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| Titel |
Open-path TDL-Spectrometry for a Tomographic Reconstruction of 2D H2O-Concentration Fields in the Soil-Air-Boundary-Layer of Permafrost |
| VerfasserIn |
Anne Seidel, Steven Wagner, Andreas Dreizler, Volker Ebert |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081099
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| Zusammenfassung |
| The melting of permafrost soils in arctic regions is one of the effects of climate change. It is
recognized that climatically relevant gases are emitted during the thawing process, and that
they may lead to a positive atmospheric feedback [1]. For a better understanding of these
developments, a quantification of the gases emitted from the soil would be required.
Extractive sensors with local point-wise gas sampling are currently used for this task, but are
hampered due to the complex spatial structure of the soil surface, which complicates the
situation due to the essential need for finding a representative gas sampling point. For this
situation it would be much preferred if a sensor for detecting 2D-concentration fields of e.g.
water vapor, (and in the mid-term also for methane or carbon dioxide) directly in the
soil-atmosphere-boundary layer of permafrost soils would be available. However, it also has
to be kept in mind that field measurements over long time periods in such a harsh
environment require very sturdy instrumentation preferably without the need for sensor
calibration. Therefore we are currently developing a new, robust TDLAS (tuneable
diode laser absorption spectroscopy)-spectrometer based on cheap reflective foils
[2]. The spectrometer is easily transportable, requires hardly any alignment and
consists of industrially available, very stable components (e.g. diode lasers and glass
fibers). Our measurement technique, open path TDLAS, allows for calibration-free
measurements of absolute H2O concentrations. The static instrument for sampling open-path
H2O concentrations consists of a joint sending and receiving optics at one side of
the measurement path and a reflective element at the other side. The latter is very
easy to align, since it is a foil usually applied for traffic purposes that retro-reflects
the light to its origin even for large angles of misalignment (up to 60°). With this
instrument, we achieved normalized detection limits of up to 0.9 ppmv-
m-
-Hz. For
absorption path lengths of up to 2 m and time resolution of 0.2 sec, we attained
detection limits of 1 ppmv. Furthermore we realized a wide dynamic range covering
concentrations between 200 ppmv and 12300 ppmv. The static spectrometer will now be
extended to a spatially scanning TDL sensor using rapidly rotating polygon mirrors. In
combination with tomographic reconstruction methods, spatially resolved 2D-fields will be
measured and retrieved. The aim is to capture concentration fields with at least 1 m2
spatial coverage with concentration detection faster than 1 Hz rate. We simulated
various measurements from typical concentration distributions (“phantoms”) and used
Algebraic Reconstruction Techniques (ART) to compute the according 2D-fields. The
reconstructions look very promising and demonstrate the potential of the measurement
method. In the presentation we will describe and discuss the optical setup of the
stationary instrument and explain the concept of extending this instrument to a spatially
scanning tomographic TDL instrument for soil studies. Further we present first
results evaluating the capabilities of the selected ART reconstruction on tomographic
phantoms.
[1] E. Schuur, J. G. Vogel, K. G. Crummer, H. Lee, J. O. Sickman, and T. E. Osterkamp, “The
effect of permafrost thaw on old carbon release and net carbon exchange from tundra.,”
Nature, vol. 459, no. 7246, pp. 556–9, May 2009.
[2] A. Seidel, S. Wagner, and V. Ebert, “TDLAS-based open-path laser hygrometer using
simple reflective foils as scattering targets,” Applied Physics B, vol. 109, no. 3, pp. 497–504,
Oct. 2012. |
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