Long-Term Size-Segregated Cloud Condensation Nuclei Counter (CCNc) Measurements in a Boreal Environment and the Implications for Aerosol-Cloud Interactions

Ambient aerosol CCN and hygroscopic properties were measured with a size-segregated CCNc in a boreal environment of Southern Finland at the SMEAR II station since February 2009. The overall median critical diameter D-c for CCN activation is reported at 75 nm, exhibiting a clear maximum in February and a minimum in July. The overall median aerosol hygroscopicity parameter kappa is reported at 0.22, indicating that ambient aerosol in Hyytiala is less hygroscopic than the global continental and European continental averages. It is, however, more hygroscopic than ambient aerosol in an Amazon rainforest, the European high alpine site or the mountainous forest. The low hygroscopicity in the boreal forest is attributed to a large organic fraction present in the aerosol mass comparative to other locations within Europe. Aerosol mass spectrometer (AMS) data were used to demonstrate a positive correlation between kappa and sulphate and ammonia, and a negative correlation between kappa and the organic mass fraction. No distinguishable effect of atmospheric new particle formation (NPF) on D-c and kappa was observed. Ambient aerosol was found to be internally mixed in the summer, and externally mixed during the rest of the year.