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+
βgas+βinc+βbel ) ] }.
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.�
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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. |
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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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