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We have developed an ensemble Kalman Filter (EnKF) to estimate 8-day regionalsurface fluxes of CO2 from space-borne CO2 dry-air mole fractionobservations (XCO2) and evaluate the approach using a series ofsynthetic experiments, in preparation for data from the NASA OrbitingCarbon Observatory (OCO). The 32-day duty cycle of OCO alternates every 16 daysbetween nadir and glint measurements of backscattered solar radiation atshort-wave infrared wavelengths. The EnKF uses an ensemble of states torepresent the error covariances to estimate 8-day CO2 surfacefluxes over 144 geographical regions. We use a 12×8-day lagwindow, recognising that XCO2 measurements include surface fluxinformation from prior time windows. The observation operator thatrelates surface CO2 fluxes to atmospheric distributions of XCO2includes: a) the GEOS-Chem transport model that relates surface fluxesto global 3-D distributions of CO2 concentrations, which are sampledat the time and location of OCO measurements that are cloud-free andhave aerosol optical depths <0.3; and b) scene-dependent averagingkernels that relate the CO2 profiles to XCO2, accounting fordifferences between nadir and glint measurements, and the associatedscene-dependent observation errors. We show that OCO XCO2measurements significantly reduce the uncertainties of surface CO2flux estimates. Glint measurements are generally better at constrainingocean CO2 flux estimates. Nadir XCO2 measurements over theterrestrial tropics are sparse throughout the year because of eitherclouds or smoke. Glint measurements provide the mosteffective constraint for estimating tropical terrestrial CO2 fluxesby accurately sampling fresh continental outflow over neighbouringoceans. We also present results from sensitivity experiments thatinvestigate how flux estimates change with 1) bias and unbiased errors,2) alternative duty cycles, 3) measurement density and correlations, 4)the spatial resolution of estimated flux estimates, and 5) reducing thelength of the lag window and the size of the ensemble.At the revision stage of this manuscript, the OCO instrument failed toreach its orbit after it was launched on 24 February 2009. The EnKFformulation presented here is also applicable to GOSAT measurements ofCO2 and CH4

atmospheric chemistry and physics

Estimating surface CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes from space-borne CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; dry air mole fraction observations using an ensemble Kalman Filter