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Mike Burton – AOPP– 26 July 2013

Introduction to open-path FTIR measurements of volcanic gases (and aerosols…) Dr. Mike Burton Istituto Nazionale di Geofisica e Vulcanologia Pisa, Italy. Mike Burton – AOPP– 26 July 2013. Talk Outline Infrared radiative transfer FTIR technology and signal processing

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Mike Burton – AOPP– 26 July 2013

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  1. Introduction to open-path FTIR measurements of volcanic gases (and aerosols…) Dr. Mike Burton IstitutoNazionale di Geofisica e Vulcanologia Pisa, Italy Mike Burton – AOPP– 26 July 2013

  2. Talk Outline • Infrared radiative transfer • FTIR technology and signal processing • Open-path FTIR applied to volcanology • Future directions for OP-FTIR in volcanology Mike Burton – AOPP– 26 July 2013

  3. Wien’s Displacement Law The peak wavenumber of the Planck curve increases with increasing temperature, following ~1.96 x T(K) Mike Burton – AOPP– 26 July 2013

  4. If a gas is in thermal equilibrium then the amount of radiation emitted must be equal to the amount of radiation absorbed (ignoring convection and conduction). Gas with temperature T absorbs 20% of radiation Ambient Temperature is T, assuming surrounding is blackbody the gas is exposed to radiation with intensity = B(T) Radiation with intensity B(T)*0.8 is transmitted and B(T)*0.2 is emitted, so B(T) is observed Gas with temperature T emits B(T)*0.20 Gas is ‘invisible’: thermal contrast with background is needed to ‘see’ gas Mike Burton – AOPP– 26 July 2013

  5. Beer-Lambert Law Transmittance is I/I0 = exp (-ecl) = t Therefore observed intensity is I0.exp(-ecl) = I0.t Adding more gases produces a multiplicative effect, e.g. I = I0.tgas1.tgas2.tgas3.tgas4… Mike Burton – AOPP– 26 July 2013

  6. Gas layers both emit and absorb radiation I(n) Atm. Layer with temperature T1 and transmittance t1(n) I(n) . t1 (n) + B(T1) . (1-t1) Atm. Layer with temperature T2 and transmittance t2(n) (I(n) . t1(n) + B(T1) . (1-t1)) . t2 + B(T2) . (1-t2) Mike Burton – AOPP– 26 July 2013

  7. Michelson Interferometer Requires collimated light Half the source radiation returns to the source Throughput is high All wavelengths measured simultaneously Mike Burton – AOPP– 26 July 2013

  8. Input and output from the interferometer

  9. Broadband source Mike Burton – AOPP– 26 July 2013

  10. Finite mirror movement distance Mike Burton – AOPP– 26 July 2013

  11. Multiplication in mirror displacement / Fourier space is a convolution in frequency space

  12. Phase errors and correction Mike Burton – AOPP– 26 July 2013

  13. Field of view As well as OPD, the field of view of the instrument can degrade spectral resolution and produce wavelength shifts. This is because off-axis rays travelling through the spectrometer travel further than the on-axis rays. Mike Burton – AOPP– 26 July 2013

  14. Field of view, resolution and instrument lineshape Spectral resolution is controlled by the distance of mirror movement within the FTIR Normal estimates for the spectral resolution are given by Resolution = 0.9 / OPD Where OPD is the optical path difference produced by moving one of the mirrors in the FTIR. In the OPAG-22 the maximum OPD is 1.78 cm, producing a resolution of 0.9/1.78 ~0.5 cm-1 FTIR spectrometers may have OPD’s of several meters, for the highest resolution spectrometers. Typical absorption line-widths at atmospheric pressure are 0.1cm-1, so an FTIR with OPD of 9cm would be optimal. Higher resolution = larger spectrometer = greater weight and less portability

  15. Applying OP-FTIR to volcanic gas • First demonstrated by Japanese team on Unzen (Mori et al., 1993) • 1996 Francis and Oppenheimer measured SiF4 on Vulcano (Francis et al., 1996) • 1998 Francis, Oppenheimer, Burton used sunlight on Etna and active on Masaya (Francis et al., 1998 Nature, Burton et al., 2000, Geology) • 1998 Love & Goff measure SO2 and CO2 in emission at Popo (Nature, 1998) • From 2000 regular measurements on Etna (Allard et al., 2005, Nature) • 2001-2003 several measurements on Stromboli (Burton et al., 2007, Science) • 2005-2007 regular measurements at Kilauea (Edmonds and Gerlach, 2008) • 2010 Review of more than 10 years results from Unzen, Usu, Aso, and Satsuma-Iwojima (Notsu and Mori, 2010) • 2012 Cerberus on Stromboli (La Spina et al., JVGR, 2012) • 2013 FTIR on LUSI mud volcano (submitted) Mike Burton – AOPP– 26 July 2013

