Using the OZIP Model Interface

This reading provides a step-by-step "how to" description for using the Web interface to the OZIP model.

Overview

The OZIP program has been installed on an SGI workstation at the Shodor Education Foundation. To run the program, a forms-based interface allows the user to specify options and commands for a run. Simply by clicking on the "Submit" button, an input file is created, the program is run, and output files of various types are created "on-the-fly" and displayed, based on the users's desires.

The input file is a text file consisting of a series of options and commands. In addition, the input file can contain the names of commonly used "include" files that are inserted into the text of the input file. The text file, with the proper syntax needed by the OZIP Fortran programs, is created through a perl script with data extracted from the submitted form.

Again, the primary purpose of the Web interface is to relieve you, the modeler, of the inconvenience of having to pay attention to coding syntax and structure. We have eliminated several of the available OZIP options in the Web interface to reduce the turn-around time needed for you to get your results. For the purposes of this scenario, please use the Web form instead of OZIP source code that you might have or be able to obtain. We wish to be able to collect statistical data on number of hits to the Web interface, run times, and other data to submit to the EPA.

The zenith set choice allows the user to vary the rates of photolytic reactions relative to the angle of solar elevation (which is related to the spectral distribution of actinic flux). The values in these tables are input for 10 zenith angles (0 through 70 in increments of 10, and 78 and 86 degrees from zenith). A cubic spline interpolation (a calculation used to determine a data point) is performed to find the factor value for the current zenith angle.

The user is not provided with the option to custom-build a zenith file from this interface; rather, one of two pre-established zenith files are available:

The modeler must then choose a reactivity set. These data sets describe percentages of the overall VOC emissions that come from specific organic groups, such as single- versus double-bonded organic structures. There are four choices:

You can read more about reactivities in a separate reading. For the customizable option, the form provides a table to be completed by the user. To include a species, click on its box, and enter the initial concentration, transported aloft concentration, and transported surface concentrations (all in ppm). Care must be taken to ensure that each concentration adds up to 1.0, or 100%. For example, in the sample form, adding the initial concentrations for each of the species yields a value of 1.0

Title and Initial Settings:

This section is fairly self-explanatory. Care should be taken to ensure that the lat/long values are correct, as the program uses these data points to calculate the photolysis rate constants from the zenith data sets.

Boundary Conditions:

This section provides the modeler with input variables which control the various concentrations, reactivities, and deposition rates that occur at the physical boundaries of the reaction column and at the beginning of the simulation.

Fraction of NO₂/NO_x is the fraction of NO₂ out of all NO_x emitted, that is present due to controllable emissions (VOCs, NO_x, and CO) at the start of the simulation. The default value for this is 25% (or 0.25). Most of the user's guides suggest that there is no need to change this value.

O₃ aloft is the amount of ozone that is transported into the column above ground at the beginning of the simulation, in ppm. The height (in meters) of this transported ozone is inputted before the concentration.

O₃ at the surface is the amount of ozone that is transported into the column at the surface at the beginning of the simulation, in ppm. The default value is 0.00 ppm, since studies have shown that surface transport of ozone is generally insignificant.

VOC at the surface is the amount of VOCs transported into the column at the beginning of the simulation, in ppm. The default value is 0.00 ppm.

VOC aloft is the amount of VOCs transported aloft into the column at the beginning of the simulation, in ppm. The default value is 0.03 ppm.

NO_x at the surface is the amount of NO_x transported into the column at the beginning of the simulation, in ppm. The default value is 0.00 ppm.

NO_x aloft is the amount of NO_x transported aloft into the column at the beginning of the simulation, in ppm. The default value is 0.02 ppm.

CO is the amount of CO transported aloft into the column at the beginning of the simulation, in ppm. The default value is 0.50 ppm.

