Balloon-borne measurement of the aerosol size distribution from an Icelandic flood basalt eruption
Introduction
Volcanoes release gases and particles into the atmosphere through continuous degassing or episodic eruptive events, and depending on the injection altitude and emission rate, they can impact both the tropospheric and stratospheric composition and climate (McCormick et al., 1995, Robock, 2000, Schmidt et al., 2012, Solomon et al., 2011). Ash-rich plumes such as that from the Icelandic Eyjafjallajökull eruption in 2010 can lead to widespread disruption of aviation (Spinetti et al., 2013). Ash-poor volcanic plumes may also strongly impact the environment and quality of life due to high concentrations of polluting gases and aerosol particles. Indeed, the recent flood basalt eruption at Holuhraun (31 August 2014–27 February 2015, 1.6 km3 of erupted lava, Gislason et al., 2015) was a major source of sulfur gases and aerosols and caused both local (Gislason et al., 2015) and European-wide (Schmidt et al., 2015) deteriorations to air quality.
Long-lasting flood basalt eruptions are one of the most hazardous volcanic scenarios in Iceland and have had enormous societal and economic consequences across the northern hemisphere (Gudmundsson et al., 2008). One of the best known examples is the Laki eruption (1783-84 CE) (Thordarson and Self, 2003) which led to deaths of >20% of the Icelandic population by environmental pollution and famine, and likely increased European levels of mortality through air pollution by sulfur-bearing gas and aerosol (Grattan, 1998, Taylor et al., 2003, Witham and Oppenheimer, 2004). Potential impacts of such an eruption on modern day Europe have been modeled by Schmidt et al. (2011) who found that PM2.5 aerosol pollution would double causing 142,000 additional cardiopulmonary fatalities in the year following the eruption onset. A Laki-type eruption scenario has been recently included in the UK National Risk Register (UK Cabinet Office, 2013). However, there are still many uncertainties about the source terms of Icelandic flood basalt eruptions that are necessary for atmospheric models and health impact assessments. The 2014–2015 Holuhraun eruption was therefore a unique opportunity to study the near-source composition of an Icelandic flood basalt eruption plume.
Direct measurements of volcanic aerosol (defined here as non-silicate particles such as sulfate) are needed to better constrain the plume sulfur chemistry and particle processes, which together with plume injection height are two key uncertainties in models used to predict the dispersion and air quality impacts from eruptions. Existing in-situ measurements of elevated volcanic plumes mostly involve interception of aged plumes that have already undergone significant chemical and physical evolutions (Marenco et al., 2011; Jégou et al., 2013). Small portable sensors placed on airborne drones or balloons offer new possibilities to characterize volcanic plumes close to source. McGonigle et al. (2008) demonstrated heli-type drone sensing of SO2 and CO2 to determine CO2 fluxes at Vulcano fumarole field (Italy). More recently, Shinohara (2013) deployed a suite of gas sensors on a drone to characterize the plume of Kirishima volcano (Japan) during an eruptive phase where ground-based sampling was too hazardous. Pieri et al. (2013) performed drone as well as balloon-based campaigns to measure gases and ash particles in the eruptive plume of Turrialba volcano (Costa Rica).
Here we present measurements made by a newly developed lightweight optical aerosol counter (LOAC) carried on a meteorological balloon through the near-source Holuhraun eruption plume. By combining size-resolved particle number concentration measurements with meteorological parameters and remote sensing of SO2 flux, we are able to provide some of the key eruption source term information.
Section snippets
Holuhraun and plume conditions
Holuhraun is located northwards of the Vatnajökull ice cap in the largest desert area of Iceland. On January 22nd 2015, visible plumes were emitted from the main crater (Baugur) and several places within the lava field (Fig. 1). It was estimated that ∼90% of the released gas volume was from Baugur. These distinct plumes merged into one main plume which was advected northeastwards. The rising plume visibly changed while being advected, with the upper part turning into a condensed, optically
Balloon instrumentation
The LOAC (Light Optical Aerosol Counter) is an optical particle counter sufficiently lightweight to be carried by a 1000 g meteorological balloon. The instrument contains a laser (650 nm) and measures the intensity of light scattered at two angles, 12° and 60° (Lurton et al., 2014; Renard et al., 2016) to discriminate the particle concentration over 19 size classes from 0.2 μm to 100 μm in diameter. Sampling is driven by a miniature pump (constant flowrate of 2 L min−1) enclosed in the gondola
Results
Fig. 4 shows the total number concentration of particles (TPC) sized between 0.2 μm and 100 μm in diameter, the vertical velocity of the balloon and the relative humidity (RH) as a function of altitude (see online Appendix 2 for size distributions as a function of altitude). On each plot we have determined 6 altitude-dependent Zones with different characteristics (Fig. 4). The 6 distinct Zones were determined based on the correlation between RH and particle number concentration profile. Fig. 5
Plume Height
We compare our plume height observation (2.0–3.1 km a.s.l., with a condensed layer at 2.3–3.1 km a.s.l.) to an independent estimate of the plume height from IASI data on Meteorological Operational (MetOp) Satellite. This novel technique of retrieving plume height from satellite data is described by Carboni et al. (2012) and has been applied to several volcanic eruptions (Carboni et al., 2016). Here we report only a summary of the algorithm. The optimal estimation technique of Rodgers (2000) is
Conclusion
This study demonstrated that the newly developed lightweight balloon-borne aerosol counter, LOAC, is an effective method to detect and characterize aerosol properties in volcanic plumes close to the eruptive source. We were able to determine with great accuracy the altitude of the plume top and bottom, identify and characterize distinct layers in the plume as a result of different hygroscopic phases of the plume based on the measured particle size distribution.
Volcanic plumes are known to
Acknowledgments
The LOAC team of the LPC2E/CNRS warmly thank all the persons from Icelandic Meteorological Office (IMO) and the University of Reykjavik for the support during this field campaign, the Cambridge scientist teams who have accepted our presence in their field campaign.
We acknowledge the ECMWF Archive product and Metview platform that enabled calculation of the backward trajectories. We also acknowledge the Icelandic team from IMO that produced the HARMONIE outputs, especially Bolli Pálmason.
This
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