Source profiling, source apportionment and cluster transport analysis to identify the sources of PM and the origin of air masses to an industrialised rural area in Limpopo

Abstract

The Greater Tubatse Municipality in Limpopo is home to three ferrochrome smelters and over fifteen operational mines which are mining chromium, platinum or silica. Source apportionment in this study was performed by combining air mass back trajectories and receptor modelling. The particulate matter (PM) samples at six sites were collected using the University of North Carolina Passive Samplers. The monthly samples were collected for a period of 4-5 weeks, except for August-September and September-October 2015 where the samples were collected for up to 6 weeks. The sampling was carried out from July 2015 to June 2016. PM chemical analysis was performed using Computer Controlled Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (CCSEM-EDS). The PM chemical analysis indicated the presence of elements such as carbon (C), calcium (Ca), chromium (Cr), iron (Fe), aluminium (Al), silicon (Si), magnesium (Mg) and lead (Pb). All the six sites except site 1 exceeded the WHO annual guidelines for PM₁₀ concentration of 20 μg/m³. The annual chromium concentrations exceeded the New Zealand limits of 0.0001 μg/m³ and 0.11μg/m³ Cr (VI) and Cr (III), respectively. The back trajectory clusters computed by the HYSPLIT model identified 5 transport clusters for each site. The main transport patterns were northerly to north-easterly, easterly to south-easterly, and south-westerly to north-westerly. The US EPA PMF model version 5.0 used in source profiling and source apportionment identified agriculture/wood combustion, coal combustion, crustal/road dust, ferrochrome smelters, and vehicle emissions as the main sources in the area. The source contributions varied across all sites indicating the existence of different microenvironments within the airshed and that the pollution can originate from either local or regional sources as indicated by back trajectory clusters.