1. Understanding the USEPA’s AERMOD Modeling System for Environmental Managers
Ashok Kumar
University of Toledo
Introduction

2. SOLUTION OF ATMOSPHERIC DISPERSION PROBLEMS
Theoretical Approach
Analytical Solution
Numerical Solution
Experimental Approach
Field Studies
Laboratory Studies

3. What is a model?
Model is a way of expressing the relationship between the different variables of a system in mathematical terms

4. AIR QUALITY MODELING
An attempt to predict or simulate, by physical or numerical means, the ambient concentrations of contaminants found within the atmosphere of a region of interest.
An Air Quality Model can be as simple as an algebraic equation or could involve solutions of coupled partial differential equations using super computers.

5. Examples of Air Modeling Problems
Release of contaminants due to agriculture, mining, industrial, and refining activities.
Evolution of toxic gases during accidents.

6. Input Data Requirements of a Dispersion Model
Source Data
Receptor Data
Site Data
Meteorological Data
Dispersion Data

7. OUTPUT OF A BASIC DISPERSION MODEL
The location and amount of maximum ground level concentration from the source(s) for various conditions of wind speed and atmospheric stability.
The amount of ground level concentration at varying distances from the sources.
The amount of ground level concentration at arbitrary locations on a grid.

8. EXAMPLES OF ATMOSPHRIC DISPERSION MODELS
USEPA's Industrial Source Complex (ISC3)
COMPLEX
SCREEN
Urban Airshed Model
AERMOD

9. AERMOD (Joint effort by AMS/EPA)
Steady state plume model
Uses PBL technology developed during 80’s
Estimates the impacts from a variety of industrial sources
Improvement over the ISC model

10. AERMOD MODELING SYSTEM
Pre-processors – AERMET and AERMAP
AERMET deals with the meteorological data
AERMAP generates receptor grids and characterizes the terrain features.
Dispersion models

11. CHARACTERISTICS OF AERMOD
Rural and urban areas
Simple as well as complex terrain
Accounts for different source types
Surface and elevated sources
Multiple sources – point, area and volume sources
Concentration distribution in stable boundary layer (SBL): Gaussian in both vertical and horizontal directions
Concentration distribution in convective boundary layer (CBL): horizontal distribution is assumed Gaussian but vertical distribution is described with bi-Gaussian function
Plume penetrates through the elevated boundary layer and re-enters into the boundary layer.
This model accounts for the vertical inhomogeneity of the planetary boundary layer (PBL)

12. INPUT DATA REQUIREMENTS
Source data
Dispersion data
Receptor and Terrain data
Meteorological data
Downwash related information

13. Data Flow in the AERMOD

14. Features of AERMOD Steady state plume model
Applied to source releases that are assumed to be steady over individual modeling periods
Computation of pollutant impacts in both the flat and complex terrain
Terrain height with respect to stack height need not be specified since the receptors at all elevations are handled with the same general methodology

15. Review of Terms Used in AERMOD
Latent Heat
ALBEDO
Bowen Ratio
Lapse Rate
Friction Velocity
Solar Radiation
Sensible Heat
Temperature Scale
Monin-Obukhov Length
Atmospheric Stability
Stefan-Boltzmann Law
Planetary Boundary Layer
Potential Temperature

16. Planetary Boundary Layer
The PBL is a region immediately above the Earth surface that is affected by horizontal pressure gradients, viscosity, and Coriolis forces.
Surface layer and Ekman layer = PBL

17. Solar Radiation Sun emits enormous amount of energy to space.
Solar radiation that reaches earth’s surface is known as insolation (for incoming solar radiation).

18. Sensible Heat
This is the amount of heat transferred via conduction and convection from the surface of Earth into troposphere.
The sensible heating can be monitored, or “sensed”, as the temperature changes.

19. Latent Heat
The amount of heat that is involved in phase change is known as latent heat.
Evaporation of water from oceans,------

20. ALBEDO
Albedo = Reflected radiation/Incident radiation

21. Bowen Ratio The ratio of sensible heating to latent heating
Typical values:
North America
Australia
Indian Ocean
Note: Lower Bowen Ratio for moist surfaces.

