Anthropogenic activities but also the biosphere lead to the emission of many compounds into the atmosphere. They evolve there according to chemistry and nonlinear physics that leads to the formation of complex secondary constituents. The ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) megaproject is an integrative, innovative and large-scale initiative aimed at improving understanding of the impacts of the mixture of urban air masses on the one hand, and biogenic, on the other hand. evolution of pollution plumes: in particular on the oxidation of organic compounds, aerosol, the formation of photo-oxidants. The ACROSS hypothesis is that this mixture of air masses leads to modifications in the production of secondary compounds whose physical properties modify the chemistry of the troposphere and the air quality. It is also based on the fact that the Paris agglomeration as an intense source of pollution inserted in a relatively sparsely urbanized region, heavily wooded and with moderate orography, constitutes a space of choice to study the impact of the forest suburban study on air pollution at regional level. ACROSS-AO is the airborne component of ACROSS. By using the ATR-42 of the National Infrastructure of research aircraft, it projects the in-situ characterization of the composition of the Parisian plume in synergy with ground measurements (outside the project). These measurements relate to a broad spectrum of carbonaceous, nitrogenous and photo-oxidant species in the gas and particulate phases. Particular interest is paid to the chemical processes leading to secondary pollutants and to the properties of aerosols. The project bringing together five partners is organized into six work packages which respectively focus on the coordination of activities, the preparation of the campaign, the use of data to answer priority scientific questions and the exploitation of this data by a community of modellers. extending beyond this project. The project will advance science through high-quality observations and analyses, and it will lead to the development of improved chemical mechanisms to eventually incorporate them into air quality models. They will lead to more reliable forecasts and more effective mitigation strategies. After more than twelve years without comparable campaigns, these results are possible due to recent advances in our understanding of atmospheric chemistry but also thanks to advances in instrumentation. They are made necessary by the recent insufficient performance of operational modelling of air quality in the context of climate change and changes in anthropogenic emissions.