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 powerinefficient,
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 400m 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 (XDClO, 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.

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