice ofusing models to determine compliance with the S02 NAAQS. While this guidance explains the use ofmodeling for NAAQS designations, 

### it does not preclude the fact that monitoring datamay be more technically appropriate than mode1ing in some cases. In cases wherethere is complete air quality data from FRMorFEM S02 monitors, that data would be considered byEPA 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 inthese initial designations toa limited degree (as could monitoring) but would likely 

### be used to a largerextent subsequently as the basis for redesignation ofnonattainment and unclassifiable areas to attainment. As thepreamble to the rule promulgatingthe new I-hour S02 NAAQS noted, EPA does notthink 

### it realistic orappropriate to expect states 10 complete modeling for all significant sources ofS02 and assess the results in time for the designationrecommendationsthe Actrequires be submitted by June 2011. (See 75 FR at 35570-71). Therefore, we 

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

I EPA publiclydisseminateda copyofthesignednoticeonJune 3,2010, and therefore treats June 2. 2010, as the dateofthe rule's promulgation, for purposesofthe deadlines in eMsection 107(d). 1L the use ofmodeling to inform the nonattainment boundaries for areas with violating ambient airquality monitors ifthepresumptive county boundaries are not used (either to expand the boundaries outside the county orshrink the boundary within the county); and 2. '{be use ofmodeling in areas without a violating monitor as evidence ofattainment ofthe NAAQS (showing no violations orcontributions to violations ofthe standard). This guidance is consistent with EPA's 

### Guideline onAir Quality Models, orAppendix W to 

### 40 CFRPart 51, and otherrelevant modeling guidanceissued to support regulatory programs. When the need tor interpretation ofthis guidance arises, the user should consult with the appropriate Regional Modeling Contace. Also as indicated in the preamble ofthe**I~hour** S02NAAQS tinal rule, we intend to issue additional guidance describing the development ofan approvablc IIO(a)(1) implementation plansfor areas designated "unclassifiable"that will include technical direction onhow to conduct refined dispersion modeling to demonstrate future NAAQS attainment.



### 2. Guidance on Air Quality Models Much orthis guidance is based onEPA's 

### Guideline onAir QualityModels, also published asAppendix W of

### 40 CFRPart 51. Appendh W is the primary source ofinformation on the 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¥ in support ofthis designations process would need to employ air quality dispersion models that properly address the source-oriented nature ofSOland, thus, should rely upon the principles and techniques in Appendix W. Appendix W was originally published in April 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 52.21 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 or expandedregulatory requirements are incorporated. Clarifications and interpretations ofmodeling procedures become official EPA guidance through several courses ofaction: 1)the procedures are published as regulations or guidelines; 2)

zList ofRegional Modeling Contacts by EPA Regional Office is availablefrom SCRAM website at:

3 Dispersion modeling uses mathematical formulations to characterizetheatmospheric processes that dispersea pollutant emitted by a source. Based onemissions and meteorological inputs,a dispersion model canbe used to predictconcentrations atselected downwind receptorlocations. 2the procedures are formally transmitted as guidance to Regional Office managers; 3) the procedures are formally transmitted as guidance to Regional Modeling Contacts as a result ofa Regional consensus on technical issues; or4) the procedures are a result ofdecisions by the EPA'sModel Clearinghouse thateffectively 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 in modeling for designations includes:

• "ApplicabilityofAppendix W Modeling Guidance for the I-hour S02 NAAQS" August 

### 23, 201O-confirmingthat Appendix W guidance isapplicable for NSRlPSD permit modeling for the new S02 NAAQS. • "Additional Clarification Regarding Application ofAppendix W Modeling Guidance for the I-hour

### NOz National Ambient AirQuality Standard" March I, 

### 20] 1- provides additional guidance regarding

### NOz permitmodeling and also relevantto S02. The following sections will provide reference to the relevant sections ofAppendix W and other existing guidance with summaries as necessary. Please refer to those original guidance documents for full discussion and consult with the appropriate EPA Regional Modeling Contact ifquestions arise aboutinterpretation 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. Ifa model is to be used for a particularapplication. the user should follow the guidance on the preferred model for that application. These models may be used without an area specific formal demonstration of applicability as long as they are used as indicated ineach model summary ofAppendix A. Further recommendations tor the application ofthese modelsto specific source problems are found in subsequent sections ofAppendix W. In 

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

4,The SupportCenter for RegulatoryAtmospheric Modeling (SCRAM) websiteis availableat:
3For area designations underthe I-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 "ApplicabilityofAppendix 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 S02 NAAQS as part ofthe NSRlPSD pernlit programs. AERMOD is appropriate to inform this designations process because S02 concentrations result from direct emissions from combustion sources so that concentrations are highest relatively close to sources and are much lower at greater distances due to dispersion. Given the source-oriented nature ofthis pollutant (See, e.g., 7S FR at 35570), dispersion models are the most appropriate air quality modelingtools to predict the ncar-field concentrations ofthis pollutant. The AERMOD modeling system includes several compunents. The regulatory components are:

• AERMOD: the dispersion model (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 characteristicsprocessor for AERMET (U.S. EI)A. 2008) • AERSCREEN: a recentlyreleased screening version ofAERMOD (U.S. EPA, 2011b) Beforerunning AERMOD. the user shouldbecome familiar with the user's guides associated with the modeling components listed above and the AERMOD Implementation Guide (AIG) (U.S. EPA, 2009). The AIG lists several recommendations for applications 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 center the modeling domain?, and 2. How large should themodeling domainbe? (i.e,. in terms ofthe number ofsources to model and size ofthe receptornetwork inorderto account for the areas ofimpact).

