1. Introduction and Fundamentals
Air Pollution Dispersion Models:
Applications with the AERMOD Modeling System
Introduction and Fundamentals
Course #423
Day 1 Morning

2. Introductions
Instructor Name(s)
Background
Student Introductions

3. Course Objectives / Overview
Basic understanding of important concepts that affect pollutant dispersion in AERMOD
To understand some of the processes in the atmospheric boundary layer as they are applied in AERMOD
Familiarize you with some of the terminology
Practical understanding of the AERMOD modeling system
How to develop the input for the various components that make up the AERMOD modeling system
Run the components and interpret the output
A third-party graphical user interface (GUI) will not be used

4. Course Schedule Day 1 Day 2 Morning AERMOD fundamentals
Introduction to hands-on activities
Afternoon
AERMET (part 1)
AERSURFACE w/ hands-on activity
Day 2
AERMINUTE w/ hands-on activity
AERMET (part 2) w/ hands-on activity
AERMAP w/ hands-on activity
BPIPPRM
Each subsequent morning, we will start with a brief review of the previous day followed by an opportunity for Q & A.
Day 1
Morning:
AERMOD fundamentals will cover basic concepts that affect pollutant dispersion in AERMOD such as stability, overview of similarity theory, turbulence, dispersion, etc.
There will be brief orientation to the hands-on activities to familiarize you with the modeling scenario the class will be working through over the next few days as well as organization of the files provided so you can find the materials easily and stay organized.
Afternoon:
AERMET part 1 will cover the data requirements for AERMET including: the types of meteorological data and the formats AERMET can process; the temporal data requirements (time period); surface characteristics; and control file structure and multistage processing.
The instruction on AERSURFACE will cover: the methods used to compute each of the surface characteristics (surface roughness, Bowen ratio, albedo); data requirements, data products and data resources; challenges and considerations; control file format and options; and a hands-on activity to dervive the surface characteristics needed for the modeling application.

5. Course Schedule Cont’d
Day 3
Morning
AERMOD set-up
Afternoon
Special topics for AERMOD
AERMOD hands-on activity
Day 4
AERSCREEN w/ hands-on activity
Exam
Wrap-up / Course evaluation
Day 3
Morning:
The morning session will cover AERMOD set-up exclusively and will include a discussion on source types (point, area, volume, etc.); data input types (met, source parameters, emission rates, building data, terrain elevations, etc.); and an overview of control file pathways (Control, Source, Receptor, Meteorology, Output) and the most commonly used options for regulatory applications.
Afternoon:
The afternoon session will include a discussion of special topics related to AERMOD such as: modeling for the NAAQS (NO2, SO2, PM2.5, Lead); NOx speciation methods; urban vs. rural; and deposition. The day will conclude with a hands-on activity to run AERMOD with utilizing the output from all previous hands-on activities (AERSURFACE, AERMINUTE, AERMET, BPIPPRM, AERMAP).

6. Day 1 – Morning - Outline Precursor Review:
Energy balance, stability, structure of the atmospheric boundary layer (ABL)
How AERMOD Characterizes the ABL
Similarity theory, surface friction velocity (u*) Monin-Obukhov length (L), convective velocity scale (w*), effective parameters
Turbulence and Dispersion in AERMOD
Turbulence, convective and mechanical mixing heights, terrain, building downwash, dispersion, plume types in AERMOD

7. Precursor Review

8. Energy balance and Heat Fluxes
Net Radiation, RN = H + L + G
H = sensible heat flux
Daytime – convective – unstable – upward (H > 0)
heat is supplied to the atmosphere from surface of earth
Nighttime – stable – downward (H < 0)
heat is drawn from the atmosphere
Inhibited by presence of clouds
L = Latent heat flux
Incoming radiation goes toward evaporating moisture at the earth’s surface
G = heat absorbed by the soil
Assumed to be 0.1 RN
Net radiation is the balance between the sensible heat flux (H), the latent heat flux (L), and the ground heat flux (G).
Sensible heat (H) is the heat we feel or can “sense.” The sensible heat flux is the heat from the ground that we feel on a sunny day that is conducted upward into the atmosphere from the surface and heats the air above. During the daytime, the sensible heat flux is away from the surface (positive), upward from the surface into the atmosphere. Typically, on a sunny day, the temperature of the air is warmer nearer the earth’s surface and decreases with height and is more amenable to unstable conditions and a convective boundary layer (CBL) due to heating of the air above the surface.
During the nighttime, as the earth’s surface cools, heat is drawn from the atmosphere and the sensible heat flux is toward the surface (negative). As the surface cools and heat is drawn from the atmosphere, the air near the surface cools more rapidly than the air aloft. Typically at night, the temperature of the atmosphere increases with height (inversion) near the earth’s surface which represents stable conditions and a stable boundary layer (SBL). Cooling can be inhibited on a cloudy night since the clouds (moisture) store and radiate energy back to the surface.
Latent heat (L) is that portion of energy utilized to evaporate moisture. This could be moisture in the soil, moisture in vegetation such as grasslands, crops, or a forest canopy, or bodies of water.
Soil heat flux (G) is the heat absorbed by the soil which and is assumed to be 1/10 of the net radiation in the AERMOD system.

