DEFINITIONS – ISC-AERMOD View (ISCST3, AERMOD, ISC-PRIME), Rammet View (PCRAMMET), Aermet View (AERMET)

Minimum Monin-Obukhov Length

The Monin-Obukhov length is a measure of atmospheric stability.  It is negative during the day when surface heating results in an unstable atmosphere and positive at night when the surface cools (stable atmosphere).  Values near zero indicate very unstable or stable conditions (depending on the sign).  In urban areas during stable conditions, the estimated value of L may not adequately reflect the less stable boundary layer.  Hanna and Chang (1991) point out that mechanical turbulence generated by obstacles (buildings) in urban areas will tend to produce a "more neutral" surface layer than that over an unobstructed site.  They suggest that a minimum value of L be set for stable hours in order to simulate this effect.  Using an approximate relation between obstacle height and the zone of flow affected by an obstacle, they suggest the following minimum values for several urban land use classifications:

Ceiling Height

The ceiling height is determined as the lowest cloud layer for which the coverage is broken or overcast.  Scattered cloud layers do not have an associated ceiling height, i.e., the ceiling is considered unlimited. 

Estimates of hourly stability class are based on Turner's (1964) method using time of day, surface wind speed, and observations of cloud cover and ceiling. (Turner, D.B., 1964: "A Diffusion Model for an Urban Area."  J. Applied Meteorology, 3:  83-91.)

DEM – Digital Elevation Model

      1-Degree DEM: The 1-degree DEM data are produced by interpolating elevations at intervals of 3 arc-seconds from contours, ridgelines, and drains digitized from 1:250,000-scale topographic maps.  Three seconds of arc represents approximately 90m in the north-south axis and a variable dimension (approximately 90m at the equator to 60m at 50° latitude) in the east-west axis due to convergence of the meridians.  The area of each map is divided into an east half and a west half to accommodate the large volume of data required to cover the 1° x 2° topographic map area. You can download 1-Degree DEMs, free of charge, from the following Web site:
http://www.webmet.com/terrain.html

You can also download files by FTP from directory pub/data/dem/250

          FTP EDCFTP.CR.USGS.GOV

7.5-Minute DEM: The 7.5-minute DEM data are produced in 7.5- x 7.5-minute blocks either from map contour overlays that have been digitized or from automated or manual scanning of photographs usually taken at an average height of 40,000 ft. (1:80,000-scale).  The data are processed to produce a DEM with a 30-m sampling interval.  You can download from the following Web site: http://edcwww.cr.usgs.gov

            You can also download files by FTP from 
            directory pub/data/dem/250

            FTP EDCFTP.CR.USGS.GOV

The spacing for the USGS terrain data and the U.S. EPA recommended spacing for the receptor grids are not the same.  However, ISC View can easily extract terrain elevations for any receptor grid points from 1-Degree and 7.5-Minute USGS digital elevation models.

Mixing Height

Mixing height is the altitude in the atmosphere where pollutants gets mixed and dispersed.  It is dependent on the local surface roughness, wind speed, and solar radiation.  The higher the mixing height the more volume there is to dilute pollutants.  The figure below demonstrates the calculation of the Maximum Mixing Depth (MMD) as the interception of the Adiabatic Lapse Rate (ADLR) and the Environmental Lapse Rate (dt/dZ).

Plume Rise

The most common type of stationary source is a stack.  Emissions from stacks may rise well above the stack height.  The main physical process causing a plume to rise is described below:

The final vertical plume position depends on the temperature difference and on the exit velocity.  This is an important parameter when designing stacks and air pollution control equipment.

Building Downwash

Building downwash occurs when the aerodynamic turbulence, induced by nearby buildings, cause pollutants emitted from an elevated source to be mixed rapidly toward the ground (downwash).  This results in higher ground-level concentrations.

“If stacks for new or existing major sources are found to be less than the height defined by EPA’s refined formula for determining GEP height, then air quality impacts associated with cavity or wake effects due to the nearby building structures should be determined.” (EPA 1986).

GEP Stack Height = H + 1.5L

(EPA’s refined formula for determining GEP stack height)

H = Building/Tier Height measured from ground to the highest point
L = Lesser of the BH or PBW
BH = Building Height
PBW = Projected Building Width
GEP = Good Engineering Practice

You should consider building downwash for point sources that are within the GEP 5L Area of Influence of a building.  See description of the GEP 5L Area of Influence below.  For point sources within the GEP 5L Area of Influence, building downwash information (direction-specific building heights and widths) should be included in your ISC3 modeling project.  Using BPIP View, you can easily calculate these direction-specific building heights and widths.

GEP 5L Area of Influence :  Each structure produces an area of wake effect influence that extends out to a distance of five times L directly downwind from the trailing edge of the structure, where L is the lesser of the BH or PBW.  As the wind rotates full circle, each direction-specific area of influence changes and is integrated into one overall area of influence termed the GEP 5L Area of Influence.  GEP wake effects, for some wind direction or range of wind directions, affect any stack that is on or within the limit line.

Simple Terrain

 When modeling simple terrain you are given the option of modeling either:

  Simple Flat Terrain: where terrain heights are assumed not to exceed stack base elevation.  Terrain height is considered to be 0.0 m.
  Simple Elevated Terrain: where terrain heights exceed stack base but are below stack height.

Complex Terrain

Complex terrain is defined as terrain with height above stack height.  The figure below describes the terminology used in complex terrain.

Anthropogenic Heat Flux [W/m²]

Anthropogenic heat flux is the surface heating caused by human activity, including automobiles and heating systems.  The anthropogenic heat flux can be set equal to zero in areas outside highly urbanized locations.  U.S. EPA OSW recommends that a value of 0.0 Watts/m² be used for rural areas.  A value of 20 Watts/m² is appropriate for large urban areas based on the annual value for Los Angeles.  Rammet View supplies you with information on anthropogenic heat flux values that have been calculated for several urban areas around the world.

Scavenging Coefficient (ISC-AERMOD View: SO Pathway: Wet Deposition)

A scavenging ratio approach is used to model the deposition of gases and particles through wet removal.  In this approach, the flux of material to the surface through wet deposition (F_w) is the product of a scavenging ratio times the concentration, integrated in the vertical:

where the scavenging ratio (7) has units of s^-1.

The scavenging ratio is computed from a scavenging coefficient and a precipitation rate (Scire et al., 1990):

where the coefficient 8 has units (s-mm/hr)^-1, and the precipitation rate R has units (mm/hr).  The scavenging coefficient depends on the characteristics of the pollutant (e.g., solubility and reactivity for gases, size distribution for particles) as well as the nature of the precipitation (e.g., liquid or frozen).  For more information see the ISCST3 User’s Guide Volume 2, available on our website under Free U.S. EPA Models.  The file is ISC3VOL2.zip.

If you have any questions about this, or any of our support services, please do not hesitate to contact us at support@weblakes.com.

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