Long Path (active) DOAS

Transcription

1 Long Path (active) DOAS -basic principle -Long path DOAS (UV/vis/IR) -instrumental improvements -Specific applications -white cell -vertical profiles -tomographic inversions

2 DOAS: Differentielle Optische AbsorptionsSpektroscopie I I exp 0 c l A) Lambert-Beersches Gesetz:

3 DOAS: Differentielle Optische AbsorptionsSpektroscopie I I exp 0 c l A) Lambert-Beersches Gesetz: Relative Intensität Lichtabschwächung durch Ozon für UV-Licht bei 300nm Schichtdicke [km] Aus der Intensitätsmessung kann die Konzentration bestimmt werden ln I I l c 0

4 DOAS: Differentielle Optische AbsorptionsSpektroscopie I I exp 0 c l A) Lambert-Beersches Gesetz: In der Realität ist es sehr ähnlich... Lichtquelle & Spektrograph Spiegel km

5 Long Path DOAS -basic principle -Long path DOAS (UV/vis/IR) -instrumental improvements -Specific applications -white cell -vertical profiles -tomographic inversions

6 DOAS: Differentielle Optische AbsorptionsSpektroscopie Lichtquelle & Spektrograph Spiegel km

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8 Typisches (früheres) LP-DOAS-Instrument Spiegel Lichtquelle

9 Basic requirements for Long path systems -divergence of the light beam should be small (diameter of a few meters over a distance of several kilometers) => Large mirrors => Light sources with high luminance (photon flux per area) d f d W W: width of the light beam at distance L sin f L For W=1m, L=5km: sin = 0.001, =0.06 For f = 2m => d =2mm

10 Diplomarbeit Thorsten Hermes, IUP Heidelberg, 1999

11 Der Druck der Xenon- Edelgasfüllung steigt während des Betriebs von etwa 8 bar im kalten Zustand auf bis zu 70 bar an.

12 Diplomarbeit Thorsten Hermes, IUP Heidelberg, 1999

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14 Diplomarbeit Jens Tschritter, IUP Heidelberg, 2007

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16 The electromagnetic spectrum UV/Vis and near IR: Electronic and vibrational transisons (typ. Absorption) Thermal IR: Vibrational transisons (typ. Emission) Microwaves: Rotational transisons (typ. Emission)

17 Example of trace gas cross section: H 2 O absorption cross section for 290K (HITRAN data base) How can spectra be determined? (depending on properties of the molecules)

18 Electronic transitions: Energy levels: -exact energy levels can be determined using (time independent) Schrödinger equation, Example: Hydrogen atom -energy levels are of the order of electron volts Example: Hydrogen atom: -Lyman series: 13.6 ev ( 95 nm) -Balmer series: 3.4 ev ( 430nm) -Paschen series: 1.5 ev ( 1282nm)

19 Energy levels for different states of vibration The distance between the energy levels is constant:

20 Energy levels for different states of vibration The distance between the energy levels is not constant.. For increasing v the distance decreases. There exist only a limited number of eneryg levels. Gv 1 1 () 0 0 v 2 v 2 x e 2

21 Absorption cross section of the OClO molecule 1.4E-17 Absorption cross section [cm²] 1.2E-17 1E-17 8E-18 6E-18 4E-18 2E Wavelength [nm]

22 Differential Optical Absorption Spectroscopy (DOAS -identification of different absorption processes by their spectral signature => Differential optical absorption spectroscopy (DOAS) -consideration of scattering processes by broad band spectral structures, e.g. low order polynomials Wavelength [nm] '[10-19 cm 2 ] O 3 Phenol SO 2 4 HONO 0 1 HCHO 0 20 ClO 0 50 BrO Benzol Toluol para-kresol 0 NO 3 IO NO2 Detection Limit 1 ppb L=5km 50 ppt L=5km 200 ppt L=5km 100 ppt L=5km 5 ppt L=5km 500 ppt L=5km 20 ppt L=12km 2 ppt L=12km 1 ppt L=16km 200 ppt L=1km 250 ppt L=1km 50 ppt L=1km 20 ppt L=1km

