Fig. 4. High-resolution (9 s) record of (a) plume ratios and (b) CO2 concentrations, showing the contrasting compositions of the passive and syn-explosive gas plume emissions. In the most favourable conditions (strong winds blowing from the N), a Strombolian explosion (grey arrow labelled “EXP”) is followed (with a time-lag of a few seconds) by a brief (lasting a few minutes) but significant increase of CO2 concentrations and CO2/SO2 ratios detected by the MultiGAS. The syn-explosive gas phase is typically H2O-poorer (and CO2-richer) than the passive plume released in between explosions (this contribution by far dominating Stromboli's bulk plume emissions in the long-term).
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