Determination of atmospheric trace gas amounts and corresponding natural isotopic ratios by means of ground-based FTIR spectroscopy in the high Arctic

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.