, 

### UNITED STATES ENVIRONMENTAL PROTECTION AGENCY**R~GION** 

### 10 1200 SixthlAvenue, Suite 900 Seattle,

### W~hington 98101-3140



## ,APR 04 2011 OFFICE OF THE REGIONALADMINISTRATOR


### The Honorable John Kitzhaber Governorofthe State ofOregon 160 State Capitol 900 Court Street Salem, Oregon 91301-4047 Dear GovernorKitzhaber: On June 3, 20]0, the U.S. Environmental Protection Agency (EPA) revised the heath- I 

### basedNational Ambient Air Quality**Standar~** 

### (NAAQS) for sulfurdioxide (SOz) by establishing anew I-hour standardat alevel of15 parts Perbillion (ppb). This new I-hourstandard will protect public health byreducing people's exPosure to high short-term (5-minutes to 24.hours) concentrations ofS02. An

### areameetsthe ne,¥ standard whenthe 3-yearaverage ofthe 99th 

### percentile ofI-hourdaily maximum concentrations does not exceed 75 ppb. Section I01(d)(1) ofthe Clean Air Act (CAA) describes the process EPA follows for designating areas as meeting ornot meeting a new orrevisedNAAQS. Within one year of establishing a new or revised standard, each governormustrecommend a designation of attainment, nonattainmentor unclassifiable, With appropriate boundaries, for areas within their jurisdiction. Therefore, we 

### askthat your stateprovide designation recommendations. including appropriate areaboundaries, for all areas by,une3, 2011. withrespectto the I-hour S02 NAAQS. ! 

### The enclosed memorandum provides guidance to state and local airpollution control agencies and tribes onthe timeline and process for designating 

### areas underthe revised S02 standard. We plan to announce ourintendeddesignations no laterthan 

### 120 days priorto promUlgating final designations. Followingthe announcement ofintended designations, states andtribes will havethe opportunity to commentonany modifications EPA proposes to their recommendations. EPA intends to complete aesignationsby no laterthan 

### June 3, 2012. I 

### Establishing area designations is akey step in

### the process ofproviding the health protectionintendedby the CAA. Should**yo~**

### have questionsregarding this matter, please do not hesitate to contactme or Steve Body ofthe State & 

### Tribal 

### AirPrograms Unit at206-553-0182 or body.steve@epa.gov. !**Sincere~y,**

# ~~ Dennis 

### J. Mclerran

## ~ Regio!Administrator Enclosure 

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



## MAR 

## 242011 OFrlGE OF AIR ()UAlirf PLANNING AND 5TANDARDS**MEMOI~NDUM** SUB.JECT: A.rea DeSign.ations for th.e 2010

### Revi2~ imnry Sulfur Dioxide National ..•. Ambient Air Quality**Sla~dard** I'

.**/~** FROM: 

### Stephen D. Page. 

### Direct;

.. , Office ofAir Quality Pla nl g and

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

'rhis memorandum provides information on the schedule and process for designating areas for the purpose ofimplementingthe 2010 revised primary sulfurdioxide (S(h) national ambient air quality standard (NAAQS). In addition. 

### it identities factors EPA intends to evaluate indetermining boundaries lor areas designated nonattainment We recommend that states and tribes consider and address these factors when identifying boundaries for their area designation recommendations. Please share this information with the state and tribal agencies in your Region. On June 2, 2010. AdministratorJackson signed the revised primary 

### SOl NAAQS (75 FR 35520, published 

### on June 22,2010) after review 

### ufthe existing two primary 

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

### S02 standard at 75 parts per billion (ppb) which is attained when the 3-year average ofthe 99th percentile of 1**~hour** 

### daily maximum concentrations does not exceed 75 ppb. The Administrator has detCn11ined that this is the level necessary to provide protection ofpublic health with an adequate margin ofsafety, especially for children, the elderly and those \:vith asthma. These groups are particularly susceptible to the health effects associatcd with breathing 

### S02.**Gen~ral** 

### approl\\;;h and schedule. Clean AirAct (CAA) section l07(d) directs statesto submit their 

### S02 designation recommendations to EPA 

### by June 3, 201 L IfEPA intcnds 10 modify any state'sboundary recommendation, EPA will notify the state no iater than 120 days priorto its action to promulgatedesignations 

### (i.e., February 2012 for designationl'l to be promulgated in June 2012), and the state will have an opportunityto comment on EPA's intended moditications and provide additional infomtationf()r EPA to consider. Section 107(d)

AecycledIRecyclable • Printed whh Ve!l8tablfJ OU Basud Inl<S (l1l100% Recy{;I;;(j P;;p;lf (40%**P".~lcon"unl6r)**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 intcnd to follow the same process for tribes, pursuant to section 30l(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 promulgatingthe revised primary S02NAAQS describe the approach EPA anticipates using for designations for the I-hour S02 standard. EPA anticipates taking an analytic approach 

### that uses both airquality monitoring and modeling information for designations. Such an approach, ifadopted, wouldbe consistent with EPA'shistoric practices for S02 NAAQS implementation. In that preamble we acknowledged that in some cases, monitoring data may be the more technically appropriate information for determining compliance with the I-hourNAAQS. (See e.g., 75 FRat 35552, n. 22). We also recognized that a single monitor may generallynot be adequate to 

### fully characterize ambient concentrations ofSOl, includingthe maximum ground level concentrations that exist around stationary S02 sources, particularly when measuring for a I-hour standard. (See 75 FR at 35551). Refined dispersion models are able to characterize 

### SOz air quality impacts from the modeled sources across the domain ofinterest on an hourly basis with a high degree ofspatial resolution, thus overcoming the limitations ofan approach based solely onmonitoring. Attachment 2 summarizes three possible designations and the criteriafor initial designations ofthe I-hour SOlprimary standard that EPA expects to apply. As stated in the preamble, we do not believe it would be realistic or appropriate to expect states to complete modeling for all significant sources ofS02 and assess the results in time for the designation recommendationsthe Act requires bc submitted to EPA by June 3, 2011. (See 75 FR at 35570­ 71). Therefore, we do not generally expeet states to provide refined dispersion modeling information along with their initial designation recommendations. However, EPA does intend to consider, as appropriate, available air quality monitoring and modeling information submitted by states ortribes in supportoftheir recommendations. States and tribes should identify areas as attainment, nonattainment or unclassifiable on the basis ofavailable information. Given the currently limited network ofS02monitors, and our expectation that states will not yet have completed appropriate modeling ofall significant S02 sources, we anticipate that most areas ofthe country will be designated "unclassifiable." Ifa state ortribe, following receipt ofan EPA 120-day letter, has additional informationthat itwants EPA to consider with respect to a designation recommendation that EPA plans to modify, we request that such information be submitted within 60 days after receiving EPA's letter. This will help ensure that EPA can fully consider any such informationprior to issuing final designations.
2Also, although not required by statute, in orderto considerpublic input in the designation process, we plan toprovide a 30-day public comment period 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 mlemaking includes a general discussion ofstates' statutory planning and emissions control responsibilities undereach ofthe three possible designations. The CAAdirects states with areas designated as "nonattainment" for S02to develop and submit a plan within 18 months after designation providing for attainment as expeditiously as practicable, but no later than 5 years afterthe initial designation date. (See CAA sections**191~193).** The CAA also directs statesto submit by June 3, 2013, a SIP demonstrating an adequateprogram to implement, maintain and enforce the S02NAAQS. Generally, these infrastructure plans for attainment areas are not expected to include an attainment demonstration. However, in light ofthe incomplete monitoring and modeling data available at the time ofdesignations, for areas designated unclassitiable, we would expect states to include in these plans demonstrations ofexpeditious attainment and maintenance ofthe S02 NAAQS. EPAis developing separate guidance ondeveloping SIP revisions for the S02standard and we intend to seek public review and comment on that guidance document. Identifying an areathat is in violation ofthe S02 NAAQS. Section 107(d)(l) ofthe CAA defines an area as "nonattainment" if

### itis violating the NAAQS or 

### ifit is contributing to a violation in a nearby area. Thus, the first step in making designations is to identify through monitoring orappropriate modeling areas violating the NAAQS. In assessing whether monitoring data indicate a violation, EPA intends to use the most recent threeconsecutive years ofquality-assured, certitied air qualitydata 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 with40CFR Parts 50 and 58. Procedures for using monitored air quality data to determine whether a violation has occurred are given in 40 CFRPart 50 Appendix T, as revised in conjunction with the final rule for the 2010 802NAAQS. We expect that in providingtheir recommendationsto EPA, states andtribes would review available S02 monitoring data from 2008 through 2010. Priorto EPA issuing letters to states and tribes concerning any intended modifications to their recommendations, data from 2011 maybecome available. Ifthis is the case, EPA intends to also consider 2011**S~** airquality 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 meetthe criteria for exclusion, as specil1ed in the final rule

