Pamela F. Faggert Vice President and ChicfEnvironmental Officer
Dominion Resources Services, Inc. 5000 Dominion Boulevard, Glen Men,VA 23060 Phone: 804-273-3467

December 2,201 1

Submitted via e-mail to: www.regulations.gov

Air Docket Docket ID No. EPA-HQ-OAR-2010-1059 US.Environmental Protection Agency Mailcode: 6 102T 1200Pennsylvania, Avenue, NW Washington,DC 20460

Attention: Docket ID No. EPA-HO-OAR-2010-1059

Subject: Draft "Guidance for 1-Hour SO2 NAAQS SIP Submissions"; Proposed Rule, 76 Federal Register 61098, October 3,2011

Dear Administrator Jackson:

Dominion appreciatesthe opportunity to provide comment on EPA's draft "Guidance for 1-Hour SO2NAAQS SIP Submissions1". The company is one of the nation's largest producers and transporters of energy in the US., with a portfolio ofmore than 28,200 megawatts of generation, and other energy assets including natural gas transmission, gathering and storagepipeline, electric transmission lines, the nation's largest natural gas storage system and serves retail energy customers in 15 states.

The Company has already taken significant steps to reduce emissions from our electric generation fleet. Sincethe mid 1990's, Dominion has spent over $2.6billion on pollution controls that have significantly reduced S02,NOx and mercury and remains in the midst of a comprehensive emission reduction program across our fossil fuel-f~ed generationfleet in the mid-Atlanticregion, the Midwest and New England to comply with federal and state-specific rules addressing S02,NOx and mercury. We expect to have spent about $3.0 billion on air pollution controls by 2015 and to reduce SO2 and NOx emissions by about 80% and mercury emissionsby about 86%. We also expect that many of the SO2 controls implemented at our facilities to achieve these reductions will also address the 1-hour SO2 standard.

According to EPA, annual SO2 emissions in the US.have declinedby over 60% since 1990 and
average SO2 ambient concentrations have declinedby 65% over the last 20 yea&. These significant air quality improvements for SO2 (as well as other pollutants) will continue with additional emission reductions achieved through the Cross State Air Pollution Rule (CSAPR)and the expected implementationof the Utility Mercury and Air Toxics Rule (Utility MACT) and the Boiler MACT rule regardless of EPA's implementationofthe new 1-hour SO2 standard.

We have reviewed the draft guidance EPA has proposed to assist the states with implementing the new 1-hour SOzstandard. While we believe this draft guidance in some respects represents

an improvement from the Agency's August 2010 internal SO2 modeling guidance memorandum3, many of the methodologies proposed in this document are overly conservative and burdensome to states and the regulated community and will require resource-intensive, detailed technical modeling analyses over a very short time frame. In addition, some of the proposed approaches are not consistent with long-standingEPA policies for implementingair quality standards. These issues are addressed below.

NAAQS Implementation Issues

1. EPA Should Complete Its Intended Formal Comment and Rulemaking Process Before Advancing the Required Use of Air Qualitv Rlodeliny to Implement the SO2 NAAQS Through ~uidance

The proposed guidance appears to confirm EPA's intent to require states to use a new hybrid approach that appliesthe use of air dispersion modeling along with ambient air monitoring for purpos& of both attainment designations and the development of Section 110(a)(l)infrastructure state implementation plans (SIPS)under the new 1-hour SO2 standard. EPA points to the lack of sufficient SO2monitoring networks across the states

as thebasis for this new requirement. In addition, the states arebeing required to evaluate through refined dispersion modeling the effectivenessof their SIPS for all areas (including attainment or unclassifiable areas), notjust nonattainment areas.

These are significant changes in EPA's approach for implementing air quality standards that EPA has not yet promulgated through a formallregulatorycomment rulemaking. It has consistently been the Agency's practice to base attainment designations on actual ambient air quality as reflected by ambient air quality monitoring data, even in cases where statemonitoring networks were deemed sparse. For example, in its recent rulemaking for implementing the 1 -hour NO2 standard, which was completed only six months prior to promulgatingthe 1-hour SO2 standard, EPA determined that areas without adequate monitoring data would initially be designated unclassifiable (as with the SO2standard), but provides states time to install and collect data from new ambient air NO2 monitors and to subsequently redesignate such areasto either attainment or nonattainmentbased solely on the results of the new monitoring data. For SOz, the proposed guidance requires states to submit an analysis as part of their infrastructure SIPS showing that federally enforceable control measures will bring unclassified areas

U.S. EPA Memorandum kom Tyler Fox (Leader,Air Quality Modeling Group) to EPA Air Division Directors: "Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient Air Quality Standard", August 23,2010. http:/lwww.eoa.rov/ttn/scramiClarificatioemo AooendixW Hourly-S02- NAAOS FINAL OX-23-2010.udfinto attainment of the NAAQS. Even if a stateprovides ambient monitoring data showing compliance with the SO2 standard, it will stillbe required to submit refined air dispersionmodeling analysis demonstrating attainmentin order to be designated attainment for the standard. EPA is effectively presuming that all unclassifiable areas are nonattainment unless the state can prove, via modeling, that the area is attainment.

