4 The record articles

A Fine Particulate Series (Part 3): PM2.5 Prevention of Significant Deterioration (i.e., Attainment) – What You Need to Know

Posted: January 19th, 2012

Author: All4 Staff 

U.S. EPA’s final rules (and policies) governing the implementation of the New Source Review (NSR) program for fine particulate matter (i.e., particulate matter with an aerodynamic diameter less than 2.5 microns, or PM2.5) are now being implemented.  In the March 2009 issue of 4 The Record, ALL4’s PM2.5 Team discussed the technical basics of PM2.5.  Then, in the April 2009 issue, ALL4’s PM2.5 Team provided details regarding PM2.5 and nonattainment New Source Review (NNSR). Part 3 of the series addresses PM2.5 and Prevention of Significant Deterioration (PSD) and includes a discussion on determining PSD applicability (or non-applicability), PM2.5 precursors, Best Available Control Technology (BACT) considerations, PM2.5 air quality modeling requirements, and timing and cost considerations when planning for PSD projects where PM2.5 emissions may be a factor.


U.S. EPA published final rules governing the implementation of the NSR program for PM2.5 in the Federal Register on May 16, 2008 (the rules became effective on July 15, 2008).  The final NSR rules were introduced to achieve compliance with the National Ambient Air Quality Standards (NAAQS) for PM2.5.  Specifically, the NSR program includes both the PSD rules for areas that are designated as being in attainment with the NAAQS for PM2.5 and the NNSR rules for areas designated as not being in attainment with the PM2.5 NAAQS.  The introduction of the PM2.5 NSR rules in addition to a recent revision to the PM2.5 24-hour NAAQS (resulting in proposed new PM2.5 nonattainment areas) will present several permitting challenges for both existing and new facilities that are located in attainment areas:

  • PM2.5 is a complex pollutant that exists in many forms: filterable, condensable, organic, inorganic, solid, and gas. It can also form in the atmosphere as a result of reactions between precursor pollutants (i.e., NOX, SO2, ammonia, and VOC).  Condensable PM2.5 emissions are not yet required to be included in applicability determinations or in permit limits in some areas. In cases where condensable PM2.5 emissions must be addressed, they are difficult to measure and quantify.
  • PM2.5 emissions data is not widely available, making quantifying PM2.5 emissions to support air quality permitting projects difficult. Revised PM2.5 emission testing methods are just becoming available.
  • BACT determinations will require facilities to evaluate air pollution control technologies that may not have been considered during previous PSD permitting activities.
  • Delegated states that do not adopt the Federal PSD rules by reference have three years to revise their State Implementation Plans (SIPs) to adopt the PM2.5 NSR provisions.  In the interim period, states must use the interim guidance policy in which PM10 is used as a surrogate for PM2.5 which could lead to confusion and delays.
  • U.S. EPA recently administratively stayed a grandfathering provision of the PM2.5 NSR rule that allowed permit applications submitted before July 15, 2008 to address PM10 emissions only.  Many permit applications will need to be reopened and revised to include and address PM2.5 emissions.

This article is a practical guide to help you understand and address these challenges now and includes steps that you can take to prepare for future PM2.5 permitting in attainment areas (even if no new projects are currently planned).

Determining PSD Applicability

The only sources subject to PSD permitting regulations are “major stationary sources” and “major modifications” located in areas designated as attainment or unclassifiable for the NAAQS.  A new source is major if it has the potential to emit (PTE) any pollutant regulated under the Clean Air Act (Act) in amounts equal to or exceeding specified major source thresholds [100 or 250 tons per year (tpy)] which are predicated on the source’s industrial category.  A major modification is a physical change or change in the method of operation at an existing major source that causes a significant “”net emissions increase”” at that source of any pollutant regulated under the Act.

PM2.5 and the PM2.5 precursor pollutants, SO2 and NOX, are regulated under the PSD regulations.  A net emissions increase of 10 tpy of PM2.5 or 40 tpy of SO2 or NOX (as precursors to PM2.5) at a major stationary source will trigger PSD requirements.  SO2 and NOX have always been regulated as PSD pollutants with a triggering threshold of 40 tpy, and their designation as PM2.5 precursors has not changed the 40 tpy PSD triggering threshold.  Currently, an emissions increase for direct PM2.5 emissions does not need to include the condensable fraction as part of a PSD-applicability analysis.  Although not required, several state regulatory agencies are strongly suggesting that applicants include condensable particulate matter in their applications to avoid delays in processing due to public comments.

The previous U.S. EPA administration issued guidance to the state agencies administering the PSD permit program to allow facilities to use PM10 emissions as a surrogate for PM2.5 until January 2011 or until U.S. EPA has approved test methods for measuring both PM2.5 filterable and condensable fractions.  However, U.S. EPA has recently announced that they are reconsidering this decision to allow PM10 to be used as a surrogate for PM2.5.  In addition, U.S. EPA is expected to approve test methods for measuring PM2.5 later this year, thereby eliminating the argument that the PM10 surrogate policy is appropriate because no approved PM2.5 test methods are available.  These factors will shorten the time frame in which the PM10 surrogate policy may be used.  The use of the PM10 surrogate policy as it relates to an individual state’s SIP status is discussed in the following section.

