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Scanning Differential Optical Absorption Spectroscopy (DOAS) is used to measure sulfur dioxide (SO₂) in volcanic plumes. The technique relies on the fact that SO₂ absorbs ultraviolet light at specific wavelengths. By comparing observed spectra with calibrated references, SO₂ concentrations along a line of sight can be derived. At Cleveland volcano, Scanning DOAS provides the first continuous SO₂ monitoring in the Aleutians, a region with frequent explosive activity. Measuring SO₂ fluxes here is critical because persistent degassing at Cleveland has been a hallmark of activity, and flux increases often precede eruptive events.

Processed scan data from the CLNE and CLES scanning DOAS instruments for a period of elevated gas emissions from June 21–July 3, 2024. SO₂ emission rates, in tons per day, are indicated by the green (CLNE) and orange (CLES) circles. Wind speeds are indicated by the grey triangles.