Low-level urban anthropogenic sources contribute to strong aerosol light-absorption on the South African Highveld

This study investigated the chemical composition and shortwave absorption coefficient, βabs(λ), of aerosols collected from sites on the Highveld, a major industrial and highly polluted region of South Africa. Local anthropogenic mineral dust was found to be the dominant chemical component, accounting for (53 ± 14)% of the aerosol mass concentrations. Carbonaceous aerosols (34 ± 12%), mainly from domestic and waste biomass burning, and secondary inorganic aerosols (13 ± 6%) from anthropogenic combustion sources were also found. Industrial emissions made limited contributions. High β abs (λ) was observed at all sites, with an estimated mass absorption efficiency, MAE(λ), from (1296 ± 472)∙10 −3 m 2 .g −1 at 375 nm to (621 ± 239)∙10 −3 m 2 .g −1 at 850 nm. The contributions of the primary light-absorbing aerosols to β abs (λ) were determined using chemical tracers. Black carbon (BC) was the major contributor to β abs (λ) at all wavelengths (> 60%), with an MAE of ~8 m 2 .g −1 at 850 nm, in agreement with the literature. Local anthropogenic mineral dust appeared to be more light-absorbing than pure desert dust and was a significant contributor to β abs (λ), constant with wavelength at ~16%. Finally, the brown carbon contribution decreased with λ from ~20% at 375 nm to ~3% at 532 nm. The levels of light-absorbing aerosols identified have implications for the radiation budget and atmospheric stability. Although BC dominates light-absorption on the South African Highveld, mineral dust contributes significantly to aerosol mass concentrations and enhanced light-absorption.