Surface area, porosity and water adsorption properties of fine volcanic ash particles

Abstract

Our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary, despite the importance of these reactions in influencing the chemistry and dynamics of a plume. In this study, six samples of fine ash (<100 mm) from different volcanoes were measured for their specific surface area, a s , porosity and water adsorption properties with the aim to provide insights into the capacity of silicate ash particles to react with gases, including water vapour. To do so, we performed high-resolution nitrogen and water vapour adsorption/desorption experiments at 77 K and 303 K, respectively. The nitrogen data indicated a s values in the range 1.1-2.1 m 2 /g, except in one case where a a s of 10 m 2 /g was measured. This high value is attributed to incorporation of hydrothermal phases, such as clay minerals, in the ash surface composition. The data also revealed that the ash samples are essentially nonporous, or have a porosity dominated by macropores with widths >500 . All the specimens had similar pore size distributions, with a small peak centered around 50 . These findings suggest that fine ash particles have relatively undifferentiated surface textures, irrespective of the chemical composition and eruption type. Adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption a s values. Enhanced reactivity of the ash surface towards water may result from (i) hydration of bulk ash constituents; (ii) hydration of surface compounds; and/or (iii) hydroxylation of the surface of the ash. The later mechanism may lead to irreversible retention of water. Based on these experiments, we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions. In addition, capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume. The total mass of water vapour retained by 1 g of fine ash at 0.95 relative water vapour pressure is calculated to be~10 2 g. Some volcanic implications of this study are discussed.

Ash iron mobilization through physicochemical processing in volcanic eruption plumes: a numerical modeling approach

“…According to this plot, we estimate the total number concentration near the vent to be approximately 10 12 particles per cubic centimeter having a total surface area of ash of 45 cm 2 cm −3 . According to previous studies, the specific surface area of fine volcanic ash is in the range 0.2-2.1 m 2 g −1 (Delmelle et al, 2005;Mills and Rose, 2010). We find 0.9 m 2 g −1 as the specific surface area of the fine ash in this study, which is well within the range mentioned above.…”

“…Fine ash is thought to represent a substantial contribution (50-97 wt %) to tephra deposits from plinian and subplinian volcanic eruptions (Rose and Durant, 2009). Particles in this size range not only have a higher surface to mass ratio (compared to the coarser particles) for interaction with the gases and aqueous phases (Delmelle et al, 2005) but can also be lifted to high altitudes and remain suspended in the atmosphere for several days before sedimentation (Sparks et al, 1997). Among others, Rose and Durant (2009) investigated the ash content of volcanic eruption plumes and suggested a typical polymodal size distribution for fine ash subdivided into 27 bins (Fig.…”

Retrieval of the Eyjafjallajökull volcanic aerosol optical and microphysical properties from POLDER/PARASOL measurements

“…The low real refractive index of 1.42 could be also a result of an average between the refractive indexes of various particles, e.g., pure volcanic and sulfate aerosols or their mixtures. This might be also due to presence of liquid water coating the dust particles as a result of water vapor condensation near the emission point (Delmelle et al, 2005;Lathem et al, 2011). This would be consistent with the presence of spherical particles near the emission point (FS ∼ 25 %), which would tend to disappear as the plume is transported and liquid water evaporates or turns into ice crystals.…”

A review of volcanic ash aggregation

“…Whilst the effect this may have on morphology is poorly constrained, the effects on binding mechanisms can be more readily investigated. Volcanic ash plumes contain abundant aerosol and gas phases which can be scavenged by silicate ash particles and can cause rapid surface acid dissolution (Rose, 1977;Varekamp et al, 1984;Oskarsson, 1980;Delmelle et al, 2005Delmelle et al, , 2007. This can alter the particle surface chemistry and result in the precipitation of sulphate and halide salts at the ash-liquid interface (Delmelle et al, 2007).…”