Aerosols and clouds are known to play important roles in the atmosphere regulation, but the scientific understanding of the associated processes is incomplete, leading to major uncertainties in climate predictions. An important contributor to the poor understanding is a lack of molecular level descriptions of heterogeneous chemical reactions and physical transformations that occur in the atmosphere’s complex multiphase environment. Significant progress has been achieved in the past decades, showing the importance of the atomistic approach to fully understand the catalytic processes in this chemically rich environment. One of the major challenges for modeling these aspects at the molecular level is being able to describe precisely both the reaction mechanism and the explicit environment. A second difficulty is to establish a direct comparison with experimental data in particular due to timescales and size discrepancies. This talk will show different applications based on nanoscale molecular simulations (Classical Molecular Dynamics, full Quantum and QM/MM approaches). Comparisons with experiments will be also addressed. Acknowledgment: The presented work benefits from the support the CaPPA project (Chemical and Physical Properties of the Atmosphere) funded by the French National Research Agency (ANR) through the PIA (Programme d’Investissement d’Avenir) under Contract No. ANR-10-LABX-005; the Région Hauts de France and the Ministère de l’Enseignement Supérieur et de la Recherche (CPER Climibio) and the European Fund for Regional Economic Development for their financial support. This work was performed using HPC resources from GENCI-TGCC (Grant No. 2022–A0110801859) and the Centre de Ressources Informatiques (CRI) of the Université de Lille. References [1] Lovric, J.; Duflot, D.; Monnerville, M.; Toubin, C.; Briquez, S. Water Induced Organization of Palmitic Acid at the Surface of a Model Sea-Salt Particle: a Molecular Dynamics Study. J. Phys. Chem. A 2016, sous presse. DOI: 10.1021/acs.jpca.6b07792. [2] Johansson, S. M.; Lovrić, J.; Kong, X.; Thomson, E. S.; Papagiannakopoulos, P.; Briquez, P.; Toubin, C.; Pettersson, J.B.C. Phys. Chem. Chem. Phys., 2019, 21, 1141. DOI: 10.1039/C8CP04151B. [3] Roose, A.; Toubin, C.; Dusanter, S.; Riffault, V.; Duflot, D. Classical molecular dynamics study of small-chain carboxylic acid aerosol particles. Earth Space and Chemistry, 33, 380-389 (2019), DOI:10/1021/acsearthspacechem.8b00172. [4] Abouhaidar, R.; Duflot, D.; Toubin ; C., Theoretical characterization of the kinetics of the multiphase ozonolysis of an aqueous maleic acid droplet, Aerosol. Sci. & Tech. under revision.