THIS PROPOSAL ADDRESSES NASA'S CRITICAL NEED TO ENSURE THE INTEGRITY AND CONTINUITY OF ITS SPACE-BASED OBSERVATIONAL SYSTEMS FOR WATER VAPOR AND OZONE BY CONTINUING THE TICOSONDE WATER VAPOR AND OZONE BALLOON SONDE LAUNCH PROGRAM WHICH WE STARTED IN COSTA RICA IN 2005. TICOSONDE CFH MEASUREMENTS ARE THE ONLY CONTINUOUS SERIES OF IN SITU WATER VAPOR MEASUREMENTS IN THE TROPICS WITH NEARLY 200 LATE VAPOR SOUNDINGS TO DATE AND TICOSONDE HAS CONTRIBUTED 500 PROFILES OF OZONE TO SHADOZ AS WELL. WE HAVE TWO MAIN OBJECTIVES IN THIS PROPOSAL. THE FIRST IS TO FULFILL NASA S CRITICAL NEED TO SUPPORT WATER VAPOR AND OZONE MONITORING IN THE TROPICS AND SECOND TO PROVIDE SUFFICIENT WATER VAPOR AND OZONE BALLOON SONDE DATA FOR VALIDATION OF THE SAGE-III ISS OMPSLIMB AND OTHER SPACE-BASED MEASUREMENTS OF THESE TRACE SPECIES IN THE UTLS. AS A COMPLEMENTARY OBJECTIVE WE WILL CONDUCT NEEDED INTER-COMPARISONS BETWEEN FROST POINT HYGROMETER WATER VAPOR MEASUREMENTS AND THE NEW VAISALA RS-41 RADIOSONDE. TO ACHIEVE THESE OBJECTIVES WE WILL UNDERTAKE THE FOLLOWING PRIMARY TASKS: REGULAR BALLOON SONDE PROFILING OF WATER VAPOR AND OZONE TO THE LOWER AND MIDDLE STRATOSPHERE. WE WILL MAKE TWICE-MONTHLY LAUNCHES IN COSTA RICA OF ELECTROCHEMICAL CONCENTRATION CELL (ECC) OZONESONDES AUGMENTED ONCE MONTHLY WITH WATER VAPOR MEASUREMENTS USING THE CRYOGENIC FROSTPOINT HYGROMETER (CFH). PROVIDE HIGH-RESOLUTION WATER VAPOR AND OZONE PROFILES FOR VALIDATION OF SAGE-III ISS AND OMPS-LIMB WITH COORDINATED BALLOON SONDE LAUNCHES. SUPPORT ONGOING ASSESSMENT AND VALIDATION OF SATELLITE MEASUREMENTS AT NOAA ESRL AND BY OTHER INVESTIGATORS. COMPARE UT/LS MEASUREMENTS OF WATER VAPOR FROM THE VAISALA RS-41 RADIOSONDE AGAINST CFH FROST POINT MEASUREMENTS DURING OUR MONTHLY WATER VAPOR/OZONE LAUNCHES. THE CONTINUED TICOSONDE OBSERVATIONS OF WATER VAPOR AND OZONE IN THE TROPICAL UT/LS PROPOSED HERE DIRECTLY ADDRESS NASA S NEED FOR MONITORING OF THESE TRACE SPECIES IN SUPPORT OF ITS SPACE-BASED PROGRAM TO OBSERVE ATMOSPHERIC COMPOSITION. COSTA RICA ALSO PROVIDES A UNIQUE SITE IN THE DEEP TROPICS FOR VALIDATING INSTRUMENTS SUCH AS SAGE-III ISS. AND IN THE EVENT OF A PREMATURE TERMINATION OF THE AURA MLS INSTRUMENT THE TICOSONDE DATA RECORD WILL BE A KEY BRIDGE TO SUCCESSOR SPACE-BASED WATER VAPOR MEASUREMENTS FOR THE TROPICAL UPPER TROPOSPHERE AND LOWER STRATOSPHERE.