For cases in which receptor elevations are lower than the base elevation of the source (i.e.,

receptors that are down-slope of the source), AERMOD will predict concentrations that are less

than what would be estimated from an otherwise identical flat terrain situation. While this is

appropriate and realistic in most cases, for cases of down-sloping terrain where expert judgment

suggests that the plume is terrain-following (e.g., down-slope gravity/drainage flow), AERMOD

will tend to underestimate concentrations when terrain effects are taken into account. AERMOD

may also tend to underestimate concentrations relative to flat terrain results for cases involving

low-level, non-buoyant sources with up-sloping terrain since the horizontal plume component

will pass below the receptor elevation. Sears (2003) has examined these situations for low-level

area sources, and has shown that as terrain slope increases the ratio of estimated concentrations

from AERMOD to ISC (which assumes flat terrain for area sources) decreases substantially.

To avoid underestimating concentrations in such situations, it may be reasonable in cases of

terrain-following plumes in sloping terrain to apply the non-DFAULT option to assume flat,

level terrain. This determination should be made on a case-by-case basis, relying on the

modeler’s experience and knowledge of the surrounding terrain and other factors that affect the

air flow in the study area, characteristics of the plume (release height and buoyancy), and other

factors that may contribute to a terrain-following plume, especially under worst-case

meteorological conditions associated with the source. The decision to use the non-DFAULT

option for flat terrain, and details regarding how it will be applied within the overall modeling

analysis, should be documented and justified in a modeling protocol submitted to the appropriate

reviewing authority prior to conducting the analysis.


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