a spontaneous rayleigh-brillouin scattering experiment for the characterization of atmospheric

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


-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

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anti-Stokes vibration-rotationRaman lines

Stokes rotational Raman lines

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[nm]

Cabannes line (Hydrodynamic regime) Cabannes line (Knudsen regime)

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filter A filter B

Molecular scattering in air - even more than 100 years after Rayleigh still some open issue

IASI 5 nm resolution


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


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


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

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2009.06.08-07 Gaussian Tenti S6 Tenti S7

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O2 1000 mbar

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Frequency [MHz]

2009.06.05-01 Tenti S6 Tenti S7

-3720 -2976 -2232 -1488 -744 0 744 1488 2232 2976 3720-5.0x10-5

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Air 1000mbar

Inte

nsity

[nor

mal

ized

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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Tenti S6 Tenti S7 Gaussian

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Tenti S6 Tenti S7

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12 Air 1000 mbar

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Frequency [MHz]

Tenti S6 (Air model) Tenti S7 (Air model) Gaussian

±12 %


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)


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


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tion [%

]

frequency [GHz]

residual 0% - 100% residual 0% - varying% residual varying - 100% signal ~ sqrt(N)

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

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.u.]

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0 % humidity

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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


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%)

a spontaneous rayleigh-brillouin scattering experiment for the characterization of atmospheric

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