Pat Arnott, ATMS 749, UNR, 2008 Chapter 9: Absorption by Atmospheric Gases Visible and UV...

Pat Arnott, ATMS 749, UNR, 2008

Chapter 9: Absorption by Atmospheric Gases

• Visible and UV Absorption: due to electronic transitions. Monatomic - polyatomic.

• IR Absorption: due to vibration and rotation transitions. Polyatomic.

• Microwave Absorption: due to rotation transitions. Polyatomic.

• Absorption cross sections depend on temperature and pressure.

• Population of energy levels depends on temperature (thermal energy, kT). Transitions between levels therefore depend on temperature.

• Temperature (Doppler) broadening of absorption lines in the mesosphere.

• Pressure broadening of absorption lines (due to molecular collisions) in the troposphere.

KEY POINTS


Absorption and Emission Lines: Three level molecule. ij=Eij/h.


Dances of the Molecules in the Atmosphere: Which dance? Depends on temperature, available IR photons.

From Liou


Atmospheric Temperature Profile: US “Standard” Atmosphere.

From Liou

Dances of the Molecules in the Atmosphere: Which dance? Depends on temperature, available IR photons.

P(Eij) ≈ exp(-hij / kT)


Transitions


Dominant Transitions

Wavelengths BandDominant Transition

< 1 µm,

(200 nm - 1000 nm)Near IR, Visible, UV Electronic

1 µm - 20 µm Near IR, Thermal IR Vibration

> 20 µm Far IR, Microwave Rotation


Rotations DescriptionMoments of

InertiaExamples

Monatomic I1=I2=I3=0 Argon, He, Xe

Linear I1=0, I2=I3>0 N2, O2, CO2, N2O

Spherical Top

I1=I2=I3 > 0 CH4

Symmetric Top

I1≠0, I2=I3 > 0NH3. CH3Cl,

CF3Cl

Asymmetric Top

I1≠I2≠I3 > 0 H2O, O3


Selection Rules: Accelerated Charges are the Source, Sinks of Electromagnetic Radiation: Dipole Moment, p.

Selection rules specify the possible transitions among quantum levels due to absorption or emission of electromagnetic radiation. Incident electromagnetic radiation presents an oscillating electric field that interacts with a transition dipole pz. A transition dipole moment is a transient dipolar polarization created by an interaction of electromagnetic radiation with a molecule. If pz is zero then a transition is forbidden. The selection rule is a statement of when pz is non-zero.

q

-q

General


Why Don’t We Worry About Rotational and Vibrational Transitions for N2, and worry only a little about O2?

Homonuclear Diatomic Molecules: N2 has no permanent electric or magnetic dipole moment due to the symmetry of positive and negative charge within the molecules. (O2 has a permanent magnetic dipole moment, rotation bands at 60 and 118 GHz.)


Why Don’t We Worry About Rotational and Vibrational Transitions for N2, and worry only a little about O2?

Homonuclear Diatomic Molecules: N2 has no permanent electric or magnetic dipole moment due to the symmetry of positive and negative charge within the molecules. (O2 has a permanent magnetic dipole moment, rotation bands at 60 and 118 GHz.)

Bonding electron ‘clouds’ (orbitals) for O2 and N2 (bottom).O2

N2


Common Triatomic Molecules CO2 and CH4.

CO2 and CH4 (carbon dioxide and methane) have no permanent electric or magnetic dipole moment and don’t have pure rotational transitions. However, bending modes associated with vibrational energy levels can induce dipole moments that couple vibrational and rotational transitions in the thermal IR.

rotation

rotation

vibration {


Vibrational Transitions for Diatomic Molecules: CO


Some Energy States of Water Molecules

http://www.lsbu.ac.uk/water/vibrat.html

... of Carbon Dioxide MoleculesVibration modes of carbon dioxide. Mode (a) is symmetric and results in no net displacement of the molecule's "center of charge", and is therefore not associated with the absorption of IR radiation. Modes (b) and (c) do displace the "center of charge", creating a "dipole moment", and therefore are modes that result from EM radiation absorption, and are thus responsible for making CO2 a greenhouse gas.

“15 um motion”


Line BroadeningNatural Broadening:Finite time, finite widths (Heisenberg is uncertain about widths, certain they are not infinitely narrow!)

Doppler Broadening:

Molecules with relative motions due to thermal energy ‘see’ doppler shifts of the light. Important in the mesosphere.

Pressure Broadening: Lorentz line shape

Molecular collisions distort energy levels for absorption and emission. Emperically determined (by measurement). Very important for the troposphere and lower stratosphere.

frequency

frequency


Absorption Cross Section per Molecule for a Single Transition

Hitran04Database:S, n, ∞

WHY?


Line Strength Temperature Dependence Summary

*** Energy levels are determined from quantum mechanics, electronic, vibration, rotation etc, as related to molecular mass, charge distribution, orientation, number of atoms, etc.

*** # of molecules in each state is determined from statistical mechanics, partition function, thermal energy. Is there sufficient thermal energy to populate the energy levels above the ground state? What is the probability molecules are in a given energy state?


Is it likely that a molecule can be in energy state El?Water Vapor must be in state El before it can absorb photon with energy h0c.

Molecules are in lower energy states at lower temperature.

