Meteorology & Air Pollution Dr. Wesam Al Madhoun
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Transcript of Meteorology & Air Pollution Dr. Wesam Al Madhoun
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Meteorology & Air Pollution
Dr. Wesam Al Madhoun
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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:a. Ambient temperatureb. Temperature of emissionsc. Roughness factorsd. Wind velocitye. Wind directionf. Humidityg. Stability
Source Receptor
Transport
Re-entrainment
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Air Pollutant Cycle
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Dispersion
• General mean air motion• Turbulent velocity fluctuations• Diffusion due to concentration gradients –
from plumes• Aerodynamic characteristics of pollution• Particles
– Size– Shape– Weight
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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.
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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
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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
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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
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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
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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
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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
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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
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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.
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Mixing Height of atmosphere
The height of the base of the inversion layer from ground surface.
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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
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Plume Types
• Plume types are important because they help us understand under what conditions there will be higher concentrations of contaminants at ground level.
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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.
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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
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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
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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.
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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.