Model Clearinghouse Information Storage and Retrieval System
Record Details

Issue:  EPA Region VIII is seeking concurrence with the Model 
Clearinghouse on the use of the BETA Adjust U* option that was a 
part of a modeling analysis recently received from the North Dakota 
Department of Health (NDDH) for the Montana-Dakota Utilities Company’s 
R.M. Heskett Station (Heskett) facility located about 10 km northwest 
of Bismarck. NDDH sought permission to use the BETA Adjust U* option 
in a formal request sent to the Regional Administrator under 40 CFR 
Part 51, specifically Section 3.2.2 of Appendix W - Guideline on Air 
Quality Models. The formal request letter from NDDH dated June 22, 2016 
and its attachment are included as part of this concurrence request 
package submitted by Region VIII on June 29, 2016.

C/H Response:  
The Model Clearinghouse concurs that a reasonable justification was 
thoroughly documented and demonstrates that the ADJ_U* Beta option 
in AERMET selected for the Heskett facility modeling demonstration 
performs better than the default regulatory version of AERMET for the 
given application where high modeled concentrations are likely to occur 
under low wind, stable conditions. In the case of the Heskett facility, 
excessive 1-hour SO2 concentrations on terrain (Crown Butte) 
approximately 15 km from the source at elevations above the lowest 
effective stack height were predicted by the regulatory default version 
of the AERMOD Modeling System, specifically during low-wind/stable 
conditions when u* values were relatively small. This parallels similar 
excessive 1-hour SO2 concentrations predicted on distant terrain during 
similar low wind/stable conditions for the Region 1 Schiller and 
Region 3 Wagner situations. Given this model response, the Region 8 
technical report and associated alternative model justification package 
from the NDDH, and our similar concurrences of ADJ_U* for a nearly 
identical circumstances that were also appropriately justified, we agree 
that it was appropriate for the ADJ_U* Beta option in AERMET to be 
considered for this regulatory modeling demonstration at the Heskett 
facility.

While not an issue in this particular case, we continue to evaluate 
the potential that the use of the ADJ_U* Beta option in combination 
with site-specific meteorological data that includes the sigma-theta 
and/or sigma-w turbulence parameters may introduce a bias toward 
concentration underprediction and caution anyone considering the use 
of both the ADJ_U* Beta option and meteorological data that includes the 
derived sigma-theta and/or sigma-w turbulence parameters in regulatory 
applications without consultation and approval from the appropriate 
permitting authority and the respective EPA Regional Office.

Memo:  Model Clearinghouse Review of the Use of the ADJ_U* 
Beta Option in the AERMET Meteorological Processor (Version 15181) for 
the Montana-Dakota Utility Company’s R.M. Heskett Station Modeling 
Demonstration
         Dated:  August 1, 2016
          From:  George Bridgers, Model Clearinghouse Director
                   Air Quality Modeling Group, EPA
            To:  Rebecca Matichuk, Environmental Scientist
                   Office of Partnerships and Regulatory Assistance, Air 
                   Program, Indoor Air, Transportation and Toxics Unit, 
                   EPA Region 8
                 Scott Jackson, Unit Chief
                   Office of Partnerships and Regulatory Assistance, Air 
                   Program, Indoor Air, Transportation and Toxics Unit, 
                   EPA Region 8

Memo:  Request for Approval of Alternative Model: Use of Surface 
Friction Velocity (ADJ_ U*) Non-Regulatory Default Option in 
AERMET/AERMOD version 15181
         Dated:  June 29, 2016

          From:  Rebecca Matichuk, Environmental Scientist
                   Office of Partnerships and Regulatory Assistance, Air 
                   Program, Indoor Air, Transportation and Toxics Unit, 
                   EPA Region 8
          Thru:  Scott Jackson, Unit Chief
                   Office of Partnerships and Regulatory Assistance, Air 
                   Program, Indoor Air, Transportation and Toxics Unit, 
                   EPA Region 8
            To:  George Bridgers, Director of Model Clearinghouse
                   Air Quality Modeling Group, Office of Air Quality
                   Planning and Standards