23 Typical DOAS-Spectrograph Spectrograph +30 C Grating Detector -35 C glass fibre Light Lense

24 Schematic of a Czerny-Turner monochromator

25 channel 4 channel nm 611 nm 316 nm 405 nm electronic box channel 1 channel 2 Satellite Instruments are usually more complex gratings beam splitter lamp calibration unit gratings Sun diffuser channel separator predisperser prism telescope mirrors nadir Scannig mirror sun

26 Absorption spectroscopy Beer- Lambert-law : Optical depth l I( ) I0 ( ) exp i ( ) i ( s) s ( ) ds 0 i i : i : s : Absorption cross section of trace gas i Concentration of trace gas i Extinction coefficient => From the knowledge of the absorption cross section it is possible to determine the trace gas concentration

27 Intensit t I( ) Meßspektrum 'differentielle' optische Dichte I 0 c ln I I' l ' 0 O3-Absorption I NO2-Absorption Cross section [arb. Units] diff ' Wellenlänge [nm] 1 2 Wavelength [arb. Units] 3

28 Example: NO 2 observation 8.E-19 Typical wavelength window absorption cross section [cm²] 6.E-19 4.E-19 2.E-19 0.E Wavelength [nm] Path length: 6km NO 2 mixing ratio: 10 ppb (parts per billion) Air concentration: 2.9e19 molec/cm³ NO 2 concentration: 2.9e11 molec/cm² l 0.07 => I/I

29 -also scattering reduces the measured intensity

30 Platt et al., 1980

31 Platt et al., 1980

32 NO 3 time series collected in the marine boundary layer of Mace Head, Ireland. The letters denote the origin of the observed air masses: A, Atlantic, P polar marine, EC, easterly continental, NC, northerly continental [Allan et al, 2000].

33 Catalytic ozone destruction mechanisms: X + O 3 XO + O 2 XO + O X + O 2 Net: O + O 3 2O 2 with: X = OH, NO, Cl, Br

34 Observation of volcanic emissions C. Kern, IUP Heidelberg

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38 Halogen compounds in coastal regions? C. Peters, PhD-thesis, IUP Heidelberg

39 IO reference optical density atmospheric spectrum Comparison of atmospheric spectrum after the removal of NO 2 and H2O absorptions. The comparison with the IO absorption cross section clearly shows the presence of IO [Alicke et al, 1999] residual wavelength [nm]

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45 K. Hebestreit, PhD-thesis thesis,, IUP Heidelberg

46 High BrO coincides with low O3

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48 Long Path DOAS -basic principle -Long path DOAS (UV/vis/IR) -instrumental improvements -Specific applications -white cell -vertical profiles -tomographic inversions

49 Retro Reflector Plane Mirror Compensation of turbulent beam dispersion by a (corner-cube) retroreflector arrangement (upper panel) in comparison to reflection by a plane mirror (lower Atmospheric panel). remote sensing thomas.wagner@mpic.de

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53 J. Stutz, PhD-thesis thesis,, IUP Heidelberg

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55 J. Stutz, PhD-thesis thesis,, IUP Heidelberg

56 R. Ackermann, PhD-thesis thesis,, IUP Heidelberg

57 J. Tschritter, Diploma-thesis thesis,, IUP Heidelberg

58 Schematic set-up of a DOAS system using a coaxial arrangement of transmitting- and receiving telescope in conjunction with a retro-reflector array [Geyer et al. 2001]. This type of set-up pioneered by Axelsson et al. [1990] has become the standard for artificial Atmospheric - light DOAS remote systems sensing for research thomas.wagner@mpic.de in the recent years.

