Characterizing light-absorbing aerosols from residential solid fuel combustion in Mpumalanga, South Africa

Abstract

Light-absorbing aerosols are an important fraction of atmospheric particulates that have large impacts on human health and the climate system. There is still limited information available on this type of aerosol-fraction in South Africa. This dissertation outlines the optical characteristics of ground-based absorbing-particulates from residential solid-fuel combustion in Kwadela township measured in winter 2014 and summer 2015. A dual and multiple aethalometers were used to measure the concentrations of light-absorbing aerosols at 5-minute time intervals. Ambient light-absorbing aerosols winter daily average mass concentrations were 2.9 times higher (1.89 (±0.5) μg/m³) than the summer levels (0.66 (±0.2) μg/m³). Hourly-averaged mass concentrations indicated a strong bimodal diurnal pattern with maxima in the morning (04h00-09h00) and evening (15h00-21h00). The proportion of absorbing aerosols to PM2.5 mass was 6.5(±1.0)% and 3.4(±1.0)% in winter and summer, correspondingly, indicating the dominance of fine absorbing particulates particularly in winter. The absorption Ångström exponent (AAE370/950) was 1.3(±0.7) during summer. The diurnal averaged AAE(370/950) indicated a strong bimodal pattern with two maxima (AAE(370/950)~1.5) suggesting the presence of black carbon (BC) and brown carbon (BrC) and according to previous literature, this AAE value (~1.5) is attributed to solid-fuel combustion-generated particulates. Therefore the analysis from absorption wavelength dependence suggested that domestic fuel combustion is one of the important sources of absorbing aerosol mass loading. Traffic-related BC was suggested by low AAE(370/950)(~1.1) and wind patterns pronounced at midday and at midnight when local fuel burning is minimal. On average, 73% of summer days were estimated to be influenced by internally-mixed BC aerosols which dominated at peak hours. These particles can have 30% higher absorption which can contribute 44% to the regional radiative forcing compared to uncoated BC particulates. It is believed that the results from the current study would provide useful information in understanding light−absorbing aerosols and thus further improve assessment of aerosol−related human health and environmental impacts on a local scale.