- Volcanogenic products from the eruption: Volcanic gasses,
derived from reactions of volcanogenic and atmospheric materials.
- Products from the ambient atmosphere, such as H20 and gaseous species and various particles from the land and sea including wind blown silicates, sea salt and others. The volcanogenic components make the clouds distinctive, and they can be tracked by satellite sensors for periods that range from minutes to weeks (Bluth et al, 1997; Schneider et al, 1995). During this time the volcanogenic particles mix and interact with meteorological and hydrospheric particles.
clouds consist of fine pyroclasts
, salts and acids in aerosol
The particles consist of two main types: (Reference to: http://www.geo.mtu.edu/~raman/GE4170.volcanicclouds.html)
- Silicate pyroclasts representing
fragments of the magma. These are glassy
pyroclasts and minerals, which represent the crystalline fraction of the magma. Their
shape is angular, and basaltic and andesitic eruptions give rise to particles that have
moderate aspect ratios (Riley et al, 1999), while rhyolitic eruptions can generate an
abundance of glassy pyroclasts with a platy geometry and extreme aspect ratios (Rose and
Chesner, 1987). The diameters of silicate pyroclasts generated during explosive
eruptions range from meters to microns. Those in volcanic clouds are smaller, generally
less than about 50 μm. The mass proportions of silicate particles with diameters
less than about 1 μm are very small (Rose et al 1980).
- Non-silicate particles which are
related to reactions among the constituents of
the volcanic gases. These particles are generally smaller than the
less than 1 micron in diameter. The commonest composition for these is sulfate, especially
H2SO4, which forms as submicron spherical droplets which also contain H20 (typically about
25% by volume--Zhao et al, 1995). A total of at least 28 different phases have been
observed also including native
sulfur, sulfates, haloids, metallic oxides, and such exotic species as silver sulfide
and even native gold (Meeker et al, 1985). Overall the analogy between the observed
phases and fumarolic incrustations and sublimates at gas vents (Stoiber and Rose,
1974; Bernard, 1985; Symonds et al, 1987) suggests that these phases originate from
reactions among the volcanic gases, sometimes involving the atmosphere and volcanic
Besides these two broad types, a wide variety of other, unexplained materials have been observed in volcanic clouds. They consist largely of phases that are amorphous and have uncertain compositions (Chuan et al, 1987). Many or most of these particles are likely to be non-volcanic in origin, and represent accidental material of surficial or extraterrestrial origin.
of volcanic gases:
- The most abundant gas released into the atmosphere from volcanic systems is water vapor (H20), carbon dioxide (CO2) and sulfur dioxide (SO2). Volcanoes also release smaller amounts of others gases, including hydrogen sulfide (H2S), hydrogen (H2), carbon monoxide (CO), hydrogen chloride (HCL), hydrogen fluoride (HF), and helium (He).
- Examples of volcanic gas compositions, in volume percent concentrations
(from Symonds et. al., 1994)
Potential effects of volcanic gases
- The volcanic gases that pose the greatest potential hazard to
people, animals, agriculture, and property are
fluoride. Locally, sulfur dioxide gas
can lead to acid rain and air pollution downwind from a volcano.
Globally, large explosive eruptions that inject a tremendous
volume of sulfur aerosols into the stratosphere can lead
to lower surface temperatures and promote depletion of the
Earth's ozone layer. Because carbon dioxide gas is heavier
than air, the gas may flow into in low-lying areas and collect
in the soil. The concentration of carbon dioxide gas in these
areas can be lethal to people, animals, and vegetation. A few
historic eruptions have released sufficient fluorine-compounds
to deform or kill animals that grazed on vegetation coated with
volcanic ash; fluorine compounds tend to become concentrated
on fine-grained ash particles, which can be ingested by animals.
Adsorption models for SO2-volcanic ash.
- Adsorption is a combination of physical and chemical processes in which a substance accumulates on a solid surface (cf. Mihelcic et al., 1997). When gas molecules are present near a solid surface, a vast number of collisions occur each second. Molecules striking the surface will either rebound or adsorb to the surface. As molecules strike repeatedly upon the solid surface and linger for a time, a higher concentration of gas molecules develops at the surface than in the bulk gas phase. This phenomenon is called adsorption (Boer, 1953).
- From a thermodynamic viewpoint, molecules always prefer to be in a lower energy state (cf. Mihelcic et al., 1997). An adsorbed molecule has traded a higher energy state (that of a gas molecule) for a lower energy state (that of an adsorbed molecule). The difference in the two energy states is called the heat of adsorption (Q) and this is the quantity of energy liberated when the molecule is adsorbed (Boer, 1953). Attraction of a molecule to a surface can be caused by physical and chemical forces. Electrostatic forces are the basic physical forces in most adsorption reactions. These forces include dipole-dipole interactions, dispersion interactions or London-van der Waals force, and hydrogen bonding (Mihelcic et al., 1997). In contrast to physical adsorption, chemisorption involves the formation of strong chemical bonds between adsorbate molecules and specific surface locations known as chemically active site (Hoffmann, 1988).
