17.1 Counting Particle Trajectories




In the trajectory section, we reviewed the analysis of multiple trajectories using frequency plots or cluster analysis. There is a procedure similar to the trajectory frequency analysis that can be utilized for dispersion simulations by counting the particle intersections with the concentration grid.

  1. Start by loading captex_control.txt and captex_setup.txt settings into the GUI menu. As some of the previous examples, we will only use the sampling data from 3-hour duration collections and therefore set the run duration to 19 hours.

  2. Open the Advanced / Configuration Setup / Concentration menu #4 for Particle/Puff Release Number Limits and set the Particles released per cycle to 10000. The maximum number can be set to any value greater than the release number. Save to close the menu and then open menu #8 the Concentration Grid Packing Method and set the output units radiobutton to Mass. Setting this button (variable CMASS=1) results in only the summation of particle mass in each concentration grid cell. The final result is no longer divided by the cell volume, resulting in output units of mass rather than mass/volume. Save to close the menus.

  3. Before running the simulation, open the Concentration / Setup Run menu and select the Pollutant, Deposition, and Grids tab and open the Pollutant Definition menu and change the emission rate from 67000 to 3333.3. Because we are emitting for 3 hours at a rate of 3333.3 per hour, a total of 10000 units of mass will be released on 10000 particles, resulting in a mass of 1 unit for each particle. Save to close the menus and then run the simulation. After the run completes, check the MESSAGE file to insure that there is one unit of mass for each particle released. Results may differ slightly due to time step variations and roundoff errors.

  4. After it completes, first open the Advanced / File Edit / Border Labels menu and change the mass units to Particles and make the Volume field blank. Save and open the concentration display menu, and set the contours to 1000+500+200+100+50+20+10+5+2+1. Note that the concentration multiplier should be set to 1.0. At the first time period the maximum is around 150 particles. Remember only the 0-100 m layer is being examined. To see all the particles, the layer depth must be increased. At the last time period the maximum region contains about 24 particles.

  5. As the particles pass through the concentration grid, their mass will be summed. Because the concentration grid was defined for 3-hour averaging, a single particle may reside in the cell for multiple time steps. For instance a "concentration" value of 10 suggests a range of possibilities. One particle may have resided in the cell for 10 integration time steps or 10 particles may have been in the cell only for one time. To narrow the possibilities, it is necessary to output instantaneous values rather than time-averages.

A method was demonstrated above to show how the particle dispersion simulation can be used to provide the equivalent of a trajectory frequency plot, thereby accounting for the effects of dispersion and stratification, as well as the wind-driven mean particle motion.

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