Investigation of the atmospheric reactivity through experimental measurement of the oxidizing capacity of the atmosphere

PhD student: M. Al Ajami

The work of this thesis concerns the experimental study of the atmospheric chemistry by the development and application of a technique able to measure radicals present in the atmosphere, namely OH, HO2 and RO2, whereby R represents an alkyl group. Indeed, these reactive species are in the centre of the degradation process of Volatile Organic Compounds (VOC) emitted either by nature (vegetation) of through human activity (traffic, industry…).

The products formed during this degradation are oxygenated, more functionalized species up to Secondary Organic Aerosols (AOS). Despite several field campaigns and the employment of more and more instruments, an important fraction of the atmospheric reactivity is still unidentified. This entails uncertainties in the chemical mechanism used in atmospheric models and limits the analysis and understanding of phenomena observed in the atmosphere.

With the goal of improving the knowledge of the atmospheric reactivity, laboratory experiments as well as measurements in the field will be carried out in the frame of this PhD thesis. A technique named FAGE (Fluorescence Assay by Gas Expansion) will be employed for the quantification of the radicals OH, HO2 and RO2 as well as for the measurement of the OH lifetime (measurement of the sum of all OH consumption paths). This technique, selective and sensitive, is based on the measurement of the Laser Induced Fluorescence of OH radicals at low pressure and is available in some 10 laboratories around the world. A FAGE has been developed in recent years in PC2A laboratory. In the frame of this thesis we will evaluate in laboratory experiments the potential of this technique to measure RO2 radicals but also characterise in a detailed manner possible interferences due to other reactive species (such as Criegee intermediates, formed during the ozonolysis of alkenes). The instrument will also be employed during field campaigns for an intercomparison with other instruments in an atmospheric simulation chamber in order to test the reliability of the measurements. A field campaign in a forested environment is also planed in order to investigate the oxidation of biogenic species and their role in the formation of SOA.

The ideal candidate is interested in experimental work with a particular interest in field campaigns. Basic knowledge of atmospheric chemistry, optics and spectroscopy are desired.

Key words
Atmospheric radicals - oxidizing capacity of the atmosphere - chemical mechanism – optical diagnostics – field campaigns

Advisors : C. Schoemaecker, C. Fittschen

Laboratory: PC2A

Financing: Doctoral School SMRE / Labex CaPPA