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| Titel |
OClO and BrO observations in the volcanic plume of Mt. Etna – implications on the chemistry of chlorine and bromine species in volcanic plumes |
| VerfasserIn |
J. Gliß, N. Bobrowski, L. Vogel, D. Pöhler, U. Platt |
| 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 ; 15, no. 10 ; Nr. 15, no. 10 (2015-05-26), S.5659-5681 |
| Datensatznummer |
250119748
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| Publikation (Nr.) |
 copernicus.org/acp-15-5659-2015.pdf |
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| Zusammenfassung |
Spatial and temporal profiles of chlorine dioxide (OClO), bromine
monoxide (BrO) and sulfur dioxide (SO2) of the volcanic plume
at Mt. Etna, Italy, were investigated in September 2012 using Multi-Axis
Differential Optical Absorption Spectroscopy (MAX-DOAS). OClO was
detected in 119 individual measurements covering plume ages up to 6 min. BrO
could be detected in 452 spectra up to 23 min downwind.
The retrieved slant column densities (SCDs) reached maximum values of
2.0 × 1014 molecules cm-2 (OClO) and
1.1 × 1015 molecules cm-2 (BrO).
Mean mixing ratios of BrO and OClO were estimated assuming a
circular plume cross section. Furthermore, ClO mixing ratios were
derived directly from the BrO and OClO-SCDs. Average abundances
of BrO = 1.35 ppb,
OClO = 300 ppt and
ClO = 139 ppt were found in the young plume (plume
age τ < 4 min) with peak values of 2.7 ppb (BrO),
600 ppt (OClO) and 235 ppt (ClO) respectively.
The chemical evolution of BrO and OClO in the plume was
investigated in great detail by analysing the OClO/SO2 and
BrO/SO2 ratios as a function of plume age τ. A marked
increase of both ratios was observed in the young plume (τ < 142 s) and a levelling off at larger plume ages showing mean SO2
ratios of 3.17 × 10-5 (OClO/SO2) and
1.65 × 10-4 (BrO/SO2). OClO was less
abundant in the plume compared to BrO with a mean
OClO/BrO ratio of 0.16 at plume ages exceeding 3 min.
A measurement performed in the early morning at low solar radiances revealed
BrO/SO2 and OClO/SO2 ratios increasing with
time. This observation substantiates the importance of photochemistry
regarding the formation of BrO and OClO in volcanic plumes.
These findings support the current understanding of the underlying chemistry,
namely, that BrO is formed in an autocatalytic, heterogeneous reaction
mechanism (in literature often referred to as "bromine explosion") and that
OClO is formed in the reaction of OClO with BrO.
These new findings, especially the very detailed observation of the
BrO and OClO formation in the young plume, were used to infer
the prevailing Cl-atom concentrations in the plume. Relatively small
values ranging from [Cl] = 2.5 × 106 cm-3 (assuming
80 ppb background O3) to
[Cl] = 2.0 × 108 cm-3 (at
1 ppb O3) were calculated at plume ages of about 2 min. Based on these Cl abundances, a potential – chlorine-induced
– depletion of tropospheric methane (CH4) in the plume was
investigated. CH4 lifetimes between 14 h (at 1 ppb
O3) and 47 days (at 80 ppb O3) were derived. While
these lifetimes are considerably shorter than the atmospheric lifetime of
CH4, the impact of gaseous chlorine on the CH4 budget in the
plume environment should nevertheless be relatively small due to plume
dispersion (decreasing Cl concentrations) and ongoing mixing of the
plume with the surrounding atmosphere (replenishing O3 and
CH4).
In addition, all spectra were analysed for signatures of IO,
OIO and BrO. None of these species could be detected. Upper
limits for IO/SO2, OIO/SO2 and
OBrO/SO2 are 1.8 × 10-6, 2.0 × 10-5
and 1.1 × 10-5 respectively. |
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