FOR THIS PROPOSAL WE PLAN TO INVESTIGATE THE IMPACT OF AEROSOL SCHEME COMPLEXITY ON THE SIMULATION OF AEROSOL LOADING COMPOSITION AND RADIATIVE PROPERTIES IN THE UPPER TROPOSPHERE/LOWER STRATOSPHERE (UTLS). WE WILL PERFORM SIMULATIONS FOCUSED ON THE PERIOD 2010 TO PRESENT WHICH OVERLAPS WITH THE AVAILABILITY LIMB SCATTERING OBSERVATIONS FROM OMPS/LP AND OSIRIS AND OCCULTATION OBSERVATIONS FROM SAGE III/ISS THAT WILL BE USED TO EVALUATE THE SIMULATIONS. SIMULATIONS WILL BE PERFORMED IN A REPLAY MODE (I.E. METEOROLOGY DRIVEN BY WINDS FROM THE MERRA-2 REANALYSIS) AT A GLOBAL 0.5 DEGREE HORIZONTAL RESOLUTION WITH 72 VERTICAL LEVELS. ALL SIMULATIONS WILL INCLUDE THE GMI TROPOSPHERIC-STRATOSPHERIC CHEMICAL MECHANISM. THE FIRST SIMULATION TO BE PERFORMED PRIOR TO THE START OF THIS PROPOSAL WILL INCLUDE THE BULK AEROSOL MODULE GOCART COUPLED TO THE RADIATION THE TWO-MOMENT CLOUD SCHEME AND THE GMI CHEMISTRY. GOCART WILL BE SET UP USING TWO TRACERS FOR SULFATE ONE WITH AEROSOL MEDIAN RADIUS APPROPRIATE FOR TROPOSPHERIC AEROSOL (0.15 M) AND ONE WITH RADIUS APPROPRIATE FOR NABRO-SIZED VOLCANIC ERUPTIONS (0.27 M). MAM AND CARMA WILL BE RUN PASSIVELY. PRIOR TO THE START OF THIS SIMULATIONS THE OCS PHOTOLYSIS WILL BE IMPLEMENTED IN GMI IN ORDER TO PROVIDE THE FORMATION OF BACKGROUND STRATOSPHERIC AEROSOL. ADDITIONALLY THE COUPLING BETWEEN GMI AND GOCART WILL BE EXTENDED TO INCLUDE NITRATES. THIS SIMULATION WILL BE COMPARED TO SIMILAR RECENT REANALYSIS INCLUDING THE MERRA-2 AEROSOL REANALYSIS (WITH DATA ASSIMILATION) AND THE MERRA-2-GMI REPLAY (WITH FULL CHEMISTRY BUT NO DATA ASSIMILATION). OUR RESULTS WILL BE MOST LIKE MERRA-2-GMI BUT DIFFER BY INCLUSION OF THE CARMA AND MAM SCHEMES AS PASSIVE TRACERS AND THE INCLUSION OF THE NATURAL BACKGROUND STATE OF STRATOSPHERIC AEROSOL.