4

### Ifthe modeling is being performed to inform the nonattainment 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 ofaviolating 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 thoughtto cause orcontribute to a potential NAAQS violation, as explained further below in Section 4.1. The determination ofsources to include in modeling is a multi-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 isrecommended.

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 greaterthan 100 tons per year. Please note, however, that sources less than 100 

### tons can be potential contributors to aNAAQS violation, especially sources with short stacks andior located in complex terrain (i.e., where receptor elevation 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.c., use ofmonitored background concentrations. Accounting for such sources through the use ofa background monitor will depend upon how well that monitorreflects impacts from those sources. 3. Sources found notto 

### be representative by monitored background should also be examined through the use ofscreening models to see ifthey should or shouldnot be included in the refined modeling. We recommend the use ofEPA's new screening model AERSCREEN (U.s. EPA, 201lb) and fol1owing recommendations based on pre-existing screening guidance (U.S. EPA, 1992). For small isolated sources, screening may be useful on a source by source basis. However, for a cluster ofsmall sources, their cumulative impact should also be assessed. Individual sources may not be 

### significantby themselves, but together they could cause a NAAQS violation or significantly contribute to aNAAQS violation. Although AERSCREEN does notoutput a design value 5conce,ntration based on the 99th percentile form ofthe l-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 AER8CREEN violates the NAAQS, it does not meanthat the source is in nonattainment, but that the source should 

### be evaluated using refined dispersion modeling (See Step 3 below for more details).

Figure 1 shows a hypothetical monitor with circles of50 km and 10 km radii centered overit. Based on this figure, an example application ofthese three steps is described below. Step 1: FibJUre 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 ten 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 thispoint, 

### itcould be initially assumed that all facilities should be included in refined mode1ing. Step 2: Determine whether any source or sources can be accounted for bya representative background monitor. In Figure 1, there are two othermonitors in the area, one north and one south ofthe violating monitor. The northern.most monitormay 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 7 below. Step 3: Screening modeling may 

### be used to determine additional sources orcombinations of sources to be excluded from refined modeling, especially smallersources 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 802 design value butdoes output the overall maximum I-hour concentration for an individual stack. Ifa facility contains more than one emissionpoint or stack, each stack should be processed in AERSCREEN and the maximum I-hourconcentrations can be added together to 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 eta1., 2008), it may be useful to also model the source in AER8CREEN using the surface characteristics ofthe meteorological site being used inthe refined modeling as well, to ensure that the source is belowde minimis impact levels with either set ofsurface characteristics.

5The 3 ppb interim SIL fornewl-hourS02 NMQS was provided by EPA for states to considerusingfor PSD program in theAugust 23, 2010memorandum "Guidance Concerningthe Implementation ofthe l-hourS02 NAAQS for the Prevention ofSignificantDeterioration Program" 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 relined dispersion modeling, • Ifthe facility's maximum I-hourconcentration is below 75 ppb but above the suggested interim I-hour significantimpact level of3 ppb orthe state's I-hour SIL, it should be included in the refined modeling, • Ifthe facility's maximum I-hourconcentration is belowthe suggested interim 1-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 notbe excluded on the sole basisofbeing belowthe SIL without Jirst looking at surrounding sources and their maximum I-hour concentrations. The case may arise when there are several small sources that singularly arc belowthe SIL buttheir cumulative impact may lead to concentrations that contribute to violations ofthe NAAQS.
In summary for the example in Figure 1. the smaller sources below 1ton ofemissions to the north ofthe monitormaybe best representedwiththe use ofbackground monitor concentrations. Other sources between I and 10 tons that are notrepresented bybackground monitorscould be excluded based on screening results. depending on their stack parameters and terrain. The smaller sources (less than 1ton) within IO km ofthe monitor location may also screenout. 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 inrefined modeling as they are likely contributingto potential NAAQS violations and are not reflected in background monitors. This isjust oneexample ofhow to determine the modeling domain and sources to model. In some cases, an analysis out to 50 km may not be needed. Please consult with the appropriate EPA Regional Office modeler ifthere is uncertainty in deciding which sources to explicitly model, which sourcestorepresent based on background monitoring, and/or which to exclude from refined modeling 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 beplaced in areas that are considered ambient air (i.e.• where the public generallyhas access) and placed outto a distance such that areas ofviolation can be detected from the model output to help determine the size ofnonattainment areas. Receptor placement should be of sufIicient density to provideresolution needed to detect significant gradients in the concentrations withreceptors placed closertogethernear the source to detect local gradients and placed farther apart away from the source. In addition. the user should placereceptors at key locations such as around facility fence lines (which definethe ambient air boundary for a particular source) ormonitor locations (forcomparison to monitored concentrations for model evaluationpurposes). The receptor network should cover the modeling domain. An example receptor grid for a single source is shown in Figure 2a with an example grid \\1Jth multiple sources shown in Figure 2b. In Figure 2a, receptors are located every 50 m within one kilometer ofthe source and then every 100 m from one to two kilometers. From two to 

### to km, the receptor spacing is 250 m andevery 500 m outside of 10 km ofthe source. The modeling domain is centered on an isolated facility and extends out to IO 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 I km (50m spacing) around each source embedded within a lOxIO 8

