NASA HAS MADE LARGE INVESTMENTS IN SATELLITE OBSERVATIONS OF ATMOSPHERIC COMPOSITION BUT THEIR USE IN THE GEOS DATA ASSIMILATION SYSTEM (DAS) AT GMAO IS STILL VERY LIMITED. THIS PROJECT WILL (1) DELIVER AND EVALUATE A COMPREHENSIVE STATE-OF-SCIENCE ATMOSPHERIC CHEMISTRY MODULE IN THE GEOS EARTH SYSTEM MODEL (ESM) AND DAS TO SIMULATE COUPLED GAS-AEROSOL CHEMISTRY FROM THE SURFACE TO THE STRATOPAUSE AND (2) APPLY THIS MODULE TO ASSIMILATE CURRENT AND NEAR-FUTURE SATELLITE OBSERVATIONS OF ATMOSPHERIC COMPOSITION IN THE GEOS DAS. IT WILL CONTINUE THE SUCCESSFUL HARVARD-GMAO COLLABORATION TO IMPLEMENT THE CHEMICAL MODULE FROM THE GEOSCHEM CHEMICAL TRANSPORT MODEL (CTM) INTO THE GEOS SYSTEM. THROUGH THIS ACTIVITY THE GEOS-CHEM CTM HAS BEEN CONVERTED INTO A GRID-INDEPENDENT ESMF-COMPLIANT MODEL SO THAT THE EXACT SAME CODE IS USED BY THE STAND-ALONE CTM AND AS A CHEMICAL MODULE WITHIN THE GEOS ESM. THIS ENSURES THAT THE CONTINUAL STREAM OF INNOVATION FROM THE LARGE WORLDWIDE GEOS-CHEM USER COMMUNITY IS STRAIGHTFORWARDLY PASSED ON TO THE GEOS ESM CHEMICAL MODULE WHICH ALWAYS REMAINS UP-TO-DATE AND REFERENCED TO THE LATEST STANDARD RELEASE OF GEOS-CHEM. THE GEOS-CHEM CHEMICAL MODULE IS NOW IN PLACE IN THE GEOS ESM AND IS BEING USED FOR VERYHIGH- RESOLUTION SIMULATIONS OF TROPOSPHERIC OZONE CHEMISTRY AND FOR ASSIMILATION OF OMI AND MLS SATELLITE OZONE DATA IN THE DAS. OUR FOCUS SO FAR HAS BEEN ON TROPOSPHERIC OZONE CHEMISTRY BUT GEOS-CHEM HAS STATE-OF-SCIENCE CAPABILITY FOR COUPLED GAS-AEROSOL CHEMISTRY IN THE TROPOSPHERE AND STRATOSPHERE. HERE WE WILL EXPLOIT THIS CAPABILITY IN THE GEOS ESM CHEMICAL MODULE AND COUPLE GEOS-CHEM AEROSOL CHEMISTRY TO AEROSOL MICROPHYSICS AND RADIATIVE TRANSFER THROUGH THE MODAL AEROSOL MODULE (MAM) ALREADY IN PLACE IN THE GEOS ESM. GEOS-CHEM WILL PROVIDE A CAPABILITY FOR COUPLED GAS-AEROSOL CHEMISTRY AT GMAO AND WILL HELP TO REVITALIZE THE GLOBAL MODEL INITIATIVE (GMI) CAPABILITY FOR STRATOSPHERIC-TROPOSPHERIC CHEMISTRY. WE WILL APPLY THE GEOS-CHEM MODULE IN THE GEOS DAS TO MULTI-SPECIES CHEMICAL DATA ASSIMILATION IN THE TROPOSPHERE EXPLOITING DATA FROM PRESENT AND NEAR-FUTURE SATELLITE MEASUREMENTS. OUR SPECIFIC OBJECTIVES ARE AS FOLLOWS: 1. EXPAND THE GEOS-CHEM CHEMICAL MODULE CAPABILITY IN THE GEOS ESM TO INCLUDE COMPLETE COUPLED GAS-AEROSOL CHEMISTRY IN THE TROPOSPHERE AND STRATOSPHERE AND DELIVER IN THIS MANNER A COMPREHENSIVE PLATFORM FOR MODELING ATMOSPHERIC CHEMISTRY IN THE GEOS ESM AND DAS; 2. EVALUATE THE GEOS-CHEM CHEMICAL MODULE IN THE GEOS ESM WITH MULTI-YEAR SATELLITE AND SUBORBITAL OBSERVATIONS AND COMPARE WITH OTHER CHEMICAL MODULES INCLUDING GMI (STRATOSPHERIC AND TROPOSPHERIC CHEMISTRY) AND GOCART (AEROSOLS); 3. CONDUCT MULTI-SPECIES CHEMICAL DATA ASSIMILATION IN THE GEOS DAS INCLUDING CURRENT SATELLITE OBSERVATIONS OF TROPOSPHERIC OZONE CO NO2 AND FORMALDEHYDE (HCHO); 4. APPLY THIS CHEMICAL DATA ASSIMILATION CAPABILITY IN THE GEOS DAS TO NEAR-FUTURE SATELLITE MEASUREMENTS OF ATMOSPHERIC COMPOSITION INCLUDING FROM TROPOMI AND THE GEOSTATIONARY CONSTELLATION WITH THE GOAL OF DEVELOPING A SYSTEM FOR REAL-TIME ANALYSIS OF GLOBAL AIR QUALITY AT GMAO. THIS PROJECT WILL CONTINUE THE PARTNERSHIP BETWEEN HARVARD AND GMAO IN DEVELOPING A STATE-OF-SCIENCE CHEMICAL DATA ASSIMILATION CAPABILITY FOR THE GEOS SYSTEM. AS SUCH IT WILL MAKE A MAJOR CONTRIBUTION TO THE DEVELOPMENT OF AN INTEGRATED EARTH SYSTEM ANALYSIS (IESA) CAPABILITY AND TO THE CORE MISSION OF GMAO.