Monitoring of acid gases at the crater of Tajogaite volcano using alkaline traps, La Palma, Canary Islands

The eruption of the Tajogaite volcano occurred between September and December 2021, being the longest one recorded at La Palma Island. During the course of the eruption, the emission of gas and pyroclasts was constant, forming a cone of approximately 200m. Once the eruption ended, the gas emission has gradually decreased with fumaroles of low and high temperatures along craters. In situ sampling techniques, close as possible to the gas emission sources, are the most suitable tools to investigate the chemical composition of primary emitted volcanic gases. For this reason, we applied the sampling technique proposed by Noguchi & Kamita (1963) to monitor C/S, S/Cl, S/F and F/Cl ratios in the air inside the crater of Tajogaite by using alkaline traps (named JB1, JB3 and JB6) spatially distributed in order to access to different volcanic gas emission spots. This method gives the advantage of an integral monitoring of the acid gases during a certain sampling interval. Initially, two redundant sampling sites were use at three selected locations in April 2022 to study whether there were significant differences in the data from each pair of traps. The results indicated no significant differences in the results of the redundant sampling sites. The alkaline solutions, consisting of 2N KOH solutions, are analyzed using both volumetric and chromatographic methods. The carbon is analyzed by volumetric titrations, whereas Cl, S and F are analyzed by Ion Chromatography (IC). Samples are collected weekly for subsequent analysis.

Measured C/S ratios varied between 0.003-0.02, 0.001-002 and 0.002-4.1 for JB1, JB3 and JB6, respectively. S/Cl ratios varied between 11.5-62.3, 64.3-241.3 and 0.1-482.0 for JB1, JB3 and JB6, respectively. S/F ratios varied between 0.8-82.1, 6.3-78.9 and 0.01-74.4 for JB1, JB3 and JB6, respectively. F/Cl ratios varied between 0.4-17.8, 1.8-22.6 and 0.6-169.7 for JB1, JB3 and JB6, respectively. Results show a progress increase on the C/S ratio during all sampling period (April-November 2022), except for the JPB1 and JB3 during October and November 2022, which is explained in base of a observed slight decreasing trend in S content, which is to be expected since the post-eruptive activity should decrease as well as the decrease in the fumarolic output. However, the variations observed in the S/Cl, S/F and F/Cl ratios show a more chaotic behavior, depending on the alkaline trap and therefore its location. The fumaroles that come into contact with the alkaline traps are not the same, and the possible effects of external variables such as wind, can considerably reduce the transport of minor fumarolic gases from their source to the traps producing these variations. S/Cl ratio seems to show higher values during the first period of monitoring, that may be related to variations in the fumarolic activity whereas S/F and F/Cl do not show in general a clear temporal tendency. Direct sampling of plume/fumarolic gases by alkaline traps is a cheap and easy technique to monitor the relationship between various chemical species and volcanic activity.

Noguchi & Kamiya (1963). Bull. Volcanol., 24, 367-378.