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| Titel |
CHARM-F: An airborne integral path differential absorption lidar for simultaneous measurements of carbon dioxide and methane columns |
| VerfasserIn |
A. Amediek, H.-C. Büdenbender, G. Ehret, A. Fix, C. Kiemle, M. Quatrevalet, M. Wirth, D. Hoffmann, J. Löhring, V. Klein |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062957
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| Zusammenfassung |
| CHARM-F (CO2 and CH4 Atmospheric Remote Monitoring – Flugzeug) is DLR’s airborne
Integral Path Differential Absorption (IPDA) lidar for simultaneous measurements of the
column-weighted average dry-air mixing ratios of atmospheric carbon dioxide and methane,
designed to be flown on DLR’s new High-Altitude, LOng-range research aircraft,
HALO. It is meant to serve as a demonstrator of the use of spaceborne active optical
instruments in inferring atmospheric CO2 and CH4 surface fluxes from total column
measurements by inverse modeling. As it will be shown, this is enabled by HALO’s high
flight altitude and its range of 8000 km, which will make it possible to produce
real-world data at truly regional scales with a viewing geometry and vertical weighting
function similar to those enabled by a space platform. In addition, CHARM-F has the
potential to be used as a validation tool not only for active but also passive spaceborne
instruments utilizing scattered solar radiation for remote sensing of greenhouse
gases.
Building on the expertise from CHARM, a helicopter-borne methane IPDA lidar for
pipeline monitoring developed in collaboration with E.ON, and WALES, DLR’s water vapour
differential absorption lidar, CHARM-F relies on a double-pulse transmitter architecture
producing nanosecond pulses which allows for a precise ranging and a clean separation of
atmospheric influences from the ground returns leading to an unambiguously defined
column. One pulse is tuned to an absorption line of the trace gas under consideration,
the other to a nearby wavelength with much less absorption. The close temporal
separation of 250 μs within each pulse pair ensures that nearly the same spot on
ground is illuminated. The ratio of both return signals is then a direct function of
the column-weighted average dry-air mixing ratio. The two laser systems, one for
each trace gas, use highly efficient and robust Nd:YAG lasers to pump an optical
parametric oscillator (OPO) level which converts the pump radiation to the desired
wavelengths.
Because typical surface CO2 and CH4 sources and sinks alter the total column only by a
few percent, the required precision and accuracy are very stringent. This puts particularly
challenging requirements on the spectral properties of the emitted pulses. To achieve single
mode operation with very high spectral purity, both pumps and OPOs are injection seeded.
Absolute stability of the emitted wavelengths is achieved by locking the seed lasers to
the same absorption lines as those used in the atmosphere by means of a single
absorption cell filled with a mixture of CO2 and CH4, and monitoring the wavelength
deviations between each outgoing laser pulse and the corresponding seed laser to
detect and correct for possible mode pulling effects. Another key requirement is the
monitoring of the relative outgoing pulse energies with high accuracy, which is based
on a specifically designed optical architecture. Assembly and laboratory tests of
the instrument are on-going, the first ground tests are planned for summer 2012. |
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