NITROUS OXIDE (N20) IS THE THIRD MOST IMPORTANT ANTHROPOGENIC GREENHOUSE GAS (GHG) IN THE ATMOSPHERE AND A MAJOR OZONE DEPLETING SUBSTANCE (0DS) IN THE STRATOSPHERE. ITS ATMOSPHERIC CONCENTRATION HAS BEEN INCREASING AS A RESULT OF HUMAN ACTIVITIES ALTERING THE NATURAL NITROGEN CYCLE. TOTAL EMISSIONS HAVE INCREASED BY 60% ABOVE PREINDUSTRIAL CONDITIONS AS DEDUCED FROM THE OBSERVED GLOBAL MEAN CONCENTRATION TREND AND ITS ATMOSPHERIC LIFETIME. ALTHOUGH MAJOR N20 SOURCES ARE KNOWN THE MAGNITUDES OF INDIVIDUAL SOURCES ARE HIGHLY UNCERTAIN. ACCURATE N20 EMISSIONS ESTIMATES FROM THESE SOURCES AND THEIR PAST CHANGES AND FUTURE EVOLUTION HAVE IMPORTANT IMPLICATIONS FOR CLIMATE AND FOR FORMULATION OF EMISSIONS MITIGATION STRATEGIES. WE PROPOSE TO MAP A 4-YEAR PROJECT WITH THE NASA GE0S-5 MODEL TO SIMULATE ATMOSPHERIC AND OCEANIC N20 AND ITS ISOTOPOLOGUES. MODELING AND ANALYSIS OF THE N20 STABLE ISOTOPOLOGUES HAS EMERGED AS A PROMISING MEANS FOR CONSTRAINING THE BUDGET SINCE N20 FROM VARIOUS SOURCES AND SINKS HAS DISTINCT ISOTOPIC SIGNATURES. THE PROPOSED MAIN TASKS ARE: 1. DEVELOP A FLUX-BASED CONFIGURATION FOR N20 AND ITS ISOTOPOLOGUES IN THE GE0S-5 MODEL SPECIFICALLY THE ABUNDANT LIGHT 14N14N160 AND THE THREE RARER HEAVY ISOTOPOLOGUES: 14N14N180 14N15N160 AND 15N14N160. 2. DEVELOP A PROCESS-BASED OCEANIC EMISSIONS SCHEME FOR N20 IN THE GE0S-5 0CEAN BIOGEOCHEMISTRY MODEL. 3. USE GROUND (AGAGE AND N0AA GMD) BALLOON (P0LARIS) AIRCRAFT (S0LVE HIPP0 ATOM) SATELLITE (AURA-MLS ACE-FTS) AND OCEANIC (MEMENT0) MEASUREMENTS TO EVALUATE KEY PHOTOCHEMICAL AND TRANSPORT PROCESSES IN GE0S-5. 4. USE N20 AND ITS ISOTOPOLOGUES MEASUREMENTS TO DERIVE A NEW N20 EMISSIONS ESTIMATE WITH IMPROVED EMISSIONS FROM OCEAN SOIL AND ANTHROPOGENIC ACTIVITIES. 5. ESTIMATE HOW OCEANIC N20 EMISSIONS MAY EVOLVE WITH FUTURE CLIMATE AND ASSESS THE CHEMICAL AND RADIATIVE IMPACTS OF N20 IN A CHEMICALLY COUPLED ATMOSPHERE-OCEAN MODEL. UPON SUCCESSFUL COMPLETION OF THE PROPOSED WORK WE WILL DELIVER: A NEW N20 EMISSIONS ESTIMATE WITH BETTER-QUANTIFIED GLOBAL EMISSIONS PARTITIONED AMONG DIFFERENT SOURCES. A GLOBAL MODEL-BASED BUDGET ESTIMATE OF N20 ISOTOPOLOGUES AND REALISTIC 3-D GEOGRAPHIC DISTRIBUTION OF THE N20 ISOTOPOLOGUE SIGNATURES IN THE ATMOSPHERE. A FLUX-BASED N20 CHEMISTRY CAPABILITY IN THE GE0S-5 RADIATIVE-CHEMICAL-DYNAMICAL 3-D MODEL WITH PROGNOSTIC OCEANIC EMISSIONS IN THE 0CEAN BIOGEOCHEMISTRY MODEL WITH ASSIGNED ISOTOPOLOGUE SIGNATURES. THIS PROPOSED INVESTIGATION WILL GREATLY IMPROVE THE GLOBAL MODEL REPRESENTATION OF N20 A MAJOR GHG AND 0DS AND ITS INTERACTION WITH CLIMATE IN THE NASA GE0S-5 MODEL. THE PROPOSED NEW N20 CONFIGURATION WILL BE A CRITICAL FIRST STEP IN FUTURE DEVELOPMENT OF THE NITROGEN CYCLE IN CHEMICALLY COUPLED ATMOSPHERE-OCEAN-LAND WITHIN THE EARTH SYSTEM MODELING FRAMEWORK. THE PROPOSED INVESTIGATION IS DIRECTLY RELEVANT TO THE RESEARCH ELEMENTS DESCRIBED IN R0SES 2016 A.13 MODELING ANALYSIS AND PREDICTION: "CONSTITUENTS IN THE CLIMATE SYSTEM: CONSTITUENTS IN THE ATMOSPHERE (AEROSOLS AND CHEMICAL SPECIES) WILL RESPOND TO CLIMATE CHANGE AND CHANGES IN CONSTITUENT CONCENTRATIONS CAN HAVE CLIMATIC CONSEQUENCES AS WELL" AND "THE DEVELOPMENT AND IMPLEMENTATION OF PHYSICALLY-BASE INTERACTIVE EMISSIONS PARAMETERIZATIONS WHICH CAN RESPOND TO CLIMATE CHANGE AND OTHER SOURCES OF VARIABILITY IN THE EARTH SYSTEM".