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
*currently at: Forschungszentrum Jülich GmbH, Institut für Energie-und Klimaforschung: Troposphäre (IEK-8), Jülich, Germany
Abstract. Recent studies have shown very high frequencies of atmospheric new particle formation in different environments in South Africa. Our aim here was to investigate the causes for two or three consecutive daytime nucleation events, followed by subsequent particle growth during the same day. We analysed 108 and 31 such days observed in a polluted industrial and moderately polluted rural environments, respectively, in South Africa. The analysis was based on two years of measurements at each site. After rejecting the days having notable changes in the air mass origin or local wind direction, i.e. two major reasons for observed multiple nucleation events, we were able to investigate other factors causing this phenomenon. Clouds were present during, or in between most of the analysed multiple particle formation events. Therefore, some of these events may have been single events, interrupted somehow by the presence of clouds. From further analysis, we propose that the first nucleation and growth event of the day was often associated with the mixing of a residual air layer rich in SO2 (oxidized to sulphuric acid) into the shallow surface-coupled layer. The second nucleation and growth event of the day usually started before midday and was sometimes associated with renewed SO2 emissions from industrial origin. However, it was also evident that vapours other than sulphuric acid were required for the particle growth during both events. This was especially the case when two simultaneously growing particle modes were observed. Based on our analysis, we conclude that the relative contributions of estimated H2SO4 and other vapours on the first and second nucleation and growth events of the day varied from day to day, depending on anthropogenic and natural emissions, as well as atmospheric conditions.