1. LINE MIXING EFFECTS OF O2 A-BAND WITH PHOTOACOUSTIC SPECTROSCOPY IN SUPPORT OF REMOTE SENSING
Thinh Bui1, Daniel Hogan1, Priyanka M. Rupasinghe1, Mitchio Okumura1
David A. Long2, Joseph T. Hodges2
Charles E. Miller3
1California Institute of Technology, Division of Chemistry, MC ,
Pasadena, CA 91125
2NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, MD
3Jet 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 CO2 concentration around the world NASA’s future OCO-2 mission will monitor global atmospheric CO2 from space.
As a precession requirement, this space based remote sensing mission must monitor the global atmospheric CO2 with an unprecedented accuracy of 0.25%.

4. Motivation cont.
To minimize systematic uncertainties OCO-2 will measure column averaged CO2 dry-air mole fraction (XCO2) as defined by,
This requires absorption line shape parameters of O2 and CO2 to be known with 0.1% accuracy.

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 O2 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. Collisional line mixing
(B) High pressure regime
The resulting spectrum can not be considered as a superposition of two isolated lines due the population exchange.

9. Collisional line mixing
( ): Predictions made with the inclusion of line mixing
( ): Predictions made without the line mixing
Pressure
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

10. Outline Motivation Collisional line mixing
Photoacoustic spectroscopy (PAS)

11. Photoacoustic spectroscopy (PAS)
Why 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)).

12. Photoacoustic spectrometerf P
Optical
transition
vfi + vmod i
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.

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

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

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. Spectrum S/N > 10 000, Fitted S/N ~ 1325
Same doublet fitted with Galatry profile (pairwise Line mixing is included)
A.U
Pressure = 4 atm
Spectrum S/N > , Fitted S/N ~ 1325
Still not close to spectrum S/N ??

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 A closer look: Pairwise line mixing approximation is no longer valid.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

19. Recent data: O2 A-Band @ 4 atm
R-branch
P-branch
A.U
Not only line mixing but also CIA is clearly visible.

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

21. Summary
We have utilized a photoacoustic spectrometer for O2 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 NASA’s Jet Propulsion Laboratory at Caltech and NSF Graduate Fellowship program.