remote sensing FTIR-technique remote sensing

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1 Ground-based remote sensing FTIR-technique technique, application for atmospheric remote sensing From lecture of Justus Notholt Institute for Environmetal Physics University of Bremen University of Bremen Ronald Macatangay (U3230; Institute of Environmental Physics

2 simple introduction to spectroscopy Michelson interferometer, - how does it work - mathematical description comparison with grating spectrometer - advantages and disadvantages - applications remote sensing methods our measurement sites current and future applications

3 Measurement of radiation Gamma rays X rays UV radiation x-rays visible light heat radio frequencies Visible Mikrowaves Infrared Radio waves characterisation wavelength (nm, cm -1 ) intensity (Watt) polarisation

4 Spectroscopy: Investigating the optical properties of light Prism spectrometer white light red orange yellow green blue violet spectrum: intensity as a function of wavelength

5 rainbow white red blue

6 Spectra of different sources light bulb street-light spirit-flame sun spectra yield information on the light source

7 Influence by atmosphere polar light in Spitsbergen, 79 N

8 Remote sensing Measurement of the modification of the solar radiation by the atmosphere

9 5 solar spectrum intensity top of atmosphere sea level black body (6500 K) wavelength (µm) (Warnecke, 1991)

10 wavelength Wellenlänge (µm) (µm) intensity (arb. Units) N 2 O O 3 O 3 O 3 O 3 N 2 O wavenumber (cm -1 ) - each trace gas has ist own fingerprint in the spectrum - infrared region contains many lines

11 Grating spectrometer Source FTS Detector intensity sin α ( ) ~ λ (nm)

12 Michelson interferometer Source Detector S δ x D intensity?? intensity retardation δ x λ (nm)

13 Signal position Signal Measured δx at detector (time average) S D high S D low S D high Intensity Conversion Hz 10 5 Hz δx (or δt because δt= δx/v)

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15 Intensity Intensity wavenumber (cm -1 ) retardation (cm) Interferogramm yields information on spectrum Mathematical relation: Fourier transformation (FT) 1 -iνx -1 B( ν ) = I(x) e 2π dx = 1 i ν e ν x I(x) B( ) d 2π ν FT FT FT { I(x) } = B( ν ) { B( ν )} = I(x) FT { I(x) } = I(x) -1

16 Where does the intensity go? D1 intensity D2 D2 D1 + D2 OPD δ (cm -1 ) D1 beamsplitter thickness: λ/4

17 Real measurement L S W L2 L1 L3 D measurement of OPD HeNe-laser (632.8 nm) and phase shift at beam splitter (90 ) determination of centerburst by Fourier transformation by white light source intensity L1 opd δ (cm -1 ) L2 opd δ (cm) intensity L3 opd δ (cm)

18 For a single monochromatic wave S x 1 2 D x 2 2 E=E 0 cos(ωt + 2πνx) ν=1/λ (wave number in cm -1 ) E=E 0 e i(ωt+2πνx) E D (δ) = E o e iωt (r m r s t s e (i2πνx 1 ) + r m r s t s e (i2πνx 2 ) ) I D (δ) = A R m R s T s e iωt e -iωt (e i2πνx 1 + e i2πνx 2) (e -i2πνx 1 + e -i2πνx 2) = A R m R s T s (1 + e +i2πν (x 2 x 1 ) + e -i2πν (x 2 x 1 ) +1) = A R m R s T s (2 + e +i2πνδ + e -i2πνδ ) = A R m R s T s 2(1+ cos (2πνδ)) = A R m M (1+ cos (2πνδ)) / 2 = B (1+ cos (2πνδ))

19 di D (δ ) = B(ν )dν (1+ cos 2πνδ ) di I D (δ ) = di = dν = B(ν) dν + B(ν) cos 2πνδ dν dν 0 For continuous light source Replace I(δ) by di(δ) I D ( δ ) ~ 0 0 B( ν ) cos 2πνδ dν 0 0 But for FT I(x) = 1 B( ν ) 2π e iνx dν

20 Relation to Fourier Transformation Michelson interferometer Fourier Transformation 1 B( ν ) = p s 2 I(δ) = B(ν) = 0 I( δ ) = B p( ν ) cos 2πνδ dν [ B ( ν ) + B (-ν )] B(ν) cos 2πνδ dν I(δ) = F[B(ν)] B(ν) = F -1 [I(δ)] I(δ) cos 2πνδ dδ In general f (x) = - For symmetric F(y) F(y) e ixy dy f(x) = F(y) cos 2πxy dy - F(y) = f(x) cos 2πxy dx -

21 Resolution Michelson interferometer -1 1 ν (cm ) = L L : maximum optical path difference (cm) grating spectrometer λ = mn λ m :order N : Number of rules -1 1 ν (cm ) = B B:size of grating (cm)