1This information is available on EPA's website atwww.cpa.gov/ttn/airs!airsaqs/. 3'Treatment ofData Influenced by Exceptional Events" (72 FR 13560; March 22, 2007) codified in 40 CFR Parts 50 and 51. In sectionVII.B ofthe S02NAAQSfinal ru1e 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 1of40 CFR 50.14 and require initial data flagging by October 1, 2010 and detailed documentation submittal by June I, 2011. This should assure that any exceptional events claim asserted by a state ortribe can 

### be fu]]y considered by EPA before final designations. States andtribes may also choose to use available air quality modeling results to indicate a violation ofthe NAAQS. Attachment 3 provides further guidance onthe 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 higher than good engineeringpractice. As noted above (and in the preamble to the final S02primaryNAAQS rulemaking), we recognize that 

### it is notrealistic to expect states or tribes to complete this type ofmodeling tor all significant sources ofS02 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 significant sources ofS02emissions, and on those sources that are most likely to contribute to a violation. 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**area~.** EPA may initially designate an areaas_attainmentjfitis clear that it meets the S02NAAQS. EPA does not believe 

### itwould beappropriate to do so without appropriate refined dispersion modeling and, where available, air quality monitoring data indicating no violations oftheNAAQS. In the absence ofinformation clearly demonstrating a designation of"attainment" or "nonattainment," EPA intends to designate the area as Hunclassifiable." Determining nonattainment area boundaries. As a pollutant that arises from direct emissions, S02 concentrations are highestrelatively close to the source(s) and much lower at greater distances due to dispersion. Thus, S02 concentration patterns resemble those ofother directly emitted pollutants like lead and differ from those ofphotochemically-formed (secondary) pollutants such as ozone. Accordingly, consistent with ourapproach under other NAAQS, we expect to consider the 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 nonattainment boundaries. We believe it is appropriate to evaluate each potential nonattainment areaon a case-by-case basis, and to recognize that area4specific analyses conducted by states, tribes andlor 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 orportions thereof) that contain emissions sources contributing to the violation. (See CAA section 107(d)( 1 )(A)(i»). Consequently, we recommend that states and tribes basc their boundary recommendations on an evaluation offive factors; 1) airquality data; 2)emissions-related data; 3) metcorology; 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 themodel user to simultaneously assess multiple factors. States and tribes may identify and evaluate otherrelevant factors orcircumstances specific to a particular area. While EPA generally believes that in the absence ofotherrelevant information it is appropriate to use county boundaries to define nonattaimnent areas, we recognize thatthe five* factor analysis and other infomlation may support designating only a portion ofa county as ''nonattainment.'' Forexample, a topographical feature may divide a county into two separate air basins, or contributing sources may be clustered in only a portion ofa county. For defining partial county boundaries, EPA recommends the use ofwell-det1nedjurisdictional 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 S02 sources is in attainment for the I-hour SOlNAAQS. An attainment area boundary cannot contain any area that exceeds the NAAQS orany area containing sources that are causing orcontributing to a violating area. (See CAA section 107(d)(l)(A)(i»). County boundaries may be appropriate tor 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 on how boundaries would be determined for S02designations, the guidance contained herein is notbinding on states. tribes the public orEPA. The final basis for determining nonattainment area boundaries\\rill 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 for the revised 2010 SalNAAQS. Attachment 2 identifies the primary five factors that EPA plans to consider in evaluating andmaking decisions on nonattainment area boundaries. Attachment 3 is the modeling guidance that states and tribes should use to support designation recommendations, 5including appropriate area boundaries. Staffmembers at EPA's Office ofAir Quality Planning and Standards are available for assistance and consultation throughout the designations process. General questions on this guidancemay be directed to Valerie Broadwell (919) 541-3310 orDoug Solomon (919) 541­ 4132. Modeling-related questions may be directed to James Thurman (919) 541-2703. Attachments: 3

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

## ooe

6ATTACHMENT 1

TIMELINEFOR 2010 

### Primary SOl NAAQS DESIGNATION PROCESS Milestone Date'" EPA promulgates S02NAAQS June 3, 2010 States and tribes flag exceptional event-influenced S02monitoring data from 2008-2009 October 1, 2010

States and tribes flag exceptional event-influenced**S~** monitoring data from 2010; provide detailed documentation to support all 2008-2010 claims No later than June 1, 2011

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

EPA notifies states and tribes concerning any intended modifications to theirrecommendations 020-dayletters) ola February 3,2012 (no later than 120 days priorto final designations)

EPApublishes public notice ofstate andtribal recommendations and EPA's intended modifications and initiates 30-day public comment period ola February 20, 2012

End of30-day public comment period 

### ola March 20,2012 States and tribes submit additional information to demonstrate why an EPA modification is inappropriate ola April 3, 2012

EPA promulgates final S02area designations No laterthan June 3, 2012 

## * o/a::= onorabout Note: This schedule assumes EPAhas sufficient information to promulgate designations within 2 years. In the event EPA determines that insufficient information is available to do so, the Clean AirAct allows EPA to extendthe designation process, but no laterthan June 3, 2013.ATTACHMENT2 DeterminingDesignations and 

### Appropriate Area Boundaries forthe I-hour, 75 ppb SOlNAAQS

,-----:----_._--, 

### Nonattainment Attainment UncJassijiab/e (all other areas) An area where monitoring data oran appropriate modeling analysis indicate a violation. An area that has no monitored violations and which has an appropriate modeling analysis, ifneeded, and any other relevant information demonstrating no violations. An area that has no monitored violations and lacks an appropriate modeling analysis, ifneeded, or other appropriate information sufficient to support an alternate designation.



### Attainment area boundaries. Areas designated as "attainment" should be supported by infonnation clearly demonstrating thatthere are no violations ofthe 802NAAQS 

### inside the area boundary. This could consist ofappropriate air quality 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 higbcr than good engineering practice. Countyboundaries may be appropriate for defining attainment areas intbc absence ofothcr infonnation that would help define a more specificboundary around the 

### modeled source(s). In the absence ofinformation clearly demonstrating adesignation of"attainment" or "nonattainment," EPA intends to designate the area as 

### "unclassifiable." Nonattainment area boulldaricf!. EPA intends to use the county as the analytical starting point lor assessing the appropriate geographic boundaries ofa S02 

### nonattainment area. As a framework for area-specific analyses to support final boundary determinations, we intend to evaluate the five factors listed below, as 

### well as 

### otherrelevant available information. The purpose ofevaluatingthese factors is

### to dctcnnine the 

### appropriate boundaries encompassing the area meeting the CAA's definition ofHnonattainment area" Le., an areaviolating the 802 

### standard and any nearby areas contributing to the 

### violating area. The modeling guidam;e in Attachment 3discusses how modeling could beused to address several ofthese factors simultaneously. When considered as 

### awhole, 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 rccent 3-yearperiod. Arcas where monitoring data indicate a 

### violation ofthe I-hour, 75 

### ppb primary S02 

### standard will be designated as "nonattainment." Source-oriented modeling may also be used to assess air quality in a

### particular location. Attachment 3 

### provides further guidance on using refined dispersion modeling for thistype ofassessment. 2. 

### Emissions-related data 

### (location ofsources and potential contributionto ambient S02 

### concentrations). We intend to examine allowable emissions of802 from sources located inand aroundthe violating area. Significant emissions levels in a nearbyarea indicate the potential for the area to contribute to observed or modeJed violations ofthe NAAQS. We intendto review data from the latest National Emissions Inventoryorother relevant sources ofthe data, such as state inventories orinventories from other federal sources. We would also consider any additional infonnation we receive ontcderally.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 detennine how weatherconditions, including wind speed and direction, affect the plume ofsources contributing to ambient SQz concentrations. Where feasible, we wouldconsider results from source-oriented dispersion modeling. 4. 

### Geography/topography (mountain ranges or other airbasin boundaries). We intendto examinethe physical features ofthe land that might affectthedistribution ofS02 over an area. Mountains orotherphysical features may affect the distribution ofemissions, and may help define boundaries. 

