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Phil. Trans. R. Soc. A (2008) 366, 4581–4595
doi:10.1098/rsta.2008.0152
Published online 25 September 2008
Volcanism and the atmosphere: the potential
role of the atmosphere in unlocking the
reactivity of volcanic emissions
B Y T AMSIN A. M ATHER *
Department of Earth Sciences, University of Oxford, Parks Road,
Oxford OX1 3PR, UK
Recent measurements of reactive trace gas species in volcanic plumes have offered
intriguing hints at the chemistry occurring in the hot environment at volcanic vents.
This has led to the recognition that volcanic vents should be regarded not only as passive
sources of volcanic gases to the atmosphere, but also as ‘reaction vessels’ that unlock
otherwise inert volcanic and atmospheric gas species. The atypical conditions created by
the mixing of ambient atmosphere with the hot gases emitted from magma give rise to
elevated concentrations of otherwise unexpected chemical compounds. Rapid cooling of
this mixture allows these species to persist into the environment, with important
consequences for gas plume chemistry and impacts. This paper discusses some examples
of the implications of these high-temperature interactions in terms of nitrogen, halogen
and sulphur chemistry, and their consequences in terms of the global fixed nitrogen
budget, volcanically induced ozone destruction and particle fluxes to the atmosphere.
Volcanically initiated atmospheric chemistry was likely to have been particularly
important before biological (and latterly anthropogenic) processes started to dominate
many geochemical cycles, with important consequences in terms of the evolution of the
nitrogen cycle and the role of particles in modulating the Earth’s climate.
Keywords: volcanology; atmospheric chemistry; tropospheric ozone;
early Earth atmospheres
1. Introduction
Volcanic activity on the surface of the Earth comes in many different
manifestations, from hydrothermal vents (‘black smokers’) deep on the ocean
floor (Butterfield 2000) to Plinian eruption columns punching high into the
stratosphere (Cioni et al. 2000). Such phenomena inspire awe and fascination,
but they have also played a key role in shaping the planet. In terms of our
atmosphere, volcanism as an agent of planetary outgassing was probably a major
contributor to its initial development (Schaefer & Fegley 2007). Over the
geological history of the planet, volcanism has subsequently played a part in
both maintaining and perturbing the atmosphere’s chemistry and physics, with
important implications in terms of the evolution of life. For example, while
*[email protected]
One contribution of 10 to a Triennial Issue ‘Earth science’.
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T. A. Mather
volcanic outgassing as a part of the inorganic carbon cycle has contributed to
maintaining long-term stability of atmospheric carbon dioxide levels, and hence
surface temperature and planetary habitability (e.g. Kasting & Catling 2003), it
has also been suggested that large-scale and long-lived basaltic eruptions (giving
rise to large igneous provinces or LIPs) have perturbed the atmosphere and
ecosystems to such a degree as to be implicated in mass extinction events such as
that associated with the demise of the dinosaurs (e.g. Courtillot & Renne 2003).
We have not experienced volcanism on the scale of which the planet is capable
during historical times. The most recent magnitude 8 explosive eruptions
(sometimes popularly known as ‘supereruptions’) were ca 74 000 and 26 500
thousand years ago (Toba, Indonesia, and Taupo, New Zealand, respectively, from
compilation by Mason et al. 2004), while the most recent LIP dates from ca 16 Myr
ago (Columbia River Flood Basalts, from compilation by Courtillot & Renne 2003).
Further, we have no direct way to make measurements of long-term atmospheric
composition and the effects of volcanic degassing over the entire geological history of
the planet (4.56 Gyr). Indeed, what we know of the composition of the atmosphere
from direct measurements extends back only over the ca 400 000 years recorded in
ice cores (Petit et al. 1999). Thus, much of our understanding of the potential
impacts and hazards of such large-scale volcanism and the role of volcanism in
atmospheric evolution comes by studying present-day examples and using them as
analogues. For example, the effects of the 1991 Mt Pinatubo (The Philippines)
eruption (including a notable decline in global average surface temperatures) have
informed our understanding of the potential effects of a much larger explosive
eruption (Robock 2000), and similarly the 1783–1784 Laki fissure eruption in Iceland
serves as a smaller-scale example of LIPs (Thordarson & Self 2003).
