(Photo Credit)

As of December 9, 2006, U.S. EPA promulgated AERMOD as a replacement for ISCST3 as the recommended dispersion model.  The AQMD required modeling options are discussed here.

Regulatory Default

AERMOD should be executed with the U.S. EPA regulatory default option, which is required by AQMD.  If the default option is not utilized, the report should contain a discussion to justify this change and include all supporting data and information. 

WARNING:     According to the AERMOD Implementation Guide (pdf, 189 kB) revised March 19, 2009, for cases in which receptor elevations are lower than the base elevation of the source, AERMOD will predict concentrations that are less than what would be estimated from an otherwise identical flat terrain situation.  While this is appropriate and realistic in most cases, for cases of down-sloping terrain where the plume is terrain-following, AERMOD will tend to underestimate concentrations when terrain effects are taken into account.  In order to avoid underestimating concentrations in such situations, AQMD recommends the following:

1)      If all receptor elevations are lower than the base elevation of the source, the non-default option within AERMOD should be applied to assume flat, level terrain.

2)      If some receptors are lower and some receptors are higher than the base elevation of the source, AERMOD should be run twice – once using the default option and the second time using the non-default option.  The maximum ground-level concentration from both runs should be reported.

Elevations

The AERMOD modeling system includes AERMAP, which is a terrain data pre-processor.  Terrain data, available from the United States Geological Survey (USGS), is used by AERMAP to produce terrain base elevations for each receptor and source and a hill height scale value for each receptor.  AERMAP must be used to develop the terrain data required for AERMOD.

Urban Dispersion Option

AERMOD should be executed using the urban modeling option, which is AQMD policy for all air quality impact analyses in its jurisdiction. All sources should be modeled with urban effects using the population of the County where the project is located. Table below lists the various County populations within AQMD jurisdiction.

If the rural modeling option is utilized, the report should include a discussion to support this change based on the U.S. EPA procedure outlined in Section 7.2.3 of 40 CFR Part 51 Appendix W (November 2005).

County Population to Use in AERMOD

Note: County Population data based on 2008 estimates from the U.S. Census Bureau.

Building Downwash

Building downwash effects must be included and should be modeled using BPIPPRIME (BPIPPRM).

Flagpole Receptor Heights

All receptors should be set to a height of 0.0 meters, so that ground-level concentrations are analyzed.

Surface Characteristics

When using AERMET to process meteorological data for AERMOD, values for three surface characteristics (surface roughness, albedo, and Bowen ratio) must be determined.  When using the AQMD provided AERMOD meteorological data, this information is not necessary as the surface characteristics have already been incorporated into the AERMET processed meteorological data.  However, if the dispersion modeler would like to use AERMET to develop a project-specific meteorological data set for use in AERMOD, then AERSURFACE should be used to determine the surface characteristics to be used in AERMET.

Averaging Times

When using AERMOD to determine the maximum annual concentration of criteria pollutants such as NO₂, PM₁₀, PM_2.5, and SO₂, AQMD requires that each calendar year is run separately.

When using AERMOD to determine cancer risks or chronic health indexes, it is appropriate to estimate the maximum annual concentration using the entire 3-years of meteorological data provided.

Receptor Grid

To identify the maximum impacted receptors, a grid spacing of 100 meters or less must be used.  All receptors should be identified in UTM coordinates.  Receptor grid points outside of the project boundary with grid spacing of 100 meters or more must be placed so that individual grid points are placed at UTM coordinates ending in “00” (e.g., grid point UTM East 572300 and UTM North 3731000).  Receptor grids with less than 100 meter spacing must include grid points at UTM coordinates ending in “00”.

Receptors on the project boundary must be placed along the boundary following the maximum spacing requirements shown in the Table below.  Sensitive receptors must be identified by exact UTM coordinates.  Elevations must be provided for all receptors.

Maximum Receptor Spacing Requirements for Fenceline Receptors

Non-default Option: Conversion of NOX to NO2

If you will be using the OLM or PVMRM option within AERMOD please contact Jillian Baker at (909) 396-3176 or by e-mail at jbaker@aqmd.gov before you begin, to ensure that you are following the most recent and updated methodology.

Hourly ozone data for the 26 meteorological sites can be found in Table 4.  The time period for the ozone data matches the time period for the AQMD’s AERMOD-ready meteorological data in Table 1.  The ozone data is listed in the Fortran format (I2,3I3,F9.3) with units of PPB.

If you have questions please contact Jillian Baker at (909) 396-3176, or by e-mail at jbaker@aqmd.gov.

Photo Credit:  Ralph Turcotte, Beverly (Massachusetts) Times