WE PROPOSE TO DERIVE BIAS CORRECTED LOWER TROPOSPHERIC (LT) PARTIAL COLUMN CO2 ESTIMATES FROM OCO2 FOR ASSIMILATION IN CO2 FLUX INVERSIONS. OUR RESULTS WILL PROVIDE OCO2 FLUX INVERSION MODELERS WITH AN ALTERNATIVE LEVEL 2 QUANTITY LT CO2 THAT IS MORE SUITED FOR INVERSIONS DUE TO ITS ROBUSTNESS TO CERTAIN TRANSPORT ERRORS (COMPARED TO BOUNDARY LAYER CO2) AND LARGER FLUX-INDUCED GRADIENTS AND VARIATIONS (COMPARED TO TOTAL COLUMN CO2). THE FINAL PRODUCT BIAS CORRECTED LT CO2 WITH QUANTIFIED ERRORS WILL BE OBTAINED IN FOUR STEPS: (A) EXPLORING THE TRADE-OFF BETWEEN REDUCED TRANSPORT UNCERTAINTY AND SMALLER FLUX SIGNALS AS ONE TRANSITIONS FROM BOUNDARY LAYER CO2 TO TOTAL COLUMN CO2 IN ORDER TO CHOOSE AN OPTIMAL LT COLUMN HEIGHT THAT BALANCES BOTH (B) ESTIMATING THE LT PARTIAL COLUMN CO2 FROM VERTICAL INFORMATION IN NASA'S OCO2 RETRIEVALS OF THE CO2 PROFILE (C) BIAS CORRECTING LT CO2 VIA CO-ASSIMILATION OF CALIBRATED IN SITU CO2 MEASUREMENTS IN A VARIATIONAL FLUX INVERSION AND (D) VALIDATING THESE BIAS CORRECTED LT CO2 DATA AGAINST COMPARABLE PARTIAL COLUMNS CONSTRUCTED FROM AIRCRAFT PROFILE MEASUREMENTS. WE RECENTLY PUBLISHED A TECHNIQUE TO DERIVE LT CO2 PARTIAL COLUMNS FROM GOSAT CO2 RETRIEVALS AND STEPS (A) AND (B) EXTEND THAT TO OCO2 RETRIEVALS WITH EMPHASIS ON DERIVING AN OCO2 LT CO2 OPTIMALLY SUITED FOR FLUX INVERSIONS. BIAS CORRECTING AND VALIDATING GOSAT LT CO2 WITH COLLOCATED AIRCRAFT PROFILES WAS A MAJOR HURDLE FOR THAT WORK SO IN STEP (C) WE PROPOSE TO DERIVE ACCURATE BIAS CORRECTIONS FOR OCO2 LT CO2 BY USING A PREVIOUSLY PUBLISHED ONLINE BIAS CORRECTION TECHNIQUE THAT DOES NOT REQUIRE STRICTLY COLLOCATED VALIDATION DATA. THE BIAS CORRECTION TECHNIQUE WAS PREVIOUSLY APPLIED TO GOSAT TOTAL COLUMN CO2 AND PRELIMINARY ANALYSES SHOW THAT IT IS APPLICABLE AS WELL FOR OCO2 TOTAL COLUMN CO2 AND LT CO2. FOR QUANTIFYING THE ACCURACY AND PRECISION OF BIAS-CORRECTED OCO2 LT CO2 IN STEP (D) WE WILL USE ONGOING ROUTINE AND CAMPAIGN FLIGHTS AS WELL AS NEW FLIGHTS PROPOSED SPECIFICALLY FOR THIS PROJECT. DELIVERABLES FROM THIS PROJECT INCLUDE (A) BIAS CORRECTED LT CO2 ESTIMATES FROM OCO2 (B) SURFACE CO2 FLUX ESTIMATES FROM ASSIMILATING SAID LT CO2 AND OTHER DATA SOURCES SUCH AS IN SITU CO2 AND OCO2 COLUMN CO2 (C) VALIDATION PROFILES OF CO2 OVER THE WESTERN US FROM THE AJAX AIRCRAFT (D) VALIDATION PROFILES OVER CENTRAL AMAZONIA FROM AN AIRCRAFT UPWIND OF MANAUS AND (E) CO2 TIME SERIES FROM A 45 M TOWER AT TAMBOPATA PERU IN WESTERN AMAZONIA. PRODUCT (A) WILL BE BENEFICIAL TO THE OCO2 FLUX INVERSION COMMUNITY AS A NOVEL LEVEL 2 PRODUCT IDEALLY SUITED TO FLUX INVERSIONS. PRODUCT (B) WILL BE EVALUATED AGAINST OTHER INVERSIONS PERFORMED BY THE COMMUNITY BOTH WITH AND WITHOUT OCO2 DATA. PRODUCT (B) WILL ALSO BE VALIDATED BY COMPARING THE POSTERIOR CO2 FIELDS WITH NON-ASSIMILATED ATMOSPHERIC CO2 DATA. IN ADDITION TO VALIDATING LT CO2 FOR OUR PROJECT PROFILES FROM PRODUCTS (C) AND (D) CAN BE USED TO VALIDATE OTHER OCO2 COLUMN CO2 RETRIEVALS. THEY GO BEYOND THE EXISTING VALIDATION PLAN FOR OCO2 LEVEL 2 DATA AND THE MANAUS PROFILES IN PARTICULAR WILL PROVIDE VALIDATION DATA IN A SEVERELY UNDER-SAMPLED BUT ECOLOGICALLY IMPORTANT PART OF THE GLOBE. PRODUCTS (C) AND (D) CAN ALSO BE USED BY THE OCO2 FLUX INVERSION GROUP AND THE LARGER INVERSION COMMUNITY TO VALIDATE THEIR INVERSIONS BY COMPARISON WITH POSTERIOR CO2 FIELDS. PRODUCT (E) PRESENTS A NEW DATA SOURCE FOR FLUX INVERSIONS OVER A DATA-POOR REGION OF THE GLOBE. OUR PROPOSAL RESPONDS TO TOPICS 2 5 AND 6 OF THE PROPOSAL CALL: "NEW RESEARCH AND INNOVATIVE ANALYSES USING DATA FROM OCO2" "ASSESSING RETRIEVAL BIASES ERRORS AND COVARIANCES IN THE OCO2 LEVEL 2 PRODUCTS" AND "VALIDATION STRATEGIES FOR LEVEL 2 PRODUCTS".