Modelling the impact of anthropogenic aerosols on the CCN concentrations in rural boreal forest environment

Abstract. The radiative properties of the clouds depend partially on the cloud droplet number concentration, which is determined by the available cloud condensation nuclei (CCN) when the clouds are formed. In turn, the CCN concentrations are determined by the atmospheric particle size distribution and their chemical composition. Here, we use a novel modelling framework (SOSAA-FP) that combines a) backward trajectories from the FLEXPART airmass dispersion model, b) a detailed description of atmospheric chemistry and aerosol dynamics from the SOSAA model, and c) global emission datasets to model the formation and lifetime of atmospheric aerosols and their potential to act as CCN at the boreal forest measurement site SMEAR II. This enables us to examine the origin and history of the gas and aerosol components observed at SMEAR II station in southern Finland. In this study, we apply the SOSAA-FP to simulate a period from March to October 2018 with one hour time resolution, focusing on the concentrations of CCN between 0.1–1.2 % maximum supersaturation as calculated by the κ-Köhler theory. We find that the model PM₁ fraction of primary particles, sulfates and secondary organic aerosol correlate well with the observed organic aerosol and sulfate trends and explain most of the observed organic aerosol and sulfate PM₁ mass. Our results show that primary particle emissions play a considerable role in CCN concentrations even at a rural site such as SMEAR II. Atmospheric cluster formation rates had a relatively weak impact on the CCN concentrations in the sensitivity runs. Enhanced nucleation increased (decreased) the CCN concentrations for the highest (lowest) maximum supersaturation. Without any new particle formation the modelled CCN concentrations dropped as much as 36 % for CCN 1.2 % and increased by 29 % for 0.1 % supersaturation, whereas omitting primary particle emissions had a decreasing effect in all calculated CCN supersaturation classes.