1Division of Atmospheric Sciences, Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland
2Max-Planck Institute for Nuclear Physics (MPIK), Atmospheric Physics Division, P.O. Box 103980, 69029 Heidelberg, Germany
3Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, 00250 Helsinki, Finland
4Department of Applied Physics, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland
Abstract. Freshly formed atmospheric aerosol particles are neither large enough to efficiently scatter incoming solar radiation nor able to act as cloud condensation nuclei. As the particles grow larger, their hygroscopicity determines the limiting size after which they are important in both of the aforementioned processes. The condensing species resulting in growth alter the hygroscopicity of the particles. We have measured hygroscopic growth of aerosol particles present in a boreal forest, along with the very hygroscopic atmospheric trace gas sulfuric acid. The focus was on days with new particle formation by nucleation. The measured hygroscopic growth factors (GF) correlated positively with gaseous phase sulfuric acid concentrations. This correlation had a strong size dependency; the smaller the particle, the more condensing sulfuric acid is bound to alter the GF due to initially smaller mass. In addition, water uptake of nucleation mode particles was monitored during new particle formation events and followed during their growth to Aitken mode sizes. As the modal diameter increased, the solubility of the particles decreased. This indicated that initially more hygroscopic particles transformed into less hygroscopic or even hydrophobic particles. A similar behavior was seen also during days with no particle formation, with GF decreasing during the evenings and increasing during early morning. This can be tentatively explained by day- and nighttime differences in the hygroscopicity of condensable vapors.