The preface to the draWproposed guidance indicates that EPA is planning to conduct a notice and comment rulemaking on this new hybrid approach for the implementation of the 1-hour SO2 standard. We encourage EPA to do so and request that upon completion of the rulemaking process, EPA provide the states a reasonable amount of time to implement any new procedures. Until such rulemaking is completed, and states are provided sufficient time to respond to any newly-established requirements, EPA should maintain its current practice ofrequiring statesto develop infrastructureSIPs only for nonattainment areas. Requiring the states to implement the measures described in the proposed guidance for unclassified areas places unreasonable burden upon the states and the facilities ultimatelyimpacted by measures imposed under the SIPs, as described below.

2. The Timcline to Completc the Proposed Modeling Analvses and State SIP Submittals is Burdcnsomc and Should be Extended

The proposed guidance requires states to develop and submit SIPS by June 2013 for areas designated unclassifiable under the 1-hour SO2standard. This will require extensive statewide refined dispersion modeling, involving in some states hundreds of sources, and subsequent establishment of federally-enforceable control measures and emission limits for these sources over the next 18 months. This timeline is extremelyaggressive and for the following reasons, it is unrealistic to assume that states will have enforceable limitationsin place and compliancedemonstrations completedby June 2013.

Following EPA's current guidance on modeling allows several avenues for facilities
to refine modeling outputs. For example, facilities may gather site specific meteorological data and may perform model evaluation studies. Such work products requirethat facilities create and submit quality assurance project plans (QAPPS) and that these plans be reviewed and approved by the state permitting agencies. The timeline for receiving the final modeling results from such efforts can easily be five years from the start of the modelingproject, which would be well beyond the timeline for submittal of the SIPSestablished under these vrovosed guidance. 

### - In addition, the development of SIPs involving federally-enforceable control

measures and/or emission limits often involves a negotiated effort between affected facilities and the state agencies, and the SIP itselfmist go through public notice and
comment prior to final development, approval at the state level and submittalto EPA. Clearly the extensive modeling effort required under this proposed guidance and the subsequent SIP developmentprocess cannot be reasonably completed within the timeline established in the proposed guidance.Evidence of the inadequacyofthat timeframe is provided from the recent SO2regulatory action involving the Portland Generating Station (PGS) located in eastern Pennsylvania. 

### In that case, the state ofNew Jersey filed allegations that emissions from PGS were causing SO2NAAQS exceedances in New Jersey. The initial petition was filed in May 2010 by New Jersey and EPA took final action on the petition in November 201 1-some 18 months later. Compared to the analyses that will be required for hundreds of sources based on EPA's proposed guidance, the effort that was required for PGS is relatively modest. First, PGS had already collected onsite meteorological data, which requires approximately 18months from initiation. Even with onsite data, those data stillrequired adjustments (madeby EPA) to suitably predict impacts. Second, the area immediately around PGS is substantially flat, while many sourcesthat will be modeled under the proposed strategy are located in elevated terrain, which adds a layer of complexityto the modeling analyses required by EPA's proposed guidance. Third, and perhaps most importantly, the PGS analysis only involved a single source. In contrast, the proposed guidance will require the modeling ofhundreds of sources and the evaluation of very complexmulti-sourceimpacts, the differentiation of source culpabilityand the development and submittal of a SIP containing source-specific emission limits by June 2013. These additional complexities are substantial and will significantly increase the resources needed by states to complete these requirements within the next 18months.