  16. Hardware • Midac • Bruker • MCT • InSb • Cerberus FLIR Photon 320 Scanner Midac M4401-S Acid-resistant sealing Mike Burton – AOPP– 26 July 2013

  17. Measurement Geometries FTIR IR Source Mike Burton – AOPP– 26 July 2013

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  21. Examples of OP-FTIR spectra

  22. Examples of OP-FTIR spectra

  23. Examples of OP-FTIR spectra

  24. Examples of OP-FTIR spectra

  25. Examples of OP-FTIR spectra

  26. Examples of OP-FTIR spectra Emission spectra: if the source of radiation is cooler than the gas itself we observe emission spectra

  27. Data Analysis The basic objective of any retrieval scheme is to retrieve quantitative information from the measured spectra. This is achieved by producing a best-fit simulated spectrum, such that y = F(x) Where y is the measured spectrum, F is a model which simulates the measured spectrum and x is a state vector containing the model parameters, such as gas amounts.

  28. Field of view, resolution and instrument lineshape The measured spectrum is affected by the instrument, primarily by smearing of the spectrum due to its finite spectral resolution.

  29. Potential problems Many measurements are made with a lower resolution spectrometer than the linewidth of the target gases. In this case care must be taken when fitting the spectrum to take account of the non-associative nature of the ILS convolution. F = exp(-ec1l)  ILS

  30. Potential problems When performing the fit it is important to re-convolve the theoretical spectrum with the ILS at each iteration, because… c2 =3 x c1 ln(exp(-ec1l)  ILS) x 3 ≠ ln(exp(-ec2l)  ILS) i.e. you cannot use a reference spectrum measured at one gas concentration to fit measured spectra with markedly different gas concentration. Using weak absorption lines is generally a good idea, as they are less affected by these problems.

  31. Potential problems

  32. Potential problems

  33. Single beam or ratio retrievals Single beam retrieval: simulating the original measured spectrum Ratio retrieval: prior to analysis divide the measured spectrum by a reference spectrum, ideally identical to the measured spectrum but without the target gases. In OP-FTIR reference (or background) spectra can be hard to come by; conditions can change rapidly. Single-beam retrievals are preferable.

  34. Simple retrieval: Masaya Volcano active source. FTIR 520 m IR Source

  35. Simple retrieval: Masaya Volcano active source. Volcanic gas temperature ~ atmospheric temperature High volcanic gas amount Stable source, hotter than gas Simple atmospheric model, single layer, no temperature retrieval

  36. Complex retrievals: Etna lava fountain

  37. Complex retrieval: Etna lava fountain Volcanic gas temperature is subject to very strong gradients Highly variable volcanic gas amounts (difficult to always use weak absorption lines) Highly unstable source (need to eliminate spectra in emission, and take account of the changing field of view) Complex retrieval: First determine volcanic gas temperature from SO2 rotational absorption structure Use two layer model, one for the atmosphere one for volcanic gases, with simultaneous fit of field of view parameter.

  38. a Transmittance b Complex retrieval: Etna lava fountain

  39. Trade-offs: Spectral resolution vs weight and SNR Higher spectral resolution: Pro – Better quality fits, less sensitive to non-linearity problems Con – Larger spectrometer, with more weight and more delicate Con – longer data acquisition time for each spectrum and lower snr Up until now typical portable open-path FTIR’s have been 0.5cm-1 resolution and weighing ~15 kg: can be improved

  40. Trade-offs: SNR vs measurement frequency Higher signal to noise ratio: achieved through longer integration times Pro – Better quality spectra Con – Lower spectrum acquisition frequency Con – potential for large variations in absorbing gas amount, leading to non-linearity problems Higher measurement frequency: Pro: resolve short-term variations Pro: can always average spectra to increase SNR after data collection Con: inferior snr on individual spectra

  41. Future Directions for OP-FTIR • Emission spectroscopy • Examine limits for lower resolution gas measurements, in order to increase measurement frequency or SNR • Mud volcanism • Aerosol and ash quantification • Satellite validation (IASI) • Gas solubility model validation

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