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New NO2 air standards prove technically challenging

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On February 9, 2010, the U. S. Environmental Protection Agency (EPA) established a new National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO2). The new NO2 NAAQS is a one-hour standard set at 100 parts per billion (ppb), which is approximately 188 micrograms per cubic meter (μg/m3). Effective on April 12, 2010, the new one-hour standard is intended to protect against peak exposures to NO2, particularly from mobile source emissions associated with major roadways. Even though the new standard is intended to protect against peak exposures from mobile sources, the new standard has significant impacts on industrial point source air quality compliance.

NO2 is the largest component of a group of gases referred to as oxides of nitrogen (NOX). EPA established the first NAAQS for NO2 in 1971 as an annual average set at 53 ppb (100 μg/m3). All areas of the country currently meet this standard. Between 1971 and the present, EPA has reviewed the annual average NO2 standard twice, with no proposed changes or additions to the NO2 NAAQS. The existing annual NO2 NAAQS remains unchanged with this proposal.

The newly promulgated NO2 standard not only establishes a new one-hour averaging period for the NO2 NAAQS, but also establishes a new “form” for the one-hour standard. The form of the new one-hour NO2 NAAQS is the three-year average of the 98th percentile of the yearly distribution of daily maximum one-hour average concentrations.

Immediate Impacts to Industrial Point Sources
With the recent promulgation of the new NO2 NAAQS, all sources (major or minor) with a proposed NO2 emissions increase that were not approved by April 12, 2010 may be impacted by the new standard. Compliance must be demonstrated by performing an air quality dispersion modeling analysis. Based on guidance provided by EPA, some states are requiring NO2 modeling analyses for all permit applications involving NO2 emissions, even for projects that previously might have been exempt from modeling requirements. Many companies that have performed air quality dispersion modeling for the one-hour standard have found demonstrating compliance with the new standard to be challenging, resulting in significant delays and hurdles in major and minor source permit processing and approvals. Some of the main challenges companies have been faced with are summarized below:

  • Higher impacts from relatively low-emitting sources
  • New form of the standard
  • Significant Impact Levels (SILs)
  • Case-by-case approval required for NO2 modeling techniques
  • Lack of clear guidance for modeling methodology

Higher Impacts from Relatively Low-Emitting Sources
Many companies have found that their largest NO2 emitting sources do not always have the largest one-hour impact in the air quality dispersion modeling analysis. Smaller sources, which may also have stacks with the least desirable dispersion characteristics (e.g., short stacks, horizontal releases, low air flows, near ambient temperature exhaust, etc.), may be the largest contributors in one-hour NO2 NAAQS demonstrations. For example, demonstrating compliance with the one-hour NO2 NAAQS at a site with diesel-driven emergency engines located near buildings and/or within close proximity to the fenceline, has been found to be problematic for many sites. Companies will need to closely evaluate emission rates, timing and duration of releases, and stack parameters for these smaller emission sources.

New Form of the Standard
The “workhorse” of near-field dispersion models and the model recommended by EPA’s Guideline on Air Quality Models, AERMOD, is currently not capable of processing results in the form of the new standard (three-year average of the 98th percentile of the yearly distribution of daily maximum one-hour average concentrations). EPA has indicated that a new AERMOD post-processor will be released that will be able to calculate results in the form of the new standard. Additionally, some modeling software, such as BREEZE, have updated post-processors that calculate modeling results in the form of the new one-hour NO2 NAAQS. In the meantime, lack of an approved method for demonstrating compliance in the form of the new standard has led to differing guidance among states in how to best approximate NO2 modeling results for comparison to the form of the standard. A suggested conservative method is to use the highest eighth high (H8H) modeled concentration from AERMOD.

Significant Impact Levels (SILs)
Initially, EPA had not defined or even proposed Significant Impact Levels (SILs) or Class I / Class II Increment Levels associated with the new one-hour NO2 NAAQS under the Prevention of Significant Deterioration (PSD) regulations. This absence of SILs has led to guidance that any increase in NO2 emissions would be considered significant; therefore, a full NAAQS analysis would be required for increases in NO2 emissions. A full NAAQS analysis includes modeling site-wide NO2 emissions, nearby off-property sources, and adding a representative background concentration. Furthermore, a SIL is necessary to determine the distance from the source that should be used in determining which off-property sources to model in the full NAAQS analysis.

On April 21, 2010, the Northeast States for Coordinated Air Use Management (NESCAUM) recommended the use of 10 μg/m3 as an interim SIL. Many states have begun to follow the proposed NESCAUM interim SIL or have established their own interim SIL for use in air quality dispersion modeling analyses. On June 28, 2010, EPA issued guidance recommending an interim SIL of 4 ppb (approximately 7.5 ug/m3). The recommendation by EPA was not intended to supersede state adopted interim SILs but can be used until a SIL has been promulgated for the one-hour NO2 standard.

Case-By-Case Approval Required for NO2 Modeling Techniques
Section 5.2.4 of EPA’s Guideline on Air Quality Models includes a multi-tiered approach method to modeling NO2 emissions based on the fact that NOx is emitted in many forms, not exclusively as NO2. EPA’s Tier 3 screening, which may include the Ozone Limiting Method (OLM) or other similar methodologies, requires the most detailed level of analysis and produces the least conservative, and presumably the most representative, results. One such method is the Plume Volume Molar Ratio Method (PVMRM), which is a modeling technique included in AERMOD that is considered to provide more realistic NO2 modeling results.

