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| Titel |
Laser Absorption spectrometer instrument for tomographic 2D-measurement of climate gas emission from soils |
| VerfasserIn |
Anne Seidel, Steven Wagner, Andreas Dreizler, Volker Ebert |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097885
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| Publikation (Nr.) |
 EGU/EGU2014-13507.pdf |
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| Zusammenfassung |
| One of the most intricate effects in climate modelling is the role of permafrost thawing during
the global warming process. Soil that has formerly never totally lost its ice cover now
emits climate gases due to melting processes[1]. For a better prediction of climate
development and possible feedback mechanisms, insights into physical procedures
(like e.g. gas emission from underground reservoirs) are required[2]. Therefore, a
long-term quantification of greenhouse gas concentrations (and further on fluxes) is
necessary and the related structures that are responsible for emission need to be
identified. In particular the spatial heterogeneity of soils caused by soil internal
structures (e.g. soil composition changes or surface cracks) or by surface modifications
(e.g. by plant growth) generate considerable complexities and difficulties for local
measurements, for example with soil chambers. For such situations, which often cannot
be avoided, a spatially resolved 2D-measurement to identify and quantify the gas
emission from the structured soil would be needed, to better understand the influence
of the soil sub-structures on the emission behavior. Thus we designed a spatially
scanning laser absorption spectrometer setup to determine a 2D-gas concentration
map in the soil-air boundary layer. The setup is designed to cover the surfaces in
the range of square meters in a horizontal plane above the soil to be investigated.
Existing field instruments for gas concentration or flux measurements are based
on point-wise measurements, so structure identification is very tedious or even
impossible. For this reason, we have developed a tomographic in-situ instrument
based on TDLAS (“tunable diode laser absorption spectroscopy”) that delivers
absolute gas concentration distributions of areas with 0.8m x 0.8m size, without any
need for reference measurements with a calibration gas. It is a simple and robust
device based on a combination of scanning mirrors and reflecting foils, so that only
very little optical alignment is necessary in the field. The measurement rate for a
complete 2D field is presently up to 2.5 Hz. The measurement field size is currently
limited only by laboratory conditions and could be extended easily to the range
of several meters, as previous tests have confirmed[3]. A fast laser tuning rate of
more than 5 kHz leads to high measurement path density, and overall more than
70% of a square shaped field area is covered. With this instrument, measurements
of H2O- and CH4 – concentration distributions have taken place so far. We are
going to discuss the instrument setup and the spectroscopic performance and present
numerical studies concerning the tomographic reconstruction quality as well as first 2D
reconstructions in the laboratory. The applicability to 2D CO2 detection and the
improvement of frame rate and reconstruction quality using faster laser tuning will be
discussed.
[1] K. M. Walter Anthony, P. Anthony, G. Grosse, and J. Chanton, “Geologic methane seeps
along boundaries of Arctic permafrost thaw and melting glaciers,” Nat. Geosci., vol. 5, no. 6,
pp. 419–426, May 2012.
[2] B. Elberling, A. Michelsen, C. Schädel, E. A. G. Schuur, H. H. Christiansen, L. Tamstorf,
M. P. Berg, and C. Sigsgaard, “Long-term CO2 production following permafrost thaw,” Nat.
Clim. Chang., vol. 3, no. 10, pp. 890–894, 2013.
[3] A. Seidel, S. Wagner, and V. Ebert, “TDLAS-based open-path laser hygrometer using
simple reflective foils as scattering targets,” Appl. Phys. B, vol. 109, no. 3, pp. 497–504, Oct.
2012. |
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