This review paper contrasts volcanic ash and mineral dust regarding their chemical and physical properties, sources, atmospheric load, deposition processes, atmospheric processing, and environmental and climate effects. Although there are substantial differences in the history of mineral dust and volcanic ash particles before they are released into the atmosphere, a number of similarities exist in atmospheric processing at ambient temperatures and environmental and climate impacts. By providing an overview on the differences and similarities between volcanic ash and mineral dust processes and effects, this review paper aims to appeal for future joint research strategies to extend our current knowledge through close cooperation between mineral dust and volcanic ash researchers.
Volcanic ash represents a major product of volcanic eruptions [
The global mineral dust cycle and its interactions with the Earth’s climate system have been studied widely [
This review contrasts the environmental and climatic effects of volcanic ash versus those of mineral dust. A stronger focus is put on the description of volcanic ash, whereas mineral dust effects are described in less detail, but with referencing the extensive literature. Similarities and differences will be emphasised (Figure which physical-chemical processes during long-range transport in the atmosphere affect the surface chemical composition of volcanic ash and mineral dust? how important is resuspension of volcanic ash from deposits on land for posteruptive climate and environmental effects? what are the reasons for the huge variability of nutrient and toxic element fluxes from volcanic ashes and mineral dust to the surface ocean? how important is volcanic iron fertilisation of the surface ocean and the associated modifications of atmospheric CO2 in comparison to that induced by mineral dust? how relevant is the role of volcanic ash and/or mineral dust for the Earth’s climate?
Schematic diagram showing the important processes controlling environmental and climate effects of volcanic ash (in grey) and mineral dust (in yellow). (CCN: cloud condensation nuclei; IN: ice nuclei).
Section
According to the “Glossary of Atmospheric Chemistry Terms” [
The main chemical elements contained in mineral dust, as well as volcanic ash, are silicium and oxygen, which constitute the main components of minerals and rocks in the Earth’s crust and mantle.
The chemical composition of the bulk volcanic ash is mainly determined by the magma from which it is generated. Generally, three types of magma are distinguished from each other (Table
Major types of magma.
Magma type | SiO2 (wt%) |
|
Viscosity and gas content |
---|---|---|---|
Basaltic | 45–55 | 1000–1200 | Low |
Andesitic | 55–65 | 800–1000 | Intermediate |
Rhyolitic | 65–75 | 650–1000 | High |
A major difference in the mineral composition of mineral dust and volcanic ash results from chemical weathering of mineral dust, generally on geological time scales. Mineral composition changes under the influence of water, oxygen, and acids. For example, feldspar will weather to clay and iron-bearing minerals can form hematite and goethite. Therefore primary iron containing minerals (e.g., amphiboles and pyroxenes) of volcanic rocks are not identified in mineral dust [
Close to the source regions, the size distribution of mineral dust particles varies from about 0.1 to over 100
The size distribution of volcanic ash is greatly dependent on the formation process, which is either an explosive volcanic eruption, a phreatomagmatic eruption, or a pyroclastic density current. In addition, secondary volcanic ash clouds result from the resuspension from volcanic ash depositions on land (see Section
Phreatomagmatic eruptions [
Pyroclastic flows occur when the eruption column or lava dome collapses leading to gas and tephra flows rushing down the flanks of a volcano at high speed, which thereby also contribute to the fragmentation process through milling by the collisional processes [
As eruption conditions may be highly variable in time, all fragmentation processes can take place simultaneously. According to [
The morphology of mineral dust particles can be assumed to be spherical with a widely used particle density of 2650 kg/m3 in mineral dust modelling, for example, [
