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Understanding the USEPA’s AERMOD Modeling System for Environmental Managers. Ashok Kumar Kanwar Siddharth Bhardwaj Abhilash Vijayan University of Toledo [email protected] Concentration Calculation. AMBIENT AIR CONCENTRATION MODELING Types of Pollutant Sources

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Understanding the usepa s aermod modeling system for environmental managers
Understanding the USEPA’s AERMOD Modeling System for Environmental Managers

Ashok Kumar

Kanwar Siddharth Bhardwaj

Abhilash Vijayan

University of Toledo

[email protected]

Concentration Calculation


  • AMBIENT AIR CONCENTRATION MODELING Environmental Managers

    • Types of Pollutant Sources

      • Point Sources e.g., stacks or vents

      • Area Sources e.g., landfills, ponds, storage piles

      • Volume Sources e.g., conveyors, structures with multiple vents

    • Factors Affecting Dispersion of Pollutants in the Atmosphere

      • Source Characteristics

        • Emission rate of pollutant

        • Stack height

        • Exit velocity of the gas

        • Exit temperature of the gas

        • Stack diameter

      • Meteorological Conditions

        • Wind velocity

        • Wind direction

        • Ambient temperature

        • Atmospheric stability


Concentration modeling
CONCENTRATION MODELING Environmental Managers

  • Plume rise calculations

  • Concentration calculations

  • Dispersion coefficients

  • Downwash conditions

  • Evaluation


BASIC SEGMENTS OF AN ELEVATED PLUME Environmental Managers


  • BASIC SEGMENTS OF AN ELEVATED PLUME Environmental Managers

  • INITIAL PHASE

    • Vertical Jet : Effluents are not deflected immediately upon entering the cross flow if (Vs / U > 4 )

    • Bent-Over Jet Section : Entrainment of the cross flow is rapid because by this time appreciable growth of vortices has taken place

    • Thermal Section : Self generated turbulence causes mixing and determines the growth of plume

  • TRANSITION PHASE

    • Plume's internal turbulence levels have dropped enough so that the atmospheric eddies in the inertial sub range determines the plume's growth

  • DIFFUSION PHASE

    • The plume's own turbulence has dropped and energy containing eddies of atmospheric turbulence determine the growth of plume


  • TYPES Of PLUME Environmental Managers

    • Continuos Plume: The release and the sampling time are long compared with the travel time

    • Puff Diffusion / Instantaneous Plume: The release time or sampling time is short when compared with the travel time

  • TYPES OF PLUME RISE

    • Buoyancy Effect: Rise due to the temperature difference between stack plume and ambient air

    • Momentum Rise: Rise due to exit velocity of the effluents (emissions)


  • CLASSICAL GAUSSIAN PLUME MODELS Environmental Managers

    • Advantages

      • Produce results that match closely with experimental data

      • Incorporate turbulence in an ad-hoc manner

      • Simple in their mathematics

      • Quicker than numerical models

      • Do not require super computers

    • Disadvantages

      • Not suitable if the pollutant is reactive in nature

      • Fails to incorporate turbulence in comprehensive sense

      • Unable to predict concentrations beyond radius of approximately 20 Km

      • For greater distances, wind variations, mixing depths and temporal variations become predominant




  • EFFECTIVE SOURCE HEIGHT METHOD Environmental Managers

    • Independent of downwind distance, x

    • Effective source height.(Screen model)

  • h = hs + Dh - ht

  • where,

  • hs = Physical chimney height

  • ht = Maximum terrain height between the source and receptor

  • VARIABLE PLUME METHOD

    • Takes into account the tilt of the plume



Plume rise calculations
PLUME RISE CALCULATIONS Environmental Managers

  • No penetration

  • Complete penetration

  • Partial penetration


Input parameters for plume rise
INPUT PARAMETERS FOR PLUME RISE Environmental Managers

  • Buoyant Flux

  • Momentum Flux

  • Brunt-Vaisala Frequency

  • Penetration Parameter


Plume rise input
PLUME RISE INPUT Environmental Managers


Plume rise input1
PLUME RISE INPUT Environmental Managers



Indirect plume
INDIRECT PLUME Environmental Managers


Plume rise for penetrated plume
PLUME RISE FOR PENETRATED PLUME Environmental Managers


Plume rise in stable boundary layer
PLUME RISE IN STABLE BOUNDARY LAYER Environmental Managers

Up and N are evaluated initially at stack height and subsequent plume estimates are made iteratively by averaging them at stack top with those at hs+ Δhs/2



Neutral atmospheric condition n 0
NEUTRAL ATMOSPHERIC CONDITION (N=0) Environmental Managers


Calm stable condition
CALM STABLE CONDITION Environmental Managers

FINAL STABLE PLUME RISE EQUATION


Source characterization
SOURCE CHARACTERIZATION Environmental Managers

  • Source can be characterized as point, area, volume.

