Spectrometer for Estimating SO2 Content in Volcanic Plumes

Abstract

This work presents the development and implementation of an autonomous portable spectrometer DEVI (Doas Expedition Volcanic Instrument), designed to measure SO₂ slant columns in volcanic plumes by remote optical method DOAS (Differential Optical Absorption Spectroscopy) in the range of 290–365 nm with a resolution of at least 1 nm. To achieve this goal, the following tasks have been solved: practical implementation of the spectrometer, including design of optical scheme; design of a spectrometer housing for reducing scattered radiation and facilitate adjustments; applying of additional sensors to record measurement conditions; laboratory measurements to determine the spectrometer's characteristics; field measurements and preliminary data processing to retrieve SO₂ slant columns in volcanic plumes. During the spectrometer design phase, numerical simulation methods in the Zemax software have been used, while DOAS was applied for processing experimental data for retrieving SO₂ slant columns. Our laboratory measurements showed that the DEVI spectrometer has a spectral resolution of 0.58 ± 0.5 nm and an angular field of view of 1 × 0.25°. To improve the signal-to-noise ratio, mathematical filter based on the experimentally determined noise parameters of the DEVI detector has been introduced, which allowed us to estimate the SO₂ slant columns in volcanic plumes. DEVI was successfully tested during expeditions to the Kuril Islands in the periods of July – August, 2021 and 2022 (31.07–13.08.2021 and 27.07–29.08.2022). Our field measurements and data processing showed the SO₂ slant column value of (7.5 ± 1.2)·10¹⁷ molecules/cm² for the volcano Chirinkotan. Obtained estimation is consistent with known results obtained for other volcanoes.