Industrial Source Complex (ISC) Description

The Industrial Source Complex, or ISC, model is a well-known model that looks to simulate air pollution due to emissions from an industrial plant. The ISC model is applicable in both urban and rural modeling areas, and looks to evaluate the effect of stack height on the behavior of the pollutant. The model is capable of running calculations for terrains that are moderately complex.

The ISC models are steady-state plume models, using the Briggs Gaussian algorithm to assess concentrations of pollutants. The model can calculate these endpoints:

  1. settling and dry deposition of particles
  2. downwash (a deflection of the pollution downward)
  3. area, line and volume sources
  4. plume rise as a function of downwind distance
  5. separation of point sources
  6. limited terrain adjustment

The EPA states that the ISC model is appropriate for the following applications:

  1. industrial source complexes
  2. rural or urban areas
  3. flat or rolling terrain
  4. transport distances less than 50 kilometers
  5. one-hour to annual averaging times
  6. continuous toxic air emissions

There are two "versions" of this model:

  1. ISCST: the ISC short term model, which computes the concentration of the pollution over a period of one to several hours.
  2. ISCLT: the long-term version, which calculates concentrations both over a larger area (several hundred square kilometers) and for a period of up to a year.

You will also notice version numbers, such as ISCST3. This refers to Version 3, which was developed early 1992. The ISC series of models is currently being "phased-out", to be replaced by AERMOD. As of this writing (Spring 2001), it is not clear when the ISC series will no longer be a "preferred" regulatory model.

This model is a good model to use to outline some of the basic concepts of the regulatory process regarding atmospheric diffusion modeling. There are six regulated hazardous air pollutants (HAPs) under the Clean Air Act:

  1. Carbon Monoxide (CO)
  2. Nitrogen oxides (NOx)
  3. Sulfur dioxide (SO2)
  4. Particulate matter (PM10)
  5. Ozone (by regulating Volatile Organic Compounds, or VOCs)
  6. Lead
A geographical area can be designated as a non-attainment area if it does not meet EPA standards for air quality. There are currently 247 designated air quality control regions (AQCRs) in the United States, each one regulated by a State Implementation Plan (SIP) in terms of staying in compliance with federal air quality standards.

Those AQCRs that are in compliance, another set of regulations -- PSDs, or "Prevention of Significant Deterioration" -- are in force. As the name suggests, these regulations serve to ensure that the region does not slip out of compliance. In addition, the PSDs work to ensure that there is "room" for future industrial growth and development. In other words, the PSDs contain margins that allow for more emissions, but only up to a point.

Under PSD regulations, there are three "classes" of regions:

  1. Class I: in this class, very little degradation is permitted. A good example of a Class I area is a national park, where air quality issues are of key importance for both the ecosystem but also for the economical health of the park (i.e., tourism)
  2. Class II: less demands are placed on ensuring continued good air quality
  3. Class III: in this class, minimal constraints are placed on control of additional emissions
Under PSD rules, a number of review requirements, such as an analysis of the BACT (Best Available Control Technology) must be undertaken. For example, suppose a community wishes to develop a new chemical plant. Under PSD requirements, a number of analyses must be undertaken. Since the primary pollutants in this case are sulfur dioxide and particulate matter, the use of a disperson model such as the ISC model are good tools. Typically two model runs would be undertaken: a "baseline" run that looks at all of the pollutant sources in the area prior to the building of the new plant, and a run that adds the predicted effects of the new plant to the existing conditions. If the second model run shows that the new plant will not exceed the air quality standards for the area, then a permit to build is issued.
Quick Quiz: ISC models using the Briggs Gaussian algorithm can calculate all but which following endpoint?
settlying and dry deposition of particles
limited terrain adjustment
downwash
amount of ozone

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