Current and emerging laser sensors for greenhouse gas sensing

and leak detection Michael B. Frish*

Physical Sciences Inc., 20 New England Business Center, Andover, MA, USA 01810-1077

ABSTRACT

To reduce atmospheric accumulation of the greenhouse gases methane and carbon dioxide, networks of continuously-operating sensors that monitor and map their sources are desirable. In this paper, we discuss advances in laser-based open-path leak detectors, as well as technical and economic challenges inhibiting widespread sensor deployment for “ubiquitous monitoring”. We describe permanently-installed, wireless, solar-powered sensors that overcome previous installation and maintenance difficulties while providing autonomous real-time leak reporting without false alarms.

Keywords: Greenhouse gas emission, Pipeline safety, Leak detection, Laser sensing, Natural gas, Open-path

1. INTRODUCTION

In the study of climate change, obtaining good data demands widely-deployed accurate and reliable sensors for identifying, understanding, and controlling the origins, sources, sinks, and fates of greenhouse gases (GHGs), especially carbon dioxide (CO2) and methane (CH4). To reduce atmospheric accumulation of these gases, emissions from natural gas extraction, transmission, distribution, and combustion processes should be controlled and minimized. Currently, handheld and airborne laser sensors are accepted industry tools for periodic surveys to detect leaks from the gas pipeline infrastructure. However, to identify intermittent leak sources and better quantify emissions, networks of permanent widely-deployed sensors are desirable to continuously monitor and map, spatially and temporally, GHG concentrations with sufficient sensitivity and resolution to distinguish local sources from ambient background, and provide fast health and safety danger alerts. The sensors must be suitable for widespread cost-effective deployment, autonomous, accurate, and reliable in compact packages.

Below, we describe recent advances in laser-based leak detectors, focusing on permanently-installed, wireless, solar-powered open-path sensors that overcome previous installation and maintenance difficulties while providing autonomous real-time leak reporting without false alarms. In one example, during more than six-months of field testing, aiming of the laser from a methane sensor’s transceiver to passive targets up to 600 ft distant was stable and required no maintenance. Leak challenges originating up to 70 ft upwind of the laser path demonstrated detection of methane emission rates as small as 0.5 scfh. We also discuss some of the technical and economic challenges that must be addressed to realize widespread sensor deployment for “ubiquitous monitoring”, including the emerging need for laser sources designed with power and cost attributes specifically intended to fulfill these sensor requirements.

2. SENSOR PURPOSES

2.1 Methane

Biogenic and anthropogenic methane sources both contribute to GHG loading. Anthropogenic sources can be controlled and limited if their origins are located. Significant sources are landfills, bovine and rice farms, and the natural gas system which, in the US, includes nearly 500,000 active wells, 300,000 miles of transmission pipelines, and over 1,200,000 miles of distribution pipelines. Natural gas leaks are potential safety risks as well as GHG sources.

With increasing national emphasis on natural gas as an abundant energy resource, there is increased emphasis on improving leak detection and mitigation, as well as characterizing other sources of natural and anthropogenic methane emissions. Recent studies show considerable uncertainty in the leakage rate from gas production sites, rang-

ing from 0.42%1 to 11%.

2 Table I, extracted from a recent study prepared by the office of U.S. Senator Edward Markey,

*frish@psicorp.com; phone 1 978 689-0003; fax 1 978 689-3232; psicorp.com

summarizes the national impact of pipeline leakage.3. High leakage rates have the potential to offset the climate benefits

of natural gas over other fossil fuels. Some, but significant, leakage is believed to be intermittent, and thus not detected with routine infrequent leak surveying.4

Table I. U.S. Unaccounted for Gas, Emissions, and Significant Incidents on Natural Gas Systems

Total U.S. Unaccounted for Gas from Natural Gas Systems from 2000-2011 (3)

2.6 trillion cubic feet of natural gas

Total U.S. Reported Emissions from Natural Gas Distribution Systems from 2010 – 2011 (4)

Equivalent to releasing 56.2 million metric tons of CO2

Significant Incidents on U.S. Natural Gas Distribution Systems from 2002-2012 (5)

