2 Meteorological Aspects

Meteorological aspects of air pollution dispersion

11/04/23Meteorological aspects of Air Pollution

The Earth’s Great Spheres Lithosphere- The lithosphere contains all of the cold, hard solid land of the planet's crust (surface), the semi-solid land underneath the crust, and the liquid land near the center of the planet

Hydrosphere- The hydrosphere contains all the solid, liquid, and gaseous water of the plane

Biosphere- The biosphere contains all the planet's living things. This sphere includes all of the microorganisms, plants, and animals of Ear

Atmosphere- The atmosphere contains all the air in Earth's system


Atmosphere •It is a mixture of gases that forms a layer of about 250 miles

thick around the earth.• -Bottom 10-12 miles (Troposphere) is most important part in

terms of –Weather

–Other aspects of Biogeochemical cycle

• -The lowest 600 meters of Troposphere: Air Quality Studies •Composition of Air - 78% nitrogen, 21% oxygen, 1% carbon

dioxide, water, other gases•Divided into four zones:

- Troposphere- Stratosphere- Mesosphere- Thermosphere





The Fog of London, The Fog of London,

Dec 5th , 1952Dec 5th , 1952

In early December 1952, an

area of high pressure settled

over London. Residents kept

piling sulfur-rich coal into

their

stoves to keep warm in the

near- freezing temperatures. In

the still air, the smoke from

these stoves and from coal-

fired power plants in the city

formed a smog laden with

sulfur dioxide and soot .

On Friday, 5 December,

schools closed and

transportation was disrupted.


Santiago in JulySantiago in July


Dust storm in East Asia




Air Pollution SystemAir Pollution System


AtmosphereAtmosphere

• Structure

• Composition

• Energy balance for earth and atmosphere

• Temperature in lower atmosphere and inversions


THE HADLEY CIRCULATION (1735): THE HADLEY CIRCULATION (1735): Global Sea BreezeGlobal Sea Breeze

HOT

COLD

COLD

Explains:• Intertropical Convergence Zone (ITCZ)• Wet tropics, dry poles

Problem: does not account for Coriolis force. Meridional transport of air between Equator and poles would result in unstable longitudinal motion.



Characteristics of Troposphere and Characteristics of Troposphere and StratosphereStratosphere

• Troposphere:Troposphere:– Ground level to 25 km.– Temp. normally decreases with altitude.– Strong vertical mixing.– Pollutants may be washed back to earth.– All weather and climate take place in the

troposphere


Characteristics Of Troposphere And Characteristics Of Troposphere And StratosphereStratosphere

• Stratosphere:Stratosphere:– 15-50 km– Temp. increases with altitude– Little vertical mixing, very slow diffusion

exchange of gases with troposphere– Pollutants entering remain here unless

attacked by light or other chemicals– Isolated from troposphere by tropopause


Half of air below 5.5 km


Stratification Of The Earth's Atmosphere Showing Changes In

Temperature And Pressure With

Altitude.


The Atmosphere

With Water Vapor

No Water Vapor Molecular weight

Gas

75.6578.0928.016 2N

20.2920.9432.0002O

3.12-18.016O2H

0.900.9339.944Ar

0.030.0344.0102CO

CommentsThese ratios are the same through most of the atmospheric height. Total - is almost 99.99%.Where the pollution goes to ???Typical atmosphere: N2 - 79%, O2 - 21% and MW - 28.85.


Minor Constituents

Conc., ppmMolecular WeightGas

0.1820.183Ne

5.24.003He

1-2.216.04CH4

1.083.8Kr

0.25-1.044.01N2O

0.00230.008NO

0.00446.0082NO

0.52.0162H

0.08131.3Xe

0.0148.0003O

Other constituents of the atmosphere include pollen, bacteria, fungi, particles (smoke, sea spray, dust), oxides of carbon, sulfur and nitrogen, and organic gases.


Blackbody RadiationBlackbody Radiation


An Example of Temperature Dependence – Black BodyAn Example of Temperature Dependence – Black Body• What is a black body?

• What law from statistical mechanics describes the amount of radiation emitted by a black body?

