Quantitative Spectroscopy and Atmospheric Measurements

Abstract

Optical measurements of atmospheric minor constituents are performed using spectrometers working in the UV-visible, infrared and microwave spectral ranges. In particular recently the satellite ENVISAT has been launched with three spectrometers on board, SCIAMACHY and GOMOS working in the UV-visible spectral region and MIPAS working in the thermal infrared. The analysis and interpretation of the atmospheric spectra require good knowledge of the molecular parameters of the species of interest as well as of the interfering species. This is true not only in the spectral domain used to retrieve the species (Thermal infrared for MIPAS for example) but also in the other spectral domains used by other instruments: Meaningful comparisons of profiles retrieved by various instruments using different spectral domains require indeed that the spectral parameters are consistent in these spectral domains. To illustrate these points we will concentrate on three molecules namely nitric acid, formaldehyde and ozone. For HNO3 we will show the difficulty to measure line intensities in the laboratory and we will describe how a comparison of MIPAS profiles with those obtained by another instrument operating in a different spectral range (Far infrared) may be used to validate the HNO3 line parameters in the mid-infrared. For the measurement of atmospheric formaldehyde concentrations, mid-infrared and ultraviolet absorptions are both used by ground, air or satellite instruments. It is then of the utmost importance to have consistent spectral parameters in these various spectral domains. Consequently the aim of the study performed at LISA was to intercalibrate formaldehyde spectra in the infrared and ultraviolet regions. The experiments were performed by acquiring simultaneously UV and IR spectra at room temperature and atmospheric pressure using a common optical cell. The reactor contains two multiple reflection optical systems interfaced to a Fourier transform infrared spectrometer and to an UV-visible absorption spectrometer. The results are discussed and compared with previous ones. In the mid-infrared range, the 10 µm ozone band is very strong and is the most widely used to derive concentration profiles. In the UV region, various bands are currently used for spectroscopic remote-sensing of ozone. In this paper we present two sets of results: First a careful comparison of four sets of ozone line intensities measured independently in the 10 µm region has been achieved. From them new and more accurate transition moment constants for the ν1 and ν3 bands of 16O3 were derived and used to generate new line positions and intensities. These new spectroscopic parameters allowed one to simulate atmospheric spectra better than the previous spectroscopic parameters showing that on a relative basis the new spectral parameters are of better quality. Second, we present intercomparisons of ozone absorption cross sections in the UV and mid-infrared regions, using the absorption of ozone at 254 nm and in the Huggins region. In the case of the Huggins region the comparison has been performed by acquiring simultaneously UV and infrared spectra at room temperature and atmospheric pressure using the same reactor as for H2CO.