Space Based Measurements of Atmospheric Carbon Dioxide with the NASA Orbiting Carbon Observatory (OCO) http://oco.jpl.nasa.gov

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Space Based Measurements of Atmospheric Carbon
Dioxide with the NASA Orbiting Carbon Observatory
(OCO) http//oco.jpl.nasa.gov
Science Enabled by New Measurements of CO2

• David Crisp (JPL/Caltech)
• OCO Principal Investigator
• April 2008

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Overview

• What are the priority science uses of the new
  measurements?
• Science objectives
• Implementation approach
• Measurement approach
• Observing strategy
• What do we need to do scientifically to use these
  new measurements and/or to get ready for the
  mission?
• Retrieving XCO2 from OCO spectra
• The OCO validation program
• Deriving CO2 sources and sinks from OCO data
• Are there any major issues to be resolved before
  this science is enabled, and if so, what are they
  and what needs to be done?
• Discussion

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The Orbiting Carbon Observatory (OCO)
OCO will acquire the space-based data needed to
identify CO2 sources and sinks on regional scales
over the globe and quantify their variability
over the seasonal cycle

• Approach
• Collect spectra of CO2 and O2 absorption in
  reflected sunlight
• Use these data to resolve variations in the
  column averaged CO2 dry air mole fraction, XCO2
  over the sunlit hemisphere
• XCO2 0.20995 CO2 / O2
• Validate measurements to ensure XCO2 accuracies
  of 1 - 2 ppm (0.3 - 0.5) on regional scales at
  monthly intervals

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Priority Science Uses of the New Measurements

• Where are the missing CO2 sinks?
• Humans have added gt200 Gt C to the atmosphere
  since 1958
• Only 58 of this CO2 is staying in the
  atmosphere
• Where are the sinks that are absorbing over 40
  of the CO2?
• Land or ocean?
• Eurasia/North America?
• Why does the CO2 buildup vary from year to year
  with nearly uniform emission rates?
• How will these CO2 sinks respond to climate
  change?

?
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What is XCO2?
Measured Spectra
CO2
O2
CO
CO
O
Column Abundance Path Dependent
Ratio
XCO2 Path Independent Mixing Ratio
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Getting Science from OCO Measurements
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Precisions of 12 ppm (0.30.5) on regional
scales needed to

• Resolve pole to pole XCO2 gradients on regional
  scales
• Resolve the XCO2 seasonal cycle in the Northern
  Hemisphere

• Resolve (8ppm) pole to pole XCO2 gradients on
  regional scales
• Resolve the XCO2 seasonal cycle in the Northern
  Hemisphere
• Reduce surface CO2 flux errors by about a factor
  of 10

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Making Precise XCO2 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 band Column CO2
• 2.06 ?m CO2 band Column CO2, Aerosols
• 0.76 ?m O2 A-band Surface pressure, clouds,
  aerosols
• Why high spectral resolution?
• Enhances sensitivity, minimizes biases

O2 A-band
CO2 2.06 ?m
CO2 1.61?m
Column CO2
Clouds/Aerosols, H2O, Temperature
Clouds/Aerosols, Surface Pressure
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The OCO Instrument

• Single instrument
• 3 bore-sighted, high resolution, grating
  spectrometers
• O2 0.765 ?m A-band
• CO2 1.61 ?m band
• CO2 2.06 ?m band

Relay Optics
Beamsplitter Pre-Filter
Detector
Polarizer
Cold Filter
Slit
Telescope/ Recolimator
Collimator
Camera
Vent Pipe
Primary Instrument Assembly (PIA)
Telescope Opening
Baffle/Calibration Assembly (BCA)
Optical Bench Assembly (OBA)
Light Trap
Remote Electronics Module (REM)
Grating
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Pre-Flight Atmospheric Spectra from OCO
Fit of convolved FTS spectra to OCO heliostat
spectra (upper panel) in the O2 A Band region
(left) and the 1.61 micron CO2 band region. The
lower panel shows the spectral residuals.
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OCO Will Fly in the A-Train
Coordinated Observations
GLORY 134
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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,
aerosols

• OCO files at the head of the A-Train, 4 minutes
  ahead of the Aqua platform
• 705 km altitude sun synchronous, 98.2?
  inclination, 98.8 minute period
• Global coverage with a 16-day(233 orbit) ground
  track repeat cycle

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On-orbit Measurement Strategy

• Optimized to minimize bias and yield high SNR
  XCO2 over the globe
• Nadir Observations views local nadir
• Small footprint (lt 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
• Validation sights/field campaigns
• Data acquisition schedule
• alternate between Nadir and Glint on 16-day
  intervals
• One target each day for validation

Glint
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Spatial Sampling Approach

• OCO Orbit Constraints
• The 705 km altitude ,98.2? inclination
• global coverage with a 16-day ground repeat cycle
• 98.8 minute period 14.57 Orbits/day
• 25? longitude offset between consecutive orbits
• 1.5? longitude offset between orbit tracks over
  16-day repeat cycle
• OCO Sampling Rate/Coverage
• Glint 75o SZA, Nadir 85o SZA
• 12-24 samples/second collected along track over
  land and ocean
• 200 to 400 samples/degree of latitude along orbit
  track on day side of the Earth
• 7 and 14 million soundings over the globe once
  every 16 days.

