Single-source and Cumulative 

Impact Analyses

11th Conference on Air Quality Modeling

RTP, NC

George M. Bridgers

OAQPS-AQAD-Air Quality Modeling Group

08/12/2015 U.S. Environmental Protection Agency 1
Proposed Revisions to the Guideline

• Throughout the previous presentations of this morning, 

the various proposed revisions to the Guideline on Air 

Quality Models (the Guideline or Appendix W) across 

Sections 1 through 7 and the meteorological input 

portions of Section 8 have been explained and 

discussed in more detail.

• The culmination of the proposed Guideline revisions and 

the application of the preferred or alternative models 

and/or techniques in a regulatory context are presented 

in Sections 8 and 9.

– Sections 8 and 10 of the current 1995 version of Appendix W.

08/12/2015 U.S. Environmental Protection Agency 2
Proposed Revisions to the Guideline (Cont.)

• Despite numerous public presentations over the past 3 

to 5 years in which the EPA has cautioned against the 

literal and uncritical application of very prescriptive 

procedures for conducting NAAQS and PSD modeling 

compliance demonstrations as described in Chapter C 

of the Draft New Source Review Workshop Manual, 

there are continual and ongoing practices in the 

regulatory modeling community that are overly 

conservative and unnecessarily complicated.

– Past practices may have “worked” given previous NAAQS standards 

(form and level of the standard), but that does not mean that many of 

the past practices were technically correct.

08/12/2015 U.S. Environmental Protection Agency 3
Proposed Revisions to the Guideline (Cont.)

• The proposed changes to Section 8 are intended to 

modify these past practices and provide more 

appropriate basis for selection and use of modeling 

inputs through the Guideline itself and supporting 

guidance.

• The proposed revisions to Section 9 more clearly 

summarize the general concepts presented in earlier 

sections of the Guideline and set the stage for 

appropriate regulatory application of models and/or, in 

rare circumstances, air quality monitoring data.

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Proposed Revisions to Section 8

• The proposed Appendix W, Section 8 provides more definitive 

definition of the appropriate modeling domain and how to best 

characterize the various contributions to air quality concentrations 

in that domain.

• Modeling Domain (Section 8.1):

– Specific requirements for NAAQS or PSD increment 

assessments.

• A radius extending from the new/modifying source to (1) the most distant 

point that a significant ambient impact will occur or (2) the nominal 50km 

distance considered applicable for Gaussian dispersion models, whichever 

is less.

– Specific requirements for SIP attainment demonstrations.

• Determined by the nature of the problem, including all major upwind source 

areas and all monitor locations current or recently violating the NAAQS in 

the nonattainment area.

08/12/2015 U.S. Environmental Protection Agency 5
Proposed Revisions to Section 8 (Cont.)

• Source Input Data (Section 8.2):

– New language regarding how to characterize direct and 

precursor emissions from modeled sources for SIP attainment 

demonstrations for ozone, PM2.5, and regional haze.

• Section 8.2.2(a).

– Revised requirements on how to characterize emissions from 

nearby sources to be explicitly modeled for purposes of a 

cumulative impact analysis.

• Section 8.2.2(b)-(d).

– Revised language on how to characterize emissions from 

mobile sources.

• Section 8.2.2(e).

08/12/2015 U.S. Environmental Protection Agency 6
Proposed Revisions to Section 8 (Cont.)

• Source Input Data (Section 8.2) (cont.):

– Most notable revision in Section 8 is with respect to the 

characterization of emissions from nearby sources to be 

explicitly modeled.

• Tables 8-1 and 8-2 supported by Section 8.2.2(b)-(c).

• Nearby sources are proposed to be characterized by “actual” 

emissions rather than “allowable” emissions.

• Emissions are based on the Emissions Limit, Operating Level, and 

Operating Factor.

• “Actual” emissions should be determined based on the most recent 

2 years of actual nominal operation.

• The new/modifying source would still be characterized by the 

proposed “allowable” or permit limited emissions.

08/12/2015 U.S. Environmental Protection Agency 7
Proposed Revisions to Section 8 (Cont.)

• Background Concentrations (Section 8.3) :

– Revised recommendations on how to determine background 

concentrations in constructing the design concentration as part 

of a cumulative impact analysis for NAAQS and PSD increment.

• Discussion on the importance of understanding what ambient 

monitoring data are available and what these data represent.

• Isolated single-source(s) in Section 8.3.2.

• Multi-source areas in Section 8.3.3.

• Emphasis on how to determine which “nearby sources” to explicitly 

model based on the concept of significant concentration gradients.

• Use of monitored background to adequately represent “other 

sources.”

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Proposed Revisions to Section 8 (Cont.)

• Meteorological Input Data (Section 8.4) :

– Incorporation of prognostic meteorology data as an option 

where there is no representative National Weather Service 

(NWS) station, and it is prohibitive or not feasible to collect 

adequately representative site-specific data. (Already presented 

earlier this morning.)

– Introduction into Appendix W of the AERMINUTE NWS 

meteorological data preprocessor. (Already presented earlier 

this morning.)

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Proposed Revisions to Section 9

• The importance of developing and vetting a modeling 

protocol is more prominently presented in Section 9.2.1.

• Information related to the design concentrations has 

been updated and unified in Section 9.2.2.

– Previously scattered between Sections 7 and 10.

– Previously worded such that it was not dynamic with any 

NAAQS revisions.

• Expanded and revised discussion on receptor sites also 

in Section 9.2.2.

– New considerations given past practices of model users tending 

to define an excessively large and inappropriate number of 

receptors.

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Proposed Revisions to Section 9 (Cont.)

• In Section 9.2.3, a complete overhaul of the 

recommendations provided in Appendix W for NAAQS 

and PSD increment compliance demonstrations to more 

clearly and accurately reflect long-standing EPA 

recommendations.

– First stage: Perform a single-source impact analysis.

– Second stage: As necessary, conduct a more comprehensive 

cumulative impact analysis.

– Revised the considerations in developing emissions limits as 

the existing recommendations were well out of date and not 

reflective of newer percentile (often referred to as probabilistic) 

based NAAQS

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Proposed Revisions to Section 9 (Cont.)

• Finally, the discussion on the Use of Measured Data in 

Lieu of Model Estimates in Section 9.2.4 is proposed to 

be revised with more details on the process for 

determining the rare circumstances in which air quality 

monitoring data may be considered for determining the 

most appropriate emissions limit for a modification to an 

existing source.

– Previously found in Section 10.2.2.

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