In the study of the atmospheric impacts of modern-day volcanism, it is
important to study both sporadic explosive eruptions as well as persistent and
often more passive emissions. This is not only because the plumes from these
different types of activities can present rather different environments (from
opaque, ash-rich plumes rapidly rising through the atmosphere (figure 1a) to
almost transparent, ash-poor plumes that drift with the wind after their emission
from the vent (figure 1b)), but also due to the different challenges associated with
making measurements at each. In terms of their time-averaged source strength,
the emissions from these two different types of volcanic activity are also likely to
be comparable (see compilation in Mather et al. 2003).
This paper seeks to summarize some exciting new developments over the past
decade in our understanding of the chemistry of volcanic plumes and their effects
upon our atmosphere. These developments have been in part due to improved
measurement technologies, for both ground-based remote sensing of volcanic gases
(e.g. Bobrowski et al. 2003) and laboratory analysis of samples (e.g. Aiuppa et al.
2005a), but have also resulted from serendipitous measurement opportunities (e.g.
the encounter of a NASA aircraft with the plume from Hekla volcano, Iceland, in
2000 (Hunton et al. 2005; Rose et al. 2006)). These new measurements build on
observations made in earlier studies to highlight the role that the atmosphere itself
plays in determining the chemistry that occurs in volcanic plumes as they
are transported downwind. The mixing of small amounts of ambient air into the
hot volcanic gas mixture in the plume has the potential to ‘unlock’ otherwise
chemically inert species into reactive forms. Electrical discharges in the plume may
also contribute to this ‘activation’ process. We must change the way that we view
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Volcanism and the atmosphere
(a)
4583
(b)
Figure 1. (a) The eruption column from Mount St Helens, USA, on 18 May 1980. (b) The persistent
plume from the summit craters of Mt Etna in August 2004 drifting away from the volcano with the
wind (the plume was quite condensed on the day that this photograph was taken, which is not
always the case).
eruptive vents, from chemically passive emission sources to reaction sites with
potentially far-reaching consequences.
2. The composition of volcanic emissions
A volcanic plume is a mixture of gas, and liquid and solid particles. For present-day
volcanism on the Earth, steam (H2O) and carbon dioxide (CO2) tend to be the most
prevalent gaseous species exsolving from a magma, followed by sulphur species (SO2
and H2S) and halogen halides (HCl, HF, HBr), and further minor species such as H2,
CO, OCS, Ar, NH4, CH4, N2 and He (Delmelle & Stix 2000), but the proportions
will vary from volcano to volcano. If the degassing is from a visible lava lake surface
or associated with the eruption of fresh magma, then it is likely that most of the
gases released have exsolved from the magma itself. However, this is not always the
case. If the volcanic emanations are emitted from fumaroles (cracks or fractures
associated with volcanic edifices), their exact origins may be more complicated, with
the interactions of groundwaters or marine fluids playing a role (figure 2). Although
high-temperature fumaroles might be conduits for gas directly from the magma, at
the opposite end of the spectrum, gases of truly magmatic origin might be almost
negligible, with the main role of the volcanism being the input of heat to vaporize
subsurface fluids. Logistically, studying fumarolic emissions can be more
straightforward than sampling other types of degassing; however, given their
potentially complex and volcano-specific origins in some cases, care must be taken
when interpreting the data obtained in terms of global volcanism as a whole
(Mather et al. 2003).
Upon release into the atmosphere, the volcanic plume gases will mix rapidly
and become diluted, with the consequence that, even shortly after emission,
components of the background atmosphere will account for significant proportions
of the species present within the plume. This mixing of the background atmosphere
(especially atmospheric oxygen) with the high-temperature volcanic gases followed
by further dilution and rapid cooling has the potential to alter the composition of
the volcanic plume from its composition when emitted from the magma (Gerlach &
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(a)
T. A. Mather
(b)
Figure 2. (a) The view from the sampling site at the crater rim of Masaya volcano in Nicaragua.
Masaya emits gas and aerosol from a vent in the floor of the active crater, a pit some 300 m deep and
500 m in diameter. The glow from the lava lake surface present approximately 10 m below the surface
down the vent is visible. (b) Sampling gases from a fumarole at Vulcano, Aeolian Islands, Italy.
