Determination of atmospheric trace gas amounts and corresponding natural isotopic ratios by means of ground-based FTIR spectroscopy in the high Arctic
Sammanfattning: Abstract In the given report, scientific results gathered at the Alfred- Wegener- Institut für Polar- und Meeresforschung (AWI) within an ScD contract are summarized. The AWI is the German large scale research facility for polar- and marine research. In January 1992 +he new research de- partment at Potsdam has been established with the section Physics und Chemistry of the Atmosphere. 1t coordinates the activities at the Ger- man polar research station Carl Koldewey on Spitsbergen (7g0N, 12OE), which is a primary site of the Network for Detection of Strutospheric Change (NDSC). Data recorded at this arctic research facility make up an important basic contribution to this report. There are a lot of different measurement techniques and gauges avail- able to detect atmospheric gases. The main topic in this work is the ground-ba.sed Fourier-Transform Infrared (FTIR) spectrometer. With the Instrument introduced in chapter 1, more than 25 different chemical species abundant in our atmosphere can be quantified with high qual- ity in terms of their total column amount. Additionally, inforrnation on the vertical distribution and on natural isotopic ratlos are derivable for selected molecules. The operational principle of an ideal FTS (Fourier transform spectrom- eter) and the limitations to real instruments are described in the first chapter. In the following chapter, the principles of the measurement and the analysis procedure are introduced. Basically, information is derived from the comparison of a simulated atmosphere with the recorded inter- ferogram that has been Fourier transformed to a spectrum. In the third chapter, improvements to the standard analysis procedure are discussed to increase the scope and quality of derivable results. This includes the correction of emission that adds to the absorption signal and becomes significant in lunar spectra. Further, the use of information available from other Instruments like radio- and ozone-sondes for the model at- mosphere is discussed as well as the formulation of strategies for deducing information on the vertical distribution of trace gases, because the vol- ume mixing ratio (VMR) profiles are usually not known a priori as has to be assumed in the standard analysis. Chapter -1: gives a description of the polar atmosphere. After a brief introduction to the basic dynamics, the principles and the efficiency of the computer programs developed to deduce information on the VMR profiles of selected trace gases are illustrated by results from the partic- ipation in tlie intercomparison experiment of the NDSC performed with synthetic spectza. In the two remaining sections of the chapter, these algorithms are applied to real data. First, the conditions in early arctic summer are described that offer the most favorable conditions for record- ing spectra and the dynamics of the atmosphere are the least complex in this period. Tlie last section deals with data obtained in early spring and include solar and lunar spectra recorded within 1 2 h. It is the first direct comparison of solar and lunar FTIR spectra reported so far. However, the comparison is not straightforward due to the very high dynamical ac- tivity of tlie atmosphere, the possibility of extensive chemical processing, and the restrictions that apply to the recording geometry due to the very low position of the sun just after the end of the polar night. In the next chapter, results from the study of natural isotopic ratios are presented. After a brief introduction to natural isotopic fraction pro- cesses, results from the study of water vapor isotopomers are presented and isotopic studies in methane and HCl are discussed. An isotopomer is a molecule that contains a rare isotope, e.g. HDO with one deuterium atom replacing a hydrogen atom. The second half of the chapter is de- voted to the detailed study of isotopic abundances of ozone. The quality of isotopic ratios is shown to have improved by one order of magnitude compared to previous works and is ascribed to the improvements intro- duced in chapter 3 and 4. This allows for the first time to quantify a significant symmetry-selective isotopic anomaly in stratospheric ozone by means of a ground-based optical Instrument. Moreover, it is so far the first report that includes observations from polar night, which reveals important details about the mechanisms causing the anomaly. The ob- served isotopic signal gives strong evidence for a new symmetry selective fractionation process in contrast to known fractionation processes that all depend exclusively On mass. Nevertheless, the theoretical understanding of ozone formation is still incomplete and the necessary modifications to theory to account for the observed isotopic signals may become relevant in the global ozone discussion, since each ozone molecule in sunlit air masses is destroyed and reformed in the middle and upper stratosphere every 15 to 30 minutes On the average. A Summary of the main conclusions is given in the final chapter. Pro- spects On future tasks conclude the main part of this report. The ap- pendices A and B give additional details on the observation site and on the software that is developed. Appendix C consists of an atlas showing some 60 spectral microwindows used in the analysis. Besides 2 figures per interval showing a typical fit and the individual contributions from inter- fering species, additional hints on parameter settings used in the retrieval are listed.
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