October 30, 1992


MEMORANDUM

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

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

TO:       Royan W. Teter, Regional Ozone Modeling Contact
          Air Planning and Development Section, Region VII 


     In response to your request, the Model Clearinghouse has reviewed the
Regional Office proposal to allow a potential modeled exceedance in the St.
Louis UAM demonstration for ozone.  The Missouri Department of Natural
Resources (DNR) is proposing to model 12 of the highest ranked ozone days
occurring during four meteorological episodes from 1987-1991.  This far
exceeds the Environmental Protection Agency's (EPA) guidance on the number
and type of days to be modeled.  Furthermore, the proposed episode days
described in the Photochemical Modeling Protocol for the St. Louis
Metropolitan Area represent the highest ranked ozone days in the St. Louis
airshed.

     Consensus has been reached that one modeled exceedance may be accepted
for the St. Louis UAM Application.  For clarification, a modeled exceedance
refers to any simulated ozone value >0.12 ppm.  Therefore, we understand
that the St. Louis UAM demonstration will indicate values ó0.12 ppm for
every hour and every grid cell for all modeled days except one.  A limit
will not be imposed on the number of values >0.12 ppm for the single day
when an exceedance occurs, consistent with the way in which ozone NAAQS are
typically interpreted.

     Since the highest ranked ozone episodes for all meteorological regimes
are proposed for modeling, we believe the attainment test remains
sufficient.  The DNR could have chosen to model a minimum of 3 days and
meet minimally acceptable criteria suggested by EPA's Guideline for
Regulatory Application of the UAM.  However, assurance that designed ozone
precursor control strategies will be successful is strengthened by
considering 12 high ozone days in the modeling demonstration. 
Consequently, we believe the procedure being followed in the St. Louis
demonstration is consistent with the spirit of Section 6.4 of the "Guidance
for Regulatory Application of the Urban Airshed Model."  In that section we
note that States may opt for more comprehensive statistical testing of
modeling results in estimating attainment.  Use of a much larger number of
episodes with some modeled exceedances is consistent with this notion.  We
understand that this optional approach has been approved by the Regional
Office and is included in the protocol.

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

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