1. Meteorology & Air Pollution Dr. Wesam Al Madhoun

2. Dispersion = Advection (Transport) + Dilution (Diffusion)
Fick’s law of diffusion J= - D * D C/Dx
Where, J= Mass Flux; D = Diffusivity coefficient,; D C/Dx = Concentration gradient
Diffusion of pollutants occur due to turbulence, which further depends upon many factors:
Ambient temperature
Temperature of emissions
Roughness factors
Wind velocity
Wind direction
Humidity
Stability
Transport
Source
Receptor
Re-entrainment

3. Air Pollutant Cycle

4. Dispersion General mean air motion Turbulent velocity fluctuations
Diffusion due to concentration gradients – from plumes
Aerodynamic characteristics of pollution
Particles
Size
Shape
Weight

5. Turbulence Not always completely understood Two types:
Atmospheric heating
Causes natural convection currents
Thermal eddies
Mechanical turbulence
Results from shear wind effects
Result from air movement over the earth’s surface, influenced by location of buildings and relative roughness of terrain.

6. Lapse Rate Important characteristic of atmosphere is
ability to resist vertical motion: stability
Affects ability to disperse pollutants
When small volume of air is displaced upward
Encounters lower pressure
Expands to lower temperature
Assume no heat transfers to surrounding atmosphere
Called adiabatic expansion

7. Adiabatic Expansion
To determine the change in temp. w/ elevation due to adiabatic expansion
Atmosphere considered a stationary column of air in a gravitational field
Gas is a dry ideal gas
Ignoring friction and inertial effects
( dT/dz)adiabatic perfect gas = - (g M/ Cp)
T = temperature
z = vertical distance
g = acceleration due to gravity
M = molecular weight of air
Cp = heat capacity of the gas at constant pressure

8. Adiabatic Expansion ( dT/dz)adiabatic perfect gas = -0.0098°C/m or
( dT/dz)adiabatic perfect gas = -5.4°F/ft
Change in Temp. with change in height

9. Lapse rate Lapse rate is the negative of temperature gradient
Dry adiabatic lapse rate =
Metric:
Γ = - 1°C/100m or
SI:
Γ = - 5.4°F/1000ft

10. Conti…. Important is ability to resist vertical motion: stability
Comparison of Γ to actual environment lapse rate indicates stability of atmosphere
Degree of stability is a measure of the ability of the atmosphere to disperse pollutants

11. Atmospheric Stability
Affects dispersion of pollutants
Temperature/elevation relationship principal determinant of atmospheric stability
Stable
Little vertical mixing
Pollutants emitted near surface tend to stay there
Environmental lapse rate is same as the dry adiabatic lapse rate

12. Stability Classes Developed for use in dispersion models
Stability classified into 6 classes (A – F)
A: strongly unstable
B: moderately unstable
C: slightly unstable
D: neutral
E: slightly stable
F: moderately stable

13. Vertical Temperature Profiles
Environmental lapse rate (ELR)
Dry adiabatic lapse rate (DALR)
If,
ELR > DALR =sub adiabatic condition, atmosphere is stable.
ELR >> DALR= Inversion conditions. Very stable atmosphere.
ELR= DALR= atmosphere is neutral.
ELR< DALR = super adiabatic condition, atmosphere is unstable.
Shapes of plumes depends upon atmospheric stability conditions.

17. Mixing Height of atmosphere
The height of the base of the inversion layer from ground surface.

18. General Characteristics of Stack Plumes
Dispersion of pollutants
Wind – carries pollution downstream from source
Atmospheric turbulence -- causes pollutants to
fluctuate from mainstream in vertical and crosswind directions
Mechanical & atmospheric heating both present at same time but in varying ratios
Affect plume dispersion differently

19. Plume Types
Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level.

20. Looping Plume High degree of convective turbulence
Superadiabatic lapse rate -- strong instabilities
Associated with clear daytime conditions accompanied by strong solar heating & light winds
High probability of high concentrations sporadically at ground level close to stack.
Occurs in unstable atmospheric conditions.

21. Coning Plume Stable with small-scale turbulence
Associated with overcast moderate to strong winds
Roughly 10° cone
Pollutants travel fairly long distances before reaching ground level in significant amounts
Occurs in neutral atmospheric conditions

22. Fanning Plume Occurs under large negative lapse rate
Strong inversion at a considerable distance above the stack
Extremely stable atmosphere
Little turbulence
If plume density is similar to air, travels downwind at approximately same elevation

23. Lofting Plume
Favorable in the sense that fewer impacts at ground level.
Pollutants go up into environment.
They are created when atmospheric conditions are unstable above the plume and stable below.

24. Fumigation
Most dangerous plume: contaminants are all coming down to ground level.
They are created when atmospheric conditions are stable above the plume and unstable below.
This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.