Error correlation between CO2 and CO as constraint for CO2 flux inversions using satellite data
1School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
2Harvard-Smithsonian Center For Astrophysics, Cambridge, MA, USA
3Department of Physics, University of Toronto, Toronto, Ontario, Canada
4School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
5Department of Geography, University of Toronto, Toronto, Ontario, Canada
6NASA Goddard Space Flight Center, Global Modeling and Assimilation Office, Greenbelt, MD, USA
Abstract. Inverse modeling of CO2 satellite observations to better quantify carbon surface fluxes requires a forward model such as a chemical transport model (CTM) to relate the fluxes to the observed column concentrations. Model transport error is an important source of observational error. We investigate the potential of using CO satellite observations as additional constraints in a joint CO2–CO inversion to improve CO2 flux estimates, by exploiting the CTM transport error correlations between CO2 and CO. We estimate the error correlation globally and for different seasons by a paired-model method (comparing CTM simulations of CO2 and CO columns using different assimilated meteorological data sets for the same meteorological year) and a paired-forecast method (comparing 48- vs. 24-h CTM forecasts of CO2 and CO columns for the same forecast time). We find strong positive and negative error correlations (r2>0.5) between CO2 and CO columns over much of the world throughout the year, and strong consistency between different methods to estimate the error correlation. Application of the averaging kernels used in the retrieval for thermal IR CO measurements weakens the correlation coefficients by 15% on average (mostly due to variability in the averaging kernels) but preserves the large-scale correlation structure. Results from a testbed inverse modeling application show that CO2–CO error correlations can indeed significantly reduce uncertainty on surface carbon fluxes in a joint CO2–CO inversion vs. a CO2–only inversion.