Habilitation à diriger des recherches de Coralie Schoemaecker

Understanding of the oxidation mechanisms involving the HOx radicals by laboratory and field campaigns approaches. Abstract : Oxidation processes are at the origin of the transformation of hydrocarbons in all reactive systems: combustion, atmosphere, indoor air, photocatalysis,…A major part of these transformations takes place in the gaseous phase and involves the HOx (OH and HO₂) radicals. In regard of the multiplicity of the hydrocarbon species present in these environments and the potential impact of their oxidation on the formation of pollutants (formation of soot, NOx in combustion processes, secondary species such as ozone or secondary aerosol in the atmosphere), the air quality and human health, but also on the global climate change (constraining the concentration of some greenhouse gas GHG such as ozone and methane), there is a strong need to better understand these transformations. As an example, improving atmospheric chemical mechanisms requires the determination of kinetic parameters in laboratory (rate constant, products yield of elementary reactions) but also the validation of these mechanisms in real environments by comparison between measured and modelled profiles of the oxidants involved in these processes. With this aim, different instruments to detect HOx radicals based on optical techniques, complementary in term of pressure range and generation/detection methods (FAGE: Fluorescence Assay by Gas Expansion and photolysis reactor/LIF: Laser Induced Fluorescence/cw-CRDS: continuous wave Cavity Ring Down Spectroscopy) have been developed at the PC2A. I am involved in their development, their validation through intercomparisons, their use for laboratory experiments or the deployment in field campaigns. I developed the UL-FAGE (University of Lille) instrument to quantify OH and HO₂ radicals during field campaigns. This instrument is one of the 10 instruments available in the world. This instrument has been continuously improved since its initial development started in 2005. It now allows also the measurement of the OH reactivity, a complementary parameter useful to better characterize the oxidative capacity of the atmosphere. But this configuration can also be used to determine kinetic parameters in the laboratory at atmospheric pressure. More recently we expanded its use to the measurement of the peroxy radicals (RO₂), key intermediate species in the oxidation processes. I am also involved in the development and use of a laboratory experimental setup (photolysis reactor/LIF/cw-CRDS) dedicated to kinetic studies for atmospheric chemistry focused on the HOx chemistry. In the laboratory, the FAGE and the reactor coupled to cw-CRDS/LIF detections are used complementarily to better understand gas phase chemical processes. They allow to better estimate the role of reactions such as RO₂+OH in the atmosphere or to quantify HOx radicals in photocatalytic systems or in combustion. To ensure the reliability of the HOx measurements in real environments, particularly difficult due to the high reactivity and the low concentration of OH, intercomparisons and interference tests are needed. In the last few years, interferences on OH and HO₂ measurements have been highlighted in different FAGE instruments and the role of trioxides have been identified as interfering for OH measurements in our instrument. Our FAGE instrument has been deployed to quantify HOx radicals in the atmosphere, in controlled conditions (smog chambers) and indoors. These measurements can be compared to modeled profiles to highlight missing reactions in the models. Our instrument was one of the first deployed indoors and highlighted the presence of OH radicals under specific conditions. In order to better quantify the impact of the oxidation processes indoors, a new model has been developed to allow the comparison of measured and modelled profiles of oxidants and hydrocarbon species. It requires representative input parameters such as emission, sorption coefficients, light distribution. It led me to extend my expertise to the characterization of surfaces, to Volatile Organic Compounds detection and to modelling to analyse as precisely as possible the driving processes of the indoor air quality.