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50 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.3.3 Absolutely calibrated solar irradiance measurements Participating scientists Lindner, A., H. Bösch, A. Butz, M. Dorf, F. Weidner, and K. Pfeilsticker Abstract Absolutely calibrated solar irradiance spectra are measured from aboard the LPMA/ DOAS balloon gondola at around 30 to 35 km altitude. After accounting for residual atmospheric extinction the findings are compared to previous observations and discussed with respect to implications for atmospheric photochemistry and the Earth’s climate. Figure 2.22: Ratio of solar irradiance spectra referenced to the solar spectrum of Kurucz et al. [1984] with recent updates of Fontenla et al. [1999] (MODTRAN 3.7) with (a) the present balloon borne measurement, (b) SORCE/SIM Harder et al. [2000], (c) SCIAMACHY/ENVISAT in channel 1, 2, 3, and 4 using the revised IUP-Bremen calibration. Background Solar radiation is the driving force for climate and thus for life on Earth. Evidently, the extraterrestrial solar irradiance Eo and its temporal evolution is of primary interest for atmospheric spectroscopy, photochemistry, climate and the solar cell industry as well. Although Eo is monitored by a large number of space-borne, airborne and ground-based instruments, to date, a consensus on Eo could only be achieved within few percents in the UV-A and visible spectral wavelength ranges mainly due to given problems with the absolute calibration of the radiation measurements and long term drifts of the various sensors. Funding The present work has been supported by ESA, BMBF, DLR and the European Union. Methods and results Here, direct Sun observations from aboard the LPMA/DOAS balloon payload are used to absolutely infer Eo in two wavelength bands in the UV/visible spectral range. After an on-ground spectro-radiometrical calibration has been performed for each balloon flight, the solar irradiance measurements are conducted at about 30 to 35 km balloon float altitude where the remaining atmospheric extinction is minimal and can be accounted for easily. Thorough tracking and removal of systematic errors results in high-accuracy measurement of the extraterrestrial solar irradiance. Comparison of our balloon borne data set with previous studies yields excellent agreement, see Figure 2.22, and suggests that, to date, the integrated Eo is uncertain by as much as 5 W/m 2 in the respective wavelength range Gurlit et al. [2005]. Outlook/Future work • Quality check of the standard Eo recommendation by further refinement of the calibration procedure • Long term validation, monitoring and recalibration of satellite-borne Eo measurements • Monitoring of natural solar variability Main publications Lindner A., Ballongestütze Messungen der extraterrestrischen spektralen solaren Irradianz, Diploma thesis, University of Heidelberg, Germany, 2005.
2.3. RADIATIVE TRANSFER 51 2.3.4 Atmospheric detection of water dimers and their contribution to solar shortwave absorption Participating scientists Lotter, A., C . Peters, J. Schwärzle, and K. Pfeilsticker Abstract The cause of the water vapor continuum and possible contributions to it by water dimers is one of the most challenging question in atmospheric spectroscopy. Field measurements by means of active long-path differential optical absorption spectroscopy (DOAS) were performed to probe the atmosphere for water vapor continuum and dimer absorption. Figure 2.23: Absorption cross section for water monomer (WM) and water dimer (WD). Due to the overlap, WD absorption is masked by strong WM absorption, except for the three marked regions, where detection may be possible. Background Disagreement in Earth’s radiation budget between observations and radiative transfer models has been a major concern for several decades, as there exists a significant level of solar shortwave (SW) cloudy-sky absorption, commonly referred to as excess absorption, beyond the ability of any model to predict. Many attempts have been made to explain the observed excess of solar SW absorption, but all potential mechanisms on their own fall far short of explaining a possible absorption deficit as much as 30 W/m 2 . One mechanism of the required additional absorption was suggested to come from a combined contribution of the water continuum and water dimers (WD). WDs may also play an important role in atmospheric physics by e.g., initiating homogeneous nucleation of water and photo-chemistry by catalyzing chemical reactions (e.g., in the formation of H2SO4). Methods Possible atmospheric WD absorption was investigated by extreme long path (up to 29 km) near ocean surface absorption measurements during 3 field campaigns conducted in Northern Germany in May 2002, France in June 2003, tropical Brazil in Dec./Jan. 2004. Since the near-IR absorption spectrum of WD is predicted to largely overlap with that of WM, 3 almost interference free WD lines were selected for the study (see Figure 2.23). Here, the dependance of WD absorption on the WM pressure squared is used as an indicator for WD detection. Results Although a WD absorption line at 13340 cm −1 was tentatively identified to be due to WD absorption (Pfeilsticker et al. [2003]), subsequent campaigns involving a larger WM concentration (e.g., in Brazil) could not confirm its existence. However, the latter measurements indicate a possible WD absorption band at 16312 cm −1 . Uncertainties, however, still remain in WD identification due to uncertainties in the predicted WD line position (up to 200 cm −1 ) and width (10– 200 cm −1 FWHM). The latter being a function as to whether WD are truly bound, meta-stable or simply free pair states. WD detection is also hindered by uncertainties in the WM data bases in particular for the WM weak lines. Funding comes through the Deutsche ForschungsGemeinschaft (PF 384/1-2 and 2-3). Outlook More sensitive detection of WD absorption will require higher quality WM and WD data than existing. Main publication Lotter A., Field measurements of water continuum and water dimer absorption by active long-path Differential Optical Absorption Spectroscopy (DOAS), PhD thesis, <strong>Universität</strong> Heidelberg, Heidelberg, Germany, in preparation.
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7.3. PHD THESES 217 PhD Theses Baye
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7.5. INVITED TALKS 221 Invited Talk
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Institut für Umweltphysik Im Neuen
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