Nordlie 1975; Martin et al. 2006). Although these changes may be subtle, recent
work has suggested that they may be important in terms of the chemistry that
occurs as the plume is transported downwind away from the vent.
The solid and liquid phases of a volcanic plume are composed of many different
components. It is well known that explosive volcanism produces solid silicate
products, some of which are fine enough to remain within the plume and be
transported downwind (mainly that classified as ash). However, recent work
(Martin et al. 2008) has built on earlier studies (Lefevre et al. 1986; Meeker &
Hinkley 1993) to show that fine silicate particles probably resulting from bubble
breaking on the magma surface can be present (although in much lower
concentrations) even in plumes associated with relatively non-explosive activity.
While emitted as a spray of liquid droplets, the silicate phase will rapidly solidify
as the plume cools. Other chemical species will be emitted from the magma in the
gas phase, or result from high-temperature reactions, and then condense as either
overgrowths or liquid envelopes on the pre-existing silicate particles or form a
separate aerosol phase (Mather et al. 2003).
The relative importance of these different processes (figure 3) will vary
depending on the nature of the volcanic emission (e.g. a strong explosive plume
punching up through the atmosphere versus a weak passive emission that will
drift away from the volcano carried by the wind).
3. Chemistry at the vent
As mentioned above, the effect on gas composition of mixing atmospheric oxygen
into hot volcanic emissions has been known for some time, and has previously
been studied with thermodynamic models (e.g. Gerlach & Nordlie 1975).
However, it is only recently, with the measurement of reactive nitrogen and
halogen trace species in volcanic plumes, that some of the implications of this
mixing in terms of atmospheric chemistry are beginning to be explored.
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Volcanism and the atmosphere
plume processing
physical conditions
silicate particles
ambient
temperature:
plume begins
transport
downwind
reactive trace gases
aerosol
silicate particles
+ overgrowths
cool magmatic +
atmospheric gases
background
atmosphere
rapid cooling
condensation
rapid mixing
and cooling
high-temperature
environment
hot magmatic +
atmospheric gases
condensation
hot magmatic gases
silicate particles
fragmentation and solidification
emission
mixing
emission
molten rock +
bubbles of volatiles
magma
Figure 3. A schematic summarizing the processes that contribute to a volcanic plume’s composition
upon measurement at the crater rim. For simplicity, potential hydrothermal processes have
been omitted.
(a ) Nitrogen chemistry
Biologically available nitrogen is essential for both animal and plant life.
Despite being the major component of the Earth’s atmosphere, nitrogen gas (N2)
is unavailable to most organisms and must be converted or ‘fixed’ to chemical
forms (such as NO, NO2, HNO3 and NH3). NOx (NOCNO2) species are formed
naturally (i.e. biotic or lightning fixation) and anthropogenically (e.g. combustion
of fuel N species). Although fixed N is necessary for life, anthropogenic emissions
have highlighted the detrimental effects of excessive releases of fixed N, on
both local and global scales, for the atmosphere, the terrestrial environment
and human health.
The fact that electrical discharges associated with explosive volcanic activity
(Mather & Harrison 2006) will fix nitrogen in a similar way to thunderstorm
lightning is no surprise, but recent measurements at volcanoes (Huebert et al.
1999; Mather et al. 2004a,b; Oppenheimer et al. 2005) have also shown that the
thermal energy released to the atmosphere at volcanic vents also fixes
atmospheric N2 into biologically available forms, even during very passive
volcanic activity, when N2 and O2 from the atmosphere are heated up in hot
volcanic environments as described above (§2).
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T. A. Mather
Table 1. Comparison of estimated present-day volcanic fixed nitrogen fluxes with those from
other sources.
release of fixed nitrogen
(Tg (N) yrK1)
volcanic sources:
thermal fixation
electrical discharges
other natural sources:
lightning
biological (marine)
biological (terrestrial)
total
total anthropogenic
0.01–0.06
references
0.02
Mather et al. (2004a,b) and
Oppenheimer et al. (2005)
Mather et al. (2004a)
!10
!30 to O300
90–140
130–450
w180
Vitousek
Vitousek
Vitousek
Vitousek
Vitousek
et
et
et
et
et
al.
al.
al.
al.
al.
