Steven P. Love

Passive infrared spectroscopy of the eruption plume at Popocatépetl volcano, Mexico

Volcanic gases provide important insights into deep-Earth processes, and gas composition and flux variations show promise as predictors of eruptive activity. But data correlating gas composition with eruptions are sparse, largely because such studies have traditionally involved direct sampling inside a volcanic crater — a hazardous operation that has resulted in numerous deaths,. Crater-rim-based spectroscopy, closed-path spectroscopy of gases sampled from aircraft, and time-averaged studies using volatile traps allow measurements to be taken from safer distances. But when a full-scale explosive eruption threatens, even these methods become dangerous as the hazard radius expands to many kilometres. Previously, only sulphur dioxide has been reliably measurable at such large distances, using correlation spectroscopy. Here we describe techniques that extend the useful range of passive infrared spectroscopy to monitor many gases at distances of over 17 km. We demonstrate the use of these techniques in a high-temporal-resolution study of short-term compositional variations associated with an explosive eruption at Mexicos Popocatépetl volcano on 25–26 February 1997. We observed a steady increase in SiF4/SO2 over several days preceding the eruption, followed by a tenfold decrease in this ratio over a few hours immediately afterwards.

Passive infrared remote sensing evidence for large, intermittent CO2 emissions at Popocatépetl volcano, Mexico

Abstract Passive infrared (FTIR) and correlation spectrometer (COSPEC) measurements were conducted at Popocatepetl volcano during February 10 to 26, 1998 from sites 4 to 17 km distant from the summit. Volcano behavior was relatively quiet and SO 2 flux averaged 1670±1420 t/day (51 measurements), relatively small for Popocatepetl. Concurrent HCl/SO 2 and HF/SO 2 ratios were 0.17±0.01 and 0.031±0.003, respectively, about the same as ratios measured from 1994 to 1997. The amount of CO 2 in the volcanic plume was quantified using FASCODE in which atmospheric CO 2 is numerically subtracted from the total infrared spectrum to obtain the residual magmatic CO 2 . Surprisingly, CO 2 /SO 2 mass ratios rose dramatically to values as high as 140, about 30 times higher than typical values of 2 to 8 measured from 1994 to 1996. These excursions in high CO 2 /SO 2 ratios were short-lived, lasting no longer than about 0.5 to 3.0 h but CO 2 flux occasionally exceeded 100,000 t/day. We estimate that the average CO 2 /SO 2 ratio for the period was about 23, yielding an average CO 2 flux of roughly 38,000 t/day. Chemical and petrographic analyses of lava and pumice erupted during explosions on June 30, 1997 and January 1, 1998 show conclusively that Popocatepetl produces mixed products formed by injection of mafic magma into a more silicic chamber at temperatures and pressures of roughly 1040°C and 5 kbar. In addition, Popocatepetl eruptive products include xenoliths of metamorphosed carbonate rocks containing wollastonite and other calc-silicate minerals indicating reaction of magma with Cretaceous limestone underlying the volcano. Using a normal CO 2 /SO 2 ratio of 4 for reference, we calculate an average excess CO 2 production of 32,000 t/day for 17 days. This would require assimilation of only 5×10 −4 km 3 of limestone, an amount easily accessible in the 3-km-thick Cretaceous section beneath the volcano. We also examine two scenarios in which excess CO 2 is produced by degassing of subjacent basalt magma, but these explanations seem less plausible to us. Because many other volcanoes are underlain by carbonate sequences, short-duration bursts of CO 2 flux, and increased CO 2 /SO 2 ratio, might be observed at other sites, if simultaneous, real-time measurements of major gas species are made.

Geochemical surveillance of magmatic volatiles at Popocatépetl volcano, Mexico

Surveillance of Popocatepetl volcanic plume geochemistry and SO 2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993. Volatile traps placed around the summit were collected at near-monthly intervals until the volcano erupted on December 21, 1994. Additional trap samples were obtained in early 1996 before the volcano erupted again, emplacing a small dacite dome in the summit crater. Abundances of volatile constituents (ppm/day of Cl, S total , F, CO 2 , Hg, and As) varied, but most constituents were relatively high in early and late 1994. However, ratios of these constituents to Cl were highest in mid-1994. δ 34 S-S total in trap solutions ranged from 1.5‰ to 6.4‰; lowest values generally occurred during late 1994. δ 13 C-CO 2 of trap solutions were greatly contaminated with atmospheric CO 2 and affected by absorption kinetics. When trap data are combined with SO 2 flux measurements made through November 1996, Popocatepetl released about 3.9 Mt SO 2 , 16 Mt CO 2 , 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H 2 O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTIR) commonly exceed human recommended exposure limits and may constitute a potential health hazard. Volatile geochemistry combined with petrologic observations and melt-inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at Popocatepetl. Minor assimilation of Cretaceous wall rocks probably occurred in mid-1994.

