4.9 DEPOSITION CLASSES

        The dry and wet deposition rates are computed as described above. A mass budget approach is used to compute the deposition of net Gaussian plume source depletion fractions as a function of distance from the release.

        The implementation of removal rates is accomplished in MEPAS using seven deposition classes which represent the constituent properties shown in Table 4.1. The air concentration and deposition patterns are computed for each of these classes based on a unit emission rate. Depending on the properties of a constituent, each constituent is assigned to a deposition class. Then the air concentration and deposition patterns for each constituent are computed using the appropriate normalized concentration patterns and the constituent emission rate. In the MEPAS user interface the deposition classes are allocated in the following manner.

        The constituent database contains a "default" entry for the deposition class for each constituent. If the material is released in a particulate form, this value is 1 indicating that one of the first three classes are appropriate. The MEPAS user interface assumes that Class 1 is most appropriate for area releases of suspended soil materials and that Class 3 is most appropriate for point releases (stack and vent releases). Class 2 is not currently available through the MEPAS user interface. If the material is released in a gaseous form, then the constituent database contains the most appropriate value from Classes 4, 5, 6, and 7.

TABLE 4.2 Definition of Deposition Classes
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Deposition Class	Class of Constituent
1	particles with average radius = 7.5 ì
2	particles with average radius = 3.0 ì
3	particles with average radius = 0.3 ì
4	gas with moderate surface resistance (s/cm)
5	nondepositing gas(a)
6	gas emission thatdeposits as a Class 3 particle
7	as with fast deposition rate (zero surface resistance)