JULY 25, 1994



MEMORANDUM

SUBJECT:  Wind Tunnel Modeling Demonstration to Determine
          Equivalent Building Dimensions for the Cape Industries
          Facility, Wilmington, North Carolina

FROM:     Joseph A. Tikvart, Chief
          Source Receptor Analysis Branch, TSD (MD-14)

TO:       Brenda Johnson, Regional Modeling Contact
          Region IV

          Douglas Neely, Chief
          Air Programs Branch, Region IV


     This memorandum is in response to your request for
additional Model Clearinghouse input to the review of the wind
tunnel modeling demonstration report for the Cape Industries
facility in Wilmington, NC.  This also serves as a followup to
Dean Wilson's February 2, 1994 memorandum to you.  The February 2
memorandum contained comments from the Clearinghouse and also 
Dr. William Snyder, Chief of the Fluid Modeling Branch, on the
wind tunnel report for the Cape Industries facility.

     Subsequent to the February 2 memorandum, we received a
request from Region V concerning the review of a wind tunnel
modeling protocol to determine equivalent building dimensions. 
We also became aware that at least three other wind tunnel
modeling protocols were being reviewed by a Regional Office or
State agency.  As a result, the Clearinghouse convened a
conference call with the Regional Modeling Contacts to discuss
technical issues pertinent to the review of the Cape Industries
report and the other wind tunnel protocols.  During the call, it
was agreed to solicit technical questions and concerns from the
Regional Modeling Contacts and appropriate State agencies
concerning the technical review of the Cape Industries report and
the wind tunnel protocols.  Also, it was agreed that a meeting
with you, other Environmental Protection Agency (EPA) technical
staff, and the consultant developing the Cape Industries report
(Cermak, Peterka, Petersen, Inc.) would be useful to address
these technical issues associated with reviewing the report.
     
     Subsequent to the conference call, we received a list of
technical issues from the Regional Offices and State agencies. 
These were discussed at the Regional/State Modelers Conference
and a final list of issues was developed.  A meeting was held
with the consultant on June 8, 1994 to discuss these and other
technical issues associated with the Cape Industries report.

     In reviewing the Cape Industries report, it is important to
note aspects of this study in context of the overall ambient air
quality modeling analysis objectives.  First, it is important to
note that the wind tunnel study does not replace an ambient air
quality analysis using a preferred air quality model (i.e.,
Appendix A of the Guideline on Air Quality Models (Revised)). 
Rather, the wind tunnel demonstration was used to develop
appropriate building dimensions for input to the Industrial
Source Complex (ISC2) model.  Thus, the analysis is viewed as a
source characterization study which generally has been considered
under the purview of the Regional Office.  As a result, the study
is considered not subject to the requirements under Section 3.2
of the Guideline (i.e., Use of an Alternative Model).

     Second, the purpose of the study is to develop appropriate
direction-dependent "equivalent building dimensions" for input to
the ISC2 model.  The Cape Industries facility consists of
lattice-type structures.  Using standard techniques, Cape
Industries would typically use the full structure height as
building height in the ISC2 model.  The Cape Industries report
states that "this building height would tend to overestimate the
downwash effect of the nearby lattice-type structures and as a
result produce unrealistically high ground-level concentration
estimates."1  The first step in the wind tunnel study is
therefore designed to simulate the actual direction-dependent
dispersion from the sources with the actual lattice-type
structures in place.  This is done by measuring downwind ground-
level concentration profiles.  Next, the structures are removed
from the wind tunnel and replaced with simplified solid structure
more typical of the structure from which the ISC2 downwash
algorithm was developed (i.e., "Huber-Snyder").  From this, the
simplified structure which matches the concentration profiles
with the site structures in place according to pre-determined
criteria is selected for input to the ISC2 model.  Provided the
wind tunnel demonstration is technically sound, this seems to be
a reasonable approach for deriving the building dimensions input
to the ISC2 model.

