Early in-flight detection of SO₂ via Differential Optical Absorption Spectroscopy: a feasible aviation safety measure to prevent potential encounters with volcanic plumes

Volcanic ash constitutes a risk to aviation, mainly due to its ability to cause jet engines to fail. Other risks include the possibility of abrasion of windshields and potentially serious damage to avionic systems. These hazards have been widely recognized since the early 1980s, when volcanic ash provoked several incidents of engine failure in commercial aircraft. In addition to volcanic ash, volcanic gases also pose a threat. Prolonged and/or cumulative exposure to sulphur dioxide (SO₂) or sulphuric acid (H₂SO₄) aerosols potentially affects e.g. windows, air frame and may cause permanent damage to engines. SO₂ receives most attention among the gas species commonly found in volcanic plumes because its presence above the lower troposphere is a clear proxy for a volcanic cloud and indicates that fine ash could also be present. <br><br> Up to now, remote sensing of SO₂ via Differential Optical Absorption Spectroscopy (DOAS) in the ultraviolet spectral region has been used to measure volcanic clouds from ground based, airborne and satellite platforms. Attention has been given to volcanic emission strength, chemistry inside volcanic clouds and measurement procedures were adapted accordingly. Here we present a set of experimental and model results, highlighting the feasibility of DOAS to be used as an airborne early detection system of SO₂ in two spatial dimensions. In order to prove our new concept, simultaneous airborne and ground-based measurements of the plume of Popocatépetl volcano, Mexico, were conducted in April 2010. The plume extended at an altitude around 5250 m above sea level and was approached and traversed at the same altitude with several forward looking DOAS systems aboard an airplane. These DOAS systems measured SO₂ in the flight direction and at &pm;40 mrad (2.3&deg;) angles relative to it in both, horizontal and vertical directions. The approaches started at up to 25 km distance to the plume and SO₂ was measured at all times well above the detection limit. In combination with radiative transfer studies, this study indicates that an extended volcanic cloud with a concentration of 10¹² molecules cm^&minus;3 at typical flight levels of 10 km can be detected unambiguously at distances of up to 80 km away. This range provides enough time (approx. 5 min) for pilots to take action to avoid entering a volcanic cloud in the flight path, suggesting that this technique can be used as an effective aid to prevent dangerous aircraft encounters with potentially ash rich volcanic clouds

Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales

Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS$_2$, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.Comment: 6 pages, 5 figure

Experimental investigation of fast electron transport in solid density matter: Recent results from a new technique of X-ray energy-encoded 2D imaging

AbstractThe development activity of a new experimental technique for the study of the fast electron transport in high density matter is reported. This new diagnostic tool enables the X-ray 2D imaging of ultrahigh intensity laser plasmas with simultaneous spectral resolution in a very large energy range to be obtained. Results from recent experiments are discussed, in which the electron propagation in multilayer targets was studied by using the Kα. In particular, results highlighting the role of anisotropic Bremsstrahlung are reported, for the sake of the explanation of the capabilities of the new diagnostics. A discussion of a test experiment conceived to extend the technique to a single-shot operation is finally given

Experimental investigation of fast electron transport through Kα imaging and spectroscopy in relativistic laser-solid interactions

Abstract The study of the basic physical processes underlying the generation of fast electrons during the interaction of high-intensity short laser pulses with solid materials and the transport of these fast electrons through the target material are of great importance for the fast ignition concept for inertial confinement fusion and for the development of ultra-short X-ray sources. We report on the experimental investigation of fast electron transport phenomena by means of the spatial and spectral characterization of the X-ray emission from layered targets using bent crystal spectrometers and a new diagnostic technique based on a pinhole-camera equipped with a CCD detector working in single-photon regime for multi-spectral X-ray imaging The experiments were carried out at relativistic laser intensities, both in the longer (≃ps) pulse interaction regime relevant for fast ignition studie

Time Resolved Photoelectron Spectroscopy of Thioflavin T Photoisomerization: A Simulation Study

The excited state isomerization of thioflavin T (ThT) is responsible for the quenching of its fluorescence in a non-restricted environment. The fluorescence quantum yield increases substantially upon binding to amyloid fibers. Simulations reveal that the variation of the twisting angle between benzothiazole and benzene groups (ϕ(1)) is responsible for the sub-picosecond fluorescence quenching. The evolution of the twisting process can be directly probed by photoelectron emission with energies ε ≥ 1.0 eV before the molecule reaches the ϕ(1)-twisted configuration (~300 fs)

