Highresolution size distributions and emission fluxes of trace elements

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, B08206, doi:10.1029/2012JB009487, 2012

High-resolution size distributions and emission fluxes of trace elements from Masaya volcano, Nicaragua R. S. Martin,1 G. M. Sawyer,1,2 J. A. Day,3 J. S. LeBlond,4,5 E. Ilyinskaya,6 and C. Oppenheimer1 Received 28 May 2012; revised 10 July 2012; accepted 12 July 2012; published 25 August 2012.

[1] Active volcanoes are significant natural sources of trace elements to the atmosphere yet the processes of emission and the impacts of deposition into terrestrial and aquatic environments remain poorly understood. The varying contributions of volatile degassing and magma ejection (i.e., spattering, spraying, extrusion and fragmentation) to the emission of trace elements from Masaya volcano (Nicaragua) are investigated through measurement of high-resolution trace element size distributions using cascade impactors in 2009 and 2010. The volatile elements (e.g., As, Cd, Tl, Cu, Pb, Zn) are strongly correlated across the size distribution and exist in the plume primarily as fine sulfate (0.6 mm diameter) with lesser amounts transported as coarse sulfates (3.5 mm diameter) and coarse chlorides (11 mm diameter). These results suggest that trace elements released from the magma as chlorides react rapidly with H2SO4 in the plume to form sulfates. In contrast, the non-volatile elements (e.g., alkali earth and rare earth) exist primarily as particles in the 1–10 mm range and show no correlation with sulfate, chloride or the volatile elements, suggesting that they are emitted primarily by magma ejection. Trace element emission fluxes from Masaya in 2010 were estimated using filter pack measurements, with emissions of Cu, Zn, As, Tl, Rb and Cd each in excess of 10 kg d1. These emission fluxes are similar to those measured in 2000–2001 suggesting notable decadal stability in the emission of trace elements from Masaya. Citation: Martin, R. S., G. M. Sawyer, J. A. Day, J. S. LeBlond, E. Ilyinskaya, and C. Oppenheimer (2012), High-resolution size distributions and emission fluxes of trace elements from Masaya volcano, Nicaragua, J. Geophys. Res., 117, B08206, doi:10.1029/2012JB009487.

1. Introduction [2] Active volcanoes are significant natural sources of trace elements to the atmosphere yet the processes of emission and the impacts of deposition into terrestrial and aquatic environments remain poorly understood [Calabrese et al., 2011]. The emission of trace elements, defined here as elements with concentrations of 103 for volatile elements (e.g., As, Cd, Pb, Se, Tl) relative to nonvolatile elements (e.g., Ca, Ba, Sr, La, Al, Be) [e.g., Zoller et al., 1983; Taran et al., 1995; Zreda-Gostynska et al., 1997; Aiuppa et al., 2003; Moune et al., 2010; Mather et al., 2012]. [5] Studies of trace elements in volcanic emissions have focused mostly on fumarolic condensates and filter pack measurements of volcanic plumes. Fumarolic condensates offer important direct insights into the degassing behavior and transport of trace elements through the hydrothermal system. The chemical compositions of fumarolic condensates often vary across the fumarole field as a function of temperature with the highest temperature fumaroles most enriched (i.e., highest EF) in volatile trace elements [e.g., Toutain et al., 1990; Symonds et al., 1992; Taran et al., 1995; Gilbert and Williams-Jones, 2008]. Analyses of fumarolic condensates however are difficult to scale to emission fluxes and filter pack measurements are more appropriate for flux estimation [e.g., Hinkley et al., 1999; Aiuppa et al., 2003; Moune et al., 2010]. Element ratios to SO2 in the volcanic plume (which sums emissions from a range of point and diffuse sources) are measured and then multiplied by the SO2 flux, which is typically estimated using ultraviolet spectroscopy [e.g., Edmonds et al., 2003; Galle et al., 2003; Bluth et al., 2007; Nadeau and Williams-Jones, 2009]. [6] The environmental behavior of trace elements in volcanic emissions depends on their solubility in water and the size distribution of particles that they occur in. Trace elements hosted within salts (i.e., high EF) are more soluble and thus likely more susceptible to wet deposition and transfer to the aquatic environment than trace elements hosted within silicates (i.e., low EF) [Mather et al., 2012]. The solubility of trace elements therefore parallels volatility. An important control on dry deposition is the size distribution of particles. The Stokes’ settling velocity of particles increase with the square of the particle size with a 1 mm (diameter) particle settling in still air at 104 m s1 [e.g., Hinds, 1999]. However, the high concentrations of particles measured 2 km below the plume of Etna [Allen et al., 2006] suggests that the actual rate of dry deposition may be enhanced further by complex wind fields around volcanoes. While few direct measurements of trace element deposition fluxes are available [Calabrese et al., 2011], spatial and temporal variability in trace element deposition can be usefully assessed, at least qualitatively, using lichens, tree leaves and grasses as bio-