  • Additional ability to account for irregular shaped areas

  • Point Source: similar to ISC3

    Input: Location, Elevation, Emission rate, Stack height, Stack inside diameter, Stack gas exit velocity, and Temperature.

  • Area Source:

    • Treatment is enhanced from that available in ISC3

    • Input as squares, rectangles, circles or polygons

    • Polygons may be defined upto 20 vertices.


Source characterization contd
SOURCE CHARACTERIZATION Environmental Managers (Contd..)

  • Volume Sources:

    • Differs from ISC3 in considering the initial plume size

    • Input includes Location, Elevation height, Height of release, Emission rate, Initial lateral and vertical plume rise

    • Unlike ISC3, AERMOD adds the square of the initial plume size to the square of ambient plume size

      σy2 = σyl2 + σyo2


Concentration
CONCENTRATION Environmental Managers

  • Concentration, C is given by the equation

    Where,

    Q Emission rate

    U Effective wind speed

    Py pdf in lateral direction

    Pz pdf in vertical direction


Concentration contd
CONCENTRATION Environmental Managers (contd..)

  • AERMOD assumesa traditional Gaussian p.d.f. for both the lateral and vertical distributions in the SBL and for the lateral distribution in the CBL.

  • The CBL’s vertical distribution of plume material reflects the distinctly non-Gaussian nature of the vertical velocity distribution in convectively mixed layers.

  • Weighting of the 2 states depends on the

    • Degree of atmospheric stability

    • Wind speed

    • Plume height relative to terrain

  • Under stable conditions horizontal plume dominates thus given greater weight, while in unstable and neutral conditions terrain rising plume is weighted more.


General structure for complex terrain
GENERAL STRUCTURE FOR COMPLEX TERRAIN Environmental Managers

  • In stable flows a stable two layer structure is used: lower layer remains horizontal while upper layer tends to rise over terrain

  • Layers are distinguished by the dividing stream line Hc. Plume below the Hc remains horizontal and the plume above Hc follows the hill and rises.

  • In neutral and unstable cases lower layer disappears and entire flow rises up the hill.


Two state approach for concentration calculations in the presence of a hill
TWO STATE APPROACH FOR CONCENTRATION CALCULATIONS IN THE PRESENCE OF A HILL

The total concentration predicted by AERMOD is the weighted sum of the two extreme possible plume states


Two layer concept
TWO LAYER CONCEPT PRESENCE OF A HILL

  • The concentrations on a hill lies between values associated with two possible extreme states of a plume:

    • Case 1:A horizontal plume that occurs under stable conditions where he flow is forced to go around the hill

    • Case 2: Terrain flowing state where the plume rises over terrain

      Note: For simple terrain the two cases are equivalent.


Concentration in the presence of a hill
CONCENTRATION IN THE PRESENCE OF A HILL PRESENCE OF A HILL

Where:

CT {xr, yr, zr} Total Concentration

Cc,s {xr, yr, zr} Concentration from the horizontal plume state

Cc,s {xr, yr, zp} Concentration from the terrain following plume state

f Plume state weighting function

zp Height of receptor above terrain

zr Elevation of receptor above stack base

zt Elevation of terrain above stack base


Dividing streamline height h c
DIVIDING STREAMLINE HEIGHT - H PRESENCE OF A HILLC

  • Hc is calculated using the algorithm in CTDMPLUS using hc from AERMAP as:

    Where:

    N Brunt-Vaisala frequency

    u(Hc) Wind speed at height Hc

    hc Receptor specific terrain scale


Dividing streamline height h c contd
DIVIDING STREAMLINE HEIGHT – H PRESENCE OF A HILLC (Contd..)

  • The fraction of the plume mass below Hc, as

  • Weighting factor f is related to the fraction by


Three plume approach fundamental feature of aermod s convective model
THREE PLUME APPROACH - FUNDAMENTAL FEATURE OF AERMOD’S CONVECTIVE MODEL

AERMOD’s Three Plume Treatment of the CBL


Concentrations in cbl
CONCENTRATIONS IN CBL CONVECTIVE MODEL

  • Downdrafts more prevalent in CBL than the updrafts; the vertical concentration distribution is not Gaussian.

  • Since larger percentage of the plume is affected by the downdrafts this ensemblage average has a general downward trend.

