Physical, chemical and optical properties of aerosol particles collected over Cape Town during winter haze episodes

Abstract

Airborne measurements were conducted in the winter months of July and August 2003 over the metropolitan area of Cape Town to characterize physical, chemical and optical properties of aerosol particles during intense brown haze episodes. Particles were collected on highly temporally and spatially resolved samples and investigated using a high-resolution scanning electron microscope (SEM). From morphology and elemental composition, particles were categorized in terms of seven groups: aggregated soot particles, mineral dust, sulphates (SO42-), sea-salt, tar balls/fly ash, rod-shaped particles associated with soot agglomerates, and those that could not be attributed to any of these groups. Refractive indices of aerosols were derived from chemical distributions obtained from SEM analysis and combined with in situ measurements of number-size distributions to determine optical properties of dry particles in the size range 0.1-3.5 μm. Particles exhibited marked spatial and temporal variability in chemical composition. They were externally mixed with highly absorbing soot particles. From number concentrations, light extinction and absorption coefficient ranges were σap = 19-755 Mm-1 and σap = 7-103 Mm-1, respectively (at wavelength λ = 550 nm). Single scattering albedo, ω0, varied from 0.61 to 0.87 with a mean value of 0.72 ± 0.08; this value was much lower than generally reported in the literature, a resuit that was attributed to high concentrations of highly absorbing soot (fractional number concentrations of up to 46% were observed in the SEM). The haze could be attributed to extinction of light by fine aerosols composed mainly of anthropogenic particles. High extinction coefficients and low single scattering albedo computed here demonstrate quantitatively the contributions of particulate matter to visibility reduction and the brown haze phenomenon in Cape Town.