Aerosol Chemical Composition and Sources over East Africa’s Background Atmosphere

Abstract

Aerosol emissions from sub-Saharan Africa are perceived to influence the regional climate, impact precipitation patterns, and impair human respiratory health. However, the magnitudes of these perturbations are ill-constrained, largely due to limited ground-based observations and uncertainties in emissions from different sources. Of particular concern are black carbon, a climate-warming agent and a more toxic component of atmospheric aerosols. To investigate the fine participate chemical composition and sources, we collected filter samples from the Rwanda climate observatory - a strategically located regional background site at the summit of Mt. Mugogo. The filter samples were analyzed for carbonaceous aerosols and water-soluble inorganic species. In addition, multi-year high-temporal resolution BC data was retrieved from an AE33 Aethalometer. Overall, we find a strong seasonal cycle for all investigated chemical species, where the maxima coincide with large-scale upwind savanna fires. The Aethalometer BC data show notable inter-annual variability with no clear long-term trend. For source apportionment, mass ratios and highly precise isotopic techniques unambiguously show highly elevated biomass burning contributions, up to 93 ± 3%, with a clear and strong savanna burning imprint. The study further provide improved relative emission factors of key aerosol components in savanna-fires-influenced background atmosphere. Taken together, this study provides quantitative source constraints on Eastern Africa aerosol emissions, with implications for parameterization of satellite fire and bottom-up emission inventories as well as regional climate and chemical transport modelling.