THIS PROPOSAL SEEKS FUNDING TO CONTINUE OUR RESEARCH TO ADVANCE OCO-2'S CAPABILITIES IN CLOUDY SCENES THROUGH DEVELOPMENT OF NEW TECHNIQUES FOR PARTIAL COLUMN RETRIEVALS. THE CURRENT OPERATIONAL OCO-2 ALGORITHM (B8) INCLUDES SENSITIVE CLOUD FILTERING ALGORITHMS AND CO2 CONCENTRATIONS ARE RETRIEVED ONLY WITHIN CLEAR SKY FIELDS OF VIEW (FOV). THE RETRIEVED CO2 PROFILES ARE INTEGRATED TO THE TOTAL COLUMN DRY AIR MOLE FRACTION (XCO2). HERE WE PROPOSE TO DEVELOP NEW OBSERVATION SELECTION FILTERING AND RETRIEVAL TECHNIQUES TO APPLY RETRIEVALS ALSO TO FOVS OVER BOUNDARY LAYER STRATUS CLOUDS. THE RESEARCH EFFORT WILL HAVE FOUR RELATED TASKS: 1) DEVELOPMENT OF A NEW RETRIEVAL ALGORITHM FOR PARTIAL COLUMN XCO2 OVER STRATUS CLOUDS: CURRENTLY OCO-2'S RETRIEVALS ARE ONLY DONE FOR CLEAR SKY SCENES AND THESE PARTIAL COLUMNS WILL EXTEND THE COVERAGE OF THE OCO-2 DATASET. PARTIAL COLUMN XCO2 INCORPORATES A NEW SOURCE OF VERTICAL INFORMATION ON CO2 CONCENTRATION DISTINCT FROM THE TOTAL COLUMN MOLE FRACTION. 2) DEVELOPMENT OF NEW RETRIEVALS FOR CLOUD PROPERTIES FROM OCO-2: IN ORDER TO ACCURATELY RETRIEVE PARTIAL COLUMN XCO2 WE MUST HAVE CLOUD PROPERTIES APPROPRIATE FOR THE SPATIAL AND RADIOMETRIC SCALE OF OCO-2. THESE CLOUD PROPERTIES MUST BE OBTAINED BY RETRIEVING THESE QUANTITIES FROM THE OCO-2 MEASUREMENTS. THUS WE WILL CREATE A NEW DATASET OF CLOUD PROPERTIES RETRIEVED FROM THE OCO-2 RADIANCES. 3) IMPROVED UNDERSTANDING OF LIMITATIONS OF PASSIVE ONLY RETRIEVALS: FOR DEVELOPMENT OF THESE ALGORITHMS WE WILL INITIALLY UTILIZE A COLOCATED DATASET WHICH COMBINES CALIOP AND OCO-2. THE HIGHLY ACCURATE CLOUD BOUNDARIES FROM THE LIDAR MEASUREMENTS WILL BE USED AS A PRIOR CONSTRAINT FOR THE OCO-2 RETRIEVALS. FURTHER RESEARCH WILL FOCUS ON THE FEASIBILITY OF THE OCO-2 MEASUREMENTS BY THEMSELVES TO QUANTIFY THE PERFORMANCE OF PASSIVE ONLY RETRIEVALS IN THESE SITUATIONS. 4) VALIDATION AND ERROR ANALYSIS: DOWNSTREAM USE OF PARTIAL COLUMN XCO2 DATA IN FLUX INVERSION MODELS WILL DEPEND ON ROBUST ERROR ESTIMATES. VALIDATION OF PARTIAL COLUMN XCO2 WILL BE DONE USING PLANNED FLIGHTS WITH THE ALPHA JET ATMOSPHERIC EXPERIMENT (AJAX) AIRCRAFT. CLOUD PROPERTIES WILL BE VALIDATED AGAINST CONCURRENT OBSERVATIONS FROM OTHER A-TRAIN SENSORS IN PARTICULAR MODIS. WE PLAN TO USE SIMULATIONS TO UNDERSTAND THE ERROR CHARACTERISTICS OF THE RETRIEVED PARTIAL COLUMN. OUR TEAM HAS EXTENSIVE RESEARCH EXPERIENCE WITH ALL ASPECTS OF THE PROPOSED RESEARCH AND HAS DEMONSTRATED EFFECTIVE COLLABORATION WITH THE OCO-2 SCIENCE TEAM. THE NEW PRODUCTS SHOULD BE OF SIGNIFICANT INTEREST TO THE BROADER REMOTE SENSING AND EARTH SCIENCE COMMUNITIES.