# 11.1 Linear Mass Conversions

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Normally when multiple pollutants are defined, each pollutant is released on its own particle and different pollutants cannot interact with each other because they follow different transport pathways. However, there is an option for a simple linear transformation from one pollutant to another by defining multiple pollutants on the same particle. The multiple pollutant single particle option should only be invoked when the pollutants have comparable transport characteristics.

1. Start by retrieving the previously saved captex_control.txt and captex_setup.txt settings into the GUI menu. Open the Setup Run menu and change the run duration to 25 h and open the Configuration menu and reduce the particle number to 20000. Both changes will speed up the calculation.

2. Before turning on the conversion option, run the model to create a baseline calculation. Create the eight concentration contour plots and save the graphical results to a unique file name (plot_base) as a reference to compare with the transformation simulations. This and all subsequent sections under pollutant transformations are just a theoretical exercise because the PMCH tracer used in CAPTEX is inert and does not deposit.

3. The pollutant transformation computation only requires defining an hourly conversion rate. The pollutant mass fraction (β) converted each time step is computed directly from the conversion rate (R) which has a default value of 0.10 hr-1:
• β = R Δt

The mass (Mr) removed from pollutant M1 when β < 0.01
• Mr = β M1

but when β > 0.01
• Mr = (1 - exp) M1

In each case, the mass on pollutant M2 is increased by
• Mr W2/W1

where W2/W1 is the ratio between the molecular weights of the two pollutants. The molecular weight adjustment factor can be used to account for other reactions not considered in the simple conversion module. For instance, if one were to define pollutants #1 as SO2 and and #2 as SO42-, then the molecular weight adjustment factor should be set to 1.5 as SO2 transforms to SO42- (the conversion picks up two additional oxygen molecules).

4. To demonstrate the conversion calculation, open the pollutant menu from the Pollutant, Deposition, Grid tab. At the top of the Pollutant column, change the num=1 to 2, which then populates the Specie 2 sub-menu with the values from Specie 1 (PMCH). Click on Specie 2 to open the emission rate menu and change the pollutant name from PMCH to POL2, and change both the emission rate and duration to 0.0. We could emit both pollutants and transform them at the same time, but for this example, POL2 mass will only come by conversion from Specie 1.

5. Now open the Advanced / Configuration / menu #10 conversion modules and select the [fraction] per hour radio-button. It is only necessary to define two different pollutants in the concentration setup menu and select the ICHEM=2 option. This combination automatically sets MAXDIM=2 in the model code (it does not need to be set in the GUI) and calls the transformation routine every time step to convert pollutant #1 to #2 at a default rate of 10% per hour. Other rates can be entered as needed. If more than two species are defined, then the single particle mass dimension needs to be explicitly set in the menu (last data entry line). Now save the changes and run the model.

6. After the simulation has completed, open the Display / Concentration / Contour menu. The menu now shows the two pollutants as display possibilities. First change the default Postscript output file name from its default concplot to a unique name, such as plot_pol1. The .ps suffix is added automatically. Then select the PMCH radio-button as the pollutant to display. As in previous examples, set the pg units conversion to 1.0E+12 and increase the zoom to 80%, then Execute the Display. You could also deselect the View On check-box because right now we only want to create all the Postscript files. The viewer is not yet required. Once the PMCH Postscript file has been created, change the options, and create another file for plot_pol2.

7. Rather than going through a frame-by-frame comparison of the 3-hour PMCH and POL2 concentration averages (the plumes must cover identical regions because both pollutants are on the same particle), just find the maximum concentration on each graphic (printed toward the bottom of the contour labels text box), and note the value for each time period for the first 24 hours, for the base simulation with no transformation shown in the first column is comparable to the sum of the PMCH and POL2 calculation. These values are also shown below and may differ slightly from your values due to differences introduced from the hardware/software dependent random number generator driving the 3D particle dispersion.

 Day/Time Base PMCH POL2 25/18-21 250000 200000 47000 25/21-00 120000 70000 47000 26/00-03 79000 41000 38000 26/03-06 57000 21000 36000 26/06-09 33000 8900 25000 26/09-12 19000 3700 15000 26/12-15 14000 2000 12000 26/15-18 9000 980 8000

The results show how much more rapidly the PMCH concentrations decrease above the base simulation due to the additional conversion and correspondingly the POL2 concentrations show only a small decline as decreasing concentration due to dispersion is compensated by increasing mass due to the conversion from PMCH. After 24-h into the simulation, the POL2 concentrations are comparable to the base calculation because most of the pollutant has been converted.

8. Normally other conversion rates or a greater number of pollutants are required for more realistic simulations. In these situations it is necessary to edit the CHEMRATE.TXT file in the local directory. This file consists of one or more records, where each record defines a pollutant conversion. The data are free-format and consist of four fields, the integer "from" and "to" pollutant index (specie) numbers, the real hourly conversion rate, and molecular weight adjustment factor. For instance, a file representing the default conversion would consist of the following single line:
• 1 2 0.10 1.0

This file is created automatically by the GUI for the conversion rate selected. Try editing the CHEMRATE.TXT file and running the conversion with a different conversion rate or mass adjustment factor.

The HYSPLIT pollutant transformation option is not intended to replace the need for more complex chemical models that account for the non-linear simultaneous interaction of multiple pollutants, but to provide some first-order guidance as to the potential effect of transformations that can be represented by just a few pollutants. There are some advanced chemical modules available for HYSPLIT that have been developed for specific applications. However, all of these are beyond the scope of this tutorial.