Pamela F. Faggert 

Vice President and Chicf Environmental 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 

1200 Pennsylvania, 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 3,201 1 

Dear Administrator Jackson: 

Dominion appreciates the opportunity to provide comment on EPA's draft "Guidance for 1-Hour 

SO2 NAAQS SIP Submissions1". The company is one of the nation's largest producers and 

transporters of energy in the US., with a portfolio of more than 28,200 megawatts of generation, 

and other energy assets including natural gas transmission, gathering and storage pipeline, 

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. Since the mid 1990's, Dominion has spent over $2.6 billion 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 generation fleet in the 

mid-Atlantic region, 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 

emissions by 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 declined by over 60% since 1990 and 

average SO2 ambient concentrations have declined by 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 implementation of the Utility Mercury and Air Toxics Rule (Utility MACT) 

and the Boiler MACT rule regardless of EPA's implementation of the new 1-hour SO2 standard. 

We have reviewed the draft guidance EPA has proposed to assist the states with implementing 

the new 1-hour SOz standard. 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-standing EPA policies for implementing air 

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 applies the 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 SO2 monitoring networks across the states 

as the basis for this new requirement. In addition, the states are being required to evaluate 

through refined dispersion modeling the effectiveness of their SIPS for all areas 

(including attainment or unclassifiable areas), not just nonattainment areas. 

These are significant changes in EPA's approach for implementing air quality standards 

that EPA has not yet promulgated through a formallregulatory comment 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 state monitoring 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 promulgating the 1-hour SO2 standard, EPA determined that areas 

without adequate monitoring data would initially be designated unclassifiable (as with the 

SO2 standard), but provides states time to install and collect data from new ambient air 

NO2 monitors and to subsequently redesignate such areas to either attainment or 

nonattainment based 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.udf 
into attainment of the NAAQS. Even if a state provides ambient monitoring data 

showing compliance with the SO2 standard, it will still be required to submit refined air 

dispersion modeling analysis demonstrating attainment in 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 of requiring states to develop infrastructure SIPs 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 ultimately impacted 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 SO2 standard. 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 extremely aggressive and for 

the following reasons, it is unrealistic to assume that states will have enforceable 

limitations in place and compliance demonstrations completed by 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 

require that 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 modeling project, which would be well beyond the timeline 

for submittal of the SIPS established 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 itself mist go through public notice and 

comment prior to final development, approval at the state level and submittal to EPA. 

Clearly the extensive modeling effort required under this proposed guidance and the 

subsequent SIP development process cannot be reasonably completed within the 

timeline established in the proposed guidance. 
Evidence of the inadequacy of that timeframe is provided from the recent SO2 regulatory 

action involving the Portland Generating Station (PGS) located in eastern Pennsylvania. 

In that case, the state of New Jersey filed allegations that emissions from PGS were 

causing SO2 NAAQS 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 18 months from initiation. Even with onsite data, those data still required 

adjustments (made by EPA) to suitably predict impacts. Second, the area immediately 

around PGS is substantially flat, while many sources that will be modeled under the 

proposed strategy are located in elevated terrain, which adds a layer of complexity to 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 of hundreds of sources and the evaluation of very 

complex multi-source impacts, the differentiation of source culpability and 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 18 months. 

Many of the sources that are likely to be impacted by the modeling requirements of 

Section 110(a)(l) SIPS are the 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 switching or even unit shutdowns) that will 

significantly reduce SO2 emissions. CSAPR, although finalized in July 201 1, is currently 

intthe process of being 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 of promulgation. These sources may not he in a 

position to negotiate new permit limits prior to the SIP submittal deadline because they 

will not have had a chance to finalize their compliance strategies for CSAPR, the Utility 

MACT or Industrial Boiler MACT by that time or to fully understand 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 of Virginia4, suggest allowing states a phased SIP 

development and submittal process 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 regulatory programs (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 strategies that address all regulatory requirements. 

Comments on Proposed Guidance - Docket ID No. EPA-HQ-OAR-2010-1059 filed by the Commonwealth of 

Virginia (November 16,201 1). 
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 complete the work 

according to the schedule or may even be forced to reject delegation of these SIP 

requirements. 

3. The Requirement to Set 1-Hour SO2 Emission Limits and to Include Source-Specific 

Limits in State SIPs Should Be Eliminated 

The proposed guidance would require the states to impose federally-enforceable 1-hour 

SO2 emission limits for all covered sources in the SIP. This overly strict requirement 

does not reflect the engineering realities associated with many types of processes that will 

be modeled as part of this SO2 NAAQS compliance exercise. 

