Pollutant Wet Deposition

The term wet deposition refers to the removal of pollutants, both gases and particles, by condensation, or rainfall, either within a cloud or below the cloud. Naturally all the calculations in this section are just examples because the PMCH tracer does not deposit. As in the previous sections, 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 as was done previously for dry deposition, 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. Then re-run the base case simulation without any deposition, wet or dry.

1. The wet deposition of gases depends upon their solubility and for inert non-reactive gases it is a function of the Henry's Law constant (H), the ratio of the pollutant's equilibrium concentration in water to that in air. A gaseous wet deposition velocity can be defined as
   • V_gas = H R T P
   
   where R is the universal gas constant, T is temperature, and P is the precipitation. Then the gaseous wet removal time constant can be defined:
   • β_gas = V_gas ΔZ ^-1
   
   Note that the wet removal of gases is applied at all levels from the ground to the top of the cloud-layer. The wet removal time constant is added to the other removal constants to compute the particle mass loss each time step.


2. Open the Setup Run / Deposition menu and set the wet deposition radio-button to Yes to populate the wet deposition removal parameters with their default values. For gases, the default value of 1.0E+5 is the Henry's constant for hydrogen peroxide (H2O2). The other two values on that line are ignored for gases. Save the changes, run the model, and after it completes, open the display menu and advance to the last frame showing the total deposition pattern. It shows two distinct rainfall areas near 41.5N 81.5W and 41.5N 79.5W. Looking at the individual time-period deposition patterns shows that wet removal ocurred on the 26th between 00-03 and 06-12. The MESSAGE file shows a total mass of 173006 grams at the end of the run.


3. Wet deposition of particles is divided into two processes, those in which the polluted air is continuously ingested into a cloud from a polluted boundary layer and those in which rain falls through a polluted layer. A scavenging ratio is assumed for pollutants located within a cloud layer and the scavenging coefficient is used for pollutant removal in rain below a cloud layer. At the grid points where it is raining, the cloud bottom is defined at the level when the RH first reaches 80% and the cloud top is reached when the RH drops below 60%. All removal amounts are adjusted by the fraction of the pollutant mass that is within the cloud layer.


4. For the wet removal of particles by within-cloud processes a scavenging ratio (S), which is the ratio of the pollutant's concentration in water to its concentration in air, is expressed as a wet deposition velocity,
   • V_inc = S P
   
   where the precipitation rate is given by P. The time constant for within-cloud removal,
   • β_inc = V_inc ΔZ ^-1
   
   where the average scavenging ratio is S=4x10⁴ by volume, and as before, ΔZ is the depth of the pollutant layer. Different scavenging ratios can be defined for different pollutants. Below-cloud removal is defined directly as a rate constant, independent of the precipitation rate. The below-cloud removal constant (s^-1) is given by,
   • β_bel = 5x10^-5
   
   
5. Open the Setup Run / Deposition menu and reconfigure the emissions for particles by setting the three values on the first line all to one. However, to avoid computing dry deposition, force the dry deposition to a very small value (1E-10) so that we can see the only the effects of wet deposition. Save the changes, run the model, and display the results. The final particle deposition pattern looks very similar to the gaseous pattern but with slightly less deposition. The MESSAGE file shows a total mass of 193846 grams at the end of the run.


6. Wet removal can be very effective when the pollutant is concentrated in one area as it would be near the source. As an example of this, move the starting location to the first precipitation area. Open the Setup Run / new location menu and enter the position 41.5 -81.5 and change the starting time to 83 09 26 00. Save the changes and run the model. There is no need to display the results, but open the MESSAGE file, which shows a mass of 151163 grams after just 3-hours and 123382 grams at the end of the simulation. Essentially 25% of the mass released was deposited by the end of the release!
   

Dry deposition rates may be small but they occur everywhere at all times resulting in large losses over longer time periods, while large losses by wet deposition are limited to areas with rainfall and over much shorter time periods. However, model derived rainfall predictions may be very uncertain and access to observed precipitation such as from NASA TRMM may be useful in the interpretation of modeling results.