### S. Jurisdictional boundaries (e.g., counties, air districts, pre-existing nonattllinment areas, reservations, metropolitanplanning 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 a clearlydefined legal boundary for carrying out the air qualityplanning 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 identified as meeting the nonattainment definition. Where existingjurisdictional boundaries are not adequate to describe the nonattainment area, otherclearly defined and pennanent landmarks or geographic coordinates may be used. EPA plans to consider these factors, along with any otherrelevant intonnation, in detennining whetherto make modifications to the boundary recommendations madeby 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 infonnationprovided to support a boundary recommendation for a nonattainment area should showthat: I) violations are not occurring in nearby portions that are excluded from the recommended nonattainment area; and 2) the excluded portions do not contain emission sources that contribute to the monitored ormodeled violation.

2

### ATTACHMENT3 Modeling Guidance for SOlNAAQS Designations 1. Purpose 

### On June 2,2010, 

### Administrator Jackson signed a final rulemaking notice that revised the primary S02 NAAQS (75 FR 35520, 

### published on June 22,2010) 

### after review ofthe existing two primary S0:2 standards, promulgated on April 30, 1971 

### (36 FR 8187). I 

### EPA established the revised primary S02standard at 75 parts per billion (Ppb) which is attained when the 3-year average ofthe 99th 

### percentile of 1-hourdaily maximum concentrations does not exceed 75 ppb. Inthe final rule preamble, EPA outlined an expected analytic approach to detennining compliance with the new NAAQS that would include the use ofbothmodeling and monitoring. EPA believes this analytic approach to detennining compliance with the new I-hour NAAQS would be the generally more technically appropriate and accurate means ofassessing peak 1­ hour S02 concentrations, and would be 

### 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 data may be more technically appropriate than modeling in some cases. In cases where there is complete air quality data from FRM orFEM S02monitors, that data would be considered by EPA in designating areas as attainment or nonattainment. (See 75 

### FRat 35570). 

### The guidance presented here is for cases where modeling is used in support of the designations process. Dispersion modeling could be used in these initial designations to a limited degree (as could monitoring) but would likely be used to a larger extent subsequently as the basis for redesignation ofnonattainment and unclassifiable**area~** 

### to attainment. As thepreamble to the rule promulgaling the new I-hour S02 NAAQS noted, EPA does not think it realistic or appropriate to 

### expect states to complete modeling for all significant sources ofS02 and assess the results in time for the designation recommendations the Act requires be submitted by June 2011. 

### (See 75 

### FR at 35570-71). 

### Therefore, we do 

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

1EPA publiclydisseminateda copy ofthesignednoticeonJune 3,2010,and therefore treatsJune 2, 2010, as thedateofthe rule's promulgation,for purposesofthe deadlines in 

### eAA section 107(d). 1I. the use ofmodeling to infonnthe nonattainment boundaries for areas with violating ambient airquality monitors ifthe presumptive countyboundaries are not used (either to expand the boundaries outsidethe county orshrinkthe boundary ",,;thin the county); and 2. The use ofmodeling in areas without a violating monitor as evidence ofattainment ofthe NAAQS (showing noviolations or contributions to violations ofthe standard). This guidance is consistent with EPA's 

### Guideline onAirQuality Models, or Appendix W to 40 CFRPart 51, and other relevant modeling guidam,'C issued to support regulatory programs. When the need for interpretation ofthis guidance arises, the user should consult with the appropriate Regional Modeling Contact2• Also as indicated in the preamble ofthe I-hour S02NAAQS final rule, we intend to issue additional guidance describing the development ofan approvablc 11O(a)(l) implementation plans for areas designated "unclassifiable" that will include technical direction on howto conduct refined dispersion modeling to demonstrate future NAAQS attainment.



### 2. Guidance on Air Quality Models Much ofthis guidance is based on EPA's 

### Guideline onAir QualityModels, also published as Appendix W of40 CFRPart 51. Appendix W is the primary source ofinformation onthe regulatory application ofairquality models for State Implementation Plan (SIP) revisions for existing sources and for New Source Review (NSR) and Prevention ofSignificant Deterioration (PSD) programs. Air quality modelin¥ in support ofthis designations process would need to employ air quality dispersion models thatproperly address the source-oriented nature ofS02 and, 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 ofSigniticantDeterioration 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 ensurethat new model developments or expanded regulatory requirements are incorporated.
Clarifications and interpretations ofmodeling procedures become official EPA guidance through several courses ofaction: 1) the procedures are published as regulations orguidelines; 2)

2 List ofRegional Modeling Contacts byEPA Regional Office is available from SCRAM website at:

3 Dispersion modelinguses mathematical formulations to characterizetheatmosphericprocesses that disperse a pollutant emitted by a source. Based on emissions and meteorological inputs, a dispersion model can beused to predictconcentrationsatselected downwind receptor locations. 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 Clearinghousethateffectively establish national precedent. Formally located in the Air Quality Modeling Group (AQMG) orEllA's Office ofAir Quality Planning and Standards (OAQPS), the Model Clearinghouse is the single EPA focal point for the review of criteria pollutantmodelingtechniques for specific regulatory applications. Model Clearinghouse and related Clarification memoranda involving decisions with respect to interpretation of modeling guidance are available atthe Support Center for Regulatory Atmospheric Modeling (SCRAM) website.4 Recently issued EPA guidance ofrelevance for consideration in modeling for designations includes:

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

### S02 NAAQS" August 23, 2010--confirming that Appendix W guidance isapplicable for NSRlPSD permit modeling for the new S02NAAQS. • "Additional Clarification Regarding Application ofAppendix W Modeling Guidance for the I-hourN02 National AmbientAir Quality Standard" March I, 2011- provides additional guidance regarding N02 permit modeling and also relevant to 

### Sal.
The following sections will provide reference to the relevant sections ofAppendix W and other existing guidance with summaries as necessary, Please referto those original guidance documents for full discussion and consult with the appropriate EPA Regional Modeling Contact ifquestions arise about interpretation on modeling techniques and procedures.



### 3. Model selection Preferred air quality models for use in regulatory applications are addressed in Appendix A ofEPA's GUIDELINE ON AIR QUALITY MODELS. Ifa model is to be used for a particular application, the user should follow the guidance on the preferred model for that application. These models may be used without an area specitic formal demonstration of applicability as long as they are used as indicated in each model summary ofAppendix A. Further recommendations for the application ofthese models to specific source problems are found in subsequent sections ofAppendix W. In 2005, the American Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) was promulgated as EPA'spreferred**near~field** dispersion modeling for a widerange ofregulatoryapplications in all types ofterrain based on extensivedevelopmental and perfonnanceevaluation.

'"The SupportCentel' for RegulatoryAtmosphericModeling (SCRAM) websiteis available at:

3For area designations underthe J-hour S02primary NAAQS, AERMOD should be used unless use ofan alternative model can bejustified (Section 3.2, Appendix W), such asthe Buoyant Line and Point Source Dispersion Model (BLP). As outlined in the August 23, 2010 clarification memo "Applicability ofAppendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard''; AERMOD is the preferred model for single source modeling to addressthe I-hour S02 NAAQS as part ofthe NSRJPSD permitprograms. AERMOD is appropriate to infonnthis designations process because S02 concentrationsresult from direct emissions from combustion sources so that concentrations are highest relatively closeto sources and are much lower at greater distances due to dispersion. Giventhe source-orientednature ofthis pollutant (See. e.g., 75 FR at 35570), dispersion models are the most appropriate airquality modelingtools to predictthe ncar-field concentrations ofthis pollutant. The AERMOD modeling system includes several components. 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 tor AERMOD (U.S. EPA, 2004c) • BPIPPRIME: the building input processor (U.S. EPA, 2004d) and non-regulatory components are:

• AERSLJRFACE: the surface characteristics processor for AERMET (U.S. EPA, 2008) • AERSCREEN: a recently released screening version ofAERMOD (U.S. EPA, 20Ilb) 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 ofhowmany sources to explicitly model and what kind ofreceptor network tocreate. Two questions may arise in model domain selection: 1. Where to centerthe modeling domain?, and 2. How large should the modeling domain be? (i.e., in terms ofthe number ofsources to model and size ofthe receptor nenvork in order to account tor 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 ofa violating monitor, the domain should be centered on the dominant source in an area, that is, the source or sources expected to contribute the most to S02 air quality levels. In both cases, the domain should then extend to include nearby sources that are thought to cause orcontribute to a potential NAAQS violation, as explained further below in Section4.1. 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 arc 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 nonattainrnent area. For these situations, consultation with the appropriate EPA Regional Modeling Contact is recommended.

