Understanding the USEPA’s AERMOD Modeling System for Environmental Managers Ashok Kumar University of Toledo akumar@utnet.utoledo.edu Introduction

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

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

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.

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

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

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.

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

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

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

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)

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

Data Flow in the AERMOD

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

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

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

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).

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.

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

ALBEDO Albedo = Reflected radiation/Incident radiation

Bowen Ratio The ratio of sensible heating to latent heating Typical values: North America 0.74 Australia 2.18 Indian Ocean 0.09 Note: Lower Bowen Ratio for moist surfaces.

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 10-8 W/m2/oK-4

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

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 .

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

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

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.

Temperature Scale

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

Data Flow In AERMOD Modeling System

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 .

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”.

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.

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

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.

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

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.

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.

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

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

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

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