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.

  1. To configure the simulation for particle deposition, it is necessary to define the particle's characteristics. The particle settling velocity (Vg) is calculated for a spherical particle from the particle diameter (dp), air density (ρ), and particle density (ρg):
    • Vg = dp2 g (ρg - ρ) (18 μ)-1

    Open the Setup Run / Deposition menu and set the particle diameter, density, and shape all to 1.0. 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 2700 at the end of the gaseous deposition simulation. Open the MESSAGE file and the end mass is now 95391 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 4 grams per particle (200,000 g / 50,000 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 < βdry Δt

    In this situation, 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 95528 grams but this time distributed over 24,173 particles (4 g/particle) rather than 50,004.

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
25/21-00110000 76000100000100000
26/00-03 87000 53000 66000 66000
26/03-06 62000 35000 47000 46000
26/06-09 32000 18000 25000 25000
26/09-12 21000 9600 14000 14000
26/12-15 13000 5500 8300 8300
26/15-18 8200 2700 4500 4800

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.