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| Titel |
Towards the retrieval of tropospheric ozone with the Ozone Monitoring Instrument (OMI) |
| VerfasserIn |
T. Mielonen, J. F. de Haan, J. C. A. Peet, M. Eremenko, J. P. Veefkind |
| 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. 2 ; Nr. 8, no. 2 (2015-02-09), S.671-687 |
| Datensatznummer |
250116127
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| Publikation (Nr.) |
 copernicus.org/amt-8-671-2015.pdf |
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| Zusammenfassung |
| We have assessed the sensitivity of the operational Ozone Monitoring
Instrument (OMI) ozone profile retrieval algorithm to a number of a priori
and radiative transfer assumptions. We studied the effect of stray light
correction, surface albedo assumptions and a priori ozone profiles on the
retrieved ozone profile. Then, we studied how to modify the algorithm to
improve the retrieval of tropospheric ozone. We found that stray light
corrections have a significant effect on the retrieved ozone profile but
mainly at high altitudes. Surface albedo assumptions, on the other hand, have
the largest impact at the lowest layers. Choice of an ozone profile
climatology which is used as a priori information has small effects on the
retrievals at all altitudes. However, the usage of climatological a priori
covariance matrix has a significant effect. Based on these sensitivity tests,
we made several modifications to the retrieval algorithm: the a priori ozone
climatology was replaced with a new tropopause-dependent climatology, the a
priori covariance matrix was calculated from the climatological ozone
variability values, and the surface albedo was assumed to be linearly
dependent on wavelength in the 311.5–330 nm channel. As expected, we found
that the a priori covariance matrix basically defines the vertical
distribution of degrees of freedom for a retrieval. Moreover, our case study
over Europe showed that the modified version produced over 10% smaller
ozone abundances in the troposphere which reduced the systematic
overestimation of ozone in the retrieval algorithm and improved
correspondence with Infrared Atmospheric Sounding Instrument (IASI)
retrievals. The comparison with ozonesonde measurements over North America
showed that the operational retrieval performed better in the upper
troposphere/lower stratosphere (UTLS), whereas the modified version improved
the retrievals in the lower troposphere and upper stratosphere. These
comparisons showed that the systematic biases in the OMI ozone profile
retrievals are not caused by the a priori information but by some still
unidentified problem in the radiative transfer modelling. Instead, the a
priori information pushes the systematically wrong ozone profiles towards the
true values. The smaller weight of the a priori information in the modified
retrieval leads to better visibility of tropospheric ozone structures,
because it has a smaller tendency to damp the variability of the retrievals
in the troposphere. In summary, the modified retrieval unmasks systematic
problems in the radiative transfer/instrument model and is more sensitive to
tropospheric ozone variation; that is, it is able to capture the tropospheric
ozone morphology better. |
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