OCO provides dense sampling along track and
coarser sampling from track-to-track. Plumes of
CO2 rich/poor air are captured by the column
measurements.
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Preparing for OCO Data

• What do we need to do scientifically to use these
  new measurements and/or to get ready for the
  mission?
• The OCO team is focusing on
• Retrieving XCO2 from OCO spectra
• Validating XCO2 retrievals from OCO
• Deriving CO2 sources and sinks from OCO XCO2 data
• Are there any major issues to be resolved before
  this science is enabled, and if so, what are they
  and what needs to be done?
• Discussion

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Estimating XCO2 from OCO SpectraThe OCO L2
Retrieval Algorithm

• Purpose To derive XCO2 from calibrated spectral
  radiances (Level 1b data products)
• Approach A hybrid approach has been adopted
• A Full Physics algorithm that incorporates
  everything known about atmospheric and surface
  optical properties that affect observed radiances
• Should be reliable over a wide range of
  conditions, providing an absolute standard
• Too slow to process all data
• A Semi-Analytical method based on correlations
  between correlated Apparent Optical Path
  Differences (AOPDs) in the O2 and CO2 bands
• Fast and accurate over training range
• Provides good initial guess for full physics
  algorithm

CO2
Altitude
xi
K ?f/?xi
XCO2
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Space-based XCO2 Validation Strategy Ensures
Accuracy and Early Acceptance
A rigorous validation approach will speed
acceptance of OCO data by the Science Community

• The space-based measurements must be validated
  against the surface CO2 standard

OCO/GOSAT
FTS
Aircraft
Tower
Flask
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OCO Data Products
Validation
L1
L3
Retrieval Algorithm
XCO2 Maps
Source/Sink Inversion Model
Spectra
Calibration
IOC6 Months (August 2009)
L4
IOC9 Months (November 2009)
Sources/Sinks
As Available
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Backup
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Footprint Shapes on Surface
Near South Pole
IFOV
S/C Motion
2.3 km
60o S of subsolar point
10 km
S/C Motion
2.3 km
IFOV
8.6 km
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Footprint Shapes on Surface (cont)
At subsolar point
30o S of subsolar point

• Notes
• On the north side of the orbit, the footprints
  switch sides (i.e. the west footprints become
  east footprints)
• Subsolar point provides excellent opportunity
  each orbit to validate calibration of footprints

S/C Motion
IFOV
S/C Motion
0.08 km
2.3 km
2.3 km
IFOV
5 km
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OCO Horizontal Footprints
Orientation and shape of horizontal contribution
functions for Nadir observations

• Each OCO sounding describes the average mixing
  ratio along
• The incoming optical path between the surface
  measurement footprint and the sun
• The outgoing optical path that extends from the
  surface footprint and the spacecraft.
• High resolution carbon data assimilation
  calculations must account for the effective
  horizontal projections of the XCO2 columns
  retrieved from OCO data

Incoming and outgoing parts of a single Nadir
sounding
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Atmospheric Footprints

• For nadir observations, the solar zenith angle
  varies from 21.5 ? at the sub-solar latitude to
  85? at the maximum polar extent.

Surface solar zenith angle, ?o Tropopause solar zenith angle ?1 Path to Tropopause ds (km) Horizontal distance , dy (km)
21.5 21.44 16.4 6.7 km
85.0 83.64 151.6 150.6

• For glint observations, the solar zenith angle
  varies from 10.75 ? at the sub-solar latitude to
  75? at the maximum polar extent, but the outgoing
  path increase the horizontal footprint

Surface solar zenith angle, ?o Tropopause solar zenith angle ?1 Path Tropopause ds (km) Horizontal distance, dy (km)
10.75 10.72 15.35 6.7km
75.0 74.51 56.57 109.9
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The Observatory

• Orbital Sciences LEOStar-2 Bus
• 0.94 m x 2.1 m
• 3-axis stabilized
• Includes propulsion system for orbit maintenance

8/07
12/07
1/08

• Spacecraft bus development and flight
  qualification testing completed
• Integration with instrument completed
• Observatory testing underway

2/08
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Our Ride Taurus 3110 Launch Vehicle

• Manufactured by the Orbital Sciences Launch
  Services Group
• Integration on Schedule

• Launch Site
• - VAFB (Site 576E ) All from VAFB
• History
• - 7 Taurus have been launched-6 Std, 1 XL, 1
  Std failure
• - 1st Launch 3/1994

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OCO Fills a Critical Measurement Gap
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NOAA TOVS
ENVISAT SCIAMACHY
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Aqua AIRS
4
3
CO2 Error (ppm)
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Aircraft
1
Flask Site
Globalview Network
OCO
Flux Tower
0
1000
10000
1
100
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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.
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Accurate XCO2 Retrievals Require Normalization
With O2

• The key OCO data product is the CO2 column
  averaged dry air mole fraction, XCO2
• XCO2 cannot be measured directly
• The OCO experiment actually measures the
  wavelength dependent intensity of NIR solar
  radiation reflected from the surface of the
  earth, or spectral radiances
• The absorption depth of the spectral radiances in
  the CO2 spectral range is proportional to CO2,
  the concentration of atmospheric CO2 molecules in
  the photon path between the sun and the
  Observatory
• O2 is used to normalize the CO2 values for the
  total number of molecules in the observed airmass
  
• The fraction of O2 is constant throughout the
  atmosphere at 0.20955
• Normalization removes the effects of varying
  surface pressure and topography
• A 50-meter change in surface elevation equals a
  0.3 (1 ppm) change in XCO2
• XCO2 is determined from independent retrievals of
  CO2 and O2
• XCO2 0.20995 x CO2 / O2

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