Infra-Red Spectroscopy of Anthropogenic Atmospheric Gases and Natural Aerosols

Webb, Sarah Marie (2012). Infra-Red Spectroscopy of Anthropogenic Atmospheric Gases and Natural Aerosols. PhD thesis The Open University.



Characterisation of the mechanisms and consequences of global warming is one of the most important topics in modern science. It requires a detailed investigation of the infrared properties of the constituents of the terrestrial atmosphere: gaseous, particulate and liquid. In this thesis Fourier-Transform Infra-Red (FTIR) Spectroscopy is used to provide data on the absorption and scattering cross sections needed to evaluate the radiative forcing of several substances present in the terrestrial atmosphere.

CF3I is being considered as replacements for some of the highly global warming and ozone depleting gases currently used in industry. High-Resolution FTIR spectra has been recorded for CF3I over a range of temperatures and pressures and used to calculate approximate values of its global warming potential.

Aerosols may affect the terrestrial radiation budget in two ways: i) they may induce global cooling due to scattering and reflection of incident solar radiation and ii) global warming due to absorption of terrestrial infrared radiation. The effects can either be directly due the aerosols themselves or due to indirect effects such as the aerosols acting as cloud concentration nuclei or nuclei for ice particles in the atmosphere. Furthermore, soluble aerosols, such as sulphates and sodium chloride, will affect the radiative properties of these water droplets once formed. To assess the net contribution of these aerosols one must know the concentration and location of any aerosol and also absorption and scattering coefficients. One method of monitoring the amount of aerosol in the atmosphere is to use satellites such as the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) or Meteostat Second Generation (MSG). Both the satellite retrieval of aerosol concentrations and the calculation of their scattering and absorption coefficients require knowledge of the optical properties of these aerosols. Studying the spectroscopy of aerosols has proven to be a difficult problem since aerosols settle out quickly under the influence of gravity and so have short residence times in aerosol chambers thus radiative properties of aerosols provide one of the largest uncertainties in atmospheric modelling.

In this thesis an ‘acoustic trap’ has been developed to counteract gravity allowing us to record spectra of some common atmospheric aerosols: volcanic ash, coal soot and sand. Descriptions of the design of this novel trap and methods for its effective use are presented along with suggestions for further improvements.

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