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Volcanism and atmospheric chemistry
Emissions from volcanic eruptions are a signi cant source of atmospheric gases (and particles) which, in uence the tropospheric and stratospheric trace-gas budgets. The quantity and nature of gases emitted from volcanoes depend on the amount and com-position of magma (see section 1.2). The quantities of various gaseous species released during an eruption vary from one eruption to another.
Volcanoes contribute to the aerosol burden by two ways, emissions of SO2 and the subsequent formation of sulphuric acid (H2SO4; which nucleate to form new aerosols or condense on pre-existing aerosols) and direct emissions of sulphate aerosols (discussed further in the next section). Volcanic ash particles have limited climatic impact due to their short lifetimes in the troposphere relative to the e cient dry deposition. Only sub-micron volcanic ash can participate to the climate forcing due to their relative smaller size and longer lifetimes (Langmann, 2014).
The most common and abundant volcanic gases released from eruptions are water vapour (H2O), carbon dioxide (CO2), sulphur dioxide (SO2) and hydrogen sulphide (H2S). H2O represents 50 – 90 % by volume of gas phase and the contribution of CO2 is estimated between 1 – 40 % by volume of gas (Textor et al. (2004) and references therein). Despite the relatively high abundances of H2O and CO2, the contribution of volcanic emissions to the global atmosphere for these two gases is limited due to their high background concentrations (Cadle, 1980, Schmincke, 1993, Textor et al., 2004).
Textor et al. (2004) report that sulphur species contribute between 2 – 35 % by volume of gas phase, SO2 and H2S being the most abundant (much lower quantities of carbon disulphide (CS2) and carbonyl sulphide (COS)). However, it might be worth noting that although COS are emitted in much lower quantities compared to SO2 and H2S, the residence time of COS in the atmosphere is of several years (Kjellstrom (1998), Steele et al. (2009), Textor et al. (2004), amongst others). COS is able to di use into the stratosphere due to it’s relative inertness and form sulphate particles upon oxidisation (contributing to reactions which involve stratospheric ozone chemistry and implications for the atmospheric radiation balance) (Andreae & Crutzen, 1997, Schlesinger & Bernhardt, 2013, Steele et al., 2009, Sturges et al., 2001, Textor et al., 2004).
This being said, SO2 is one species emitted from volcanoes which has clearly been shown to have the potential to a ect the climate system through it’s reaction with OH (hydroxyl) radical in the gas phase or through oxidation in clouds (Robock, 2000). These reactions form a sulphate (H2SO4) aerosol cloud. Gaseous SO2 is converted into H2SO4 aerosols within about 30 days (Co ey, 1996). SO2 species are also an important oxidant in volcanic plumes (Robock, 2000). von Glasow (2010) show how the lifetimes of OH radicals are drastically reduced (to the point of being virtually absent) in a volcanic plume which contain high concentrations of SO2. Sulphate aerosols have a lifetime of a few days in the troposphere (Mather et al., 2003), whereas in the stratosphere these volcanic aerosols can persist for years (Robock, 2000) as there is no wet deposition.
The detection of bromine monoxide (BrO) by Bobrowski et al. (2003) from a non-explosive volcanic plume at Soufriere hills volcano, Montserrat, has directed great at-tention of the scienti c community to the possible contributions of bromine oxides in the troposphere (Theys et al., 2009). Furthermore, halogens are considered to play an important role in volcanic plume chemistry (von Glasow, 2010). BrO since it’s de-tection by Bobrowski et al. (2003) has been measured in several other volcanic plumes (quiescent degassing) (Bobrowski et al. (2007), Kern et al. (2009), Oppenheimer (2006), Theys et al. (2009), amongst others). The inorganic halogen compounds are mainly emitted as hydrogen chloride (HCl; reported by Textor et al. (2004) to contribute 1 – 10 % by volume of gas phase), hydrogen bromide (HBr; contributes 10 3 % by volume of gas phase) and hydrogen uoride (HF , although HF is not always present; Delmelle & Stix (2000)) (Gerlach, 2004). A study by von Glasow (2010) show how high levels of reactive bromine in the plume (leading to extensive ozone (O3) destruction) can be maintained for several days (Boichu et al., 2011, Kelly et al., 2013).
