1. ODU_ISC3 Interface Usage (= How to run the program),
2. Short introduction to EPA's ISC3 Dispersion Air Quality models, and
3. Sources of further model information and other EPA Air Quality models.

Regulatory Applicability

The U.S. Environmental Protection Agency (EPA) maintains the Guideline on Air Quality Models which provides the agency's guidance on regulatory applicability of air quality dispersion models in the review and preparation of new source permits and State Implementation Plan (SIP) revisions. Regulatory application of the ISC3 models should conform to the guidance set forth in the Guideline, including the most recent Supplements. Any non-guideline application of the models should meet the requirements of the applicable reviewing agency, such as an EPA Regional Office, a State or a local air pollution control agency.

In general, regulatory modeling applications should be carried out in accordance with a modeling protocol that is reviewed and approved by the appropriate agency prior to conducting the modeling. The modeling protocol should identify the specific model, modeling options and input data to be used for a particular application.

ISC Short Term Dispersion Model (ISCST3)

The Industrial Source Complex Short Term model (ISCST3) provides options to model emissions from a wide range of sources that might be present at a typical industrial source complex.

The basis of the model is the straight-line, steady-state Gaussian plume equation, which is used with some modifications to model simple point source emissions from stacks, emissions from stacks that experience the effects of aerodynamic downwash due to nearby buildings, isolated vents, multiple vents, storage piles, conveyor belts, and the like.

Emission sources are categorized into four basic types of sources, i.e., point sources, volume sources, area sources, and open pit sources. The volume source and the area source may also be used to simulate line sources. The model algorithms used to model

1. each of four source types,
2. calculating dry deposition for point, volume, area and open pit sources,
3. calculating wet deposition,
4. calculations for simple terrain (defined as terrain elevations below the release height).

The ISC Short Term model accepts hourly meteorological data records to define the conditions for plume rise, transport, diffusion, and deposition. The model estimates the concentration or deposition value for each source and receptor combination for each hour of input meteorology, and calculates user-selected short-term averages. For deposition values, either the dry deposition flux, the wet deposition flux, or the total deposition flux may be estimated. The total deposition flux is simply the sum of the dry and wet deposition fluxes at a particular receptor location. The user also has the option of selecting averages for the entire period of input meteorology.

ISC Long Term Dispersion Model (ISCLT3)

The Industrial Source Complex Long Term model (ISCLT3) provides options to model emissions from a wide range of sources that might be present at a typical industrial source complex. The long-term model provides options for modeling the same types of sources as provided by the short-term model, ISCST3.

The basis of the model is the straight-line, steady-state Gaussian plume equation, which is used with some modifications to model simple point source emissions from stacks, emissions from stacks that experience the effects of aerodynamic downwash due to nearby buildings, isolated vents, multiple vents, storage piles, conveyor belts, and the like.

Emission sources are categorized into four basic types of sources, i.e., point sources, volume sources, area sources, and open pit sources. The volume source and the area source may also be used to simulate line sources. The model algorithms used to model

1. each of four source types,
2. calculating dry deposition for point, volume, area and open pit sources,
3. calculating wet deposition,
4. calculations for simple terrain (defined as terrain elevations below the release height).

The ISC long-term model uses input meteorological data that have been summarized into joint frequencies of occurrence for particular wind speed classes, wind direction sectors, and stability categories. These summaries, called STAR summaries for STability ARray, may include frequency distributions over a monthly, seasonal or annual basis. The long term model cna be used to calculate concentration or dry deposition values for each separate STAR summary input and/or for the combined period covered by all available STAR summaries. Since the wind direction input is the frequency of occurrence over a sector, with no information on the distribution of winds within the sector, the ISC long-term model uses a Gaussian sector-average plume equation as the basis for modeling pollutant emissions on a long-term basis.

Basic Input Data Requirements and Options

There are two basic types of inputs that are needed to run the ISC models. They are

1. the input runstream file, and
2. the meteorological data file.

The runstream setup file contains the selected modeling options, as well as source location and parameter data, receptor locations, meteorological data file specifications, and output options. The ISC models offer various options for file formats of the meteorological data. A third type of input may also be used by the models when implementing the dry deposition and depletion algorithm. The user may optionally specify a file of gridded terrain elevations that are used to integrate the amount of plume material that has been depleted through dry deposition processes along the path of the plume from the source to the receptor.