Many of the sourcesthat are likely to be impacted by the modeling requirements of Section 110(a)(l) SIPSarethe same sources that are subject to CSAPR, the Utility MACT and the Industrial Boiler MACT rules. manv of which will reauire pollution control measures/options (technologies, fuel switchingor even unit shutdowns) that will significantly reduce SO2 emissions. CSAPR, although finalized in July 201 1, is currently inttheprocess ofbeing modified and the Utility 

### MACT and Industrial Boiler MACT are not yet final. Even when they do become final, the corresponding compliance dates will be up to three to four years after the date ofpromulgation. These sourcesmay not he in a position to negotiate new permit limitsprior to the SIP submittal deadline because they will not have had a chance to finalize their compliance strategiesfor CSAPR, the Utility MACT or Industrial Boiler MACT by that time or to fullyunderstand how these strategies will impact their SO2 emissions on a short-term basis.

EPA should allow states additional time to complete the required SIPS. Some states, including the Commonwealth ofVirginia4, suggest allowing states a phased SIP development and submittalprocess up until at least January 2017, rather than requiring the submittal of all demonstrations in the 2013 SIP, which is an extremely challenging goal and will not account for the effects of other regulatoryprograms (identified above). This will allow states the time needed to evaluate the expected impacts and effectiveness of these federal programs and to prioritize the required modeling workload. It will also give affected sources the time they need to develop comprehensive and cost-effective compliance strategiesthat address all regulatory requirements.

Comments on Proposed Guidance -Docket ID No. EPA-HQ-OAR-2010-1059filedby the Commonwealthof Virginia (November 16,2011).A more appropriate deadline for this NAAQS compliance demonstration is the statutory 2017 deadline. Accordingly, EPA should revise the guidance to reflect a 2017 deadline for submittal of the state SIPs. Otherwise, states may not be able to completethe work according to the schedule or may even be forced to reject delegation ofthese SIP requirements.

3. The Requirement to Set 1-Hour SO2 Emission Limits and to Include Source-Specific Limits in State SIPs ShouldBe Eliminated

The proposed guidance would requirethe states to impose federally-enforceable 1-hour SO2emission limits for all covered sourcesin the SIP. This overly strict requirement does not reflect the engineering realities associated with many types ofprocesses that will be modeled as part of this SO2NAAQS compliance exercise.

EPA should revise the proposed guidance to eliminatethe requirement that emission limits to demonstrate compliance with the 1-hour SO2NAAQS must be expressed as 1- hour average limits. To the extent EPA requires states to set 1-hour limits, such a limit should be based on full-load conditions and must include exemptionsfor start-up, shutdowns and malfunctions. Limits shouldbe set on a case-by-casebasis, with larger averaging times allowed to demonstrate compliance to allow facilities some flexibilityin the design and operation of controls.

In addition, EPA should not require enforceable limits to be incorporated into the SIP for

sourceslocated in unclassified areas as proposed in the guidance. These limits will be highly source-specific (unlike RACT limits in nonattainment areas that generally cover many sources with similarrequirements), and the inclusion of such limits into a SIP for

an unclassified area would necessitate a SIP-revision for even minor future changes to the emission limit or any of the associated record-keeping, reporting or monitoring requirements,which is often a lengthy, multi-year process. While the requirement to include enforceable limits in SIPSmay be needed to meet Clean Air Act mandates in nonattainment areas, EPA should allow states the use of alternativeways to enforce limits such as state operatingpermits, Title V permits or consent decreesin unclassified areas.

4. EPA Should EstablishDefault Off-ramps for model in^ Analyses

EPA should provide an exemption from modeling for sources willing to take a federally enforceable limitation on their PTE to less than 100tpy SO2, prior to 2017. Facilities that

are willing to do so, either through operational curtailments, use of lower sulfur fuels, operational changes, or installation of controls, should not be required to evaluate air quality impacts.

EPA should also establish an exception from the requirement to complete a modeling analysis for facilities where all significant SO2 sources are scrubbed or already have BACT-equivalent controls. 

### In the proposed guidance, EPA refers to a qualitative assessment based on the results ofpreliminary modeling of sample facilities that indicates that "effective SO2 source control, such as the installation ofSO2 scrubbers, should

### generally sufice for areas to meet the 1-hour SO2NAAQS"'. We concur with this assessment. However, given the very conservative modeling approachesbeing advanced under this proposed guidance (discussed below), it is not unreasonableto expect that in

some situations, the modeling may show that state-of-the-art controls may not sufficefor clusters of facilities (e.~.. 

### in urban areas) or facilities with short stacks and/or comvlex

\ -, terrain issues. While the proposed guidance recognizes this possibility, it does not providemeaningful recourse for sources that have installed state-of-the-art controls at significant cost but may stillbe faced with modeling analyses that predict exceedances of the standards largely due to the very conservative approaches required under the guidance that could force a reduction in the source's allowableemission rate to a level that is technically unachievable at any cost and could even necessitate shutdown of the source. EPA should carefully consider whether the proposed multiple layers of conservatism are appropriate and what might occur if a source is simply unable to meet the reduced emission limit dictated by an overly conservative model.

