Cavity Ring Down Measurements of Atmospheric Molecules

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Cavity Ring Down Measurements of Atmospheric
Molecules
support SRON, FOM, TNO, EU-network
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Cavity Ring-Down Spectroscopy (CRDS) Principle
empty cavity
advantages -intensity independent -long
effective pathlength (up to 100 km) -high
sensitivity (up to 10-9 cm-1) -cross section
cavity filled with absorbing gas

• problems
• non-exponential decay
• bandwidth effects
• mode structure of the resonator

absorption coefficient cross section
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CRD Measurement of the water spectrum 555-604 nm
1830 absorption lines 800 not in Hitran 243 not
in Reims data 111 newly assigned lines
mirror effect
not in hitran
Naus, Ubachs, Levelt, Polyansky, Zobov,
Tennyson J. Mol. Spectr. 205 (2001) 117
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Water vapour column density retrieval 585-600 nm
window
Rüdiger Lang Spectral Sampling Techniques
for Atmospheric Modeling Thesis 31 Oct. 2002
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The peculiar oxygen molecule
lowest electronic states O(3P) O(3P)
in electric dipole approximation all transitions
forbidden
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CRD measurements on small samples
oxygen B-band, 17O enriched
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The O2-O2 collisional complex
two molecules absorb one photon
X3Sg-, v02 hn ? a1Dg(v1) b1Sg(v2)
collisional flyby vs. Van-der-Waals complex
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Advantages of CRDS measurements
Simultaneous fit
b b0 kRayp kO4p2
unraveling contibutions in pressure curves
residuals
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Results
Blue band ? OMI
Naus Ubachs, Applied Optics 38 (1999) 3423
Sneep Ubachs, JQRST (2002) in press
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Rayleigh Scattering
J.W. Strutt, 3rd Baron of Rayleigh
1899 full theory based on Electromagnetism
No distinction of fine structure Raman, Brillouin
cross-section connected to index of refraction
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Results Rayleigh scattering
Cavity Loss bn/c lnR/L snN
including blue data
SF6
Ar
sn sn4e
for N2 sth 22.80(0.23) 10-45
CO2 overestimated King factor ?
sexp 23.05(0.12) 10-45