Fig. 2. An example of a 1800 sMultiGAS acquisition at Stromboli (acquisition frequency, 9 s). Whilst small erratic variations of H2O concentrations are typically measured when the plume is condensing (curve a), more systematic variations (curve b) are observed in dry weather conditions and when the plume fumigates the Pizzo Sopra la Fossa area. These are correlated with variations of CO2 (curve c) and SO2 (curve d) concentrations. In such circumstances, volcanic H2O was derived from the raw data (b) by subtracting background air H2O content; this required fitting a polynomial function (shown as a dotted line) to H2O measurements for which a SO2 content of nearly 0 was consistently detected.
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