114 research outputs found
Tomographic MAX-DOAS observations of sun illuminated targets: a new technique providing well defined absorption paths in the boundary layer
A novel experimental procedure to measure the surface-near distribution of
atmospheric trace gases using passive Multi-Axis-Differential Absorption Optical
Spectroscopy (MAX-DOAS) is proposed. The idea consists of pointing the receiving
telescope of the spectrometer to non-reflecting surfaces or to âbrightâ targets placed at known
distances from the measuring device, which are illuminated by sunlight. We show that the
partial trace gas absorptions between the top of the atmosphere and the target can be easily
removed from the measured total absorption. Thus it is possible to derive the average
concentration of trace gases like e.g. NO2, HCHO, SO2, H2O, Glyoxal, BrO and others along
the line of sight between the instrument and the target like for the well-known long-path
DOAS observations (but with much less expense). If tomographic arrangements are used,
even two- or three-dimensional trace gas distributions can be retrieved. The basic assumptions
of the proposed method are confirmed by test measurements across the city of Heidelberg
Towards imaging of atmospheric trace gases using FabryâPĂ©rot interferometer correlation spectroscopy in the UV and visible spectral range
Many processes in the lower atmosphere including transport,
turbulent mixing and chemical conversions happen on timescales of the order
of seconds (e.g. at point sources). Remote sensing of atmospheric trace gases
in the UV and visible spectral range (UVâVis) commonly uses dispersive
spectroscopy (e.g. differential optical absorption spectroscopy, DOAS). The
recorded spectra allow for the direct identification, separation and
quantification of narrow-band absorption of trace gases. However, these
techniques are typically limited to a single viewing direction and limited by
the light throughput of the spectrometer set-up. While two-dimensional imaging
is possible by spatial scanning, the temporal resolution remains poor (often
several minutes per image). Therefore, processes on timescales of seconds
cannot be directly resolved by state-of-the-art dispersive methods.
We investigate the application of FabryâPĂ©rot interferometers (FPIs) for
the optical remote sensing of atmospheric trace gases in the UVâVis spectral range. By
choosing a FPI transmission spectrum, which is optimised to correlate with
narrow-band (ideally periodic) absorption structures of the target trace gas,
column densities of the trace gas can be determined with a sensitivity and
selectivity comparable to dispersive spectroscopy, using only a small number
of spectral channels (FPI tuning settings). Different from dispersive optical
elements, the FPI can be implemented in full-frame imaging set-ups (cameras),
which can reach high spatio-temporal resolution. In principle, FPI
correlation spectroscopy can be applied for any trace gas with distinct
absorption structures in the UVâVis range.
We present calculations for the application of FPI correlation spectroscopy
to SO2, BrO and NO2 for exemplary measurement
scenarios. In addition to high sensitivity and selectivity we find that the spatio
temporal resolution of FPI correlation spectroscopy can be more than 2
orders of magnitude higher than state-of-the-art DOAS measurements. As proof
of concept we built a 1-pixel prototype implementing the technique for
SO2 in the UV. Good agreement with our calculations and conventional
measurement techniques is demonstrated and no cross sensitivities to other
trace gases are observed.</p
New insight into halogen release from experimental studies on BrO/Br ratios in volcanic plumes
On the link between Earth tides and volcanic degassing
Long-term measurements of volcanic gas emissions conducted during the last decade suggest that under certain conditions the magnitude or chemical
composition of volcanic emissions exhibits periodic variations with a period
of about 2 weeks. A possible cause of such a periodicity can be attributed
to the Earth tidal potential. The phenomenology of such a link has been
debated for long, but no quantitative model has yet been proposed. The aim of
this paper is to elucidate whether a causal link between tidal forcing and variations in volcanic degassing can be traced analytically. We model the
response of a simplified magmatic system to the local tidal gravity
variations and derive a periodical vertical magma displacement in the conduit
with an amplitude of 0.1â1 m, depending on the geometry and physical state of
the magmatic system. We find that while the tide-induced vertical magma
displacement presumably has no significant direct effect on the volatile
solubility, the differential magma flow across the radial conduit profile may
result in a significant increase in the bubble coalescence rate at a depth of
several kilometres by up to several multiples of 10 %. Because bubble coalescence
facilitates separation of gas from magma and thus enhances volatile
degassing, we argue that the derived tidal variation may propagate to a
manifestation of varying volcanic degassing behaviour. The presented model
provides a first basic framework which establishes an analytical
understanding of the link between the Earth tides and volcanic degassing.</p
Reactive halogen chemistry in volcanic plumes
Bromine monoxide (BrO) and sulphur dioxide (SO2) abundances as a function of the
distance from the source were measured by ground-based scattered-light Multi AXis
Differential Optical Absorption Spectroscopy (MAX-DOAS) in the volcanic plumes of
Mt. Etna on Sicily, Italy in August-October 2004 and May 2005 and Villarica in Chile in
November 2004. BrO and SO2 spatial distributions in a cross section of Mt. Etnaâs plume
were also determined by Imaging DOAS. We observed an increase in the BrO/SO2 ratio
in the plume from below the detection limit near the vent to about 4.5 x 10-4 at 19 km
(Mt. Etna) and to about 1.3 x 10-4 at 3 km (Villarica) distance, respectively. Additional
attempts were undertaken to evaluate the compositions of individual vents on Mt. Etna.
