Abstract
ABSTRACT Light‑absorbing aerosols, particularly black carbon (BC), have significant impacts on human health and the climate. They are also the least‑studied fraction of atmospheric particles, particularly in residential areas of southern Africa. The optical characteristics of ground‑based light‑absorbing aerosols from Kwadela Township in South Africa are investigated in this study. Daily averaged ambient PM2.5 highest levels were 51.39 µg m‑3 and 32.18 µg m‑3, whereas hourly averages peaked at 61.31 µg m‑3 and 34.69 µg m‑3 during winter and summer, respectively. Levels of daily averaged light–absorbing aerosols were 2.9 times higher (1.89 ± 0.5 μg m‑3) in winter 2014 than in summer 2015 (0.66 ± 0.2 μg m‑3). In both seasons, hourly averaged levels showed bimodal diurnal cycles, which correlated with the PM2.5 diurnal patterns that indicated distinct peaks in the morning and evening. These diurnal cycle peak periods corresponded with the times of increased solid domestic fuel usage, road traffic, and also shallower boundary layer. On average, light‑absorbing aerosols contributed a larger proportion of total ambient PM2.5 levels in winter (6.5 ± 1.0 %) than in summer (3.4 ± 1.0 %). The winter average Absorption Angstrӧm exponent AAE(370/880 nm) (1.7± 0.5), indicated the dominance of brown carbon (BrC) from biofuel/biomass burning and/or low‑quality coal combustion emissions. In summer, the average AAE(370/950 nm) (1.3 ± 0.7), suggested the presence of BC and BrC in the mornings and evenings possibly from fossil fuel combustion sources. At midday and at night in summer, the AAE was close to 1, suggesting more BC contributions from sources such as diesel emissions during this time. A combination of BC and BrC particulates dominated on 50 % and 5 % of the summer days, respectively, whereas fresh BC were only measured in summer days (23 %). Residential solid‑fuel and/biomass combustion are important sources of light‑absorbing aerosols in this study region, with concomitant human health and environmental impacts.
Highlights
Residential solid-fuel burning has been increasingly highlighted as an important source of atmospheric pollution
Averaged ambient PM2.5 maximum levels in winter and summer were 51.39 μg m–3 and 32.18 μg m–3, respectively, whereas hourly averaged mass concentrations peaked at 61.31 μg m–3 and 34.69 μg m–3 during winter and summer, respectively
Results from the measured light-absorbing aerosols indicated that the black carbon (BC) proportion correlated more with fine particles (PM2.5) than with PM10, and were similar to the results reported by Feig et al (2015)
Summary
Residential solid-fuel burning has been increasingly highlighted as an important source of atmospheric pollution. The importance of domestic fuel combustion to aerosol mass loading was recognized by Maritz et al (2015) who found that BC and organic particles can contribute up to 12% and 24% to total particulate matter, respectively, at five South African sites in the Deposition of Biogeochemical Important Trace Species-IGAC DEBITS in Africa (DEBITS-IDAF) network. There is limited information on absorption wavelength dependence ( in South Africa), a parameter that is key for estimation absorbing aerosol radiative impacts on regional climate and understanding the sources of light-absorbing aerosols (Praveen et al, 2012; Backman et al, 2014). This study examines light-absorbing aerosols from residential solid-fuel combustion in South Africa, adding novel findings to the understanding of this important particle source in this region. Part of improving aerosol characterization includes understanding aerosol types and sources, those that are strongly influenced by human activities (Cazorla et al, 2013)
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