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| Titel |
First retrievals of methyl chloride from ground-based high-resolution FTIR solar observations |
| VerfasserIn |
Emmanuel Mahieu, Jeremy Harrison, Peter Bernath, Geoffrey Toon, Curtis Rinsland, Philippe Demoulin, Pierre Duchatelet, Bernard Lejeune, Christian Servais, Martine De Mazière |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046437
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| Zusammenfassung |
| Methyl chloride (CH3Cl) is one of the most abundant chlorine-bearing gas in the Earth’s
troposphere and a significant contributor to the organic chlorine budget. Measurements by in
situ networks indicate a mean volume mixing ratio of 550 pptv, with a significant seasonal
cycle of about 80 pptv, peak to peak. This species also exhibits inter-annual variability, but no
long-term trend. Major sources are from tropical and sub-tropical plants and dead leaves, the
oceans and biomass burning. Some industrial processes and waste incineration further add
to the emissions. Oxidation by the hydroxyl radical is by far the largest CH3Cl
sink, followed by soil uptake. Although balanced, its atmospheric budget is still
affected by large uncertainties and contributions from unidentified sources and
sinks cannot be ruled out. Methyl chloride has an atmospheric lifetime of 1Â year, a
global warming potential of 13 (100-yr horizon) and an ozone depleting potential of
0.02.
The retrieval of methyl chloride from ground-based infrared (IR) spectra is very
challenging. Indeed, numerous interferences by strong water vapor and methane
lines complicate the detection of small CH3Cl absorptions, close to 1%, near 3
microns. In addition, and although weak, ethane features contribute to the difficulty, in
particular since a significant number of ethane branches were absent until very recently
from official HITRAN compilations. Therefore, the scientific literature does not
report thus far about any investigations of CH3Cl from ground-based remote sensing
observations.
In this contribution, we will present first CH3Cl total column retrievals, using the SFIT-2
algorithm (v3.94) and high-resolution Fourier Transform Infrared (FTIR) solar
absorption observations recorded with a Bruker 120HR instrument, at the high altitude
station of the Jungfraujoch (46.5Ë N, 8Ë E, 3580 m asl), within the framework of the
Network for the Detection of Atmospheric Composition Change (NDACC, visit
http://www.ndacc.org).
In our retrievals, we use new ethane absorption cross sections recorded at the Molecular
Spectroscopy Facility of the Rutherford Appleton Laboratory (Harrison et al., 2010). They
were calibrated in intensity by using reference low-resolution spectra from the Pacific
Northwest National Laboratory (PNNL) IR database. These new cross sections were recently
released as a HITRAN update (see http://www.hitran.com). Pseudoline parameters fitted to
these ethane spectra have been combined with HITRAN 2004 line parameters (including all
the 2006 updates) for all other species encompassed in the selected microwindows, including
our target CH3Cl.
We will evaluate the improvement brought by the new ethane line parameters on the
fitting residuals, and characterize the quality, the precision and the reliability of the retrieved
product. If successful, a long-term CH3Cl total column time series will be produced using the
Jungfraujoch observational database, and we will perform preliminary investigations of the
seasonal and inter-annual variations of methyl chloride total columns at northern
mid-latitudes.
Acknowledgments
The University of Liège contribution to the present work has primarily been supported by
the SSD and PRODEX programs funded by the Belgian Federal Science Policy Office,
Brussels. We thank the International Foundation High Altitude Research Stations
Jungfraujoch and Gornergrat (HFSJG, Bern) for supporting the facilities needed to
perform the observations. The Molecular Spectroscopy Facility of the Rutherford
Appleton Laboratory is funded by the Natural Environment Research Council,
UK.
References
Harrison, J.J., N.D.C. Allen, and P.F. Bernath, Infrared absorption cross sections for
ethane (C2H6) in the 3 μm region, J. Quant. Spectrosc. Radiat. Transfer, 111, 357-363,
2010.
Rothman, L.S., D. Jacquemart, A. Barbe et al., The HITRAN 2004 molecular
spectroscopic database, J. Quant. Spectrosc. Radiat. Transfer, 96, 139-204, 2005.
WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion:
2006, Global Ozone Research and Monitoring Project—Report No. 50, 572 pp., Geneva,
Switzerland, 2007. |
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