1. Linhan Shen1, Thinh Bui1, Lance Christensen2, Mitchio Okumura1
MID-IR CAVITY RINGDOWN SPECTROSCOPY FOR ATMOSPHERIC ETHANE ABUNDANCE MEASUREMENTS
Linhan Shen1, Thinh Bui1, Lance Christensen2, Mitchio Okumura1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA
Jet Propulsion Laboratory, Pasadena, CA

2. Environmental Impact of Fugitive Methane
US Greenhouse Gas
Emission in 2013
US Methane Emission
By Source
Graphic Courtesy of John Bellamy/ Stanford University
Methane is a very important greenhouse gas.
Second highest emitted greenhouse gas in the US (~10%) from human activities.
It has ~25 times more impact on the climate change than CO2 in 100 year period.
Natural gas and petroleum systems contribute to 29% of anthropogenic CH4 emission, including a large contribution from fugitive (~ 1.5% of gross production, 28 million tons a year).
Inventory of U.S. Greenhouse gas emissions and sinks. (EPA 430-R )

3. Methods of Fugitive Methane Detection
Direct methane detection
Good at mapping out overall methane emissions.
Not efficient at identifying the source.
Measure abundance of different methane isotopologues
Good at identifying methane sources.
Requires high precision measurements for both δ13C-methane and δD-methane in order to identify sources.
Measure abundance of methane co-emitting gases (ethane, propane, butane)

4. Ethane Abundances from Different Methane Sources
Biogenic Sources
Gas Well NG
Ethane/Methane ratio is a good indicator for methane source identification.
[C2H6]/[CH4] < 0.2%
[C2H6]/[CH4] > 30%
NG Processing Plants
Pipeline Grade NG
[C2H6]/[CH4] > 30%
[C2H6]/[CH4] < 15%
T. I. Yacovitch et al. Environ. Sci. Technol. 2014, 48, 8028−8034

5. Atmospheric Trace Gas Detection Methods
Tunable infrared laser direct absorption spectroscopy (TILDAS)
Aerodyne System:
Heriot multi-pass cell
Pathlength = 76m
1s noise range between 19 to 210 ppt
T. I. Yacovitch et al. Environ. Sci. Technol. 2014, 48, 8028−8034

6. Atmospheric Trace Gas Detection Methods
Cavity enhanced absorption spectroscopy (CEAS)
Advantages of CRDS:
Long pathlength and high sensitivity.
Not sensitive to laser intensity fluctuation.
Cavity ringdown spectroscopy (CRDS)

7. Mid-IR Cavity Ringdown Spectrometer
AOM
R1 ~ 99.98%
R2 ~ 99.98%
L ~ 80cm
3.36 μm IC Laser
MCT Detector
Lehmann, K. K. & Romanini, D, J. Chem. Phys. 105 (23) 10263

8. JPL Manufactured 3.36 μm ICL Laser
High power ICL manufactured by JPL Microdevices Laboratory (MDL)
Laser line center: 2977 cm-1
Tunable range: 2972 cm-1 to 2984 cm-1
Peak power: > 15 mW
Side-mode attenuation: 25dB
C. Borgentun, C. Frez, R. M. Briggs, M. Fradet, S. Forouhar, Optics Express, 23, (2015)

9. Heterodyne Measurements of the Laser Linewidth
Assume two lasers have the same linewidth, the linewidth of individual laser is ~3MHz

10. Allan Deviation Analysis
For Ethane detection at ~ 2980cm-1:
δτ/τ ~ 0.2 %
αmin ~ 2.85x10-9 cm-1
Data acquisition rate ~ 20 Hz
NEA ~ 2.02x10-8 cm-1Hz-1/2
Slope =
αmin = 2.85 × 10-9 cm-1
σAllan
nringdowns

11. Spectra of 6ppb C2H6 in 250 Torr N2
Spectrometer was tested with a ~ 6ppb ethane mixture in N2 balance.

12. Spectra of 6ppb C2H6 in 250 Torr N2
Spectrometer was tested with a ~ 6ppb ethane mixture in N2 balance.
C2H6
H2O

13. Spectra of 6ppb C2H6 in 250 Torr N2
Spectrometer was tested with a ~ 6ppb ethane mixture in N2 balance.

14. Results from Spectra of 6ppb C2H6 in 250 Torr N2
Spectrometer was tested with a ~ 6ppb ethane mixture in N2 balance.
Band Center
(cm-1) S
(cm-1/(molecule cm-2))
Retrieved Ethane Mixing Ratio
(ppb)
4.05E-19
6.29 ± 0.65
3.611E-19
6.38 ± 2.38
3.969E-19
5.88 ± 0.10
Contains 372 lines.
Center of the laser, with the most power.
Clear of water interference.

15. Results from Spectra of 6ppb C2H6 in 250 Torr N2
Spectrometer was tested with a ~ 6ppb ethane mixture in N2 balance.
Band Center
(cm-1) S
(cm-1/(molecule cm-2))
Retrieved Ethane Mixing Ratio
(ppb)
4.05E-19
6.29 ± 0.65
3.611E-19
6.38 ± 2.38
3.969E-19
5.88 ± 0.10
Contains 47 lines.
High laser power.
Large water interference. Large error in measured concentration.

16. Results from Spectra of 6ppb C2H6 in 250 Torr N2
Spectrometer was tested with a ~ 6ppb ethane mixture in N2 balance.
Band Center
(cm-1) S
(cm-1/(molecule cm-2))
Retrieved Ethane Mixing Ratio
(ppb)
4.05E-19
6.29 ± 0.65
3.611E-19
6.38 ± 2.38
3.969E-19
5.88 ± 0.10
Contains 48 lines.
Edge of the laser. Very low laser power
Small amount of water interference.

17. Future Work
Measure ethane abundance in dry air sample from different sources.
Samples will be collected in a 1L glass flask and expanded into the cavity to reach 250 Torr cavity pressure.
Retrieve [C2H6]/[CH4] by comparing CH4 abundance measurements by a commercial methane analyzer.
Stabilize the cavity length by locking it to a stabilized HeNe Laser.
Stabilize the MIR laser.
Try to use liquid nitrogen cooled InSb detector instead of TEC cooled MCT detector.

18. Acknowledgement Funding:
Grant: This project is funded by JPL and Caltech President’s and Director’s Fund (PDF)
Student Fellowships: Linhan Shen and Thinh Q. Bui are both supported by NASA NESSF fellowship.
Support:
JPL microdevice laboratory (MDL) for manufacturing the high power MIR IC Laser.