796 incidents / 116 fatalities / 465 injuries / $810,677,757 in property damage

Maintaining the pipeline system’s security and integrity currently is a continual process of monitoring pipeline parameters (e.g. pressure and flow rate) to recognize abnormal events that may indicate leaks and ruptures, supplemented by scheduled periodic walking, driving, or aerial surveys using methane detectors for locating and repairing leaks. However, routine but infrequent periodic surveys may overlook sporadic leaks. Continually-operating cost-effective methane detector networks, when located judiciously, can help identify leak sources that are intermittent or develop abruptly. Continuous monitoring can also provide early warning of abnormal gas concentrations that may indicate a potential safety hazard. The July 2012, U.S. Department of Transportation’s Government & Industry Pipeline R&D Forum’s Leak Detection Working Group identified the lack of sensors intended to specifically fulfill this need as a key technology gap inhibiting widespread deployment of such real-time leak detection networks. Novel cost-effective and low-power mass-produced sensors are needed to fill this gap.

2.2 Carbon Dioxide

CO2 is created by combusting fossil fuels, including natural gas, and is participating in climate change.5 To slow or

reverse the trend of increasing atmospheric concentration of CO2, Geologic Carbon Sequestration (GCS) is emerging as a strategy for restraining the anthropogenic CO2 flux. GCS involves separating CO2 from combustion effluent, transporting it via pipeline to storage sites, and injecting it at high pressure into storage reservoirs several thousand feet deep. In essence, the process is the opposite of natural gas extraction but utilizes very similar pipeline infrastructure. The

current 3900 miles of CO2 pipeline in the U.S. is projected to grow to 120,000 miles to support GCS deployment,6 and

these pipelines will be subjected to leak detection and repair much like the natural gas pipeline infrastructure. Furthermore, monitoring CO2 at sequestration sites is critical for assuring that the sequestration process is safe and performing its intended purpose, i.e storing CO2 without leakage. Much like natural gas leaks, any leaks from underground sequestration infrastructure, including reservoirs, are likely to be diffusely spread through the ground to emerge from a relatively broad area and be transported by the wind. Thus, economical distributed surface sensor networks with high sensitivity and precision are needed to distinguish leaking CO2 from ambient CO2. The measurement technologies must be sensitive enough to discern CO2 leakage signals that may barely rise above the variations in natural background CO2 concentrations occurring on diurnal to inter-annual time scales, and provide spatial resolution sufficient to localize leak origins.

3. TDLAS SENSORS

Sensors based on near-IR (NIR, 1.0 – 2.5 µm) midwave-IR (MWIR, 3 – 12 µm) Tunable Diode Laser Absorption Spectroscopy (TDLAS) are emerging to fulfill these GHG sensing needs. TDLAS sensors provide fast, highly-sensitive measurement of a selected gas or set of gases in complex mixtures. They couple telecommunications-style diode lasers

with straightforward spectroscopic principles and detection techniques.7

Near-IR TDLAS emerged from the laboratory as a practical analytical tool about 20 years ago,8 and is now a proven

mature technology9,10

commercially successful in industrial markets such as process monitoring and control, quality

assurance, environmental sensing, plant safety, and infrastructure security that demand sensitive and gas-specific detection of abnormal conditions with minimal false positive occurrences and minimal down time. Indeed, some of the largest present applications of TDLAS are in the natural gas industry, for pipeline leak surveying as well as monitoring

pipeline gas quality.11

Enabled by the state-of-the-art low-cost low-power TDLAS platforms, like that of Figure 1,

handheld standoff leak detectors based on backscatter TDLAS are now in use worldwide for walking surveys of natural gas transmission and distribution pipeline networks. Recently, these detectors have been reconfigured to detect CO2

pipeline leakage.12

Surveying from vehicles and aircraft has been demonstrated, as has the ability to sense methane

emissions from landfills.

Figure 1. Battery-powered TDLAS sensor in a 6” x 7” x 1.5” package, consumes < 1W for continuous operation at a 10 Hz data reporting rate, and provides digital interfaces.

