1.

INTRODUCTION

Methane (CH4) is the second most important anthropogenic greenhouse gas (GHG) with a higher radiative forcing potential than Carbon Dioxide (CO2) on a per molecule basis¹, making anthropogenic CH4 a critical target for mitigation. The current CH4 global mixing ratio is 1852 parts per billion (ppb)^{2, 3}. Anthropogenic CH4 is responsible for a significant portion of the global warming produced by all well-mixed greenhouse gases and contributes to the formation of ozone⁴, another GHG and air pollutant.

The existing CH4 observing network has proven inadequate to constrain global, regional, and sectoral sources, and explain observed trends and variation in atmospheric CH4 over the last few decades. Therefore, there is a critical need for CH4 observations for constraining the strength and distribution of methane’s sources, including natural (e.g., wetlands) and anthropogenic (e.g., energy sector) ones. Much of the year-to-year variations in methane’s global growth rate are likely from variations in wetland emissions and part of the recent increasing trend in methane’s growth rate may be associated with increased energy extraction activities^{5, 6}. An adequate CH4 observing network is necessary to monitor the interaction between the carbon cycle and climate change, such as the potential release of CH4 from stored carbon reservoirs (e.g., Arctic and boreal soils) and changes in natural emissions. The current CH4 observing network does not provide the necessary data to understand and constrain methane’s sources, such as from permafrost thaw, wetlands, which challenges our ability to make confident projections of future climate. The importance of measuring CH4 is also reflected in the recent Earth Science Decadal Survey⁷ and the recent report by the Carbon Climate Workshop⁸.

Our current understanding of CH4 distributions and processes is founded mostly on precise and accurate ground-based, in-situ measurements from global monitoring networks^{9, 10}. The location and frequency of these measurements is, however, very sparse on a global scale and is even sparser at high latitudes where the thawing Arctic permafrost is of particular concern. Large quantities of organic carbon are stored in the Arctic permafrost and a warming climate can induce drastic changes in carbon emissions and a subsequent positive feedback mechanism that can significantly accelerate climate change¹¹.

Global measurements from satellites are available from passive optical sensors AIRS¹², SCIAMACHY¹³, ¹⁴, TES¹⁵, IASI¹⁶, and GOSAT¹⁷, but currently lack the required sensitivity to derive regional CH4 sources. Passive sensors measuring reflected sunlight are limited to sunlit areas of the planet and their sensitivity falls off at low sun angles, increasing cloud cover, aerosol scattering, and low surface reflectivity. Recent observations indicate that the thawing Arctic permafrost is active even during the cold season18 highlighting the need for continuous sampling at high latitudes even in the winter months.

These measurements need to be able to resolve CH4 concentration uncertainties over global and regional scales, at all latitudes throughout the year to address important science issues, such as how the large terrestrial carbon reservoir will respond in a warming world (e.g., permafrost thaw), the need to constrain large emission variations from tropical wetlands, and the impact of energy extraction activities on methane’s recent global growth.

Active (laser) remote sensing technology will be a key step in obtaining CH4 measurements to supplement existing observations from passive satellite instruments and the sparse surface monitoring network. Active measurements will enable accurate CH4 observations at high latitudes, during all seasons and at night, over land and water surfaces, and in the presence of scattered or optically thin clouds and aerosols, and with higher coverage than passive instruments. Together, data from an observational suite of active, passive and in situ instruments will provide sufficient coverage, sampling, and precision to constrain emissions on global and regional scales and potentially some sectoral CH4 emission sources (e.g., oil and natural gas exploration in the central U.S.).

The only active trace gas mission currently in development is the Franco-German MEthane Remote sensing LIdar missioN (MERLIN) by the French Centre National d’Etudes Spatiales (CNES) in collaboration with the German Aerospace Centre (DLR) scheduled for launch in 2021^{19, 20}. The MERLIN mission targets an 8-36 ppbv relative random error in the methane column abundance with a 50 km horizontal resolution.

