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/ Controlled Programs Operations Branch (MD-15) TO: Bruce Miller, Chief Air Programs Branch, Region IV My staff and I have discussed the five situations involving the definition of ambient air that you sent on December 18, 1986. The following comments represent 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 scenario follow: Scenario One: We agree with you that the road and the unfenced property are ambient air and could be locations for the controlling receptor. Scenario Two: We agree with your determination in this case also. Scenario Three: We agree with you that the road is ambient air. However, Area B is not ambient air; it is land owned or controlled by the company and to which public access is precluded by a fence or other physical boundary. Scenario Four: We do not think that any of the barriers mentioned here are sufficient to preclude public access so as to allow the source to dispense with a fence. An example of an unfenced boundary that would qualify is a property line along a river that is clearly posted and regularly patrolled by security guards. Any area, such as grassy areas that might even remotely be used by the public, would have to be fenced even in this situation. We would not think that a drainage ditch would meet these criteria. Scenario Five: Both fenced pieces of plant property, even though noncontiguous, would not be considered ambient air (see Scenario Three). The road, of course, would be ambient air. Again, ownership and/or control of the property and public access are the keys to ambient air determination. I hope that these comments are helpful to you and your staff. This memorandum was also reviewed by the Office of General Counsel. Please call me if you have any comments. cc: S. Schneeberg P. Wyckoff R. Rhoads D. Stonefield Air Branch Chiefs, Regions I-X (Incoming Request Follows) United States Environmental Protection Agency DATE: December 18, 1986 SUBJECT: EPA Definition of Ambient Air FROM: Bruce P. Miller, Chief /s/ Air Programs Branch Air, Pesticides & Toxics Management Division TO: Tom Helms, Chief Control Programs Operation Branch (MD-15) SUMMARY The North Carolina Division of Environmental Management has asked for a clarification of ambient air in regards to a certain source located in North Carolina. The Regional Meteorologist's memorandum dated May 16, 1985, provides that for modeling purposes receptors are located everywhere outside of the continuous property of a plant to which the public is precluded due to a fence or other effective physical barriers. Attached are a number of scenarios for the source where we request a response on whether the receptors at certain locations are considered ambient air and whether the calculated modeling result at these receptors are to be considered in establishing an emission limit if one or more of these receptors is controlling. The Region IV opinion for each scenario is provided. Most of the scenarios we believe are dealt with adequately in the May 16, 1985 memorandum, however, there is a major concern on our part about how to interpret the modeling results in scenario numbers three, four and five. Please provide us with a written response by January 27, 1987. Please contact me or Mr. Lewis Nagler of my staff at FTS 257- 2864 if you require additional information. Enclosure (1) cc: Joseph Tikvart (MD-14) RTP, NC NORTH CAROLINA AMBIENT AIR SCENARIOS Scenario One The plant property is divided by a public road. The portion of the property on which a point source is located (Area A) is completely fenced. The property on the other side of the road (Area B) is unfenced. The Region IV position is that the road and the unfenced property are ambient air and if air quality modeling locates the controlling receptor in Area B, the emission limit will be determined based on the calculated concentration at that receptor. Scenario Two This scenario is the same as scenario one except that Area B is fenced except for the property along the public road. The Region IV position is identical to that provided in scenario one. Scenario Three This scenario is the same as scenario one except that all of Area B is fenced. The Region IV position is that the road is ambient air and that Area B should have receptors located there for modeling purposes. We also believe that since Area B is not continuous to that property that is needed for plant operation, even though fenced, Area B is ambient air. We further believe that if a receptor located in Area B is found to contain the controlling receptor for establishing the source emission rate then that receptor value must be used. There is a concern on our part that the May 16, 1985 memorandum could be interpreted to allow the Air Quality Management officials to discard the calculated concentrations within Area B. We believe a clarification of the ambient air policy on this point is needed. Scenario Four Area A is fenced except for the property along the public road. The Region IV position is that Area A is ambient air unless the source can demonstrate that the public is precluded to entry by an effective physical barrier. However, since a physical barrier other than a fence is subject to various interpretation, we are seeking advise on what we can accept as meeting that requirement. For instance, a drainage ditch alongside a road with no shoulder for parking or the use of "NO PARKING" signs could be considered an effective barrier. As you can see, the concept can be quite subjective and we require additional guidance in this area. For this actual situation, would you concur or non concur that no parking signs in association with no shoulder to park upon constitute a physical barrier? The Region IV position is that this situation does not constitute an effective physical barrier, but the addition of a drainage ditch would constitute an effective barrier. Scenario Five (Hypothetical) The entire plant is fenced. As a result of the county or state's power of eminent domain, a road is built through the property. Does the area that is no longer contiguous to the plant operation area lose its exemption from the ambient air definition even if the source fences off the area taken by the road? The Region IV position is that the area should be grandfathered in that situation.