Chap.7. Raman Systems

Transcription

1 DE. OF SIGAL TEORY AD COMMUICATIOS Chap.7. Raman Systems Francesc Rocadenbosch ETSETB, Dep. TSC, EEF Group Campus ord, D4-016

2 RAMA LIDAR DE. OF SIGAL TEORY AD COMMUICATIOS MEASUREMETS: Direct: Concentration of chemical species in the atmosphere such as SO 2, O, CO, 2 S, C 2 4, C 4, 2 CO, 2 O, 2, O 2... Indirect: Temperature, umidity, α, β, S M LASER TYES: Ruby (λ nm, nm) 2 (λ 337 nm) d:yag (λ 1060 nm, 532 nm, 266 nm) Excimer (λ ~ 350 nm) IELASTIC ITERACTIO hν VIRTUAL LEVEL * hν GROUD LEVEL VIBRATIOALLY EXCITED LEVEL 2

3 RAMA LIDAR DE. OF SIGAL TEORY AD COMMUICATIOS OERATIOAL RICILE 1) In contrast to elastic systems, the return wavelength, λr, is shifted from the incident one, λ0. 2) Wavelength shift, κ, depends on each molecular species. λ0 λ R 1 κλ 3) Very faint returns. requires photon counting very often, night-time operation 0 Fig. ADATED FROM: Inaba,. Detection of Atoms and Molecules by Raman Scattering and Resonance Fluorescence. In Laser Monitoring of the Atmosphere, inkley, E. D., Ed.; Springer-Verlag: ew York, 1976; Chap. 5,

4 RAMA LIDAR DE. OF SIGAL TEORY AD COMMUICATIOS RAMA SECTRUM CARACTERISTICS The Raman shift, κ: 1) does not depend on the excitation wavelength λ 0 and, 2) it is specific of the chemical species A) Laser need not be tunable B) The Raman spectrum is characteristic of each molecule C) Conveys temperature info. Anti-Stokes Rayleigh and Mie Scattering Stokes lines Overview of the lidar backscatter signals for 532-nm laser excitation wavelength. Fig. SOURCE: Behrendt, A., et al., Combined Raman lidar for the measurement of atmospheric temperature, water vapor, particle extinction coefficient, and particle backscatter coefficient, Appl. Opt. 41 (36), , (2002). 4

5 RAMA LIDAR DE. OF SIGAL TEORY AD COMMUICATIOS KEY COCETS Common Raman shifts: cm -1 2 O 3654 cm -1 Fig. SOURCE: Inaba,. Detection of Atoms and Molecules by Raman Scattering and Resonance Fluorescence. In Laser Monitoring of the Atmosphere, inkley, E. D., Ed.; Springer- Verlag: ew York, 1976; Chap. 5, p ) Raman components Stokes lines molecule gains energy from the radiation field scattered radiation is at λ R > λ 0 Anti-Stokes lines (λ R <λ 0 ) 2) Motivation for the wavenumber concept (with κ, the Raman shift): 1 1 [ ] 1 κ, κ cm λ λ R 3) Raman cross-sections dependency λ -4 O 1556 cm -1 2 dσ ( ) dσ ( ) d π Ω Raman , 4 d π Ω Ray 5

6 RAMA LIDAR LIK-BUDGET 6 DE. OF SIGAL TEORY AD COMMUICATIOS

7 TEMERATURE MEASUREMET (I) METODS 1) Rotational Raman (RR) DE. OF SIGAL TEORY AD COMMUICATIOS KEY Raman signatures are direct measures of the relative populations among the internal molecular modes (In termal equilibrium) fundamental def. of temperature Comparison of the envelope shape of all the lines Intensity ratio of selected spectral regions of the band Suitable for atmospheric profiling 2) Vibrational Raman (VR) + Intensity ratio between Stokes and anti-stokes components Width of a specific Q-branch Suitable for high-temperature diagnostics (e.g. flames) Fig. SOURCE: Inaba,. Detection of Atoms and Molecules by Raman Scattering and Resonance Fluorescence. In Laser Monitoring of the Atmosphere, inkley, E. D., Ed.; Springer-Verlag: ew York, 1976; Chap. 5,

