Long-term assessment of aerosol chemical composition in the interior of South Africa

PhD (Science with Atmospheric Chemistry), North-West University, Potchefstroom Campus, 2019

The impacts of atmospheric particulate matter (PM) are co-determined by chemical composition. PM with aerodynamic diameter ≤ 2.5 μm (PM2.5) typically contains a significant fraction of organic- (OC), elemental carbon (EC), water-soluble inorganic ions and organic acids (OA), which can affect climate, air quality, human health, acid deposition and visibility. Exceedances of PM with aerodynamic diameter ≤ 10 μm (PM10) associated standard limits are frequent in many regions in the South African interior (SAAQIS, 2018). However, very little data have been published regarding PM’s chemical composition. This research presents a multi-year aerosol dataset for South Africa, as determined at two regional background sites (Skukuza and Louis Trichardt, i.e. SK and LT), and two sites that are more directly impacted by nearby industrial emissions (Vaal Triangle and Amersfoort, i.e. VT and AF) operated within the Deposition of Biogeochemical Important Trace Species (DEBITS) project. 24-hour PM2.5 and PM10 aerosol samples were collected on quartz and Teflon filters, once a month from March 2009 to December 2015, at each of the four sites. The quartz filters were analysed on a Desert Research Institute (DRI) analyser 2001 Model and a Sunset OCEC Dual Optical Lab Instrument (Version 6.4) for OC and EC contents, while the Teflon filters were analysed with an ion chromatograph (IC) to obtain the water-soluble inorganic and OA contents. Results indicated that the mass fractions of organic carbon (OC) and elemental carbon (EC) at all four sites were lower than what is typical within a developed world context; not due to lower concentrations, but due to the larger fractional contributions from especially sulphate (SO42-). Open biomass burning was found to contribute to elevated OC and EC levels on a regional scale. However, household combustion for space heating in semi- and informal settlements made a substantial local (e.g. at VT) and noticeable regional (e.g. at AF and SK) OC and EC contribution. Additionally, industrial and/or vehicle emissions contribute to the baseline OC and EC levels year-round, while oxidation of volatile organic compounds (VOCs) during the wet season will also contribute to OC levels. The highest concentration of water-soluble ions was reported for the PM2.5 size fraction, and in this size fraction SO42- had the highest concentrations, followed by OA. Spatial assessment showed that the VT had the highest SO42- and NH4+ concentrations, followed by AF, SK and LT, while the NO3- concentrations were the highest at VT, followed by SK, AF and LT. NH4+ was found to be the most probable cation to neutralise the acidic ions in the PM2.5 size fraction at all the sites. Back trajectories, diagonal correlation graphs, principal component analysis (PCA), calculations of sea-salt fractions (SSFs) and non-sea-salt fractions (nSSFs), as well as other empirical calculations were used to determine possible source contributions from marine, terrigenous, fossil fuel use, biomass burning, NH4+-associated and OA-associated sources. At all sites in the PM2.5 size fraction, fossil fuel use, NH4+-associated and terrigenous sources contributed most to the water-soluble ion and OA content. However, the fractional marine influence at SK and LT were higher than at the other sites. Aerosol mass closure indicated that organic matter (OM), derived from the OC mass, was the most significant contribution to the PM2.5 aerosol mass percentage at all sites, with SO42- making the second largest contribution at all sites

National Research Foundation (NRF)

Doctoral