Devil in the Details: NASA Satellite Joins The Search For Carbon Sinks and Sources

3 January 2009

by Jack Rosebro

Artist’s visualization of the Orbiting Carbon Observatory (OCO) in orbit approximately 483 miles above the Earth’s surface. Source: NASA. Click to enlarge.

NASA is preparing for next month’s launch of its Orbiting Carbon Observatory (OCO), a 441 kg (970 lb) satellite designed to map geographic distribution and seasonal variations of Earth’s natural and anthropogenic carbon sinks and sources at a resolution of approximately one square mile per measurement. The mission is part of NASA’s ongoing study of the planet’s carbon and water cycles under the auspices of NASA’s Earth System Science Pathfinder (ESSP) program.

The Orbiting Carbon Observatory will circle the Earth every 98.8 minutes in a sun-synchronous orbit, collecting about half a million measurements per day and mapping the Earth’s surface approximately once per month.

OCO Instrument
The OCO telescope has  an 11 cm aperture, with a primary and a secondary mirror. The relay optics assembly includes fold mirrors, dichroic beam splitters, band isolation filters and re-imaging mirrors. Each spectrometer consists of a slit, a two-lens collimator, a grating, and a two-lens camera. Each of the three spectrometers has an essentially identical layout. Minor differences among the spectrometers, such as the coatings, the lenses and the gratings, account for the different bandpasses that are characteristic of each channel.
OCO instrument. Click to enlarge. Source: NASA/JPL.
To implement an optically fast, high-spectral-resolution measurement system, the OCO instrument combines refractive and reflective optical techniques. Since the light in the common telescope and relay optics assembly has not yet been separated into the three distinct wavelength bands, these instrument subsystems primarily use reflective optics. On the other hand, the extremely narrow channel bandpasses make potential chromatic aberrations in the spectrometers negligible, which enables the use of refractive optics.

The OCO experiment requires the measurement of three relatively small bands of electromagnetic radiation, the spectral wavelength ranges of which are widely separated. To accomplish this task economically, the OCO instrument incorporates three classical grating spectrometers.

Each spectrometer detects the intensity of radiation within a very specific narrow band at Near Infrared (NIR) wavelengths. The three spectrometers have independent optics and signal processing electronics. They share a common structure, a cryogenic cooler, and an input telescope.

The first spectrometer measures concentrations of carbon dioxide near the Earth’s surface, while the second provides data on atmospheric concentrations of carbon dioxide at higher altitudes. The third measures molecular A-band oxygen.

The oxygen measurements are used to calculate the effects of clouds, aerosols, and water vapor on exchanges of CO₂, while providing a reference for the first two measurements. Differences in trace gases of as little as one part per million can be detected by the satellite’s spectrometers.

The light detectors for the instrument’s three cameras are chilled by the cryocooler, which maintains detector temperatures at or near -150 °C (-240 °F) to eliminate measurement errors caused by the absorption of energy from unwanted sources.

To enhance the quality and verify the accuracy of mission data, the Orbiting Carbon Observatory will collect spectrometer data in three standard observational modes: nadir, glint and target.

• Nadir mode produces the highest resolution for a given area of measurement. In this mode, the satellite points the instrument straight down to the ground, returning a higher percentage of usable data in regions that are mountainous or partially obscured by clouds.  Nadir observations, however, are not as effective over dark ocean surfaces or areas covered by snow.

• In glint mode, the OCO points the instrument at the spot on the Earth’s surface where the sun’s reflection is most intense, which reflects back up to 100 times more signal than nadir mode and improves observations over dark ocean surfaces. The spacecraft will alternate between nadir and glint modes over each sequential 16-day global ground track repeat cycle, so the entire Earth is mapped in both modes approximately once per month.

• Target mode is used to validate observatory data against ground calibration sites by locking the OCO telescope onto a single specific surface location as it flies overhead. Up to one target observation can be taken per day to validate collected data.

Once OCO enters the Earth’s orbit, it will lead a group of five existing satellites, collecting data on aerosols, clouds, cloud ice, carbon sinks, carbon sources, ozone, particulates, and atmospheric water vapor. The group, which will map the entire sunlit hemisphere of Earth with a full suite of active and passive instruments every sixteen days, is referred to as the Afternoon Constellation, and has been nicknamed the “A-Train”. A seventh satellite, Glory, is scheduled to join the Afternoon Constellation next summer.

