March 21, 1994



MEMORANDUM

SUBJECT:  Denver Carbon Monoxide Attainment Demonstration

FROM:     John S. Seitz, Director
          Office of Air Quality Planning and Standards (MD-10)

TO:       Patricia D. Hull, Director
          Air, Radiation and Toxics Division (8ART)


     In response to your request, the Office of Air Quality Planning and
Standards has reviewed the Region VIII position regarding the attainment
demonstration for the Denver Carbon Monoxide (CO) State implementation plan
(SIP).  I understand the Region VIII position to be that an approvable CO
SIP attainment demonstration for Denver needs to show no exceedances of the
National Ambient Air Quality Standards (NAAQS) for each of the two episodes
being modeled.  While I greatly appreciated the opportunity to meet with
members of the Regional Air Quality Commission (RAQC) and other interested
groups during my recent visit to Region VIII, I continue to support your
position for the reasons stated below.

     In a previous correspondence on this SIP (copy attached), my staff
provided rationale on why the applicable regulatory guidance is indeed
appropriate for the Denver case.  The expectation was that three episodes
would be modeled, each with distinctly different meteorological regimes. 
Our conclusion at that time was essentially the same as now.  Even with
three meteorological regimes modeled, we are far from having enough data
points to be able to conclude that the episodes containing both the highest
and second highest days in the future have been included.  Any case for
disregarding the high day prediction is even less viable now, since only
two days are being modeled.

     The analysis included in your Technical Support Document (TSD) shows
no compelling technical reasons that support a departure from national
guidance on this issue.  For example, it appears to us that using
meteorological conditions for the high day, as part of modeling, results in
a conclusion that is consistent with that indicated by monitoring data. 
Your TSD clearly points out the factors supporting this conclusion,
including:  1) there is strong evidence that even more adverse
meteorological conditions can occur than those on the so-called "worst case
day," 2) a recent saturation monitoring study indicates that the current
monitors are not located at points of maximum concentration, 3) rollback
from the second highest monitored day in 1992 results in a more stringent
control requirement than the second highest modeled day, and 
4) monitoring data suggest that downward trends in 8-hour average CO
concentrations and number of violations have flattened over the past 5
years. 

     Based on these facts, I agree that straightforward application of
regulatory modeling guidance, as advocated by Region VIII, is appropriate
in order to ensure that the carbon monoxide ambient standards are attained
in a timely manner.

     Again, I want to express my appreciation for the opportunity to meet
with you and various groups representing the interests of the City of
Denver and the State of Colorado on this issue.  These meetings provided me
with an excellent perspective on the various viewpoints and on the
significance of our decision.  

Attachment

cc:  T. Helms
     M. Payne
     D. Skie
     J. Tikvart


                                Attachment

                        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