United States Environmental Protection Agency
                     Washington, D. C. 20460

                       September 16, l982



MEMORANDUM

SUBJECT:  Milwaukee SO2 Nonattainment Designation

FROM:     Sheldon Meyers, Director  /s/
          Office of Air Quality Planning and Standards (ANR-443)

TO:       David Kee, Director
          Air Management Division, Region V


     Thank you for your August 9, l982, memorandum to Assistant
Administrator Bennett regarding Wisconsin's request for a
redesignation attainment of the sulfur dioxide (SO2) standard for
the Milwaukee area.  

     You asked four (4) separate questions in your memo.  Those
questions are repeated in full below along with my responses.  

Q)   In nonattainment areas with no emission limits, what is
required to support a redesignation to attainment?  (It does not
appear to be sufficient to accept eight quarters of data showing
no violations, even if the monitors were located in the expected
high concentration areas.)  

A)   The fact that no Federally enforceable emission limits are in
place does not affect the criteria applied in determine the area's
attainment status.  In general, Section 107 designation changes
should utilize all available data, including both monitoring and
modeling data.  Whatever is available should certainly be used. 
Monitoring data should be used only within the limits of being
representative for a specific geographic area.  The object of any
designation should be to make the best decision based upon the
maximum amount of available information.

Q)   What is the role of modeling in redesignations?

A)   The need for dispersion modeling for Section 107 designation
purposes is especially important when dealing with areas dominated
by point sources of SO2.  In these cases, a small number of ambient
air quality monitors will not be able to tell the whole story. 
Modeling is essential to evaluate comprehensively and thoroughly
the sources' impacts as well as identify the areas of highest
concentrations.  It must be included in a redesignation analysis
where feasible.

     For all other areas, if modeling already exists, it should be
considered.  However, dispersion modeling is generally not required
to be performed strictly for the purposes of Section 107
redesignation requests for such areas.

Q)   Is a redesignation to attainment acceptable if there are eight
quarters of monitored data showing no violations but there is
modeling that predicts violations?  (Note, this is not to say that
the modeling contradicts the monitoring since the modeling shows
attainment at the monitor locations, but nonattainment at other,
nonmonitored locations.)

A)   There is no answer that fits all possible situations. 
However, where valid dispersion modeling has been performed, such
modeling results should set the designation status.  When the
appropriateness of the model is of some concern, Regional Offices
must exercise judgment after considering such things as how many
monitors are in the network; is complex terrain (terrain greater
than stack height) involved; what model is being used; is it a
guideline model, if not, has it been demonstrated to be
appropriate; does the model tend to over- or under-predict for the
situation at hand?

     Again, it should be emphasized that the objective is to make
the best determination possible using all relevant information as
to what the attainment status of an area really is.  

Q)   Mr. Barber's letter says that adequate monitored data are
necessary.  How is "adequate" defined?  (We suggest that a
determination of adequate monitoring data involve reference
modeling.  That is, monitors must be located in the areas of
expected high concentrations, based on a reference modeling
analysis.)

A)   Your suggestion is what ideally should be required.  However,
monitors are seldom sited at the locations shown by later
dispersion modeling to be those of maximum impact.  

     Again, the responsibility lies with the Regional Office to
make the necessary judgments as to whether or not the existing
monitor locations are sufficient both in number and spatial
arrangement to allow them to be representative of the air quality
for the area.  Some judgment as to whether the potential problem
is of a localized or more general areawide nature should be made. 
This judgment will influence whether modeling or monitoring should
be given preference in the particular situation in question.  How
much information is needed before such a judgment can be made is
subject to the complexity of the situation.  

     I would like to add the following comments regarding the
particular situation in Milwaukee, Wisconsin, as described in the
background portion of your August 9, l982, memo.

     In a situation where an area was originally designated
nonattainment based on measured violations but subsequently has air
quality measurements less than the ambient air quality standard,
common sense would recognize the need for a study of the situation,
including modeling.  It could not reasonable be expected that
violations would disappear by themselves.  If a source has
voluntarily made some emission reduction changes that eliminate
violations, these changes need to be embodied into regulation and
then be made part of the approved State Implementation Plan (SIP)
control strategy.  The approval of such emission limits as part of
a SIP must be based on an adequate demonstration that ambient air 
quality standards will be protected.  Such a demonstration must
include a dispersion modeling analysis under worst case conditions.

     If you have any other questions, please let me know.