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PM2.5 Air Quality Modeling Updates

Posted: April 30th, 2013

Author: All4 Staff 

There has been much activity over the past six (6) months that will impact how air quality modeling analyses will be conducted for Prevention of Significant Deterioration (PSD) permit applications with respect to particulate matter less than 2.5 microns (PM2.5).  As the resident air quality modeling expert at ALL4, I thought it was a good time to walk through each of these changes and discuss the potential challenges and yes, opportunities.  Here is a list of what’s been happening over the last six (6) months:

  • December 14, 2012 – U.S. EPA announced revised annual average PM2.5 National Ambient Air Quality Standards (NAAQS) reduced from 15 mg/m3 to 12 mg/m3,
  • December 17, 2012 – U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS) releases regulatory modeling update to AERMET and AERMOD (aka Version 12345),
  • January 22, 2013 – DC Circuit Court vacated and remanded PM2.5 Significant Monitoring Concentrations (SMCs) and Significant Impact Levels (SILs),
  • March 4, 2013 – U.S. EPA OAQPS issues draft PM2.5 modeling guidance for Regulatory Applications.

Each is briefly discussed below.

Updated Annual PM2.5 NAAQS

There are really only challenges associated with the reduction of the annual PM2.5 NAAQS from 15 mg/m3 to 12 mg/m3.  This is especially true for NAAQS demonstrations for PSD permitting projects which require the inclusion of background PM2.5 concentrations from representative monitoring stations.  I have examined PM2.5 monitoring data across the country, and typically the monitors are showing concentrations at 75% or more of the 24-hour and annual PM2.5 NAAQS.  Recommendations regarding what to do about high PM2.5 monitoring data are presented in the DC Circuit Court decision summary below.  Looking on the bright side, U.S. EPA concluded that the existing 24-hour standard of 35 mg/m3 will remain, and that means per the Clean Air Act Section 112 requirements the PM2.5 NAAQS won’t be evaluated again for five (5) years.  One recommendation I can give around the annual PM2.5 standard related to PSD permit modeling is to be absolutely sure that you are modeling annual emission rates.  This likely means developing both short-term and annual PM2.5 emission rates to model against the respective PM2.5 NAAQS averaging periods.  Also, be aware that any PM2.5 emission rate that is modeled as part of a PSD permitting project has the potential to become a limit in your Title V Operating Permit.

Updated AERMOD Air Dispersion Model and AERMET Meteorological Preprocessor

OAQPS released a new version of the U.S. EPA preferred AERMOD [Version 12345 (it is alleged that OAQPS waited until the 345th day of 2012 just to get that version)] air dispersion model used for near-field (i.e., < 50 kilometers) air quality modeling analyses.  The new version addresses some nagging “bugs” and also has added some BETA (non-default) options to address concerns regarding model performance under low wind speed conditions.  Specifically, the beta options allow modelers to do the following:

  • Increase the minimum value of sigma-v from 0.2 to 0.5 meters per second (m/s) and turn off the meander component,
  • Increase the minimum value of sigma-v from 0.2 to 0.3 m/s while maintaining the meander component,
  • Adjust the minimum sigma-v and minimum wind speed values within certain ranges.

What does this mean for all the non-modelers out there?  Essentially, these options allow modelers to evaluate issues of elevated predicted AERMOD concentration that result from periods of low wind speed events.  This issue has been under the microscope ever since the 1-hour sulfur dioxide (SO2) and nitrogen dioxide (NO2) NAAQS were introduced.  While these options are non-default,(meaning that approval from the U.S. EPA Region is required for their use), U.S. EPA is taking recommendations on how to update these equations for possible incorporation into a 40 CFR Part 51 Appendix W – Guideline on Air Quality Modeling proposed update scheduled for late 2014 early 2015.

I encourage everyone to keep in mind that the AERMOD air dispersion model is only as good as the data you supply it and onsite meteorological data [versus National Weather Service (NWS)] will always result in more accurate (not necessarily lower) predicted concentrations.  For more information on setting up an on-site meteorological monitoring system, check out ALL4’s capabilities here.

