THE OCO-3 INSTRUMENT SCHEDULED FOR DEPLOYMENT TO THE INTERNATIONAL SPACE STATION (ISS) WILL CONTINUE THE LEGACY OF THE OCO-2 MISSION WHILE FURTHER ADVANCING THE CAPABILITY OF URBAN EMISSION OBSERVATION. A GREAT ADVANTAGE OVER THE OCO-2 LIES IN THE OBSERVING STRATEGY OF OCO-3 FOCUSING ON CO2 PLUMES OVER CITIES (100 TARGETS PER DAY) USING ITS MAP MODE OBSERVATION CAPABILITY. FROM PREVIOUS SUPPORT THE PROPOSAL TEAM HAVE FOUND THAT THE OCO-2 UNLIKE JAPANESE GREENHOUSE GAS OBSERVING SATELLITE (GOSAT) KEPT MISSING MAJOR CITIES DUE TO THE LACK OF THE POINTING OBSERVATION CAPABILITY AND ONLY PROVIDED VERY FEW SCENES WHERE CLEAR URBAN PLUMES AND/ OR CO2 CONCENTRATION ENHANCEMENTS WERE CAPTURED. ALSO THE OCO-2 SWATH (~8 KM) IS OFTEN TOO NARROW TO CAPTURE CO2 PLUMES WHOSE SPATIAL AND TEMPORAL STRUCTURES ARE HIGHLY VARIABLE AND CAN DEPEND ON A COMPLEX MIX OF EMISSION PATTERNS AND LOCAL METEOROLOGICAL CONDITIONS. WITH THE MAP-MODE OBSERVATIONS THE OCO-3 SHOULD PROVIDE SIGNIFICANTLY MORE URBAN SOUNDINGS AND ALLOW US TO IMPROVE QUANTIFICATION OF URBAN CO2 EMISSIONS FROM GLOBAL CITIES. THE VARIABILITY OF OCO-3 FOOTPRINT SCALE CO2 OBSERVATIONS WILL BE THE KEY INFORMATION USED TO CONSTRAIN EMISSIONS. TO FULLY UTILIZE THESE FINE SCALE SPATIAL CO2 GRADIENTS FROM THE SPATIALLY-DENSE MAP-MODE RETRIEVALS THE RETRIEVAL TEAM WILL LIKELY NEED TO IMPLEMENT THE RETRIEVAL EVALUATION IN A DIFFERENT WAY THAN HAS BEEN DONE FOR OCO-2. THE RETRIEVAL OVER URBAN AREAS ARE OFTEN CHALLENGING DUE TO THE PRESENCE OF OPTICALLY THICK CLOUDS AND AEROSOLS WHICH RESTRICT VIEWING AND TOPOGRAPHY THAT IS UNIQUE TO EACH CITY AND REGION. CO2 FIELDS FROM ATMOSPHERIC TRANSPORT MODELS HAVE BEEN A VALUABLE TOOL FOR EVALUATING THE OCO-2 RETRIEVALS AND IDENTIFYING LARGE SCALE BIASES. THOSE MODELS USED ARE HOWEVER TYPICALLY RUNNING AT SEVERAL TENDS TO HUNDREDS OF KM HORIZONTAL RESOLUTIONS AND ARE NOT ABLE TO RESOLVE THE FINE SCALE URBAN CO2 VARIABILITIES AT THE OCO-2/3 FOOTPRINT SCALE IN TERMS OF METEOROLOGY OR SURFACE EMISSIONS. THIS WORK PROPOSES THE USE OF HIGH-RESOLUTION SIMULATIONS AS AN EVALUATION TOOL FOR THE OCO-3 DATA TO ENSURE THE REALISM OF URBANSCALE SPATIAL GRADIENTS. LEVERAGING THE LOCALLY CALIBRATED WRF AND HIGH-RESOLUTION GLOBAL SIMULATIONS FROM NASA'S GEOS MODEL WE WILL SUPPORT EFFORTS OF THE RETRIEVAL TEAM TO QUANTIFY BIASES AND ERRORS IN A MORE ADVANCED WAY SUPPORTING USE OF THE LEVEL 2 RETRIEVAL CODE WITH HIGH-RESOLUTION URBAN SCALE SIMULATIONS OF CO2 CLOUD AND AEROSOL FIELDS. TO EFFICIENTLY USE THE LIMITED OBSERVATION RESOURCE THIS WORK ALSO STUDIES THE OPTIMAL OBSERVATION STRATEGY BY OPTIMIZING THE GEOMETRY OF THE OCO-3 OBSERVATIONS FOR FOCUSED CITIES USING A STATE-OF-THE-ART HIGH-RESOLUTION URBAN INVERSION SYSTEM. FINALLY WE WILL PROTOTYPE A GLOBAL URBAN EMISSION INVERSION LEVERAGING ONGOING NASA'S HIGH-RESOLUTION SIMULATION CAPABILITIES AND OCO-3 DATA. THIS PROPOSAL IS DIRECTLY RESPONSIVE TO THE CALL BY PROPOSING WORK THAT PERFORMS AN ANALYSIS OF OCO-2 AND OCO-3 DATA SETS FOR UNDERSTANDING DISCRETE SOURCES OF CO2 EMISSION PARTICULARLY FOCUSING ON THE OCO-3 CAPABILITY. THROUGH THE MODEL-BASED EVALUATION THIS WORK WILL ENSURE CONSISTENT DATA QUALITY AND BETTER SCIENTIFIC INTERPRETATIONS OF THE LEVEL 2 DATA PRODUCT BETWEEN THE TWO MISSIONS BY ASSESSING RETRIEVAL BIASES ERRORS AND COVARIANCE IN THE TWO PRODUCTS RELATIVE TO KEY VARIABLES SUCH AS OBSERVATIONAL GEOMETRIES CLOUDS AND AEROSOLS. THIS WORK ALSO PERFORM NEW RESEARCH AND INNOVATIVE ANALYSES USING DATA FROM OCO-2 AND 3 COMBINED WITH OTHER SENSORS (SUOMI-NPP/VIIRS AND CALIPSO) TO ADVANCE THE SCIENCE GOALS OF OCO-2 AND 3 IN ORDER TO SIGNIFICANTLY ADVANCE OUR UNDERSTANDING OF ANTHROPOGENIC EMISSIONS WHICH IS A KEY COMPONENT IN THE CARBON CYCLE.