Attractive features of commercial NIR TDLAS are: reliable laser sources, high precision, very low power consumption (< 1 W), compact, minimal maintenance, and acceptable cost of ~$10,000 per sensor unit. Utilizing sensitive detection techniques such as Wavelength Modulation Spectroscopy (WMS), NIR TDLAS probes overtones of fundamental molecular rotational-vibration energy-state transitions (Figure 2), and can sense over a dozen simple molecules at ppm concentrations (Table II). The noise equivalent absorption (NEA) for NIR TDLAS sensors is typically on the order of 0.00001 @ 0.1 Hz.

Figure 2. CH4 absorption spectra in the MWIR fingerprint region and NIR overtone, illustrating the 200x linestrength advantage of the fundamental.

Table II. Some Gases Measured by Near-infrared TDLAS

Gas

Detection

Limit

(ppm-m) Gas

Detection

Limit

(ppm-m)

HF 0.2 HCN 0.2

H2S 20.0 CO 40.0

NH3 5.0 CO2 1.0

H2O 1.0 NO 30.0

CH4 1.0 NO2 0.2

HCl 0.2 O2 50.0

H2CO 5.0 C2H2 0.2

During the past decade, semiconductor quantum cascade lasers (QCLs) and interband cascade lasers (ICLs) have emerged as miniature industrial quality MWIR laser sources suitable for sensing in the molecular fingerprint spectral

region,13,14

where absorption linestrengths are orders-of-magnitude stronger than the NIR. The MWIR offers

opportunities for sensing complex molecules that are difficult to detect with NIR TDLAS, as well as sensor miniaturization by enhancing sensitivity to simple molecules over shorter optical pathlengths. Ongoing research efforts to reduce MIR sensor noise and power requirements by use of uncooled pulsed or low-power laser sources and small area uncooled detectors, and to reduce MIR laser cost by high-volume production, is starting to make MIR TDLAS sensors commercially attractive in niche markets that cannot be addressed by NIR.

4. TDLAS CONFIGURATIONS

The TDLAS platform provides a technical foundation for a suite of sensors configured specifically for GHG sensing. Figure 3 illustrates the fundamental segments of a TDLAS sensor, comprising: The Laser Transmitter; the Measurement Path; and the Signal Processor. Usually the Laser Transmitter and Signal Processor functions are combined in a “Control Unit”. The Laser Transmitter unit generates a wavelength agile (i.e. tunable) laser beam. An optical fiber may conduct the laser beam from its origin at the Laser Transmitter to the Measurement Path. At the beginning of the Measurement Path, the laser beam launches into a gas mixture sample. At the opposite end of the Measurement Path, the laser beam impinges upon a photodetector which converts the laser beam’s power, attenuated by target gas absorption, into an electrical signal. The Signal Processor unit interprets the electrical signal and outputs information via a local display and a remote communications signal.

Figure 3. Fundamental components of a laser-based gas analyzer.

Some common measurement path configuration examples are: Extractive, Point, Long-Path, In-situ, and Standoff. Each of these is relevant to GHG sensing. They are described and illustrated briefly here:

Point and Extractive (Figure 4): An optical cell installed in a measurement chamber through which gas flows either passively or by active pumping. Optical path length is designed to provide the required sensitivity. Multipass optics, which reflect the laser beam many times between a pair of mirrors, can provide an extended optical path length within a small volume. Uses: Permanent or portable sampling to measure local target gas concentrations or locate leaks.

Figure 4. Example point sensor.

Long Path (Figure 5): A transceiver projects the laser beam onto a remote surface, and receives laser light returned from the surface. A simple reflective highway sign is a suitable target for path lengths ~1000 ft. Uses: Continuous and

permanent pipeline health and safety monitoring; Fugitive emission localization via multi-sensor networks.

Figure 5. Long-path sensor configuration.

In-situ (Figure 6): Optical configurations like long-path, but laser beam transits a process gas stream, e.g. combustion effluent. Use: Monitoring and controlling CO2 separation processes.

