Application of NIR Laser Spectroscopy to the Monitoring of Volcanic Plumes: Principles and Practicalities

Abstract:

The major volatile species in volcanic plume emissions (i.e., H₂O, CO₂, SO₂, HCl, HF) are all strongly infrared (IR)-active, and lend themselves to infrared spectroscopic analysis. However, physical/optical access to plume gases along pathways which include a suitable natural or active IR radiation source is often difficult or impossible to achieve, particularly for timeframes extending beyond short campaign periods.  In this study, we present results from preliminary tests conducted on three volcanic CO₂ plume emissions using a tunable diode NIR laser system (TDL, Boreal Laser Inc.).  

The approach is proving itself as a good candidate for continuous monitoring of volcanic plume CO₂, and by default all other IR-active constituents for which lasers of appropriate wavelength are available.  The CO₂ system is configured with a TDL in a transceiver generating laser light which can be tuned to coincide with one of several absorption lines in the CO₂ absorption band between 1575 nm and 1585 nm. This beam propagates through the atmosphere (and plume) to a retro-reflector, which returns the beam to a photodiode detector in the transceiver which processes the signal to report real time CO₂ column densities.

 

The CO₂ absorption line at 1579.1 nm was used to good effect on Mt Ruapehu (NZ) where volcanic gases emanate through a 100 m deep crater lake, resulting in CO₂ concentrations of > 78 ppm above background in the mixing zone varying from 4 to 30 m above the lake surface.  Subsequent tests on the main plume at White Island, however, generated only poor results with indicated CO₂ amounts being less than atmospheric. We concluded that this was the result of interference from a neighboring but comparatively minor H₂O absorption band which in the proximal, higher temperature plume (estimated 50-70 °C), had H₂O concentrations some 4-5 times greater than ambient. A change to a less sensitive absorption line further removed from potential H₂O band interference (1567.9 nm) appears to have solved this problem, and yielded maximum CO₂ concentrations along the 730 m pathway in excess of 500 ppm.

This approach holds promise for continuous, real-time monitoring of volcanic plume chemistry, and we will now turn our focus to the detection of SO₂, HCl and HF plume species.