air pollution meteorologyclass.atmos.ucla.edu/as2/scrns/pdfnotes/03.aos2... · 2015. 7. 11. ·...
TRANSCRIPT
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Air Pollution Meteorology
The solution to pollution is dilution
Air Pollution Meteorology
• Weather
Winds and Breezes
• Dispersion Processes
• Convective Dispersion
Air Parcel Dynamics
Adiabatic Process
Lapse Rate
Atmospheric Stability
Stability and Dispersion
• Temperature Inversions
Stability
Formation/Types
Mixing Height
• Daily and Seasonal Smog Variation
• Application: Chimney Plumes
Plume Type vs. Stability
Enhancing Plume Dispersion
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Winds
• Winds blow from high to low atmospheric pressure
H LWind
Sea Breeze
• Sea breezes blow from the cool ocean region toward the warmer land
H LSea breeze
Cool ocean Warm land
Sea Breeze
• Sea breezes blow from the cool ocean region toward the warmer land
H Sea breeze Upsl
ope
bree
ze
Cool ocean Warm land
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Fig. 6.20
Fig. 6.22
Fig. 6.23
Dispersion Processes
Defn.: A substance mixes in and becomes diluted within a larger volume of another substance.
Molecular Diffusion
Turbulence
Convection
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Molecular Diffusion
• Molecules drift from regions of high concentration to regions of lower concentration
The larger the concentration gradient, the higher the diffusion rate
Length scale of motion = molecular
— Slow!!
Turbulence
• Bulk air motion in random directions
Pollution
Strong, gusty winds generate the most turbulence
Plume Core
Convection
• Mass transport of pollutants by winds
➔ Advection: horizontal motion
➔ Convection: vertical motion
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Air Parcel Mechanics
• Air Parcel: a specified volume of air (ex.: bubble, balloon)
Pout
Pin• Constraints:
Pinside = Poutside
at all times
No mixing of air across boundary
Parcel Buoyancy
• Buoyancy: up- or downward force from combination of atmospheric pressure and gravity
Air Parcel
Gravity
Atmos. Pressure
Pressure buoyancyMotion
Up- or downward motion of air parcel depends on buoyancy vs. gravity
Air Density vs. Temperature
ρ ∝ 1
Tfor P = constant
Cold air is more dense
than warm air
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Buoyancy in Fluids
Negative Buoyancy
Positive Buoyancy
Tparcel < Tenvir Tparcel > Tenvir
Parcel colderthan
environment
Parcel warmerthan
environment
ρparcel > ρenvir ρparcel < ρenvir
Adiabatic Process
No heat exchanged between a system and its surroundings
Heat
Adiabatic
Compression: P, TExpansion: P, T
Parcel Temperature
P P
P
P
P
P Rising air parcels undergo adiabatic expansion
Rising air parcels lose temperature
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Lapse Rate
Defn.: Rate at which temperature decreases as altitude increases
z2, T2
z1, T1
Lapse rate = γ or Γ
(“gamma”)
=T2 – T1
z2 – z1
– – ΔT
Δz
= γ
Making Parcels Buoyant
• Need: parcel temp. > envir. temp.
Heat up air parcel at the ground
Then, positively buoyant parcel rises
But, rising parcel loses temperature through adiabatic expansion…
T
z
Parcel
(Γ = 10°C/km)
Environment
γ < Γ
γ = 0°C/km
γ = Γ
γ > Γ
Γ Γ Γ
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Atmospheric Stability
• Related to behavior of an air parcel after it has been disturbed
• Indicates atmosphere’s ability to mix vertically
• Related to air parcel buoyancy after parcel is disturbed
Stability Behavior
Behavior after disturbance of equilibrium characterizes stability
Stable
Unstable
Neutral
Stability Behavior
Behavior after disturbance of equilibrium characterizes stability
Stable
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Stability Behavior
Stable equilibrium: suppresses disturbances; restores original configuration of system
Stable
T
z Γ γ < Γ
Stability Behavior
Behavior after disturbance of equilibrium characterizes stability
Unstable
Stability Behavior
Unstable: encourages or accelerates disturbances
Unstable
T
z Γ
γ > Γ
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Stability Behavior
Behavior after disturbance of equilibrium characterizes stability
Neutral
Stability Behavior
Neutral: neither suppresses nor accelerates disturbances; new equilibrium configuration results
Neutral
T
z Γγ = Γ
Stability Criteria
Stable: γ < Γ
Unstable: γ > Γ
Neutral: γ = Γ
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Stability vs. Dispersion
• Turbulence gives parcel an initial push
• Stability vs. vertical mixing:
Stable—vertical motion suppressed—vertical dispersion discouraged
Unstable—vertical motion encouraged—vertical dispersion enhanced
Temperature Inversions
Defn.: Temperature increases with
increasing altitude (γ < 0)
z
T
• Inversions are extremely stable
Temperature Inversion Aloft
z
T
Γ γ < 0
γ = ΓInversion
base
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Inversion Types
• Radiation Inversion
• Advection Inversion
• Regional Subsidence Inversion
• Large-scale Subsidence Inversion
Radiation InversionInfrared radiation from ground to space
Ground cools off at night
— Cools air next to it
Shallow
Advection Inversion
Cool ocean
Warm air inland
Cool marine air
Cool air flows underneath warm air
In Southern California, called the “Marine Layer”
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Regional Subsidence Inversion
N.E. deserts
Weak Santa Ana wind
Air compresses adiabatically while descending
Air flows from high altitude to low altitude
Large-scale Subsidence Inversion
H
L.A. has a semi-permanent large-scale subsidence inversion
Air aloft sinks (subsides) and warms from adiabatic compression
Mixing Heights
Mixing HeightMixed Layer
Pollution concentration ∝
Mixing Height
1
z
T
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Mixing Height vs. Season
MH ∝ ground temperature
z
T
MHs
MHw
Winter
Summer
Fig. 6.14
Mixing Height vs. Time of Day
Daily Smog Variation
Sunrise Noon Sunset
NO, CO
Rush hours: high primary pollutant source rate
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Daily Smog Variation
Sunrise Noon Sunset
NO, CO NO2
Daily Smog Variation
Sunrise Noon Sunset
Ozone accumulates over time
No sun at night—no ozone production
NO, CO NO2
O3
Daily Meteorology
• Inversions affect daily concentration
Large-scale subsidence inversion
Marine layer inversion
Diurnal variation of mixing height
Daytime, afternoon: Coast Inland
Sea breeze
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Seasonal Variation
q
Winter Spring Summer Fall
CO O3
Low MH means high concen. of primary pollt.
High sun intensity means high O3
production rate
SunlightIntensity
AirTemperature
Winter Low Cold
Summer High Warm
Seasonal Meteorology
• Winter
Low morning mixing height → high
[CO] early in morning
Occasional storm helps clear the air
• Summer
Inversions very strong (LSSI + MI)
Warm, sunny afternoons → high [O3]
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Chimney Plume Dispersion
—Elevated source
Plume
Stack
Stack height Wind
Chimney Plume Types
Wind
Γ γFanning
Γ
γLooping
Γ γ
Γγ
Γγ
Coning
Fumigation
Lofting
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Plume Dispersion Enhancement
Tall stacks increase mixing volume
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Artificial Enhancement
Higher Exit Temperature (higher plume buoyancy)
Higher Exit Velocity (higher plume momentum)
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