Figure 6. Example in-situ sensor transceiver.

Standoff (Figure 7): Like Long-Path but projects laser beam onto a non-cooperative surface and collects a fraction of the passively scattered laser light. Deduces the amount of target gas in the laser path between the transceiver and the illuminated surface. Use: Surveying pipelines.

Figure 7. Standoff or backscatter TDLAS.

Mobile (Figure 8): Standoff sensor installed on a vehicle or aircraft.15

Uses: Mapping gathering fields, sequestration

sites and landfills; surveying pipelines.

Figure 8. Experimental aerial standoff TDLAS on a small unmanned quadrotor.14

The TDLAS configurations described above can be included in a spatially-distributed network of sensors that communicate among each other or with a centralized network interface to enable spatial and temporal mapping of gas concentrations. The

use of fiber optics enables multiplexing TDLAS sensors to monitor multiple target gases in each Measurement Path.16

5. CONTINUOUS MONITOR DEMONSTRATIONS

Figure 9 illustrates an example long-path sensor network under consideration for continuous monitoring of natural gas and carbon dioxide pipelines. The eye-safe laser beams traverse across and above a pipeline to detect leaks and, upon detection, the sensor transmits alerts to the pipeline operator. The sensors ignore temporary disruptions due to passing vehicles and are insensitive to vehicle emissions.

Figure 9. Pipeline Monitor Concept: a) Example installation of open-path pipeline leak monitor. b) Bird’s eye view of network protecting extended length of pipeline. Red arrows illustrate laser beam paths.

In recent early testing of this configuration, near-IR backscatter TDLAS technology was adapted into permanently-installed continuous and autonomous long-path sensor for CO2 and CH4 pipeline leak detection and alarm

reporting.17,18,19,20

Pictured in Figure 10, the transceiver, installed in a weatherproof NEMA 4 enclosure, mounted to a

post with a co-located solar panel and a local RF link to a remote computer that stored data, processed the data to recognize leak signatures, and transmitted alarms via a cellular internet connection.

Figure 10. Long-path CH4 sensor at test site.

These sensors operated continuously for months at three field test locations: the PSI facility in Andover, MA, the Illinois Basin – Decatur Project (IBDP) GCS validation site, and the Pacific Gas and Electric training facility in Livermore CA. At each site, target gas leaks were manufactured artificially or resulted from maintenance of pipeline infrastructure. Data were acquired to demonstrate sensor efficacy, evaluate sensor response to leaks in typical operating scenarios and weather conditions, and verify sensor freedom from false alarms including any resulting from sensitivity to other ambient gases. The data reveal that CO2 and CH4 leaks create distinct statistical signatures: turbulent leak plumes cause rapid fluctuations that are readily distinguished from relatively slow natural background variations (e.g. diurnal fluctuations). Figure 11 shows example data from the CH4 sensor. The CO2 sensor data are similar. These wireless, solar-powered sensors, combined with proprietary statistical processing algorithms, have: a) demonstrated the capability to discern the signatures of small leaks, b) detected CH4 leaks as small as 0.5 scfh, comparable to the gas flow rate of a pilot light, and c) activated real-time notification of leakage. When fully deployed, the envisioned pipeline protection network will facilitate identifying fugitive leaks that contribute to greenhouse gas loads and pose potential personnel safety hazards.

Figure 11. Example long-path CH4 sensor data illustrating: a) diurnal variation of ambient CH4; b) period of leakage from a source upwind of the laser path.

6. SENSOR-ON-A-CHIP PLATFORM CONCEPT

The quest for “ubiquitous” CH4 and CO2 monitoring remains confounded by a lack of sensors having the sensitivity and specificity characteristics of TDLAS, while being amenable to economical mass-production and permanent low-maintenance installation and continuous operation in networked configurations. Current TDLAS sensors are not yet suitable for ubiquitous because of size, complexity, and power usage.

