Traditionally, the interactions between aerosols, clouds, weather and climate has focused on sulphate aerosols. However, in the last 20 years it has become apparent that secondary organic aerosols are also highly abundant in the troposphere. These could represent a major coupling in the earth system between the biosphere and the atmosphere, and thus climate because forests are known to emit large quantities of biogenic VOCs that are known to produce secondary organic aerosols. However selectively studying their influence on the free troposphere is difficult as it requires in situ measurements aboard scientific aircraft. This study observes the role of biogenic secondary organic aerosols on the abundance of cloud condensation nuclei in the upper troposphere above the Amazon, and compares it with a state-of-the-art predictive model. This further supports the importance of these processes in earth system models and gives confidence that the current level of understanding will produce accurate predictions.