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

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 Hyytiala, 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 standard deviations 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 nonevent 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.