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doi:10.5194/amt-2015-380
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
18 Jan 2016
Review status
A revision of this discussion paper for further review has not been submitted.
iFit: An intensity-based retrieval for SO2 and BrO from scattered sunlight ultraviolet volcanic plume absorption spectra
M. R. Burton1 and G. M. Sawyer2 1School of Earth and Environment, University of Manchester, Manchester, UK
2Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont Ferrand, France
Abstract. iFit is a new intensity-based retrieval algorithm for direct fitting of measured UV spectra, specifically developed for use in volcanology. It has been designed with a focus on minimising processing of the measured spectra prior to analysis. Here, we report a detailed presentation of the iFit algorithm, and test it in 4 case studies, examining clear sky spectra, SO2 calibration cell experiments and volcanic SO2 and BrO retrievals from traverse measurements performed on Mt. Etna volcano, Italy. We show that the major source of fit error in the intensity fitting come from air mass factor independent solar spectrum errors, which are, however, easily characterised and removed by dividing the measured spectrum by a pre-calculated solar spectrum residual. Furthermore, we have quantified the magnitude of the flat spectrum in two spectrometers, and shown that this spectrum is strongly spectrometer dependent but temperature independent, opening the possibility for robust analysis and and BrO without the need for temperature stabilisation of quantification of both SO2 the spectrometer. We find that iFit can be robustly and easily applied to traverse measurements of volcanic plumes, producing bias-free profiles of SO2 , and high quality SO2 /BrO ratios without the need for clear sky background spectra. Fit residuals are typically pure instrumental noise when the residual solar spectrum is removed. We believe that the iFit approach, which avoids the need for a clear sky spectrum and which achieves noise-limited fits, is ideally suited to the automated analysis of spectra produced by networks of scanning UV spectrometers around volcanoes.

Citation: Burton, M. R. and Sawyer, G. M.: iFit: An intensity-based retrieval for SO2 and BrO from scattered sunlight ultraviolet volcanic plume absorption spectra, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2015-380, in review, 2016.
M. R. Burton and G. M. Sawyer
M. R. Burton and G. M. Sawyer

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Short summary
In this work we present a new approach to the retrieval of the amounts of volcanic gas species from ultraviolet spectra. This work was motivated by the desire to extend previous work on the analysis of such spectra to avoid the use of a clear sky background spectrum, which can be cumbersome or unpractical in many scenarios. We succeeded in producing a new approach through identification of the key role of the pixel-by-pixel detector response in commonly used UV spectrometers.
In this work we present a new approach to the retrieval of the amounts of volcanic gas species...
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