October 30, 1993


MEMORANDUM 

SUBJECT:  Attainment Demonstrations Using the Empirical Kinetics Modeling
          Approach (EKMA)

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

TO:       Brenda Johnson, Ozone Modeling Contact
          Region IV


     The Model Clearinghouse has reviewed your request to allow more than 3
years as a basis for using EKMA to estimate precursor controls necessary to
demonstrate attainment of the National Ambient Air Quality Standard (NAAQS)
for Ozone in Miami.

     Guidance for the use of EKMA (prepared prior to the passage of the
Clean Air Act) states that the most recent 3 years for which measurements
are made generally should be chosen to estimate controls needed to meet the
NAAQS.  However, the 1990 Clean Air Act (CAA) (Section 181(a)) classifies
severity of an area's ozone problem using information from 1987-1989.  This
implies that this period should be considered within the period used to
assess control needs.  Furthermore, extension of the EKMA guidance to treat
more than 3 years of data (e.g., n years where n = 4 or more years) with
the attendant control target based on the highest nth + 1 site could lead
to a situation where "no modeling" is required.

     After discussing this matter with the Regional Office Ozone Modeling
Contacts, Model Clearinghouse staff, and the Ozone/CO Policy Branch, we
have determined that EKMA modeling should be based on the 3-year period
consistent with the time period on which nonattainment classifications were
determined for the CAA.  In most cases, this time period is 1987-1989.

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

cc:  D. Grano, MD-15
     T. Helms, MD-15
     N. Meyer, MD-14
     R. Scheffe, MD-14
     D. Wilson, MD-14