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     In many applications, the removal of material from the plume may be extremely small, so that FQ(x) and P(x,z) are virtually unity.  When this happens, the vertical term is virtually unchanged (Vd = V, see Equation (1-54)).  The deposition flux can then be approximated as vdχ rather than vdχd.  The plume depletion calculations are optional, so that the added expense of computing FQ(x) and P(x,z) can be avoided.  Not considering the effects of dry depletion results in conservative estimates of both concentration and deposition, since material deposited on the surface is not removed from the plume.

1.1.7 The Decay Term (D)

     The Decay Term in Equation (1-1) is a simple method of accounting for pollutant removal by physical or chemical processes.  It is of the form:

where:

      ψ

=  the decay coefficient (s‑1) (a value of zero means decay is not considered)

      x

 

=  downwind distance (m)

 

For example, if T1/2 is the pollutant half life in seconds, the user can obtain ψ from the relationship:

     The default value for ψ is zero.  That is, decay is not considered in the model calculations unless ψ is specified.  However, a decay half life of 4 hours (ψ = 0.0000481 s‑1) is automatically assigned for SO2 when modeled in the urban mode.

 

 

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