Stable gas plume composition measured by OP‐FTIR spectroscopy at Masaya Volcano, Nicaragua, 1998–1999

We report non‐eruptive plume gas composition data collected by open‐path Fourier transform infrared (OP‐FTIR) spectroscopy at the summit of Masaya Volcano, Nicaragua, in 1998 and 1999. Masaya's plume is characterised by SO 2 /HCl and HCl/HF mass ratios of 2.8–3.0 and 8.1–8.2 respectively. When combined with COSPEC SO 2 emission rate data, these ratios indicate average emission rates of HCl and HF of 7.3 kgs −1 and 0.9 kgs −1 . The gas composition is remarkably consistent, showing little temporal variation on the scale of days, weeks or even between the two observation periods, one year apart. Such striking uniformity is best explained by steady‐state open system degassing.