0.00000001

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

0 500 1000 1500 2000 2500

Lower Energy State E

l

(cm

-1

)

Relative Population at 296 K

Relative Population at 196 K

exp[(-E

l

/(k

b

T)]=exp[(-hc

l

/(k

b

)]T

: Probability Thermal Energy sufficient to mingle with Photon energy

h

0

c

E

l

= h

l

c

E

H

= ( h

+l

0

) c


Additional Interactions

Continuum Absorption (e.g. water vapor in window region)Broad, weak absorption.From poor model of spectral lines? From water vapor clusters of 2 or more molecules? Both?VERY IMPORTANT!!! ‘Dirties’ the window region in the thermal IR.

Photoionization (Gamma, X-ray)Continuum absorption, electrons ejected with kinetic energy.

Photodissociation (UV, Vis?) e.g. NO2 and < 400 nm.Molecules are broken and leave with kinetic energy.NO2 + h --> NO + OVERY IMPORTANT FOR ATMOSPHERIC CHEMISTRY!!!


Water Vapor Continuum Absorption Very Important in Window Region!!!! (Example from 6 Nov 08 Measurements of downwelling radiance at UNR)

Weak absorption, continuum baseline with a few weak lines, easily overshadowed by the much larger emission amounts from clouds!!


Number of Lower Energy States for Water Molecules in Wavenumber bins for the Wavenumber Range 500-750 cm-1.

0

10

20

30

40

50

60

70

80

Lower Energy State (cm

-1

)

h

0

c

E

l

= h

l

c

E

H

= ( h

+l

0

) c

Lower Energy States of Water Vapor Associated with Transitions in the wavenumber range

500 750 -1)to cm


Line Strength Temperature Dependence Water Vapor: Weak Line

0.0E+00

5.0E-25

1.0E-24

1.5E-24

2.0E-24

2.5E-24

3.0E-24

190 210 230 250 270 290

Temperature (K)

Line Strength (cm

2 molecule

-1 cm

-1)

0.032

0.033

0.034

0.035

0.036

0.037

0.038

0.039

g Air Broadened Half Width (cm-1 / atm)

S Line Strength at 296 K

Air Broadened Half Width g to get gammap @296K (cm-1 / atm)

0 = 500.035137 -1cm


Line Strength Temperature Dependence Water Vapor: Strong Line

0.0E+00

2.0E-21

4.0E-21

6.0E-21

8.0E-21

1.0E-20

1.2E-20

1.4E-20

1.6E-20

1.8E-20

190 210 230 250 270 290

Temperature (K)

Line Strength (cm

2 molecule

-1 cm

-1)

0.000

0.010

0.020

0.030

0.040

0.050

0.060

g Air Broadened Half Width (cm

-1 /

S Line Strength at 296 K

Air Broadened Half Width g to get gammap @296K (cm-1 / atm)

0 = 525.959891 cm -1


Line Strength Temperature Dependence Water Vapor

0.007 0.015 0.030 0.053 0.0910.148

0.232

0.350

0.511

0.724

1.000

0.200.26

0.320.39

0.470.55

0.630.72

0.810.91

1.00

-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.5

190 210 230 250 270 290

Temperature (K)

Relative Line Strength

Relative Line Strength, nu0=500.035 cm-1

Relative Line Strength, nu0=525.960 cm-1

Relatively Strong Line, lower energy state

l = 920.2 cm -1 .

Relatively Weak Line lower energy state

l = 2248.1 cm -1 .

(P l) ≈ [-EXP hc l/( )]kT


Line Strength and Lower Energy States and Temperature

Water Vapor, 500 - 750 cm-1

1.E-33

1.E-32

1.E-31

1.E-30

1.E-29

1.E-28

1.E-27

1.E-26

1.E-25

1.E-24

1.E-23

1.E-22

1.E-21

1.E-20

1.E-19

0 1000 2000 3000 4000 5000 6000

Lower State Energy l (cm-

( Line Strength cm

2 molecule

-1 cm

-1)

296 S Line Strength at K

196 S Line Strength at K

Detection?Threshold

( ) (290 ) S T grossly related to S K exp [(-hc l/(k b )]T


Electronic, Vibrational, energy levels and the big break up (dissociation level)

From Liou


Absorption cross sections of O3 and O2 in the UV and Visible.

Strongly affects atmospheric chemistry, thermal structure, and amount of deadly UV that doesn’t make it to the surface.


Depth for abs=[Babs (Ztoa-H)]=1 as a function of wavelength, and the gases responsible for absorption.

H(km)


Classical Stratospheric Ozone Theory of Chapman (1930) (from Liou)


Ozone Number Density: Theory and Measurements.


Solar Spectrum, Top of the Atmosphere and at the Surface

Shaded region is solar irradiance removed by Rayleigh scattering and absorption by gases as indicated. (from Liou).


ERBE View of the radiation

story (Wallace and Hobbs CH4)

Note the IR cold spots near the Equator and the cold poles.


ERBE View of the radiation story (Wallace and Hobbs CH4)

Pat Arnott, ATMS 749, UNR, 2008 Chapter 9: Absorption by Atmospheric Gases Visible and UV...

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Transcript of Pat Arnott, ATMS 749, UNR, 2008 Chapter 9: Absorption by Atmospheric Gases Visible and UV...