The Orbiting Carbon Observatory Mission: Effects of Polarization on Retrievals

1

• The Orbiting Carbon Observatory Mission Effects
  of Polarization on Retrievals
• Vijay Natraj
• Advisor Yuk Yung
• Collaborators Robert Spurr (RT Solutions, Inc.),
  Hartmut Boesch
• (JPL), Yibo Jiang (JPL)

2
Outline

• Introduction
• Retrieval Strategy
• Radiative Transfer Essentials
• O2 A Band Results
• Sensitivity Analysis
• Outlook

3
Introduction
Since 1860, global mean surface temperature has
risen 1.0 C with a very abrupt increase since
1980.
Atmospheric levels of CO2 have risen from 270
ppm in 1860 to 370 ppm today.
Does increasing atmospheric CO2 drive increases
in global temperature? Do increasing temperatures
increase atmospheric CO2 levels?
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Where are the Missing Carbon Sinks?

• Only half of the CO2 released into the atmosphere
  since 1970 has remained there. The rest has been
  absorbed by land ecosystems and oceans
• What are the relative roles of the oceans and
  land ecosystems in absorbing CO2?
• Is there a northern hemisphere land sink?
• What are the relative roles of North America and
  Eurasia?
• What controls carbon sinks?
• Why does the atmospheric buildup vary with
  uniform emission rates?
• How will sinks respond to climate change?
• Reliable climate predictions require an improved
  understanding of CO2 sinks
• Future atmospheric CO2 increases
• Their contributions to global change

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Why Measure CO2 from Space?Improved CO2 Flux
Inversion Capabilities

• Current State of Knowledge
• Global maps of carbon flux errors for 26
  continent/ocean-basin-sized zones retrieved from
  inversion studies
• Studies using data from the 56 GV-CO2 stations
• Flux residuals exceed 1 GtC/yr in some zones
• Network is too sparse
• Inversion tests
• global XCO2 pseudo-data with 1 ppm accuracy
• flux errors reduced to lt0.5 GtC/yr/zone for all
  zones
• Global flux error reduced by a factor of 3.

Flux Retrieval Error GtC/yr/zone
Rayner OBrien, Geophys. Res. Lett. 28, 175
(2001)
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OCO Mission

• First global, space-based observations of
  atmospheric CO2
• high accuracy, resolution and coverage
• geographic distribution of CO2 sources and sinks
  and variability
• High resolution spectroscopic measurements of
  reflected sunlight
• NIR CO2 and O2 bands
• Remote sensing retrieval algorithms
• estimates of column-averaged CO2 dry air mole
  fraction (XCO2)
• accuracies near 0.3 (1 ppm)
• Chemical transport models
• spatial distribution of CO2 sources and sinks
• two annual cycles

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Spectroscopy

• Column-integrated CO2 abundance gt Maximum
  contribution from surface
• High resolution spectroscopic measurements of
  reflected sunlight in near IR CO2 and O2 bands

O2 A band Clouds/Aerosols, Surface Pressure
weak CO2 band Column CO2
strong CO2 band Clouds/Aerosols, H2O,
Temperature
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Retrieval Strategy
9
Radiative Transfer Essentials
Fundamental Equation of RT
Beers Law
Source Function (Emission, Scattering)
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Polarization and the Stokes Vector

• Electromagnetic radiation can be described in
  terms of the Stokes Vectors I, Q, U V
• I - total intensity
• Q U - linear polarization
• V - circular polarization
• Degree of Polarization
• (for OCO)

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Atmospheric and Surface Setup

• 11-layer plane-parallel atmosphere (4 in
  stratosphere)
• Urban, tropospheric and stratospheric aerosols
• Lambertian surface albedos of 0.05, 0.1, 0.3
• SZA 10, 40, 70
• VZA 0, 35, 70
• Azimuth 0, 45, 90, 135, 180
• Aerosol extinction optical depth 0, 0.025, 0.25

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Extinction Processes
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Results for O2 A Band with Rayleigh Scattering
0.0113
0.818
103.539
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Varying Solar Zenith Angle
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Varying Aerosol Loading
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Varying Surface Albedo
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Linear Sensitivity Analysis

• Park Falls, Wisconsin
• Geometry
• Nadir viewing
• SZA 75.1 (Jan), 34.8 (Jul)
• Azimuth 210.9 (Jan), 240.0 (Jul)
• Lorentzian ILS
• Resolving Powers
• O2 A Band 17000
• CO2 Bands 20000
• Errors
• July 0.3 ppm
• January 10 ppm

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Outlook

• Polarization significant part of retrieval error
  budget
• Full vector retrieval too time-consuming and not
  practical
• Ways to handle polarization?
• Orders of Scattering
• Spectral Binning
• Look-up tables