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Near-source observations of aerosol size distributions in the eruptive plumes from Eyjafjallajökull volcano, March–April 2010
Atmospheric Environment 45 (2011) 3210e3216
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Short communication
Near-source observations of aerosol size distributions in the eruptive plumes from Eyjafjallajökull volcano, MarcheApril 2010 E. Ilyinskaya a, *, V.I. Tsanev a, b, R.S. Martin c, d, C. Oppenheimer a, J. Le Blond a,1, G.M. Sawyer a, M.T. Gudmundsson e a
Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UK Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK School of Biological and Chemical Sciences, Queen Mary, University of London, London, E1 4NS, UK d Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK e Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavík, Iceland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2010 Received in revised form 25 February 2011 Accepted 2 March 2011
Near-source observations of aerosol size distributions (<15 km from the vent) were made during three eruptive phases of Eyjafjallajökull in 2010: Phase I - basaltic lava fountaining; Phase II - andesitic phreatomagmatic explosions (mass discharge rate of the order of 105 kg s1); Phase III - andesitic magmatic explosions (mass discharge rate of the order of 103 kg s1). Two methods were used, photometric measurements of suspended aerosol mass (with size distributions retrieved by inverse modelling); and sampling of ash fallout during Phases II and III (with size distributions measured by laser diffraction). The suspended aerosol in Phase I plume was dominated by particles sized <0.4 mm in diameter. During Phase II, the suspended aerosol mass contained a high number of fine (<1 mm) particles (an internal mixture of soluble aerosol and very fine ash) and coarse (>1 mm) ash particles. The number ratio of fine and coarse particles fluctuated strongly within short intervals of time, indicative of changes in the fragmentation energy. The near-source ash fallout was poorly sorted (interquartile range 16e80 mm); ash grains sized <1 mm contributed up to 7% of total volume. The high water content and electrical charge in the plume are believed to have enhanced the deposition through particle aggregation. In Phase III the eruption became drier and less explosive. The suspended aerosol mass was strongly bimodal with high particle abundances at w0.2 and 1e4 mm. The ash fallout was better sorted (interquartile range 60e120 mm; no grains <1 mm), attributed to limited particle aggregation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Volcanic plume Aerosol Ash Sun photometry Eyjafjallajokull
1. Introduction The Eyjafjallajökull eruption in Iceland in 2010 disrupted aviation and revealed gaps in operational capacity for measurement and forecasting of volcanic plume dispersion. A key requirement for the modelling of plume trajectory is the particle size distribution at source. Here we present the measurements of size distributions in near-source volcanic aerosol (<15 km from the vent) collected between April 1 and April 24, 2010 (Fig. 1). ‘Aerosol’ is used here as an umbrella term for all particles transported in the volcanic plume, while ‘ash’ refers only to silicate particles. * Corresponding author. Present address: Icelandic Meteorological Office, Bustadavegi 9, 150 Reykjavik, Iceland. E-mail address: [email protected] (E. Ilyinskaya). 1 Present address: Department of Mineralogy, Natural History Museum, London, SW7 5BD, UK. 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.03.017
Eyjafjallajökull (63.63 N 19.65 W; 1670 m a.s.l.) is an icecovered volcano surmounted by a 2500 m-diameter summit caldera where the thickness of the glacial cover reaches 250 m (Strachan, 2001). Eyjafjallajökull has erupted four times in the last 1400 years (the previous eruption was in 1821e23), all of which have been of low magnitude (w0.1 km3 of erupted material, Sturkell et al., 2010). Eruptions of Eyjafjallajökull pose a great hazard due to the proximity of populated centres and, as shown recently, may lead to severe disruption of international air traffic. The 2010 eruption continued for more than two months, and in the first six weeks it exhibited three distinct phases of activity (Gudmundsson et al., 2010): (I) Basaltic fissure eruption on the volcano flank (no glacial cover) started on March 20. The eruption included lava fountaining and flows. The associated plume was transparent in the daytime and visibly ash-poor. This phase ceased on April 12.
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(II) Ash-rich eruption at the summit crater started on April 14. This phase was associated with extensive melting of glacial ice. Between April 14e17, phreatomagmatic explosions produced an opaque, dark 4e9 km high plume with abundant ash of intermediate composition (w58 wt% SiO2; Óskarsson, 2010). Ash fallout thickness reached a few centimetres on the lowlands south of the volcano. The plume was advected by upper tropospheric and lower stratospheric northwesterly winds and carried towards Europe, where it caused unprecedented air traffic disturbance. The eruptive plume was monitored using satellites (e.g., MODIS, OMI) and sensors across Europe (including EARLINET LIDARs, the German ceilometer network, AERONET stationary CIMEL Sun photometers, ozone sondes; Ansmann et al., 2010; Flentje et al., 2010; Schumann et al., 2010). (III) Mildly explosive eruption between April 18 and May 5, with much reduced ash production. Explosions were intermittent with quiescent intervals. The plume was dense and white, commonly reaching a height of w3 km above the crater. Incandescence in the crater became visible around April 21, and a lava flow began around the same time (Petersen, 2010). Following Phase III, the explosivity and ash production increased again; the eruption continued until May 22 alternating between ash-rich and ash-poor stages. This activity period is not covered by this report. 2. Methods 2.1. Sun photometry Microtops II Sun photometer measures the solar radiance in five spectral bands centred at 380, 440, 675, 879 and 1020 nm
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(bandwidth 10 nm, except the 380 nm channel where bandwidth is 4 nm) and automatically calculates the aerosol optical thickness (AOT) (Morys et al., 2001). The hand-held instrument is compact and extremely well suited for field measurements of volcanic plumes near to the source. Microtops II measurements are taken in a rapid sequence at a rate of 3 s. The measurements are then automatically collated in scans; one scan consists of 32 measurements from each of the five channels, yielding an overall sampling rate of w0.3 Hz. To investigate the aerosol properties of a plume, it is necessary to observe the Sun both through the plume and in the absence of plume (i.e., background). The aerosol optical thickness of the plume is estimated by subtracting the background AOT from the AOT measured through the plume (i.e., plume þ background). These background measurements represent the minimum AOT across all wavelengths, and typically the plume AOT measurements are >>10 background AOT measurements (Fig. 1). Field measurement details are listed in Table 1 and the recorded AOT scans are reported in the Appendix. The standard deviation of AOT in each individual scan is negligibly small. Therefore we group individual scans into optically homogenous ‘scan groups’ (listed in Appendix) and evaluate the mean and standard deviations for each group. These values are used in inversion modelling. This methodology has been previously used to study volcanic plumes (e.g., Mather et al., 2004; Martin et al., 2009). The aerosol number, surface and volume distributions were retrieved by inverse modelling of the measured AOT in each scan group (software by Tsanev and Mather, 2007). The optimal retrieval, i.e., the best match between measured AOT and AOT calculated from the retrieved size distribution, is achieved with diameters 0.16e8.0 mm following the procedure proposed by King et al. (1978). The algorithm is explained in detail by Tsanev and Mather (2007). The main sources of error in inversion modelling of spectral optical depths are the uncertainty in the index of
Fig. 1. Map showing the eruptive centres in Eyjafjallajökull, the direction of plume transport (approximate locations), and the measurement/sampling points. AOT - photometric measurements of aerosol optical thickness; BG - background AOT measurements; (*) - ash samples. Exact times and locations of AOT measurements are listed in Appendix. Base map courtesy of the National Land Survey of Iceland.
