November 6, 1992


MEMORANDUM

SUBJECT:  Proposal to Use ISCRDT to Model Intermediate Terrain
          (Boise Cascade, Rumford, Maine)

FROM:     Dean A. Wilson, Meteorologist/Model Clearinghouse Coordinator
          Techniques Evaluation Section, SRAB (MD-14)
          
TO:       Brian Hennessey, Regional Modeling Contact
          Techniques Assistance Section, Region I


     In response to your request, the Model Clearinghouse has reviewed your
position with respect to the equivalency between applying the proposed
ISCRDT model and applying the Industrial Source Complex Short Term (ISCST)
model and the Rough Terrain Dispersion Model (RTDM) in accordance with the
Intermediate Terrain Policy (ITP).  It is our understanding that you
believe equivalency between ISCRDT and the ITP has been demonstrated for
application to Boise Cascade, but only for assessing the impacts of
increment consuming sources and the impacts of sources that are in the
existing baseline.  For sources which are shutting down or reducing
emissions, you believe that the ISCRDT model needs to be modified such that
the lesser of the simple terrain and complex terrain concentration
"credits" are chosen (on an hour-by-hour, source-by-source and receptor-by-
receptor basis).

     The Model Clearinghouse agrees with your position, but suggests that
there may be some room for flexibility with respect to concentration
credits for sources reducing emissions.  We have also reviewed the
replicability aspects of the equivalency and agree that the ISCRDT model is
indeed reproducing the concentration estimates of ISCST and RTDM and that
it is making the appropriate choice of concentration estimates,
notwithstanding the issue of what choice should be made in the case of
concentration credits.  We also agree that the appropriate cases have been
chosen to make the replicability demonstration.

     Regarding the issue of concentration credits, we agree that the source
should not be given credit for increment expansion or for reductions in the
total concentration by choosing the higher of the simple terrain or complex
terrain modeled concentration credits.  As you have pointed out, doing so
would result in a nonconservative credit, i.e., a larger credit than is
really available if a refined model yielding unbiased estimates were
applied.  For this reason you conclude that for ambient credits due to
emission reductions, the lower of the two estimates (in absolute value)
should be chosen, the result being that concentration credits would be
conservative.

     As indicated above, we agree that the lower of the two credits would
indeed be conservative but we believe that we can offer an alternative
scheme, one that is still conservative but does not penalize the source
quite so much.  Our reasoning is as follows.  RTDM is a screening model,
designed to produce conservative estimates within its applicability domain. 
ISCST is a refined model, designed to yield unbiased estimates, but again
within its applicability domain.  The problem in intermediate terrain is
that both models are being applied outside of their stated limitations. 
However, when applying ISCST in intermediate terrain, following the ITP,
the terrain is "cut off" at stack height and the estimate is made at that
elevation and applied at a higher elevation.  Since it is postulated by the
guidance that: (1) ISCST is unbiased at stack top, and (2) as one goes up
in elevation above stack top, the true concentration would be expected to
increase, it follows that the true concentration should be at least as high
as the ISCST estimate.  Following this rationale we believe that crediting
the ISCST estimate as increment expansion or as a concentration offset
would still be protective of air quality in intermediate terrain.  If the
source wants to go back and modify their program to reflect this concept
and again demonstrate equivalency to the ITP, we believe that the
Environmental Protection Agency should accept such an approach.

     It should be made clear however, that the above suggestion of allowing
ambient credit for emissions reductions according to ISCST can only apply
in intermediate terrain.  For receptors above plume height, i.e., in true
complex terrain, crediting the ISCST estimate could easily be
nonconservative.  Also, crediting RTDM or Complex I estimates at such
receptors would also be nonconservative since such screening estimates
would normally be higher in absolute magnitude than the true concentration. 
If the true concentration could be estimated from an unbiased refined
model, it could be credited.  Also, an estimate from a model that was
designed to yield a minimum concentration (hour-by-hour) could also be
credited as being conservative.  Since neither a refined model nor a
minimum estimate model are currently available for application to the Boise
Cascade problem, there is really no other choice than to allow no credit
for emission reductions from sources impacting on complex terrain.  Again,
if the source wants to develop a technique for making minimum estimates in
complex terrain, and demonstrate that such estimates would be lower than a
refined model estimate, we would be willing to entertain such a proposal. 
However, such an undertaking is not a small task.  We have thought about
the problem some within the Clearinghouse; it is not a trivial problem. 

     If the source chooses to modify their program to allow for ISCST
concentration credits for receptors in intermediate terrain and no credit
for receptors in complex terrain, they should also be cognizant of the
following.  The "classification" of some receptors with respect to a given
source can switch between intermediate terrain and complex terrain because
the plume height will vary with stability and wind speed.  For hours when
the receptor is in intermediate terrain, the ISCST credit would be
acceptable, whereas when the receptor is in complex terrain, a zero
concentration credit would be appropriate.

     There are a couple of other points worth mentioning with respect to
the Boise Cascade permit.  First, as we discussed on the telephone, for
purposes of increment expansion the source reducing emissions only gets
credit for actual emissions decreases.  Second, given the results of the
petition for reconsideration in the Hadson case, it is advisable to provide
notice and opportunity for public hearing for the application of a
nonguideline model. 

     If you have any questions, please contact me at 919-541-5683.

cc:  R. Lee
     D. deRoeck