HELIOS is a large hemispherical outdoor simulation chamber (volume of 90 m3) positioned on the top of ICARE-CNRS building in Orléans.HELIOS is dedicated mainly to the investigation of the gas phase processes and radical chemistry under different conditions (sunlight, artificial light and dark). It is equipped with a large set of instrumentation: in-situ FTIR (up to 500 m path length), PTR-ToF-MS (Ionicon 8000), Aerodyne ToF-CIMS, ATD-GC-MS, UHPLC, IC, SMPS, Lopap, HCHO (Aerolaser), Spectroradiometer, Monitors (O3 and NOx), …. Other instruments for radicals measurements are under development (FAGE for HOx and CRDS for NO3).