Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 2 2006 10.5194/acpd-6-3135-2006 http://www.atmos-chem-phys-discuss.net/6/3135/2006/ http://www.atmos-chem-phys-discuss.net/6/3135/2006/acpd-6-3135-2006.html http://www.atmos-chem-phys-discuss.net/6/3135/2006/acpd-6-3135-2006.pdf 3135 3174 2006-04-18 Factors of air ion balance in a coniferous forest according to measurements in Hyytiälä, Finland H. Tammet U. Hõrrak L. Laakso M. Kulmala Institute of Environmental Physics, University of Tartu, Ülikooli 18, 50090, Tartu, Estonia Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00014, Finland A new theoretical model includes dry deposition of ions onto the forest canopy and takes into account different parameters of positive and negative ions. Explicit algebraic solution of the air ion balance equations allows calculating the ionization rate and the average charge of aerosol particles according to air ion and aerosol measurements, and parameters of forest. The transformation of direct measurements to the values of the ionization rate does not bring along amplification of measurement errors. The model was used to estimate the ionization rate at the Hyytiälä forest station, Finland, and it solved the controversy of different estimates in the earlier study. The ionization rate during one-day measurements proved to be 5.6±0.8 cm^−3 s^−1 at the standard measuring height of 2 m and 3.9±0.2 cm^−3 s^−1 at the height of 14 m between the tops of the trees. The height variation should be considered when the ionization rate is used as a parameter in models of ion-induced nucleation. The recombination sink of cluster ions on the ions of opposite polarity made up 9–13%, the sink on aerosol particles 65–69%, and the sink on forest canopy 18–26% of the total sink of cluster ions. The average lifetime of cluster ions was about 130 s for positive ions and about 110 s for negative ions. About 70% of the space charge of the air was carried by aerosol particles at the height of 2 m and about 84% at the height of 14 m.