9. Energy Balance and Heat Fluxes, cont’d
Bowen Ratio, Bo = H/L
Smaller values – moist conditions
Larger values – dry conditions
Substituting
The Bowen ratio (Bo) relates the sensible heat flux to the latent heat flux and is computed as the ratio of the two, sensible to latent heat.
In very moist conditions such as over water or a forest canopy, the amount of latent heat may be far greater than the amount of sensible heat, in which case the Bowen ratio is < Conversely, in very dry conditions, where there is little moisture, such as a desert shrubland, the Bowen ratio might be much greater than 1.0 and as high as 10.
Smaller values of Bowen ratio indicate greater amounts of moisture while larger values indicate dryer conditions.
With the Bowen ratio, we can estimate sensible heat flux from the equation on the previous slide, assuming the ground heat flux is about 1/10 of the net radiation.
The Bowen ratio is a value required by AERMET and is derived by the user based on land cover.

10. Atmospheric Stability
Stability - determined by the temperature difference between an air mass (parcel) and the surrounding environment
Three basic conditions of stability
Stable: vertical movement is supressed; a parcel tends to return to its original position
Unstable: vertical movement is not supressed; a parcel tends to continue in the direction of its initial motion
Neutral: when conditions neither suppress nor encourage a parcels’ vertical movement
Inversion
extremely stable, cooler air trapped by a layer of warmer air above it; virtually no vertical motion ; surface based or elevated
Consider a parcel of air to be a theoretical uniform volume of dry air that does not mix with the air around it (environment), similar to the air in a balloon but without the balloon skin.
In a state of equilibrium, the temperature of the air parcel is the same temperature as the air around it. In this state, the air parcel remains where it is, it neither sinks or rises without some external force or influence.
Stable: If we lift a dry air parcel to some height, it will cool adiabatically (no heat exchange via evaporation or condensation) due to a decrease in pressure of the surrounding air. The dry adiabatic lapse rate is about 1 degree Celsius per 100 meters (i.e., A parcel lifted 100 meters will cool about 1 degree Celcius.) After it is lifted, if the temperature of the parcel is colder than the surrounding air, the parcel will want to sink back to where it was originally, where it was the same temperature and pressure as the surrounding air, where it is in equilibrium with the surrounding air. Likewise, a dry parcel forced downward will heat adiabatically. After it is forced downward, if the parcel is warmer than the surrounding air, it will want to rise back to the height where it was originally, where it is in equilibrium with the environment.
Unstable: After lifting our parcel, if the temperature of the parcel is warmer than the temperature of the surrounding air, the parcel will want to continue to rise due to a positive buoyancy. Similarly, when the parcel is forced downward, if the temperature of the parcel is colder than the temperature of the surrounding air, the parcel will tend to continue to sink due to negative buoyancy.
Neutral: After being lifted or forced downward, if our parcel is the same temperature as the surrounding air, it will want to remain where it is and neither rise nor sink because it is in equilibrium with the environment.

11. Atmospheric Stability Cont’d
Potential Temperature
Height
Stable
Unstable
Temperature of Environment w/ Height
Temperature of Air Parcel w/ Height
Air Parcel
These diagrams illustrate stable and unstable conditions.
The slanted green line in the diagrams represent the potential temperature of the ambient air with height (i.e., the environmental lapse rate). The dashed blue lines represents the potential temperature of the parcel with height. For simplicity, we’ll assume the air is dry and the parcel only undergoes dry adiabatic cooling or heating if lifted or forced downward, respectively. (Potential temperature is the temperature of the parcel when taken to 1000 millibars of pressure, adiabatically, and is used to compare the temperature at different levels in the atmosphere.)
In the diagram on the left the potential temperature of the environment increases with height (i.e., the air gets warmer with height = inversion). As the parcel is lifted to some height, though it cools adiabatically, the potential temperature of the parcel remains the same with height. Because the potential temperature of the parcel is colder than the potential temperature of the environment, the actual temperature of the parcel is also colder than the environment and the parcel wants to sink back to its origin to be in equilibrium with the environment. This is a simple illustration of stable conditions.
In the diagram on the right, we see the opposite behavior. The potential temperature of the environment decreases with height. Though the parcel cools adiabatically when lifted, it is warmer than the surrounding air at the height to which it is lifted. Because the parcel has buoyancy at that height, the parcel wants to continue rising until it is once again in equilibrium with the environment. This is a simple illustration of an unstable environment.

