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Remotely sensed measurements of volcanic plumes have been undertaken for 30 years with instruments such as the correlation spectrometer, and more recently, open-path Fourier transform infrared (OP-FTIR) spectrometers. Observations are typically made several kilometres from the source, by which time chemical reactions may have occurred in the plume, overprinting the source composition and flux. Volcanological interpretations of such data therefore demand an understanding of the atmospheric processes initiated as gases leave the volcanic vent. Ground-based remote sensing techniques offer the temporal resolution, repeatability and quantitative analysis necessary for investigation of these processes. Here we report OP-FTIR spectroscopic measurements of gas emissions from Volcano, Nicaragua, between 1998 and 2001, and examine the influence of atmospheric processes on its tropospheric plume. Comparisons of observations made at the summit and down-wind, and in different measurement modes confirm that tropospheric processes and local meteorology have only minor impact on gas composition after the plume has left the crater. This study demonstrates that plume monitoring downwind provides a reliable proxy for at-crater sampling, and that volcanological information content is not obscured by the intervening transport. From February 1998 to May 2000, Masaya's plume composition was strikingly stable and characterized by SO2 /HCI and HCl/HF molar ratios of 1.6 and 5.0, respectively. Departures from this stable background composition are likely to signify changes in the volcanic system or degassing regime, as identified in April-May 2001.

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