Other major options for ISC3 models are:

• Dispersion Options
• Source Options
• Receptor Options
• Meteorology Options
• Output Options
• Source Contribution Analyses

  Dispersion Options

  Since the ISC3 models are especially designed to support the EPA's regulatory modeling programs, the regulatory modeling options are the default mode of operation for the models. These options include the use of stack-tip downwash, buoyancy-induced dispersion, final plume rise (except for sources with building downwash), a routine for processing averages when calm winds occur, default values for wind profile exponents and for the vertical potential temperature gradients, and the use of upper bound estimates for super-squat buildings having an influence on the lateral dispersion of the plume. The user can easily ensure the use of the regulatory default options by selecting a single keyword on the modeling option input card.

  To maintain the flexibility of the model, the non-regulatory default options have been retained, and by using descriptive keywords to specify these options it is evident at a glance from the input or output file which options have been employed for a particular application. The user may select either rural or urban dispersion parameters, depending on the characteristics of the source location. The user also has the option of calculating concentration values or deposition values for a particular run.

  TOC - Basic Input Data Requirements and Options

  Source Options

  The model is capable of handling multiple sources, including point, volume, area and open pit source types. Line sources may also be modeled as a string of volume sources or as elongated area sources. Several source groups may be specified in a single run, with the source contributions combined for each group. This is particularly useful for Prevention of Significant Deterioration (PSD) applications where combined impacts may be needed for a subset of the modeled background sources that consume increment, while the combined impacts from all background sources (and the permitted source) are needed to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS). The models contain algorithms for modeling the effects of aerodynamic downwash due to nearby buildings on point source emissions, and algorithms for modeling the effects of settling and removal (through dry deposition) of large particulates.

  The model also contains an algorithm for modeling the effects of precipitation scavenging for gases or particulates. Source emission rates can be treated as constant throughout the modeling period, or may be varied by month, season, hour-of-day, or other optional periods of variation. These variable emission rate factors may be specified for a single source or for a group of sources.

  TOC - Basic Input Data Requirements and Options

  Receptor Options

  The ISC3 models have considerable flexibility in the specification of receptor locations. The user has the capability of specifying multiple receptor networks in a single run, and may also mix Cartesian grid receptor networks and polar grid receptor networks in the same run. This is useful for applications where the user may need a coarse grid over the whole modeling domain, but a denser grid in the area of maximum expected impacts. There is also flexibility in specifying the location of the origin for polar receptors, other than the default origin at (0,0) in x,y, coordinates.

  The user can input elevated receptor heights in order to model the effects of terrain above (or below) stack base, and may also specify receptor elevations above ground level to model flagpole receptors. For simple terrain calculations, any terrain heights input above the release height for a particular source are "chopped-off" at the release height for that source's calculations. The Short Term model includes the complex terrain algorithms from the COMPLEX1 screening model. If these algorithms are used, the model will calculate impacts for terrain above the release height. The Long Term model does not include any complex terrain algorithms.

  TOC - Basic Input Data Requirements and Options

  Meteorology Options

  The Short Term model can utilize the unformatted, sequential files of meteorological data generated by the PCRAMMET and the MPRM preprocessors, provided the data file was generated by the same Fortran compiler as was used for the model, and provided the deposition algorithms are not being used.

  The user also has considerable flexibility to utilize formatted ASCII files that contain sequential hourly records of meteorological variables. For these hourly ASCII files, the user may use a default ASCII format, may specify the ASCII read format, or may select free-formatted reads for inputting the meteorological data. A utility program called BINTOASC is provided with the ISC3 models to convert unformatted meteorological data files of several types to the default ASCII format used by ISCST and ISCEV. This greatly improves the portability of applications to different computer systems. The model will process all available meteorological data in the specified input file by default, but the user can easily specify selected days or ranges of days to process.

  The Short Term model includes a dry deposition algorithm and a wet deposition algorithm. The dry deposition algorithm requires additional meteorological input variables, such as Monin-Obukhov length and surface friction velocity, that are provided by the PCRAMMET preprocessor. The wet deposition algorithm in the Short Term model also needs precipitation data, which is optionally available in the PCRAMMET preprocessed data. When using the dry deposition or wet deposition algorithms in ISCST, the meteorological data must be a formatted ASCII file.

