mike burton – aopp– 26 july 2013 introduction to open-path ftir measurements of volcanic gases...
TRANSCRIPT
Mike Burton – AOPP– 26 July 2013
Introduction to open-path FTIR measurements of volcanic gases (and aerosols…)
Dr. Mike BurtonIstituto Nazionale di Geofisica e Vulcanologia
Pisa, Italy
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
Wien’s Displacement Law
The peak wavenumber of the Planck curve increases with increasing temperature, following ~1.96 x T(K)
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400°K
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1200°K
Peak 1200 K at ~2400 cm-1
Peak 1000 K at ~2000 cm-1
Peak 800 K at ~1600 cm-1
Peak 600 K at ~1200 cm-1
Peak 400 K at ~800 cm-1
Mike Burton – AOPP– 26 July 2013
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).
Ambient Temperature is T, assuming surrounding is blackbody the gas is exposed to radiation with intensity = B(T)
Gas with temperature T absorbs 20% of radiation
Radiation with intensity B(T)*0.8 is transmitted and B(T)*0.2 is emitted, so B(T)
is observed
Gas is ‘invisible’: thermal contrast with background is needed to ‘see’ gas
Mike Burton – AOPP– 26 July 2013
Gas with temperature T emits B(T)*0.20
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
Gas layers both emit and absorb radiation
Mike Burton – AOPP– 26 July 2013
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)
Michelson Interferometer
Mike Burton – AOPP– 26 July 2013
Requires collimated lightHalf the source radiation returns to the sourceThroughput is highAll wavelengths measured simultaneously
Input and output from the interferometer
Broadband source
Mike Burton – AOPP– 26 July 2013
Finite mirror movement distance
Mike Burton – AOPP– 26 July 2013
Multiplication in mirror displacement / Fourier space is a convolution in frequency space
Phase errors and correction
Mike Burton – AOPP– 26 July 2013
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.
Field of view
Mike Burton – AOPP– 26 July 2013
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
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
Hardware
• Midac
• Bruker
• MCT
• InSb
• Cerberus
Mike Burton – AOPP– 26 July 2013
FLIR Photon 320
Scanner
Acid-resistant sealing
Midac M4401-S
Measurement Geometries
Mike Burton – AOPP– 26 July 2013
FTIR
IR Source
Mike Burton – AOPP– 26 July 2013
Mike Burton – AOPP– 26 July 2013
Mike Burton – AOPP– 26 July 2013
Examples of OP-FTIR spectra
Lava Source
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Lava Source
Examples of OP-FTIR spectra
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700°K
1200°K
Lava Source
Examples of OP-FTIR spectra
Passive FTIR spectrum of volcanic gases
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Wavenumber (cm-1)
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un
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SO2 1+3 combination band
P-branch of HCl
Examples of OP-FTIR spectra
Examples of OP-FTIR spectra
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Solar
Lava Source
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Lava Source
Examples of OP-FTIR spectra
Emission spectra: if the source of radiation is cooler than the gas itself we observe emission spectra
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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.
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.
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2746 2747 2748 2749 2750 2751 2752 2753 2754 2755 2756
Wavenumber
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Measured HCl Spectrum Theoretical HCl spectrum
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
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Measured HCl Spectrum Theoretical HCl spectrum
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.
Potential problems
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HCl 3e17 correct
HCl 3 * 1e17 xsec
Potential problems
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HCl 3e18 correct
HCl 3 x 1e18 xsec
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.
Simple retrieval: Masaya Volcano active source.
FTIR
IR Source
520 m
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
Complex retrievals: Etna lava fountain
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.
Complex retrieval: Etna lava fountain
Tran
smit
tan
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a
b
Trade-offs: Spectral resolution vs weight and SNR
Higher spectral resolution:
Pro – Better quality fits, less sensitive to non-linearity problemsCon – Larger spectrometer, with more weight and more delicateCon – 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
Trade-offs: SNR vs measurement frequency
Higher signal to noise ratio: achieved through longer integration times
Pro – Better quality spectraCon – Lower spectrum acquisition frequencyCon – potential for large variations in absorbing gas amount, leading to non-linearity problems
Higher measurement frequency:
Pro: resolve short-term variationsPro: can always average spectra to increase SNR after data collectionCon: inferior snr on individual spectra
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