ABSTRACT

Volcanic gas emissions from the main low-temperature fumaroles (around 100°C) of one of the most dangerous volcanoes in the Lesser Antilles, La Soufrière de Guadeloupe, are measured routinely using direct sampling with NaOH bottles (for H2O, CO2, H2Stot, H2, CH4, CO, N2, He, Ar, O2), P2O5 bottles (for H2O, CO2, H2S, SO2, H2) and MULTIGAS (for H2O, CO2, H2S, SO2, H2). This allows us to perform a study about an intercomparison of volcanic gas monitoring methologies. The results show some good overlap and similar temporal evolution between the dataset of the different methologies, which is of utmost of importance for monitoring purposes. However, there is no good agreement between the three methodologies for the same chemical ratio. The differences could be explained by a time-dependent advective/effusive phenomenon that affects the concentrations of sampled gases for two methods of the three (P2O5 bottles and MULTIGAS). P2O5-based analyses are affected because effusion on rapid decompression and fast sampling affects the gas volume in the pipeline connecting the vent outlet to the sampling. Although MULTIGAS is a continuous, flowthrough, sampling method, results are likely affected because of the sampling architecture involving filter and pump. Moreover, sulphur reaction (precipitation of elemental S upon exit from the fumarole, conversion of SO2 to H2S in P2O5 bottles) and water condensation and consequent SO2 scrubbing can also explain some discrepancies.

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