Refined Modeling

AERMAP is designed to utilize two different resolutions of DEM data: 1-degree and 7.5-minute data; however, USEPA recommends that the higher resolution 7.5 minute data be used. The USGS has provided links on their website where DEM files can be downloaded for free at Geocomm.com. AERMOD requires the source DEM data to be in the Spatial Data Transfer Standard (SDTS format) format.

Once you have navigated to the correct county and map that you will need, you can then download the necessary DEM data file(s). Typically, there will be two files associated with each DEM file, one that contains the DEM-related data and one text file. As an example, for the Augusta 1:24,000 DEM, the following files are available for download:

1643784.DEM.SDTS.TAR.GZ

1643784.DEM.SDTS.TXT

The first file ( 1643784.DEM.SDTS.TAR.GZ ) contains the zipped DEM data files, while the second file ( 1643784.DEM.SDTS.TXT ) is a text file that contains valuable information about the corresponding DEM data file (name, datum, resolution, projection type, map boundary coordinates, etc.). Be sure to download both files to verify that the data contained in the text file is the required data for your specific application.

The zipped file can then easily be unzipped using WINZIP or similar type archiving/compression program.

It will be necessary to convert all of the individual files contained in the *.GZ (zipped) file into a single *.DEM that AERMOD will accept for processing. This can be accomplished using the SDTS2DEM utility program. Most commercially available software packages will likely have this conversion utility program built in. If the utility is not provided in the software, a small DOS-based utility program can be downloaded at any of the following sites:

http://data.geocomm.com/dem/sdts2dem.html

http://www.gisdatadepot.com/dem/sdts2dem.html

After downloading, the SDTS2DEM program will need to be saved in the same directory as the unzipped DEM data. Navigate to the folder that contains the DEM data and SDTM2DEM program and double click on the SDTS2DEM executable. If done correctly, a DOS-based window will be displayed. When prompted, enter the first four numerical characters of the SDTS file names that is to be converted. Note that you should have a number of files in your directory with the same four leading digits such as 3931CATD, 3931CATS, 3931CEL0, etc. In this case, the Augusta DEM file is 3931.

Enter the output file name (the name must be 8 characters or less) of the DEM file that you wish to create (you do not need to provide an extension to the filename, as the *.DEM extension will automatically be assigned to the filename). The SDTS2DEM utility will then execute for several seconds. Once completed, a *.DEM file with the provided name should be successfully created in your directory.

At this point, it is best to verify that the converted *.DEM file just created is compatible with the modeling software being used.

TROUBLESHOOTING TIP: Certain modeling software will not recognize the *.DEM file, as *.DEM files created using the SDTS2DEM utility are designed to work in a UNIX (i.e., non-PC based) format. This is because the *.DEM file does not have record delimiters, which some DOS-based modeling software needs to correctly read the file. It may be necessary to run the CRLF (Carriage Return Line Feed) utility which adds a carriage return/line feed to each record of the *.DEM file to make it DOS compatible. Most commercially available software packages will likely have this conversion utility program built in.

DEFINING A MODELING DOMAIN IN AERMAP

The AERMAP terrain preprocessor requires the user to define a modeling domain, which is defined as the area that contains all the receptors being modeled and is the geographic extent in which the effects of terrain will be considered. The domain must be a quadrilateral in the same orientation as the DEM data. It is important to note that the size of the domain can significantly affect the receptor height scales calculated because AERMAP determines the largest terrain height difference in the domain as a part of the process of determining height scales.

Anchor locations are used to anchor a local coordinate system (e.g., user location of 0, 0) to a UTM coordinate system. If you are using UTMs for all data (stacks, receptors, etc.), make the anchor location equal to the primary source location.

The modeling domain must be large enough so that the AERMOD receptor network will be both sufficiently detailed and extensive enough so as to fully represent the immediate surrounding terrain and the entire domain being modeled.

While the domain (which can span multiple DEM files) can be specified either in the UTM or the latitude/longitude coordinate system, MEDEP prefers that the domain be in UTM coordinates.

DEFINING A RECEPTOR GRID

As an initial starting point, construct a grid centered on the source with 50 or 100 meter spacing out to a distance of 2 kilometers (km), aligned with the Universal Transverse Mercator (UTM) grid marks. From 2 km to 5 km, construct a grid with 500 meter spacing, and beyond 5 km, use a 1 km spacing. For calculating impacts in the near-wake and cavity regions of structures, a receptor spacing of no more than 10 meters is recommended in the immediate vicinity of the stacks and nearby buildings.

In certain cases, additional special-purpose receptors may be required by MEDEP-BAQ for inclusion in the analysis. Some examples of special-purpose receptors are:

• Fenceline receptors at the closest occurrence of "ambient air", as defined by Chapter 116;
• Receptors within the wake region of the controlling building;
• Receptors at state/international borders;
• Receptors in Class I areas; and
• Receptors at any additional designated sensitive locations.

The above approach for selecting a receptor network must completely represent the surrounding terrain in the area of expected maximum impact of the proposed source, the area of maximum impact of nearby sources and the area where all sources will combine to cause maximum impact. It is possible that the applicant may need to identify these locations through a trial and error process. If in doubt about receptor spacing, consult with the project meteorologist as to what receptor spacing would be appropriate for the proposed modeling analysis.

