Publication Date:
2017-04-04
Description:
Improving the constraints on the atmospheric fate and depletion rates of acidic compounds
persistently emitted by non-erupting (quiescent) volcanoes is important for
quantitatively predicting the environmental impact of volcanic gas plumes. Here, we
present new experimental data coupled with modelling studies to investigate the chemical
processing of acidic volcanogenic species during tropospheric dispersion. Diffusive
tube samplers were deployed at Mount Etna, a very active open-conduit basaltic volcano
in eastern Sicily, and Vulcano Island, a closed-conduit quiescent volcano in the
Aeolian Islands (northern Sicily). Sulphur dioxide (SO2), hydrogen sulphide (H2S),
hydrogen chloride (HCl) and hydrogen fluoride (HF) concentrations in the volcanic
plumes (typically several minutes to a few hours old) were repeatedly determined at
distances from the summit vents ranging from 0.1 to 10 km, and under different environmental
conditions. At both volcanoes, acidic gas concentrations were found to
decrease exponentially with distance from the summit vents (e.g., SO2 decreases from
10 000 μg/m3 at 0.1 km from Etna’s vents down to 7 μg/m3 at 10 km distance),
reflecting the atmospheric dilution of the plume within the acid gas-free background
troposphere. Conversely, SO2/HCl, SO2/HF, and SO2/H2S ratios in the plume showed
no systematic changes with plume aging, and fit source compositions within analytical
error. Assuming that SO2 losses by reaction are small during short-range atmospheric
transport within quiescent (ash-free) volcanic plumes, our observations suggest that,
for these short transport distances, atmospheric reactions for H2S and halogens are
also negligible. The one-dimensional model MISTRA was used to simulate quantitatively
the evolution of halogen and sulphur compounds in the plume of Mt. Etna. Model
predictions support the hypothesis of minor HCl chemical processing during plume
transport, at least in cloud-free conditions. Larger variations in the modelled SO2/HCl
ratios were predicted under cloudy conditions, due to heterogeneous chlorine cycling
in the aerosol phase. The modelled evolution of the SO2/H2S ratios is found to be
substantially dependent on whether or not the interactions of H2S with halogens are
included in the model. In the former case, H2S is assumed to be oxidized in the atmosphere
mainly by OH, which results in minor chemical loss for H2S during plume
aging and produces a fair match between modelled and measured SO2/H2S ratios. In
the latter case, fast oxidation of H2S by Cl leads to H2S chemical lifetimes in the early
plume of a few seconds, and thus SO2 to H2S ratios that increase sharply during plume
transport. This disagreement between modelled and observed plume compositions
suggests that more in-detail kinetic investigations are required for a proper evaluation
of H2S chemical processing in volcanic plumes.
Description:
Published
Description:
11653–11680
Description:
open
Keywords:
tropospheric processing
;
volcanic gas plumes
;
01. Atmosphere::01.01. Atmosphere::01.01.04. Processes and Dynamics
;
01. Atmosphere::01.01. Atmosphere::01.01.07. Volcanic effects
;
03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
Permalink
