Tamsin A. Mather

Tropospheric Volcanic Aerosol

Volcanic emissions represent an important source of aerosol to the global troposphere, and have important implications for the Earth’s radiation budget at various temporal and spatial scales. Volcanogenic aerosol can also transport trace metals and other pollutants, with impacts on terrestrial ecosystems and human health. We provide here a primer on the current understanding of the origins and transformations of volcanogenic particles in the troposphere, covering their fluxes, size distribution, composition and morphology, and focusing on sulfur, halogen, and trace metal compounds. Such an understanding is essential to investigations of the atmospheric, environmental and human health impacts of volcanic volatile emissions.

Characterization and evolution of tropospheric plumes from Lascar and Villarrica volcanoes, Chile

Chile, reveal that both are significant and sustained emitters of SO2 (28 and 3.7 kg s � 1 , respectively), HCl (9.6 and 1.3 kg s � 1 , respectively), HF (4.5 and 0.3 kg s � 1 , respectively) and near-source sulfate aerosol (0.5 and 0.1 kg s � 1 , respectively). Aerosol plumes are characterized by particle number fluxes (0.08–4.0 mm radius) of � 10 17 s � 1 (Lascar) and � 10 16 s � 1 (Villarrica), the majority of which will act as cloud condensation nuclei at supersaturations >0.1%. Impactor studies suggest that the majority of these particles contain soluble SO4� . Most aerosol size distributions were bimodal with maxima at radii of 0.1–0.2 mm and 0.7–1.5 mm. The mean particle effective radius (Reff) ranged from 0.1 to 1.5 mm, and particle size evolution during transport appears to be controlled by particle water uptake (Villarrica) or loss (Lascar) rather than sulfate production. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 8409 Volcanology: Atmospheric effects (0370); 8494 Volcanology: Instruments and techniques; KEYWORDS: volcanoes, degassing, aerosol sulphur dioxide, sulphate, Llaima

Size-Resolved Characterisation of Soluble Ions in the Particles in the Tropospheric Plume of Masaya Volcano, Nicaragua: Origins and Plume Processing

We present the first application of a multi-stage impactor to study volcanic particle emissions to the troposphere from Masaya volcano, Nicaragua. Concentrations of soluble SO42−,Cl−, F−, NO3−, K+, Na+,NH4+, Ca2+ and Mg2+ were determined in 11 size bins from ∼0.07 μm to >25.5 μm. The near-source size distributions showed major modes at 0.5μm (SO42−, H+,NH4+); 0.2 μm and 5.0 μm (Cl−) and 2.0–5.0 μm(F−). K+ and Na+ mirrored the SO42− size-resolvedconcentrations closely, suggesting that these were transported primarily asK2SO4 and Na2SO4 in acidic solution, while Mg2+ andCa2+ presented modes in both <1 μm and >1 μm particles. Changes in relative humidity were studied by comparing daytime (transparent plume) and night-time (condensed plume) results. Enhanced particle growth rates were observed in the night-time plume as well as preferential scavenging of soluble gases, such as HCl, by condensed water. Neutralisation of the acidic aerosol by background ammonia was observed at the crater rim and to a greater extent approximately 15 km downwind of the active crater. We report measurements of re-suspended near-source volcanic dust, which may form a component of the plume downwind. Elevated levels ofSO42−, Cl−, F−,H+, Na+, K+ and Mg2+ were observed around the 10 μm particle diameter in this dust. The volcanic SO42− flux leaving the craterwas ∼0.07 kg s−1.

Ozone depletion in tropospheric volcanic plumes

Ground based remote sensing techniques are used to measure volcanic SO2 fluxes in efforts to characterise volcanic activity. As these measurements are made several km from source there is the potential for in-plume chemical transformation of SO2 to sulphate aerosol (conversion rates are dependent on meteorological conditions), complicating interpretation of observed SO2 flux trends. In contrast to anthropogenic plumes, SO2 lifetimes are poorly constrained for tropospheric volcanic plumes, where the few previous loss rate estimates vary widely (from 99% per hour). We report experiments conducted on the boundary layer plume of Masaya volcano, Nicaragua during the dry season. We found that SO2 fluxes showed negligible variation with plume age or diurnal variations in temperature, relative humidity and insolation, providing confirmation that remote SO2 flux measurements (typically of approximate to500-2000 s old plumes) are reliable proxies for source emissions for ash free tropospheric plumes not emitted into cloud or fog.

Nitric acid from volcanoes

Abstract Atmospheric cycling of nitric acid and other nitrogen-bearing compounds is an important biogeochemical process, with significant implications for ecosystems and human health. Volcanoes are rarely considered as part of the global nitrogen cycle, but here we show that they release a previously unconsidered flux of HNO3 vapour to the atmosphere. We report the first measurements of nitric acid vapour in the persistent plumes from four volcanoes: Masaya (Nicaragua); Etna (Italy); and Villarrica and Lascar (Chile). Mean near-source volcanic plume concentrations of HNO3 range from 1.8 to 5.6 μmol m−3, an enrichment of one to two orders of magnitude over background (0.1–1.5 μmol m−3). Using mean molar HNO3/SO2 ratios of 0.01, 0.02, 0.05, and 0.07 for Villarrica, Masaya, Etna, and Lascar respectively, combined with SO2 flux measurements, we calculate gaseous HNO3 fluxes from each of these volcanic systems, and extend this to estimate the global flux from high-temperature, non-explosive volcanism to be ∼0.02–0.06 Tg (N) yr−1. While comparatively small on the global scale, this flux could have important implications for regional fixed N budgets. The precise mechanism for the emission of this HNO3 remains unclear but we suggest that thermal nitrogen fixation followed by rapid oxidation of the product NO is most likely. In explosive, ash-rich plumes NO may result from, or at least be supplemented by, production from volcanic lightning rather than thermal N fixation. We have calculated NO production via this route to be of the order of 0.02 Tg (N) yr−1.

