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Practical demonstration of FSF FTIR by Chris Maclellan FTIR workshop group photo Prof. Martin Wooster demonstrating FTIR

The Field Spectroscopy Facility held a two day workshop, hosted at KCL, on the theory of FTIR, the practicalities of field measurements and data processing and analytical techniques. Practical demonstrations, using the FSF's field portable FTIR, were also held.

FTIR Workshop Speakers:

Wednesday: 7th June 2007 Introduction to FTIR

John Remedios, University of Leicester, gave an overview of the principles of FTIR spectrometers, covering the Michelson Interferometer at the heart of the technique and the means by which, through the interferogram and Fourier transformation, a high resolution spectrum of thermal emission across a wide spectral region is attained. The pros and cons of the different detector types available were also discussed as well as other issues associated with potential measurement error. Examples of systems included the MIPAS FTIR instrument on the Envisat satellite platform.

There followed three talks on the use of FTIR in open path configuration:

Pete Zemek, Midac Corporation, gave a presentation on behalf of Bob Yokelson from the University of Montana on the use of FTIR to measure the influence of biomass fires on the atmosphere in both in situ and experimental set ups. This included not only ground-based measurement but those made with an airborne FTIR system using an enclosed cell in which extracted air is circulated. The talk showed examples from fires around the world and discussed the implications of the results of such fires to atmospheric chemistry and modeling.

The use of a number of open path FTIR configurations for volcanic emission measurement were illustrated by Clive Oppenheimer, University of Cambridge. These included the use of lamps as radiation sources, solar occultation, and the use of lava as a hot source reference. Examples of gas retrievals from a number of volcanoes (Erebus, Etna, Masaya, Yasur) were given as was the use of ratios of concentrations of different gases that showed how new conclusions can be drawn on inner volcanic structure and behaviour.

Mike Burton, Italian National Institute of Geophysics and Volcanology. gave an overview of the main factors that need to be considered in Open path-FTIR. He discussed the physical principles by which measurements of temperature can be made through the atmosphere, and showed examples of practical measurements. He further examined the trade-offs that need to be made between resolution, instrument size and gas concentration retrieval. Issues associated with the retrieval of gas amounts from FTIR data were illustrated with examples of simple and complex retrieval approaches. The key factors to be considered in the in designing FTIR measurement campaigns in the field were also discussed.

Anu Dudhia, University of Oxford, described the Reference Forward Model (RFM), a radiative transfer model used at the heart of a number of model-based retrievals of gas concentration. It can be used in a number of different measurement geometries. He also outlined the future plans for RFM.

Jolyon Reburn, Rutherford Appleton Laboratories, also spoke about gas retrievals from spectroscopic measurements, the principles use of the forward model approach, fitting using least squares and optimal estimation. Examples shown included retrievals from gas cell measurements and from data produced from a new laser heterodyne instrument under development at RAL.

Peter Bernath, University of York, illustrated ground-based solar measurement with the PARIS-IR FTIR system. The principles behind this Bowmem-built FTIR instrument were outlined as were its characteristics. He also discuseed the use of a suntracker for continuous measurement of gas concentrations. He outlined retrieval theory using the SFIT2 retrieval code for trace gas retrievals, considered errors in estimation and illustrated the use of FTIR in a number of measurement campaigns.

Suzana Briz, European University of Madrid, presented a comparison of different retrieval methods of open-path FTIR spectra. These included: classical least squares, the line by line non linear method, and the line method. The principles behind each of these techniques was outlined. A comparison of retrievals using the different approaches was made during a measurement campaign made in Breschia as part of the ICAROS project. The results were validated against an extractive method. Her results showed that potential errors in estimation were associated with the classical least squares compared to the other approaches. The line by line method was the most reliable. She outlined further retrieval approaches being explored, including the use of artificial neural networks.

Thursday: 8th June 2007 Focussed on use of FTIR in measurement of emissivity.

Vitchko Tsanev, University of Cambridge outlined the measurement of emissivity and hemispherical radiance. Nomenclature was introduced and measurement discussed as were the mathematical principles. Examples of measurement were shown. He outlined the use of an enclosed box attachment to an FTIR system in which emissivity of most surfaces can be accurately measured. He also discussed the problem of coping with mixed classes.

Beatriz Ribeiro da Luz, from the USGS discussed field emissivity and attenuated total reflectance (ATR) measurements of plants. Comparison of optical and thermal behaviour in plant tissues were made. ATR gives spectra specific to plant species and identification of many compounds and components present in leaves. Examples of approaches to direct measurement in the field and factors for consideration were given for different plant species. Field emissivity measurements of leaves show that there is useful spectral information that may be detectable by passive remote sensing in the thermal infrared. However, in order to minimize spectral contrast losses due to canopy voids and multiple scattering, sensors having both high signal-to-noise and an IFOV on the scale of individual leaves will be required. Beatriz also showed that atmospheric compensation methods and spectral analysis algorithms will also require refinement to permit the extraction of plant emissivity features. As technical capabilities improve, understanding of the TIR spectral contributions of leaves will become increasingly important, and ultimately, may enable new types of remote sensing observations over vegetation canopies.

Tim Nightingale, Rutherford Appleton Laboratory, presented a paper on his experience in the radiometric calibration of field spectrometers. In particular he outlined the use of calibration using high and low black bodies (the so called ‘two point calibration method’). Issues associated with black body design were discussed n detail. Examples of different types of black bodies were also shown and measurement geometry considerations discussed. Issues associated with the accurate measurement of temperature of blackbodies in the field were also touched on.

Pete Zemek, Midac Corporation, discussed real-time in-situ FTIR emissions measurements in a fluidized bed reactor. In this novel application he was able to show that rapid estimation of the CO/CO2 ratio could be made, which is a strong estimator of fluid bed conditions. He also discussed the benefits of FTIR in this application. In a more interactive session, Pete also outlined recent and future developments in MIDAC FTIR instrumentation, the use of retroreflectors, and advances in detector technologies. 2-D and 3-D approaches to measurement of emissions over landfill sites were also discussed.

Amdrew Wilson, Centre for Ecology and Hydrology, outlined ideas for the development of a mirror-based scanning system for the FSF FTIR instrument and the fifferent approaches that could be taken. The need for a lightweight and adaptable solution and opinions on the idea sought from the audience. It prompted a lively discussion.