February 22, 1993


MEMORANDUM

SUBJECT:  Carbon Monoxide State Implementation Plan                         
          Attainment Demonstrations

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

TO:       Kevin Golden, Regional Meteorologist 
          Region VIII          
          
          Larry Svoboda, Chief
          Assessment, Modeling, and Emissions Section, 
          Region VIII                                                       


     In response to your request, the Model Clearinghouse has reviewed your
position regarding carbon monoxide (CO) State implementation plan (SIP)
attainment demonstrations.  We support your position that the CO SIP
attainment demonstrations for both Provo and Denver not allow modeled
exceedances for any of the three episode days chosen for modeling.  This is
in accordance with the Colorado and Utah modeling protocols which committed
to showing attainment on each of the three episodes selected for modeling. 
The episode selection is in accordance with the June 1992 Guideline for
Regulatory Application of the Urban Airshed Model for Areawide Carbon
Monoxide and we concur with the episodes selected for modeling.  For a
deterministic standard such as CO, it is clear that a large number of the
highest days comprising the entire tail of the frequency distribution would
need to be modeled before we could have confidence that both the highest
and second highest days had been included.  Clearly, the highest day cannot
be thrown out when a subset of only three days are modeled.

     If you have any questions, please contact Tom Braverman of my staff at
919-541-5383.

cc:  E. Baldridge, MD-14
     T. Braverman, MD-14
     K. Scavo, MD-15
     D. Wilson, MD-14