Since 1992 and the first Earth summit, different countries have recognized that climate is strongly impacted by human activity and they planned to tackle it under an international convention. In this context that, under the United Nations aegis, COPs (Conference of parties) bringing together many countries to make commitments. However, in order to take meaningful action, it is important that scientists around the world come together to provide the useful data to policy. It is in this context that the REACTE project, involving internationally recognized French and German researchers in respective highly complementary scientific fields, is proposed. The atmosphere is a complex and highly reactive system in which many bio-physicochemical processes take place. It is therefore crucial to have a good understanding of this system and its evolution according to the different pressures it is subjected to. One of the key points is therefore to have a good understanding of the capacity of such a system to react according to the species present. Redox reactions are one of the major transformation pathways that should be carefully considered in order to better understand the evolution of the atmosphere. The REACTE project will focus on the (photo)chemistry of transition metal ions (TMIs) which represents a major source of highly reactive species in aerosols and in the water phase of tropospheric clouds. Indeed, very little data currently exists on the exact role and reactivity of these metals, which are currently almost considered almost exclusively in free form, whereas they are known to be present as complexes in natural environments. The REACTE project will focus on answering the following questions: i) How will the complexation of TMIs influence their photo-reactivity and redox reactions directly through the metal and/or with H2O2 as "Fenton" type reactions; ii) What will be the reactive species associated with these reactions, H2O2, HO●, HO2●/O2●- and their formation yields? What will be their impact on the oxidative capacity of the atmosphere and thus on, more generally, its chemical composition? The obtained results will be then implemented in a multiphase atmospheric chemistry models involving the chemical mechanisms of radicals in aqueous phase (CAPRAM) to predict their influence on the organic matter transformation, the HOx balance and the valence states of TMs in atmospheric droplets or aerosols. The REACTE project, which associates complementary scientific competences, will allow to better understand the chemistry of the TMIs complexes present in the atmosphere and thus to understand their role on atmospheric chemistry. More generally, it will provide data to better understand/assess their impact on climate and air pollution, impact that is currently strongly underestimated.