4.1 Determining 

### sources to model 

### As stated above, the determination ofsources to explicitly model is a multi-step process: 1. The spatial distribution ofall sources within 50 km ofthe violatingmonitor or dominant source should be analyzed and initially assumed to be included in refincd dispersion modeling. Forthe purposes ofdesignations it is reasonable to initially tbcus 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 and/or 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, Le., use ofmonitored background concentrations. Accounting for such sources through the use ofa background monitor will depend upon how well that monitor reflects impacts from those sources. 3. Sources found not to be representative by monitored background should also be examined through the use ofscreening models to see ifthey should or should. not be included in the refined modeling. We recommend the use ofEPA's new screeningmodel AERSCREEN (U.s. EPA, 2011b) and following recommendations based on pre-existing screening guidance (U.S. EPA, 1992). For small isolated sources, screening may be useful on a source by source basis. However, for a cluster ofsmall sources, their cumulative impact should also be assessed. Individual sources may not be significantby themselves, but together they could cause aNAAQS violation or significantly c,ontribute to aNAAQS violation. Although AERSCREEN does not output a design value
5conce:ntration based on the 99th percentile form ofthe I-hour S02 standard, it**do~s** output the overall maximum I-hour concentration which could be used as a conservative estimate for comparison with the NAAQS and EPA's suggested interim significant

.impact level (SIL) for the I-hour S02 NAAQS of3 ppb5• Ifthe maximum I-hour concentration output from AERSCREEN violates the NAAQS, it does not mean that the source is in nonattainment, but that the source should be evaluatedusing refined dispersion modeling (See Step 3 belowfor more details).

Figure 1 shows a hypothetical monitor with circles of50 km and 10 km radii centered over it. Based on this figure, an example application ofthese three steps is described below. Step 1: Figure 1 shows facility emissions ranging from less than one ton to over 100 tons per year within 50 km ofthe violating monitor. Most ofthe smaHer facilities (less than ten tons) are located north ofthe violatingmonitor. There are two 100+ ton emitters near the monitor and two 100+ton emitters west-southwest ofthe monitor. Atthis point, it could be initially assumed that all facilities should be included in refined modeling. Step

### 2: Determine whether any source or sources can be accounted for by a representative background monitor. In Figure 1, there aretwo othermonitors in the area, one north and one south ofthe violating monitor. The northernmost monitor may be representative ofthe facilities north (white and yellow dots) ofthe violating monitor and the southern monitor may be representative ofthe sources southeast (white and blue dots) ofthe violating monitor. Background concentrations should be calculated following the guidance in Section 7 below. Step 3: Screening modeling may beused to determine additional sources orcombinations of sources to be exc1uded from refined modeling, especially smaller sources whose impacts may be largely dependent on their stackparameters (height, exit velocity, etc.). AERSCREEN could be used to eliminate such sources through screening modeling. AERSCREEN does not output an S02 design value but does output the overall maximum I-hourconcentration for an individual stack. Ifa facility contains more than one emissionpoint orstack, 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 et at, 2008), it may be useful to also model the source in AERSCREEN using the surface characteristics ofthe meteorological site being used in the refined modeling as well. to ensure that the source is below de minimis impact levels with either set ofsurface characteristics.

5The 3 ppb interim SIL for new 1-hourS02 NAAQS was provided by EPA for statesto considerusing for PSD program in theAugust23.2010 memorandum "GuidanceConcerningthe Implementation ofthe 1-hour502 NAAQS for the PreventionofSignificantDeterioration 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:

• !fthefacility's maximum I-hour concentration exceeds 75 ppb. then the source should be included inrefined dispersion modeling. • Ifthe facility's maximum I-hourconcentration is below 75 ppb but above the suggested interim I-hour significant impact level of3ppb or the state's I-hour SIL., it should be included in the refined modeling. • Ifthe facility's maximum I-hourconcentration is below the suggested interim ]-hour significant impact level orthe state's I-hour SIL, that source may not have to be
7included in refined modeling. However, the facility should not be excluded onthe sole basisofbeingbelowthe SIL without first looking at surrounding sources and their maximum]-hour concentrations. The case may arise when there are several small sources that singularly arebelowthe SIL buttheircumulative impact may lead to concentrations that contribute to violations ofthe NAAQS.
In summary for the example in Figure 1, the smaller sources below I ton ofemissions to the north ofthe monitormay be bestrepresented with the use ofbackground monitor concentrations. Othersources between 1 and 10 tonsthat are not represented by background monitors could be excluded based on screening results, depending on their stack parameters and terrain. '{be smaller sources (less than 1ton) within 10 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 in refined modeling asthey are likely contributing to potential NAAQS violations and are not reflected in background monitors. Thisisjust one example ofhow to detcnnine 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 ifthereis uncertainty in deciding which sources to explicitly model, which sources to represent based on background monitoring. andlor which to exclude from refined modelingusing 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 ofmodelcd sources, and complexity ofthe terrain. Receptors should be placed in areas that are considered ambient air (i.e., where the public generally has access) and placed out to a distance such that areas ofviolation can be detected from the model output to help dctennine the size ofnonattainment areas. Receptor placement should be of sufficient density to provide resolution neededto detect significant gradients in the concentrations with receptors placed closertogethernear the source to detect local gradients and placed farther apart away from the source. In addition, the user should place receptors at key locations such as around facility fence lines (which define the ambient air boundary for a particular source) or monitor locations (torcomparison to monitored concentrations for model evaluation purposes). The receptornetwork should cover the modeling domain. An example receptor grid for a single source is shown in Figure 2awith an example grid with multiple sources shownin Figure 2b. In Figure 2a, receptors are located every 50 m within one kilometer ofthe source and thenevery 100 m from one to two kilometers. From two to 10 km, thereceptor spacing is 250 m and every 500 m outside of 10 

### km ofthe source. The modeling domain is centered on an isolated facility and extends out to 10km intheeast-west and north-south direction. Figure 2b shows an exa.mple grid for a multi-source area. Two sources are modeled with a fine grid ofreceptors 1km (50m spacing) around each source embedded within a lOx 10 8

### kIn grid (250 mspacing). The lOxI 0 km grid is then embedded within a 20x20 km grid with coarser spacing (500 m). Ifmodeling indicates elevated levels ofSOl 

### (ncar the standard) near the edge ofthe receptor grid, consideration ofexpandingthe grid or conducting an additional modeling run centered on the area ofconcern should be investigated. As noted above, terrain complexity should also be 

### considered when setting up the receptor grid. Ifcomplex terrain is included in the model calculations, AERMOD requires that receptor elevations be included in the model inputs. In those cases, the AERMAP terrain processor (U.S. EPA, 2004b) should be used to generate the receptorelevations and hill heights. The latest version ofAERMAP (09040) can process either Digitized Elevation Model (DEM) orNational Elevation Data(NED) data files. The AIG recommends the use ofNED data since it is more up to date than DEM data, which is no longer updated (Section4.3 ofthe AIG).

5. Source inputs 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 on urban/rural determination ofthe sources, and Section 5.5 provides general guidance on source grouping, which may be important fer design value calculations.

5.1 Allowable vs. Actual emissions Consistent with past S02 modeling guidance (Section 4.5.2 orU.S. EPA (1994» and regulatory modeling for other programs (Appendix W, Section 8.1), dispersion modeling for the purposes ofdesignations should be based on the use ofmaximum allowable emissions or federally enforceable permit limits. Also consistent with past and current guidance, in the absence ofallowable emissions orfederally enforceable permit limits, potential to emit emissions (i.e., design capacity) should be 

### used. Because ofthe short-term nature ofthe new S02 NAAQS, the maximum short term or hourly emission rate should be input into AERMOD for each modeled hour. As stated in the August 23, 2010 memo, "Since short-term S02 standards (:'5 

### 24 hours) have been in existence for decades, existing S02 emission inventories used to support modeling for compliance with the 3-hourand 24-hour S02 standards should serve as a useful starting point, and may be adequate in many cases t'Or use in assessing compliance with the new I-hour S02 standard since issues identified in Table 8-2 ofAppendix Wrelated to short-term vs. long-term emission estimates may have already been addressed."
9

# a

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

### 10The existing S02 inventories used for permitting or SIP demonstrations should contain the necessary emissions information for designations-related modeling. Ifshort-term emissions arenot readily available, they may becalculated using the methodology shown in Table 8-2 of Appendix W. Fot an example calculation ofshortterm emissions, see theJune 28, 20 10 memorandum "ApplicabilityofAppendix W Modeling Guidance for the I-hourN02National Ambient Air Quality Standard." Although the example is for N02, the calculationmethodology would be the same for S02. Regarding the use ofallowable emissions and modeling ofintermittent emissions sources, from such sources as emergency generators and startup/shutdovm emissions, the inclusion ofsuch emissions for thepurpose ofmodeling for S02 designations should follow the recommendations in the March 1,2011 memo "Additional Clarification Regarding Application ofAppendix W Modeling Guidance for the l-hourN02National Ambient Air Quality Standard." As stated in this memo, EPA beJieves the most appropriate data to use for compliance demonstrations for the I-hour N02NAAQS are those based on emissions scenarios that are continuous enough or frequent enough to contribute significantlytothe annual distribution ofmaximumdaily I-hourconcentrations. Although the referenced guidance inthis memo is for N02permit modeling, thecommon 1hour averaging time and form ofboth the N02 and S02 standards makes this modelingguidance applicable to the I-hour S02 NAAQS and, thus, applicable to S02modelingin support ofdesignations. Formore details, refer to the N02 memo. Ifany questions ariseregarding preparation ofemissions inputs for dispersions modeling including intermittent emissions from sources, then users shouldconsult the appropriate EPA Regional Modeling Contact.



