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Optics For Dissolved Oxygen

Optics For Dissolved Oxygen. Bridget Benson Ocean Optics Class May 30, 2007. Outline. What is Dissolved Oxygen? Why measure Dissolved Oxygen? Electro-Chemical Based Measurements Optical Based Measurements

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Optics For Dissolved Oxygen

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  1. Optics For Dissolved Oxygen Bridget Benson Ocean Optics Class May 30, 2007

  2. Outline • What is Dissolved Oxygen? • Why measure Dissolved Oxygen? • Electro-Chemical Based Measurements • Optical Based Measurements • Advantages of Using Optical Based Measurements and studies that have proven these advantages • The Argo-Oxygen Program

  3. What is Dissolved Oxygen? • Two main sources of oxygen found in water • Atmosphere • Waves and tumbling water mix air into water where oxygen readily dissolves until saturation occurs • Photosynthesis • Oxygen is produced by aquatic plants and algae as a by-product of photosynthesis • Amount of DO depends on water Temperature, Salinity, & Atmospheric Pressure. Colder, Fresher water at low altitudes holds more oxygen

  4. Why Measure Dissolved Oxygen? • Indication of Water Quality • Low levels of DO cannot support the aerobic bacteria that purify the water. Low levels of DO also result in extensive fish kills • High levels of DO may cause fish to suffer from gas bubble disease • Allow for detection of the oceanic impact of global warming on ocean biogeochemistry and circulation • Allow for improved estimates of the oceanic uptake of anthropogenic CO2.

  5. Why Measure Dissolved Oxygen? • Plattner et al. (2002) predict by 2100 a 17% reduction of the ocean’s oxygen content. • This is accompanied with a substantial increase in the extent of the oxygen minimum zones. Some model simulations (Matear and Hirst, 2003) suggest a doubling of regions with suboxia (O2 < 10 μmol kg-1), i.e. a doubling of the marine “deadzones” with severe implications for all higher life-forms and for long-term nutrient inventories and cycles

  6. ElectroChemical Methods Polarographic Clark Cell (1956) 1) Battery source provides 800mV 2) Oxygen passes through the gas permeable Membrane 3) Cathode consumes the oxygen and creates a partial pressure across the membrane 4) Oxygen diffuses into the electrolyte solution 5) Current is generated directly proportional to the oxygen consumed

  7. ElectroChemical Methods • Galvanic Cell • Similar to Clark cell but does not need external power supply because uses two dissimilar medals

  8. ElectroChemical Methods • (1) Units can require frequent calibration and maintenance. • (2) Mechanical Stirring is necessary for most models • (3) The gas permeable (Teflon) membrane can be punctured by aquatic insects, improper handling, and waterborne debris • (4) Fouling of the membrane by algae and fine waterborne materials can significantly affect measurement quality

  9. Based on Dynamic Quenching of Luminescence Sensor is coated with a thin layer of oxygen-sensitive fluorescent dye. Polycyclic aromatic hydrocarbons RU(II), Os(II), Rh(II) Phosphorescent porphyrins A LED shines blue light on the dye layer, causing the dye to emit red fluorescent light Oxygen concentration in the sample is related to luminescence intensity and luminescence lifetime Optical Measurements

  10. Optical Measurements

  11. Optical Measurements • Measure luminescence lifetime by comparing the phase difference between excitation and emitted signal • Decay time not dependenton fluctuations in intensity light source • Photo-bleaching has noinfluence on the measuringsignal

  12. Advantages of Optical Measurements • Optical Measurements eliminate the need for frequent membrane changes, stirring, and frequent calibration during deployments • USGS conducted a field experiment to compare Clark cell based instruments to optical instruments in the Tualatin River. Results indicated that the optical sensors were less prone to fouling drift and calibration drift

  13. Advantages of Optical Measurements PERFORMANCE OF OPTICAL DISSOLVED OXYGEN SENSORS IN A SEVEN SITE, MIXED MATRIX STUDY Robert J. Mooney & Tony Arnerich

  14. ARGO OXYGEN PROGRAM To determine seasonal to decadal-time variability in sub-surface ocean oxygen storage and transport on a global scale.

  15. References • THE ARGO-OXYGEN PROGRAM • Basics in Optical Sensing (Presens Appendix 1) • RDO Optical Dissolved Oxygen Sensor • Eutech Instruments: Introduction to Dissolved Oxygen • Matthew W. Johnston and John S. Williams. Field Comparison of Optical and Clark Cell Dissolved Oxygen Sensors in the Tualatin River, Oregon, 2005 • Robert J. Mooney & Tony Arnerich. Performance of optical dissolved oxygen sensors in a seven site, mixed matrix study

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