United States Environmental Protection Agency
          Office of Air Quality Planning and Standards
          Research Triangle Park, North Carolina 27711

                         April 30, 1987


MEMORANDUM

SUBJECT:  Ambient Air

FROM:     G. T. Helms, Chief  /s/
          Control Programs Operations Branch (MD-15)

TO:       Steve Rothblatt, Chief
          Air Branch, Region V


     My staff and I have discussed the five ambient air cases which
you submitted for our review on January 16, 1987.  The following
comments are our interpretation of the ambient air policy. 
However, this memorandum is not a discussion of the technical
issues involved in the placement of receptors for modeling.  

     Our comments on each of the cases follow:

     Case 1 (Dakota County, MN):  This case involves two
noncontiguous pieces of fenced property owned by the same source,
divided by a public road.  We agree that the road is clearly
ambient air and that both fenced pieces of plant property are not.

     Case 2 (Warrick County, IN):  This case involves two large
sources on both sides of the Ohio River.  We agree that receptors
should be located over the river since this is a public waterway,
not controlled by the sources.  We also agree that the river does
indeed form a sufficient natural boundary/barrier and that fencing
is not necessary, since the policy requires a fence or other
physical barrier.  However, some conditions must be met.  The
riverbank must be clearly posted and regularly patrolled by plant
security.  It must be very clear that the area is not public.  Any
areas where there is any question--i.e., grassy areas, etc.--
should be fenced and marked, even if there is a very remote
possibility that the public would attempt to use this property.  

     However, we also feel that current policy requires that
receptors should be placed in ALCOA and SIGECO property for
modeling the contribution of each source's emissions to the other's
ambient air.  Thus, ALCOA's property--regardless of whether it is
fenced--is still "ambient air" in relation to SIGECO's emissions
and vice-versa.  

     Case 3 (Wayne County, MI):  This case involves the air over
the Detroit River, the Rouge River and the Short-cut Canal.  We
agree that the air over all three of these is ambient air, since
none of the companies owns them or controls public access to
them.  Note, however, that one source's property--regardless of
whether it is fenced--is the "ambient air" relative to another
source's emissions.

     Case 4 (Cuyahoga County, OH):  This case involves LTV
Steel's iron and steel mill located on both sides of the Cuyahoga
River.

     We do not feel that LTV Steel "controls" the river traffic
in that area sufficiently to exclude the public from the river,
whether it be recreational or industrial traffic.  The fact that
there is little or no recreational traffic in that area is not
sufficient to say that all river traffic there is LTV traffic. 
The public also includes other industrial users of the river that
are not associated with LTV.

     It is difficult to tell from the map whether the railroad
line is a through line or not.  If the railroad yard serves only
the plant then it would not be ambient but the railroad entrance
to the plant would have to be clearly marked and patrolled. 
However, if the line is a through line then that would be ambient
air.  We would need additional information to make a final
determination.  

     The unfenced river boundaries should meet the same criteria
as in Case 2 above.

     Case 5 (involves the placement of receptors on another
source's fenced property):  As mentioned above in Case 2, we feel
that present policy does require that receptors be placed over
another source's property to measure the contribution of the
outside source to its neighbor's ambient air.  To reiterate,
Plant A's property is considered "ambient air" in relation to
Plant B's emissions.

     I hope that these comments are helpful to you and your
staff.  This memorandum was also reviewed by the Office of
General Counsel.  

cc:  S. Schneeberg
     P. Wyckoff
     R. Rhoads
     D. Stonefield
     Air Branch Chiefs, Region I-X