### S.2 Good 

### Engineering Practice «;EP) stack height Consistent with previous S02 modeling guidance (U.S. EPA, 1994) and Section 6.2.2 of Appendix W, for stacks with heights that are within the limits ofGood Engineering Practice (GEP), actual heights should 

### be used in modeling. UnderEPA's regulations at40 CFR 51.100, GEP height, Hg, is determined to be the greaterof:

• 65 m, measured from the ground-level elevation atthe base ofthe stack; • For stacks in existence onJanuary 12, 1979, and for which the owneror operatorhad obtained all applicable permits orapprovals required under40 CFRParts 51 and 52 Hg=2.5H provided the owner or operatorproduces evidence that this equation was actually relied on in designing the stack orestablishing an emission limitation to ensure protection against downwash;

11Forall other stacks, 

### Hg=H + 

### 1.5L, 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

• theheight demonstrated by a fluid model or a field study approved by EPA orthe State/local agency which ensures that the emissions from a stack do not result in excessive concentrations ofany airpoJlutant as a result ofatmospheric**down~'ash,** wakes, eddy effects created by the source itself, nearby structures or nearby terrain features. For more details about GEP, see the Guidelinefor Determination ofGood Engineering Practice Stack Height Technical Support Document (U.S. EPA, 1985). Ifstack heights exceed GEP, then OEPheights should be used with the individual stack's otherparameters (temperature, diameter, exit velocity). For stacks modeled with actual heights below GEP, building downwash should 

### be considered as this can impact concentrations near the source (Section 6.2.2b, Appendix W). Ifbuilding downwash is being considered, the BPIPPRIME program (U.S. EPA, 2004d) should be used to input building parameters for AERMOD. More information about buildings and stacks is in Section 5.3.



### 5.3 Source configurations and source types Anaccurate characterization ofthe modeled facilities is critical for refined dispersion modeling, including accurate stack parameters and physical plantlayout. Accurate stack parameters should bedetermined 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 exit velocity shouldreflect 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 affect the impact ofan emission source onreceptors, determination ofstack base elevation, and relative location to any nearby building structures. Not only are accurate stack locations needed, but accurate information for any nearby buildings is important. This information would include location and orientation relative to stacks and building size parameters (height, and comer coordinates oftiers) as these parameters are input into BPIPPRIMEto ca1culate building

1\ Latitudes andlongitudesto fOUT decimal places pOSition a stack within 30feet ofits actual location and five decimal places place a stackwithin threefeetofits actual location. Users should usethegreatestprecision available. 

### 12parameters for AERMOD. Ifstack locations and orbuilding information are not accurate, downwash will not be accurately accounted for in AERMOD. Emission sourcetype characterization within the modeling envirorunent is also important. As stated in the AERMOD User'sGuide (U.S. EPA, 2004a), emissions sources can be characterized as several different source types: POINT sources, capped stacks (POINTCAP), horizontal stacks (POINTHOR), VOLUME sources, OPENPIT sources, rectangular AREA sources, circular area sources (AREACIRC), and irregularly shaped area sources (AREAPOLV). Note that POINTCAP and POINTHORare not part ofthe regulatory default option in AERMOD because the user must invoke the BETA optionin the model options keyword MODELOPT while not including the "DFAULT" modeling option for theseoptions to work properly. While most sources canbe characterized as POINT sources, some sources, such as fugitive releases or nonpaint sources (emissions from ports, airports, or smaller point 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 U rban/rural determination

For any dispersion modeling exercise, the urban orrural determination ofa source is important in detemlining the boundary layer characteristics that affectthe model's prediction of downwind concentrations. Figure 3 gives example maximum I-hourconcentration 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 concentrationfor distances less than 750 m**fro~** the stack but then drops below the rural concentration beyond 750 m. Forthe taller stack in Figure 3b, the urban concentration is much higher thantherural concentration even as distances increase from the source. These profiles show that the urban orrural designation ofa source can be quite important. In addition, for S02**madelin~** the urban/rural determination is important because AERMOD invokes a 4-hour half

### life for urban S02 sources. This would onlybe done for urban sources when the POLLlJTlD keyword in AERMOD is set to "S02" and the MODEI"OPT keyword includes the DFAULT option. Rural sources withinthe same AERMODrun would not 

### be afIected. Ifthe DFAULT option is not included with the MODELOPTkeyword, the 4-hour halflife would not be used and the user would specify the4-hour halflife using the HALFLlFE 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-bourperiod,**SO~** concentrations decrease by halffrom theinitial value. 

### 13the modeling would need to be run in separate AERMOD runs so that they are not subjectto the 4-hour halflife. Note that 

### ifthe OFAULT option is used, the rural sources would not need to be in a separate run from the urban sources. Determining whether a source is urban or rural can 

### be done using the methodology outlined in Section 7.2.3 ofAppendix Wand recommendations outlined in Sections 5.1 through 5.3 in the AIG (U.S. EPA, 2009). In summary, there are two methods ofurbani rural classification described in Section 7.2.3 ofAppendix W. Thefirst method ofurban determination is a land use method (Appendix W, Section 7.2.3c). In the landuse method, the user analyzesthe 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 consideredurban ifthe land use types, 

### It (heavy industrial), 12 Oight-moderate industrial), Cl (commercial), R2 (common residential), and R3 (compact residential) are 50%or more ofthe area within the 3 kmradius 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 

### kIn radius is used. 

### Ifthe population density within the circle is greater than 750people/km2

, then the source is considered urban. Otherwise, the source is modeled as a rural source. Ofthe two methods, the land use method is considered more definitive (Section 7.2.3e, Appendix W). Caution should be exercised with either classification method. As stated in Section 5.1 of the AIG (U.S. EPA, 2009), when using the land use method, a source may be in an urban area butlocated close enough to a body ofwater orother non-urban land use category to result in an erroneous rural classification for the source. The AIG in Section 5.1 cautions users against using the land use scheme on a sourceby source basis, but advises considering the potential for urban heat island influences across the full modeling domain. When using the population density method, Section 7.2.3e ofAppendix W states, "Population density should be used with caution and should not be applied to highly industrialized areas where thepopulationdensity may be low and thus a rural classification would be indicated, butthe area is sufficiently built-up so that the urban land use criteria would be satisfied

..," With either method, Section 7.2.3(0 ofAppendix W recommends modeling all sources within an urban complex as urban, even ifsome sources within the complex would be considered rural using either the land use orpopulation density method. Another consideration that may need attention by the user and is discussed in Section 5.1 ofthe AIG relates to tall stacks located within oradjacent to small to moderate size urban areas. In such cases, the stack height oreffective plume height for very buoyant sources may extend above the urban boundary layerheight. The application ofthe urban option in AERMOD for these types ofsourcesmay artificially limit the plume height. The use ofthe urban option may not be appropriate forthese sources, since the actual plume is likely to 

### be transported overthe urbanboundary layer. Section 

### 5.) ofthe AIG gives details on determiningifa tall stack should be modeled as urban or rural, based on comparing the stack oreffective plume height to the urban boundary layer height. The 100 m stackillustrated 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 

# a 1000 

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100

0 1 0 5QO 1500 2000 2500 lOOO 3500 4000 4500 sooo DliII_lln, 

### 100m 

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j. • l" 1 J 1500 2000 2500 lOOO 3500 4000 4500 5000 Distance (In)
FIGURE 3. URBAN (RED) AND RURAL (BLUE) CONCENTRATION PROFILES FOR (A) 10 M BUOYANT STACK RELEASE, AND (B) 100 M BUOYANT STACK RELEASE. 

