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U.S. EPA also Proposing Secondary Visibility NAAQS With Updated Annual Primary PM2.5 NAAQS

Posted: August 2nd, 2012

Author: All4 Staff 

In July, ALL4 posted a blog regarding U.S. EPA’s June 14, 2012 proposal to strengthen the National Ambient Air Quality Standards (NAAQS) for fine particle (PM2.5) pollution.  As part of the proposed rule which is proposing to strengthen the annual PM2.5 NAAQS within a range of 12 micrograms per cubic meter (µg/m3) to 13 µg/m3, U.S. EPA is also proposing a secondary visibility NAAQS within a range of 28 to 30 deciviews (dv).  Unlike a primary standard which provides public health protection, including the protection of health of “sensitive” populations such as asthmatics, children, and the elderly, the secondary standard provides public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.  Many people may remember the term dv from the Best Available Retrofit Technology (BART) requirements from a couple of years ago that were part of the Regional Haze Program.  For those of you lucky enough not to be involved in this program, a dv is a unit of visibility proportional to the logarithm of the atmospheric extinction or more simply put, it is a measure of a noticeable change in visibility.  The dv scale is near zero (0) for a pristine atmosphere and a “just noticeable” change in visibility would be approximately one (1) or (2) dv.  Visibility is measured by the total light extinction of the sum of light scattering and light absorption components.  PM2.5 causes the vast majority of light scattering along with atmospheric, or Rayleigh scattering which is natural scattering not related to air pollution, and light absorption due to particles and gases. 

The proposed secondary visibility standard is different from the BART requirements and Prevention of Significant Deterioration (PSD) Air Quality Related Value Analysis (AQRV) requirements because it will affect all counties in the U.S. not just Class I areas.  U.S. EPA has indicated that only three (3) counties would not meet the proposed 30 dv secondary visibility NAAQS and an additional six (6) would not meet the proposed 28 dv secondary visibility NAAQS.  Additionally, U.S. EPA believes that by 2020 only three (3) counties would not meet the proposed 28 dv secondary NAAQS based on expected reduction of visibility impairing pollutants from several current and upcoming federal programs including the Regional Haze Regulations and Guidelines for BART determinations.  For the primary proposed annual PM2.5 NAAQS U.S. EPA anticipates making attainment/nonattainment designations by December 2014, with those designations likely becoming effective in early 2015.  However, the Clean Air Act does not specify a date for states to meet secondary NAAQS; U.S. EPA and States determine that date through the NAAQS implementation planning process. 

U.S. EPA initially intends to rely on a surrogate policy for the proposed secondary visibility NAAQS under the PSD program.  In their opinion, there is sufficient evidence to suggest that a demonstration that a source will not cause or contribute to a violation of the mass-based 24-hour primary PM2.5 NAAQS serves as a suitable surrogate for demonstrating that a source does not cause or contribute to a violation of the proposed secondary visibility NAAQS under the PSD program.  However, past history suggests that surrogate policies are temporary and it appears likely that the use of complex visibility modeling and its inherent uncertainties will at some point become routine for projects triggering PSD.


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