air pollution dispersionclass.atmos.ucla.edu/as2/scrns/web notes (turco... · air parcel mechanics...

Air Pollution Dispersion

• Dispersion Processes

• Convective Dispersion

➔ Air Parcel Dynamics

➔ Adiabatic Process

➔ Lapse Rate

➔ Equilibrium and Stability

➔ Atmospheric Stability

➔ Stability and Dispersion

• Temperature Inversions

➔ Stability

➔ Formation/Types

➔ Mixing Height

• Application: Chimney Plumes

➔ Plume Type vs. Stability

➔ Enhancing Plume Dispersion

DispersionProcesses

TemperatureInversion

ChimneyPlumes

Buoyancy

Lapse Rates

AdiabaticProcesses

Stability

Diffusion

Turbulence

Convection

StableUnstableNeutral

RadiationAdvectionSubsidence

MixingHeight

Air Pollution Dispersion

Air ParcelMotion

TemperatureInversions

RegionalLarge-scale

TypesRadiationAdvectionSubsidence

Sinking air

Winds

Nighttimecooling

Seasonal

VentilationFactor

Mixing Height Chimney PlumesTypes

DispersionEnhancement

TurbulenceExit VelocityExit Temperature

Air Pollution Dispersion

TallStacks

Dispersion Processes

Defn.: A substance mixes in andbecomes diluted within alarger volume of anothersubstance.

Molecular Diffusion

Turbulence

Convection/Advection

Molecular Diffusion

➪ Molecules drift from regions ofhigh concentration to regions oflower concentration

Larger concentration gradient→ higher diffusion rate

Length scale of motion = molecular

— Slow!!

Turbulence

➪ Bulk air motion in random directions

Pollution

Wind

x

yPlumeCore

PlumeEdge

Strong, gusty windsgenerate the mostturbulence

Plume}

Convection/Advection

Mass transport of pollutantsby winds

➔ Advection: horizontal motion

➔ Convection: vertical motion

Air Parcel Mechanics

• A specified volume ofair (ex.: bubble, bal loon)

• Constraints:

Pout

Pin

➔ P inside = P outside atall times

➔ No mixing of air

Parcel Buoyancy

AirParcel

Gravity

• Buoyancy: up- or downward forcefrom combination of atmosphericpressure and gravity

Pressure

?Up- or downwardmotion of airparcel depends onbuoyancy

Air Density vs. Temperature

ρ ∝ for P = constantT1

Cold air is more dense than warm air

Buoyancy in Fluids

Negativebuoyancy

Positivebuoyancy

ρ > ρ

Parcelcolder

than envir.

parcel envirρ < ρ

Parcel

warmerthan envir.

parcel envir

Adiabatic Process➜ No heat exchanged between a

system and its surroundings

Compression: P ↑, T ↑Expansion: P ↓, T ↓

Heat

Adiabatic

Lapse Rate

Lapse rate = γ or Γ(“gamma”)z

z1 T1

z2 T2T – T2 1

z – z2 1– =

∆T

∆z–

= γ

T ↓ as z ↑ : positive γ

Defn.: Rate of temperature decrease asaltitude increases.

Atmospheric Stability

• Describes behavior of air after ithas been disturbed

• Indicates atmosphere’s ability tomix vertically

• Related to air parcel buoyancyafter perturbing parcel

Stabil ity Behavior

➜ Behavior after disturbance ofequilibrium characterizes stability

Stable

Stabil ity Behavior

➜ Behavior after disturbance ofequilibrium characterizes stability

Unstable

Stabil ity Behavior

➜ Behavior after disturbance ofequilibrium characterizes stability

Neutral

Making Air Parcels Buoyant• Need: parcel temp. ≠ envir. temp.

➔ Heat up air parcel at the ground

➔ Or, force parcel upward so it losestemp. via adiabatic expansion

z

T

z

T

ParcelEnvironment

γ = 0

Γ (= 10°C/km)

γ < 0γ > 0

z

T

ParcelEnvironment

Γ Γ ΓStableγ < Γ

Unstableγ > Γ

Neutralγ = Γ

Stability Criteria

Unstable: γ > Γ

Neutral: γ = Γ

Stable: γ < Γ

Stability vs. Dispersion

• Turbulence gives parcel initial push

• Stabil ity vs. mixing:

➔ Stable—vertical motion suppressed—vertical dispersion discouraged

➔ Unstable—vertical motion encouraged—vertical dispersion enhanced

Temperature Inversions➜ Defn: Temp. increases with

increasing altitude (γ <0)

z

T

• I nversions are extremely stable

Temperature Inversion Aloft

z

T

Γ γ < 0

Inversionbase

• Radiation Inversion

• Advection Inversion

• Regional Subsidence Inversion

• Large-scale Subsidence Inversion

Radiation Inversion

Infrared radiation from ground to space

Ground cools offat night; cools airnext to it

Shallow

Advection Inversion

Cool ocean

Warm airinland

Cool marine air

Cool air flowsunderneath warm air

➜ Occurs as Marine Layer alongSoCal coast

Regional SubsidenceInversion

N.E. Deserts

S.W. CoastS.W. Coast

Weak SantaAna Wind

Air flows fromhigh altitudeto low altitude

Air compressesadiabaticallywhile descending

Large-Scale SubsidenceInversion

H

Air aloft sinks (subsides) and warms fromadiabatic compression

➜ L.A. has semi-permanentsubsidence inversion

Mixing Heights

T

zInversion or stable layer

NeutralMixedLayer Mixing Height

Pollutionconcentration ∝ Mixing

Height

1

Mixing Height vs. Season

MH ∝ ground temperature

Winter: low temp. Summer: high temp.

z

T(Morning, calm air)

Summer γ

Winter γ

MHW

MH S

Ventilation Factor∝ Volume

Mixingheight

Wind speed

VF = MH × WS

Chimney Plume Dispersion

Γd

γ

Fanning

(Top view)

Γd

γ Looping

Γd

γ Coning

γ

Γd Lofting

γ

Γd

Fumigation

Dispersion Enhancement

Turbulence generated inflow around buildings

Top

Wake Eddy

Downwash

• T all smokestacks

Larger volume formixing pollutant

Taller chimney → lower smokeconcentration at ground

Inversion Layer

Fumigation Plume

Fanning plume

Inversion base

Fumigation PlumeInversion Layer

Fanning Plume

Lofting Plume

Inversion top

Increase Exit Velocity

Low Vexit

Higher exit velocity→ lower pollution

concentration atground

High Vexit

How to Increase ExitVelocity

Pump out faster with fan

Fan usesenergy

$$

Reduce diameterof chimney tip

Restricted flow

Increase Exit Temperature

➜ Hotter smoke is more buoyant

Cool plume

Hot plume

How to Increase ExitTemperature

Heat up smoke

Hotter combustion

Uses energy

$$ for energy,maintenance

Uses energyAlters smokecomposition