Review of Molecular Absorption Basic properties of molecular absorption (how & why)
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Review of Molecular Absorption
Basic properties of molecular absorption (how & why)
Absorption Spectrum
Absorption coefficient and pressure broadening
Transmission
Application: basis for estimating column CO2 and O2 (TCCON)
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Gas Concentration Forms:• Number concentration
Molecules/m3
• Mass Densitykg gas / m3
• Dry Air Mole Fraction (volume mixing ratio)Molecules gas / molecules dry air
• Mass mixing ratiokg gas / kg dry air
Gas Column Forms:• Number Column:
Molecules/m2
• Mean Dry Air Mole FractionParts per million, etc.
• Mass Column: kg/m2 (mm)
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Properties of atmospheric gases.
Atmospheric gases are highly selective in their ability to absorb and emit radiation.
Each radiatively active gas has a specific absorption spectrum- its own signature.
An atmosphere is the mixture of gases and thus the abundance of gases in the atmosphere controls the overall spectral absorption.
Radiatively active gases in the Earth’s atmosphere can be highly variable in space and time (especially if chemically active!)
The ability of a molecule to absorb (emit) radiation is determined by its structure which controls whether the molecule has a dipole.
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UV/Vis/NIR Gas Absorption
[Gottwald et al., The SCIAMACHY Book, 2006]
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3.6 mm15 mm
[Clerbaux et al., ACP 2009]
Gas absorption spectra in the thermal IR
All CurrentlyMeasured!
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Microwave
Absorption spectra in the microwave
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Fact: Molecules must have a permanent or induced electric (or magnetic) dipole moment to significantly interact with radiation.
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• Some molecules have permanent dipole moment
• Others (eg CO2) only have induced dipole moments
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Types of Absorption Spectra:
Sharp Individual Lines
Collections of many lines called a band.
Spectral Continuum extending over broad range of wavelengths.
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Transition Types
• Electronic: Larger energies. UV. Important for Ozone, O2, O.
• Rotational/Vibrational: Most of the visible and IR.
• Pure rotational : some transitions in microwave
• Continuum absorption is NOT discretized – not fully understood (thought to be molecular interactions).
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THE OVERALL PICTURE
Most Energetic:Shortest Wavelengths
Least Energetic:Longest Wavelengths
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P-Branch
R-Branch
Q-Branch
Example Band:15 μm ν2 CO2 Transition
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Absorption from a Single LineThree important factors define the properties of an absorption line
• spectral position - o• strength - S• shape - f(-o)
10
df )(
Defined by factors already described - ieproperties of the given molecule
These three properties in turn define the absorption coefficient
β= S f(-o)
where
The lines are not sharp but are ‘fuzzy’ - measure the fuzzinessin terms of width of the line
Depends on temperature, quantum-mechanical factors
SHAPE = VOIGT:Lorentz x Gaussian
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The Lorentz pressure broadening model
where L = Lorentz Width:
Pressure broadening is the most importantbroadening mechanism for the problems of interest to this class. The proportionality ofthe line width on pressure is fundamental
121
21
070
cmTT
PP
TT
PP
s
s
s
sLoL
/
/
.Pressure of colliding molecule (e.g N2, O2)which defines foreign broadeningin contrast to self broadening
220
0L
LLf
)(
/)(
wing
center
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Sensitive to lower atmosphere
Sensitive to upper atmosphere
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Applications• Gas measurements.
o Thermal IR emission measurements provide ways to “see” gas concentrations at different atmospheric layers, assuming a known temperature profile. (ie, temperature variations must be known much better than gas concentrations)
o Vis/NIR reflected sunlight measurements. Only really sensitive to total column, but don’t need to know temperature nearly as well. Generally need to know surface pressure.
OR• Temperature profile measurements
o Using a gas of known concentration, can measure a temperature profile via thermal emission. (must know gas concentration much better than small temperature variations, and gas should be well-mixed). Generally this is O2, but CO2 is sometimes also used.
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Aircraft Spectrograph Measurements
Basic question: can we separate atmospheric scattering from surface reflection? If so with what sensitivity?Analysis procedure:m=1/µo +1/µs [1-(p/p0)2]convenient to introduce adjusted airmassm’ m (p0/p)2
X am’+bm’ cX+dFrom X (ratio of spectral radiances), obtain m’ and thus p. Compare with measured p. Two ‘models’Coarse band (model A); m’= cX +d, where X is ratio of 2 3 nm-mean radiancesNarrow band (model B):m’= fjXj + gj + cX +dwhere Xj are narrow band radiance ratios (0.03nm).
Thin Cirrus
Aerosol
Aircraftpressure p
These define X for model A
O’Brien et al, 1998
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Same idea: measuring CO2 from Space
ACTUAL XCO2 (ppm)345 350 355 360 365 370 375
RE
TRIE
VE
D XC
O2 (
ppm
)
345
350
355
360
365
370
375
XCO2-4 0 4
SZA ac
75 0.05 35 0.15 35 0.05
N = 250 EACH CASE
Level 1
Level 2
Level 3
XCO2 Retrieval
Calibration
Source/SinkRetrieval
• Inverse Models• Assimilation Models
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The OCO design• Sampling Approach - High spatial resolution
– Isolates cloud-free scenes– Minimizes scene inhomogeneity
• Single space-based instrument incorporating three passive near-infrared spectrometers– CO2: 1.58 mm band
• CO2 column abundances– CO2: 2.06 mm band
• CO2, cloud, aerosol, water vapor, temperature– O2: 0.76 mm A-band
• Clouds, aerosols, and surface pressure
2.06
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Spectrometer Systems
• refraction (prism spectrometers)• diffraction (grating spectrometers)• interference (interferometer, etalons)
Elementary Prism Spectrometer