12. Diurnal Variation of the Atmospheric Boundary Layer
The atmospheric boundary layer evolves over the course of a day in response to the presence or absense of solar heating of the earth’s surface.
Here once again is a diagram of the diurnal variation of the atmospheric boundary layer based a similar diagram from An Introduction to Boundary Layer Meteorology by Roland Stull (1988).
It illustrates the change in the structure and height of the boundary layer over the course of a 24-hour period:
beginning at noon when the boundary layer is well developed with an established mixed layer that continues to grow via convection (convective mixed layer),
through sunset when solar heating shuts off and the mixed layer rapidly collapses leaving a shallow mixed layer and a deeper residual layer capped by an inversion,
then to sunrise when solar heating begins again and the convective mixed layer redevelops.
In general, during the daytime the conditions are more likely to be unstable (convective boundary layer, CBL) while at night they tend to be stable (stable boundary layer, SBL) due to the inversion that develops from the lack of incoming solar radiation.
Throughout the day and night, there is a shallow surface layer.
The surface layer, SBL, and CBL will be discussed in more detail later in during this session.
NikNaks (2012). Atmospheric Boundary Layer, Wikimedia Commons
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13. Structure of the Atmosphere ObservationsσW σV u θ
zim
Sfc
Layer
zic
STABLE
CONVECTIVE M I X E D L A Y R
These two graphics depict the vertical profiles through the ABL of potential temperature (θ), mean horizontal wind speed (U), and turbulent velocities in the horizontal and vertical (σV and σW, respectively) in both stable (night) and convective (day) conditions. The turbulent velocities are the standard deviations of the mean horizontal and vertical wind speeds.
Incidentally, neutral conditions never truly exists, although the ABL might be ‘near-neutral’.
Zim and Zic represent the mixing height where Zim indicates the shallow mixing height is primarily from mechanical mixing under stable conditions and Zic indicates the mixing height is due to convection.
Characteristics of the stable boundary layer:
Potential temperature and mean horizontal wind speed increase with height with sharp increases nearer the surface and a more gradual increase at higher altitudes.
In contrast, the turbulent velocities, which are primarily mechanically driven by the interaction with the earth’s surface (obstacles, land cover), decrease with height. In other words, though the wind speed increases with height, the turbulence is greater nearer the surface where the wind speeds are lighter but the flow is interrupted or obstructed by obstacles at the surface or the roughness of the land cover.

14. Turbulence
Turbulence, in general, is irregular, random, chaotic motion.
Turbulence in the atmosphere is responsible for mixing which results in dispersion.
Turbulence is generated and maintained by two primary mechanisms:
Buoyancy from radiative heating at the ground
(convective turbulence)
Wind shear from variation of wind speed with height (mechanical turbulence)
We think of turbulence as the random, chaotic motion of air as a result of a disruption to the air flow from what it would be if it were uninhibited.
Turbulence in the atmosphere is respsonsible for mixing the air which in turn disperses pollutants and dilutes concentration of the pollutants in the air.
There are two primary mechanisms responsible for generating turbulence. We’ve already talked briefly about one of these, buoyancy from the solar heating of the earth’s surface, which in turn heats the air just above the ground through conduction (i.e., sensible heat flux) causing convection or vertical motion (rising air) from instability.
Turbulence from wind shear, mechanical turbulence, is the result of the variation in the wind speed with height. In general, wind speed increases with height from the earth’s surface upward. The earth’s surface has a certain amount of roughness associated with it (terrain and land cover). This roughness interferes with air flow at the surface due to friction. As you go up in height, you get farther away from the influence of the earth’s surface on the wind speed. Mechanical turbulence, then, is generated from the shear stress of air moving faster than the air just beneath it.
Turbulence will be discussed in greater detail later in this session, but the basic concept is important to the understanding of some of the terms and parameters important to AERMOD discussed next.

15. Characterizing the ABL in AERMOD

16. Similarity Theory
Similarity theory is a method for developing empirical equations to characterize the physical processes in the atmosphere
The goal is to define universal relationships that apply under a variety of atmospheric conditions
Similarity relationships are usually designed to apply to steady-state , time-independent situations (equilibrium)
AERMOD uses similarity relationships to characterize the atmospheric boundary layer
Many of the physical processes in the atmosphere cannot be computed or derived based on the established laws of physics (first principles). That is where similarity theory plays an important role.
NOTE: Similarity theory is not limited to the atmospheres – it is applicable in other fields as well.
Similarity theory uses a method known as dimensional analysis to develop logical non-dimensional groupings of atmospheric variables. Similarity theory only gives us the form of the non-dimensional groups. Combined with experimental or observational data, universal relationships between non-dimensional groups are developed.
AERMOD is an advancement over older steady state dispersion models in that it uses similarity relationships to construct vertical profiles of meteorological variables such as wind speed, wind direction, turbulence, and temperature through the surface and mixed layers of the ABL. Detailed vertical profiles are constructed using similarity relationships and measurements of meteorological parameters taken at as few as one height (commonly 10 meters) or multiple heights.

17. Similarity Theory, cont’d
Various scaling parameters have been derived that are important to the similarity relationships that characterize the ABL. A few include:
Surface Friction Velocity (u*)
Monin-Obukhov Length (L)
Convective Velocity Scale (w*)
Computed by AERMET from measured meteorological parameters
Scaling parameters or variables are those that are found to be key ones that appear in common classes of similarity problems (Stull). While there are many, here we list a few of the common scaling parameters important in AERMOD: surface friction velocity, Monin-Obukhov length, and the convective velocity scale.
Hourly values of each of these are computed by AERMET using the meteorological data supplied. These will be seen in the surface file generated by AERMET which is read by AERMOD.
We’ll discuss each of these in the next few slides.