Balancing the “Carbon Budget”

NASA hopes to use the Orbiting Carbon Observatory to identify and quantify so-called “missing carbon sinks” and to close gaps in the understanding of anthropogenic influences on Earth’s natural carbon cycle. At present, carbon cycle input-output inventories, known as carbon budgets, are unable to account for all carbon that is known to be absorbed from the Earth’s atmosphere. The Woods Hole Research Center in Massachusetts estimates, for example, that sinks for 2.9 of the 8.5 petagrams (PtC) of carbon emitted annually in the 1990s are as of yet unaccounted for. One petagram equals one billion metric tons.

Approximately 100 ground-based sites currently measure concentrations of carbon dioxide near the Earth’s surface, supplemented by a growing network of instruments mounted on tall towers, as well as instruments carried in aircraft and high-altitude balloons. Cumulative data from such sources, however, lacks the coverage, resolution, sampling rates, and altitude variations necessary to identify all land- and ocean-based sinks.

Example of carbon cycle, showing estimated emission and uptake fluxes that constitute a carbon budget. Source: Jorge L. Sarmiento, Princeton University and Nicolas Gruber, University of California, Los Angeles, 2002 Click to enlarge.

As man-made greenhouse gases are expelled into the atmosphere, some carbon sinks increase their uptake of greenhouse gases. However, not all observed carbon uptake can be attributed to known sinks, which indicates the presence of unknown sinks. Accurate assessments of the world’s carbon sinks is a prelude to monitoring the ongoing ability of ecosystems to absorb man-made carbon dioxide. Although many emissions models project the future degradation of some carbon sinks into carbon sources, the timing and severity of such transformations is unclear.

Sinks: Forests and Soils

While the missing carbon sink is usually attributed to unknown land-based sources, pinpointing those sources has been difficult at best. A 2001 review of satellite and forest inventory data^[1] from 1981 to 1999 concluded that northern forests in Europe, Russia, and North America absorbed nearly 700 million metric tons of carbon a year, or about 12% of annual global carbon emissions from industrial activities, during the 1980s and 1990s, and further speculated that another 1 to 2 billion tons of carbon was stored in underlying soils. Field studies did not, however, confirm a corresponding uptake of carbon in northern soils.

A 2007 study led by Britton Stephens from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado^[2] found that conventional carbon models had not sufficiently accounted for vertical mixing of CO₂ through the atmosphere as a result of convection and storm systems. Using atmospheric CO₂ samples collected by aircraft twice a week at 12 locations worldwide for up to 27 years, Stephens’ team of 22 researchers concluded that tropical forests were absorbing about one billion tons more carbon than previously thought, and that northern mid-latitude forests were absorbing 0.9 billion tons, or 38% less than previously assumed.

The results of their findings indicated “strong carbon uptake in undisturbed ecosystems.” However, last year’s Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) found that during 2004, the contribution of deforestation—primarily in the tropics—and the decay of biomass to global warming was 17.3% of total global greenhouse-gas emissions. Subsequent studies and observations have indicated that much of the world’s tropical deforestation continues unabated.

Recent research has also raised the possibility that specific desert soils can, under the right conditions, function as carbon sinks, absorbing as much carbon dioxide for a given area as temperate forests. A June 2008 article in the journal Science noted that researchers in China and Nevada deserts have separately measured significant uptakes of carbon in arid and semi-arid soils, suggesting that deserts may function as dry oceans, becoming less alkaline as they absorb carbon in the form of carbonic acids.

Researchers conducting a two-year study^[3] of desert soil carbon uptake at the Mojave Global Change Facility in Nevada surmised that “desert ecosystem CO₂ exchange may be playing a much larger role in global carbon cycling and in modulating atmospheric CO₂ levels than previously assumed, especially since arid and semiarid biomes make up more than 30% of Earth’s land surface.” Results, however, are preliminary, and it is not known if such activity is continuous, or if the stored carbon is later released to the atmosphere through natural processes.

Sinks: Oceans

One recent area of scientific concern has been the Southern Ocean, which may be responsible for as much as half of all uptake of anthropogenic carbon. Although the magnitude of the ocean’s uptake is in dispute, researchers from eight countries found that the ocean’s ability to absorb carbon dioxide was weakening (earlier post). Oceanic carbon uptake may be linked to declining sea surface salinity (SSS), which is presently measured by a network of over 3,000 Argo floating instruments. However, the Southern and Arctic Oceans are sparsely populated with these floats.