     Attachment 1 contains a list of the technical issues
identified at the Modelers Conference and responses based on
discussions from the June 8 meeting.   These responses should be
helpful in your review of the Cape Industries report.  (Note that
Attachment 1 references Attachments 2, 3 and 4).  Below are some
additional comments concerning the technical issues described in 
Attachment 1.

     Issue 1 addresses which structures to include in the wind
tunnel modeling.  Procedures used in past experiments are
provided although no generic guidance can be provided at this
time to cover all scenarios.  As noted in Attachment 1, use of a
uniform roughness across the entire tunnel floor seemed to be the
simplest and a reasonable approach according to the meeting
participants.  However, it was noted that another approach might
be to replace the actual site configuration on the turntable with
a uniform characteristic surface roughness - similar to the
approach used at Cape Industries.  The issue of which structures
to include/exclude in the tunnel demonstration for the equivalent
building would need to be addressed on a case-specific basis.

     Issue 2 addresses surface roughness in the wind tunnel. 
Surface roughness is important in the tunnel both in
characterizing the upwind and downwind fetch from the site and
characterizing the buildings removed in determining the
equivalent building for the site.  Based on experience gained
thus far, larger magnitudes of surface roughness used in the
tunnel simulations tended to yield larger equivalent building
dimensions, other factors being equal.

     Issue 3 describes the shape of the equivalent building.  The
wind tunnel demonstrations thus far are appropriate for building
dimensions equivalent to "Huber/Snyder" type structures.  That
is, a structure with a crosswind dimension approximately double
the building height.  There are cases where this type of building
when used in the wind tunnel simulations does not provide an
adequate characterization of the ground-level concentrations.  As
noted in Attachment 1, one resolution for such cases might be to
use Building Profile Input Program (BPIP) or some other
equivalent technique to define the building dimensions for input
into the ISC2 model.

     Issue 9 addresses the criteria for demonstrating
equivalency.  Described are methods that have been suggested in
previous protocols.  As more experience is gained in these wind
tunnel demonstrations, these criteria will likely continue to
evolve.  The criteria used for Cape Industries was to determine
the equivalent building dimensions that yielded maximum ground-
level concentrations in the wind tunnel within 10 percent of the
maximum observed ground-level concentrations with the actual site
buildings in place.  You may wish to review these criteria for
Cape Industries with the State and Cape Industries to evaluate
the appropriateness of this approach.

     Another issue not specifically listed in Attachment 1 is the
use of zero equivalent building dimensions as input into the ISC2
model for wind directions where downwash is not expected to
occur.  Some wind tunnel protocols have a provision that if the
increase in the wind tunnel simulated ground-level concentrations
is less than 40 percent with the site structures in place as
compared to the structures removed, then the building dimensions
would be zero for input to the ISC2 model for that wind
direction.  This 40 percent is based on the procedures used in
wind tunnel studies to derive Good Engineering Practice (GEP)
stack height.  It was suggested in the meeting that, to simplify
the modeling demonstration, the equivalent building dimensions be
identified for all wind directions independent of the increase in
ground-level concentrations.  These building dimensions could be
determined either using BPIP or equivalent processors, guidance,
or wind tunnel results, and allow the model to determine the
effects on the predicted concentration values.  It was noted
however that this simplification may not likely change the
conclusions from the ISC2 modeling.  However, it seemed that this
simplification may avoid unnecessary complexity in the wind
tunnel simulation and subsequent regulatory agency review.  In
the case of Cape Industries where the 40 percent criterion was
applied, results from the ISC2 modeling are not expected to
change even if equivalent building dimensions were included for
all directions.

     At this time, it would be premature to provide generic
guidance on how to conduct wind tunnel studies to determine
equivalent building dimensions.  Much of the information
described thus far is based on recent experience and continues to
evolve.  Hopefully as more experience is gained in the review and
application of wind tunnel demonstrations, more specific guidance
can be provided.  As a general comment, you may wish to suggest
to your State agencies that prospective sources submit complete
wind tunnel modeling protocols and receive approval by the State
agency and Regional Office prior to initiating any wind tunnel
modeling demonstrations.  

