Dispersion modeling— New downwash calculations change the playing field

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Courtesy of Trinity Consultants

For many people, the terms “stack height” and “dispersion model” do not ring a bell. Others grapple with these terms as they can be challenging factors in the process of obtaining an air construction permit. Have you ever constructed a stack at a specific height due to EPA guidelines and regulations surrounding Good Engineering Practice (GEP) as it relates to air dispersion modeling? Or, have you ever been told that the height of a stack at your facility is a source of high model-predicted concentrations that may be resolved by increasing the stack height? If so, be aware that any stacks historically constructed at specific heights for the purpose of showing model-predicted compliance with air quality standards, a common practice tied to the process of obtaining a construction permit, may present a problem in future modeling analyses.

In April 2011, EPA updated the approved regulatory version of AERMOD.1 Some of the changes were expected, including coding changes to allow for the determination of 1-hour design concentrations for pollutants with new 1-hour NAAQS. Further, most of the changes did not impact concentrations predicted by the model. One change that was not expected, but that has the potential to increase concentrations, was a change to the way dispersion is simulated for stack plumes that are influenced by building downwash. The change has not been widely publicized, which is puzzling considering the potential implications resulting from an increase in model-predicted concentrations for some facilities.

Building Downwash and GEP Stack Height
The term building downwash describes the effect that wind flowing over or around buildings has on plumes released from nearby stacks. Essentially, buildings create a cavity of recirculating winds in the area near the buildings, and these building cavities cause increased vertical dispersion of plumes emitted from stacks on or near the buildings. Building downwash often leads to elevated concentrations downwind of affected stacks.

Cavity regions near buildings are characterized based on building geometry and wind direction. Stacks may be revised to a height such that plumes released from the stacks are outside the cavity region and thus not influenced by building downwash. The stack height at which a plume released from a stack is not excessively affected by downwash is referred to as the GEP stack height. The GEP stack height is the tallest height that may be utilized in the model. Although the actual height of the stack may be taller than GEP height, the stack height entered into the model cannot exceed GEP height.

65 meters, measured from the ground-level elevation at the base of the stack
(i) For stacks in existence on January 12, 1979, and for which the owner or operator obtained all applicable permits or approvals required under 40 CFR Parts 51 and 52,

Hg = 2.5H,

provided the owner or operator produces evidence that this equation was actually relied on in establishing an emission limitation (ii) For all other stacks,

Hg = H + 1.5L

Where:

Hg = GEP stack height, measured from the ground-level elevation at the base of the stack,
H = height of nearby structure(s) measured from the ground-level elevation at the base of the stack,
L = lesser dimension, height or projected width, of nearby structure(s)
The height demonstrated by a fluid model or a field study approved by the EPA, State or local control agency, which ensures that the emissions from a stack do not result in excessive concentrations of any air pollutant as a result of atmospheric downwash, wakes, or eddy effects created by the source itself, nearby structures or nearby terrain features.

The equation in 40 CFR 51.100(ii)(2)(ii) stated above is also known as the “EPA formula height” or “Equation 1 stack height.” In pre-2011 versions of AERMOD, the effects of building downwash were “turned-off” for plumes released from stacks at EPA’s formula height on a wind direction-specific basis. Thus, historically, facilities received a benefit by raising their stacks to EPA formula height to avoid downwash. Absent dispersion techniques or other limitations, raising a stack height to the formula height was an acceptable means of demonstrating compliance in air dispersion modeling analyses.

April 2011 AERMOD Change
Historically, the turning off of downwash at the EPA formula height introduced a discontinuity in the dispersion profile (i.e., a stepwise decrease in concentration for stacks just above or just below the formula height). This discontinuity in the dispersion profile has long been recognized by EPA as an area for improvement. In a new version of AERMOD, released in April 2011, EPA implemented a change to the way that building downwash is considered for stacks at or above the EPA formula height. Specifically, AERMOD no longer turns off downwash above the EPA formula height. Rather, the AERMOD code allows the Plume Rise Model Enhancement (PRIME) algorithms within the model to determine when and how to apply downwash.

Impact of Change
The impacts of the change to the criteria for applying downwash are illustrated with a few simple examples. Consider a facility with a single existing stack at a height just above the EPA formula height. The maximum 1-hour model-predicted concentrations based on both the previous version of AERMOD and the April 2011 version of AERMOD are summarized in the table below:

The maximum concentration predicted by the April 2011 version of AERMOD is much higher than the maximum concentration predicted by the previous version. The reason for the increase in the maximum predicted concentration is that building downwash is not applied to the stack plume in the pre-April 2011 version of AERMOD (since the stack is taller than the EPA formula height and the pre-April 2011 version of AERMOD turns off downwash above the EPA formula height), but downwash is applied in the new version.

Next, consider a facility with a single building and single stack, where the actual stack height is 43 meters and the EPA formula height is 61 meters. Since the existing stack is below the EPA formula height, the plume released from the stack is affected by building downwash. The owner/operator has been contemplating an expansion for a number of years and recently decided to move forward with the expansion. The environmental manager recognized that the expansion would require an air construction permit and that dispersion modeling would be required as part of obtaining the air permit.

