April 26, 1991


MEMORANDUM

SUBJECT:  Use of Nonguideline Versions of Urban Airshed Model

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

TO:       Regional Modeling Contact
          Regions I-X


     As you are aware we will soon be applying the Urban Airshed
Model (UAM) to a number of ozone nonattainment areas across the
country.  One such application will be in the Lower Lake Michigan
area, including Chicago.  The Lake Michigan Air Directors
Consortium, who will be doing the modeling, has proposed to
conduct an extensive study to improve on UAM for application to
this area.  They have asked Region V for permission to use the
modified UAM that comes out of the study for the ozone State
Implementation Plan.  

     Attached is a letter from Region V to the Consortium,
granting them permission to use the model that comes out of the
study.  Note that the permission to use this nonguideline model
is granted without the usual requirement for a side-by-side
(Interim Procedures) type of model evaluation because of the
extensive study on which this application will be based. 
However, the letter also indicates that if the model is to be
used outside of the study area, the standard performance
evaluation would be required.

     This letter is being furnished to you in the anticipation
that you may also receive requests to modify UAM, or perhaps to
apply the Lower Lake Michigan version of the model.  Unless the
State plans to conduct an extensive model development study,
comparable in scope to the Lake Michigan study, any nonguideline
version of UAM will need to undergo a performance evaluation,
pursuant to Section 3.2.2 of the Guideline on Air Quality Models
(Revised).

Attachment

cc:  D. Grano
     E. Meyer