Seasonal variations of trace gases, meteorological parameters, and formation of aerosols in boreal forests

The data from observations made in the Hyytiälä background station over the period 1997–2003 were analyzed in order to investigate the possible links between physical, chemical and meteorological parameters for event and nonevent days during different seasons. The seasonal patterns of average trace gas (O3, H2O, NOx = NO + NO2, SO2) concentrations, UV-A irradiation intensity, temperature, relative humidity and condensation sink were established and used for evaluating their influence on the seasonal dependence of the formation and growth rates of particles in the nucleation mode. For the forest region at Hyytiälä, an estimate was made on the contribution of the oxidation products of SO2 and volatile organic compounds (VOCs) by O3 and OH to the formation and growth of the nucleation mode particles. The condensation sink was shown to play a crucial role in the chemical dynamics of gaseous precursors and aerosol particles. The variability of the sink was attributed mainly to the humidification of the pre-existing aerosol particle population, depending thus strongly on the relative humidity. Our results indicate that the observed growth rate of new particles is connected with condensation of secondary organic compounds originating from the oxidation of terpenes.