ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-1-239-2001 Nucleation events in the continental boundary layer: Influence of physical and meteorological parameters Boy M. 1 Kulmala M. 1 Dept. of Physics, University of Helsinki, P.O. Box 64, FIN-00014, UHEL, Finland 08 10 2001 1 2 239 276 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/1/239/2001/acpd-1-239-2001.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/1/239/2001/acpd-1-239-2001.pdf

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.