Retrieval of CO 2 Column Abundances from Near-Infrared Spectroscopic Measurements Vijay Natraj

Welcome-2 Outline Introduction Retrieval Strategy Case Study: Retrieval of Aircraft Measurements Future Work Conclusion

Welcome-3 Introduction Atmospheric levels of CO 2 have risen from ~ 270 ppm in 1860 to ~370 ppm today. Since 1860, global mean surface temperature has risen ~1.0 °C with a very abrupt increase since Does increasing atmospheric CO 2 drive increases in global temperature? Do increasing temperatures increase atmospheric CO 2 levels?

Welcome-4 Where are the Missing Carbon Sinks? Only half of the CO 2 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 CO 2 ? 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 CO 2 sinks Future atmospheric CO 2 increases Their contributions to global change

Welcome-5 Why Measure CO 2 from Space? Improved CO 2 Flux Inversion Capabilities Rayner & O’Brien, Geophys. Res. Lett. 28, 175 (2001) 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-CO 2 stations Flux residuals exceed 1 GtC/yr in some zones Network is too sparse Inversion tests global X CO2 pseudo-data with 1 ppm accuracy flux errors reduced to <0.5 GtC/yr/zone for all zones Global flux error reduced by a factor of ~3. Flux Retrieval Error GtC/yr/zone

Welcome-6 OCO Mission First global, space-based observations of atmospheric CO 2 with accuracy, resolution and coverage needed to characterise the geographic distribution of CO 2 sources and sinks and quantify their variability High resolution spectroscopic measurements of reflected sunlight in near IR CO 2 and O 2 bands Remote sensing retrieval algorithms will process these data to yield estimates of column-averaged CO 2 dry air mole fraction (X CO2 ) with accuracies near 0.3% Chemical transport models will use OCO X CO2 data and other measurements to retrieve the spatial distribution of CO 2 sources and sinks on regional scales over two annual cycles

Welcome-7 Problem Description Aim: to develop and test algorithms for the retrieval of X CO2 from spectrometric measurements in three NIR bands. Any retrieval problem can be broadly divided into two main components, viz., a forward model and an inverse method. Forward model: describes the radiative transfer in the atmosphere –computation of absorption coefficients –key parameters to compute radiances: scattering and absorption optical depths, single scattering albedo and surface reflectance –convolution function to simulate instrument response Inverse method: compare the measured spectrum with the computed spectrum, and iteratively improve the computed spectrum to best match the observed spectrum –Optimal Estimation Theory [Rodgers, 2000]

Welcome-8 Fundamentals of Atmospheric RT Fundamental Equation of RT μ: cosine of zenith angle I: specific intensity J: source function (multiple scattering) τ: optical depth

Welcome-9 Key Parameters Influencing RT optical depth: amount of extinction a beam of light experiences travelling between two points surface reflectance: ratio of intensity reflected from surface to that incident on it –function of wavelength –can depend on the zenith and azimuthal angles (BRDF) single scattering albedo: fraction of energy scattered to that removed from radiance stream –conservative scattering: ω 0 = 1 –pure absorption: ω 0 = 0 –function of optical depth phase function: describes amount of light scattered from incident direction into scattered direction –function of scattering angle

Welcome-10 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

Welcome-11 Retrieval Strategy

Welcome-12 Inverse Method x a : a priori state vector S ε : measurement error covariance S a : a priori error covariance Measurement Description y: measurement vector x: state vector f(x): forward model ε: measurement error Cost Function

Welcome-13 Inverse Method … Continued A priori constraints obtained from –Climatological data –Measurements –Markov descriptions Levenberg-Marquardt method dx: state vector update K: weighting function (Jacobian) γ: Levenberg-Marquardt parameter

Welcome-14 Case Study: Retrieval of Aircraft Measurements High precision, high resolution O 2 A-band spectra of sunlight reflected from ocean surface (O’Brien et al., J. Atmos. Oceanic Tech., Feb and Dec. 1998) Retrieve column O 2 with precisions required for OCO Retrieval Strategy –Forward Model: multistream, multiple scattering RT model with BRDF at the surface –ILS provided by O'Brien –Inverse method based on optimal estimation theory

Welcome-15 Retrieval: First Cut rms residual = 8.8%

Welcome-16 Retrieval: Second Cut Wavelength Scaling rms residual = 2.3%

Welcome-17 Retrieval: Third Cut Continuum Level, Tilt, Zero Offset, ILS Width Fits rms residual = 1.4%

Welcome-18 Retrieval: Fourth Cut Line Mixing, Solar Feature Removal rms residual = 1.1%

Welcome-19 Future Work Polarisation Surface Types Other RT Models Analytic Weighting Functions Sensitivity Tests Speed Improvements

Welcome-20 Conclusion Algorithm developed to retrieve X CO2 from spectroscopic measurements of absorption in NIR bands Retrieved column O 2 with precision ~ 1% Demonstrates potential to retrieve column O 2 with precisions around 0.1% by averaging sufficient soundings Indicates feasibility of retrieving X CO2 with precisions better than 0.3%

Welcome-21 Acknowledgements California Institute of Technology –Yuk Yung, Run-Lie Shia, Xun Jiang, Zhonghua Yang Jet Propulsion Laboratory –Hartmut Boesch, Geoff Toon, Bhaswar Sen, David Crisp, Charles Miller Commonwealth Scientific and Industrial Research Organisation –Denis O’Brien University of Washington –Zhiming Kuang