Observations on Aging of Biomass Burning Aerosols in Southern Africa. VILLE VAKKARI, Veli-Matti Kerminen, Johan Paul Beukes, Petri Tiitta, Pieter G. van Zyl, Miroslav Josipovic, Andrew D. Venter, Kerneels Jaars, Douglas Worsnop, Markku Kulmala, Lauri Laakso, Finnish Meteorological Institute, Helsinki, Finland