Articles | Volume 12, issue 9
Atmos. Chem. Phys., 12, 3951–3967, 2012
Atmos. Chem. Phys., 12, 3951–3967, 2012

Research article 04 May 2012

Research article | 04 May 2012

Characterisation of sub-micron particle number concentrations and formation events in the western Bushveld Igneous Complex, South Africa

A. Hirsikko1,2, V. Vakkari2, P. Tiitta3,4, H. E. Manninen2, S. Gagné2,*,**, H. Laakso2, M. Kulmala2, A. Mirme5, S. Mirme5, D. Mabaso6, J. P. Beukes4, and L. Laakso1,4 A. Hirsikko et al.
  • 1Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101, Finland
  • 2Department of Physics, University of Helsinki, P.O. Box 64, University of Helsinki, 00014, Finland
  • 3University of Eastern Finland, PL 1627, 70211, Kuopio, Finland
  • 4School of Physical and Chemical Sciences, North-West University, Potchefstroom, South Africa
  • 5Institute of Physics, University of Tartu, 18 Ülikooli Str., 50090 Tartu, Estonia
  • 6Rustenburg Local Municipality, Rustenburg, South Africa
  • *now at: Environment Canada, Toronto, Ontario, M3H5T4, Canada
  • **now at: Dalhousie University, Halifax, Nova Scotia, B3H3J5, Canada

Abstract. South Africa holds significant mineral resources, with a substantial fraction of these reserves occurring and being processed in a large geological structure termed the Bushveld Igneous Complex (BIC). The area is also highly populated by informal, semi-formal and formal residential developments. However, knowledge of air quality and research related to the atmosphere is still very limited in the area. In order to investigate the characteristics and processes affecting sub-micron particle number concentrations and formation events, air ion and aerosol particle size distributions and number concentrations, together with meteorological parameters, trace gases and particulate matter (PM) were measured for over two years at Marikana in the heart of the western BIC. The observations showed that trace gas (i.e. SO2, NOx, CO) and black carbon concentrations were relatively high, but in general within the limits of local air quality standards. The area was characterised by very high condensation sink due to background aerosol particles, PM10 and O3 concentration. The results indicated that high amounts of Aitken and accumulation mode particles originated from domestic burning for heating and cooking in the morning and evening, while during daytime SO2-based nucleation followed by the growth by condensation of vapours from industrial, residential and natural sources was the most probable source for large number concentrations of nucleation and Aitken mode particles. Nucleation event day frequency was extremely high, i.e. 86% of the analysed days, which to the knowledge of the authors is the highest frequency ever reported. The air mass back trajectory and wind direction analyses showed that the secondary particle formation was influenced both by local and regional pollution and vapour sources. Therefore, our observation of the annual cycle and magnitude of the particle formation and growth rates during nucleation events were similar to results previously published for a semi-clean savannah site in South Africa.

Final-revised paper