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| Titel |
Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO2, CH4 and CO over concentrations representative of "clean air" and "polluted plumes" |
| VerfasserIn |
T. E. L. Smith, M. J. Wooster, M. Tattaris, D. W. T. Griffith |
| 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. 1 ; Nr. 4, no. 1 (2011-01-26), S.97-116 |
| Datensatznummer |
250001570
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| Publikation (Nr.) |
 copernicus.org/amt-4-97-2011.pdf |
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| Zusammenfassung |
| When compared to established point-sampling methods, Open-Path Fourier
Transform Infrared (OP-FTIR) spectroscopy can provide path-integrated
concentrations of multiple gases simultaneously, in situ and near-continuously. The
trace gas pathlength amounts can be retrieved from the measured IR spectra
using a forward model coupled to a non-linear least squares fitting
procedure, without requiring "background" spectral measurements unaffected
by the gases of interest. However, few studies have investigated the
accuracy of such retrievals for CO2, CH4 and CO, particularly
across broad concentration ranges covering those characteristic of ambient
to highly polluted air (e.g. from biomass burning or industrial plumes).
Here we perform such an assessment using data collected by a field-portable
FTIR spectrometer. The FTIR was positioned to view a fixed IR source placed
at the other end of an IR-transparent cell filled with the gases of
interest, whose target concentrations were varied by more than two orders of
magnitude. Retrievals made using the model are complicated by
absorption line pressure broadening, the effects of temperature on
absorption band shape, and by convolution of the gas absorption lines and
the instrument line shape (ILS). Despite this, with careful model
parameterisation (i.e. the optimum wavenumber range, ILS, and assumed gas
temperature and pressure for the retrieval), concentrations for all target
gases were able to be retrieved to within 5%. Sensitivity to the
aforementioned model inputs was also investigated. CO retrievals were shown
to be most sensitive to the ILS (a function of the assumed instrument field-of-view),
which is due to the narrow nature of CO absorption lines and their
consequent sensitivity to convolution with the ILS. Conversely, CO2
retrievals were most sensitive to assumed atmospheric parameters,
particularly gas temperature. Our findings provide confidence that
FTIR-derived trace gas retrievals of CO2, CH4 and CO based on
modeling can yield results with high accuracies, even over very
large (many order of magnitude) concentration ranges that can prove
difficult to retrieve via standard classical least squares (CLS) techniques.
With the methods employed here, we suggest that errors in the retrieved
trace gas concentrations should remain well below 10%, even with the
uncertainties in atmospheric pressure and temperature that might arise when
studying plumes in more difficult field situations (e.g. at uncertain
altitudes or temperatures). |
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