The recent eruption of Iceland Eyjafjallajokull volcano and the massive shutdown of air traffic in Europe have attracted the attention of travellers and worldwide news agencies for weeks. The volcano initially erupted in March. On April 14, the volcano emitted a huge cloud of ash into the atmosphere. This generated a certain concern. A huge area of the European airspace was shut down and air travel was interrupted for several days. The officials were concerned because there was a considerable amount of atmospheric volcanic ash that could damage the aircrafts. The fact that volcanic ash can damage aircraft engines is not in dispute.
There are several examples of engine failures due to volcanic ash clouds. However, the damage of the aircrafts is only one of the problems created by the introduction of volcanic ash into the atmosphere. Ash can also have negative effects on human health, agriculture, and ecology. The occurrence of these negative effects depends a lot on the physical characteristics of the ash that include particle size, shape, surface area, porosity, density, and hardness.
The Impact of Volcanic Ash
SiO2-rich glass shard particles, formed when rocks are pulverized by an eruption, contain a key contaminant in a volcanic ash plume discharge. Silica is very hard. The combination of the increased hardness and irregular shape of these particles makes them highly abrasive. Engine teardowns after ash exposure clearly show the damage caused by these particles.
The effects of volcanic ash are listed below:
- Volcanic ash may clog air filters of turbine engines, block cooling air passages and erode the gas path components and protective paint on casings.
- Silicate ash entering the engine at a significantly high relative speed can melt in the hot section of the engine and then re -solidify on the high pressure turbine blades and guide vanes, almost choking the turbine airflow and leading to surging and to an in-flight shutdown.
- Falling ash particles far away from an eruption can pose a large number of hazards to the environment, human and animal health, agriculture, appliances, communications, power generating facilities and water supply systems.
- Fine ash particles that are smaller than 10 μm in size can enter the pulmonary section of the lungs and cause severe respiratory problems.
- In an eruption, fluorine aerosols can become attached to fine ash particles. Due to their increased surface area, fine particles can transport significant amounts of soluble fluorine onto pastures far downwind from an erupting volcano.
- A thin layer of fine ash only 1 millimeter thick can contain potentially toxic amounts of fluorine. Fluorosis can kill livestock who graze on these ash-contaminated pastures.
- Fruit and vegetables ready for harvesting and covered with ashfall can be hard to clean, resulting in the destruction of the crop. The abrasive and corrosive nature of ash can damage machinery, appliances, computers, electrical and mechanical systems.
- Considerable quantities of electrically charged ash can contribute to the interference of radio waves and render radio and telephone systems inoperative.
Composition of Fragmented Volcanic Material
The general term for fragmented volcanic material is “tephra.” There are three classifications based on grain size. The three types include the following:
Bombs and blocks comprise the largest classification, measuring more than 64 mm. ‘Lapilli’ makes up the middle range of particles, measuring from 2 to 64 mm. Volcanic ash is a superior grade of ejected solid debris containing particles ranging from 2 mm down to micrometer-scale. Particles that are comparatively close to the eruption plume have a larger particle size range and higher apparent density then that in downwind ash clouds.
Volcanic ash particles have a relatively low apparent density for rock materials due to a large number of pores and voids, even for the finest particles. These particles can rise to the higher levels of the plume at the site of the eruption.
Atmospheric Dispersal of Airborne Volcanic Ash
Remaining in suspension at prevailing ambient air densities, upper winds transport these particles to eventual dispersal in an ash cloud. The atmospheric dispersal of airborne volcanic ash depends on the magnitude and the type of eruption, wind direction and the size and apparent density of the ash particles.
It is normally accepted that ash clouds far downwind from the eruption contain fine ash particles with a particle size distribution of less than 200 micrometers that have a much smaller settling rate.
Study of Volcanic Ash
Many models for studying volcanic ash transport include a range of measurement parameters such as an accurate description of the shape and explosivity of the ash plume, thermodynamic cloud modelling to study cloud destabilization through water phase changes and wind field data. However, with most of these models, size, shape, surface area, and porosity of the particles are all important factors determining how far fine volcanic ash will travel. Many studies have indicated a quick thinning and fining of collected particle subpopulations with distance from the volcano. Hence, particle size distribution measurements of pristine volcanic ash at a range of distances from an eruption offer a significant piece of information to determine ash transport in future events. Particle shape and porosity impact the velocity with which a particle will fall from the atmosphere.
BET surface area analyses indicate that irregular shape and porosity contribute to the high surface area of fine ash particles. This irregular shape and porosity forms a drag and therefore affects how far a particle can be transported by wind and how quickly it will fall out of the atmosphere. These characteristics can slow the terminal velocity of particles below 500 μm by several orders of magnitude.
Instruments offered by Micromeritics
Micromeritics offers a number of analytical instruments for determining physical characteristics of volcanic ash particles:
The SediGraph III determines the size of the particles, from 300 to 0.1 μm and the sedimentation velocity, based upon sedimentation through a known fluid. Complete particle accountability assures that the entire introduced sample is taken into account. The SediGraph III measures the particle mass by X-ray absorption and the particle size by sedimentation– no modelling is needed.
The Particle Insight determines up to 28 distinct shape parameters analyzed in real-time for a range of particle shape models, from spherical, through elliptical, to fiber and rod shaped, as well as multi-faceted polyhedral.
The Saturn DigiSizer II digital, high-definition laser light scattering particle size analyzer can offer a very high level of resolution and sensitivity for size measurements at the upper end of the “ash” range, for particles as large as 2500 μm.
Micromeritics TriStar II, Gemini VII, ASAP 2020 and ASAP 2420 can all be used to determine both BET surface areas and total pore volume distributions, from pores as small as 0.4 nm to as large as 400 nm.
Micromeritics AccuPyc II 1340 Gas Pycnometer enables skeletal volume and density measurements on materials having volumes from 0.01 to 350 cm3.
Micromeritics GeoPyc 1360 Pycnometer can determine the bulk volume and density of ash materials.
Micromeritics AutoPore IV mercury porosimeter can calculate apparent and skeletal densities as well as the porosity characteristics of the larger ash particles that include lapilli ash up to approximately 25 mm diameter.
It may be surprising to many, but, globally, there are over 530 active volcanoes. Extensive geologic regions that include most of the planet are at risk from volcanic ash. As noted previously, volcanic ash particles can be carried high into the atmosphere by an eruption and be transported for long distances by a variety of factors.
The damage and economic loss caused by volcanic plume eruptions are staggering. Precise prediction of the transport and fallout of this ash will help to limit the destructive and economic impact of an eruption event.
As a result, it is essential to understand and eventually predict with some accuracy the transport of ash particles from a volcanic plume eruption event. Physical characteristics of ash particles are some of the most significant factors targeted in ongoing research aiming to develop prediction models.
Micromeritics expertise and its innovative materials characterization instrumentation have already been, and will continue to be fundamental in providing important measurement tools required for establishing models for the transport and fallout of volcanic ash.
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
For more information on this source, please visit Micromeritics Instrument Corporation.