We recommend that EPA grant an exemption from modeling for facilities that have installed or are installing state-of-the-art SO2control technology, such as scrubbers. This would provide assurances to sources that have already taken significant measures to reduce SO2 emissions or arein the process of or planning to install controls to achieve compliance with current and expected federal rules (such as CSAPR and the Utility MACT rule) would not be faced with the possibility ofupgrades, redesigns or at worst case the shutdown of a well-controlled unit. It would also provide a mechanism for streamlining the SIP submittal requirementsby avoidingunnecessary and costly modeling exercises. Moreover, facilitiescouldbe exempted from modeling requirements if the post-control emissions, achievedby either the installation of control technology or alternative options such as lower sulfur fuel, arebelow the established de minimis level (e.g. 

### 100tpy) no later than the compliance date of August 2017.

The Keference to Stiitc Reallocation of CSAPK Allowances to Address Local-Scale Compliance with the SO2 Standard Is Problematic and Should be Removcd

The proposed guidance envisions that control measures that sources use to comply with the requirements of federal rules, such 

### as CSAPR, will facilitateattainment and maintenanceof the 1-hour SO2 standard, but then speaks to options the states have to 

### '>pursue a distribution ofcontrols under CSAPR that optimize the achievementof attainment of

### the SO2 

### standard'I6,including the option to revise the allocation of allowances to CSAPR units to reflect a distribution of emissions that best serves the purpose of attaining the SO2 standard. CSAPR is designed as a limited trading program
to address long-rangetransport that provides for annual SO2 allowances and does not impose any unit-specific short-terns02 limits. To the extent it deems it necessary to establish short-term limits on a source to address the 1-hour SO2standard, which is more of a local-scale issue, a statehas other optionsunder the Clean Air Act to do so without interferingwith a regional trading program. This suggestionthat states reallocate

httP:ll~.e~a.novlairaualitvisul~dioxide/~dfsraSO2Guidce 9-22-11.pdf (p. 11) htt~:llwww.e~a.novlairaualit~l~~1~dioxidel~dfslDraSO2Guice 9-22-1l.~df (p. 20)CSAPR allowances to address local-scale compliance with the SO2 standard is problematic and shouldbe removed from the guidance.

Technical Modeling Issues

EPA's proposal in large part employs the same modeling protocols for purposes of SO2NAAQS SIP modeling that are used under the Clean Air Act's Prevention of Significant Determination ("PSD) program. This carries through a number of very conservative assumptions on which those protocols are based. To the extent EPA intends to require states to use modeling in place of (andlor in the absence of) ambient monitoring for SIP modeling demonstrations, the modeling assumptions shouldreflect in as much aspossible atmospheric and physical conditions that can be expected to occur over a period of time and not be based on or drivenby a series of simultaneously-occuningworst-case events. If modeling is to be used for attainment demonstrations, EPA must develop adequate tools andlorprocedures to result in modeled results that are realistic and provide a reasonable assurance of compliance. As proposed the guidance would result in predicted impacts that are completelyunrealistic and not in line with the expected monitored values.

1. Modeling Should Be Conducted Using Actual Emissions and Stack Heights

The draft guidancerequires that each source included in the SIP modeling be modeled assumingthe source emits at its maximum potential SO2 emission rates for 8,760 hours per year. Sincemaximum emissions (either though permit limitations or unit capacity) in
many instances far exceed actual emissions, modeling with maximum emissions as proposed by EPA will lead to large over-predictions of ambient air qualityimpacts. This deficiency for single sourcesbecomes greatly magnified when considering multiple
sources, as the likelihood of each of the sources emitting at the maximum emissionrate at the samehour is nil. The guidance must be modified to allow a more realistic approach for modeling source emission rates for purposes of attainment demonstrations.

Normally, a network of monitors continuously records the ambient air conditions within a given region within which sources are emitting SOzat actual levels,ranging from zero to maximum potential at any given moment. Therefore,monitors arerecording the impacts from actual emissions of SO2resulting in the normal ambient conditions of the area. Modeling should be conducted using approachesthat better simulatethe real time variability in source emission rates. For example, the use of actual emissions from some period concurrent with the meteorological data used in the modeling shouldbe allowed as

an option. The actual emissions can even be considered on an hour-by-hour basis for larger sources that record either the emissionsusing continuous emissions monitoring systems (CEMs) or by using fuel usage. This would be particularlyuseful for intermittent Sources, where modeling at the maximum potential for 8,760 hours per year will result in totally unrealistic and highly conservative results.

