Haris Riris1, Kenji Numata, Stewart Wu1, Stephan R. Kawa1, and James Abshire1
1NASA Goddard Space Flight Center, Greenbelt, Maryland
2Department of Astronomy, University of Maryland, College Park, Maryland
Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas, with approximately 25 times the radiative forcing of carbon dioxide (CO2) per molecule .åÊ CH4 also contributes to pollution in the lower atmosphere through chemical reactions leading to ozone production. Lack of understanding of the processes that control CH4 sources and sinks and its potential release from stored carbon reservoirs contributes significant uncertainty to our knowledge of the interaction between carbon cycle and climate change. The importance of this problem is clearly reflected in the Intergovernmental Panel on Climate Change report  and the National Research Council Decadal Survey for Earth Science . The increase in atmospheric concentrations of CH4 from their pre-industrial levels can largely be attributed to anthropogenic sources: industrial and fossil fuel production, rice farming, livestock, and landfills. Over recent decades, a major challenge has been to understand the variation of the global trends in CH4. The reasons for the observed changes and the implications for future changes in atmospheric CH4 are not well understood.
Natural sources of CH4 are dominated by wetland emissions in the tropics and sub-Arctic boreal regions, with additional contributions from termites, ocean biology, and a geological source of unknown significance. Natural sources account for about one-third of the emission total. The wetland source is particularly variable, linked to temperature, precipitation, and surface hydrological changes. Better characterization of the wetland sources requires reliable CH4 measurements in the often-cloudy tropics and over partially inundated land surfaces and open water. A related and equally important question is the potential release of large amounts of organic carbon, stored as CH4 and CO2, from thawing Arctic permafrost soils. This is a major cause for concern as a rapid, positive greenhouse gas/climate feedback. In addition, large but greatly uncertain amounts of CH4 are sequestered as gas hydrates in shallow oceans and permafrost soils that are also subject to potentially rapid release. Although these boreal, phase-change driven sources are not yet estimated to be large, their potential magnitude and rapid growth dictate that measurement systems need to be put in place for early detection. Because CH4 and CO2 emissions are often closely coupled in processes such as biomass burning and anaerobic respiration, coordinated measurement of both is vital.
At the Goddard Space Flight Center, we have been developing the technology needed to remotely measure CH4 from orbit using lasers. Over the past several years, we developed a strong capability using different lasers, detectors and spectroscopic detection techniques for the remote measurements of trace gases in absorption cells, in open paths, and from airborne platforms. We have already demonstrated CO2 and O2 detection from airborne platforms [3, 4] using Erbium fiber technologies. Our concept for a CH4 lidar is a nadir-viewing instrument that uses the strong laser echoes from the Earth’s surface to measure CH4 (Figure 1). The instrument has a tunable, narrow-frequency light source and photon-sensitive detector to make continuous measurements from orbit in sunlight and darkness at all latitudes. It can be relatively immune to errors introduced by scattering from clouds and aerosols.
A laser transmitter consisting of a seed and a pump laser and an optical parametric generator (OPG) generates tunable laser radiation at 1.65 åµm. Our measurement technique uses integrated path differential absorption (IPDA), which measures the absorption of laser pulses by a trace gas when tuned to a wavelength coincident with an absorption line . Using the instrument in a sounding (i.e., surface reflection) mode enables integrated trace gas measurements from orbit with modest laser power. It also allows us to minimize bias errors caused by atmospheric scattering by isolating the surface echo with a time gate. It substantially improves the receiver’s signal-to-noise ratio (SNR) by reducing the amount of noise from the detector and the solar background.
We have already demonstrated ground-based and airborne CH4 detection using optical parametric amplifiers (OPA) [5, 6]. We are currently funded by NASA’s Earth Science Technology Office to advance the technology readiness level TRL of the CH4 laser transmitter. Our current laser transmitter uses an OPA to generate near infrared laser radiation at 1651 nm, coincident with a CH4 absorption. In an OPA, a photon from a high-power pump laser pulse is overlapped with a photon from a low-power seed laser in a nonlinear crystal and converted into two photons: signal and idler. The signal beam is the same wavelength as the seed laser, but the output power is significantly higher. The wavelength of the signal is rapidly tuned over the CH4 absorption by tuning the seed laser to sample the CH4 absorption line at several wavelengths (Figure 2).
The OPA is well suited for this application since it offers a large wavelength tuning range and high pulse energy and generates near infrared laser radiation in a wavelength region that is not readily covered by other laser sources. We have been able to improve our OPA and generate more than 170 åµJ/pulse at 1651 nm. However, when the seed and pump energies are grossly mismatched, as is the case in our OPA, it is difficult to amplify the seed with the desired spectral characteristics (e.g., narrow linewidth). Even though we were successful in scaling the energy to 170 åµJ, our current OPA linewidth is 10 GHz, too wide for accurate CH4 measurements. We are currently trying to amplify the seed laser power and reduce the OPA linewidth.
In addition to OPAs, we have investigated the use of optical parametric oscillators (OPO).åÊ An OPO is similar to an OPA but uses an optical resonator cavity to enhance the energy of the non-linear conversion and reduce the linewidth of the signal (Figure 3).
The OPA approach is simpler than the OPO, but it is more difficult to obtain narrow linewidth emission. OPOs, on the other hand, are difficult to align and tune. We have been successful in demonstrating a two-wavelength, high power, narrow linewidth OPO. With our OPO, we were able to obtain 210 åµJ/pulse at 5 KHz with a narrow linewidth . However, the use of only two wavelengths for a trace gas lidar is not optimum. Multiple wavelengths can adequately sample instrumental and systematic errors that may be undersampled with the two wavelength approach. They may also allow the retrieval of additional spectroscopic parameters such as the pressure shift and spectral linewidth. For these reasons, we would like to add several more wavelengths to the OPO approach. We are in the process of upgrading our OPO system to accommodate four seed lasers. We expect that these upgrades will enable us to demonstrate CH4 measurements with a 1 percent precision (10-20 ppb).
Intergovernmental Panel on Climate Change Report (IPCC), 2007, available at: http://www.ipcc.ch/index.htm
National Research Council Decadal Survey: Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, National Academic Press, 2007.
J.B. Abshire, J., Riris, H., Allan, G. R., et.al., Pulsed airborne lidar measurements of atmospheric CO2 column absorption, Tellus (2010), 62B, 770ÛÒ783.
H. Riris, Michael Rodriguez, Graham R. Allan, William Hasselbrack, Jianping Mao, Mark Stephen, and James Abshire, Applied Optics, Vol. 52, Issue 25, pp. 6369-6382 (2013).
K. Numata, H. Riris, S. Li, S. Wu, S. R. Kawa, M. Krainak, J. Abshire Ground demonstration of trace gas lidar based on optical parametric amplifier, Journal of Applied Remote Sensing 063561-1 Vol. 6, (2012).
H. Riris, K. Numata, S. Li, S. Wu, A. Ramanathan, M. Dawsey, J. Mao, R. Kawa, and J. B. Abshire, Airborne measurements of atmospheric methane column abundance using a pulsed integrated-path differential absorption lidarÛ, Applied Optics , Vol. 51, No. 34, (2012).
Kenji Numata, Stewart Wu, Haris Riris, Fast-switching methane lidar transmitter based on a seeded optical parametric oscillator, submitted to Applied Physics B.