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Table 1 Number of AOT scans and ash fallout samples. Time of AOT measurements is reported in local time. During Phase II, the plume was mostly opaque so that AOT measurements had to be made at the plume’s edge where the Sun was clearly visible. Approximate age of ash fallout samples is given in hours. Phase Date
Sun photometry
Ash fallout
n of scans Local time (hh:mm) n of samples Sample age (h) I II III
April April April April April
1 16 17 23 24
180 e 90 220 e
14:30e15:20 e 17:50e19:30 12:20e13:30 e
No ash 2 3 1 2
e <12 0.25e12 <12 4e8
refraction, and the assumption that all particles are spherical and exist as an internal mixture. The errors of the retrieval are estimated analytically by King et al. (1978) and numerically by Jorge and Ogren (1996). A difference of 0.1 in the real part of the refractive index was found to cause up to 82%, 30% and 32% error in the total particle number, surface area and volume, respectively (Jorge and Ogren, 1996). However, the general shape of the size distribution was retained. The index of refraction m was selected based on results discussed in Section 2 and available data on the ash composition (Óskarsson, 2010) as follows. Phase I - 1.44 þ 0.0i, a value applicable for an atmospheric mixture of water and sulphate (Krotkov et al., 1997); the composition of Phase I aerosol was measured independently (Ilyinskaya et al., in preparation). Phase II - 1.55 þ 0.01i, a plume rich in trachyandesitic and poorly porphyritic ash (Horwell, 2007). Retrievals for Phase II were also made with m values of 1.50 þ 0.01i and 1.55 þ 0.015i with negligible impact on the shape of the size distribution. Phase III - 1.55 þ 0.001i; due to much decreased ash content, the plume was assumed to be much less absorbing than in Phase II. Retrievals for Phase III were also made with m values of 1.50 þ 0.001i and 1.55 þ 0.0015i with negligible impact on the size distribution. 2.2. Ash sampling Ash fallout samples (Table 1) were collected by laying out pristine polypropylene bags, and by brushing off the few top millimetres off thick layers that had collected on man-made surfaces (e.g., bridge railings). All collected ash samples were presumably non-weathered, as they were dry and collected soon after deposition. The sample bags were sealed and kept dry. The grain size distribution of each sample was determined by laser diffraction using a Malvern Mastersizer (Hydro Mu2000 with ultrasonics) at the Department of Geography, University of Cambridge (method by Horwell, 2007).
Fig. 2. Average total optical thickness of the aerosol in three eruptive phases plotted as a function of wavelength (l). The total optical thickness in Phase I is an average of 180 measurements in 27 scan groups; Phase II - 90 measurements in 15 scan groups; Phase III - 220 measurements in 9 scan groups. Background optical thickness has been subtracted from each wavelength; standard error is evaluated from all scan groups in each Phase.
shortest (380e440 nm) and the longest (1020 nm) of the measured wavelengths, indicative of a bimodal size distribution (King et al., 1978). The Ångström equation (Ångström, 1964) s(l) ¼ b/la offers a simple, empirical framework to interpret the results shown in Fig. 2. A possible source of error is the assumption that the background AOT is known precisely; however in volcanic plumes, the background AOT is much smaller (<<10) than the plume AOT and does therefore not influence the results significantly. The scatter plots a vs. b of the estimated Ångström parameters are presented in Fig. 3. The parameter a represents particle size: a ¼ 0 suggests aerosol dominated by coarse particles, while a ¼ 1e2 indicates mainly fine particles (Baltensperger et al., 2003). The parameter
3. Aerosol optical thickness The wavelength-dependent AOT, s(l), varied between the three eruptive phases (Fig. 2). Phase I plume had the greatest extinction in the shortest wavelengths; and its s(l) was typical of ash-free volcanic plumes (Watson and Oppenheimer, 2000, 2001; Mather et al., 2004). This type of s(l) usually indicates either Junge (number of particles decreases near-linearly with size) or twoslope size distributions (King et al., 1978). The Phase II plume had a more flattened AOT with the extinction increasing slightly with wavelength. This indicates that the plume had a large number of coarse particles, consistent with the high ash content of the plume observed during Phase II. Size distributions of particles are retrievable provided that the AOT shows wavelength-dependence (King et al., 1978). Phase III showed enhanced extinction in the
Fig. 3. Ångström parameters (a and b) calculated for the Phases I, II and III from the measured AOT (individual scans). Phase II is divided into 3 time-dependent subgroups which have distinct a and b-values. Background measurements for each Phase are represented by (þ) symbols.
