1Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101, Finland
2Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
3Fine Particle and Aerosol Technology Laboratory, Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
4School of Physical and Chemical Sciences, North-West University, Potchefstroom, South Africa
Abstract. Two or three consecutive daytime nucleation events followed by subsequent growth were observed in a polluted industrial and moderately-polluted rural environment in South Africa on 108 and 31 days, respectively, based on two years of measurement at each site. In order to describe and understand these observations we analysed particle size distribution data together with air mass back-trajectories, forecasts of the atmospheric mixing layer evolution as well as trace gas and meteorological data. After rejecting days having notably changing air mass origin and wind directions, we were able to investigate selected nucleation events affected by temporal changes in air from specific origins. As a result of our analysis, we propose that the first nucleation and growth event of the day was driven by mixing of a residual layer rich with SO2, oxidized to sulphuric acid, into the surface coupled boundary layer. The second event of the day started usually before midday and was sometimes associated with SO2 emissions from the industrial origin. However, our analysis indicates that also vapours other than sulphuric acid, most likely oxidation products of biogenic organic vapours, seem to be required to initiate and sustain especially the second event. We demonstrate that analysis of multiple nucleation events during a single day offers a new way to investigate the factors driving atmospheric nucleation.