     We recommend that, if you think necessary, you meet with the
State and perhaps the technical representatives for Cape
Industries.  Review the current results in light of the
information provided and ascertain whether any additional
clarification or studies are needed.  We believe that this matter
is best resolved at the Regional Office and State level.  

     If we can be of further assistance please contact me at
(919) 541-5562 or Dennis Doll at (919) 541-5693.

Attachments

cc:  D. Doll
     J. Irwin






                          ATTACHMENT 1


Technical Issues Concerning Wind Tunnel Modeling to Determine
Equivalent Building Dimensions (EBD) for input to the Industrial
Source Complex (ISC2) Model

Introduction.  The following represents a final list of technical
issues developed at the 1994 Regional Office/State Modelers
Conference associated with the review of wind tunnel modeling
protocols to determine equivalent building dimensions (EBD) for
input to the Industrial Source Complex (ISC2) model.  It is
important to note aspects of these wind tunnel studies in context
of the overall objectives of the ambient air quality modeling
analyses. First, these wind tunnel studies to determine EBD do
not replace ambient air quality analyses based on a preferred air
quality model (i.e., Appendix A of the Guideline on Air Quality
Models (Revised)).  Rather, the wind tunnel studies are used to
develop appropriate building dimensions for input to the
Industrial Source Complex (ISC2) Model downwash algorithm.  Thus,
the analyses are viewed as source characterization studies which
generally have been considered under the purview of the Regional
Offices.  As a result, these studies are considered not subject
to the requirements under Section 3.2 of the Guideline (i.e., Use
of an Alternative Model).

     Second, the purpose of the study is to develop appropriate
direction-dependent EBD for input to the ISC2 model.  Typically
using standard techiques the full structure height would be input
as building height into the ISC2 model.  The wind tunnel
protocols have reported that for "lattice-type" building
configurations and structures this building height would tend to
overestimate the downwash effect and as a result produce
unrealistically high ground-level concentration estimates.  The
first step in the wind tunnel studies is therefore designed to
simulate the actual direction-dependent dispersion from the
sources with the actual lattice-type building configurations or
structures in place.  This is done by measuring downwind ground-
level concentration profiles.  Next, the structures are removed
from the wind tunnel and replaced with simplified solid structure
more typical of the structure from which the ISC2 downwash
algorithm was developed (i.e., "Huber-Snyder").  From this, the
simplified structure which matches the concentration profiles
with the site structures in place according to pre-determined
criteria is selected for input to ISC.  Provided the wind tunnel
demonstrations are technically sound, this seems to be a
reasonable approach for deriving the building dimensions input to
the ISC2 model.


1.   Q. What buildings need to be adjusted?  Should BPIP be used
to determine which buildings that influence the source?  What is
the maximum area for which this technique may be appropriate,
e.g., should it include all structures within 200-, 800-, 1200-,
etc. meters?  Is it preferable to include all structures when a
stack is within five building heights/widths of the structures?

     A. This question is related to how the wind tunnel is
configured to characterize concentrations associated with 1) the
actual site configuration, and 2) the equivalent building
configuration.  A typical wind tunnel configuration is shown in
Figure 1.  Tests of the actual site configuration are conducted
with a model of the site on the turntable. (See Figure 2 which is
an example site configuration discussed in the meeting for
District Energy St. Paul, Inc.)  The remainder of the tunnel
floor is covered with a uniform density of randomly distributed
roughness elements ("uniform roughness") to simulate the actual
aerodynamic roughness upwind and downwind of the site.  
     In the meeting, we discussed three approaches that have been
used in past wind tunnel demonstrations to configure the wind
tunnel to simulate the equivalent building flows.  These
discussions included which buildings have been included/excluded
in the actual site configuration and equivalent building
configuration in past demonstrations.  The approaches discussed
were 1) removal of selected buildings on the turntable, 2) use of
a turntable of uniform roughness matching the roughness of the
actual site, and 3) use of a uniform roughness across the entire
tunnel floor.  In 1) the tunnel is configured as with the actual
site simulation except selected buildings are removed.  The
intent is to remove the buildings which contribute to the
downwash phenomena.  This is illustrated in Figures 2 and 3.  In
2) the actual site buildings are removed from the turntable and a
surface roughness representative of the actual site configuration
is uniformally placed on the turntable.  This representative
surface roughness may be direction-specific.  See Figures 4 and
5.  In 3)  the actual site buildings are removed from the
turntable and a uniform surface roughness matching the remaining
tunnel floor is installed on the turntable (Figure 6).  These
approaches are described more fully in Attachment 2 under Wind
Tunnel Configurations for Tests.

