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.

SEAMLESS DEM FILE PROCESSING

Seamless elevation data can also be used in AERMAP in the same manner as individual DEM files. Using seamless data has several major advantages over using individual DEM files; seamless data eliminates any continuity problems at edges of individual DEM files, seamless data files can encompass a much larger area in one continuous GEOTIFF file, seamless data eliminates handling of multiple files, etc.

Seamless DEM data can be extracted for a user-defined domain at: http://nationalmap.gov/viewer.html

You can then choose the resolution of DEM data (1, 1/3 and 1/9 arc-second data) on the left side of the screen. The user can then use the on-line tool to select the DEM domain and download the data in the GEOTIFF (*.TIF) format for inclusion into AERMAP.

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).

AERMOD requires two types of meteorological data files which must be pre-processed by AERMET:

• Surface met data file - (*.SFC)
• Profile met data file - (*.PFL)

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 (*.SFC) 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 (*.PFL) 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 ASOS 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.

When pre-processing the meteorological data using AERMET, users are usually faced with the lack of proper meteorological data.

AERMET minimally requires the following parameters to be present in the hourly surface met data file for succesful processing:

• Year, Month, Day, Hour
• Wind Speed
• Wind Direction
• Dry Bulb Temperature
• Cloud Cover (tenths)

It is typically considered to be helpul to include any additional surface data that might be available.

Note that when one of the above parameters is missing, AERMET will still produce the *.SFC and the *.PFL files with missing indicators applied to all missing values (e.g., -99 for windspeed, etc).

AERMOD will typically run succefully with missing values in the surface file, however AERMOD will not produce concentration impacts (all concentration values will be 0.0) for missing data periods. A warning message will be displayed in the message section of the AERMOD main output file stating the number and percentage of missing hours.

For deposition calculations, the following additional meteorological parameters are required:

• Station Pressure
• Relative Humidity
• Precipitation (required for wet deposition calculations)

AERMET will apply a default station pressure value of 1013.25mb if this parameter is missing.

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 work in close consultation with the MEDEP project meteorologist when preparing a meteorological database for use in dispersion modeling.

AERSURFACE & DEFINING SURFACE CHARACTERISTICS

In AERMET, one can specify monthly 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 these parameters, the monthly surface characteristics will need to be calculated on a sector-by-sector basis for the measurement site by running the AERSURFACE program. AERSURFACE uses the 1992 USGS Land Cover Datasets to determine the land cover types for user-specified locations.

The AERSURFACE program, user's guide and other related programs can be downloaded at the SCRAM website.

More information on the 1992 Land Cover Datasets can be found at the USGS Land Cover Institute.

The NLCD92 data can be downloaded in two different forms:

• complete files by state;
• files for a user-specified domain of arbitrary size and location from a “seamless data server.”

AERSURFACE supports both forms of the NLCD92 data.

ADDITIONAL AERSURFACE NOTES

Per USEPA Guidance:

1. The determination of the surface roughness length should be based on an inverse distance weighted geometric mean for a default upwind distance of 1 kilometer relative to
the measurement site. For all analyses, divide the 1 kilometer ring 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°.
2. The determination of the Bowen ratio should be based on a simple unweighted
geometric mean (i.e., no direction or distance dependency) for a representative domain,
with a default domain defined by a 10x10 kilometer domain centered on the measurement site.
3. The determination of the albedo should be based on a simple unweighted arithmetic
mean (i.e., no direction or distance dependency) for the same representative domain as
defined for Bowen ratio, with a default domain defined by a 10x10 kilometer domain
centered on the measurement site.

MEDEP-BAQ recommends that the surface characteristics be developed on a month-by-moth basis in the following manner:

CATEGORY	DEFINITION	MONTHS
1	Midsummer with lush vegetation	June through August
2	Autumn with unharvested cropland	Sepetmber, October
3	Late autumn after frost and harvest, or winter with no snow	November
4	Winter with continuous snow on the ground	December through March
5	Transitional spring with partial green coverage or short annuals	April, May

Further guidance on AERMET surface characteristics can be found in the AERMET User's Guide.

Interactive Sources

• Tier 1 is performed by obtaining the maximum average concentration and assuming a total conversion of NO to NO₂. If the concentration exceeds MAAQS, NAAQS and/or increment standards, a tier 2 analysis may be explored.
• Tier 2 uses the Ambient Ratio Method (ARM) where the maximum 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, although this will likely involve USEPA approval on a case-by-case basis. If the concentration exceeds MAAQS, NAAQS and/or increment standards, a tier 3 analysis may be explored.
• Tier 3 uses the Plume Volume Molar Ratio Method (PVMRM), a non-regulatory default option available in AERMOD, to model the NO-to-NO₂ conversion chemistry in exhaust plumes. The PVMRM is recognized as a detailed Tier-3 screening approach for NO₂ modeling as outlined in USEPA’s memorandum from Tyler Fox (OAQPS) to EPA Regional Air Division Directors, "Applicability of Appendix W Modeling Guidance for the 1-hour NO₂ National Ambient Air Quality Standard" (June 28, 2010).

When exhaust plumes from sources are emitted, they usually contain mostly nitric oxide (NO). As the plume is transported downwind, the NO is converted to NO₂, typically through the interaction with ozone (O₃):

NO + O₃→ NO₂ + O₂

This chemical transformation, know as titration, is considered to be the primary mechanism for the conversion of NO to NO₂.
The PVMRM option calculates the moles of NO_x (NO and NO₂) using the stack emission rate and travel time of the plume. The moles of O₃ are determined using hourly representative background ambient O₃ concentrations and plume size. The NO₂/NO_x ratio is then calculated and used to scale the predicted NOx concentrations within AERMOD.

Detailed information on the application of the PVMRM method is available from MEDEP.

Use of the Tier-3 PVMRM method is considered to be a non-regulatory default option and requires written approval by MEDEP and USEPA on a case-by-case basis.