Aerosol particles are important in air quality, climate, and human health. They can affect the earth’s energy budget directly by scattering and absorbing solar radiation and serve as cloud condensation nuclei (CCN), thus influencing cloud properties and lifetime. Atmospheric aerosols are formed through a wide variety of natural and anthropogenic sources. This dissertation presents a comprehensive investigation into the behavior of water-soluble organic molecules on atmospheric aerosol surfaces using Second Harmonic Scattering (SHS). The study focuses on understanding the partitioning of these molecules at the aerosol surface, a crucial aspect in atmospheric chemistry impacting cloud formation, radiation balance, and air quality. The research is divided into three main parts. Initially, the study explores the disposition of organic molecules on aerosol surfaces, utilizing a modified Langmuir model to describe their behavior. This part emphasizes the predominant residence of these molecules on the aerosol surface, highlighting the surface's significant role in atmospheric reactions. The second part examines the interactions between salts and organic molecules on the aerosol surface. A series of experiments with varying salts reveal how different ions influence the partitioning behavior of organic molecules, underscoring the importance of ionic species in governing aerosol surface dynamics. The final part of the study reveals a significant difference between the aerosol and planar air-water interfaces. The equilibrium rate constant for aerosols is found to be tenfold faster, indicating a larger Gibbs free energy, contrasting with the planar air-water interface scenario. And aerosol surfaces exhibit lower molecular density due to the finite availability of organic molecules. These findings highlight aerosol surfaces' unique kinetic and thermodynamic behaviors compared to their planar counterparts. This work significantly advances our understanding of aerosols, their surfaces, and the various factors influencing their behavior in the atmosphere. The findings have crucial implications for our comprehension of climate change, air quality, and aerosols' environmental and health impacts. The introduction of a novel in-situ technique for detecting organic molecules at aerosol surfaces marks a breakthrough in aerosol research, offering insights into the distribution and interactions of these molecules within atmospheric particles. / Chemistry

Ce travail de thèse a porté sur l’étude de la génération de second harmonique par lesnanoparticules métalliques. La présence de résonances de plasmon de surface localisées dans lesnanoparticules métalliques accroit la section efficace non linéaire et le but de ce travail a été decomprendre comment ce couplage peut être mis à profit. D’un point de vue fondamental, lacomparaison entre expérience et théorie a permis de mettre en évidence le mode d’émissionoctupolaire ainsi que de pondérer les contributions de surface et de volume au signal total émis.Il a également été démontré que des profils de Fano non linéaire pouvaient être observés dans lecas de nano-objets très simples. Des applications pratiques, telle que les capteurs plasmoniques,ont été abordées montrant que les propriétés intrinsèques de la génération de second harmoniqueoffrent plusieurs avantages et augmentent la sensibilité de tels capteurs. Des mesuresexpérimentales ont également été effectuées sur des nanoparticules d’or uniques enfermées dansune matrice homogène, ouvrant la voie pour une compréhension plus fine de l’impact de lamorphologie des nanoparticules sur leur réponse non linéaire. / This work is devoted to the study of the second harmonic scattering from spherical metallicnanoparticles. The optical properties of noble metal nanoparticles are known to be dominated bythe localized surface plasmon resonances which increase the second harmonic intensity. From afundamental point of view, the octupolar emission mode is revealed for the first time bycomparison between experiments and theory. The origin of the second harmonic generation(SHG) in metallic nanoparticles is discussed showing that bulk contributions are needed. It wasshown that Fano profiles can be observed in a very simple configuration. Practical applicationsare also addressed demonstrating that the properties of SHG increase the plasmonic sensorperformances. The second harmonic generation form single gold nanoparticles embedded in ahomogeneous medium is also reported for the first time paving the way toward a betterunderstanding of the morphology impact on the nonlinear response of metallic nanoparticles.