Predicting cloud formation and the arrival of precipitation has always been a hot topic in meteorology. With global warming and the many ongoing drought events, this has become a major issue. Environmental scanning and transmission electron microscopy (respectively ESEM and ETEM) can provide valuable information to understand how clouds are formed by condensation of water on submicronic aerosol particles. Indeed, Environmental Electron Microscopy allow following the structural changes during the condensation of water droplets, while controlling the relative humidity, from the dry state up to fully hydrated conditions. We will first present results from an ESEM study in a QuattroS instrument from FEI/TFS. A commercial Peltier stage has been used to characterize the deliquescence of pure micrometer-sized salt particles used as model aerosols. Water uptake clearly occurs first at around 50% relative humidity, with the gradual disappearance of surface steps and crystal corners. At 76% relative humidity, deliquescence occurs, in agreement with the literature [1]. Further analysis of smaller crystals is performed using a home-made Peltier stage allowing (3D) bright-field and dark-field STEM imaging. Then, observations performed in a Titan ETEM from FEI/TFS will be discussed and compared to literature results [2]. The hygroscopic behavior of a mixture of atmospheric and synthetic aerosols will be described, using either a commercial liquid-nitrogen cryo-holder from Gatan/Ametek, or a home-made system based on a Peltier micro-cooler [3]. Both approaches enable the analysis of salt particles not only in 2D, but also in 3D, for different values of relative humidity. Quantitative results related to the crystal shape and size will be presented. Moreover, as irradiation damage can have large influence on the observed behavior and kinetics, the electron dose received by the sample will be quantified and its effects discussed. References: [1] Langlet, M. et al., Chemical Engineering Science 86 (2013) 78. [2] Wise, M.E. et al., Aerosol Science and Technology, 42 (2008), 281. [3] Vas, J. et al., Microsc. Microanal., 28, S1 (2022), 818. Acknowledgements: This work was funded by the French National Research Agency (project ANR-20-CE42-0008-02). The authors thank the Consortium Lyon Saint-Etienne de Microscopy (CLYM) for the access to the microscope. José Ferreira, Erwan Allain, Christophe Goudin (MATEIS), Mimoun Aouine and Emmanuel Landrivon (IRCELYON) are acknowledged for their experimental contribution.