### kIn grid (250 m spacing), The lOx 1 0 km grid is then embedded within a 20x20 km grid with coarser spacing (500 m). 

### Ifmodeling indicates elevated levels ofS02 (ncar the standard) nearthe edge ofthe receptor grid. consideration ofexpanding the grid orconductingan additional modeling run centered on the area ofconcern shouldbe investigated. As noted above, terrain complexity should also be considered when setting up the receptorgrid. Ifcomplex terrain is included inthe model calculations, AERMOD requires that receptor elevations be included in the model inputs. In those cases, the AERMAPterrain processor (U.S. EPA, 2004b) should be used to generate the receptor el.evations and hill heights. The latest version ofAERMAP (09040) can process either Digitized ElevationModel (DEM) orNational Elevation Data(NED) data files. The AIG recommends the use ofNED data since 

### it is more up todate than OEM data, which is no longer updated (Section4.3 ofthc AlG).

5. Sourceinputs This section provides guidance on source characterization to develop appropriate inputs for dispersion modeling with the AERMOD modeling system. Section 5.1 provides guidance on use ofallowable vs. actual emission levels, Section 5.2 covers guidance on Good Engineering Practice (GEP) stack heights, Section 5.3 provides details on source configuration and source types, Section 5.4 provides details onurban/rural determination ofthe sources, and Section 5.5 provides general guidance on source grouping, which may be important tordesign value calculations.

5.1 Allowable \'5. 

### Adual emissions Consistent with past S02 modeling guidance (Section4.5.2 ofU.S. EPA (1994)) and regulatory modeling for otherprograms (Appendix W, Section 8.1), dispersion modeling for the purposes ofdesignations should 

### be basedonthe use ofmaximum allowable emissions or federally enforceable permitlimits. Also consistent with past and current guidance, in the absence ofallowable emissions orfederally enforceable permit limits, porential to emit emissions (Le., design capacity) should be used. Because oflhe short-term nature oflhe new S02 NAAQS, the maximum short term orhourly emissionrate should beinputinto AERMOD for each modeled hour. As stated in the August 23, 2010 memo, "Since short-term S02 standards (S24 hours) have been in 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 inTable 8-2 ofAppendix W related to short-term vs. long-term emission estimates may have already been addressed." 9

# a



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,**~** .. •
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FIGURE 2, EXAMPLE RECEPTOR GRIDS WITH (A) A GRID CENTERED ON AN ISOLATED SOURCE WITH FENCEUNE RECEPTORS SHOWN IN BLUE AND THE EMISSION POINTS SHOWN IN BLACK. AND (B) A GRID WITH MULTIPLE SOURCES, 

### 10The existing 

### SOz inventories used for pennitting or SIP demonstrations should contain the necessary emissions infonnation 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. Fatanexample calculation ofshort tenn emissions, see the June 28, 20J0 memorandum "Applicability ofAppendix W Modeling Guidance for the I-hour N02 National Ambient Air Quality Standard." Although the example is for N02, the calculation methodology would be the same for 

### SOz. Regarding the use ofallowable emissions and modeling ofintennittent emissions sources, from such sources as emergency generators and startup/shutdown emissions. the inclusion ofsuch emissions for the purpose ofmodeling for SOl designations should follow the recommendations inthe March 1,2011 memo "Additional Clarification Regarding Application ofAppendix W Modeling Guidance forthe l-hourN02National Ambient Air Quality Standard." As stated in this 

### memo, EPA believes the most appropriate data to use for compliance demonstrations for the I-hourN02NAAQS are those based on emissions scenarios that are continuous enough or frequent enough to contribute significantly to the annual distribution ofmaximum daily I-hourconcentrations. Although the referenced guidance in this memo is for NOz pennitmodeling, the common I hour averaging time and fonn ofboth the N02 and S02 standards makes this modeling guidance applicable to the I-hour S02 NAAQS and, thus, applicable to S02 modeling in support ofdesignations. For more details, refer tothe NOz memo. Ifany questions arise regarding preparation ofemissions inputs for dispersions modeling including intennittent emissions from sources, then users shouldconsult the appropriate EPA Regional Modeling Contact.



### 5.2 Good Engineering Practice 

### «;EP) stackheight Consistent with previous SOz modeling guidance (U.S. EPA, 1994) and Section 6.2.2 of Appendix W, for stacks withheights that are within the limits ofGood Engineering Practice (GEP), actual heights shOuld be used in modeling. Under EPA'sregulations at 40 CFR 51.100, GEP height, Hg, is determined to be the greater of:

• 65 m, measured from the ground-level elevation atthe base ofthe stack; • For stacks inexistence on January 12, 1979, and for which the owner or operatorhad obtained all applicable permits or approvals required under

### 40 CFRParts 51 and 52 Hg=2.5H provided the oVvner or operatorproduces evidence that this equation was actually relied on in designing the stack orestablishing an emission limitation toensure protection against downwash;

11Forall other stacks, 

### Hg=H 

### + 1.51.., 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 thatthe emissions from a stack do not result in excessive concentrations ofany airpollutant as a result ofatmospheric downwash, wakes, eddy effects created by the source itself, nearby structures or nearby terrain features. For more details about GEP, see the Guideline for Determination ofGood Engineering Practice Stack Height Technical Support Document (U.S. EPA, ]985). Ifstack heights exceed GEP, then GEP heights should beused with the individual stack's otherparameters (temperature, diameter, exit velocity). For stacks modeled with actual heights below GEP, buildingdownwash shouldbe considered as this can impact concentrations near the source (Section 6.2.2b, Appendix W). Ifbuilding downwash is being considered, the BPIPPRIME program (U.S. EPA, 2004d) should 

