Volcanic Ash Concentrations Research Articles

Visualizing Volcanic Ash Forecasts: Scientist and Stakeholder Decisions Using Different Graphical Representations and Conflicting Forecasts

Abstract During volcanic eruptions, Volcanic Ash Advisory Centres issue ash advisories for aviation showing the forecasted outermost extent of the ash cloud. During the 2010 Icelandic volcano Eyjafjallajökull eruption, the Met Office produced supplementary forecasts of quantitative ash concentration, due to demand from airlines. Additionally, satellite retrievals of estimated volcanic ash concentration are now available. To test how these additional graphical representations of volcanic ash affect flight decisions, whether users infer uncertainty in graphical forecasts of volcanic ash, and how decisions are made when given conflicting forecasts, a survey was conducted of 25 delegates representing U.K. research and airline operations dealing with volcanic ash. Respondents were more risk seeking with safer flight paths and risk averse with riskier flight paths when given location and concentration forecasts compared to when given only the outermost extent of the ash. Respondents representing operations were more risk seeking than respondents representing research. Additionally, most respondents’ hand-drawn no-fly zones were larger than the areas of unsafe ash concentrations in the forecasts. This conservatism implies that respondents inferred uncertainty from the volcanic ash concentration forecasts. When given conflicting forecasts, respondents became more conservative than when given a single forecast. The respondents were also more risk seeking with high-risk flight paths and more risk averse with low-risk flight paths when given conflicting forecasts than when given a single forecast. The results show that concentration forecasts seem to reduce flight cancellations while maintaining safety. Open discussions with the respondents suggested that definitions of uncertainty may differ between research and operations.

3D VISUALIZATION OF VOLCANIC ASH DISPERSION PREDICTION WITH SPATIAL INFORMATION OPEN PLATFORM IN KOREA

Abstract. Visualization of disaster dispersion prediction enables decision makers and civilian to prepare disaster and to reduce the damage by showing the realistic simulation results. With advances of GIS technology and the theory of volcanic disaster prediction algorithm, the predicted disaster dispersions are displayed in spatial information. However, most of volcanic ash dispersion predictions are displayed in 2D. 2D visualization has a limitation to understand the realistic dispersion prediction since its height could be presented only by colour. Especially for volcanic ash, 3D visualization of dispersion prediction is essential since it could bring out big aircraft accident. In this paper, we deals with 3D visualization techniques of volcanic ash dispersion prediction with spatial information open platform in Korea. First, time-series volcanic ash 3D position and concentrations are calculated with WRF (Weather Research and Forecasting) model and Modified Fall3D algorithm. For 3D visualization, we propose three techniques; those are 'Cube in the air', 'Cube in the cube', and 'Semi-transparent plane in the air' methods. In the 'Cube in the Air', which locates the semitransparent cubes having different color depends on its particle concentration. Big cube is not realistic when it is zoomed. Therefore, cube is divided into small cube with Octree algorithm. That is 'Cube in the Cube' algorithm. For more realistic visualization, we apply 'Semi-transparent Volcanic Ash Plane' which shows the ash as fog. The results are displayed in the 'V-world' which is a spatial information open platform implemented by Korean government. Proposed techniques were adopted in Volcanic Disaster Response System implemented by Korean Ministry of Public Safety and Security.

3D VISUALIZATION OF VOLCANIC ASH DISPERSION PREDICTION WITH SPATIAL INFORMATION OPEN PLATFORM IN KOREA

Visualization of disaster dispersion prediction enables decision makers and civilian to prepare disaster and to reduce the damage by showing the realistic simulation results. With advances of GIS technology and the theory of volcanic disaster prediction algorithm, the predicted disaster dispersions are displayed in spatial information. However, most of volcanic ash dispersion predictions are displayed in 2D. 2D visualization has a limitation to understand the realistic dispersion prediction since its height could be presented only by colour. Especially for volcanic ash, 3D visualization of dispersion prediction is essential since it could bring out big aircraft accident. In this paper, we deals with 3D visualization techniques of volcanic ash dispersion prediction with spatial information open platform in Korea. First, time-series volcanic ash 3D position and concentrations are calculated with WRF (Weather Research and Forecasting) model and Modified Fall3D algorithm. For 3D visualization, we propose three techniques; those are 'Cube in the air', 'Cube in the cube', and 'Semi-transparent plane in the air' methods. In the 'Cube in the Air', which locates the semitransparent cubes having different color depends on its particle concentration. Big cube is not realistic when it is zoomed. Therefore, cube is divided into small cube with Octree algorithm. That is 'Cube in the Cube' algorithm. For more realistic visualization, we apply 'Semi-transparent Volcanic Ash Plane' which shows the ash as fog. The results are displayed in the 'V-world' which is a spatial information open platform implemented by Korean government. Proposed techniques were adopted in Volcanic Disaster Response System implemented by Korean Ministry of Public Safety and Security.