In comparatively lesser amounts, eruptions also emit mercury (Hg) vapour, nitrogen oxides (N Ox) (von Glasow et al., 2009) and even gold (Meeker et al., 1991). Only a few measurements are available for emissions of Hg and N Ox. The volcanic plume chemistry of these volatiles is also an active area of research (von Glasow et al., 2009), along with their atmospheric impacts.
Volcanic eruptions as a climate forcing agent
Franklin (1784) and Mitchell (1961) were amongst the rst to discuss the impact of volcanic eruptions on our climate (Robock, 2000). Since, countless research studies have been conducted and a brief summary of the known e ects are outlined here.
Studies by Symons (1888) on the 1883 Krakatau eruption and Robock & Mass (1982) on the 1980 Mt. St. Helens eruption have shown how small amounts of tephra can remain in the stratosphere for up to a few weeks. However, very small climatic impacts are associated to this release (Wegmann, 2012).
On the other hand, the volatiles ejected during an eruption, especially SO2 deriva-tives have been shown to considerably impact the climate and remain in the stratosphere for years (compared to a residence time of a few days in the troposphere) (Robock, 2000). The impact on climate is achieved through the reaction of SO2 with H2O and the OH (hydroxyl) radical, forming a sulphate (H2SO4) aerosol cloud. Due to the characteristic of the stratosphere and the associated winds, the aerosol cloud is ad-vected around the globe in a few weeks (Robock, 2000, Wegmann, 2012). This process depends on the timing and geographical position of the eruption (Bluth et al., 1992, Robock & Matson, 1983, Wegmann, 2012). The perturbation caused by the H2SO4 cloud through radiative e ects is the main reason for climatic variations. The incom-ing short-wave solar radiation is scattered by this H2SO4 cloud, while a signi cant proportion is backscattered to space (Robock, 2000, Wegmann, 2012), thus increasing the Earth’s albedo. The amount of scattering that takes place is dependent on two factors, the wavelength of the incoming radiation and the size of the scattering particle (in this case the H2SO4 aerosol). Some solar radiation does get forward scattered by the aerosol clouds and de ected by the aerosols, it continues toward the Earth’s surface at a di erent angle. Overall, there is a local solar heating through absorption as solar radiation is retained by a substance and converted into heat. Heat resulting from the absorption of incoming shortwave radiation is emitted as longwave radiation by the Earth and the atmosphere. Most of this outgoing radiation is absorbed by the aerosol cloud. On the whole, these processes have a net cooling e ect on the lower atmosphere, while a local net heating is observed in the altitude of the aerosol cloud (Robock, 2000, Wegmann, 2012). The classic idea described here is depicted in Fig.(2.1).
Apart from the e ects on the Earth’s albedo, volcanic aerosols in the stratosphere, can also in uence the O3 chemistry (Wegmann, 2012). O3 is created through the process of photolysis, whereby, the solar radiation strikes oxygen (O2) molecules causing the two O2 atoms to split apart. When this free atom interacts with another O2 molecule, they join and form O3 (also known as the Chapman’s cycle). Furthermore, O3 is also naturally broken down in the stratosphere by sunlight and through chemical reaction with various reaction surfaces such as compounds containing nitrogen, hydrogen and chlorine (Robock, 2000). However, all these factors are altered by volcanic aerosols (Wegmann, 2012). In general it is found that volcanic aerosol concentrations decrease O3 concentrations. For example, H2SO4 aerosols act as a surface for chlorine activation and O3 depletion. On the other hand, volcanic H2SO4 aerosols lead to an increase in surface UV (ultra violet) radiation. The net e ect is considered such that, O3 depletion lets more UV through the atmosphere than is backscattered by volcanic aerosols (Robock, 2000, Wegmann, 2012).
In the following sub-sections the most important e ects of this perturbed radiation regime on temperature and precipitation are brie y discussed.