(1997)
(1997)
(1997)
(1997)
(1997)
While these fluxes of fixed nitrogen are relatively insignificant on the present-day
planet (table 1), they may be important on a local or regional scale. Nitrogen species
may play an important role in other chemistry associated with volcanic plumes, and
these fluxes would have been more significant at other times during the geological
history of the Earth (see §4c). It should also be noted that there is still much to be
understood about volcanic nitrogen fixation, especially about the suggested rapid
conversion of NOx to HNO3 in the first few minutes after an emission from a volcanic
vent (Mather et al. 2004a; Martin et al. 2006).
(b ) Halogen chemistry
For many years, measurements of halogens in volcanic plumes have been
restricted to measurements of HCl and HF alone. However, recent developments in
remote sensing technology have led to the first measurements of BrO in volcanic
plumes (Bobrowski et al. 2003), and prompted similar efforts to detect the other
oxidized trace halogen species such as ClO (Lee et al. 2005) and OClO (Bobrowski
et al. 2007). Further the application of more sophisticated mass spectroscopic and
gas chromatographic techniques to gas samples collected has led to the detection of
HBr and HI in high-temperature volcanic emissions (Aiuppa et al. 2005a; Witt et al.
2008) and the detection of halocarbons in the lower-temperature emissions from
some other systems (Schwandner et al. 2004; Frische et al. 2006).
Measurements of halogen oxides (XO, XZCl, Br and I) in volcanic plumes are
important as an indicator of halogen-catalysed ozone destruction. It is actually
monatomic halogen species (i.e. Br, Cl, I) that have the potential to participate in
catalytic cycles for ozone destruction (via XO); however, these atomic halogens are
not measurable by current remote spectroscopic techniques. Here again it appears
that the mixing of atmospheric oxygen into hot volcanic gases plays a key role in
initiating this chemistry. While Gerlach (2004) showed that insufficient BrO itself
is generated in such mixtures, various modelling studies have shown that sufficient
Br might be generated to account for the observed BrO following low-temperature
oxidation during transport downwind (Gerlach 2004; Martin et al. 2006;
Bobrowski et al. 2007). This theory is supported by BrO levels being below
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Volcanism and the atmosphere
+
Br
HO
O3, OH, HO2
+h
a ero
sol
HBr
2Br
HOBr
BrO + O2
H
ozone destruction
O
+
+
~25°C
H2O + Br2
+ O3
HBr + Br
re-entry of Br into
reaction cycles
+ O2
HBr
~ 600°C
O2 + N2
O2 + N2
~ 800 −1000°C
Figure 4. A schematic summarizing some of the bromine chemistry proposed to occur in
volcanic plumes.
detection limits near the vents of Mt Etna, Italy, and Villarrica volcano, Chile,
but then increasing downwind (Bobrowski et al. 2007). If reaction of halogen
radicals (i.e. X) generated in or near the hot volcanic vent were the only process
involved in this volcanic plume chemistry, then only a finite amount of BrO
generation and O3 destruction would be possible, as the initial concentration of Br
generated would be limiting. However, reactive halogen species can also be involved
in autocatalytic reactions on the surface of acid aerosols, increasing the total
reactive halogens via generation from HX. These reactive halogens can then
re-enter the autocatalytic cycle to generate more of themselves or act to destroy
ozone, generating BrO. Thus ozone destruction (and BrO generation) is only
limited by HX in the plume, which is a much larger reservoir. The balance between
these different cycles as plumes are transported away from the vent downwind requires
much further investigation. This chemistry is summarized for bromine in figure 4.
So far there are limited observations of halocarbons in volcanic plumes, and
their formation mechanisms remain highly uncertain. Suggested mechanisms are:
catalytically activated radical reactions occurring in the gas phase with light
alkanes (e.g. CH4, produced at shallow depth in the volcanic and hydrothermal
systems by thermal decomposition of organic matter); thermal cracking of larger
hydrocarbons to form smaller hydrocarbon free radicals that then react with
halogens; or catalytic cracking on rock surfaces in the presence of magmatic
hydrogen halides (Schwandner et al. 2004; Frische et al. 2006).