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b equals s(l ¼ 1 mm) and increases with the number of particles. The plume in Phase I had the largest a values (between 1.6 and 2.5), revealing a plume dominated by relatively fine particles (<1 mm); and b were negatively correlated with a. A similar trend has been observed in other ash-free plumes (e.g., Mather et al., 2004). AOT during Phase II reveals a from about 0.5 to 0 which indicates a higher abundance of coarse particles. Three time-dependent ‘clusters’ (a, b, c) are distinguishable in Phase II, indicative of strong fluctuations in the particle number. The Phase III a value was w0 and had a weak positive correlation with b. Aerosol in Phase III had much lower b-values than during Phase II, consistent with the plume being less opaque. Background measurements had low b-values with large fluctuations in a. The plume in Phase III was progressively moving away due to wind shift, resulting in a weaker plume signal - a decrease in b and more scattered a. 4. Aerosol size, volume and surface area distribution 4.1. Suspended aerosol Retrieval results are presented in Fig. 4, illustrating the differences in aerosol properties between the three eruptive phases. Table 2 shows ratios of particle numbers with different diameters, in order to illustrate the variation in relative abundances of fine and coarse size modes. Phase I was dominated by the finest particles (w0.1e0.2 mm, Table 2, Fig. 4). This agrees well with distributions measured in other ash-free plumes, which were mainly composed of water droplets and sulphate (e.g., Watson and Oppenheimer, 2000, 2001; Porter et al., 2002). The difference in particle number distribution between Phases II and III is very intriguing. The number of the finest particles (<0.4 mm) is very similar between the two Phases, but Phase II plume also contains a large number of particles enhanced in every size bin above 0.4 mm. This indicates that the eruptive plume of Phase II contained a large quantity of <1 mm ash grains, reflecting very high fragmentation energy (due to the interaction of magma with external water). Presence of <1 mm ash was confirmed by laser diffraction analysis of the ash fallout. The explosivity of the eruption dropped in Phase III, which was reflected not only in the decreased abundance of <1 mm ash but also in the mass discharge rate (see Section 4.2). The number ratio of fine and coarse particles in Phase II fluctuated between 2 and 15 within short intervals of time (Table 2). It is noteworthy that the number of fine particles was relatively stable (within an order of magnitude) and the significant fluctuations (1e2 orders of magnitude) occurred only in the coarse mode. This highlights dynamic changes in the fragmentation energy of the eruption. The fluctuations in the fragmentation energy appear, however, to have had little influence on the nucleation rate of the aerosol (i.e., the number density of the finest mode). Phase III had a bimodal particle distribution (Fig. 4), with higher fine/coarse aerosol number ratio than Phase II (Table 2). There is a very distinct particle number minimum between 0.2 and 1 mm; this strongly indicates that the fine and the coarse mode have different chemical composition (e.g., Greenfield, 1957; Wang et al., 1978). Aerosol particles which nucleate and grow through gas absorption and condensation tend to fall in a narrow size mode (accumulation mode, <1 mm), as their growth is intra-competitive. The coarse particle mode (>1 mm) is primarily composed of ash (Ilyinskaya et al., in preparation). Far-field observations of particle size distributions in the Eyjafjallajökull plume were made across Europe (several 100s and 1000s of km from source) by several research groups (Ansmann et al., 2010; Flentje et al., 2010; Harrison et al., 2010; Schumann et al.,
Fig. 4. Size-dependent distribution of mean particle number distribution (dN), surface area (dA) and volume (dV) during three eruptive phases (averaged between all scan groups). Phase II is divided into 3 time-dependent subgroups.
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Table 2 Particle column density N(d) and ratios N(di)/N(dk) measured during the three phases of the eruption estimated for given diameters (mm). Phase II is subdivided into 3 time-dependent subgroups (a, b, c). N(0.37 mm) refers to the columnar number density of fine particles whilst the ratios N(0.37 mm)/N(1.1 mm) and N (0.37 mm)/N(2.0 mm) reflect the local minima between fine and coarse particles, and the relative amount of particles with diameter d ¼ 1 mm. Phase
N(0.37 mm)[cm2]
I II-a II-b II-c III
(1.4 (3.1 (7.8 (7.5 (5.3
0.8) 2.4) 0.1) 3.6) 3.9)
109 107 107 107 106
N(0.37 mm)/N(1.1 mm)
N(0.37 mm)/N(2.0 mm)
240 1.0 0.77 0.93 7.6
6100 3.5 2.1 15 6.2
2010). Measurements made by the research aircraft Falcon of a four-day old Phase II plume (Schumann et al., 2010) revealed that the particle number distribution was close to a Junge-type distribution. The abundant coarse particles (>2 mm) which were measured near the source had mostly settled out and the plume was composed of soluble aerosol (water droplets and/or sulphate). No particles larger than 30 mm were found in the far-field plume, and the ash mass concentration was found to be below the newly established aviation safety limit of 0.2 mg m3 (Schumann et al., 2010). Measurements of Phase III plume on May 2 over the North Atlantic (w450 km from source) revealed a significantly different particle size distribution compared to Phase II (Schumann et al., 2010). In agreement with the near-source observations the particle number distribution was bimodal, with a large number of fine (soluble) particles and a coarser (ash) mode of 4e10 mm. 4.2. Ash fallout A rough estimate of mass discharge of the plume for Phases II and III at the time of the sampling was estimated from plume height. At 18:00e18:20 on 17 April the plume was 6 km high (observations from inspection flight), while it was 3e3.5 km high on April 23e24 (Iceland Met Office radar and inspection flight data). Using empirical formulas relating plume height and magma discharge (Sparks et al., 1997; Mastin et al., 2009) and magma density of 2500 kg m3 we obtained a discharge rate of about 1 105 kg s1 for the time of measurement on 17 April and 2e3 103 kg s1 for 23e24 April. The first value was probably an underestimate since the plume was very dark in colour, heavily loaded with tephra and partly collapsing on 17 April. If we assume that ash density does not significantly vary with grain size, the measured grain volume distribution in the fallout samples (Fig. 5) is indicative of the mass distribution. Ash fallout during Phase II was fairly poorly sorted (the interquartile range of grain size distribution was 16e80 mm). A large proportion consisted of small-sized grains (on average, 30% was <18 mm; Fig. 5). The terminal fall velocity for this grain size fraction (from 2000 to 0 m height) is w0.02 m s1 (Rose and Durant, 2009), which would give an atmospheric residence time of 27 h. This is clearly much longer than the age of the ash from Phase II considering the sampling distance from the vent (<11 km) and the prevailing wind speed (5e10 m s1; Petersen, 2010). However, volcanic ash has previously been found to settle out much quicker than predicted (summary in Rose and Durant, 2009). Ash particles in water-rich plumes (such as the phreatomagmatic Phase II plume) tend to aggregate by surface water tension, and adsorption of liquid water increases grain density (e.g., Delmelle et al., 2005), thereby increasing the settling velocity. Electrical charging (particularly enhanced in phreatomagmatic eruptions; James et al., 2008) also causes attraction and aggregation of fine particles. Volcanic lightning was frequent during Phase II and positive charge was measured in the plume
Fig. 5. Size-dependent volume distribution of ash fallout from Phases II (5 samples) and III (3 samples). Each distribution curve represents one ash sample and is an average of three consecutive measurements by laser diffraction. Samples labelled with a symbol were collected simultaneously with the Sun photometry measurements.