DC Circuit Court vacated and remanded PM2.5 Significant Monitoring Concentrations (SMC) and Significant Impact Levels (SIL)

The DC Circuit Court vacated and remanded the PM2.5 SMC and SILs on January 22, 2012.  For a summary of this Court decision check out All4’s February 7, 2013 blog post.  Before I talk about the implications of this Court decision, let’s explain what SMCs and SILs are.  U.S. EPA established a 24-hour average 4 mg/m3 PM2.5 SMC in the 2008 PM2.5 PSD rule.  This meant that if a PSD permit application could show that predicted PM2.5 concentrations modeled from project related PM2.5 emissions were below 4 mg/m3, no pre-construction monitoring would be required for the project.  The vacatur of the PM2.5 SMC means that if a project exceeds the PM2.5 PSD significant emissions increase threshold(i.e., 10 tons per year), one (1) year of pre-construction ambient monitoring is required.  Let’s not get ahead of ourselves; the reason most people haven’t heard of SMCs is because usually an argument can be made to use representative ambient monitoring data from a State-run ambient monitoring station when SMCs were exceeded in the past.  This is still true; however, be aware that the State and possibly public reviewers (i.e., environmental groups) are going to pay closer attention to whether the State ambient monitoring stations are representative of the conditions at the applicant’s site.  Conversely, preconstruction PM2.5 monitoring might just be beneficial to a project.  As I alluded to during my discussion on the recent PM2.5 NAAQS revision above, PM2.5 ambient monitoring levels are typically very close to the NAAQS, leaving little room for applicants to model their facilities below the PM2.5 NAAQS.  Pre-construction ambient monitoring can be used to develop site specific background levels for your project.  It is important to always site a meteorological monitoring station with an ambient monitor, because onsite background levels for your site will be determine when winds are blowing away from your PM2.5 ambient monitor and not influencing measured concentrations.  The goal is to not double-count in a NAAQS air quality modeling demonstration, and the best way to do that is to have site specific monitoring data.

PM2.5 SILs were also established in the 2008 PM2.5 PSD rule.  The PM2.5 SILs were/are 0.3 mg/m3 and 1.2 mg/m3 for the annual and 24-hour average PM2.5 NAAQS, respectively.  SILs are used to determine if a full multi-source air quality modeling analysis would be required for a PSD permit application.  If predicted concentrations of project-related PM2.5 emissions are below the SILs, no NAAQS or PSD increment analysis is required.  The Court decision for the SILs is slightly different than the SMC as they were remanded and not vacated.  The intent of the remanded SILs is that a permit applicant cannot rely alone on the SILs as the sole reason for not conducting a NAAQS and PSD increment analysis.  The applicant must also show that predicted concentration below the SILs also will not lead to a violation of the PM2.5 NAAQS.  Essentially, the concern is due to elevated PM2.5 monitoring levels because an increase below the SILs could still result in a NAAQS violation when combined with high monitored levels.  Therefore, applicants can still use the SILs but will have to supply further justification to show that no PM2.5 NAAQS violation will occur.  It is important to note that SILs also still exist for the other pollutants modeled as part of a PSD permit applications [i.e., PM10, NO2, SO2, and carbon monoxide (CO)]. However, SILs for these pollutants were never published in a rulemaking. Instead, they were issued in OAQPS guidance memorandums.  While they weren’t addressed in the recent Court decision, you can be sure that the same scrutiny will be applied to them as well.  A question and answer document prepared by OAQPS going into further detail about the Court decision ramifications can be found here.

Draft Guidance for PM2.5 Permit Modeling

On March 4, 2013 OAQPS released a “Draft Guidance for PM2.5 Permit Modeling” memorandum.  The memorandum provides additional clarification on addressing both primary and secondary formed PM2.5 in NAAQS and PSD Increment air quality modeling demonstrations.  I gave a summary of the memorandum in a March 4, 2013 blog post.  Since then the OAQPS has taken comments during the Air and Waste Management Association’s (AWMA) Specialty Modeling Conference that ALL4 attended on March 19-21, 2013 and the Regional, State, and Local (RSL) Modelers’ workshop on April 21-25.  It was announced after the AWMA conference that a 45-day extension has been made to the comment period originally scheduled to end May 31, 2013.  All comments should be e-mailed to George Bridgers at OAQPS.

What Does This All Mean?

Plan Ahead!!  A number of changes and uncertainties have the potential to significantly extend the time needed to obtain a PSD construction permit for a project.  In some cases this time period can be as much as a year to collect monitoring and meteorological data before the application process can even begin.  Know where your facility stands with the PM2.5 NAAQS.  Even if a project is not currently in the works it’s always helpful to see where things stand to determine if undertaking an ambient monitoring or meteorological monitoring program would be beneficial for future projects.  Determine where there are potential decreases in PM2.5 emissions at your facility.  The best case scenario for a project is to not have a significant emission increase in PM2.5 emissions and not have to go down any of these roads.


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