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Proposed revisions to the Regional Haze Rule (RHR) (40 CFR Parts 51 and 52) were published in the Federal Register on May 4, 2016.  This proposed rule directly impacts state, local, and tribal governments as well as Federal Land Managers (FLMs) responsible for protection of visibility in Class I areas.1  Indirectly, this rule may potentially affect sources that emit visibility-impairing pollutants, including particulate matter less than or equal to 10 and 2.5 microns in diameter (PM10/PM2.5), sulfur dioxide (SO2), nitrogen oxides (NOX), and volatile organic compounds (VOC).

EQ_Spring 2016_Class I Areas Map

Defining Haze


Regional haze (RH) is a reduction in visibility caused by the cumulative impact of air pollutant levels in the atmosphere resulting from human activities (“anthropogenic”) and natural events such as wild fires, which can be transported over a wide geographic area.  Anthropogenic and natural particles and gases impede the amount of visible light that reaches an observer by scattering and absorbing sunlight as it travels through the atmosphere.  This detectable reduction in visible sunlight by an observer is “haze.”  Haze is measured in terms of “light extinction” (the amount of light reduced per distance in the atmosphere), and is also expressed in terms of “visual range” (the distance 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 utilizes the haze index in 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 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, including national parks and wilderness areas, to pristine conditions by preventing any future, and remedying any existing, man-made visibility impairment, or “regional haze.”  Protection of visibility 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 Regional Haze Rule.3  These regulations have evolved over the course of the past few decades, as outlined below.



  • July 1, 1999 – The Environmental Protection Agency (EPA) promulgated the RHR (40 Parts 51 and 52) to address regional haze.  The RHR included criteria defining sources that may be subject to Best Available Retrofit Technology (BART) requirements and BART guidelines, i.e., emission controls for industrial facilities emitting air pollutants that reduce visibility by causing or contributing to regional haze.  States were also required to establish regional haze State Implementation Plans (SIPs) that must establish goals, the rate of progress, and long-term strategies for attaining natural visibility conditions by 2064.  State SIPs were due to EPA in 2007 and were required to describe the strategy for achieving reasonable progress goals through 2018.    
  • July 6, 2005Amendments to the 1999 RHR were finalized.4  These amendments (often called the BART rule) applied to the provisions of the RHR that require BART emission controls and included definitions for BART applicability criteria and guidance for making source-specific BART determinations.
  • January 15, 2009A notice was published in the Federal Register identifying several states that failed to submit SIPs under the 1999 RHR.  Only 13 states submitted a complete SIP by the 2007 deadline.5
  • 2007-Present – States completed source-specific BART determinations.  BART analyses are still being conducted due to the limited disapproval of some states’ SIPs and Federal Implementation Plans (FIPs) finalized in the June 7, 2012 rulemaking (see below).
  • June 7, 2012 – Revisions to RHR and BART rules were finalized.6   Key highlights are as follows:   
    • EPA’s Cross-State Air Pollution Rule (CSAPR) trading programs may be used by participating states in lieu of BART for SO2 and/or NOX emissions from power plants.
    • Limited disapprovals of the regional haze SIPs were finalized for 14 states that previously relied on the Clean Air Interstate Rule (CAIR), which was vacated due to legal challenges, to satisfy BART.
    • FIPs for 12 states were finalized to replace reliance on CAIR with CSAPR in the SIPs.
  • May 4, 2016 – EPA proposed revisions to the RHR for various requirements, including actions that states must take for their regional haze SIP and progress report submittal.7



Proposed Revisions to RHR


In accordance with the RHR, states must develop a long-term strategy to reduce regional EQ_Spring 2016_Haze photohaze from all types of anthropogenic sources.  A state’s plan must address the following factors:  1) emission reductions due to ongoing air pollution control programs, 2) measures to mitigate the impacts of construction activities, 3) emissions limitations and schedules for compliance to achieve the reasonable progress goal, 4) source retirement and replacement schedules, 5) smoke management techniques, 6) enforceability of emissions limitations and control measures, and 7) the anticipated net effect on visibility due to changes in source emissions.   


The May 4, 2016 proposed amendments address requirements for the second planning period (2018 - 2028) with several proposed updates.  These updates include, but are not limited to, those listed below.



  • Rolling back the due date for SIPs for the second implementation period to July 31, 2021; the end date will remain 2028, meaning SIPs will still focus on measures to achieve reasonable progress by 2028.
  • Removing the requirement that progress reports be submitted as SIP revisions, but adding the requirement for states to consult with FLMs and obtain public comment on their progress reports prior to submitting to EPA.
  • Changing the due dates for progress reports per state to a fixed schedule.  The second and all subsequent progress reports would be due by January 31, 2025; July 31, 2033;  and every 10 years thereafter.
  • Revising the way in which some days are being selected for purposes of tracking progress towards natural visibility conditions in order to focus on anthropogenic emissions and provide the option to exclude emissions from natural events (e.g., fires).  
  • Adding the requirement that all states (not just those with Class I areas) develop long-term strategies using the four statutory factors to ensure reasonable progress.  



Written comments on the proposed rule changes must be received before July 5, 2016.  Public hearings were held in Washington, DC on May 19, 2016, and in Denver on June 1, 2016.  Some participants at the May 19th hearing objected to the delayed SIP due date, arguing that faster progress is needed to meet the visibility goal.  According to a notice posted on the website on May 24, 2016, EPA sent draft guidance to the Office of Management and Budget’s (OMB) Office of Information and Regulatory Affairs for states on implementation of the second phase of the RHR.  The draft has not been made public but will be released once the OMB review is complete.


For more information, please visit the EPA website.  Contact your local Trinity office at (800) 229-6655 for assistance in preparing comments on FIPs and conducting visibility assessments using the CALPUFF Modeling system and/or the Comprehensive Air-quality Model with extensions(CAMx). 



1 Class I federal areas include national parks (over 6,000 acres), national wilderness areas (over 5,000 acres), and national memorial parks (over 5,000 acres) that were in existence as of August 1977.  These areas are granted special air quality protections under Section 162(a) of the federal Clean Air Act.

2 40 CFR Part 51, Subpart P 1980 Rulemaking - Protection of Visibility.  See 45 Federal Register 80084 (December 2, 1980).

3 40 CFR Part 51, Subpart P, July 1, 1999 Amendments - Regional Haze Rule.  See 64 Federal Register 35713 (July 1, 1999).

4 40 CFR Part 51, Final Rule.  Regional Haze Regulations and Guidelines for Best Available Retrofit Technology (BART) Determinations.  See 70 Federal Register 39104 (July 6, 2005). 

5 40 CFR Part 52, Final Rule.  Finding of Failure to Submit State Implementation 

Plans Required by the 1999 Regional Haze Rule.  See 74 Federal Register 2392 (January 15, 2009).

6 40 CFR Parts 51 and 52, Final Rule.  Regional Haze: Revisions to Provisions Governing Alternatives to Source-Specific BART Determinations, Limited SIP Disapprovals, and Federal Implementation Plans.  See 77 Federal Register 33642 (June 7, 2012).

7 40 CFR Parts 51 and 52, Proposed Rule.  Protection of Visibility:  Amendments 

to Requirements for State Plans.  See 81 Federal Register 26942 (May 4, 2016).