PSD Permit Application Preparation

One of the first steps you should take when trying to determine whether a project may trigger PSD applicability is to determine whether your state agency is a “SIP-approved” or “delegated” authority.  Most PSD permits are issued by state or local air pollution control agencies. States may develop unique PSD requirements and procedures tailored for the air quality needs of each area as long as the program is at least as stringent as U.S EPA’s requirements. A state’s PSD program is defined and codified in its SIP. In some cases, state or local air pollution control agencies have not developed a unique PSD program and rely completely on U.S. EPA’s PSD program. These states are delegated the authority to issue permits on behalf of U.S. EPA and are often referred to as “”delegated states.””  In these states, the new PM2.5 rules, with respect to PSD, were effective immediately on July 15, 2008 when the Federal rules were finalized.

SIP-approved states may or may not have updated their SIP to include PM2.5 and its precursors into their PSD permitting programs and may not require the applicant to address PM2.5 emissions as part of their application.  In these states, the PM10 surrogate policy will likely remain in use until their SIP is revised to account for PM2.5 emissions (provided that U.S. EPA does not discontinue the use of the PM10 surrogate policy first).  If you are in doubt about the status of your state’s PSD permitting program, contact the state agency directly and seek guidance.

During the engineering/project definition phase of a project, facilities should begin to take steps in preparation of a PSD applicability review.  Questions to ask early on in the project planning process include, but are not limited to, the following:

  • What data is available for developing my PM2.5 emission rates?
  • Should I conduct testing to determine PM2.5 emission rates?
  • What test methods are most suitable for my process?
  • How do I find a testing company that is experienced in testing for PM2.5?
  • Do I need to determine and include emissions of both PM2.5 filterable and condensable fractions?
  • Can I submit emissions of PM10 instead of PM2.5 (review the applicable state’s regulatory status)?

These questions were all addressed in ALL4’s Practical Guide to PM2.5, and should be discussed with the appropriate state agency prior to initiating any permitting exercise.

PSD Permitting Requirements

A project triggering PSD applicability is required to obtain a PSD air permit prior to beginning actual construction.  The PSD air permitting requirements apply to direct PM2.5 emissions as well as to SO2 and (and possibly NOX) as individual PM2.5 precursors. Please note that a major modification for a PM2.5 precursor does not trigger PSD requirements for direct PM2.5 (e.g., BACT, modeling).  The PSD permitting program is more rigorous than most state air permitting programs and among other requirements must include the following:

  • Best Available Control Technology (BACT) – A BACT analysis is a case-by-case analysis of potential emissions control devices or technologies that must be performed for each new or modified emission unit for each individual pollutant it emits that is PSD significant.  The analysis must address the technical feasibility of end control option and include a determination of the annual cost for the control system or technology per ton of regulated PSD pollutant controlled.  If a control device offers control of more than one regulated pollutant (e.g., a wet scrubber controlling SO2, PM2.5, and acid gases), some agencies may require that the control cost evaluation consider the removal of all regulated pollutants, and not just the removal of the PSD pollutant triggering the need for a BACT analysis.  This can result in a control technology that will be more likely to be considered cost-effective and therefore required for installation as part of the PSD-triggering project.  Because PM2.5 can include both filterable and condensable fractions, ALL4 anticipates that “non-traditional” particulate matter controls such as condensers and wet scrubbers will need to be considered as part of any PM2.5 BACT analysis.
  • Air Quality Modeling – An applicant must conduct an ambient air quality impacts analysis for the proposed new source or modification.  Air quality modeling requirements are discussed in the following section.

PSD Air Quality Modeling Requirements

Projects that trigger PSD permitting requirements must address the potential impacts of the project on local and regional ambient air quality.  Two of the required analyses include a Significant Impact Analysis (SIA) and a NAAQS evaluation. Both analyses require the use of specific air quality dispersion models and representative meteorological data to complete. These analyses must now address direct PM2.5 and PM2.5 precursor emissions for major modifications involving these pollutants, thereby complicating the application process. Each analysis is described below.

In the SIA, the net emissions increases associated with the project are modeled1 and the results are compared to the appropriate significant impact level (SIL) for the specific pollutant.  If the resulting increase in ambient concentrations exceeds the SILs, the applicant is required to continue the evaluation to demonstrate compliance with the NAAQS and the PSD Increments.  If the resulting concentrations are below the SILs, then no further evaluation is required.

In the NAAQS evaluation, the PTE of the entire facility (including emission units that are not impacted by the proposed project) in addition to other local emission sources that emit the PSD significant pollutant are modeled. The estimated cumulative concentrations resulting from your facility and other local facilities are summed with the ambient background concentrations for the PSD significant pollutant.  Ambient background concentrations are typically obtained from a nearby U.S. EPA or state agency-operated ambient monitoring station.  The sum of the modeled cumulative concentrations and the ambient background concentrations are then compared to the NAAQS level for the PSD regulated pollutant.