Furthermore, we detected the halogen species ClO and OClO. This is the first time that
OClO could be detected in a volcanic plume. Using calculated thermodynamic
equilibrium compositions as input data for a oneâdimensional photochemical model, we
could reproduce the observed BrO and SO2 vertical columns in the plume and their ratio
as function of distance from the volcano as well as vertical BrO and SO2 profiles across the plume with current knowledge of multiphase halogen chemistry, but only when we
assumed the existence of an âeffective source regionâ, where volcanic volatiles and
ambient air are mixed at about 600°C (in the proportions of 60% and 40%, respectively
High-spectral-resolution Fabry-Perot interferometers overcome fundamental limitations of present volcanic gas remote sensing techniques
Remote sensing (RS) of volcanic gases has become a central tool for studying volcanic activity. For instance, ultraviolet (UV) skylight spectroscopy with grating spectrographs (GS) enables SO2 (and, under favourable conditions, BrO) quantification in volcanic plumes from autonomous platforms at safe distances. These measurements can serve volcanic monitoring and they cover all stages of volcanic activity in long measurement time series, which substantially contributes to the refinement of theories on volcanic degassing. Infrared (IR) remote sensing techniques are able to measure further volcanic gases (e.g., HF, HCl, CO2, CO). However, the employed Fourier transform spectrometers (FTSs) are intrinsically intricate and, due to limited resolving power or light throughput, mostly rely on either lamps, direct sun, or hot lava as light source, usually limiting measurements to individual field campaigns. We show that many limitations of grating spectrographs and Fourier transform spectrometer measurements can be overcome by Fabry-Perot interferometer (FPI) based spectrograph implementations. Compared to grating spectrographs and Fourier transform spectrometers, Fabry-Perot interferometer spectrographs reach a 1-3 orders of magnitude higher spectral resolution and superior light throughput with compact and stable set-ups. This leads to 1) enhanced sensitivity and selectivity of the spectral trace gas detection, 2) enables the measurement of so far undetected volcanic plume constituents [e.g., hydroxyl (OH) or sulfanyl (SH)], and 3) extends the range of gases that can be measured continuously using the sky as light source. Here, we present measurements with a shoe-box-size Fabry-Perot interferometer spectrograph (resolving power of ca. 150000), performed in the crater of Nyiragongo volcano. By analysing the light of a ultraviolet light emitting diode that is sent through the hot gas emission of an active lava flow, we reach an OH detection limit of about 20 ppb, which is orders of magnitude lower than the mixing ratios predicted by high-temperature chemical models. Furthermore, we introduce example calculations that demonstrate the feasibility of skylight-based remote sensing of HF and HCl in the short-wave infrared with Fabry-Perot interferometer spectrographs, which opens the path to continuous monitoring and data acquisition during all stages of volcanic activity. This is only one among many further potential applications of remote sensing of volcanic gases with high spectral resolution
Abel Symposia
Discrete Morse theory has recently lead to new developments in the theory of random geometric complexes. This article surveys the methods and results obtained with this new approach, and discusses some of its shortcomings. It uses simulations to illustrate the results and to form conjectures, getting numerical estimates for combinatorial, topological, and geometric properties of weighted and unweighted Delaunay mosaics, their dual Voronoi tessellations, and the Alpha and Wrap complexes contained in the mosaics
New Physics in Bs -> J/psi phi: a General Analysis
Recently, the CDF and D0 collaborations measured indirect CP violation in Bs
-> J/psi phi and found a hint of a signal. If taken at face value, this can be
interpreted as a nonzero phase of Bs-Bsbar mixing (beta_s), in disagreement
with the standard model, which predicts that beta_s ~= 0. In this paper, we
argue that this analysis may be incomplete. In particular, there can be new
physics (NP) in the bbar -> sbar c cbar decay. If so, the value of beta_s is
different than for the case in which NP is assumed to be present only in the
mixing. We have examined several models of NP and found that, indeed, there can
be significant contributions to the decay. These effects are consistent with
measurements in B -> J/psi K* and Bd -> J/psi Ks. Due to the NP in the decay,
polarization-dependent indirect CP asymmetries and triple-product asymmetries
are predicted in Bs -> J/psi phi.Comment: 28 pages, JHEP, no figures. Considerable changes made. Abstract and
main text of paper modified to alter presentation. Appendix added. References
added. Conclusions unchanged
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