History of AERMOD Development

1. History of AERMOD Development • Collaboration between American Meteorological Society (AMS) and EPA, starting from 1991. • To introduce Planetary Boundary Layer (PBL) concept into regulatory dispersion models: • CBL (Convective Boundary Layer): a mixed layer characterized by vigorous turbulence tending to stir and uniformly mix • SBL (Stable Boundary Layer): a cool layer of air adjacent to a cold surface of the earth, where temperature within that layer is statically stably stratified. • Also include plume interaction with terrain, surface releases, building downwash and urban dispersion Aerosol & Particulate Research Lab

2. Based on ISC3 • Developed based on the EPA’s regulatory platform: • Adopt ISC3’s (Industrial Source Complex) input/output computer architecture • Update antiquated ISC3 model algorithms with state-of-the art modeling techniques • Ensure the source and atmospheric processes modeled by ISC3 can continue to be handled, albeit in an improved manner Aerosol & Particulate Research Lab

3. New/Improved Algorithms relative to ISC3 • Dispersion in both the convective and stable boundary layers • Plume rise and buoyancy • Plume penetration into elevated inversions • Computation of vertical profiles of wind, turbulence and temperature • Urban night-time boundary layer • Treatment of receptors on all types of terrain from the surface up to and above the plume height • Treatment of building wake effects • An improved approach for characterizing the fundamental boundary layer parameter • Treatment of plume meander Aerosol & Particulate Research Lab

4. Modeling System Structure Aerosol & Particulate Research Lab

5. Two Pre-Processors • AERMET: uses meteorological data and surface characteristics to calculate boundary parameters (e.g. mixing height, friction velocity, etc) • AERMAP: uses gridded terrain data for the modeling area to calculate a representative terrain-influence height associated with each receptor location Aerosol & Particulate Research Lab

6. How AERMOD Models Terrain • No need to specify the terrain type (flat, simple or complex) • Treat a plume as a combination two limiting cases: • horizontal plume (terrain impacting) • terrain-following plume Aerosol & Particulate Research Lab

7. Aerosol & Particulate Research Lab

8. Sum of Two States • Total is the weighted sum of the concentrations from these two states (f: plume state weighting factor) • Plume state weighting factor depends on fraction of plume mass (φp) below critical dividing streamline (Hc) Conc. from horizontal plume Conc. from terrain-following plume Total conc. for concentration in the absence of the hill for stable conditions Aerosol & Particulate Research Lab

9. Hc Q: When the plume is entirely below Hc, how much is f? Q: When the plume is entirely above Hc, how much is f? Aerosol & Particulate Research Lab

10. Concentrations in the CBL In the CBL, vertical velocity (w) is positively skewed due to higher frequency of occurrence of downdrafts than updrafts Aerosol & Particulate Research Lab

11. Bi-Gaussian Vertical Distribution Approximates the skewed distribution by superimposing 2 Gaussian distributions, the updraft and downdraft Aerosol & Particulate Research Lab

12. Combined probability density function • Air parcel height • Horizontal distribution is still conventional Gaussian λ: weighting coefficient subscripts 1, 2: updraft and downdraft hs: physical stack height Δh: plume rise Aerosol & Particulate Research Lab

13. 3-Plume Treatment of the CBL • Direct source, Indirect source and Penetrated Source Aerosol & Particulate Research Lab

14. Total concentration is the sum from the 3 sources • Direct source: • No “final” plume rise assumed. Plume trajectory is determined by the addition of a distance-dependent plume rise and random vertical displacement caused by vertical distribution of w. • Ground level concentration first appears when the downdraft velocities are sufficiently large to overcome the plume rise velocity • The direct source handles the downdraft portion to first reach the ground and all subsequent reflections of the mass at z = zi and 0. direct indirect penetrated Aerosol & Particulate Research Lab

15. Indirect source: • Added to address the initial quasi-reflection of plume at z = zi. • Not a conventional image source for a buoyant plume • A plume rise is added to delay downward dispersion of material from the CBL top to mimic lofting behavior (tendency of buoyant plumes to remain temporarily near zi and resist downward mixing) • For non-buoyant sources, it reduces to the conventional image source (i.e. reflection at z = zi) • Penetrated source: • To account for material that initially penetrates the elevated inversion but is subsequently re-entrained by and disperses in the growing CBL Aerosol & Particulate Research Lab

16. Concentrations in the SBL • Use the conventional steady-state plume model but with an “effective mixing lid”, zieff, that retards but does not prevent plume material from spreading into the region above the estimated mechanical mixing layer, zim. • When the plume is below zim yet the upper edge (plume height plus 2.15 zs) reaches zim, zieffis allowed to increase and remain at a level near the upper edge of the plume. Thus, only the extreme tail of the vertical plume distribution reflects back Aerosol & Particulate Research Lab

17. Treatment of Lateral Plume Meander • Meander: slow lateral back and forth shifting of the plume • Treat by interpolating between two concentration limits: • Coherent plume limit: wind direction is distribute about a well-defined mean direction with variations solely due to lateral turbulence • Random plume limit: • wind direction is uniformly distribution through an angle of 2 nywea.org Aerosol & Particulate Research Lab

18. Total concentration • Horizontal wind energy • Random wind energy • For CBL, different r2 and h2 are needed for direct, indirect and penetrated sources coherent plume random plume (alongwind and crosswind fluctuations assumed equal) (Tr: 24 hr, timescale when mean wind information at the source is no longer correlated with the downwind receptor) Q: What’s the initial value of r2? What’s the value of r2 after a while? Aerosol & Particulate Research Lab

