Size-resolved chemical characterisation of aerosols in low-income settlements in South Africa and their health-related effects

Abstract

The impacts of naturally and anthropogenically emitted aerosols, which are determined by their physical and chemical properties, are related to climate change and air quality. Health related impacts (e.g. respiratory and cardiovascular disease mortality) associated with exposure to atmospheric particulate matter (PM) are globally considered important and are still not completely understood. An important source of atmospheric particulates in South Africa is household combustion for space heating and cooking, which predominantly occurs in low-income urban settlements. Therefore, the main aims of this study were to conduct a size-resolved chemical characterisation of aerosols in low-income urban settlements in the north-eastern interior of South Africa through the collection of outdoor (ambient) and indoor particulates in order to quantify the extent of the impacts of atmospheric pollution within these low-income urban settlements, as well as assess the potential impacts of PM on health through determining their oxidative potential (OP) as a proxy of toxicity. Outdoor and indoor aerosols in different size fractions were collected through summer and winter sampling campaigns conducted at four low-income urban settlements located in the highly industrialised and densely populated north-eastern interior of South Africa. Indoor particulates had the highest mass concentrations, and higher aerosol mass concentrations were measured for samples collected during winter. The PM1 size fraction had the highest mass concentrations in all outdoor aerosol samples collected during winter and summer, as well as in indoor samples collected during summer. The highest aerosol mass concentration was, however, determined in the PM2.5-10 size fraction of aerosols sampled during the winter indoor campaign. Significantly higher concentrations were determined for SO4-2 outdoor and indoor particulates, with NH4+ and NO3- being the second most abundant. SO4-2 and NH4+ almost exclusively occurred in the PM1 size fraction, while NO3- was the major constituent in the larger size fractions. The highest SO4-2 levels were recorded for the winter and summer outdoor campaigns conducted at Zamdela, while higher NO3- and NH4+ concentrations were determined for the winter outdoor campaigns. The highest total trace element concentrations were determined for aerosols collected during the indoor campaign, while total trace element levels in PM1 were substantially higher than levels thereof in the two larger size fractions of particulates collected during all sampling campaigns. No distinct seasonal trend was observed for trace element concentrations. Na, Ca and Cr had the highest concentrations in particulates collected during respective sampling campaigns. Ni concentrations in outdoor and indoor aerosols exceeded the annual average European standard,

with indoor Ni levels being an order of magnitude higher than the levels determined within proximity of large pyrometallurgical smelters. PM1 collected during all sampling campaigns in low-income urban settlements had the highest OC and EC concentrations, while the highest OC and EC levels were determined in PM1 collected during the winter indoor campaign. OC and EC levels revealed significantly higher concentrations during winter, which can be attributed to changes in meteorological patterns and increased biomass burning. Low OC/EC ratios determined for particulates collected in low-income urban settlements are indicative of OC and EC being mainly associated with local sources of these species. OC concentrations determined in this study were an order of magnitude lower than OC concentrations determined for ambient aerosols collected in other areas of the north-eastern interior of South Africa (e.g. Vaal Triangle), while similar EC levels were measured. Briefly, dust was estimated to be the major constituent in all size ranges of particulates collected in this study, while trace element species were the second most abundant. However, trace elements made the highest contribution to indoor PM1 and PM1-2.5 mass. No clear seasonal pattern was observed in the chemical composition of particulates. Mass concentrations and chemical concentrations determined for aerosols collected in low-income urban settlements reflect the regional impacts of anthropogenic sources in the north-eastern interior of South Africa, as well as the influence of local sources. In this study, the dithiothreitol (DTT) assay was used to quantify OP, as it is the most commonly used technique due to its relative simplicity, cost efficiently and reproducibility. The first step was to modify the DTT methodology based on previous applications, which entailed the choice of extraction procedure and setup. Then, the redox activity of samples collected in three low-income urban settlements i.e. Jouberton, Kwazamokuhle and Zamdela was evaluated in different size fractions and was related to their chemical composition through correlation analysis. Furthermore, an attempt was made to analyse seasonal variations of DTT redox activity normalised on PM mass (DTTm) and sampled volume (DTTv) for outdoor and indoor environments. The results indicated higher redox activity per PM mass and per sampled volume basis on the finest (<1 μm) particles compared to the finer/coarser particles (1 – 10 μm) for both outdoor and indoor environments. DTT redox activity of PM was highly correlated with elemental (EC) and organic carbon (OC), as well as overall trace elements and water-soluble inorganic species, mainly in the PM1-10 particle fraction, for both outdoor and indoor samples. Possible atmospheric aerosol emission sources suggested from these correlations include domestic- and biomass burning, vehicular and industrial emissions, as well as secondary aerosol formation (e.g. sulphate) from other combustion sources.

The results highlighted that industrial activities in South Africa have a significant influence on aerosol chemical composition and human health at both outdoor and indoor sites in this study. Health of people living in these study locations are at risk as they are exposed to air pollutants from domestic- and open biomass burning, vehicle and industrial emissions and dust from unpaved roads. It is therefore imperative that legislators introduce measures to reduce emissions of particulate species in the low-income urban settlements.