By Florian M. Schwandner, Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA, USA
NASA’s Orbiting Carbon Observatory 2 (OCO-2) was finally successfully launched on 2 July 2014 from Vandenberg Air Force base in California. This event marks not only the successful completion of unfinished business after the tragic January 2009 loss of the first OCO (the fairing protecting the observatory atop the launch vehicle didn’t separate). It also marks the culmination of a success story of more than 5 years of international collaboration in dedicated satellite-based atmospheric carbon research.
One month prior to the 2009 launch, the Japan Aerospace Exploration Agency (JAXA) and its partners launched the Greenhouse gases Observing SATellite, GOSAT (nicknamed “Ibuki”), which measures carbon dioxide, methane, oxygen, and water vapor. Following the loss of the first OCO, the Japanese GOSAT Project Team invited the OCO Team to collaborate on a variety of tasks, including instrument calibration, algorithm development, and data product validation, leading to extremely important lessons learned.
Among the important results from this unique opportunity was a much more mature ground-based vicarious calibration approach  and a far more capable algorithm for retrieving total column mixing ratios of carbon dioxide and methane. Both are essential to meet the daunting 0.25% accuracy requirements for OCO-2 [2, 3]. Others include advances in treatment of aerosol retrievals , the retrieval of solar-induced chlorophyll fluorescence as an indicator of net and gross biospheric fluxes [5, 6], advanced quality filtering routines using artificial intelligence and machine learning , fine-tuned point-source target mode strategies , and sophisticated regional and global inversion and transport models [9,10, 11]. Larger scale sources and sinks like oceans, terrestrial biosphere, and megacities have seen detection and refinement thresholds superseded, with very respectable science results. However, attempts to quantify and detect single deep carbon point source emission targets such as volcanoes and natural gas seeps have remained somewhat elusive to date. The lessons learned from the openness and collaboration within the international carbon climate science community may serve as a luminary example of how the Deep Carbon Observatory research community might benefit from the unique opportunities offered by these two excellent carbon observing satellite systems now in orbit: GOSAT and OCO-2.
We now live in a 400ppmv CO2 world, and governments at various levels (international, national, state/province, and cities) are discussing and implementing measures to cope and counteract the increasing concentration of atmospheric carbon dioxide. Policies to enact emission regulations, mitigation, and adaptation require highly complex global models of emission, dispersion, and sequestration. To quantify sources and sinks, complex inversion modeling necessitates coherent databases of emission variabilities in space and time (emission inventories). The complexity of the models mirrors the complexity of interplay of natural and anthropogenic factors. In the ecosystem and anthropogenic carbon flux world, global databases and emission inventories exist which permit inclusion into these complex models. After all, a single estimate of global fluxes of one species from one type of source (e.g., volcanoes) by itself doesn’t serve any specific purpose beyond inclusion in a textbook diagram. The volcanic gas community has recently made great progress in our understanding of global volcanic CO2 fluxes , which is an essential starting point toward a more complex emissions inventory. Complex and accessible databases and emission inventories open to the community will ensure a lasting impact and bridge between the deep carbon and carbon climate science communities.
Both OCO-2 and GOSAT offer unique opportunities for deep carbon research, especially in the field of CO2 emissions. Both are polar orbiters and their instruments observe the same weak and strong SWIR (short-wave infrared) CO2 bands, and the O2 A-band. They complement each other in their measurement approach: (a) GOSAT has a fast 3-day repeat cycle with agile pointing capabilities, but provides no chemical spatial context. It is excellent to precisely target small point sources with its single-shot, 4-second exposures covering a circular field of view of 10km diameter. However, no continuous quasi-mapping capability exists. (b) OCO-2 is not principally a target-mode mission: It collects data continuously at 24 samples per second along a narrow (< 10.6 km wide) path either along the orbit track, or in the direction of the “glint spot,” where sunlight is specularly reflected from the surface. Its strength is a “mapping” approach with 8 pixels across-track, and continuous sampling along-track, each footprint having a ground resolution of about 0.1 to 1.3 km by 2.25km, providing some spatial context. OCO-2 has limited targeting capabilities because the spacecraft bus points the instrument and takes 10 to 20 minutes (20 to 40% of a day side orbit) to acquire a target, while GOSAT has an agile pointing mirror system. OCO-2 target observations are principally used for observing ground-based validation and calibration targets. Consequently, direct overflight of the nadir or glint track over deep carbon sources such as seeps and volcanoes is unlikely. Close incidental flybys within ~20km of a source might intercept a plume and might detect a CO2 anomaly. CO2 concentration in plumes is known to dissipate exponentially, and each nominal OCO-2 pixel requires an excess total column loading of ~ 42t of CO2, for an anomaly to be detectable at a 1ppmv pixel average contrast (enhancement) . This threshold may be slightly more favorable under certain conditions, like a high surface albedo, higher ground altitude, lower wind speeds, and the smaller OCO-2 footprints at sub-solar latitudes. In addition, the spatial context along-track may provide the contrast required to validate a suspected anomaly. Optically thick cloud and aerosols (including eruptive volcanic plume aerosols) will interrupt continuous measurements, but we expect to have the ability to retrieve at least 6% of all soundings.
