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High Precision DOAS: New Long-Path-DOAS System

This presentation discusses the development of a new Long-Path-DOAS system with reduced noise and a new light source. The system aims to improve usability, automation, and detection limits for long-term measurements.

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High Precision DOAS: New Long-Path-DOAS System

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  1. 7th International DOAS Workshop Brussels 2015 “High Precision DOAS”A new Long Path-DOAS system with reduced noise and new light source Jan-Marcus Nasse, Denis Pöhler, Philipp Eger, Stefan Schmitt, Udo Frieß and Ulrich Platt Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  2. Motivation • Advantages of Long-Path DOAS: • analysis directly yields (path-averaged) concentrations • independent of solar radiation • own light source allows measurements in a very broad spectral range • Operation of LP-DOAS instruments is complex: • regular exchange of Xe-light bulbs • Optical adjustments “Classical” configuration of the fibre bundle • Goals of instrument design: • improved usability and automation (  long term measurements) • better detection limits ( necessary for a number of research topics) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  3. Origin of residual structures 1. Photon noise  Linked to intensity of light source and transmission 2. Lamp structures  unstable lamp spectrum 3. Insufficient mode mixing inhomogeneous illumination of CCD 5. Imprecise literature cross sections 4. Stray light  creates residual structures in particular in the UV Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  4. Laser-Driven Light Source (LDLS) • Standard light source for LP-DOAS: • Xenon arc lamps • short lifetimes • unstable • New light source: xenon lamp driven by infrared laser (LDLS) • Advantages: • Reduced abrasion of electrodes •  long life time • small and stable arc spot •  improved optical coupling Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  5. Comparison between xenon lamps and LEDs Size of the arc spot: Coupling into 800 μm opticalfibre Coupling into 200 μm opticalfibre Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  6. New fibre configuration Eger (2014) residual residual “classical” Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  7. Origin of residual structures Photon noise  Linked to intensity of light source and transmission Lamp structures  Unstable light arc induces Xe structures LDLS Insufficient mode mixing inhomogeneous illumination of CCD Stray light  creates residual structures in particular in the UV Imprecise literature cross sections Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  8. Mode mixing Eger (2014) from Platt and Stutz (2008) - adapted from Stutz and Platt (1997) New method: fibre roughened with 12 μm grit polishing sheet  Like diffuser but less light loss Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  9. Mode mixing - Comparison no mixing vibration diffuser roughened Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  10. Origin of residual structures Photon noise  Linked to intensity of light source and transmission Lamp structures  Unstable light arc induces Xe structures Insufficient mode mixing inhomogeneous illumination of CCD Stray light  creates residual structures in particular in the UV Imprecise literature cross sections Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  11. Stray light reduction I – “lens filter” Exploiting the chromatic aberration of the lens coupling the into the fibre Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  12. Stray light reduction II -filters Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  13. LDLS with stray light reduction lamp lens moving stage laser shutter fibre filter wheel Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  14. Origin of residual structures Photon noise  Linked to intensity of light source and transmission Lamp structures  Unstable light arc induces Xe structures Insufficient mode mixing inhomogeneous illumination of CCD Stray light  creates residual structures in particular in the UV Imprecise literature cross sections Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  15. Combined reduction of residuals chromatic aberration filter band pass filter LDLS new fibre configuration improved mode mixing Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  16. Comparison with conventional Xe-lamp reversed reversed classical classical classical classical Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  17. Origin of residual structures Photon noise  Linked to intensity of light source and transmission Lamp structures  Unstable light arc induces Xe structures Insufficient mode mixing inhomogeneous illumination of CCD Stray light  creates residual structures in particular in the UV Imprecise literature cross sections  Recording of own differential X-sections e.g. O2, O3 Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  18. Conclusions • Laser Driven Light Source (LDLS) offers important advantages in comparison with conventional Xenon lamps  small and stable arc spot; long lifetime • Optimal configuration: LDLS+ “reversed” coupling into 200 μm fibre + mode mixing by roughened fibres • Improvement of the residuals by a factor of 4 to 5 x 10-5 RMS • Good temporal stability (in particular with LDLS)  adding up spectra is possible • Available differential cross sections are no longer sufficient  recording of own differential cross sections • Autonomous Long Path DOAS measurements are possible Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  19. Outlook: Long term measurements at Neumayer III/Antarctica • Investigation of chlorine, iodine and bromine chemistry • continuous measurements (full diurnal cycles and Polar night) • operation for at least one year Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

  20. 7th International DOAS Workshop Brussels 2015 Thank you for your attention! Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

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