Heat treatment effect on solid particle erosion properties of polyphenylene sulfide composites reinforced with silane coupled volcanic ash particles

In this study both heat treatment and silane coupling effects on solid particle erosion behavior of volcanic ash (VA) particles reinforced polyphenylene sulfide (PPS) composites were investigated. Firstly, samples with various concentrations of pristine VA (0, 5, 10, 15, 20 and 25 wt%) were subjected to solid particle erosion tests for determining the optimum pristine VA concentrations. Secondly, VA particles with optimum concentrations (10 and 15 wt%) were silane coupled with 3‐aminopropyltriethoxysilane (3‐APTS). At last, PPS composites with silane coupled VA particles at optimum concentrations were heat treated at 200°C for 2 hours. After erosion tests, all eroded VA/PPS composite surfaces were scanned with non‐contact 3D profilometer for roughness investigation. And also scanning electron microscopy (SEM) analysis was performed. It was reported that pure PPS and all kind of VA/PPS composites exhibited ductile erosion behavior. Erosion mass loss and roughness values were increased with the increase of pristine VA content significantly for the impingement angles of 15° and 30°. It was determined that silane coupling had negative effect on erosion properties but had positive effect on roughness properties. Heat treatment of silane coupled VA/PPS composites achieved half mass loss and roughness value of pristine VA/PPS composites. POLYM. COMPOS., 39:1638–1646, 2018. © 2016 Society of Plastics Engineers

Measurements of Particle Distribution and Ash Fluxes in the Plume of Sakurajima Volcano with Optical Particle Counter

Volcanic eruptions have caused very costly disturbances of international air traffic. This problem has been dealt with by simulating the formation and migration of dangerous ash plumes. However, the results of the simulations have sometimes been too safe, producing ash clouds that are too large. This was especially the case for the North Atlantic in 2010 (Eyjafjallajökull) and 2011 (Grímsvötn). Since 2012, an international cooperation team led by the Disaster Prevention Research Institute (DPRI) of Kyoto University has conducted airborne measurements of volcanic ash concentrations in the plume from Mount Sakurajima in Kagoshima Prefecture, Japan. This volcano was chosen because of its frequent but limited eruptions, which allow close observation. These measurement campaigns have provided data showing gravitational flattening of the plume, a new and previously unknown dispersion process of volcanic plumes. A new and previously unknown fallout process, called streak fallout, also has been measured. Results concerning plume flux, concentration distributions, aerosol (PM10) content of the plume, and content of very fine particles (PM2.5 and PM1) are presented, and the ways by which the observational methods can be used to produce reliable initial data and boundary values for simulations of plume dispersion are discussed.

Investigation and Separation of Turbulent Fluctuations in Airborne Measurements of Volcanic Ash with Optical Particle Counters

The science of measuring airborne volcanic ash concentrations supports research in such fields as atmospheric environmental science and the modeling of atmospheric pollution from volcanoes, and is thus very valuable to the aviation industry. These measurements show large scatter directly traceable to turbulent fluctuations responsible for diffusing volcanic dust. Before semistationary components in observations can be compared to each other or to simulation results, they must be separated from fluctuations. In the design of the separation process, however, neither seasonal or diurnal periodicity nor random disturbance with known properties exists to serve as a guideline. It has been suggested that fluctuations could be eliminated through repeated convolutions of a simple 3-point filter enough times. The number of convolutions is chosen from the change in the rate of increase of a special variability parameter. When semistationary concentrations are separated from fluctuations, their statistics are compared to turbulence parameters and the autocorrelation of the series. The method is demonstrated using three measurement series from Sakurajima, Japan measured in 2013. It is concluded that this new method is simple and trustworthy where knowledge and experience of the environmental parameters can be utilized to support the results. They indicate a variability of 40% in the relative fluctuations of the PM10 and around 20% of the PM2.5. The relative fluctuations may be considered completely random, but normally distributed rather than a white noise with an evenly distributed variance spectrum.