With OCO-2 in orbit, opportunities abound to develop algorithms and databases, and to maximize the use and exposure of distinct research products (such as ground-based measurements of CO2 flux) for both the Deep Carbon and climate science communities. Building stronger bridges between these two fields can only lead to a win-win situation. Two great examples of already ongoing efforts are the Deep Carbon Observatory's DECADE (Deep Carbon Degassing) initiative of volcanic emission data, and the efforts at the Jet Propulsion Laboratory and its partners to improve detection of volcanic CO2 emissions from space. First, the DECADE database initiative (Brendan McCormick, Smithsonian Institution) and other DCO-funded database ventures will be instrumental in building two-way data-sharing bridges between the Deep Carbon and carbon climate science communities. Second, in 2009 we began to develop techniques to observe volcanic CO2 emissions from space using GOSAT, together with our Japanese colleagues. Since 2010 we have conducted these observations in target-mode, at up to 40 volcanic targets per repeat cycle (PIs F. Schwandner, JPL, and S. Carn, MTU). A team of DCO scientists from the Smithsonian Institution’s Global Volcanism Program joined this effort in early 2013 (PI Christoph Popp).
“We're really excited about our collaboration with scientists at JPL and the ever-increasing prospects for satellite monitoring of global carbon emissions from point sources such as volcanoes,” said DCO’s Christoph Popp. “We've begun the process of analyzing data from GOSAT, and look forward to exploiting the increased capabilities of the OCO-2 instrument for the remote sensing of volcanic degassing.”
What does the future hold? GOSAT is still working well, despite the unfortunate failure of one of the two solar paddles in late May 2014, and has sufficient fuel for at least another 4 years of operation. GOSAT-2 is in the planning stage, with a similar system, and will measure CO as well as CO2 and CH4. OCO-2 has a nominal mission lifetime of 2 years, but fuel and funds permitting, might see an extension. Planning has started to install the OCO-2 flight spare instrument on the International Space Station (ISS) as the OCO-3 mission. Like GOSAT, OCO-3 employs an agile pointing system, thus permitting “city mode,” which maps an area on the order of 60x60km. This observing mode, combined with the low-inclination orbit of the ISS, which precesses in local time, is ideal for extended point sources such as cities, volcanoes, power plants etc., whose emissions vary in complex temporal multimodal cycles. At the European Space Agency, a mission proposal under consideration for the Earth Explorer 8 opportunity (CarbonSat) provides the next major step forward. It will allow mapping of carbon dioxide and methane over the entire globe at high resolution (2 km by 3 km) every two weeks. As mission development and life cycles develop, the hope is that the various carbon communities will build further bridges and databases and open up to enhanced data sharing. NASA and the GOSAT operating agencies share their data products freely, and research progress will benefit greatly from global deep carbon information involving database efforts such as DECADE and WOVOdat (The World Organization of Volcano Observatories (WOVO): Database of Volcanic Unrest).
"While the DECADE database is still under construction, it will ultimately integrate space- and ground-based measurements of volcanic volatile emissions, including carbon,” said DCO’s Brendan McCormick. “Our aim is for the database to become a valuable tool for scientists from multiple disciplines, not just deep carbon science."
What is OCO-2 up to today, in orbit? The various spacecraft functions are being checked out (including the X-Band transmission system, attitude control, on-board calibrators and propulsion system) before finally ascending into its final orbit at the leading position of the “A-train” satellite constellation, planned for early August. At that time, the instrument will be cooled down to operating temperatures, and we will conduct “first light” observations, followed by first in-flight calibrations. Soon after, (~Fall 2014) first calibrated spectra will be made available. The first Level 2 data (total column mixing ratios of CO2) will be delivered for distribution to the science community and public in early Spring 2015.
Published on the web 21 July 2014.
Government funding by NASA’s OCO-2 project at the Jet Propulsion Laboratory, California Institute of Technology, and a thorough review by David Crisp (JPL) are gratefully acknowledged.
About the author:
Florian Schwandner is an analytical volcanic gas geochemist by training, and is currently an OCO-2 Science Team Member involved in volcano surveillance and carbon climate science as part of the Atmospheric Observations Group at NASA Jet Propulsion Laboratory. He is also a member of DCO’s DECADE initiative, a project underway in the Reservoirs and Fluxes Community. His expertise lies in applied ground- and satellite-based geochemical monitoring techniques in extreme and remote environments. He also develops autonomous real-time networks and data management systems, including designing and building his own sensor instrumentation solutions.
JPL/NASA (Top Image: OCO Spacefraft. Bottom Image: A-Train Constellation with Details Artist depiction of A-train constellation with time they are separated when they fly. Source)
Karen Yuen (Portrait of Florian Schwandner)
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© 2014 California Institute of Technology. Government sponsorship acknowledged.