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| Titel |
The tropospheric processing of acidic gases and hydrogen sulphide in volcanic gas plumes as inferred from field and model investigations |
| VerfasserIn |
A. Aiuppa, A. Franco, R. Glasow, A. G. Allen, W. D'Alessandro, T. A. Mather, D. M. Pyle, M. Valenza |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 7, no. 5 ; Nr. 7, no. 5 (2007-03-13), S.1441-1450 |
| Datensatznummer |
250004756
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| Publikation (Nr.) |
 copernicus.org/acp-7-1441-2007.pdf |
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| Zusammenfassung |
| 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/m3at 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. |
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