Pollutant Dry Deposition

The term dry deposition in the context of how it is implemented within HYSPLIT refers to the depletion of pollutant mass from gases and particles when they interact with the ground surface in the absence of rainfall. Gases or particles within the surface layer will loose mass at a rate proportional to the deposition velocity. Also particles (but not gases) will gravitationally settle downward toward the surface where they can then be subject to dry deposition.

1. Naturally all the calculations in this section are just examples because the PMCH tracer does not deposit. Start by retrieving the previously saved captex_control.txt and captex_setup.txt settings into the GUI menu. We will only focus on the first day's results, so set the run duration to 25 h and 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. Save the MESSAGE file to a unique name. From the MESSAGE file, note that at the end of the 25-h simulation the system mass is still 200968 grams with the profile showing about 18% of the mass within 200 m of the ground.


2. The dry deposition velocity is defined as the ratio of the deposition flux (mass/area/time) to the air concentration (mass/volume):
   • V_d = D_dry / C
   
   Dry removal is computed when the bottom of the puff or the particle center position is within the surface layer (Z_sfc), defined in the model as the top of the second meteorological data level. The deposition velocity can be expressed as a time constant:
   • β_dry = V_d ΔZ_sfc^-1
   
   The total deposition from all removal processes can be expressed in terms of the time constants. The time constants can be added and hence the total deposition for particle mass m over a time step becomes:
   • D_dry = m {1-exp[-Δt (β_dry + β_... )]}


3. To compute gaseous dry deposition, open the Setup Run / Pollutant-Deposition-Grids / Deposition menu and change the deposition velocity, the first number in the second line, from 0.0 to 0.01 (1 cm/s a relatively high removal rate). The other entries on this line will not be discussed here but are required if the resistance method is going to be used to compute the deposition velocity instead of explicitly setting a value. One other change is required. To be able to view the deposition amounts, rather than just its effect on air concentration, a deposition level needs to be added to the concentration grid.


4. Open the Setup Run / concentration grid menu and change the number of vertical levels from 1 to 2 and add the level height of 0 preceeding the 100 on the height of levels line. The deposited mass will be accumulated on level zero. Save the changes and run the model. When it completes, open the display menu and insure that the bottom level radio-button for height 0 and top-level height 100 is selected. On the deposition multiplier line check the Total radio-button. This will show the deposition accumulate between sampling time periods rather that just showing the deposition for each time period. As in all previous examples, set the concentration multiplier and units label.


5. Execute the display and sequence through each frame and note the maximum concentration each time period. The last time period shows a maximum concentration of 5900. The total deposition frame is at the end and shows the deposition footprint as the plume passed over the region. Check the MESSAGE file which shows a mass of 109328 grams and only 9% of the mass within the three lowest model levels. The non-depositing run showed a maximum concentration of 25,000 at the end of the run. With half the mass lost to deposition and the lowest layers showing the greater mass losses (9% versus 18%), deposition accounts for the factor of 4 decrease in the maximum concentration.


6. 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. 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 the changes and run the model.


7. When the run completes, display the concentrations and note the maximum value each time period. At the end of the run the maximum concentration is now 11000 rather 5900 at the end of the gaseous deposition simulation. Open the MESSAGE file and the end mass is now 70381 grams, considerably less that the gas run but the concentrations are higher. The answer lies in the profile, which shows 23% of the mass in the lowest three 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.


8. All the previous simulations ended with the same number of particles as were released. However, instead of 4 grams per particle, each may only end up with considerably less mass by the end of the simulation. There is 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 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, again note the maximum concentrations each time period. Note they are identical to the previous simulation. Open the MESSAGE file which shows a mass of 70572 grams but this time distributed over 18,000 particles (4 g/particle) rather than 50,000.

The maximum concentration simulation results are summarized in the table shown below.

Day/Time	Base	GAS	PART	PROB
25/18-21	440000	280000	360000	370000
25/21-00	130000	78000	87000	84000
26/00-03	56000	34000	34000	33000
26/03-06	50000	27000	32000	33000
26/06-09	42000	19000	24000	24000
26/09-12	32000	13000	17000	17000
26/12-15	33000	10000	17000	17000
26/15-18	25000	5900	11000	11000