In the previous section, to determine the location or magnitude of an unknown source, the dispersion model was integrated backwards in time from all the measurement locations to determine the geographic area of the potential source. In this section, we will run the model forward from a grid of potential source locations and then in the next section, determine which location gives the best fit with the measurements.
- If not continuing from the previous section, after pressing reset, retrieve the saved geol_control.txt and geol_setup.txt settings into the GUI menu. Insure that the run starts on 83 09 01 00 for a duration of 60 hours using the RP198309.gbl meteorological data file. Using the sampling domain map as guidance and following the same approach as was used in the trajectory matrix, open the source location menu for 3 locations and enter the lower left (40,-78) as the first location, upper right (45,-70) as the second, and the first internal grid point (40.5,-77.5) as the third location. This will create a regular grid of emission points at 0.5 degree resolution.
- Then open the concentration grid menu and if required, set the sampling start and stop times to 83 09 01 00 and 83 09 03 12 00 and three-hour averaging. Provide a unique ouput file name such as srm.bin. Save the changes and open the pollutant menu and change the emission rate to a unit value, in this case 1.0 so that one unit of mass is released each hour for the duration of the simulation, 60.0 hours. Save the change and close the Setup Run menu.
- Now open the Advanced / Concentration Setup / Concentration / Conversion Modules menu #10 and check the radio-button to restructure the concentration grid to the source-receptor format. The effect of this switch is that instead of all sources contributing mass to a single concentration grid, each source has its own concentration grid, which can be treated as a matrix with each row representing a source location and the columns representing the receptors (the concentration grid points). Plotting the values on a row gives a concentration map for that source. Plotting the values in a column gives the contribution of each source to that receptor.
- Also open the Advanced / Concentration Setup / Concentration / Release Number Limits menu #4 and set the particle release number at 500000 because it will be divided over the release location matrix (187). By removing particles after 30 hours, the maximum number of particles will never exceed 300000 and the simulation should run very quickly.
- Save the changes to close the menu and then run the model from the Special Runs / Matrix menu tab. When the run completes, press the Display / Source-Receptor / View menu tab, to open the display menu. This menu calls a special program to extract the source or receptor information from the matrix output file. It writes out a single binary file that can be displayed with the normal concentration plotting program or any other concentration utility. Rename the binary output file to give it a unique name, such as SRM_fwrd.bin. The extraction method radio-button should be set to source, which means we are going to extract the row computed from the tracer source location entry box values: 43.0 -75.0. Then execute to view the plume dilution factors at the end of the sampling period, the 3rd from 0900-1200.
- Go back and check the receptor radio-button and enter the location near the center of the previously created forward plume, such as 44.0 -70.0 and give the output file a unique name, such as SRM_back.bin The resulting source display map now at the beginning of the sampling shows the dilution factors for sources contained within the contoured region.
- The forward dilution factor from 43N 75W to 44N 70W is approximately 10-12 and it is the same as the backward dilution factor from 44N 70W to 43N 75W. Naturally the forward and backward dilution factors must be identical because they are all derived from the same forward calculation. This can be checked by using the Grid to Station analysis as was done in the forward-backward tutorial.
- For future reference save the CONTROL file to matrix_control.txt and the SETUP.CFG file to matrix_setup.txt.
The results shown here are another approach to source location from the source perspective, relying only on a forward calculation from all potential source locations. Simply selecting a receptor location will show the contributions of all sources contributing to that location. The computation can be very resource demanding, in terms of time and memory, because a unique concentration grid is required for each source.
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