19. Estimation of Dispersion Coefficients • Treat standard deviation of concentration distribution as a combination of the dispersion resulting from ambient turbulence and plume buoyancy induced turbulence (subscript a for ambient and b for buoyancy) (Also, yb = zb) Aerosol & Particulate Research Lab

20. Dispersion from ambient turbulence • Lateral dispersion from ambient turbulence • Other release heights ( = 78 and p = 0.3, obtained by fitting with Prairie Grass data; good for both CBL and SBL) Non-dimensional distance Prairie grass release height Aerosol & Particulate Research Lab

21. Vertical dispersion from ambient turbulence • For sources in the SBL and those in the CBL that are emitted directly into the stable layer above the mixed layer, it is composed of an elevated component (zes) and a near-ground component (zgs) • For hes < zi, : vertical turbulence due to mechanical mixing N: Brunt-Vaisala frequency u*: surface friction velocity L: Monin-Obukhov length Q: How should we treat z for direct sources in CBL differently? Aerosol & Particulate Research Lab

22. For direct and indirect sources in the CBL, the ambient portion (zaj) is composed of updraft (j=1) and downdraft (j=2) components. zaj is also composed of an elevated component and a near-ground component • The parameterization is designed (1) to agree with field tracer studies, and (2) to decrease with source height in the surface layer and ultimately vanish for above the surface layer. Hp: plume centroid height (m) Aerosol & Particulate Research Lab

23. Buoyancy induced dispersion • Follows Pasquill form Δhis the plume rise appropriate for each of the plume types (direct, indirect, penetrated or stable plumes) Q: How would building downwash affect dispersion? Aerosol & Particulate Research Lab

24. Treatment of Building Downwash • Incorporates Plume RIse Model Enhancements (PRIME) algorithms for estimating enhanced plume growth and restricted plume rise for those affected by building wakes • PRIME partitions plume mass between cavity circulationregion and a dispersion enhanced wake region based on the fraction of plume mass intercepting the cavity boundary. • Dispersion of the re-circulated cavity mass is based on building geometry and is assumed to be uniformly mixed in the vertical. Aerosol & Particulate Research Lab

25. At the boundary of the cavity region, cavity mass is “emitted” into the wake region and combined with plume mass not captured by the cavity. • The combined plume mass is dispersed at an enhanced rate based on source location, release height and building geometry. • The enhancement of turbulence within the wake decays gradually with distance, allowing for a smooth transition to ambient levels of turbulence in the far-field. • Two models are used for dispersion estimates in the near-wake and far-wake regions, respectively. • Plume rise for sources influenced by a building is estimated using a numerical model that includes (a) effects from streamline deflection near the building, (b) vertical wind speed shear, (c) enhanced dilution from the turbulent wake and (d) velocity deficit. Aerosol & Particulate Research Lab

26. Transition of PLUME estimate to AERMOD estimate: exponential decrease with vertical, lateral and downwind distance from the wake to ensure concentrations can approach AERMOD estimates for regions far beyond the wake where building influences should be insignificant longitudinal dimension of the wake distance from the building centerline to lateral edge of the wake height of the wake at the receptor location Aerosol & Particulate Research Lab

27. Plume Rise in the CBL • Direct source: source momentum and buoyancy effects following Briggs Stack momentum flux Stack buoyant flux Stack radius corrected for stack tip downwash Wind speed for plume rise Q: How should an indirect source be treated differently? How about a penetrated source in the stable layer? Aerosol & Particulate Research Lab

28. Indirect source: height adjustment (for the delay in vertical mixing due to plume lofting at the top of the boundary layer) • Penetrated source: calculated as the equilibrium plume rise in a stratified environment Lofting plume half-widths in the lateral and vertical directions Convective velocity scale Stack height corrected for stack tip downwash Complete penetration Partial penetration Aerosol & Particulate Research Lab

29. Plume Rise in the SBL • Decreased plume rise due to positive potential temperature gradient • For downwind distances less than that to final rise • N and up evaluated initially at stack height. Subsequent plume rise estimates are made by averaging the N and up calculated at stack top and those at hs+Δhs/2. Iteration until convergence. Aerosol & Particulate Research Lab

30. When the atmosphere is close to neutral • When wind speed approaches zero (<1 m/s) Neutral length scale Aerosol & Particulate Research Lab

31. Source Characterization • Allows point source, area source, volume source and irregular area sources • Point source: location, elevation, emission rate, stack height, stack gas temperature, gas exit velocity and stack inside diameter • Volume source: • Area source: also input as circles or polygons. A polygon of up to 20 vertices. A circle is created as a nearly circular polygon of 20 sides. Q: What additional inputs are needed for a volume source compared to a point source? Plume size before account for the initial size Initial lateral plume size Aerosol & Particulate Research Lab

32. Adjustments for Urban Boundary Layer • The effect of urban surface characteristics is largest at night and relatively absent during the day. • Add a convective-like urban contribution to that found in the rural SBL (to be the total turbulence in the urban SBL). http://atmoz.org/blog/2007/06/17/surface-energy-budget-urban-heat-island/ Aerosol & Particulate Research Lab

33. The convective contribution is a function of the convective velocity scale, which depends on surface heat flux and urban mixed layer height • Upward surface heat flux • Mixing height in the nighttime urban boundary layer Q: What does the upward heat flux depend on? Empirical data from Canadian cities population ziuo = 400 m Aerosol & Particulate Research Lab

34. Urban nighttime convective velocity scale • Total nighttime turbulence in the urban boundary layer is the sum of the convective and mechanical terms • Potential temperature gradient Results in a near neutral plume rise formulation. Q: How should daytime dispersion in urban areas be handled? Aerosol & Particulate Research Lab

35. Summary Take 2 minutes to summarize here what you have learned from this section Aerosol & Particulate Research Lab