Investigating stratospheric chlorine chemistry by SCIAMACHY measurements in limb and occulation geometry: Retrieval and intepretation of vertical OCIO profiles
Final Report Abstract
Algorithms for the retrieval of vertical profiles of NO 2 , BrO and OClO from the SCIAMACHY limb observations were developed. The whole SCIAMACHY dataset (August 2002 to April 2012) was processed and the results were validated (mainly to correlated balloon measurements) and compared to other existing retrievals and model results. Global datasets for NO 2 , BrO and OClO profiles were created and stored in a relational database together with meteorological and EMAC simulation data. All validation studies performed showed a good to very good agreement. For NO 2 the agreement to balloon profiles is excellent to very good and for BrO very good to good, depending on latitude and season. For OClO, there are no correlated balloon measurements but the comparison to previous data in similar conditions also showed a good agreement regarding the magnitude of the number densities and the altitude of the peak. Also the comparison studies to other datasets revealed that the vertical distribution and the seasonal cycles of NO 2 , BrO and OClO are retrieved with large accuracy and precision. Thus, the quality of the retrievals is equal to that from “full retrieval” methods which need much more computational resources. During the course of the project, two major improvements were developed that also lead to the good quality of the profiles: The tomographic retrieval approach significantly reduces the systematic error for situations with large horizontal gradients. The Taylor series expansion for the optical depth of ozone largely improves the DOAS results in the UV region. Both improvements are not only important for the retrieval from SCIAMACHY limb measurements, but also might be of great benefit for future missions. The Taylor series method might also have advantages for ground based retrievals of BrO and other absorbers that are masked by strong absorptions by e.g. ozone. The global NO 2 profiles from SCIAMACHY measurements of backscattered light are - along with those from OSIRIS - a valuable addition to occultation measurements which generally provide more accurate profiles but have much less global coverage. While SCIAMACHY is one of a very few instruments that provides BrO profiles on a long time basis and with global coverage, the dataset of daytime OClO profiles created in this project is the first global and long term dataset from observations. It constitutes a new possibility for monitoring the stratospheric chlorine activation and - together with the observations of ClO profiles and OClO columns - largely improves the understanding of stratospheric chlorine chemistry in particular during polar winter time. The retrieved datasets were applied in various scientific studies. For NO 2 studies on the location of stratospheric transport barriers and related processes are ongoing. Also, the NO 2 profiles could be applied in a “limb nadir matching” approach that significantly improves the retrieval of tropospheric NO 2 from the SCIAMACHY nadir observations. For BrO an inter-comparison study of different retrievals and their results was performed, which reduced the uncertainty regarding the total bromine budget and thereby indirectly confirmed the contribution of Very Short Lived Substances (VSLS). For OClO a strong dependence on the meteorology (T, PV) was found, in particular strong chlorine activation occurs only for temperatures below T NAT. Also a very clear relation to the observed ozone depletion, the area for possible PSC formation (A PSC ) and a good agreement of the nadir and limb observations could be found. Also the datasets were compared to results of the EMAC simulation. While there is a very good agreement for NO 2 and BrO globally, larger differences occur for the polar region, especially for the Arctic. These discrepancies are even larger for OClO and could be attributed to short-comings of the simulation of the dynamics, causing too high temperatures during winter and thus too small chlorine activation and OClO. This issue could be solved by a stronger nudging of the EMAC stratospheric meteorology to ECMWF values. The remaining discrepancies are much smaller and now investigated in detail.
Publications
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Accounting for the effect of horizontal gradients in limb measurements of scattered sunlight. Atmos. Chem. Phys., 8, 3045-3060, 2008
Pukite, J., Kühl, S., Deutschmann, T., Platt, U., and Wagner, T.
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SCIAMACHY Limb Measurements of NO2, BrO and OCIO. Retrieval of vertical profiles: Algorithm, first results and validation. Adv. Space Research, 42, 1747-1764, 2008
Kühl, S., J. Pukite, T. Deutschmann, U. Platt, and T. Wagner
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Extending differential optical absorption spectroscopy for limb measurements in the UV. Atmos. Meas. Tech., 3, 631-653, 2010
Pukite, J., Kühl, S., Deutschmann, T., Platt, U., and Wagner, T.
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Retrieval of tropospheric column densities of NO2 from combined SCIAMACHY nadir/limb measurements. Atmos. Meas. Tech., 3, 283-299, 2010
Beirle, S., Kühl, S., Puķīte, J., and Wagner, T.
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Retrieval of vertical profiles of stratospheric trace gases from limb observations of the SCIAMACHY instrument on ENVISAT. PhD Thesis, Universität Heidelberg, Heidelberg, 2010
Janis Pukite
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The effect of horizontal gradients and spatial measurement resolution on the retrieval of global vertical NO2 distributions from SCIAMACHY measurements in limb only mode. Atmos. Meas. Tech., 3, 1155-1174, 2010
Pukite, J., Kühl, S., Deutschmann, T., Dörner, S., Jöckel, P., Platt, U., and Wagner, T.
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BrO vertical distributions from SCIAMACHY limb measurements: comparison of algorithms and retrieval results. Atmos. Meas. Tech., 4, 1319-1359, 2011
Rozanov, A., Kühl, S., Doicu, A., McLinden, C., Puķīte, J., Bovensmann, H., Burrows, J. P., Deutschmann, T., Dorf, M., Goutail, F., Grunow, K., Hendrick, F., von Hobe, M., Hrechanyy, S., Lichtenberg, G., Pfeilsticker, K., Pommereau, J. P., Van Roozendael, M., Stroh, F., and Wagner, T.