Future data from the Orbiting Carbon Observatory will be used to complement data from NASA’s Aquarius satellite, which is scheduled to be launched in 2010. Aquarius will evenly map sea surface salinity of the world’s oceans for the first time, and is expected to produce more data in its first sixty days of operation than has been collected by conventional means (ships, buoys, floats) in the past hundred years. About a quarter of the world’s ice-free oceans have never been measured for sea surface salinity, which is in decline as oceans absorb more carbon from the atmosphere.

Aligning with the Vulcan Project

Data from the Orbiting Carbon Observatory will also be compared to data from the Vulcan Project (earlier post), a land-based inventory of carbon dioxide emissions which NASA is co-sponsoring with the US Department of Energy.

Vulcan Project carbon emissions intensity map of the United States for calendar year 2002 (in kilotons of carbon, with 1 kiloton equivalent to about two million pounds), showing emissions from urban centers (larger red patches), widely scattered point sources like remote power stations or smelters (small red dots), and highways. Source: Jesse Allen, NASA/Earth Observatory, based on data from the Vulcan Project Click to enlarge.

Prior to the Vulcan Project, overall and distributive US carbon emissions were estimated from records of coal, oil, and natural gas sales. The Vulcan Project uses public data as diverse as population records, state motor vehicle registrations, and county records of the square footage of commercial buildings to calculate intensity and seasonal variability of carbon emissions. EPA point-source carbon monoxide databases were also used to make combustion models from which carbon dioxide emissions could be calculated.

“Vulcan estimates the movement of carbon dioxide through the combustion of fossil fuels at very small scales,” explained project leader Kevin Gurney “with Vulcan estimating movements in the atmosphere from the bottom up, and the Orbiting Carbon Observatory estimating sources from the top down.”

“If you visualize a column of air that stretches from Earth’s surface to the top of the atmosphere,” added Gregg Marland, an environmental scientist with Oak Ridge National Laboratory in Tennessee, “the Orbiting Carbon Observatory will identify how much of that vertical column is carbon dioxide, with an understanding that most is emitted at the surface.” The Oak Ridge National Laboratory is home to the Carbon Dioxide Information Analysis Center (CDIAC), which is among the data analysis centers that are expected to evaluate OCO data.

Leading the Afternoon Constellation

Flying order of the six-satellite Afternoon Constellation (A-Train) after the Orbiting Carbon Observatory (OCO) moves into position to lead the formation in February. The seventh satellite, Glory, is scheduled to join the A-Train in June. Source: NASA Click to enlarge.

The seven satellites that constitute the Afternoon Constellation—so named because it will pass over observers at any point on the equator at approximately 1:30 PM local time—as well as their functions and instruments are, in order of launch dates:

Aqua (4 May 2002): cycling, evaporation, and precipitation of water (microwave scanning radiometer, spectroradiometer, microwave sounding unit, atmospheric infrared sounder, humidity sounder).

Data from the Aqua satellite’s Atmospheric Infrared Sounder (AIRS) instrument is used by scientists from NASA, the National Oceanic and Atmospheric Administration (NOAA), the European Center for Medium-Range Weather Forecasts, University of Maryland, Princeton University, and the California Institute of Technology (Caltech) to measure ozone, carbon monoxide, carbon dioxide, methane, sulfur dioxide, and suspended dust particles. Once the Orbiting Carbon Observatory joins the Afternoon Constellation, it will fly about 15 minutes ahead of Aqua, so that data from the two satellites can be compared with each other, as well as with ground, low altitude, and high altitude measurements of carbon dioxide.

Aqua’s Advanced Microwave Scanning Radiometer (AMSR-E) was furnished by Japan’s Aerospace Exploration Agency, while its humidity sounder was supplied by Brazil’s space agency, the Instituto Nacional de Pesquisas Espaciais.

Data from the Tropospheric Emission Spectrometer (TES) on NASA’s Aura satellite: heavy and light water vapor molecules over Earth’s tropics, 7 October 2006. Red illustrates heavy water vapor, indicating recent evaporation or plant exhalation. Blue and purple show lighter water vapor that has undergone significant condensation. Source: NASA/JPL Click to enlarge.

Aura (15 July 2004): ozone layer, air quality, aerosols, cloud ice, climate (two limb sounders, two spectrometers).

Aura’s four instruments, which collect data sources, radicals, and reservoir gases active in ozone chemistry, are as follows:

• High Resolution Dynamics Limb Sounder (HIRDLS), measuring infrared radiation from ozone, water vapor, CFCs, methane and nitrogen compounds. HIRDLS was developed jointly with the United Kingdom Natural Environment Research Council, but has been hampered by a piece of protective plastic film which became lodged in the instrument’s optics during launch, reducing visibility by 80%. Scientists have developed data algorithms and re-calibrated the instrument to partially compensate for the flaw.

