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EVERGREEN Workshop. Measuring CO 2 Sources and Sinks from Space: The Orbiting Carbon Observatory (OCO) Ilse Aben (SRON) David Crisp (JPL/Caltech) January 2006. What Processes Control Atmospheric CO 2 ?. Atmospheric Carbon dioxide (CO 2 )

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Measuring CO2 Sources and Sinks from Space: The Orbiting Carbon Observatory (OCO)

Ilse Aben (SRON)

David Crisp (JPL/Caltech)

January 2006

what processes control atmospheric co 2
What Processes Control Atmospheric CO2?
  • Atmospheric Carbon dioxide (CO2)
    • Primary man-made greenhouse gas
    • Mixing ratios have increased by ~25% since the beginning of the industrial age (280 to 375 ppm)
    • Only half of the CO2 from fossil fuel combustion is staying in the atmosphere – the rest is being absorbed by the oceans and land biosphere
  • Outstanding Issues:
    • Where are the CO2 sinks?
      • Oceans vs land ecosystems
      • North American and Eurasian sinks?
    • Why does the atmospheric buildup vary with uniform emission rates?
    • How will CO2 sinks respond to climate change?
oco fills a critical measurement gap
OCO Fills a Critical Measurement Gap











CO2 Error (ppm)

















Spatial Scale (km)

OCO will make precise global measurements of XCO2 over the range of scales needed to monitor CO2 fluxes on regional to continental scales.

the o rbiting c arbon o bservatory oco
The Orbiting Carbon Observatory (OCO)

OCO will acquire the space-based data needed to identify CO2 sources and sinks and quantify their variability over the seasonal cycle


  • Collect spatially resolved, high resolution spectroscopic observations of CO2 and O2 absorption in reflected sunlight
  • Use these data to resolve spatial and temporal variations in the column averaged CO2 dry air mole fraction,XCO2over the sunlit hemisphere
  • Employ independent calibration and validation approaches to produce XCO2 estimates with random errors and biases no larger than 1 - 2 ppm (0.3 - 0.5%) on regional scales at monthly intervals
making precise co 2 measurements from space

CO2 2.06 m

CO2 1.61m

O2 A-band

Clouds/Aerosols, Surface Pressure

Column CO2

Clouds/Aerosols, H2O, Temperature

Making Precise CO2 Measurements from Space
  • High resolution spectra of reflected sunlight in near IR CO2 and O2 bands used to retrieve the column average CO2 dry air mole fraction, XCO2
    • 1.61 m CO2 bands – Column CO2 with maximum sensitivity near the surface
    • O2 A-band and 2.06 m CO2 band
      • Surface pressure, albedo, atmospheric temperature, water vapor, clouds, aerosols
  • Why high spectral resolution?
    • Enhances sensitivity, minimizes biases

The OCO spatial sampling strategy has been designed to provide precise, bias-free estimates of XCO2 on regional scales at monthly intervals

  • Contiguous sampling not needed
    • Chemical Transport Models infer sources and sinks from spatial and temporal gradients in XCO2
      • Have resolutions of 1o to 5o
    • Winds transport CO2 over large areas as it is mixed through the column
  • XCO2 soundings must be collected at high spatial resolution
    • Maximizes the number of cloud-free samples in partly cloudy regions
    • Minimizes errors due to spatial inhomogeneities within each footprint

OCO Spatial Sampling Strategy

oco observing strategy

Glint Spot

Ground Track

Local Nadir

OCO Observing Strategy
  • Nadir Observations: tracks local nadir
    • + Small footprint (< 3 km2) isolates cloud-free scenes and reduces biases from spatial inhomogeneities over land
    • Low Signal/Noise over dark ocean
  • Glint Observations: views “glint” spot
    • + Improves Signal/Noise over oceans
    • More interference from clouds
  • Target Observations
    • Tracks a stationary surface calibration site to collect large numbers of soundings
  • Data acquisition schedule:
    • alternate between Nadir and Glint on 16-day intervals
    • Acquire ~1 Target observation each day
oco will fly in the a train


OCO Will Fly in the A-Train

Coordinated Observations

CloudSat – 3-D cloud climatology

CALIPSO – 3-D aerosol climatology

aerosols, polarization

AIRS – T, P, H2O,

CO2, CH4

MODIS – cloud, aerosols, albedo

TES – T, P, H2O, O3, CH4, CO

MLS – O3, H2O, CO

HIRDLS – T, O3, H2O, CO2, CH4

OMI – O3, aerosol climatology

OCO - - CO2

O2 A-band

ps, clouds,


  • OCO files at the head of the A-Train, 12 minutes ahead of the Aqua platform
    • 1:18 PM equator crossing time yields same ground track as AQUA
    • Near noon orbit yields high SNR CO2 and O2 measurements in reflected sunlight
    • CO2 concentrations are near their diurnally-averaged values near noon
    • Maximizes opportunities of coordinated science and calibration activities
the oco instrument


