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.

The Amazonian rainforest is one of the largest carbon sinks on Earth, sequestrating annually 0.42–0.65 PgC, but its status is questioned by the rapid disruption caused by climate change. In this region, the productivity of the Amazon rainforest may be limited by the low availability of nutrients provided by the highly weathered soils on which the vegetation grows. The role of external atmospheric inputs in sustaining the Amazon rainforest for millions of years is still debatable. These aerosols are characterized by their richness in essential elements such as K, P, and metals. On a geological timescale, the influence of atmospheric fallout on the functioning of the critical zone is also questioned, particularly as these contributions are often ignored in the geochemical balance of erosion and weathering fluxes on a watershed scale. The ATMO-GEO project is merging a unique scientific consortium, critical zone geochemists, and scientists in atmospheric chemistry and physics to quantify the impact of atmospheric inputs on the geochemical functioning of the Amazon basin. In South America, the most intense period of atmospheric inputs prevails during the boreal winter through the easterlies winds when the Intertropical Convergence Zone is at its southern position, transferring massive amounts of dust from the Saharan-Sahelian region, together with biomass burning soot from Northern tropical Africa. During the rest of the year, inputs are lower, but the atmospheric flux is not negligible, particularly regarding inputs of easily soluble and, therefore, bioavailable elements via soot. Within the ATMO-GEO project, aerosols and deposition will be targeted through an ambitious and unique sampling. Aerosols, total deposition, and the soluble-insoluble fractions of rainfall at the scale of a rain event will be collected simultaneously at two separate sites, one coastal in French Guiana and the other continental, 1000 km to the southeast, near Manaus in Brazil. Integrated over several seasons, several years, and at different frequencies (from a single rainfall event to a full year), these data will enable us to integrate, over the long term, the temporal variability of aerosol and deposition composition, as well as the potential changes affecting them during their transit from coastal to continental regions of the Amazon basin. Since this aerosol composition can vary according to the season, to the dust emission sources in North Africa, or to aerosol penetration into continental areas, extensive geochemical, isotopic, and mineralogical studies will be applied to characterize the different types of aerosols, determine the main sources of North African dust and target their solubility patterns. In addition, total and element-specific deposition will be quantified at both observatories, along with the soluble and insoluble fractions of these elements in rainfall, to estimate annual deposition fluxes. Extrapolation of deposition rates from two single sites to the Amazon basin scale will be carried out using an updated version of the GEOS-Chem chemical transport model, which will consider the deposition's chemical composition as a model input. The importance of atmospheric inputs in the Amazonian geochemical equilibrium will be estimated by comparing them with the rates of denudation (erosion + weathering) documented throughout the Amazon basin. In addition to its scientific dimension, this project also has a local dimension, with a strong desire from public authorities to understand these atmospheric input dynamics better to adapt their policies, particularly regarding health.