  The Long Term model uses joint frequency distributions of wind speed class, by wind direction sector, by stability category, known as STAR (STability ARray) summaries. These STAR summaries are available from the National Climatic Data Center in Asheville, North Carolina. They may also be generated from sequential data files using the STAR utility program available on EPA's SCRAM Bulletin Board System or by the MPRM meteorological processor for on-site data. The meteorological data for ISCLT are read in from a separate data file, and the user may use a default ASCII format or may specify the ASCII read format for the data.

  TOC - Basic Input Data Requirements and Options

  Output Options

  The basic types of printed output available with the Short Term model are:

  1. Summaries of high values (highest, second highest, etc.) by receptor for each averaging period and source group combination;

  2. Summaries of overall maximum values (e.g., the maximum 50) for each averaging period and source group combination; and

  3. Tables of concurrent values summarized by receptor for each averaging period and source group combination for each day of data processed. These "raw" concentration values may also be output to unformatted (binary) files.

  For the Long Term model, the user can also select output tables of values for each receptor, and/or tables of overall maximum values. The tables by receptor and maximum value tables can be output for the source group values or for the individual source values, or both. In addition, when maximum values for individual sources are output, the user has the option of specifying whether the values are to be the maximum values for each source independently, or the contribution of each source to the maximum group values, or both.

  In addition to the tabular printed outputs, the ISC models provide options for several types of file output products. One of these options for ISCST is to output an unformatted ("binary") file of all concentration and/or deposition values as they are calculated. These files are often used for special postprocessing of the data. In addition to the unformatted concentration files, ISCST provides options for three additional types of file outputs. One option is to generate an ASCII formatted file with the same results that are included in the unformatted postprocessing file.

  Another option is to generate a file of (X,Y) coordinates and design values (e.g., the second highest values at each receptor for a particular averaging period and source group combination) that can be easily imported into many graphics plotting packages to generate contour plots of the concentration and/or deposition values. Separate files can be specified for each of the averaging period and source group combinations of interest to the user.

  Another output file option of the ISCST model is to generate a file of all occurrences when a concentration or deposition value equals or exceeds a user-specified threshold. Again, separate files are generated for only those combinations of averaging period and source group that are of interest to the user. These files include the date on which the threshold exceedance occurred, the receptor location, and the concentration value.

  TOC - Basic Input Data Requirements and Options

  Source Contribution Analyses

  In air quality dispersion modeling applications, the user may have a need to know the contribution that a particular source makes to an overall concentration value for a group of sources. This section provides a brief introduction to how these types of source contribution (sometimes referred to as source culpability) analyses are performed using the ISC3 models.

  Recognizing that source contribution information is important to many short term modeling analyses, the ISCST model has been designed to facilitate performing this type of analysis. This is accomplished with an additional model, referred to as the ISC Short Term - EVENT model (ISCEV). The ISCST model treats source groups independently. The ISCEV (EVENT) model is set up specifically to provide the contributions from individual sources to the concentration values for particular events. These events may be the design concentrations (e.g., the high-second-high 24-hour average concentration for a particular group of sources) that were generated from an execution of the ISCST model.

  Other events of interest might be occurrences of violations of a particular standard, for which it is necessary to determine whether the source being permitted contributes above a significance level. The models are set up in such a way that both of these types of events can be passed directly from an execution of the ISCST model to an input file for the EVENT model. The user is thus able to run the models in a batch mode to obtain the overall design value results from ISCST and the source contribution information from ISCEV in a single step. The EVENT model can also be run separately and accepts user-specified events for source contribution processing.

  In the ISCLT model, the user has an option to have the highest 10 values for each source and source group reported independently, or to have the 10 highest values from the combined source group and the contributions from the individual sources to those highest group values.

  TOC - Basic Input Data Requirements and Options
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	EPA-454/B-95-003a
	EPA-454/B-95-003b
	U.S. ENVIRONMENTAL PROTECTION AGENCY
	Office of Air Quality Planning and Standards
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