when it reached Scotland, although near-vent and distal electrification were likely to be generated by different mechanisms (Harrison et al., 2010). No lightning was observed during Phase III (Petersen, 2010). Ash fallout in Phase III consisted of better sorted grains than in Phase II (interquartile range is 60e120 mm). The size distribution tends to be more uniform when the eruption is driven by magmatic gas exsolution rather than interaction with external water (e.g., Zimanowski et al., 2003). In the drier scenario, the grain size distribution at a given location is better sorted because much finer grains no longer form clusters and are transported further. The upper limit of ash grain size was fairly similar between Phases II and III; this was most likely controlled by the settling velocity, where coarser grains were removed from the plume closer to the vent. The change in grain sizes from a high proportion of fines in Phase II to a coarser population in Phase III occurred despite the fact that Phase III was sampled at a greater distance. This suggests that the total content of fine ash decreased markedly between the two phases.
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5. Conclusions We present unique near-source measurements of suspended and deposited particle matter from the eruptive plume of Eyjafjallajökull. Phase I of the eruption (basaltic lava fountaining) had high concentrations of very fine particles (0.2e0.4 mm), which are soluble particles such as sulphate salts, and water droplets with dissolved components (Ilyinskaya et al., in preparation). Phase II was highly explosive and associated with extensive melting of the glacial cover; it is highly likely that interaction with external water contributed to the extensive fragmentation of the magma. Phase III plume was distinctively ‘drier’ and dominated by smaller intermittent explosions with quiescent intervals. Suspended aerosol mass in Phases II and III was multimodal, with enhanced number of very fine aerosol (<0.2 mm), as well as a coarse size mode (>1 mm). Phase II plume had very high concentrations of ash sized <1 mm. The ratio of coarse/fine particles showed strong short-interval fluctuations, indicative of the fluctuations in the fragmentation energy. These fluctuations did not appear to significantly affect the number concentration of aerosol <0.4 mm which is most likely generated through gas condensation. Drop in fragmentation energy during Phase III produced a strongly bimodal particle size distribution, with much smaller amounts of <1 mm ash. This agreed very well with far-field measurements of the plume (Schumann et al., 2010). The near-source ash fallout during Phase II was poorly sorted and included a large amount of <1 mm ash. The high water content and electric charge in the plume are believed to have enhanced the deposition of the fine ash through particle aggregation. The fallout during Phase III was much better sorted and lacked <1 mm ash, attributed to limited grain aggregation in the drier plume. The implication of this is that a larger proportion of fine ash may be transported further away from the volcano; this is supported by aircraft measurements in the far-field Phase III plume by Schumann et al. (2010). Near-source measurements are invaluable for ground truthing of satellite data and meaningful interpretations of far-field data. Our results combine measurements of both the suspended aerosol mass and near-source fallout and give insights into the dynamic eruptive behaviour. Acknowledgements EI is grateful to the Gates Cambridge Trust for research funding and the Institute of Earth Sciences, University of Iceland for fieldwork support. VIT and CO acknowledge generous support via NERC grant NE/F001487/1 and the National Centre for Earth Observation “Dynamic Earth and Geohazards” project (http://comet.nerc.ac.uk/). Alasdair Mac Arthur, Christopher MacLellan and the NERC Field Spectroscopy Facility are thanked for arranging the rapid deployment of the MICROTOPS II. Appendix. Supplementary data Supplementary data related to this article can be found online, at doi:10.1016/j.atmosenv.2011.03.017. References Ångström, A., 1964. The parameters of atmospheric turbidity. Tellus 16, 64e75. Ansmann, A., Tesche, M., Groß, S., Freudenthaler, V., Seifert, P., Hiebsch, A., Schmidt, J., Wandinger, U., Mattis, I., Müller, D., Wiegner, M., 2010. The 16 April 2010 major volcanic ash plume over central Europe: EARLINET lidar and AERONET photometer observations at Leipzig and Munich, Germany. Geophys. Res. Lett. 37 (L13810). doi:10.1029/2010GL043809 2010.