8 TEMERATURE MEASUREMET (II) RASC (GKSS) lidar 1) T (temperature) DE. OF SIGAL TEORY AD COMMUICATIOS Fig. SOURCE: Behrendt, A., et al., Combined Raman lidar for the measurement of atmospheric temperature, water vapor, particle extinction coefficient, and particle backscatter coefficient, Appl. Opt. 41 (36), , (2002). 2) WV (water vapor mixing ratio) 3) α, β, S M 4) R (humidity) 8

9 UC OLYCROMATOR TEST LAYOUT 9 DE. OF SIGAL TEORY AD COMMUICATIOS

10 TEMERATURE MEASUREMET (III) DISCUSSIO ARAMETERS (RASC lidar LAYOUT) DE. OF SIGAL TEORY AD COMMUICATIOS ote: D filters are used to cope with saturation effects in the RR channels in the lower troposfere (correction of photon-counter receiver dead-time effects). Tab. SOURCE: Behrendt, A., et al., Combined Raman lidar for the measurement of atmospheric temperature, water vapor, particle extinction coefficient, and particle backscatter coefficient, Appl. Opt. 41 (36), , (2002). 10

11 DE. OF SIGAL TEORY AD COMMUICATIOS Specific RR Temperature approaches: TEMERATURE MEASUREMET (IV) Use of two RR channels with opposite temperature dependency + 3rd RR channel (isosbestic point) as reference or, combine them to obtain a temperature-indep. reference RR2 ( ) ( T ) RR1( T ) ( z) ( z) + c ( z) Q T ref RR1 RR2 calibrate Q(T) with a radiosonde find c for minimum temp. variation Fig. SOURCE: Behrendt, A., et al., Combined Raman lidar for the measurement of atmospheric temperature, water vapor, particle extinction coefficient, and particle backscatter coefficient, Appl. Opt. 41 (36), , (2002). 11

12 TEMERATURE MEASUREMET (V) DE. OF SIGAL TEORY AD COMMUICATIOS roblem: Elastic cross-talk with the RR channel Action: Calibrate on a cirrus cloud using RR2 ( ) ( T ) Q T T ε z RR1 ( ) ( ) El Fig. SOURCE: Behrendt, A., et al., Combined Raman lidar for the measurement of atmospheric temperature, water vapor, particle extinction coefficient, and particle backscatter coefficient, Appl. Opt. 41 (36), , (2002). 12

13 MOLECULAR SECIES (GAS) DETECTIO (I) DE. OF SIGAL TEORY AD COMMUICATIOS COCETS: 1) The absolute concetration of each molecular species can be performed by comparing the Raman backscattered intensity with that of the Raman line from 2 which occupy the same volume. 1A) Raman-backscattered signal: K O R ( T, λ ) dσλr dω ( π) exp { [ ( ) ( )] } R tot tot α ξ + α ξ dξ λ λ λ F R R 2 R R R 0 0 λr 1B) (Oversimplified) Gas-to- 2 normalised ratio λ X λ O O X where F F τ X solve for x (R) ( T, λ X ) ( T, λ ) 2 -normalised cross section X dσ dσ λ X λ ( π) ( π) dω ξ dω ξ ( λ X ) ( λ ) { [ ] } dξ R tot tot ( λ X, λ, R) exp αλ ( ξ) αλ ( ξ) X 0 τ ( λ, λ, R) known (US-std model, radiosonde) X, 13

14 MOLECULAR SECIES (GAS) DETECTIO (II) DE. OF SIGAL TEORY AD COMMUICATIOS 1C) (Estimation of the) Differential Transmission term: Molecular extinction US-std. atmosphere model + radiosonde Aerosol extinction Cooperative elastic-raman channel (2) Only in hazy conditions (See Elastic-Raman inversion Sect. in Chap.7) Angström coefficient E.g. Sun photometer calibration 2) The (VR) spectrum is preferred to the (RR) RR lines of major atmospheric constituents overlap, large Rayleigh-Mie cross-talk. In contrast, VR crosssections are usually lower than RR ones. 14

15 MOLECULAR SECIES (GAS) DETECTIO (III) SOME MEASUREMET EXAMLES Fig.2 DE. OF SIGAL TEORY AD COMMUICATIOS Fig.1 Raman spectroscopy from the ordinary atmosphere Fig.2 Molecular species in an oil smoke plume Fig.1 Fig. SOURCE: Inaba and Kobayasi, Opto-Electron 4, 101 (1972). 15