•What happens as T changes? K

m 2897mode T

Wien’s displacement law:

1-23B

B5

2

K s W 1038.1

1/hexp

h2

k

Tkc

cTB

Planck’s law:

[Kittel and Kroemer, 1980, p. 95]

– absorbs all incident radiation, emission is maximum


Radiative Equilibrium Slab Model of EarthRadiative Equilibrium Slab Model of Earth

• At top of atmosphere:

• At surface:

• To calculate need to integrate Planck’s law over

Stefan-Boltzmann law:

2-solar 0 m W 241F

rmalearth theF

4-2-8

45

2

thermal

K m W 1067.5

1/exp

2

TTkhc

hcdTF

B

cold! C18- K 255

o4

1

solar 0earth

4earthsolar 0

FT

TF

solar 0Frmalearth theF

solar 0F rmalearth theF


Add the AtmosphereAdd the Atmosphere

• In atmosphere:

• At surface:

effect" greenhouse" C31 K 3042

2

1

2

1

o4

1

solar 0earth

4earth

4earth

4earthsolar 0

FT

TTTF

4atm T

4earth T

4atm T

2-solar 0 m W 241F

4earth T 4

atm 2 Tsolar 0F 4

earth T 4atm T

BUT too hot for global mean temperature


Approximate the Atmosphere as a Grey BodyApproximate the Atmosphere as a Grey Body

• In atmosphere:

• At surface:

raturemean tempe global C17 K 290

21

2

1 2

1

o

41

solar 0earth

4earth

4earth

4earthsolar 0

eF

T

Te

TeTF

solar 0F 4earth T 4

atm Te

4atm Te

4earth T

4atm Te

2-solar 0 m W 241F

4atm Ta

8.0 emissivity 4atm thermalatm eTeF

4earth Ta 4

atm 2 Te4earth Te

Kirkhoff’s law0.8ty absorptivi a





The Global Energy BudgetThe Global Energy Budget

•31% of the incoming solar radiation is reflected or scattered back to space – the ALBEDO

•235 Wm-2 warms the Earth and atmosphere, 168 Wm-2 of which warms the surface.

•235 Wm-2 corresponds to a black body temperature of -19 C, thermal emission at 10 um.

•This is colder than Earth’s surface and is reached at around 5 km.•Thus the peak terrestrial emission is in the atmospheric window in the IR.

•This fraction is transmitted directly to space, but majority is intercepted and interacts.

•Clouds can absorb and emit thermal radiation but are also reflectors of solar radiation andso act to cool the surface.

•Strong cancellation between these effects the global net effect appears to be a slight cooling at the surface.


The Atmospheric The Atmospheric Oxygen Cycle.Oxygen Cycle.


Temperature Lapse Rates & Stability

Air pollutants emitted from anthropogenic sources must first be transported and diluted in the atmosphere before these undergo various physical and photochemical

transformations and ultimately reach their receptors .Otherwise, the pollutant concentrations reach dangerous levels near the source of emission.

Hence, it is important that we understand the natural processes that are responsible for their dispersion. Effective dispersion of pollutants in the atmosphere depends primarily on the degree of stability of the atmosphere and on its turbulent structure. In the broadest sense dispersion is controlled by meteorological conditions prevailing in the atmosphere.


The degree of stability of the atmosphere in turn depends on the rate of change of ambient temperature with altitude.

The relationship between temperature and the altitude can be obtained by considering air to be an ideal gas. The change of pressure in the vertical direction may be represented by the relation:

dp/dz = - ρg -------(1)Where p is the atmospheric pressure, z is the altitude and ρ is the atmospheric density.


The perfect gas equation is given by

p = ρRT -------(2)

Holds at any point in the atmosphere where R is the gas constant for air, and T is the absolute temperature

Substituting eqa.(2) in eqa.(1) gives the general expression of for the variation of pressure with altitude

dp/dz = - ρg/RT-------(3)


If we consider the simple case of isothermal atmosphere, eqa. (3) could be integrated directly to give

p = poexp(-gz/RT)

Where po is the pressure at ground level(1.013 bars).

The above equation gives an exponentially decreasing pressure with altitude, which is a typical characteristic of compressible fluids such as air.


A better model for the atmospheric dependence of p and T is the polytrophic atmosphere which obeys the relation

T = To(p/po)n-1/n-------(5)Substituting eqa. 5 in eqa. 3 and differentiating, we get

dT/dZ = -[(n-1)/n]g/R--------(6)This equation represents the variation of temperature with altitude for a polytropic model where the temperature decreases with altitude linearly with a slope of

)–n-1(g/nR.


