Césaire FOTSING KWETCHE

PhD in Physics, Discipline Milieux dilués et optique
Thesis defended on 6th December 2018
Laboratoire de Physique des lasers, Atomes et Molécules (PhLAM)

Title and thesis summary
Publications
Oral & posters presentation

Future

 


Title and thesis summary

Molecular modeling of the  atmospheric aerosols reactivity

This thesis contributes to the molecular level understanding of atmospheric aerosol chemistry. The objective is to set up an hybrid classical/quantum methodology to treat the heterogeneous chemical mechanisms occurring at the surface of model aerosols attacked by various reactive species, either radicals (Cl, OH or RO2 peroxyl) or saturated (oxygen, ozone). Firstly, the quantum approach used to treat the reactivity between chlorine and a carboxylic acid molecule (valeric acid) or fatty acid (palmitic acid) has been validated in the gas phase. These calculations made it possible, on the one hand, to highlight the spontaneity of the abstraction of the hydrogen atoms of the acid by the chlorine radical and, on the other hand, the increase of the reaction rate constant as the chain of the acid molecule gets longer. In the second part dedicated to the simulation of a model submicrometer aerosol, we have built an aggregate of palmitic acid using classical molecular dynamics and analysed its structural and energetic properties. Then, addition of water molecules leads to the formation of water islands at the surface of the aggregate. Finally, we studied the heterogeneous reactivity at the surface of the aerosol by dividing the system into two regions treated differently (QM/MM approach, Quantum Mechanics/Molecular Mechanics). We have identified a sample of hydrogen atoms at the surface of the aggregate that can be captured by chlorine and calculated for each of them the reaction rate constant within the QM/MM scheme. We have finally been able to determine for the first time a theoretical value of the reactive uptake coefficient, which can be in principle compared to experimental data.

Thesis supervisor :
Denis Duflot (University of Lille)

Thesis co-supervisor :
Céline Toubin (University of Lille)

Examiners :
Denis Petitprez (University of Lille)
Aude Simon (Université Paul Sabatier)

Referees :
Manuel Ruiz-Lopez (Université de Lorraine)
Sylvain Picaud (Université de Franche-Comté)

Laboratory : PhLAM

Financing : Région / Labex CaPPA

 

 

Publications

 

 

Oral and posters presentation

 

 

Future