18. Similarity Example
Log wind profile in statically neutral conditions is an example of a simple similarity relationship
Where:
U = mean horizontal wind speed
u* = surface friction velocity
z = height
zo = surface roughness length
k = von Karman constant = 0.4
One example of a similarity relationship derived using this method is the log wind profile in statically neutral conditions.
The equation is shown here where U is the mean horizontal wind speed, u* is the surface friction velocity, z is the height, zo the surface roughness length, and k is the von Karman constant.
The surface roughness length is based on the roughness of the surface due to the land cover. Like Bowen ratio, discussed earlier, it is a required input to AERMET and can be derived from the land cover.
We can see from the equation that a profile of mean wind speeds can be developed if we first have an estimate of u* and z0

19. Surface Friction Velocity, u*
Represents the characteristic velocity of the mechanically induced eddies near the surface
where τ is the shear stress and ρ is density of the air
On the order of 0.1 to 1.0 m/s
Shear
As we mentioned recently, mechanical turbulence is produced by shear stress related to the variation in the wind speed with height.
Friction velocity, also called the shear velocity, is a scaling parameter that describes shear stress in terms of velocity. Or, in other words, characterizes the shear-related motion. The surface friction velocity characterizes the shear at the surface which influences the wind speed above the surface. Remember, the wind speed is zero right at the surface and increases with height until reaching some height at which surface forcings do not have an influence.
Friction velocity is an important parameter in AERMOD to characterize the ABL. It is used to build the wind speed profile, compute the Monin-Obukhov length which is discussed next, and compute mechanical the mechanical mixing height, to name a few in which it is used.
Rule of thumb for QA’ing data: Friction velocity is roughly 1/7 – 1/10 the wind speed

20. Monin-Obukhov Length, L
Represents the height (in meters) above the surface where the mechanical production of turbulence is balanced with the buoyant production
For stable nighttime conditions: L > 0 (since H < 0)
very stable: 0 < L < 10
For unstable daytime conditions: L < 0 (since H > 0)
very unstable: < L < 0
Approaches neutral for very large values of |L|
|L| > 1000
The Monin-Obukhov length (L) is a length scale parameter that represents the height above the surface of the earth where the mechanical (or shear generated) turbulence equals the buoyant or convective turbulence. In other words, the height at which the friction (shear) and buoyant forces are balanced.
As such, the Monin-Obukhov length is an indication of stability. Theoretically, L can range from -∞ to ∞ (negative to positive infinity), though AERMET imposes limits on the value of L. More realistic values for the absolute value of L (|L|) are on the order of 1 to 1000 meters.
While theoretically, L can equal 0, this is not realistic. In AERMET, u* is set to 0 when the wind speed = 0. AERMOD does not compute a concentration for the hour if the wind speed = 0 (calm).
Generally, L > 0 and L < 10 indicates very stable conditions (nighttime).
L > -10 and L < 0 indicates very unstable conditions.
Very large values of |L| (> 1000) indicate that conditions are approximately neutral.

21. Convective Velocity Scale, w*
Related to the vertical velocities in the large thermals (i.e., convective eddies)
Depends on
Magnitude of buoyant turbulent energy
Scaling height for the eddies (i.e., zi)
Magnitude on the order of 1-2 meters/second
Observational data show that large convective eddies in the convective boundary layer have vertical velocities that are proportional to w*. Therefore, w* is a scaling parameter for the vertical velocities for these thermals and is used to estimate turbulence in the convective boundary layer produced from buoyancy.
Note the magnitude of the w* is normally 1-2 meters/second (1 m/s = 2.2 mph).

22. Profiles in AERMOD
AERMOD uses similarity relationships and meteorological observations to develop wind, temperature, and turbulence profiles up to 4000 meters for each modeled hour
Compares height at which meteorological variable is to be calculated to the observations
Below the lowest measurement or above the highest measurement, the interface computes an appropriate value from ABL similarity profiling relationships
Between upper and lower measurement heights, the observations are interpolated to the gridded height while maintaining the shape of the similarity profile
Finer resolution closer to the ground, coarser resolution higher up in the atmosphere
In some cases data is available at only one height, e.g., when only NWS data are used. In this case the profiles will resemble the similarity relationships for each variable.
For details on how these profiles are constructed, which is beyond the scope of this course, see the AERMOD Model Formulation Document.

23. AERMOD – Effective Parameters
ABL is vertically and horizontally inhomogeneous
AERMOD is a steady-state plume model
Uses a single value of each meteorological parameter to represent the layer through which these parameters are varying
AERMOD accounts for the vertical inhomogeneity of the PBL in its dispersion calculations by "averaging" the parameters of the actual ABL into "effective" parameters of an equivalent homogeneous ABL
The atmosphere is not uniform in structure and composition (e.g. winds, temperature) neither vertically nor horizontally. As we have discussed winds vary with height. In the precursor materials, there are figures of the general structure of atmospheric variables, none of which are uniform with height.
A steady-state plume model is one in which there is no time dependency. The conditions are assumed to be constant over the entire domain of interest for the period under consideration (usually one hour). It assumes that concentrations at all
distances during a modeled hour are governed by the temporally averaged meteorology of the hour.
The steady state assumption yields useful results since the statistics of the concentration distribution are of primary concern rather than specific concentrations at particular times and locations. In other words, we cannot expect AERMOD to predict concentrations for a specific hour or location.
AERMOD accounts for the vertical inhomogeneity of the ABL in its dispersion calculations by "averaging" the parameters of
the actual ABL into "effective" parameters of an equivalent homogeneous ABL.