Volcanic ash concentration during the 12 August 2011 Etna eruption

AbstractMount Etna, in Italy, is one of the most active volcanoes in the world and an ideal laboratory to improve volcano ash monitoring and forecasting. During the volcanic episode on 12 August 2011, an eruption column rose up to several kilometers above sea level (asl), and the volcanic plume dispersed to the southeast. From the video‐surveillance system, we were able to estimate variations in the column height (peak value of 9.5 ± 0.5 km above sea level) with time. We derived the time‐varying discharge rate (peak value of 60 m3 s−1) and determined the ash concentration using a volcanic ash dispersal model. The modeled ash concentration was compared with lidar measurements using different particle effective radius, and differences are within the error bars. Volcanic ash concentrations range from 0.5 to 35.5 × 10−3 g m−3. The comparison highlights that to improve volcanic ash forecasting during volcanic crises it is necessary to take into account the time‐varying discharge rate of explosive eruptions.

Solid Particle Erosion Behavior of Volcanic Ash/PVC Composites

In this study an attempt has been made to evaluate the solid particle erosion behavior of PVC based composites with various volcanic ash (VA) concentrations (5, 10, 15, 20 and 25 wt.%). VA/PVC composites were prepared by using a twin screw micro-compounder and an injection molding machine. The erosion testing of VA/PVC composites was performed in accordance with the ASTM G76-95 standard at 6 di erent particle impingement angles (15◦, 30◦, 45◦, 60◦, 75◦ and 90◦) and under the pressure of 1.5 bar by using abrasive Al2O3 particles. The morphology of eroded surfaces was examined by using a scanning electron microscope (SEM), and possible wear mechanisms were discussed. Moreover, the surface roughness values of eroded VA/PVC composites were analyzed by using a non-contact laser optical pro lometer. 3D surface topographies were also examined.

Airborne Measurement in the Ash Plume from Mount Sakurajima: Analysis of Gravitational Effects on Dispersion and Fallout

Volcanic ash concentrations in the plume from Sakurajima volcano in Japan are observed from airplanes equipped with optical particle counters and GPS tracking devices. The volcano emits several puffs a day. The puffs are also recorded by the Sakurajima Volcanological Observatory. High concentrations are observed in the puffs and fallout driven by vertical air current, called streak fallout. Puffs dispersion is analyzed by the classical diffusion-advection method and a new gravitational dispersion method. The fluid mechanic of the gravitational dispersion, streak fallout, and classical diffusion-advection theory is described in three separate appendices together with methods to find the time gravitational dispersion constant and the diffusion coefficient from satellite photos. The diffusion-advection equation may be used to scale volcanic eruptions so the same eruption plumes can be scaled to constant flux and wind conditions or two eruptions can be scaled to each other. The dispersion analyses show that dispersion of volcanic plumes does not follow either theories completely. It is most likely diffusion in the interface of the plume and the ambient air, together with gravitational flattening of the plumes core. This means larger boundary concentration gradients and smaller diffusion coefficients than state of the art methods can predict.

The role of in-situ measurements of volcanic ash concentrations in preventing economic disasters due to volcanic ash clouds

During the Eyjafjallajökull eruption in 2010 and the Grimsvotn eruption in 2011, a lot of airports where closed because ash cloud predictions were much on the safe side compared to the actual ash clouds. This created a staggering economic disaster. An extent of predicted ash clouds, up to 40 times the size of the real clouds, are documented. Modern methods in disaster prevention can probably not eliminate airport closures altogether, but including in-situ airborne measurements of volcanic ash concentrations with OPC technology and other scientific methods of ash cloud assessment can reduce unnecessary airport closings to insignificant levels. Airborne measurements by a team of German, Icelandic and Japanese scientists have shown that important methods, additional to computer simulations by source models, are available. An example of parameter and ash volume predictions is shown, supported by measurements from Iceland, Germany and Japan. It is concluded, that point source diffusion models can only have a limited value in predicting scenarios of ash clouds, unless supported by in-situ observations of good quality.

