UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
               Office of Air Quality Planning and Standards
               Research Triangle Park, North Carolina 27711

                           June 28, 1989




MEMORANDUM

SUBJECT:    Clarification of Stack-Structure Relationships

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

TO:     Regional Modeling Contacts
        Regions I-X
    
    Recently there have been inquiries from a few Regional
Modelers and the private sector concerning a specific aspect of
implementing the wind direction specific downwash algorithm in
the ISC Model.  A letter from Trinity Consultants, Incorporated
to Russell Lee, of my staff, is attached which describes the
question.

    Our guidance on determining the area of building wake
influence for direction-dependent wake calculations (the
5L/2L/0.5L scheme described in the Model Clearinghouse memo and
attachment dated May 11, 1988) was meant to be as simple as
possible while accounting more realistically for building edge
effects assumed to occur within roughly 0.5L crosswind.  The
simplest approach was to assume a rectangular area, even though
cavity and wake regions do not have "corners".

    The definition of and procedure for determining when a stack
will be within the wake region of a nearby structure is contained
in the GEP stack height guideline.  There is no intention that
any procedure in that guideline should be modified or expanded by
the procedure in the revised ISC user's guide.  The application
of the GEP guidance should come first, i.e., is the stack within
the area of influence of the structure in question where the area
is based on five times the lesser of the height or width
dimension downwind from the trailing edge of the structure?  If
the stack is not within this area of influence, one would
probably not proceed with any downwash modeling analysis.  So as
not to preclude elimination of adjacent wind direction sectors
from the analysis that accounts for the edge effects of
structures, our guidance does imply that a dimension 0.5L be
added laterally to include these wind directions.  However, this
should not be used to extend the area of influence beyond the GEP
stack height definition.

    If you have further questions, please contact Jim Dicke (FTS
629-5682). 

Attachments

cc: R. Lee, SRAB/TSD (MD-14)
    J. Dicke, SRAB/TSD (MD-14)                                  
    D. Wilson, SRAB/TSD (MD-14)