PhD 2023-2026 Iodine fate in atmospheric droplets: heterogeneous reactivity and role of interface

Description        

PhD position in experimental physical chemistry: 3 years from October 2023

Inscription to Doctoral School: Sciences Chimiques University of Bordeaux

Doctoral contract financed by IRSN : salary 1800 net + Tuition fees (380 /year) are supported by IRSN

Location : 18 months at L2EC laboratory - IRSN / Cadarache Saint Paul lez Durance and 18 months at Institut des Sciences Mol culaires (ISM) UMR CNRS 5255 University of Bordeaux (https://gsm.ism.u-bordeaux.fr/)

Supervisors : Dr Sophie Sobanska (ISM-Bordeaux) sophie.sobanska@u-bordeaux.fr Dr Anne C cile Gr goire (IRSN Cadarache) - anne-cecile.gregoire@irsn.fr

Context

The importance of halogens in the atmospheric compartment is well established since many decades especially for their oxidative reactivity and their implication in the ozone budget control. Of the atmospheric very short-lived halogenated species, brominated and iodinated species are predominantly of oceanic origin, while chlorinated species have significant additional anthropogenic sources (GIEC 2021). So, much attention was paid to chlorine most abundant halogen found in the atmosphere - and to bromine - very efficient to cause ozone depletion in the stratosphere and upper troposphere. Both Cl and Br gas phase reactivity and interactions with atmospheric aerosols (including aqueous aerosols) have been largely studied at the laboratory scale and through field measurements, allowing to build up comprehensive models capable of reproducing field observation. The fate of tropospheric iodinated species aroused the interest of atmospheric chemistry community but has also gained much interest in the field of nuclear safety. Indeed, in case of incident or accident in a nuclear power plant or in a radioisotope production facility, the release of gaseous radioiodine compounds to the atmosphere must be considered. The knowledge of its reactivity is a key issue to predict its dispersion for efficient current accident management plans. The iodine chemistry in the atmosphere received increasing interest since the late 1990 s, and large progresses have been achieved regarding gas phase (photo)reactivity and the formation of iodine-containing aerosols. However, heterogenous reactivity of iodine gases remains still poorly documented. This gap prevents the achievement of a comprehensive atmospheric iodine model. In the same way, to the best of our knowledge, the formation of secondary iodinated species as products of heterogeneous reactivity is not considered in any of global iodine model. However, these are crucial data to properly quantify iodinated species in the atmosphere and to evaluate iodine dispersion, in case of accidental radioactive iodine releases and be able to explain its long distance transport. In that context, the main objective of the PhD project is to contribute to a better modeling of the global iodine cycle in the atmosphere by filling the gap in the heterogeneous reactivity of iodine in focusing on inorganic iodine aqueous phase interaction. The originality of the project lies in the study of heterogeneous reactivity at the single particle scale.

Research program

In the PhD project is divided in two complementary parts:

1- (at IRSN-Cadarache) Determination of the uptake coefficients of gaseous inorganic iodine (I₂ and HOI) by aqueous droplets as models of atmospheric aerosols. Laboratory experimental studies of gas-aerosol interaction will be conducted under controlled conditions using open droplet train flow reactor that is commonly employed to determine gaseous uptake coefficients The role of droplets composition will be investigated with respect to I₂ and HOI uptake: (i) pure water reference case, (ii) including carboxylated compounds (formic acid, oxalic acid) may contribute to the release of stable iodinated organic volatile compounds into the atmosphere (CH₃I for instance) and (iii)inorganic salts (NaCl, (NH₄)₂SO₄, NaSO₄, NaNO₃ and NH₄NO₃) potentially impacting reactivity. These compounds are representative of organic and inorganic loading of atmospheric aerosols. State-of-the art detection techniques that include gas chromatography and mass spectrometry will be used to monitor on-line the reactant concentration and/or the formed reaction products in liquid phase at low level of detection. Thus, iodine uptake coefficients and composition evolution of particles will be addressed. The influence of the particle size will be explored with the generation of particles ranging between 10 and 100 m. The influence of the pH will be studied.

2- (at ISM-Bordeaux) Heterogeneous reactivity between gaseous iodinated species and aqueous aerosols and the physico-chemical evolution of single particles exposed to iodine species will be carried out using acoustic levitation since this single particle technique can be easily coupled with various analytical techniques. The setup consists in a commercial acoustic levitator device which is included within an environmental jacket to control gaseous environment. This levitation cell is coupled routinely with an optical camera and a Raman microspectrometer for in-situ and on-line characterization of chemical composition and size of single particles. Influence of the particle size, the effect of humidity, the UV-visible irradiation on the chemical (photo)evolution of the particles during gaseous iodinated species exposure and the role of the interface will be investigated. All experiments will be carried out under carefully controlled experimental conditions (temperature, relative humidity, UV radiation intensity, initial reactant mixing ratios).