Development of a portable active long-path differential optical absorption spectroscopy system for volcanic gas measurements

Active long-path differential optical absorption spectroscopy (LP-DOAS) has been an effective tool for measuring atmospheric trace gases for several decades. However, instruments were large, heavy and power-inefficient, making their application to remote environments extremely challenging. Recent developments in fibre-coupling telescope technology and the availability of ultraviolet light emitting diodes (UV-LEDS) have now allowed us to design and construct a lightweight, portable, low-power LP-DOAS instrument for use at remote locations and specifically for measuring degassing from active volcanic systems. The LP-DOAS was used to measure sulfur dioxide (SO₂) emissions from La Fossa crater, Vulcano, Italy, where column densities of up to 1.2 × 10¹⁸ molec cm⁻² (~ 500 ppmm) were detected along open paths of up to 400 m in total length. The instrument's SO₂ detection limit was determined to be 2 × 10¹⁶ molec cm⁻² (~ 8 ppmm), thereby making quantitative detection of even trace amounts of SO₂ possible. The instrument is capable of measuring other volcanic volatile species as well. Though the spectral evaluation of the recorded data showed that chlorine monoxide (ClO) and carbon disulfide (CS₂) were both below the instrument's detection limits during the experiment, the upper limits for the X / SO₂ ratio (X = ClO, CS₂) could be derived, and yielded 2 × 10⁻³ and 0.1, respectively. The robust design and versatility of the instrument make it a promising tool for monitoring of volcanic degassing and understanding processes in a range of volcanic systems.