Multiphase cloud chemistry perturbs homogeneous gas phase chemistry as well as physicochemical properties of aerosol particles. Formation of clouds is consequently modified and this effect remains one of the main uncertainties in climate models assessing the Earth's radiative balance. This research project is directly connected to this problematic as it will help to better understand how cloud chemistry impacts on climate change which is one of the main concerns of our society. It has been known since a long time that the aqueous phase photochemical reactions of constituents present in atmospheric water such as H2O2, NO3-, NO2- and Fe(III) aquacomplexes or organic complexes can form •OH within the water drop. However, the literature lacks of precise data concerning the rate of •OH formation and the relative contribution of the photochemical sources of •OH; also the organic speciation of Fe in cloud aqueous phase is still unknown. The concept of biocatalysed reactions contributing to atmospheric chemistry as an alternative route to photochemistry is new; it emerged from the recent discovery by our group of metabolically active microorganisms in clouds. Microorganisms could interact with oxidant species (or their precursors) thanks to their oxidative and nitrosative stress metabolism that will act directly on these species and to their interactions with Fe (metalloproteins and siderophores). To assess the role of cloud chemistry at a local scale, interactions between gases, cloud droplets and aerosol particles have to be taken into account in numerical models. The main issue is to evaluate the relative contribution of photochemistry and microbial activity on the cloud oxidant capacity. Because this topic is still new, we first need :1) to describe in detail the unexplored aspects of cloud water composition, 2) to assess •OH photochemical production and 3) to describe the biochemical mechanisms leading to the production or the degradation of oxidant species in cloud aqueous phase, 4) to build cloud chemistry models integrating biological and photochemical pathways impacting the cloud oxidant capacity, 5) to assess the overall effect of clouds processing chemical compounds at a regional scale using a chemistry/transport model. These scientific questions are novel and need a strong interdisciplinary approach involving microbiologists and biogeochemists (ICCF, team SEESIB and LEMAR), photochemists (ICCF, team Photochimie), physicists and modelers (LaMP). The present consortium has unique and complementary specificities to achieve successfully this program. ICCF and LaMP have worked together (14 common papers) on photo- and biotransformation of organic compounds in cloud water collected at the puy de Dome station; LEMAR has a long experience on studying interaction (and speciation) of iron complexes with living cells at the ocean-atmosphere interface. The project is constructed around 6 interconnected tasks: Task 1: coordination; Task 2: Cloud sampling. Microbiological and chemical characterization; Task 3: Evaluation of the photoproduction of •OH radical and of the relative contribution to the main sources in cloud water; Task 4: Microbial interactions with oxidant species; Task 5: Photo-biochemical interactions with oxidant species; Task 6: Atmospheric chemistry modeling. Five meetings will be held, 4 reports written, a consortium agreement signed and an international workshop will be organized. Our results will be highlighted in scientific publications and communications in conferences but also though specific actions to the public outreach. This project will also participate to the training of students. Cloud microbiological and chemical data and description of the project will be found on a dedicated web site. Finally siderophore producing bacteria could be applied for industrial purposes. The development of the WRF-Chem model will be used for the setting up of the meteorological conditions in Auvergne.