Clouds play an important role in the Earth's radiative budget (i.e. the amount of energy lost to and gained from space by the Earth). The concentration of droplets present in clouds is a critical factor in determining their albedo so any factor which influences the formation of droplets in clouds will affect the Earth's radiative budget. Cloud droplets are formed by some aerosol particles known as cloud condensation nuclei (CCN) given appropriate ambient conditions. Secondary organic aerosols (SOAs) are one such component which is abundant in the atmosphere. Globally, SOAs have a large semi-volatile component (i.e. material which partitions between the gas and aerosol phases) and have been found in modelling work to co-condense with water, enhancing their CCN activity. In this thesis, the first chamber based evidence for CCN activity enhancement of SOA via co-condensation is presented. Experiments have been conducted in a controlled chamber environment to generate SOA from 1,3,5-trimethylbenzene, limonene, β-caryophyllene and alpha-pinene. These aerosols were then transferred to a cloud chamber where evacuations were conducted on the samples in order to produce clouds. The activation observed in these clouds has been compared to modelled data (which does not include co-condensation) and a discrepancy has been observed with SOA samples generated from β-caryophyllene and alpha-pinene which suggests enhancement from co-condensation. This conclusion is further supported by additional modelling tests which rule out the possibility of uncertainties in the volatility bin distribution or in the hygroscopicity parameter kappa being responsible for the discrepancy between chamber and model data. Agreement can be reached however, by including plausible concentrations of co-condensing material. These findings are placed within the broader context of SOA properties and may explain some of the discrepancies observed concerning the value of the hygroscopicity parameter kappa.