Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 3 2008 10.5194/acp-8-655-2008 http://www.atmos-chem-phys.net/8/655/2008/ http://www.atmos-chem-phys.net/8/655/2008/acp-8-655-2008.html http://www.atmos-chem-phys.net/8/655/2008/acp-8-655-2008.pdf 655 675 2008-02-12 Variation and balance of positive air ion concentrations in a boreal forest U. Hõrrak urmas.horrak@ut.ee P. P. Aalto J. Salm K. Komsaare H. Tammet J. M. Mäkelä L. Laakso M. Kulmala Institute of Physics, University of Tartu, 18 Ülikooli St., 50090 Tartu, Estonia Department of Physical Sciences, Division of Atmospheric Sciences P.O. Box 64, 00014 University of Helsinki, Finland Tampere University of Technology, Institute of Physics, P.O. Box 692, 33101, Tampere, Finland Air ions are characterized on the basis of measurements carried out in a boreal forest at the Hyytiälä SMEAR station, Finland, during the BIOFOR III campaign in spring 1999. The air ions were discriminated as small ions (charged molecular aggregates of the diameter of less than 2.5 nm), intermediate ions (charged aerosol particles of the diameter of 2.5–8 nm), and large ions (charged aerosol particles of the diameter of 8–20 nm). Statistical characteristics of the ion concentrations and the parameters of ion balance in the atmosphere are presented separately for the nucleation event days and non-event days. In the steady state, the ionization rate is balanced with the loss of small ions, which is expressed as the product of the small ion concentration and the ion sink rate. The widely known sinks of small ions are the recombination with small ions of opposite polarity and attachment to aerosol particles. The dependence of small ion concentration on the concentration of aerosol particles was investigated applying a model of the bipolar diffusion charging of particles by small ions. When the periods of relative humidity above 95% and wind speed less than 0.6 m s^−1 were excluded, then the small ion concentration and the theoretically calculated small ion sink rate were closely negatively correlated (correlation coefficient −87%). However, an extra ion loss term of the same magnitude as the ion loss onto aerosol particles is needed for a quantitative explanation of the observations. This term is presumably due to the small ion deposition on coniferous forest. The hygroscopic growth correction of the measured aerosol particle size distributions was also found to be necessary for the proper estimation of the ion sink rate. In the case of nucleation burst events, the concentration of small positive ions followed the general balance equation, no extra ion loss in addition to the deposition on coniferous forest was detected, and the hypothesis of the conversion of ions into particles in the process of ion-induced nucleation was not proved. 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