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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.