The decrease in temperature with altitude is known as lapse rate.

Based on meteorological data in the troposphere up to about 10 Km, where the temperature decreases linearly with altitude, the environmental lapse rate is found to be about 6.5oC/ 1000metres.

Putting this value for dT/dZ in the equation for polytropic model, we have

)dT/dZ(env = -6.5oC/ 1000metres = -[(n-1)/n]g/R--------(7)


The value of n comes to be 1.23. The lower atmosphere ends at the tropopause which is at an altitude of about 12 Km .

Above the tropopause is the stratosphere. In the stratosphere there are two distinct regions of temperature variation.The lower region, extending up to 25 Km, has a temperature which is essentially constant, and in the upper region the temperature increases with altitude as a result of ozone formation. In the lower region of the stratosphere the isothermal model based on n = 1 is applicable.


Adiabatic Lapse Rate (ALR)

The adiabatic decrease in temperature with altitude is especially important in the vertical movement of air pollutants and can be explained by utilising the concept of an air parcel.

As the air parcel rises in the atmosphere, it goes through a region of decreasing pressure and expands to accommodate the decreasing pressure. As it expands, it does work on surroundings. Since the process is usually rapid, there is no heat transfer between the air parcel

and the surrounding air .


The temperature lapse rate for dry parcel of air moving upwards adiabatically and is

known as the dry adiabatic lapse rate is

)dT/dZ(adia = -9.86oC/ 1000metres

Thus, the dry adiabatic lapse rate, which is denoted by T, is given by

T = -(dT/dZ)adia ≈ 10oC/ 1 Km


Super adiabatic lapse rate (SALR)

When the prevailing lapse rate or ambient lapse rate or environmental lapse rate is greater than the dry adiabatic lapse rate, the atmospheric condition is known as super adiabatic. For such a case , the rising parcel of air , cooling at adiabatic rate, will be warmer and less dense than the surrounding environment.

On a clear summer day, rapid heating of the earth by the sun warms the air near the surface, to the point where the lapse rate is super adiabatic.

As a result, air becomes more buoyant and tends to continue its upward motion, resulting in unstable equilibrium, due to this, marked vertical mixing takes place and pollutants are dispersed rapidly.


Sub-adiabatic or Negative lapse rate

When the environmental lapse rate is less than the dry adiabatic lapse rate a rising air parcel becomes cooler and more dense than its surroundings and tends to fall back to its original position.

Such an atmospheric condition is known as stable and the lapse rate , which is sub adiabatic is called the negative lapse rate.


Atmospheric Stability

The ability of the atmosphere to disperse the pollutants emitted in to it depends to a large extent on the degree of its stability.

A comparison of the adiabatic lapse rate with the environmental lapse rate gives an idea of the stability of the atmosphere .


UNSTABLE ATMOSPHERE

When the environmental lapse rate is greater than the dry adiabatic lapse rate (super adiabatic), a rising parcel of air , cooling at the adiabatic rate, will be warmer and less dense than the surrounding environment.

As a result it becomes more buoyant and tends to continue its upward motion. Since vertical motion is enhanced by buoyancy, such an atmosphere is called unstable. In the unstable atmosphere, the air from different altitudes

mixes thoroughly .This is very desirable from the point of view of preventing pollution, since the effluents will be rapidly dispersed throughout the atmosphere.


STABLE ATMOSPHERE

On the other hand, when the environmental lapse rate is less than the dry adiabatic lapse rate (sub adiabatic), a rising parcel of air becomes cooler and dense than the surroundings and tends to fall back to its original position. Such an atmospheric condition is known as stable.

Under stable conditions there is very little vertical air mixing and pollutants can only disperse very slowly. As a result, their levels can buildup very rapidly in the environment.


Inversion

The extreme case of a stable atmosphere is called an inversion, occurs when the temperature increases with altitude.

During inversion vertical air movement is stopped and the pollution is concentrated beneath the inversion layer, i.e. the denser air at ground level.

Thus atmospheric inversion influence the dispersion of pollutants by restricting vertical mixing, hence pollutants in the air do not disperse.

Inversion is a frequent occurrence in the autumn and winter months.