16 WATER-VAOR MEASUREMET (I) DE. OF SIGAL TEORY AD COMMUICATIOS WATER-VAOR (WV) KEYS: Influences convective stability (likelihood of storm initiation) Most active green-house gas it absorbs terrestrial radiation more strongly than does CO 2 Water-vapor mixing ratio (w) w MW MW where MW x stands for molecular weight ( 18g/mol for 2 O and g/mol for dry air) and x stands for molecule number concentration. Importance of the mixing ratio: 2O / MW 2O 2O 2O MW 0.78 DryAir DryAir DryAir It is conserved in atmospheric processes that do not involve condensation or evaporation Serves well as a tracer of the movement of air parcels in the atmosphere 2 2O 2 16

17 WATER-VAOR MEASUREMET (II) DE. OF SIGAL TEORY AD COMMUICATIOS DERIVATIO FROM TE RAMA CAELS From Eq.(1B) in Slide 12 λ λ O O F F ( T, λ R ) ( T, λ ) R dσ dσ and the definition of the mixing ratio (w) w ( π) ( π) ( λ ) ( λ ) λ λ ( π) ( π) ( T, λ ) ( T, λ ) dω ξ dω ξ ( λ ) ( λ ) O dσ d λ Ω ξ F 0,485 τ λ O dσλ dω ξ F System s calibration factor, k*(r) τ ( λ, λ, R) ( λ, R), Temperaturedependent ratio where x (R) are background-substracted quantities. In summary, ( T, λ ) ( T, λ ) * F w kl Rw τ( λ, λ, R) Rw F Measurement factor 17

18 WATER-VAOR MEASUREMET (III) DE. OF SIGAL TEORY AD COMMUICATIOS EXAMLE In the UV (λ L 355 nm), Raman channels: λ 387 nm, λ 408 nm w k F F ( T, λ ) ( T, λ ) kappa lambdar (1) lambdar (2) SECIES (cm-1) (nm) (nm) Air 0 354,7 532,1 O ,4 580, ,7 607,4 2O ,5 660,5 Excitation wavelength (lambda0) (1) 354,7 nm (d:yag, TG) (2) 532,1 nm (d:yag, SG) ( λ, R) * 355 τ λ, Water-Vapor Mixing Ratio Error σ w 2 w 2 σ k 2 * k *2 + σ R σ R σ w τ Rw + ; Rw w τ R w k * O O ( λ ) ( λ ) Cross sections are computed assuming a λ-4 dependency i.e. higher in the UV than in the IR ξ ξ 18

19 DE. OF SIGAL TEORY AD COMMUICATIOS WATER-VAOR MEASUREMET (IV) ERROR SOURCES AD UCERTAITIES Water-Vapor Mixing Ratio Error σ 2 2 σw * σ k R σ σ R w τ Rw + + ; R w k R R 2 * w k * w τ w : Calibration factor Can be considered to be small Calibrated using a radiosonde (e.g. VAISALA RS80-A) or a MW radiometer R w : Signal-induced statistical error dominates the error budget ( ) τ λ,λ R Differential Transmission (DT)., λ and λ experience different amounts of attenuation on their return trips, caused mainly by Rayleigh scattering. The DT can be calibrated by using 1) A radiosonde estimate the molecular number density (i.e, T(z), (z)) 2) For hazy atmospheres (DT < 0.9 for τ BL > 2), the 2 -Raman channel is used to estimate the aerosol extinction (typ., λ -1 dependence) aer aer See Sec. Inversion of Optical arameters / Extinction inversion, α λ, α λ ote: WV absorbs weakly at λ 660 nm λ L 355 nm preferred to 532 nm ( ) Fig. SOURCE: Inaba,. Detection of Atoms and Molecules by Raman Scattering and Resonance Fluorescence. In Laser Monitoring of the Atmosphere, inkley, E. D., Ed.; Springer-Verlag: ew York, 1976; Chap. 5, p

20 WATER-VAOR MEASUREMET (V) DE. OF SIGAL TEORY AD COMMUICATIOS Fig. SOURCE: Goldsmith, J.E.M., et al., Turn-key Raman lidar for profiling atmospheric water vapor, clouds, and aerosols, Appl. Opt. 27 (21), , (1998). 20