In uence on temperature
We have just seen how outgoing long-wave and incoming short-wave radiations are absorbed and de ected by the aerosol cloud. On the whole this leads to changes in the earth’s temperature on shorter times scales, reducing Earth’s surface temperature (Wegmann, 2012). Robock (2000) based on observational data states that this e ect is limited to one to four days after the eruption.
Table of contents :
1 Volcanic eruptions in the Earth’s system
1.1 Types of volcanoes: Volcanic eruption variability
1.2 What controls eruption styles?
1.2.1 Magma composition
1.2.1.1 Volcanic rocks – mineral composition
1.2.1.2 Volcanic rocks – magma cooling rate
1.2.1.3 Volcanic rocks – classications
1.2.2 Magma viscosity, temperature and gas content
1.2.3 Controls on explosivity
1.3 General volcanic eruption dynamics
1.3.1 Magma column
1.3.2 Eruption column
1.4 Man and volcanism
1.4.1 Vesuvius eruption and Pompeii
1.4.2 Laki eruption, Iceland
1.4.3 Eruption of Mount Tambora
1.4.4 Pinatubo eruption
1.4.5 Soufriere Hills, Montserrat
1.4.6 Eyjafjallajokull, Iceland
1.5 Outline
2 Volcanic eruption impacts and consequences to models
2.1 Volcanism and atmospheric chemistry
2.1.1 Volcanic eruptions as a climate forcing agent
2.1.1.1 Inuence on temperature
2.1.1.2 Inuence on water cycle
2.1.2 A focus on the tropospheric sulphur cycle
2.1.3 Volcanic eruptions and air pollution
2.1.3.1 SO2uxes and measuring tools
2.1.3.2 Volcanic smog, Laze and Acid rain
2.1.3.3 The particular case of Piton de la Fournaise
2.2 Volcanic columns and ash cloud: models and challenges
2.2.1 Atmospheric dispersal processes: high energy plume models
2.2.2 Buoyant volcanic column and proximal dynamics: buoyant column and dispersal models
2.2.3 Ash dispersal at regional, continental and global scale
2.2.4 Overview
2.3 On the importance of plume heights
2.4 Thesis objectives
3 1D idealised simulation and parameterisation of January 2010 PdF eruption
3.1 The atmospheric model
3.1.1 Parametrisations for shallow convection
3.1.1.1 Parameterised turbulent ED terms
3.1.1.2 The Mass-Flux (MF) scheme
3.1.2 Entrainment through turbulent mixing
3.2 Abstract of the research article
3.3 Introduction
3.4 Volcanic plume parameterisation and model congurations
3.4.1 January 2010 summit eruption of Piton de la Fournaise
3.4.2 Description of the volcanic plume parameterisation
3.4.2.1 Sub-grid cloud parameterisation as per Pergaud et al.
3.4.2.2 Modied EDMF – updraft initialisation
3.4.2.3 Modied EDMF { basal lateral mass exchange
3.4.3 Simulation set-up and conguration
3.4.3.1 Common features to all simulations
3.4.3.2 3-D spin-up simulation to generate background proles
3.4.3.3 LES simulations
3.4.3.4 SCM simulations
3.5 Results and analysis
3.5.1 Demonstration of the need of specic heat source to generate deep plumes
3.5.2 Inuence of entrainment/detrainment at the base of the updraft
3.6 Supporting analysis to the research article
3.7 Conclusions
3.8 Appendix
3.8.1 Volcanic mass and energy sources in the LES expressed as surfaceuxes
3.8.1.1 Massuxes (H2O and SO2)
3.8.1.2 Sensible heatux
4 First 3D application of Modied EDMF parameterisation
4.1 Strategy
4.2 Conguration of the 3D simulations
4.3 Results and analysis
4.3.1 Volcanic plume representation
4.3.2 SO2 measurements by ORA
4.3.3 Volcanic plume transport
4.4 Downwind chemistry
4.5 Conclusion
General conclusions and perspectives
