Abstract. The relationship between nucleation events and numerous physical and meteorological parameters was analysed using data collected at the Station for Measuring Forest Ecosystem-Atmosphere Relations (SMEAR II) in Hyytiälä, Finland. To do this, measurements of solar radiation (ultraviolet [UV], global, photosynthetically active radiation [PAR], net, reflected global radiation and reflected PAR), gas concentrations, temperature, humidity, wind direction, horizontal and vertical wind speed, horizontal and vertical wind variances and particle concentrations were collected over a 4 year period. For the year 1999 a detailed analysis of data were completed by examining parameters in order to determine the physical and meteorological conditions favourable to the formation of new particles. A comparison of different wavelength bands during the bursts of new particles led to the suggestion, that UV-A solar radiation seems to be the most probable radiation band concerning the photochemical reactions involved in the production of condensable vapours. Furthermore a high correlation between the daily curves of UV-A irradiance and the concentration of 3 - 5 nm particles was found throughout the year and examples will be given for two days. During the whole year the concentration of H₂O is very low at times nucleation occurs compared to the average of the corresponding month. Especially in June and July many non-event days with high solar irradiance show high amounts of water molecules. To combine these results a "nucleation parameter" was calculated for the year 1999, by dividing UV-A solar radiation by the concentration of H₂O and temperature. Throughout the year nearly all nucleation event days reach a value of the "nucleation parameter" of at least 5.4 x 10^-25 W m molecules^-1 K^-1.  Non-event days with high values (> 2.7 x 10^-25 W m molecules^-1 K^-1) are mostly accompanied by high concentrations of existing particles.