Year

2001

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Nitrous oxide is the third most important anthropogenic greenhouse gas after CO2 and CH4 and contributes 6% of the total terrestrial radiative forcing due to greenhouse gases. It is closely involved in the depletion of stratospheric ozone by providing one of the main sources of NO radicals. Biological processes such as nitrification and denitrification are primarily responsible for N2O production. Despite its importance and years of research, the estimates of the global size of N2O sources and sinks remain highly uncertain, and its budget is not yet fully balanced.

Analysis of N2O isotopes can aid in reducing the large uncertainties in source and sink estimates by providing information that is complementary to the N2O mixing ratio. Analysis of the mean 815N and mean 8180 has already lead to some insight on the N2O budget. Until very recently the intramolecular 15N positional isotopes 14N15N16O and 15N14N16O were not measurable by any analytical technique. The intramolecular difference 814N15N16O-815N14N16O is an additional isotopic discriminator than the mean 815N alone, as it directly describes the processes forming the N-N bond in N2O production processes such as nitrification and denitrification.

This thesis describes the development of a high resolution Fourier transform infrared technique for the measurement of the N2O isotopomers 14N15N16O, and 15N14N16O, 14N14N18O and 14N14N17O. The FTIR technique utilises 0.012 cm-1 resolution FTIR spectroscopy, a 2.4 m optical pathlength, 120 mL sample cell, with precise control of sample temperature and pressure. The typical analytical precision of the 14N15N16O and 15N14N16O isotopomers is of the order of 1-2. 0/00, and approximately 3-4 %0 for the oxygen isotopomers 14N14N18O and 14N14N17O. Two independent chemometric multivariate analytical methods were developed for determination of high resolution N2O spectra: multi-micro-window classical least squares, and non-linear least squares. The strengths and limitations of the FTIR technique are analysed and compared to those of the complementary isotope ratio mass spectrometry technique. The FTIR technique is analytically robust and serves as an independent and complementary technique to N2O analysis by isotope ratio mass spectrometry.

The FTIR technique was used to analyse N2O from several contexts. The isotopic fractionation factors in the laboratory photolysis of N2O at three wavelengths were determined by analysis of the unphotolysed N2O fraction. Samples of nitrous oxide were extracted from whole air at an urban location over the period of approximately one year and isotopically characterised. The emissions of N2O from a pig effluent fertilized crop field have been isotopically characterised. In each of the three contexts, results are interpreted in relation to the processes involved.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.