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 Ångstrӧm
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.