24. AERMOD – Effective Parameters
Effective parameters are functions of:
Vertical profiles
Plume height
Downwind distance
Vertical plume size
Mixing height
AERMOD uses the profiles to develop the “effective” parameters for its estimates of turbulence, dispersion, and eventually pollutant concentration
Effective parameters are determined by ‘averaging’ their values over that portion of the layer that contains plume material between the plume centroid height (a surrogate for the height of the plume’s center of mass) and the receptor height. The profiles generated each hour by AERMOD are used for this purpose.

25. Turbulence and Dispersion in AERMOD

26. Turbulence and Dispersion in the Atmospheric Boundary Layer
STABLE
L > 0
CONVECTIVE
L < 0
zim zo w* u*
zic
This slide illustrates the relationship between the scaling parameters (Monin-Obukhov length, surface friction velocity, convective velocity), surface roughness, mixing height, and turbulence for stable and unstable conditions.
Here we see a degree of roughness at the surface and mechanical turbulence from shear which results in a shallow mixed layer under stable conditions. Zim represents the mechanical mixing height. During unstable (convective) conditions, we see a similar shallow layer alsong with a deep mixed layer dominated by large thermal eddies from convection due to the heating of the earth’s surface. Zic represents the convective mixing height.

27. Turbulence Plume dispersion is due to atmospheric turbulence
Turbulence is generated by
Shear (both convective and stable BL)
Lateral
Vertical - elevated & surface formulations
Buoyancy (convective BL only)
Effects of turbulence varies with height in AERMOD using the (hourly) profiles developed within the interface
As we have already highlighted in previous slides, dispersion occurs as a result of atmospheric turbulence.
The two types of turbulence are mechanical turbulence generated from shear and convective turbulence generated from buoyancy due to the heating of the air at the earth’s surface. Mechanical (shear) turbulence occurs in both the convective and stable boundary layers while convective turbulence occurs only in the convective boundary layer.
Note: Plumes are transported by the mean wind
Note: There is a third element – building-induced turbulence to be discussed shortly

28. Turbulence Cont’d In the Convective Boundary Layer Shear component
A function of wind speed, surface roughness, and stability (L)
Scales with friction velocity (u*)
Effects decrease with height – dominant near the earth’s surface
Convective component
A function of stability, friction velocity, and mixing height
Scales with convective velocity scale (w*)
Effects increase with height
In the CBL, there is both a shear and convective component to turbulence. The shear component is a function of the wind speed, surface roughness and stability and relates to the friction velocity. The shear component is more dominant near the surface and decreases with height.
The convective component is a function of stability, friction velocity and mixing height and scales with the convective velocity scale. The effects of the convective turbulence increase with height.
The Monin-Obukhov length is the height above which buoyancy dominates the production of turbulence and below wind shear dominates.

29. Turbulence Cont’d In the Stable Boundary Layer Shear component
Stable stratification suppresses vertical turbulence
Profiles tend to be independent of height
AERMOD assumes little change with height
No convective component
Exception: Convective contribution for urban conditions
In the SBL, only the shear component is considered since there is no incoming solar radiation to produce convection at the surface. The stable atmosphere suppresses vertical turbulence.
There is an exception in an urban setting in which there continues to be some convection due to the thermal capacity of the urban subsurface (urban heat island). AERMOD makes adjustments for the SBL in an urban setting to account for this convective component. The convective contribution is a function of the convective velocity scale, which depends on the surface heat flux. The upward heat flux is a function of the urban-rural temperature difference.

30. Mechanical Mixing Height
Computed for all hours of the day
Based on the calculated height (zie)
Smoothed to get the mechanical mixing height (zim)
where
As we have said, turbulence is responsible for dispersion in the atmosphere which is the result of the mixing of the air caused by the turbulence. The height at which the atmosphere is mixed from the turbulence is referred to as the mixing height.
Just as there are two primary sources of turbulence in the atmospheric boundary layer, shear (mechanical) and buoyancy (convective), there are mixing heights associated with each type of turbulence. These are the mechanical mixing height (Zim) and the convective mixing height (Zic).
The mechanical and convective mixing heights are computed by AERMET and can be found in the AERMET surface file that is input to AERMOD.
The mechanical mixing height is computed for all hours of the day and is based on the current hour’s calculated/unsmoothed height (zie) (defined by the first equation) and the previous hour’s smoothed mixing height.
The current hour’s smoothed mixing height (left side of second equation) is a weighted (exponential) function of the previous hour’s smoothed mechanical mixing height (first term on the right side) and the current hour’s calculated/unsmoothed mixing height (second term on the right side); here Δt = 3600 seconds = 1 hour
If the previous hour’s smoothed mechanical mixing height was not computed, the current hour’s mechanical mixing height is given by the first equation and the smoothing process restarts with the next hour
During day when both Zic and Zim are computed, AERMOD selects larger of the two.

31. Convective Mixing Height
Compares hourly cumulative heat flux (when H>0) over course of daytime hours to the area under the potential temperature profile from the ‘appropriate’ sounding
Equate the areas
Morning
Sounding
zic z θ H
time
The graphics displayed illustrate the potential temperature with height from the appropriate sounding (ideally a morning sounding just before sunrise in the U.S.), and the cumulative heat flux over the course of the daytime hours for the same day.
In general, for a given time of day, the height of the convective mixing height is the height at which the area beneath the potential temperature curve is equal to the area beneath the cumulative heat flux curve.
After the method developed by Weil and Brower (1983): Estimating Convective Boundary Layer Parameters for Diffusion Applications, Martin Marietta Corp., Baltimore, MD, PPSP-MP-48.
The convective mixing height is only computed for ‘daytime’ hours, when the net radiation and heat flux are upward. We will address hours around sunrise and sunset on the next slide.
The ‘appropriate’ sounding to use will be discussed during the presentations for AERMET.