X-RAY ABSORPTION STUDIES OF MIXED-VALENT RARE EARTH CLUSTERS

The transition from atomic to metallic valences in small matrixisolated clusters of Sm, Nd and Pr has been studied by XANES- and EXAFS-measurements at the corresponding L3-edges. The observed valence of the Rare Earth clusters depends on the atomic concentrations and on the type of metal atom. Hints on the cluster size are given by annealing the samples to temperatures near the melting point of the Rare Gas matrix. In Nd and Pr samples a complete valence change from 2 to 3 has been observed within an atomic concentration range of 0.1 to 0.25 % metal content. Clusters containing more than 4 - 6 atoms are totally trivalent. For Sm a valence transition is observed at 1 % metal concentration, the valence rising from 2.0 to 2.6. From EXAFS data a critical cluster size of K = 13 atoms at v = 2.6 has been elaborated. Sm clusters with K > 13 are mixed valent

Spatial characteristics of K alpha X-ray emission from relativistic femtosecond laser plasmas

The spatial structure of the Kalpha emission from Ti targets irradiated with a high intensity femtosecond laser has been studied using a two-dimensional monochromatic imaging technique. For laser intensities I<5x10(17) W/cm(2), the observed spatial structure of the Kalpha emission can be explained by the scattering of the hot electrons inside the solid with the help of a hybrid particle-in-cell/Monte Carlo model. By contrast, at the maximum laser intensity I=7x10(18) W/cm(2) the half-width of the Kalpha emission was 70 mum compared to a laser-focus half-width of 3 mum. Moreover, the main Kalpha peak was surrounded by a halo of weak Kalpha emission with a diameter of 400 mum and the Kalpha intensity at the source center did not increase with increasing laser intensity. These three features point to the existence of strong self-induced fields, which redirect the hot electrons over the target surface

A Fabry–Perot interferometer-based camera for two-dimensional mapping of SO₂ distributions

We examine a new imaging method for the remote sensing of volcanic gases, which relies on the regularly spaced narrow-band absorption structures in the UV–VIS of many molecules. A Fabry–Perot interferometer (FPI) is used to compare the scattered sunlight radiance at wavelengths corresponding to absorption bands with the radiance at wavelengths in between the bands, thereby identifying and quantifying the gas. In this first theoretical study, we present sample calculations for the detection of sulfur dioxide (SO₂). Optimum values for the FPI setup parameters are proposed. Furthermore, the performance of the FPI method is compared to SO₂ cameras. We show that camera systems using an FPI are far less influenced by changes in atmospheric radiative transfer (e.g., due to aerosol) and have a great potential as a future technique for examining emissions of SO₂ (or other gases) from volcanic sources and other point sources

Plume propagation direction determination with SO₂ cameras

SO2 cameras are becoming an established tool for measuring sulfur dioxide (SO2) fluxes in volcanic plumes with good precision and high temporal resolution. The primary result of SO2 camera measurements are time series of two-dimensional SO2 column density distributions (i.e. SO2 column density images). However, it is frequently overlooked that, in order to determine the correct SO2 fluxes, not only the SO2 column density, but also the distance between the camera and the volcanic plume, has to be precisely known. This is because cameras only measure angular extents of objects while flux measurements require knowledge of the spatial plume extent. The distance to the plume may vary within the image array (i.e. the field of view of the SO2 camera) since the plume propagation direction (i.e. the wind direction) might not be parallel to the image plane of the SO2 camera. If the wind direction and thus the camera–plume distance are not well known, this error propagates into the determined SO2 fluxes and can cause errors exceeding 50 %. This is a source of error which is independent of the frequently quoted (approximate) compensation of apparently higher SO2 column densities and apparently lower plume propagation velocities at non-perpendicular plume observation angles.Here, we propose a new method to estimate the propagation direction of the volcanic plume directly from SO2 camera image time series by analysing apparent flux gradients along the image plane. From the plume propagation direction and the known location of the SO2 source (i.e. volcanic vent) and camera position, the camera–plume distance can be determined. Besides being able to determine the plume propagation direction and thus the wind direction in the plume region directly from SO2 camera images, we additionally found that it is possible to detect changes of the propagation direction at a time resolution of the order of minutes. In addition to theoretical studies we applied our method to SO2 flux measurements at Mt Etna and demonstrate that we obtain considerably more precise (up to a factor of 2 error reduction) SO2 fluxes. We conclude that studies on SO2 flux variability become more reliable by excluding the possible influences of propagation direction variations