Greenhouse gases (GHG) have the potential to cause catastrophic climate changes, and the atmospheric concentrations of the gases, mainly CO2, CH4, and N2O, are increasing over time. However, the natural “sinks” that remove or absorb these natural and man-made gases are not understood.
For example, CO2 is being absorbed by oceans and land areas much faster than models predict, and no one knows whether the current absorption rate will be sustained in the future.
Mike Dobbs of ITT, instrument co-principal investigator for the ASCENDS project, pours liquid nitrogen that will cool a laser detector during one of the test flights. Photo courtesy Sean Smith, NASA.
As industrialized nations attempt to grapple with this uncertain situation, a need has arisen for monitoring GHG concentrations and emission rates globally. The emergence of cap-and-trade schemes has only heightened the need for verifiable monitoring techniques.
Monitoring GHG emission rates is difficult for several reasons. The gases have a long lifetime in the atmosphere; their characteristic spectra are in the infrared; and they are emitted on spatial scales ranging from point sources such as smokestacks all the way to regional areas.
Their long lifetimes imply that measurements of emissions have to be done in the presence of large background signals.
For example, the global average CO2 concentration is about 390 parts per million by volume (ppmv) but the maximum variations from one location to another are only about 10% of the average value, or about 40 ppmv. In order to accurately monitor the variability of CO2, the measurement accuracy must be about ±1 ppmv, which is only about 0.25 % of the total. That’s very demanding for a remotely sensed atmospheric measurement.
Choosing the right technology
All greenhouse gases absorb IR light; this is the phenomenon responsible for the greenhouse effect, and so sensors for GHG tend to exploit their IR absorption spectra.
NOAA’s measurements of CO2 in the atmosphere at an altitude of 3400 meters. The red line represents the monthly mean values of CO2. The black line represents the same, after correction for the average seasonal cycle. The 2010 data was preliminary as of press time.
Unfortunately, the technologies of both IR laser sources and detectors provide poor performance and fewer options than the ultraviolet and visible regions. Off-the-shelf solutions will not provide the required accuracy. For that reason, active IR remote sensing for GHG requires pioneering technology development.
The need to monitor GHG emissions on a huge range of spatial scales, from point sources to regions spanning hundreds of kilometers, presents another problem. The solution is a suite of techniques including point sensors, ground-based open-path and remote sensors, and spaceborne remote sensors.
The international lidar community is rising to these challenges. Lidar, or laser radar, is often used to provide range-resolved profiles of trace gases such as ozone. However, due to the challenges outlined above, a variation on the technique known as integrated path differential absorption (IPDA) is most often proposed for GHG.
Photo courtesy NASA-Goddard Space Flight Center Conceptual Image Lab
IPDA uses a pulsed-laser transmitter but it relies on a strong backscattered signal from terrain or water (rather than atmospheric aerosols), and it only provides one data point: the number of trace gas molecules in a column from the lidar to the terrain. To get the mixing ratio, a measurement of atmospheric density is also required, and so a corresponding measurement of another gas, usually O2, is required.
Combination of platforms
Remote-sensing systems often go through evolutionary phases, starting out as ground-based (usually in mobile vans), later becoming airborne, and finally spaceborne. All three of these measurement platforms have applications in GHG monitoring.
Ground-based GHG remote sensors are useful for monitoring emissions from point sources or the concentrations in downwind plumes from area sources, which may be as much as 1 kilometer wide. Airborne nadir-viewing sensors can be flown across such plumes to infer emission rates, if the wind speed can be estimated or measured. This technique has been previously demonstrated by NOAA for ozone plumes, using an ozone lidar and a wind-sensing lidar aboard the same aircraft.
In its most recent decadal survey for earth science, the U.S. National Research Council recommended a future space mission called ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons), with the goal of monitoring unbiased global atmospheric column CO2 amounts (See Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, The National Academies Press, 2007).
Scientists have recorded increasing atmospheric CO2 off Hawaii since 1958. The black curve represents seasonally corrected data.
Graphics courtesy of Pieter Tans, NOAA/ESRL.