32. Convective Mixing Height, cont’d
Computed for those hours when the net radiation and heat flux are positive
Around sunset, atmosphere may switch between stable and convective
Two consecutive stable hours at sunset – AERMET considers the atmosphere to have transitioned to stable nighttime conditions
Stable hour(s) during daytime – no convective mixing height
During the daytime, there are two estimates of the mixing height – the stable mixing height Zim and the convective mixing height Zic. AERMET writes both to its output file. In AERMOD, the mixing height (Zi) for the hour is assumed to be the larger of the mechanical and convective mixing heights.
If the heat flux during the daytime is missing for an hour, AERMET will interpolate the heat flux between the hour before and the hour after. This interpolation is done only for a one-hour ‘gap’.
If for some reason the heat flux is downward (negative), as could happen when the out-going long wave radiation is greater than the incoming radiation, AERMET will persist the cumulative heat flux from the previous hour (i.e. the cumulative heat flux does not decrease).
At night, Zi is set equal to Zim since there is no incoming solar radiation and no convective boundary layer.
What do we mean by ‘stable hours during daytime’? It is possible that the net radiation and heat flux is downward shortly after the transition from ‘nighttime’ to ‘daytime’ or shortly before the transition from ‘daytime’ to ‘nighttime’

33. Surface Layer Lower 10% of the atmospheric boundary layer
Dominated by frictional forces
Fluxes are constant (constant flux layer)
Surface layer similarity applies
The lower 10% of the atmospheric boundary layer is referred to as the surface layer. It is present both day and night as part of the stable boundary layer and the convective boundary layer.
Turbulence is dominated by frictional forces due to the roughness of the earth’s surface based on topography and land cover that are obstacles to the wind.
Turbulent fluxes and stresses are near constant, varying by less than 10% of their magnitude.
The surface layer can be described using surface layer similarity theory.

34. Stable Boundary Layer Turbulence in the SBL is driven by shear
Turbulence is largest at surface and decays with height
Very weak mixing
Height of mixed layer is shear driven
In the stable boundary layer (nighttime), the dominant production of turbulence is from shear (mechanical turbulence) since there is little if any heating of the atmosphere at the surface (an exception is with the urban boundary layer).
Remember, wind speeds typically increase with height unless there are interruptions to the flow. Mechanical turbulence is larger at the earth’s surface where shear stress is the greatest. Mechanical turbulence weakens or decays with height.
Mechanical turbulence from shear generally equates to smaller eddies than with convective turbulence. Vertical mixing is weak which results in a shallow mixed layer compared to the convective boundary layer.

35. Convective Boundary Layer
Growth is due to surface heating
Turbulence in the CBL is driven by strong vertical mixing
Convective cells grow during the day creating areas of updrafts and downdrafts – non-Gaussian vertical structure
60% downdrafts vs. 40% updrafts
AERMOD accounts for non-Gaussian vertical structure of dispersion in the CBL
Portions of plume released into updrafts are simulated separately from those released into downdrafts
Full or partial penetration of plume through top of mixed layer is also simulated
The convective boundary layer grows throughout the day due to surface heating from solar radiation, which in turns heats the air above the surface, causing large, rising thermals. This convective turbulence causes strong vertical mixing through a deep layer of the atmosphere.
Areas of updrafts and downdrafts develop which result in a non-Gaussian distribution through the boundary layer.
AERMOD accounts for these separate updrafts and downdrafts by partitioning the plume between areas of updraft and downdraft then computing a total concentration based on probability density functions.

36. Convective Boundary Layer
This figure illustrates the effects of updrafts and downdrafts on a plume and an ensemble-averaged plume.
Since a larger percentage (60%) of the instantaneous plume is effected by downdrafts, the ensemble average has a general downward trend. Since downdrafts are more prevalent, the average velocity of the downdrafts is correspondingly weaker than the average updraft velocity to insure that mass is conserved.
EPA, AERMOD: Description of Model Forumulation, 2004

37. Transition – CBL to SBL
When the ABL transitions from convective to stable conditions the heat flux changes sign from a positive to a negative value
AERMET determines a critical solar elevation angle corresponding to the transition between convective and stable
On average, for clear and partly cloudy conditions, the transition from stable to convective conditions occurs when the angle reaches approximately 13°; for overcast conditions the angle increases to about 23°
When the atmospheric boundary layer transitions from convective to stable conditions (daytime to nighttime), the sign of the heat flux changes from positive to negative (i.e., directed away from the surface to directed toward the surface), atmospheric heating and atmospheric cooling, respectively. Thus, net radiation is usually positive during the day and negative at night.
The solar elevation angle is the angle at a point on the earth between the sun and the horizon and is a function latitude, time of day, and time of year.
The critical solar elevation angle, according to AERMET, is the angle that defines the transition between convective (unstable, daytime) conditions to stable, nighttime conditions.
The critical solar elevation angle varies based on sky conditions. For clear to partly cloudy conditions, the critical angle is approximately 13 degrees. For overcast conditions, the critical angle increases to about 23 degrees.

