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LP-DOAS meets FTIR: Open path measurements of Greenhouse Gases

LP-DOAS meets FTIR: Open path measurements of Greenhouse Gases. David Griffith University of Wollongong, Australia & Denis Pöhler , Stefan Schmitt, Sam Hammer, Sanam Vardag , Ingeborg Levin, and Ulrich Platt University of Heidelberg, Germany. DOAS Workshop July 2015.

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LP-DOAS meets FTIR: Open path measurements of Greenhouse Gases

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  1. LP-DOAS meets FTIR: Open path measurements of Greenhouse Gases David Griffith University of Wollongong, Australia & Denis Pöhler, Stefan Schmitt, Sam Hammer, SanamVardag, IngeborgLevin, and Ulrich Platt University of Heidelberg, Germany DOAS Workshop July 2015

  2. FTIR meets DOAS in the NIR • Accurate greenhouse gas measurements are mostly point-based • Using calibrated in situ analysers • Or flask samples • Open path spectroscopy has some advantages • Spatial averaging, better match to regional-scale model resolution • How spatially representative are point measurements? • How accurate are open path measurements? • TCCON provides precise measurements of GHGs: • Solar absorption spectroscopy, 8-20 km-atm atmospheric path

  3. Remote sensing of total column trace gases: CO2, CH4, N2O, CO, H2O Satellite SCIA, GOSAT, OCO-2 TCCON • Infrared absorption spectroscopy in the near IR • Retrieve total column amounts of trace gases • Derive column average dry air mole fractions (e.g. XCO2) • TCCON precision/accuracy 0.1 – 0.2% (0.4 – 0.8 ppm)

  4. TCCON solar FTS & CO2 spectraeffective path ~ 10 km-atm

  5. FTIR meets DOAS in the NIR • Accurate greenhouse gas measurements are mostly point-based • Using calibrated in situ analysers • Open path spectroscopy has some advantages • Spatial averaging, better match to regional-scale model resolution • How representative are point measurements? • TCCON provides precise measurements of GHGs: • bright source, high resolution, 8-20 km-atm atmospheric path • CO2 precision & accuracy ~ 0.1-0.2 % • How well can we measure GHGs at the ground in the NIR? • Low resolution (portable FTS) • Weaker source than the sun (quartz lamp) • 2-6 km path

  6. Long open path measurement of trace gases: CO2, CH4, N2O, CO, H2O FT spectrometer& telescope Retroreflectorarray • Infrared absorption spectroscopy in the near IR • Long open horizontal path (km scale)

  7. History: NIR LP-DOAS collimating mirrors grating NIR-PDZ Spectrometer: Acton 500 f=500mm InGaAs Sensor: 0.9μm to 1.7μm Dispersion: 0.15nm/Pixel ~1nm spectral resolution

  8. History: Problems of NIR LP-DOAS 4.7*10-2  to high uncertainty for most applications • Example of CH4 measurements • Large remaining spectral structures • Origin are: • Inhomogenious illumination • the optical fibres • limited mode mixing • weak light sources • detector • Limited accuracy of 30% • Improvement to 10% with optimised fibre and mode mixing

  9. Long path FTS setup  30 cm FL 150 cm 1-3 km 17 x 70mm retroreflectors interferogram source spectrum FT Resolution: ~0.1nm FT Interferometer 0.5 cm-1resoln BrukerIrcube detector Illuminating bundle Receiving fibre

  10. IUP Retrorefl.

  11. FTIR-DOAS setup

  12. NIR long path spectrum3.1 km total path IUP - PhilosophenWeg CH4 Strong CO2 Weak CO2 O2 H2O H2O 1.6 2 1.4 Microns 2.5 1.25

  13. “Strong” CO2 band 4900-5000 cm-1 CO2 H2O CO2

  14. CH4 is weaker… CH4

  15. O2 O2

  16. Spectrum analysis : MALT least squares fitCO2

  17. Spectrum analysis : O2

  18. O2and temperature

  19. O2and temperature

  20. O2and temperature Mean O2 = 0.217 (+3.5%)

  21. CO2 – 3 days in July Germany wins World Cup! Cal adjust16.2 ppm

  22. CO2 4 monthsJuly-October 2014

  23. CH4July-October 2014

  24. CO2 OP-in situ differences

  25. Summary: CO2 precision and accuracy • Precision/repeatability • ~1 ppm (0.25%, 1s) for 5 min averages • limited mainly by spectrum SNR • 300mm telescope (f=1500mm), 17 retros, 50 W source • ~ 40 ppm p-p differences to in situ measurements • Real differences, larger at low wind speeds • Accuracy/bias • ~3% uncalibrated(~ 13 ppm) • Limited by • HITRAN, simulation ~ 2-4% • Temperature ~ 3C(1%) • Pressure <1 hPa, (0.1%) • Pathlength <3 m (0.1%) • Fibre residual ~1%

  26. Now – the residuals …

  27. Co-fitting fibre residualDCO2 in retrieval <0.5 ppm

  28. Current setup  30 cm FL 150 cm 1-3 km 17 x 70mm retroreflectors interferogram source spectrum FT FT Interferometer 0.5 cm-1resoln BrukerIRcube detector Illuminating bundle Receiving fibre

  29. A better setup 1-3 km detector interferogram Stray and scattered radiation (e.g. sunlight) is not modulated, appears only as DC and does not appear in the FT spectrum FT interferometer source

  30. Potential improvements& future work • Pre-modulate IR source before transmission • Removes stray light • More photons • Precision is detector noise limited • Remove or co-fit fibre spectral structures • Higher resolution? • Better discrimination against fibre residuals • Lower SNR => lower precision • Less portable • Temperature measurement • Improve retrieval

  31. Thank you !

  32. An interesting artefact … 18:15 every sunny day!

  33. Solar beam travels through ~17 km path - Apparent increase in CO2, CH4, O2

  34. Some applications • Distributed measurements cf in situ point measurements • Validation of point measurements in inhomogeneous environment • Fluxes (conc * windspeed = flux) • Eg cross-Neckar path, perpendicular windspeed • Fluxes – with backward Lagrangian model (eg. STILT, Windtrax) • Mass balance methods • Paths surround source/sink, measure path concentrations and windspeed • Tracer studies • Eg cattle CH4

  35. Co-fitting residuals • ~80-90% of the variance in the residual is constant • => due to the fibre • ~1% of the residuals correlate with CO2 spectrum • R2 (residual-CO2)= 0.01 • Co-fitting fibre structure changes retrieved CO2 by < 1 ppm

  36. Co-fitting residuals • ~90% of the variance in the residual is approx. constant • => due to the fibre • ~1% of the residuals correlate with CO2 spectrum • R2 (residual-CO2)= 0.01 • Co-fitting fibre structure changes retrieved CO2 by < 1 ppm • Residual after co-fitting fibre spectrum reduced but not white noise • => better estimate of random uncertainty from the fit • Note: this method is also used in OCO-2 retrieval • More work required!!!

  37. History: open path FTIR – mid IRMeasuring greenhouse gases from agriculture wind Retroreflector Tracer release (N2O) FTIR beam FTIR FTIR & telescope

  38. OP-FTIR-tracer: results

  39. Because we are in Canada…Alberta 2013

  40. ILS: short path H2OFit H2O 7000-7400 cm-1 (Frey, Hase et al., AMTD 2015) Hamming Apodisation ME ~ 0.7

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