Application of Fabry-Perot Interferometer for atmospheric HCl remote sensing

One of the most important reservoirs of chlorine in the stratosphere is HCl, which sequesters active species and affects the rate of catalytic reactions with ozone. HCl presents a vibro-rotational absorption spectrum, in the near infrared (3-4 micron); for this reason it is possible to use a Fabry-Perot Interferometer (FPI) as a multiple narrow band filter with an appropriate free spectral range so its transmission bands overlap the KCI absorption lines. The model MAES (Mathematical Algorithm for Etalon Sensor) was developed for simulating the response of a sensor based on a plane FPI. This instrument utilises the direct sun radiation collected by a solar tracker to detect total path atmospheric HCl. MAES has been used to study the main optical characteristics of the FPI (free spectral range, finesse, transmission lines number) and to optimise the instrumental response over a wide range of atmospheric conditions. The MAES simulations show that the remote sensing system is able to detect the atmospheric HCl column amount. Moreover MAES allows us to minimise the interfering gases contribution in the spectral region analysed, The main optical characteristics of an FPI and its deployment for atmospheric sensing are discussed; the result of model simulation and the layout of the HCl sensor are presented as well as some preliminary laboratory tests.