Volcanic CO2 measurements at Campi Flegrei by Infrared Tunable Diode Laser absorption Spectroscopy

Abstract

Gas studies add information for the interpretation of fluid circulation dynamics at dormant volcanoes and can contribute to eruption forecasting. Direct in-situ and remote-sensing techniques were used in order to improve volcanic gas monitoring, essential for hazard assessment. In the last decades, near-infrared diode lasers have increasingly been used in atmospheric research and, though in an experimental phase, are now finding applications in volcanic gas studies. The Tunable Diode Laser Spectroscopy technique (TDLS) relies on measuring the absorbance at specific wavelengths due to the absorption of IR radiation by a target gas. Here, we report on the application of the GasFinder 2.0, an infrared laser unit operating in the 1.3-1.7 μm wavelength range, to measuring CO2 mixing ratios in volcanic gas emissions. Three different campaigns were carried out at Campi Flegrei volcano (near Pozzuoli, Southern Italy) in the attempt to obtain novel information on the current degassing unrest of Solfatara and Pisciarelli fumarolic fields. At each site, we used the GasFinder unit and several retro-reflector mirrors, to scan the plumes from different angles and distances. From post-processing of the data, by using a tomographic Matlab routine, we resolved, for each of the manifestations, the contour maps of CO2 mixing ratios in their atmospheric plumes. From their integration (and after multiplication by the plumes’ transport speeds) we evaluated the CO2 fluxes. The so-calculated flux (about 490 Mg/day) supports a significant contribution of fumaroles to the global CO2 budget. The cumulative (fumaroles [this study] +soil [1]) CO2 output from Campi Flegrei is finally evaluated at 1600 Mg/day. The application of lasers to volcanic gas studies is still an emerging (though intriguing) research field, and requires more testing and validation experiments.