The Far-Infrared Radiation Mobile Observation System (FIRMOS), characterization of the measurements and retrieval studies.

The Far-Infrared Radiation Mobile Observation System (FIRMOS), is a spectro-radiometer developed by CNR-INO with the support of the European Space Agency and the Italian Space Agency for the preparation of the FORUM Earth Explorer 9 mission, planned for launch in 2027. FIRMOS measurements cover most of the atmospheric emission in the far-infrared (the spectral region of wavelengths longer than 15 µm) since it can measure the wavelengths from 6.25 to 100 µm (100 to 1600 cm−1 in wavenumber), with a maximum spectral resolution of 0.25 cm−1. The instrument is very versatile and was deployed in ground-based field campaigns (Zugspitze, Germany 2018-19) and flew on stratospheric balloons flights (Timmins, Canada 2022 and Kiruna, Sweden 2024). The observations from stratospheric platforms mimic very well the coming FORUM observations. Considering the paucity of top of the atmosphere FIR measurements the spectra of atmospheric radiance measured by FIRMOS are of great value per se, but they are also useful to consolidate the data processing algorithms of the FORUM mission. In this contribution, we present the instrument design and its characterization and discuss the geophysical products obtained by inverting the atmospheric spectral radiance measured during the campaigns: profiles of temperature and water vapor in all-sky conditions (above the cloud in case of cloudy scenes), cloud properties and spectral emissivity of natural surfaces. In addition, we demonstrate the results of the characterization of the spectral emissivity of passive radiative cooling materials in the field and in the lab performed in the framework of the on-going PaRaMetriC project.