Thesis abstract:

Volatile organic compounds (VOCs) from natural and anthropogenic sources play a crucial role in tropospheric chemistry, producing by-products such as secondary organic aerosols (SOAs) and ozone, which can have adverse effects on human health and the ecosystem. This PhD thesis focuses on highly oxygenated organic molecules (HOMs), first identified in the gas phase in ambient air in 2010. HOMs form rapidly from the autoxidation of peroxyl radicals (RO2) formed during the oxidation of VOCs. Due to their low volatility, HOMs contribute significantly to the formation and growth of AOS. The measurement of HOMs in ambient air was made possible by a NO3¯ ToFCIMS. The aims of this thesis were therefore to optimize the instrument in the laboratory, to develop a direct calibration protocol, and to study HOMs in the Rambouillet forest during the ACROSS 2022 summer campaign. To achieve these objectives, laboratory tests were carried out to find the optimum settings for the instrument. Calibration methods were tested with organic acids to assess the instrument's sensitivity, compared with conventional calibration with H2SO4. Simulation chamber experiments were carried out involving the oxidation of biogenic and anthropogenic VOCs, to help identify potential products of HOMs in the air during the campaign. Finally, a principal component analysis (PCA) was applied to HOM measurements obtained by the NO3¯ ToFCIMS deployed during ACROSS. This analysis identified seven principal components, each characterized by unique temporal profiles, linked to the two main biogenic VOCs, isoprene and monoterpenes.