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indicators [Varrica et al., 2000; Bellomo et al., 2007; Watt et al., 2007; Martin et al., 2009b, 2010a, 2012]. [7] Composition resolved size distributions measured using scanning electron microscopic (SEM) imaging with energy dispersive X-ray (EDX) analyses of filter pack substrates [e.g., Martin et al., 2008, 2009a] are limited by the imaging and analytical resolution and the uncertain relationship between size distributions on the filter and in the plume. Specific issues are overlapping particles on the filter surface and the challenge of obtaining a representative ensemble from a small number of single particle analyses. Automated analyses address the latter problem but may exacerbate the former. Spectroscopic methods such as Sun photometry [Mather et al., 2004; Ilyinskaya et al., 2011] are used to estimate the overall size distribution of airborne particles but remote sensing of particle composition is not currently possible. These limitations mean that compositionresolved size distributions are best measured directly, by aerodynamic segregation of particles at the point of sampling followed by subsequent laboratory analysis of the deposits. [8] Segregation of particles into a fine and coarse fraction is possible using filtration [e.g., Allen et al., 2000, 2002, 2006] but higher size resolution is offered by cascade impaction [Hinds, 1999]. Previous studies have used a range of cascade impactors with varying size resolution, flow rate, power requirements and portability to sample particles in volcanic emissions. Measurements in the quiescent plume (i.e., ash-poor and white or transparent in appearance) of Masaya volcano (Nicaragua) using cascade impactors with 5 size intervals (manufactured by Sioutas; from >2.5 mm to 18 mm to >0.054 mm) and 14 size intervals (nano-MOUDI; from >18 mm to >0.01 mm) suggest high concentrations of 1 mm Mg-Ca chloride [Mather et al., 2003; Martin et al., 2011b]. Similar observations were made at Láscar and Villarrica [Mather et al., 2004] suggesting commonalities between different volcanoes. In contrast, particles in quiescent plumes from Etna [Martin et al., 2008] are coarser (i.e., >1 mm Na-K sulfate) with variations between the different summit vents, while particles in the quiescent plume of Erebus [Ilyinskaya et al., 2010], and those in the plume from the 2010 fissure eruption of Eyjafjallajökull [Ilyinskaya et al., 2012], are finer and dominated by 1 mm) Pb and Al were also reported at Erebus [Ilyinskaya et al., 2010] although the concentrations of most other trace elements were below the analytical detection limit. A recent study of the quiescent plume of Halema’uma’u [Mather et al., 2012] used a high flow rate cascade impactor with 5 size intervals (HFI; from >2.5 mm to >0.26 mm), followed by inductively coupled plasma – mass spectrometry (ICP-MS) analysis, to offer the first comprehensive measurements of trace element size distributions in volcanic emissions. This study suggested that volatile trace elements (e.g., As, Cd, Tl) are associated with 1 mm chloride. However, the low size range and resolution of the cascade impactor and the complexity of the Halema’uma’u plume limited the interpretation of trends. [9] In this work, we present results from cascade impactor sampling at Masaya volcano in 2009 using a 14-stage nano-MOUDI impactor (n = 5 samples) and in 2010 using

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Table 1. Sample Details From Masaya Volcano in 2009 and 2010a Sample

Site

Date

Local Time

D (min)

M9/1 M9/2 M9/3 M9/4 M9/5 S10/1 S10/2 S10/3 S10/4 S10/5 F10/1 F10/2 F10/3 F10/4 F10/5 F10/6 F10/7 F10/8 F10/9 F10/10 F10/12 F10/13 F10/14 F10/15 F10/17 F10/18