EPA should allow the use of modeling with up to 

### 5 years of actual emissions (if available and representative of current conditions) as an option. This will ensurethat a largesample of various emission cases is included in the modeling. EPA should also
encourageand consider alternative approaches such as long-term frequencydistributions
to factor into the process the realizationthat emissionrates are variable.

In a similarway, EPA's suggestionthat modeled stack heightsbe cut off at the GEP formula height (limited to GEP stackheight) will result in hypothetical air quality impactsthat do not actually occur. The attainment status of an area should be based on actual air quality (i.e., as measured by ambient monitors) rather than on hypothetical ambient concentrations predicted by modeling that isbased on artificial upper limit emissions from truncated stacks, neither ofwhich reflect reality. To the extent air dispersion models are used in place of (or in combination with) ambient air monitors to determine attainment status or to establish control measures for existing sourcesunder Section 110(a)(l) SIPS,the model inputs should as best as possible represent actual conditions. Sources should be modeled at their actual stack heights.

2. Thresholds for Modeling ShouldBe Based on Actual Emissions

EPA notes in the proposed guidancethat sources emitting over 100 tpy SO2 actual
account for greater than 

### 99% of total SO2 emissions across the U.S, but is unclear regarding the proposed threshold for consideration in modeling analyses and whether the threshold shouldbe determined by actual emissions or potential (annualized PTE) emissions. For example, elsewherein the guidance, EPA suggests that modeling of sources below 100 tpy could be required. Based on EPA's own assertion that nearly all SO*stationary source emissions arise from sourceswith actual emissions over 100tpy, EPA should limit the analyses to only those facilitieswith greater than 100tpy emissions. Such an approach would help to more reasonably balance the effort and costs required by the analyses and better link the potential benefits with the effort by focusing

on facilities that emit the majority of SO2 emissions. Using potential emissions as the threshold for consideration needlesslybroadens the universe of sourcesrequired to undergo the substantial efforts requiredby the proposed analyses without a commensurate environmentalbenefit.

3. Modeling of Various Load Conditions is not Necessary and Would be a Burdensome Requirement

The EPA guidance suggests that sourcesbe modeled at various operating conditions (partial loads) in addition to peak load. However, EPA should recognizethat refined sitespecificmodeling is a burden that is compoundedby directing modeling of multiple configurations or load conditions. To limit the number of configurations to be evaluated, EPA should allow an affected facility andlor stateto determine the operating configuration that combinesload and emissionrate assumptions to generate a single reasonable worst case emission scenario for model inputs.4. Modeling of Multiple Sources Should Not Extend to 50 km.

The proposed guidance would require an AERMOD analysis to evaluate sources as far as 50 

### km away from the sourceiarea being evaluated. Although AERMOD's stated applicability limit is 50

### km, this assumes that a plume reaches this distance within 1hour. To do this, the wind speed at plume height would need to be about 13.9mls, or about 31 miles per hour. This conditionis rarely achieved at most sites in the United States. 

### In guidance issued on March 1,20117, EPA recognized that plumes will rarely travel more than 10km 

### in one hours. Thus, for assessing 1-hour ambient concentrations at a particular point, it is not necessary to evaluate sourceslocated more than 10

### km away; those farther-away sources will have only an insignificant concentration gradient and thus need not be modeled. Instead, they shouldbe incorporated into the regionalbackground estimate available from monitoring data. EPA should return to the position it took in its March 1,2011 Guidance and should not require the modeling of other sources located more than 10

### km from the sourceiarea being evaluated.



### .

5. EPA ShouldAllow Time for Sources to Install and Collect OnsiteMonitoringData

EPA should acknowledge that there are circumstanceswhere measured data may be needed to validate model results or used in lieu of modeling as described in Section 10.2.2of the Guideline on Air Quality Models (40 CFR Part 5 1, Appendix W)9where no applicable model is appropriate. Although the prefemed model, AERMOD, has been shown to perform adequately using a limited set of model validation databases, there will undoubtedly be circumstanceswhere theprefened model is not appropriate or does not perform well. Some of the reasons may include 1) the lack of available or representative meteorological data to drive the model; 2) the use of National Weather Service (NWS) data in lieu of onsitemeteorological data, resulting in model performance degradation; and 3) the interactionof plumes with complex building configurations that are not adequately characterized by the Building Profile Input Program (BPIP). Section 10.2.2of the Guideline on Air Quality Models clearly contemplatesthese scenarios. EPA has long acknowledged that models have limitations and continue to evolve. Accordingly, the guidance should recognizethe importance of ambient air monitoring data and its critical role in the evaluationof model performance and to supplement attainment demonstrations.

