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

                             November 9, 1983




MEMORANDUM

SUBJECT:  Use of Composite Quarterly Meteorological Data with Industrial
          Source Complex Long Term (ISCLT) Modeling 

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

TO:       Jack Divita, Chief
          Air Branch, Region VI

     This memorandum responds to your staff's request for guidance on
quarterly composite meteorological data as input to the ISCLT model for
developing lead SIP's.  By quarterly composite we mean meteorological data
for the same quarter of each of five years, averaged together.  This
results in four quarters of composite data.

     Using composite data will necessarily underpredict the worst
meteorological regime that occurred during the five years and therefore
will underpredict air pollutant concentrations.  Therefore, in modeling
lead sources, the recommended approach is to run ISCLT using the
meteorological data from each of the individual quarters for the five-year
period (i.e., twenty quarters of data).

     This same guidance applies when using other long term models and when
modeling the annual mean for a pollutant for which an annual ambient
standard exists.  Since the annual standard is never to be exceeded with
the current form of our standards, attainment should be demonstrated by
modeling five individual years of National Weather Service data and
checking for the highest estimate rather than using a five-year composite
data base as input to the model.

     If you have any questions concerning this matter, please contact Dean
Wilson at 629-5681.

cc:  Air Branch Chief, Regions I-V, VII-X
     Air Lead Contact, Regions I-X
     Regional Modeling Contact, Regions I-X
     R. Rhoads
     D. Tyler





bcc:  R. Bauman
      J. Calcagni
      J. Dicke
      T. Helms
      B. Nicholson
      J. Silvasi
      D. Wilson