Cloud droplets in the Earth’s atmosphere form on ubiquitous aerosol particles. At present, predictions of cloud droplet size and number concentration derived from aerosol properties are still poor, leading to large uncertainties in the radiation budget and climate projections. Cloud droplet formation on cloud condensation nuclei (CCN activation) is often investigated in closure studies, where the number of activated particles derived from their hygroscopic growth is compared with the one directly measured with a CCN counter. Many of these studies resulted in poor agreement, most probably due to effects related to the organic aerosol fraction: lowered surface tension of the growing droplets compared to pure water due to surface-active substances (or surfactants), solution non-ideality affecting hygroscopic growth due to sparingly soluble organic substances, and co-condensation of semi-volatile organic substances from the gas phase. ORACLE aims to fundamentally improve the understanding of the role organics play in CCN activation through combined experimental and modelling work. The main objectives are: - First, to investigate the evolution of surface tension in a growing solution droplet - Second, to elucidate the effect of co-condensation on particle growth and surface tension. The core of the ORACLE project are tank experiments, where a monodisperse particle population will be equilibrated with an organic vapour over longer time periods at different relative humidity (RH) closely mimicking the atmosphere. The equilibration process will be monitored by measuring the concentration of the semi-volatile species in the gas and the condensed phase. The influence of co-condensation on hygroscopic growth will be assessed by sizing the equilibrated particles at different RH and measuring their CCN activity. To assess the loss of semi-volatile species due to the heating within the commercial CCN counter, an in-house-built continuous-flow thermal-gradient diffusion chamber will be run in parallel. To single out the effect of surface-active species on CCN activation, recently-developed techniques to measure the surface tension of single particles will be optimized and applied to growing droplets. The experiments will be accompanied by thermodynamic and kinetic modelling of surface tension, solution non-ideality and co-condensation effects on CCN activation. These experiments will lead to - an improved theoretical understanding of the contribution of semi-volatile and/or surface-active species to cloud droplet activation for realistic atmospheric conditions; - the proposition of new experimental techniques that are suited to capture contributions of semi-volatile and surface-active species to cloud droplet activation in field experiments; - input for chemical transport and global climate models on how to treat semi-volatility and surface activity of organic species to improve predictions of hygroscopic growth, CCN activation and cloud properties. In the long term, ORACLE will improve our ability to predict the number and sizes of cloud droplets, two parameters paramount to precipitation forecasts in numerical weather prediction models and the climate impact of aerosol particles in future climate projections