Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 5 2006 10.5194/acpd-6-9297-2006 http://www.atmos-chem-phys-discuss.net/6/9297/2006/ http://www.atmos-chem-phys-discuss.net/6/9297/2006/acpd-6-9297-2006.html http://www.atmos-chem-phys-discuss.net/6/9297/2006/acpd-6-9297-2006.pdf 9297 9314 2006-09-26 Waterfalls as sources of small charged aerosol particles L. Laakso A. Hirsikko T. Grönholm M. Kulmala A. Luts T.-E. Parts Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland Institute of Environmental Physics, University of Tarto, U¨ likooli 18, 50090, Tarto, Estonia In this study, we measured (to our knowledge, for the first time) the mobility distributions of cluster and intermediate ions with an ion spectrometer near a waterfall. We observed that the concentration of negative 1.5–10 nm ions was one-hundred fold compared to a reference point 100 m away from the waterfall. Also the concentration of positive intermediate ions was found to be higher by a factor of ten compared to the reference point. The increased concentration of negative intermediate ions is assumed to be due to the so-called waterfall effect. In the waterfall effect, autoionization causes free charges inside water droplets. These charges fluctuate and cause surface protrusions which produce free ions when a droplet collides with an obstacle. Differences between the polarities are supposed to be caused by the different interaction volumes of positive and negative ions and the formation of magical clusters in water. In addition to the waterfall effect, we assume that some of the ions of both polarities are formed in droplet breakup due to uneven distribution of charges inside the droplet.