EPA should revise the proposed guidance to eliminate the requirement that emission 

limits to demonstrate compliance with the 1-hour SO2 NAAQS 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 exemptions for start-up, 

shutdowns and malfunctions. Limits should be set on a case-by-case basis, with larger 

averaging times allowed to demonstrate compliance to allow facilities some flexibility in 

the design and operation of controls. 

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

sources located 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 similar requirements), 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 SIPS may be needed to meet Clean Air Act mandates in 

nonattainment areas, EPA should allow states the use of alternative ways to enforce limits 

such as state operating permits, Title V permits or consent decrees in unclassified areas. 

4. EPA Should Establish Default 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 100 tpy 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 of preliminary modeling of sample facilities that indicates 

that "effective SO2 source control, such as the installation of SO2 scrubbers, should 
generally sufice for areas to meet the 1-hour SO2 NAAQS"'. We concur with this 

assessment. However, given the very conservative modeling approaches being advanced 

under this proposed guidance (discussed below), it is not unreasonable to expect that in 

some situations, the modeling may show that state-of-the-art controls may not suffice for 

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 

provide meaningful recourse for sources that have installed state-of-the-art controls at 

significant cost but may still be 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 allowable emission 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 SO2 control technology, such as scrubbers. This 

would provide assurances to sources that have already taken significant measures to 

reduce SO2 emissions or are in 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 of upgrades, redesigns or at worst 

case the shutdown of a well-controlled unit. It would also provide a mechanism for 

streamlining the SIP submittal requirements by avoiding unnecessary and costly 

modeling exercises. Moreover, facilities could be exempted from modeling requirements 

if the post-control emissions, achieved by either the installation of control technology or 

alternative options such as lower sulfur fuel, are below the established de minimis level 

(e.g. 100 tpy) 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 facilitate attainment and 

maintenance of the 1-hour SO2 standard, but then speaks to options the states have to 

'>pursue a distribution of controls under CSAPR that optimize the achievement of 

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-range transport 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 SO2 standard, which is more 

of a local-scale issue, a state has other options under the Clean Air Act to do so without 

interfering with a regional trading program. This suggestion that states reallocate 

httP:ll~.e~a.novlairaualitvisul~dioxide/~dfsraSO2Guidce 9-22-1 1 .pdf (p. 11) 

htt~:llwww.e~a.novlairaualit~l~~1~dioxidel~dfslDraSO2Guice 9-22-1 l.~df (p. 20) 
CSAPR allowances to address local-scale compliance with the SO2 standard is 

problematic and should be removed from the guidance. 

Technical Modeling Issues 

EPA's proposal in large part employs the same modeling protocols for purposes of SO2 NAAQS 

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 should reflect in as much as possible atmospheric and physical conditions that can 

be expected to occur over a period of time and not be based on or driven by a series of 

simultaneously-occuning worst-case events. If modeling is to be used for attainment 

demonstrations, EPA must develop adequate tools andlor procedures 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 completely unrealistic and not in line with the expected 

monitored values. 

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

The draft guidance requires that each source included in the SIP modeling be modeled 

assuming the source emits at its maximum potential SO2 emission rates for 8,760 hours 

per year. Since maximum 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 quality impacts. This 

deficiency for single sources becomes greatly magnified when considering multiple 

sources, as the likelihood of each of the sources emitting at the maximum emission rate at 

the same hour 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 SOz at actual levels, ranging from zero to 

maximum potential at any given moment. Therefore, monitors are recording the impacts 

from actual emissions of SO2 resulting in the normal ambient conditions of the area. 

Modeling should be conducted using approaches that better simulate the 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 should be allowed as 

an option. The actual emissions can even be considered on an hour-by-hour basis for 

larger sources that record either the emissions using continuous emissions monitoring 

systems (CEMs) or by using fuel usage. This would be particularly useful 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 ensure that a large 
sample of various emission cases is included in the modeling. EPA should also 

encourage and consider alternative approaches such as long-term frequency distributions 

to factor into the process the realization that emission rates are variable. 

In a similar way, EPA's suggestion that modeled stack heights be cut off at the GEP 

formula height (limited to GEP stack height) will result in hypothetical air quality 

impacts that 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 is based on artificial upper limit 

emissions from truncated stacks, neither of which 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 sources under 

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 Should Be Based on Actual Emissions 

EPA notes in the proposed guidance that 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 should be determined by actual emissions or potential (annualized PTE) 

emissions. For example, elsewhere in 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 sources with actual emissions over 100 tpy, 

EPA should limit the analyses to only those facilities with greater than 100 tpy 

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 needlessly broadens the universe of sources required to 

undergo the substantial efforts required by the proposed analyses without a 

commensurate environmental benefit. 

