11.3 Dry Deposition of Particles

Dry deposition of particles is a continuation of the previous section on gaseous dry deposition. Complete the simulations in that section before starting these particle deposition examples. Otherwise start by retrieving the previously saved deposit_control.txt and deposit_setup.txt settings into the GUI menu and re-run the base case simulation. Follow the same process as in the previous section and note the maximum concentration for each of the first eight time periods.

1. To configure the simulation for particle deposition, it is necessary to define the particle's characteristics. The particle settling velocity (V_g) is calculated for a spherical particle from the particle diameter (d_p), air density (ρ), and particle density (ρ_g):
   • Vg = d_p² g (ρ_g - ρ) (18 μ)^-1
   
   Open the Setup Run / Deposition menu and set the particle diameter, density, and shape all to 1.0. The shape factor ranges from 1 for a sphere to 2. A value of 2 reduces the settling velocity by half. Setting these to non-zero values defines the pollutant as a particle rather than a gas. Getting the density and diameter correct is essential in computing the correct settling velocity. However, for this exercise we will take a shortcut and force the model to use a fixed value, regardless of the particle settings. This means that the previously entered value of 0.01 (1 cm/s) will be used to compute the deposition, but more important this value will be added to the vertical velocity in the particle trajectory advection computation. Save to close the setup menu and run the model.


2. When the run completes, display the concentrations and note the maximum value each time period. As before, save the graphical results to a unique file name such as plot_part. At the end of the run the maximum concentration is now 4500 rather 3000 at the end of the gaseous deposition simulation. Open the MESSAGE file and the end mass is now 95861 grams, considerably less that the gas run, but the concentrations are higher. The answer lies in the profile, which shows 20% of the mass in the lowest four layers. Although the deposition rate is the same between the gas and particle run, the additional gravitational settling of the particles moves more mass into the lower layers, providing more mass to be removed, and counter-intuitively, higher concentrations near the surface, at least during the earlier time periods.


3. All the previous simulations ended with the same number of particles as were released. However, instead of 10 grams per particle (200,000 g / 20,000 p), each may only end up with considerably less mass by the end of the simulation. There is an alternative approach, where instead of removing a fraction of the particle’s mass, we can compute the probability that a particle will deposit all of its mass and then only a fraction of the particles in the deposition layer will be deposited each time step. In this situation, if R is a random number from 0 to 1, then a particle will deposit if
   • R < β_dry Δt
   
   In this particle deposition approach, the particle number goes down as the particles stick to the surface. To set this option, open the Advanced / Configuration / Conversion Modules Menu #10 and set the Deposit Particles ... radio-button. Save the changes and run the model. The simulation log will indicate that the dry deposition probability option has been selected. When the run has completed, save the graphical results to a unique file name such as plot_prob and again note the maximum concentrations each time period. Note they are almost identical to the previous simulation. Open the MESSAGE file which shows a mass of 95761 grams but this time distributed over 9685 particles (10 g/particle) rather than 95861 grams over 20,000 particles (5 g/particle) in the mass deposition approach.

The maximum concentration simulation results for no deposition, gaseous deposition, explicit particle deposition, and particle probability deposition are summarized in the table shown below.

Day/Time	Base	Gas	Part	Prob
25/18-21	250000	190000	220000	190000
25/21-00	120000	79000	100000	99000
26/00-03	79000	47000	69000	73000
26/03-06	57000	32000	45000	49000
26/06-09	33000	18000	25000	26000
26/09-12	19000	9200	13000	15000
26/12-15	14000	6200	7700	8200
26/15-18	9000	3000	4500	4600

Normally the default would be to just remove particle mass rather than reducing the number of particles during a simulation. Although removing particles may make long duration simulations with continuous emissions more computationally efficient by eliminating particles, there may be issues at greater downwind distances of insufficient particle density to provide for smooth concentration or deposition patterns.