Dispersion modeling, as implemented in the mannerproposed by EPA, would not determine actual air quality, but rather the theoreticalmaximum potential levels of 

# 'US.EPA Memorandum fiom Tyler Fox (Leader, Air Quality Modeling Group) to EPA Air Division Directors: "Additional Clarification Reeardine A~olicabilitv of Avvendix W Modeline Guidance for the 1-hourNO? 

# - 

# -

. . 

# - NAAQS", March 1,2011. http:l/www.e~a.~ov/ttdscramiAdditional Clarifications A~pendixW Hourlv-N02- NAAOS FINAL 03-01-201 1.~df

htt~~:llwww.eva.~ovlttd~~ram/Additional Clarifications ApvendixW Hourlv-N02-NAAOS FINAL 03-01- 

### Wf.

Guideline on Air QualityModels 40 CFR Part 51, Appendix W (Revised,November 9,2005), httll:llwww.ena.eov/scramOO1/euidance vermit.htmpollution. This approachis inappropriate as it will lead to areas that are well in compliance with NAAQS, as would be demonstrated through monitoring, as being designated nonattainrnent. Thus, compliancewith the NAAQS for areas with existing

sources should eitheruse monitoring, or if monitoring data is unavailable, modeling should be applied in a manner that realisticallyestimates the present state of ambient air quality; i.e., modeling shouldbe designed to provide results are as close to monitoring results as possible.

Modeling with the currentversion of AERMOD and existingmodeling guidance should be regarding as "preliminary" or "screening", especiallyif individual sources elect to gather new site-specificmeteorological and/or air quality data. Since the June 2013 deadlinefor maintenance SIP does not allow sufficient time for sourcesto complete the collection of site-specificmeteorological data or to fullyimplement alterative modeling approaches, EPA should provide an extension of time for such activities to be completed.



### 6. Elements of Support

We support the following components of the draft document that EPA should affirm in the final guidance:

Treatment of Intermittent Sources: The draft guidance follows the approach EPA established for NO2 in its March 1,2011NO2modeling guidance on how to treat intermittent sources such as emergency generators and startup-shutdown emissions. We concur with this approachto base compliance demonstrations only on emissions which occur frequentlyenough to realistically contribute significantlyto the annual distribution of daily maximum 1 -hour concentrations and allows for a source whose operation is sufficientlyintermittent that it cannot cause a violation to be omitted from modeling. However, the March 201 1NOx guidance does not define what EPA considers "sufficiently intermittent". EPA should explicitlyexcludethese intermittentlinfrequentsources from anyrequired modeling analyses. Absent an exclusion, EPA should allow the use of annualized emission rates for these sources as allowed in the March 1,2011NOx midance. 

## - Background concentrations: EPA has backed away from its August 23,2010 SO2 modeling guidance memo that conservatively recommended use of the highest 1 

### -hr backgro&d value to set background conditions and instead refers to the March 1, 2011NO2modeling guidance and the March 24,201 1 SO2 NAAQS designation guidancelo whch suggest using the form of the standard for the latest 3-year period

as a first-tier approach. At a minimum, the background value shouldbe consistent with the form of the standard. The draft guidance also allows use of more refined background concentration approaches, such as temporarily varying background concentrationsby hour of day and season. These alternativesshould be retained as optionsin the final guidance. In addition, periods when sources that are explicitlymodeled, either individuallyor collectively,may be impacting the monitor should be excluded from the background.

0 Form of Standard for Modeling Purposes: We agree with EPA's recommendationin the draft guidancethat the modeled impact shouldbe consistent with the form of the standard. For 1-hour S02, which is a 99' percentile impact averaged acrossthe years ofmeteorological data, the appropriate modeled form is the highest fourth-high (H4H) averaged across the years of meteorological dataused in the model.

We appreciate this opportunity to comment. If you have any questions, please contact me or Lenny Dupuis @ 804-273-3022 or 

### leonard.du~uis(iidom.com.

S' erely, 

# k&k&

Pamela 

### F. Faggert

Ecc: Larry D. Wallace, Ph.D. (U.S. EPA - OAQPS)