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SiF4 is probably a common trace component of fumarole gases on volcanoes, arising from interactions between magmatic HF and wall rocks. It is not distinguished from HF in conventional analyses. A strong absorption band at 1032 cm−1 makes SiF4 well suited to remote open‐path FTIR spectroscopic techniques. We report the first measurements of SiF4 in volcanic gases at Etna and Vulcano (Sicily). SiF4 concentrations were low at Etna, but higher at Vulcano, where the SO2: SiF4 ratio was consistently around 1.7×10−2: 1. Thermodynamic considerations show that the HF : SiF4 ratio increases sharply with increasing temperature. Thus, remote measurements of SiF4 may offer a qualitative way of constraining fumarole temperatures. We use our FTIR measurements of HCl: SiF4 in combination with conventional measurements of HCl: HF to infer an HF :SiF4 ratio for Vulcano of ∼10:1, consistent with gas temperatures in the range 200–450°C.

Volcanic gases have important climatic and environmental effects, and provide insights into magmatic processes. Direct sampling of volcanic gases is inherently difficult and often hazardous. Here, we report the results of long path measurements of SO2 and HCl from Mt. Etna and Vulcano (Italy) obtained by active mode Fourier Transform InfraRed (FTIR) spectroscopy. Spectra recorded in September 1994 over path lengths of up to 2 km indicate SO2∶HCl ratios of 3–5∶1 for Etna, and 0.7–1.4∶1 for Vulcano, consistent with their different styles of activity. Combined with contemporaneous Correlation Spectrometer (COSPEC) SO2 flux measurements, these ratios indicate an HCl flux for Etna of about 1700 t/d (about 16% of the present global anthropogenic flux) and for Vulcano of about 13 t/d. We also report the first remote spectroscopic detection of volcanic SiF4.

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