At NASA Goddard Space Flight Center (GSFC), we have also been developing an active, airborne lidar to measure atmospheric methane using Integrated Path Differential Absorption (IPDA) as a precursor to a space mission to measure CH4 from orbit.

2.

GSFC MEASUREMENT APPROACH

An IPDA lidar measures the absorption of laser pulses by a trace gas when tuned to a wavelength coincident with an absorption line^21-31. Using the instrument in a sounding (surface reflection) mode which enables integrated column trace gas measurements from orbit with relatively modest laser power. Although in principle, only two wavelengths (“on” and “off” the absorption line) are needed to determine the transmittance through the atmospheric column, our technique uses multiple wavelengths to probe the absorption feature.

CH4 has absorption lines in the near infrared spectral region at 3.3, 2.4 and 1.65 μm. The lines at 1.65 μm are best suited for active remote sensing from space because they have the right linestrengths and there is very little interference from other molecules. Unfortunately, there are no commercially available lasers in this spectral region.

There are two primary candidate lines can be used for CH4 monitoring: One at 1650.96 nm and one at 1645.55 nm. The GSFC IPDA lidar uses a tunable, narrow-linewidth light source and a photon-sensitive detector coincident with the CH4 absorption at 1650.96 nm. The CH4 line is mostly isolated from adjacent CO2 lines and there is very little water (H2O) vapor interference. The MELRIN line at 1645.55 nm is not as well less suited to our technique because it is interfered with by H2O vapor at ~1645.47 nm and it is wider than our line (~56 pm vs. ~36 pm), an important consideration because it increases the tuning requirement for our laser transmitter. Fig. 1 shows the two-way atmospheric transmittance spectrum for the two lines from a 400 km orbit using the 2008 HITRAN database³² and a US standard atmosphere.

Figure 1

Atmospheric transmittance at 1645.5 nm (left) and 1650.9 nm (right) wavelengths using a US Standard Atmosphere and the HITRAN 2008 database.

The GSFC lidar (Figure 2) uses multiple wavelengths to probe the CH4 absorption feature. Our IPDA approach has been validated experimentally over several years in multiple airborne campaigns and extensive ground tests with our CO2, O2, and CH4 IPDA lidars. Using multiple wavelengths can reduce errors that may affect the measurement precision³³, measure the spectral shift of the line with changing atmospheric pressure³⁴, generate atmospheric backscatter profiles of the entire column³⁵, and enable retrievals of trace gas mixing ratios above and below the planetary boundary layer³⁶.

Figure 2.

(Left) An Integrated Path Differential Absorption (IPDA) lidar measures the absorption of laser pulses by a trace gas at multiple wavelengths. (Right) Simplified functional block diagram of the CH4 IPDA lidar.

In late 2015, we flew the CH4 lidar instrument on NASA’s DC-8 airborne laboratory. The airborne IPDA lidar used two different laser transmitters. The first was an Optical Parametric Amplifier (OPA)^{37, 38} and the second was an Optical Parametric Oscillator (OPO)³⁹. Only one laser transmitter was used at a time by using a movable mirror to select the desired transmitter.

The OPA, used 20 wavelengths, but was simpler to implement than the OPO, because it did not require an optical resonator cavity, was easier to align and tune, and used only two seed lasers. The OPO used 5 wavelengths and required a separate seed laser for each wavelength. Table 1 summarizes the major parameters of the 2015 airborne IPDA lidar and Figure 3 shows the instrument in the DC-8 prior to flight. Both the OPO and OPA used a master seed laser locked on the CH4 absorption peak at ~ 1650.96 nm. The locking technique is the same for both the OPA and OPO, and is described by Numata⁴⁰. It is based on the technique used by Pound–Drever–Hall⁴¹ and is similar to the technique used by Fix⁴². The detector is an e-APD that has flown in multiple airborne missions^43-45.

Table 1

2015 CH4 Airborne Lidar

Figure 3

The 2015 GSFC CH4 IPDA Lidar in the DC-8 prior to flight.

