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

Abstract

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.

• author: Meier, Arndt
• supervisor: Uwe Keyser
• opponent: Professor Keyser, Uwe, Technical University of Braunschweig; Professor Weidelt, Peter, Technical University of Braunschweig; Professor Larink, O., Technical University of Braunschweig
• publishing date: 1997
• type: Thesis
• publication status: published
• subject: Earth and Related Environmental Sciences
• pages: 309 pages
• defense location: Technical University of Braunschweig, Germany
• defense date: 1997-03-24 10:00:00
• language: English
• LU publication?: no
• id: 86e93387-3ad6-44d8-8651-9168742d1704 (old id 5266380)
• alternative location: http://epic.awi.de/33164/1/BerPolarforsch1997236.pdf
• date added to LUP: 2016-04-01 15:36:40
• date last changed: 2018-11-21 20:35:22