Types of Inversions

INVERSION

RADIATIONINVERSION

SUBSIDENCEINVERSION

DOUBLEINVERSION


THERMAL INVERSIONTHERMAL INVERSION


Types of Thermal InversionsTypes of Thermal Inversions

• Radiative: Earth cools during night by radiating thermal energy into space. In morning, air near surface will be cooler than air above creating thermal inversion. More frequent, but less problematic and persistent.


• High pressure subsidence: high pressure mass of air moves towards earth. Is compressed and heated, causing thermal inversion some distance

above ground.


Double Inversion

•This phenomenon happens when both

the radiation inversion and subsidence inversion occur simultaneously.


.constT

pV

1MV

.w.mM

n T*R.w.m

MpV

TRT.w.m

*Rp a

ThermodynamicsThermodynamicsGases Laws:

In the Atmosphere there is no define volume of air and therefore it is advisable to use the parameter instead. If a volume of air V has a mass M, then:

R* =8.314107 erg/k*mol, and Ra=2.87106 erg/ gram ok


dWdUdQ

Adn dVpA F A

Fp

First Law of Thermodynamics:Heat is equivalent to energy, and both are conservative properties.

Heat added = Internal heat + Work perform by the gas

For per unit mass the Equation becomes:

dq = du + dwdq = du + pd

dW= Fdn = pAdn = pdV


pp dT

dqC

dq = du + pd = Cv dT

dq = CvdT = du

dq = Cv dT + pd

Differential of p = RaT is

pd + dp = RadT

dq = CvdT + RadT- dp = CpdT - dp

vv dT

dqC


For adiabatic case dq=0

Cp d T= dp

Cp d T

Cp d log T = Ra d log p

dpp

TRa

pCaR

oo PP

TT

1

p

a 286.0C

R

Integration will yield:

Ra=2.87 erg / gram degree K


For adiabatic caseCp d T = dp

if we divide by dz (small vertical distance), we will get:

Cp dT/dz = dp/dz

A

gmdzzpzpdp

)()(

A

gdzA

gdz

dp Or

This is known as the Hydrostatic equation


dz

dp

dz

dTCp

m

C

C

g

dz

dT o

p 10098.0

'ZT

gdzdp

From first Law of Thermodynamics for adiabatic case dq = du + dw = 0

Since Cp = 1.005

Cv = 0.716 Joule/gram*Degree K


Time Scales For Horizontal TransportTime Scales For Horizontal Transport(Troposphere)(Troposphere)

2 weeks1-2 months

1-2 months

1 year


Vertical Transport: BuoyancyVertical Transport: Buoyancy

• What is buoyancy?

Object (z

z+zFluid (’)

buoyancy P gradient gravity

g

Note: Barometric law assumed a neutrally buoyant atmosphere with T = T’

P gradient gravity T T’ would produce bouyant acceleration


Atmospheric Lapse Rate And StabilityAtmospheric Lapse Rate And Stability

T

z

= 9.8 K km-1

Consider an air parcel at z lifted to z+dz and released.It cools upon lifting (expansion). Assuming lifting to be adiabatic, the cooling follows the adiabatic lapse rate :

z

“Lapse rate” = -dT/dz

-1/ 9.8 K kmp

gdT dz

C

ATM(observed)

What happens following release depends on the local lapse rate –dTATM/dz:

• -dTATM/dz > upward buoyancy amplifies initial perturbation: atmosphere is unstable

• -dTATM/dz = zero buoyancy does not alter perturbation: atmosphere is neutral

• -dTATM/dz < downward buoyancy relaxes initial perturbation: atmosphere is stable

• dTATM/dz > 0 (“inversion”): very stable

unstable

inversion

unstable

stable

The stability of the atmosphere against vertical mixing is solely determined by its lapse rate


Effect Of Stability On Vertical StructureEffect Of Stability On Vertical Structure


What Determines The Lapse Rate Of The What Determines The Lapse Rate Of The Atmosphere?Atmosphere?

