Source apportionment of ambient PM2.5 using positive matrix factorization (PMF) model in Cape Town, South Africa

Abstract

Background and Aim: Particulate matter (PM) with an aerodynamic diameter <2.5µm (PM2.5) is an indicator of air pollution that threatens global population health. In 2017, exposure to PM2.5 was attributed to 2.9 million premature deaths and 83 million DALYs globally. The dearth of data on PM2.5 sources and quantification presents a significant gap in knowledge for Africa and South Africa in particular. This study identified the sources and their contributions to ambient PM2.5 in Cape Town, South Africa. Methods: PM2.5 samples were collected every third day for 121 days in an urban background site in the Kraaifontein suburb in Cape Town. Filters were analyzed for black carbon, organic carbon, and trace elements using Model OT21 Optical Transmissometer and a Xepos 5 Energy-dispersive x-ray fluorescence (EDXRF) spectrometer, respectively. Concentration and uncertainty data for PM2.5, BC, OC, and trace elements (S, Cl, Si, K, Ca, Ti, V, Fe, Ni, Cu, Zn, As, Se, Br, Sr, Sb, Ba, Pb, U) were used in the PMF, version 5.0 from the US Environmental Protection Agency to identify sources by markers. Results: The mean PM2.5 concentration was 13.4 ± 8.2 µg/m3 (1.2 to 39.1µg/m3) above the stipulated WHO air quality guideline of 5µg/m3. The PM2.5 mean seasonal concentration were 17.37µg/m3 (spring), 16.1 µg/m3 (winter), 11.25 µg/m3 (autumn) and 9.12 µg/m3 (summer). Sulfur had the highest mean concentration of 346.95ng/m3 followed by chlorine (288.21ng/m3) then silicon (204.52ng/m3). Uranium had the lowest mean concentration of 0.62ng/m3. Seven sources of PM2.5 were quantified including point source emission (24.6%), biomass burning (22.3%), traffic emission (16.8%), secondary sulphate (13.1 %), sea salt (9.3%), diesel combustion (8.4%) and crustal sources (5.5%). Conclusion: The main PM2.5 emitters are combustion sources. Therefore abatement strategies should focus on improving the combustion processes. Keywords: Air pollution, PM2.5, Trace elements, Source apportionment, South Africa