May 10, 1990
 
 
 

 
MEMORANDUM 
 
SUBJECT:  Four Billings Montana Modeling Proposals 
 
FROM:     Joseph A. Tikvart, Chief 
          Source Receptor Analysis Branch (MD-14) 
 
TO:       Douglas M. Skie, Chief 
          Air Programs Branch, Region VIII (8AT-AP) 

     In response to your request, the Model Clearinghouse has reviewed John 
Notar's evaluation of the four proposals for modeling SO2 in Billings/Laurel,
MT.  Our conclusion is that John has done a thorough review and we agree with
his comments and conclusions. 
 
     In our estimation, with the possible exception of the fumigation 
phenomenon, the Billings/Laurel situation can be handled well by models and 
screening techniques recommended in the Guideline on Air Quality Models 
(Revised).  The situation is not unlike many we have dealt with in the Ohio 
Valley and in Appalachia with sources located in river valleys with 
surrounding marginally complex terrain.  These situations generally require 
the application of two models, a simple terrain model such as ISC and a 
complex terrain model such as COMPLEX I.  Intermediate terrain between stack 
height and plume height needs to be evaluated with both of these models and 
the higher of the two estimates chosen. 
 
     Regarding fumigation, we suggest that the technique contained in the EPA
SCREEN model be tried before spending a lot of contract monies in developing,
testing and applying a nonguideline technique.  If the SCREEN results show 
underestimates of the fumigation concentrations, and such concentrations are 
associated with the design value, then perhaps a more rigorous technique could
be pursued. 
 
     We agree with John that it is important to carefully determine whether 
the area is urban or rural before selecting the models to be applied.  We also
agree that priority for spending contract monies should be on the emission 
inventory, including building dimension data needed for downwash calculations.
Also, it might be useful to require collection of very short term, e.g. 3-5
minute, emissions and air quality data so we can understand the potential for
emergency episodes, which has recently been raised as an issue in this area. 
 
     If you have any questions, please contact Dean Wilson at FTS 629-5683.
 
cc:  D. Grano 
     W. Laxton 
     D. Wilson