     The meeting participants generally agreed that approach 3,
use of a uniform surface roughness across the entire tunnel
floor, while the least complicated approach, seems to provide a
reasonable EBD as input to ISC.  As noted in Attachment 2, this
approach is most consistent with the uniform roughness comprising
the "universe" of ISC.  Also, this approach may avoid some of the
complications associated with the other two more complex
configurations.  However, as improvements in dispersion models
progresses in terms of accommodating directional-dependent
surface roughness, approaches such as in 1) and 2) above may be
more feasible. 

     Another aspect of the question concerning which buildings to
include on the turntable concerns the size of the model domain. 
The participants discussed the suggestions in the Guideline for
Fluid Modeling of Atmospheric Diffusion (EPA, 1981) where
cubical-shaped structures should be included if the stack being
modeling is within 20 structure heights.  A structure much wider
than its height should be included if the stack is within
approximately 100 structure heights.  Past experience suggests
that the tunnel model generally includes structures within a 400m
to 1000m radius of the stack (Attachment 2).  This may be refined
based on professional judgement to remove/exclude some buildings.

     Attachment 2 provides more detailed description of the model
design issues discussed at the meeting. 


2.  Q. What are the appropriate similarity requirements to
determine EBD in a wind tunnel if they are different from those
for GEP stack height determinations?  Is buoyancy an important
factor (to match Froude number)?  Or does buoyancy produce a more
conservative EBD?  How can we determine from the data presented
that the modeling was done properly?  For example, how do we know
that valid Reynolds and Froude numbers were considered? 
Appropriate surface roughness? etc.?  What are the parameters the
must be considered?

     A. The most important similarity requirements were discussed
as illustrated in Attachment 3.  These requirements basically
follow the recommendations contained in Guideline for Use of
Fluid Modeling to Determine Good Engineering Practice Stack
Height (EPA, 1981) and Guideline for Fluid Modeling of
Atmospheric Diffusion (Snyder, 1981).  Snyder (1981) provides
information on the capabilities and limitations of fluid modeling
studies recommendation to follow to conduct such studies.  EPA
(1981) provides information and recommendations to conduct fluid
modeling studies for Good Engineering Practice (GEP) stack height
determinations.  

     It was noted in the discussions that in general conducting
the wind tunnel demonstration with a nonbuoyant plume will tend
toward a larger effective building height for the equivalent
building.  This is because buoyancy effects are more important
farther downwind from the source than in the near-field.  This
was illustrated in Attachment 4 which shows the affect of
increased stack height (to simulate buoyant plume rise) on
equivalent building height.  Based on the results shown, it is
anticipated that neglecting buoyancy effects would have minimal
effect on the estimated building dimensions.  However, it was
recognized that as experience is gained, some testing may be
considered useful to explore the sensitivity of the determined
building dimensions to simulated release height.  In this regard,
it was recommended that raising or lowering the simulated stack
height was preferred to actually attempting to simulate the
buoyancy of the plume.  When such tests are needed could not be
defined with the information available.  We anticipate
clarification of this issue as more cases are examined.  

