Effects of humidity on aerosols in southern Africa during the biomass burning season

[1] Measurements of the aerosol total light scattering coefficient, sigma(sp), and the aerosol hemispheric backscatter ratio, beta(1), as functions of relative humidity (RH in %) (so-called humidographs) at three wavelengths (450, 550, and 700 nm) were obtained in the dry season in southern Africa (1) in ambient air in South Africa, Botswana, Mozambique and Zambia, (2) at various distances downwind in identifiable smoke plumes from biomass fires, and (3) along the southwest coast of Africa. The ratio of sigma(sp) at an RH of 80% to that at 30% provides a measure of the effects of RH on sigma(sp), and similarly for beta(1). For the three broad sampling categories given above, and at a wavelength of 550 nm, these ratios varied from 1.42 +/- 0.05 to 2.07 +/- 0.03 for sigma(sp), and from 0.69 +/- 0.05 to 0.99 +/- 0.14 for beta(1). In general, humidographs for the ambient air samples showed a greater dependence on RH than those for smoke from identifiable biomass fires. During a period when dense, aged smoke was transported to the region from the north, the humidographs for sigma(sp) for ambient air samples were similar to those for identifiable smoke plume samples just similar to10- to 50-min old. This suggests that as far as the effects of RH on sigma(sp) are concerned the effects of aging of smoke and its mixing with ambient air are realized within less than about an hour.