22. Stefan-Boltzmann Law
The total energy radiated by an object is proportional to the fourth power of its absolute temperature.
E = σ T4
σ is the Stefan-Boltzmann constant
= 5.67 x W/m2/oK-4

23. Lapse Rate
The lapse rate is the rate of change of temperature with height.
Γ = -T/z

24. Potential Temperature
The temperature air would have if it was compressed, or expanded, adiabatically from a given state (P, T) to a pressure of 1000mb is defined as potential temperature .

25. Friction Velocity
u* = (Shear stress/Density)0.5

26. Atmospheric Stability
Atmospheric stability is defined as the ability of the atmosphere to enhance or to resist atmospheric motions.

27. Monin-Obukhov Length
A constant, characteristic length scale. It is negative in unstable conditions (upward heat flux), positive for stable conditions, and approach infinity as the actual lapse rate for ambient air reaches the dry adiabatic lapse rate.

28. Temperature Scale

29. AERMOD - AERMIC DISPERSION MODEL
AERMOD, designed by the AERMIC committee to implement state-of-the-art modeling concepts into the EPA's local-scale air quality models
AERMOD, developed as a new platform for regulatory steady-state plume modeling

30. Data Flow In AERMOD Modeling System

31. AERMOD - Input File Format Description
Wide range of options available for modeling air quality impacts of pollution sources
Use of an input data file called a “Run Stream File”
Run stream file is divided into five functional sets, each called a “ Run Stream Image”.
Each run stream image describes the dispersion data, source data,
receptor data, meteorological data and output data respectively .

41. AERMOD – Run Stream Image Description
Each run stream image starts with
i. A pathway ID
ii. An 8 character keyword
iii. A parameter list
A pathway ID describes the type of data being input
If the input data is “Control data,” then the ID is “CO”.
Source data is indicated by “SO”.

42. AERMOD – Run Stream Image Description
The 8- character keyword describes the nature of the input.
For example, MODELOPT says that the model options of the “Dispersion option” are being entered.
The parameters falling under that particular pathway and 8-character keyword follow the 8-character keyword.
A simple example of how a run stream image starts is shown in the next slide.

43. AERMOD – Run Stream Image Description
Example: CO MODELOPT DEFAULT CONC’
( Parameters )
( 8 – character Keyword )
( 2 – character Pathway ID )

44. Advantages of Keyword Approach
Descriptive of options and inputs being used
Considerable flexibility in structuring the input files to improve their readability
Gives easy notation of the input - output parameters and data used.

45. Dispersion Input Data Options
The regulatory modeling options will be the default mode of operation for the model.
Use of stack-tip downwash and a routine for processing averages when calm winds or missing meteorological data occurs.
Includes the use of non-default options
Calculates concentration values (dry and wet depositions review copy is available on the USEPA website).
Short term averages in a single run and also the overall period averages

46. Source Input Data Options
Capable of handling multiple sources ( point, area, volume )
Line source also (as elongated area source or as string of volume sources)
Several source groups may be specified in a single run with combined source contributions for each group.

47. Source Input Data Options (Contd.)
Capable of modeling the effects of aerodynamic downwash due to nearby buildings on point source
Emission rate can be assumed as constant or varied by month, hour, other options. The variable emission rate factors can be specified for a single source or group of sources.
Separate file of hourly emission rates for some or all sources.

48. Receptor Input Data Options
Designed to handle all types of terrain, from flat to complex
Requires information about the surrounding terrain for the modeling of receptors in elevated or complex terrain
Includes a height scale and base elevation for each receptor in the run stream file
Terrain preprocessor (AERMAP) helps to obtain the base elevation and height scale for a receptor

49. Receptor Input Data Options
Considerable flexibility in the specification of receptor locations
Capability to specify multiple receptor networks in a single run
Can mix Cartesian grid receptor networks and polar grid receptor networks in the same run

50. Receptor Input Data Options
Flexibility in specifying the location of the origin for polar receptors
Flexibility in input of elevated receptor heights to model the effects of terrain above stack base or ground level
No distinction between the elevated terrain below and above the release height

51. Terrain & Receptor Data from AERMAP
Uses gridded terrain data to calculate a representative terrain-influence height (hc ), also referred to as the terrain height scale
Gridded data needed are selected from DEM data
Creates receptor grids
Automatically assigns an elevation to each specified receptor
Passes the receptor’s location (xr , yr), its height above mean sea level (zr ), and the receptor specific terrain height scale (hc ) to AERMOD