SCP SCP SCP SCP UW SCP SCP SCP DW SCP SCP SCP SCP SCP SCP SCP SCP SCP SCP SCP SCP UW UW SCP SCP SCP

20/03/09 21/03/09 22/03/09 23/03/09 24/03/09 07/04/10 08/04/10 09/04/10 10/04/10 11/04/10 31/03/10 31/03/10 01/03/10 01/03/10 01/03/10 01/03/10 01/03/10 01/03/10 01/03/10 02/03/10 02/03/10 03/03/10 03/03/10 08/03/10 09/03/10 09/03/10

0900–1250 0900–1310 0900–1240 0900–1220 0900–1210 0930–1700 1100–1700 0930–1700 0800–1000 1230–1800 1140–1240 1140–1210 1425–1515 1425–1515 1425–1555 1555–1655 1715–1835 1715–1835 1715–1815 1010–1610 1010–1610 1505–1550 1505–1550 1800–1900 1930–2000 2010–2040

230 250 220 200 190 450 360 450 120 330 60 30 50 50 90 60 60 60 60 360 360 45 45 60 30 30

a SCP = Sapper car park (i.e., in-plume) UW = Upwind, DW = Downwind. See Figure 1 for sampling sites.

a 5-stage Sioutas impactor (n = 5 samples). Samples were analyzed using ICP-MS for a wide range of trace elements. These new results are combined with ion chromatography + +  (IC) analyses for major ions (SO2 4 , Na , K , Cl ) in the same samples [Martin et al., 2011b]. In addition, filter pack samples collected at Masaya in 2010 (n = 16 samples) are analyzed using ICP-MS, IC and SEM. The main aims of the study are: (i) to assess trace element size distributions in Masaya’s volcanic plume relative to the more abundant

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sulfate and chloride particles, (ii) to infer the various processes of emission by calculation of enrichment factors, and (iii) to offer a new high quality data set of trace element emission fluxes from Masaya. This work reports the first high-resolution size distributions of trace elements in volcanic emissions and provides detailed new insights into the processes of trace element emission.

2. Methodology [10] Masaya (elevation 600 m, 11 59′04″ N, 86 10′06″ W) is a basaltic volcano in Nicaragua that sustains a vigorous and persistent plume from its currently active Santiago crater. Eruptions are rare at Masaya and generally limited to minor vent clearing eruptions which eject minimal amounts of juvenile material [e.g., Duffell et al., 2003]. In contrast, the quiescent gas and aerosol emissions are among the most prodigious of the Central American arc volcanoes [Mather et al., 2006]. Over the last two decades, the total volatile flux (H2O, CO2, SO2, HCl, HF, etc.) from Masaya has varied in the range of 10,000–30,000 Mg d1 [Martin et al., 2010b]. The samples analyzed in this study were collected during two campaigns at Masaya: 20 March–24 March 2009 and 29 March–12 April 2010 (Table 1). The plume was sampled from the Sapper Car Park on the southwest rim of Santiago crater (Figure 1) using a nano-MOUDI impactor in 2009 and a Sioutas impactor in 2010. The Sapper Car Park is frequently exposed to concentrated emissions (e.g., >1 ppm SO2) as prevailing winds in the dry season (December–April) transport the plume to the southwest. Ion chromatographic analyses of these samples are reported in Martin et al. [2011b], with emission fluxes for gases and major ions from the 2009 campaign reported in Martin et al. [2010b]. Filter pack samples were also collected from the Sapper Car Park in 2010 to estimate the emission fluxes of trace elements. In addition, samples were collected from a sheltered location not exposed to plume (M9/5, F10/13, F10/14) or

Figure 1. Aerial imagery showing Masaya volcano (Nicaragua). The nested craters of Santiago (S), Nindiri (N) and San Pedro (SP) are shown, along with Old Masaya (OM) crater to the East. The active degassing vent is found at the bottom of Santiago crater. In-plume sampling was performed from the Sapper car park (SCP) on the southwest rim. Background samples were collected from a sheltered location near the crater rim (M9/5, F10/13 and F10/14) and from 5 km downwind (S10/4). The Main car park (MCP), which is accessible to the public, is typically upwind of the vent. The aerial image is dated 31st March 2010 and was sourced from Google Earth. (Google Earth imagery ©Google Inc. Used with permission.) 3 of 12