- Physical adsorption is characterized by relatively small heats of adsorption-about 5 kcal per mole or less. All gases exhibit physical adsorption but some also exhibit chemisorption, which involves forces and energies about 10 to 100 kcal per mole. Although physical adsorption occurs in all cases, chemisorption is observed only when some kind of chemical interaction is possible (Boer, 1953).
- When a surface is initially exposed to a gas, the average number of molecules on the surface increases with time. After some time, the rate at which molecules are leaving the surface (desorption) will equal the rate at which molecules are being adsorbed and the average number of molecules per unit area of surface per interval of time will become constant. This condition is called dynamic equilibrium (Boer, 1953).
- Adsorption models
- There are many types of adsorption models. The classical adsorption models for isothermal conditions are the Langmuir, Fruendlich, and Brunauer, Emmett and Teller (BET).
The earliest model of gas adsorption wassuggested by Langmuir (1916). The classical Langmuir model is limited to monolayer adsorption. It is assumed that gas molecules striking the surface have a given probability of adsorbing. Molecules already adsorbed similarly have a given probability of desorbing. At equilibrium, equal numbers of molecules desorb and adsorb at any time. The probabilities are related to the strength of the interaction between the adsorbent surface and the adsorbate gas (cf. Swinkels, 1999).
The Langmuir model is usually expressed as:
V= volume of gas adsorbed at pressure P;
Vm = volume of gas which could cover the entire adsorbing surface with a monomolecular layer;
P0 = saturation pressure of the gas, i.e., the pressure of the gas in an equilibrium with bulk liquid at the temperature of the measurement;
x = P/P0 = relative pressure (0£
C = a constant for the gas/solid combination.
The Langmuir model is applicable when there is a strong specific
interaction between the surface and the adsorbate so that a single adsorbed layer forms and no multi-layer adsorption occurs (cf. Swinkels, 1999).
The Fruendlich isotherm model is valid for heterogeneous surfaces, monolayer coverage (cf. Hand, 1997). The Fruendlich isotherm equation can be expressed as (Valsaraj et al., 1992):
a and n are experimentally determined parameters.
When 1/n =1, the reaction is linear and called "partitioning". When 1/n<1, the reaction is said to be "favorable" as the incremental change in amount sorbed decreases with increasing concentrations. While 1/n>1 is called "unfavorable" because the reverse is true. Most natural adsorbents exhibit either linear or favorable adsorption. The Langmuir and the Fruendlich model for 1/n<1 are concave downwards, so both models can be calibrated to similar data..
Brunauer, Emmett and Teller (BET) developed several models
gas adsorption on solids which have become the effective standard for surface area measurements. The models are valid for multiple layers on homogeneous surfaces (cf, Hand, 1997).
The assumptions underlying the simplest BET isotherm are:
- Gas adsorbs on a flat, uniform surface of the solid with a uniform heat of adsorption due to van der Waals forces between the gas and the solid.
- There is no lateral interaction between the adsorbed molecules.
- After the surface has become partially covered by adsorbed gas molecules, additional gas can adsorb either on the remaining free surface or on top of the already adsorbed layer. The adsorption of the second and subsequent layers occurs with a heat of adsorption equal to the heat of liquefaction of the gas (Swinkels, 1999).
The single-solute BET isotherm is:
Where: C is analogous to the C parameter for Langmuir. The BET simplifies to the Langmuir when relative pressure x < 0.01 and C>100 (Valsaraj et al., 1992).
For dry ash, the single component BET isotherm may describe the adsorption of SO2. For wet ash, a two-component BET isotherm may be more appropriate. The two-component BET isotherm expression is given by (Valsaraj et al., 1992):
xA=PA/P0,A & xB=PB/P0,B are the relative partial pressures of solute A & B, respectively.
CA= BET isotherm constant for solute A (e. g., SO2).
CB= BET isotherm constant for solute B (e. g., water vapor).
4. A Laboratory study of SO2 Adsorption onto the Volcanic ash.
Nucleation plays a fundamental role whenever condensation, precipitation, crystallization, sublimation, boiling, or freezing occur.
Homogeneous nucleation is the nucleation of vapor on embryos comprised of vapor molecules only, in the absenece of foreign substance.
Heterogeneous nucleation is the nucleation on a foreign substance or surface, such as an ion or a solid particle.
2. Ice Nucleation
First and foremost, cloud condensation nuclei (CCN
) are particles suspended in the air which support the growth of
cloud droplets or ice on their surface. Of all of the CCN floating in the air, a low percentage act as ice forming nuclei
(IN) that have the ability to act as a surface for ice growth to initiate (from water in the vapor or liquid phase). Lets
take a simple case of a solo cloud in the sky with no others around it. Also, we assume the cloud has a temperature of
<0C and is composed of all supercooled liquid drops and water vapor (NO ICE). The only way to have ice form is by
growing it on an IN particle's surface. [Again this assumes no ice comes from anywhere outside the cloud - which can
happen (known as the seeder-feeder mechanism)]. Once the ice growth begins on the nuclei, the IN particle is said to
Since not all CCN particles are IN, or said another way - not all CCNs promote the growth of ice themselves - there
must be something special about them. This "something special" is their chemical makeup. Water changing phase to
grow as ice in a cloud is very particular as to the chemical composition of the particle on which it would like to grow
It also depends on the relative humidity and temperature of the cloud. CCN particles have a better chance of being an
IN as the temperature decreases and the relative humidity increases. In fact, no IN's can be activated (or have ice begin
to grow on them) above the temperature of -4C even if the cloud is supersaturated (relative humidity > 100%).
