Earth’s lower atmosphere is oxidative and its chemistry consists of reactions of short-lived chemical compounds called radicals, controlling the abundance of key compounds, such as of ozone and greenhouse gases. In spite of decades of investigation many atmospheric observations indicate that important aspects of this radical chemistry remain ill-understood, in particular in the reactions of the multitude of organic radicals present in the atmosphere, peroxy radicals (“RO2”) and Criegee Intermediates (“CI”). Detailed investigations of these reactions have been limited for over 50 years by instrumental challenges, in particular by the lack of technique detecting individual radicals (”speciation”). Recently, the Nozière group achieved an important milestone by developing the first technique, based on Chemical Ionization Mass Spectrometry or “CIMS”, speciating the volatile RO2 that are important for the atmospheric oxidation cycles. The objective of EPHEMERAL is to solve key questions in atmospheric radical chemistry by overcoming the long-standing instrumental challenges with cutting-edge developments based on this new technique. Three strategic directions will be pursued: - in laboratory, the detection of CI will be investigated and gas-phase reactions of RO2 and CI studied, - an instrument will be built to detect individual RO2 and CI in the atmosphere, thus achieving a historical milestone, - the reactivity of these radicals at/with surfaces will be investigated in laboratory, assessing for the first time the role of clouds and aerosols in the oxidation cycles and setting the bases for next-generation challenges. The fundamental knowledge and atmospheric observations resulting from this project will dramatically improve the understanding of the oxidation cycles and the predictions of the atmospheric chemical composition, from the regional to the climate scale. EPHEMERAL will thus place Europe at the leading edge in the discipline, scientifically and technologically.