1. Understanding the USEPA’s AERMOD Modeling System for Environmental Managers
Ashok Kumar
Kanwar Siddharth Bhardwaj
Abhilash Vijayan
University of Toledo
Concentration Calculation

2. AMBIENT AIR CONCENTRATION MODELING
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

3. CONCENTRATION MODELING
Plume rise calculations
Concentration calculations
Dispersion coefficients
Downwash conditions
Evaluation

4. BASIC SEGMENTS OF AN ELEVATED PLUME

5. BASIC SEGMENTS OF AN ELEVATED PLUME
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

6. TYPES Of PLUME TYPES OF PLUME RISE
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)

7. CLASSICAL GAUSSIAN PLUME MODELS
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

8. SOURCES OF ERROR IN A CLASSICAL GAUSSIAN MODEL

9. METHODS TO INCORPORATE PLUME RISE
The effective Source Height Method
The variable Plume Model Method

10. EFFECTIVE SOURCE HEIGHT METHOD
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

11. PLUME DISPERSION PARAMETERS
Release Height
Terrain Features
Velocity Field
Sampling Time

12. PLUME RISE CALCULATIONS
No penetration
Complete penetration
Partial penetration

13. INPUT PARAMETERS FOR PLUME RISE
Buoyant Flux
Momentum Flux
Brunt-Vaisala Frequency
Penetration Parameter

14. PLUME RISE INPUT

15. PLUME RISE INPUT

16. PLUME RISE IN THE CONVECTIVE BOUNDARY LAYER

17. INDIRECT PLUME

18. PLUME RISE FOR PENETRATED PLUME

19. PLUME RISE IN STABLE BOUNDARY LAYER
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

20. THE MAXIMUM FINAL RISE OF STABLE PLUME

21. NEUTRAL ATMOSPHERIC CONDITION (N=0)

22. FINAL STABLE PLUME RISE EQUATION
CALM STABLE CONDITION
FINAL STABLE PLUME RISE EQUATION

23. SOURCE CHARACTERIZATION
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.

24. SOURCE CHARACTERIZATION (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

25. CONCENTRATION 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

26. CONCENTRATION (contd..)
AERMOD assumes a 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.

27. GENERAL STRUCTURE FOR COMPLEX TERRAIN
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.

28. 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

29. TWO LAYER CONCEPT
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.

30. CONCENTRATION IN THE 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

31. DIVIDING STREAMLINE HEIGHT - HC
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

32. DIVIDING STREAMLINE HEIGHT – HC (Contd..)
The fraction of the plume mass below Hc, as
Weighting factor f is related to the fraction by

33. AERMOD’s Three Plume Treatment of the CBL
THREE PLUME APPROACH - FUNDAMENTAL FEATURE OF AERMOD’S CONVECTIVE MODEL
AERMOD’s Three Plume Treatment of the CBL

34. Instantaneous and corresponding ensemblage-averaged plume in the CBL
CONCENTRATIONS IN CBL
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

35. CONCENTRATIONS IN CBL (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

36. THREE PLUME APPROACH (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

37. CONCENTRATION IN CBL
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.

38. CONCENTRATION IN SBL
Equation for concentration in SBL

39. PLUME SIMULATION IN AERMOD
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.

40. ESTIMATION OF DISPERSION COEFFICIENTS
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

41. DISPERSION COEFFICIENT IN CBL

42. DISPERSION COEFFICIENT FOR A PENETRATED PLUME

43. DISPERSION COEFFICIENT IN SBL(Injected Sources)

44. LATERAL DISPERSION DUE TO AMBIENT TURBULENCE (CBL & SBL)

45. LATERAL DISPERSION DUE TO AMBIENT TURBULENCE (PENETRATED SOURCE)

46. BUOYANCY INDUCED DISPERSION COEFFICIENTS (DIRECT SOURCE)

47. BUOYANCY INDUCED DISPERSION COEFFICIENTS (STABLE PLUME RISE)

48. BUOYANCY INDUCED DISPERSION COEFFICIENTS (PENETRATED SOURCE)

49. VERTICAL DISPERSION DUE TO AMBIENT TURBULENCE (SBL)

50. CONCENTRATION CALCULATIONS UNDER DOWNWASH
x Downwind distance from the upwind of the building to the receptor y Crosswind distance from the building centerline to the receptor z Receptor Height above ground σxg Longitudinal dimension of the wake σyg Distance from the building centerline to the lateral edge of the wake σzg Height of the wake at the receptor location
CONCENTRATION CALCULATIONS UNDER DOWNWASH

51. CONCENTRATION CALCULATIONS UNDER DOWNWASH
Within the wake
Use PRIME algorithm
Beyond wake
Use of PRIME and AERMOD
CTotal = γ CPrime + (1- γ) CAERMOD
When :

52. TREATMENT OF BUILDING DOWNWASH
Use of numerical plume rise model
Use of AERMOD dispersion coefficients

53. AERMOD-How it is different from other models
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