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| Titel |
Strategy for high-accuracy-and-precision retrieval of atmospheric methane from the mid-infrared FTIR network |
| VerfasserIn |
R. Sussmann, F. Forster, M. Rettinger, N. Jones |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 4, no. 9 ; Nr. 4, no. 9 (2011-09-20), S.1943-1964 |
| Datensatznummer |
250002100
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| Publikation (Nr.) |
 copernicus.org/amt-4-1943-2011.pdf |
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| Zusammenfassung |
We present a strategy (MIR-GBM v1.0) for the retrieval of column-averaged
dry-air mole fractions of methane (XCH4) with a precision <0.3%
(1-σ diurnal variation, 7-min integration) and a seasonal bias
<0.14% from mid-infrared ground-based solar FTIR measurements of the
Network for the Detection of Atmospheric Composition Change (NDACC,
comprising 22 FTIR stations). This makes NDACC methane data useful for
satellite validation and for the inversion of regional-scale sources and
sinks in addition to long-term trend analysis. Such retrievals complement
the high accuracy and precision near-infrared observations of the younger
Total Carbon Column Observing Network (TCCON) with time series dating back
15 years or so before TCCON operations began.
MIR-GBM v1.0 is using HITRAN 2000 (including the 2001 update release) and 3
spectral micro windows (2613.70–2615.40 cm−1, 2835.50–2835.80 cm−1,
2921.00–2921.60 cm−1). A first-order Tikhonov constraint
is applied to the state vector given in units of per cent of volume mixing
ratio. It is tuned to achieve minimum diurnal variation without damping
seasonality. Final quality selection of the retrievals uses a threshold for
the goodness of fit (χ2 < 1) as well as for the ratio of
root-mean-square spectral noise and information content (<0.15%).
Column-averaged dry-air mole fractions are calculated using the retrieved
methane profiles and four-times-daily pressure-temperature-humidity profiles
from National Center for Environmental Prediction (NCEP) interpolated to the
time of measurement.
MIR-GBM v1.0 is the optimum of 24 tested retrieval strategies (8 different
spectral micro-window selections, 3 spectroscopic line lists: HITRAN 2000,
2004, 2008). Dominant errors of the non-optimum retrieval strategies are
systematic HDO/H2O-CH4 interference errors leading to a seasonal
bias up to ≈5%. Therefore interference errors have been
quantified at 3 test sites covering clear-sky integrated water vapor levels
representative for all NDACC sites (Wollongong maximum = 44.9 mm, Garmisch
mean = 14.9 mm, Zugspitze minimum = 0.2 mm). The same quality ranking of the
24 strategies was found for all 3 test sites with one optimum, i.e. MIR-GBM v1.0.
Seasonality of XCH4 above the Zugspitze (47° N) shows a minus-sine
shape with a minimum in March/April, a maximum in September, and an
amplitude of 16.2 ± 2.9 ppb (0.94 ± 0.17%). This agrees well with
the WFM-DOAS v2.0 scientific XCH4 retrieval product.
A conclusion from this paper is that improved spectroscopic parameters for
CH4, HDO, and H2O in the 2613–2922 cm−1 spectral domain
are urgently needed. If such become available with sufficient accuracy, at
least two more spectral micro windows could be utilized leading to another
improvement in precision. The absolute inter-calibration of NDACC MIR-GBM
v1.0 XCH4 to TCCON data is subject of ongoing work. |
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