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Schumann, U., Weinzierl, B., Reitebuch, O., Schlager, H., Minikin, A., Forster, C., Baumann, R., Sailer, T., Graf, K., Mannstein, H., Voigt, C., Rahm, S., Simmet, R., Scheibe, M., Lichtenstern, M., Stock, P., Rubal, H., Schauble, D., Tafferner, A., Rautenhaus, M., Gerz, T., Ziereis, H., Krautstrunk, M., Mallaun, C., Gayet, J.-F., Lieke, K., Kandler, K., Ebert, M., Weinbruch, S., Stohl, A., Gasteiger, J., Olafsson, H., Sturm, K., 2010. Airborne observations of the Eyjafjalla volcano ash cloud over Europe during airspace closure in April and May 2010. Atmos. Chem. Phys. Discuss. 10, 22131e22218. doi:10.5194/acpd-10-22131-2010. Sparks, R.S.J., Bursik, M.I., Carey, S.N., Gilbert, J.S., Glaze, L.S., Sigurdsson, H., Woods, A.W., 1997. Volcanic Plumes. John Wiley & Sons, Chichester. 574 p.. Strachan, S., 2001. A Geophysical Investigation of the Eyjafjallajökull Glaciovolcanic System, South Iceland, Using Radio Echo Sounding. University of Edinburgh, PhD thesis (unpublished), 200 p.. Sturkell, E., Einarsson, P., Sigmundsson, F., Hooper, A., Ófeigsson, B.G., Geirsson, H., Ólafsson, H., 2010. Katla and Eyjafjallajökull volcanoes. Dev. Quaternary Sci. 13, 5e21. doi:10.1016/S1571-0866(09)01302-5. Tsanev, V.I., Mather, T.A., 2007. MicrotopsII Inverse: Software Package for Retrieving Aerosol Columnar Size Distributions Using MicrotopsII Data Available from: http://www.fsf.nerc.ac.uk 380p.
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Short communication
Near-source observations of aerosol size distributions in the eruptive plumes from Eyjafjallajökull volcano, MarcheApril 2010 E. Ilyinskaya a, *, V.I. Tsanev a, b, R.S. Martin c, d, C. Oppenheimer a, J. Le Blond a,1, G.M. Sawyer a, M.T. Gudmundsson e a
Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UK Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK School of Biological and Chemical Sciences, Queen Mary, University of London, London, E1 4NS, UK d Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK e Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavík, Iceland b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2010 Received in revised form 25 February 2011 Accepted 2 March 2011
Near-source observations of aerosol size distributions (<15 km from the vent) were made during three eruptive phases of Eyjafjallajökull in 2010: Phase I - basaltic lava fountaining; Phase II - andesitic phreatomagmatic explosions (mass discharge rate of the order of 105 kg s1); Phase III - andesitic magmatic explosions (mass discharge rate of the order of 103 kg s1). Two methods were used, photometric measurements of suspended aerosol mass (with size distributions retrieved by inverse modelling); and sampling of ash fallout during Phases II and III (with size distributions measured by laser diffraction). The suspended aerosol in Phase I plume was dominated by particles sized <0.4 mm in diameter. During Phase II, the suspended aerosol mass contained a high number of fine (<1 mm) particles (an internal mixture of soluble aerosol and very fine ash) and coarse (>1 mm) ash particles. The number ratio of fine and coarse particles fluctuated strongly within short intervals of time, indicative of changes in the fragmentation energy. The near-source ash fallout was poorly sorted (interquartile range 16e80 mm); ash grains sized <1 mm contributed up to 7% of total volume. The high water content and electrical charge in the plume are believed to have enhanced the deposition through particle aggregation. In Phase III the eruption became drier and less explosive. The suspended aerosol mass was strongly bimodal with high particle abundances at w0.2 and 1e4 mm. The ash fallout was better sorted (interquartile range 60e120 mm; no grains <1 mm), attributed to limited particle aggregation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Volcanic plume Aerosol Ash Sun photometry Eyjafjallajokull
1. Introduction The Eyjafjallajökull eruption in Iceland in 2010 disrupted aviation and revealed gaps in operational capacity for measurement and forecasting of volcanic plume dispersion. A key requirement for the modelling of plume trajectory is the particle size distribution at source. Here we present the measurements of size distributions in near-source volcanic aerosol (<15 km from the vent) collected between April 1 and April 24, 2010 (Fig. 1). ‘Aerosol’ is used here as an umbrella term for all particles transported in the volcanic plume, while ‘ash’ refers only to silicate particles. * Corresponding author. Present address: Icelandic Meteorological Office, Bustadavegi 9, 150 Reykjavik, Iceland. E-mail address: [email protected] (E. Ilyinskaya). 1 Present address: Department of Mineralogy, Natural History Museum, London, SW7 5BD, UK. 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.03.017
Eyjafjallajökull (63.63 N 19.65 W; 1670 m a.s.l.) is an icecovered volcano surmounted by a 2500 m-diameter summit caldera where the thickness of the glacial cover reaches 250 m (Strachan, 2001). Eyjafjallajökull has erupted four times in the last 1400 years (the previous eruption was in 1821e23), all of which have been of low magnitude (w0.1 km3 of erupted material, Sturkell et al., 2010). Eruptions of Eyjafjallajökull pose a great hazard due to the proximity of populated centres and, as shown recently, may lead to severe disruption of international air traffic. The 2010 eruption continued for more than two months, and in the first six weeks it exhibited three distinct phases of activity (Gudmundsson et al., 2010): (I) Basaltic fissure eruption on the volcano flank (no glacial cover) started on March 20. The eruption included lava fountaining and flows. The associated plume was transparent in the daytime and visibly ash-poor. This phase ceased on April 12.
