Size Distributions and Mineralogy of Ash Particles in the Stratosphere from Eruptions of Mount St. Helens

Mie scattering from optically levitated mixed sulfuric acid-silica core-shell aerosols: observation of core-shell morphology for atmospheric science

Sulfuric acid is shown to form a core-shell particle on a micron-sized, optically-trapped spherical silica bead. The refractive indices of the silica and sulfuric acid, along with the shell thickness and bead radius were determined by reproducing Mie scattered optical white light as a function of wavelength in Mie spectroscopy. Micron-sized silica aerosols (silica beads were used as a proxy for atmospheric silica minerals) were levitated in a mist of sulfuric acid particles; continuous collection of Mie spectra throughout the collision of sulfuric acid aerosols with the optically trapped silica aerosol demonstrated that the resulting aerosol particle had a core-shell morphology. Contrastingly, the collision of aqueous sulfuric acid aerosols with optically trapped polystyrene aerosol resulted in a partially coated system. The light scattering from the optically levitated aerosols was successfully modelled to determine the diameter of the core aerosol (±0.003 μm), the shell thickness (±0.0003 μm) and the refractive index (±0.007). The experiment demonstrated that the presence of a thin film rapidly changed the light scattering of the original aerosol. When a 1.964 μm diameter silica aerosol was covered with a film of sulfuric acid 0.287 μm thick, the wavelength dependent Mie peak positions resembled sulfuric acid. Thus mineral aerosol advected into the stratosphere would likely be coated with sulfuric acid, with a core-shell morphology, and its light scattering properties would be effectively indistinguishable from a homogenous sulfuric acid aerosol if the film thickness was greater than a few 100 s of nm for UV-visible wavelengths.

Persisting volcanic ash particles impact stratospheric SO2 lifetime and aerosol optical properties

Volcanic ash is often neglected in climate simulations because ash particles are assumed to have a short atmospheric lifetime, and to not participate in sulfur chemistry. After the Mt. Kelut eruption in 2014, stratospheric ash-rich aerosols were observed for months. Here we show that the persistence of super-micron ash is consistent with a density near 0.5 g cm-3, close to pumice. Ash-rich particles dominate the volcanic cloud optical properties for the first 60 days. We also find that the initial SO2 lifetime is determined by SO2 uptake on ash, rather than by reaction with OH as commonly assumed. About 43% more volcanic sulfur is removed from the stratosphere in 2 months with the SO2 heterogeneous chemistry on ash particles than without. This research suggests the need for re-evaluation of factors controlling SO2 lifetime in climate model simulations, and of the impact of volcanic ash on stratospheric chemistry and radiation.

Passive Earth Observations of Volcanic Clouds in the Atmosphere

Current Earth Observation (EO) satellites provide excellent spatial, temporal and spectral coverage for passive measurements of atmospheric volcanic emissions. Of particular value for ash detection and quantification are the geostationary satellites that now carry multispectral imagers. These instruments have multiple spectral channels spanning the visible to infrared (IR) wavelengths and provide 1 × 1 km2 to 4 × 4 km2 resolution data every 5–15 min, continuously. For ash detection, two channels situated near 11 and 12 μ m are needed; for ash quantification a third or fourth channel also in the infrared is useful for constraining the height of the ash cloud. This work describes passive EO infrared measurements and techniques to determine volcanic cloud properties and includes examples using current methods with an emphasis on the main difficulties and ways to overcome them. A challenging aspect of using satellite data is to design algorithms that make use of the spectral, temporal (especially for geostationary sensors) and spatial information. The hyperspectral sensor AIRS is used to identify specific molecules from their spectral signatures (e.g., for SO2) and retrievals are demonstrated as global, regional and hemispheric maps of AIRS column SO2. This kind of information is not available on all sensors, but by combining temporal, spatial and broadband multi-spectral information from polar and geo sensors (e.g., MODIS and SEVIRI) useful insights can be made. For example, repeat coverage of a particular area using geostationary data can reveal temporal behaviour of broadband channels indicative of eruptive activity. In many instances, identifying the nature of a pixel (clear, cloud, ash etc.) is the major challenge. Sophisticated cloud detection schemes have been developed that utilise statistical measures, physical models and temporal variation to classify pixels. The state of the art on cloud detection is good, but improvements are always needed. An IR-based multispectral cloud identification scheme is described and some examples shown. The scheme is physically based but has deficiencies that can be improved during the daytime by including information from the visible channels. Physical retrieval schemes applied to ash detected pixels suffer from a lack of knowledge of some basic microphysical and optical parameters needed to run the retrieval models. In particular, there is a lack of accurate spectral refractive index information for ash particles. The size distribution of fine ash (1–63 μ m, diameter) is poorly constrained and more measurements are needed, particularly for ash that is airborne. Height measurements are also lacking and a satellite-based stereoscopic height retrieval is used to illustrate the value of this information for aviation. The importance of water in volcanic clouds is discussed here and the separation of ice-rich and ash-rich portions of volcanic clouds is analysed for the first time. More work is required in trying to identify ice-coated ash particles, and it is suggested that a class of ice-rich volcanic cloud be recognized and termed a ‘volcanic ice’ cloud. Such clouds are frequently observed in tropical eruptions of great vertical extent (e.g., 8 km or higher) and are often not identified correctly by traditional IR methods (e.g., reverse absorption). Finally, the global, hemispheric and regional sampling of EO satellites is demonstrated for a few eruptions where the ash and SO 2 dispersed over large distances (1000s km).