21 RELATIVE UMIDITY MEASUREMET DE. OF SIGAL TEORY AD COMMUICATIOS KEY Water-vapor mixing ratio (w2o) + Temperature profile R Derivation of the R profile emerges from specific physical refs.1,2 e ( z) ( z) w ( ) 2O z + w ( z) R, ( z) e e e ( z) ( z) ( z) where e(z) is the WV pressure, and e w (z) is the saturation pressure, O w w M A exp M B + [ T ( z) 273] [ T ( z) 273] M A 17.84, and M B 245.4, for T < and > 273 K, respectively. REFERECES: 1) R.R. Rogers and M.K. Yau, A Short Course in Cloud hysics (ergamon, ew York,1988). 2) R.J. List, ed., Smithsonian Meteorological Tables (Smithsonian Institution, Washington, D.C., 1951). 21

22 A COMLETE RAMA SYSTEM LAYOUT DE. OF SIGAL TEORY AD COMMUICATIOS 3 unshifted returns (1064, 532, 355 nm), O polarization 4 returns (Stokes and anti-stokes portions) of the 2 RR spectrum 3 vibrational Raman returns ( 2 at 387, 607 nm and 2 O at 407 nm) 2 returns from the parallel and cross-polarized unshifted 532 nm Fig. SOURCE: Matthis, I, Ansmann, A et al., Relative-humidity profiling in the troposphere with a Raman lidar, Appl. Opt. 41 (30), , (2002). 22

23 COMOSITE OUTUTS A COMLETE RAMA SYSTEM DE. OF SIGAL TEORY AD COMMUICATIOS Fig. SOURCE: Matthis, I, Ansmann, A et al., Relative-humidity profiling in the troposphere with a Raman lidar, Appl. Opt. 41 (30), , (2002). 23

24 IVERSIO OF OTICAL ARAMETERS (I) RICILE DE. OF SIGAL TEORY AD COMMUICATIOS 1) Two receiving channels: Besides the ELASTIC receiver, which is only sensitive to the elastic return, another receiver -i.e., the RAMA receiveris spectrally tuned to the Raman-shifted wavelength (Q-branch) of any abundant species of known relative concentration (usually 2 ). 2) From: radiosoundings or ground-level measurements of pressure and temperature + assumption of a standard atmosphere, the 2 concentration -as a function of the range to the lidar- is inferred. 24

25 IVERSIO OF OTICAL ARAMETERS (II) DE. OF SIGAL TEORY AD COMMUICATIOS Raman-backscattered signal: λ α ( z) K O () z ( z) dσ λ dω ( π) exp R R λr 2 R 0 λ0 λ0 aer () z d dz z Scattering wavelength dependency: λ -κ λ0 ln λ R () z () z z R α 2 λ λ λ R mol 0 k λ λ Raman-channel inverted extinction: α α aer λ0 aer λr { [ () () () ()] } z mol aer mol aer α ξ + α ξ + α ξ + α ξ dξ mol () z α () z λr 0 R κ λr λr IVERTED ATMOSERIC OTICAL ARAMETERS 25

26 IVERSIO OF OTICAL ARAMETERS (III) DE. OF SIGAL TEORY AD COMMUICATIOS Backscatter inversion requires: combination of lidar returns from both elastic and Raman channels β β aer λ0 mol aer mol ( z0 ) ( z) ( z) 0 ( z) ( z) ( z0 ) ( z0 ) λr λ R β + β + β λ0 λ0 λ 0 λ ( z0 ) ( z) ( z0 ) 0 λr R { [ () ()] } z aer mol exp α ξ + α ξ dξ z λ 0 R λr ; z z aer mol exp α () ξ + α () ξ dξ aer ( ) ( ) ( ) ( ) aer mol R0 >>β R β + β λ 0 R λ 0 R 0 0 λ0 S aer λ ( z 0 ) { [ ] } 0 z λ 0 0 λ0 a backscatter calibration at some height R 0 so that mol λ 0 0 The lidar ratio is found as β α β aer λ0 aer λ0 aer λ 0 ( ) aer R f λ, λ, αλ,, {, 0 0 R T R ( z) () z channel returns z Rayleigh comp. 26

27 4. IVERTED OTICAL ARAMETERS Range [km] 27 DE. OF SIGAL TEORY AD COMMUICATIOS d/dr[ ln(.) ] [a.u.] ln(/[r 2 z(r)]) [a.u.] -0.3 ML estim ML estim S ˆ S i σ 1 σ 2 2 i i