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| Titel |
Reactive bromine chemistry in Mount Etna's volcanic plume: the influence of total Br, high-temperature processing, aerosol loading and plume–air mixing |
| VerfasserIn |
T. J. Roberts, R. S. Martin, L. Jourdain |
| 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 ; 14, no. 20 ; Nr. 14, no. 20 (2014-10-23), S.11201-11219 |
| Datensatznummer |
250119118
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| Publikation (Nr.) |
 copernicus.org/acp-14-11201-2014.pdf |
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| Zusammenfassung |
Volcanic emissions present a source of reactive halogens to the troposphere,
through rapid plume chemistry that converts the emitted HBr to more reactive
forms such as BrO. The nature of this process is poorly quantified, yet is
of interest in order to understand volcanic impacts on the troposphere, and infer
volcanic activity from volcanic gas measurements (i.e. BrO / SO2 ratios).
Recent observations from Etna report an initial increase and subsequent
plateau or decline in BrO / SO2 ratios with distance downwind.
We present daytime PlumeChem model simulations that reproduce and explain the
reported trend in BrO / SO2 at Etna including the initial rise and
subsequent plateau. Suites of model simulations also investigate the
influences of volcanic aerosol loading, bromine emission, and plume–air
mixing rate on the downwind plume chemistry. Emitted volcanic HBr is
converted into reactive bromine by autocatalytic bromine chemistry cycles
whose onset is accelerated by the model high-temperature initialisation.
These rapid chemistry cycles also impact the reactive bromine speciation
through inter-conversion of Br, Br2, BrO, BrONO2, BrCl, HOBr.
We predict a new evolution of Br speciation in the plume. BrO, Br2, Br
and HBr are the main plume species near downwind whilst BrO and HOBr are
present further downwind (where BrONO2 and BrCl also make up a minor
fraction). BrNO2 is predicted to be only a relatively minor plume
component.
The initial rise in BrO / SO2 occurs as ozone is entrained into the plume
whose reaction with Br promotes net formation of BrO. Aerosol has a modest
impact on BrO / SO2 near-downwind (< ~6 km,
~10 min) at the relatively high loadings considered. The
subsequent decline in BrO / SO2 occurs as entrainment of oxidants
HO2 and NO2 promotes net formation of HOBr and BrONO2, whilst
the plume dispersion dilutes volcanic aerosol so slows the heterogeneous
loss rates of these species. A higher volcanic aerosol loading enhances
BrO / SO2 in the (> 6 km) downwind plume.
Simulations assuming low/medium and high Etna bromine emissions scenarios
show that the bromine emission has a greater influence on BrO / SO2 further
downwind and a modest impact near downwind, and show either complete or
partial conversion of HBr into reactive bromine, respectively, yielding BrO
contents that reach up to ~50 or ~20%
of total bromine (over a timescale of a few 10 s of minutes).
Plume–air mixing non-linearly impacts the downwind BrO / SO2, as shown by
simulations with varying plume dispersion, wind speed and volcanic emission
flux. Greater volcanic emission flux leads to lower BrO / SO2 ratios near
downwind, but also delays the subsequent decline in BrO / SO2, and thus
yields higher BrO / SO2 ratios further downwind. We highlight the
important role of plume chemistry models for the interpretation of observed
changes in BrO / SO2 during/prior to volcanic eruptions, as well as for
quantifying volcanic plume impacts on atmospheric chemistry. Simulated plume
impacts include ozone, HOx and NOx depletion, the latter
converted into HNO3. Partial recovery of ozone occurs with distance
downwind, although cumulative ozone loss is ongoing over the 3 h
simulations. |
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