22 I(δ) = I obs (δ) = I (δ) A(δ) Influence of optical path difference B(ν) cos 2πνδ dν 0 B obs (ν)=f -1 [I obs (δ)] =F -1 [I (δ) A(δ)] =F -1 [F{B (ν)} F{sinc(ν)}] =F -1 F[B (ν) * sinc(ν)] =B (ν) * sinc(ν) = B (ν') sinc(ν-ν') dν' = B (ν-ν') sinc(ν') dν') δ (cm) 1/δ (cm -1 ) FTS (F) PC (F -1 ) ν 0 ν (cm -1 ) L x (cm) ν 0 ν (cm -1 ) limited optical path difference FWHM = L (cm -1 )

23 rectangular pulse FT (N=2, 3, 5,...) FT -1 ( rectangular pulse)

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26 Instead of dδ only δ I obs (x)=i con (x) Π x (x) Discrete Fourier transformation We know: F [ Π (x)] = Π ( ν ) = Π ( ν ) x ν 1/ x B obs (ν)=f -1 { I obs (x) } B obs (ν)=f -1 { I con (x) Π x (x) } B obs (ν)=f -1 {F(B con (ν)} F{Π ν (ν)}] B obs (ν)=f -1 F{B con (ν) * Π ν (ν)} B obs (ν)=b con (ν) * Π ν (ν) B obs (ν)= B con (ν') Π ν (ν-ν') dν' x 1/ x x F and F -1 ν Spectrum repeats every ν=1/ x Measurement requirement: x 1 2ν max = λ max 2

27 Nyquist theorem

28 Example: Temperature measurement in autumn 24 h sampling 11 h sampling T ( C) 25 h sampling T ( C) 25 h sampling 13 h sampling 23 h sampling Day of year Day of year

29 Influence of the aperture δ 2 = δ cosα 2 2 α α δ 2 δ (1- ) for smallα : cosα δ r r δ 2 δ - for smallα : α 2 2 F F δ = n λ constructive interference: n N 2 δ r λ = δ - 2 F n 2 2 constructive intererence for n N

30 Quantification ( δ ) = cos (2πνδ ) I 2 di = dω cos (2πνδ cos α ) Ω = πα 2 I I Ω ( δ, Ωm ) = di = ( δ, Ω ) m = Ω m 0 m νδω sinc 2π 2 α cosα α di = dω cos (2πνδ (1- )) 2 Ω di = dω cos (2πνδ (1- )) 2π Ω cos (2πνδ (1- ))dω 2π m cos 2πνδ 1 Ωm 2π... mathematics... effect on spectra by FT -1 of sinc (νδω m /2π); line broadening frequency shift by (- Ω m /2π)

31 effect of aperture and OPD on line width and shape FTS (F) PC (F -1 ) aperture ν 0 ν (cm -1 ) L x (cm) ν 0 ν (cm -1 ) OPD ν 0 ν (cm -1 ) L x (cm) ν 0 ν (cm -1 ) both ν 0 ν (cm -1 ) L x (cm) ν 0 ν (cm -1 )

32 Optimum aperture envelope of interferogram for δ = L νδω 2 I( ) (sinc ) sin νδω m m Ω Ω = m m 2π νl 2 maximum intensity for δ = L large Ω νlω sin m = 1 2 νlω m π = 2 2 π Ωm = νl n=1 fringe amplitude small Ω δ = L δ = 2 L

33 Comparison with OPD I ( δ, Ω ) m = Ω m sinc δ 2L cos πδ 2πνδ 1 2L - δ sinc = M for ν 2 L F 1 1 L Extended optimum aperture Limited optical path difference 1.0 FWHM = (cm 2 L FWHM = 2 L -1 ) (cm -1 )

34 incoming spectrum Michelson interferometer FT grating with N slits interferogram N single waves computer FT -1 interference measured spectrum A measured spectrum B

35 Comparison with grating spectrometer, A: Theory FTS Grating FTS Prism FT by computer FTS: 2 beams, Nyquist theorem Grating spectrometer: N beams, grating order, overlapping spectra Prsim spectrometer: beams, only one spectrum FT on prism grating

36 Spectral line shape FTS grating spectrometer I(ν) ~ sinc (ν) I(ν) ~ sinc 2 (ν) ν (cm -1 ) ν (cm -1 ) F F -1 retardation (cm) slit width (cm)

37 Comparison with grating spectrometer; B: Application 1. Light troughput (Jaquinot advantage) circular aperture 2. Multiplex principle (Fellget advantage) Single detectors array detector 3. Wavelength accuracy (Connes advantage) Laser as reference, linear relationship

38 Light troughput A 1Ω 1 = A 2 Ω 2 Use of a telescope (?)

39 1. Light troughput advantage (Jaquinot advantage) Source FTS Grating Detector FTS Grating Circular aperture more light Digilab FTS Beckman 4240

40 2. Multiplex principle (Fellget advantage) All wavelengths at once more light (for single detectors)