• Microwave Limb Sounder (MLS) which measures emissions from ozone, chlorine and other trace gases, and clarifies the role of water vapor in global warming.

• Ozone Monitoring Instrument (OMI), a nadir viewing spectrometer that uses ultraviolet and visible radiation to produce daily high resolution maps. OMI was developed by the Finnish Meteorological Institute and the Netherlands Agency for Aerospace Programmes.

• Tropospheric Emission Spectrometer (TES), which measures tropospheric ozone in infrared wavelengths, as well carbon monoxide, methane and nitrogen oxides.

PARASOL (18 December 2004): aerosols and clouds (wide-field imaging radiometer/polarimeter).

Launched from the Kourou, French Guiana spaceport on an Ariane 5 G+ rocket by French space agency Centre National d’Études Spatiales (CNES), PARASOL carries the POLDER (Polarization and Directionality of the Earth’s Reflectances) radiometer/polarimeter, which assesses the radiative and microphysical properties of clouds and aerosols by measuring the directionality and polarization of solar radiation reflected by the Earth-atmosphere system.

CloudSat (26 April 2006): altitude and properties of clouds (94-GHz cloud-profiling radar).

CloudSat is designed to provide the first global survey of cloud profiles and properties with seasonal and geographic variations from space, including:

• Vertically-resolved estimates of how much water and ice are in Earth’s clouds;
• Detection of snowfall from space;
• Estimates of how efficiently the atmosphere produces rain from condensates; and
• Measurements of the percentage of Earth’s clouds that produce rain.

CALIPSO (28 April 2006): aerosols and thin clouds (wide-field camera, three-channel infrared radiometer, high-resolution orthogonally polarized cloud-aerosol LIDAR).

A joint project of NASA and CNES, CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) is designed to carry out three tasks from Earth’s orbit for the first time:

• Measure the precise altitudes and overlaps of clouds and aerosol layers;
• Map the three-dimensional distribution of aerosols, as well as the geographic distribution of their sources; and
• Detect polar stratospheric clouds (on which chlorine compounds are converted into chlorine radicals that destroy ozone) as well as invisible clouds in the upper troposphere.

Both CloudSat and CALIPSO will generate data on the vertical structure of clouds worldwide, as well as indirect estimates of the effects of clouds and aerosols on atmospheric warming.

Glory (15 June 2009 at earliest): aerosols, black carbon/soot (LIDAR-based aerosol polarimetry sensor with integrated cloud camera, four-radiometer solar irradiance monitor).

Glory will collect data on the properties and distributions of atmospheric aerosols and particulates, including black carbon, as well as on solar irradiance for the long-term Earth climate record. Although black carbon is not a greenhouse gas, it has received increasing attention for its ability to absorb heat and reduce albedo. A March report (earlier post) reported observations that black carbon could be contributing to atmospheric warming at three to four times the magnitude previously thought, a conclusion that paralleled earlier modelling studies from Caltech, NASA, and Stanford. A July report from the University of Colorado and NOAA estimated that emissions of black carbon from large ships was about twice as much per unit of fuel as other commercial ships (earlier post).

The Orbiting Carbon Observatory is scheduled for launch aboard an Orbital Sciences Taurus XL rocket from the Vandenberg Air Force Base spaceport, about 130 miles northwest of Los Angeles, as soon as 23 February between 01:50 and 01:57 PST/PDT. The Glory satellite is scheduled for a June launch.

^[1] R.B. Myneni et al. (2001) A large carbon sink in the woody biomass of Northern forests. Proceedings of the National Academy of Sciences, Volume 98, Number 26
^[2] Britton Stephens et al. (2007) Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO₂. Science, Volume 316
^[3] Georg Wohlfahrt et al. (2008) Large annual net ecosystem CO₂ uptake of a Mojave Desert ecosystem. Global Change Biology, Volume 14 Issue 7

Additional Resources

• Georg Wohlfahrt et al.: Large annual net ecosystem CO₂ uptake of a Mojave Desert ecosystem. Presented 15 April at European Geosciences 2008, Vienna

• Hartmut Aumann and Joao Teixeira: Temperature Dependence of the Frequency of Severe Storms. Presented 14 October 2008 at Atmospheric Sounding Science Team Meeting, Greenbelt, Maryland