Determination of atmospheric ozone solar infrared, ultraviolet and visible spectroscopy

The importance of ozone in the chemistry and the radiative balance of the atmosphere is well established. Several ground and satellite based methods are available for measuring total column or the vertical distribution of ozone. The most common ground based instruments that measure ozone are the Dobson and the Brewer instruments, while ozone sondes measure vertical distribution of ozone. There are six Dobson instruments located in Australia, and ozone sondes are launched from one location (Laverton, Victoria). Solar atmospheric spectroscopy is an established technique to measure the total column amounts of trace atmospheric gases and aerosols. Ozone has a rich spectrum extending from the far ultraviolet to the microwave region. At Wollongong University a high resolution Fourier transform infrared spectrometer (HR-FTIR), a diode array ultraviolet spectrometer and a diode array visible spectrometer have been coupled to the same solar tracker to ensure all three instruments are viewing the same atmospheric column. One aim of this thesis is to develop a mgged, reliable inexpensive solar ultraviolet and visible spectrometer system, suitable to determine total column atmospheric ozone, based on the diode array spectrometer. The shape of the absorption features of the infrared spectrum is influenced by the temperature and pressure environment of the absorbing molecule, and therefore potentially contains information on the vertical distribution of the absorbing molecule. In contrast, in the ultraviolet and visible regions of the spectrum, the environment of the absorbing molecules has limited influence on the shape of the absorption features, and there is greater potential for retrieving accurate total column atmospheric ozone. The thesis also aims to develop the techniques required to determine total column atmospheric ozone from ground based solar high resolution FTIR, and to assess if the complementary information available from solar infrared, ultraviolet and visible spectra can be used to determine some vertical information on atmospheric ozone. The analysis algorithm requires some prior knowledge of the pressure-temperature and ozone vertical profile. Monthly averaged profiles, compiled from data collected remote from the measurement site (Laverton, 1000km south, and Mascot airport, 80km north) are used. Total column ozone has been retrieved from spectra collected between 1996 and 1999 on the HR-FTIR and diode array spectrometers, and compared with TOMS data. The ratio of ultraviolet to TOMS data is 0.85±0.027. The major contribution to the offset between TOMS and the ultraviolet data is the differences in the absorption coefficients used in the two retrievals and in the scattered light in the ultraviolet spectrometer. The ratio of the visible data to TOMS is 1.01+0.084. The strong absorption by water and oxygen and the weak absorption by ozone in the analysis region contributed to the high standard deviation for the retrieval from the visible spectra. The standard deviation of ultraviolet TOMS data is comparable to the ratio between Dobson instruments, located at Brisbane and Melbourne, and TOMS (0.988 ± 0.023 and 0.973±0.028 respectively). The spectrometers provide continuous data on cloud free days, making it possible to follow changes in atmospheric ozone concentrations, with the spectrometer only requiring 1 second of unobscured direct sunlight to collect a spectrum suitable for analysis. The ratio of the infrared to TOMS data is 0.92+0.054. The offset and scatter in the infrared retrieval compared with TOMS data is due, in part, to the lack of sensitivity of the infrared retrieval from the intervals of the spectrum used in this work (1162.875cm’1 to 1162.952cm'1 and 1163.382cm'1 to 1163.457cm'1) to tropospheric ozone. The error in the HITRAN line parameters (+5%) will also contribute to the offset. In this work the instrumental line shape has not been modelled and any broadening of the absorption feature due to the instrument line shape will be incorporated into the algorithm as a downward shift in the ozone vertical profile, and will result in an error in the retrieval. The analysis of the high-resolution infrared spectra has been proven to be sensitive to changes in vertical distribution of atmospheric ozone in the altitude range of 20 to 25km. This range includes the altitude of the ozone maximum and the altitudes at which ozone depletion occurs. Once the instrumental lineshape has been stabilised and modelled, using the total column ozone information from the ultraviolet and visible spectra and the vertical distribution information from the infrared spectra, it will be possible to retrieve ozone vertical profile information at this altitude range, with an estimated error of 3.8%, and to possibly identify changes in ozone concentrations as being of stratospheric or tropospheric origin.