Ab initio study of the reactivity of adsorbed molecules : application to aerosols

PhD student: Césaire FOTSING

Context
Due to the complexity of its exchanges with the ground and/or the oceans’ surface, the low troposphere contains a huge number of chemical species, either in gas phase or as particles called aerosols. Because of their various sizes and origin, their impact on global warming remains difficult to quantify and suffer from large uncertainties (2013 IPCC report). Moreover, the surface of these particles may harbor chemical reactions between minor gaseous species and serve as a sink or source for these compounds. These phenomena influence the chemistry of the atmosphere and may have impact on human health in case of inhalation. In order to gain a better understanding of these processes, laboratory experiments in well controlled conditions are performed in several groups involved in the Cappa project (PC2A & LASIR, Lille 1, SAGE, Mines de Douai). However, these experiments are performed at the macroscopic level, so that very few information is available on the detailed mechanisms leading to the apparition or destruction of these species. Thus, numerical simulations at the molecular level are essential for a better understanding of these phenomena.

Goal
The objective is to calibrate a quantum mechanical methodology allowing to study the reactivity of typical molecules (fatty acids, saturated or not) at the surface of aerosol (marine and/or mineral), attacked by reactive species, either radicalar (chlorine atom, hydroxyl radical or peroxy RO2) or neutral (dioxygen, ozone). The so called “multi-scale” approach, allowing the treatment of large systems, will be used. The basic idea is to cut the system of interest into several zones, whose computational level of treatment will be more or less sophisticated, depending on their importance in the process. Typically, the core of the system will be treated at the quantum level, while the rest will be treated classically (QM/MM methods). In addition to structural information, reaction paths will be obtained,  from those reaction rates can be determined.

Keywords
Atmosphere, Aerosol, Reactivity, theoretical and computational chemical physics

Period
October 2015-October 2018

Director: Denis Duflot (PhLAM)
Co-director: Céline Toubin (PhLAM)

Laboratory: PhLAM

Financing: Region / Labex CaPPA