Specifying the atmospheric pollutant distribution forecast using the mathematical modeling and measurement data

Considered is a possibility of specifying the forecast of volcanic aerosol distribution in the atmosphere using the data of ground-based actinometric measurements. To simulate the aerosol distribution, the FlexPart software (Norwegian Institute for Air Research) was used. Using the FlexGraph software (Planeta Research Center for Space Hydrometeorology), the visualization and interactive analysis were carried out of aerosol concentration fields at different time moments. The prognostic concentration of volcanic ash from Grimsvotn volcano (May 2011) is bound to the results of measurements of aerosol optical depth in Hamburg at one of the points of AERONET global network. The comparison of the corrected aerosol concentration with the measurements at other AERONET points demonstrated the competence of the proposed approach. To specify the aerosol cloud distribution forecast on the territory of Russia, the Roshydromet actinometric network data can be used.

Aircraft observations and model simulations of concentration and particle size distribution in the Eyjafjallajökull volcanic ash cloud

Abstract. The Eyjafjallajökull volcano in Iceland emitted a cloud of ash into the atmosphere during April and May 2010. Over the UK the ash cloud was observed by the FAAM BAe-146 Atmospheric Research Aircraft which was equipped with in-situ probes measuring the concentration of volcanic ash carried by particles of varying sizes. The UK Met Office Numerical Atmospheric-dispersion Modelling Environment (NAME) has been used to simulate the evolution of the ash cloud emitted by the Eyjafjallajökull volcano during the period 4–18 May 2010. In the NAME simulations the processes controlling the evolution of the concentration and particle size distribution include sedimentation and deposition of particles, horizontal dispersion and vertical wind shear. For travel times between 24 and 72 h, a 1/t relationship describes the evolution of the concentration at the centre of the ash cloud and the particle size distribution remains fairly constant. Although NAME does not represent the effects of microphysical processes, it can capture the observed decrease in concentration with travel time in this period. This suggests that, for this eruption, microphysical processes play a small role in determining the evolution of the distal ash cloud. Quantitative comparison with observations shows that NAME can simulate the observed column-integrated mass if around 4% of the total emitted mass is assumed to be transported as far as the UK by small particles (< 30 μm diameter). NAME can also simulate the observed particle size distribution if a distal particle size distribution that contains a large fraction of < 10 μm diameter particles is used, consistent with the idea that phraetomagmatic volcanoes, such as Eyjafjallajökull, emit very fine particles.

Volcanic ash over the Russian Federation territory after the volcanic eruption in Iceland on April 14, 2010 from the data of model simulations and observations

After the volcanic eruption in Iceland on April 14, 2010, the volcanic ash cloud spread over Europe including Russia, that resulted in the air traffic collapse for several days. Presented are the results of computations of the volcanic ash spreading obtained with the FLEXPART Lagrangian dispersion model, as well as the results of the lidar sounding over Troitsk (Moscow region). The obtained model and experimental results agree well with each other and indicate the existence of settling layer of volcanic ash at the height of 4–6 km, that is corroborated by the data of observations in Europe. The largest uncertainty in the model computations is caused by the lack of information on the volume of the emission of micron particles transported with the wind to the large distance. The comparison of model results with the observational data can enable to correct the source function and to obtain more realistic estimates of volcanic ash concentrations, that is a key factor of the air traffic control.

AGU Chapman Conference 2012: Of ash and airplanes

<xhtml:span xmlns:xhtml="http://www.w3.org/1999/xhtml" xml:lang="en"> Scientists share ideas about how to model and forecast volcanic ash concentrations and how to pronounce Icelandic names. </xhtml:span>

Ash3d: A finite‐volume, conservative numerical model for ash transport and tephra deposition

We develop a transient, 3‐D Eulerian model (Ash3d) to predict airborne volcanic ash concentration and tephra deposition during volcanic eruptions. This model simulates downwind advection, turbulent diffusion, and settling of ash injected into the atmosphere by a volcanic eruption column. Ash advection is calculated using time‐varying pre‐existing wind data and a robust, high‐order, finite‐volume method. Our routine is mass‐conservative and uses the coordinate system of the wind data, either a Cartesian system local to the volcano or a global spherical system for the Earth. Volcanic ash is specified with an arbitrary number of grain sizes, which affects the fall velocity, distribution and duration of transport. Above the source volcano, the vertical mass distribution with elevation is calculated using a Suzuki distribution for a given plume height, eruptive volume, and eruption duration. Multiple eruptions separated in time may be included in a single simulation. We test the model using analytical solutions for transport. Comparisons of the predicted and observed ash distributions for the 18 August 1992 eruption of Mt. Spurr in Alaska demonstrate to the efficacy and efficiency of the routine.

