A Review of 1-Hour NO2 NAAQS Air Quality Modeling

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Courtesy of Courtesy of All4 Inc.

During the development of this 4TR article, proposed revisions to Appendix W to 40 CFR Part 51 – Guideline on Air Quality Models (Guideline) were published in the Federal Register. As a result of revisions which propose to establish Tier 3 nitrogen dioxide (NO2) atmospheric chemical reaction screening techniques as default methods, applicants will no longer need to gain U.S. EPA approval after the Guideline revisions are finalized. In the meantime, the guidance provided below related to the Tier 3, NO2 atmospheric chemical reaction screening techniques must be provided for U.S. EPA regional approval for permits issued before the Guideline is finalized. The Guideline is anticipated to be finalized in the spring of 2016.

I’ve been involved in quite a few natural gas combined-cycle power plant projects and it seems like the critical issue is related to showing compliance with the 1-hour NO2 National Ambient Air Quality Standard (NAAQS) [188 micrograms per cubic meter (μg/m 3 ) or 100 parts per billion (ppb) based on the average (5 years or 1 year depending on whether you using NWS or onsite data) of the 98 th percentile of maximum daily 1-hour concentrations]. This is especially challenging when evaluating start-up related emissions. As natural gas combined-cycle power plants have become more efficient, it has led to the ability to start-up faster to quickly meet energy demand. Even though these units are more fuel efficient than ever, the fact that they can now be started up in less than one hour (usually daily, Monday through Friday) to meet peak energy demand has actually made complying with the 1-hour NO2 NAAQS more difficult for two (2) reasons:

  1. Since these more efficient units start-up in less than one hour, the start-up emissions can’t be averaged across multiple hours and
  2. The total number of start-ups that occur in a year. 

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