Excerpted from Rose et al, 2000, Philos Trans Royal Society London, 358: 1433-1729

Clouds are suspensions of particles in the atmosphere. Meteorological clouds contain particles that are mainly liquid or solid H20 (hydrometeors) which are smaller than about 100 microns in diameter and which fall through the atmosphere in a laminar regime at velocities of less than about 0.1 m/sec. Larger meteorological particles fall much faster, in the turbulent regime, and are called precipitation (Rogers and Yau, 1989; Houze, 1993). Because of the slow fall speeds of their particles, clouds can persist in the atmosphere for periods of hours to weeks or longer, although they often are dynamic.

Volcanic clouds are much rarer features than meteorological clouds. They are initiated by explosive eruptions which release volcanic gases and hot silicate fragments called pyroclasts and form vertical buoyant columns or plumes which rise to heights of up to 50 km as heat is transferred from the hot pyroclasts to entrained air from the surrounding atmosphere (Sparks et al, 1997; Gilbert and Sparks, 1998). Large amounts of lower tropospheric air are entrained in these plumes (Woods, 1993; Glaze and Baloga, 1996; Glaze et al, 1997), and this air typically contains water vapor which saturates the rising air as it rises and condenses, forming hydrometeors. Large pyroclasts fall out of the eruption column margins quickly (Ernst et al, 1996)--lapilli (pyroclasts >2mm in diameter) fall back to earth within less than about 30 minutes (Walker et al, 1971; Wilson and Huang, 1979; Lane et al, 1993). Ash (pyroclasts < 2mm in diameter) particles fall out more slowly and fine ash (<50 microns in diameter) falls out in the laminar flow regime (Rose, 1993; Bonadonna et al, 1998) at slow velocities like the particles in meteorological clouds. So, like meteorological clouds, volcanic clouds can persist in the atmosphere for days to weeks or longer. Typically they detach from plumes before or after the eruption stops and drift in response to the 3 dimensional wind patterns (Servranckx, et al, 1996).

What volcanic clouds are made of.

Volcanic clouds contain a variety of components including

1. volcanogenic products from the eruption: volcanic gases, pyroclasts, and aerosol particles derived from reactions of volcanogenic and atmospheric materials; and

2. 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.

Volcanogenic particles in volcanic clouds consist of fine pyroclasts, salts and acids in aerosol form. Direct sampling of volcanogenic particles has been accomplished by balloon studies (eg Rietmeijer, 1993) and a variety of research aircraft. Volcanogenic particles in volcanic clouds have been examined in a number of studies (Table 1) where a research aircraft with a particle collection system was flown through the clouds. Because of safety, only plumes and relatively small volcanic clouds have been directly sampled in this way. The particles consist of two main types:

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 m. The mass proportions of silicate particles with diameters less than than about1 m 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 silicates, usually 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 (see Table 3 in Rose et al, 1982 for a partial list) 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 silicates.

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.

Direct sampling and analysis of gases in volcanic clouds has been done only rarely (eg Cadle et al, 1979) although analysis of CO2 has been done much more extensively during CO2 flux determination surveys (Harris et al, 1981; Gerlach et al 1999). Other information about gases has been collected from extensive airborne remote sensing of volcanic plumes using the correlation spectrometer (COSPEC) instrument. These results show that the volcanic gases in volcanic clouds are mixed and highly diluted by the ambient atmosphere, and the concentrations of volcanic SO2 and CO2 are less than a few ppmv (McGee, 1992).

Table 1: Direct sampling studies of small particles in volcanic clouds.

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