Clouds profoundly influence weather and climate. The brightness2 and lifetime of clouds is determined by cloud droplet number concentration, in turn dictated by the number of available seed particles. The formation of cloud droplets on non-volatile atmospheric particles is well understood. However, fine particulate matter in the atmosphere ranges widely in volatility. Co-condensation of semi-volatile compounds with water increases a particle’s propensity for cloud droplet formation, with potential consequences for feedbacks between the terrestrial biosphere and climate. Here we systematically study cloud droplet formation, using a cloud parcel model extended to include co-condensation of semi-volatile organic compounds under a broad variety of realistic conditions. Read more… (subscription required)