• An atmosphere left to evolve adiabatically from an initial state would eventually tend to neutral conditions (-dT/dz = G ) at equilibrium

• Solar heating of surface disrupts that equilibrium and produces an unstable atmosphere:

Initial equilibriumstate: - dT/dz =

z

T

z

T

Solar heating ofsurface: unstableatmosphere

ATM

ATM

z

Tinitial

final

buoyant motions relaxunstable atmosphere to –dT/dz =


In Cloudy Air Parcel, Heat Release In Cloudy Air Parcel, Heat Release From HFrom H22o Condensation Modifies o Condensation Modifies

RH > 100%:Cloud forms

“Latent” heat releaseas H2O condenses

9.8 K km-1

W2-7 K km-1

RH

100%

T

z

W

Wet adiabatic lapse rate W = 2-7 K km-1


VERTICAL PROFILE OF TEMPERATUREVERTICAL PROFILE OF TEMPERATUREMean Values For 30Mean Values For 30oon, Marchn, March

Alt

itu

de,

km

Surface heating

Latent heat releaseRadiativecooling (ch.7) - 6.5 K km-1

2 K km-1

- 3 K km-1Radiativecooling (ch.7)

Radiative heating:O3 + hO2 + OO + O2 + M O3+M

heat


Subsidence InversionSubsidence Inversion

typically 1- 2 km altitude


Diurnal Cycle Of Surface Diurnal Cycle Of Surface Heating/Cooling:Heating/Cooling:

z

T0

1 km

MIDDAY

NIGHT

MORNING

Mixingdepth

Subsidenceinversion

NIGHT MORNING AFTERNOON


FrontsFronts

WARM FRONT:

WARM AIR COLD AIR

WIND Front boundary;inversion

COLD FRONT:

COLD AIRWARM AIR

WIND

inversion


Typical Time Scales For Vertical MixingTypical Time Scales For Vertical Mixing

• Estimate time t to travel z by turbulent diffusion:

2

5 2 -1 with 10 cm s2 z

z

zt K

K

0 km

2 km

1 day“planetaryboundary layer”

tropopause

5 km

10 km

1 week

1 month

10 years


The Electromagnetic Spectrum, Showing The The Electromagnetic Spectrum, Showing The Visible Portion Of The Spectrum In Color.Visible Portion Of The Spectrum In Color.


Representation Of Representation Of BlueBlue, , GreenGreen And And RedRed Photons, Photons, Demonstrating Their Relative Wavelengths.Demonstrating Their Relative Wavelengths.


PLUME BEHAVIOUR

The behavior of plume emitted from an elevated source such as a tall stack depends on the degree of instability of the atmosphere and the prevailing wind

turbulence .


Looping PlumeIt occurs under super adiabatic conditions with light to moderate wind speeds on a hot summer afternoon when large scale thermal eddies are present.

The plume has wavy behavior since it occurs in a highly unstable atmosphere.

The high turbulence helps in rapid dispersion of the plume, but high concentration may occur close to the stack if the plume touches the ground.


Coning PlumeIt occurs on cloudy day or nights with strong winds (velocity at 32 Km/hr) when the lapse rate is near neutral (adiabatic condition).

The plume shape is vertically symmetrical about the plume line. How ever, the plume reaches the ground at greater distance than with looping.


Fanning PlumeThis occurs under surface inversion conditions, in the presence of light wind.Most of the vertical dispersion is suppressed by extremely stable condition, and the plume fans out in the horizontal direction.Strong concentrations at plume height are exhibited down wind of the stack.A fanning plume is often observed at a height and in the early morning in all seasons.


Lofting PlumeThis occurs under strong super adiabatic lapse rate above a surface inversion.In such a condition, down ward motion and mixing is prevented by surface inversion but the upward mixing will be quite turbulent and rapid .

The emission will there fore not reach the ground surface.Lofting is the most favorable plume type as far as ground level concentrations are concerned and is one of the major goals of tall stack operation.


Fumigating PlumeThe conditions for fumigation are just the inversion of lofting plume.Fumigation takes place when an inversion layer occurs at a short distance above the top of the stack and they are brought down rapidly near ground due to turbulence in the region above the ground and below the inversion, caused by strong lapse rate.Fortunately, this condition is usually of short duration lasting for about 30 minutes .

Fumigation represents quite a bad case of atmosphere.


Trapping PlumeThis condition is achieved when the plume is caught between two inversion layers.

Hence the emitted plume can neither go up nor down and will be trapped between the two levels.

The diffusion of the effluent will be severely restricted to the unstable layer between the two stable regions.

A trapping plume is considered to be a bad condition for dispersion.


Thank you

Documents

2 Meteorological Aspects