### 15

### population of 100,000) and equation 104 ofthe AERMOD formulation document (Cimorelli, et a1.,2004). This equation is:

(1) 

### where Ziuo 

### is a reference height of400 m corresponding to a reference population Poof2,000,000 people. Given that the stack is abuoyant release, the plume may extend above the urban boundary layer and may be best characterized as a rural source, even 

### ifit

### were near an urban complex. Exclusion ofthese elevated sources from appJication ofthe urban option would need to bejustified on a**case-by~case** 

### basis in consultation with the appropriate EPA Regional Modeling Contact. AERMOD requires the input ofurban population when utilizing the urban option. Population can be entered to one or two 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 URBANOPT keyword in the runstream file (U.S. EPA, 2004a). Ifmultiple urban areas are entered, AERMOD requires that each urban source be associated with a particular urban area or AERMOD model calculations 

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

5.5 Source groups 

### In AERMOD, individual emission sources' concentration results can be combined into groups using the SRCGROUP keyword (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 SRCGROUP ALL keyword. For the purposes ofdesignations and design value calculations, source group ALL should be used, especially ifall sources in the modeling domain are modeled in one AERMOD run. Design values should be calculated from the total concentrations (all sources and background). Forthe purposes ofdesignations modeling, individual source contributions outputs to the total concentration may not be necessary. However, ifindividual facility contributions are needed for deciding which facilities to include in the nonattainrnent or attainment area, source groups by facility should be used. To avoid any confusi.on, source groups that are used to calculate the design value concentrations or determine source contributions to design values should be mutually exclusive (i.e. an emission source should not be in two source groups). This wou1d be especially important ifthe design value concentrations are calculated outside ofAERMOD by adding the individual groups togetherto calculate a total concentration (See Section 8.1 ofthis document for examples). If 

### individual source groups that 

### are used in design value concentrations are not mutually exclusive, there would be double
16counting ofconcentrations when calculating design values either in AERMOD oroutside of AERMOD.

6. Meteorological data Section 6 gives guidance onthe selection ofmeteorological data for input into AERMOD. Much ofthe guidance from Section 8.3 ofAppendix W is applicable to designations modeling and is summarized here. In Section 6.2.1, the usc ofa new tool, AERMINUTE (U.S. EPA, 2011a), is introduced. AERMINUTE is an 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 consideredcarefully. The selection ofdata should be based on spatial and climatological (temporal) representativeness (Appendix W, Section 8.3). The representativeness ofthe data is based on: 1) the proximity ofthe meteorological monitoring site to the area under consideration, 2) the complexityofterrain, 3) the exposure ofthemeteorological site, and 4) theperiod oftime during which data arc 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.aand 8.3.c. Spatial representativeness ofthe meteorological datacan be adversely affected by large distances betweenthe source and receptors ofinterest and the complex topographic characteristics ofthe area (Appendix W, Section 8.3.a and 8.3.c). Ifthe modeling domain is large enough such that conditions vary drastically across the domain then the selection ofa single station to represent the domain should be carefully considered. Also, care should be taken when selecting a station 

### ifthe area has complex terrain. While a source and meteorological station may be in close proximity, there maybecomplexterrain between them such that conditions at the meteorological station may notbe representative ofthe source. An example would be a source located on the windward side ofa mountain chain with a meteorological station a few kilometers away onthe leeward side ofthe mountain. Spatial representativeness for off-site data should also be assessed by comparing the surface characteristics (albedo, Bowen ratio, and surface roughness) ofthe meteorological monitoring site and the analysis area. When processing meteorological data in AERMET (U.S. EPA, 2004c). the surface characteristics ofthe meteorological site should be used [Section 8.3.c ofAppendix Wand the AERSURFACE User's Guide (U.S. EPA 2008)]. Spatial representativeness should also be addressed for each meteorological variable separately. Forexample. temperature data from a meteorological station several kilometers from the analysis areamaybe cons.idered adequately representative, while it may be necessary to collect wind data nearthe plume height (Section 8.3.c ofAppendix W). 17Surface characteristics can be calculated in several ways. For details see Section 3.1.2 of the AIG (U.S. EPA, 2009). EPA has developed a tool. AERSURFACE (U.s. EPA, 2008) to aid in the detennination ofsurface characteristics. Thecurrent version ofAERSURf'ACE 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 followed unless an alternative method can bejustilied.



### 6.2 Meteorological inputs Appendix W states in Section 8.3.1.1 thatthe user should acquire enough meteorological data to ensure that worst-case conditions are adequately represented in the model results. Appendix W states that 5 years ofNWS meteorological data orat least one year ofsite-speciiic 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-specilic dataare available, those data are preferred. While the fonn ofthe S02 NAAQS contemplates obtaining three years of monitoringdata, this does not preempt the use of

### 5 years ofNWS dataorat least one year of site-specific data in the modeling. The 5-year average based on the use ofNWS data, oran avel"'d.ge across one ormore years ofavailable site specific data, serves as an unbiased estimate of the 3-year average for purposes ofmodeling demonstrations ofcompliance 

### ''''ith the NAAQ (See the August 23, 2010 Clarification Memorandum on "Applicability ofAppendix W Modeling Guidance for the 1-hour SOl National Ambient Air Quality Standard"). Seethe memorandum for more details on the use of5 years ofNWS data orat least oneyearofsite-specilic data and applicability to the NAAQS.

6.2.1 NWS data NWS dataare available from the National Climatic Data Center (NCDq in many formats, with the most commonone in recent yearsbeingthe Integrated Surface Hourly data (ISH). Most available formats canbe processedby AERMET. As stated in Section 6.1, when using data from an NWS station alone or in conjunction with site-specific data, the data should 

### be spatially and temporally representative ofconditions at the modeled sources. A recently discovered issue with ASOS is that 5-second ""rind data that are used to calculatethe 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 ofV2 knot or 0.26 mls is added to the winds in stage 3 AERMETprocessing. Formore details refer to the AERMET User'sGuide (U.S. EPA, 2004c) and/ortheappropriate EPA Regional Modeling Contact.



### 186.2.1.1 AERMINUTE In AERMOD, concentrations are not calculated for variable wind (Le., missing wind direction) and calmconditions, resulting in zero concentrations for those hours. Since the S02 NAAQS is a one hour standard, these lightwind conditions maybe the contro1ling meteorological circumstances in some cases because ofthe limited dilutionthat occurs under low wind speeds which can lead to higher concentrations. Theexclusion ofa greaternumber of instances ofnear-calm conditions from the modeled concentration distribution may therefore lead to underestimation ofdaily maximum I-hourconcentrations for calculation ofthe design value. To address the issues ofcalm and variable winds associated with the use ofNWS meteorological data, EPA has developed a preprocessor to AERMET.caBed AERMINUTE (U.S. EPA, 2011a) that can read 2-minute ASOS winds and calculate anhourly average. Beginning with year 2000 data, NCDC has made freely available, the I-minutewinds, reported every minute from the ASOS network. The AERMINUTE program reads these 2-minute winds and calculates an hourly average wind. In AERMET (U.S. EPA, 2004c). these hourly averaged winds replace the standard observation time winds read from the archive ofmeteorological data. This results in a lower number ofcalms and missing winds and an increase in the number of hours used in averagingconcentrations. Formore details regarding the use ofNWS data in regulatory applications see Section 8.3.2 ofAppendix W and for more infonnation about the processing ofNWS data in AERMETand AERMlNUTE. seethe AERMET (U.S. EPA, 2004c) and AERMINUTE User'sguides (U.S. EPA, 2011a).

6.2.2 Site-specific data The use ofsite-specific meteorological data is the bestway to achieve spatial representativeness. AERMET can process a variety offonnats and variables for site-specific data. The use ofsite-specific data for regulatory applications is discussed in detail inSection 8.3.3 ofAppendix W. Due to the range ofdata that can be collected onsite and the range of formats ofdata inputto AERMET, the user should consultAppendix 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 data lbranurban application, Section 3.3 ofthe AERMOD Implementation Guide offers recommendations for data processing, In summary, the guide recommends that site-specific turbulence measurements should not be used when applying AERMOD's urban option, in order to avoid double counting the effects ofenhancedturbulence due to the urban heat island.



### 19

### 6.2.3 Upperairdata AERMET requires full upper air soundings to calculate the convective mixing height. For AERMOD applications inthe U.S., the early morning sounding, usually the 1200 UTC (Universal Time Coordinate) sounding, is typically used for this purpose. Upper air soundings can be obtained from the Radiosonde Data ofNorth America CD for the period 1946-1997. Upper air soundings for 1994 through the present are also available for free download from the Radiosonde Database Access website. Users should choose all levels or mandatory and significant pressure levelss when selecting upper air data. Selecting mandatory levels only would not be adequate for input into AERMET as the use of just mandatory levels would not provide an adequate characterization ofthe potential temperature profile.



