[2] Recent observations have provided impressive evidences that the particle formation processes and their further growth can be detected in different geographic locations and in absolutely diverse conditions. Long term data set on particle formation at a continental forest site (Hyytiälä, Southern Finland) display a strong seasonal variation on the number of occurrences of these events with a maximum during spring time and a minimum in midsummer. Annual variations of the particle growth rate during nucleation events in forest and rural locations also show a strong seasonal trend [Kulmala et al., 2004] with maximal and minimal particle growth rate 4 -17 nm/h (summer time) and 0.5 - 2 nm/h (winter time) respectively.
[3] The particle formation in the atmosphere is a dynamic process. The combination of chemical, physical, and meteorological processes define the production rate of non-volatile substances that can then either nucleate or condense onto the surfaces of foreign particles. Because the chemical composition of newly born particles with the sizes 3 - 10 nm is still unknown, an information on the mechanisms, the sources, and the actual conditions of the particle formation can be only derived from the studies on correlations of various factors and the attempts of their subsequent classification.
[4] The major goal of this presentation is to analyze the diurnal variations in different seasons and the long-term behavior of the trace gases concentrations (SO2, NO, NOx ), ozone level, relative and absolute humidity, and the meteorological parameters for examining the influence of these seasonal dependence on the aerosol concentration, the formation and growth rates of the particles in the nucleation mode.
[5] Our analysis uses the data set on the parameters obtained during the period 1996 - 2003 at the Hyytiälä station. The station is located in the extended area of Scots pines dominant forests with the conditions typical for a background location. The hour average values of the studied concentrations were calculated for different seasons of each year and for the whole period of measurements. The average concentrations of trace gases, SO2, NOx, water vapor, ozone, and short wavelength irradiance UV-A (320-400 nm) as well as all necessary meteorological parameters were determined as averages over different periods for all days of observations and separately, for event and nonevent days.
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