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Class I area impact analyses – recent guidance and updates

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As recommended in Appendix W of 40 Code of Federal Regulations (40 CFR) Part 51 (Guideline on Air Quality Models), the CALPUFF modeling system has been widely used for long-range transport dispersion modeling analyses, especially for applications related to Class I Area analyses (e.g., regional haze evaluation and Class I Prevention of Significant Deterioration (PSD) increment analysis). As U.S. EPA, Federal Land Managers (FLMs), and state agencies have become more knowledgeable on the use of CALPUFF for Class I Area impact analyses, regulators have made case-by-case determinations regarding when analyses are required and what user-defined model inputs are appropriate. However, requirements for Class I Area analyses remain widely disparate across the U.S.

At the 2009 Regional, State, and Local Modelers Workshop, held on May 11–14, 2009 in Philadelphia, Pennsylvania, the FLMs provided an update including topics related to the application of CALPUFF for Class I Area impact analyses and the Draft 2008 Federal Land Managers’ Air Quality Related Values Work Group (FLAG 2008) guidance. The update addressed many issues applicants have been facing in recent submittals including how a site can screen out of a Class I Area impact analysis, selection of user-defined input parameters for CALMET data preparation, and acceptable methods for post processing. On August 31, 2009, the EPA posted a clarification memo, [EPA-FLM Recommended Settings for CALMET], on the Support Center for Regulatory Atmospheric Modeling (SCRAM) website. Each of these topics is addressed in more detail below.

When Should a Class I Impact Analysis Be Conducted?
FLAG 2008 guidance incorporates findings from recent scientific studies and methodologies for conducting visibility analyses based on experience gained through implementation of the Regional Haze Rule. The most substantial change to the guidelines is the setting of a threshold ratio of emissions to distance, below which AQRV review is not required. Specifically, if

Q (tpy)/d (km) < 10, no AQRV analysis is required


  • Q is the emissions increase of sulfur dioxide (SO2), oxides of nitrogen (NOx), particulate matter less than 10 microns (PM10), and sulfuric acid mist (H2SO4), combined in tons per year (tpy)
  • d is the nearest distance to a Class I Area in kilometers (km)

This proposed screening approach (10 D Rule) was discussed at the 2009 Workshop with a note that applicants must convert maximum 24-hour emissions (lb/hr) of SO2 + sulfates (SO4) + NOx + PM Coarse (PMC) + PM Fine (PMF) + secondary organic aerosols (SOA) + Elemental Carbon (EC) into tons per year (tpy) before applying the 10 D Rule. If Q/d is less than 10 for a Class I Area, then presumptively, there is no adverse impact and a project “screens out” of a Class I AQRV analysis. If Q/d results in a value above 10, a Class I analysis is required. Once FLAG 2008 is finalized, this new screening approach will assist applicants in determining if a Class I AQRV analysis is required. However, there may still be instances when an AQRV analysis is not required, but a Class I increment analysis is still requested by EPA.  

Meteorological Data Preparation
A primary concern regarding input meteorological data preparation using CALMET is the user selection of non-default values. Some of the parameters most scrutinized recently are the horizontal receptor grid spacing, radius of influence (RMAX 1, RMAX 2), and the weighting provided in the Step 1 wind field (R1 and R2) when an applicant is preparing CALMET data using the Diagnostic Wind Module.

The developer of the CALPUFF modeling system has provided guidelines for determining these values, and recently, BART protocols have included requirements for particular regions of the country. Despite these resources, FLMs can have varying opinions on these issues, especially in the case of PSD analyses. The meteorological data setup is highly contingent on the modeler with respect to selection of appropriate RMAX 1, RMAX 2, R1 and R2 values. Modelers should not select input values just because they were used by a previous applicant. In addition, the FLMs have communicated that CALPUFF-ready data sets provided by regional planning organizations is not acceptable for PSD modeling. Rather, an applicant can use existing BART-related MM5 data sets along with the settings for RMAX 1, RMAX 2, R1 and R2 detailed in EPA Clearinghouse memos to create a CALMET data set for a specific project.

Post-Processing Methods
There are numerous methods available in the CALPOST post-processing module to calculate background light extinction using the original FLM background extinction equation. The Interagency Workgroup on Air Quality Models (IWAQM) Phase 2 guidance recommends “Method 2,” which uses hourly relative humidity adjustment applied to background and modeled sulfate and nitrate with the relative humidity factor (f(RH)) capped at 95 percent to compute visibility impairment in terms of percent change of the light extinction coefficient (bext) from modeled pollutant concentrations.

Class 1 Area sshot 2An alternative approach, “Method 6,” computes bext using a monthly average relative humidity adjustment particular to each Class I Area applied to background and modeled sulfate and nitrate. Because a monthly average is used, no cap on f(RH) is necessary since the function is not used in Method 6 and the results tend to be smoothed out since peak short-term humidity events are not considered. In addition, the Interagency Monitoring of Protected Visual Environments (IMPROVE) workgroup proposed a more robust equation that was used for some BART projects. The Eastern regional planning organization, VISTAS, developed an initial revised IMPROVE algorithm spreadsheet for an applicant to manually calculate light extinction by entering the results from the CALPOST Method 6 output file, along with NOx concentration data and several other parameters specific to the location of the project in the U.S., resulting in a newly calculated light extinction value. Values for the natural background conditions at Class I Areas to be modeled can be obtained on an annual average basis from EPA’s Guidance for Estimating Natural Visibility Conditions under the Regional Haze Rule. This algorithm is now available in CALPOST as “Method 8.

Recently, applicants have been requested by FLMs to follow the historical method of post-processing (Method 2) and include the two newer methods (Method 6 and Method 8) as supplemental results. Until FLAG 2008 is approved, Method 2 is required for all PSD projects. If any other methods are submitted, they are reviewed as supplemental information. The thresholds for determining a no-impacts determination varies greatly between these methods.

FLAG 2008 proposes to compare the 98th percentile of the daily values per year to the threshold, allowing for seven days per year, to exceed the threshold before additional review of impacts is warranted. This method was reconfirmed at the 2009 Workshop. Applicants should be advised this is not the 21st high or a 3-year average. If one year exceeds the 98th percentile, it fails the FLAG 2008 test. This new guidance will help remove subjectivity from visibility analyses for Class I Area impact analyses.  

Under FLAG 2008, if an adverse impact is determined, additional refined analyses can be conducted using hourly (instead of daily) data and fundamental aerosol and visibility theory to assess impacts when an adverse impact is predicted using the standard prescribed methods. These analyses should be conducted in close consultation with the reviewing authority.

Although requirements for dispersion modeling for Class I Area analyses continue to be somewhat variable in different geographies, the FLMs, EPA, and state regulators are working to standardize guidance to the extent possible. However, facilities that must conduct Class I Area Impact analyses using CALPUFF should continue to work closely with pertinent regulatory personnel in order to verify modeling requirements.

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