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The relationship between nucleation events and numerous physical andmeteorological parameters was analysed using data collected at the Stationfor 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 detailedanalysis of data were completed by examining parameters in order to determine the physical andmeteorological conditions favourable to the formation of new particles. A comparison of different wavelength bands during the bursts of new particlesled to the suggestion, that UV-A solar radiation seems to be the most probable radiation band concerning the photochemical reactions involved inthe production of condensable vapours. Furthermore a high correlation betweenthe 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 twodays. During the whole year the concentration of H2O is very low at timesnucleation occurs compared to the average of the corresponding month. Especially in June and July many non-event days with high solarirradiance show high amounts of water molecules. To combine these results a"nucleation parameter" was calculated for the year 1999, by dividing UV-A solar radiationby the concentration of H2O and temperature. Throughout the year nearlyall nucleation event days reach a value of the "nucleation parameter" of atleast 5.4 x 10-25 W m molecules-1 K-1. Non-event days with high values (> 2.7 x10-25 W m molecules-1 K-1) are mostly accompanied by high concentrations of existing particles

Nucleation events in the continental boundary layer: Influence of physical and meteorological parameters