Atmospheric Black Carbon Loadings and Sources over Eastern Sub-Saharan Africa Are Governed by the Regional Savanna Fires

Abstract

Vast black carbon (BC) emissions from sub-Saharan Africa are perceived to warm the regional climate, impact rainfall patterns, and impair human respiratory health. However, the magnitudes of these perturbations are illconstrained, largely due to limited ground-based observations and uncertainties in emissions from different sources. This paper reports multiyear concentrations of BC and other key PM 2.5 aerosol constituents from the Rwanda Climate Observatory, serving as a regional receptor site. We find a strong seasonal cycle for all investigated chemical species, where the maxima coincide with large-scale upwind savanna fires. BC concentrations show notable interannual variability, with no clear long-term trend. The Δ 14 C and δ 13 C signatures of BC unambiguously show highly elevated biomass burning contributions, up to 93 ± 3%, with a clear and strong savanna burning imprint. We further observe a near-equal contribution from C3 and C4 plants, irrespective of air mass source region or season. In addition, the study provides improved relative emission factors of key aerosol components, organic carbon (OC), K + , and NO 3 − , in savanna-fires-influenced background atmosphere. Altogether, we report quantitative source constraints on Eastern Africa BC emissions, with implications for parameterization of satellite fire and bottom-up emission inventories as well as regional climate and chemical transport modeling.

Black carbon aerosols over Indian Ocean have unique source fingerprint and optical characteristics during monsoon season

“…Plumes from such have been reported to disperse eastbound over the southern Indian Ocean (“rivers of smoke”) ( 23 ). A clear seasonal pattern (July to November) of pollution originating from biomass-burning plumes located in Africa and Madagascar has been established by both remote and in-situ observations of trace gases such as by measurements at the Reunion Island, southwest Indian Ocean ( 24 , 25 , 26 ). During the boreal summer when the intertropical convergence zone is at its northernmost point, it is conceivable that this pollution is intercepted and recorded by the measurements achieved at MCOH.…”

Sources and Long-term Variability of Carbon Monoxide at Mount Kenya and in Nairobi

“…The elevated summertime (June -August) CO mole fractions are linked to the arrival of south-easterly airmasses, coinciding with large-scale savanna fires in southern Africa and Madagascar. The air masses shift north-easterly during winter (December -March), and coincide with savanna fires in northern Africa (Andersson et al, 2020;Kirago et al, 2022a). Although the intercepted air masses do not directly flow over West-Central Sub-Saharan Africa, where most fires occur, the atmospheric residence time of CO is sufficient for regional and intercontinental mixing.…”

Quantifying the Light-Absorption Properties and Molecular Composition of Brown Carbon Aerosol from Sub-Saharan African Biomass Combustion

“…Brown carbon (BrC), a subset of light-absorbing organic carbon in CA, is a critically important yet poorly understood component of BB aerosols emitted from solid fuels during smoldering combustion in both natural and residential sectors. − The positive direct radiative forcing of BB-derived BrC aerosols in subtropical Africa varies considerably, yet it potentially matches the cooling effect of nonabsorbing organic aerosols (OAs) . Common reasons for negative model bias in the CA warming effect in southern/sub-Saharan Africa are the neglect of light absorption by BrC aerosols − and the scarcity of observational BrC data. − In addition, BrC aerosol degrades visibility and CA poses human health risks upon inhalation, , including increase in noncommunicable diseases …”

“… 14 Common reasons for negative model bias 15 in the CA warming effect in southern/sub-Saharan Africa are the neglect of light absorption by BrC aerosols 16 − 18 and the scarcity of observational BrC data. 19 − 22 In addition, BrC aerosol degrades visibility and CA poses human health risks upon inhalation, 23 , 24 including increase in noncommunicable diseases. 25 …”