Off-beam lidar: an emerging technique in cloud remote sensing based on radiative green-function theory in the diffusion domain

Abstract Atmospheric lidars do not penetrate directly most boundary-layer clouds due to their large optical density. However, the lidars photons are not absorbed but scattered out of the beam. Typically, about half are actually transmitted through the cloud and the other half escape the cloud by reflection in extended diffuse patterns that evolve in time. For all practical purposes, these are the clouds space-time Green functions (GFs). In a Fourier-Laplace expansion of the space-time GF, the leading term is representative of solar remote-sensing (i.e., steady/uniform source) while higher-order terms correspond to active approaches with temporal- and/or spatial- resolution capabilities. Radiative GF theory is tractable within the limits of photon-diffusion theory and homogeneous clouds. Monte Carlo simulations with realistically variable cloud models are used to extend the range of validity of analytical GF theory with minor modifications. GF theory tells us that physical and optical cloud thicknesses can be retrieved from off-beam cloud lidar returns.

Near-infrared Raman spectroscopy of solid C60. Raman activation of silent modes by 13C and sample disorder

Abstract High-resolution Raman spectra excited from 1.45 to 1.75 eV are presented for solid C 60 films and crystals prepared by four methods. Intersample comparison shows that imperfections introduced in sample preparation can lower symmetries sufficiently to activate silent modes, while oxygen has no observable effect. Unusual resonance and broadening effects observed near 1.6 eV indicate the existence of a weak electronic transition. Explicit calculations for C 60 molecules having one or two 13 C atoms reproduce the observe spectra and show that isotopic symmetry lowering is manifested in lifting of H g degeneracies and activation of silent modes through mixing with nearly Raman-active modes.

Wide-Angle Imaging Lidar Deployment at the ARM Southern Great Plains Site: Intercomparison of Cloud Property Retrievals

Abstract The Wide-Angle Imaging Lidar (WAIL), a new instrument that measures cloud optical and geometrical properties by means of off-beam lidar returns, was deployed as part of a multi-instrument campaign to probe a cloud field at the Atmospheric Radiation Measurement (ARM) Southern Great Plain (SGP) site on 25 March 2002. WAIL is designed to determine physical and geometrical characteristics using the off-beam component of the lidar return that can be adequately modeled within the diffusion approximation. Using WAIL data, the extinction coefficient and geometrical thickness of a dense cloud layer is estimated, from which optical thickness is inferred. Results from the new methodology agree well with counterparts obtained from other instruments located permanently at the SGP ARM site and from the WAIL-like airborne instrument that flew over the site during our observation period.

Remote sensing of cloud thickness and liquid water content with Wide-Angle Imaging Lidar

We describe a new type of lidar instrument, Wide-Angle Imaging Lidar (WAIL), designed to study and directly make use of multiple scattering in clouds. Providing time-resolved imagery over a 60° field of view, the new instrument captures returns at virtually all orders of scattering in a ground-based measurement. We report the first retrievals of cloud properties using this system, from measurements of a moderately opaque altostratus. Following insights from photon diffusion theory, we are able to infer the physical thickness and optical depth of the cloud layer, and, from there, obtain an estimate of the volume-averaged liquid water content. Performance of the new instrument is discussed and it is compared with other active techniques in cloud remote sensing.

Control of selected physical properties of MX solids: an experimental and theoretical investigation