Figure 1 represents the current state-of-the-art for compact TDLAS sensor packages. It is comparable in size to a smoke detector, consumes < 1W for continuous operation at a 10 Hz data reporting rate, and provides interfaces to alarm

communications devices. It samples the ambient air along a 2” (5 cm) optical path. When configured to sense methane, its expected sensitivity is approximately 200 ppm. Unlike electrochemical and broadband spectroscopic sensors (e.g. LEDs and NDIR), it is insensitive to ambient gases other than methane. Yet, although this compact TDLAS sensor can achieve the basic performance and data reporting goals for widely-deployed methane detector networks (sensitivity ~200 ppm methane, insignificant cross-sensitivity to other vapors, 60 second response to abnormal methane), its cost (exceeding $5,000 per unit) and power demand (~1W) must both diminish by at least an order-of-magnitude to practically serve the envisioned ubiquitous natural gas leak detection needs. The high cost results from using: 1) laser packages designed in the early 1990’s by and for the telecommunications industry, that are produced in relatively low volumes and cost about $1000 each, even though the laser chips alone could cost only tens of dollars each if produced in quantities of tens of thousands; 2) bulk optical components, often hand-polished with sophisticated coatings; 3) control and data processing electronics built from commercial discrete components.

A typical laser package includes a thermo-electric cooler (TEC) platform, a thermistor, a microlens to project laser light onto an optical fiber facet, an optical isolator to prevent optical fiber backreflections from disrupting laser performance, and a monitor photodiode for measuring laser output. Because this laser package style enabled industrial quality TDLAS sensors, it has become a de facto TDLAS standard. However, for the ubiquitous GHG sensing applications, the sophisticated laser packaging is unnecessary, detrimental to performance, and an inhibition to high-volume production. Eliminating bulk optical components, i.e, lenses and optical isolators, enables assembly of the sensor as a single manufactured unit using standard semiconductor fabrication and high-volume production techniques. The power consumption results primarily from laser thermal control, wherein laser temperature is maintained near 300 K (room

temperature) with the ±10mK precision needed for sensitive gas detection. The laser’s efficiency is about 10%, thus about 90% of the power supplied to it is discarded as waste heat. The TECs in the laser packages consume additional power and create additional waste heat to transport the laser’s waste heat to a heat sink external to the laser package. Since the TEC draws and wastes considerable power, its control electronics must withstand currents ~1 A, forcing use of discrete and inefficient electronic components. Eliminating the laser TEC is a key to reducing sensor power consumption as well as cost.

We are developing a novel sensor platform, conceptually illustrated by Figure 12, intended to meet these challenges. It utilizes a silicon substrate for supporting the laser chip, detector photodiode, and thermal control components. The platform is designed to work with common fabrication tools, e.g. passive precision automated pick-and-place guided by machine vision and fiducial markings etched or printed onto the wafers. It eliminates the TEC by operating the laser in heated mode such that its own waste heat contributes to temperature stabilization, thus reducing system power

consumption for thermal control by more than 80% (Figure 13).21

Precise temperature stabilization is maintained by a

simple resistive heater with feedback control requiring little power. Bulk optical components are eliminated. Preliminary data acquired in our laboratory, using the components acquired for building the envisioned sensor platform, have shown that the bulk optical components can indeed be eliminated while maintaining sensor performance suitable for detecting methane at the concentrations of interest.

Figure 12. Miniature TDLAS sensor concept. The integrated optics sensor head is shown attached to the electronics control and signal processing board in an enclosure similar to a smoke detector.

Figure 13. Power required to stabilize laser at selected temperature. Note that with virtually no power provided to the TEC,

the laser stabilizes near 32°C.

7. CONCLUSION

From the perspective of sensing requirements for both methane and carbon dioxide as representative greenhouse gases, we have provided an overview of TDLAS sensing technologies that are currently available, utilized, and in development in the near infrared for open path and point leak detection applications. These sensors are starting to provide important data that can be used to better understand sources of anthropogenic, biogenic and abiogenic greenhouse gases. These data are useful in guiding future developments of low-cost sensors that can be combined into network topologies to enhance safety and advance our understanding of greenhouse gas sources and fates.