(c ) Near-source particles
The role of acid aerosols for providing surfaces for chemical reactions during
transport downwind in the plume was alluded to in §3b. The exact nature and
extent of the role of particle surfaces in mediating plume chemistry and deposition
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T. A. Mather
is poorly understood and requires further work (Delmelle et al. 2007). Volcanic
particle emissions to the atmosphere also have important atmospheric consequences of their own (see §4b). However, continuing studies are helping to
elucidate the nature of the particles in volcanic plumes (summarized in Mather
et al. (2003) and more recently in Martin et al. (2008)), and in terms of acid aerosol,
sulphuric acid tends to be the dominant species. This acid aerosol is emitted
straight from the mouth of the high-temperature volcanic vents (see haze in
figure 2a) and hence constitutes a source of new particles to the atmosphere (Allen
et al. 2002). Field measurements have shown that these particles are not produced
by the same pathways of sulphur dioxide oxidation that occur in the plumes as they
are transported downwind (Mather et al. 2004c). Thermodynamic modelling
(Martin et al. 2006) and isotope studies (Mather et al. 2006) have suggested that
again the mixing of atmospheric oxygen into the hot volcanic environment might be
the key, with high-temperature thermodynamic equilibrium predicted to generate
sufficient levels of SO3 (which can then react with H2O to form H2SO4) to account
for the observed levels of sulphate measured near the vent in high-temperature
volcanic plumes. In cooler emissions, equilibria within the volcanic gases themselves
have the potential to generate observed sulphate (Mather et al. 2004d ).
4. Consequences
Much remains to be done to understand the full implications of our new
understanding of volcanic plume reactivity described above. Some of the initial
studies and results are discussed in §4a–c.
(a ) Ozone chemistry
Field campaigns have shown a clear anticorrelation between BrO and O3 in both
the polar (Tuckermann et al. 1997) and mid-latitude troposphere (Matveev et al.
2001). Observing localized ozone holes associated with persistently degassing
volcanoes is challenging, in part due to the difficulty of deconvolving chemical ozone
loss from the effects of the dynamic history of the air parcel and distinguishing a
signal above that due to stratospheric ozone when using solar radiation as a source
for remote sensing. Lee et al. (2005) presented preliminary data at sea level and at
more than 5 km from the summit crater of Sakurajima volcano, Japan, but the data
were not conclusive. Stratospheric ozone levels were observed to decrease after the
1991 Pinatubo eruption, but it has been suggested that this was the result of
the increase in the surface area available for heterogeneous reactions (due to the
sulphate aerosol particles associated with the eruption) activating anthropogenic
halogens (Robock 2000 and references therein). Conversely, the localized upper
troposphere–lower stratosphere ozone hole associated with the 2000 Hekla eruption
(documented by the serendipitous measurements taken by a NASA aircraft) has
been shown to be due to the combination of volcanogenic compounds within the
plume, including halogen and nitrogen species (Millard et al. 2006; Rose et al. 2006).
Interestingly, modelling suggested that nitrogen chemistry played a key role,
implying that without volcanic nitrogen fixation (either by high temperature or
electrical discharges in the plume) the localized impacts of volcanic plumes on ozone
may be very much less.
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(b ) Direct and indirect effects of volcanic particles on the Earth’s
radiative balance
Aerosol particles interact with solar radiation both directly and indirectly by
modifying cloud properties and extent and therefore affect the Earth’s radiative
balance (e.g. Kiehl & Briegleb 1993). Owing to the altitude and location of their
emissions, the volcanic contribution to these effects may be disproportionately
large compared with their source strength (e.g. Graf et al. 1997). Emissions of
new particles to the atmosphere may be especially important in terms of these
effects as, by contrast, the production of condensable material from gaseous
precursors (e.g. oxidation of SO2 to sulphate) in ageing plumes tends to increase
the size of existing particles rather than form new particles.