### 7. Background concentration The inclusion ofambient background concentrations to the model results is important in determiningcumulative 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 WModeling Guidance for the 1­ hour S02 National Ambient Air Quality Standard." This memo suggested a "firsttier" approach to including a uniform monitored background contributionbased on adding the overall highest hourly background S02 concentration from a representative monitor to the modeled design value. We recognize that this approach could be overly conservative in many cases andmay also be prone to reflecting source-oriented impact'), increasingthe potential for double·counting of modeled and monitored contributions. As discussed in EPA's March 1,2011 memo "Additional Clarification Regarding Application ofAppendix W Modeling Guidance for the I-hour N O2 Ambient Air Quality Standard," we recommend a less conservative "first tier" approach for a uniform monitored background concentration based onthe monitored design values for the latest 3-yearperiod, regardless ofthe years ofmeteorological dataused in the modeling. Adjustments to this approach may be considered in consultation with the appropriate EllA Regional 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. pne option is, as described in Section 8.2.2.b:

8 Byinternational convention. mandatorylevels are in millibars: 1,000,850,700,500,400,300,200, 150, 100,50,30,20,10,75,3,2,and 1. Significantlevelsmay varydepending on the meteorological conditions at the upper-airstation 20"Use airquality data in the vicinity ofthe source to determine the background concentration for the averaging times ofconcern. Determine the mean background concentration al each monitor by excluding concentrations when the source inquestion is impacting the monitor

... For shortertime 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 averagingtime to determine the average background value. Monitoring sites inside a 90° degree sector downwind ofthe source may beused to determine the area ofimpact:' When no monitors are located in the vicinity ofthe sources being modeled a "regional site" (Le., one that is located away from the area ofinterest but is impacted by similar natural and distant man-made sources) may be used to determine background (Section 8.2.2.c, Appendix W). In multi-source areas, background includes two components, nearby sources and other sources (Section 8.2.3 ofAppendix W). Nearby sources arc those sources that arc expected to cause a significant concentration gradient inthe vicinity ofthe source or sources under consideration, and should be explicitly modeled. Identificationofnearby sources calls forprofessional judgment and consultation with the appropriate EPA Regional Modeling Contact. Forother sources, such as natural sources, minor sources and distant major sources, themethodology of Section 

### 8.2.2 should be used. EPA's March 

### 1,2011 memo "Additional Clarification Regarding Application of Appendix W Modeling Guidance for the I-hour N02 Ambient Air Quality Standard," describes an appropriate methodology ofcalculating temporally varying background monitored concentrations by hour ofday and season (excludingperiodswhen the source in question is expected to impact the monitored concentration). The methodology for N02 is to use the 98th percentile concentration for each hourofthe day by season and average across three years. This same methodology is applicableto S02 designations modeling based on use ofthe 99th percentile by hour ofday and season for background concentration excludingperiods when the dominant source(s) are influencing the monitored concentration (i.e., 99th percentile, or 4th highest, concentrations for hour 1for January or winter, 99th percentile concentrations for hour 2 for January orwinter, etc.). Recent updatesincluded in AERMOD allow for the inclusion of temporally varying background concentrations in the design value calculation in combination with modeling results.
An illustrative example is shown in Figure 4. Shown are the NMQS standard concentration, the monitor's**3~year** average design value, and 3-year averages ofthe 99th percentile concentrations by season and hour 0

### f day. To calculate the 99th percentile concentration for a season andhourofday combination, the second highest concentration for thatcombination shouldbe selected. Also shown are 3-year averages ofthe 99th percentile concentration by hour ofday (across all seasons), andthe 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,

1 2 3 4 5 6 7 S 91011121314151617181920212223 Hourof day

FIGURE 4. S02 MONITORED CONCENTRATIONS FOR VARIOUS AVERAGING TIMES.



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

9Modelers should use the 1'1-highestvalue for moredetailed pairings. such as month byhour-of-dayor season byhour-of-day and day-of-week 

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



### 8. Determiningdesign value metries Designations modeling will provide predictions ofS02 design values at eachreceptor that includes contributions Jrom 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 valuecalculation methodology Whether design values are calculated within AERMOD or outside ofAERMOD, to calculate a design value to compare against the standard, the fol1owing steps should 

### be followed: 

### 1. At each receptor, for eachhour ofthe modeled period, calculate a total concentration across all sources including background concentrations ifapplicable. This can be done in AERMOD using SRCGROUP ALL or by adding individual source groups outside of AERMOD, using hourly POSTFJLEs. Ifthe user is totaling the concentrations outside of AERMOD, the source groups need to be mutually exclusive, i.e. no onc source should be in multiple source groups. 2. From the total concentrationscalculated 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 concentration at each receptor. Ifmodeling 5 years of meteorological data, this results in**fiv~** 99th percentile concentrations at each receptor. 4. Average the 99th percentile (or4th highest) concentrations acrossthe modeled years to obtain a design value at each receptor. 5. Modeled source contributions to aNAAQS violation can be determined by analyzingthe hourly concentrations from the individual source groups corresponding to the same hour as the 4th daily maximum i-hourconcentration from each year. (See 75 FR at 35540). Forexample, a receptor has a 5-year average design value of200.8 mg/m3 (or approximately 77 ppb) and AERMOD wasmodeled for the period January 1. 2005 through December 31, 2009 tor four source groups. From the AERMOD output, the user can determinethe date ofthe 4th highest daily maximum I-hourconcentrationsthat are used to calculate the 5-year average design value. Table 1showsthe 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 (IJG1Ml ) FOR 2005-2009. Date Concentration**~_YMMDD!:IH)** 05080101 200.1 06073105 201.5



## - 07080403 207.1 08072705 197.1 09080104 198.1 5-YEARAVG. 200.8


### Ifoutput by source group is available, the user can extract each source group's concentration at each ofthe hours listed in Table 1. Table 2 shows example source contributions for each hour shown in Table 1and indicates that Source I is the main contributor to the design value for all hours.

TABLE 2. SOURCE CONTRIBUTIONS TO 4TH HIGHEST DAILY MAXIMUM 1-HOUR CONCENTRATIONS (jJG1M;\) AND 5-YEAR AVERAGE DESIGN VALUES. Date TOTAL SOURCE 3 SOURCE 1 SOURCE 2 SOURCE 4 

# I (VYMMDDHH)

05080101 200.1 155.1 25.1 1.5 06073105 201.5 157.4 26.2 0.5 17.4 07080403 i 20.5 2.1 23.1 1.7

# 18~**~~,** 207.1 . 161.5 23.0 08072705 13.1'­ 09080104 197.1 159.2 198.1 2.0 155.3 22.6



### - 5-YEARAVG. 1.6 200.8 157.7 23.5

# ~:~~~

8.2 Running 

### AERMOD and 

### implications for design value calculations Recent enhancements to AERMOD include options to aid ill the calculation ofdesign values for comparison with the SOl NAAQS. 'Ibese enhancements include:

• The output ofdaily maximum 1-hour concentrations by receptor for each day in the modeled period for a specified source group. This is the MAXDAILY output option in AERMOD. • The output, for each rank specified on the RECTABLE output keyword, ofdaily maximum l-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.c. 4111 

### highest, and a lowerthreshold value, such as the NAAQS for the target source group. The model will 24process each rank within the range specHied, but will stop afterthe 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 on the RECTABLE keyword). 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 

### all modeled years. In this case, the use ofthe one ofthe above output options can be used in AERMOD to calculate design values for comparisonto the NAAQS and determine the area's attainment status andJor inform attainmentlnonattainment boundaries. The use ofthese options in AERMOD allows AERMOD to internally calculate concentration metrics that canbe used to calculate design values andtherefore lessen the need for large output files, i.e. hourly POSTFILES. However, there may be situations where a single AERMOD run with all expHcitly modeled sources is notpreferred. These situations often arise due to runtime or storage space considerations duringthe AERMOD modeling. Sometimes separate AERMOD runs are done for each facility or group oftacilities, orby year, orthe receptornetwork is divided into separate sub-networks. In some types ofthese situations, the MAXDAILY, MXDYBYYR, or MAXDCONT output option may not 

### be an option for design value calculations, especially ifall sources are not included in a single run. Ifthe user wishes to utilize one ofthe three output options, then care should be taken in developing the model inputs to ensure accurate design value calculations. Situations that would effectively preclude the use ofthe MAXDAILY, MXDYBYYR, and MAXDCONT option to calculate meaningful AERMOD design value calculations include the following examples:

• Separate AERMOD nms for each source or groups ofsources.