     Surface roughness issues were discussed as described above
and in Attachment 2 under Wind Tunnel Configurations for Tests.
As noted above, the participants generally agreed that a uniform
surface roughness across the tunnel seemed adequate for the
tests.  Representative values for surface roughness for the wind
tunnel modeling can be found in Snyder (1981) and the On-Site
Meteorological Program Guidance for Regulatory Modeling
Applications (EPA, 1987).  It was also noted that based on past
wind tunnel demonstrations for EBD, larger magnitudes of surface
roughness used in the tunnel simulations tend toward larger
resulting equivalent building dimensions, other factors being
equal.   


3.   Q. What shape should the equivalent building be?  Does it
have to be a unique, predetermined shape such as Huber/Snyder
type?  Can more than two types of single buildings qualify for
the equivalent building for the same case, i.e., if, or when,
they produce the same concentration field?

     A. It was noted that the tunnel experiments conducted to
date have focussed on developing equivalent building dimensions
similar to "Huber/Snyder"-type buildings, i.e., a structure with
crosswind dimensions approximately double the building height. 
This was done to be consistent with the type of building used in
the wind tunnel experiments to construct the ISC downwash
algorithm.  Thus, it seemed reasonable that the wind tunnel
demonstrations focus on these types of "Huber/Snyder" structures. 
However, there were cases where this type of structure when used
in the wind tunnel simulations did not provide an adequate
characterization of the ground-level concentration profiles. 
Also, there may be actual site configurations where
"Huber/Snyder"-type structures are not appropriate (e.g, tall,
narrow structures).  Thus, It was suggested that a resolution to
such cases would be to use BPIP or some other equivalent
technique to define the building dimensions for ISC input.

     
4.   Q. Where should the single equivalent building be?  In
front, behind, or at the middle of the source?

     A. It was stated that the purpose of the wind tunnel
simulation was to develop an equivalent building dimension
similar to "Huber/Snyder"-type structure to be consistent with
the experiments done to develop the ISC downwash algorithm.  In
these experiments, the stack was placed midway on the downwind
side of the building.  Thus, it seemed reasonable that the wind
tunnel simulations to determine equivalent building dimensions be
done similarly with the stack on the downwind side of the
equivalent building. (See Figure 5 for illustration).
     

5.   Q. The stack height velocity (2%) (wind speed) in the field
in the protocol we reviewed was calculated from the measurements
at other than stack height through log-law wind speed profile
although the stack height velocity measurements are available.

     The stack height (wind speed) velocities calculated from
different heights, in this case, may result in a 20% difference
which causes a 20% difference in velocity ratio which is one of
the similarity parameters.  Therefore, the stack downwash could
be underestimated and, in turn, the building downwash could be
overestimated.  Should a general method be set up for the stack
height velocity determination?

     A. The 2% criteria (i.e., 98th percentile wind speed based
on the climatological records of a site) is a guideline for wind
tunnel modeling to determine GEP stack height.  In the meeting it
was suggested that the 98th percentile wind speed is not a
necessary criterion for determining EBD.  It was suggested that
wind speeds ranging from the 94th to 99th percentiles be used in
the tunnel simulations.  The wind speed should not exceed the
maximum observed from the appropriate climatological records.  It
was also suggested that the wind speed should be set so that the
ratio of a realistic exit velocity to simulated wind speed is
greater than 1.5 to avoid stack tip downwash.  

     In a June 28 conference call with the EPA Regional Office
Modeling Contacts and technical staff, an alternative approach
for modeling the stack top wind speed in the tunnel was discussed
and was determined to be reasonable.  It was noted that it is
more important to simulate in the tunnel a wind speed that
maximizes ground-level concentrations than some specified
percentile range of wind speeds.  It was also suggested that the
actual stack exit velocity should be simulated instead of some
undefined "realistic" velocity.  This alternative approach is as
follows;
     1. Simulate the actual stack exit velocity or plume momentum
in the wind tunnel.
     2. Simulate the highest allowable wind speed at stack top
that just avoids stack tip downwash.
     3. The highest simulated stack top wind speed should not
exceed the maximum observed from appropriate records.


