Atmospheric Radiation - Atmospheric Chemistry - Lecture Slides, Slides of Chemistry

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ATMOSPHERIC RADIATION

Here

is the radiation flux emitted in [

is the flux distribution function characteristic of the object Total radiation flux emitted by object:

EMISSION OF RADIATION

•^

Radiation is energy transmitted by electromagnetic waves; allobjects emit radiation

-^

One can measure the radiation flux spectrum emitted by a unitsurface area of object:

^ 

  

^0

d

KIRCHHOFF’S LAW:

Emissivity

T ) = Absorptivity

For any object:

…very useful!

Illustrative example: Kirchhoff’s law allowsdetermination of theemission spectrum ofany object solely fromknowledge of itsabsorption spectrumand temperature

SOLAR RADIATION SPECTRUM: blackbody at 5800 K

RADIATIVE EQUILIBRIUM FOR THE EARTH

Solar radiation flux intercepted by Earth = solar constant

FS

=^

1370 W m

Radiative balance

effective temperature of the Earth:

= 255 K

where

A

is the albedo (reflectivity) of the Earth

ABSORPTION OF RADIATION BY GAS MOLECULES^ •^

…requires quantum transition in internal energy of molecule.

-^

THREE TYPES OF TRANSITION^ –

Electronic transition: UV radiation (<0.

m)

•^

Jump of electron from valence shell to higher-energy shell,sometimes results in dissociation (example: O

+h 3

O^2

+O)

–^

Vibrational transition: near-IR (0.7-

m)

•^

Increase in vibrational frequency of a given bond requires change in dipole moment of molecule

-^

Rotational transition: far-IR (20-

m)

•^

Increase in angular momentum around rotation axis

Gases that absorb radiation near the spectral maximum of terrestrialemission (

m) are called

greenhouse gases;

this requires

vibrational or vibrational-rotational transitions

GREENHOUSE EFFECT:

absorption of terrestrial radiation by the atmosphere

-^ Major greenhouse gases: H

O, CO 2

, CH 2

, O 4

, N 3

O, CFCs,… 2

-^ Not greenhouse gases: N

, O^2

, Ar, … 2

SIMPLE MODEL OF GREENHOUSE EFFECT

Earth surface (

To

Absorption efficiency 1-

A^

in VISIBLE1 in IR

Atmospheric layer (

T^1

abs. eff. 0

for solar (VIS) f for terr. (near-IR)

/ 4 S Incomingsolar F

/ 4 S F

Reflectedsolar

/ 4 F AS

/ 4 F AS

4 T o

Surface emission^ 

4

(

)^

o

f^

T  

Transmittedsurface

(^41)

f^

T 

(^41)

f^

T 

AtmosphericemissionAtmosphericemission

Energy balance equations:•^ Earth system

4

(^41)

(^

) / 4

(^

)

S^

o

F^

A^

f^

T^

f^

T

^



^

^

^



-^ Atmospheric layer

4

(^41)

2 o f^

T^

f^

T



 

Solution:

1 4

(^

)

4(

) 2 S o

F^

A

T^

f^  ^



^

 

 

  ^



^

T 

=288 Ko 

f=0. T^1

= 241 K

VISIBLE

IR

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The ultimate modelsfor climate research

EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?

1.^

1. Initial state

2.^

2. Add to atmosphere a GGabsorbing at 11

m;

emission at 11

m

decreases (we don’t seethe surface anymore atthat

but the atmosphere)

3. At new steady state, totalemission integrated over all

’s

must be conserved ^

Emission at other

’s must

increase ^

The Earth must heat!

Example of a GG absorbing at 11

m

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EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING

The efficient GGs are the ones that absorb in the “atmospheric window” (8-13 m). Gases that absorb in the already-saturated regions of the spectrum arenot efficient GGs.

CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE

Positive feedback from water vapor causes rough doubling of

CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS

convection

Tcloud^ To

≈^

To

Clouds reflect solar radiation (

 A >

cooling;

…but also absorb IR radiation (

 f

warming

WHAT IS THE NET EFFECT?

 T

4  o  T

cloud

4 ≈^

 T

(^4) o

LOW CLOUD: COOLING

 T

cloud

 T

(^4) o

 T

4  o

HIGH CLOUD: WARMING