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

o 10 facilities are modeled for 5 years ofNWS data. Each facility is modeled separately toreach year, resulting in 

### fifty individual AERMODruns. Inthe two situations listed above. the MAXDAILY, MXDYBYYR, or, MAXDCONT option would not be usetul as the different AERMOD runs do not include a total concentration ,,"'ith 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, MAXDCONT option but may necessitate some external post-processing afterwards to calculate a design value include:

• The receptor network is divided into sections and anAERMOD run, with all sources and years, is made for each network. o 	 A receptor network of20,000 receptors is divided into four 5,000 receptor subnetworks. Ten facilities are modeled with five years ofNWS data in one AERMOD run for each receptor network. resulting in four AERMOD runs. After the AERMOD runs are complete. the MAXDAILY, MXDYBYYR, or, MAXDCONTresults for each network can be re-combined into the larger network.

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

• Ten facilities are modeled with five years ofNWS data. All facilities are modeled with all receptors for each year individually, resulting in five AERMOD runs. MAXDAILY, MXDYBYYR, or, MAXDCONT output can beused and post-processedto generate the necessary design value concentrations. The receptor network is divided and each year is modeled separately for each sub-network with all sources. Ten facilities are modeled withfive 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 eachyear separately, resulting intwenty AERMOD runs. MAXDAILY. MXDYBYYR, or, MAXDCONT outputcan be used and post-processed to generate the necessary design value concentrations.



### 9. Use ofmodeling results to inform nonattainmentlattainment boundaries Dispersion modeling is a tool that could be used to examinethe spatial extent ofpotential violations ofthe I-hourS02 NAAQS. Thus, in accordance with this guidance. refined dispersion modeling could 

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

### 2. For an area without a violating monitor, modeling could be used as evidence ofan area's attainment status and also to inform decisions on the appropriate (attainment or nonattainment) boundary. The shape and size ofthe nonattainment or attainment area is recommendedby the state and eitheradopted ormodified by EPA. For initial designations, it is expected that states will focus on areas with violating monitors. Ifa county contains a violating monitor, that county would be 26considered in nonattainment. Ifthere are no violating monitors and no dispersion modeling results to show attainment ornonattainment, that county would generally be considered unclassifiable.



### 9.1 Nonattainment area boundaries For nonattainment areas (those witha violating monitor), modeling could be used to refine the nonattainrnent area boundaries from the presumptive county boundaries in conjunction with other factors such as those listed in Attachment 2. This could include reducing the nonattainment area from the presumptive county to a smallerarea orexpanding the boundary beyond the county ifsources outside the county contribute to a NAAQS violation in the county. A nonattainment area boundary should contain the areathat exceeds the NAAQS and include sources that may cause orcontribute to a NAAQS exceedance. Figure 4 shows a hypothetical example ofmodeling ofan area that exceedsthe NAAQS (eitherthrough monitoring or modeling). In each panel ofFigure 5, the black dot representsthe emission source. In Figure Sa, the contours inorange 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 are 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. Finally, Figure 5d shows a hypothetical nonattainment boundary as an irregularpolygon in shape, perhapsbased onjurisdictional boundaries or other landmarks such as roads. Figure 6 illustrates a hypothetical exanlple for a multi-source situation that is in nonattainrnent. In the example, there are five sources (denoted by blue dots) in a modeling domain that covers four counties (A, B, C, and D). Themodeling domain 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 nonattainment area could be circular, rectangular, or irregularly shaped usingjurisdictional boundaries. Inthis example, the hypotheticalnonattainment boundary would be defined bythe northern portion ofCounty A and the southernportion ofCounty C. Since multiple sources are involved, the hypothetical nonattainment boundary should be extended to cover those sources that cause or contribute to a NAAQS violation. Inthis 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 NAAQS VIOLATION (RED AND ORANGE CONTOURS) AND POSSIBLE NONATTAINMENT AREA BOUNDARIES DEFINED BY (B) CIRCLE, (C) RECTANGLE, AND (D) AN IRREGULAR POLYGON.

28

# c a

B

A

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



### 9.2 Attainment area boundaries In areas without a violating monitor, modeling couldbe used to help determine that an area with 

### S02emitting sources isin attainment for the I-hour 

### S02 NAAQS. An attainment area boundary could not contain any area that exceeds the NAAQS or any area containing sources that are causing or contributingto a violating area. When considering attainment area boundaries, there will 

### be nopredicted area ofviolation from dispersion modeling so thatother factors would need to beconsidered ifthe boundary is not determined by using the county presumptive boundary. Figure 7 illustrates a group ofsources where a monitored design value does not exceed the NAAQS and modeling also does not show any concentration levels in excess ofthe NAAQS. In this case, the slate could recommend thatcounty A beconsidered 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

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 It is expected thatthe state would submit a modeling and analysis protocol that details the methodology and model inputs before commencement ofthe modeling exercise. This information should support the states' recommended designations, and provide a basis for EPA's evaluation ofthe recommendations. The protocol should include the fol1owing:

30• Characterizationofthe nonattainment problem orcharacterization ofthe modeled area in absence ofaviolating monitor, • Anemissions analysis around the violating monitor orareaunderconsiderationfor designations in absence ofa violating monitor, and • Methodology for preparing airquality and meteorology inputs including choice of meteorological data and representativeness ofthe data. Additionally, the documentation should include:

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



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

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

### Guideline onAir Quality

### Models, also published as Appendix W of

### 40 CFRPart 51.
• Sources should be modeled with maximum allowable I-hour or short-term emission rates in the designations modelingbased 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 canbe included as: o "Firsttier" approachbased onmonitored design values added to modeled design values; or o Temporally varying based onthe 99th percentile monitored concentrations by hour ofday and season added to modeled design values.
• Dispersion modeling results could be usedto inform the nonattainment or attainment areas in conjunction with otherdesignations factors.
• States should submit a modeling and analysis protocol that details the methodology and model inputs before commencement ofthe modeling exercise. This infbrmation should support the states' recommended designations, and provide a basis for EPA'sevaluation ofthe recommendations.·



### 31• At any time duringthe designations process whenthere are questions regarding modeling orinterpretation ofthis guidance, the appropriate EPA Regional Modeling Contact should be consulted.

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

### J. Thurman, and C. Tmerson, 2008: AERMOD Sensitivity to the Choice of Surface Characteristics, Paper 811, Air And Waste Management Association Annual Conference. Cimorelli, A. 1., 

### S. G. Perry, A. Venkatram, 

### 1. C. Weil, 

### R. J. Paine, 

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

### R. W. Brode, and 

### 1. O. Paumier, 2004: AERMOD: Description ofModel Formulation, EPA-454/R-03-004. U.S. Environmental Protection Agency, Research Triangle Park, NC.
U.S. EPA, 1985: Guideline for Determination ofGood Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised. EJ>A-450/4­ 80-023R. U.S. Environmental Protection Agency, Research Triangle Park, NC 2771 

### J. U.S. EPA, 1992: Screening Procedures for Estimating the Air Quality Impact ofStationary Sources. EPA-454/R-92·()19. U.s. Environmental Protection Agency, Research Triangle Park, NC 27711. U.s. EPA, 1994: S02 Guideline Document. EPA-452/R-95-008. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. U.S. EPA, 2000: Meteorological Monitoring Guidance for Regulatory Modeling Applications. EPA-454/R-99-005. 

## U.s. Environmental Protection Agency, Research Triangle Park, NC 27711. U.S. EPA, 2004a: User's Guide for the AMSIEPA Regulatory Model- AERMOD. EPA-454/B03-001. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. U.S. EPA, 2004b: User's Guide for the AERMOD Terrain Preprocessor(AERMAP). EPA454/B-03-003. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711. U.S. EPA, 2004c: User's Guide for the AERMOD Meteorological Preprocessor (AERMET). EPA-454/B-03-002. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711.

32U.S. EPA, 2004d: User's Guide to the Building Profile Input Program. EPA-454/R-93-038. U.S. Environmental Protection Agency, ResearchTriangle Park, North Carolina 277] 

### t. U.S. EPA, 2005. 

### Guideline onAir Quality Models. 40 CPR .Part 51 Appendix W. U.S. EPA, 2008: AERSURFACE User's Guide. EPA-454IB-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 2771 L 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-l1-001. U.S. Environmental Protection Agency, Research Triangle Ilark, North Carolina 27711.



### 33