As mentioned above (§3c), probably due to interactions between magmatic
and atmospheric gases, even passively degassing volcanoes are significant sources
of new particles to the atmosphere. Fluxes of approximately 1015–1018 sK1 per
volcano have been measured from volcanoes during non-explosive volcanic
activity, comparable to large coal-fired power plants (Stith et al. 1978; Hobbs
et al. 1980, 1982; Radke 1982; Mather et al. 2004c). These emissions often impact
on otherwise relatively clean parts of the atmosphere. The presence of suitable
particles in air greatly reduces the supersaturation needed to form water droplets
and hence clouds. The ability of a particle to act as a nucleus for water droplet
formation (i.e. to become activated as a cloud condensation nucleus, CCN) will
depend on its size, chemical composition and the local supersaturation. Simple
modelling shows that the majority of the particles released in passive volcanic
degassing will act as CCN at typical atmospheric supersaturations (Mather et al.
2004c,d ), suggesting that volcanoes play a potentially important role in
modulating cloud cover and properties.
Despite their potential importance, there are few fully quantitative studies of the
extent of the volcanic aerosol’s direct or indirect effects on the troposphere. Graf
et al. (1997) suggested that time-averaged volcanic sulphate accounts for 33 per cent
of the total global direct negative radiative effect due to sulphate aerosol. Gassó
(2008) used satellites to observe phenomena akin to ship tracks downwind of active
volcanoes, with the volcanically influenced clouds having decreased droplet effective
radius and liquid water content and increased brightness. Further studies of these
effects are to be encouraged.
(c ) The early Earth
As noted above (§3a), the present-day global budgets to the atmosphere of many
species are dominated by biological and anthropogenic processes. However, on the
early Earth in the absence of biological and anthropogenic emissions, volcanism had
the potential to have a much more important influence on atmospheric chemistry
and composition. Here again we see the potential importance not only of volcanic
degassing as a major agent of planetary outgassing but also of the processing of the
atmosphere due to mixing into the hot volcanic environment.
There are some important differences when considering the effects of this
processing on the early atmosphere. The early Earth may have exhibited higher
rates of volcanism (e.g. Richter 1985) and hence the rate of atmospheric
processing by volcanic activity is potentially much greater. Furthermore, higher
magmatic temperatures (Herzberg et al. 2007) would suggest that mixtures of
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magmatic and atmospheric gases would equilibrate at higher temperatures (i.e.
1400–16008C) than on the modern Earth. It can also be speculated that there
may have been more extensive subaerial high-temperature magmatism than at
present, for example, if parts of the oceanic ridges were exposed above sea level
(de Wit & Hynes 1995) or during the emplacement of early flood-basalt provinces
(Ernst & Buchan 2003). The nature and evolution of the early Earth atmosphere
are poorly constrained, but also probably had some important differences from
those present today. Although some earlier studies (e.g. Urey 1952) suggested a
more reduced atmosphere, the current consensus is that the early Earth is likely
to have had a CO2-rich atmosphere since at least 4 Gyr ago until the rise of
oxygen ca 2 Gyr ago (Kasting & Catling 2003). However, there is much
uncertainty about the exact atmospheric composition and about the contents of
trace species such as CH4 (e.g. Pavlov et al. 2000).
Taking these differences between modern-day and early Earth volcanism into
account, a recent study (Martin et al. 2007) has used thermodynamic modelling
to elucidate some of the potential effects of high-temperature volcanic processing
of the atmosphere on the chemistry of the early Earth atmosphere. The model
results predicted that the fluxes of NO, OH, Cl, Br and I from early Earth
volcanism exceeded those from modern Earth volcanism as the higher
temperature of the early Earth emissions compensates for lower levels of O2 in
the atmosphere, compared with the modern Earth. A key consequence of this is
that, under certain conditions, the volcanic NO flux from early Earth volcanism
may have been comparable to other sources of reactive N such as lightning NO
and photochemical HCN. This may have alleviated the postulated Archaean
nitrogen crisis suggested to have triggered the emergence of biological nitrogen
fixation (Navarro-González et al. 2001). The thermodynamic model also reveals
that production of SO3 (the likely precursor for near-source volcanic sulphate
and hence volcanic aerosol, see §§3c and 4b) is likely to be significantly lower
from early Earth volcanism, with consequences for the haziness and hence
radiative balance of the early Earth atmosphere.
