QUAntitative Spectroscopy for Atmospheric Remote Sensing: from IR/THz simultaneous measurements to satellite remote sensing – QUASARS

Measuring trace gases having a notable impact on human health, climate and the stability of the ozone layer constitutes an extremely important challenge. In the coming years, new, higher sensitivity satellite instruments will improve atmospheric sounding only if the necessary spectral parameters are available. This is not the case for many reactive species, which are the subject of this project: HONO, HOBr, ClONO2, BrONO2 and O3. The challenge is to obtain accurate spectroscopic parameters for chemically unstable species via a transfer of the precision from the molecular dipole moment (MW region) onto the infrared (IR) measurement. We propose an original experimental development involving the simultaneous use of two instruments: an IR/THz dual beam experiment to obtain absolute IR intensity for lines used for satellite data retrievals. We intend to build a new, coolable cell (200-300 K), made of inert materials, capable of accommodating a 0.1-1 THz spectrometer and coupled to a high resolution (HR) FTS (0.001 cm-1 resolution). The 0.1-1 THz range should permit to select rotational lines for any molecule to be considered. Both measurement channels are planned with variable path lengths to accommodate molecules with variable dipole and rovibrational transition moments. The FT spectrometer will allow collecting the IR signature at HR simultaneously with the THz channel measurements used for pressure determination. The adopted strategy relies on the fact that the intensities of the rotational spectra simply depend on the permanent dipole moment of the molecule of interest, determined with high accuracy from Stark effect measurements. The pure rotational spectrum of the molecular species, in the THz region, will then be used to determine the partial pressure of the species in the sample mixture. Since the IR spectrum of the same sample is measured simultaneously, then absolute intensities in the IR region can be obtained.
Our project, if successfully carried out, aims at furnishing the scientific community with datasets for HONO, HOBr, ClONO2, BrONO2 and O3 permitting the quantification of these species in satellite-based IR surveyor systems, mainly IASI and MIPAS data. For these molecules, linelists, when available, are mostly incomplete, or of a low accuracy. For example, for O3 and HONO, the results obtained in this project can improve the quality of chemistry-transport models.
Within this project, four French groups and two European teams will collaborate to create the needed synergy to perform spectroscopic measurements and to apply the data directly to remote sensing observations. The strength of our consortium is that experimental and theoretical molecular spectroscopy experts, able to handle specific experimental developments and advanced theoretical calculations, will work together with remote sensing experts able to develop innovative methodologies for retrieving atmospheric composition from satellite measurements. Besides novel spectroscopic measurements on reactive molecules of atmospheric interest, our project involve state-of-the art theoretical modelling, line by line analysis and the validation of line lists, their use for analyses and interpretation of atmospheric observations: radiative transfer simulation of atmospheric spectra and, finally, determination of the role of the target species on the atmospheric chemistry.
The requested support would enable to develop novel equipment, improve software tools and engage additional manpower.