REFERENCES

[1] D.T. Allen, et.al (2013), “Measurements of methane emissions at natural gas production sites in the United States,” www.pnas.org/cgi/doi/10.1073/pnas.1304880110

[2] A. Karion, et al. (2013), “Methane emissions estimate from airborne measurements over a western United States natural gas field”, Geophys. Res. Lett., 40, doi:10.1002/grl.50811

[3] “America Pays for Gas Leaks”, Report Prepared for Senator Edward J. Markey, July 2013. http://www.markey.senate.gov/documents/markey_lost_gas_report.pdf

[4] A. R. Brandt, et al. (2014), “Methane Leaks from North American Natural Gas Systems,” Science 343, 733 (14 February 2014)

[5] Intergovernmental Panel on Climate Change (IPCC) (2007), IPCC Fourth Assessment Report: Climate Change 2007, Cambridge University Press, Cambridge UK.

[6] J.J. Dooley, R.T. Dahowski, and C.L. Davidson, (2008), “Comparing Existing Pipeline Networks with the Potential Scale of Future U.S. CO2 Pipeline Networks,” PNNL-17381 Pacific Northwest National Laboratory.

[7] D.E. Cooper, and R.U. Martinelli, “Near-infrared diode lasers monitor molecular species,” Laser Focus World, (November 1992)

[8] D.S. Bomse, “Diode Lasers: Finding Trace Gases in the Lab and the Plant,” Photonics Spectra, 29(6) (1995). [9] M.B. Frish, and F. Klein, “Trace Gas Monitors based on Tunable Diode Laser Technology: An Introduction and

Description of Applications,” 5th International Symposium on Gas Analysis by Tunable Diode Lasers, Freiburg, Germany, VDI Berichte 1366 (1998).

[10] M. Druy, M.B. Frish, and W.J. Kessler, W.J., “From Laboratory Technique to Process Gas Sensor - The Maturation of Tunable Diode Laser Absorption Spectroscopy,” Spectroscopy 21(3), 14-18 (March 2006).

[11] M.B. Frish, R.T. Wainner, J. Stafford-Evans, B.D. Green, M.G. Allen, S. Chancey, J. Rutherford, G. Midgley, and P. Wehnert, “Standoff Sensing of Natural Gas Leaks: Evolution of the Remote Methane Leak Detector (RMLD),” Invited Paper in Conference on Lasers and Electro-optics/Quantum Electronics and Laser Science and Photonic Applications, Systems and Technologies 2005, Optical Society of America, Washington DC (2005).

[12] J.W. Zimmerman, R.A. Locke II, C.S. Blakley, M.B. Frish, M.C. Laderer, and R.T. Wainner, “Initial testing of prototype tunable diode laser absorption spectrometers for CO2 monitoring applications at the Illinois Basin - Decatur Project, USA,” 13th Annual Conference on Carbon Capture Utilization & Sequestration, Pittsburgh PA (April 2014).

[13] D.M. Sonnenfroh, E.W. Wetjen, M.G. Allen, C. Gmachl, F. Capasso, A.L. Hutchinson, D.L.Sivco, J.N. Baillargeon, and A.Y. Cho, “Mid-IR Gas Sensors Based on Quasi-CW Room-Temperature Quantum Cascade Lasers,” Paper No. 2000-0641, AIAA 38th Aerospace Sciences Meeting, January (2000).

[14] D.M. Sonnenfroh, M.B. Frish, R.T. Wainner, and M.G. Allen, “Mid-IR Quantum Cascade Laser Sensor for Tropospherically Important Trace Gases,” Final Report prepared for U.S. Environmental Protection Agency under Order No. 4C-R348-NASA, PSI-2857/TR-1971, November (2004).12-13

[15] D. Picciaia, G. Zazzeri, M.S. Gimberini, and P. Andreussi, “A New Remote Sensing Method for Landfill Emissions Quantification,” Proceedings Sardinia 2011, Thirteenth International Waste Management and Landfill Symposium, S. Margherita di Pula, Cagliari, Italy; 3-7 (October 2011).

