7.3 Single Particle Trajectory

To continue the simplification, we will reduce the particle number release from 2500 to just one, but keep the total mass released the same, at one unit. This will illustrate the influence of a single particle trajectory on the air concentration.

1. The test calculation release was one unit for a duration of one hour. To change this value, press the large bar labeled Pollutant, Deposition, and Grids setup under the meteorology files section of the Concentration Setup menu. Then under the left most column labeled Pollutant, press the Specie 1 radio-button (there is only one pollutant). This then opens the emission menu where you will change the emission rate from 1.0 to 100.0 and the emission duration from 1.0 to 0.01. Notice that although we have changed the mass release rate, the total mass released is still one unit, because the rate is 100 units per hour but only for 0.01 hours. Effectively this means all the mass is released in one time step. Save to close out the Pollutant menu.


2. Now open the middle menu, labeled Grids where we can configure the concentration grid which accumulates the mass of each particle as its trajectory takes it through the computational domain. At this point we will change only one line. Change the height of the level (averaging layer) from 100 to 5000 meters. Because we will be following only one particle trajectory, we need to make sure the sampling layer is deep enough not to miss the particle as the particle moves through different levels in the boundary layer due to vertical mixing. Then Save to close the menus


3. Before running the model, one more modification is required: we need to insure that only one particle is released over the emission period. The default is to release 2500 particles over the emission period. That is we would follow 2500 trajectories, each with a 1/2500 of pollutant mass. These changes are made through the namelist file: SETUP.CFG. Press the Advanced / Configuration Setup / Concentration menu tab and select menu #4 to change the release number limits from 2500 to 1. Then Save to close out all menus.


4. Now from the main concentration tab, Run Model. You will be prompted again that the SETUP.CFG namelist file found!, which we created in the previous step, so this time select Run using Setup File. When the run has completed, again select the contour display menu. Note no changes are required, as the new layer height of 5000 m has been passed to the menu. Then Execute Display which will show a very narrow plume, essentially one concentration grid cell wide and very similar to the trajectories calculated in the first few sections of the tutorial.


5. Try the calculation again, but this time reverting back to a layer depth of 500 meters. Can you explain the missing plume?

The results shown here illustrate how a particle trajectory is used to compute air concentrations. This particular example, using one particle, is insufficient for a realistic simulation.