Our retrieval algorithm used a least squares fit to minimize the root mean squared error between the IPDA lidar measurements and the model prediction and is similar to the approach used by Abshire et.al.⁴⁶ in their CO2 retrievals. The remaining design details of our instrument and results of our airborne demonstration were summarized in a recent publication.⁴⁷

3.

LASER TECHNOLOGY

Most of the key technologies for a space CH4 lidar have a long heritage, and most have been demonstrated in previous space missions. Telescopes of various sizes (1.0 and 0.7 m diameter) for lidar missions have flown on the Ice, Cloud, and land Elevation Satellite (ICESat-I and soon ICESat-II), Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (Calipso), and Cloud-Aerosol Transport System (CATS). The command & control and data acquisition electronics will be similar to other lidar missions. The only remaining low technology readiness level component of the instrument is the laser transmitter.

The laser transmitter must have high energy, narrow linewidth and must be rapidly tunable over the absorption line. Depending on the receiver size and other instrument parameters we calculate that approximately 600 μJ is needed to obtain a measurement with a 0.5% precision from space.

The primary means to generate 1.65-μm laser radiation has been to use non-linear optical parametric techniques: optical parametric amplifiers (OPA) or oscillators (OPO)⁴⁸. MERLIN uses a two-wavelength OPO. Our airborne OPA, which used 20 wavelengths, produced better fits, was simpler to implement than the OPO because it did not require an optical resonator cavity, and was easier to align and tune. However, it is extremely difficult to scale the OPA energy to that needed for space (~ 600 μJ depending on the receiver size and other instrument parameters) and maintain a narrow linewidth. The highest energy we obtained in the laboratory with our OPA was 290 μJ using a two-stage OPA, and the burst-mode Yb fiber laser amplified by a custom solid-state amplifier as a pump. However, at high energies, the OPA output spectrum typically consists of sharp peak near the seed wavelength and a broad side lobe, when the parametric gain is high. In that case, we cannot clearly define the linewidth but it is generally too wide for accurate CH4 IPDA lidar measurements. In addition, for a space mission we are aiming for a simple and efficient single stage - not a complex multi-stage - OPA based on quasi-phase matching (QPM). In this configuration, we have observed that the OPA output linewidth does not fully converge to the seed linewidth, giving wide side lobes, especially when pump and seed fluences are high and low, respectively⁴⁹. Back-conversion and parametric amplification of the seed’s side lobes are possible causes. If the seed laser power can be significantly scaled up then it may be possible to achieve energies of 600 μJ out of the OPA with a narrow linewidth. With the existing seed and pump laser technology, we do not see a path to space for the OPA in the near future. However, it remains a viable transmitter for CH4 measurements from an airborne platform.

In the OPO, narrow linewidth was achieved by using an optical resonator cavity, which also enhances the energy of the non-linear conversion. Our 5-wavelength OPO used a 1.2 mJ GSFC-built solid-state pump laser and a triangular optical ring cavity. We have since replaced the GSFC-built solid-state pump laser with a smaller, compact Yb fiber laser and redesigned the OPO cavity to improve stability (Figure 4).

Figure 4

Our airborne 5-wavelength OPO used a 1.2 mJ GSFC-built solid-state pump laser and a triangular optical ring cavity. We have since replaced the GSFC-built solid-state pump laser with a smaller, compact Yb fiber laser and used a single DBR seed laser that step-tunes over the CH4 absorption line.

We also replaced the five seed diode lasers with a single custom-made DBR laser that is stepped tuned over the absorption line. We now use sixteen wavelengths to tune over the CH4 absorption line. That simplified the OPO design considerably. In the laboratory we demonstrated energies of ~250 μJ at 5 kHz with a narrow (transform limited) linewidth. The multi-wavelength OPO energy could be scaled to space and remains our baseline approach. However, it still requires optical phase-lock loop and cavity control.