Meteorological Conditions:

The meteorological section is relatively simple. Some of the more important inputs to be included are the values needed for OZIP to calculate the amount of dilution that occurs within the column as it moves from its starting position to the position of the peak ozone concentration. The most important concept here is the mixing height, or the height of the column as measured from the surface. The default height of the column (at 0800, or 8 am) is 250 meters, while the final mixing height is typically a function of geography. The modeler must also input the time that the mixing height begins to increase (almost always 0800) and the time it reaches a maximum, typically 1500 (3 pm).

The other numbers are self-explanatory. For each of the options of temperature, pressure, and relative humidity, the user inputs the data point corresponding to the beginning of the hour. The default data on the form suggests that this model runs for 12 hours (0800 to 2000). Care must be taken to ensure that the correct units (Kelvin, Celsius, atmospheres, etc.) are "dialed in". Relative humidities are in percentages.

Emissions Inventories:

There are two options for this category: Emissions Fractions and Mass Emissions Inventory. In OZIP, all emissions that are input to the program on an hourly basis (post-0800) must be converted into fractions, as a function of an initial concentration in parts per million and the mixing height in meters.

Emissions Fractions Option

The modeler has the option of converting these numbers through the use of a pop-up calculator (using the three calculators) or providing the appropriate numbers to the model for conversion:

In this option, the only species that can be included are VOCs, NO_x, and CO. Care must be taken to ensure that the correct number of fractions are entered, corresponding to the number specified in the emissions fraction option. Commas must separate each fraction.

We have provided two options for this section -- the use of a default emissions fractions inventory or the input of a customized fractions profile.

Mass Emissions Inventory Option

This section allows the user to input hourly emission mass for up to 10 species or groups in units of kg/km² per hour. An initial concentration (in units of ppm) that is used by the program to convert hourly emission densities into emission fractions, must be included on the species line. For VOC, NO_x and CO, OZIP compares this value to the concentration input in the single simulation calculation, EKMA calculation, or isopleth profile for consistency. A warning message will print if there is a mismatch, but in some cases a different value for VOCs, NO_x, and/or CO is useful so the program does not stop. In other words, it is not necessary to have the same concentrations of VOCs, NOx, and CO in the emissions inventories as is entered into the single calculation or EKMA options. There are occasions when having different values might allow the model to perform more simulations, but in the majority of cases the use of the same emissions concentrations is recommended.

Because the calculation of column concentration of emissions also depends on the molecular density of air and the volume of the column, the ppm emission ratio factors printed on output files will differ if the temperatures and mixing height values vary from 298.15 K and the initial mixing height, respectively. If there are not enough hours of data given to fill the simulation period, values of 0.0 are assumed after the emission period ends.

The concentration units are ppmC for VOC and ppmV for all other species, including individual organics that are part of the VOC group.

We have provided two options for this section -- the use of a default mass emissions inventory or the input of a customized mass emissions profile.

Calculation Options:

The modeler has two choices for calculations:

single simulation calculation
EKMA calculation

Both options can be selected, but typically only one is chosen. We have configured the Web interface to allow the choice of one, both, or no options. It is possible to run the model with just the isopleths graphical output options selected, but this choice only generates a simple accounting of the species identified in the emissions inventories, and does not generate simulation data. It is recommended, therefore, that one or both of the calculation options always be selected.

Single Simulation Calculation

The single calculation option allows the user to perform one simulation with all the input options previously described. The user has a choice of using a default calculations file or using the form to input a custom calculations set.

VOC is mandatory. It is the concentration of VOC (ppmC) in the initial surface-layer mixture that is due to emissions and subject to control. This mass is added to any transported or other uncontrollable VOC sources to total the actual concentration that a simulation will begin with. The default value on the form is set to 1.269 ppm (the same value as the default calculation file).
NO_x is mandatory. It is the concentration of NO_x (ppm) in the initial surface-layer mixture that is due to emissions and subject to control. At the beginning of a simulation this mass is added to any uncontrollable NO or NO₂. The default value on the form is set to 0.114 ppm (the same value as the default calculation file).
CO is mandatory. It is the concentration (ppm) in the initial surface-layer mixture that is due to emissions and subject to control. This mass is added to any transported or other uncontrollable CO sources to total the actual concentration that a simulation will begin with. The default value on the form is set to 1.7 ppm (the same value as the default calculation file).