PVMRM adjusts NOx emissions to estimate more realistic ambient NO2 concentrations by modeling the conversion of NOx to NO2. Additional information needed to use PVMRM includes the NO2/NO ratio within each NO2 emitting stack (typically 0.05-0.3), the ambient NO2/NOx ratio (0.75-0.9), and background ozone concentrations.

The use of PVMRM as a Tier 3 screening method requires case-by-case approval for use in a modeling analysis. Applicants must obtain approval to use PVMRM for all projects triggering federal permitting requirements, and, for some states, for all projects triggering state permitting requirements.

More conservative modeling techniques, such as modeling all NOx emissions as NO2 or using the Ambient Ratio Method (ARM), which assumes 75 percent of NOx is converted to NO2, are Tier 1 and Tier 2 techniques that would not typically require EPA approval for use in major or minor source permit applications. However, these modeling techniques may overestimate actual NO2 impacts and may be too conservative for a source to demonstrate compliance with the new NO2 one-hour NAAQS.

As an example, the figure below presents a case study of an electric utility generating station that has two main boilers, combustion turbines, emergency engines, and auxiliary boilers that are located inside a large fenced property owned by the utility. While the site-wide modeled average NOx concentration is 20 percent of the annual NO2 NAAQS, the same emissions result in an exceedance of the new one-hour NO2 NAAQS by a factor of two with a Tier 1 analysis. Using a Tier 2 analysis (ARM), site-wide impacts still exceed the one-hour NO2 NAAQS (even prior to the addition of a background concentration). Only by utilizing a Tier 3 analysis (PVMRM) will modeled concentrations meet the one-hour NO2 NAAQS.

However, NO2 background concentrations vary significantly throughout the country (between 11-164 μg/m3 based on 2006-2008 data). Therefore, additional refinements may be necessary to demonstrate that site-wide emissions with an appropriate NO2 background concentration and the addition of nearby off-property sources are compliant with the new NO2 one-hour NO2 NAAQS.

The requirement to obtain EPA approval for PVMRM has begun to slow down the permitting process for many applicants. Estimates of the additional review time needed for PVMRM approval are currently unknown. However, as more applicants submit requests to use PVMRM, agency review times may increase. In addition, the use of lower in stack NO2/NOxratios (<0.3) are expected to require site-specific testing or monitoring data to support the use of the proposed NO2/NOx ratio. This requirement for site-specific data will increase the time needed to prepare a request for use of PVMRM.

Lack of Clear Guidance for Modeling Methodology
Without clear guidance from EPA, the implementation of the new one-hour NO2 standard has been incongruous between states and challenging for industrial sources. In some areas, applicants have found that guidance on demonstrating compliance with the new standard changes on a weekly, or even daily, basis. EPA issued new one-hour NO2 modeling guidance on June 28, 2010. However, the guidance does not provide answers so much as identifies additional questions that will need to be addressed when modeling hourly NO2 emissions.

Additional Impacts from the New NO2 NAAQS

New NO2 Monitors Required

With the new standard, EPA has added requirements for the establishment of new NO2 monitors, summarized below. All new NO2 monitors must begin operation by January 1, 2013.

  • Approximately 126 near-road monitors in 102 urban areas will be added based on 2 criteria:
  • At least one monitor located near a major road in an urban area with a population ≥500,000 people, and
  • A second monitor may be located near a major road if an urban area:
  • has a population ≥2.5 million people, or
  • one or more road segments have an annual average daily traffic count ≥250,000 vehicles
  • Approximately 53 community-wide monitors will be added in urban areas with a population ≥ one million people
  • EPA will identify locations for 40 additional monitors in communities that are determined to be susceptible and vulnerable to NO2 related health effects

Anticipated new NO2 monitoring locations are shown in the following maps.

NO2 Emissions
Based on the current network of NO2 monitors and data from 2006 to 2008, only one monitor in Cook County, Illinois, showed a violation of the new one-hour standard. However, EPA anticipates that new monitors added near major roads could show NO2 concentrations approximately 30-100 percent higher than monitors at non-road locations. According to the U.S. EPA, mobile sources account for 58 percent of all NOx emissions. Utilities are the next largest contributor of NOx emissions, making up 22 percent of all NOx. A pie chart depicting NOx emission sources is shown below. The U.S. EPA expects overall NOx concentrations to decrease as new NOx standards for heavy-duty engines are phased in for 2007-2010 model years.

States have until January 22, 2011 to submit proposed designations under the new one-hour NO2 NAAQS based on the current monitoring network. However, a second designation recommendation will be required after all new NO2 monitors have begun operation (approximately 2015). The implementation schedule for the new NO2 NAAQS is summarized below.

The implementation of the new one-hour NO2 NAAQS will have widespread impacts on industrial point sources that must demonstrate air quality compliance. Compounding the challenge for companies is EPA and state agencies’ struggle with providing clear and consistent guidance to air permit applicants. For the latest guidance in your state, contact your nearest Trinity Consultants office at (800) 229-6655.

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