11.2 Dry Deposition of Gases




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. Confirm that the maximum concentration at the last time period is the same as in the previous section: 8200. 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 200993 grams with the profile showing about 15% of the mass within the four layers up to 265 m above ground level.

  2. The dry deposition velocity is defined as the ratio of the deposition flux (mass/area/time) to the air concentration (mass/volume):
    • Vd = Ddry / C

    Dry removal is computed when the bottom of the puff or the particle center position is within the surface layer (Zsfc), defined in the model as the top of the second meteorological data level. The deposition velocity can be expressed as a time constant:
    • βdry = Vd ΔZsfc-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:
    • Dtotal = m {1-exp[-Δt (βdry + βwet + 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 preceding the 100 on the height of levels line. The deposited mass will be accumulated on level zero.

  5. Save to close and then 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 for these cases the deposition multiplier, the units label, and set the contours to Dyn-Exp if they had been fixed for one of the previous examples. As before, save the graphical results to a unique file name such as plot_dry.

  6. 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 2700. 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 129000 grams and only 11% of the mass within the four lowest model levels. The non-depositing run showed a maximum concentration of 8200 at the end of the run. With only 35% of the mass lost to deposition, the deposition accounted for the factor of 3 decrease in the maximum concentration.

The maximum concentration simulation results for the base case with no deposition and gaseous deposition, are summarized in the table shown below.

Day/Time Base Gas
25/21-00110000 76000
26/00-03 87000 53000
26/03-06 62000 35000
26/06-09 32000 18000
26/09-12 21000 9600
26/12-15 13000 5500
26/15-18 8200 2700

Dry deposition of gases removes mass due to the pollutant's interaction with elements of the surface such as vegetation, buildings, or the ground. Particles may interact with the surface the same way, but particles also move into the surface layer due to gravitational settling. This will be discussed in more detail in the next section.