Laboratory based FT spectroscopy in support of atmospheric science

Abstract

Many aspects of atmospheric science rely on the availability of suitably accurate information which describes the interactions between electromagnetic radiation and atmospheric constituents. Examples include measurements of the atmosphere by remote sensing methods, calculations of the radiative forcing of the atmosphere by greenhouse gases, and studies of atmospheric processes such as ozone depletion. The number of gases whose spectral properties must be known is large and includes many unstable and reactive species. This, together with the requirement to understand the spectral properties of other atmospheric components such as aerosols, presents a difficult challenge to the spectroscopist. The development of the high resolution Fourier transform spectrometer (FTS) has been central to meeting this challenge. FTS is widely used in the determination of many spectral parameters of atmospheric relevance, such as absolute absorption cross sections, line positions, line strengths and pressure broadening coefficients. Other developments in sample handling and sample containment have allowed realistic simulation in the laboratory of the physical conditions which pertain in terrestrial and planetary atmospheres. In this paper some of the capabilities for and results from spectroscopic studies carried out at the Rutherford Appleton Laboratory (RAL), and elsewhere, in support of atmospheric science are described. Examples will include long optical path spectroscopy of weak absorbers such as molecular oxygen (relevant to satellite measurements of ozone), spectroscopy of greenhouse gases (relevant to radiative forcing and global warming potential calculations), and spectroscopy of gases and aerosol particles (relevant to ozone depletion). In addition to discussing the role that FTS plays in these investigations, experimental techniques which may be used to generate suitable sample conditions and a method for extracting accurate line parameters from measured spectra will also be described.