Reconstructing the history of mining and remediation in the Coeur d'Alene, Idaho Mining District using lake sediments

Mining that began in the late 1800s intensified during World War II contaminating Lake Coeur d'Alene sediments with potentially toxic elements. We used 80y of the sediment record to reconstruct metal(loid) loadings to the lake and quantitatively evaluate the effectiveness of tailings management. Sediment core analysis for pollen, chronological markers, and metal(loid)s permitted stratigraphic reconstruction showing that contaminant loading decreased after tailings pond construction, but that most metal(loid) concentrations exceed recommended limits. Arsenic concentrations (250-450 mg kg(-)(1)) at the sediment-water interface are potentially toxic; however, low P concentrations in recent sediments (1.0-1.4 mg kg(-)(1)) inhibit eutrophication and the concomitant release of soluble As. Zinc (3 g kg(-)(1)), Cd (10 mg kg(-)(1)), Ag (10 mg kg(-)(1)), and Cu (90 mg kg(-)(1)) concentrations are now lower than in sediments deposited during active mining, but remain an environmental concern. Sedimentary Cr and Pb concentrations have not changed in the last 50y, because tailings continue to enter the lake. Although modern Cr concentrations (40 mg kg(-)(1)) are unlikely to cause toxicity, current Pb concentrations (4 g kg(-)(1)) exceed acceptable limits, creating challenges for remediation. Strategies to manage other mining-contaminated watersheds should include consideration of elemental differences when evaluating remediation effectiveness.

The great tambora eruption in 1815 and its aftermath

Quantitative analytical methods are used to reconstruct the course of events during and after the cataclysmic eruption of Mount Tambora, Indonesia, on 10 and 11 April 1815. This was the world's greatest ash eruption (so far as is definitely known) since the end of the last Ice Age. This synthesis is based on data and methods from the fields of volcanology, oceanography, glaciology, meteorology, climatology, astronomy, and history.

Possible role of volcanic ash-gas clouds in the Earth's prebiotic chemistry

Volcanic ash-gas clouds represent versatile local atmospheric environments appropriate for abiotic synthesis of rather complex organic molecules due to the simultaneous presence of various gaseous reagents, catalytically active inorganic particles, electric discharges, pressure and temperature gradients. They are relatively readily attainable for the scientists, contrary to objects or events of space origin (interstellar and planetary dust, meteoritic/cometary impacts, etc.), providing excellent opportunities for in situ studies and grounded simulating experiments. This paper reviews the available data on this environment, its most important chemical and physical parameters. Based on this analysis, it is suggested in brief experimental conditions for the simulation.

Environmental Effects of an Impact-Generated Dust Cloud: Implications for the Cretaceous-Tertiary Extinctions

A model of the evolution and radiative effects of a debris cloud from a hypothesized impact event at the Cretaceous-Tertiary boundary suggests that the cloud could have reduced the amount of light at the earth's surface below that required for photosynthesis for several months and, for a somewhat shorter interval, even below that needed for many animals to see. For 6 months to 1 year, the surface would cool; the oceans would cool only a few degrees Celsius at most, but the continents might cool a maximum of 40 Kelvin. Extinctions in the ocean may have been caused primarily by the temporary cessation of photosynthesis, but those on land may have been primarily induced by a combination of lowered temperatures and reduced light.