| Assessment of the impact of tropospheric volcanic gas and aerosol emissions requires
integration of observation and modelling. Knowledge and understanding is rapidly advancing
in both areas, particularly due to the development of kinetic plume models of reactive
halogen chemistry, and due to recent advances in measurement techniques for collecting in
situ measurements of plume physico-chemical properties (i.e. using meteorological balloon
and aircraft platforms), as well as a proliferation of remote sensing DOAS measurements.
Here, we demonstrate this synergic relationship through model-observation plume
studies.
Volcanoes are a large natural source of SO2 and sulphate to the atmosphere, as is well
demonstrated from both observational and model studies. In a recent study that deployed
quasi-Lagrangian balloons in emissions at Kilauea volcano, Hawaii, both H2O(g) and SO2(g)
were measured in situ, in the downwind plume. The observations showed periods of both
correlation and anti-correlation between SO2 and water-vapour, implying the occurrence of
both source and sink processes. Co-emission of volcanic H2O with SO2 accounts
for the correlation. We use a thermodynamic model along the plume transect to
assess how H2O-sulphate interactions might account for H2O anti-correlation with
SO2 within the plume to elucidate in-plume sulphate formation, both near-vent (as
predicted by high-T thermodynamic models) and downwind (as predicted by kinetic
models).
Volcanoes are a source of halogens (HBr, HCl) to the atmosphere, and volcanic plumes are
highly reactive zones, not only in the high-temperature region near the vent, but also in the
downwind plume where autocatalytic chemistry cycles produce reactive halogens such as
BrO, first discovered from DOAS observations.
The rapid formation of BrO can be reproduced through modelling which predicts high
concentrations (reaching ppbv) on short formation timescales (minutes). Simulations using
the PlumeChem model (developed to analyse volcanic plume chemistry during
atmospheric dispersion) indicate that Br-content, sunlight and plume-air mixing exert
important controls on plume chemistry, in agreement with DOAS observations. NOx is
predicted to play a significant role, accelerating reactive bromine chemistry, and thereby
converting NOx into nitric acid. Thus the simulations support a mechanism for
elevated HNO3 observed in volcanic plumes, and highlight additional impacts of
plume chemistry. Furthermore, significant ozone depletion was predicted in the
downwind plume that could previously only be compared to limited observations.
Ozone depletion has recently been characterised by in-situ aircraft measurements of
Redoubt (and Erebus) plumes. Our direct comparison of simulated and observed ozone
depletion, in good agreement, supports and further constrains estimates of downwind
impacts.
The recent advances in in-situ measurements in downwind plumes enable us to test and
further develop process-based models of volcanic plume chemistry and impacts, ultimately
on a global scale. Conversely the model studies provide interpretation of observations, predict
new phenomena and provide spatial simulations of plume chemistry, essential to guide future
measurement-strategies. |