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(II) Ash-rich eruption at the summit crater started on April 14. This phase was associated with extensive melting of glacial ice. Between April 14e17, phreatomagmatic explosions produced an opaque, dark 4e9 km high plume with abundant ash of intermediate composition (w58 wt% SiO2; Óskarsson, 2010). Ash fallout thickness reached a few centimetres on the lowlands south of the volcano. The plume was advected by upper tropospheric and lower stratospheric northwesterly winds and carried towards Europe, where it caused unprecedented air traffic disturbance. The eruptive plume was monitored using satellites (e.g., MODIS, OMI) and sensors across Europe (including EARLINET LIDARs, the German ceilometer network, AERONET stationary CIMEL Sun photometers, ozone sondes; Ansmann et al., 2010; Flentje et al., 2010; Schumann et al., 2010). (III) Mildly explosive eruption between April 18 and May 5, with much reduced ash production. Explosions were intermittent with quiescent intervals. The plume was dense and white, commonly reaching a height of w3 km above the crater. Incandescence in the crater became visible around April 21, and a lava flow began around the same time (Petersen, 2010). Following Phase III, the explosivity and ash production increased again; the eruption continued until May 22 alternating between ash-rich and ash-poor stages. This activity period is not covered by this report. 2. Methods 2.1. Sun photometry Microtops II Sun photometer measures the solar radiance in five spectral bands centred at 380, 440, 675, 879 and 1020 nm
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(bandwidth 10 nm, except the 380 nm channel where bandwidth is 4 nm) and automatically calculates the aerosol optical thickness (AOT) (Morys et al., 2001). The hand-held instrument is compact and extremely well suited for field measurements of volcanic plumes near to the source. Microtops II measurements are taken in a rapid sequence at a rate of 3 s. The measurements are then automatically collated in scans; one scan consists of 32 measurements from each of the five channels, yielding an overall sampling rate of w0.3 Hz. To investigate the aerosol properties of a plume, it is necessary to observe the Sun both through the plume and in the absence of plume (i.e., background). The aerosol optical thickness of the plume is estimated by subtracting the background AOT from the AOT measured through the plume (i.e., plume þ background). These background measurements represent the minimum AOT across all wavelengths, and typically the plume AOT measurements are >>10 background AOT measurements (Fig. 1). Field measurement details are listed in Table 1 and the recorded AOT scans are reported in the Appendix. The standard deviation of AOT in each individual scan is negligibly small. Therefore we group individual scans into optically homogenous ‘scan groups’ (listed in Appendix) and evaluate the mean and standard deviations for each group. These values are used in inversion modelling. This methodology has been previously used to study volcanic plumes (e.g., Mather et al., 2004; Martin et al., 2009). The aerosol number, surface and volume distributions were retrieved by inverse modelling of the measured AOT in each scan group (software by Tsanev and Mather, 2007). The optimal retrieval, i.e., the best match between measured AOT and AOT calculated from the retrieved size distribution, is achieved with diameters 0.16e8.0 mm following the procedure proposed by King et al. (1978). The algorithm is explained in detail by Tsanev and Mather (2007). The main sources of error in inversion modelling of spectral optical depths are the uncertainty in the index of
Fig. 1. Map showing the eruptive centres in Eyjafjallajökull, the direction of plume transport (approximate locations), and the measurement/sampling points. AOT - photometric measurements of aerosol optical thickness; BG - background AOT measurements; (*) - ash samples. Exact times and locations of AOT measurements are listed in Appendix. Base map courtesy of the National Land Survey of Iceland.
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Table 1 Number of AOT scans and ash fallout samples. Time of AOT measurements is reported in local time. During Phase II, the plume was mostly opaque so that AOT measurements had to be made at the plume’s edge where the Sun was clearly visible. Approximate age of ash fallout samples is given in hours. Phase Date
Sun photometry
Ash fallout
n of scans Local time (hh:mm) n of samples Sample age (h) I II III
April April April April April
1 16 17 23 24
180 e 90 220 e
14:30e15:20 e 17:50e19:30 12:20e13:30 e
No ash 2 3 1 2
e <12 0.25e12 <12 4e8
refraction, and the assumption that all particles are spherical and exist as an internal mixture. The errors of the retrieval are estimated analytically by King et al. (1978) and numerically by Jorge and Ogren (1996). A difference of 0.1 in the real part of the refractive index was found to cause up to 82%, 30% and 32% error in the total particle number, surface area and volume, respectively (Jorge and Ogren, 1996). However, the general shape of the size distribution was retained. The index of refraction m was selected based on results discussed in Section 2 and available data on the ash composition (Óskarsson, 2010) as follows. Phase I - 1.44 þ 0.0i, a value applicable for an atmospheric mixture of water and sulphate (Krotkov et al., 1997); the composition of Phase I aerosol was measured independently (Ilyinskaya et al., in preparation). Phase II - 1.55 þ 0.01i, a plume rich in trachyandesitic and poorly porphyritic ash (Horwell, 2007). Retrievals for Phase II were also made with m values of 1.50 þ 0.01i and 1.55 þ 0.015i with negligible impact on the shape of the size distribution. Phase III - 1.55 þ 0.001i; due to much decreased ash content, the plume was assumed to be much less absorbing than in Phase II. Retrievals for Phase III were also made with m values of 1.50 þ 0.001i and 1.55 þ 0.0015i with negligible impact on the size distribution. 2.2. Ash sampling Ash fallout samples (Table 1) were collected by laying out pristine polypropylene bags, and by brushing off the few top millimetres off thick layers that had collected on man-made surfaces (e.g., bridge railings). All collected ash samples were presumably non-weathered, as they were dry and collected soon after deposition. The sample bags were sealed and kept dry. The grain size distribution of each sample was determined by laser diffraction using a Malvern Mastersizer (Hydro Mu2000 with ultrasonics) at the Department of Geography, University of Cambridge (method by Horwell, 2007).
Fig. 2. Average total optical thickness of the aerosol in three eruptive phases plotted as a function of wavelength (l). The total optical thickness in Phase I is an average of 180 measurements in 27 scan groups; Phase II - 90 measurements in 15 scan groups; Phase III - 220 measurements in 9 scan groups. Background optical thickness has been subtracted from each wavelength; standard error is evaluated from all scan groups in each Phase.
shortest (380e440 nm) and the longest (1020 nm) of the measured wavelengths, indicative of a bimodal size distribution (King et al., 1978). The Ångström equation (Ångström, 1964) s(l) ¼ b/la offers a simple, empirical framework to interpret the results shown in Fig. 2. A possible source of error is the assumption that the background AOT is known precisely; however in volcanic plumes, the background AOT is much smaller (<<10) than the plume AOT and does therefore not influence the results significantly. The scatter plots a vs. b of the estimated Ångström parameters are presented in Fig. 3. The parameter a represents particle size: a ¼ 0 suggests aerosol dominated by coarse particles, while a ¼ 1e2 indicates mainly fine particles (Baltensperger et al., 2003). The parameter
3. Aerosol optical thickness The wavelength-dependent AOT, s(l), varied between the three eruptive phases (Fig. 2). Phase I plume had the greatest extinction in the shortest wavelengths; and its s(l) was typical of ash-free volcanic plumes (Watson and Oppenheimer, 2000, 2001; Mather et al., 2004). This type of s(l) usually indicates either Junge (number of particles decreases near-linearly with size) or twoslope size distributions (King et al., 1978). The Phase II plume had a more flattened AOT with the extinction increasing slightly with wavelength. This indicates that the plume had a large number of coarse particles, consistent with the high ash content of the plume observed during Phase II. Size distributions of particles are retrievable provided that the AOT shows wavelength-dependence (King et al., 1978). Phase III showed enhanced extinction in the
Fig. 3. Ångström parameters (a and b) calculated for the Phases I, II and III from the measured AOT (individual scans). Phase II is divided into 3 time-dependent subgroups which have distinct a and b-values. Background measurements for each Phase are represented by (þ) symbols.
