Six-year observations of aerosol optical properties at a southern African grassland savannah site

Abstract

Atmospheric aerosols have a significant effect on earth's radiative budget, particularly on regional scales. This paper presents a ~6 year, in situ, ground level aerosol scattering and absorption dataset, measured at a background site strategically positioned to enable differentiation of the effect of anthropogenic, population density and open biomass burning activities on a regional scale. Relatively well-defined seasonal and diurnal patterns were observed for all the aerosol optical properties, i.e. scattering coefficient (σSP), absorption coefficient (σAP), single scattering albedo (ω0) and Ångström exponent of scattering (αSP). These patterns were explained by considering southern African specific sources and metrological conditions. Using a receptor modelling method (auto-generated source maps) it was found that air masses that had higher σSP, σAP and ω0, and lower αSP, if compared with the relatively clean background, passed over source regions with significant industrial or other anthropogenic activities, higher population density, re-circulation of polluted air masses and higher open biomass burning frequency. To quantify differences, four source regions were defined, i.e. Karoo, Kalahari, anti-cyclonic recirculation pattern and the industrial hub of South Africa. Air masses that had passed over the Karoo source region represented the cleanest regional background conditions, while air masses that had passed either over the industrial hub and/or the anti-cyclonic recirculation pattern represented the most significant anthropogenically impacted, as indicated by the aerosol optical properties. The ω0 medians of air masses that had passed over the Karoo (0.80–0.86) were 9, 12 and 7% lower than in air masses that had passed over source regions with the highest ω0 median, in the warmest/wettest, coldest, and driest, peak open biomass burning periods, respectively