The Regional Haze Rule (RHR) establishes a comprehensive visibility protection program for Class I areas and requires states to set reasonable progress goals (RPGs) towards achieving natural visibility conditions in all Class I areas by 2064. EPA released Technical Guidance on Tracking Visibility Progress for the Second Implementation Period of the Regional Haze Rule (Guidance)1 outlining the methods by which states should develop their State Implementation Plans (SIPs) for the 2nd planning period (2018-2028). The guidance includes methods previously not allowed in the 1st planning period.

This article discusses the key differences between the 1st planning period and the 2nd planning period, including how to account for international sources and prescribed wildfires in recalculating the Uniform Rate of Progress (URP) for each Class I area, and how to evaluate the control measures in a state's Long Term Strategy (LTS). The discussion is based on experience from 1st planning period evaluations for Best Available Retrofit Technology (BART) and Reasonable Progress (RP) controls, which pushed the envelope and incorporated several of the technical guidance methods in assessments completed for industrial sites in response to EPA SIP denials and/or Federal Implementation Plans (FIPs).

Defining Haze

Class I Are website linkRegional Haze (RH) is visibility impairment caused by the cumulative impact of air pollutant levels in the atmosphere resulting from human activities ("anthropogenic") and natural events such as wildfires and windblown dust, which can be transported over a wide geographic area. Haze is measured in terms of "light extinction" (the amount of light reduced per distance) and is also expressed in terms of "visual range" (the distance at which an observer can see an object). Since these two terms are not linear, due to the differences in baseline visibility conditions for any particular scene, the RHR defines and utilizes a "haze index," in units of deciviews (dv), designed such that uniform changes in haziness produce similar "perceptible changes" and are constant regardless of the baseline conditions. Pristine skies are classified as having a haze index of zero dv; as the haze index increases, so does the degradation in visibility.

Regional Haze Rule to Date

The 1977 Amendments to the Clean Air Act (CAA) set a national goal to restore the 156 federally mandated Class I Areas to pristine conditions by preventing any future, and remedying any existing, man-made visibility impairment, or "regional haze."

Protection of visibility impairment from new sources was established in a 1980 rulemaking where requirements for visibility impairment in terms of "reasonably attributable" impairment and regional haze were addressed.2 Regulations specifically targeting visibility impairment in Class I Areas were finalized in the 1999 RHR.3 Over the course of the past two decades, these regulations have evolved.

Regional Haze timeline

2nd Planning Period

EPA's Guidance for the 2nd planning period (2018-2028) includes several key items that differ from the 1st planning period.

Natural vs. Anthropogenic Contributions

The flowchart in Figure 2 illustrates the methodology adopted in the 2nd planning period for estimating visibility impairment. Light extinction, which is based on data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) visibility monitoring network, is divided into two components: natural contribution and anthropogenic (or manmade) contribution.

  1. Natural contribution is further broken down to episodic (primarily from events like wildfires and/or dust storms) and routine (typically biogenic sources and sea salts) occurrences. Episodic contributions can be removed to calculate the routine fraction.
  2. Anthropogenic contribution is attributed to national or international sources. The guidance allows the URP to be adjusted based on impacts from international sources.

RH_Figure 2

For example, natural windblown dust emissions, which are particularly prevalent in the western U.S., should be accounted for in the natural contribution assessment. If an emission inventory does not account for these dust emissions in the modeling for the region, which is conducted using a Photochemical Grid Model (PGM), this can be evaluated through additional PGM runs. Assessing the basis for inclusion of natural events, e.g., wildfires, in a preliminary assessment is important to determine if the emissions inventory adequately reflect natural fluctuations.

20% Most Impaired Days Based on Anthropogenic Impairment

States must select the 20% most impaired days each year at each Class I area based on daily anthropogenic impairment. Therefore, unlike the 1st planning period, where the haziest days were evaluated, states should now look at the worst impaired days caused by manmade sources.

RH_Figure 3

EPA's guidance provides a seven-step method for assessing anthropogenic impacts (see Figure 3). Steps 1-4 establish the light extinction thresholds to identify extreme events and classify these as natural-episodic events, leaving the remaining light extinction to natural-routine. This is based on the estimates of natural conditions at each Class I area (referred to as "NC-II estimates") used in the 1st implementation period, and anthropogenic light extinction.

Step 5 estimates the anthropogenic light extinction from monitored data, and the calculated natural light extinction. Step 6 estimates the deciview impact of anthropogenic light extinction, and Step 7 then identifies the 20% most impaired days.

Because the focus in the 2nd planning period is now on anthropogenic impacts, the relative contribution from point sources will appear larger. Therefore, the emissions inventory used as the basis of the screening modeling should be reviewed carefully to ensure it accurately accounts for regulatory changes, recent BART determinations, announced shutdowns, and appropriate boundary conditions in PGM.

URP Adjustment for International Anthropogenic Sources

States can propose an optional adjustment to the URP "glidepath" to account for anthropogenic international sources, as illustrated in the example provided in Figure 4. If international anthropogenic emission visibility impacts are added to the end point, the URP slope changes. A slower slope means fewer controls on U.S. sources are needed to meet the new end goal.