38. Terrain Influences
Plume is modeled as a combination of two limiting cases:
a horizontal plume (terrain impacting) and
a terrain-following (terrain responding) plume
AERMAP uses gridded terrain data to obtain a representative terrain-influence height (or hill height scale, hc) for each receptor
Actual terrain height which most influences the flow in the vicinity of the receptor
Not necessarily the height of a particular hill
Inherent assumptions
Terrain influence decreases with distance from receptor
Terrain influence decreases with decreasing elevation
As we have alluded to but not specifically addressed, air flow and dispersion is largely affected by terrain. AERMOD was formulated to be applicable for all types of terrain including flat, simple (stack top above terrain), and complex (stack top below terrain). Unlike previous models, AERMOD uses the same formulation for simple and elevated terrain.
When a plume encounters a hill (or mountain), the plume may impact the terrain, or flow around or over (follow) the terrain.
In AERMOD, a plume is modeled as a combination of two limiting extreme states: horizontal impacting plume and a terrain-following plume which flows over the hill.
AERMAP, the terrain preprocessor, reads gridded elevation data to extract elevation heights for each source and receptor location. AERMAP also derives for each receptor what is termed the hill height scale which is considered the terrain height that most influences flow in the vicinity of the receptor. The hill height scale is either the elevation of the receptor or the highest elevation found in the domain for which the slope of the difference in elevation between receptor and a data node is 10% or greater. If no elevation is found that meets this 10% slope criteria, the elevation at the receptor is used as the hill height scale. The domain is defined by the user in the AERMAP control file. Additional information will be provided when we present AERMAP.
It is assumed that terrain influence decreases with distance from the receptor and with decreasing elevation.

39. Terrain Influences
AERMOD uses the hill height scale (hc) to compute a critical dividing streamline (Hc) that defines the physical height of the two plume states
AERMOD’s total concentration is calculated as a weighted sum of the concentrations associated with these two plume states
AERMOD uses the hill height scale derived by AERMAP to compute a critical dividing streamline height (Hc) that defines the physical hdight of the two limiting plume states (impacting and terrain-following).
Dividing streamline height (Hc) is the height at which kinetic energy = potential energy; above Hc, plume material passes over terrain; below Hc, plume impacts the hill
The total concentration is a weighted sum of the concentrations associated with the two plume states.
Note: Unlike ISCST, impacts in both simple and complex terrain are handled within the same modeling framework (recall that the terrain algorithms from COMPLEX1 were incorporated into ISCST3)

40. EPA, AERMOD: Description of Model Forumulation, 2004
Terrain Influences
This figure illustrates the dividing streamline concept in relation to the receptor and the plume. The top diagram identifies the receptor on a hill, the dividing streamline, and mass fractions of the plume that are above (Ma) and below (Mb) the dividing streamline. The bottom diagram shows how the weighting factor (f) is constructed.
Note that in the formula for the weighting factor and how it is applied in computing the total concentration, the plume is never allowed to completely approach the terrain-following state. Even if the entire mass of the plume is above the dividing streamline, f = 0.5, and the total concentration is an average of the contributions from the two states.
EPA, AERMOD: Description of Model Forumulation, 2004

41. Building Influences / Downwash
Cavity or
Near Wake
Far Wake
Buildings are obvious obstacles to air flow and influence dispersion in the vicinity of an emission source. Pressure gradients induced by the separation of airflow as the wind impinges on the building result in upwash over the building and downwash on the lee side of the building creating a cavity of recirculation into which the plume is entrained. This are recirculation cavity is referred to as the “Cavity” or “Near Wake” region.
The structure of the cavity region is determined by the building height and the projected building width and length of the building across and along the flow, respectively.
Note: AERMOD computes concentrations in the cavity. ISCST3 did not compute concentrations in cavity and a separate model run (SCREEN3 with cavity algorithm) was needed

42. Building Influences / Downwash
Major effects of downwash on plume:
Enhanced vertical and lateral turbulence
Plume rise
Suppression due to increased turbulence and descending streamlines
Enhancement due to velocity deficit and ascending streamlines
Magnitude of effects depend on source location
Max concentration for sources located just downwind
Max concentration decreases as source location moves downwind
Plumes rise higher for stacks located upwind than downwind of the building
Building downwash enhances vertical and lateral turbulence causing the plume to disperse nearer the location of the release than if the building was not present.
Downwash effects have the potential to both suppress and enhance plume rise based on the location and height of the source. Suppression can occur due to increased turbulence and descending streamlines which carry the plume downward. However, ascending streamlines can initially carry the plume upward. Plumes located upwind of the building can be carried higher.
Maximum concentrations due to building downwash are typically higher from sources located just downwind of a building. Plume rise is suppressed and the plume is entrained into the cavity.
Max concentrations decrease as the source location moves farther downwind from the building and the streamlines become closer to the original mean flow, less entrainment and less suppression of plume rise.