Improving forecasts of volcanic ash concentrations

Volcanic ash can severely damage airplanes, and eruptions such as the 2010 Eyjafjallajökull eruption may result in major disruption to air travel. Improved forecasting of ash cloud locations and concentrations could benefit the aviation industry and reduce delays, but forecasting is challenging because eruptions and atmospheric transport of volcanic ash are complex processes. The UK Met Office had improved its modeling procedures prior to the 2010 Eyjafjallajökull eruption, enabling peak ash concentrations to be estimated during that event. Webster et al. describe the Met Office's method of ash concentration forecasting and how it has evolved from simply predicting regions of ash to also estimating peak ash concentrations.

A case study of observations of volcanic ash from the Eyjafjallajökull eruption: 1. In situ airborne observations

On 17 May 2010, the FAAM BAe‐146 aircraft made remote and in situ measurements of the volcanic ash cloud from Eyjafjallajökull over the southern North Sea. The Falcon 20E aircraft operated by Deutsches Zentrum für Luft‐ und Raumfahrt (DLR) also sampled the ash cloud on the same day. While no “wingtip‐to‐wingtip” co‐ordination was performed, the proximity of the two aircraft allows worthwhile comparisons. Despite the high degree of inhomogeneity (e.g., column ash loadings varied by a factor of three over ∼100 km) the range of ash mass concentrations and the ratios between volcanic ash mass and concentrations of SO2, O3 and CO were consistent between the two aircraft and within expected instrumental uncertainties. The data show strong correlations between ash mass, SO2concentration and aerosol scattering with the FAAM BAe‐146 data providing a specific extinction coefficient of 0.6–0.8 m2 g−1. There were significant differences in the observed ash size distribution with FAAM BAe‐146 data showing a peak in the mass at ∼3.5μm (volume‐equivalent diameter) and DLR data peaking at ∼10μm. Differences could not be accounted for by refractive index and shape assumptions alone. The aircraft in situ and lidar data suggest peak ash concentrations of 500–800 μg m−3with a factor of two uncertainty. Comparing the location of ash observations with the ash dispersion model output highlights differences that demonstrate the difficulties in forecasting such events and the essential nature of validating models using high quality observational data from platforms such as the FAAM BAe‐146 and the DLR Falcon.

Performance assessment of a volcanic ash transport model mini‐ensemble used for inverse modeling of the 2010 Eyjafjallajökull eruption

The requirement to forecast volcanic ash concentrations was amplified as a response to the 2010 Eyjafjallajökull eruption when ash safety limits for aviation were introduced in the European area. The ability to provide accurate quantitative forecasts relies to a large extent on the source term which is the emissions of ash as a function of time and height. This study presents source term estimations of the ash emissions from the Eyjafjallajökull eruption derived with an inversion algorithm which constrains modeled ash emissions with satellite observations of volcanic ash. The algorithm is tested with input from two different dispersion models, run on three different meteorological input data sets. The results are robust to which dispersion model and meteorological data are used. Modeled ash concentrations are compared quantitatively to independent measurements from three different research aircraft and one surface measurement station. These comparisons show that the models perform reasonably well in simulating the ash concentrations, and simulations using the source term obtained from the inversion are in overall better agreement with the observations (rank correlation = 0.55, Figure of Merit in Time (FMT) = 25–46%) than simulations using simplified source terms (rank correlation = 0.21, FMT = 20–35%). The vertical structures of the modeled ash clouds mostly agree with lidar observations, and the modeled ash particle size distributions agree reasonably well with observed size distributions. There are occasionally large differences between simulations but the model mean usually outperforms any individual model. The results emphasize the benefits of using an ensemble‐based forecast for improved quantification of uncertainties in future ash crises.

