Retrieving Co₂ From Orbiting Carbon Observatory-2 (Oco-2) Spectra
Creators: name: Crisp, David
Publisher: name: Zenodo
Year: 2014
Subjects: Remote Sensing and Radiative Transfer

description: Fossil fuel combustion, deforestation, and other human activities are currently adding almost 40 billion tons of carbon dioxide (CO<sub>2</sub>) to the atmosphere each year. These emissions have increased by roughly a factor of 3 over the past half century and are still growing by more than 2% per year. The developing world is now responsible for the majority (57%) of these emissions and their rapid growth rates. Precise measurements collected by a global network of surface stations show that these emissions have contributed to a 25% increase in the atmospheric CO<sub>2 </sub>concentration over the past half century. Surprisingly, comparisons of these measurements with fossil fuel emission inventories indicate that only about half of the CO<sub>2 </sub>emitted into the atmosphere by human activities stays there. The rest is apparently being absorbed by natural CO<sub>2 </sub>”sinks” at the Earth’s surface. Measurements of the pH of the ocean indicate that it absorbs roughly one quarter of these emissions. The remainder has been attributed to the land biosphere, but the identity and location of the land sinks is still unknown. In addition, the fraction of the anthropogenic CO<sub>2 </sub>absorbed by these natural sinks has varied dramatically from year to year, but has remained near 1/2 on decadal time scales as the emissions have steadily increased. Uncertainties in the nature, location, and processes controlling these natural sink largely preclude reliable predictions of future atmospheric CO<sub>2 </sub>buildup rates.

The existing greenhouse gas monitoring network can accurately track CO<sub>2 </sub>changes on hemispheric to global scales, but does not have the resolution or coverage needed to quantify emission sources on regional scales or to identify the natural sinks responsible for absorbing CO<sub>2</sub>. One way to improve the measurement density is to retrieve precise, spatially-resolved estimates of the column-averaged CO<sub>2 </sub>dry air mole fraction, X<em><sub>CO</sub></em><sub>2</sub>, from satellites. Surface-weighted estimates of X<em><sub>CO</sub></em><sub>2 </sub>can be retrieved from measurements of reflected sunlight in near infrared CO<sub>2 </sub>and O<sub>2 </sub>bands. However, this is among the most challenging space-based remote sensing applications because even the largest CO<sub>2 </sub>sources and sinks produce changes in the background X<em><sub>CO</sub></em><sub>2 </sub>distribution no larger than 1%, and most are smaller 0.25% (∼1 ppm).

This approach was pioneered by the European Space Agency’s EnviSat SCIAMACHY and Japanese GOSAT TANSO-FTS instruments. These sensors have provided valuable insights into space based X<em><sub>CO</sub></em><sub>2 </sub>measurement techniques, but still do not have the sensitivity, resolution, and coverage needed to quantify CO<sub>2 </sub>sources and sinks on regional scales. The Orbiting Carbon Observatory-2 (OCO-2) is the first NASA spacecraft designed to exploit this measurement approach. This spacecraft carries and points a 3channel, imaging, grating spectrometer that collects high resolution spectra of reflected sunlight in the 765 nm O<sub>2 </sub>A-band and in the 1610 and 2060 nm CO<sub>2 </sub>bands. Coincident O<sub>2 </sub>and CO<sub>2 </sub>spectra are combined into ”soundings” that are analyzed with a full-physics retrieval algorithm to yield estimates of X<em><sub>CO</sub></em><sub>2</sub>. Each spectrometer channel will collect 24 spectra per second, yielding up to a million soundings per day over the sunlit hemisphere. Between 10 and 30% of these soundings are expected to be sufficiently cloud free to yield full-column estimates of X<em><sub>CO</sub></em><sub>2</sub>.

OCO-2 is currently scheduled for launch from Space Launch Complex 2 at Vandenberg Air Force Base in California on a United Launch Alliance Delta-II 7320-10 Launch Vehicle at 02:56:44 AM PDT (12:56:44 GMT) on 1 July 2014. The nominal spacecraft checkout and orbit raising plan will take about 37 days to insert the observatory into the 705-km Afternoon Constellation (A-Train). This 98.8-minute, sun-synchronous orbit has a 98.2-degree inclination, a 1:36:30 PM mean ascending equator crossing time time, and a 16-day (233 orbit) ground track repeat cycle. Once in the A-Train, the instrument’s optical bench and detectors will be cooled to their operating temperatures, and a ∼7-day instrument check-out period will commence. OCO-2 will then start routinely collecting and returning science data.

For routine science operations, the instrument’s bore sight will be pointed to the local nadir or at the ”glint spot,” where sunlight is specularly reflected from the Earth’s surface. Nadir observations provide the best spatial resolution and are expected to yield more cloud-free X<em><sub>CO</sub></em><sub>2 </sub>soundings over land. Glint observations will have much better signal-to-noise ratios (SNR) over dark, ocean surfaces. As often as once each day, the satellite will target a selected surface calibration and validation site and collect thousands of observations as the spacecraft flies overhead. The instrument’s rapid sampling, relatively small (<em>&lt; </em>3 km<sup>2</sup>) sounding footprint, and high SNR, combined with the spacecraft’s ability to point the instrument’s bore sight toward the glint spot over the entire sunlit hemisphere, are expected to provide more complete coverage of the ocean, cloudy regions, and high latitude continents than earlier CO<sub>2 </sub>monitoring spacecraft. The first calibrated, geo-located spectral radiances will be delivered to the NASA Earth Sciences Data and Information Services Center (GES DISC) approximately 90 days after nominal science operations begin. The first X<em><sub>CO</sub></em><sub>2 </sub>products will start being delivered 90 days after that.

The OCO-2 mission is required to return estimates of X<em><sub>CO</sub></em><sub>2 </sub>with accuracies of 0.3% on regional scales at monthly intervals. To meet this stringent requirement, the OCO2 team has developed a ”full-physics” retrieval algorithm that incorporates a forward radiative-transfer model based on a spectrum-resolving multiple scattering algorithm, an OCO-2 instrument model, and an inverse model based on Optimal Estimation. To yield X<em><sub>CO</sub></em><sub>2 </sub>estimates with accuracies of 0.3%, the forward radiative-transfer model must simulate reflected solar radiances with biases no larger than ∼ 0<em>.</em>1%. The accuracy of our X<em><sub>CO</sub></em><sub>2 </sub>estimates retrieved from spectra collected by the GOSAT TANSO-FTS and the ground based Total Carbon Column Observing Network (TCCON) has improved steadily over the past 5 years, but these retrievals still yield biases as large as 1%. Persistent biases and spectrally-dependent residuals in fits to the O<sub>2 </sub>A-band and the two CO<sub>2 </sub>bands sampled by OCO-2 indicate that shortcomings in gas absorption cross sections are a leading cause of these errors. To address this issue, the OCO-2 project has supported an ambitious molecular spectroscopy measurement and analysis effort. Laboratory spectra of CO<sub>2 </sub>and O<sub>2 </sub>collected with cavity ringdown and photoacoustic techniques are being combined with new low-temperature Fourier transform spectra and analyzed with multi-spectral fitting techniques to yield a new, self-consistent description of the line positions, strengths, shapes, and mixing in these bands. This presentation will provide a quick overview of the OCO-2 mission and summarize the recent progress in our molecular spectroscopy effort.
description: Session I: Remote Sensing and Radiative Transfer. June 23, 2014