43. Downwash - AERMOD
Incorporates the Plume Rise Model Enhancements (PRIME) algorithms
Partitions plume mass between cavity recirculation and a dispersion enhanced wake region based on fraction of plume that intercepts cavity boundary
To insure a smooth transition between concentrations estimated by PRIME within the wake and AERMOD estimates in the far field, concentrations beyond the wake are estimated as the weighted sum of the two calculations
AERMOD incorporates the Plume Rise Model Enhancements (PRIME) algorithms, originally developed for the Electric Power Research Institute, to model dispersion from buildings.
Improvements to building downwash over previous methods include:
1. enhanced plume dispersion coefficients from the turbulent wake
2. reduced plume rise from descending streamlines on the lee side of the building and entrainment into the wake
From the Addendum to the ISC3 User’s Guide, 1997

44. Dispersion in AERMOD Three components
Ambient dispersion (environment, mechanical and convective)
Lateral
Vertical
Buoyancy induced dispersion (BID) (i.e., plume buoyancy)
Building induced dispersion (downwash)
Total dispersion is a combination of the dispersion resulting from ambient turbulence, buoyancy induced dispersion, and building downwash
Now that we have talked about the sources and types of turbulence in the ABL and the relationship between turbulence and dispersion, here is a simplified look at how AERMOD treats dispersion.
There are three components of dispersion computed independently by AERMOD:
Ambient Dispersion – this includes dispersion from the shear and buoyancy produced turbulence in the environment that we have referrenced in previous slides on turbulence.
Ambient dispersion includes both a lateral and vertical component, sigma-y and sigma-z, respectively.
Dispersion directly related to the buoyancy of the plume – this is called Buoyancy Induced Dispersion or BID. Similar to the convective turbulence from solar heating of the earth’s surface, a buoyant plume also generates turbulence as it ascends which enhances mixing and dispersion.
Dispersion from building downwash – as we just discussed, the presence of buildings, which disrupt flow, is a source of turbulence which has dispersive effects though we often see higher maximum ground level concentrations due to the entrainment of the plume into the the cavity and wake regions near the source.
These three components are additive in the sense that the total dispersion coefficient (sigma), is computed as the square root of the sum of the squares of each of these three components.
Actually, separate coefficients for total dispersion are computed for the horizontal (sigma-y) and vertical (sigma-z).

45. AERMOD – Plume Types Convective conditions (L < 0)
Horizontal concentration distribution is Gaussian
Vertical concentration distribution results from a combination of three plume types:
1) the direct plume material within the mixed layer that initially does not interact the top of the mixed layer,
2) the indirect plume material within the mixed layer that rises up and tends to initially remain near (loft) the mixed layer top, and
3) the penetrated plume material that is released in the mixed layer but, due to its buoyancy, penetrates into the elevated stable layer; can be entrained back into the mixed layer
Injected plume the stack top (or release height) is greater than the mixing height
Modeled as in stable conditions, however the influence of the turbulence and the winds within the mixed layer are considered in the inhomogeneity calculations as the plume material passes through the mixed layer to reach receptors
We previously mentioned that daytime convection leads to updrafts and downdrafts that result in a non-Gaussian distribution vertically, and AERMOD accounts for this by partitioning the plume into updrafts and downdrafts. A total concentration is computed using probability density functions for the two compartments.
Defining the state, behavior, and location of all of the plume mass in the vertical at any point in time and distance from the source is further complicated by the fact that the plume can be deflected off the top of the mixed layer and some portion of the plume may penetrate the top of the mixed layer. To account for the mixture of states a plume may transition through, AERMOD can models as many as 5 different plume types. Under convective conditions (L>0), AERMOD models 3 plumes simultaneously which will each contribute to the total concentration computed at a given distance from the source. These include:
Direct plume which does not interact with the top of the mixed layer
Indirect plume which rises up and tends to initially loft near the top of the mixed layer. The direct and indirect plumes are further split into separate updraft and downdraft components as discussed previously. a portion of plume mass, released from a buoyant source, rises to and remains near the top of the
boundary layer before becoming mixed into the CBL
Pentrated plume which accounts for any part of the plume that penetrates the top of the mixed layer. It is possible in AERMOD for the penetrated material to be entrained back into the mixed layer.
AERMOD also models a plume that is released above the mixing height. This is referred to as an injected plume. During unstable conditions, the injected plume can pass through the mixed layer to receptors.

46. AERMOD – Plume Types zi Mixed Layer Direct Material Height w* Surface
Penetrated Material
Indirect Material
Direct Material w*
This illustration demonstrates the three types of plumes modeled during convective conditions discussed on the previous slide (direct, indirect, and penetrated). Here we see the penetrated plume above the mixing height and the indirect plume just beneath the mixing height.

47. AERMOD – Plume Types Stable conditions (L > 0)
Horizontal concentration distribution is Gaussian
Vertical concentration distribution is Gaussian
Whereas in unstable conditions the vertical pollutant concentration distribution is non-Guassian due to the updrafts, downdrafts and possible penetration of some part of the plume material above the mixed layer; in stable conditions both the horizontal and vertical distributions are considered Gaussian.

48. Summary In this session, we covered the following topics:
Structure of the atmospheric boundary layer
Parameters important to AERMOD’s dispersion
Concepts that affect how dispersion is handled in AERMOD
We examined the structure of the convective and stable boundary layers, generation of turbulence, parameters important for dispersion in AERMOD, how AERMOD constructs profiles that are used to estimate ‘effective parameters’, and the influences of terrain and buildings on dispersion.