    Instantaneous and corresponding ensemblage-averaged plume in the CBL


Concentrations in cbl contd
CONCENTRATIONS IN CBL CONVECTIVE MODEL (contd..)

  • The instantaneous plume is assumed to have a Gaussian concentration distribution about its randomly varying centerline

  • The mean concentration is found by summing the concentrations due to random centerline displacements. This results in a skewed distribution which AERMOD presents as a bi-Gaussian p.d.f.

  • AERMOD approach extends Gifford’s model to account for plume rise.

  • The p.d.f. of the plume centerline height zc is

    Where hs is the stack height, u is the mean wind speed and x is the downwind distance, ∆h is the plume rise including source momentum and buoyancy effects


Three plume approach contd
THREE PLUME APPROACH CONVECTIVE MODEL (contd..)

  • Direct or Real Source - describes the dispersion of the plume material that reaches ground directly from source via downdrafts

  • Indirect Source - treats the plume sections that initially rise to the CBL top in updrafts and return to the ground via downdrafts

  • Penetrated Source - accounts for the material that initially penetrates the elevated inversion height


Concentration in cbl
CONCENTRATION IN CBL CONVECTIVE MODEL

  • The total concentration in the CBL for the horizontal plume state is

    Where:

    Cc {xr, yr, zr} Total concentration in CBL

    Cd {xr, yr, zr} Direct Source concentration contribution

    Cr {xr, yr, zr} Indirect Source concentration contribution

    Cp {xr, yr, zr} Penetrated Source concentration contribution

    The total concentration for the terrain responding state has the form of the above equation by replacing zr with zp.


Concentration in sbl
CONCENTRATION IN SBL CONVECTIVE MODEL

  • Equation for concentration in SBL


Plume simulation in aermod
PLUME SIMULATION IN AERMOD CONVECTIVE MODEL

  • 5 different plume typed simulated based on the atmospheric stability and on the location and in and above the boundary layer

    • Direct

    • Indirect

    • Penetrated

    • Injected

    • Stable

  • During stable conditions, plumes are modeled with the familiar horizontal and vertical Gaussian formulations

  • During convective conditions (L<0) the horizontal distribution is still Gaussian; the vertical concentration distribution results from a combination of the first three plume types.

  • During convective conditions, AERMOD also handles a speaicl case referred to as an injected source where the stack top (or release height) is greater than the mixing height.


Estimation of dispersion coefficients
ESTIMATION OF DISPERSION COEFFICIENTS CONVECTIVE MODEL

σy Standard deviation for lateral concentration

σz Standard deviation for vertical concentration

Case 1: Without a building

  • Ambient turbulence

  • Turbulence due to buoyancy

    Case 2: Presence of a building

  • Building wake effects






Lateral dispersion due to ambient turbulence penetrated source
LATERAL DISPERSION DUE TO AMBIENT TURBULENCE CONVECTIVE MODEL(PENETRATED SOURCE)


Buoyancy induced dispersion coefficients direct source
BUOYANCY INDUCED DISPERSION COEFFICIENTS CONVECTIVE MODEL(DIRECT SOURCE)


Buoyancy induced dispersion coefficients stable plume rise
BUOYANCY INDUCED DISPERSION COEFFICIENTS CONVECTIVE MODEL(STABLE PLUME RISE)


Buoyancy induced dispersion coefficients penetrated source
BUOYANCY INDUCED DISPERSION COEFFICIENTS CONVECTIVE MODEL(PENETRATED SOURCE)



CONCENTRATION CALCULATIONS UNDER DOWNWASH CONVECTIVE MODEL

x Downwind distance from the upwind of the building to the receptory Crosswind distance from the building centerline to the receptorz Receptor Height above groundσxgLongitudinal dimension of the wake σygDistance from the building centerline to the lateral edge of the wakeσzgHeight of the wake at the receptor location


Concentration calculations under downwash
CONCENTRATION CALCULATIONS UNDER DOWNWASH CONVECTIVE MODEL

  • Within the wake

  • Use PRIME algorithm

  • Beyond wake

    • Use of PRIME and AERMOD

    • CTotal = γ CPrime + (1- γ) CAERMOD

    • When :


Treatment of building downwash
TREATMENT OF BUILDING DOWNWASH CONVECTIVE MODEL

  • Use of numerical plume rise model

  • Use of AERMOD dispersion coefficients


Aermod how it is different from other models
AERMOD-How it is different from other models CONVECTIVE MODEL

  • Air dispersion fundamentally based on the planetary boundary layer turbulence structure and scaling concepts

  • The treatment of both surface and elevated sources in included

  • Both simple and complex terrains are treated with the same set of equations


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