4 The record articles

Not So Fast – Getting That Air Permit Won’t Be As Easy As You Think

Posted: January 18th, 2012

Authors: Colin M.  Dan D. 


Most of the air regulations promulgated by U.S. EPA are developed and implemented to achieve compliance with the National Ambient Air Quality Standards (NAAQS).  The NAAQS represent ambient air concentration levels that are established by U.S. EPA to ensure that the general public and sensitive subgroups of the general public are not adversely affected by air pollution.  The various NAAQS arguably represent the “backbone” of the air quality regulatory system in the United States.

Over the past 35 years the ambient air has become progressively cleaner as air pollution control programs and technological improvements to reduce emissions from industrial facilities and motor vehicles have evolved.  Although our air is becoming cleaner, U.S. EPA is gaining more knowledge about what levels and types of air pollutants actually affect people.  As a result, it is U.S. EPA’s belief that adverse health effects still occur even at the current low ambient air concentration levels, and that if the NAAQS were lowered further, fewer people would experience air-quality related health issues.

The Clean Air Act (CAA) requires U.S. EPA to conduct periodic reviews of the NAAQS and, if necessary, update each NAAQS to ensure that they are protective of the general public and sensitive subgroups (e.g., children and elderly).  U.S. EPA is in various stages of reviewing and revising the NAAQS for five (5) pollutants (lead, ozone, fine particulate, sulfur dioxide, and nitrogen dioxide).  The revisions to the NAAQS will significantly affect all types of industry.  It is critical for the regulated community to understand what changes are occurring and what the ramifications are with respect to planning successfully for future operations.  Ultimately, these new standards will force emission reductions via the installation of controls or the use of lower pollutant-emitting fuels.   Keep in mind that the changes to the NAAQS are occurring on top of all of the additional regulatory activity involving air emissions.

Of particular importance are the recently promulgated NAAQS levels for particulate matter with an aerodynamic diameter of less than 2.5 microns (PM2.5) and nitrogen dioxide (NO2) and the proposed NAAQS for sulfur dioxide (SO2).  These new NAAQS represent concentration levels that are significantly lower than the previously existing NAAQS and in many cases will impact the feasibility of projects and the way that projects need to be designed.  When the new NAAQS levels are evaluated within the requirements of the New Source Review (NSR) air quality permitting program, they can serve as a mechanism for state agencies and U.S. EPA to use to minimize emissions increases that are allowable from a new project.  As a result, the new NAAQS levels will force industry to rethink currently proposed projects.

For example, in the past, we never hesitated to present a 400 ton per year (tpy) facility-wide increase of oxides of nitrogen (NOX) to a state agency, so long as control requirements meeting the level of Best Available Control Technology (BACT) were applied for the pollutant.  Such a 400 tpy increase typically included a comfortable cushion for compliance and may have been well below the previous emission limit for the source.  The permitting strategy for such a project was developed with the understanding that we would be able to demonstrate compliance with the annual NAAQS.  Now, a 400 tpy emissions increase would be an immediate cause for concern with the 1-hour NO2 NAAQS and would require further consideration and planning, particularly when the changes are occurring at a major facility. [Note: For NSR applicability purposes, NOX emissions are the sum of all oxides of nitrogen, NO, NO2, etc.  For air quality modeling purposes, regulatory precedent has been to consider all NOX emissions as NO2 emissions.]

Of further immediate importance is that U.S. EPA has recently established that no application grandfathering will be allowed for the new NAAQS levels.  If a construction permit was still pending on the date that the 1-hour NO2 standard was promulgated, for example, then the applicant would have been required to re-open the application and address the new NAAQS level, even if that application had resulted in a draft construction permit.  Therefore, the PM2.5, NO2, and SO2 (scheduled for June) NAAQS must all be addressed as part of NSR permit applications that are being prepared right now.  The remainder of this article describes the challenges associated with the new NAAQS levels for permitting in attainment areas, and provides recommendations on how to plan to address them to minimize surprises down the road.

PSD Air Quality Modeling Process

The Prevention of Significant Deterioration (PSD) program requirements apply to projects that are located in areas classified as in attainment with the NAAQS.  Two (2) of the primary requirements for obtaining a PSD construction permit include (a) demonstrating through air quality modeling that the project will not cause or contribute to a violation of the NAAQS, and (b) demonstrating that the project will not consume more of the available increment than allowed by the rule. The NAAQS demonstration typically consists of two (2) air quality modeling evaluations:

  • PSD Significant Impact Area (SIA) air quality modeling analysis.
  • NAAQS air quality modeling analysis.