6.   Q. What meteorological conditions should be considered in
the fluid modeling, e.g., wind speeds?  How many wind directions
should be considered?  (It is assumed that 36 sectors for ISCST
and 16 sectors for ISCLT with appropriate surface roughness
replicated will be modeled.  Should only mid-sector directions be
considered, or should additional directions within each sector be
evaluated to determine the dimensions that give the highest
concentrations?

     A. It was noted that previous wind tunnel experiments have
used both 10 degree and 20 degree sectors to determine EBD.  For
the 20 degree sectors, the highest EBD determined from either
side of the sector was used for the 10 degree midpoint.  In other
cases, EBD have been determined every 10 degree for critical wind
directions (e.g., 90-180 degrees) while BPIP or some other
technique was used to determine the building dimensions for wind
directions outside this range.  These techniques seemed
reasonable and the issue of appropriate wind direction sectors
for the wind tunnel demonstration seemed to be case-specific. 
Given the limitations in science for accommodating building wake
effects on dispersion processes, it was recognized that sectors
less than 10 to 20 degrees are likely beyond the state of science
and currently are not recommended.

     For the ISCLT model, it seemed reasonable to select the
largest EBD within each 22.5 degree sector as input (consistent
with ISCLT).

     Another meteorological condition discussed concerned the
representation of the turbulent structure of the boundary layer
in the model.  It was generally agreed that configuring the wind
tunnel to simulate near-neutral stability was the most
appropriate approach.  This is also consistent with the turbulent
structure of the wind tunnel experiments used to develop the ISC
downwash algorithm.


7.   Q.  Should sources of heat be simulated, e.g., buildings
that have processes that release considerable heat to the
atmosphere?

     A.  This question concerns the simulation in the wind tunnel
of enhanced dispersion associated with heat flux from these
buildings.  This is unrelated to question (2) above concerning
the simulation of plume buoyancy in the tunnel.  In the
discussions, there seemed to be questionable value for simulating
these heat sources in the tunnel.  However, it may be reasonable
to consider these sources if warranted on a case-by-case basis. 


8.   Q. What type of Quality Assurance/Quality Control
requirements are needed for these studies?

     A. CPP agreed to provide some QA/QC procedures used in
previous test programs.  An outline of these procedures are
provided in Attachment 2.  It was suggested that a wind tunnel
protocol and report provide adequate information describing these
procedures.  


     Also discussed were repeatability tests of the wind tunnel
simulations.  This is also described in Attachment 2.  The value
of some demonstration of repeatability was noted.  These tests
may demonstrate the variations in concentration profiles that may
result due to slight differences in model configuration from one
test to a repeat test.  An example of a repeatability test
discussed could be as follows:  After all EBD have been
determined for all wind directions, go back and repeat the test
for the wind direction that yielded the concentration profile
containing the overall maximum observed concentration.  Do this
with all structures in place.  Then repeat the test with the EBD
in place and compare the concentration profiles.



9.   Q. How do we define equivalency?
          * centerline
          * distance to max. concentrations downwind
          * lateral/vertical profile
          * density of profile
          * precision

     A. Methods for determining equivalent building dimensions
from past wind tunnel studies were discussed.  Various criteria
have been used in these studies to select the EBD and have been
evolving as these studies continued.  The approaches used were
noted as follows:

     * Measure the ground-level longitudinal (alongwind)
     concentration profile with site structures in place for the
     wind directions specified in the protocol.  This is
     illustrated in Figure 2 of Attachment 2.  Ground-level
     concentrations are measured at each longitudinal distance
     using an array of receptors lateral (crosswind) to the
     tunnel axis.

     * Measure the ground-level longitudinal concentration       
     profile with the site structures removed according to  the
     procedures specified in the protocol.   Measure ground-level
     concentrations for various equivalent building              
     dimensions.

     * Compare the longitudinal concentration profiles.  From
     this comparison, alternative criteria have been used in past
     experiments to determine the EBD.  The procedure was to
     select the lowest EBD that meets the criteria.  [Note:
     Figure 7 is an illustration of the longitudinal
     concentration profiles from several EBD tests and with all
     site structures in place (i.e., "actual site" in Figure 7).]

