Institut für Physik der Atmosphäre
Rayleigh-Brillouin Scattering in N2, O2, and Air
Oliver Reitebuch1, Benjamin Witschas1, Ofelia Vieitez2, Eric-Jan van Duijn2, Willem van de Water3, Wim Ubachs2
1DLR Oberpfaffenhofen, 2Vrije Universiteit VU Amsterdam, 3Eindhoven University
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
A SPONTANEOUS RAYLEIGH-BRILLOUINSCATTERING EXPERIMENT FOR THE
CHARACTERIZATION OF ATMOSPHERICLIDAR BACKSCATTER
Ofelia Vieitez, Eric-Jan van Duijn, Wim UbachsLaser Centre Vrije Universiteit Amsterdam, Netherlands
Afric Meijer, Nico Dam, Institute for Molecules and Materials, Radboud University Nijmegen
Willem van de WaterEindhoven University of Technology, Netherlands + RUN
Ad Stoffelen, Jos de KloeKNMI, de Bilt, Netherlands
Anne Straume, Oliver Le RilleESA
Benjamin Witschas, Oliver ReitebuchDLR
Spontaneous RayleighBrillouin scatteringexperiments
Coherent RayleighBrillouin scatteringexperiments
Theory and TENTImodelling; new code
RB measurements on air, + water vapor
Consequences for ALADINError budgetLook up Tables
Study was funded by ESA under ITT AO/1-5467/07/NL/HEResearch stay of Witschas at VU Amsterdam was funded by EU FP7/2007-2013
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30
-1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
-3,5 -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
anti-Stokes rotational Raman lines
Stokes vibration-rotationRaman lines
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nsity
[a.u
.]
[nm]
anti-Stokes vibration-rotationRaman lines
Stokes rotational Raman lines
inte
nsity
[a.u
.]
[nm]
Cabannes line (Hydrodynamic regime) Cabannes line (Knudsen regime)
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nsity
[a.u
.]
pm]
filter A filter B
Molecular scattering in air - even more than 100 years after Rayleigh still some open issue
IASI 5 nm resolution
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
B. Rye (1998), Appl. Opt., 6321-6328
355 nm
2.1 µm
10.6 µm
What is the exact lineshape in air? Fiocco and DeWolf (1968) pointed out the difference of Gaussian to Rayleigh-Brillouin lineshape to lidar community
Lidar techniques using molecular backscatter and narrow instrumental bandwidths are affected, e.g. wind, T, HSRL
Errors for wind retrievals of ADM-Aeolus would be 3% (10 km) to 10% (ground) if Gaussian is used (Dabas et al. 2008) and thus exceeding specification of 0.7%
Widely used models for lineshape from Boley et al. (1972) and Tenti et al. (1974) - the Tenti S6 model - are valid for single species, but not for mixtures like air (N2+O2)
No experimental validation of Tenti S6 model for air; even Tenti S6 was not compared to N2 for atmospheric pressures
Most (or probably all) use Tenti S6 with N2 gas parameters => Is there a difference between N2 and air lineshape?
What is the influence of the water vapour molecule (up to 4% in atmosphere)?
3 km10 km
± 4 GHz
± 0.6 GHz
± 0.1 GHz
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
enhancement cavity reference laser
scattered light
L3 L4
PM
Fabry-Perot etalon
L7
L1
L2 S1
laser beam “cleaning”
M1
L5 L6
light “cleaning”
S2
S3
Choice 3FSR ~ 7.4 GHzResolution ~ 0.23 GHz
Choice 1Laser radiation366.5 nmnarrowband
Choice 2:90o scattering
“Choice 4”:No polarizing optics on detection
Setup for spontaneous Rayleigh-Brillouin scattering experiment at VU Amsterdam
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
Measurements and Tenti models at 1000 hPa
-3720 -2976 -2232 -1488 -744 0 744 1488 2232 2976 3720-5.0x10-5
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N2 1000 mbar
Inte
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Frequency [MHz]
2009.06.08-07 Gaussian Tenti S6 Tenti S7
-3720 -2976 -2232 -1488 -744 0 744 1488 2232 2976 3720-5.0x10-5
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O2 1000 mbar
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a]
Frequency [MHz]
2009.06.05-01 Tenti S6 Tenti S7
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Air 1000mbar
Inte
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[nor
mal
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to a
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Frequency [MHz]
2009.06.16-01 Tenti S6 (Air model) Tenti S7 (Air model) Gaussian
N2 O2Air
±3.7 GHz
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N2 1000 mbar
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Frequency [MHz]
Tenti S6 Tenti S7 Gaussian
-4000 -3000 -2000 -1000 0 1000 2000 3000 4000-12
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O2 1000 mbar
Diff
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Frequency [MHz]
Tenti S6 Tenti S7
-4000 -3000 -2000 -1000 0 1000 2000 3000 4000-12
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-2
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12 Air 1000 mbar
Diff
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erce
ntag
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Frequency [MHz]
Tenti S6 (Air model) Tenti S7 (Air model) Gaussian
±12 %
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
Measurements and Tenti S6 model at higher pressures
For higher pressure 3 spectral features become visible: the central Gross line and the Brillouin doublet Tenti S6 is able to resolve these features Gas parameter with highest uncertainty within Tenti model is bulk viscosity, which is obtained from sound absorption measurements (grey curve) Bulk viscosity was used as fit-parameter to minimize measurement-model difference in this study (black curve)
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
Is there a difference between lineshape of N2 and air?
Rel. difference between lineshape for N2 and air from measurements and Tenti S6 model at 2000 hPa, 297 K,normalized to maximum of lineshape.
Most (or all) use Tenti S6 model with N2 parameters
No measurements of line-shape were performed up to now for air (79% N2, 21% O2)
There is a measurable difference between N2 and air
The difference can be modeled with Tenti S6 using appropriate gas parameters as input for
molar mass shear viscosity bulk viscosity thermal conductivity
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
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frequency [GHz]
residual 0% - 100% residual 0% - varying% residual varying - 100% signal ~ sqrt(N)
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.u.]
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0 % humidity
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.u.]
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0 % humidity varying humidity Atmosphere can contain up to
4 % water vapor (100 % rel. humidity at 37 °C)
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0 % humidity 100 % humidity varying humidity
Is there an effect of humidity?
Measurements showed no significant difference between lineshape of dry and humid air up to 2.6% water vapor
No need to consider water vapor within lineshape modelfor atmospheric conditions
Institut für Physik der Atmosphäre 33rd Lidar Working Group, Destin (FL), 2-4 Feb 2010
Summary Spontaneous Rayleigh-Brillouin scattering was measured at 366.5 nm, at a scattering angle of 90°, at ambient temperature (≈300 K), and for pressures of 0.3 bar to 3 bar for N2, O2, and air by a setup at VU Amsterdam with a narrowband laser, a scattering cell within an enhancement cavity, and a confocal Fabry-Perot interferometer.
For the first time the Cabannes lineshape was measured for dry air (N2+O2) and for humid air with up to 2.6 % water vapor content.
For the first time the lineshapes of N2 and air were compared to Tenti S6 and S7 model
Conclusion
It was confirmed that Tenti S6 is (slightly) better than Tenti S7
Bulk viscosity parameter was varied as model input to minimize measurement to model difference => resulted in a factor of 2 higher bulk viscosity values as reported from sound absorption experiments in literature
Deviations between Tenti S6 model and measurements are below +-2% for atmospheric pressures, if appropriate gas parameters are used as model input
Water vapor does not influence lineshape - at least up to 2.6% content (compared to a maximum atmospheric content of 4%)