59 C. Hak, Dissertation, IUP Heidelberg, 2007

60 J. Tschritter, Diploma- thesis,, IUP Heidelberg Mode mixer has still to be used, but measurement of lamp reference spectrum is much easier: => Just put a reflecting surface in front of the fibre bundle

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63 DOAS instrument used during the SOS field campaign in Nashville, TN, 1999, picture : Cathy Burgdorf, en/research/doas/doas.html

64 Long Path DOAS -basic principle -Long path DOAS (UV/vis/IR) -instrumental improvements -Specific applications -white cell -vertical profiles -tomographic inversions

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67 -Multi-Reflektions-System (White-Zelle): Lichtquelle & Spektrograph Hohlspiegel m

68 A. Geyer PC Quartz fibre with mode-mixer Transfer optics 500 W Xe -arc lamphouse Schematic optical set-up of the 'White' multi-pass system with a base path of 15 m as used during a field campaign in Pabstthum/Germany.

69 Schematic of the DOAS setup in the EUPHORE chamber in Valencia, Spain [Volkamer et al. 2002].

70 Long Path DOAS -basic principle -Long path DOAS (UV/vis/IR) -instrumental improvements -Specific applications -white cell -vertical profiles -tomographic inversions

71 B. Alicke 4 m Sonic Anemometer 2.45 m DOAS 1.57 m 2.1m 1.8 m 1.25 km Set-up of the experiment to measure gradients and fluxes of NO 2 and HONO during the PIPAPO experiment. The DOAS instrument aimed sequentially at the three retroreflectors mounted on the tower at 1.25 km distance.

72 B. Alicke NO 2 and HONO gradients during the night of May 29, 1998 in Milan, Italy. [NO 2 ] (ppb) Gradient (ppb m -1 ) 5/29/ :00 5/30/ :00 5/30/ :00 80 a upper LP 60 middle LP 40 lower LP b 360 HONO (ppb) gradient (ppb m -1 ) 5/29/ :00 5/30/ :00 5/30/ : a b upper LP middle LP lower LP

73 J. Stutz 750m 1.9 km 6.1 km DOAS Systems ME 2m WT 44m RTU 115m RTM 99m RTL 70m Setup during the TEXAQS 2000 experiment. Five retroreflector arrays were mounted at different distances and altitudes. The measurements were performed by two DOAS instruments [Stutz et al, 2004].

74 J. Stutz altitude (m) O 3 (ppb) NO 2 (ppb) NO 3 (ppt) Vertical mixing ratio profiles during the night of 8/31 9/1 at four different times (noted on top of the graphs). The N 2 O 5 mixing ratios shown are calculated from the steady state of measured NO 2,NO 3, and N 2 O 5 [Stutz et al., 2004].

75 H.-J. Veitel, IUP Heidelberg

76 H.-J. Veitel, IUP Heidelberg

77 H.-J. Veitel, IUP Heidelberg

78 H.-J. Veitel, IUP Heidelberg

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82 Long Path DOAS -basic principle -Long path DOAS (UV/vis/IR) -instrumental improvements -Specific applications -white cell -vertical profiles -tomographic inversions

83 A. Hartl,, Dissertation, IUP Heidelberg, 2007

84 tomographic measurement setup during the BAB II campaign at the motorway BAB 656 between Heidelberg and Mannheim

85 Wind direction North South

86 Modelled concentration distribution of NO 2 and measurement setup Measured (reconstructed) concentration distribution of NO 2

87 Kai-Uwe Mettendorf, Dissertation, IUP Heidelberg, 2006

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89 Measurement setup for the validation measurements

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92 Distribution of light paths Example of a reconstructed trace gas distribution

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94 Summary (I) Long Path (active) DOAS -direct application of the Lambert-Beer law -long absorption path is needed to become sensitive even for small trace gas concentrations -many trace gases were first observed by LP DOAS -measurements possible also during night -only the averaged trace gas concentration along the light path can be obtained (from simple LP DOAS) -expensive and complicated instrumental set-up

95 Summary (II) Long Path (active) DOAS -recently many instrumental improvements were introduced, e.g. fibre optics, LEDs as light sources, which make instruments much cheaper and easier to operate -many specialisations of LP-DOAS exist for specific applications: -White (multi-reflection) system -light paths at different altitudes -balloon-borne reflectors -tomographic inversions