The notation “emission” describes the release of material from outside the atmosphere into the atmosphere, where the location outside the atmosphere represents a source for the atmosphere. Mineral dust source areas are generally located in semiarid or arid areas where the surface is sparsely vegetated and dry. Here, fine grained material can accumulate and be mobilised into the atmosphere by wind. Numerical models for mineral dust mobilisation usually define dust emission areas based on, for example, soil moisture [
Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions, or pyroclastic density currents (see Section
Tephra mass release in DRE (Dense Rock Equivalent) of well-known volcanic eruptions since 1900 given for VEI values from 4 to 6 in three mass ranges: 0.1–1 km3, 1–10 km3, and 10–100 km3 ([
0.1 km3 | 1 km3 | 10 km3 | 100 km3 | |||
---|---|---|---|---|---|---|
VEI = 4 | VEI = 5 | VEI = 6 | ||||
Nabro 2011 and Puyehue-Cordón Caulle 2011 | ||||||
Grimsvötn 2011 and Merapi 2010 | ||||||
Eyjafjallajökull 2010 and Sarychev Peak 2010 | ||||||
Kasatochi 2008 and Chaiten 2008 | ||||||
Reventator 2002 and Ulawun 2000 | ||||||
Lascar 1993 and Mt. Spurr 1992 | ||||||
Kelud 1990 and Kiluchevkoi 1987 | Mt. Hudson 1991 | Pinatubo 1991 | ||||
Chikurachki 1986 and Mount Augustine 1986 | ||||||
Colo 1983 and Galunggung 1982 | El Chichon 1982 | |||||
Pagan 1981 and Alaid 1981 | ||||||
Mount Augustine 1976 and Tolbachik 1975 | Mt. St. Helens 1980 | |||||
Volcan de Fuego 1974 and Tiatia 1973 | ||||||
Fernandina 1968 and Mount Awu 1966 | ||||||
Kelud 1966 and Taal 1965 | ||||||
Shiveluch 1964 and Carran-Los Venados 1955 | Agung 1963 | |||||
Mount Spurr 1953 and Bagana 1952 | Bezymianny 1956 | |||||
Kelud 1951 and Mount Lamington 1951 | ||||||
Ambrym 1950 and Hekla 1947 | ||||||
Sarychev Paek 1946 and Avachinsky 1945 | ||||||
Paricutin 1943–1952 and Suoh 1933 | ||||||
Volcan De Fuego 1932 and Mont Aniakchak 1931 | Kharimkotan 1933 | |||||
Kliucheskoi 1931 and Komagatake 1931 | Cerro Azul 1932 | |||||
Komagatake 1929 and Avachinsky 1926 | ||||||
Raikoko 1924 and Manam 1919 | ||||||
Kelud 1919 and Agrhan 1917 | Katla 1918 | |||||
Tungurahua 1916 and Sakurajima 1914 | ||||||
Mount Lolobau 1911 and Grimsvötn 1903 | Colima 1913 | Katmai/Novarupta 1912 | ||||
Monut Pelee 1902 | Ksudach 1907 | St. Maria 1902 |
It should be considered as well that fresh volcanic ash may be remobilised into the atmosphere from ash deposits on land, similar to what is observed for mineral dust, particularly in arid and semiarid regions. This contradicts with the general assumption that volcanic ash environmental and climate effects are restricted only to the duration of a volcanic eruption with time scales of days to weeks. Reference [
Volcanic ash resuspension event as seen from satellite: (a) September 21, 2003: Katmai/Novarupta, Alaska; (b) November 27, 1991: Cerro Hudson, Chile; (c) May 27, 2010: Eyafjallajökull, Iceland; (d) approximate location of (a)–(c). Courtesy of NASA.
Atmospheric concentrations of mineral dust and volcanic ash are subject to considerable temporal and spatial variability. Seasonal variability, for example, rainy and dry seasons, determines to a great extent the mineral dust load in the atmosphere, whereas volcanic ash atmospheric load is mainly dependent on the occurrences of sporadic and usually unpredictable volcanic eruptions.
Measurements in the Sahelian belt of West Africa [
During the eruption of Eyjafjallajökull on Iceland in 2010, maximum ash concentrations up to 4000
Mineral dust and volcanic ash are removed from the atmosphere by gravitational settling, turbulent dry deposition, and wet scavenging by rain called wet deposition [
Reference [
Gravitational settling of volcanic ash has been observed to exceed the terminal settling velocity of single ash particles [
Before atmospheric processing occurs at ambient temperatures, volcanic ash undergoes extreme temperature gradients (from about 1000°C to less than 0°C) in extreme short periods of time (few minutes) in the volcanic eruption plume [
Leaching experiments with pristine volcanic ash in water have been performed for decades [
After a volcanic plume reaches neutral buoyancy conditions, the volcanic cloud spreads out more horizontally (Figure
SO2 emissions from major volcanic eruptions observed from satellite. Arc eruptions with SO2 emissions exceeding 1000 kt: Mt. St. Helens 1980 (slightly less), Alaid 1981, El Chichon 1982, Mt. Pinatubo 1991, Mt. Hudson 1991, Raboul 1994, and Kasatochi 2008 (Courtesy of NASA).