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b equals s(l ¼ 1 mm) and increases with the number of particles. The plume in Phase I had the largest a values (between 1.6 and 2.5), revealing a plume dominated by relatively fine particles (<1 mm); and b were negatively correlated with a. A similar trend has been observed in other ash-free plumes (e.g., Mather et al., 2004). AOT during Phase II reveals a from about 0.5 to 0 which indicates a higher abundance of coarse particles. Three time-dependent ‘clusters’ (a, b, c) are distinguishable in Phase II, indicative of strong fluctuations in the particle number. The Phase III a value was w0 and had a weak positive correlation with b. Aerosol in Phase III had much lower b-values than during Phase II, consistent with the plume being less opaque. Background measurements had low b-values with large fluctuations in a. The plume in Phase III was progressively moving away due to wind shift, resulting in a weaker plume signal - a decrease in b and more scattered a. 4. Aerosol size, volume and surface area distribution 4.1. Suspended aerosol Retrieval results are presented in Fig. 4, illustrating the differences in aerosol properties between the three eruptive phases. Table 2 shows ratios of particle numbers with different diameters, in order to illustrate the variation in relative abundances of fine and coarse size modes. Phase I was dominated by the finest particles (w0.1e0.2 mm, Table 2, Fig. 4). This agrees well with distributions measured in other ash-free plumes, which were mainly composed of water droplets and sulphate (e.g., Watson and Oppenheimer, 2000, 2001; Porter et al., 2002). The difference in particle number distribution between Phases II and III is very intriguing. The number of the finest particles (<0.4 mm) is very similar between the two Phases, but Phase II plume also contains a large number of particles enhanced in every size bin above 0.4 mm. This indicates that the eruptive plume of Phase II contained a large quantity of <1 mm ash grains, reflecting very high fragmentation energy (due to the interaction of magma with external water). Presence of <1 mm ash was confirmed by laser diffraction analysis of the ash fallout. The explosivity of the eruption dropped in Phase III, which was reflected not only in the decreased abundance of <1 mm ash but also in the mass discharge rate (see Section 4.2). The number ratio of fine and coarse particles in Phase II fluctuated between 2 and 15 within short intervals of time (Table 2). It is noteworthy that the number of fine particles was relatively stable (within an order of magnitude) and the significant fluctuations (1e2 orders of magnitude) occurred only in the coarse mode. This highlights dynamic changes in the fragmentation energy of the eruption. The fluctuations in the fragmentation energy appear, however, to have had little influence on the nucleation rate of the aerosol (i.e., the number density of the finest mode). Phase III had a bimodal particle distribution (Fig. 4), with higher fine/coarse aerosol number ratio than Phase II (Table 2). There is a very distinct particle number minimum between 0.2 and 1 mm; this strongly indicates that the fine and the coarse mode have different chemical composition (e.g., Greenfield, 1957; Wang et al., 1978). Aerosol particles which nucleate and grow through gas absorption and condensation tend to fall in a narrow size mode (accumulation mode, <1 mm), as their growth is intra-competitive. The coarse particle mode (>1 mm) is primarily composed of ash (Ilyinskaya et al., in preparation). Far-field observations of particle size distributions in the Eyjafjallajökull plume were made across Europe (several 100s and 1000s of km from source) by several research groups (Ansmann et al., 2010; Flentje et al., 2010; Harrison et al., 2010; Schumann et al.,
Fig. 4. Size-dependent distribution of mean particle number distribution (dN), surface area (dA) and volume (dV) during three eruptive phases (averaged between all scan groups). Phase II is divided into 3 time-dependent subgroups.
