THE STRATOSPHERIC AEROSOL AND GAS EXPERIMENT III ON THE INTERNATIONAL SPACE STATION (SAGE III-ISS) CONTINUES THE LONG HISTORY OF SAGE INSTRUMENTS RETRIEVING AEROSOL EXTINCTION PROFILES FROM SPACE. A CRITICAL ELEMENT OF THIS LONG-TERM DATA RECORD IS THE VALIDATION OF THE RETRIEVED AEROSOL EXTINCTION AND SURFACE AREA DENSITY PROFILES AGAINST IN SITU MEASUREMENTS OF AEROSOL PROPERTIES. THE WORK PROPOSED HERE WILL EXTEND AND ENHANCE AN EQUALLY LONG-TERM RECORD OF IN SITU MEASUREMENTS OF STRATOSPHERIC AEROSOL SIZE DISTRIBUTION FROM BALLOONS AND APPLY THESE ESTABLISHED MEASUREMENTS TO THE VALIDATION OF THE SAGE III-ISS AEROSOL EXTINCTION AND SURFACE AREA DENSITY MEASUREMENTS. THE PRIMARY ACTIVITY OF THE PROPOSED WORK IS TO MEASURE IN SITU PROFILES OF STRATOSPHERIC AEROSOL SIZE DISTRIBUTIONS USING NEWLY DEVELOPED OPTICAL PARTICLE AND CONDENSATION NUCLEI COUNTERS DEPLOYED ON HIGH ALTITUDE BALLOONS AND TIMED TO COINCIDE WITH SAGE IIIISS OVERPASSES OF THE TWO MEASUREMENT SITES BOULDER COLORADO AND SAN JOSE COSTA RICA. THIS NEW IN SITU AEROSOL INSTRUMENTATION PROVIDES A SINGULAR CAPABILITY TO MEASURE STRATOSPHERIC AEROSOL SIZE DISTRIBUTIONS OF RADIUS 0.125 - 10 MICROMETERS AS WELL AS THE TOTAL AEROSOL POPULATION WITH RADIUS>0.01 MICROMETERS AT EXTREMELY LOW NON-VOLCANIC PARTICLE DENSITIES AND WITH A VERTICAL RESOLUTION OF BETTER THAN 50M. THE MEASURED AEROSOL SIZE DISTRIBUTIONS WILL BE USED TO CALCULATE AEROSOL EXTINCTION AT THE SAGE III WAVELENGTHS AND THUS PROVIDE AN INDEPENDENT VALIDATION OF THE SAGE III-ISS AEROSOL EXTINCTION PRODUCTS IN RESPONSE TO THE REQUIREMENTS IN SECTION 3.1 OF THE SOLICITATION. THE MEASUREMENTS WILL PROVIDE A PRIORI AEROSOL SIZE DISTRIBUTION INFORMATION TO INFORM LIMB SCATTER ALGORITHM DEVELOPMENT PROVIDING INFORMATION TO TEST VALIDATE AND REFINE LIMB SCATTER ALGORITHMS ADDRESSING REQUIREMENTS IN SECTION 3.2 OF THE SOLICITATION. THE IN SITU MEASUREMENTS WILL CONTINUE A 45-YEAR RECORD OF STRATOSPHERIC AEROSOL MEASUREMENTS WHICH SPANS THE HISTORY OF THE SAGE/SAM INSTRUMENTS AND CAN BRIDGE THE 12-YEAR GAP BETWEEN SAGE III-METEOR AND SAGE III-ISS. PERFORMING AEROSOL PROFILE MEASUREMENTS AT THE SHADOZ SITE IN COSTA RICA WILL: INCREASE THE LATITUDINAL COVERAGE OF IN SITU AEROSOL PROFILES PROVIDE AEROSOL MEASUREMENTS CORRELATED WITH MEASUREMENTS OF OZONE WATER VAPOR AND SULFUR DIOXIDE AND INTEGRATE THE AEROSOL MEASUREMENTS INTO A LONG-TERM TROPICAL RECORD. THESE MEASUREMENTS CAN AUGMENT SAGE III-ISS AEROSOL EXTINCTION PROFILES DURING FUTURE VOLCANIC ERUPTIONS WHEN THE LOWER STRATOSPHERE MAY BE OPAQUE AT SAGE III WAVELENGTHS. TYING THE SAGE III-ISS MEASUREMENTS INTO TWO MULTI-DECADAL DATA SETS AND INSURING THE CONTINUITY OF THESE DATA IN THE CASE OF AN ABRUPT GEOPHYSICAL EVENT ADDRESSES SECTION 3.3 OF THE SOLICITATION. MEASUREMENTS OF STRATOSPHERIC AEROSOL SIZE DISTRIBUTIONS CAN PROVIDE CONTEMPORARY BACKSCATTER-TO-EXTINCTION RATIOS AND ANGSTROM EXPONENTS NECESSARY TO COMPARE SAGE III WITH LIDAR AND WITH EXTINCTION MEASUREMENTS FROM OTHER SATELLITE SENSORS. THIS FACILITATES THE COMPARISON AND SYNTHESIS OF MEASUREMENTS FROM MULTIPLE SENSORS ADDRESSING SECTION 3.6 OF THE SOLICITATION.