Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 1 2 2001 10.5194/acpd-1-239-2001 http://www.atmos-chem-phys-discuss.net/1/239/2001/ http://www.atmos-chem-phys-discuss.net/1/239/2001/acpd-1-239-2001.html http://www.atmos-chem-phys-discuss.net/1/239/2001/acpd-1-239-2001.pdf 239 276 2001-10-08 Nucleation events in the continental boundary layer: Influence of physical and meteorological parameters M. Boy M. Kulmala Dept. of Physics, University of Helsinki, P.O. Box 64, FIN-00014, UHEL, Finland 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<br> 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&ndash;5 nm particles was found throughout the year and examples will be given for two days. During the whole year the concentration of H<sub>2</sub>O is very low at times nucleation occurs compared to the average of the corresponding month. Especially<br> 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 H<sub>2</sub>O and temperature and for clarity all values of the "nucleation parameter" have been divided by the maximum value of the year. Throughout the year nearly all nucleation event days reach a value of 0.2, which means that at this time the "nucleation parameter" reaches 20% of its yearly maximum and non-event days with high values (&gt; 0.1) are mostly accompanied by high concentrations of existing particles.