Using measurements of the aerosol charging state in determination of the particle growth rate and the proportion of ion-induced nucleation
1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
2Department of Physics, P.O. Box 64, 00014, University of Helsinki, Finland
3School of Physical and Chemical Sciences, North-West University, Private Bag x6001, Potchefstroom 2520, South Africa
4Finnish Meteorological Institute, Kuopio Unit, and University of Eastern Finland, Department of Applied Physics, P.O. Box 1627, 70211 Kuopio, Finland
*now at: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, B3H 3J5, Canada and at Environment Canada, Downsview, Toronto, M3H 5T4, Canada
Abstract. The fraction of charged nucleation mode particles as a function of particle diameter depends on the particle growth rate and the proportion of particles formed via ion-induced nucleation. In this study we have tested the applicability of recent data analysis methods to determine the growth rate and the proportion of ion-induced nucleation from the measured charged fractions. For this purpose we have conducted a series of aerosol dynamic simulations covering a wide range of atmospheric conditions. The growth rate and initial fraction of charged particles were estimated from simulated data using these methods and compared with the values obtained directly from the simulations. We found that the data analysis methods used in this study should not be used when the nuclei growth rate is less than ~3 nm h−1, or when charged particles grow much more rapidly than neutral ones. Furthermore, we found that the difference in removal rates of neutral and charged particles should be taken into account when estimating the proportion of ion-induced nucleation. Neglecting the higher removal rate of charged particles compared with that of neutral ones could result in an underestimation of the proportion of ion-induced nucleation by up to a factor of 2. This underestimation is further increased if charged particles grow more rapidly than neutral ones. We also provided a simple way of assessing whether these methods are suitable for analyzing data measured under specific conditions. The assessment procedure was illustrated using a few examples of actual measurement sites with a more detailed examination of the typical conditions observed at the SMEAR II station in Hyytiälä, Finland.