3. Possible Influence of Cloud Condensation Nuclei (CCN) on Climate
The possible influence of CCN on cloud droplet size distributions and consequent radiation transfer remains a very active area of cloud physics research as the role of clouds in the radiative
balance of the earth becomes increasingly recognized. The recent books entitled ``Aerosol-Cloud-Climate Interactions'' and ``Aerosol Effects on Climate'' edited by Hobbs  and Jennings
 respectively, provide a good summary of present day knowledge of the possible influence of CCN and other aerosol particles on clouds and climate. The recent review by Hudson 
provides the current state of the art on Cloud Condensation Nuclei, and shows that CCN knowledge is still inadequate for understanding global climate change, and suggests that the knowledge
base for CCN be significantly expanded.
4. Aggregation and fallout
How Ice-Crystals Grow In a Supercooled Liquid Cloud
- Growth by deposition - a big one! [also called diffusion deposition]
Growth by deposition, physically, is the change from water in the vapor form to water in the solid form (or water vapor to ice). This
process is governed by the Bergeron-Findeisen Process which states that ice crystals will grow at the expense of liquid droplets in an
environment where the relative humidity is 100% (or the environment is saturated with respect to (wrt) water). The saturation vapor
pressure over ice is less than that of water and therefore the vapor will want to move toward the ice or ice nuclei versus a liquid drop in
the same environment. Deposition occurs by water vapor depositing on the ice in a liquid form and immediately freezing, or directly
depositing as a solid. Once this water vapor changes to a liquid/solid, the relative humidity of the surrounding air falls slightly below
100%, and more water drops can evaporate. This is the common process of the ice crystal growing at the expense of the water droplets.
Growth by Accretion
Commonly, this is the growth of an ice particle accomplished when it overtakes or captures supercooled liquid droplets. It follows that it
should occur more readily after the ice-phase particle has grown to a sufficient size to begin to fall and collect the supercooled droplets.
Thus, initially the ice grows via the diffusion method at the top or mid level of the cloud and later by this process.
The collection of liquid supercooled droplets is best for ice particles which fall the most rapid. These are graupel (which are really a
collection of frozen drops), needles of snow, and finely dendritic or powder snows, in order of decreasing fall speeds.
While deposition dominates ice formation and growth in the upper to middle portion of the cloud, accretion dominates the lower portion
of the cloud. This is the main growth mechanism for ice crystals - see riming below.
Growth by Aggregation
Aggregation is the coming together of multiple ice particles to form one main snowflake. Although not a great deal is known about this
behavior due to many "unknown variables", the process is maximized when temperatures are warmer than -10C. This allows for
effective sticking and refreezing of ice crystals. In figure 14-19 below (from Pruppacher and Klett, 1978), you can see that the largest
snowflakes occur when temperatures are near 0C (red box). Thus, cloud layers which have extended regions where the wet-bulb
temperature is near 0C near the surface will produce larger flakes. Remember this for accumulation amounts.
4. Accumulation of particles in fallout
The complete description of all microphysical processes in a volcanic eruption plume is rather complex. Silicate particles as well as volcanic gasses should be included: Water vapor can condense or can deposit on ash surfaces. Silicate particles can interact with hydrometeors and form clusters with increased fall speed compared with that of the constituents. Volcanic gasses like HCL can be dissolved into liquid droplets and thereby substantially lower the freezing temperature.
A typical gas-particle-mixture erupted at the volcanic vent is charactered by gas fraction of 3-5 wt. % and temperature between 1000 and 1400 K. The exit velocity is determined by the speed of sound of the mixture and ranges between 250-300 m/s (Woods, 1995). The ejecta experience a shock-like temperature decrease leading to crystallisation of microscopic salt particles 9primarily chlorides , fluorides, and sulphates of alkali metals and calcium). The salt crystals form either homogeneously or on surfaces of preexisting ash particles.
Temperature decrease in the rising plume allows for condensation of water vapor; the amount of the water determines the microphysical processes. From the numerical simulations the amount of entrained water in the plume is about 3 times larger than the environmental conditions. (Herzog, 1998; Graf et al . 1999). Latent heat release from the condensation of water vapor is important for the plume dynamics during the eruption (Herzog et al., 1999).
The interaction between hydrometers and volcanic ash leads to coagulation of moist particles. Larger aggregates exhibit an increased fall speed and influencing the height and the shape of the plume. Hydrometers are able to remove volcanic volatiles from the gas phase. Scavenging processes are most important to prevent volcanic volatiles and ash from being injected into the stratosphere.
Particle Size Distribution.
1. Gamma Distribution
2. Normal Distribution
3. Log Normal Distribution
Two ways of characterizing lognormal distributions, in terms of the original data (a) and after log-transformation (b).
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