1. Page 1 1 of 16, NATO ASI, Kyiv, 9/15/2010 Vijay Natraj (Jet Propulsion Laboratory) Collaborators Hartmut Bösch (Univ Leicester) Rob Spurr (RT Solutions) Yuk Yung (Caltech) NATO ASI on Polarimetry and Remote Sensing September 15, 2010 A Two Orders of Scattering Approach to Account for Polarization in CO 2 Retrievals from Space

2. Page 2 2 of 16, NATO ASI, Kyiv, 9/15/2010 Importance of Polarization Polarization is a result of scattering The Earth’s atmosphere contains molecules, aerosols and clouds, all of which contribute to scattering Surfaces can also polarize, in some cases significantly (e.g., ocean) Polarization depends on solar and viewing angles and will therefore introduce spatial biases in retrieved trace gas column densities if unaccounted for The satellite instrument could be sensitive to polarization

3. Page 3 3 of 16, NATO ASI, Kyiv, 9/15/2010 Orbiting Carbon Observatory (OCO) Mission First global, space-based observations of atmospheric CO 2 –high accuracy, resolution and coverage –geographic distribution of CO 2 sources and sinks and variability High resolution spectroscopic measurements of reflected sunlight – NIR CO 2 and O 2 bands Remote sensing retrieval algorithms –estimates of column-averaged CO 2 dry air mole fraction (X CO2 ) –accuracies near 0.3% (1 ppm) Chemical transport models –spatial distribution of CO 2 sources and sinks

4. Page 4 4 of 16, NATO ASI, Kyiv, 9/15/2010 OCO Spectroscopy O 2 A band Clouds/Aerosols, Surface Pressure “strong” CO 2 band Clouds/Aerosols, H 2 O, Temperature “weak” CO 2 band Column CO 2 Column-integrated CO 2 abundance => Maximum contribution from surface High resolution spectroscopic measurements of reflected sunlight in near IR CO 2 and O 2 bands

5. Page 5 5 of 16, NATO ASI, Kyiv, 9/15/2010 Polarization in the O 2 A Band continuum line core gas absorption od ~ 1 SZA = 10° (solid); 40° (dotted); 70° (dashed) Natraj et al., JQSRT, 103, 245–259, 2007

6. Page 6 6 of 16, NATO ASI, Kyiv, 9/15/2010 Why Two Orders of Scattering (2OS)? Full multiple-scattering vector RT codes (e.g. VLIDORT) are too slow to meet large-scale operational processing requirements Scalar computation causes two kinds of errors –polarized component of the Stokes vector is neglected –correction to intensity due to polarization is neglected Major contribution to polarization comes from first few orders of scattering (multiple scattering is depolarizing) Single scattering does not account for the correction to intensity due to polarization

7. Page 7 7 of 16, NATO ASI, Kyiv, 9/15/2010 2OS gives a first order approximation to the intensity correction Multiple scattering angular integrals involve computations for the following sets of angles: –Solar-quadrature –Quadrature-viewing –Quadrature-quadrature 2OS does NOT need quadrature-quadrature computations O(N) rather than O(N 2 ) computation (N: # of quadrature angles) Why 2OS?

8. Page 8 8 of 16, NATO ASI, Kyiv, 9/15/2010 2OS Model Schematic Scenario 1 Scenario 2 scatterer Scenario 3 scatterer Scenario 4 scatterer 1 scatterer 2 Natraj and Spurr, JQSRT, 107, 263–293, 2007

9. Page 9 9 of 16, NATO ASI, Kyiv, 9/15/2010 2OS Approximation Overview Retrievals will only be applied to optically thin scattering ( < 0.3) Intensity will still be calculated with full multiple scattering scalar model (for OCO) Fast correction to standard scalar code Exact through second order Supports analytic Jacobians

10. Page 10 10 of 16, NATO ASI, Kyiv, 9/15/2010 Scenarios (Nadir Mode) DarwinPark FallsNy Alesund AlgeriaSouth PacificLauder DarwinPark FallsNy Alesund AlgeriaSouth PacificLauder Natraj et al., JGR, 113, D11212, 2008

11. Page 11 11 of 16, NATO ASI, Kyiv, 9/15/2010 Scenarios (Nadir Mode) DarwinPark FallsNy Alesund AlgeriaSouth PacificLauder DarwinPark FallsNy Alesund AlgeriaSouth PacificLauder Two seasons (Jan/Jul) Three AOD (0, 0.01, 0.3) Tropospheric aerosol: Kahn et al., 2001 Stratospheric aerosol: 75% H 2 SO 4, modified gamma size distribution Surface reflectances from ASTER library

12. Page 12 12 of 16, NATO ASI, Kyiv, 9/15/2010 Spectral Residuals Scalar Model 2OS Model AOT ↑ Scalar model errors order of magnitude larger than 2OS model errors

13. Page 13 13 of 16, NATO ASI, Kyiv, 9/15/2010 Retrieval error dominated by incorrect estimation of surface pressure X CO2 and Surface Pressure Errors X CO2 Error (ppm) Surface Pressure Error (Pa)

14. Page 14 14 of 16, NATO ASI, Kyiv, 9/15/2010 X CO2 Errors (Glint Mode) Scalar Model 2OS Model X CO2 errors from 2OS model < 1 ppm; scalar model errors as high as 5 ppm Wind speed = 4 m/s AOT ↑

15. Page 15 15 of 16, NATO ASI, Kyiv, 9/15/2010 X CO2 Errors (Target Mode) Scalar Model 2OS Model X CO2 errors from 2OS model < 1 ppm; scalar model errors as high as 30 ppm

16. Page 16 16 of 16, NATO ASI, Kyiv, 9/15/2010 Summary Ignoring polarization could lead to significant (as high as 30 ppm) errors in X CO2 retrievals 2OS approach to account for polarization works very well (in and out of principal plane), giving X CO2 errors that are typically smaller than 1 ppm Errors dominated by errors in retrieved surface pressure 2OS model two orders of magnitude faster than a full vector calculation