Updated Guidance for Sulfur Dioxide Area Designations: Part 2- Dispersion Modeling Option

EPA’s Updated Guidance for Sulfur Dioxide Area Designations makes reference to two options to determine  SO2 area designations in areas with large SO2 sources.  These choices include dispersion modeling and ambient monitoring.  Of these two options, dispersion modeling is the fastest and most cost effective one to characterize SO2 air quality.  However, this compliance demonstration can be very challenging given that, in many cases, AERMOD tends to produce overly conservative concentration estimates. Facilities that meet the criteria in Round 2 (i.e., sources that either: emitted more than 16,000 tons of SO2 in 2012 or; emitted more than 2,600 tons of SO2 and had  an annual emission rate of at least 0.45 lbs SO2/MMBtu in 2012; and have not been announced for retirement) should take a proactive approach and perform dispersion modeling of their facility to assess compliance with the 1-hour SO2 NAAQS. The sooner a facility starts working on their modeling analysis, the more time it will have to address and mitigate any potential issues.

Even if compliance cannot be achieved through modeling, this work will not be in vain since the results can be used for siting of ambient monitors which is the last option available for facilities not able to model compliance. In either case, the following dates must be kept in mind:

Dispersion modeling option:

  • 09/18/2015: Deadline for states to provide updated recommendations and supporting information to EPA
  • 04/08/2016: Deadline for states to provide additional information that it wants EPA to consider before final designations are issued

Monitoring option:

  • 07/01/2016: Annual monitoring network plans due to EPA regional administrator
  • 01/07/2017: SO2 monitors are required to be operational

December 2013 Modeling TAD 

According to the December 2013 Modeling TAD, dispersion modeling for SO2 attainment designations is an appropriate tool.  This modeling could include the use of AERMOD or other appropriate dispersion model. Additionally, the four main changes from the previous modeling guidance (issued March 2011) are as follows:

  1. Use of actual instead of allowable emissions (i.e., PTE) to assess violations of the standard
  2. Use of 3 years of meteorological data instead of 5
  3. Receptor placement only in locations where monitor could be placed
  4. Use of actual stack height instead of GEP stack height

These changes are aimed at obtaining more representative results that can be comparable to the monitor.  It is worth mentioning the difference between attainment designations modeling and attainment plan modelingAttainment designation modeling addresses current air quality (simulating a monitor) while attainment plan modeling is used to assure attainment will occur in the future.  This is the reason EPA provides to justify these four recommendations that deviate from past guidance.

Advanced Modeling Techniques

EBD: Facilities finding it challenging to achieve compliance through modeling should evaluate the building profile output file generated by BPIP. In many cases the building parameters generated by BPIP tend to overestimate the downwash effects in the model significantly.  This is especially true for complex and elongated buildings, streamlined structures (e.g., hyperbolic cooling towers), and lattice structures.  For these cases, building parameters generated with Equivalent Building Dimensions (EBD) can have a significant improvement in predicted concentrations- reductions of more than half are common. EBDs are the dimensions (height, width, length and location) that are input into AERMOD in place of BPIP dimensions to more accurately predict building wake effects. EPA considers wind tunnel EBD studies to be source characterization studies not subject to the requirements of alternative modeling (i.e., Section 3.2 from Appendix W, “Use of Alternative Models”).  This advanced modeling technique is able to refine the inputs to AERMOD with the best available science. Therefore, EBD is a better and more economical option to achieve compliance while still protecting ambient air quality.  Watch a video on how EBD works here.

HYWINMOD: Another option available to overcome some of the overstated results from AERMOD is the use of HYWINMOD.  This is a model that combines the theoretical merging of wind tunnel model predictions and AERMOD plume rise and dispersion algorithms to allow for accurate concentration estimates for any averaging time for direct comparison with health limits, odor thresholds and/or NAAQS. An evaluation against two field data bases was presented in a 2013 AWMA paper. The results show that HYWINMOD agrees as well with field observations as AERMOD, and in some case better than AERMOD. The use of this method is ideally suited for complex building or terrain configurations where AERMOD is not appropriate (i.e., urban area, very complex building configuration, upwind terrain wakes, etc.). HYWINMOD can be used to better characterize the SO2 emissions expected. Therefore, according to the December 2013 Modeling TAD, this may be a more appropriate tool that can be used to characterize SO2 designations instead of AERMOD.

Don’t hold your breath for the SO2 DRR

A critical element in the SO2 designation process is the guidance for states to carry out either of the two options for SO2 designations (i.e., modeling or monitoring). This guidance is the anticipated SO2 Data Requirements Rule (DRR).

However, the Updated Guidance for Area Designations states the following about the SO2 DRR.

“The last two deadlines for completing remaining designations are December 31, 2017, and December 31, 2020. The designations completed by these later deadlines are expected to be informed by information provided by states pursuant to the anticipated SO2 DRR.3 However, even if the SO2 DRR is not finalized, the EPA must still complete designations pursuant to these deadlines.“ 

The footnote to this also adds:

“As a legal matter, the deadlines in the court order apply even if the EPA does not finalize the SO2 DRR. However, the general approach described in this memorandum reflects our expectation that the EPA will in fact finalize the SO2 DRR.”

This situation is troubling because it means that there is the possibility that EPA will have to make SO2 attainment designations without final guidance.  Needless to say, this would create confusion and uncertainty for many facilities that choose either modeling or monitoring to evaluate 1-hour SO2 NAAQS.  Also, even if the SO2 DRR is finalized, if it is not done promptly, it could invalidate the modeling approach followed by facilities should significant changes be included.

**June 25, 2015  UPDATE**

Mike Koerber, Associate Director, EPA Office of Air Quality Planning and Standards (OAQPS) mentioned that EPA hopes to get the final DRR by fall of this year (2015). Comments made during a panel session at the A&WMA’s 108th Annual Conference & Exhibition in Raleigh, NC.  

Updated Guidance for Sulfur Dioxide Area Designations: http://epa.gov/airquality/sulfurdioxide/pdfs/20150320SO2designations.pdf


Major SO2 facilities that meet the criteria in Round 2 need to be proactive and work with their state regulatory agency in the SO2 attainment designation process. There are only two opportunities rapidly approaching for the states to provide modeling information to EPA to inform SO2 designations in this 2nd Round. These dates are: September 18, 2015 and April 8, 2016. SO2 sources that are not able to model compliance with the 1-hour SO2 NAAQS will have ambient monitoring as a last resort to prove compliance.  The costs of such monitoring option will be significant compared to dispersion modeling. However, before going down the ambient monitoring road, facilities should consider the use of EBD tunnel analyses and HYWINMOD. These advanced modeling methods can provide more realistic results that can avoid a costly monitoring network to prove compliance with the NAAQS.

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