The concentrations listed for VOC, NO_x , and CO in the output file represent all the total initial mass of these species that is included in the calculation. That is, the values printed at each time step (times during the simulation period at which the model calculates an ozone concentration) in the output file include the VOC, NO_x , and CO concentrations listed in the single calculation option as well as any VOC, NO_x , and CO transported in the surface layer.

EKMA Option

The choice of the EKMA option provides the modeler with a method of running multiple simulations (default is 8). The modeler invokes the EKMA methodology by specifying a base case condition followed by a future conditions simulation. The OZIP program uses these values to evaluate a number of scenarios and reports back various reductions and changes needed to meet a specified air quality standard.

In the section on base conditions, the following data points are required:

O₃ concentration (in ppm): the daily design value for the simulation. The form uses a default of 0.158 ppm.
CO concentration (in ppm): the form uses a default of 1.7 ppm.
Design ratio (ratio of VOCs/NO_x): default is 11:1.

In lieu of the design ratio, the modeler can choose to input the concentrations of controllable VOCs and NO_x, and a design ratio will be calculated by the program. The defaults for VOC and NO_x are 1.269 and 0.1143, respectively, giving a design ratio of 11.1

In the section on future conditions, the following values are entered:

Decrease in NO_x emissions, as a percentage. This is the amount of decrease anticipated in the future.
Decrease in CO emissions (as a percentage)
Decrease in O₃ aloft (in ppm). This future value of O₃ is determined based on an EPA table. A pop-up calculator is available for determing the correct value for this option.
Decrease in VOC emissions aloft (in ppm)

There are other options available for OZIP, but these have been removed for the purposes of this educational scenario.

There are some extra options available in the EKMA calculations block:

Simulation attempts: use this entry to change the default value from 8 to another user-specified value
VOC reduction level (as a percentage): this allows the modeler to specify one level of VOC control to be performed. Using a negative value, this data point represents a percent reduction from the base year level.
Print VOC table: this option generates a useful table of change in ozone concentrations as a function of change in VOC emissions.
Print NO_x table: this option generates a useful table of change in ozone concentrations as a function of change in NO_x emissions.
Print CO table: this option generates a useful table of change in ozone concentrations as a function of change in CO emissions.

Output Options:

The modeler has the option of producing a graphical isopleth ("equal concentration"), as described in the readings about the EKMA approach. If this option is not chosen, the form provides two results files: a copy of the input file, and text-only copy of the run results. If selected, the form provides those two files plus a copy of raw data set (file.iso) and a graphical display of the isopleths.

For generation of the graphic, the modeler must input the levels to be plotted, in units of parts per million. Default values are 0.08 through 0.24 in increments of 0.04.

The modeler should also specify the size of the graphical output. This will be determined as a function of the type and size of monitor at the remote sites. Defaults are a width of 600 pixels and a height of 400 pixels.

Lastly, the modeler must set the abscissa and ordinate of the graph. The maximum values for VOCs, NO_x, and CO can be modified on the Web interface form. If the isopleth option is chosen, simulations are performed at decade percentages (increments of 10) of these options.

Run the Model:

Once you have determined that the options are set to your specifications, running the model is a simple step of clicking on the "Run" button. You should expect for the page to not change for some number of minutes, before changing to a "Results" page. If you have chosen the isopleth option, and your results graphic shows a broken image, this means that there was an error in your input file. You should look at the input file to determine where errors might be. If you encounter difficulties in making this determination, you can send a copy of the input file to the OZIP Modeling Help Desk.

Likewise, if you choose not to generate an isopleth chart, and your text-only results file is empty, this signifies an error in your input file. Follow the same procedure of proofing your input file for errors and/or sending it to the Help Desk for evaluation.

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