In late 2010, the facility was modeled using AERMOD. At that time, the maximum model-predicted concentration for a criteria pollutant based on the existing stack height of 43 meters was higher than an applicable air quality standard. Thus, the modeler considered raising the stack to reduce the maximum model-predicted concentration to less than the standard. The modeler determined that as long as the stack exceeded the EPA formula height of 62 meters, the maximum model-predicted concentration dropped significantly and was less than the standard.

However, recently the facility was modeled again, this time using the updated version of AERMOD. The goal of the modeling was to determine if the stack height determined in late 2010 would still result in modeled concentrations below the standard. Using the updated version of AERMOD, the 62 meter height previously identified as being the height above which the maximum model-predicted concentration dropped below the air quality standard, no longer resulted in concentrations less than the standard. The difference between the two AERMOD versions is the result of downwash. In the previous version, at the EPA formula height of 62 meters, downwash was turned off. In the new version, downwash is still on at 62 meters.

The modeling results described above are summarized in the table below.

The maximum concentration predicted by the model for the 60 meter tall stack is the same for both versions of AERMOD. However, for the 62 meter tall stack, the older version of AERMOD predicts a much lower concentration that the April 2011 version. The example illustrates the benefit that existed when relying on the pre-2011 version of AERMOD by raising the example stack 2 meters, such that the stack is taller than the EPA formula height. The example also highlights the increase in modeled concentrations that can occur when transitioning to the April 2011 version of AERMOD. Had the facility raised its stack to 62 meters in late 2010 to yield compliant model results, future modeling for the facility could yield unacceptable concentrations simply based on the change in the model version.

The complexity of the downwash algorithms and the direction-dependent nature of downwash effects (due to differing building-stack orientations for different wind directions) are such that the magnitude of the differences between the pre- and post-April 2011 versions of AERMOD vary by scenario. One thing that is clear is the change to the treatment of downwash with respect to stack heights in the currently approved version of AERMOD has the potential to cause large increases in model-predicted concentrations.

Regulations vs. Model Change
EPA has positioned the revision to the downwash criteria as a necessary change to eliminate the discontinuity in the dispersion profile for plumes released from stacks that are taller than the EPA formula height. While logic may support the removal of the discontinuity, the process by which EPA made the change is inconsistent with the existing regulations for GEP. The inconsistency is in the regulations that govern the application of dispersion models (Appendix W of 40 CFR Part 51). Appendix W indicates that cavity or wake effects should be determined for stacks below the EPA formula height and is silent in regards to stacks above the EPA formula in weight. The April 2011 version of AERMOD considers cavity or wake effects for stacks of any height, including stacks that are taller than the EPA formula height.

Focusing more closely on the GEP definition, Appendix W suggests that GEP is the height necessary to insure that emissions from the stack do not result in excessive concentrations as a result of downwash.Excessive concentrations is defined in 40 CFR 51.100(kk) which states:

  1. For sources seeking credit for stack height exceeding that established under §51.100(ii)
  2. a maximum ground-level concentration due to emissions from a stack due in whole or part to downwash, wakes, and eddy effects produced by nearby structures or nearby terrain features which individually is at least 40 percent in excess of the maximum concentration experienced in the absence of such downwash, wakes, or eddy effects and which contributes to a total concentration due to emissions from all sources that is greater than an ambient air quality standard.

Simply stated, excessive concentration is a 40% increase in concentration from the concentration that would exist without downwash. The examples provided demonstrate that concentrations predicted using the April 2011 version of AERMOD are more than 40% higher than concentrations predicted using the previous version of AERMOD due to the change in the criteria for applying downwash. Since it has been established that stacks taller than the EPA formula height are needed to avoid excessive concentrations caused by downwash, stacks taller than the formula height should likely be considered GEP, even if they exceed GSM.

Overall, the change to the downwash criteria in the April 2011 version of AERMOD goes beyond the intended application of downwash in Appendix W of 40 CFR Part 51. However, despite the inconsistency, it appears that EPA is moving forward with the new AERMOD version. EPA has indicated it intends to publish a guidance memorandum related to the downwash change.

Conclusion
By changing the downwash criteria used in AERMOD, EPA dramatically changed the playing field for some facilities with respect to the use of AERMOD to determine compliance with air quality standards, as the change can significantly impact model-predicted concentrations for individual facilities. This can cause concern from an air permitting standpoint and for other aspects of air quality analyses, not the least of which is the AERMOD dispersion modeling that states must conduct to assess regional compliance with the 1-hour NAAQS for SO2.

Facilities most likely to be affected by the change include facilities with stacks above the EPA formula height, where plumes emitted from the stack were previously not subject to the influence of building downwash but, as of April 2011, are subject to downwash. There is a growing list of facilities that obtained air permits based on stack heights that resulted in modeled compliance with air quality standards prior to the change in downwash criteria that now have model-predicted concentrations in excess of the standards based solely on the change in model version. For the benefit of those affected by the change, the much anticipated clarification memo from EPA related to the issue may shed some light on this complex situation.

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