### be used to input building parameters for AERMOD. More infonnation aboutbuildings and stacks is in Section 5.3.



### 5.3 Source configurations and source types An accurate characterization ofthe modeled facilities is critical for renned dispersion modeling, including accurate stack parameters and physical plantlayout. Accurate stack parameters should be detennined for the emissions being modeled. Since modeling would be done with maximum allowable orpotential emissions levels at each stack, the stack'sparameters such as exit temperature, diameter, and exitvelocity should reflect those emissions levels. Accurate locations (i.e. latitude and longitude or Universal Transverse Mercator (UTM) coordinates and datum)6 ofthe modeled emission sources are also important, as this can afrect the impact ofan emission source onreceptors, determination ofstack base elevation, and relative locationto any nearby building structures. Not only are accurate stack locations needed, but accurate information for any nearby buildings is important. This infonnation would include location and orientation relative to stacks and building size parameters (height, and comer coordinates oftiers) as these parameters are input into BPIPPRIMEto calculate building

{, Latitudes and longitudes to four decimalplaces position a stack within 30feet ofits actual location and five decimal placesplacea stack within threefeetofits actual location. Users shoulduse thegreatestprecision available. 

### 12parameters for AERMOD. Ifstack locations and or building information are not accurate, downwash will not be accurately accounted for in AERMOD. Emission source type characterization within the modeling environment is also important. As stated in the AERMOD User's Guide (U.S. EPA, 2004a), emissions sources can be characterized as several different source types: POINT sources, capped stacks (POINTCAP), horizonta1 stacks (pOINTHOR), VOLUME sources, OPENPIT sources, rectangular AREA sources, circular area sources (AREACIRC), and irregularly shaped area sources (AREAPOLY). Note that POINTCAP and POINTHOR are not part ofthe regulatory default option in AERMOD because the user must invoke the BETA option in the model options keyword MODELOPT while not including the "DFAULT"modeling option for these options to work properly. While most sources canbecharacterized as POINT sources, some sources, such ali fugitive releases or nonpoint sources (emissions from ports, airports, or smallerpoint sources with no accurate locations) may be best characterized as VOLUME or AREA type sources. Ifquestions arise about proper source characterization ortyping, users should consult the appropriate EPA Regional Modeling Contact.

5.4 Urban/rural determination Forany dispersion modeling exercise, the urban orrural determination ofa source is important in determining the boundary layer characteristics that affect the modcl'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 below the rural concentration beyond 750 m. For the tal1er stack in Figure 3b, the urban concentration is much higher thanthe rurdi concentration even a') distances increase from the source. These profiles show that the urban orrural designation ofa source can be quite important. In addition, for S02**modelin~,** the urban/rural determination is important because AERMOD invokes a 4-hour halflife for urban S02 sources. This would only be done for urban sources when the POLLUTID keyword inAERMOD is setto "S02" and the MODELOPT keyword includes the DFAULT option. Rural sourceswithinthe same AERMOD run would not be affected. Ifthe DF AULToption isnot included with the MODELOPT keyword, the**4~hour** halflife would not be used and the user would specify the 4-hour halflife using the HALPLlFE orDCAYCOEFF keywords in orderto account for the chemical transformation. See Section 3.2.6 ofthe AERMOD User's Guide (U.S. EPA, 2004a) for more details about these keywords. Ifthe user invokes the HALFLIFE orDCAYCOEEF option, then any rural sources included in

7 Overa 4-hourperiod, 502 concentrations decreaseby halffrom theinitial value. 

### 13

### the modeling would need to be run in separate AERMOD runs so that they are not subject to the 4-hourhalflife. Note that ifthe DFAULT option is used, the rural sources would not need to be in a separate run from the urban sources. Detennining whether a source is urban or rural can be done using the methodology outlined in Section 7.2.3 ofAppendix Wandrecommendations outlined in Sections 5.1 through 5.3 in the AIG (U.S. EPA, 2009). In summary, there are two methods ofurbanirural 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 land use method, the user analyzes the land use within a 3 km radius ofthe source using the meteorological land use scheme described by Auer (1978). Using this methodology, a source is considered urban ifthe land use types, Jl 

### (heavy industrial), 12 (light-moderate industrial), Cl (commercial), R2 (common residential), and R3 (compact residential) are 50% or more ofthe area within the 3km radius circle. Otherwise, the source is considered a rural source. The second method uses popUlation density and is described in Section 7.2.3d of Appendix W. As with the land use method, a circle of3 km radius is used. Ifthe population densitywithin the circle is greater than 750 people/km2

, 

### then the source is considered urban. Otherwise, the source is modeled as arural source. Ofthe two methods, the land use method is considered more definitive (Section 7.2.3e, Appendix W). Caution should be exercised with eitherclassification method. As 

### stated in Section 5.1 of the AlG (U.S. EPA, 2009). when using the land use method, a source may be 

### in an urban area but located close enough to abody ofwater orother non-urban land use category to result in an erroneous rural classification for the source. The AlG in Section 5.1 cautions users against using the land use scheme on a source by source basis, hut 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 should be used with caution and should not be applied to highly industrialized areas where the populationdensity may be low and thus arural classification would be indicated, but the area is sufficiently built-up so that the urban land use criteriawould 

### be satisfied

..." With either method, Section 7.2.3(1) ofAppendix Wrecommends modeling all sources within an urban complex as urban, even ifsome sources within the complex would be considered rural using eitherthe land use orpopUlation density method. Anotherconsideration that may need attention by the user and is discussed in Section 5.1 ofthe AIG relates to tall stacks located within or adjacent to small to moderate size urban areas. In such cases, the stack height or effective plume height for very buoyant sources may extend above the urban boundary layerheight. The application ofthe urban option in AERMOD for 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 determining ifatall stack should be modeled as urban or rural, based on comparing the stack or effective plume height to the urban boundary layer height. The 100 m stack illustrated in Figure 3b, may be such an example as the urban boundary layer height for this stack would be 189 m (based on a
14

