UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
          Office of Air Quality Planning and Standards
          Research Triangle Park, North Carolina 27711

                        December 2, 1986




MEMORANDUM

SUBJECT:  Urban/Rural Determinations

FROM:     Dean A. Wilson  /s/
          Techniques Evaluation Section

TO:       Michael Koerber
          Regional Meteorologist, Region V


    In response to your request, I have considered your
concerns, and those of your States, regarding guidance on
determining whether an area is urban or rural for modeling
purposes.  We have also had subsequent telephone conversations
and you discussed the subject at the November 1986 Regional
Modelers Mini-Workshop.  My recommendation is that we not modify
our urban/rural guidance at this time but continue to entertain
exceptions to the guidance on a case-by-case basis.
  
    You have listed several possible criteria for justifying
exceptions to the guidance: heat flux, surface roughness,
supporting monitoring data and overall classification of the
area.  Of these various criteria, a justification based on a
comparison between monitored data and model estimates would
probably be the most defensible.  The overall classification of
the area, while supportable from a consistency standpoint, could
be a problem for justifying urban modeling of large sources in
rural areas adjacent to urban areas.  The first two criteria have
been espoused in the past but generally have not prevailed,
primarily because of their qualitative nature.  

    With these considerations in mind, let me comment briefly on
your shoreline power plant and refinery problems.  First, a much
more complete justification would be needed in each case before
we could offer an opinion.  Second, I concur with your suggestion
that monitoring data could be used to support a deviation from
guidance.  As we discussed on the telephone, while the
spatial/temporal coverage of monitors may not necessarily be as
rigorous as is the case for a performance evaluation, I believe
that the same principles hold, e.g., monitors should exist at
points of estimated maximum concentration for both the urban and
rural versions of the applicable model.  Finally, I suggest that
the justification in each case discuss the discrepancy between
monitored data and estimates as attributable to the urban/rural
question and not to some other phenomena such as downwash,
fumigation, etc.  

    In summary, I have been reminded that the current policy,
based on land use, was decided upon by consensus with the Regions
playing a major role.  I understand that many of the criteria you
mention were considered at that time but were not included in the
final policy by our mutual agreement.  However, because they were
considered does suggest that they have merit and, as with all of
our guidance, there is room for a different interpretation,
supported by these or other criteria, on a case-by-case basis.  

    If you have any further questions, please contact me. 

cc:  Reinders
     Rhoads
     Tikvart

bcc:  Regional Modeling Contact, Regions I-V, VI-X