High-temperature mixtures of magmatic and atmospheric gases

Recent measurements of BrO, NOx, and near-source sulfate in volcanic plumes suggest that volcanic vents might not simply act as point sources of emissions into the troposphere, but may also act as high-temperature reaction sites where mixtures of magmatic and ambient atmospheric gases may combine, giving new and previously unexpected reaction products. The detection of such species demands that a more complex model be developed for the interaction of volcanoes and atmospheres. We show that general thermodynamic models can be applied successfully to volcanic gas equilibria by comparing the results from HSC Chemistry with those from two volcanic gas equilibrium models (Solvgas and Gasmix). Using a thermodynamic model optimized for volcanic gas chemistry (C-O-S-H-F-Cl-Br-I-N-Ar speciation), we show that the volume ratio of atmospheric gas to magmatic gas in a high-temperature mixture is an important parameter of the volcanic plume chemistry, and our results suggest that even small amounts of air (a few % for an H2O-rich magmatic gas) in the high-temperature mixture are sufficient to yield elevated levels of reactive nitrogen, halogen (Cl, Br, and I), and sulfur species within the volcanic plume. Further modifications of the plume chemistry may also occur due to low-temperature reactions, and chemical schemes for the modification of halogen (Cl, Br, I), nitrogen, and sulfur chemistry are suggested, within the constraints imposed by recent measurements.

Global link between deformation and volcanic eruption quantified by satellite imagery

A key challenge for volcanological science and hazard management is that few of the world’s volcanoes are effectively monitored. Satellite imagery covers volcanoes globally throughout their eruptive cycles, independent of ground-based monitoring, providing a multidecadal archive suitable for probabilistic analysis linking deformation with eruption. Here we show that, of the 198 volcanoes systematically observed for the past 18 years, 54 deformed, of which 25 also erupted. For assessing eruption potential, this high proportion of deforming volcanoes that also erupted (46%), together with the proportion of non-deforming volcanoes that did not erupt (94%), jointly represent indicators with ‘strong’ evidential worth. Using a larger catalogue of 540 volcanoes observed for 3 years, we demonstrate how this eruption–deformation relationship is influenced by tectonic, petrological and volcanic factors. Satellite technology is rapidly evolving and routine monitoring of the deformation status of all volcanoes from space is anticipated, meaning probabilistic approaches will increasingly inform hazard decisions and strategic development.

Self-charging of the Eyjafjallajökull volcanic ash plume

Volcanic plumes generate lightning from the electrification of plume particles. Volcanic plume charging at over 1200 km from its source was observed from in situ balloon sampling of the April 2010 Eyjafjallajokull plume over Scotland. Whilst upper and lower edge charging of a horizontal plume is expected from fair weather atmospheric electricity, the plume over Scotland showed sustained positive charge well beneath the upper plume edge. At these distances from the source, the charging cannot be a remnant of the eruption itself because of charge relaxation in the finite conductivity of atmospheric air.

Volcanic source for fixed nitrogen in the early Earth's atmosphere

Hot volcanic vents promote the thermal fixation of atmospheric N2 into biologically available forms. The importance of this process for the global nitrogen cycle is poorly understood. At Masaya volcano, Nicaragua, NO and NO2 are intimately associated with volcanic aerosol, such that NOx levels reach as much as an order of magnitude above local background. In-plume HNO3 concentrations are elevated above background to an even greater extent (#50 mmol·m 23 ). We estimate the production efficiency of fixed nitrogen at hot vents to be ;3 3 10 28 mol·J 21 , implying present-day global production of ;10 9 mol of fixed N per year. Although conversion efficiency would have been lower in a preoxygenated atmosphere, we suggest that subaerial volcanoes potentially constituted an important source of fixed nitrogen in the early Earth, producing as much as ;10 11 mol·yr 21 of fixed N during major episodes of volcanism. These fluxes are comparable to estimated nitrogen-fixation rates in the prebiotic Earth from other major sources such as bolide impacts and thunderstorm and volcanic lightning.

Sulphur dioxide fluxes from Mount Etna, Vulcano, and Stromboli measured with an automated scanning ultraviolet spectrometer

We report here SO 2 flux measurements for the southern Italian volcanoes: Mount Etna, Vulcano, and Stromboli made in July 2002 from fixed positions, using an automated plume scanning technique. Spectral data were collected using a miniature ultraviolet spectrometer, and SO 2 column amounts were derived with a differential optical absorption spectroscopy evaluation routine. Scanning through the plume was enabled by a 45° turning mirror affixed to the shaft of a computer controlled stepper motor, so that scattered skylight from incremental angles within the horizon-to-horizon scans was reflected into the field of view of the spectrometer. Each scan lasted ∼5 min and, by combining these data with wind speeds, average fluxes of 940, 14, and 280 Mg d - 1 were obtained for Etna, Vulcano, and Stromboli, respectively. For comparative purposes, conventional road and airborne traverses were also made using this spectrometer, yielding fluxes of 850, 17, and 210 Mg d - 1 . The automated scanning technique has the advantage of obviating the need for time-consuming traverses underneath the plume and is well suited for longer-term telemetered deployments to provide sustained high time resolution flux data.