VALIDATION AND APPLICATION OF OCO-2 DATA IN THE NORTHEASTERN UNITED STATESURBAN AREAS REPRESENT A KEY TARGET FOR OCO-2 SCIENCE. WE PROPOSE TO VALIDATE OCO-2 DATA WITH PARTICULAR FOCUS ON THE URBAN-RURAL GRADIENTS OF XCO2 IN THE NORTHEASTERN US AND TO DEVELOP AND DEMONSTRATE METHODS FOR DETERMINING URBAN CO2 EMISSIONS USING OCO-2 DATA. ANALYZING OCO-2 DATA IN AN URBAN DOMAIN IS CHALLENGING ESPECIALLY FOR CITIES CONTAINING AND SURROUNDED BY SIGNIFICANT VEGETATION. THE ENHANCEMENT OF XCO2 IN THE "URBAN DOME" IS 2-10 PPM AND EVEN IN DENSELY POPULATED REGIONS THE DAILY EXCHANGE OF CO2 FROM VEGETATION EQUALS OR EXCEEDS FOSSIL FUEL EMISSIONS. THE NORTHEAST US A HEAVILY VEGETATED DENSELY POPULATED URBAN-RURAL DOMAIN WITH A UNIQUE COMPREHENSIVE CO2 OBSERVATION NETWORK OFFERS A SCIENTIFICALLY AND LOGISTICALLY COMPELLING LOCUS FOR VALIDATION OF OCO-2 DATA AND FOR DEVELOPING INVERSE MODELING FRAMEWORKS TO USE OCO-2 FOR DETERMINING URBAN EMISSIONS IN LESS DATA-RICH ENVIRONMENTS AROUND THE GLOBE. THE VEGETATED LANDSCAPE OF THE NORTHEAST US IS MORE HETEROGENEOUS WITH LOWER REFLECTIVITY AND HAS A MORE ACTIVE BIOSPHERE WITH MUCH GREATER INFLUENCE ON CO2 FLUXES THAN IN LOS ANGELES THE OTHER MAJOR SITE FOR URBAN VALIDATION OF XCO2 FROM OCO-2. HAVING A VALIDATION REGION IN THE NORTHEASTERN US TARGETS ESSENTIAL CHALLENGES FOR INVERSE MODELING OF URBANEMISSIONS USING OCO-2 SATELLITE DATA. OUR PROPOSED WORK WILL PROVIDE TO THE OCO-2 INSTRUMENT TEAM DIRECT ASSESSMENT OF THE XCO2 GRADIENTS ACROSS THE URBAN DOMAIN TRACEABLE TO WMO STANDARDS USING SURFACE MEASUREMENTS TOTAL COLUMN MEASUREMENTS OF XCO2 USING TWO BRUKER EM27SUN FTIRSYSTEMS AT SEVERAL SITES DATA FROM TWO PANDORA UV-VIS SPECTROMETERS AND LIDAR DATA. ALL OF OUR GROUND-BASED SENSORS ARE COMPACT AND MOBILE WHICH PROVIDES SPATIAL FLEXIBILITY FOR THE VALIDATION. PROJECT DELIVERABLES INCLUDE (1) ASSESSMENT AND VALIDATION OF THE CAPABILITY FOR OCO-2 TO ACCURATELY RESOLVE REGIONAL SCALE GRADIENTS OF XCO2 IN A MAJOR URBAN DOMAIN USING OUR NETWORK OF SOLARTRACKING FTSS AND LIDARS INCLUDING RESULTS FROM TEMPORARY DEPLOYMENTS DIRECTLY ON OCO-2 TRACKS; (2) BASIC DATA FOR REFINEMENT OF AEROSOL CORRECTIONS FOR OCO-2 MEASUREMENTS VIA VERTICAL PROFILES OF AEROSOLS BY LIDARS AND TOTAL AEROSOL OPTICAL DEPTH MEASURED BY THE PANDORA UV-VIS SPECTROMETER; (3) ACCURATE REGIONAL CO2 EMISSIONS ESTIMATES BY COMBINING GROUND-BASED SURFACE AND COLUMNDATA WITH OCO-2 XCO2 DATA IN A DATA-MODEL INVERSION FRAMEWORK; (4) IMPROVED CHARACTERIZATION OF URBAN VEGETATION COVER AND ACTIVITY AS NEEDED FOR OCO-2 VALIDATION AND FOR THE TOTAL URBAN CARBON BUDGET; THIS WORK INCLUDES USE OF HIGHLY SELECTIVE OCO-2 FLUORESCENCE DATA; AND (5) DEVELOPMENT OF A GENERALIZED FRAMEWORK TO DERIVE URBAN/REGIONAL SCALE EMISSIONS FROM CITIES AND ASSESSTHE CAPABILITY OF OCO-2 DATA ALONE TO QUANTIFY CO2 EMISSIONS IN DATA-POOR REGIONS. TAKEN AS A WHOLE OUR PROPOSED VALIDATION AND DEVELOPMENT OF A MODEL-DATA INVERSION FRAMEWORK WILL PROVIDE A COMPREHENSIVE BASIS FOR QUANTIFYING URBAN EMISSIONS FILLING MAJOR GAPS IN EXISTING URBAN VALIDATION EFFORTS FOR OCO-2 AND PROVIDING CRITICAL NEW INSIGHTS FOR A REGION THAT INCLUDES SIGNIFICANT BIOGENICAND ANTHROPOGENIC CONTRIBUTIONS.