The Basic Concentration Calculation

The first step in understanding the nature of the air concentration dispersion calculation will be to modify the test example and reduce the simulation down to its most basic component, a single particle trajectory, which we will then enhance step-by-step until the calculation is optimized to represent a realistic concentration simulation.

1. First from the main menu press the Reset button to set the menu variables for the test calculation and then press the Concentration / Setup Run menu tab to open the concentration setup menu to set all the concentration run parameters. Change the starting time to 83 09 25 17, the run duration to 12 hours, and the starting location to the tracer release point 39.90 -84.22 10.0. Clear the meteorology files and then add captex2_narr.bin from the Tutorial/captex directory. When finished with the changes, Save to close the menu window.


2. Then go ahead and press the Run Model tab under the Concentration tab. If the SETUP.CFG namelist file found! message appears, select Delete file and then Run because we have not yet created a namelist file for this example and it is probably left over from a previous calculation. When complete, Exit and continue to the next step.


3. To create the concentration graphic, press the Concentration / Display / Concentration / Contours menu tab to open the display menu, where you should just accept the defaults, press Execute Display to open the plume graphic which shows color filled contours of 12-hour average air concentrations at factor of 10 intervals ranging from 10^-12 to 10^-15 mass units per cubic meter. This calculation is identical to the original test case except we moved the location and time to be consistent with CAPTEX release number two.


4. 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.


5. 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


6. 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.


7. 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.


8. To examine the details of the calculation we need to eliminate the temporal averaging. Open the Concentration / Setup menu and the open the grids menu and change the last line from 00 12 00 to 01 01 00. This changes the particle mass accumulation from a 12 hour average to a snapshot value once an hour. The term "now" equals a one-time-step accumulation.


9. Save the change, run the model, and open the display menu. There will now be 12 frames in the graphic, one for each hour, the first frame shows the concentration pattern after one hour. Note the concentration grid cell size is represented by the red square and although it looks like the pattern covers several grid cells, a single particle calculation can only be contributing to one cell during a time step. The bleeding of the concentration contours into adjacent cells is just an artifact of the smoothing and contouring process.


10. To see all the frames in a single animated graphic, similar to a trajectory, press the Concentration / Utilities / Convert Postscript menu tab to open the ImageMagick conversion menu. Select the Animate and crop radio-buttons, and move the slider bar to a higher resolution (140) for a better quality image. After executing the conversion, the window closes with no message, but you will find the animated GIF, concplot.gif in the working directory. The animation is a little jumpy due to the atmospheric turbulence affecting this single particle trajectory.

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. These issues will be explored in more detail in the following sections.