1. Jet Propulsion Laboratory, Pasadena, CA
MID-IR CAVITY RINGDOWN SPECTROSCOPY FOR ATMOSPHERIC ETHANE TO METHANE RATIO
Linhan Shen1, Thinh Bui1, Lance Christensen2, Sally Newman3, Mitchio Okumura1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA
Jet Propulsion Laboratory, Pasadena, CA
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA

2. Anthropogenic Methane Emissions
8%: Other
29%: Natural Gas and Petroleum Systems
10% Manure Management
[C2H6]/[CH4] < 1%
[C2H6]/[CH4] ~ 15 – 30%
[C2H6]/[CH4] ~ 2 - 8%
Ethane/Methane ratio is a good indicator for methane source identification.
10%: Coal Mining
26%: Enteric Fermentation
18%: Landfills
Inventory of U.S. Greenhouse gas emissions and sinks. (EPA 430-R )

3. Goals and Difficulties
Goal: Measure ethane to methane ratio
([C2H6]/[CH4])
[C2H6]
[CH4]
Low Atmospheric Abundance
~ 3-15 ppb
Hard to Standardize Concentration
Atmospheric Abundance ~ 2ppm
Relatively Easy to Measure
Poorly Constraint Line Intensities
Lack of Concentration Standards

4. Mid-IR Cavity Ringdown Spectrometer
Mg(ClO4)2 Filter
AOM
IC Laser
MCT Detector
R1 ~ 99.98%
R2 ~ 99.98%
L ~ 80cm
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
Lehmann, K. K. & Romanini, D, J. Chem. Phys. 105 (23) 10263
C. Borgentun, C. Frez, R. M. Briggs, M. Fradet, S. Forouhar, Optics Express, 23, (2015)

5. Spectrometer Sensitivity Test
For Ethane detection at ~ 2978cm-1:
δτ/τ ~ 0.2 %
αmin ~ 3.5 × cm-1
Data acquisition rate ~ 100 Hz
NEA ~ × 10-9 cm-1Hz-1/2

6. HITRAN Simulation of C2H6 Spectrum

7. Spectra of 6ppb C2H6 in 250 Torr Zero Air
Band Center: cm-1.
Summed Line Strength: 4.05 E-19 cm-1/ molecule cm-2
Contains 372 lines.
Center of the laser, with the most power.
Clear of water interference. 20
(GHz)
Scanning Time: 45min
Fit: Sum of 2 Lorentzian Distributions.

8. Spectra of 6ppb C2H6 in 250 Torr Zero Air15
(GHz)
Scanning Time: 30min
Fit: Lorentzian Distribution.
Band Center: cm-1.
Summed Line Strength: E-19 cm-1/ molecule cm-2
Contains 48 lines.
Edge of the laser. Very low laser power
Small amount of water interference.

9. Measurement of 6ppb C2H6 in 250 Torr Zero Air
cm-1 Line
cm-1 Line 1
6.29 ppb
5.88 ppb 2
6.15 ppb
6.04 ppb 3
6.32 ppb
5.93 ppb 4
6.22 ppb
6.11 ppb 5
6.07 ppb
6.06 ppb
Average
6.21 ± 0.10 ppb
6.00 ± 0.10 ppb

10. Goals and Difficulties
Goal: Measure ethane to methane ratio
([C2H6]/[CH4])
[C2H6]
[CH4]
Low Atmospheric Abundance
~ 3-15 ppb
Hard to Standardize Concentration
Atmospheric Abundance ~ 2ppm
Relatively Easy to Measure
Poorly Constraint Line Intensities
Lack of Concentration Standards

11. Solution for Concentration Standardization
0.5% [C2H6]: [CH4] Ethane Methane Mixtures in Zero Air
Measure Ethane Abundance Using Two Lines in the MIR
Measure Methane Abundance Using MIR and NIR CRDS
Cross Calibrate Methane Abundance Measurement with Picarro
Standardized Ethane Abundance Measurement

12. Atmospheric Sample Collections
Caltech Site Urban Air
Samples collected in an evacuated 2L glass bulb.
Water in the samples is removed by passing through a Mg(ClO4)2 filter.
Each bulb is filled up to atmospheric pressure.
Beach Site Clean Air

13. MIR Measurement of Air Sample
Measurement of cm-1 line.
A multi-Lorentzian fit was done to calculate abundances of CH4 and C2H6.
C2H6
Cavity Pressure: 350 Torr
CH4 35
(GHz)
H2O

14. MIR Measurement of Air Sample
Measurement of the cm-1 line
A 2-Lorentzian fit was done to calculate abundance of C2H6.
Cavity Pressure: 350 Torr
C2H6 20
(GHz)

15. Correlation between Ethane and Methane Abundance
[C2H6] vs. [CH4]

16. Atmospheric Carbon Sources
Source Correlation Plot
Increasing Natural Gas Emission
Increasing Fossil Fuel Burning

17. Future Work
Increase scan efficiency. Decrease scanning duration from 30 – 45 min to less than 5 min.
Frequency stabilize the cavity to increase sensitivity.
Redesign sample introduction system to adapt the spectrometer to a mobile measurement system.

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.