### 10m

300
200
100

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# . I I) 500 1000 1500 :ZOLLO 2500 3000 3500 4000 4500 5QOO OIlIl1WlcItIml 

### 100m 

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FIGURE 3. URBAN (RED) 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 aI., 2004). This equation is:

(1) where Ziuo is a reference height of400 m 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 bebest characterized as a rural source, even ifit were near an urban complex. Exclusion ofthese elevated sources from application ofthe urban optionwould need to bejustified on a case-by-casebasis 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 (i.e., an urban population of 1,674,365 can be entered as 1,700,000). Users canentermultiple urban areas and populations using the URBANOPTkeyword in the runstream file (U.S. EPA, 2oo4a). Ifmultipleurban areas are entered, AERMOD requires that each urban source be associated with a particular urban area or AERMOD model calculations 

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

5.S 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 user can automatically calculate a total concentration (from all sources) using the SRCGROUPALL keyword. Forthe purposes ofdesignations and design value calculations, source group ALL should be used, especially ifall sources in the modeling domain are modeled in oneAERMODrun. Designvalues should be ca1culated from the total concentrations (all sources and background). For the purposes ofdesignations modeling, individual source contributions outputsto the total concentration may nol be necessary. However, ifindividual facility contributions are needed for deciding which facilities to include in the nonattainrnent or attainment area, source groups by facility should be used. To avoid any confusion, source groups that are used to calculate the design value concentrations ordetermine source contributions to design values shouldbemutually exclusive (i.e. an emission source should not 

### be in two source groups). This would be especially importantifthe design value concentrations are calculated outside ofAERMOD by addingthe individual groups togetherto calculate a total concentration (See Section 8.1 ofthis document 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 oroutside of AERMOD.



### 6. Meteorological data Section 6 gives guidance on the 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 use ofa new tool, AERMINUTE (U.S. EPA, 2011a), is introduced. AERMINUTE is an AERMETpre-processor that calculates hourly averaged winds from ASOS (Automated Surface Observing System) I-minute winds.



### 6.1 Surface characteristics and 

### representativeness The selection ofmeteorological data that are input into a dispersion model should be considered carefully. The selection ofdata shouldbebased on spatial and climatological (temporal) representativeness (Appendix W, Section 8.3). Therepresentativeness ofthe data is based on: 1) the proximity ofthe meteorological monitoring site to the areaunder consideration, 2)the complexity ofterrain, 3) theexposure ofthe meteorological site, and 4) the period oftime during which data 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 dataean be adversely affected by large distances between the source and receptors ofinterest and the complex topographic characteristics ofthe area (Appendix W, Section 8.3.a and 8.3.c). Ifthe modelingdomain is large enough such that conditions vary drastically across thedomain then the selection ofa single station to representthe domain should be carefully considered. Also, care should bc taken when selecting a station ifthe areahas complex terrain. While a source andmeteorological station may be in close proximity, there may becomplex terrain between them such that conditions at the meteorological station may not be representative ofthe source. An exanlple would be a source located onthe 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 for each meteorological variable separately. Forexample, temperature data from a meteorological station several kilometers iromthe analysis area may be considered adequately representative, while 

### it may benecessary to collectwind data nearthe plumeheight (Section 8.3.c ofAppendix W). 

### 17Surface characteristics canbe calculated in several ways. Fordetails see Section 3.1.2 of the AIG (U.S. EPA, 2009). EPA has developed a tool, AERSURFACE (U.S. EPA, 2008) to aid inthe determination ofsurface characteristics. The current version ofAERSlJRFACE uses 1992 National Land Cover Data. Note that the use ofAERSURFACE is not a regulatory requirement but the methodology outlined in Section 3.1.2 ofthe AIG should be folJowed 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 datato ensure that worst-case conditions are adequately represented in the model results. Appendix W states that 5 years ofNWSmeteorological data orat least one year ofsite-speci1ic data should be 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 form ofthe S02 NAAQS contemplates obtaining three years of monitoring data, this does not preempt the use of5 years ofNWS dataoralleast one yearof site-specific data in the modeling. The 5-yearaverage based on the use ofNWS data, or an average across one or more years ofavailable site specific data, serves as an unbiased estimate of the 3-year average for purposes ofmodeling demonstrations ofcompliance with the NAAQ (See the August 23, 2010 Clarification Memorandum on "Applicability ofAppendix W Modeling Guidance for the I-hour S02National Ambient Air Quality Standard"). Seethe memorandwn for more details on the use of5 years ofNWS data orat least oneyearofsite-specific data and applicability to the NAAQS.

6.2.1 NWS data NWS data are available from the National Climatic Data Center (NCDC) in many formats, with the mostcommon one in recent years being the Integrated Surface Hourly data (ISH). Most available fonnats 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,thedata should 

