Thesis of Maria Chehab

Defense of thesis  Maria Chehab - laboratory LOA/PC2A

Abstract :

Forest fires inject into the atmosphere very large quantities of absorbing aerosols. Quantifying these aerosols, from remote-sensing techniques, is crucial to monitor their atmospheric transport and understand their environmental impact. Nonetheless, these techniques require accurate information about the optical properties, specifically the Complex Refractive index (CRI).
Due to the complexity of the burning process, little information about their optical properties is known. Additionally, previous studies focused on scattering aerosols while some forest fires aerosols tend to absorb in the UV. These drawbacks limit their representativeness for aerosols study by remote-sensing techniques. In this project, several experimental protocols have been implemented allowing optical, microphysical and chemical measurements for two different stages of the burning process ; pyrolysis and residual ash.
We were able to accurately determine CRI from Far-Infrared to UV for kaolinite, one of the main clays found in dust which was chosen as a test sample. For residual ash, a mechanical agitation technique was used to produce aerosols in suspension followed by the extinction and size measurements. Using multiple chemical measurements, we managed to find the inorganic composition of the ash sample. From these measurements, we were able to use the methodology previously tested on kaolinite to retrieve CRI data.
As for pyrolysis, a new experimental system using a furnace for particle formation was tested. The extinction spectra of the pyrolyzed particles was distinguished from that of the gases, where some bands in the infrared region have been proven to come from particle phase. As for the UV region, particle absorption was evident as well as seen to be wavelength and size dependent. The chemical composition of the pyrolyzed particles was also measured. They were formed primarily of absorbing oxygenated species where several biomass tracers were found. Some optical properties have been derived such as the Mass Absorption Coefficient (MAC) between 300 nm and 1 μm.

Keywords : Complex Refractive Index, Biomass Fires Aerosols, Extinction spectra