[16] M.B. Frish, “Overview of Sensitive Detection and Multiplexing Techniques for Tunable Diode Laser Absorption Spectroscopy,” in OSA Trends in Optics and Photonics Vol. 31, Advanced Semiconductor Lasers and Their

Applications, Leo Hollberg and Robert J. Lang, eds. Optical Society of America, Washington DC (2000). [17] M.B. Frish, and B.A. Cummings, “Laser Sensors for Gas Pipeline Explosion Protection,” Final Report prepared

for the California Energy Commission under Grant No 57238A/11-01G, PSI-1802/TR-2926, November 2013. [18] M.B. Frish, M.C. Laderer, J.S.G. Stafford-Evans, J. Lewicki, and R. Locke, “Networked TDLAS Sensors for

Sequestration MVA,” DoE Carbon Storage Program Infrastructure Annual Review Meeting, Pittsburgh, PA, PSI VG11-186. (November 2011).

[19] M.B. Frish, and D.M. Sonnenfroh, “TDLAS Analyzers for Energy Production, Transmission, and Storage,” Field Applications in Industry and Research (FLAIR) 2011 Industry Session, Murnau, Germany, PSI VG11-154. (September, 2011).

[20] J. Zimmerman, C. Blakley, R. Locke and M. Frish, “Final Report of TDLAS CO2 Sensor Field Testing at the Illinois Basin – Decatur Project,” Illinois State Geological Survey, Champaign, Illinois (February 20, 2014).

[21] M.B. Frish, R. Shankar, I. Bulu, I. Frank, M.C. Laderer, R.T. Wainner, M.G. Allen, and M. Lončar, “Progress Toward Mid-IR Chip-Scale Integrated-Optic TDLAS Gas Sensors,” SPIE Photonics West, San Francisco, CA, 2-7 February 2013, Paper No. 8631-9 (January 2013).

ACKNOWLEDGMENT

The author gratefully acknowledges the financial support, technical contributions and participation of the U.S. Department of Energy, U.S. Army, Physical Sciences Inc., Heath Consultants Inc., Pacific Gas and Electric Co., Illinois State Geological Survey, and the California Energy Commission.

ENDNOTE REFERENCES: copied to end of Section 6 – Delete this page from pdf file

1. D.T. Allen, et.al (2013), “Measurements of methane emissions at natural gas production sites in the United States,”

www.pnas.org/cgi/doi/10.1073/pnas.1304880110

2. A. Karion, et al. (2013), “Methane emissions estimate from airborne measurements over a western United States natural gas field”, Geophys. Res. Lett., 40, doi:10.1002/grl.50811

3. “America Pays for Gas Leaks”, Report Prepared for Senator Edward J. Markey, July 2013. http://www.markey.senate.gov/documents/markey_lost_gas_report.pdf

4. A. R. Brandt, et al. (2014), “Methane Leaks from North American Natural Gas Systems”, Science 343, 733 (14 February 2014)

5. Intergovernmental Panel on Climate Change (IPCC) (2007), IPCC Fourth Assessment Report: Climate Change 2007, Cambridge University Press, Cambridge UK.

6. J.J. Dooley, R.T. Dahowski, and C.L. Davidson, (2008), “Comparing Existing Pipeline Networks with the Potential Scale of Future U.S. CO2 Pipeline Networks, PNNL-17381 Pacific Northwest National Laboratory.

7. D.E. Cooper, and R.U. Martinelli, “Near-infrared diode lasers monitor molecular species,” Laser Focus World, (November 1992)

8. D.S. Bomse, “Diode Lasers: Finding Trace Gases in the Lab and the Plant,” Photonics Spectra, 29(6) (1995).

9. M.B. Frish, and F. Klein, “Trace Gas Monitors based on Tunable Diode Laser Technology: An Introduction and Description of Applications,” 5th International Symposium on Gas Analysis by Tunable Diode Lasers, Freiburg, Germany, VDI Berichte 1366 (1998).