     The alternative criteria used in past experiments are as
     follows:



     First Define:
         CS = maximum ground-level concentration with all site
          structures in place determined at each longitudinal    
          distance (See Figure 2, Attachment 2).

         CE = maximum ground-level concentration for the
          equivalent building dimension test with site structures
          removed determined at each longitudinal distance.

         CSmax = overall maximum CS value with all site
          structures in place.  Determined from among all
          longitudinal distances measured.

         RSmax = Longitudinal distance to overall maximum CS
          value (i.e., CSmax).

         CEmax = overall maximum CE value for the equivalent     
          building test with site structures removed.  Determined
          from among all longitudinal distances measured.


     Alternative Criteria used in Previous Demonstrations to
     Select the Equivalent Building Dimensions:
     - The EBD that produces an overall maximum concentration 
          within 10 percent of the overall maximum observed with
          all site structures in place (i.e., CEmax + 10% of
          CSmax).

     - The EBD with a longitudinal concentration profile 90
          percent or greater of the concentration profile from
          all structures in place (i.e., CE  90% of CS at each
          longitudinal distance).  [Note: This is illustrated in
          Figure 7.  EB3 was chosen for this particular wind
          direction.]

     - The EBD that produces 1) an overall maximum concentration
           the overall maximum concentration from all structures
          in place, and 2) concentrations > 80 percent of the
          concentrations from all structures in place at all
          other longitudinal distances [i.e., 1) CE  CSmax at
          RSmax; 2) CE > 80% of CS at all other distances].

     - The EBD that produces 1) an overall maximum concentration
           the overall maximum concentration from all site
          structures in place, and 2) concentrations  90 percent
          at all other distances. [i.e., CE  CSmax at RSmax;
          2) CE  90% of CS at all other distances].

     Another more recent procedure was discussed at the meeting
and noted in Attachment 2 as follows;

      Select the EBD that 1) produces an overall maximum
concentration exceeding 90 percent of the overall maximum
concentration observed from the all site structures in place
(i.e., CE > 90% of CSmax at RSmax), and 2) at all other
longitudinal distances, produces ground-level concentrations
which exceed the ground-level concentration observed from all
site structures in place less 20 percent of the overall maximum
ground-level concentration from all site structures in place
[i.e., CE > (CS-.20CSmax) at each longitudinal distance].  This
is in Figure 8.  Note that in this illustration, EB2 would be
selected over EB1 according to these criteria.

     In discussing this more recent criteria in the June 28
conference call, the appropriateness of the 90 percent criterion
for the overall maximum concentration was questioned (i.e., CE >
90% of CSmax at RSmax).  It was noted that because larger EBD
yield higher ground-level concentrations, it may be appropriate
to require 100% or greater matching (CE  CSmax at RSmax) for the
EBD test. 

     Criteria that tests at all locations on the longitudinal
concentration profile are more comprehensive than comparisons
only of overall maximum concentration values.  An examination of
the criteria illustrates the fact that a progression of thought
has occurred on acceptable criteria as more experience has been
gained.  It is anticipated these criteria will become more
refined as more cases are examined.

     It was suggested that these criteria be applied at downwind
distances beyond the cavity region of the actual site downwash
structure.  One suggestion was to apply these criteria to
distances greater than 3L from the source and further downwind so
long as the longitudinal maximum concentrations resulting with
the site structures in place exceed 30% of the overall maximum
concentration with the site structures in place.  Where L is the
lessor of the height or projected width of the dominant downwash
structure for that site configuration and wind direction.
After some discussion, the participants generally agreed that the
3L criteria might be too restrictive and that all sampling points
be included downwind of the structure having concentrations
greater than 30% of the overall maximum concentration with site
structure in place.  Some points may need to be excluded because
of cavity region phenomena and will need to be determined on a
case-by-case basis.

                     **********************

NOTE:  Additional figures and attachments 2, 3, and 4 are available from
       the Model Clearinghouse Coordinator.