Atmospheric chemistry is an essential component of the functioning of the Earth's climate. It determines the atmospheric lifetime of many climate forcing agents, such as the nature and concentration of aerosols, the concentration of greenhouse gases, or the mechanism of cloud formation. Determining how this chemical reactivity has evolved in the past is essential both for evaluating chemistry-climate models (CCMs) and for determining future climate trajectories. Atmospheric chemical activity is driven by highly reactive atmospheric compounds that have very short atmospheric lifetimes. Because of this transient nature, they are not archived in the paleoclimate record. Reconstructing this chemical activity over time remains a difficult exercise that has not been successful to date. Using ice cores, the multidisciplinary DOC-PAST project proposes to develop new tracers of this chemical activity, taking advantage of the revolution introduced by clumps and isotopic anomalies. The aim is to use a variety of ice cores covering all latitudes to highlight key elements of the chemical reactivity of the atmosphere. This will be done by 1) determining in the laboratory the isotopic characteristics of key atmospheric oxidation reactions of compounds preserved in the ice, 2) documenting in the ice archives these isotopic compositions and deriving the associated chemical reactivity of the atmosphere, 3) incorporating these changes into the LMDz-INCA CCM and measuring their impact on climate. These new isotopic proxies will require the development of new analytical approaches based on the reorientation of an orbitrap towards isotopic measurements and the construction of a very sensitive infrared spectrometer, paving the way for the use of clumped isotopes in broad disciplinary fields using stable isotopes. DOC-PAST will, for the first time, provide an in situ "surgical level" view of the formation of atmospheric species with unprecedented mechanistic detail, setting new standards in geochemistry and spectroscopy.
