In-cloud measurements highlight the role of aerosol hygroscopicity in cloud droplet formation

The relationship between aerosol hygroscopicity and cloud droplet activation was studied at the Puijo measurement station in Kuopio, Finland, during the autumn 2014. The hygroscopic growth of 80, 120 and 150 nm particles was measured at 90 % relative humidity with a hygroscopic tandem differential mobility analyzer. Typically, the growth factor (GF) distributions appeared bimodal with clearly distinguishable peaks around 1.0‒1.1 and 1.4‒1.6. However, the relative contribution of the two modes appeared highly variable reflecting the varying presence of fresh anthropogenic particle emissions. The hygroscopicity-dependent activation properties were estimated in a case study comprising three separate cloud events with varying characteristics. At 120 and 150 nm, the activation efficiencies within the low- and high-GF modes varied between 0‒0.33 and 0.66‒0.86, respectively, indicating that the less hygroscopic particles remained almost non-activated, whereas the more hygroscopic mode was predominantly scavenged into cloud droplets. By modifying the measured GF distributions, it was estimated how the cloud droplet concentrations would change if all the particles belonged to the more hygroscopic group. According to the κ-Köhler simulations, the cloud droplet concentrations increased up to 70 % with increasing hygroscopicity. This is an indirect but clear illustration of the sensitivity of cloud formation to aerosol chemical composition.