The study of climate change, in order to best respond to it, requires a detailed knowledge of the physico-chemical properties of the atmosphere. In particular, the calculation of the radiation balance requires knowledge of the transmission and reflection of the atmosphere in the solar spectral band (direct forcing) and the IR spectral band of thermal emission from the earth (indirect forcing). Current methods require precise knowledge of the real and imaginary parts of the aerosol refractive indices. However, these complex refractive indices (CRI) are very 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 allow the measurement of ICR of suspended particles according to their size in a controlled environment. Transportable, it will be able to participate in measurement campaigns coupled with other spectral techniques.