In recent years, resonantly pumped Erbium (Er) doped YAG, Er:YAG and Er:YGG, lasers, which directly emit at 1645.5 and 1650.96 nm, respectively offer another option for a CH4 transmitter. Using Er:YAG for CH4 detection dates back to 1972⁵⁰. In recent years, successful demonstrations and commercialization of high power and high spectral brightness pump sources at 1470 nm and 1532 nm, driven in large part by the various industries including defense, telecommunications and medical, have afforded the realization of resonant pumping of of Er:YAG ^51-54 and Er:YGG ^{55, 56} lasers. The emission cross-section of Er:YAG crystal is centered near 1645.3 nm and falls off rapidly at 1650.96 nm. It is near the MERLIN lines at 1645.55 nm which are relatively wide (~56 pm). Our CH4 line at 1650.96 nm is narrower (~36 pm) which makes fast tuning easier. Unfortunately, Er:YAG cannot be used as a gain medium at 1650.96 nm but Er:YGG can be used as a potential medium for lasing at that wavelength. Our preliminary radiative transfer calculations show that both lines (Er:YAG at 1645.55 nm and Er:YGG at 1650.96 nm) have similar temperature sensitivity and are well suited for space born CH4 measurements. Recent high accuracy spectroscopic measurements indicate that line mixing effects in the Er:YAG 1645.55 nm line⁵⁷ should also be taken into account. We expect similar effects to be present for the Er:YGG line at 1650.96 nm.

Both materials are good candidates for a space CH4 laser transmitter but Er:YAG is more readily available. Power scaling for both materials, and most importantly multi-wavelength operation and tuning considerations remain. For both materials, the gain peak is slightly off the CH4 absorptions lines requiring the use of a tuning element, such as an etalon, in the cavity. However, for a multi-wavelength IPDA that introduces yet another element that needs to be tuned and controlled at high precision.

SUMMARY

Anthropogenic CH4 is responsible for a significant portion of the global warming produced by all well-mixed greenhouse gases. Despite the critical importance of CH4 for climate, the existing CH4 observing network has proven inadequate to constrain global, regional, and sectoral sources, and explain observed trends and variation in atmospheric CH4 over the last few decades. Part of the increasing trend in methane’s growth rate over the last decade may be associated with anthropogenic sources, including increased energy extraction activities. Identifying methane “super emitters” and quantifying regional-scale emissions from specific industry sectors (e.g., domestic natural gas production and storage) which are projected to increase significantly in the next few decades will require substantial investments in CH4 monitoring technology. At GSFC, we developed a multi-wavelength IPDA lidar and demonstrated CH4 column measurements using an optical parametric oscillator (OPO) and optical parametric amplifier (OPA) laser transmitter and sensitive avalanche photodiode detector. The lidar measured the atmospheric CH4 absorption at multiple, discrete wavelengths near 1650.96 nm. The stability and reliability of the laser transmitters need to improve considerably but the basic measurement approach has been demonstrated. However, many technical challenges remain to scale the space.

Generally, an OPA is simpler to implement than an OPO because it does not require an optical resonator cavity and is easier to align and tune. However, it is extremely difficult to scale the power of the OPA and maintain a narrow linewidth. After extensive testing we have concluded that it is not feasible to scale the OPA laser energy to the level needed for space without significant improvements considerable investments in the seed and pump laser.

In an OPO, a narrow linewidth is achieved by using an optical resonator cavity, which also enhances the energy of the non-linear conversion. Our new multi-wavelength OPO is scalable to space, but still requires phase-locking loops and cavity tuning. The OPO will remain the baseline option until solid-state approaches (Er:YAG/Er:YGG) can be convincingly demonstrated.

It would seem that the simplest and most promising technology at this wavelength range is Er:YAG/Er:YGG. The lasing occurs near (but unfortunately not exactly) at the CH4 absorptions 1645 and 1651 nm. Power scaling has already been demonstrated with the desired spectral characteristics but without fast tuning and only at one or two wavelengths. The Er:YAG/Er:YGG still require a tuning element in the cavity to lase at the right wavelength. If the tuning problem can be solved this technology may be the most promising.