Thesis of Mirna Shamas

Thesis of Mirna Shamas - laboratory PC2A

Abstract :

In the atmosphere, organic pollutants such as Volatile Organic Compounds (VOCs) from biogenic or anthropogenic sources are oxidized by OH radicals leading to the formation of peroxy radicals RO2 and HO2, which play a major role in tropospheric chemistry. Their reactivity controls the oxidative capacity of the atmosphere (cycling of reactive radicals) and the formation of tropospheric ozone and other secondary pollutants. While the reactivity of these peroxy radicals is well known in polluted environments (high NOX concentrations), it is still poorly known in a remote environments containing low concentrations of NOx (ex: tropical forests, marine boundary layer).
The aim of the present work is to study the kinetics of some of these peroxy radicals to better understand the radical + radical reactions in clean atmosphere. Two experimental set-ups have been used. First a fast discharge flow reactor, originally designed to complement an existing photolysis cell set-up, was continued to be developed in the frame of this work. This fast flow reactor is coupled to three complementary techniques: the continuous-wave Cavity Ring-Down Spectroscopy (cw-CRDS) for the measurement of the HO2 radical, the Laser Induced Fluorescence (LIF) for the detection of the OH radical and Mass spectrometry with Molecular Beam sampling (MB/MS) for the measurement of stable reaction products and radical species. An unexplained OH reformation phenomenon has been observed in the previous work done during the validation of this recently developed experimental set-up. In this work, different tests were carried out to explain this OH reformation phenomenon and will be described here. Meanwhile a laser photolysis cell coupled to a double cw-CRDS paths for the simultaneous measurements of RO2 and HO2 radicals was used to determine the absorption cross-section of C2H5O2 radical, the rate constant of C2H5O2 + HO2, and the rate constant and branching ratios for the C2H5O2 self-reaction.

Keywords : Atmospheric chemistry,reactivity,peroxy radicals,laser techniques,mass spectrometry