October 30, 1992


MEMORANDUM

SUBJECT:  Demonstrating Attainment of the Ozone National Ambient Air
          Quality Standards (NAAQS) with the Urban Airshed Model (UAM) for
          Detroit 

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

TO:       Rebecca Calby, Regional Meteorologist
          Regulation Development Branch, Region V (AR-18J)


     In response to your request, the Model Clearinghouse has reviewed the
proposed Detroit UAM demonstration.  The Michigan Department of Natural
Resources (DNR) has identified two principal meteorological regimes and is
proposing to model 4 of the 5 highest ranked days from these regimes.  One
of the proposed modeling periods, July 5-7, 1988, also includes a day (July
6) which recorded a higher peak ozone value.  However, based on criteria in
Section 3.1 of the "Guideline for Regulatory Application of the Urban
Airshed Model", the Michigan DNR is proposing to disregard the day for the
purpose of the modeling demonstration.

     Consensus has been reached that July 6 may be discarded and for the
other 4 days that no modeled exceedance would be accepted for the Detroit
UAM Application.  For clarification, a modeled exceedance refers to any
simulated ozone value >0.12 ppm.  Therefore, the Detroit UAM demonstration
should indicate values ó0.12 ppm for every hour and every grid cell for the
4 remaining modeled days.  

     We believe it is important to encourage use of the most technically
viable available tools in performing State implementation plan attainment
analyses.  Therefore, the consensus was influenced by the fact that Detroit
is an intrastate moderate area and Michigan is not required to apply the
UAM.  However, the DNR and participating groups believe that the UAM
application provides valuable information for air quality planning purposes
and has opted to apply the model in lieu of the Empirical Kinetics Modeling
Approach (EKMA).  Furthermore, consensus that July 6 may be disregarded
appears to be within bounds of the Environmental Protection Agency's
guidance on selection of episode days.

     We understand that the proposed approach to selection of days to be
modeled has been approved by the Regional Office and is included in the
modeling protocol.

     If you have any questions, please contact Ned Meyer at 919-541-5594.

cc:  P. Dolwick, Region V
     D. Grano, MD-15
     N. Meyer, MD-14
     R. Scheffe, MD-14
     D. Wilson, MD-14