Reactive bromine chemistry in Mount Etna's volcanic plume: the influence of total Br, high-temperature processing, aerosol loading and plume–air mixing
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,...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/11201/2014/acp-14-11201-2014.pdf |
| Summary: | 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 / SO<sub>2</sub> ratios).
Recent observations from Etna report an initial increase and subsequent
plateau or decline in BrO / SO<sub>2</sub> ratios with distance downwind.
<br><br>
We present daytime <i>PlumeChem</i> model simulations that reproduce and explain the
reported trend in BrO / SO<sub>2</sub> 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, Br<sub>2</sub>, BrO, BrONO<sub>2</sub>, BrCl, HOBr.
<br><br>
We predict a new evolution of Br speciation in the plume. BrO, Br<sub>2</sub>, Br
and HBr are the main plume species near downwind whilst BrO and HOBr are
present further downwind (where BrONO<sub>2</sub> and BrCl also make up a minor
fraction). BrNO<sub>2</sub> is predicted to be only a relatively minor plume
component.
<br><br>
The initial rise in BrO / SO<sub>2</sub> occurs as ozone is entrained into the plume
whose reaction with Br promotes net formation of BrO. Aerosol has a modest
impact on BrO / SO<sub>2</sub> near-downwind (< ~6 km,
~10 min) at the relatively high loadings considered. The
subsequent decline in BrO / SO<sub>2</sub> occurs as entrainment of oxidants
HO<sub>2</sub> and NO<sub>2</sub> promotes net formation of HOBr and BrONO<sub>2</sub>, whilst
the plume dispersion dilutes volcanic aerosol so slows the heterogeneous
loss rates of these species. A higher volcanic aerosol loading enhances
BrO / SO<sub>2</sub> in the (> 6 km) downwind plume.
<br><br>
Simulations assuming low/medium and high Etna bromine emissions scenarios
show that the bromine emission has a greater influence on BrO / SO<sub>2</sub> 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).
<br><br>
Plume–air mixing non-linearly impacts the downwind BrO / SO<sub>2</sub>, as shown by
simulations with varying plume dispersion, wind speed and volcanic emission
flux. Greater volcanic emission flux leads to lower BrO / SO<sub>2</sub> ratios near
downwind, but also delays the subsequent decline in BrO / SO<sub>2</sub>, and thus
yields higher BrO / SO<sub>2</sub> ratios further downwind. We highlight the
important role of plume chemistry models for the interpretation of observed
changes in BrO / SO<sub>2</sub> during/prior to volcanic eruptions, as well as for
quantifying volcanic plume impacts on atmospheric chemistry. Simulated plume
impacts include ozone, HO<sub>x</sub> and NO<sub>x</sub> depletion, the latter
converted into HNO<sub>3</sub>. Partial recovery of ozone occurs with distance
downwind, although cumulative ozone loss is ongoing over the 3 h
simulations. |
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| ISSN: | 1680-7316 1680-7324 |