Volcanic lightning has also been suggested to have contributed to the
development of our atmosphere and life on the Earth in a number of ways (see
Mather & Harrison 2006 for a summary).
5. Conclusions and the future
Passively degassing or weakly erupting volcanic plumes are showing themselves
to be a fascinating natural laboratory to study an increasing array of interesting
atmospheric chemistry. There is much to suggest that the near-source processes
(described above) mixing atmospheric gases into the hot volcanic environment
and unlocking otherwise chemically inert species into reactive trace gases have
important consequences both on the present-day planet, for example, in terms of
its oxidation capacity (via destruction of ozone) and near-source particle
production, and on the pre-biotic planet, for example, in terms of the production
of biologically available nitrogen for the evolution of early life. These processes
may also represent another way that volcanoes impact upon the environment
during periods of heightened volcanic activity such as the emplacement of LIPs.
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However, there are many outstanding mysteries about the nature of this nearsource plume chemistry. To name but a few: Does it occur at every hightemperature degassing volcano? What effects might the different dynamic
degassing regimes operating at different volcanoes have on this chemistry (e.g.
the shape and permeability of the edifice and vent)? How can, for example, Mt
Etna’s plume appear to have coexisting reduced species (e.g. measured H2S;
Aiuppa et al. 2005b) and oxidized species (e.g. Br, NOx implied by the chemistry;
Martin et al. 2006; Bobrowski et al. 2007)?
There is also still much to understand about the medium-range (tens to
hundreds of kilometres) processes in operation. For example, chemical reactions
on ash and aerosol particles from volcanoes are largely unconstrained but, due to
their high concentrations in volcanic plumes, these reactions could have
important consequences for the chemistry that occurs (e.g. in ozone destruction
as described in §3b). So far our models have not been able to account for the
positive night-time perturbation of the HOx cycle observed in the Hekla plume
(Rose et al. 2006). Answering these questions will require the integration of
laboratory studies, modelling and longer-range measurements, often of trace
species, making the use of downwind in situ measurement platforms such as
aircraft or weather balloons imperative.
Finally, we need to understand the potential larger-scale regional and global
impacts of this chemistry. This will require the application of atmospheric
chemistry models on a range of scales suitably initiated using the outputs from
medium-range model and measurement studies. We are at an exciting point in
our understanding of the tropospheric chemistry of volcanic plumes. The stage is
set for this framework to be used to unravel how different elements have been
processed through volcanoes over geological time and their impacts upon our
planet’s atmosphere and its evolution.
The author is supported by a Royal Society Dorothy Hodgkin Research Fellowship and wishes to
thank Rob Martin, David Pyle and Roland von Glasow for their useful discussions. The Mount St
Helens image in figure 1 is used with the courtesy of USGS/Cascades Volcano Observatory. David
Pyle, Sarah Collins and Clive Oppenheimer are thanked for the use of the other photographs
in figures 1 and 2. Terry Gerlach and Marie Edmonds are gratefully acknowledged for reviewing
the manuscript.
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AUTHOR PROFILE
Tamsin A. Mather
Tamsin A. Mather grew up in Bristol and graduated from the University of
Cambridge with an MSci degree in chemistry in 1999 and an MPhil degree in the
history and philosophy of science in 2000. After spending a year out of academia,
among other things travelling and working for the EU Commission in Brussels, she
returned to Cambridge to study for a PhD in the Department of Earth Sciences
on the atmospheric chemistry of volcanic plumes. After finishing her PhD in 2004,
she spent three months on secondment to the Parliamentary Office of Science
and Technology in Westminster writing a report on carbon capture and storage.
She then returned to Cambridge as a Royal Society Dorothy Hodgkin Research
Fellow, continuing her work on the role that volcanism plays in cycling material
between the solid Earth and its atmosphere and the impacts of volcanic activity on
the environment. In July 2006, she moved to the Department of Earth Sciences at
the University of Oxford to combine her Royal Society Research Fellowship with a
Research Councils UK Academic Fellowship in the Physics and Chemistry of the
Earth and Environment. She is also currently a fellow of University College.
Photograph: at the crater rim of Villarrica volcano, Chile.
Phil. Trans. R. Soc. A (2008)