8. Summary, Conclusions and Further Work

In this thesis an investigation of the type and extent of the errors in retrieved vertical profiles that may be expected to arise as a result of common line shape defects is reported (chapter 4), and the results assessed with reference to the Bruker IFS 120M. A forward model and retrieval scheme were developed for the purpose.

It has been shown that variations in the size of the field stop that are within the manufacturing tolerances can give rise to an artefactual subsidence of up to 10% in the retrieved profile, and that for a signal-to-noise level of 300, any measurement of the instrument line shape must be accurate to better than 0.6% with respect to width (section 4.6.1). Similar studies have been carried out to investigate the effects of asymmetry in the instrument line shape with the result that the maximum tolerable asymmetry in the line shape for a measurement with a signal-to-noise ratio of 300 is 2.4% (section 4.6.2).

The results of these simulations have been considered with respect to the requirements of atmospheric models. These impose the conditions that the vertical profile of such species as hydrogen chloride be known to better than 10% but also that there is no additional gain to be derived from reducing the errors below 5%. This places upper limits of 2.5% and 5.6% respectively, on the asymmetry and broadening of the line shape, and lower limits of 1.3% and 3.6%.

The development of a portable laser system for measuring instrument line shapes, simultaneously with the acquisition of atmospheric and low pressure cell spectra, has been reported. Laser and hydrogen bromide spectra recorded at Table Mountain, California, have been presented along with the results of attempts to fit the measured hydrogen bromide lines with simulations based on the laser measurements and on calculated instrument line shapes. Calculation of the line shapes involved simulation of the hyperfine spectra of hydrogen bromide; this has been appended.

Laser system temperature and power data recorded at Lauder, New Zealand, have been presented and show evidence of temperature drift in the laser cavity giving rise to frequency drift and mode hopping in the infrared laser. These data have been used to assess the frequency drift rate of the infrared laser and, hence, estimate the degree of asymmetry that might be expected in the measured laser line.

The possibility that a system such as this could be used to provide valid simultaneous measurements of the instrument line shape of Fourier spectrometers has been demonstrated although such measurements were not obtained in this case. It has been shown that the use of a low pressure gas cell provides a useful check on the laser measurement.

Difficulties were experienced in the use of this system leading to a lack of reproducibility, and unacceptable levels of asymmetry in the line shape measurements. This asymmetry could be caused by:

- error in the phase correction for the spectrum;
- internal misalignment of the interferometer;
- misalignment of the infrared laser to the spectrometer;
- changes in the frequency of the spectrometer's internal reference laser during a measurement;
- changes in the frequency of the infrared laser during a measurement;
- non-uniform illumination of the field stop.

The possible causes of these effects have been discussed.

The conclusions, with respect to the declared aims of this work (section 1.5) are therefore:

- the errors in retrieved vertical profiles resulting from deviations of the instrument line shape from the theoretical have been quantified and used to define the necessary accuracy of any line shape measurement system;

- a portable means of measuring the instrument line shape of Fourier spectrometers simultaneously with making atmospheric measurements for routine quality assurance and to reduce the errors on retrieved vertical profiles has been developed;

- to date, this system has been used as part of the National Physical Laboratory's programme of instrument intercomparisons for the validation of NDSC FTIR measurements at Table Mountain, California and at Lauder, New Zealand, but thermal drift and alignment problems have seriously hampered measurements.

Suggestions for Further Work

It follows from the work described here that further development of such a laser system as this is worthwhile and necessary if this equipment is to provide reliable, direct measurements of high resolution instrument line shapes, but it must be borne in mind that:

- the infrared laser must remain adequately stable during measurements as frequency drift will cause artefactual asymmetry in the measured line;

- the alignment of the infrared laser system to the spectrometer is highly critical to the enterprise, and further work should be done to attempt to improve this;

- if such components as moving field stops in the spectrometer are to be used in the alignment process, they should be treated with great suspicion;

- the profile of the infrared laser beam should be established by some direct means that cannot not impose any artificial symmetry or uniformity on the resulting beam profile;

- work should be undertaken to quantify the requirements for uniformity on illumination of the field stop by the infrared laser;

- work should be undertaken to quantify the requirements for colinearity of the solar and infrared laser beams.

Summary
Acknowlegements
Contents
Chapters: 1, 2, 3, 4, 5, 6, 7, 8
Appendices
References