### be spatially and temporal1y representative ofconditions at the modeled sources. A recently discovered issue with ASOS is that 5-second wind datu that are used to calculate the 2-minute average winds are truncated rather than rounded to whole knots. For example, a wind of2.9knots 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 112 knot or 0.26 mls is added to the winds in stage 3 AERMETprocessing. For more details refer to the AERMET User's Guide (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 calm**conditions~** resulting in zero concentrations for those hours. Since the S02 NAAQS is a one hour standard, these light windconditions may be the controlling meteorological circumstances in some cases because ofthe limited dilution that occurs under low wind speeds which can lead to higherconcentrations. The exclusion ofa greater numberOf instances ofnear-calm conditions from the modeled concentration distribution may therefore lead to underestimation ofdaily maximum I-hour concentrations for calculation ofthe design value. To address the issues ofcalmand variable winds associated with the use ofNWS meteorological data, EPA has developed a preprocessorto AERMET, called AERMINlJTE (U.S. EPA, 2011a) that can read 2-minute ASOS winds andcalculate an hourly average. Beginning with year 2000 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. Thisresults 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 for 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-specitic meteorological data is the best way to achieve spatial representativeness. AERMETcan 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 thatcan becollected onsite and the range of formats ofdata input to AERMET. the user should consult Appendix W, the AERMET User's Guide (U.S. EPA, 2004c), and Meteorological Monitoring Guidance for Regulatory Modeling Applications (U.S. EPA, 2000). Also, when processing site-specific datafor an urban application, Section 3.3 ofthe AERMOD Implementation Guide offers recommendations for dataprocessing. In summary, the guide recommends that site-specific turbulencemeasurements should not be used when applying AERMOD's urban option, in order to avoid double counting the effects ofenhanced turbulence due to the urban heat island.

196.2.3 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 canbe obtained from the Radiosonde Data ofNorthAmerica CD for the period 1946-1997. Upper air soundings for 1994 through the present are also available for free**dO~1110ad** from the Radiosonde Database Access website. Users should choose all levels ormandatory and significant pressure levels8 when selectingupper air data. Selecting mandatory levels only would notbe adequate for input into AERMET as the use of just mandatory levels would not provide an adequate characterization ofthe potential temperature prof1le.

7. Background concentration The inclusionofambient backgroundconcentrationsto the 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 "Applicability ofAppendix W Modeling Guidance for the 1­ hour S02 National AmbientAir Quality Standard." This memo suggested a "first tier" approach to including a uniform monitored background contributionbased on adding the overall highest hourly background S02concentration from a representative monitorto the modeled design value. We recognize that this approach could be overly conservative in many cases and may also beprone to reflecting source-oriented impacts, increasingthepotential 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-hourN02 Ambient AirQuality Standard," we recommend a less conservative "first tier" approach tor a uniform monitored background concentration based on the 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 EPARegional Modeling Contact with adequatejustification and documentation ofhow the 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..One option is, as described in Section 8.2.2.b:

II Byinternational 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 mayvarydependingon the meteorological conditions at the upper-air station 20"Use air quality 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 shorter time periods, the meteorological conditions accompanying concentrations ofconcern should be identified. Concentrations for meteorological conditions ofconcern, at monitors, not impacted by the source in question, should be averaged for separate averaging time to determine the average background value. Monitoring sites inside a 90° degree sector dowmvind 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 are expected to cause a significant concentration gradient inthe vicinity ofthe source or sources under consideration, and should be explicitlymodeled. Identification ofnearby sources calls for professional judgmentand consultation withthe appropriate EPA Regional Modeling Contact. Forother sources, such as natural source.<;, 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 l·hourN02 Ambient Air Quality Standard," describes an appropriate methodology ofcalculating temporally varying background monitored concentrations by hour ofday and season (excluding periods when the source in question is expected to impact the monitored concentration). The methodology for N02 is to use the 98th 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 influencingthe monitored concentration (i.e., 99th percentile, or 4th highest, concentrations for hour 1for January orwinter, 991h percentile concentrations f'Or hour 2 for January orwinter, etc.). Recent updates included inAERMOD allow for the inclusion of temporally varying background concentrations in the design value calculation in combination with modelingresults. An illustrative example is shown in Figure 4. Shown arc the NMQS standard concentration, the monitor's 3-year average design value, and 3·yearaverages ofthe 99th percentile concentrations by season and hour ofday. To calculatethe 99111 percentile concentration for a seasonand hour ofday combination, the second highest concentration for thatcombination 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 each ofthe other seasons.

70

60

50

20· 

### ..

6 9 1011121314151617181920212223 Hour ofday**~~~~~~v~-"~**

>
... Winter i

.... Spring



### -+- Summer 

## I i- Fall 

## I HourlyAverage 99th**L.~-1::!0urly A~Qe** 

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 onthe hour-beginning convention, and meteorological monitoring where the observation is based on the hour-ending time. Thus, ambient monitoring data reported for hour 00 should be paired with meteorological data for hour 01, etc. This is important when incorporating time-varying background

9 Modelers should use the 1s!.highest value for moredetailed pairings. suchas monthbyhour-of-dayor season by hour-of-day and day-of-week 

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



### 8. Determining design value metrics Designations modeling will provide predictions of802 design values at eachreceptor that includes contributions from all modeled sources and background. Based on the form ofthe 1­ hour S02NAAQS, the design value should be 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 against the standard, the following steps shouldbe followed: 1. At eachreceptor, for eachhour ofthe modeled period, calculate a tuta) concentration across all sources including background concentrations 

### ifapplicable. 'Ibis 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, i.e. no one source should be in multiple source groups. 2. From the total concentrations calculated in step 

### I.obtain the I-hourmaximum concentration at each receptorfor eachmodeled day. 3. From the output ofstep 2, for each yearmodeled, calculate the 99th percentile (4th highest) daily maximum I-hour concentrationat each receptor. Ifmodeling 5 years of meteorological data, this results in five 99th percentile concentrations at each receptor. 4. Average the 99th percentile (or 4th highest) concentrations across the modeled years to obtain a design value ateach receptor. 5. Modeled source contributions to a NAAQS violation can be determined by analyzing the hourly concentrations from the individual source groups corresponding to the same hour as the 4th daily maximum I-hour concentration from each year. (See 75 FR at 35540). For example, a receptor has a 5-yearaverage design value of200.8 mg/m 3 (or approximately 77 ppb) and AERMOD was modeled for the periodJanuary 1,2005 through December 31, 2009 (or four source groups. From the AERMOD output, the user can determinethe date ofthe 4th highest daily maximum I-hourconcentrations that arc used tocalculate the 5-year average design value. Table 1 shows the 4th highest daily maximum I-hourconcentrations for each year and associated dates that are used in the design value calculation.