10. M. Druy, M.B. Frish, and W.J. Kessler, W.J., “From Laboratory Technique to Process Gas Sensor - The Maturation of Tunable Diode Laser Absorption Spectroscopy,” Spectroscopy 21(3), 14-18 (March 2006).

11. M.B. Frish, R.T. Wainner, J. Stafford-Evans, B.D. Green, M.G. Allen, S. Chancey, J. Rutherford, G. Midgley, and P. Wehnert, “Standoff Sensing of Natural Gas Leaks: Evolution of the Remote Methane Leak Detector (RMLD),” Invited Paper in Conference on Lasers and Electro-optics/Quantum Electronics and Laser Science and Photonic Applications, Systems and Technologies 2005, Optical Society of America, Washington DC (2005).

12. J.W. Zimmerman, R.A. Locke II, C.S. Blakley, M.B. Frish, M.C. Laderer, and R.T. Wainner, “Initial testing of prototype tunable diode laser

absorption spectrometers for CO2 monitoring applications at the Illinois Basin - Decatur Project, USA”, 13th Annual Conference on Carbon

Capture Utilization & Sequestration, Pittsburgh PA (April 2014).

13. D.M. Sonnenfroh, E.W. Wetjen, M.G. Allen, C. Gmachl, F. Capasso, A.L. Hutchinson, D.L.Sivco, J.N. Baillargeon, and A.Y. Cho, "Mid-IR Gas Sensors Based on Quasi-CW Room-Temperature Quantum Cascade Lasers," Paper No. 2000-0641, AIAA 38th Aerospace Sciences Meeting, January (2000).

14. D.M. Sonnenfroh, M.B. Frish, R.T. Wainner, and M.G. Allen, “Mid-IR Quantum Cascade Laser Sensor for Tropospherically Important Trace Gases,” Final Report prepared for U.S. Environmental Protection Agency under Order No. 4C-R348-NASA, PSI-2857/TR-1971, November (2004).12-13

15. D. Picciaia, G. Zazzeri, M.S. Gimberini, and P. Andreussi, “A New Remote Sensing Method for

Landfill Emissions Quantification”, Proceedings Sardinia 2011, Thirteenth International Waste Management and Landfill Symposium, S. Margherita di Pula, Cagliari, Italy; 3-7 (October 2011).

16. M.B. Frish, “Overview of Sensitive Detection and Multiplexing Techniques for Tunable Diode Laser Absorption Spectroscopy,” in OSA Trends in Optics and Photonics Vol. 31, Advanced

Semiconductor Lasers and Their Applications, Leo Hollberg and Robert J. Lang, eds. Optical Society of America, Washington DC (2000).

17. M.B. Frish, and B.A. Cummings, “Laser Sensors for Gas Pipeline Explosion Protection,” Final Report prepared for the California Energy Commission under Grant No 57238A/11-01G, PSI-1802/TR-2926, November 2013.

18. M.B. Frish, M.C. Laderer, J.S.G. Stafford-Evans, J. Lewicki, and R. Locke, “Networked TDLAS Sensors for Sequestration MVA”, DoE Carbon Storage Program Infrastructure Annual Review Meeting, Pittsburgh, PA, PSI VG11-186. (November 2011).

19. M.B. Frish, and D.M. Sonnenfroh, “TDLAS Analyzers for Energy Production, Transmission, and Storage”, Field Applications in Industry and Research (FLAIR) 2011 Industry Session, Murnau, Germany, PSI VG11-154. (September, 2011).

20. J. Zimmerman, C.Blakley, R. Locke and M. Frish, “Final Report of TDLAS CO2 Sensor Field Testing at the Illinois Basin – Decatur Project”, Illinois State Geological Survey, Champaign, Illinois (February 20, 2014).

21. M.B. Frish, R. Shankar, I. Bulu, I. Frank, M.C. Laderer, R.T. Wainner, M.G. Allen, and M. Lončar, “Progress Toward Mid-IR Chip-Scale Integrated-Optic TDLAS Gas Sensors,” SPIE Photonics West, San Francisco, CA, 2-7 February 2013, Paper No. 8631-9 (January 2013).