During setup processing, AERMAP will check all of the sources and receptors coordinates to ensure that they lie within the specified domain, and therefore, within the area covered by the DEM file(s). If a receptor is found to lie outside the domain, or if the domain extends beyond the area covered by the DEM data, AERMAP generates a fatal error message, and further processing of the data is aborted.

Further guidance on the use of AERMAP can be found in the User's Guide For The AERMOD Terrain Preprocessor (AERMAP) (EPA, 1998).

AERMET

AERMET processes commercially available or on-site meteorological data, representative of the modeling domain, for use in AERMOD. AERMET uses meteorological measurements of several boundary layer parameters to compute vertical profiles of: wind direction, wind speed, temperature, vertical potential temperature gradient, vertical turbulence (sigma-w) and horizontal turbulence (sigma-v).

AERMET processes meteorological data in three stages:

1. The first stage (Stage1) extracts meteorological data from archived data files and processes the data through various quality assessment checks.
2. The second stage (Stage2) merges all data available for 24-hour periods (surface data, upper air data, and on-site data) and stores these data together into a single file.
3. The third stage (Stage3) reads the merged meteorological data and estimates the necessary boundary layer parameters for use by AERMOD.
4. AERMET then passes this data along for use in AERMOD in two files:
   
   • A surface file of hourly boundary layer parameters estimates. The surface file also contains the surface characteristics (noontime albedo, Bowen ratio and roughness length) of the area being modeled.
   
   • A profile file of multiple-level observations of wind speed, wind direction, temperature, and standard deviation of the fluctuating wind components.

At the present time, AERMET is designed to accept data from any for the following sources:

• standard hourly National Weather Service (NWS) data from the most representative site;

• hourly on-site wind, temperature, turbulence, pressure, and radiation measurements (if available);

• morning soundings of winds, temperature, and dew point from the nearest NWS upper air station.

Before beginning to process any meteorological data, it is very important to determine that the meteorological data is representative. Representative is defined as the extent to which a set of measurements taken at the collection site (met tower) reflects the actual conditions at the application site (source/facility). The collected meteorological data should closely mimic the conditions affecting the transport and dispersion of pollutants in the area of interest as determined by the locations of the sources/receptors being modeled. It is recommended to consult with the project meteorologist when proposing a meteorological database for use in modeling.

AERSURFACE & DEFINING SURFACE CHARACTERISTICS

In AERMET, one can specify seasonal variations of three surface characteristics for up to 12 different contiguous non-overlapping sectors. These surface characteristics include are listed below. Each wind sector can have a unique noon-time albedo, Bowen ratio, and surface roughness value.

• Noon-time Albedo (r) : the fraction of total incoming solar radiation reflected by the surface. Typical values range from 0.1 for thick forests to almost 1.0 for fresh snow.

• Bowen Ratio (Bo) : the ratio of the sensible heat flux to the latent (evaporative) heat flux.

• Surface Roughness Length (Zo) : the height at which the mean horizontal wind speed is zero . Values range from less than 0.001 meter over calm water surface to 1 meter or more over a forest or urban area.

In order to account for the surface characteristics at the collection site, the monthly/seasonal values, on a sector-by-sector basis, for each of these parameters will need to be entered into AERMET.

Pending data availability, the MEDEP-BAQ can typically provide, free of charge, the necessary surface characteristics on a project-by-project basis. The surface data for each of the three parameters will be calculated from the 2004 MELCD data and provided in an EXCEL spreadsheet.

The following methodology is used to derive surface characteristics:

A three kilometer circle is scribed around the meteorological collection site and divided into twelve 30 degree sectors (with the sectors extending from 0 - 30 degrees, 30 - 60 degrees, etc). Sectors are defined clockwise, as the direction from which the wind is blowing with north at 0°/360°.

For each of the twelve sectors, the various land use data points (pixels) are summed and the percentage of occurrence for each of the following eight AERMET categories is calculated:

Water
Deciduous Forest
Coniferous Forest
Swamp
Cultivate Land
Grassland
Urban
Pavement

Each percentage is then entered into an EXCEL spreadsheet, where the appropriate factors (taken from tables 4-1, 4-2 and 4-3 in the AERMET User's Guide) are applied for each of the three parameters on a seasonal basis. A weighted average is then computed based on the area of each land use category. The final values are then tabulated and organized into a sector-by-sector, season-by-season table. The data can then be entered into the AERMET processor.

Although a season-by-season surface characteristics table will be developed/provided, MEDEP-BAQ recommends that the surface characteristics be entered on a monthly basis, with the seasons defined as follows:

WINTER: December, January, February, March
SPRING: April, May
SUMMER: June, July, August
FALL: September, October, November

Further guidance on the use of AERMET can be found in the AERMET User's Guide (EPA, 2004).

Interactive Sources

• Tier 1 is performed by obtaining the maximum annual average concentration and assuming a total conversion of NO to NO₂. If the concentration exceeds MAAQS and/or increment standards, a tier 2 analysis may be used.

• Tier 2 uses the Ambient Ratio Method (ARM) where the maximum annual average concentration (Tier 1 estimate) by an empirically-derived NO₂/NO_x value of 0.75 (national annual default). A ratio differing from 0.75 may be used if the source provides sound data that is representative of the area where the maximum annual impact occurred.