3D tomography for SCIAMACHY limb and nadir measurements: retrieval of stratospheric NO2, BrO and OClO profiles and their application for the investigation of stratospheric chemistry
Zusammenfassung der Projektergebnisse
The 2D tomographic trace gas retrieval algorithm from SCIAMACHY measurements in limb geometry was improved by considering also gradients in across track direction and by introducing a 3D tomographic algorithm, which accounts for horizontal inhomogeneities not only along satellite track but also across track. It was found that the consideration of 3D effects is especially important for measurements at very high solar zenith angles (close to 90°), especially for the very reactive trace gas OClO (which is used as an indicator for Chlorine activation). Here, also accounting for the across track scanning features is very important. Further, also nadir measurements were incorporated as additional information source in the retrieval algorithm. While a significant correction effect on the retrieved trace gas profiles was obtained, the improvement is largely limited to measurements at high solar zenith range because 1) high amounts of OClO are only observed there, 2) NO2 can have significant tropospheric contributions, for which the nadir measurements are sensitive. Thus they can largely interfere with the stratospheric signal. For BrO the scheme seems not to be applicable because of the often occuring strong tropospheric signals, especially at high solar zenith angles. Also the feasibility of a “spectroscopic tomography” was investigated. Here, limb measurements at different wavelength regions were combined in one retrieval utilizing their different spatial sensitivities caused by wavelength dependency of Rayleigh-scattering. Consistency checks and comparison with result obtained by the ‘spatial tomography’ showed very promising agreement for the lowermost stratosphere both in the magnitude of the obtained profile correction as well as for the regions where the horizontal gradient is expected to be strongest. Concerning the spectral evaluation part of the retrieval, the consideration of nonlinearities in cases of strong absorption was improved. The quantitative parameterization of these higher order effects was introduced by expanding the radiative transfer equation in Taylor series, defining higher order absorption derivatives and implementing their calculation in the radiative transfer model. Since the nonlinearity is just a linear scaling of these derivatives with the (mathematical) products of the absorber vertical column densities, the formalism is easy to use in applications of Differential optical absorption spectroscopy (DOAS). Besides quantifying the effects of the Taylor series approach implemented before, it allowed improving the retrieval of trace gas profiles by means of an iterative algorithm, also significantly reducing the effect of the a-priori information. The algorithm was kept time-efficient because these terms can be calculated at zero absorption background thus showing only a smooth wavelength dependency which can be approximated by a polynomial. Unfortunately, it was impossible to process the announced reprocessing of the whole SCIAMACHY dataset because the delivery of the final and consistent level 1b spectra was postponed several times by ESA. Despite several promises by ESA it was still not available even after several extensions of the project. Consequently it was also impossible to start with the scientific interpretation of the whole time series and to compare the results of the new retrieval algorithm with chemistry models. A comparison of the already existing SCIAMACHY BrO and NO2 dataset were performed while validating ground based zenith sky DOAS observations over Kiruna showing a pretty good agreement (around 20% for NO2, 50% for BrO). Last but not least also an aerosol retrieval algorithm for SCIAMACHY limb observations was developed in our group based on the onion peeling approach. Since during large volcanic eruptions aerosols can propagate to high altitudes (even into the stratosphere) and such plumes naturally show strong horizontal gradients by far exceeding those of trace gases, it was interesting to also investigate 3D effects on the aerosol retrieval. It was shown that the retrieved extinction profiles are largely affected. It was demonstrated that the correct consideration of the horizontal distribution is important in order to derive the correct vertical profile, for example to assess whether a volcanic plume has reached the stratosphere or not.
Projektbezogene Publikationen (Auswahl)
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Characterisation of a stratospheric sulfate plume from the Nabro volcano using a combination of passive satellite measurements in nadir and limb geometry, Atmos. Chem. Phys., 14, 8149-8163
Penning de Vries, M. J. M., Dörner, S., Puķīte, J., Hörmann, C., Fromm, M. D., and Wagner, T.
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Stratospheric aerosol extinction retrieved from SCIAMACHY measurements in limb geometry, PhD Thesis, Max Planck graduate centre with University Mainz
Dörner Steffen
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Quantification and parametrization of non-linearity effects by higherorder sensitivity terms in scattered light differential optical absorption spectroscopy, Atmos. Meas. Tech., 9, 2147-2177
Puķīte, J. and Wagner, T.
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Long term trends of Stratospheric trace gases from ground-based DOAS observations of Kiruna, Sweden, PhD Thesis, University Heidelberg
Gu, Myojeong
