Pollutant Deposition

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    HIGHLIGHTS
  • Defining deposition
  • Dry deposition velocity
  • Gravitational settling
  • Deposition probability conversion

The deposition (D) from a particle is expressed as a fraction of the mass (m) computed as the sum of different time constants,

Dwet+dry = m {1-exp[Δt(βdry+ βgasincbel ) ] }.

Configure the model to run for 24 hours with default values, from the previous Florida location of 28.5N, 80.7W using NAM 12 km data, releasing one unit over one hour. Reset and save the settings in the Advanced / Configuration Setup / Concentration menu. Set the concentration grid for 0.05 degrees resolution with one output level at zero meters (for deposition). This simulation will yield no results because deposition has not been activated. Open up the Deposition menu from the Pollutant, Deposition & Grids Setup menu (right) and set the deposition velocity to 0.006 m/s, leaving everthing else as default values. When the deposition is entered directly as a velocity (Vd), then
βdry = Vd ΔZp-1.

Deposition Velocity Setup Menu.

The results (below left) show the deposition left by the puff as it moved across the domain.

The dry deposition of particles due to gravitational settling can also be computed from the particle diameter and density:
Vg = dp2 g(ρg - ρ) (18 )-1.

Enter a density of 5 g/cc and a diameter of 6 um, which should result in a settling velocity close to previous 0.6 cm/s. The results (below right) from this configuration are almost identical to the previous calculation.

Deposition Velocity

Settling Velocity

The default approach is for the number of particles and puffs to remain the same but lose mass due to deposition. If the Deposit particles rather than reducing the mass of each particle option is selected in the Advanced / Configuration Setup / Concentration / In-line chemical conversion modules menu, then particles will be lost if R < βdryΔt, and where R is a random number between 0 and 1. The model must be configured for 3D particles to use this option. If a sufficient number of particles are released the results will be identical to the other deposition options. In this case, more than 15,000 particles are needed to produce a similar deposition pattern.


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