The first step of the NAAQS demonstration is the SIA air quality modeling analysis.  In this analysis only the project-related emissions are modeled and compared to Significant Impact Levels (SILs).  SILs are ambient concentration levels that below which the project is not assumed to contribute to or cause a violation of a NAAQS.  The following table presents the SILs for PM2.5, NO2, and SO2:

(a) These are the ranges of the SILs that have been proposed by U.S. EPA.

(b) The 1-hour NO2 SIL is currently expected to be in the range of 1 to 5% of the new 1-hour NO2 NAAQS.

(c) The 1-hour SO2 SIL will be proposed after promulgation of the 1-hour SO2 NAAQS in June 2010.

If the modeled ambient concentrations associated with the project-related emissions increases are less than the SILs, then a full NAAQS evaluation is usually not required.  If ambient concentrations exceed the SILs, then a full NAAQS analysis that accounts for facility-wide emissions and other local industrial emission sources (typically within 50 to 60 kilometers of the project location) must be conducted.  The cumulative modeled pollutant concentration associated with the project facility and the other local industrial sources is determined.  In addition, a representative background concentration associated with the project facility and the other local industrial sources is determined, which must be added to the cumulative modeled concentration. The background concentration is obtained from nearby ambient air monitoring stations and should be representative of the “background” sources (i.e., uninventoried pollutant sources).  The sum of the modeled concentration and the background concentration is then compared to the appropriate NAAQS levels which are presented in the following table:

The requirement to add the background monitored concentration is critical since it dictates the ambient concentration level that the facility and the other local industrial sources can contribute without resulting in a potential violation of a NAAQS.  The following subsections describe how the new NAAQS levels relate to the PSD air quality modeling process.

PM2.5 Implications

As described above, a project that triggers PSD permitting requirements for PM2.5 will require a PM2.5 NAAQS demonstration.  The first goal in the evaluation should be to model the project-related emissions increase and to demonstrate that the resulting pollutant concentrations are below the low end of the proposed SIL range for PM2.5.  If the proposed SILs are exceeded, then a full NAAQS demonstration is triggered.  The current PM2.5 NAAQS level presents a challenge for demonstrating NAAQS compliance due to the high background monitored concentrations that are being measured at ambient monitors across the country.  As discussed previously, the sum of the modeled concentration and background monitored concentration is compared to the NAAQS levels.  Therefore, the background monitored concentration will dictate the concentration that can be modeled from the project facility and local sources without resulting in a NAAQS violation.  The following are the NAAQS and typical corresponding background monitored concentrations for PM2.5:

  • 24-hour NAAQS:  35 µg/m3, background concentrations between 25 and 35 µg/m3.
  • Annual NAAQS:  15 µg/m3, background concentrations between 10 and 15 µg/m3.

For both averaging times, the background monitored concentrations represent a majority of the NAAQS (and the entire NAAQS at the high end), leaving little room to add modeled concentrations and still demonstrate NAAQS compliance.  Considering that the permitted emission rates of PM2.5 are modeled as part of a NAAQS analysis, it will be difficult in some cases to demonstrate compliance with the 24-hour and annual PM2.5 NAAQS.

NO2/SO2 Implications

U.S. EPA has established new short-term (1-hour) NO2 NAAQS and proposed a range for a new 1-hour SO2 NAAQS.   The new 1-hour NO2 NAAQS became effective on April 12, 2010 and the new 1-hour SO2 NAAQS is scheduled to become effective on June 2, 2010.  These new NAAQS are, and will be, significantly more restrictive than the currently established standards.  The short-term standards were developed to help protect against adverse health effects related to short-term (i.e., 5-minute) exposure.

The new NO2 and SO2 short-term standards will be difficult to demonstrate compliance with for two (2) primary reasons:

  • Both the new NO2 and SO2 NAAQS are measured on a 1-hour basis, which means that worst-case conditions will dominate the modeled concentrations.  Worst-case conditions include adverse dispersion conditions as well as peak short-term emission rates.  Adverse dispersion conditions could reflect severe meteorological conditions or extreme aerodynamic building downwash influences.  Peak short-term emission rates reflect 1-hour NOX and SO2 emission rates.  The use of a peak 1-hour NOX emission rate will be significantly different than the annual average peak emission rate used for the annual NO2 NAAQS demonstration.  Since SO2 emission rates representative of a 3-hour period are required, there will be less of a difference between the peak short-term emission rates.  Higher emission rates and the potential for adverse 1-hour dispersion conditions to cause high modeled concentrations will make demonstrating compliance with the 1-hour standards difficult.
  • The new 1-hour NO2 NAAQS level of 188 µg/m3 is an extremely stringent standard, as will be the proposed 1-hour SO2 NAAQS of between 100 and 300 µg/m3 (i.e., the promulgated 1-hour NAAQS is not even double the NO2 annual NAAQS and a new 1-hour SO2 standard could be less than the current 24-hour standard).  Since the 1-hour standards are so stringent, the background ambient air concentrations are a significant percentage of the NAAQS and will make NAAQS demonstrations difficult.

