The study of climate change, in order to react best, requires a detailed knowledge of the physicochemical properties of the atmosphere. In particular, the calculation of the radiative balance which requires the knowledge of the transmission and the reflection of the atmosphere in the solar spectral band (direct forcing) and the spectral range IR of thermal emission of the earth (indirect forcing). Current methods require precise knowledge of the real and imaginary parts of aerosol refractive indices. However, these complex refractive index (CRI) are poorly known in the lower frequency bands and in particular in the TeraHertz (100 GHz-30THz). It is therefore essential to propose experiments to measure them. STEPSON will measure CRI of suspended particles based on their size in a controlled environment. Transportable, it will be able to participate in measurement campaigns coupled with other spectral techniques.