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

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

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 4. Stray light  creates residual structures in particular in the UV 5. Imprecise literature cross sections Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

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

Comparison between xenon lamps and LEDs Size of the arc spot: “EQ-99” (experimental) XBO-75 (new) LED 0,014 mm2 0,125 mm2 1 mm2 Coupling into 800 μm optical fibre Coupling into 200 μm optical fibre Eger (2014) Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

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

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

Mode mixing New method: 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

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

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

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

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

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

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

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

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

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

Improvement of the residuals by a factor of 4 to 5 x 10-5 RMS 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

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

Thank you for your attention! 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