December 15, 1992


MEMORANDUM

SUBJECT:  The Ozone Attainment Test in the State Implementation Plan (SIP)
          Modeling Demonstrations

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

TO:       Brenda Johnson, Ozone Modeling Contact
          Region IV


     This is in response to your September 28 memorandum to Richard Scheffe
concerning an appropriate modeled air quality value to demonstrate
attainment of the National Ambient Air Quality Standard (NAAQS) for ozone. 
In order to focus the problem, we have discussed the issue further with you
as it applies to specific SIP's in Region IV.  Based on these discussions,
we understand that, for example, the State of Georgia would like for their
SIP demonstration to be acceptable if the design concentration is reduced
to 0.124 ppm, after controls are applied.  In the intervening weeks, we
have consulted with the staff of the Air Quality Management Division
(AQMD), surveyed Regional Office Modeling Contacts, reviewed related past
AQMD decisions on other pollutants and reviewed portions of the 1990 Clean
Air Act (Act) which we believe to be pertinent.  The recommendation
provided below is consistent with the results of this review.

     As you know, the level specified in the ozone NAAQS is 0.12 ppm.  The
"Guideline for Regulatory Application of the Urban Airshed Model" ("UAM
Guideline") is clear (p.63) that to demonstrate attainment, predicted ozone
must be 0.12 ppm or less throughout the modeling domain.  Thus, a
prediction of 0.13 ppm is not sufficient, whereas a prediction of 0.12 ppm
is so.  The question you raise is whether a value of 0.121 ppm or greater
is acceptable in showing attainment in a SIP demonstration.

     More specifically, our review of prior work addressing this issue for
ozone and other criteria pollutants included:

     1.  section 181(a) of the Clean Air Act, in which Congress defines a
"marginal area" as one having an ozone design value of 0.121 - 0.138 ppm
(Attachment 1); nonattainment areas having modeled values within this range
are consistent with the Act's classification as a marginal nonattainment
area.

     2.  past correspondence from AQMD concerning interpretations of
control targets for carbon monoxide and lead (Attachments 2 and 3); model
estimates for comparison to the target value should not be rounded. 

     3.  a survey of Regional Office Modeling Contacts (Attachment 4) in
which the most common practice, with regard to rounding of model estimates,
appears to interpret values like 0.121 ppm of ozone as not meeting the
target concentration level.

     Based on the attachments it appears in the majority of situations
involving various criteria pollutants that for NAAQS demonstrations the
numerical value of the standard is the "target" level to which model
estimates should be reduced.  For ozone this implies that for application
of a model that estimates concentrations to three significant figures, the
target level is actually 0.120 ppm.  Attachments 2 and 3 provide a more
elaborate discussion of this point.

     Based on this analysis, we conclude that the State of Georgia's
proposal to use 0.124 ppm as the target level would not be acceptable. 
Moreover, any value between 0.121 and 0.125 would also generally not meet
the target for an acceptable attainment demonstration.  However, I realize
that until now, resolution of the issue you raise has not been clear. 
Thus, if the State has previously developed a protocol that specifically
contains a value greater than 0.120 ppm as the SIP demonstration target
level, and the Environmental Protection Agency has approved that protocol,
it may qualify for grandfathering.

     Please contact Ellen Baldridge (919-541-5684), or Dean Wilson (919-
541-5683), respectively for ozone and for other criteria pollutants, if
this matter requires further clarification.

Attachments 

cc:  Air Branch Chiefs, Regions I-X
     Tom Helms, MD-15