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 σ 0 c l A) Lambert-Beersches Gesetz: ( )

3 DOAS: Differentielle Optische AbsorptionsSpektroscopie I= I exp σ 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 c = ln I I l σ 0

4 DOAS: Differentielle Optische AbsorptionsSpektroscopie I= I exp σ 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 α f sinα = d = f W L W: width of the light beam at distance L For W=1m, L=5km: sinα = 0.001, α =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 Lecture on atmospheric remote sensing

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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 Different kinds of molecular vibrations Example: : CO 2 (Degree of freedom for vibration: : 4) Symmetric vibration Assymmetric vibration Deforming vibration

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

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

22 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] Lecture on atmospheric remote sensing

23 Solution: -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 σ'[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 Wavelength [nm] 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

24 Typical DOAS-Spectrograph Spectrograph +30 C Grating Detector -35 C glass fibre Light Lense Lecture on atmospheric remote sensing

25 Schematic of a Czerny-Turner monochromator

26 Typical DOAS-Spectrograph Spectrograph +30 C nm 611 nm 316 nm 405 nm Grating channel 4 channel 3 beam splitter gratings electronic box channel 1 channel 2 gratings lamp calibration unit Sun diffuser channel separator predisperser prism telescope mirrors nadir Scannig mirror sun Detector -35 C glass fibre Light Lense Lecture on atmospheric remote sensing

27 Absorption spectroscopy Beer- Lambert-law : Optical depth τ l I( λ) = I0 ( λ) exp σ i ( λ) ρi ( s) + ε s ( λ) ds 0 i σ i : ρ i : ε s : Absorption cross section of trace gas i Concentration of trace gas i Scatter coefficient => From the knowledge of the absorption cross section it is possible to determine the trace gas concentration Lecture on atmospheric remote sensing

28 Example: NO 2 observation Lecture on atmospheric remote sensing 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: 10ppb (parts per billion) Air concentration: 2.9e19 molec/cm³ NO 2 concentration: 2.9e11 molec/cm² τ = σ ρ l τ 0.07 => I/I

29 -also scattering reduces the measured intensity

30 Intensit t I( ) Meßspektrum 'differentielle' optische Dichte I 0 c = ln I I' l σ ' 0 O3-Absorption I NO2-Absorption Cross section [arb. Units] σ 2 σ 1 σ 3 σ diff σ ' Wellenlänge [nm] Lecture on atmospheric remote sensing λ 1 Wavelength [arb. Units] λ 2 λ 3

31 Platt et al., 1980

32 Platt et al., 1980

33 pixel atmos. spekt O 3 HCHO SO 2 NO Example of a spectral analysis of O 3, NO 2, SO 2, and HCHO in the polluted air of Heidelberg. The absorptions identified in the atmospheric spectrum (top trace) were: O 3 : 21.1 ± 0.5 ppb, SO 2 :0.64 ± 0.01 ppb, and HCHO: 3.7 ± 0.1 ppb. The NO 2 mixing ratio was not determined since this wavelength interval is not optimal for its analysis. The missing area around 312 nm was excluded from the analysis procedure. residual wavelength [nm] Lecture on atmospheric remote sensing

34 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] Lecture on atmospheric remote sensing

35 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].

36 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

37 Observation of volcanic emissions C. Kern, IUP Heidelberg

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

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

48 High BrO coincides with low O3

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

52 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 panel). Lecture on atmospheric remote sensing

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

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

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

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

62 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 - light DOAS systems for research in the recent years. Lecture on atmospheric remote sensing

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65 C. Hak, Dissertation, IUP Heidelberg, 2007

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

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

70 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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73 -Multi-Reflektions-System (White-Zelle): Lichtquelle & Spektrograph Hohlspiegel m

74 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. Lecture on atmospheric remote sensing

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

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

77 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.

78 B. Alicke NO 2 gradients during the night of May 29, 1998 in Milan, Italy. Gradients well above the detection limit were observed continuously for many hours during this night. NO 2 flux [NO 2 ] (ppb) Gradient (ppb m -1 ) wind speed (m s -1 ) [molec/cm 2 s] z/l K (m 2 s -1 ) 5/29/ :00 5/30/ :00 5/30/ :00 80 a upper LP 60 middle LP 40 lower LP x x c b d z/l u * e f v dep NO 2 flux measured calculated wind dir u * (m s -1 ) v dep (cm s -1 ) 5/29/ :00 5/30/ :00 5/30/ :00

79 B. Alicke HONO gradients during the night of May 29. (a) shows the mixing ratios on the individual light paths (LP). The gradient is displayed in (b). To remove the influence of direct HONO emissions we calculated HONO corr by subtracting 0.65% of the NO x (not shown here) from the HONO mixing ratios (d). The ratio of HONO to NO 2 mixing ratios is displayed in (e). The fluxes and net deposition velocities is shown in (f).. HONO (ppb) gradient (ppb m -1 ) NO (ppb) HONO corr (ppb) [HONO] [NO 2 ] HONO flux (molec cm -2 s -1 ) 5/29/ :00 5/30/ :00 5/30/ : Lecture on atmospheric remote sensing x x10 10 a b c e f d upper LP middle LP lower LP net v dep HONO flux 5/29/ :00 5/30/ :00 5/30/ : v dep (cm s -1 )

80 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 weree performed by two DOAS instruments [Stutz et al, 2004].

81 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].

82 H.-J. Veitel, IUP Heidelberg

83 H.-J. Veitel, IUP Heidelberg

84 H.-J. Veitel, IUP Heidelberg

85 H.-J. Veitel, IUP Heidelberg

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

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

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

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

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

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97 Measurement setup for the validation measurements

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102 Summary (I) Long Path (active) DOAS -most 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 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

103 Summary (II) Long Path (active) DOAS -recently many instrumental improvements were introduced, e.g. fibre optics, LED 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

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105 Leuchtdichte von LEDs Zum Vergleich: Sonnenoberfläche 1,5 x 10 9 cd/m 2 Xenon Hochdrucklampe typ 4x 10 8 cd/m 2 Glühdraht einer Glühlampe 5 bis 35x 10 6 cd/m 2 moderne Leuchtstofflampe 0,3 bis 1,5x 10 4 cd/m 2 Lecture on atmospheric remote sensing Nachthimmel etwa cd/m 2

106 Lichtausbeute (Effizienz) Lecture on atmospheric remote sensing Die Lichtausbeute ist ein Maß für die effektive Umwandlung elektrischer Energie in Lichtenergie. Die Effizienz der LED liegt zur Zeit bei bis zu 55 lm/w.

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