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New Flag guidance for class I area modeling

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In October 2010, final guidance was issued for assessing the impacts of new and modified sources of air pollution on Air Quality Related Values (AQRV) in Class I areas, consisting of visibility (including plume impairment and regional haze), ozone effects, and nitrogen and sulfur deposition. This guidance, which was issued in draft form on July 8, 2008, makes technical revisions to the previous guidance issued in December 2000. The new guidance addresses key regulatory updates affecting air resource management and how Class I area impacts are assessed. These regulatory developments included EPA’s promulgated revisions to Appendix W of 40 Code of Federal Regulations (40 CFR) Part 51 (Guidelines on Air Quality Models) in which CALPUFF was adopted as the preferred long range transport model and EPA’s 2005 Best Available Retrofit Technology (BART) guidelines for the Regional Haze Rule. In addition, agencies have redefined their assessments of AQRVs.

The Clean Air Act Amendments of 1977 gave Federal Land Managers (FLMs) an “affirmative responsibility” to protect the natural and cultural resources of Class I areas from the adverse impacts of air pollution. Class I areas are afforded special protection under the Clean Air Act and are defined as certain national parks (over 6,000 acres), wilderness areas (over 5,000 acres), national memorial parks (over 5,000 acres), and international parks that were in existence on August 7, 1977. The Federal Land Managers’ Air Quality Related Values Workroup (FLAG) was formed to provide a consistent and objective approach for evaluating air pollution effects on AQRVs and to provide a consistent methodology for State permitting authorities and permit applicants to assess the impacts of new and existing sources on AQRVs, primarily those in Federal Class I areas, but in some instances, in Class II areas. FLAG members include representatives from the National Park Service (NPS), in cooperation with the U.S. Fish and Wildlife Service (FWS) and the U.S. Department of Agriculture Forest Service (FS).

New Guidance
The new guidelines, The Federal Land Managers AQRV Workgroup (FLAG) Phase I Report – Revised (FLAG 2010) make specific technical revisions while retaining the background material and general information provided in the Phase I document issued in 2000. The 2010 guidance incorporates recent scientific studies and methodologies for conducting visibility analyses based on knowledge developed through implementation of the Regional Haze Rule. Key elements of the new guidance include the following:

Initial Screening Criteria for Requiring AQRV Analyses for Sources Greater than 50 km from a Class I Area
The most substantial change to the guidelines is the setting of a threshold ratio of emissions distance, below which AQRV review is not required for new or modified sources located greater than 50 km from a Class I area. Specifically, if

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

Q is the combined emissions increase from a source of sulfur dioxide (SO2), oxides of nitrogen (NOx), particulate matter less than 10 microns (PM10), and sulfuric acid mist (H2SO4) in tons per year (tpy) based on 24-hour maximum allowable emissions (which are annualized) and d is the nearest distance to a Class I area in kilometers (km) from the source. This approach is limited to sources that operate year round. Emissions from sources that operate intermittently or seasonally must be adjusted to account for year round operation if the applicant uses the Q/d Initial Screening approach.

First Level Screening Model
FLAG 2000 included a Screening option (CALPUFF “Lite”) whereby applicants could use meteorological data from a single station as an initial analysis. This approach is no longer recommended as a first step for Class I area modeling. Instead, applicants should use the CALPUFF Modeling System with a minimum of three years of gridded meteorological data.

Model Settings
A number of recommended model settings were updated in FLAG 2010 including but not limited to the following:

  • Background Visibility Conditions - The threshold for an adverse impact is relative to the “natural” background. FLAG 2010 provides updated background values for all Class I areas based on annual average natural conditions and 20% best natural conditions. Unless otherwise recommended by the FLM or permitting agency, FLAG 2010 prefers the use of the annual average natural background values.
  • Relative Humidity Adjustment Factors - The relative humidity adjustment factors impact the sulfate and nitrate components of the visibility extinction coefficient, i.e., as water droplets are added, the scattering of light is enhanced. FLAG 2010 recommends the use of monthly average relative humidity adjustment factors versus hourly factors. The latter reduces potential “spikes” in visibility impairment due to short-term weather events.
  • Visibility Assessment Criteria - The previous 2000 guidance compared the highest daily visibility change to the adverse impact threshold (5% extinction change). FLAG 2010 uses the 98th percentile of the daily values over a year, allowing for seven days per year to be above the threshold before additional review of impacts is required.
  • Refined Analyses for Visibility - FLAG 2010 allows for additional refined analyses to be conducted using fundamental aerosol and visibility theory to assess impacts when an adverse impact is predicted using the standard prescribed methods. These analyses are to be conducted in close consultation with the reviewing authority.
  • Deposition Analysis Thresholds - FLAG 2010 provides specific nitrogen and sulfur deposition analysis thresholds (DATs) that are recommended as screening level values, below which impacts are deemed negligible. These values were not included in the 2000 FLAG document, but instead were defined in a separate NPS memo. This update in FLAG 2010 consolidates the guidance into a single document.

As PSD Class I modeling undergoes another transition with the final FLAG 2010 guidelines, affected permit applicants must be aware of the new guidelines and familiar with the extensive modeling requirements that accompany them.

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