In addition to the listed evaluations, you must also demonstrate compliance with PSD Increment levels and air quality related values (AQRVs) at nearby Class I areas (i.e., national parks and other protected areas), and ensure that a site-specific pollutant monitoring network is not required prior to the project.  PM2.5 emissions have historically been accounted for as part of Class I evaluations, and will continue to be included in future air quality modeling demonstrations.  The following subsection focuses exclusively on the challenges associated with the SILs and NAAQS evaluations.

  • Air Quality Demonstration Challenges

Many facilities are accustomed to completing the air quality modeling demonstration described above for long established NSR-regulated pollutants (NOX, SO2, etc.).  PM2.5 presents a number of unique challenges related to air quality modeling.

Significant Impact Levels (SILs)

U.S. EPA proposed SILs for direct PM2.5 emissions in September 2007 that have not been finalized.  The SILs are an important piece of the air quality modeling demonstration, since they dictate whether a full NAAQS and PSD Increment evaluation are required.  These additional evaluations extend the permit application submittal schedule, increase the complexity and costs of completing the air quality modeling demonstration, and increase the difficulty of demonstrating compliance for the proposed project.  U.S. EPA proposed three approaches to establish 24-hour and annual averaging time SILs for PM2.52.

The proposed SILs are so restrictive that a relatively minor direct PM2.5 emissions increase could trigger costly and complex NAAQS and PSD Increment modeling.  In a selected example air quality modeling exercise, a 20 tpy direct PM2.5 emissions increase from a 50 foot, 150 °F, 10,000 acfm stack resulted in a predicted concentration slightly higher than the highest of the proposed annual PM2.5 SILs.  Although these stack parameters represent only an approximate example, it is enough to indicate that a relatively small direct PM2.5 emissions increase could result in a SIL exceedance under specific combinations of exhaust parameters, building downwash, and meteorological and terrain conditions.

NAAQS Demonstration

As described above, a NAAQS demonstration requires you to sum the following concentrations for comparison to the NAAQS:

  • The cumulative predicted concentrations resulting from the PTE of PM2.5 from your facility and other surrounding facilities.
  • The background ambient PM2.5 concentrations obtained from a nearby ambient monitoring station.

This approach means that the modeled PM2.5 concentrations from your facility combined with the local emission sources cannot exceed the difference between the PM2.5 NAAQS and the background ambient PM2.5 concentrations.  For other PSD-regulated pollutants, the background ambient concentrations typically represent only a fraction of the NAAQS (i.e., less than 50% of the NAAQS).  PM2.5 is unique because the background ambient concentrations in many PM2.5 attainment areas represent a much larger percentage of the PM2.5 NAAQS.

A sample of 10 listed PM2.5 attainment counties across Pennsylvania and Ohio, in which PM2.5 monitoring data were available, indicated that annual PM2.5 2008 background concentrations were equal to between 68% and 84% of the annual PM2.5 NAAQS (i.e., annual concentrations between 10.2 and 12.6 ug/m3).  This example illustrates the small margin within which facilities can model PM2.5 concentrations (including nearby facilities).  In these particular counties, the contribution of the project facility and local facilities could contribute PM2.5 concentrations anywhere from 2.4 to 4.8 ug/m3 without resulting in a modeled NAAQS violation.  This range of contributions is not significantly higher than the proposed PM2.5 annual SILs.  A collection of facilities that emit PM2.5 from stack and fugitive sources could easily exceed these thresholds (in this case the annual threshold), making demonstrating NAAQS compliance difficult in the absence of refined emissions control or operational strategies.  Also note that annual PM2.5 NAAQS is being reviewed and could become more stringent.  In short, there is not much room to grow.


Like other aspects of the PM2.5 NSR rules, attainment area permitting presents a number of unique challenges.  The best way to address these challenges is to plan ahead.  Planning should include understanding the PM2.5 data that you have available, and understanding how any potential projects will interact with the challenges described in this article.

End Notes

1 The emissions increase that is modeled for an SIA must include any emissions increases from new emission units, modified emission units (i.e., physical changes or changes in the method of operation of units), and affected emission units (i.e., a modification to one unit impacts the throughput to another unchanged unit).  The modeled emissions increase must also include increases associated with contemporaneous projects that are unrelated to the current change.

2 The following summarizes the three (3) proposed PM2.5 SILs options:

  • Set the PM2.5 SILs equal to the current PM10 SILs, consistent with the proposed 1996 NSR Reform rules.  In this case, the annual and 24-hour averaging time SILs would equal 1 and 5 micrograms per cubic meter (ug/m3), respectively.
  • Scale the PM2.5 SILs using the ratio of PM2.5 to PM10 emissions from the 2001 National Emission Inventory (NEI) database (annual SIL: 0.08 ug/m3; 24-hour SIL: 4.0 ug/m3).
  • Scale the PM2.5 SILs using the ratio of the PM2.5 NAAQS to the PM10 NAAQs (annual SIL: 0.3 ug/m3; 24-hour SIL: 1.2 ug/m3).

Please contact ALL4’s Colin McCall at 610.933.5246 x20 or cmccall@all4inc.com with any questions related to PM2.5 permitting in attainment areas.


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