A FREQUENCY MODULATED UV SOURCE FOR MEASURING TROPOSPHERIC HYDROXYL RADICAL

THE HYDROXYL RADICAL (OH) PLAYS A KEY ROLE IN THE PHOTOCHEMISTRY OF BOTH THE TROPOSPHERE AND THE STRATOSPHERE. IT IS THE PRIMARY OXIDIZING AGENT RESPONSIBLE FOR THE REMOVAL OF MOST TRACE GASES INCLUDING CO AND CH4. SINCE THE OH RADICAL DETERMINES THE LIFETIMES AND, THUS, THE CONCENTRATIONS OF MANY CHEMICAL SPECIES, ACCURATE DETERMINATION OF OH CONCENTRATION IS PIVOTAL IN UNDERSTANDING ATMOSPHERIC CHEMISTRY. THE TECHNIQUE UTILIZES A TUNABLE, WAVELENGTH MODULATED UV BEAM NEAR 308 NM, GENERATED BY SUM FREQUENCY MIXING THE OUTPUT OF A HIGH POWER SINGLE MODE DIODE LASER WITH THE SINGLE FREQUENCY OUTPUT FROM AN AR+ LASER IN A NONLINEAR CRYSTAL, TO ACCESS THE OH A 2+SIGMA X(2)II BAND. THIS APPROACH YIELDS GOOD TEMPORAL AND SPATIAL RESOLUTION IN TROPOSPHERIC OH MEASUREMENTS DUE TO IMPROVED DETECTION SENSITIVITY BY HIGH FREQUENCY WAVELENGTH MODULATION DETECTION. LABORATORY EXPERIMENTS ARE CONDUCTED TO GENERATE THE FREQUENCY MODULATED UV BEAM AND TO MEASURE OH RADICALS IN A DISCHARGE-FLOW REACTOR. THE RESULTS OF THESE EXPERIMENTS PROVIDE A BASIS FOR CRITICAL ASSESSMENT OF THE FEASIBILITY OF THIS TECHNIQUE FOR TROPOSPHERIC MEASUREMENTS AND ESTABLISHES THE CRUCIAL DESIGN PARAMETERS FOR DEVELOPMENT OF FIELD INSTRUMENTATION.