### 23TABLE 1. 4TH HIGHEST DAILY MAXIMUM 1-HOUR CONCENTRATIONS (IJG/M3) FOR 2005-2009. Date Concentration 

### I**(YYMMDD~H)** 

### . 05080101 200.1 06073105 201.5 07080403 207.1 08072705 09080104 197.1 198.1 ..

### ­ 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 1. Table 2 shows example source contributions for each hour 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 (IJG/M 3) AND 5-YEAR AVERAGE DESIGN VALUES.

,---­ Date

..(YVMMDDHH) TOTAL SOURCE 1 SOURCE 2 SOURCE 3 SOURCE 4 05080101 200.1 155.1 25.1 1.5 18.4**~973105** 07080403 201.5 157.4



## -­ 26.2 0.5 17.4 207.1 161.5 20.5 2.1 1.7 -._. 23·9

__ 13.1 08072705 197.1 159.2 23.1 09080104 198.1 155.3 22.6 2.0**18.~** __ 18.0 5-YEARAVG. 200.8 157.7 23.5 ._. 1.6



### 8.2 Running 

### AERMOD and implications for design value 

### calculations Recent enhancements to AERMOD include options to aid in the calculation ofdesign values for comparison with the S02 NAAQS. These enhancements include:

• The output ofdaily maximum l-hour concentrations by receptor for each day in the modeled period for a specified source group. This is the MAXDAILY output option in AERMOD. • The output, for each rank specified on the RECTABLE output keywnrd, 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 bound rank, i.e. 41h 

### 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 aticr the first 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 onthe REcrABLEkeyword). This option may be needed to aid in determining which sources to include in a nonattainmcnt area. Ideally, all explicitly modeled sources, receptors, and background should be modeled in one AERMOD run for aU 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 determine thc area's attainment status and/or inform attainmentlnonattainment boundaries. The use ofthese options in AERMOD allows AERMOD to internally calculate concentration metrics thatcan be used to calculate design values and therefore lessenthe 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 considerationsduringthe AERMOD modeling. Sometimes separate AERMOD runs arc done for each facility or group oftacilities, or by year, or the receptor network is divided into separate sub-networks. In some types ofthese situations, the MAXDAILY, MXDYBYYR, or MAXDCONToutput option maynot 

### 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 bc taken in developingthe model inputs to ensure accurate design value calculations. Situations that would effectively preclude the use ofthe MAXDAfLY, MXDYBYYR, and MAXDCONT option to calculate meaningful AERMOD design value calculations include the following examples:

• Separate AERMOD runs for each source or groups ofsources.
o Designations modeling includes 10 facilities for five years ofNWS data and each facility is modeled for tive 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 tor each year, resultingin 

### fifty individual AERMODruns. In the two situations listed above, the MAXDAILY, MXDYBYYR, or, MAXDCONT option would not be useful as the different AERMODruns do not include a total concentration with contributions from all facilities. In these situations the use ofhourly POSTFILES, which can be quite large, and external post*processing would 

### be needed to calculate design values.

25Situations that may use the MAXDAILY. MXDYBYYR, or, MAXDCONToptionbut 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 receptornetwork of

### 20,000 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 MAXDAfLY, MXDYBYYR, or, MAXDCONT results for each network can 

### be re-combined into the larger network.

• All sources and receptors are modeled in an AERMOD run for each year.

• Ten facilities arc modeled with five years ofNWS data. All facilities are modeled with all receptors for each year individually, resulting in five AERMOD runs. MAXDAILY, MXDYBYYR, or, MAXDCONT output can be used and post-processed to generate the 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 receptor network is divided into four 

### 5,000 receptor networks. Foreach subnetwork, all ten facilities are modeled for each year separately, resulting intwenty 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 nonattainment/attainment boundaries Dispersion modeling is a tool that could be used to examine the spatial extent ofpotential violations ofthe I-hour S02 NAAQS. Thus, in accordance with this guidance, refined dispersion modeling could 

### be used to inform boundary detenninations in support ofthe S02 designations process, i.e, 1. For an areathat contains a violating monitor, modeling could be used to infonn decisions on the appropriate nonattainment boundary in conjunction with other factors listed in Attachment 2. 

### 2. For an areawithout a violating monitor, modeling could be used as evidence ofan area's attainment status and also to inform decisions onthe appropriate (attainment or nonattainment) boundary. The shape and size ofthe nonattainment orattainment area is recommended by the state and eitheradopted ormodified by EPA. For initial designations, it is expected that states will focus on areas withviolating monitors. Ifa county contains a violating monitor, that county would be 

### 26considered in nonattainment. Ifthere are no violating monitors and no dispersion modeling results to show attainment or nonattainment, that county would generally be considered unclassifiable.



