Model Clearinghouse Information Storage and Retrieval System
Record Details

Issue:  Region IV is seeking concurrence from the Model 
Clearinghouseon on the use of an approach proposed by the State of 
South Carolina Department of Health and Environmental Control 
(SCDHEC) on behalf of Nucor Steel in Darlington, South Carolina, 
to address two issues within AERMOD (version 16216r) related to 
buoyant line sources. Region IV staff have reviewed the proposed 
approach and corresponding equivalency demonstration submitted 
by SCDHEC and has determined that it is justifiable and appropriate.

C/H Response:  The Model Clearinghouse has reviewed Region 4’s technical 
summary and recommendations for approving the use of an alternative model 
technique proposed by the SCDHEC to address two identified issues with 
modeling buoyant line sources using the preferred near-field model, 
AERMOD for the PSD compliance demonstration of the Nucor Steel facility 
in Darlington, South Carolina. First, the current version of AERMOD, 
version 16216r, is limited to a single group of buoyant line sources that 
are described with identical characteristics (e.g., requires an average 
building width, line length, and buoyancy parameter and assumes all lines 
are parallel to each other with an average separation distance), which 
creates a limitation in modeling a facility with multiple buoyant line 
sources with dissimilar characteristics in one model run. Second, the 
modeling consultant for the Nucor Steel facility identified a coding 
issue or bug within AERMOD that adds the modeled NO2 impacts from buoyant 
lines sources to other modeled NO2 impacts after the Tier 2 and 3 NO2 
methodologies are applied to the modeled NOx concentrations from other 
source types. This results in an inappropriate NOx to NO2 conversion 
calculations for the modeling simulation. Both of these modeling issues 
are subsequently addressed through a multi-step alternative modeling 
technique using the AERPOST program as summarized by Region IV in your 
concurrence memorandum and more fully described in the documentation and 
equivalency demonstration provided by the SCDHEC. Considering that both 
Region IV and the Model Clearinghouse found equivalency consistent with 
Appendix W, Section 3.2.2(b)(1) for the alternative modeling technique 
proposed by the SCDHEC, we concur with Region IV on the alternative model 
approval.

Memo:  Model Clearinghouse Review of Alternative Model Techniques to 
Address Two Issues with Modeling Buoyant Line Sources in AERMOD 
for the PSD Compliance Demonstration of the Nucor Steel Facility 
in Darlington, South Carolina
          Dated: March 13, 2018
          From:  George Bridgers, Model Clearinghouse Director
                   Air Quality Modeling Group, EPA
            To:  Christopher M. Howard, Physical Scientist
                   Air Analysis and Support Branch, Region 4

Memo:  Model Clearinghouse Concurrence Request - AERMOD Buoyant Line 
Source Issue Resolution and Model Equivalency Demonstration Nucor 
Steel, Darlington, South Carolina PSD Air Quality Modeling
          Dated: February 27, 2018
          From:  Christopher M. Howard, Physical Scientist
                   Air Data and Analysis Section
                   Air Analysis and Support Branch
                   Air, Pesticides & Toxics Management Division
                   US EPA Region 4
            To:  George Bridgers, Director
                   Model Clearinghouse
                   Air Quality Modeling Group
                   Office of Air Quality Planning and Standards
                   US EPA

Memo:  Proposed Workaround Related to Buoyant Line Source 
Implementation Issues in AERMOD
          Dated: December 13, 2017
          From:  Richard Hamel
                   Environmental Resources Management (ERM)
            To:  John Glass
                   South Carolina Department of Health and Environmental 
                   Control (SCDHEC)