The chemistry of reactive species and their role on the oxidative capacity in polar regions

Albane Barbero (2018-2021)

Directors : Roberto Grilli and Joël Savarino
Funding : ministerial grant

Summary :
High southern latitude regions present specificities that are highly relevant for studying the climate in connection with the atmospheric chemistry. These regions are very sensitive to the global change : appearance and growth of the Antarctic ozone hole (1), rapid change of the sea-ice (2), rapid increases of temperature (3), changes in precipitation mass balance (4), etc. Contrary to regions of the rest of the world, Antarctica is still considered as a pristine environment not yet influenced by predominant anthropogenic emissions and thus represents the last continental-size natural laboratory. To understand the impact of the global change on the fragile equilibrium of these regions, a good knowledge of their specific characters is needed. The desire to unravel climate connections has motivated unprecedented technical and scientific endeavors for ice core retrieval and interpretation from Antarctica. The continent’s ice shield constitutes a true scientific treasure for studying the Earth’s past climate, with precious atmospheric archives providing staggering record dating back some 740,000 years in time. High reactive species (such as nitrogen oxides, NOx, hydroxyl ad hydroperoxy and peroxy radicals, HO, HO2, RO2, and halogen oxide radicals, IO, BrO) are playing a key role in such a remote environment, controlling the oxidative capacity and the atmospheric chemistry at this location. Today, two main scientific questions are still open to discussion, and further investigations will be carried out in the frame of this PhD thesis : 1) The role of the halogen oxide radicals (XO) at coastal regions but also in the Antarctic plateau : (i) at what levels they are present (ii) How they are related to the presence of sea ice (on the coast) and to the snow chemistry (in the continent) ; (iii) Is there a real difference between the chemistry at the East and West coast as recent works highlighted, with NOx and OH chemistry dominating on the East coast, and XO at the West ? (iv) Which are the sources and sinks of XO and how they take part to particles formation ? For the measurement of the halogen oxide radicals an already existing optical spectrometer based of a femtosecond laser couple with an optical cavity will be employed. The instrument has been developed by the LIPhy laboratory in the frame of the ANR project, “MOCAMAR” in collaboration with the IGE. The instrument has been tested at Roscoff in 2011 and employed at Dumont D’Urville station in Antarctica in 2011/12, providing measurements of NO2, IO and BrO. While measurements of NO2 are today validated, the one of the halogen oxide radicals required further comparisons with other existing techniques, mainly due to doubts related to the performance of the sample line to carry the high reactive species into the measurement cell. Therefore in the first part of the PhD, a validation/calibration of the instrument will be carried out, with a comparison campaigns either in the field or in simulation chambers. Afterwards, the instrument will be employed to answer the scientific questions highlighted above on the chemistry of XO at Polar Regions. 2) Constrain the balance of the NOx on the Antarctic plateau : the snow represents a principal source of NOx, and NO and NO2 control the level of other major oxidants such as O3, OH, RO2. Previous measurements in the continent at Dome C conducted in the frame of SUNITEDC and the OPALE project (2011/12) revealed a ratio NO2/NO up to 4 times larger than what can be explained by the measured level of RO2 and expected BrO concentrations. The first suspect is related to artefacts of the measurement (which was carried out using a chemiluminescent device). Further investigations are therefore required for better constraint de oxidative capacity in the Antarctic continent. For this propose, in parallel to this PhD thesis, a novel instrument based on the IBB-CEAS (Incoherent Broad-Band Cavity Enhanced Absorption Spectroscopy) for measuring NO2 and NO will be developed by a hired engineer in the framework of the ANR/BNP-Paribas project EAIIST. The PhD student will take part to the validation of the instrument in the laboratory, and will conduct measurements of NOx at Concordia station, at the Dome C. The main aspect of this PhD thesis will be related to analytical chemistry, however, strong connection with atmospheric chemistry and modelling will be required for understanding and interpreting the data.

Keywords : nitrogen, snow, oxidants, halogens, polar regions