In contrast, volcanic ash, which is remobilised from ash deposits, can be assumed to undergo very similar atmospheric processing as mineral dust. The importance of photochemistry for mineral dust under atmospheric conditions is highlighted in several studies [
At ambient temperature, when clouds are present in the atmosphere, cloud processing is assumed to provide the main mechanism for the uptake of acid gases in the atmosphere by aerosols in general [
Atmospheric processing of mineral dust and volcanic ash particles at freezing temperatures is a process, which has been rarely studied. However, indications for increased iron solubility are presented [
Atmospheric and volcanic processing with modifications of the surface chemical composition of mineral dust and volcanic ash particles has implications on their behaviour to act as cloud condensation or ice nuclei (CCN or IN; see Section
For the sake of completeness, mechanical and biogeochemical weathering that takes place at the Earth’s surface under atmospheric conditions is mentioned here as well. These processes are important in the generation of mineral dust and decomposition of volcanic ash on geological time scales [
Aeolian dust episodes represent a major health concern for humans due to elevated atmospheric concentrations (see Section
Strong mineral dust storms mainly affect take-off and landing of aircraft due to poor visibility. However, aircraft can operate in environments with high mineral dust concentration without any engine problems. Mineral dust which typically melts at temperatures of around 1700°C does not melt when it is ingested in jet engines. However, jet engines may have problems with volcanic ash as the melting temperatures are at or below the operating temperatures of high-performance jet engines, which are around 1400°C [
Heavy volcanic ash or mineral dust deposition completely buries vegetation and soil. Plant survival is dependent on deposit thickness, chemistry, compaction, rainfall, and duration of burial. Slight deposition of volcanic ash and mineral dust can affect vegetation and soil positively and negatively. Although thin volcanic ash fall inhibits transpiration and photosynthesis and alters growth, buried plants may survive [
Short-term effects during mineral dust storms, volcanic eruptions, and volcanic ash resuspension events considerably reduce visibility and solar irradiation reaching the Earth’s surface, whereas long-term effects occur in a more diluted environment. These effects can be measured by an increase in atmospheric optical depth, represented by enhanced absorption and/or scattering of solar and thermal radiation and modifications in surface temperature. During the first two days following the 1980 eruption of Mount St. Helens, [
Direct radiative effects of mineral dust have been studied widely [
Due to the relatively short residence time of mineral dust and volcanic ash in the atmosphere (in the order of a few days), their direct radiative effects (and indirect radiative effects; see Section
With about 60% of the Earth being cloud covered, clouds represent an important factor in regulating the Earth’s radiation budget [
In general, a greater quantity of cloud droplets are formed with typically smaller size if more aerosols are available to act as cloud condensation nuclei (CCN). High cloud droplet number concentrations (CDNC) reduce the diffusional droplet growth. Therefore droplet sizes cannot be reached which are large enough for an efficient growth by droplet collision. Thus, changes in CDNC can influence cloud albedo (first indirect aerosol effect) [
Super-cooled clouds are abundant in the atmosphere, which contain metastable water that freezes as soon as suitable ice nuclei are available. In the presence of particulate material, such as mineral dust, volcanic ash, or pollen [
In the past 20 years, iron-enrichment experiments ranging from bottle incubations to open-ocean amendment studies in regions of 50–100 km2 have demonstrated that iron supply stimulates phytoplankton growth in High-Nutrient-Low-Chlorophyll (HNLC) waters [
Ocean fertilisation by mineral dust has been studied extensively, as mineral dust has long been assumed to be the main component of atmospheric deposition of minerals into the open ocean [
Volcanic ash deposition into the ocean represents another external and largely neglected source of iron. However, its significance and impact on climate has long been considered negligible. The major climate forcing effect following volcanic eruptions is widely assumed to occur due to the reduction of solar radiation through volcanic sulfate aerosols [
The first direct evidence for iron fertilisation in an HNLC ocean area by volcanic ash emerged after the eruption of the Kasatochi volcano, situated on the Aleutian Islands in August 2008. Atmospheric and oceanic conditions in the NE Pacific were ideal for generating a massive and large-scale phytoplankton bloom, which was observed by satellite instruments [
After the eruption of Kasatochi in 2008 on the Aleutian Islands, atmospheric CO2 decreased slightly by ~0.01 Pg C as diatoms and mesozooplankton increased export of organic carbon from the surface to the deeper ocean [
Reduced atmospheric CO2 concentrations were observed in the years following the 1991 Pinatubo eruption [
Another interesting event is the eruption of Huaynaputina in Peru in 1600, which produced more than 9.6 km3 of volcanic ash [
Ocean iron fertilisation may also affect the climate relevant exchange of trace gases between the ocean and the atmosphere. An increase of the MPP is accompanied by an increased contribution of organic carbon (OC) to submicron marine aerosols [
Although there are substantial differences in the history of mineral dust and volcanic ash particles before they are released into the atmosphere (see Sections
Model parameterisations of volcanic ash remobilisation from its deposits on land build on mineral dust mobilisation schemes [
The extreme conditions for multiphase chemistry in volcanic plumes (see Section
During a volcanic eruption, ash particles are easily injected into atmospheric regimes where freezing temperatures prevail, and therefore a better understanding of the processes affected by freezing temperatures, like IN formations or Fe mobilisation and their climate impacts [
For paleoclimate research, the results from terrestrial and marine environmental archives, namely, ice, peat, sea, and ocean sediment cores for mineral dust [
The financial support through the Cluster of Excellence “CliSAP” (EXC177), University of Hamburg, funded through the German Science Foundation (DFG) is gratefully acknowledged. The author thanks Michael Hemming, Gholamali Hoshyaripour, and Matthias Hort for their comments on the paper.