April 6, 1993


MEMORANDUM

SUBJECT:  Nonmethane Organic Compounds (NMOC) and Nitrogen Oxides (NOx)
          Monitoring Required for the Empirical Kinetics Modeling Approach
          (EKMA) for Nonattainment Areas in Ohio

FROM:     Ellen Baldridge, Computer Specialist
          Model Application Section, SRAB, TSD (MD-14)

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


     In response to your request on the above subject, the Model
Clearinghouse has reviewed your position on the application of EKMA in
Ohio.

     The most recent guidance on the use of EKMA (Environmental Protection
Agency, 1989) is silent on the use of default ratios in EKMA State
implementation plan (SIP) analyses.  This implies that the use of such
ratios is not generally recommended.  Therefore, we endorse the approach
you mention which entails collecting NMOC and NOx data in Toledo and Dayton
consistent with monitoring guidance and requirements of the EKMA and
subsequently using these data in the model.  We believe this represents the
most defensible modeling procedure and is the approach most consistent with
the 1989 guidance for using EKMA.

     We understand, however, the concern you raise over the timing issue
requiring a SIP remission to be in place by November 1993.  We agree that
prior to the availability of NMOC and NOx  monitored data in Toledo and
Dayton, it may be necessary to resort to other procedures for estimating
the NMOC/NOx ratio.

     We agree that use of a default ratio is the only viable interim
procedure open to Ohio for use in Toledo and Dayton.  A question then
arises as to what default value should be used.  As you have noted, our
earlier guidance suggested "~9.5:1" as a default value.  More recent data
compiled by Baugues (1991)--(See table 1) suggest typical 50th percentile
ratios to be slightly lower (~8.8:1).  This appears to be more in line with
the middle of the distribution of ratios found in nearby cities.  For
example, the range of 25-75 percentile values of the ratios in Cleveland,
Cincinnati, Detroit, and Indianapolis is 7.0-9.9; 6.4-9.1; 6.5-10.0; and
7.5-10.7 respectively.  Thus, we recommend use of a default using more
recent data.

Attachment (Table 1)

cc:  P. Dolwick, Region V
     D. Grano, MD-15
     T. Helms, MD-15
     A. Van Meter, MD-14
     D. Wilson, MD-14