The NASA centers are preparing for this planned mission with several lidar development efforts. For example, the Goddard Space Flight Center has developed an airborne IPDA lidar that measures the shape of a CO2 absorption line near 1.57 microns at up to 20 discrete wavelengths. The main reason for resolving the line shape is that different parts of it are sensitive to different altitudes in the atmosphere.
That lidar was flown nine times in 2009 at altitudes of 3-13 km, and an extrapolation of the technology’s performance showed that a future space-based sensor with the desired accuracy appears to be feasible.
The Langley Research Center has developed and flown a different technology known as the multi-functional fiber laser lidar (MFLL), which measures at the center, on one side, and off of the CO2 line. MFLL was flown in eight campaigns in 2009 at altitudes of 2–8 km and achieved a typical accuracy of about ±0.6 % in CO2 concentration.
Goddard researchers are investigating a broadband approach. Using this method, the CO2 spectrum appears as green. The Fabry-Perot instrument response is in brown. Graphic courtesy of Bill Heaps, NASA.
In addition to these sensors using precisely tuned lasers, other researchers at Goddard are investigating a novel broadband approach, in which a Fabry-Perot spectrometer is tuned to match the wavelength of a whole group of CO2 absorption lines (See above and "Goddard Program for Measurement of Carbon Dioxide Using a Broadband Lidar."). The main potential advantage of this idea is that one broadband laser replaces a suite of lasers with precisely controlled wavelengths. A big disadvantage is that most of the laser photons are not useful for the measurement, and those photons are expensive, especially in space.
The European remote sensing community is also pursuing lidar technology for greenhouse gases. Investigators at DLR, the German Aerospace Center, recently performed a sensitivity analysis for spaceborne measurements [G. Ehret et al., Applied Physics B 90, 593-608 (2008)], predicting that ±0.4 – 0.6% accuracy could be obtained for total column amounts of all three GHGs with an IPDA lidar satellite.
In addition, DLR has developed an airborne lidar known as CHARM-F (CO2 and CH4 Atmospheric Remote Monitoring–Flugzeug), and a joint German-French satellite mission known as MERLIN (Methane Remote Sensing Lidar Mission) is scheduled for launch in 2014. The Merlin investigators point out that the greenhouse effect of CH4 is 25 times that of CO2 and that CH4 concentrations in the atmosphere have more than doubled since the start of the industrial revolution (CO2 has increased by about 30%).
The European Space Agency is considering a future mission called A-SCOPE (Advanced Space Carbon and Climate Observation of Planet Earth) to improve our understanding of the global carbon cycle and regional carbon dioxide fluxes. Like the NASA systems, A-SCOPE will measure total column carbon dioxide with a nadir-looking IPDA lidar. Work on critical scientific and technology underpinnings for A-SCOPE is ongoing, and European lidar researchers intend to propose it for a future Earth Explorer mission.
Researchers in Japan are also developing lidar technology for monitoring CO2, including a multi-function system operating near 1.57 microns to measure atmospheric pressure, temperature, and wind speed, as well as a 2-micron lidar for both CO2 and winds.
–SPIE Fellow Gary Gimmestad is the Glen Robinson Chair in Electro-Optics in the Electro-Optical Systems Laboratory at Georgia Tech Research Institute (USA) where he leads the lidar team.
Where is the CO2?
While scientists can accurately measure the amount of carbon dioxide in the atmosphere, much about the processes that govern its atmospheric concentration, where it comes from, and where it goes remains a mystery.
Learning more about the magnitudes and distributions of carbon dioxide’s sources and the places it is absorbed (sinks) will help improve critical forecasts of increases in atmospheric CO2 for evaluating options for mitigating or adapting to climate change.
The Orbiting Carbon Observatory (OCO) was a NASA mission designed to make precise, time-dependent global measurements of atmospheric CO2 from an Earth-orbiting satellite. On 24 February 2009, OCO failed to reach orbit, however. NASA now hopes to launch its replacement, OCO-2, in February 2013.
See a recent New York Times article about the history of CO2 measurements from Mauna Loa Observatory and the scientist, Charles David Keeling, who sounded the first alarm about the consequences of increased carbon dioxide in the atmosphere.
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