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LINE MIXING EFFECTS OF O 2 A-BAND WITH PHOTOACOUSTIC SPECTROSCOPY IN SUPPORT OF REMOTE SENSING Thinh Bui 1, Daniel Hogan 1, Priyanka M. Rupasinghe 1, Mitchio.

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Presentation on theme: "LINE MIXING EFFECTS OF O 2 A-BAND WITH PHOTOACOUSTIC SPECTROSCOPY IN SUPPORT OF REMOTE SENSING Thinh Bui 1, Daniel Hogan 1, Priyanka M. Rupasinghe 1, Mitchio."— Presentation transcript:

1 LINE MIXING EFFECTS OF O 2 A-BAND WITH PHOTOACOUSTIC SPECTROSCOPY IN SUPPORT OF REMOTE SENSING Thinh Bui 1, Daniel Hogan 1, Priyanka M. Rupasinghe 1, Mitchio Okumura 1 David A. Long 2, Joseph T. Hodges 2 Charles E. Miller 3 1 California Institute of Technology, Division of Chemistry, MC , Pasadena, CA NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, MD Jet Propulsion Laboratory, California Institute of Technology, MS , Pasadena, CA 91109

2 OUTLINE Motivation Collisional line mixing Photoacoustic spectroscopy (PAS) Data/Analysis Summary

3 MOTIVATION In order to control rapidly growing atmospheric CO 2 concentration around the world NASAs future OCO-2 mission will monitor global atmospheric CO 2 from space. As a precession requirement, this space based remote sensing mission must monitor the global atmospheric CO 2 with an unprecedented accuracy of 0.25%.

4 To minimize systematic uncertainties OCO-2 will measure column averaged CO 2 dry-air mole fraction (X CO 2 ) as defined by, This requires absorption line shape parameters of O 2 and CO 2 to be known with 0.1% accuracy. MOTIVATION CONT.

5 MOTIVATION CONT. However, as of today, the current knowledge of these parameters (widths, shifts…) with this accuracy is very limited or not known. In order to achieve this goal an accurate modeling of data will be required including line mixing as well as other non-Voigt features, collision induced absorption (CIA), Dicke narrowing and speed dependence (SD). In this talk we will present our effort to measure line mixing effect in O 2 A-band using photoacoustic spectroscopy.

6 OUTLINE Motivation Collisional line mixing

7 COLLISIONAL LINE MIXING (A) Low pressure regime At low pressures, lines can be considered as isolated each other.

8 (B) High pressure regime The resulting spectrum can not be considered as a superposition of two isolated lines due the population exchange. COLLISIONAL LINE MIXING

9 Typical evolution of the line shape of a doublet of collisionally coupled transitions with increasing pressure. Collisional effects on molecular spectra: laboratory experiments and models, consequences for applications / Jean-Michel Hartmann, Christian Boulet, Daniel Robert. Amsterdam; Boston: Elsevier Science, 2008 Pressure ( ): Predictions made with the inclusion of line mixing ( ---- ): Predictions made without the line mixing COLLISIONAL LINE MIXING

10 OUTLINE Motivation Collisional line mixing Photoacoustic spectroscopy (PAS)

11 PHOTOACOUSTIC SPECTROSCOPY (PAS) Compared to direct absorption spectroscopy, this method is benefited by zero background measurement. Optically broadband devices (acoustic properties of the system do not depend on the spectral distribution of the absorbed radiation). High precision (0.01%), high resolution (200 MHz) and a large dynamic range (Unsaturated lines for (0.1 – 5 atm)). Why PAS?

12 PHOTOACOUSTIC SPECTROMETER A non-radiative de-excitation mechanism is considered. Absorbed photon energy is released via translational energy by increasing the temperature (heating). The heat dissipation to the surrounding environment creates a pressure wave. When the laser is modulated, this pressure variation is also periodic and results a acoustic wave. This acoustic wave can be detected by using a sensitive microphone. i f v fi + v mod Optical transition P

13 PAS SPECTROMETER AT CALTECH In order to reduce background noise the spectrometer is located at sound proof environment. K.A.Gillis et al. Rev. Sci. Instrum. 81, (2010)

14 EXPERIMENTAL SETUP WM Diode Laser Amp. AOM AOM Driver Lock-in Amp FG PD Preamp PAS cell 50 mW 800 mW 400 mW 1.6 kHz Mic.

15 OUTLINE Motivation Collisional line mixing Photoacoustic spectroscopy Data/Analysis

16 A P-BRANCH DOUBLET FITTED WITH GALATRY PROFILE (NO LINE MIXING) Spectrum S/N > , Fitted S/N ~ 600

17 SAME DOUBLET FITTED WITH GALATRY PROFILE (PAIRWISE LINE MIXING IS INCLUDED) Still not close to spectrum S/N ?? Spectrum S/N > , Fitted S/N ~ 1325 Pressure = 4 atm A.U

18 A CLOSER LOOK: Solution: Multi-spectrum fit with line mixing is required! Pairwise line mixing approximation is no longer valid. This is a limitation of our current data analysis software

19 RECENT DATA: O 2 4 ATM P-branch R-branch Not only line mixing but also CIA is clearly visible. A.U

20 OUTLINE Motivation Collisional line mixing Photoacoustic spectroscopy (PAS) Data/Analysis Summary

21 SUMMARY We have utilized a photoacoustic spectrometer for O 2 A-band at 760 nm to meet precision requirements of current (ACOS/GOSAT/TCCON) and future (OCO-2/OCO-3/ASCENDS) remote sensing applications. This can be used to study line mixing effect as well as collision induced absorption for remote sensing relevant pressures (0.1-5 atm). In order to fit the spectra at relatively high pressures, more complete line mixing model must be included in the fitting procedure.

22 ACKNOWLEDGMENT This work was performed at California Institute of Technology, and was supported by NASAs Jet Propulsion Laboratory at Caltech and NSF Graduate Fellowship program.


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