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| Titel |
Pointing errors in solar absorption spectrometry – correction scheme and its validation |
| VerfasserIn |
A. Reichert, P. Hausmann, R. Sussmann |
| 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 ; 8, no. 9 ; Nr. 8, no. 9 (2015-09-16), S.3715-3728 |
| Datensatznummer |
250116575
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| Publikation (Nr.) |
 copernicus.org/amt-8-3715-2015.pdf |
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| Zusammenfassung |
| A method for quantification of sun-pointing inaccuracies in solar absorption
spectrometry is presented along with a correction scheme for the resulting
errors in trace gas vertical column or profile retrievals. A posteriori
correction of pointing errors requires knowledge of both coordinates of the
mispointing vector on the solar disk. In principle, quantitative information
on the mispointing can be retrieved from Doppler shifts of solar lines
derived from measured spectra. However, this yields only one component of the
mispointing vector, namely the one which is perpendicular to the solar
rotation axis. Missing information on the second vector component has
hindered a posteriori correction of mispointing errors so far. Our idea of how to
overcome this problem is to obtain estimates of both coordinates of the
mispointing by combining subsequent measurements with differing orientations
of the solar rotation axis relative to the zenith direction. The proposed
concept is suitable in the case of systematic mispointing, i.e., if the
mispointing is approximately constant within a given set of measurements. An
implementation of this original concept is demonstrated using measurements
from the solar absorption Fourier transform infrared (FTIR) spectrometer at
the Zugspitze (47.42° N, 10.98° E, 2964 m a.s.l.).
Soundings in the September 2012–September 2014 time interval were impacted
by mispointing problems due to a non-optimum solar tracking optics
configuration. They show a mean mispointing in the zenith direction of
−0.063°. This causes biases in vertical soundings of trace gases,
e.g., −2.82 ppb in monthly means of dry-air column-averaged mole
fractions of methane (XCH4). Measurements made with the more stable
pre-September 2012 and post-September 2014 optics configurations show
considerably smaller mispointing effects. Applying the mispointing
correction, the April 2006–March 2014 XCH4 trend determined from
Zugspitze measurements is reduced from 6.45 [5.84, 7.04] to 6.07 [5.55,
6.59] ppb yr−1. The correction thereby restores consistency with
results from the nearby Garmisch FTIR site (47.48° N,
11.06° E, 743 m a.s.l.). The mispointing correction is
applicable to solar absorption measurements in the mid-infrared and near
infrared. It will be of particular benefit for refining existing records of
high-accuracy-and-precision greenhouse gas soundings for the purpose of
improved trend analysis or source–sink inversions. |
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