January 17, 1995


MEMORANDUM

SUBJECT:  Request for Clarification on CTDMPLUS Model Inputs
          (Merrimack Generating Station, BOW NH)

FROM:     Joseph A. Tikvart, Group Leader
          Air Quality Modeling Group, EMAD, OAQPS (MD-14)

TO:       Brian Hennessey, Regional Modeling Contact
          Air Permits & Assessment Section, Region I


     We have reviewed your January 9, 1995 memorandum to the
Model Clearinghouse.  In this memorandum, you requested
Clearinghouse concurrence on one of the Region's draft comments
concerning the proposed modeling protocol for Public Service of
New Hampshire's Merrimack Generating Station.  In this comment,
the Region noted that PSNH did not specify which sigma theta
values will be input to the CTDMPLUS model for use in calculating
hourly values of lateral plume dispersion (sigma y) under
neutral/stable conditions.  Apparently, PSNH has available both
hourly and 15-minute values of sigma theta obtained from on-site
measurements.  In its comment the Region is recommending that the
hourly values be used as input to the model.

     We concur with your comment to recommend the hourly values
of sigma theta as input to the CTDMPLUS model.  As you noted, the
User's Guide to CTDMPLUS (EPA-600/8-89-041) is not explicit in
defining the averaging time for the sigma theta values input to
the model.  However, CTDMPLUS is a steady-state plume dispersion
model designed to estimate hourly-average concentrations.  Also,
it is noted in the User's Guide that the meteorological data file
used as input to the METPRO meteorological preprocessor (i.e.,
PROFILE) presumes hourly values of several meteorological values
including sigma theta.  Thus, your recommendation is consistent
with the input data requirements and lateral plume dispersion
formulations within the CTDMPLUS model.

     In response to your request for any other comments
concerning the modeling protocol, your additional comments on the
protocol noted in your draft basically cover any other comments
we have. 

     If you have any other comments or questions, please contact
Dennis Doll at (919) 541-5693.

cc:  D. Bailey
     D. Doll
     J. Irwin
     D. Wilson