Linear Transformation of Pollutants

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 and re-run the simulation if the results have not already been saved from one of the previous exercises. Create the concentration contour plots as before and save the graphical results to a unique file name 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.


2. The pollutant transformation computation only requires defining an hourly conversion rate. The pollutant mass fraction converted each time step (β) is computed directly from the conversion rate (R) which has a default value of 0.10 hr^-1:
   • β = R Δt
   
   The mass (M_r) removed from pollutant M₁ when β < 0.01
   • M_r = β M₁
   
   but when β > 0.01
   • M_r = (1 - exp^-β) M₁
   
   In each case, the mass on pollutant M₂ is increased by
   • M_r 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 SO₄, then the molecular weight adjustment factor should be set to 1.5 as SO₂ transforms to SO₄ (the conversion picks up two additional oxygen molecules).


3. To demonstrate the conversion calculation, open the Setup Run menu and then 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.


4. Now open the Advanced / Configuration / menu #10 conversion modules and select the 10% per hour radio-button. It is only necessary to define two different pollutants in the concentration setup menu and select the 10% /hr option. This combination automatically sets MAXDIM=2 in the model code and calls the transformation routine every time step to convert pollutant #1 to #2 at a rate of 10% per hour. 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.


5. 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 pmch. 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 un-select 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 POL2.


6. 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, 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	440000	350000	83000
   25/21-00	130000	81000	52000
   26/00-03	56000	27000	29000
   26/03-06	50000	18000	34000
   26/06-09	42000	11000	30000
   26/09-12	32000	5900	25000
   26/12-15	33000	4800	29000
   26/15-18	25000	2600	23000

   
   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. As an independent exercise, rerun the display, but this time select pollutant All, which will add together all the pollutants prior to display, which will result in concentrations similar (but not identical due to rounding and interpolation errors) to the base calculation. Another exercise can be to select the fixed contour (Fix-Exp) rather than the dynamic contour radio-button. With this option the contours are not rescaled with each plot and stay the same between time periods, which will more dramatically illustrate the decrease of PMCH concentrations compared with POL2.


7. Normally other conversion rates or a greater number of pollutants are required for more realistic simulations. In these situations it is necessary to create a 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
   
   Try creating the CHEMRATE.TXT file and running the conversion with a different conversion rate.

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.