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 127-72, Pasadena, CA 911252NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, MD 20899-10703Jet Propulsion Laboratory, California Institute of Technology, MS 183-901, Pasadena, CA 91109

OUTLINE

Motivation

Collisional line mixing

Photoacoustic spectroscopy (PAS)

Data/Analysis

Summary

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%.

http://oco.jpl.nasa.gov/

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.

2095.0][,][ 22

2

2

2 OwhereO

N

NX

O

COCO

MOTIVATION CONT.

http://oco.jpl.nasa.gov/

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.

OUTLINE

Motivation

Collisional line mixing

COLLISIONAL LINE MIXING

(A) Low pressure regime

At low pressures, lines can be considered as isolated each other.

(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

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

OUTLINE

Motivation

Collisional line mixing

Photoacoustic spectroscopy (PAS)

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?

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

vfi + vmodOpticaltransition P

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, 064902 (2010)

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.

OUTLINE

Motivation

Collisional line mixing

Photoacoustic spectroscopy

Data/Analysis

A P-BRANCH DOUBLET FITTED WITH GALATRY PROFILE (NO LINE MIXING)

Spectrum S/N > 10 000, Fitted S/N ~ 600

SAME DOUBLET FITTED WITH GALATRY PROFILE (PAIRWISE LINE MIXING IS INCLUDED)

Still not close to spectrum S/N ??

Spectrum S/N > 10 000, Fitted S/N ~ 1325

Pressure = 4 atmA.U

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

RECENT DATA: O2 A-BAND @ 4 ATM

P-branch

R-branch

))0,0(( 31 gg Xa

Not only line mixing but also CIA is clearly visible.

A.U

OUTLINE

Motivation

Collisional line mixing

Photoacoustic spectroscopy (PAS)

Data/Analysis

Summary

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.

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.