Atmospheric nucleation events have received increasing attention as a potentially important source of aerosol particles affecting climate and human health. Here, we report 2.5 years of continuous nucleation observations from San Pietro Capofiume, Italy (44¿39'N, 11¿37'W). We show that nucleation events occur frequently in the Po Valley region, even though the region is rather polluted with high pre-existing particle concentrations. The nucleation events are often very intensive, and the newly formed particles can grow to sizes as large as 100--200 nm in diameter within a few hours. Our calculations of the enhancement of particle number concentrations in different size ranges and comparison with steady-state and regional emission estimates indicate that the nucleation events produce a significant fraction of particles larger than 100 nm, and thus constitute an important source of cloud condensation nuclei (CCN).