Tackling the CO2 Measurement Challenge

February 10, 2009

The challenge: very precisely measure carbon dioxide in Earth’s atmosphere all over the world, especially near Earth’s surface.

For Orbiting Carbon Observatory Principal Investigator David Crisp of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and his team, the logical solution was an Earth-orbiting spacecraft. But shopping for a science instrument that could accomplish these objectives was no easy task.

In this case, “shopping” meant finding the right technology to meet the mission’s demanding requirements. The observatory contains a custom-built instrument designed to make what Deputy Principal Investigator Charles Miller of JPL calls “the most difficult atmospheric trace-gas measurement that’s ever been made from space.” To put that measurement challenge into perspective, consider that all of Earth’s trace gases combined, including carbon dioxide, make up less than one percent of Earth’s atmosphere. In addition, carbon dioxide levels vary by only about two percent from pole to pole. To substantially increase our understanding of how carbon dioxide sources (places where carbon dioxide is emitted) and sinks (places where it is absorbed, or stored) are geographically distributed on regional scales and study how their distribution changes over time, the new mission needed to be able to resolve differences in atmospheric carbon dioxide as small as 0.3 percent on regional scales every month.

While one spaceborne instrument is already making carbon dioxide measurements from space-the Atmospheric Infrared Sounder on NASA’s Aqua satellite-it sees the gas high up in the atmosphere, not near the surface, where it is emitted and where some of it is absorbed into land systems and the ocean. An instrument designed from the start to measure carbon dioxide down to Earth’s surface was clearly needed.

Enter NASA’s team of experts in atmospheric science, remote sensing instrumentation and the optical properties of the atmosphere’s components.



“You can see a good example of how a grating spectrometer works by looking at the back of a compact disc illuminated by a bright light,” said Crisp. “The narrow circular tracks that record the information on the disk are very effective at splitting light into different colors.”


The resulting spectra look something like bar codes, with dark lines showing where carbon dioxide or oxygen have absorbed specific colors. “By measuring the fraction of the light that has been absorbed in each of these dark lines, we can count the number of carbon dioxide or oxygen molecules in the atmosphere,” said Crisp.

Three separate digital detectors, one for each spectrometer, record a spectrum three times each second as the observatory flies above Earth’s surface. Fast exposures are essential because the spacecraft moves at more than four miles per second along its orbit track. “We don’t want long exposures that could include clouds as well as clear sky within individual exposures,” says Crisp. “We also want to take the data fast and get more clear views to the surface.”

While similar to the digital detectors in an ordinary camera, the observatory’s detectors take advantage of advances from the world of astronomy to achieve the greatest possible sensitivity. “These detectors were originally developed to measure objects that are faint, fuzzy and far-away,” said Crisp. “Here, we use them to measure very fine details in the spectrum of sunlight reflected from Earth.”

The three spectral ranges measured by the observatory’s spectrometers are in the near-infrared part of the electromagnetic spectrum, invisible to the human eye. Each provides a critical piece of information. One provides precise information about changes in the amount of carbon dioxide present in the atmosphere, while the others show just how much of the atmosphere is being measured. “We need all three of these measurements to do the job,” said Crisp.







“One of the most challenging aspects of the mission was not inventing components, but fitting a big instrument into a small spacecraft about the size of a phone booth and designing it to use very little power,” said JPL’s Randy Pollock, the mission’s instrument systems engineer. The observatory’s instrument uses only about 100 watts of electricity, while the entire spacecraft uses only 400-500 watts, about half the amount used by most microwave ovens.

Once received back on Earth, the observatory’s data will be analyzed to yield estimates of the carbon dioxide concentration over Earth’s sunlit hemisphere at spatial resolutions as small as one square mile using complex mathematical algorithms. Scientists will then analyze these carbon dioxide estimates using global transport models similar to those used for weather prediction to quantify carbon dioxide sources and sinks.

“Carbon dioxide is the primary human-produced greenhouse gas and, therefore, the primary human-caused driver of global warming,” said Crisp. “To estimate the rate of global warming, we have to understand the processes controlling the buildup of carbon dioxide in Earth’s atmosphere. Global, space-based monitoring systems like the Orbiting Carbon Observatory are essential tools for this task. The technology we validate on this mission will be used to develop future carbon dioxide monitoring missions.”

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