iFit: An intensity based retrieval for volcanic SO₂ from scattered sunlight UV spectra

Abstract. Accurate quantification of the sulphur dioxide (SO₂) flux from volcanoes provides both an insight into magmatic processes and a powerful monitoring tool for hazard mitigation, with miniature ultraviolet spectrometers becoming the go-to method for SO₂ flux measurements globally. The most common analysis method for these spectrometers is Differential Optical Absorption Spectroscopy (DOAS), in which a reference spectrum taken outside the plume is used to quantify the SO₂ column density inside the plume. This can lead to problems if the reference spectrum is contaminated with SO₂ as this leads to systematic underestimates in the retrieved SO₂ column density. We present a novel method, named “iFit”, which retrieves the SO₂ column density from UV spectra by directly fitting the measured intensity spectrum using a high resolution solar reference spectrum. This has a number of advantages over the traditional DOAS method, primarily by eliminating the requirement for a measured reference spectrum. We show that iFit can accurately retrieve SO₂ column densities in a series of test cases, finding excellent agreement with existing methods without the use of a reference spectrum. We propose that iFit is well suited to application to both traverse measurements and permanent scanning stations, and shows strong potential for integration into volcano monitoring networks at observatories.