Frequently Asked Questions (FAQs) on AERMOD
Q: How will AERMOD-predicted concentrations compare to ISCST3-predicted concentrations?
A: The comparison of AERMOD concentrations to ISCST3 concentrations will vary considerably depending on the source configuration, meteorological conditions and terrain influences. Based on evaluations performed during development of AERMOD and consequence analyses comparing ISCST3 and AERMOD, some general conclusions can be drawn, such as:
Q: Why does AERMOD run slower than ISCST3?
A: While AERMOD is a steady-state plume model like ISCST3, many of the algorithms in AERMOD are more complex than corresponding algorithms in ISCST3. For example, the probability density function (PDF) approach used in AERMOD for convective (daytime) conditions to account for a non-Gaussian vertical distribution, requires significantly more computations than a simple Gaussian plume algorithm as used in ISCST3.
Q: What source types does AERMOD handle?
A: AERMOD can be used to model impacts from point, volume, and area sources. Area sources can be defined as rectangular, circular or as an irregularly shaped polygon.
Q: Can AERMOD be used for sources subject to building downwash influences?
A: PES recently completed the implementation and evaluation of the improved building downwash algorithms from the ISC-PRIME model into the AERMOD model. A beta test version of AERMOD-PRIME is available from the EPA's SCRAM website. This implementation will allow the AERMOD-PRIME model to be used for all sources subject to building downwash influences, and also includes concentration estimates for the building cavity region.
Q: Is there a screening version of AERMOD?
A: Not at this time. A screening approach for AERMOD is still under development by AERMIC.
Q: Does AERMOD include deposition algorithms?
A: Not at this time.
Q: Is the AERMOD modeling system Y2K compliant?
A: Yes, the current version of the AERMOD modeling system is Y2K compliant.
Q: Does AERMOD use Pasquill-Gifford (PG) stability categories to characterize the dispersive state of the atmosphere?
A: No. AERMOD includes a continuous treatment for characterizing dispersion based on similarity theory, as compared to ISCST3’s use of discrete stability classes. The Monin-Obukhov length (L) is used as the stability parameter, and is computed by the AERMET meteorological preprocessor. This is one of the significant improvements in AERMOD’s formulation as compared to the ISCST3 model.
Q: Can I use my PCRAMMET-generated meteorological data with the AERMOD model?
A: No. The PCRAMMET-generated data files, which are designed for the ISCST3 model, do not include sufficient information to run the AERMOD model, and some of the input parameters used in AERMOD are determined differently than corresponding parameters for ISCST3, such as mixing height. The AERMET meteorological preprocessor program should be used to generate the meteorological input data for AERMOD.
Q: Does AERMOD handle calms the same as ISCST3?
A: The AERMOD model uses the same routines for processing calm hours as ISCST3, namely, hourly concentrations are not considered valid and are treated as missing, and concentrations for 3-, 8-, and 24-hour averages are calculated by dividing the sum of the hourly concentrations for the period by the number of valid (non-calm) hours. If the total number of valid hours is less than 18 for 24-hour averages, less than 6 for 8-hour averages or less than 3 for 3-hour averages, then the total concentration is divided by 18 for the 24-hour average, 6 for the 8-hour average and 3 for the 3-hour average. For annual averages, the sum of all valid hourly concentrations is divided by the number of non-calm hours during the year. However, the NOCALM option available in ISCST3, which models the calm hour by setting the wind speed to 1.0 m/s, is not available in AERMOD, since AERMOD uses a full profile of wind speeds, and is considered valid for cases when the wind speed is below 1.0 m/s but above the instrument threshold. A calm hour in AERMOD is identified by a reference wind speed of 0.0 m/s in the surface meteorological data file generated by AERMET.
Q: Does AERMOD require on-site (or site-specific) meteorological data?
A: Not necessarily. While AERMOD can use a wide range of site-specific meteorological data, it can also be run using only National Weather Service (NWS) surface and upper air data. As with any modeling application, the meteorological data should be representative of the site.
Q: Can I use SODAR data with the AERMOD modeling system.
A: Yes, multiple levels of wind, temperature and turbulence data can be input to the AERMOD modeling system. The wind and turbulence data may come from SODAR measurements or from an instrumented tower. The AERMET meteorological processor also requires at least one wind measurement at or below 100 meters for use in boundary layer scaling.
Q: What meteorological data formats does AERMET process?
A: AERMET can process the following data formats:
CD-144 – 80-character card image format formerly available from the National Climatic Data Center (NCDC)
SCRAM – available from EPA’s SCRAM web site
SAMSON – data retrieved from the Solar and Meteorological Surface Observation Network CD-ROM
TD-3280 – an element-based format that is made available from NCDC
TD-6201 – available from NCDC, both fixed and variable block length are supported
The format of on-site meteorological data is specified by the user through the READ and FORMAT keywords on the ONSITE pathway in the AERMET input control file.
Q: Does AERMOD include intermediate terrain processing, as implemented in the ISCST3 model?
A: AERMOD can be considered an "all terrain" model. It applies to all terrain situations, including flat terrain applications, as well as terrain above stack base (elevated terrain) and terrain above stack height (complex terrain). However, there is no distinction made in AERMOD between elevated simple terrain and complex terrain, as in the ISCST3 model. Instead, the AERMOD terrain algorithm provides a continuous treatment of terrain influences across the stack height demarcation. As a result, there is no need for intermediate terrain processing as performed in the ISCST3 model.
Q: What does AERMAP do?
A: AERMAP preprocesses terrain data for input to the AERMOD model. AERMAP currently supports both 7.5-minute and 1-degree Digital Elevation Model (DEM) terrain data available from the U.S. Geological Survey (USGS). The input control (runstream) file for AERMAP uses the same pathway/keyword approach as AERMOD. The user identifies the input terrain data, defines a terrain domain, and identifies the source locations (optional) and receptor locations for input to the AERMOD model. The receptor locations are defined using the same keywords as used on the receptor pathway in ISCST3. AERMAP calculates a height scale for each receptor location for use in the terrain algorithms of AERMOD. AERMAP can also be used to extract source (optional) and receptor elevations from the DEM data, although terrain elevations can also be provided by the user. AERMAP creates a source location file (optional) and a receptor file that can then be referenced in the AERMOD input control (runstream) file by use of the INCLUDED keyword.
Q: Does AERMOD require digitized terrain contours for complex terrain applications like CTDMPLUS?
A: No. The complex terrain algorithms in AERMOD do not require digitized terrain contours. The model requires a receptor elevation and a height scale. The height scale is used to determine the critical dividing streamline height, and is generated by the terrain preprocessor, AERMAP, from USGS DEM data (see the previous question).
For more information about AERMOD, or to submit a question not answered here, contact PES at aermod@rtp.pes.com.