Airborne in-situ investigations of the Eyjafjallajökull volcanic ash plume on Iceland and over north-western Germany with light aircrafts and optical particle counters

During the time period of the eruption of the Icelandic volcano Eyjafjallajökull in April/May 2010 the Duesseldorf University of Applied Sciences has performed 14 research flights in situations with and without the volcanic ash plume over Germany. In parallel to the research flights in Germany three measurement flights have been performed by the University of Iceland in May 2010 over the western part of Iceland. During two of these flights the outskirts of the eruption plume were entered directly, delivering most direct measurements within the eruption plume during this eruptive event. For all the measurement flights reported here, light durable piston-motor driven aircrafts were used, which were equipped with optical particle counters for in-situ measurements. Real-time monitoring of the particle concentrations was possible during the flights. As different types of optical particle counters have been used in Iceland and Germany, the optical particle counters have been re-calibrated after the flights to the same standard using gravimetric reference methods and original Eyjafjallajökull volcanic ash samples. In-situ measurement results with high spatial resolution, directly from the eruption plume in Iceland as well as from the dispersed and several days old plume over Germany, are therefore presented here for the first time. They are normalized to the same ash concentration calibration standard. Moreover, airborne particles could be sampled directly out of the eruption plume in Iceland as well as during the flights over Germany. During the research flights over Iceland from 9 May 2011 to 11 May 2011 the ash emitted from the vent of the volcano turned out to be concentrated in a narrow well-defined plume of about 10 km width at a distance of 45–60 km away from the vent. Outside this plume the airborne ash concentrations could be proved to be below 50 μg m −3 over western Iceland. However, by entering the outskirts of the plume directly the research aircraft could detect ash concentrations of up to 2000 μg m −3 . On the other hand, the ash plume, which was analysed by research flights over Germany several thousand km away from the eruption vent, appeared to be significantly structured in horizontal and vertical directions. Different sub-plumes could be found. Peak concentrations of more than 330 μg m −3 could be detected. The results of the measurements within the ash plume over Germany were compared with the predictions of the London VAAC model. The range of ash concentrations found by the research aircraft in Germany were not in conflict with the calculations of concentration regimes by the London VAAC model. However, the in-situ measurements performed by the research aircraft were able to deliver information about the structure and composition of the ash plume, which could not be covered by the dispersion model. Therefore, light piston-motor driven aircrafts equipped with optical particle counters proved to be a very versatile tool for the real-time in-situ determination of the spatial extension of volcanic ash plumes, the ash particle size distributions and the particle mass concentrations. Moreover, all these parameters could be measured with a high horizontal and vertical spatial resolution. Therefore, these kinds of measurements can deliver immediate data for the validation and verification of dispersion models and can give direct in-situ information additional to LIDAR measurements and satellite observations. As the piston-motor driven aircrafts are able to operate even at elevated volcanic ash concentrations they can provide valuable ash concentration results for air traffic safety. ► Aircraft in-situ measurements of the Eyjafjalla ash plume with optical particle counters. ► Direct measurements up to 2000 μg m −3 in the outskirts of the ashplume on Iceland. ► Ash plume mapping over northern Germany with concentrations up to 330 μg m −3 . ► Comparison with London VAAC model. ► Calibration of the optical Particle counters with original Eyjafjalla ash.

Modelling concentrations of volcanic ash encountered by aircraft in past eruptions

Abstract Prolonged disruption to aviation during the April–May 2010 eruption of Eyjafjallajokull, Iceland resulted in pressure to predict volcanic ash plume concentrations for the purpose of allowing aircraft to fly in regions with low ash contamination. Over the past few decades there have been a number of incidents where aircraft have encountered volcanic ash resulting in damage to the aircraft and loss of power to engines. Understanding the volcanic ash concentrations that these aircraft have encountered provides important input to determining a safe concentration limit. Aircraft encounters with six volcanic eruption plumes have been studied and ash concentrations predicted using the atmospheric dispersion model NAME. The eruptions considered are Galunggung 1982, Soputan 1985, Redoubt 1989, Pinatubo 1991, Hekla 2000 and Manam 2006. Uncertainties in the eruption source details (start time, stop time and eruption height) and in the aircraft encounter location and flight path are found to be major limitations in some cases. Errors in the driving meteorological data (which is often coarse in resolution for historic studies) and the lack of eruption plume dynamics (e.g. umbrella cloud representation) results in further uncertainties in the predicted ash concentrations. In most of the case studies, the dispersion modelling shows the presence of ash at the aircraft encounter location. Maximum ash concentrations in the vicinity of the aircraft are predicted to be at least 4000 μg m −3 although confidence in the estimated concentrations is low and uncertainties of orders of magnitude are shown to be possible.