As described above, a full NAAQS demonstration is only required if a project triggers PSD requirements and the project-related emission increases result in a modeled concentration that is greater than a respective SIL.  At this time, no SILs have been established for the 1-hour NO2 and SO2 NAAQS.  With respect to NO2, discussions with several state agencies have indicated that a SIL between 1.9 and 9.5 µg/m3 will be promulgated.  Even using the upper bound of the possible NO2 SIL range means that it will be increasingly difficult to model below the SILs because of the conservative nature of modeling a 1-hour average concentration.  Therefore, a full NAAQS analysis would need to be performed, which would include all NOX sources at the facility along with other local industrial NOX sources.  Due to the stringency of the NAAQS levels, it is this full NAAQS evaluation that will present the biggest obstacle to new projects.


The new NAAQS levels are far reaching and have the potential to impact a wide range of new projects across every industry.  As a result, air quality modeling considerations need to be accounted for earlier than ever in the planning process for a new project.  Environmental personnel need to make managers aware of the potential NAAQS issues at the outset of the project planning to minimize surprises during the later stages of project design.  The following steps outline some recommendations to consider during the process in order to comply with the new and proposed NAAQS.

Step 1:

A first step in the NSR/PSD process would be to avoid the NSR/PSD permitting by limiting emissions or offsetting emissions.  For example, will the emissions increases from the project, as preliminarily designed, result in triggering PSD requirements for PM2.5, NO2, or SO2?  If so, are emissions decreases available elsewhere at the facility that could offset the emissions increases?  Emissions decreases could include:

  • Installation of a cost effective control technology that will result in emission reductions, particularly if the control technology will be helpful in addressing another regulatory program (e.g., the proposed new Boiler MACT).
  • Removal of equipment that will no longer be required to operate as a result of the project.
  • Switching to lower emitting fuels or raw materials if it is cost effective to do so.

Step 2:

These considerations can be made early in the process with the goal of avoiding PSD applicability entirely.  If there is no feasible way to avoid PSD requirements, then the focus needs to shift to the NAAQS air quality modeling demonstration.  Think about the following areas that will minimize ambient concentrations and increase the possibility of modeling the project at concentrations below the SILs:

  • Consider emission limits that establish an adequate margin of compliance but are not unnecessarily conservative such that when the emission limits are modeled, the resulting concentrations are above the SILs.
  • Calculate the good engineering practice (GEP) stack height for all stacks being modeled as part of the project.  Stack heights can be modeled at a height up to the GEP height.  If stacks are less than GEP height, an investment in raising the stacks will result in better dispersion (i.e., lower ambient concentrations) and improve the possibility to model below the SILs.
  • If the project includes emission reductions from emission units that are being removed, quantify (with supportable data) the peak 1-hour emission rates of the removed units.  Using continuous emission monitoring system (CEMS) data or short-term records of operation to determine emission rates from decommissioned units may help offset the concentrations from emission increases at new or modified emission units.
  • For modified emission units that expect no increase in short-term emission rates, but that will realize an annual emission increase, document to the appropriate regulatory agencies using actual operating data that there will be no short-term emission increase.

Step 3:

If no refinements can be made to avoid modeling concentrations greater than the SILs, then planning must occur on a facility-wide basis to address the requirement to perform a full NAAQS analysis.  The entire facility would be modeled at its maximum (i.e., permitted) emission rates, so the dispersion from all stacks needs to be considered:

  • Do any emission units typically emit a PSD pollutant at a small fraction of the existing emission limit?  If so, then a tighter emission limit can be taken to reduce the ambient impacts of that emission unit.
  • The same stack height considerations described in Step 2 for keeping modeled concentrations below the SILs should be extended to all emission units at the facility with relatively short stack heights.

ALL4 recommends that major facilities consider performing exploratory NAAQS modeling using the current configuration of the facility to establish whether modifications to permit limits or equipment will be necessary to meet the NAAQS levels in the event that an evaluation is required.  Understanding these issues well in advance of new projects will ensure that no time is lost when a new project or opportunity presents itself.  Before undertaking any air quality modeling study, legal opinion and environmental consultant advice should be considered.


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