WE WILL UTILIZE AIRCORE AND AIRCRAFT MEASUREMENTS THAT ARE DIRECTLY TRACEABLE TO WORLD METEOROLOGICAL ORGANIZATION (WMO) STANDARDS TO IMPROVE: (I) THE BIAS CORRECTIONS OF RAW XCO2 DATA VIA SCALING OF AUXILIARY VARIABLES; (II) LINKAGE OF XCO2 DATA TO THE WMO STANDARDS AND (III) FLUX ESTIMATES FROM THE INVERSION MODELS OF OCO-2 TOTAL COLUMN CO2 (XCO2) RETRIEVALS. THIS PROPOSAL BUILDS ON THE SEVERAL YEARS OF AIRCORE SAMPLING SYSTEM DEVELOPMENTS AND DATA THAT HAVE PLAYED A SIGNIFICANT ROLE IN SCALING TCCON SITES TO WMO STANDARDS AND HENCE OCO-2 RETRIEVAL COMPATIBILITY WITH IN SITU MEASUREMENTS. THE PROPOSED STUDY HAS TWO PRIMARY ACTIVITIES:1) AIRCORE FLIGHTS AND CALIBRATION OF OCO-2: WE PROPOSE MORE THAN 45 DAYS OF AIRCORE FLIGHTS OVER A 3-YEAR PERIOD THAT WILL CAPTURE CO2 CH4 AND CO PROFILES FROM THE SURFACE TO>25 KM (>95% OF THE ATMOSPHERIC COLUMN). THE MAJORITY OF THE FLIGHTS WILL BE IN COLORADO WITH NEAR-MONTHLY LAUNCHES AND WILL BE CO-LOCATED WITH A PORTABLE BRUKER EM27SUN SPECTROMETER. THE EM27SUN HAS BEEN SHOWN TO GIVE COMPARABLE PRECISION AND REPEATABILITY TO THE TCCON SPECTROMETERS THAT ARE USED AS THE PRIMARY SOURCE OF CALIBRATION FOR OCO-2 XCO2 RETRIEVALS. IN ADDITION TO THE BOULDER FLIGHTS WE WILL CONDUCT TWO 5-DAY CAMPAIGNS IN SODANKYLA FINLAND; TWO 10-DAY CAMPAIGNS IN DARWIN AUSTRALIA; AND A 5-DAY CAMPAIGN AT EACH OF THE TCCON NETWORK SITES IN PARK FALLS WI; LAMONT OK; AND EDWARDS AIR FORCE BASE IN CALIFORNIA. THESE CAMPAIGNS WILL ESTABLISH INTRA-TCCON COMPATIBILITY AND IMPROVE THE ACCURACY OF THE SCALING FACTOR THAT LINKS TCCON MEASUREMENTS TO THE WMO SCALE. THE RESULTING WMO-TRACEABLE XCO2 FROM THE TCCON OR BRUKER EM27SUN SPECTROMETER WILL ACT AS A "TRANSFER STANDARD" FOR OCO-2 XCO2 MEASUREMENTS. THE FLIGHTS FROM DARWIN AUSTRALIA PLANNED FOR AUGUST 2018 AND JANUARY OF 2020 WILL BE THE FIRST AIRCORE FLIGHTS THAT SAMPLE THE TROPICAL STRATOSPHERE. 2) VERTICAL PROFILE DATA/MODEL COMPARISON: THE SECOND PRIORITY OF THIS PROPOSAL WILL BE TO LEVERAGE A LARGE ARRAY OF AIRCRAFT DATA IN ADDITION AIRCORE DATA COLLECTED BY NOAA AND OTHER COLLABORATORS FOR COMPARISON TO THE INVERSION MODELS USED TO ESTIMATE FLUXES FROM OCO-2 RETRIEVALS. THIS ANALYSIS WILL ASSESS HOW WELL THE INVERSE MODELS WHICH ARE INGESTING GROUND-BASED IN SITU AND OCO-2 DATA REPRESENT VERTICAL AND HORIZONTAL GRADIENTS FROM REGIONAL TO GLOBAL SCALES. PRELIMINARY COMPARISONS WITH AIRCORE SHOW THAT MOST MODELS UNDERESTIMATE THE CO2 GRADIENTS IN THE LOWER STRATOSPHERE SIGNIFYING THE POTENTIAL FOR A BIAS IN THE FLUX ESTIMATES AND MISINTERPRETATION OF OCO-2 RETRIEVALS. SIMILAR MODEL BIASES EXIST IN THE SIMULATED INTERHEMISPHERIC GRADIENT. BY MAKING TRACERS SUCH AS SF6 AVAILABLE TO MODELERS IT WILL BE POSSIBLE INDEPENDENTLY DIAGNOSE THE ACCURACY OF THE TRANSPORT FIELDS IN EACH MODEL TO BETTER UNDERSTAND THE SOURCE OF THE INTERHEMISPHERIC GRADIENT BIAS. THE PROPOSED STUDY ADDRESSES THE OCO-2 SCIENCE TEAM SOLICITATION WITH CONTINUED VALIDATION OF THE LEVEL 2 OCO-2 RETRIEVALS FROM AIRCORE PROVIDING THE BEST POSSIBLE WAY TO DIRECTLY COMPARE THE TOTAL COLUMN CO2 RETRIEVALS WITH IN SITU MEASUREMENTS. BY ALSO UTILIZING HIGH-QUALITY AIRCRAFT DATA THE NOAA/CU TEAM WILL SIGNIFICANTLY EXPAND THE IMPACT OF MEASUREMENTS THAT ARE DIRECTLY TRACEABLE TO THE WMO STANDARD SCALE AND SURFACE-BASED IN SITU MEASUREMENTS OVER A WIDE RANGE OF TEMPORAL AND SPATIAL SCALES. THIS SAME SET OF DATA CAN BE USED TO DIRECTLY ADDRESS THE SOLICITATION'S STATED NEED TO IMPROVE THE FLUX ESTIMATES RESULTING FROM OCO-2 XCO2 RETRIEVALS BY PROVIDING EVALUATION METRICS FOR THE OCO-2 MODELING TEAMS. A STRINGENT SET OF QUANTITATIVE BENCHMARKS GENERATED FROM MODEL-DATA COMPARISONS WILL ENABLE THE OCO-2 INVERSE MODELING GROUPS TO IMPROVE MODEL FIDELITY A PREREQUISITE FOR ACCURATE FLUX ESTIMATES.