<p>On September 19, 2021, a volcanic eruption began at the west flank of Cumbre Vieja, La Palma, the most northwestern of the Canary Islands. The lava flows caused the evacuation of thousands of residents living in the vicinity of the volcano, and 1,219 hectares were covered by lava flows. After 85 days of activity, the eruption ended on December 13, 2021. Since visible volcanic gas emissions (fumaroles, hot springs, etc.) do not occur at the surface environment of Cumbre Vieja, the geochemical program for the volcanic surveillance has been focused mainly on diffuse (non-visible) degassing studies. Since 2001, diffuse CO<sub>2</sub> emission surveys have been yearly performed in summer periods to minimize the influence of meteorological variations. Measurements of soil CO<sub>2</sub> efflux have been performed following the accumulation chamber method in about 600 sites and spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse CO<sub>2</sub> emission from the studied area. In the period 2001-2016, the diffuse CO<sub>2</sub> output released to the atmosphere from Cumbre Vieja volcano ranged between 320 to 1,544 t·d<sup>-1</sup>. During pre-eruptive period (2016-2021), time series of the diffuse CO<sub>2</sub> emission showed a change with an increasing trend from 788 t·d<sup>-1 </sup>up to 1,870 t·d<sup>-1</sup>, coinciding with the beginning of the seismic swarms. This increase of diffuse CO<sub>2</sub> emission is interpreted as a geochemical precursory signal of volcanic eruption of Cumbre Vieja, on September 19, 2021. The observed increase on the diffuse CO<sub>2</sub> emission during this time window suggests that in October 2017 a process of magma ascent began from the upper mantle to depths between 35-25 km, at which the seismic swarms were recorded for four years. During eruption period, diffuse CO<sub>2</sub> emission showed strong temporal variations with a minimum value of the diffuse CO<sub>2</sub> emission in October 21, followed by an increase trend of up to 4,435 t·d<sup>-1 </sup>on December 14, the highest of time series and coinciding with the end of the eruption. During the post-eruptive period, the diffuse CO<sub>2</sub> emission has shown a descending trend. Our results demonstrate that periodic surveys of diffuse CO<sub>2</sub> emission are extremely important in the volcanic surveillance tools of Cumbre Vieja to improve the detection of early warning signals of future volcanic unrest episodes.</p><p> </p><p> </p><p> </p>

<p>Cumbre Vieja is the most active volcano of the Canary Islands since it has been the scenario of  8 of 17 historical eruptions in this archipelago. A recent magmatic reactivation started at Cumbre Vieja volcano on October 2017, and 9 additional seismic swarms occurred until the recent eruption which started on September 19, 2021, and ended on December 13, 2022 after 85 days of eruption. Since the first day of the eruption, extending to current days, INVOLCAN performed the monitoring of SO<sub>2</sub> realesed by this eruption using a miniDOAS on terrestrial (car), sea (ship) and air (helicopter) mobile position. More than 360 measurements of SO<sub>2</sub> emission rates were carried out daily. The standard deviation of the estimated values obtained daily was ~ 20%. During the first days of the eruption, estimated SO<sub>2</sub> emission rates reached more than 30,000 tons/day, and maintaining weekly average values above 10,000 tons/day until the end of the eruption. After a final paroxysmal phase with an eruptive column of 8,500 m altitude, decreased significantly to averages values of 250 tons/day. Estimated SO<sub>2</sub> emission rates from the 2021 Cumbre Vieja eruption became a powerful tool to contribute to the understanding of eruptive dynamics.</p>

<p>Cumbre Vieja (220 km<sup>2</sup>) is the most active volcano in the Canary Islands. It has been the location of 8 of the 17 historical eruptions in the archipelago during the last 600 years. The establishment of a geochemical monitoring program by our research group for the volcanic surveillance of Cumbre Vieja started in 1997. This program was mainly focused on diffuse degassing monitoring because of the absence of visible volcanic degassing manifestations (fumaroles, plumes, etc.) as well as other obvious geothermal features at Cumbre Vieja up to the 2021 eruption which started on September 19, ended on December 13 and lasted 85 days.</p> <p>The INVOLCAN’s soil degassing monitoring at Cumbre Vieja is carried out by means of a geochemical instrumental permanent network (soil CO<sub>2</sub> efflux, soil gas <sup>222</sup>Rn and soil C isotope ratio) and regular geochemical surveys covering the entire area of Cumbre Vieja (diffuse CO<sub>2</sub>, He and H<sub>2</sub> emissions). Several soil degassing anomalies have been observed and some of them years before the 2021 eruption, which illustrates the importance of diffuse degassing monitoring for volcanic surveillance. The single visible manifestation of volcanic degassing at La Palma is a cold CO2-rich site at Taburiente volcano. Regular helium-3 emission monitoring of this observation site has been carried out since 1991 in collaboration with Tokyo Univ., and provided a clear early warning signal of the 2021 Cumbre Vieja eruption. Because of the registration of seismic swarms, and to strengthen the INVOLCAN geochemical monitoring program of Cumbre Vieja volcano,  regular sampling of groundwater for chemical and isotopic analysis started in October 2017. The results of this hydrogeochemical monitoring also showed significant changes related to the recent volcanic unrest of Cumbre Vieja.</p> <p>Since the 2021 eruption onset, INVOLCAN performed daily observations of SO<sub>2</sub> emissions using a miniDOAS in traverse mode, on terrestrial (car), sea (ship) and air (helicopter) mobile position recording relatively high SO<sub>2</sub> emissions (> 50.000 t/d). Static scanners and satellite instruments were used also to monitoring the SO<sub>2</sub> emission released by this eruption; a task lead by the volcano research group of Manchester University. Additional plume geochemical monitoring was carried out using OP-FTIR spectrometers and UAV, helicopter and ground-base MultiGas units to characterize the chemical composition of the plume degassing in collaboration with scientists from Manchester Univ., Palermo Univ., UCL, INGV, IPGP and Azores Univ.  Carbon isotope analysis of the CO<sub>2</sub> gas plume was also undertaken in collaboration with New Mexico Univ. Analysis of pristine ash leachates has been also performed in collaboration with Durham Univ. and Tokyo Institute of Technology since it is often used to estimate the composition of the gas phase during volcanic eruptions and provides important information on the eruption processes was also performed.</p> <p>The results of these geochemical observations during the inter-eruptive, pre-eruptive, eruptive and post-erupive phases have been tremendously useful to understand the recent magmatic reactivation of Cumbre Vieja volcano.</p> <p> </p>