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downwind in very dilute plume (S10/4). SO2 fluxes from Masaya were estimated (using the traverse method and ultraviolet spectroscopy) to be 690 Mg d1 during the 2009 campaign and 500 Mg d1 during the 2010 campaign [Martin et al., 2010b, 2011b]. These SO2 fluxes are relatively low compared to those measured at Masaya in 2005 and 2006 (>1000 Mg d1; [Nadeau and Williams-Jones, 2009]) but comparable those measured in 2000 and 2001 [Duffell et al., 2003], consistent with an (approximately) four year cycle in quiescent degassing recognized at Masaya [Williams-Jones et al., 2003]. 2.1. Sampling [11] Cascade impactors size and collect particles through inertial impaction onto a series of stages [Hinds, 1999]. The cut-off diameters for the nano-MOUDI are >19, >11, >6, >3.5, >1.9, >1.1, >0.6, >0.35, >0.19, >0.11, >0.06, >0.035, >0.019, >0.011 mm for a mean measured flow rate of 8.4 L min1. The cut-off diameters for the Sioutas were >2.4, >0.95 and >0.47 for a flow rate of 10 L min1 (an additional stage of >0.24 mm was damaged during an earlier field campaign). PTFE tweezers and nitrile gloves were used to load filters onto each impaction stage (nano-MOUDI: PTFE, 47 mm, 0.2 mm pore size, Sioutas: laminated PTFE, 25 mm, 0.5 mm pore size), and also onto a final 2 h), the pumps were disconnected and the impactor inlets were sealed. The exposed filters were subsequently transferred using PTFE tweezers to polypropylene sample bags. The filter packs [e.g., Martin et al., 2010b] comprised of a polycarbonate particle filter (Millipore, 47 mm, AAWP, pore size 0.8 mm) followed by two base-treated acidic gas filters (Whatman 41 ashless circles impregnated with 5% K2CO3 and 1% glycerol in distilled deionized water). PTFE tweezers and nitrile gloves were used to load and disassemble the filter packs, with exposed filters transferred into polypropylene sample bags. Air was pumped through each filter pack for >30 min by a Capex V2 DE pump powered by a 12V car battery. It was not possible to monitor the flow rate although previous experience in similar field conditions suggests a typical flow rate of 10–20 L min1. [12] Recent studies have tended to use much higher flow rates for impactor (e.g., 100 L min1; [Mather et al., 2012]) and filter pack (e.g., >130 L min1; [Gauthier and Le Cloarec, 1998; Moune et al., 2010]) sampling of trace elements in volcanic plumes. However, earlier studies indicate that the high concentration of trace elements in volcanic emissions allows measurements to be made at much lower flow rates and/or sampling times (e.g., 10–40 L min1 for 1 h; [Hinkley et al., 1999]). This is advantageous in terms of portability and power requirements and also allows for the use of the 14-stage nano-MOUDI impactor to measure high-resolution size distributions.

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2.2. Analysis [13] In a general laboratory environment, the filters were transferred into metal-free polypropylene vials pre-cleaned with ultra-pure deionized distilled (DI) water. The hydrophobic PTFE filters used by the nano-MOUDI and Sioutas were wet with a few drops of propan-2-ol and extracted with 15 ml (2009) or 5 ml (2010) ultra-pure DI water. The polycarbonate particle filters were extracted with 5 ml ultra-pure DI water. The solutions were placed on a mechanical shaker for at least 30 min. The filters were analyzed shortly after + + wards for major ions (i.e., SO2 4 , Cl , Na , K ) (as reported in Martin et al. [2011b]) and then stored at room temperature in an airtight container for approximately two years. The solutions were then analyzed undiluted in triplicate using inductively coupled plasma spectrometry (Perkin Elmer Elan DRC II, University of Cambridge) for a wide range of trace elements (Al, As, Ba, Ca, Cd, Ce, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ho, La, Li, Lu, Mn, Mo, Nd, Pb, Pr, Rb, Se, Sm, Sr, Tb, Tl, Tm, U, V, Y, Yb, Zn). The instrument was calibrated by dilution of standard solutions (BCR-2; 1 ppb As, Cd, Cs, Tl) with most elements showing a recovery close to 100%. The standards were re-analyzed every 10 samples to correct for instrumental drift. A 1 ppb solution of In, Re and Rh was used as an internal standard. Field blanks of each type of filter from each campaign were extracted, stored and analyzed similarly to allow for blank correction. This blank correction was typically small (i.e.,