Variability in the gas composition of the popocatépetl volcanic plume

Abstract

Long-term time series of volcanic plumes composition constitute valuable indicators of the evolution of the magmatic and volcanic systems. We present here a 4-years long time series of molecular ratios of HF/HCl, HCl/SO2, SiF4/SO2, HF/SiF4 measured in the Popocatépetl′s volcanic plume using ground-based solar absorption FTIR spectroscopy. The instrument, based in the Altzomoni NDACC (Network for the Detection of Atmospheric Composition Change) station, facing the Popocatépetl volcano, provides an unrivaled precision. The computed mean and standard deviation of the HF/HCl and HCl/SO2 ratios for this period were found to be 0.24 ± 0.03 and 0.11 ± 0.03, respectively. SiF4 was detected in three occasions and the SiF4/SO2 ratios ranged between (1.9 ± 0.5) × 10−3 and (9.9 ± 0.4) × 10−3. The HBr/HCl and HBr/SO2 ratios remained below their detection limits (1.25 × 10−4 and 1.25 × 10−5, respectively), given that a part of the HBr has already been converted to other bromine species (e.g., BrO, Br2) a few kilometers downwind of the crater. Combining our time series with satellite SO2 fluxes and seismic data, we explain the significant long-term HCl/SO2 variations by changes in the conduit and edifice permeabilities, impacting the deep and shallow degassing processes. The high temporal resolution of the data also allows capturing the variation of the volcanic plume composition preceding and induced by a common moderate explosion at Popocatépetl volcano. We interpret the observed variations of the HCl/SO2 ratio during the explosion in terms of changes in the contribution of the deep/shallow degassing. We additionally report the detection of an increase of SiF4 after the explosion, likely explained by in-plume HF-ash interaction. During this event, SiF4/HCl vs. HF/HCl was found to have a linear relation with a slope of −1/4, which implies a conservation of fluorine.