A workshop is being organized in Marseille from 16 to 17 July 2025, on the initiative of the Specialized commission of Atmosphere and Ocean (CSOA) by the LEFE/CHAT (Atmospheric Chemistry) action of CNRS-INSU's national programs, with the support of the CNRS, University of Aix-Marseille, the ICR and LCE laboratories, and the Cross-Disciplinary Project AREA of the University of Lille.

Chemical compounds emitted into the atmosphere from biogenic or anthropogenic sources are transformed in situ by physico-chemical mechanisms taking place in homogeneous (gaseous, particulate or aqueous) or heterogeneous phases. The community of atmospheric physical chemists has extensive expertise in a wide range of methods, including laboratory experiments, simulation chamber or in situ measurements, data analysis, and a range of models from 0D to 3D. Nevertheless, one of the major current difficulties remains the reduction of uncertainties linked to the role of heterogeneous and multiphase chemistry, and the transfer of knowledge acquired in the laboratory / the field (through multiple approaches) to atmospheric models.

The aim of this workshop is to bring together researchers, engineers and PhD students involved in theoretical physical chemistry and atmospheric sciences to make progress on these issues. Theoretical calculations, at the molecular scale are used to support experimental results obtained from laboratory and in situ studies, by providing a more in-depth interpretation of the findings and a better understanding of processes. In this way, they provide answers to fundamental investigations regarding (i) processes of molecular aggregation and aerosol nucleation at the heart of our understanding of secondary aerosol formation, and the role of the environment and light on these processes; (ii) (photo)chemical and physical ageing processes from the molecular to the aggregate scale; (iii) understanding the role of interfaces, (iv) ways to generalize and predict the behaviour of analogous chemical systems.

Through this workshop, we aim to bring together communities of theoreticians, laboratory / field scientists and modellers, to foster the emergence of new scientific ideas and/or new collaborations.