Annual and size dependent variation of growth rates and ion concentrations in boreal forest

Number size distributions of particles and ions were measured with a differential mobility particle sizer and two different ion spectrometers at the SMEAR II station in southern Finland during April 2003–April 2004. Cluster ion (diameter < 1.6 nm) concentrations varied between 200 and 1500 cm–3, while the concentrations of intermediated ions (1.6–6.3 nm) remained usually below 200 cm–3. During the 70 observed nucleation event days, particle growth rates were strongly dependent on their size. The median diameter growth rates of particles in size classes 7–20 nm, 3–7 nm and 1.3–3 nm were 4–5 nm h–1, 2–4 nm h–1 and < 2 nm h–1, respectively. The growth rates of the smallest ions/particles were almost independent of the season having a minimum during the summer, whereas the growth rates of larger particles had a clear annual cycle with the highest values during the summer and lowest values during the winter. The results indicate that in addition to sulphuric acid, compounds related to photosynthesis participate in the particle growth, especially in the size class 7–20 nm.