Collimator lens






Camera lens

The OCO Instrument
  • Three bore-sighted, high resolution, grating spectrometers
    • CO2 1.61 m band
    • CO2 2.06 m band
    • O2 0.765 m A-band
  • Similar optics and electronics
    • Common 200 mm f/1.9 telescope
    • Spectrometers cooled to <0 oC
    • Resolving Power ~20,000
    • Common Read-out Integrated Circuits and electronics for focal plane arrays
  • Existing Designs For Critical Components
    • Detectors: WFC-3, Deep Impact (RSC)
    • Cryocooler: TES flight spare (NGST)
  • Provided by Hamilton Sundstrand Sensor Systems (Pomona CA)
    • Provided last 4 TOMS instruments






oco retrieval algorithm


H2 O





K = f/xi





H2 O



OCO Retrieval Algorithm
  • Simultaneously retrieves CO2 and O2 columns and other environmental properties that can introduce random error or bias
    • surface pressure, clouds, aerosols, temperature and humidity
  • Consists of 3 major components
    • Spectrum resolving multiple scattering model
    • Instrument simulator model
    • Inverse models: optimal estimation theory
  • Requires reliable input
    • Accurate laboratory spectroscopy
    • A priori state structure
retrieval algorithm validation
Retrieval Algorithm Validation
  • Validation in end-to-end Observation System Simulation Experiments (OSSE’s):
  • XCO2 retrieval errors vs albedo and aerosol optical depth for 3 solar zenith angle, o
  • Single-sounding XCO2 retrieval errors exceed 2 ppm (0.5%) only for
    • High zenith angle (o>75)
    • High optical depth ( > 0.3)
    • Dark surface (a <0.1)
  • Validation Using ENVISAT SCIAMACHY Data
  • SCIAMACHY: Low resolution (0.05 to 0.1 nm) spectrometer (~0.2 to 1.5 nm)
  • Spectra provide excellent test of OCO retrieval algorithm and validation concept
  • • Good overall agreement in between FTS and SCIAMACHY XCO2
    • Precision of ~7 to 8 ppm, not far from expected given its spectral resolution
total column carbon network tccn

Site selection :

  • environmental conditions (flat terrain removed from local sources)
  • Ancillary observations (aerosols or CO2)
  • local liaison
  • accessibility and security

Pallas, Finland 68N

Kiruna, Sweden 68N

Poker Flat, Alaska 65N

Tsukuba, Japan 36N

Tenerife Island 28N

Mauno Loa 19N

Christmas Island 2N

Wollongong, Australia 34S

Arrival Hts, Antarctica 78S


Total Column Carbon Network (TCCN)

Ny Ålesund, Norway 79N

Bremen, Germany 53N

Park Falls, Wisconsin 46N

Billings, Oklahoma 37N

Darwin, Australia 12S

Lauder, New Zealand 45S


space based x co2 validation strategy
Space-based XCO2 Validation Strategy
  • Validate space-based XCO2 using:
    • Measurements of XCO2 from a ground based network of Fourier transform spectrometers (FTS)
    • FTS and space-based XCO2 processed using same retrieval code
  • FTS XCO2 compared to
    • Surface in situ CO2
    • Tall tower in situ CO2
    • Column CO2 integrated from in situ aircraft + sondes

0.1% precision, accuracy 0.68% offset !!

  • FTS XCO2 performance tracked via continuous monitoring of:
    • Instrument Line Shape (HCl gas cell)
    • Pointing (Doppler shift, telluric vs solar features)
    • Surface pressure and temperature

Observations at 79°N (Spitsbergen) FTS

Notholt et al., GRL, 2005

mission schedule
Mission Schedule

OCO Schedule

  • 7/2001: Step-1 Proposal Submitted
  • 2/2002: Step-2 Proposal Submitted
  • 7/2003: Selected for Formulation
  • 7/2004: System PDR
  • 5/2005: Mission Confirmed for Implementation
  • 10/2005: Instrument CDR
  • 2/2006: Spacecraft CDR
  • 7/2006: System CDR
  • 11/2006: Instrument Pre-Environmental Review

testing and calibration

  • 4/2007: Instrument Delivery
  • 9/2008: Launch
  • 10/2010: End of Nominal Mission

CO2 is the principal anthropogenic driver of climate change

Accurate forecasting of future climate requires an improved understanding of the global carbon cycle and its interaction with the Earth System

OCO will make the first space-based measurements of CO2 with the accuracy needed to quantify sources and sinks of this important greenhouse gas.