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Table 2 Particle column density N(d) and ratios N(di)/N(dk) measured during the three phases of the eruption estimated for given diameters (mm). Phase II is subdivided into 3 time-dependent subgroups (a, b, c). N(0.37 mm) refers to the columnar number density of fine particles whilst the ratios N(0.37 mm)/N(1.1 mm) and N (0.37 mm)/N(2.0 mm) reflect the local minima between fine and coarse particles, and the relative amount of particles with diameter d ¼ 1 mm. Phase
N(0.37 mm)[cm2]
I II-a II-b II-c III
(1.4 (3.1 (7.8 (7.5 (5.3
0.8) 2.4) 0.1) 3.6) 3.9)
109 107 107 107 106
N(0.37 mm)/N(1.1 mm)
N(0.37 mm)/N(2.0 mm)
240 1.0 0.77 0.93 7.6
6100 3.5 2.1 15 6.2
2010). Measurements made by the research aircraft Falcon of a four-day old Phase II plume (Schumann et al., 2010) revealed that the particle number distribution was close to a Junge-type distribution. The abundant coarse particles (>2 mm) which were measured near the source had mostly settled out and the plume was composed of soluble aerosol (water droplets and/or sulphate). No particles larger than 30 mm were found in the far-field plume, and the ash mass concentration was found to be below the newly established aviation safety limit of 0.2 mg m3 (Schumann et al., 2010). Measurements of Phase III plume on May 2 over the North Atlantic (w450 km from source) revealed a significantly different particle size distribution compared to Phase II (Schumann et al., 2010). In agreement with the near-source observations the particle number distribution was bimodal, with a large number of fine (soluble) particles and a coarser (ash) mode of 4e10 mm. 4.2. Ash fallout A rough estimate of mass discharge of the plume for Phases II and III at the time of the sampling was estimated from plume height. At 18:00e18:20 on 17 April the plume was 6 km high (observations from inspection flight), while it was 3e3.5 km high on April 23e24 (Iceland Met Office radar and inspection flight data). Using empirical formulas relating plume height and magma discharge (Sparks et al., 1997; Mastin et al., 2009) and magma density of 2500 kg m3 we obtained a discharge rate of about 1 105 kg s1 for the time of measurement on 17 April and 2e3 103 kg s1 for 23e24 April. The first value was probably an underestimate since the plume was very dark in colour, heavily loaded with tephra and partly collapsing on 17 April. If we assume that ash density does not significantly vary with grain size, the measured grain volume distribution in the fallout samples (Fig. 5) is indicative of the mass distribution. Ash fallout during Phase II was fairly poorly sorted (the interquartile range of grain size distribution was 16e80 mm). A large proportion consisted of small-sized grains (on average, 30% was <18 mm; Fig. 5). The terminal fall velocity for this grain size fraction (from 2000 to 0 m height) is w0.02 m s1 (Rose and Durant, 2009), which would give an atmospheric residence time of 27 h. This is clearly much longer than the age of the ash from Phase II considering the sampling distance from the vent (<11 km) and the prevailing wind speed (5e10 m s1; Petersen, 2010). However, volcanic ash has previously been found to settle out much quicker than predicted (summary in Rose and Durant, 2009). Ash particles in water-rich plumes (such as the phreatomagmatic Phase II plume) tend to aggregate by surface water tension, and adsorption of liquid water increases grain density (e.g., Delmelle et al., 2005), thereby increasing the settling velocity. Electrical charging (particularly enhanced in phreatomagmatic eruptions; James et al., 2008) also causes attraction and aggregation of fine particles. Volcanic lightning was frequent during Phase II and positive charge was measured in the plume
Fig. 5. Size-dependent volume distribution of ash fallout from Phases II (5 samples) and III (3 samples). Each distribution curve represents one ash sample and is an average of three consecutive measurements by laser diffraction. Samples labelled with a symbol were collected simultaneously with the Sun photometry measurements.
when it reached Scotland, although near-vent and distal electrification were likely to be generated by different mechanisms (Harrison et al., 2010). No lightning was observed during Phase III (Petersen, 2010). Ash fallout in Phase III consisted of better sorted grains than in Phase II (interquartile range is 60e120 mm). The size distribution tends to be more uniform when the eruption is driven by magmatic gas exsolution rather than interaction with external water (e.g., Zimanowski et al., 2003). In the drier scenario, the grain size distribution at a given location is better sorted because much finer grains no longer form clusters and are transported further. The upper limit of ash grain size was fairly similar between Phases II and III; this was most likely controlled by the settling velocity, where coarser grains were removed from the plume closer to the vent. The change in grain sizes from a high proportion of fines in Phase II to a coarser population in Phase III occurred despite the fact that Phase III was sampled at a greater distance. This suggests that the total content of fine ash decreased markedly between the two phases.
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5. Conclusions We present unique near-source measurements of suspended and deposited particle matter from the eruptive plume of Eyjafjallajökull. Phase I of the eruption (basaltic lava fountaining) had high concentrations of very fine particles (0.2e0.4 mm), which are soluble particles such as sulphate salts, and water droplets with dissolved components (Ilyinskaya et al., in preparation). Phase II was highly explosive and associated with extensive melting of the glacial cover; it is highly likely that interaction with external water contributed to the extensive fragmentation of the magma. Phase III plume was distinctively ‘drier’ and dominated by smaller intermittent explosions with quiescent intervals. Suspended aerosol mass in Phases II and III was multimodal, with enhanced number of very fine aerosol (<0.2 mm), as well as a coarse size mode (>1 mm). Phase II plume had very high concentrations of ash sized <1 mm. The ratio of coarse/fine particles showed strong short-interval fluctuations, indicative of the fluctuations in the fragmentation energy. These fluctuations did not appear to significantly affect the number concentration of aerosol <0.4 mm which is most likely generated through gas condensation. Drop in fragmentation energy during Phase III produced a strongly bimodal particle size distribution, with much smaller amounts of <1 mm ash. This agreed very well with far-field measurements of the plume (Schumann et al., 2010). The near-source ash fallout during Phase II was poorly sorted and included a large amount of <1 mm ash. The high water content and electric charge in the plume are believed to have enhanced the deposition of the fine ash through particle aggregation. The fallout during Phase III was much better sorted and lacked <1 mm ash, attributed to limited grain aggregation in the drier plume. The implication of this is that a larger proportion of fine ash may be transported further away from the volcano; this is supported by aircraft measurements in the far-field Phase III plume by Schumann et al. (2010). Near-source measurements are invaluable for ground truthing of satellite data and meaningful interpretations of far-field data. Our results combine measurements of both the suspended aerosol mass and near-source fallout and give insights into the dynamic eruptive behaviour. Acknowledgements EI is grateful to the Gates Cambridge Trust for research funding and the Institute of Earth Sciences, University of Iceland for fieldwork support. VIT and CO acknowledge generous support via NERC grant NE/F001487/1 and the National Centre for Earth Observation “Dynamic Earth and Geohazards” project (http://comet.nerc.ac.uk/). Alasdair Mac Arthur, Christopher MacLellan and the NERC Field Spectroscopy Facility are thanked for arranging the rapid deployment of the MICROTOPS II. Appendix. Supplementary data Supplementary data related to this article can be found online, at doi:10.1016/j.atmosenv.2011.03.017. References Ångström, A., 1964. The parameters of atmospheric turbidity. Tellus 16, 64e75. Ansmann, A., Tesche, M., Groß, S., Freudenthaler, V., Seifert, P., Hiebsch, A., Schmidt, J., Wandinger, U., Mattis, I., Müller, D., Wiegner, M., 2010. The 16 April 2010 major volcanic ash plume over central Europe: EARLINET lidar and AERONET photometer observations at Leipzig and Munich, Germany. Geophys. Res. Lett. 37 (L13810). doi:10.1029/2010GL043809 2010.
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