### 9.1 Nonattainment area boundaries For nonattainment areas (those with a violating monitor), modeling could beused to refine the nonattainment area boundaries from the presumptive countyboundaries in conjunction with other factors such as those listed in Attachment 2. This could include reducing the nonattainment areafrom thepresumptive county to a smallerarea orexpandingthe boundary beyond the county ifsources outside the county contribute to a NAAQS violation in thecounty. A nonattainment area boundary should contain the areathat exceeds the NAAQS and include sources that may cause orcontribute to a NAAQS exceedance. Figure 4 shows a hypothetical example ofmodeling ofan area that exceeds the NAAQS (eitherthrough 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 establishing the nonattainment boundary so that the orange and red contours arc within the boundary. In Figure 5b, the hypothetical nonattainment boundary is a circle, centered on the area shown as violating the NAAQS. while Figure 5c shows the hypothetical nonattainment boundary as a rectangle. FinaIJy, Figure 5d shows a hypothetical nonattainment boundary as an irregular polygon in shape, perhapsbased onjurisdictional boundaries or other landmarks such as roads. Figure 6 illustrates a hypothetical example for a multi-source situation that is in 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 modelingdomain is centered onthe violating monitor (star). The orange contour represents concentrations above the NAAQS. As in the single source example shown in Figure 5, the nonattainrnent areacould be circular, rectangular, or irregularly shaped usingjurisdictional boundaries. In this example. the hypothetical nonattainment boundary would be defined bythe northern portion ofCounty A and the southernportion ofCounty C. Since multiple sources arc involved, the hypothetical nonattainment boundary should be extended to cover those sources that cause or contribute to a NAAQS violation. In this hypothetical example, Sources 2 and 5 arc the largest contributing sources to the potential NAAQS violation so the nonattainment boundaries would include those two sources.



### 27

# c a 

# b

d

FIGURE 5. HYPOTHETICAL EXAMPLE OF A MODELED NMQS VIOLATION (RED AND ORANGE CONTOURS) AND POSSIBLE NONATTAINMENT AREA BOUNDARIES DEFINED BY (B) CIRCLE, (C) RECTANGLE, AND (0) 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 8Y (8) CIRCLE, (C) RECTANGLE, AND (D) AN IRREGULAR POLYGON.

9.2 Attainment area boundaries In areas without a violating monitor, modeling couldbe used to help determine that an area with S02emitting sources is in attainment for the I-hour S02NAAQS. An attainmentarea 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 thatother factors would need tobe considered ifthe boundary is not determined by using the county presumptive boundary. Figure 7 illustrates a group ofsources where a monitored design value does not exceedthe NAAQS and modeling also does not show any concentration levels in excess ofthe NAAQR 

### In this case, the state couldrecommend that 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 (i.e., B, C, or D) and their modeled concentration levels do not show violations ofthe NAAQS, then these counties could also 

### be recommended as part of the attainment area.


## D 

## B



# A
4

Kilometer o 25 '5 

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



### 10. Documentation Itis expected that the state would submit a modeling and analysis protocol. that details the methodology and model inputs before commencementofthe modelingexercise. 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• Characterization ofthe nonattainment problem orcharacterization ofthe modeled area in absence ofa vIolating monitor, • Anemissions analysis around the violating monitor orarea under consideration for designations in absence ofa violating monitor, and • Methodology tor preparing air quality and meteorology 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.



### ll.

### Summary In summary, we emphasizethe following key points ofthis modeling guidance:

• AERMOD is EPA'spreferred near-field dispersionmodel for regulatory applications and is applicable for S02 designations modeling consistent with EPA's Guideline onAir Quality Models, also published as Appendix W of40 CFRPart 51. • Sources should be modeledwith maximum allowable**l~hour** or short-term emissionrates 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 Temporally varying based on the 99lh percentile monitored concentmtions by hour ofday and season added to modeled design values. • Dispersionmodeling result') could be used to inform the nonattainment orattainment areas in conjunction with other designations factors.
• States should submit a modeling and analysis protocol that details the methodology and model inputs before commencement ofthe modelingexercise. This information should supportthe states' recommended designations. and provide a basis for EPA'sevaluation ofthe recommendations.



### 31

### • At any time during the 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.o 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. J., S. O. Perry, A. Venkatram, 1. C. Weil, R. 1. Paine, R. 

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

### D. Peters, R. W. Brode, and J. 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 ofOood Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations). Revised. EPA-45 0/4­ 

### 80-023R. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. U.s. EPA, 1992: Screening Procedures for Estimating the Air Quality Impact ofStationary Sources. EPA-454/R-92-019. U.s. Environmenta.1 Protection Agency, Research Triangle Park, NC 27711. U.S. EPA, 1994: S02 Guideline Document. EPA-452/R-9S·008. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. U.S. EPA, 2000: Meteorological Monitoring Guidance for Regulatory Modeling Applications. EPA-4S4/R-99-00S. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. U.s. EPA, 2004a: User's Guide for the AMSIEPA Regulatory Model AERMOD. EPA-4541B03-001. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. 

### U.S. EPA, 2004b: User's Guide for the AERMOD Terrain Preprocessor(AERMAP). EPA4541B-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 2771 

### L
32U.S. EPA, 2004d: User's Guide to the Building Profile Input Program. EPA454/R-93-038. U.S. Environmentru Protection Agency, Research Triangle Park, North Carolina 27711. U.S. EPA, 2005. Guideline onAir Quality Models. 40 CFR Part 51 Appendix W. U.S. EPA, 2008: AERSURFACE User's Guide. EPA-4S4/B-08-001. 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 27711. U.S EPA, 2011a: AERMINUTE User's Guide.

# u.s. Environmental Protection Agency, Research Triangle Park, North Carolina 27711. U.S EPA, 2011b: AERSCREEN User's Guide. EPA-454-/B-II-001. U.S. Enviromnental Protection Agency, Research Triangle Park, North Carolina 27711.



### 33