RH_Figure 4

Two key factors should be considered when quantifying international anthropogenic impacts.

  1. Are more enhanced international emission inventory data available?
  2. How was the PGM used to estimate international impacts?
    • A "brute-force" method with an all-sources-included model run, and a second model run with international anthropogenic emissions zeroed out. This method, while conceptually simple, results in inaccuracies of the model.
    • The Particulate Source Apportionment Tool (PSAT) probing tool from within PGM.

General 2nd Planning Period Steps

In accordance with the RHR, states must develop a LTS to reduce regional haze from all types of anthropogenic sources. EPA's guidance document redefines visibility impairment with no de minimis threshold of impact and requires PGM for visibility assessments. Unlike the 1st planning period, which focused on BART-subject sources, the 2nd planning period can target any large source of oxides of nitrogen (NOX), sulfur dioxide (SO2), and/or particulate matter (PM) across industries, including sources previously evaluated under BART. Each Regional Planning Organization (RPO) and state must develop a targeted plan, which may vary across RPOs and states.

Each state and RPO will generally follow the steps outlined below; however, as noted previously, specific steps can be modified by each jurisdiction.

  1. Source Screening. Each state/RPO will develop a screening analysis to identify sources above a particular threshold for which a four-factor analysis will be required.4 The threshold and methodology to be used for this screening assessment is not specified in the guidance for the 2nd planning period. At the time of this article, the Western Regional Air Planning (WRAP) RPO has released its methodology for screening, which utilizes the "Q/d" ratio to determine whether sources could potentially impact a Class I area based on the magnitude of emissions released (tons per year [tpy]) divided by distance (kilometers [km]) to the nearest Class I area.5

For example, the Montana Source Screening Proposal is as follows:

If Q, NOx + SO2 (tpy) / d (km) > 4, 4-Factor Analysis Needed

  1. Most Effective Control Applied. The RH July 2016 Guidance6 provides an additional screening option to identify sources that are subject to federally enforceable emission limits and apply the most effective control technology.
  1. Four-Factor Analysis. A four-factor analysis is required to be conducted if a source is not screened out to determine if there are "reasonable" controls available for reducing visibility impairing emissions. A four-factor analysis of a reasonable progress analysis considers four statutory factors: cost of compliance, time necessary for compliance, energy and non-air quality environmental impacts, and remaining useful life of the source.

The following steps must be followed in conducting the four-factor analysis:

  1. Identify all available control technologies;
  2. Eliminate technically infeasible options;
  3. Rank the remaining options based on effectiveness;
  4. Analyze the most effective measure and document the results; and
  5. Establish federally enforceable emission limits and/or other requirements.

How Trinity Can Help

Trinity worked with clients during the 1st planning period, which included involvement in BART and Reasonable Progress analyses, litigation support across the U.S., and numerous strategy discussions, and negotiations with states and EPA. From this experience, we recommend industrial sites be involved in the stakeholder process for the 2nd planning period, and ensure the following steps are evaluated. Let us provide guidance for your site(s) through this process:

  • Analyze IMPROVE data by species to determine impact per species on Class I Areas;
  • Compare actual visibility impairment as measured for a Class I Area to URP;
  • Determine source contribution to impairment by type;
  • Assess impact of international sources on Class I Areas;
  • Assess individual source contributions; and
  • Conduct Four-Factor Analyses.

As the 2nd planning period evolves, Trinity will continue to follow the RPOs/states to remain current with regional developments. For more information, contact your local Trinity office at (800) 229-6655 for assistance in determining what your RPO/state is planning, helping to decipher RPO methodologies and how they may impact your site, preparing comments on proposed methodologies and photochemical modeling, and conducting site-specific evaluations.


1 Technical Guidance on Tracking Visibility Progress for the Second Implementation Period of the Regional Haze Rule. (December 20, 2018).
2 40 CFR Part 51, Subpart P 1980 Rulemaking - Protection of Visibility. 45 FR 80084 (December 2, 1980).
3 40 CFR Part 51, Subpart P, July 1, 1999 Amendments - Regional Haze Rule.64 FR 35713 (July 1, 1999).
4 Under the 2017 Revisions to the RHR, Federal Land Managers may request an assessment of sites even if they are below the RPO/State screening threshold. 40 CFR Parts 51 and 52, Protection of Visibility: Amendments to Requirements for State Plans. 82 FR 3078 (January 10, 2017)
5 Draft Guidance on Progress Tracking Metrics, Long-term Strategies, Reasonable Progress Goals and Other Requirements for Regional Haze State Implementation Plans for the Second Implementation Period. EPA-457/P-16-001 (July 2016).
6 Draft Guidance on Progress Tracking Metrics, Long-term Strategies, Reasonable Progress Goals and Other Requirements for Regional Haze State Implementation Plans for the Second Implementation Period. EPA-457/P-16-001 (July 2016).