1Finnish Meteorological Institute, P.O. Box 64, 00014 Helsinki, Finland
2School of Physical and Chemical Sciences, North-West University, Potchefstroom, South Africa
3Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
4Department of Physics, North-West University, Private Bag X 2046, Mmabatho, South Africa
5Rustenburg Local Municipality, Rustenburg, Republic of South Africa
6School of Earth and Environment, University of Leeds, United Kingdom
Abstract. The South African savannah region is complex environment of air pollution and natural emissions influenced by a strong seasonal cycle in biomass burning and strong precipitation. However, the scarcity of long-term observations means that our knowledge of controlling aerosol processes in this environment is very poor. Here we use a new dataset of 18 months of aerosol observations to understand the factors that control aerosol properties, and in particular cloud condensation nuclei. We find that biomass burning produces a strong source of primary CCN-sized particles during the dry winter season. However, measured CCN-sized particle concentrations remain high during the wet summer season despite the lack of burning and high wet removal rates. We show that during the wet season, a substantial fraction of CCN-sized particles originate from boundary layer new particle formation, whereas primary sources dominate during the dry winter season. The large contribution of boundary layer nucleation to CCN concentrations during the wet season is found to be due to high particle formation and growth rates and low pre-existing particle concentration in the beginning of particle formation. Based on the estimated seasonal cycle of condensable sulphuric acid and organic vapours, higher growth rates during the wet season are attributed to vapours of biogenic origin. Global model results for this region have the same seasonal cycle in nuclei growth rates but the opposite cycle in particle formation rates, and both rates are much lower than observed. In contrast, the same model tends to capture the seasonal cycle in particle concentrations at many other global sites where nucleation is an important process. These results point to deficiencies in our understanding of biogenic emissions and the factors controlling nucleation and growth in such dynamic environments.