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

Requirement 

The EPA guidance suggests that sources be modeled at various operating conditions 

(partial loads) in addition to peak load. However, EPA should recognize that refined sitespecific modeling is a burden that is compounded by directing modeling of multiple 

configurations or load conditions. To limit the number of configurations to be evaluated, 

EPA should allow an affected facility andlor state to determine the operating 

configuration that combines load and emission rate 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 1 hour. 

To do this, the wind speed at plume height would need to be about 13.9 mls, or about 31 

miles per hour. This condition is rarely achieved at most sites in the United States. In 

guidance issued on March 1,201 17, EPA recognized that plumes will rarely travel more 

than 10 km in one hours. Thus, for assessing 1-hour ambient concentrations at a 

particular point, it is not necessary to evaluate sources located more than 10 km away; 

those farther-away sources will have only an insignificant concentration gradient and thus 

need not be modeled. Instead, they should be incorporated into the regional background 

estimate available from monitoring data. EPA should return to the position it took in its 

March 1,201 1 Guidance and should not require the modeling of other sources located 

more than 10 km from the sourceiarea being evaluated. . 

5. EPA Should Allow Time for Sources to Install and Collect Onsite Monitoring Data 

EPA should acknowledge that there are circumstances where measured data may be 

needed to validate model results or used in lieu of modeling as described in Section 

10.2.2 of the Guideline on Air Quality Models (40 CFR Part 5 1, Appendix W)9 where 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 circumstances where the prefened 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 onsite meteorological data, resulting in model performance degradation; 

and 3) the interaction of plumes with complex building configurations that are not 

adequately characterized by the Building Profile Input Program (BPIP). Section 10.2.2 of 

the Guideline on Air Quality Models clearly contemplates these scenarios. EPA has long 

acknowledged that models have limitations and continue to evolve. Accordingly, the 

guidance should recognize the importance of ambient air monitoring data and its critical 

role in the evaluation of model performance and to supplement attainment 

demonstrations. 

Dispersion modeling, as implemented in the manner proposed by EPA, would not 

determine actual air quality, but rather the theoretical maximum 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-hour NO? 

NAAQS", March 1,201 1. http:l/www.e~a.~ov/ttdscramiAdditional Clarifications A~pendixW Hourlv-N02- 

NAAOS FINAL 03-01-201 1 .~df 

Wf. 

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

Guideline on Air Quality Models 40 CFR Part 51, Appendix W (Revised, November 9,2005), 

httll:llwww.ena.eov/scramOO1/euidance vermit.htm 
pollution. This approach is 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, compliance with the NAAQS for areas with existing 

sources should either use monitoring, or if monitoring data is unavailable, modeling 

should be applied in a manner that realistically estimates the present state of ambient air 

quality; i.e., modeling should be designed to provide results are as close to monitoring 

results as possible. 

Modeling with the current version of AERMOD and existing modeling guidance should 

be regarding as "preliminary" or "screening", especially if individual sources elect to 

gather new site-specific meteorological and/or air quality data. Since the June 2013 

deadline for maintenance SIP does not allow sufficient time for sources to complete the 

collection of site-specific meteorological data or to fully implement 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 1,201 1 NO2 modeling guidance on how to treat 

intermittent sources such as emergency generators and startup-shutdown emissions. 

We concur with this approach to base compliance demonstrations only on emissions 

which occur frequently enough to realistically contribute significantly to the annual 

distribution of daily maximum 1 -hour concentrations and allows for a source whose 

operation is sufficiently intermittent that it cannot cause a violation to be omitted 

from modeling. However, the March 201 1 NOx guidance does not define what EPA 

considers "sufficiently intermittent". EPA should explicitly exclude these 

intermittentlinfrequent sources from any required modeling analyses. Absent an 

exclusion, EPA should allow the use of annualized emission rates for these sources as 

allowed in the March 1,201 1 NOx 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, 

201 1 NO2 modeling 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 should be consistent 

with the form of the standard. The draft guidance also allows use of more refined 

background concentration approaches, such as temporarily varying background 

concentrations by hour of day and season. These alternatives should be retained as 

options in the final guidance. In addition, periods when sources that are explicitly 
modeled, either individually or collectively, may be impacting the monitor should be 

excluded from the background. 

0 Form of Standard for Modeling Purposes: We agree with EPA's recommendation in 

the draft guidance that the modeled impact should be consistent with the form of the 

standard. For 1-hour S02, which is a 99' percentile impact averaged across the years 

of meteorological data, the appropriate modeled form is the highest fourth-high 

(H4H) averaged across the years of meteorological data used 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)