Received 17 January 1995; accepted in final form 2 May 1995 Abstract A series of eight materials of stoichiometry [Pt(L-L)2X2][Pt(L-L)2]Y 4 (X is Cl, Br; L-L is 1,2-diaminoethane (en) or 1,2- diaminocyclohexane (chxn); Y is CIO4, X-) were synthesized. Crystal structures were determined for the compounds [Pt(chxn)zC12][Pt(chxn)2](C104) 4 1, [Pt(chxn)2Brz][Pt(chxn)2](CIO4)4 2, and [Pt(chxn)zBrz][Pt(chxn)z]Br 4 4. All three of these compounds crystallize in the orthorhombic space group I222. Compound 1 has a = 5.711(1) ,~, b = 7.804(1) A, c = 24.101(7) A,, Z = 1, dx = 2.033 g cm 3. Compound 2 has a = 5.781(1) A, b = 7.720(1) ,~, c = 24.036(5) ,~, Z = 1, d× = 2.174 g cm -3. Compound 4 has a = 5.379(1) ,~, b = 7.028(1) A, c = 23.884(4) ,~, Z = 1, dx = 2.440 g cm 3. These solids contain pseudo one-dimensional chains with a charge-density-wave (CDW) ground state structure: X-Pt(IV)- X...Pt(II)...X. Single crystal resonance Raman experiments were performed on all compounds to measure the sym- metric X Pt X stretching frequency u~ and the band edge. It is shown that the optical and electronic properties and, therefore, the CDW strength of these one-dimensional materials may be systematically varied over a wide range by employing different combinations of L-L and Y; templates composed of hydrogen bonded networks of L-L and Y were found to control the metal-metal separation, thereby controlling the X-Pt(IV)-X...Pt(II)...X chain geometry. Rela- tionships between the CDW strength, measured as the ratio of the short M(IV) X distance to the long M(II) X distance, the band gap energy ul and the Pt-Pt separation are developed. The reaction coordinate is found to be dominated by changes in the M-M and Pt(II)-X separations over most of the range studied, with contributions from changes in the ptlV-x bonds becoming important only at the smallest M-M separations. Direct evidence demonstrating that MX systems are true Peierls distorted systems is also presented. These results are consistent with modeling based on Peierls- Hubbard hamiltonians. This work explains the unusual pressure and temperature dependences that have been observed for the structures and optical properties of this class of materials and also provides a wealth of information to benchmark many-body theoretical calculations modeling electron electron and electron-phonon interactions in one-dimensional materials. 1. Introduction There has been recent interest in halogen bridged * Corresponding authors.

Programmable matched filter and Hadamard transform hyperspectral imagers based on micro-mirror arrays

Hyperspectral imaging (HSI), in which each pixel contains a high-resolution spectrum, is a powerful technique that can remotely detect, identify, and quantify a multitude of materials and chemicals. The advent of addressable micro-mirror arrays (MMAs) makes possible a new class of programmable hyperspectral imagers that can perform key spectral processing functions directly in the optical hardware, thus alleviating some of HSIs high computational overhead, as well as offering improved signal-to-noise in certain important regimes (e.g. when using uncooled infrared detectors). We have built and demonstrated a prototype UV-Visible micro-mirror hyperspectral imager that is capable not only of matched-filter imaging, but also of full hyperspectral imagery via the Hadamard transform technique. With this instrument, one can upload a chemical-specific spectral matched filter directly to the MMA, producing an image showing the location of that chemical without further processing. Target chemicals are changeable nearly instantaneously simply by uploading new matched-filter patterns to the MMA. Alternatively, the MMA can implement Hadamard mask functions, yielding a full-spectrum hyperspectral image upon inverting the transform. In either case, the instrument can produce the 2D spatial image either by an internal scan, using the MMA itself, or with a traditional external push-broom scan. The various modes of operation are selectable simply by varying the software driving the MMA. Here the design and performance of the prototype is discussed, along with experimental results confirming the signal-to-noise improvement produced by the Hadamard technique in the noisy-detector regime.

Full-frame programmable spectral filters based on micromirror arrays

Abstract. Rapidly programmable micromirror arrays, such as the Texas Instruments Digital Light Processor (DLP®) digital micromirror device (DMD), have opened an exciting new arena in spectral imaging: rapidly reprogrammable, high spectral resolution, multiband spectral filters that perform spectral processing directly in the optical hardware. Such a device is created by placing a DMD at the spectral plane of an imaging spectrometer and by using it as a spectral selector that passes some wavelengths down the optical train to the final image and rejects others. Although simple in concept, realizing a truly practical DMD-based spectral filter has proved challenging. Versions described to date have been limited by the intertwining of image position and spectral propagation direction common to most imaging spectrometers, reducing these instruments to line-by-line scanning imagers rather than true spectral cameras that collect entire two-dimensional (2-D) images at once. Here, we report several optical innovations that overcome this limitation and allow us to construct full-frame programmable filters that spectrally manipulate every pixel, simultaneously and without spectral shifts, across a full 2-D image. So far, our prototype, which can be programmed either as a matched-filter imager for specific target materials or as a fully hyperspectral multiplexing Hadamard transform imager, has demonstrated over 100 programmable spectral bands while maintaining good spatial image quality. We discuss how diffraction-mediated trades between spatial and spectral resolution determine achievable performance. Finally, we describe methods for dealing with the DLP’s 2-D diffractive effects and suggest a simple modification to the DLPs that would eliminate their impact for this application.