41 3. Wavelength accuracy (Connes advantage) FTS FTS Grating He-Ne laser nm sin α ~ λ Intensity nm δx Precise linear wavenumber scale (Connes advantage)

42 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

43 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

44 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

45 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

46 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

47 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

48 Remote sensing methods Measurement of the modification of the solar radiation by the atmosphere and earth surface emission of the atmosphere and earth surface

49 Jungfraujoch, Switzerland (47 N, 3850 m) Intensität Intensität Intensity Intensity CFC-12 CF 2 Cl wavelength (µm) wavelength (µm)

50 Optimum resolution for atmospheric observations intensity HCl wavenumber (cm -1 ) Optimum resolution for smallest part of line Spectral line at high altitudes dominated by Doppler broadening (Gaussian profile) intensity f( ν -ν ) = 0 ln 2 π - exp ν ν - ln 2 0 γ D γ D 2 wavenumber (cm -1 )

51 OPD (cm) ratio line peak noise /2L resolution (Luc Delbouille) FWHM Optimum resolution when FWHM 1/2L (FWHM resolution of FTS)

52 Measurement sites of Uni Bremen with AWI Paramaribo Bremen (120 (120 HR/125 M) Since HR) 2004 Meteorological Since 2000 Biscarrosse Polarstern (120 Service Cruise M) May/June of (120 Suriname M) 2005 Institute for Environ. physics Carbo Europe Since Regional 1994 Experiment University of Bremen Centre Ny Alesund D Essais (120 Des M/120 Landes HR) Since 1992

53 Measurements in Bremen

54 Instrument in Bremen

55 Observations in Spitsbergen (79 N)

56 Observations onboard of Polarstern

57 Observations in Antarctica (71 S)

58 Trace gas measurements from the ground total columns of trace gases (mol. cm -2 ) analysis of spectral line shape: concentration profiles (ppbv) (IR: ~ 30 km, MW ~ 70 km) 1. constant N 2 2. long lived CO 2, N 2 O, CH 4, CFC-11, CFC-12, CFC troposphere C 2 H 2, C 2 H 6, CH 2 O, CO, HCN, COS, SF 6, NH 3, H 2 O 4. stratosphere O 3, HCl, ClO, ClONO 2, HNO 3, NO 2, NO, COF 2 altitude (km) mm 1000 cm -1 Lorentz halfwidth Doppler halfwidth at 300 nm FWHM (cm -1 )

59 ATMOS on Space Shuttle

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62 Synthetic ATMOS and ground-based spectra CO 2 O 3 H 2 O CO 2 H 2 O

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64 ATMOS results

65 Mechanical requirements accuracy during scan: mm (max. OPD is 2000 mm) max. acceptabel difference between rays ~ λ/7: beam diameter = 6 cm α = OPD/2 α

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67 Advantages and disadvantages - light throughput (Jaquinot advantage) times more light - multiplex principle (Fellget advantage) single detector: advantage ( times more light) array detector: no advantage, neutral atmospheric fluctuations: disadvantage (by ) - wavelength coverage beamsplitter thickness ~ λ/4, broad coverage 3 beamsplitters for cm -1 (20 µm 250 nm) - spectral resolution (1/L or 1/B) OPD max : 10 cm - 4 m; grating max : 10 cm - 1 m - instrumental line shape FTS: can be calculated grating: slit function must be measured

68 Current and future applications Infrared: Michelson interferometer, Jaquinot advantage, large spectral region Microwave: heterodyne radiometers higher spectral resolution, small spectral regions required UV/Vis: grating spectrometers, arrays multiplex disadvantage, low spectral resolution required Infrared: - numerous trace gases simultaneously - long-term monitoring (recovery of stratospheric ozone layer) - concentration profiles in troposphere (tropics) - high precision for long lived trace gases: CO 2, CH 4 - isotopes

69 Distribution of COS and CO on the Atlantic altitude (km) COS CO altitude (km) COS CO latitude ( ) latitude ( )

70 High precision measurements of CO 2 O 2 CO 2 Single channel cm cm cm Wavenumber cm-1 Analyse ratio CO 2 /O 2 : minimize most systematic errors \\Ftirserver\Ftir4\spectr14\ Spitzbergen cm cm-1 22/05/2002

71 CO 2 (molecules/cm 2 ) 8.2E E E E E+21 CO 2 in Ny-Ålesund 4.2E+24 O2 (molecules/cm 2 ) 4.1E E E E E-03 CO 2 / O2 1.90E E E E

72 CH 4 in Ny-Ålesund

73 Isotopes intensity 686 O O O 3 Messung CO 2 H 2 O mass spectrometer extreme accuracy ( ) in-situ Method spektroskopy remote sensing method separation of 668 O 3 and 686 O 3 weak precision (%) wavenumber (cm -1 )

74 Acknowledgements University Duisburg, Prof. Dr. Axel Lorke John Hopkins University Bruker Optics Alfred-Wegener-Institute for Polar and Marine Research Luc Delbouille, Jim Brault